Showing papers in "AIAA Journal in 2008"

Formulation of the k-w Turbulence Model Revisited

Abstract: This paper presents a reformulated version of the author'sk-ω model of turbulence. Revisions include the addition of just one new closure coefficient and an adjustment to the dependence of eddy viscosity on turbulence properties. The result is a significantly improved model that applies to both boundary layers and free shear flows and that has very little sensitivity to finite freestream boundary conditions on turbulence properties. The improvements to the k-ω model facilitate a significant expansion of its range of applicability. The new model, like preceding versions, provides accurate solutions for mildly separated flows and simple geometries such as that of a backward-facing step. The model's improvement over earlier versions lies in its accuracy for even more complicated separated flows. This paper demonstrates the enhanced capability for supersonic flow into compression corners and a hypersonic shock-wave/ boundary-layer interaction. The excellent agreement is achieved without introducing any compressibility modifications to the turbulence model.

Efficient Global Reliability Analysis for Nonlinear Implicit Performance Functions

Abstract: Many engineering applications are characterized by implicit response functions that are expensive to evaluate and sometimes nonlinear in their behavior, making reliability analysis difficult. This paper develops an efficient reliability analysis method that accurately characterizes the limit state throughout the random variable space. The method begins with a Gaussian process model built from a very small number of samples, and then adaptively chooses where to generate subsequent samples to ensure that the model is accurate in the vicinity of the limit state. The resulting Gaussian process model is then sampled using multimodal adaptive importance sampling to calculate the probability of exceeding (or failing to exceed) the response level of interest. By locating multiple points on or near the limit state, more complex and nonlinear limit states can be modeled, leading to more accurate probability integration. By concentrating the samples in the area where accuracy is important (i.e., in the vicinity of the limit state), only a small number of true function evaluations are required to build a quality surrogate model. The resulting method is both accurate for any arbitrarily shaped limit state and computationally efficient even for expensive response functions. This new method is applied to a collection of example problems including one that analyzes the reliability of a microelectromechanical system device that current available methods have difficulty solving either accurately or efficiently.

General Framework for Dynamic Substructuring: History, Review and Classification of Techniques

Abstract: Four decades after the development of the first dynamic substructuring techniques, there is a necessity to classify the different methods in a general framework that outlines the relations between them. In this paper, a certain vision on substructuring methods is proposed, by recalling important historical milestones that allow us to understand substructuring as a domain decomposition concept. Thereafter, based on the dual and primal assembly of substructures, a general framework for the classification of the methods is presented. This framework allows us to indicate how the various classes of methods, proposed along the years, can be derived from a clear mathematical description of substructured problems. Current bottlenecks in experimental dynamic substructuring, as well as solutions found in literature, will also be briefly discussed.

Interpolation Method for Adapting Reduced-Order Models and Application to Aeroelasticity

Abstract: Reduced-order models are usually thought of as computationally inexpensive mathematical representations that offer the potential for near real-time analysis. Although most reduced-order models can operate in near real-time, their construction can be computationally expensive, as it requires accumulating a large number of system responses to input excitations. Furthermore, reduced-order models usually lack robustness with respect to parameter changes and therefore must often be rebuilt for each parameter variation. Together, these two issues underline the need for a fast and robust method for adapting precomputed reduced-order models to new sets of physical or modeling parameters. To this effect, this paper presents an interpolation method based on the Grassmann manifold and its tangent space at a point that is applicable to structural, aerodynamic, aeroelastic, and many other reduced-order models based on projection schemes. This method is illustrated here with the adaptation of computational-fluid-dynamics-based aeroelastic reduced-order models of complete fighter configurations to new values of the freestream Mach number. Good correlations with results obtained from direct reduced-order model reconstruction, high-fidelity nonlinear and linear simulations are reported, thereby highlighting the potential of the proposed reduced-order model adaptation method for near real-time aeroelastic predictions using precomputed reduced-order model databases.

Flapping Wing Aerodynamics: Progress and Challenges

Abstract: It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.

