Showing papers in "AIAA Journal in 2013"

Multidisciplinary design optimization: A survey of architectures

Abstract: Multidisciplinary design optimization is a field of research that studies the application of numerical optimization techniques to the design of engineering systems involving multiple disciplines or components. Since the inception of multidisciplinary design optimization, various methods (architectures) have been developed and applied to solve multidisciplinary design-optimization problems. This paper provides a survey of all the architectures that have been presented in the literature so far. All architectures are explained in detail using a unified description that includes optimization problem statements, diagrams, and detailed algorithms. The diagrams show both data and process flow through the multidisciplinary system and computational elements, which facilitate the understanding of the various architectures, and how they relate to each other. A classification of the multidisciplinary design-optimization architectures based on their problem formulations and decomposition strategies is also provided, a...

Review and Unification of Methods for Computing Derivatives of Multidisciplinary Computational Models

Abstract: This paper presents a review of all existing discrete methods for computing the derivatives of computational models within a unified mathematical framework. This framework hinges on a new equation, the unifying chain rule, from which all the methods can be derived. The computation of derivatives is described as a two-step process: the evaluation of the partial derivatives and the computation of the total derivatives, which are dependent on the partial derivatives. Finite differences, the complex-step method, and symbolic differentiation are discussed as options for computing the partial derivatives. It is shown that these are building blocks with which the total derivatives can be assembled using algorithmic differentiation, the direct and adjoint methods, and coupled analytic methods for multidisciplinary systems. Each of these methods is presented and applied to a common numerical example to demonstrate and compare the various approaches. The paper ends with a discussion of current challenges and possib...

Experimental and numerical investigation of turbulence-airfoil noise reduction using wavy edges

Abstract: A passive leading-edge treatment based on sinusoidal serrations aimed at reducing turbofan interaction noise has been recently studied in the framework of a European project (FLOCON). The turbulence-airfoil interaction mechanism is achieved using a turbulence grid located upstream of an isolated NACA airfoil tested in the Institute of Sound and Vibration Research anechoic open jet wind tunnel. The experimental setup with several airfoils designed and manufactured by ONERA is first presented with main acoustic results, highlighting the sound power level reductions obtained for all studied flow speeds (about 3–4 dB reduction) without altering the aerodynamic performances (as shown by available measurements and Reynolds-averaged Navier–Stokes calculations). The experimental investigations are supplemented by numerical predictions in order to assess the acoustic performances of the serrations. The method described in the second part of the paper is based on a computational aeroacoustics code solving the nonli...

Noise-Reduction Mechanism of a Flat-Plate Serrated Trailing Edge

Abstract: This paper presents the results of an experimental investigation exploring the noise-reduction potential of sawtooth trailing-edge serrations on a flat plate at low-to-moderate Reynolds number (1.6×105

Multimodality and Global Optimization in Aerodynamic Design

Abstract: Two optimization algorithms are presented that are capable of finding a global optimum in a computationally efficient manner: a gradient-based multistart algorithm based on Sobol sampling and a hybrid optimizer combining a genetic algorithm with a gradient-based algorithm. The optimizers are used to investigate multimodality in aerodynamic-shape-optimization problems. The performance of each algorithm is tested on an analytical test function as well as several aerodynamic-shape-optimization problems in two and three dimensions. In each problem the primary objectives are to classify the problem according to the degree of multimodality and to identify the preferred optimization algorithm for the problem. The results show that multimodality should not always be assumed in aerodynamic-shape-optimization problems. Typical two-dimensional airfoil-optimization problems are unimodal. Three-dimensional shape-optimization problems may contain local optima. The number of local optima tends to increase with increasin...

Differential Interferometric Measurement of Instability in a Hypervelocity Boundary Layer

Abstract: The prediction of laminar–turbulent transition location in high-speed boundary layers is critical to hypersonic vehicle design because of the weight implications of increased skin friction and surface heating rate after transition. Current work in T5 (the California Institute of Technology’s free piston reflected shock tunnel) includes the study of problems relevant to hypervelocity boundary layer transition on cold-wall slender bodies. With the ability to ground-test hypervelocity flows, the study of energy exchange between the boundary layer instability and the internal energy of the fluid is emphasized. The most unstable mode on a cold-wall slender body at zero angle of incidence is not the viscous instability (as in low-speed boundary layers) but the acoustic instability. Quantitative characterization of this disturbance is paramount to the development of transition location-prediction tools.

