Search

Two- and three-dimensional low-aspect-ratio (AR = 4) hovering airfoil/wing aerodynamics at a low Reynolds number (Re = 100) are numerically investigated. Regarding fluid physics, in addition to the well-known leading-edge vortex and wake-capture mechanisms, a persistent jet, induced by the shed vortices in the wake during previous strokes, and tip vortices can significantly influence the lift and power performance. While in classical stationary wing theory the tip vortices are seen as wasted energy, here, they can interact with the leading-edge vortex to contribute to the lift generated without increasing the power requirements. Using surrogate modeling techniques, the two- and three-dimensional time-averaged aerodynamic forces were predicted well over a large range of kinematic motions when compared with the Navier-Stokes solutions. The combined effects of tip vortices, leading-edge vortex, and jet can be manipulated by the choice of kinematics to make a three-dimensional wing aerodynamically better or worse than an infinitely long wing. The environmental sensitivity during hovering for select kinematics is also examined. Different freestream strengths and orientations are imposed, with the impact on vortex generation and wake interaction investigated.

The extension of Li-ion batteries, from portable electronics to hybrid and electric vehicles, is significant. Developing a better understanding of the role of material properties and manipulating the morphology of the particle clusters comprising Li-ion electrodes could lead to potential opportunities for attaining higher performance goals, for which the effect of both material properties and morphology needs to be considered in a physics-based model. In this work, different particle packing arrangements are analyzed for the calculation of effective transport properties and reaction density that appear in the porous-electrode formulation due to the volume-averaging process. Surrogate-based analysis is used to systematically construct and validate reduced-order models for species transport at the particle-electrolyte interface. The low effective solid transport predicted through microscale modeling indicates the effect of packing arrangement and tortuosity, an aspect not captured by the Bruggeman’s relation. Particle cluster simulations reveal a Li-ion flux quantitatively different than that predicted by the porous-electrode model due to the variation of overpotential at the microscale. The present study offers a first-step towards integration of the effect of microstructure into a macroscale simulation.

Aiming at developing an effective tool to unveil key mechanisms in bio-flight as well as to provide guidelines for bio-inspired micro air vehicles (MAVs) design, we propose a comprehensive computational framework, which integrates aerodynamics, flight dynamics, vehicle stability and maneuverability. This framework consists of (1) a Navier–Stokes unsteady aerodynamic model; (2) a linear finite element model for structural dynamics; (3) a fluid-structure interaction (FSI) model for coupled flexible wing aerodynamics aeroelasticity; (4) a free-flying rigid body dynamic (RBD) model utilizing the Newtonian-Euler equations of 6DoF motion; and (5) flight simulator accounting for realistic wing-body morphology, flapping-wing and body kinematics, and a coupling model accounting for the nonlinear 6DoF flight dynamics and stability of insect flapping flight. Results are presented based on hovering aerodynamics with rigid and flexible wings of hawkmoth and fruitfly. The present approach can support systematic analyses of bio- and bio-inspired flight.

Micro air vehicles (MAVs) have the potential to revolutionize our sensing and information gathering capabilities in areas such as environmental monitoring and homeland security. Flapping wings with suitable wing kinematics, wing shapes, and flexible structures can enhance lift as well as thrust by exploiting large-scale vortical flow structures under various conditions. However, the scaling invariance of both fluid dynamics and structural dynamics as the size changes is fundamentally difficult. The focus of this review is to assess the recent progress in flapping wing aerodynamics and aeroelasticity. It is realized that a variation of the Reynolds number (wing sizing, flapping frequency, etc.) leads to a change in the leading edge vortex (LEV) and spanwise flow structures, which impacts the aerodynamic force generation. While in classical stationary wing theory, the tip vortices (TiVs) are seen as wasted energy, in flapping flight, they can interact with the LEV to enhance lift without increasing the power requirements. Surrogate modeling techniques can assess the aerodynamic outcomes between two- and three-dimensional wing. The combined effect of the TiVs, the LEV, and jet can improve the aerodynamics of a flapping wing. Regarding aeroelasticity, chordwise flexibility in the forward flight can substantially adjust the projected area normal to the flight trajectory via shape deformation, hence redistributing thrust and lift. Spanwise flexibility in the forward flight creates shape deformation from the wing root to the wing tip resulting in varied phase shift and effective angle of attack distribution along the wing span. Numerous open issues in flapping wing aerodynamics are highlighted.

The dielectric barrier discharge (DBD) plasma actuator offers promising opportunities for flow control because of its fast response and non-moving parts. In this work, surrogate modelling is adopted to better understand the impact of the materials and operational parameters on the actuator performance, and to provide an efficient approach for performance estimation. The DBD model based on 2-species helium chemistry engages three design variables: operating frequency, polarity (positive/negative) time ratio of the applied voltage and the dielectric constant of the insulator. Two objectives are identified: the net force generated and the power requirement. Multiple surrogate models are used, which identify two branches of the Pareto front with opposite net force direction and substantially different parametric sensitivities. Global sensitivity analysis indicates that the voltage frequency and polarity ratio are important in different regions of the design space, while the dielectric constant is always important.