Parametric Study of an Oscillating Airfoil in a Power-Extraction Regime

Abstract: A wing that is heaving and pitching simultaneously may extract energy from an oncoming flow, thus acting as a turbine. The theoretical performance of such a concept is investigated here through unsteady two-dimensional laminar-flow simulations using the commercial finite volume computational fluid dynamics code FLUENT. Computations are performed in the heaving reference frame of the airfoil, thus leaving only the pitching motion of the airfoil to be dealt with through a rigid-body mesh rotation and a circular nonconformal sliding interface. Unsteady aerodynamics basics of the oscillating airfoil are first exposed, with a description of the operating regimes. Effects of unsteadiness are stressed and the inadequacy of a quasi-steady approach to take them into account is exposed. We present a mapping of power-extraction efficiency for a single oscillating airfoil in the frequency and pitching-amplitude domain: 0 55deg in which efficiencies are higher than 20%. Results from a parametric study are then provided and discussed. It is found that motion-related parameters such as heaving amplitude and frequency have the strongest effects on airfoil performances, whereas geometry and viscous parameters turn out to play a secondary role.

Current Status of Jet Noise Predictions Using Large-Eddy Simulation

Abstract: A survey of the current applications of large-eddy simulation for the prediction of noise from single stream turbulent jets is given. After summarizing the numerical techniques used, the data predicted by the simulations are given at conditions from subsonic, heated jets to supersonic, unheated jets. Mach numbers between 0.3 and 2.0 are considered. Following the data presentation, an analysis of the trends exhibited by the data is given, with special attention paid to relationship between numerical and/or modeling choices and the prediction accuracy. The data support the conclusion that the most limiting factor in current large-eddy simulations is the thickness of the initial shear layer, which is commonly one order of magnitude thicker than what is found experimentally. There is also a large amount of uncertainty regarding the influence of the subgrid scale model on the predictions. The influence of inflow conditions is discussed in depth. Uncertainties in the inflow conditions currently prohibit the simulations from reliably predicting the potential core length. The centerline evolution of the mean and fluctuating axial velocity is strongly coupled to the resolution of the initial shear layers, but can be made to agree within experimental uncertainty when sufficiently thin initial shear layers are used. The maximum achieved Strouhal number of the sound in the acoustic far field is 1.5-3.0, depending on flow condition; this limit is due to numerical resources. A listing of some of the open questions and future directions concerning jet noise predictions using large-eddy simulation concludes the survey.

Plasma Actuators for Cylinder Flow Control and Noise Reduction

Abstract: In this paper, the results of flow-control experiments using single dielectric barrier discharge plasma actuators to control flow separation and unsteady vortex shedding from a circular cylinder in crossflow are reported. This work is motivated by the need to reduce landing gear noise for commercial transport aircraft via an effective streamlining created by the actuators. The experiments are performed at Re D = 3.3 x 104. Using either steady or unsteady actuation, Karman shedding is totally eliminated, turbulence levels in the wake decrease significantly, and near-field sound pressure levels associated with shedding are reduced by 13.3 dB. In the case of unsteady plasma actuation, an actuation frequency of St D = 1 is found to be most effective. The unsteady actuation has the advantage that total suppression of shedding is achieved for a duty cycle of only 25%. However, because unsteady actuation is associated with an unsteady body force and produces a tone at the actuation frequency, steady actuation is more suitable for noise-control applications.

Variable Kinematic Model for the Analysis of Functionally Graded Material plates

Abstract: This work addresses the static analysis of functionally graded material plates subjected to transverse mechanical loadings. The unified formulation and principle of virtual displacements are employed to obtain both closed-form and finite element solutions. The use of the unified formulation permits a large variety of plate models with variable kinematic assumptions to be compared in the same framework. These differ according to the order of the expansion in the thickness direction and the variable description: the order of the expansion ranges from 1 to 4, thus covering first-order as well as higher-order theories; the description of the unknowns can be equivalent single layer or layerwise. The dependence of the material data on the thickness direction was introduced by employing thickness functions that are derived from the Legendre polynomials that are used in the layerwise case. The proposed approach is independent of the used transition function, and as a result, any continuous variations of the material properties in the thickness direction may be easily implemented. The obtained solutions (closed form and finite element) are validated through comparison with three-dimensional exact solutions and other available solutions. These show the limitations of classical plate theories as well as the convenience of the use of the proposed variable kinematic models for the analysis of functionally graded material plates.

