Bluff body wakes

Local field correction particle image velocimetry (LFCPIV), which was first presented in 1997, is the only correlation PIV method able to resolve flow structures smaller than the interrogation window. It presents advantages over conventional systems and thus offers an alternative in the field of super-resolution methods. Improvements of the initial version are likely to promote its application even further. The issues defining some of these possible improvements were already indicated in the paper that originally introduced LFCPIV, but not developed. This work presents refinements and also simplifications of the technique, so that it can be applied using current algorithms of advanced correlation PIV systems. Furthermore, these refinements reduce the measurement error and enlarge the range of application of LFCPIV. In particular, the application of the system is no longer constrained to images with mean distances between particles larger than 4 pixels. Besides that, the use of interrogation windows smaller than in its previous version is evaluated. This allows multigrid LFCPIV implementations. The results show how multigrid LFCPIV can obtain better measurements than can the usual multigrid PIV, but still the refined version of the LFCPIV technique performs even better, at the expense of a larger computing time. The performance of these methods is evaluated for synthetic and real images. This includes examples in which the ability to cope with gradients in velocity, gradients in seeding density and the presence of boundaries is highlighted.

Particle image velocimetry with local field correction (LFC PIV) has been tested in the past to obtain two components of velocity in a two dimensional domain (2D2C). When compared to conventional correlation based algorithms, this advanced technique has shown improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the stereo technique, which is able to obtain three components of velocity in a plane (2D3C PIV). In this work this combination is implemented and its performance is evaluated carrying out the following two different tasks: -Comparison of robustness and accuracy for large and small scale flow structures. This is carried out using three techniques: the conventional Stereo PIV, the Stereo-LFC PIV and the Stereo-Multigrid PIV enhanced with image distortion. -Insight on the limit of resolvable scales for the Stereo-LFC. This task is relevant because the resolution attainable by this combination is higher than what has been obtained by the rest of the herein used algorithms. The first task has been achieved using synthetic images. Afterwards the coherence of the results has been checked with real images. The results show improvement of Stereo-LFC PIV in respect to Stereo-Multigrid PIV enhanced with image distortion. The performance of Stereo-LFC when only large scales are involved shows an increase of the dynamic range of measurable vorticity. When small scales are analysed, the magnitude of the error resulting when using Stereo-LFC is about half of the one obtained for the Stereo-Multigrid measurements. Results with errors below 20% have been achieved for some of the cases with peak vorticities as large as 1.8 Dt -1 (in the absence of out-ofplane displacements), out-of-plane loss of particle pairs of 65% (with a low peak vorticity of 0.06 Dt -1 ) and peak vorticities as large as 1.5 Dt -1 with 50% particle pair loss. For the second task most of the information has been obtained using real images. It has been found that the resolution limit is very dependent on the robustness of the algorithms against image defects and variability. The results show a remarkable improvement when using the Stereo-LFC PIV processing, although a full quantification and characterization would need further study because of the variety of noise sources possible in a real image.

The analysis of the internal flow structure and performance of a specific fluidic diverter actuator, previously studied by time-dependent numerical computations for subsonic flow, is extended to include operation with supersonic actuator exit velocities. The understanding will aid in the development of fluidic diverters with minimum pressure losses and advanced designs of flow control actuators. The selfinduced oscillatory behavior of the flow is successfully predicted and the calculated oscillation frequencies with respect to flow rate have excellent agreement with our experimental measurements. The oscillation frequency increases with Mach number, but its dependence on flow rate changes from subsonic to transonic to supersonic regimes. The delay time for the initiation of oscillations depends on the flow rate and the acoustic speed in the gaseous medium for subsonic flow, but is unaffected by the flow rate for supersonic conditions.

Experimental Study of the Power Profile Airfoil Equipped with Plasma Flow Control LIBIN DANIEL, JAMEY JACOB, Oklahoma State University -This presentation discusses results from an experimental study of the power profile airfoil at low Reynolds number. The power profile airfoil was developed by AMO Smith and consists of a blunt trailing edge shape with two wall jets near the trailing edge. The replacement of streamlining with properly designed blowing is used to prevent flow separation and additionally offers potential applications as a powered high-lift system, propulsive system, or low inertia control device. The 2D wind-tunnel model consists of the 22.5% thick power profile airfoil equipped with a movable trailing edge plug to direct flow along the trailing edge streamline. Compressed air was passed into the model via a plenum with flow conditioning devices to create pressure backdrop to allow uniform blowing at the trailing edge. The effects of varying jet momentum coefficient and trailing edge positioning on the aerodynamic characteristics are observed with both wake surveys and PIV. The impact of plasma synthetic jet actuators (PSJA) placed along the trailing edge of the power profile airfoil is also discussed. PSJA operation is compared to the baseline power profile airfoil both alone and working with the blowing to provide additional control authority.

Thrust Vectoring Flow Control Using Plasma Actuators JAMEY JACOB, MICHAEL BOLITHO, Oklahoma State University -Thrust vectoring flow control is demonstrated using plasma synthetic jet actuators (PSJA). The PSJA is a geometric variant of a plasma actuator, consisting of a symmetric electrode array that results in a circular region of dielectric barrier discharge plasma. Quiescent flow PIV measurements of the PSJA reveal that the flowfield on actuation resembles that of a zero-mass flux or synthetic jet that is useful for flow control, particularly separation reduction. Like synthetic jets, unsteady pulsed actuator operation results in formation of multiple vortex rings. The output jet momentum is found to be affected by the power input, actuator dimension and pulsing frequency. While increasing the input power increases the maximum jet velocity, an optimum range of pulsing frequencies and actuator dimensions are observed to exist in order to maximize jet momentum. By asymmetrically varying the plasma input parameters, such as frequency, amplitude and duty cycle, it is possible to control the jet angle. Vectoring using high frequency pusling akin to synthetic jets is more effective than vectoring by modifying steady control inputs and differences in control effectiveness are due primarily to the time scales associated with the vortex formation.