Much progress has been made in modeling the lithium-ion battery technology. There exists a critical need to establish a framework to assess the role of various physical, geometrical, and operating parameters and their relative influence on the energy and power capability of batteries. In this study, a surrogate modeling framework has been introduced to map the effect of design-related parameters on the performance of a lithium-ion cell. In particular, the effects of cycling rate, cathode particle size, and diffusion coefficient and electrical conductivity of the solid cathode material, on the specific energy and power have been studied using a cell-level model in conjunction with tools such as kriging, polynomial response, and radial-basis neural networks. Through global sensitivity analysis the relative impact of the various parameters are quantified under different scenarios. Specifically, the design space can be split into distinct regions based on the discharge and diffusion time scales for separate, more refined analysis. It is shown that the cathode performance becomes independent of the diffusion coefficient above a critical value. A Pareto-optimal front was constructed to quantify the tradeoff between maximum achievable energy and power levels. Such an analysis can provide guidelines for the optimization of the positive electrode design.

Vanadium redox flow batteries (VRFBs) have attracted great attention for large-scale energy storage to stabilize the fluctuated and intermittent renewable energies, attributed to their advantages including decoupled energy and power, fast response, excellent reversibility, no cross-contamination and long cycle life [1,2]. However, the widespread commercial application of VRFBs is greatly hindered by the high capital cost. One commonly used way to decrease the high capital cost is to operate the battery at a high current density with high energy efficiency, because the enhanced current density can lead to the reduced stack size, which in turns decreases the usage of stack components. In this regard, tremendous efforts have been taken to modify the graphite felt to enhance the battery performances. Especially, etching the graphite felt surface shows great promise to fabricate the high-performance electrode, because it can effectively increase the active surface area and oxygen-functional groups. However, most of the reported works use high-temperature method to activate the graphite felt surface, which needs a great deal of additional energy to achieve the high temperature and has some special requirements towards devices, such as good temperature resistance, corrosion resistance and leakproofness, greatly increasing the cost to manufacture the electrodes. In addition, the high-temperature activation process is complicated and sometimes the N2 or argon protective atmosphere is required, making it difficult to mass production. Therefore, other works try to activate the graphite felt at room temperature, such as by Fenton’s reagent [3], modified Hummers method [4] and electrochemical activation [5]. Although promising, the additional energy, such as electrical energy, is still essential in many room-temperature activation methods. In addition, the adoption of concentrated acid inevitably leads to the corrosion of devices and safety concerns. More importantly, the state-of-art room-temperature activation methods are hard to compete with high-temperature activation methods, primarily due to their poor battery performance, which greatly limits the commercial application. In this work, we propose a novel room-temperature activation method, which is simple, no need for additional energy, no special requirements for devices and suitable for mass production, to fabricate the cost-effective, highly active and stable electrodes for vanadium redox flow batteries. The XPS and BET tests show that the activated graphite felt has increased oxygen-functional groups and effective surface areas. Then, the activation time and concentrations are systematically optimized. In the battery tests, the VRFBs with the prepared graphite felt electrodes deliver an energy efficiency of 80.9% at the high current density of 250 mA cm-2, and can be stably cycled for more than 500 cycles with a ~70% capacity retention. Further experimental results will be disclosed at the meeting. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) References [1] G.L. Soloveichik, Chem. Rev. 115 (2015) 11533-11558. [2] H. Jiang, W. Shyy, M. Wu, L. Wei, T. Zhao, J. Power Sources. 365 (2017) 34-42. [3] C. Gao, N. Wang, S. Peng, S. Liu, Y. Lei, X. Liang, S. Zeng, H. Zi, Electrochim. Acta. 88 (2013) 193-202. [4] X. Wu, H. Xu, Y. Shen, P. Xu, L. Lu, J. Fu, H. Zhao, Electrochim. Acta. 138 (2014) 264-269. [5] W. Zhang, J. Xi, Z. Li, H. Zhou, L. Liu, Z. Wu, X. Qiu, Electrochim. Acta. 89 (2013) 429-435.

