Showing papers in "AIAA Journal in 2012"

Separation control with nanosecond-pulse-driven dielectric barrier discharge plasma actuators

Abstract: : The efficacy of dielectric barrier discharge (DBD) plasmas driven by high voltage (approximately 15 kV) repetitive nanosecond pulses approximately 100 ns FWHM) for flow separation control is investigated experimentally on an airfoil leading edge up to Re=1x106 (62 m/s). Unlike AC-DBDs, the nanosecond pulse driven DBD plasma actuator transfers very little momentum to the neutral air, but generates compression waves similar to localized arc filament plasma actuators. A complex pattern of quasi-planar and spherical compression waves is observed in still air. Measurements suggest that some of these compression waves are generated by discharge filaments that remain fairly reproducible pulse-to-pulse. The device performs as an active trip at high Re pre-stall angles of attack and provides perturbations that generate coherent spanwise vortices at post-stall. These coherent structures entrain freestream momentum thereby reattaching the normally separated flow to the suction surface of the airfoil. Coherent structures are identified at all tested frequencies, but values of F(subponent c, exponent +)=4-6 are most effective for control. Such devices which are believed to function through thermal effects could be an alternative to AC-DBD plasmas that rely on momentum addition.

Hierarchical Kriging Model for Variable-Fidelity Surrogate Modeling

Abstract: The efficiency of building a surrogate model for the output of a computer code can be dramatically improved via variable-fidelity surrogate modeling techniques. In this article, a hierarchical kriging model is proposed and used for variable-fidelity surrogate modeling problems. Here, hierarchical kriging refers to a surrogate model of a highfidelity function that uses a kriging model of a sampled lower-fidelity function as a model trend. As a consequence, the variation in the lower-fidelity data is mapped to the high-fidelity data, and a more accurate surrogate model for the high-fidelity function is obtained. A self-contained derivation of the hierarchical kriging model is presented. The proposed method is demonstrated with an analytical example and used for modeling the aerodynamic data of an RAE 2822 airfoil and an industrial transport aircraft configuration. The numerical examples show that it is efficient, accurate, and robust. It is also observed that hierarchical kriging provides a more reasonable mean-squared-error estimation than traditional cokriging. It can be applied to the efficient aerodynamic analysis and shape optimization of aircraft or any other research areas where computer codes of varying fidelity are in use.

Wall-Pressure Spectral Model Including the Adverse Pressure Gradient Effects

Abstract: 4, in both internal (channel) and external (airfoil) flows. A review of the boundary-layer parameters characterizing the pressure gradient effects is provided, and the more relevant ones are introduced as new variables in the model. The method is then compared to the zero pressure gradient model it is derived from. The influence of the pressure gradient on the wall-pressure spectrum is discussed.Finally,themethodisappliedtoprovideinputdataofaradiatedtrailing-edge noisemodelbymeansofan aeroacoustic analogy, namely Amiet’s theory of turbulent boundary layer past a trailing edge. The results are compared to experimental data obtained in an open-jet anechoic wind tunnel.

Alternative Cokriging Method for Variable-Fidelity Surrogate Modeling

Abstract: Surrogate modeling plays an increasingly important role in different areas of aerospace engineering, such as erodynamic shape optimization, aerodynamic data production, structural design, and multidisciplinary design optimization of aircraft or spacecraft. Cokriging provides an attractive alternative approach to conventional kriging to improve the efficiency of building a surrogate model. It was initially proposed and applied in the geostatistics community for the enhanced prediction of less intensively sampled primary variables of interest with the assistance of intensively sampled auxiliary variables. As the underlying theory of cokriging is that of two-variable or multivariable kriging, it can be regarded as a general extension of (one-variable) kriging to a model that is assisted by auxiliary variables or secondary information. In an attempt to apply cokriging to the surrogate modeling problems associated with deterministic computer experiments, this article is motivated by the development of an alternative cokriging method to address the challenge related to the construction of the covariance matrix of cokriging [7]. Earlier work done by other authors related to this study can be found in the statistical community. For example, Kennedy and O’Hagan (KOH) proposed an autoregressive model to calculate the covariances and crosscovariances in the covariance matrix and developed a Bayesian approach to predict the output from an expensive high-fidelity simulation code with the assistance of lower-fidelity simulation codes. This Bayesian approach is identical to a form of cokriging suitable for computer experiments. Later, Qian andWu proposed a similar method, in which a random function (Gaussian process model) was used to replace the constant multiplicative factor of KOH’s method to account for the nonlinear scale change. KOH’s method was applied to multifidelity analysis and design optimization in the context of aerospace engineering by Forrester et al. and Kuya et al. More recently, Zimmerman and Han proposed a cokriging method with simplified cross-correlation estimation. In this article, we propose an alternative approach for the construction of the cokriging covariance matrix and develop a more practical cokriging method in the context of surrogate-based analysis and optimization. The developed cokriging method is validated against an analytical problem and applied to construct global approximation models of the aerodynamic coefficients as well as the drag polar of an RAE 2822 airfoil.