Two independent spheres were placed in a side by side arrangement and flow structure in the wake region of the spheres was investigated with a Particle Image Velocimetry (PIV) system when the spheres were in a boundary layer over a flat plate as a special case. Reynolds number was 5000 based on the sphere diameter which was 42.5 mm. Boundary layer was tripped 8mm away from the leading edge of the flat plate with a 5 mm trip wire. The thickness of the hydrodynamically developed boundary layer was determined as 63mm which was larger than the sphere diameter of D=42.5mm. Wake region of the spheres was examined from point of flow physics for the different sphere locations in the ranges of 0G/D 1.5 and 0S/D 1.5 where G and S were the distance between the spheres and the distance between the bottom point of the spheres and the flat plate surface, respectively. Depending on the different sphere locations, instantaneous and time averaged vorticity data, scalar values of time-averaged velocity components and their root mean square (rms) values and time averaged vorticity data are presented in the study for the evaluation of wake region of the spheres. It is demonstrated that the gap between the two spheres and the interaction between the gap and the boundary layer greatly affects flow pattern, especially when spheres are located near to the flat plate surface, i.e. S/D=0.1 for 0G/D 1.5. Different distances between the spheres resulted in various flow patterns as the spheres were approached to the flat plate. The distance S/D=0.1 for all gap values has the strongest effect on the wake structures. Beyond G/D=1.0, the sphere wakes tend to be similar to single sphere case. The instantaneous vorticity fields of the side by side arrangements comprised wavy structures in higher level comparing to an individual sphere case. The gap flow intensifies the occurrence of small scale eddies in the wake region. The submersion rate of the spheres actually determines the characteristics of the wake region and is affected from boundary layer flow in a gradually decreasing manner.

Smart structures sense external stimuli, process the sensed information, and respond with active control to the stimuli in real or near-real time. The response to external stimuli can be through deforming or deflecting the structure, or through communicating the sensed information to another control center. Smart materials enact a deformation or deflection of the structure by changing their physical properties when subject to either an electric, magnetic or thermal load. Multifunctional structures (MFS), in addition to supporting loads, incorporate sensors to detect and evaluate loads or failure, and to interact with the surrounding electromagnetic environment. MFS represent a new manufacturing and integration technology by which communications and electronics equipment are integrated into conformal load-bearing structures. The technology is enabled by advances in large-scale integrated electronics packaging, lightweight composite structures and high-conductivity materials. In multifunctional structures, electronic assemblies (multichip modules), miniature sensors and actuators are embedded into load-carrying structures, along with associated cabling for power and data transmission. This level of integration effectively eliminates traditional boards, boxes, large connectors, bulky cables, and thermal base plates, thereby yielding major weight, volume and cost savings, over traditional structural and electronic packaging techniques. The Aeronautics Enterprise works closely with it customers and partners to ensure that its technology, products and services are developed to levels where customers can confidently make decisions regarding the application of those technologies. This strategy allows the enterprise to support the national aeronautics goals: Maintain the superiority of U.S. aircraft and engines; Improve safety, efficiency and affordability of the global air transportation system; Ensure the long-term environmental compatibility of the aviation system; and Achieve aircraft-like operations and costs for launch systems. Research programs are in place to develop technology that provides benefits in the following areas: Performance a Efficiency and affordability Survivability Safety and security Capacity and efficiency Environment, and Cost per pound to place payload into low-earth orbit. System Benefit Framework for the National Goals National Goals Maintain the S ' ' of U.S. Aircraft an NASA R&T BASE LEAD ROLES To take advantage of the unique capabilities that exist at each of the research centers, the Aeronautics Research and ~e c h n o l o ~~ Base consists of six programs: Airframe Systems Aviation Operations Systems Flight Research Information Technology Propulsion Systems, and Rotorcraft. Each element is led where the center of gravity of relevant expertise exists. The Airframe Systems Program is led at Langley Research Center. Airframe systems are those systems that determine or characterize the performance of an aircraft, including airframe-propulsion system integration. Key elements of airframe systems include the following: Conceptual design Aerodynamic design, development, and testing Structural design, development, and testing (including airframe materials) Flight crew station design, integration, and testing, and Airborne systems (including flight mechanics, controls, electromagnetics, and flight-crucial systems) design, integration, and testing. The Agency Center of Excellence for Structures and Materials, one of the key elements of airframe systems, is located at Langley Research Center. ASP0 LEVEL-HI PROGUMS Airframe systems technology development for some specific vehicle classes are addressed in other Aeronautics Enterprise programs. The Advanced Subsonic Technology (AST) Program has focused technology efforts for general aviation, civil tiltrotor, and subsonic transport aircraft. In addition, Ames Research Center leads the Research and Technology Base portion of the Rotorcraft Research Program. The High-Speed Research (HSR) Program is developing technologies that will allow the decision on the feasibility of building an economically viable supersonic civil transport aircraft. Reusable launch vehicle (WV) technology is being developed in the X-33 Program. While technology developments in the focused programs are aimed at relatively nearterm applications, the Airframe Systems Program has the following characteristics: Breakthrough technologies High-rismigh-payoff * Revolutionary Expanding Frontiers * Pioneering research for the twenty-first century Requires paradigm shifts A sound investment in America's aeronautics future, and Complemented by focused programs. The Airframe Systems Program Office (ASPO) has planned a set of innovative Level-I1 research programs that are focused along advanced conceptual (systems) studies, transport aircraft, high-performance aircraft, and fundamental aeronautical concepts and methods. These programs are shown in the figure. In order to assure that the research planning and implementation is responsive to customers' needs, the management of the Airframe Systems Program centers around vehicle classes and long-term aeronautics research needs. A Systems Studies and Analysis element provides independent assessments, high-payoff research and concepts, and data for deciding investment strategies. The Level-I1 programs are managed within the Aeronautics Systgms Analysis Division, the Civil Transportation Office, the High-Performance Aircraft Office, and the Fundamental Concepts and Methods Office. The Rotorcraft, Airspace Operations, and Information Systems managers serve as the Langley points of contact for those programs. The figure shows how the Airframe Systems Level-I1 programs align with the national goals. This slide presents a brief description of the following Level-I1 programs: Megaliner Error-Proof Flight and Aircraft Electronic Systems * Advanced Life Extension (ALEX) Integral Airframe Structures (L4S) Alternate Civil Capacity, Environment, Efficiency, and Safety Solutions (ACCESS), and Advanced Subsonic Transport Aircraft Research (ASTAR). This presentation contains details about the following programs: Megaliner ALEX * IAS. These programs involve a significant amount of structures research. Airframe Systems Programs v Revolutionary subsonic long-haul transports Error-Proof Error-Proof flight deck and mission critical systems Systems (Error-Proof) Advanced Life Airworthiness of thick, metallic structural Extension (ALEX) components typical of wing structure, in the aging commercial transport fleet Integral Airframe Low cost large, integral, metallic structures Structures (IAS) Alternate Civil Capacity, Cooperative activities with industry or other Environment, Efficiency, government agencies 81 Safety Solutions (ACCESS)