Some issues of He-Chen-Zhang lattice Boltzmann equation (LBE) method (referred as HCZ model) (J. Comput. Physics 1999; 152:642-663) for immiscible multiphase flows with large density ratio are assessed in this paper. An extended HCZ model with a filter technique and mass correction procedure is proposed based on HCZ's LBE multiphase model. The original HCZ model is capable of maintaining a thin interface but is prone to generating unphysical oscillations in surface tension and index function at moderate values of density ratio. With a filtering technique, the monotonic variation of the index function across the interface is maintained with larger density ratio. Kim's surface tension formulation for diffuse-interface method (J. Comput. Physics 2005; 204:784-804) is then used to remove unphysical oscillation in the surface tension. Furthermore, as the density ratio increases, the effect of velocity divergence term neglected in the original HCZ model causes significant unphysical mass sources near the interface. By keeping the velocity divergence term, the unphysical mass sources near the interface can be removed with large density ratio. The long-time accumulation of the modeling and/or numerical errors in the HCZ model also results in the error of mass conservation of each dispersed phase. A mass correction procedure is devised to improve the performance of the method in this regard. For flows over a stationary and a rising bubble, and capillary waves with density ratio up to 100, the present approach yields solutions with interface thickness of about five to six lattices and no long-time diffusion, significantly advancing the performance of the LBE method for multiphase flow simulations.

Cavitation typically occurs when the fluid pressure is lower than the vapor pressure at a local thermodynamic state. The goal of our overall efforts is to establish a predictive tool for turbulent cavitating flows, including those under cryogenic conditions with noticeable thermal effect associated with the phase change. The modeling framework consists of a transport-based cavitation model with ensemble-averaged fluid dynamics equations and turbulence closures. To date, the reported experimental investigations contain little information about the turbulent characteristics in the flow field. It is noted that the exchange between static and dynamic pressures has a dominant impact on the cavitation dynamics, and the viscous effect can modify the effective shape of a solid object to cause noticeable variations in the predicted multiphase flow structures. The uncertainty of the inlet turbulent flow variables at the inlet can affect the prediction outcome. A filter-based approach is utilized along with two-equation turbulence closures so that one can assess the local numerical resolution with the computed turbulence length scale, and reduce the impact of the inlet boundary conditions of the eddy viscosity. Experimental information covering both isothermal and cryogenic cavitation with different geometries is utilized to aid the model validation. Furthermore, it is shown that for cryogenic cavitation, the thermal effect on the evaporation and condensation dynamics appears via modifying the local cavitation number and the liquid/vapor density ratio.

We consider a combined experimental (based on flow visualization, direct force measurement and phaseaveraged 2D particle image velocimetry in a water tunnel), computational (2D Reynolds-averaged Navier-Stokes) and theoretical (Theodorsen’s formula) approach to study the fluid physics of rigid-airfoil pitch-plunge in nominally two-dimensional conditions. Shallow-stall (combined pitch-plunge) and deep-stall (pure-plunge) are compared at a reduced frequency commensurate with flapping-flight in cruise in nature. Objectives include assessment of how well attached-flow theory can predict lift coefficient even in the presence of significant separation, and how well 2D velocimetry and 2D computation can mutually validate one another. The shallow-stall case shows promising agreement between computation and experiment, while in the deepstall case, the computation’s prediction of flow separation lags that of the experiment, but eventually evinces qualitatively similar leading edge vortex size. Dye injection was found to give good qualitative match with particle image velocimetry in describing leading edge vortex formation and return to flow reattachment, and also gave evidence of strong spanwise growth of flow separation after leadingedge vortex formation. Reynolds number effects, in the range of 10,000-60,000, were found to influence the size of laminar separation in those phases of motion where instantaneous angle of attack was well below stall, but have limited effect on post-stall flowfield behavior. Discrepancy in lift coefficient time history between experiment, theory and computation was mutually comparable, with no clear failure of Theodorsen’s formula. This is surprising and encouraging, especially for the deep-stall case, because the theory’s assumptions are clearly violated, while its prediction of lift coefficient remains useful for capturing general trends.

AEROSPACE LETTERS are brief communications (approximately 2000 words) that describe new and potentially important ideas or results, including critical analytical or experimental observations that justify rapid publication. They are stringently prescreened, and only a few are selected for rapid review by an Editor. They are published as soon as possible electronically and then appear in the print version of the journal.

Numerical investigation of vortex dynamics in near wake of a hovering hawkmoth and hovering aerodynamics is conducted to support the development of a biology-inspired dynamic flight simulator for flapping wing-based micro air vehicles. Realistic wing-body morphologies and kinematics are adopted in the numerical simulations. The computed results show 3D mechanisms of vortical flow structures in hawkmoth-like hovering. A horseshoe-shaped primary vortex is observed to wrap around each wing during the early down- and upstroke; the horseshoe-shaped vortex subsequently grows into a doughnut-shaped vortex ring with an intense jet-flow present in its core, forming a downwash. The doughnut-shaped vortex rings of the wing pair eventually break up into two circular vortex rings as they propagate downstream in the wake. The aerodynamic yawing and rolling torques are canceled out due to the symmetric wing kinematics even though the aerodynamic pitching torque shows significant variation with time. On the other hand, the time-varying the aerodynamics pitching torque could make the body a longitudinal oscillation over one flapping cycle.