Robust Gust Alleviation and Stabilization of Very Flexible Aircraft

Abstract: Robust linear control, combined with model-reduction methodologies, is investigated for gust rejection on large, very flexible aircraft using trailing-edge control surfaces. Controllers are designed on linearizations of the nonlinear aeroelastic equations, and the closed-loop response of both the linearized and nonlinear system to discrete gust distributions is compared to the open-loop dynamics. Results show that an ℋ∞ controller performs well on a relatively large linearized system, with 9% load alleviation in root bending moments for the critical gust length. When applied to the nonlinear model of the same vehicle, the robust controller gives a good performance in response to short gusts, including the critical length, with even better load reductions than the linear case. However, this performance gap decreases as the gust length is increased. It is also shown how standard model-reduction techniques can provide metrics for the selection of a minimum size of the aeroelastic system. Finally, it is shown...

Derivative-Enhanced Variable Fidelity Surrogate Modeling for Aerodynamic Functions

Abstract: In this paper a derivative-enhanced variable-fidelity surrogate model approach is developed based on a cokriging formulation. In this approach the absolute values of a high-fidelity function as well as the trends obtained by low-fidelity function values are utilized to develop an accurate surrogate model. Derivative information of arbitrary fidelity levels can be also utilized to develop a more accurate surrogate model. The efficiencies of the developed approaches are investigated by analytic function-fitting, aerodynamic data modeling and two-dimensional airfoil-drag-minimization problems. In the aerodynamic problems low-fidelity levels are defined by a different physical model or coarser computational mesh. The numerical examples show that the developed surrogate-model approach is shown to be useful for efficient aerodynamic data modeling and accurate uncertainty analysis with low computational cost. An efficient aerodynamic shape optimization is also realized with the variable fidelity Kriging model. F...

Strain-Based Deformation Shape-Estimation Algorithm for Control and Monitoring Applications

Abstract: A new deformation shape-sensing methodology is investigated for the purposes of real-time condition assessment, control, and health monitoring of flexible lightweight aerospace structures. The fiber optic strain sensing technology was recently proposed by the NASA Dryden Flight Research Center. The methodology implements the use of fiber optic sensors to obtain strain measurements from the target structure and to estimate the corresponding displacement field. In this paper, the methodology is investigated through an experimental aluminum winglike swept-plate model. The proposed algorithm is implemented for three distinct loading cases and compared to a well-established modal-based shape-estimation algorithm. The estimation results from both methods are also compared to reference displacements from photogrammetry and computational analyses. The estimation error for each method is quantified using the root-mean-square measure, and the range of validity of the approach for damage detection is established. Fu...

Aerodynamic Control of Low-Reynolds-Number Airfoil with Leading-Edge Protuberances

Abstract: This paper presents an experimental investigation of the control of airfoil aerodynamics at a low Reynolds number of 5×104 within a wide range of attack angle α using a leading-edge-protuberance technique. The essence of the technique is to manipulate flow around the airfoil by replacing the straight leading edge of a baseline airfoil with a sinusoidal wavy airfoil. Whereas the lift and drag forces and the lift-to-drag ratio were measured using a three-component force balance, the flow was mainly measured using a particle-image velocimetry. The sinusoidal protuberances effectively suppressed airfoil stall, and the corresponding aerodynamic performance was impaired to some extent. Meanwhile, control significantly improved the airfoil aerodynamics in the poststall α region, for example, 16<α<70 deg, leading to a maximum 25.0 and 39.2% increase in lift coefficient and lift-to-drag ratio, respectively, and a maximum 20.0% decrease in drag coefficient. The protuberances may influence control performance in a ...

Robust Topology Optimization: Minimization of Expected and Variance of Compliance

Abstract: Robust topology optimization has long been considered computationally intractable as it combines two highly expensive computational strategies. This paper considers simultaneous minimization of expectancy and variance of compliance in the presence of uncertainties in loading magnitude, using exact formulations and analytically derived sensitivities. It shows that only a few additional load cases are required, which scales in polynomial time with the number of uncertain load cases. The approach is implemented using the level set topology optimization method. Shape sensitivities are derived using the adjoint method. Several examples are used to investigate the effect of including variance in robust compliance optimization.

Numerical Simulation of Fully Passive Flapping Foil Power Generation

Abstract: A fully passive flapping foil turbine was simulated using a two-dimensional Navier–Stokes solver with two-way fluid-structure interaction at a Reynolds number based on freestream flow Re=1100 and 1.1×106 with a NACA 0012 foil. Both pitch angle and angle-of-attack control methodologies were investigated. Efficiencies of up to 30% based on the Betz criterion were found using pitch control, which is commensurate with values reported in the literature for prescribed motion studies. Nonsinusoidal foil pitching motions were found to be superior to sinusoidal motions. Efficiencies exceeding 41% were found using angle-of-attack control, and nonsinusoidal angle-of-attack profiles were found to be superior. The key to improving the efficiency of energy extraction from the flow is to control the timing of the formation and location of the leading-edge vortex at crucial times during the flapping cycle and the interaction of the vortex with the trailing edge. Simulations using Reynolds-averaged Navier–Stokes turbulenc...