of motivation, accomplishments, and future plans This two-month visit at CTR was devoted to investigating possibilities in LES modeling in the context of the 3-D vortex particle method (=vortex element method, VEM) for unbounded flows. A dedicated code was developed for that purpose. Although O(N _) and thus slow, it offers the advantage that it can easily be modified to try out many ideas on problems involving up to N ,_ 104 particles. Energy spectrums (which require O(N 2) operations per wavenumber) are also computed. Progress was realized in the following areas: particle redistribution schemes, relaxation schemes to maintain the solenoidal condition on the particle vorticity field, simple LES models and their VEM extension, possible new avenues in LES. Model problems that involve strong interaction between vortex tubes were computed, together with diagnostics: total vorticity, linear and angular impulse, energy and energy spectrum, enstrophy. More work is needed, however, especially regarding relaxation schemes and further validation and development of LES models for VEM. + :: ()) vol',,_t__ V' ¢ ' (7)

numerical investigations were unable to fully capture all flow regimes using the k-ω model (Guilminineau and Queutey 2003). It is observed that the SST model is able to cover all three branches of the flow-induced oscillation of the cylinder. Based on the numerical results of the SST model, the maximum obtained theoretical power coefficient of the VIV power is evaluated to be approximately 10% for a single cylinder.

The Gradient Scheme framework provides a unified analysis setting for many different families of numerical methods for diffusion equations. We show in this paper that the Gradient Scheme framework can be adapted to elasticity equations, and provides error estimates for linear elasticity and convergence results for non-linear elasticity. We also establish that several classical and modern numerical methods for elasticity are embedded in the Gradient Scheme framework, which allows us to obtain convergence results for these methods in cases where the solution does not satisfy the full H 2 -regularity or for non-linear models.

numerical investigations were unable to fully capture all flow regimes using the k-ω model (Guilminineau and Queutey 2003). It is observed that the SST model is able to cover all three branches of the flow-induced oscillation of the cylinder. Based on the numerical results of the SST model, the maximum obtained theoretical power coefficient of the VIV power is evaluated to be approximately 10% for a single cylinder.

The methods employed by Martin [1971] and Chandna et al. [1982] to steady plane flows have been applied to the plane viscous flows in a porous medium using the Darcy - Brinkman - Lapwood equation. Various flows and corresponding geometries have been investigated.

Significant advancements have been made in the experimental investigation of drag reduction techniques for bluff bodies. This study explores various methods, including active and passive flow control strategies, surface modifications, and innovative aerodynamic shapes, aimed at minimizing aerodynamic drag. Extensive wind tunnel testing and computational fluid dynamics (CFD) simulations were conducted to analyze the effects of these techniques on drag reduction. The results demonstrated that specific surface modifications, such as vortex generators and riblets, effectively delayed flow separation and reduced drag coefficients. Additionally, active flow control methods, such as synthetic jets and plasma actuators, showed promise in dynamically altering flow characteristics to achieve drag reduction. The findings of this research provide valuable insights for the design and optimization of bluff bodies in automotive, aerospace, and civil engineering applications, ultimately contributing to improved energy efficiency and performance.

In the state-of-the-art PIV, image sensors are growing in size and PIV algorithms are increasing in spatial resolution capabilities. These technological advances allow for the simultaneous measurement of increasingly larger spatial scales ranges in a single PIV snapshot. To take fully advantage of the available spatial scales range, PIV setup constraints establish a coupling between the laser sheet thickness and the time between laser pulses. This condition did not apply with older technologies. The resulting value for the optimum laser sheet thickness is in a range where the superposed flow information in the direction of the imaging sensor introduces special characteristics errors at the small scale limit. This may compromise the measurement of flow quantities like dissipation and turbulent kinetic energy (tke), for which the contribution of the small and medium scales may be relevant. Within this scenario the assessment of the smallest scale resolved in the PIV measurement is nec...