Building Efficient Response Surfaces of Aerodynamic Functions with Kriging and Cokriging

Abstract: In this paper, we compare the global accuracy of different strategies to build response surfaces by varying sampling methods and modeling techniques. The aerodynamic test functions are obtained by deforming the shape of a transonic airfoil. For comparisons, a robust strategy for model fit using a new efficient initialization technique followed by a gradient optimization was applied. First, a study of different sampling methods proves that including a posteriori information on the function to sample distribution can improve accuracy over classical space-filling methods such as Latin hypercube sampling. Second, comparing kriging and gradient-enhanced kriging on two- to six-dimensional test cases shows that interpolating gradient vectors drastically improves response-surface accuracy. Although direct and indirect cokriging have equivalent formulations, the indirect cokriging outperforms the direct approach. The slow linear phase of error convergence when increasing sample size is not avoided by cokriging. Thus, the number of samples needed to have a globally accurate surface stays generally out of reach for problems considering more than four design variables.

Aeromechanics of Membrane Wings with Implications for Animal Flight

Abstract: Bats and other flying mammals are distinguished by thin, compliant membrane wings. In an effort to understand the dependence of aerodynamic performance on membrane compliancy, wind-tunnel tests of low-aspect-ratio, compliant wings were conducted for Reynolds numbers in the range of 0.7-2.0 x 10 5 . The lift and drag coefficients were measured for wings of varying aspect ratio, compliancy, and prestrain values. In addition, the static and dynamic deformations of compliant membrane wings were measured using stereo photogrammetry. A theoretical model for membrane camber due to aerodynamic loading is presented, indicating that the appropriate nondimensional parameter describing the problem is a Weber number that compares the aerodynamic load to the membrane elasticity. Excellent agreement between the theory and experiments is found. Measurements of aerodynamic performance show that, in comparison with rigid wings, compliant wings have a higher lift slope, maximum lift coefficients, and a delayed stall to higher angles of attack. In addition, they exhibit a strong hysteresis both around a zero angle of attack as well as around the stall angle. Unsteady membrane motions were also measured, and it is observed that the membrane vibrates with a spatial structure that is closely related to the free eigenmodes of the membrane under tension and that the Strouhal number at which the membrane vibrates rises with the freestream velocity, coinciding with increasing multiples of the natural frequency of the membrane.

Surrogate-Based Optimization Using Multifidelity Models with Variable Parameterization and Corrected Space Mapping

Abstract: Surrogate-based-optimization methods provide a means to achieve high-fidelity design optimization at reduced computational cost by using a high-fidelity model in combination with lower-fidelity models that are less expensive to evaluate. This paper presents a provably convergent trust-region model-management methodology for variable-parameterization design models: that is, models for which the design parameters are defined over different spaces. Corrected space mapping is introduced as a method to map between the variable-parameterization design spaces. It is then used with a sequential-quadratic-programming-like trust-region method for two aerospace-related design optimization problems. Results for a wing design problem and a flapping-flight problem show that the method outperforms direct optimization in the high-fidelity space. On the wing design problem, the new method achieves 76% savings in high-fidelity function calls. On a bat-flight design problem, it achieves approximately 45% time savings, although it converges to a different local minimum than did the benchmark.