Provably Convergent Multifidelity Optimization Algorithm Not Requiring High-Fidelity Derivatives

Abstract: functions, does not alter the con- vergence proof of the optimization algorithm, and is shown to be robust to poor low-delity information. The algorithm is compared with an unconstrained single-delity quasi-Newton algorithm and two

Refined One-Dimensional Formulations for Laminated Structure Analysis

Abstract: This paper proposes one-dimensional formulations based on hierarchical expansions of the unknown displacement variables for the analysis of multilayered structures made of anisotropic composite layers. The hierarchical technique shows variable kinematic properties and it is based on the Carrera unified formulation. Two different classes of refined theories are proposed: the first expands the unknown variables in terms of power polynomials of the cross-sectional coordinates (it consists of a Taylor-like expansion); the second class of onedimensional theories uses Lagrange polynomials (Lagrange expansion) and subdomain discretizations of the cross section, and it leads to only pure displacements as the unknown variables. Taylor-like expansion is used to develop equivalent single-layer formulations, and Lagrange expansion is used to construct both equivalent single-layer and layerwise descriptions. The finite element method is employed to develop numerical applications. Using the Carrera unified formulation, finite element matrices are obtained in terms of a few fundamental nuclei that are formally independent of all the considered one-dimensional formulations. A number of numerical examples are given concerning on beams, plates, and more complex structures. Comparisons with results from plate and solid models are provided. The following has been concluded: 1) The proposed formulation represents a reliable, compact, and accurate method to develop refined one-dimensional models. 2) The present one-dimensional models are very effective at detecting both global and local responses of composite structures. 3) Shell-and solidlike results are obtained with a significant reduction in the computational costs.

Reynolds-Averaged Navier-Stokes Simulations of the HyShot II Scramjet

Abstract: The internal flow in the HyShot II scramjet is investigated through numerical simulations. A computational infrastructure to solve the compressible Reynolds-averaged Navier–Stokes equations on unstructured meshes is introduced. A combustion model based on tabulated chemistry is considered to incorporate detailed chemical– kinetics mechanics while retaining a low computational cost. Both nonreactive and reactive simulations have been performed, and results are compared with ground test measurements obtained at DLR, German Aerospace Center. Different turbulence models were tested, and the dependence on the mesh is assessed through grid refinement. The comparison with experimental data shows good agreement, although the computed heat fluxes at the wall are higher thanmeasurements for the reactive case. A sensitivity analysis on the turbulent Schmidt and Prandtl numbers shows that the choice of these parameters has a strong influence on the results. In particular, variations of the turbulent Prandtl number lead to large changes in the heat flux at the walls. Finally, the inception of thermal choking is investigated by increasing the equivalence ratio, whereby a normal shock is created locally and moves upstream, leading to a large increase in the maximum pressure. Nevertheless, a large portion of the flow is still supersonic.

Evaluation of Dynamic Derivatives Using Computational Fluid Dynamics

Abstract: This paper focuses on the evaluation of the dynamic stability derivative formulation. The derivatives are calculated using the Euler and Reynolds-averaged Navier–Stokes equations, and a time-domain solver was used for the computation of aerodynamic loads for forced periodic motions. To validate the predictions, two test cases are used. For the standard dynamic model geometry, a database of dynamic simulations illustrates the effects of the systematic variation of motion and fluid parameters involved. A satisfactory agreement was observed with available experimental data, and the dependency of dynamic derivatives on a number of parameters, such as Mach number, mean angle of attack, frequency, and amplitude, was assessed. For the transonic cruiser wind-tunnel geometry, static and unsteady aerodynamic characteristics were validated against experimental measurements. The ability of models based on the dynamic derivatives to predict large-amplitude motion forces and moments was assessed. It was demonstrated that the dynamic derivative model does not represent all of the important effects due to aerodynamics.