Discrete Adjoint-Based Design for Unsteady Turbulent Flows on Dynamic Overset Unstructured Grids

Abstract: A discrete adjoint-based design methodology for unsteady turbulent flows on three-dimensional dynamic overset unstructured grids is formulated, implemented, and verified. The methodology supports both compressible and incompressible flows and is amenable to massively parallel computing environments. The approach provides a general framework for performing highly efficient and discretely consistent sensitivity analysis for problems involving arbitrary combinations of overset unstructured grids that may be static, undergoing rigid or deforming motions, or any combination thereof. General parent–child motions are also accommodated, and the accuracy of the implementation is established using an independent verification based on a complex-variable approach. The methodology is used to demonstrate aerodynamic optimizations of a wind-turbine geometry, a biologically inspired flapping wing, and a complex helicopter configuration subject to trimming constraints. The objective function for each problem is successful...

Classical, refined and component-wise analysis of reinforced-shell structures

Abstract: This paper compares early and very recent approaches to the static analysis of reinforced-shell wing structures. Early approaches were those based on the pure semimonocoque theory along with the beam assumptions of the Euler–Bernoulli and Timoshenko type. The recent approaches are based on a hierarchical, one-dimensional formulation. These are obtained by adopting various polynomial expansions of the displacement field above the cross-section of the structure according to the unified formulation which was recently proposed by the first author. Two classes were developed in the unified formulation framework. In the first class, Taylor expansion models were developed by exploiting N-order Taylor-like polynomials; classical beam theories (Euler–Bernoulli and Timoshenko) were obtained as special cases of Taylor expansion. In the second class, Lagrange expansion models were built by means of four- and nine-point Lagrange-type polynomials over the cross-section of the wing. The component-wise approach was obtai...

Green–Gauss/Weighted-Least-Squares Hybrid Gradient Reconstruction for Arbitrary Polyhedra Unstructured Grids

Abstract: T HE gradient calculation for the reconstruction of dependent variables is one of the critical issues for the accuracy and robustness of computational fluid dynamics (CFD) methods. There are many choices for the reconstruction for arbitrary polyhedra or polygons in unstructured meshes [1–4], which are represented by a family of weighted-least-squares (WLSQ) methods (including unweighted-least-squares [LSQ]) and a Green–Gauss (G–G) method. The WLSQ methods give exact gradients for a linear distribution of thevariables.On the other hand, theG–Greconstruction has this property only on symmetric and uniform meshes, because this method requires variables to be exactly on face centers, which are not generally obtained in a simple manner on Cartesian grids having hanging nodes (Fig. 1), for instance. Meanwhile, on thin-and-curved mesh that often appears in boundary layers for highReynolds number flow simulations, LSQ reportedly gives totally erratic gradients [1]; WLSQwith a properly chosen weighting function or G–G has better performance, albeit associated with certain errors, as shown in [5]. Therefore, each cell type/geometry has its own favorite gradient reconstructionmethods, for example Cartesian grids preferWLSQ to G–G, whereas thin-and-curved mesh does the opposite. Then, it is a natural question how to deal with the mixed grids of different types of cells. In recent years, body-fitted/Cartesian hybrid grids (sometimes called viscous Cartesian grids) [4,6–9] have been recognized as one of the standard types of unstructured grids, because they can resolve boundary layers as well as structured grids do, while saving the number of cells away from the wall. Thus, in this study, we will propose a robust and (second-order) accurate hybrid reconstruction method of WLSQ and G–G that is suitable for, but not limited to, those mixed grids in a unified manner, which overcomes the abovementioned difficulties encountered by existing methods. Our discussions are based on cell-centered schemes but are extendable to the cell-vertex counterpart by the simple replacement of the word “cell” with “control volume.” We point out here that many CFD practitioners still desire secondorder accuracy in space within the framework of an unstructured grid finite volume method (FVM) [10–13]. This has motivated us to pursue a second-order-accurate reconstruction method that is applicable to wide-ranging grid types and/or geometries, in spite of the growing attention to more sophisticated, higher order methods, such as in [14], spectral volume [15], or residual distribution [16,17] in the past several years.