Multigrid particle image velocimetry (PIV) is an open path in the search for high-resolution PIV methods. It is based on an iterative scheme that uses the information of initial processing to adapt the method parameters in order to improve the measurements. This is mainly performed by reducing the size of the interrogation windows and shifting them. In multigrid PIV, two sources of error can significantly affect the final measurement quality: (1) the error coming from the amplitude response of the initial large interrogation windows to spatial frequencies; (2) the error originating from the truncation of particles at the borders of the final small interrogation windows. By applying weighting functions and using symmetric direct correlation both errors can be reduced, respectively. These techniques have been separately tested in the past, but a joint implementation has not yet been analysed. This task is fulfilled and both sources of error are further clarified. For this purpose, a one-dimensional single wavelength displacement field is used. This gives us the opportunity to analyse the non-linear behaviour of PIV, together with the influence of basic parameters on it. In addition to this, the multigrid method, so far described, is enhanced by compensation of the particle pattern deformation. The metrological performance of this advanced method is tested using synthetic images and the results are compared with those delivered by established PIV methods. Coherence between these results and those obtained in a real image is also detailed.

Conventional particle image velocimetry (PIV) processing techniques based on correlation show a limited range of application when they are used to describe a concentrated vorticity field. This work focuses on the evaluation of these range limits. In this frame, the application of an advanced PIV technique, namely Local Field Correction Particle Image Velocimetry (LFCPIV) is also evaluated, showing a significant performance enhancement. The magnitude of the measurement error is obtained for conventional and advanced PIV by means of synthetic images of an analytical vortex flow. This gives information on the behaviour to be expected when operating beyond the classical limits of applicability of conventional PIV algorithms. The study of relevant effects, like group-locking, allows for further understanding the sources of the measurement error. The knowledge of these characteristics helps in taking decisions regarding the experimental setup as well as the kind of PIV method to use for a certain application. Real PIV images obtained by partners within Europiv 2 are also analysed and the results commented.

Particle image velocimetry (PIV) is now applied with confidence in industrial facilities such as large wind tunnels, yielding data not possible before. Despite the difficulties that arise in such environments, the conventional PIV methods can provide high quality data, especially when dealing with spatial and temporal slowly varying values of the flow magnitudes. Obtaining highly spatially resolved velocity fields is still challenging due to the inherent difficulties in these industrial facilities, such as large velocity gradients, background light and reflections. This work has focused on: (i) the behaviour of conventional PIV when the conventional limits of the processing algorithm are approached or surpassed and (ii) which kind of advanced methods can reduce the main sources of error that are characterized in this paper. The focus is on the description of vortex flows. Group locking, a major source of error, is introduced, modelled and metrologically characterized. The part of the study devoted to advanced methods deals with one that has already shown to be of profit in images from industrial facilities, local field correction PIV (LFC-PIV). This is a robust and an accurate method, able to obtain a high yield of measurements in these environments, without the need of external adjustment to each particular situation. To illustrate this point, some examples of processing of real images are given. The conclusions of this work suggest guidelines about the error figures when measuring flows with embedded vortex using conventional and advanced methods in industrial facilities.

Particle Image Velocimetry with Local Field Correction (LFC-PIV) has been tested in the past to obtain two components of the velocity in a two dimensional domain (2D2C). When compared with conventional correlation based algorithms, this advanced technique has showed improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the Stereo technique, able to obtain three components of the velocity (2D3C PIV). To successfully apply stereoscopy, the laser sheet width is usually larger than in 2C velocimetry, in order to acquire the out of plane displacement of the particles. This peculiarity can make questionable the need for improved spatial resolution. In particular, if some factors concur the measurement of small scale features of the velocity along the laser sheet can be spoiled by the velocity variations across it, caused by the flow structures. Being the spatial resolution capabilities of LFC its main advantage, this paper focuses in establishing if there are advantages left for LFC-PIV in situations where only large scale features are of interest, thus making it difficult for this technique to show its attractiveness. The remaining benefits still to be analyzed are the robustness and the ability to cope with large displacement gradients. For both aspects, the performances of 2D3C-LFC-PIV are explored. These performances are characterized and compared with conventional and other advanced algorithms, when applied to synthetic PIV images. In addition, the coherence between these results and those coming from experimental PIV images is presented and discussed.

Two independent spheres were placed in a side by side arrangement and flow structure in the wake region of the spheres was investigated with a Particle Image Velocimetry (PIV) system when the spheres were in a boundary layer over a flat plate as a special case. Reynolds number was 5000 based on the sphere diameter which was 42.5 mm. Boundary layer was tripped 8mm away from the leading edge of the flat plate with a 5 mm trip wire. The thickness of the hydrodynamically developed boundary layer was determined as 63mm which was larger than the sphere diameter of D=42.5mm. Wake region of the spheres was examined from point of flow physics for the different sphere locations in the ranges of 0G/D 1.5 and 0S/D 1.5 where G and S were the distance between the spheres and the distance between the bottom point of the spheres and the flat plate surface, respectively. Depending on the different sphere locations, instantaneous and time averaged vorticity data, scalar values of time-averaged velocity components and their root mean square (rms) values and time averaged vorticity data are presented in the study for the evaluation of wake region of the spheres. It is demonstrated that the gap between the two spheres and the interaction between the gap and the boundary layer greatly affects flow pattern, especially when spheres are located near to the flat plate surface, i.e. S/D=0.1 for 0G/D 1.5. Different distances between the spheres resulted in various flow patterns as the spheres were approached to the flat plate. The distance S/D=0.1 for all gap values has the strongest effect on the wake structures. Beyond G/D=1.0, the sphere wakes tend to be similar to single sphere case. The instantaneous vorticity fields of the side by side arrangements comprised wavy structures in higher level comparing to an individual sphere case. The gap flow intensifies the occurrence of small scale eddies in the wake region. The submersion rate of the spheres actually determines the characteristics of the wake region and is affected from boundary layer flow in a gradually decreasing manner.