Benchmarking a Coupled Immersed-Boundary-Finite-Element Solver for Large-Scale Flow-Induced Deformation

Abstract: CD = dimensionless mean drag coefficient, 2F D= fU meanD F D = drag force per unit spanwise length on the cylinder and plate, Nm 1 D = diameter of the cylinder, m E = dimensionless Young’s modulus, E = fU mean f = dimensionless oscillation frequency of the plate, f D=Umean Re = Reynolds number, fUmeanD= Umean = mean velocity at the left boundary of the channel, ms 1 V = dimensionless dilatational wave speed inside the structure, E= s= f p

Reynolds-Averaged Navier-Stokes Study of the Shock-Buffet Instability Mechanism

Abstract: A study of shock-buffet onset and instability mechanism via Reynolds-averaged Navier―Stokes simulations on several airfoils is presented. The numerical setup and the Spalart―AUmaras turbulence closure are validated based on wind-tunnel data from NACA 0012 and RA16SC1 airfoils. The paper presents simulations of the flow past three • airfoils: the subsonic NACA 0012, the supercritical RA16SC1, and the thin, transonic/supersonic NACA 64A204, at pre- and postbuffet conditions, and within a cycle of developed shock buffet. The shock-buffet cycle is found to be »■• similar in nature for all airfoils, originating in unstable interaction of the shock and the separation bubble. Simulation results support the notion that buffet onset is not related to the bursting of the separation bubble behind the shock. Shock-buffet categorizing is posited as a transonic prestall instability phenomenon that depends on the shock strength and location. Shock-buffet onset conditions occur when the shock position is behind and sufficiently close to the upper-surface maximum curvature location. Additionally, it is suggested that offset conditions are when the shock is at an upstream location and the flow aft of it is fully separated.

Formation of Turbulent Vortex Breakdown: Intermittency, Criticality, and Global Instability

Abstract: This study provides quantitative insight into the formation of vortex breakdown and the onset of global instability in a turbulent swirling jet. A water jet is guided through a rotating honeycomb that imparts the rotational motion, passed through a contraction, and discharged into a large water tank. The flow states evolving at increasing swirl are mapped out via time-resolved particle image velocimetry. The experimental results scale properly with the swirl number based on the axial momentum flux when the commonly used boundary-layer approximations are omitted. The instantaneous velocity field reveals that vortex breakdown occurs intermittently at a wide range of swirl numbers before it appears in the mean flow. At this intermittent state, the evolving breakdown bubble oscillates heavily between two streamwise locations where the vortex core is subcritical. Upon further increasing the swirl, the breakdown oscillations decay and a region of reversed flow appears in the mean flowfield. The formation of this socalled axisymmetric breakdown state is accompanied by a supercritical-to-subcritical transition of the inflowing vortex core. The reversed flow region is found to grow linearly with increasing swirl until the flow undergoes a supercritical Hopf bifurcation to a global single-helical mode, and vortex breakdown adopts a spiral shape. The global mode shape is extracted from the particle image velocimetry snapshots by means of proper orthogonal decomposition and Fourier analysis. The present experiment reveals that, at gradually increasing swirl, the jet first transitions to an axisymmetric breakdown state that remains globally stable until a critical swirl number is exceeded. This sequence of flow states agrees well with the transient formation of vortex breakdown observed in laminar flows.