Compressibility and Rarefaction Effects on Drag of a Spherical Particle

Abstract: A review of compressibility and rarefaction effects on spherical particle drag was conducted based on existing experimental data, theoretical limits, and direct simulation Monte Carlo method results. The data indicated a nexus point with respect to effects of Mach number and Knudsen number. In particular, it was found that a single drag coefficient (of about 1.63) is obtained for all particle conditions when the particle Reynolds number is about 45, and is independent of compressibility or rarefaction effects. At lower Reynolds numbers, the drag is dominated by rarefaction, and at higher Reynolds numbers, it is dominated by compressibility. The nexus, therefore, allows construction of two separate models for these two regimes. The compression-dominated regime is obtained using a modification of the Clift-Gauvin model to specifically incorporate Mach number effects. The resulting model was based on a wide range of experimental data and showed superior prediction robustness compared with previous models. For the rarefaction-dominated regime, the present model was constructed to directly integrate the theoretical creeping flow limits, including the incompressible continuum flow limit (Stokes drag), the incompressible weak rarefaction limit (Basset-Knudsen correction), and the incompressible free-molecular flow limit (Epstein theory). Empirical correlations are used to extend this model to finite particle Reynolds numbers within the rarefaction-dominated regime.

High-Order-Accurate Numerical Method for Complex Flows

Abstract: A numerical method employing high-order-accurate (higher than third) upwind discretizations for solving the compressible Navier-Stokes equations on structured grids is discussed. The inviscid fluxes are computed by a procedure based on a weighted essentially nonoscillatory interpolation of the characteristic variables and the Roe scheme. Application of the numerical method to a number of test cases of increasing complexity, that are prototypical for several of the key aspects of practical flows, demonstrates the accuracy and robustness of the method even when computing on distorted curvilinear grids. Significant reductions in computer time are possible when a second-order-accurate implicit Adams-Moulton scheme is employed for time integration. The combination of implicit time integration and high-order-accurate spatial discretization is shown to lead to significant savings in compute time as the grid resolution requirement is lowered and the time step can be increased.

Parallel Newton-Krylov Solver for the Euler Equations Discretized Using Simultaneous-Approximation Terms

Abstract: 0mesh continuity at block interfaces, accommodates arbitrary block topologies, and has low interblock-communication overhead. The resulting discrete equations are solved iteratively using an inexact-Newton method. At each Newton iteration, the linear system is solved inexactly using a Krylov-subspace iterative method, and both additive Schwarz and approximate Schur preconditioners are investigated. The algorithm is tested on the ONERA M6 wing geometry. We conclude that the approximate Schur preconditioner is an efficient alternative to the Schwarz preconditioner. Overall, the results demonstrate that the Newton–Krylov algorithm is very efficient: using 24 processors, a transonic flow on a 96-block, 1-million-node mesh requires 12 minutes for a 10-order reduction of the residual norm.

Experimental Study for Momentum Transfer in a Dielectric Barrier Discharge Plasma Actuator

Abstract: The momentum-transfer performance of a plasma actuator was investigated experimentally for the effects of ambient-gas pressure, ambient-gas species, and electrode configuration. For measurement of the momentum-transfer performance, the force exerted by the actuator, in addition to the induced velocity, was measured; the consistency between both measurement methods was demonstrated. Results showed that the ambient-gas pressure under which a plasma actuator operates has a considerable effect on the momentum-transfer performance. In fact, the performance does not decrease in a linear manner with decreasing ambient-gas pressure; rather, it initially increases and then decreases. The chemical species of the ambient gas also has a considerable effect on the momentum-transfer performance. The momentum transfer in air is greater than that in nitrogen gas at pressures of less than 1 atm, which suggests a considerable contribution of oxygen molecules in the air. The momentum-transfer performance in carbon dioxide gas is slightly greater than that in nitrogen gas for pressures of less than 1 atm, although they are comparable at 1-atm pressure. Furthermore, the electrode configuration was found to strongly affect the momentum-transfer performance of the plasma actuator. In particular, a mesh-type electrode can improve the performance markedly, compared with the performance of a tape electrode with similar thickness, in an ambient-gas pressure of 1 atm. However, the performance difference attributable to the electrode configuration is greatly reduced with a decrease in the ambient-gas pressure; for example, it almost disappears at pressures of less than approximately 50 kPa.