Motivated by growing demands for aircraft noise reduction and for revolutionary new aerovehicle concepts, the late twentieth century witnessed the beginning of a shift from singlediscipline research, toward an increased emphasis on harnessing the potential of flow and noise control as implemented in a more fully integrated, multidisciplinary framework. At the same time, technologies for developing radically new aerovehicles, which promise quantum leap benefits in cost, safety and performance benefits with environmental friendliness, have appeared on the horizon. Transitioning new technologies to commercial applications will also require coupling further advances in traditional areas of aeronautics with intelligent exploitation of nontraditional and interdisciplinary technologies. Physics-based modeling and simulation are crucial enabling capabilities for synergistic linkage of flow and noise control. In these very fundamental ways, flow and noise control are being driven to be more closely linked during the early design phases of a vehicle concept for optimal and mutual noise and performance benefits.

Direct numerical simulations of spatially developing turbulent boundary layers over riblets are conducted to examine the effects of riblets on skin friction at supersonic speeds. Zero-pressure gradient boundary layers with an adiabatic wall, a Mach number of M∞ = 2.5, and a Reynolds number based on momentum thickness of Re θ = 1720 are considered. Simulations are conducted for boundary-layer flows over a clean surface and symmetric Vgroove riblets with nominal spacings of 20 and 40 wall units. The DNS results confirm the few existing experimental observations and show that a drag reduction of approximately 7% is achieved for riblets with proper spacing. The influence of riblets on turbulence statistics is analyzed in detail with an emphasis on identifying the differences, if any, between the drag reduction mechanisms for incompressible and high-speed boundary layers.

We consider flow of dry granular matter down an inclined chute with a localized contraction. Measurements and analysis show that changes in particle volume fraction are important, especially across granular bores. For fixed upstream conditions and depending on the nozzle width of the contraction, we observe either small oblique jumps, a reservoir with a steady jump, or a reservoir with an upstream traveling bore. Shallow layer theory extended to include porosity changes qualitatively predicts these regimes. Implications for volcanic debris flows are discussed.

This study investigates the local and global flow structures and mass transfer characteristics for a group of interacting side-by-side cylinders in unbounded flow. Configurations with 2, 3, 4, and 5 members are considered for a range of pitch-ratios (1.05≤S/D≤4) at the Reynolds number Re = 90. The focus is laid on the time-averaged and instantaneous local flow features including wake field, jet flow, vortical structures, λ2, pressure coefficient, and mass transfer coefficient as well as integral variables including hydrodynamic forces. Four flow regimes are identified based on the vortical structures and average stream-wise velocity field. At low pitch-ratio, the whole structure behaves like a single bluff body, while each member in the configuration behaves like an independent isolated bluff body when the pitch-ratio is large. Between these two regimes, asymmetrically and symmetrically deflected wake regimes are observed. Flow regimes dictate hydrodynamic and mass transfer characte...

This work is devoted to the numerical solution of the three dimensional (3D) compressible Navier-Stokes system on unstructured mesh. The numerical simulation are performed using a fractional step method treating separately the convection and the diffusion parts. The discretization is done by the finite volume method for the two parts. The convective flux is computed by the Godunov method and a new scheme is presented for the treatment of the diffusive flux. The technique is illustrated by solving various numerical problems.

We analyse numerical errors (dissipation and dispersion) introduced by the discretisation of inviscid and viscous terms in energy stable discontinuous Galerkin methods. First, we analyse these methods using a linear von Neumann analysis (for a linear advection-diffusion equation) to characterise their properties in wave-number space. Second, we validate these observations using the 3D Taylor-Green Vortex Navier-Stokes problem to assess transitional/turbulent flows. We show that the dissipation introduced by upwind Riemann solvers affects primarily high wave-numbers. This dissipation may be increased, through a penalty parameter, until a critical value. However, further augmentation of this parameter leads to a decrease of dissipation, reaching zero for very large values. Regarding the dissipation introduced by second order derivatives, we show that this dissipa- tion acts at low and medium wave-numbers (lower wave-numbers compared to upwind Riemann solvers). In addition, we analyse ...

The direct numerical simulation of the flow over a sphere is performed. The computations are carried out in the sub-critical regime at Re = 3700 (based on the free-stream velocity and the sphere diameter). A parallel unstructured symmetry-preserving formulation is used for simulating the flow. At this Reynolds number, flow separates laminarly near the equator of the sphere and transition to turbulence occurs in the separated shear layer. The vortices formed are shed at a large-scale frequency, St = 0.215, and at random azimuthal locations in the shear layer, giving a helical-like appearance to the wake. The main features of the flow including the power spectra of a set of selected monitoring probes at different positions in the wake of the sphere are described and discussed in detail. In addition, a large number of turbulence statistics are computed and compared with previous experimental and numerical data at comparable Reynolds numbers. Particular attention is devoted to assessing...

In this paper a flexible finite element computational tool developed to investigate fluid-structure interaction applications in two dimensions is briefly described. We consider problems which can be modelled as a viscous incompressible fluid flow and a rigid body-spring system interacting nonlinearly with each other. The coupling is dealt using an interface approach, in which each physics involved is solved with different schemes and the required information are passed through the interface of both system. This approach is, at least in principle, very flexible and computationally efficient as the best available scheme can be adopted to solve each physics. Here, a stabilized FEM considering the "ALE" (Arbitrary Lagrangian-Eulerian) formulation with Crank-Nicholson time-integration is employed for the fluid-dynamics analysis, and the Newmark Method is used for the structural analysis. Several important tools were incorporated into our system including different possibilities...