How much compression should a scramjet inlet Do

Abstract: The supersonic combustion ramjet, or scramjet, is the engine cycle most suitable for sustained hypersonic flight in the atmosphere This paper examines a key decision in the design of the inlet or intake of these hypersonic airbreathing engines, namely, the level of compression to be performed Too much compression can lead to onerous system level issues including the need for bleed or variable geometry, while too little compression can result in low cycle efficiency and poor combustion of fuel An analysis of the important factors that affect the choice of scramjet inlet compression ratio has been performed for hydrogen-fueled scramjets at Mach 6, 8, 10, and 12 It was found that contrary to classical thermodynamic analyses, scramjet cycle efficiency reaches an optimum at a relatively low compression ratio between 50 and 100 for all Mach numbers Practical constraints related to nonequilibrium flow effects, inlet starting, and boundary-layer separation were also shown to prompt a desire for low compression ratio The lower limit on compression was found to be set by the need to complete the combustion reaction in the available engine length and is therefore dependent on engine scale On the basis of these factors it is recommended that scramjet inlet compression ratio be set to the minimum that satisfies the robust combustion requirement, with the caveat that it not be below 50 in order to maintain high cycle efficiency For typical wind-tunnel-scale engines, this results in a requirement for the inlet to compress airflow entering the combustor to a pressure of approximately 1/2 atm, regardless of the flight Mach number

Continuous adjoint approach for the Spalart−Allmaras model in aerodynamic optimization

Abstract: a new variable, solving the Eikonal equation. The accuracy of the sensitivity derivatives obtained with the complete turbulent approach is assessed by comparison with finite difference computations and the classical continuous adjoint with frozen viscosity, showing substantial improvements in the convergence properties of the method and in the quality of the obtained gradients. The validity of the overall methodology is illustrated with several design examples, including the optimization of three-dimensional geometries in combination with advanced freeform techniques for mesh deformation.

Model Reduction of Computational Aerothermodynamics for Hypersonic Aerothermoelasticity

Abstract: A primary challenge for aerothermoelastic analysis in hypersonic flow is accurate and efficient computation of unsteady aerothermodynamic loads. This study examines two model reduction strategies with the goal to enable the use of computational fluid dynamics within a long time-record, dynamic, aerothermoelastic analysis. One approach seeks to exploit the quasi-steady nature of the flow by using steady-state computational fluid dynamics to capture primary flow features, and simple analytical approximations to account for unsteady effects. The second approach seeks to minimize the computational cost of steady-state computational fluid dynamics flow analysis using either kriging or proper orthogonal decomposition-based modeling techniques. These model reduction strategies are assessed, both individually and combined, in the context of a three-dimensional hypersonic control surface. Results computed over a wide range of operating conditions and reduced frequencies indicate that when combined, the considered approaches yield an aerothermodynamic model that is tractable within a dynamic aerothermoelastic analysis, and generally has less than 5% maximum error relative to computational fluid dynamics.

Computational Investigation into the Use of Response Functions for Aerodynamic-Load Modeling

Abstract: The generation of reduced-order models (ROM) for the evaluation of unsteady and nonlinear aerodynamic loads are investigated. The ROM considered is an indicial theory based on the convolution of step functions with the derivative of the input signal. The step functions are directly calculated using the results of RANS simulations and a grid movement tool. Results are reported for a two dimensional airfoil and a UCAV configuration. Wind tunnel data are first used to validate the prediction of static and unsteady coefficients at both low and high angles of attack, with good agreement obtained for all cases. The generation of the aerodynamic models is described. The focus of the paper shifts to assess the validity of studied ROMs with respect to new maneuvres. This is accomplished by comparison of the model output with time-accurate CFD simulations. The results show that the ROMs can accurately model the unsteady loads in response to slow and fast pitch and plunge motions.

Fuel Options for Next-Generation Chemical Propulsion

Abstract: The state of research on developing fuel options for next-generation chemical propulsion is reviewed for aviation fuels and energetic fuels. For aviation fuels, the development is based on considerations of cost, energy security, and climate change, with Fischer–Tropsch synthetic fuels and biofuels hold potential as alternative aviation fuels. The need for basic research to develop predictive capability for the oxidative chemistry of evolving fuels in evolving engine designs is emphasized, illustrated by the intricate reaction pathways and the enormity of the reaction mechanisms involved. Recent research activities toward achieving the goal of fuel design are discussed through the development of detailed mechanisms, reduced mechanisms, and surrogate fuels. For the development of highenergy-density propellants, advances in several classes of materials are discussed, including metallized and hypergolic propellants and propellants with strained and functionalized molecules, as well as nanoparticle addition. The impact of the recent progress in chemical synthesis, materials science, and nano science on these advances is noted.