Kriging Hyperparameter Tuning Strategies

Abstract: Response surfaces have been extensively used as a method of building effective surrogate models of high-fidelity computational simulations. Of the numerous types of response surface models, kriging is perhaps one of the most effective, due to its ability to model complicated responses through interpolation or regression of known data while providing an estimate of the error in its prediction. There is, however, little information indicating the extent to which the hyperparameters of a kriging model need to be tuned for the resulting surrogate model to be effective. The following paper addresses this issue by investigating how often and how well it is necessary to tune the hyperparameters of a kriging model as it is updated during an optimization process. To this end, an optimization benchmarking procedure is introduced and used to assess the performance of five different tuning strategies over a range of problem sizes. The results of this benchmark demonstrate the performance gains that can be associated with reducing the complexity of the hyperparameter tuning process for complicated design problems. The strategy of tuning hyperparameters only once after the initial design of experiments is shown to perform poorly.

Investigation by Particle Image Velocimetry Measurements of Oblique Shock Reflection with Separation

Abstract: The organization and length scales of turbulent structures and unsteadiness generated in a shock-wave-induced separation at Mach number of 2.3 are investigated experimentally using particle image velocimetry. Processing of the velocity fields displays and demonstrates the existence of structures in the mixing layer developed in the separation bubble. Moreover, we show in evidence a link between the reflected shock excursions and the size of the separated flow. This overview of the spatial organization of the interaction provides a more comprehensive picture of the flow. Nomenclature C f = friction coefficient L = length of the interaction M = Mach number Re = Reynolds number based on the momentum thickness S L = Strouhal number T t = stagnation temperature U 0 = upstream external velocity U ‡ = V=u u = friction velocity V = Van Driest transformed velocity X = x X 0 †=L X 0 = mean position of the reflected shock x = longitudinal coordinate Y = y= 0 y = normal to the wall coordinate y ‡ = yu = 0 = upstream boundary-layer thickness = incidence angle of the shock generator = kinematic viscosity

Insect-Based Hover-Capable Flapping Wings for Micro Air Vehicles: Experiments and Analysis

Abstract: This paper addresses the aerodynamics of insect-based, biomimetic, flapping wings in hover. An experimental apparatus, with a biomimetic flapping mechanism, was used to measure the thrust generated by a number of wing designs at different wing pitch settings. To quantify the large inertial loads acting on the wings, vacuum chamber tests were conducted. Results were obtained for several high-frequency tests conducted on lightweight aluminum and composite wings. The wing mass was found to have a significant influence on the maximum frequency of the mechanism because of a high inertial power requirement. All the wings tested showed a decrease in thrust at high frequencies. In contrast, for a wing held at 90-deg pitch angle, flapping in a horizontal stroke plane with passive pitching caused by aerodynamic and inertial forces, the thrust was found to be larger. To study the effect of passive pitching, the biomimetic flapping mechanism was modified with a passive torsion spring on the flapping shaft. Results of some tests conducted with different wings and different torsion spring stiffnesses are shown. A soft torsion spring led to a greater range of pitch variation and produced more thrust at slightly lower power than with the stiff torsion spring. The lightweight and highly flexible wings used in this study had significant aeroelastic effects which need to be investigated. A finite element based structural analysis of the wing is described, along with an unsteady aerodynamic analysis based on indicial functions. The analysis was validated with experimental data available in literature, and also with experimental tests conducted on the biomimetic flapping-pitching mechanism. Results for both elastic and rigid wing analyses are compared with the thrust measured on the biomimetic flapping-pitching mechanism.

Dielectric Barrier Discharge Flow Control at Very Low Flight Reynolds Numbers

Abstract: Experiments were performed on a flat-plate airfoil and an Eppler E338 airfoil at very low flight Reynolds numbers (3000 ≤ Re ≤ 50,000), in which dielectric barrier discharge plasma actuators were employed at the airfoil leading edges to effect flow control. The actuators were driven in a high-frequency (kilohertz) steady mode and a pulsed mode in which pulse frequency and duty cycle were varied in a systematic fashion. Optimum reduced frequencies for generating poststall lift were approximately between 0.4 and 1, and this was broadly consistent with zero-mass-flux slot-blowing data acquired at Reynolds numbers that were approximately 200 times higher. Nevertheless, profound differences in the response to reduced frequency and duty cycle were observed between the flat-plate and E338 airfoils. In general, actuation produced considerable performance improvements, including an increase in maximum lift coefficient of 0.4 to 0.8 and maintained elevated endurance at significantly higher lift coefficients. Actuation in the steady mode resulted in circulation control at Re = 3000. Pulsed actuation also exerted a significant effect on the wake at prestall angles of attack, in which control of the upper-surface flat-plate bubble shedding produced significant differences in wake spreading and vortex shedding. The flat plate was also tested in a semispan-wing configuration (AR = 6), and the effect of control was comparable with that observed on the airfoil.