Supercavitation around a hydrofoil is studied based on flow visualization and detailed velocity measurement. The main purpose of this study is to offer information for validating computational models, and to shed light on the multiphase transport processes. A high-speed video camera is used to visualize the flow structures under different cavitation numbers, and a particle image velocimetry (PIV) technique is used to measure the instantaneous velocity and vorticity fields. It is shown that the cavitation structure depends on the interaction of the water–vapor mixture and the vapor among the whole supercavitation stage. As the cavitation number is progressively lowered, three supercavitating flow regimes are observed: first, fluctuating cavity with periodic vortex shedding, then, vapor and water–vapor mixture coexist inside the cavity with a turbulent wake, and finally, a cavity largely filled with vapor and with a two-phase tail and distinct phase boundaries in the wake region. Even though the overall cavity boundary seems to be quite steady, the unsteadiness of the pressure fluctuation and mass transfer process between the vapor and the two-phase regions is substantial. Furthermore, in the cavitating region, strong momentum transfer between the higher and lower flow layers takes place, resulting in a highly even velocity distribution in the core part of the cavitating region, and the lower velocity area becomes smaller and, as the cavitation number lowers, moves toward the downstream.

PurposeThe purpose of this paper is to develop an empiricism free, first principle‐based model to simulate fluid flow and heat transfer through porous media.Design/methodology/approachConventional approaches to the problem are reviewed. A multi‐scale approach that makes use of the sample simulations at the individual pore levels is employed. The effect of porous structures on the global fluid flow is accounted for via local volume averaged governing equations, while the closure terms are accounted for via averaging flow characteristics around the pores.FindingsThe performance of the model has been tested for an isothermal flow case. Good agreement with experimental data were achieved. Both the permeability and Ergun coefficient are shown to be flow properties as opposed to the empirical approach which typically results in constant values of these parameters independent of the flow conditions. Hence, the present multi‐scale approach is more versatile and can account for the possible changes in flow characteristics.Research limitations/implicationsFurther validation including non‐isothermal cases is necessary. Current scope of the model is limited to incompressible flows. The methodology can accommodate extension to compressible flows.Originality/valueThis paper proposes a method that eliminates the dependence of the numerical porous media simulations on empirical data. Although the model increases the fidelity of the simulations, it is still computationally affordable due to the use of a multi‐scale methodology.

To better understand the multiphase fluid dynamics and associated transport processes of cavitating flows at the capillary number of 0.74 and 0.54, and to validate the numerical results, a combined computational and experimental investigation of flows around a hydrofoil is studied based on flow visualizations and time-resolved interface movement. The computational model is based on a modified RNG k-e model as turbulence closure, along with a vapor–liquid mass transfer model for treating the cavitation process. Overall, favorable agreement between the numerical and experimental results is observed. It is shown that the cavitation structure depends on the interaction of the water–vapor mixture and the vapor among the whole cavitation stage, the interface between the vapor and the two-phase mixture exhibits substantial unsteadiness. And, the adverse motion of the interface relates to pressure and velocity fluctuations inside the cavity. In particular, the velocity in the vapor region is lower than that in the two-phase region.

Error estimation measures are useful for assessing uncertainty in surrogate predictions. We use a suite of test problems to appraise several error estimation measures for polynomial response surfaces and kriging. In addition, we study the performance of cross-validation error measures that can be used with any surrogate. We use 1,000 experimental designs to obtain the variability of error estimates with respect to the experimental designs for each problem. We find that the (actual) errors for polynomial response surfaces are less sensitive to the choice of experimental designs than the kriging errors. This is attributed to the variability in the maximum likelihood estimates of the kriging parameters. We find that no single error measure outperforms other measures on all the problems. Computationally expensive integrated local error measures (standard error for polynomials and mean square error for kriging) estimate the actual root mean square error very well. The distribution-free cross-validation error characterized the actual errors reasonably well. While the estimated root mean square error for polynomial response surface is a good estimate of the actual errors, the process variance for kriging is not. We explore a few methods of simultaneously using multiple error measures and demonstrate that the geometric means of several combinations of error measures improve the assessment of the actual errors over individual error measures.

For surrogate construction, a good experimental design (ED) is essential to simultaneously reduce the effect of noise and bias errors. However, most EDs cater to a single criterion and may lead to small gains in that criterion at the expense of large deteriorations in other criteria. We use multiple criteria to assess the performance of different popular EDs. We demonstrate that these EDs offer different trade-offs, and that use of a single criterion is indeed risky. In addition, we show that popular EDs, such as Latin hypercube sampling (LHS) and D-optimal designs, often leave large regions of the design space unsampled even for moderate dimensions. We discuss a few possible strategies to combine multiple criteria and illustrate them with examples. We show that complementary criteria (e.g. bias handling criterion for variance-based designs and vice versa) can be combined to improve the performance of EDs. We demonstrate improvements in the trade-offs between noise and bias error by combining a model-based criterion, like the D-optimality criterion, and a geometry-based criterion, like LHS. Next, we demonstrate that selecting an ED from three candidate EDs using a suitable error-based criterion helped eliminate potentially poor designs. Finally, we show benefits from combining the multiple criteria-based strategies, that is, generation of multiple EDs using the D-optimality and LHS criteria, and selecting one design using a pointwise bias error criterion. The encouraging results from the examples indicate that it may be worthwhile studying these strategies more rigorously and in more detail. Copyright © 2007 John Wiley & Sons, Ltd.