X-HALE: A Very Flexible Unmanned Aerial Vehicle for Nonlinear Aeroelastic Tests

Abstract: The University of Michigan has designed and built an unmanned aerial vehicle denoted X-HALE, which is aeroelastically representative of very flexible aircraft. The objective of this test bed is to collect unique data of the geometrically nonlinear aeroelastic response coupled with the flight dynamics to be used for future code validation. The aircraft presents specific aeroelastic features (e.g., coupled rigid/elastic body instability, large wing deflection during gust) that can be measured in flight. Moreover, the airframe construction choice is such that the elastic, inertial, and geometric properties can be well characterized. These are requirements driven by the need of the collected data to be used to support validation of coupled nonlinear aeroelastic/flight dynamics codes.

Effects of Flow Structure Dynamics on Thermoacoustic Instabilities in Swirl-Stabilized Combustion

Abstract: The thermoacoustic coupling caused by dynamic flow/flame interactions was investigated in a gas-turbine model combustor using high-repetition-rate measurements of the three-component velocity field, OH laser-induced fluorescence, and OH* chemiluminescence. Three fuel-lean, swirl-stabilized flames were investigated, each of which underwent self-excited thermoacoustic pulsations. The most energetic flow structure at each condition was a helical vortex core that circumscribed the combustor at a frequency that was independent of the acoustics. Resolving the measurement sequence with respect to both the phase in the thermoacoustic cycle and the azimuthal position of the helix allowed quantification of the oscillatory flow and flame dynamics. Periodic vortex/flame interactions caused by deformation of the helices generated local heat-release oscillations having spatially complex phase distributions relative to the acoustics. The local thermoacoustic coupling, determined by statistically solving the Rayleigh integral, showed intertwined regions of positive and negative coupling due to these vortices. In the quietest flame, the helical vortex created a large region of negative coupling that helped damp the oscillations. In the louder flames, the shapes of the oscillating vortices and flames were such that large regions of positive coupling were generated, driving the instability. From these observations, flame/vortex configurations that promote stability are identified.

Numerical Investigation of Energy Extraction in a Tandem Flapping Wing Configuration

Abstract: A number of flying insects make use of tandem-wing configurations, suggesting that such a setup may have potential advantages over a single wing at low Reynolds numbers. Dragonflies, which are fast and highly maneuverable, demonstrate well the potential performance of such a design. In this paper, a tandem-wing flapping configuration is simulated at a Reynolds number of 10,000 using an incompressible Navier–Stokes solver and an overlapping gridmethod. The flappingmotion consists of a simple sinusoidal pitch and plungemotionwith a spacing of one chord length between both wings. The arrangement was tested at a Strouhal number of 0.3 for three different phase angles: 0, 90, and 180 deg. The aerodynamics of the hindwing was compared in detail to a single wing, with the same geometry and undergoing the same flapping kinematics, to determine the effect of vortex shedding from the forewing on the hindwing, as well as how the phase angle affects the interaction. The average lift, thrust, and power coefficients and the average efficiency of the foreand hindwings were comparedwith a single wing to determine how the tandem-wing interaction affects performance. The results show that adjusting the phase angle allows the tandem wing to change the flight mode. At 0 deg phase lag, the tandem wing produces high thrust at high propulsive efficiency, but low lift efficiency. Switching to 90=180 deg phase lag decreases the thrust production and propulsive efficiency but greatly increases the lift efficiency. At 90=180 deg, the power coefficient is much lower than at 0 deg, due to the hindwing extracting energy from the wake of the forewing.

Aerodynamic Shape Optimization of Wings Using a Parallel Newton-Krylov Approach

Abstract: ANewton–Krylov algorithm for aerodynamic shape optimization in three dimensions is presented for both singlepoint andmultipoint optimization. An inexact Newtonmethod is used to solve the Euler equations, a discrete adjoint method is used to compute the gradient, and an optimizer based on a quasi-Newtonmethod is used tofind the optimal geometry. Theflexible generalizedminimal residualmethod is usedwith approximate Schur preconditioning to solve both the flow equation and the adjoint equation. Thewing geometry is parameterized byB-spline surfaces, and a fast algebraic algorithm is used for grid movement at each iteration. An effective strategy is presented to enable simultaneous optimization of planform variables and section shapes. Optimization results are presented with up to 225 design variables to demonstrate the capabilities and efficiency of the approach.

