Aerodynamics

Wing-In-Ground Effect (WIG) Craft (Russian ekranoplans) offers a new transport solution somewhere between boat and aircraft and may revolutionize the future fast sea transportation in terms of its characteristics. Aerodynamic characteristics of WIG craft near curved ground are investigated. Computational fluids dynamics (CFD) method is applied into the study due to limitation of wind tunnel test in simulating curved ground. Numerical techniques including sliding meshing and dynamic meshing are established and compared in the study to analyze effect of curved ground on WIG craft. Aerodynamics and flow features of WIG craft with and without course angles in cruise are taken into consideration in the current work. The study will hopefully clarify the understanding of the aerodynamics of WIG craft.

The longitudinal stability characteristics of a wing-in-ground effect (WIG) craft are quite different from that of conventional airplane due to the existence of ground effect. The effects of design parameters including wing section, stabiliser, wing planform and endplate on longitudinal static stability of WIG craft were studied based on numerical results. Contributions of these parameters on stabilising the WIG craft by shifting the aerodynamic centres in pitch and height were discussed respectively. The present study resulted in deep understanding of relationship between longitudinal static stability and WIG craft configuration, provided references for designers and operators of WIG craft theoretically.

The paper presents an unsteady high order Discontinuous Galerkin (DG) solver that has been developed, verified and validated for the solution of the two-dimensional incompressible Navier-Stokes equations. A second order stiffly stable method is used to discretise the equations in time. Spatial discretisation is accomplished using a modal DG approach, in which the inter-element fluxes are approximated using the Symmetric Interior PenaltyGalerkin formulation. The non-linear terms in the Navier-Stokes equations are expressed in the convective form and approximated through the Lesaint-Raviart fluxes modified for DG methods.

Verification of the solver is performed for a series of test problems; purely elliptic, unsteady Stokes and full Navier-Stokes. The resulting method leads to a stable scheme for the unsteady Stokes and Navier-Stokes equations when equal order approximation is used for velocity and pressure. For the validation of the full Navier-Stokes solver, we consider unsteady laminar flow pasta square cylinder at a Reynolds number of 100 (unsteady wake). The DG solver shows favourably comparisons to experimental data and a continuous Spectral code.

The Harvard Microrobotics Lab has previously
demonstrated the world’s first at-scale robotic insect capable
of vertical takeoff with external power. Both of the robot’s
wings were driven by a single power actuator and 1-DOF
mechanical transmission – making independent control of both
wings, and therefore asymmetric flapping and the generation
of a net body torque, impossible. This paper presents a
method to modulate body torques by altering the kinematics
of each wing transmission independently, via the introduction
of two additional control actuators. Theoretical kinematic and
dynamic predictions based on a pseudo-rigid body model are
compared to the observed wing trajectories. Controllable body
torques are necessary for the development of control algorithms
for eventual stable hovering and free flight.

This paper continues the exploration of the design
space for an insect-sized autonomous flapping-wing MAV with
the goal of stable hovering. Previous work has focused on the use
of a large primary power actuator to generate flapping motion
and smaller “control” actuators to asymmetrically alter wing
kinematics. Here a new iteration of this concept is presented,
merging the two actuator types to create a “hybrid” powercontrol
actuator. Kinematic and dynamic models for wing
motion are presented, and the predictions of these models are
compared to experimental results from a prototype design.
Controllable asymmetry in wing kinematics can be mapped
into controllable body torques via an aerodynamic model, and
this information can be used for the generation of control laws
for stable hover and eventually highly agile aerial vehicles.

Using detailed mechanisms to include chemical kinetics in computational fluid dynamics simulations is required for many combustion applications, yet the resulting computational cost is often extremely prohib- itive. In order to reduce the resources dedicated to this stage, we investigated the coupling of the dynamic adaptive chemistry (DAC) reduction scheme with the in situ adaptive tabulation (ISAT) algorithm. This paper describes the tabulation of dynamic adaptive chemistry (TDAC) method which takes advantage of both ISAT and DAC to reduce the impact of the mesh and the oxidation mechanism on the computa- tional cost, particularly for unsteady applications like internal combustion engines. In the context of homo- geneous charge compression ignition (HCCI), we performed simulations on simplified 2D cases using various n-heptane mechanisms and on a real case mesh using a detailed 857-species iso-octane mechanism. Compared to the direct integration of the combustion reactions, results are in very good agreements and a speed-up factor above 300 is obtained. This is significantly better than what was reported for ISAT and DAC which illustrates the synergy of the two methods. In addition, an experimental validation has also been performed with low load HCCI data. Accordingly, the TDAC method is a significant improvement for the computation of the combustion chemistry in engine simulations and allows the use of detailed mechanisms with practical case meshes in simulations that are inconceivable using direct integration.