Integrative Model of Drosophila Flight

Abstract: This paper presents a framework for simulating the flight dynamics and control strategies of the fruit fly Drosophila melanogaster. The framework consists of five main components: an articulated rigid-body simulation, a model of the aerodynamic forces and moments, a sensory systems model, a control model, and an environment model. In the rigid-body simulation the fly is represented by a system of three rigid bodies connected by a pair of actuated ball joints. At each instant of the simulation, the aerodynamic forces and moments acting on the wings and body of the fly are calculated using an empirically derived quasi-steady model. The pattern of wing kinematics is based on data captured from high-speed video sequences. The forces and moments produced by the wings are modulated by deforming the base wing kinematics along certain characteristic actuation modes. Models of the fly’s visual and mechanosensory systems are used to generate inputs to a controller that sets the magnitude of each actuation mode, thus modulating the forces produced by the wings. This simulation framework provides a quantitative test bed for examining the possible control strategies employed by flying insects. Examples demonstrating pitch rate, velocity, altitude, and flight speed control, as well as visually guided centering in a corridor are presented.

Space-Time Correlations in Two Subsonic Jets Using Dual Particle Image Velocimetry Measurements

Abstract: half-velocity diameter in reference to length in the shear layer and on the jet axis, respectively. From these results, a discussion on the modeling of turbulence in jets is addressed. The self-similarity of some space correlation functions in the shear layer and on the jet axis is shown. Furthermore, far enough downstream in the shear layer, some of the ratios between the space and time scales are relatively close to the values expected in homogeneous and isotropic turbulence. It is also found that the ratio between the integral length and the time scales in the fixed frame is of the order of the local mean flow velocity. In the convected frame, the appropriate scaling factor is the rms velocity.

Unsteady Discrete Adjoint Formulation for Two-Dimensional Flow Problems with Deforming Meshes

Abstract: A method to apply the discrete adjoint for computing sensitivity derivatives in two-dimensional unsteady flow problems is presented. The approach is to first develop a forward or tangent linearization of the nonlinear flow problem in which each individual component building up the complete flow solution is differentiated against the design variables using the chain rule. The reverse or adjoint linearization is then constructed by transposing and reversing the order of multiplication of the forward problem. The developed algorithm is very general in that it applies directly to the arbitrary Lagrangian-Eulerian form of the governing equations and includes the effect of deforming meshes in unsteady flows. It is shown that an unsteady adjoint formulation is essentially a single backward integration in time and that the cost of constructing the final sensitivity vector is close to that of solving the unsteady flow problem integrated forward in time. It is also shown that the unsteady adjoint formulation can be applied to time-integration schemes of different orders of accuracy with minimal changes to the base formulation. The developed technique is then applied to three optimization examples, the first in which the shape of a pitching airfoil is morphed to match a target time-dependent load profile, the second in which the shape is optimized to match a target time-dependent pressure profile, and the last in which the time-dependent drag profile is minimized without any loss in lift.

Time-Domain Spectral Element Method for Built-In Piezoelectric-Actuator-Induced Lamb Wave Propagation Analysis

Abstract: An investigation was performed to develop a computational model based on a spectral element method to simulate piezoelectric-actuator-induced acousto-ultrasonic wave propagation in a metallic structure. The model solves the coupled electromechanical field equations simultaneously in both three-dimensional and two-dimensional plane strain conditions, and so it can accept any arbitrary waveform of electrical voltage as input to any piezoelectric transducer and produce piezoelectric sensor output in voltage as a result of the excitation generated by the transducer. Basically, the model inputs electrical voltage to actuators and outputs electrical signals of sensors. To visualize the transient dynamic wave motions in the structure generated by the transducer, the code is integrated with commercial pre/postprocessing software to provide graphical outputs of the dynamic deformations of the structure. The code was verified by comparison with experimental results. Performance of the model was examined in terms of solution convergence compared with the finite element method.