General Reduced-Order Model to Design and Operate Synthetic Jet Actuators

Abstract: Synthetic jets are used in various applications from flow control to thermal management of electronics. Controlling the jet operating point using a simple voltage to velocity calibration becomes unreliable in case of external pressure field disturbances or varying actuator characteristics. This paper presents a general lumped parameter model for a synthetic jet actuator with an electromagnetic or piezoelectric driver. The fluidic model accurately predicts the synthetic jet operating point (i.e., Reynolds number and stroke length) based on the measured cavity pressure. The model requires only two empirical coefficients characterizing nozzle fluid damping and inertia. These can be obtained via calibration or estimated from pressure loss correlations and the governing acoustic radiation impedance. The model has been validated experimentally for circular and rectangular orifices. The effect of nozzle damping on the nonlinear system response is discussed. Analytical expressions are given for the two resonance frequencies characterizing the system response as a function of the diaphragm and Helmholtz resonance frequencies. The optimal design of an impinging synthetic jet actuator is discussed in terms of the thermal and acoustic efficiencies. Guidelines for selecting the optimum combination of diaphragm and Helmholtz resonance frequency are presented and compared with previous studies.

Large-Eddy Simulation of Shock/Boundary-Layer Interaction

Abstract: This work considers numerical simulations of supersonic flows when shock/turbulent boundary layer interaction occurs. Such flows reveal the existence of complex mechanisms, which need to be carefully investigated for efficient design of propulsion systems. In this study, large-eddy simulation is used to investigate unsteady mechanisms. Since a shock-capturing scheme is used, a hybrid numerical scheme has been developed to reduce its dissipative properties. The issue of the generation of coherent turbulent inlet boundary conditions is also addressed. To avoid introducing artificial low-frequency modes that could affect the interaction, a method based on a digital-filter approach originally developed by Klein et al. (2003) and modified by Xie & Castro (2008) and Touber & Sandham (2009) is used to provide a synthetic-inflow condition over a relatively short distance. The obtained results are analyzed and discussed in terms of mean and turbulent quantities. Excellent agreement between LES and experimental data is obtained for both the undisturbed boundary layer and the shock impingement region. In the latter case, oscillations of the reflected shock occurring at low frequencies are observed, in agreement with previous numerical and experimental findings. Moreover, simulations reveal the presence of such frequencies mainly near the shock foot and within the recirculation bubble. This point gives credit to the hypothesis that the instabilities of the reflected shock are due to the intrinsic low-frequency movement of the shock/bubble acting dynamically as a coupled system.

Liquid Jet Injection into a Supersonic Flow

Abstract: T study of a liquid jet injected normal to a supersonic gas crossflow is one of significant practical import and stimulating theoretical interest. There has been, however, a relative scarcity of experimental data pertaining to many details of the process, particularly the primary decomposition mechanism. The immediate stimulus and sources of the initial direction for this study were the experiments of Sherman and Schetz, wherein the presence on the jet of large axial waves, which appeared to create the dominant means of jet decomposition by gross detachment of large masses, was noted.

Dynamic Response Topology Optimization in the Time Domain Using Equivalent Static Loads

Abstract: Most topology optimization techniques find the optimal layout of a structure under static loads. Some studies are focused on dynamic response topology optimization because the dynamic forces act in the real world. Dynamic response topology optimization is solved in the time or frequency domain. A method for dynamic response topology optimization in the time domain is proposed using equivalent static loads. Equivalent static loads are static loads that generate the same displacement field as dynamic loads at each time step. The equivalent static loads are made by multiplying the linear stiffness matrix and the displacement field from dynamic analysis and used as multiple loading conditions for linear static topology optimization. The results of topology optimization are utilized in dynamic analysis again and a cyclic process is utilized until the convergence criterion is satisfied. The paradigm of the method was originally developed for size and shape optimizations. A new objective function is defined to minimize the peaks of the compliance in the time domain and a convergence criterion is newly defined considering that there are many design variables in topology optimization. The developed method is verified by solving some examples and the results are discussed.