Unsteady Behaviors of a Hypersonic Inlet Caused by Throttling in Shock Tunnel

Abstract: A two-dimensional hypersonic inlet/isolator model that exhibits self-starting characteristics is tested with different exit throttling ratios at a freestream Mach number of 59 in a shock tunnel Various flow characteristics are observed and measured by applying simultaneous high-speed Schlieren imaging and surface pressure measurements The results indicate that the backpressure generated by the throttling device can be tolerated, and the inlet can maintain the starting mode at low throttling ratios, whereas unstart flows are initiated from the near-choke throttling ratios, and a shock wave oscillation appears The Schlieren movie demonstrates that the upstream-propagating shocks in the duct play important roles during the oscillation cycles and that the formations of the upstream-propagating shocks are related to the downstream-propagating compression waves/shock waves that encounter the throttling section The frequency of the shock wave oscillation increases with increasing exit throttling ratios, pri

Three-Dimensional Large-Scale Aerodynamic Shape Optimization Based on Shape Calculus

Abstract: Large-scale three-dimensional aerodynamic shape optimization based on the compressible Euler equations is considered. Shape calculus is used to derive an exact surface formulation of the gradients, enabling the computation of shape gradient information for each surface mesh node without having to calculate further mesh sensitivities. Special attention is paid to the applicability to large-scale three dimensional problems like the optimization of an Onera M6 wing or a complete blended-wing–body aircraft. The actual optimization is conducted in a one-shot fashion, in which the tangential Laplace operator is used as a Hessian approximation, thereby also preserving the regularity of the shape.

Self-Noise Produced by an Airfoil with Nonflat Plate Trailing-Edge Serrations

Abstract: This paper represents the results of an experimental study aimed at reducing the airfoil self-noise by the trailing-edge serration of four different sawtooth geometries (defined in the serration angle and length). These serrations have a common feature: all of the sawtooth patterns are cut directly into the trailing edge of a realistic airfoil. This configuration offers better structural strength and integrity. For the sawtooth trailing edges investigated here, the radiation of the extraneous vortex shedding noise in a narrowband frequency due to the partial bluntness at the serration roots is unavoidable. However, this narrowband component tends to be less significant provided that the serration angle is large and the serration length is moderate. Sound power was measured, and some of the sawtooth geometries have been shown to afford significant boundary-layer instability tonal noise and moderate turbulent broadband noise reductions across a fairly large velocity range. This paper demonstrates that a non...

Dynamic Stall Control by Passive Disturbance Generators

Abstract: Passive cylindrical disturbance generators mounted near the leading edge of an airfoil significantly improved its performance under dynamic stall conditions. Time-resolved particle image velocimetry and simultaneous pressure measurements were conducted at the midchord of a pitching airfoil equipped with passive disturbance generators. The disturbance generators were effective in reducing the strength of the dynamic stall vortex and therefore the negative pitching moment peak and hysteresis effects. When the disturbance generators were applied, the flow separation type was altered from leading- to trailing-edge stall. In contrast to the clean case, reattachment was initiated immediately after the separation reached the leading-edge region. In addition to the circular shape, also backward- and forward-wedge-shaped disturbance generators were investigated. Although the backward wedge also showed favorable results, the forward wedge was less successful. The shape of the disturbance generators appears to have a strong influence on the effectiveness of reducing the negative impact of dynamic stall, depending on the sense of rotation of a pair of weak trailing vortices.

New Fluidic-Oscillator Concept for Flow-Separation Control

Abstract: A novel fluidic oscillator, particularly suitable to serve as the key element in actuators for control of fluid flows, was proposed and experimentally investigated. Its core part, as in known oscillators, is a fluidic bistable jet-deflection diverter amplifier. The unique feature of the new layout is the absence of the closed feedback loop channels. Instead, there is a quarter-wave Helmholtz resonator tube connected to one of the amplifier control terminals. The acoustic waves traveling in it and reflected from its open end cause the phase-delayed switching of the amplifier. The main advantage is high achievable oscillation frequency and no dependence on the flow rate.

Surrogate-Based Aerodynamic Shape Optimization by Variable-Resolution Models

Abstract: A surrogate-based optimization algorithm for transonic airfoil design is presented. The approach replaces the direct optimization of an accurate, but computationally expensive, high-fidelity computational fluid dynamics model by an iterative reoptimization of a physics-based surrogate model. The surrogate model is constructed, during each design iteration, using the low-fidelity model and the data obtained from one high-fidelity model evaluation. The low-fidelity model is based on the same governing fluid flow equations as the high-fidelity one, but uses coarser mesh resolution and relaxed convergence criteria. The shape-preserving response prediction technique is utilized to predict the high-fidelity model response, here, the airfoil pressure distribution. In this prediction process, the shape-preserving response prediction employs the actual changes of the low-fidelity model response due to the design variable adjustments. The shape-preserving response prediction algorithm is embedded into the trust reg...