Dynamics of Airfoil Separation Control using Zero-Net Mass-Flux Forcing

Abstract: Zero-net mass-flux jet based control of flow separation over a stalled airfoil is examined using numerical simulations. Two-dimensional simulations are carried out for a NACA 4418 airfoil at a chord Reynolds number of 40,000 and angle of attack of 18 deg. Results for the uncontrolled flow indicate the presence of three distinct natural time scales in the flow corresponding to the shear layer, separation bubble, and wake regions. The natural frequencies are used to select appropriate forcing frequencies, and it is found that forcing frequencies closer to the separation bubble frequency elicit the best response in terms of reduction of separation extent and an improvement in aerodynamic performance. In contrast, higher forcing frequencies closer to the natural shear layer frequency tend to enhance separation. The vortex dynamics and frequency response of flow are examined in detail to gain insight into mechanisms underlying the observed behavior.

Fast p-Multigrid Discontinuous Galerkin Method for Compressible Flows at All Speeds

Abstract: Ap-multigrid (wherep is the polynomial degree) discontinuous Galerkin method is presented for the solution of the compressible Euler equations on unstructured grids. The method operates on a sequence of solution approximations of different polynomial orders. A distinct feature ofthisp-multigrid method is the application of an explicit smoother on the higher-level approximations (p > 0) and an implicit smoother on the lowest-level approximation (p = 0), resulting in a fast (and low) storage method that can be efficiently used to accelerate the convergence to a steady-state solution. Furthermore, this p-multigrid method can be naturally applied to compute the flows with discontinuities, in which a monotonic limiting procedure is usually required for discontinuous Galerkin methods. An accurate representation of the boundary normals based on the definition of the geometries is used for imposing slip boundary conditions for curved geometries [Krivodonova, L., and Berger, M., "High-Order Implementation of Solid Wall Boundary Conditions in Curved Geometries," Journal of Computational Physics, Vol. 211, No.2,2006, pp. 492-512; and Luo, H., Baum, J. D., and Lohner, R., "On the Computation of Steady-State Compressible Flows Using a Discontinuous Galerkin Method," International Journal for Numerical Methods in Engineering, Vol. 73, No.5,2008, pp. 597-623]. A variety of compressible flow problems for a wide range of flow conditions from low Mach number to supersonic in both two-dimensional and three-dimensional configurations are computed to demonstrate the performance ofthisp-multigrid method. The numerical results obtained strongly indicate the order-independent property of this p-multigrid method and demonstrate that this method is orders-of-magnitude faster than its explicit counterpart The performance comparison with a finite volume method shows that using this p-multigrid method, the discontinuous Galerkin method, provides a viable, attractive, competitive, and probably even superior alternative to the finite volume method for computing compressible flows at all speeds.

Kinetic Ignition Enhancement of Diffusion Flames by Nonequilibrium Magnetic Gliding Arc Plasma

Abstract: Kinetic ignition enhancement of CH 4 -air and H 2 -air diffusion flames by a nonequilibrium plasma discharge of air was studied experimentally and numerically through the development of a well-defined counterflow system. Measurements of ignition temperatures and major species, as well as computations of rates of production and sensitivity analyses, were performed to understand the kinetic enhancement pathways for ignition by plasma discharge of air. It was found that plasma discharge of air led to significant kinetic ignition enhancement illustrated by large decreases in the ignition temperatures for a broad range of strain rates. Examination of the radical and NO x production in the plasma showed that the enhancement was caused primarily by the catalytic effect of NO x . The results of numerical simulations of the counterflow burner with preheated air and NO, addition showed the existence of different ignition regimes, which appeared due to the competition between radical production by NO x and other pathways, as well as heat release. There were two ignition regimes for small concentrations of NO, and three ignition regimes for large concentrations of NO,. Numerical simulations agreed well with the experimental measurements and suggested a new strategy for plasma-assisted ignition in supersonic flow, where a combination of thermal and nonthermal plasma would work more efficiently for ignition enhancement.