Micro air vehicles (MAV’s) have the potential to revolutionize our sensing and information gathering capabilities in environmental monitoring and homeland security areas. Due to the MAV’s’ small size, flight regime, and modes of operation, significant scientific advancement will be needed to create this revolutionary capability. Aerodynamics, structural dynamics, and flight dynamics of natural flyers intersects with some of the richest problems in MAV’s, including massively unsteady three-dimensional separation, transition in boundary layers and shear layers, vortical flows and bluff body flows, unsteady flight environment, aeroelasticity, and nonlinear and adaptive control are just a few examples. A challenge is that the scaling of both fluid dynamics and structural dynamics between smaller natural flyer and practical flying hardware/lab experiment (larger dimension) is fundamentally difficult. In this paper, we offer an overview of the challenges and issues, along with sample results illustrating some of the efforts made from a computational modeling angle.

The performance of a diffuser in a pump stage depends on its configuration and placement within the stage. The influence of vane shape on the hydrodynamic performance of a diffuser has been studied. The goal of this effort has been to improve the performance of a pump stage by optimizing the shape of the diffuser vanes. The shape of the vanes was defined using Bezier curves and circular arcs. Surrogate model based tools were used to identify regions of the vane that have a strong influence on its performance. Optimization of the vane shape, in the absence of manufacturing, and stress constraints, led to a nearly nine percent reduction in the total pressure losses compared to the baseline design by reducing the extent of the base separation.

Fixed wing micro air vehicles (wingspan between 10 and 15 cm) are aerodynamically challenging due to the low Reynolds number regime (104–105) they operate in. The low aspect ratio wings (typically used to maximize area under a size constraint) promote strong tip vortices, and are susceptible to rolling instabilities. Wind gusts can be of the same order of magnitude as the flight speed (10–15 m/s). Standard control surfaces on an empennage must be eliminated for size considerations and drag reduction, and the range of stable center of gravity locations is only a few millimeters long. Membrane aeroelasticity has been identified as a tenable method to alleviate these issues: flexible wing structures with geometric twist (adaptive washout for gust rejection, delayed stall) and aerodynamic twist (adaptive inflation for high lift, larger stability margins) are both considered here. Recent investigations in static aeroelastic characterization, including flight loads, wing deformation, flow structures, aeroelastic-tailoring studies through laminate orientation, as well as unconventional techniques based on membrane pre-tension, are reviewed. Multi-objective optimization aimed at improving lift, drag, and pitching moment considerations is also discussed.

A dielectric barrier discharge, operating at kHz and kV conditions, can generate largely isothermal surface plasma and induce wall-jet-like fluid flow. It can serve as an aerodynamic actuator, and has advantages of no moving parts. In order to better understand the mechanism of the momentum coupling between the plasma and the fluid flow, both computational modeling and experimental information are presented. Furthermore, the impact of such athermal, non-equilibrium plasma discharges on low-speed aerodynamics and heat transfer is discussed. The plasma and fluid species are treated as a two-fluid system exhibiting decades of length and time-scale disparities. For Reynolds numbers of 104–105, the time-scales ratios between those characterizing the discharge physics (convection, diffusion, and reaction/ionization) and the fluid flow mechanisms are separated by several decades, allowing the effect of plasma on the fluid dynamics modeled via a one-way body force treatment. At a phenomenological level, the plasma model can be established using a linearized force distribution to approximate the discharge structure. A high-fidelity approach using a first-principle-based hydrodynamic-plasma model is also reviewed. Numerical techniques such as operating splitting are introduced in order to handle the computational stiffness resulting from the time and length scale variations. The goal is to use time-step sizes in the range of the fluid dynamics level while treating the fast varying ones statistically. The momentum coupling is discussed in the context of discharge chemistry; species transport properties, insulator and electrode materials, and dielectric barrier discharge (DBD) geometry. Parametric studies conducted on the operating variables such as voltage, frequency and geometric arrangements offer substantial insight into the plasma physics, as well as a basis to explore thermal management and flow control applications.

A major issue in surrogate model-based design optimization is the modeling fidelity. An effective approach is to employ multiple surrogates based on the same training data to offer approximations from alternative modeling viewpoints. This approach is employed in a compressor blade shape optimization using the NASA rotor 37 as the case study. The surrogate models considered include polynomial response surface approximation, Kriging, and radial basis neural network. In addition, a weighted average model based on global error measures is constructed. Sequential quadratic programming is used to search the optimal point based on these alternative surrogates. Three design variables characterizing the blade regarding sweep, lean, and skew are selected along with the three-level full factorial approach for design of experiment. The optimization is guided by three objectives aimed at maximizing the adiabatic efficiency, as well as the total pressure and total temperature ratios. The optimized compressor blades yield lower losses by moving the separation line toward the downstream direction. The optima for total pressure and total temperature ratios are similar, but the optimum for adiabatic efficiency is located far from them. It is found that the most accurate surrogate did not always lead to the best design. This demonstrated that using multiple surrogates can improve the robustness of the optimization at a minimal computational cost.