Traditionally turbulence modeling of industrial flows in complex geometries have been solved using RANS models and unstructured meshes based solvers. The lack of precision of RANS models in these situations and the increase of computer power, together with the emergence of new high-efficiency sparse parallel algorithms, make posible the use of more accurate turbulent models such as Large Eddy Simulation models (LES). Recently, relevant improvements on turbulence modeling based on symmetry-preserving regularization models for the convective (non-linear) term have been developed. They basically alter the convective terms to reduce the production of small scales of motion by means of vortex-stretching, preserving all inviscid invariants exactly. To do so, symmetry and conservation properties of the convective terms are exactly preserved. This requirement yields a novel class of regularizations that restrain the convective production of smaller and smaller scales of motion by means of vortex stretching in an unconditional stable manner, meaning that the velocity can not blow up in the energy-norm (in 2D also: enstrophynorm). The numerical algorithm used to solve the governing equations must preserve the symmetry and conservation properties too. At this stage, results using regularization models at relatively complex geometries and configurations are of extreme importance for further progress. The main objective of the present paper is the assessment of regularization models on unstructured meshes. To do this, three different test cases have been studied: the impinging jet flow, the flow past a circular cylinder and a simplified Ahmed car. In order to analyse the influence of the filter, the cases have been solved using the Gaussian and the Helmholtz filters. Furthermore, the performance of the model considering the influence of the grid parameters and the filter ratio are also analysed.

The global stability analysis of the two-dimensional incompressible unsteady flow past a circular clinder cascade is studied using linear stability analysis. A new numerical technique is used to find the value of critical Reynolds number that causes the flow to undergo Hopf bifurcation. An attempt has also been made to study the blockage effect on critical Reynolds number and associated Strouhal number.

This paper presents the design of an active flow control (AFC) system for commercial aircraft based on a dense wired/wireless sensor and actuator network. The goal is to track gradients of pressure across the surface of the fuselage of commercial aircraft. This collected information will be used to activate a set of actuators that will attempt to reduce the skin drag effect produced by the separation between laminar and turbulent flows. This will be translated into increased lift-off forces, higher speeds, longer ranges and reduced fuel consumption. The paper describes the architecture of the system in the context of the European research project DEWI (dependable embedded wireless infrastructure) using the concept of the DEWI Bubble. A simulator architecture is also proposed to model each process of the AFC system and the DEWI Bubble. To the best of our knowledge this is the first approach towards the use of wireless sensor technologies in the field of active flow control.

Flow-induced vibration (FIV) of a flexible cylinder with an upstream wake interference at a subcritical Reynolds number is numerically investigated in this study. Two cylinders are installed in a tandem arrangement with the tandem separation between the cylinder centers set at 5.0 diameters. The downstream cylinder is flexible and placed in the wake of the stationary rigid upstream cylinder. A quasi-three-dimensional fluid-structure interaction (FSI) numerical methodology that couples the strip theory-based Lagrangian discrete vortex method with the finite-element method (FEM) for structural dynamics is developed to simulate the FIV response of the flexible cylinder with the upstream wake interference. The vortex-induced vibration (VIV) of an identical isolated cylinder is also numerically simulated as a contrast. This numerical study characterizes the dynamic response of the cylinder FIV with the upstream wake interference and sheds light on the FSI mechanisms responsible for the s...

Fixed-vane vortex generators (VGs) have been in existence for over 50 years and are still among the most effective available flow control devices. However once such fixed VGs have been configured to improve performance in one regime, they often penalize the performance in other conditions. This paper will summarize results of an effort making VGs deployable "on demand". The technology enabling this improvement is the use of Shape Memory Alloy (SMA) based actuators to deploy Pop Up Vortex Generators (PUVG). Because of the favorable force/stroke characteristics of SMA actuators, it is possible to design PUVGs so that they retract flush to the wing surface and so have negligible aerodynamic penalty when not deployed. In addition, novel self-locking actuation devices enable deployment of PUVGs to be maintained with no power expenditure. This paper summarizes the design, construction, and demonstration of practical PUVG devices. Wind tunnel tests of a near full scale wing with an array of PUVGs are described, illustrating a substantial mitigation of flow separation and demonstrating enhanced lift as well as improved lift/drag ratio at flow conditions representative of general aviation aircraft. Nomenclature c wing chord, ft. cl max maximum lift coefficient h vortex generator device height, ft. Re x boundary layer Reynolds number, Vx V free stream speed, ft./sec. x distance along boundary surface, ft. wing angle of attack, rad. clmax angle of attack for maximum lift, rad. 0l angle of attack for zero lift, rad. boundary layer thickness, ft. kinematic viscosity, ft./sec.