Large-eddy simulations are performed of a turbulent Coanda jet separating from a rounded trailing edge of a simplified circulation control airfoil model. The freestream Reynolds number based on the airfoil chord is 0.49 million, the jet Reynolds number based on the jet slot height is 4470, and the ratio of the peak jet velocity to the freestream velocity is 3.96. Three different grid resolutions are used to show that their effect is very small on the mean surface pressure distribution, which agrees very well with experiments, as well as on the mean velocity profiles over the Coanda surface. It is observed that the Coanda jet becomes fully turbulent just downstream of the jet exit, accompanied by asymmetric alternating vortex shedding behind a thin (but blunt) jet blade splitting the jet and the external flow. A number of “backward-tilted” hairpin vortices (i.e., the head of each hairpin being located upstream of the legs) are observed around the outer edge of the jet over the Coanda surface. These hairpins create strong upwash between the legs and weak downwash around them, contributing to turbulent mixing of the high-momentum jet below the hairpins and the low-momentum external flow above them. The probability density distribution of velocity fluctuations is shown to be highly asymmetric in this region, consistent with the observation that the hairpin vortices create strong upwash and weak downwash. Turbulent structures inside the jet, its spreading rate, and self-similarity are also discussed.

A recent LES study of flow around a circulation control airfoil (Nishino et al., AIAA Paper 2010-347) is further extended, with a particular focus on the characteristics of a Coanda wall jet separating from a rounded trailing edge of the airfoil. A number of backward-tilted hairpin vortices are observed in the outer shear layer of the wall jet; these hairpins create a strong upwash between their legs and thereby lift the high-momentum jet flow below the hairpins upward. The LES results are then compared with 2D RANS simulations employing the Spalart-Allmaras model and Menter’s SST model. The Reynolds shear stresses predicted by the two models are both found to be too small in the outer shear layer of the wall jet, resulting in smaller jet spreading rates, larger peak jet velocities, and hence stronger circulations around the airfoil compared with the LES.

Two- and three-dimensional numerical simulations are performed of the flow around a circulation control airfoil (using a Coanda jet blowing over a rounded trailing edge) placed in a rectangular wind-tunnel test section. The airfoil model spans the entire tunnel and the span-to-chord ratio of the model is 3.26. The objective of this numerical study, in which we solve the compressible Reynolds-averaged Navier–Stokes equations in a time-resolved manner (but the solutions eventually converge to steady states), is to investigate the physical mechanisms of wind-tunnel sidewall effects on the flow, especially in the midspan region. The three-dimensional simulations predict that the Coanda jet flow is quasi-two-dimensional until the flow separates from the trailing edge of the airfoil; however, the spanwise ends of this Coanda jet sheet then three-dimensionally roll up on the side walls of the wind tunnel to form two large streamwise vortices downstream. Careful comparisons between the two- and three-dimensional simulations reveal that the wind-tunnel stream goes below the airfoil more in the three-dimensional cases than in the two-dimensional cases due to the presence of these two streamwise vortices downstream. This results in smaller lift and larger drag being produced at the midspan of the airfoil in the three-dimensional cases than in the two-dimensional cases.

The objective of this paper is to describe a methodology to design control laws in the context of a computational aeroelasticity environment. The technical approach involves employing a systems identification technique to develop an explicit state-space model for control law design from the output of a computational aeroelasticity code. As a control law design techniques, the standard LQG technique is employed. The computational aeroelasticity code is modified to accept control laws and perform closed-loop simulations. Numerical results for flutter suppression of the BACT wind-tunnel model are given. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lur'e systems represent feedback interconnections of a Linear Time-Invariant system with a nonlinear operator.
The Zames-Falb criterion is a powerful tool to determine the stability of a Lur'e system when the nonlinear operator is defined by a monotonic, odd and single-valued function.
The article provides a generalization of the Zames-Falb criterion for the analysis of stability of a Lur'e system for nonlinear static operators that are ``approximately'' monotonic and odd.
This result extends the standard Zames-Falb criterion with an additional notion of ``robustness''.

The three-dimensional stability dynamics of a separation bubble over a flat plate has been studied in both linear and nonlinear conditions. Using a global eigenvalue analysis, two centrifugal global modes are identified: an asymptotically unstable three-dimensional weakly growing mode which appears to be originated by a Rayleigh instability; a marginally stable three-dimensional steady mode which is originated by a convective Gortler instability. Direct numerical simulations show that both modes play a role in the route to transition toward the turbulent flow. A structural sensitivity analysis is used to investigate the mechanism of selection of the path toward transition when small perturbations are considered. Finally, a scenario of transition via Gortler modes breakdown is studied in detail, revealing the formation of trains of hairpin vortices in streamwise succession.