SUMMARY Computational aeroacoustics requires numerical techniques capable of yielding low artificial dispersion and dissipation to preserve the amplitude and the frequency characteristics of the physical processes. Furthermore, for engineering applications, the techniques need to handle irregular geometries associated with realistic configurations. We address these issues by developing an optimized prefactored compact finite volume (OPC-fv) scheme along with a Cartesian cut-cell technique. The OPC-fv scheme seeks to minimize numerical dispersion and dissipation while satisfying the conservation laws. The cut-cell approach treats irregularly shaped boundaries using divide-and-merge procedures for the Cartesian cells while maintaining a desirable level of accuracy. We assess these techniques using several canonical test problems, involving different levels of physical and geometric complexities. Richardson extrapolation is an effective tool for evaluating solutions of no high gradients or discontinuities, and is used to evaluate the performance of the solution technique. It is demonstrated that while the cut-cell method has a modest effect on the order of accuracy, it is a robust method. The combined OPC-fv scheme and the Cartesian cut-cell technique offer good accuracy as well as geometric flexibility. Copyright q 2008 John Wiley & Sons, Ltd.

A low-aspect-ratio, low-Reynolds-number membrane wing has been identified as a viable platform for micro air vehicle applications. Desirable flying qualities include high lift and larger stability margins. Several challenges are associated with the numerical modeling of such a wing, including highly three-dimensional flows, separation bubbles, and nonlinear membrane behavior. A thorough model validation and system identification effort is therefore required. A novel experimental setup integrates a wind tunnel with a visual image correlation system for simultaneous measurement of wing displacements, strains, and aerodynamic loads. These three metrics are used for a direct comparison of numerical and experimental data for both pre- and poststall angles of attack. Suitable correspondence is demonstrated for moderate angles of attack; methods for increasing the model fidelity can be made for angles with poor predictive capability. Computed flow structures reveal further information concerning the aeroelastic behavior of membrane wings.

For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.

A multiple surrogate-based optimization strategy in conjunction with an evolutionary algorithm has been employed to optimize the shape of a simplified hydraulic turbine diffuser utilizing three-dimensional Reynolds-averaged Navier–Stokes computational fluid dynamics solutions. Specifically, the diffuser performance is optimized by changing five geometric design variables to maximize the average pressure recovery factor for two inlet boundary conditions with different swirl, corresponding to different operating modes of the hydraulic turbine. Polynomial response surfaces and radial basis neural networks are used as surrogates, while a hybrid formulation of the NSGA-IIa evolutionary algorithm and a ϵ-constraint strategy is applied to construct the Pareto front from the two surrogates. The proposed optimization framework drastically reduces the computational load of the problem, compared to solely utilizing an evolutionary algorithm. For the present problem, the radial basis neural networks are more accurate near the Pareto front while the response surface performs better in regions away from it. By using a local resampling updating scheme the fidelity of both surrogates is improved, especially near the Pareto front. The optimal design yields larger wall angles, nonaxisymmetrical shapes, and delay in wall separation, resulting in 14.4% and 8.9% improvement, respectively, for the two inlet boundary conditions.

Glow discharge at atmospheric pressure using a dielectric barrier discharge can induce fluid flow, and can be used for active control of aerodynamics and heat transfer. In the present work, a modeling framework is presented to study the evolution and interaction of such athermal nonequilibrium plasma discharges in conjunction with low Mach number fluid dynamics and heat transfer. The model is self-consistent, coupling the first-principles-based discharge dynamics with the fluid dynamics and heat transfer equations. Under atmospheric pressure, the discharge can be simulated using a plasma–fluid instead of a kinetic model. The plasma and fluid species are treated as a two-fluid system coupled through force and pressure interactions, over decades of length and time scales. The multiple-scale processes such as convection, diffusion, and reaction/ionization mechanisms make the transport equations of the plasma dynamics stiff. To handle the stiffness, a finite-volume operator-split algorithm capable of conserving space charge is employed. A body force treatment is devised to link the plasma dynamics and thermo-fluid dynamics. The potential of the actuator for flow control and thermal management is illustrated using case studies.