This work presents a numerical study on self-induced flapping dynamics of an inverted flexible foil in uniform flow. The inverted foil considered in this study is clamped at the trailing edge and the leading edge is allowed to oscillate. A high-order coupled FSI solver based on CFEI formulation has been used to present the flapping response results for a wide range of nondimensional bending rigidity using a fixed Reynolds number of 1000 and a mass-ratio of 0.1. As a function of bending rigidity four flapping regimes have been discovered: fixed point, inverted limit-cycle oscillation, deflected flapping, and flipped flapping. The inverted foil configuration undergoes flapping motion more readily and experiences very large amplitude oscillations than the conventional foil. A wide variety of vortex wakes with a maximum of 14 vortices per oscillation cycle have been observed. The inverted limit-cycle flapping generate novel 4P+6S (14 vortices) and 2P+6S (10 vortices) wakes. On the other hand, the flipped flapping regime is characterized by a von Kármán wake. We also observe that inverted foil can extract 1000 times more energy from the surrounding fluid compared to the conventional foil

This report is a collection of documents written as part of the Laboratory Directed Research and Development (LDRD) project A Mathematical Framework for Multiscale Science and Engineering: The Variational Multiscale Method and Interscale Transfer Operators. We present developments in two categories of multiscale mathematics and analysis. The first, continuum-to-continuum (CtC) multiscale, includes problems that allow application of the same continuum model at all scales with the primary barrier to simulation being computing resources. The second, atomistic-to-continuum (AtC) multiscale, represents applications where detailed physics at the atomistic or molecular level must be simulated to resolve the small scales, but the effect on and coupling to the continuum level is frequently unclear. The local/nonlocal AtC interface issue also complicates any practical scheme for coarse-graining molecular dynamics into classical continuum mechanics (CM), for instance when the finite element method is used for the classical CM discretization. In Chapter 6 we describe a method for representing a collection of atoms at finite temperature as a peridynamic body. The PD representation

A flexible inverted flag immersed in a Poiseuille flow with heated walls was numerically modeled to investigate the dynamics of the flag and its effect on convective heat transfer. An immersed boundary method was used to analyze the interaction between the fluid and the inverted flag. This inverted flag readily becomes self-oscillating because of its configuration, in which the leading edge is free to move and the trailing edge is clamped. The inverted flag has three dynamic modes according to the characteristics of the surrounding fluid and the flag flexibility: deflected, flapping, and straight. In the flapping mode, nearly 6 pairs of vortical structures are generated in the wake of the inverted flag, which include counter vortical structures formed near the walls as well as structures generated by the interaction between the flag and the surrounding fluid. These vortical structures affect the thermal boundary layer near the walls and the temperature field in a manner that enhances the heat transfer performance of the channel flow.

Turbulent fluid flows are ubiquitous in nature and technology, and are mathematically described by the incompressible Navier-Stokes equations. A hallmark of turbulence is spontaneous generation of intense whirls, resulting from amplification of the fluid rotation-rate (vorticity) by its deformationrate (strain). This interaction, encoded in the non-linearity of Navier-Stokes equations, is non-local, i.e., depends on the entire state of the flow, constituting a serious hindrance in turbulence theory and even establishing regularity of the equations. Here, we unveil a novel aspect of this interaction, by separating strain into local and non-local contributions utilizing the Biot-Savart integral of vorticity in a sphere of radius R. Analyzing highly-resolved numerical turbulent solutions to Navier-Stokes equations, we find that when vorticity becomes very large, the local strain over small R surprisingly counteracts further amplification. This uncovered self-attenuation mechanism is further shown to be connected to local Beltramization of the flow, and could provide a direction in establishing the regularity of Navier-Stokes equations.

Design of geotechnical systems is often challenging as it requires the understanding of complex soil behaviour and its influence on field-scale performance of geo-structures. To advance the scientific knowledge and the technological development in geotechnical engineering, a Scottish academic community, named Scottish Universities Geotechnics Network (SUGN), was established in 2001, composing of eight higher education institutions. The network gathers geotechnics researchers, including experimentalists as well as centrifuge, constitutive, and numerical modellers, to generate multiple synergies for building larger collaboration and wider research dissemination in and beyond Scotland. The paper will highlight the research excellence and leading work undertaken in SUGN emphasising some of the contribution to the geotechnical research community and some of the significant research outcomes.

The correction method is developed to correct the distribution of pressure coefficient around an airfoil for the aim of the post processing of measurement data from wind tunnel. The measured pressure coefficient is numerically corrected to compensate walls interference effects for wind tunnel test section. The airfoil NACA0012, in this paper, is simulated by segments with linear vortex, which are then mirrored due to the floor and ceiling of the wind tunnel test section using sufficient number of images, to simulate the flow around an airfoil in the test section with walls. The perforated wall is approximated by constant source segments. Flow calculations, both in the free stream and with the presence of tunnel walls, are then performed. The numerically pressure coefficient difference obtained between these two cases should be superimposed to the measured pressure coefficient distribution in the wind tunnel for the same airflow conditions and corresponding points, resulting in the experimental pressure coefficient distribution corrected. With the relative height reduction of the wind tunnel test section, the correction values increase. The verification of lift coefficient corrections in this paper is concentrated, where very good agreements have been obtained comparison with several well-known analytical methods.

Feedback control is applied to symmetrize the bimodal dynamics of a turbulent blunt body wake. The flow is actuated with two lateral slit jets and monitored with pressure sensors at the rear surface. The physicsbased controller is inferred from preliminary open-loop tests and is capable of symmetrizing the wake. A slight pressure recovery is achieved due to the net balance between the favourable effect of wake symmetrization and adverse effect of shear-layer mixing and vortex shedding amplification.

This paper describes the structural design of an active flow-control experiment. The aim of the experiment is to investigate the increase in efficiency of an internally blown Coanda flap using unsteady blowing. The system uses tailor-made microelectromechanical (MEMS) pressure sensors to determine the state of the oncoming flow and an actuated lip to regulate the mass flow and velocity of a stream near a wall over the internally blown flap. Sensors and actuators are integrated into a highly loaded system that is extremely compact. The sensors are connected to a bus system that feeds the data into a real-time control system. The piezoelectric actuators using the d 33 effect at a comparable low voltage of 120 V are integrated into a lip that controls the blowout slot height. The system is designed for closed-loop control that efficiently avoids flow separation on the Coanda flap. The setup is designed for water-tunnel experiments in order to reduce the free-stream velocity and the system's control frequency by a factor of 10 compared with that in air. This paper outlines the function and verification of the system's main components and their development.