Control of a Shock/Boundary-Layer Interaction by Using a Pulsed-Plasma Jet Actuator

Abstract: S HOCK wave/boundary-layer interactions (SWBLIs) are a common feature of supersonic/hypersonic flight, and the unsteadiness of strongly separated interactions can lead to rapid fatigue of structural panels as well as inlet instability and unstart. To mitigate these problems, there is interest in developing techniques for controlling the separatedflowunsteadiness by using both passive and active control techniques. Previous SWBLI control work has focused on reducing the size of the separated flow and/or shifting the frequency of the interaction unsteadiness to a band that does not coincide with the resonant frequency of structural panels. A detailed survey of the variousmeans used for controlling SWBLI until the late 1980s is given in [1]. Recently, plasma-based actuators have been used by researchers for active control of SWBLI, since these actuators have several inherent desirable features such as high bandwidth and no moving parts. For example, previous researchers have used surface-mounted arc discharges [2] and glow discharges with external magnetic fields [3,4] to achieve control of reflected SWBLI. Wang et al. [5] used surface-mounted arc discharges with external magnetic fields and demonstrated the weakening of the separation shock strength in front of a compression ramp. Recently, arc discharges have been employed by Grossman et al. [6] and subsequent researchers [7–11] to generate a pulsed synthetic jet, which they termed a spark jet. The spark-jet design was modified by Narayanaswamy et al. [12] to extend the pulsing frequency to the kilohertz range. They termed the actuator a pulsed-plasma jet since the term spark implies a thermal discharge, which was not the case at the pressures used in their study (and in the current work). Narayanaswamy et al. [12] performed a detailed parametric study of the velocity and temperature characteristics of the pulsed-plasma jets. They reported a jet-exit velocity of about 300 m=s and a bulk gas temperature in the range 600–1000 K for the range of discharge currents tested. The same pulsed-plasma-jet array actuator is used in the present work to control the separation shock of a SWBLI generated by a compression ramp in a Mach 3 flow.

Derivatives for Time-Spectral Computational Fluid Dynamics Using an Automatic Differentiation Adjoint

Abstract: sensitivities for an oscillating ONERA M6 wing. The sensitivities are shown to be accurate to 8–12 digits, and the computational cost of the adjoint computations is shown to scale well up to problems of more than 41 million state variables.

Low-Reynolds-Number Effects in Passive Stall Control Using Sinusoidal Leading Edges

Abstract: The objective of the presentwork is to investigate the application of a sinusoidal leading edge to the design ofmicro air vehicles. Wind-tunnel tests of wings with low aspect ratios of 1 and 1.5, rectangular planforms, and five distinct leading edges [four sinusoidal leading edges and one baseline (straight) leading edge for each aspect ratio] have been conducted. The Reynolds numbers of 70,000 and 140,000 have been analyzed. For the higher Reynolds number, a proper combination of amplitude andwavelength can lead to a substantial increase in lift for angles of attack greater than the baseline stall angle. Maximum lift coefficient gains of the order of 45% were achieved by combining both large amplitude and large wavelength. At the lower Reynolds number, the benefits can be extended to low angles of attack, leading to a dramatic increase in the range of operation. The results depend strongly on the aspect ratio.

Plasma Jet for Flight Control

Abstract: The need for high-speed flight-control devices has surged with recent interest in hypersonic flight. Plasma-based devices offer actuation times that are orders of magnitude smaller than conventional mechanical actuators. The plasma jet, which uses energy deposition to generate a high-speed jet, is evaluated for flight control. The jet is created by pulsing a cavity with energy deposition. The gas expands through a converging nozzle, inducing a jet flow. This research focuses on characterizing the forces generated by a single pulse of the plasma jet, assuming energy is deposited uniformly throughout the cavity and the jet exits to quiescent flow. A one-dimensional analytical model is developed to predict the force and impulse generated by the jet as well as the temporal evolution of the pressure, density, and temperature in the cavity. A relation between the dimensionless energy deposition and the dimensionless impulse is developed and verifiedwith computational results. It is shown that the dimensionless impulse generated by the jet is essentially independent of the dimensionless geometric parameters of the cavity. Additionally, a simplified analysis shows that the force from an array of plasma jets is sufficient to replace a conventional aerodynamic flap.