In lattice Boltzmann simulations, variable viscosity can complicate the truncation error analysis and create additional interaction between the truncation error and the boundary condition error. In order to address this issue, two boundary conditions for the lattice Boltzmann equation (LBE) simulations are used, including an exact, but narrowly applicable scheme previously proposed by Noble et al. (Phys. Fluids 1995; 7(1):203–209) and the popular bounce-back-on-link scheme. Using a 2-D laminar channel flow with a specified variable viscosity as a test case, it is shown that the boundary treatment error does not have a significant interaction with the truncation error associated with variable viscosity. The truncation error behaviour of the LBE for flows with variable viscosity is further investigated through a comparison between the LBE solution and the Navier–Stokes solution, showing that in the presence of strong variable viscosity the truncation error behaviour of the LBE solution is consistent with that of the Navier–Stokes solution, indicating that the LBE model closely matches the Navier–Stokes model for fluid flows with large viscosity variation. Copyright © 2006 John Wiley & Sons, Ltd.

A systematic approach is presented to approximate the Pareto optimal front (POF) by a response surface approximation. The data for the POF is obtained by multi-objective evolutionary algorithm. Improvements to address drift in the POF are also presented. The approximated POF can help visualize and quantify trade-offs among objectives to select compromise designs. The bounds of this approximate POF are obtained using multiple convex-hulls. The proposed approach is applied to study trade-offs among objectives of a rocket injector design problem where performance and life objectives compete. The POF is approximated using a quintic polynomial. The compromise region quantifies trade-offs among objectives. � 2006 Elsevier B.V. All rights reserved.

Lithium-ion batteries have attracted considerable academic and commercial interests because of their higher energy and power density relative to other rechargeable electrochemical systems. Recently, they have been used to power an increasing range of appliances, ranging from implantable medical devices to electric vehicles. Since the commercialization of lithium-ion batteries by Sony in 1991, there has been an increasing amount of research carried out to further improve their energy density, life cycle, and thermal management. In addition to experimental studies, numerical models have offered new insights into physical phenomena and processes occurring in the cell and have supported the design optimization of cells and modules. However, given the multiscale and multiphase nature of a cell, the detailed modeling of every aspect of the cell has been difficult to achieve. While thermomechanical issues play a key role in battery design, the importance of broader issues related to cell degradation, safety, manufacturing, and battery management cannot be ignored. As the achievable performance edges closer to the theoretical limits, further gains have to come from major changes, such as new electrode materials, new cell architectures, or transitioning from intercalation to conversion chemistry. Proper consideration of the wide variety of issues that impact battery performance is needed to support further investigation to meet these challenges. Keywords: energy-storage systems; electric vehicles; lithium-ion batteries; optimization; multiphysics modeling; engineering design; electrochemical engineering

The aim of the present study is to understand the mechanism of propulsion in biological flyers, initially starting from rest, using a computational analysis of the plunging motion of a flexible wing. Simulations are performed on a self-propelled flexible wing unconstrained to move in direction perpendicular to the heaving motion. A two-dimensional lumped torsional flexibility model has been developed for a multi-component system which mimics a real insect wing. Using the lattice-Boltzmann method, this model is used to investigate the role of chordwise flexibility on fluid mechanics associated with forward propulsion. The aerodynamics and vortical wake patterns at different frequencies are investigated for a range of Reynolds number and bending stiffnesses. In addition, the behavior of input power and efficiency with stiffness has been examined. This work is anticipated to pave the way towards an improved understanding of propulsion in biological flyers employing flexible wings and thereby contribute to development of micro-air vehicles employing the same concept.

A translating discontinuous-grid-block model for moving boundaries of finite thickness based on multi-relaxation time version of lattice Boltzmann method has been developed. The implementation of this model to simulate moving boundary flows has been demonstrated for the cases of a cylinder in simple shear flow, a single rigid wing executing ‘clap and fling’ motion, and the propulsion of a plunging flat plate. A number of interpolation schemes of linear, quadratic and cubic natures are assessed around the discontinuous grid interface. It is shown that the implementation of a body-fitted refined mesh that moves along with the object reduces the spurious oscillations registered in the force and velocity measurements compared to a single coarse grid block. Moreover, use of multiple relaxation times helps overcome stability issues at high Reynolds number, normally encountered in the single-relaxation time model. Significantly, in the former model the same base grid could handle flows with good accuracy for 10 ≤ Re ≤ 1000. The proposed technique offers significant advantage in terms of capturing flow around moving solids at lower computational cost and simulation time as compared to the stationary discontinuous-grid-block method.

Multiphase and multidomain fluid flows associated with interfacial dynamics, steep jump in fluid properties, and moving boundaries between different phases and materials pose substantial computational challenges. In this paper, recent progresses made in numerical simulation using continuum and lattice–Boltzmann methods are highlighted, with special attention paid to issues related to time varying geometries, adaptive grid refinement, interfacial and geometric tracking, topological changes, and reduced numerical dispersion and dissipation. Illustrative physical applications including: (1) multiple rising bubbles interacting with carrier phase and with each other; (2) wave reflected on an inclined wall; and (3) the Rayleigh–Taylor instability problem, are presented to highlight the various aspects of the techniques developed.