Effects of Reynolds Number and Flapping Kinematics on Hovering Aerodynamics

Abstract: Motivated by our interest in micro and biological air vehicles, Navier-Stokes simulations for fluid flow around a hovering elliptic airfoil have been conducted to investigate the effects of Reynolds number, reduced frequency, and flapping kinematics on the flow structure and aerodynamics. The Reynolds number investigated ranges from 75 to 1700, and the reduced frequency from 0.36 to 2.0. Two flapping modes are studied, namely, the "water-treading" hovering mode, and the normal hovering mode. Although the delayed-stall mechanism is found to be responsible for generating the maximum lift peaks in both hovering modes, the wake-capturing mechanism is identified only in the normal hovering mode. In addition to the strong role played by the kinematics, the Reynolds number's role has also been clearly identified. In the low Reynolds number regime, 0(100), the viscosity dissipates the vortex structures quickly and leads to essentially symmetric flow structure and aerodynamics force between the forward stroke and backward strokes. At higher Reynolds numbers (300 and larger), the history effect is influential, resulting in distinctly asymmetric phenomena between the forward and backward strokes.

Aeroelastic and Aerothermoelastic Behavior in Hypersonic Flow

Abstract: The testing of aeroelastically and aerothermoelastically scaled wind-tunnel models in hypersonic flow is not feasible; thus, computational aeroelasticity and aerothermoelasticity are essential to the development of hypersonic vehicles. Several fundamental issues in this area are examined by performing a systematic computational study of the hypersonic aeroelastic and aerothermoelastic behavior of a three-dimensional configuration. Specifically, the flutter boundary of a low-aspect-ratio wing, representative of a fin or control surface on a hypersonic vehicle, is studied over a range of altitudes using third-order piston theory and Euler and Navier-Stokes aerodynamics. The sensitivity of the computational-fluid-dynamics-based aeroelastic analysis to grid resolution and parameters governing temporal accuracy are considered. In general, good agreement at moderate-to-high altitudes was observed for the three aerodynamic models. However, the wing flutters at unrealistic Mach numbers in the absence of aerodynamic heating. Therefore, because aerodynamic heating is an inherent feature of hypersonic flight and the aeroelastic behavior of a vehicle is sensitive to structural variations caused by heating, an aerothermoelastic methodology is developed that incorporates the heat transfer between the fluid and structure based on computational-fluid-dynamics-generated aerodynamic heating. The aerothermoelastic solution procedure is then applied to the low-aspect-ratio wing operating on a representative hypersonic trajectory. In the latter study, the sensitivity of the flutter margin to perturbations in trajectory angle of attack and Mach number is considered. Significant reductions in the flutter boundary of the heated wing are observed. The wing is also found to be susceptible to thermal buckling.

Delayed Detached-Eddy Simulation and Analysis of Supersonic Inlet Buzz

Abstract: Supersonic inlet buzz in a rectangular, mixed-compression inlet has been simulated on a 20 x 10 6 points mesh using the delayed detached-eddy simulation method, a version of detached-eddy simulation that ensures the attached boundary layers are treated using Reynolds-averaged Navier-Stokes equations. The results are compared with experimental data obtained during a previous campaign of wind-tunnel experiments. The comparison of unsteady data is performed thanks to phase averages, Fourier transforms, and wavelet transforms. The buzz observed at Mach 1.8, which occurred at a frequency of 18 Hz, is well reproduced. The shock oscillations, as well as the different flow features experimentally observed, are present in the simulation. The buzz frequency, as well as higher frequencies existing in the experimental pressure signals, are correctly predicted. The data issued from the simulation (time history of pressure fluctuations, pseudo-Schlieren, and three-dimensional visualizations) allow a better investigation of the inlet flowfield during buzz and a detailed description and physical analysis of this phenomenon. A description and an explanation of the mechanism at the origin of secondary oscillations that occur at a higher frequency during buzz are proposed. The crucial role of acoustic waves moving through the duct is shown.