The aim of this chapter is to demonstrate an enterprise risk management system in action for an aircraft maintenance service department of an international airline. This service unit is in charge of aircraft maintenance, repair and overhauls for its own aircraft and other international airlines’ fleets. Within the case organisation, the strategic plan is already in place, and annually revised and updated. Failure to meet specific goals of the strategic plan should ideally be one of the set of risks to be managed by the enterprise risk management system. The work
covered in this chapter, however, focuses on the operational risk as a pilot study.
The Australian/New Zealand Standard for Risk Management AS/NZS 4360: 1999 is used as the basic framework, together with the Failure Mode and Effects Analysis (FMEA) as the risk assessment tool. As a quantitative analytical
method, FMEA can assist the AS/NZS 4360 in analysing the risks, giving a higher level of accuracy in risk assessment.
A cross-functional team of the key personnel was in charge of conducting the ERM workshop, which may be divided into four parts. Firstly, the risks were identified and ranked. Secondly, the potential risks were assessed by FMEA. The results of risk
assessment then led to the formulation of risk mitigation plan. Finally, the risk early warning system was established, giving a linkage between ERM, performance measurement, and strategic planning and control, and enabling the case organisation to
continually monitor the potential risks.
The conclusion from this case application is that it is essential for service organisations to include ERM in its arsenal of managerial tools. Also, it is found that such a standard as AS/NZS 4360 is useful as a guideline to provide consistent terminology in the ERM
implementation process.

The three-dimensional global optimal dynamics of a flat-plate boundary layer is studied by means of an adjoint-based optimization in a spatial domain of long – but finite – streamwise dimension. The localized optimal initial perturbation is characterized by a pair of streamwise-modulated counter-rotating vortices, tilted upstream, yielding at the optimal time elongated streaks of alternating sign in the streamwise direction. This indicates that perturbations with non-zero streamwise wavenumber have a role in the transient dynamics of a boundary layer. A scaling law is provided, describing the variation of the streamwise modulation of the optimal initial perturbation with respect to the streamwise domain length and to the Reynolds number. For spanwise-extended domains, a near-optimal three-dimensional perturbation is extracted during the optimization process; it is localized also in the spanwise direction, resulting in a wave packet of elongated disturbances modulated in the spanwise and streamwise directions. The nonlinear evolution of the ptimal and near-optimal perturbations is investigated by means of direct numerical simulations. Both perturbations are found to induce transition at lower levels of the initial energy than local optimal and suboptimal perturbations. Moreover, it is observed that transition occurs in a well-defined region of the convected wave packet, close to its centre, via a mechanism including at the same time oscillations of the streaks of both quasi-sinuous and quasi-varicose nature. Hairpin vortices are observed before transition; they have an active role in the breakdown of the streaks and result in a turbulent spot which spreads out in the boundary layer.

Authors: Mohamed M.S. Nasser, A.H.M. Murid & N.S. Amin

Based on the recently discovered second kind Fredholm integral equation for the exterior Riemann problem, a boundary integral equation is developed in this paper for the two-dimensional, irrotational, incompressible fluid flow around an airfoil without a cusped trailing edge. The solution of the integral equation contains one arbitrary real constant, which may be determined by imposing the Kutta-Joukowski condition. Comparisons between numerical and analytical values of the pressure coefficient on the surface of the NACA 0009 and NACA 0012 airfoils with zero angle of attack show a very good agreement

This third part contains these chapters:

17 Anatomy: Fluorescence Microscopy
18 Aerodynamics: Video Analysis of Pigeon Flight
19 Mathematics: Visual Solutions to a Logic Problem
20 Applied Social Studies: Masks in Social Work
21 Pathology: Diagnosis of a Kidney Disease
22 Epigraphy: Three-Dimensional Laser Scanning of Inscribed Stones
23 Geochemistry: Deformation of Grains in Sandstone
24 Food Science: Electrophoresis Gels of Cheddar Cheese 25 Zoology: Automated Recognition of Individual Cetaceans
26 Art History: Political Meanings of John Heartfield’s Photographs
27 Microbiology: Visualizing Viruses
28 Oceanography: Imaging the Sea Bed Using Side-Scanning Sonar
29  Philosophy: Arabic and Russian Visual Tropes
30  Legal pedagogy: Teaching Visual Rhetoric to Law Students

This file also contains the Afterword, which is an essay on the politics of publishing in different countries. In the Afterword, I justify my choice of a German publisher, and write about which presses are considered appropriate for young scholars in the U.S., U.K., and German-speaking countries. The politics is a sensitive issue, and as far as I know it hasn't been addressed elsewhere.