In this work, we considered the flow around two circular cylinders of equal diameter placed in tandem with respect to the incident uniform flow. The upstream cylinder was fixed and the downstream cylinder was completely free to move in the cross-stream direction, with no spring or damper attached to it. The centre-to-centre distance between the cylinders was four diameters, and the Reynolds number was varied from 100 to 645. We performed two-and three-dimensional simulations of this flow using a Spectral/hp element method to discretise the flow equations, coupled to a simple Newmark integration routine that solves the equation of the dynamics of the cylinder. The differences of the behaviours observed in the two-and three-dimensional simulations are highlighted and the data is analysed under the light of previously published experimental results obtained for higher Reynolds numbers.

In the present work, the laminar flow through a circular cylinder with two crossed splitter plates is analyzed. The characteristic-based method has been used along with the unstructured grid. The current research has been done to detect the proper conditions according to the geometrical parameters in which the optimal heat transfer is taking place. Geometric control parameters are the angle of splitter plates (\theta) and the ratio of length of the splitter plate to cylinder radius (n=L/D). It was found that the use of a two-branched splitter plate is not wise in Reynolds number less than 100 due to its insignificant effect in flow properties. In angle 30° between two plates, the least drag force is witnessed with respect to other angles. Application of double branched splitter with angles more than 60° is not recommended, which will increase the total drag significantly. Since the splitter plate increases, the overall heat transfer from the cylinder and splitter set is enhanced. Mi...

Direct and large-eddy numerical simulations have been performed for a heated sphere at Reynolds numbers 200, 300, 500, 750, 1000 and 104, in the context of the development of a spherical wind sensor for Mars atmosphere intended for missions after Rover 2020. A lowdissipation ?nite element scheme implemented in the multi-physics code Alya has been used to do so. The main heat transfer and aerodynamic parameters are presented, followed by detailed analysis of the wake and boundary layer development and the local heat transfer coe?cients. Viscous and thermal boundary layer thicknesses decrease with the Reynolds number, the thermal one being thicker than the viscous layer. However, in all cases shape factor is the same in the zone with the favourable pressure gradient. The local Nusselt number is found to be asymmetric in the rear zone of the sphere for the laminar cases and recover its statistical symmetry once the wake transition to turbulent ?ow. It is also found that the stagnation ...

Accurate prediction of laminar-turbulent transition is a critical element of computational fluid dynamics simulations for aerodynamic design across multiple flow regimes. Traditional methods of transition prediction cannot be easily extended to flow configurations where the transition process depends on a large set of parameters. In comparison, neural network methods allow higher dimensional input features to be considered without compromising the efficiency and accuracy of the traditional data-driven models. Neural network methods proposed earlier follow a cumbersome methodology of predicting instability growth rates over a broad range of frequencies, which are then processed to obtain the N-factor envelope, and then, the transition location based on the correlating N-factor. This paper presents an end-to-end transition model based on a recurrent neural network, which sequentially processes the mean boundary-layer profiles along the surface of the aerodynamic body to directly predi...

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence Newcastle University ePrints-eprint.ncl.ac.uk Assi GRS, Srinil N, Freire CM, Korkischko I. Experimental investigation of the flow-induced vibration of a curved cylinder in convex and concave configurations.

This study aims at clarifying the factors that cause transmission line galloping and the conditions influencing it, and at determining response evaluation indices of gallopings in turbulent flows. This is done using a three-dimensional analytical method considering a large deformation. The analysis is based on four-bundle transmission lines. Obtained results are as follows: 1) The occurrence of galloping in the smooth flow is limited by the combination of the following parameters: the initial angle of wind attack, the initial icing angle, and the wind speed. The galloping predominates mainly with one or two of the lowest in-plane, out-of-plane, and torsional modes for the free vibration under the conditions that the transmission line is subject to dead load as well as static wind force. However, the galloping always occurs with torsional vibration. 2) The shape of the Lissajous figure for displacement depends on the initial angle of wind attack and the initial icing angle, as well as wind speed. The main shapes are vertically elliptic, horizontally elliptic, and a configuration having the shape of a horizontally rotated figure of eight. 3) The predominant frequency components of gallopings in turbulent flows are amplified and controlled by the turbulence intensity. Vibration frequency components unrelated to galloping increase linearly with rise in turbulence intensity. 4) There is a time lag of 30 s between galloping vibration and the fluctuating wind speed. The relationships between mean wind speeds and both trend components and standard deviations of galloping in turbulent flows closely correspond to those relationships during the smooth flow, and they can be obtained using the average time of 10 times the shortest vibration period of the transmission line. That is, the response values of transmission lines in the smooth flow can be utilized to estimate gallopings in turbulent flows. To estimate the maximum amplitude of a galloping, a peak factor of approximately 2.5 can be used.

Over the last few years, superhydrophobic (SH) surfaces have been receiving an increasing attention in many scientific areas by virtue of their ability to enhance flow slip past solid walls and reduce the skin-friction drag. In the present study, a global linear-stability analysis is employed to investigate the influence of the SH-induced slip velocity on the primary instability of the 2D flow past a circular cylinder. The flow regions playing the role of 'wavemaker' are identified by considering the structural sensitivity of the unstable mode, thus highlighting the effect of slip on the global instability of the considered flow. In addition, a sensitivity analysis to slip-induced base-flow modifications is performed, revealing which areas of the cylinder surface provide a stabilising/destabilising effect when treated with a SH coating.