Strength Prediction in Open Hole Composite Laminates by Using Discrete Damage Modeling

Abstract: The present paper addresses the issue of direct simulation of complex local failure patterns in laminated composites. A model capable of discrete modeling of matrix cracking, delamination, and the interaction of these two damage modes is proposed. The analytical technique uses the eXtended Finite Element Method (X-FEM) for the simulation of matrix crack initiation and propagation at initially unknown locations, as well as a cohesive interface model for delamination. The model is capable of representing the complex kinematics of crack networks in composite laminates without previous knowledge of the crack locations or user intervention. An important feature of the technique is that it uses independently measured standard ply-level mechanical properties of the unidirectional composite (stiffness, strength, fracture toughness). Failure simulations of composites containing open holes are presented. Although the process of crack initiation is impossible to capture precisely due to local material variations, the proposed method exhibits excellent agreement with experimental data for matrix crack growth in unidirectional graphite-epoxy composites.

Comparison of Adaptive Sampling Methods for Generation of Surrogate Aerodynamic Models

Abstract: A surrogate modeling strategy, using effective interpolation and sampling methods, facilitates a reduction in the number of computational fluid dynamics simulations required to construct an aerodynamic model to a specified accuracy. In this paper, two adaptive sampling strategies are compared for generating surrogate models, based on Kriging and radial basis function interpolation, respectively. The relationships between the two model formulations are discussed, and three test cases are considered, including analytic functions and recovery of aerodynamic coefficients for two example applications: longitudinal flight mechanics analysis for the DLR-F12 aircraft and structural loads analysis of an RAE2822 airfoil. For the airfoil example, models of CL, CD, and CM were constructed with the two sampling strategies using Euler/boundary-layer-coupled computational fluid dynamics and a three-dimensional flight envelope of incidence, Mach, and Reynolds number. The two sampling approaches direct some samples toward...

Broadband shock-associated noise in screeching and non-screeching underexpanded supersonic jets

Abstract: The effect of screech tones on the broadband shock-associated noise of underexpanded jets is investigated experimentally. Screech is removed by means of a notched nozzle, and the properties of the broadband shock-associated noise in the screech-free configuration are compared to that in a screeching flow. It is first demonstrated that the suppressing technique used is nonintrusive in that it does not alter the shock-cell structure of the jet plume. It is then shown that screech has an effect on the aerodynamics of the jet, which induces changes in the broadband shock-associated noise. Indeed, screech accelerates the damping of the shock-cell pattern, leading to an attenuation of the broadband shock-associated noise and a shifting of this noise component to higher frequencies. Moreover, a tuning between the peak frequency of the broadband shock-associated noise and the screech frequency is observed. It is also deduced from the directivity of the broadband shock-associated noise in the far field that the co...

Induced-Drag Calculations in the Unsteady Vortex Lattice Method

Abstract: α angle of attack, rad Γ circulation, m2s−1 λ wake wavelength, m ρ∞ free-stream air density, kgm −3 τ panel tangential vector ω angular velocity, rad s−1 a non-dimensional distance between aerofoil mid-chord and centre of rotation A panel area, m b semi-chord, m ∆b panel span, m B wingspan, m c aerofoil chord, m ∆c panel chord, m C Theodorsen’s function, C(k) = F (k) + iG(k) Cd sectional drag coefficient CD wing drag coefficient Cl sectional lift coefficient Cs sectional leading-edge suction coefficient ∗Graduate Student, Department of Aeronautics. AIAA Student Member. †Senior Lecturer, Department of Aeronautics. E-mail: rpalacio@imperial.ac.uk. AIAA Member. ‡Lecturer, Department of Mechanical Engineering Sciences. AIAA Member.

New Nonpolynomial Shear-Deformation Theories for Structural Behavior of Laminated-Composite and Sandwich Plates

Abstract: In the present study, new nonpolynomial shear-deformation theories are proposed and implemented for structural responses of laminated-composite and sandwich plates. The theories assume nonlinear distribution of transverse shear stresses, and also satisfy the traction-free boundary conditions at the top and bottom layers of the laminates. The governing differential equations are derived for a generalized shear-deformation theory by implementing the dynamic version of principle of virtual work and calculus of variations. A generalized closed-form solution methodology of the Navier type is implemented to ensure the validity and efficiency of the present theories for bending, buckling, and free-vibration responses of the laminated-composite and sandwich plates. It is observed that the proposed formulation in conjunction with the solution methodology is capable of handling all existing five-degree-of-freedom-based shear-deformation theories. The comparison of results also shows that the adequate choice of shea...