A class of distributed systems with a cyclic interconnection structure is considered. These systems arise in several biochemical applications and they can undergo diffusion-driven instability which leads to a formation of spatially heterogeneous patterns. In this paper, a class of cyclic systems in which addition of diffusion does not have a destabilizing effect is identified. For these systems global stability results hold if the "secant" criterion is satisfied. In the linear case, it is shown that the secant condition is necessary and sufficient for the existence of a decoupled quadratic Lyapunov function, which extends a recent diagonal stability result to partial differential equations. For reaction-diffusion equations with nondecreasing coupling nonlinearities global asymptotic stability of the origin is established. All of the derived results remain true for both linear and nonlinear positive diffusion terms. Similar results are shown for compartmental systems.

A class of equilibria of the Euler equations is derived describing a vortex dipole of non-zero circulation interacting with the interface between a uniform shear layer and an irrotational region. The flow field and the shape of the deformed shear layer profile are given in terms of explicit mathematical formulae. Properties of the solutions are discussed. They share many qualitative features with a new translating quasi-geostrophic V-state solution recently found by .

A continous two-parameter family of analytical solutions to the Euler equations are presented representing a class of steadily rotating vortex arrays involving N + 1 interacting vortex patches where N > 3 is an integer. The solutions consist of a central vortex patch surrounded by an N-fold symmetric alternating array of satellite point vortices and vortex patches. One of the parameters governs the size of the central patch, the other governs the size of the N satellite patches. In the limit where the areas of the satellite vortex patches tend to zero, the solutions degenerate to the exact solutions of Crowdy (J. Fluid Mech. vol. 469, 2002, p. 209). Limiting states are found in which cusps form only on the central patch, only on the satellite patches, or simultaneously on both central and satellite patches. Contour dynamics simulations are used to check the mathematical solutions and test their robustness. The linear stability of a class of 'point-vortex models' (in which the patches are replaced by point vortices) are also studied in order to examine the stability of the distributed-vorticity configurations to pure-displacement modes. On the other hand, a desingularization of all point vortices to Rankine vortices leads to a class of 'quasi-equilibria' consisting purely of interacting vortex patches close to hydrodynamic equilibrium.

Analytical formulae for the first-type Green's function for Laplace's equation in multiply connected circular domains are presented. The method is constructive and relies on the use of a special function known as the Schottky-Klein prime function associated with multiply connected circular domains. It is shown that all the important functions of potential theory, including the first-type Green's function, the modified Green's functions and the harmonic measures of a domain, can be written in terms of this prime function. A broad range of representative examples are given to demonstrate the efficacy of the method as well as a quantitative comparison, in special cases, with the results obtained using other independent methods.

We present numerical simulation of 2D turbulent flow using a new model for the subgrid scales which are computed using a dynamic equation linking the subgrid scales with the resolved velocity. This equation is not postulated, but derived from the constitutive equations under the assumption that the non-linear interactions of subgrid scales between themselves are equivalent to a turbulent viscosity. This results in a linear stochastic equation for the subgrid scales, which can be numerically solved by a decomposition of the subgrid scales into localized wave-packets. These wave-packets are transported by the resolved scale velocity and have wavenumbers and amplitude which evolve according to the resolved strain and the stochastic forcing. The performances of our model are compared with Direct Numerical Simulations of decaying and forced turbulence. For a same resolution, numerical simulations using our model allow for a significant reduction of the computational time (of the order of 100 in the case we consider), and allow the achievement of significantly larger Reynolds number than the direct method.

We study the behavior of an elastic loop embedded in a flowing soap film. This deformable loop is wetted into the film and is held fixed at a single point against the oncoming flow. We interpret this system as a two-dimensional flexible body interacting in a two-dimensional flow. This coupled fluid-structure system shows bistability, with both stationary and oscillatory states. In its stationary state, the loop remains essentially motionless and its wake is a von Kármán vortex street. In its oscillatory state, the loop sheds two vortex dipoles, or more complicated vortical structures, within each oscillation period. We find that the oscillation frequency of the loop is linearly proportional to the flow velocity, and that the measured Strouhal numbers can be separated based on wake structure.

By immersing a compliant yet self-supporting sheet into flowing water, we study a heavy, streamlined, and elastic body interacting with a fluid. We find that above a critical flow velocity a sheet aligned with the flow begins to flap with a Strouhal frequency consistent with animal locomotion. This transition is subcritical. Our results agree qualitatively with a simple fluid dynamical model that predicts linear instability at a critical flow speed. Both experiment and theory emphasize the importance of body inertia in overcoming the stabilizing effects of finite rigidity and fluid drag.