The custom in surrogate-based modeling of complex engineering problems is to fit one or more surrogate models and select the one surrogate model that performs best. In this paper, we extend the utility of an ensemble of surrogates to (1) identify regions of possible high errors at locations where predictions of surrogates widely differ, and (2) provide a more robust approximation approach. We explore the possibility of using the best surrogate or a weighted average surrogate model instead of individual surrogate models. The weights associated with each surrogate model are determined based on the errors in surrogates. We demonstrate the advantages of an ensemble of surrogates using analytical problems and one engineering problem. We show that for a single problem the choice of test surrogate can depend on the design of experiments.

This paper proposes a generalized pointwise bias error bounds estimation method for polynomial-based response surface approximations when bias errors are substantial. A relaxation parameter is introduced to account for inconsistencies between the data and the assumed true model. The method is demonstrated with a polynomial example where the model is a quadratic polynomial while the true function is assumed to be cubic polynomial. The effect of relaxation parameter is studied. It is demonstrated that when bias errors dominate, the bias error bounds characterize the actual error field better than the standard error. The bias error bound estimates also help to identify regions in the design space where the accuracy of the response surface approximations is inadequate. It is demonstrated that this information can be utilized for adaptive sampling in order to improve accuracy in such regions.

In developing suitable numerical techniques for computational aero-acoustics, the dispersion-relation-preserving (DRP) scheme by Tam and co-workers and the optimized prefactored compact (OPC) scheme by Ashcroft and Zhang have shown desirable properties of reducing both dissipative and dispersive errors. These schemes, originally based on the finite difference, attempt to optimize the coefficients for better resolution of short waves with respect to the computational grid while maintaining pre-determined formal orders of accuracy. In the present study, finite volume formulations of both schemes are presented to better handle the nonlinearity and complex geometry encountered in many engineering applications. Linear and nonlinear wave equations, with and without viscous dissipation, have been adopted as the test problems. Highlighting the principal characteristics of the schemes and utilizing linear and nonlinear wave equations with different wavelengths as the test cases, the performance of these approaches is documented. For the linear wave equation, there is no major difference between the DRP and OPC schemes. For the nonlinear wave equations, the finite volume version of both DRP and OPC schemes offers substantially better solutions in regions of high gradient or discontinuity.

Tip vortex induces downwash movement, which reduces the effective angle of attack of a wing. For a low-aspect. ratio, low-Reynolds-number wing, such as that employed by the micro air vehicle (MAV), the induced drag by the tip vortex substantially affects its aerodynamic performance. In this paper we use the endplate concept to help probe the tip-vortex effects on the MAV aeredynamic characteristics. The investigation is facilitated by solving the Navier-Stokes equations around a rigid wing with a root-chord Reynolds number of 9 x 10 4

The sharp interface method (SIM) utilizing the Cartesian grid and cut-cell approach can solve fluid flow problems involving multiphase and/or complex geometry with second-order accuracy. Due to the algorithm and data management requirements, the SIM is computationally intensive. In this study, two issues are investigated to help further advance the SIM. First, the conservation-law property at the interface is examined in detail; an artificially assigned interface inside a single-phase, homogeneous fluid domain is devised to examine SIM's performance. Second, an interface-averaging multigrid method, accounting for the presence of irregularly shaped moving boundaries, is developed to speed up the pressure solver. The present work has demonstrated that the SIM satisfies the conservation laws numerically and has identified ways to reduce the computing time.

Thermal effects substantially impact the cavitation dynamics of cryogenic fluids. The present article reviews recent progress made toward developing modeling and computational strategies to simulate cryogenic cavitation relevant to liquid rocket propulsion applications. We re-examine previously developed cavitation models, including thermal effect and turbulence closures. The experimentally observed “frosty” appearance within the cavity is modeled as a mushy phase boundary. The impact of model parameters and material properties on the prediction is probed by global sensitivity techniques. Performance of the reported cavitation models is compared against the existing cavitation models and experimental data, under both non-cryogenic and cryogenic conditions. Time-dependent computations for various cases of cryogenic cavitation are further reviewed. Impact of the cryogenic environment and inflow perturbations on the flow structure and instabilities is explained via the simulated flow fields and the reduced order strategy of proper orthogonal decomposition (POD).

Micro air vehicles (MAVs) with a wingspan of 15cm or shorter, and flight speed around 10m∕s have attracted substantial interest in recent years. There are several prominent features of MAV flight: (i) low Reynolds number (104-105), resulting in degraded aerodynamic performance, (ii) small physical dimensions, resulting in certain favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance, and (iii) low flight speed, resulting in order one effect of the flight environment and intrinsically unsteady flight characteristics. Flexible wings utilizing membrane materials are employed by natural flyers such as bats and insects. Compared to a rigid wing, a membrane wing can better adapt to the stall and has the potential for morphing to achieve enhanced agility and storage consideration. We will discuss the aerodynamics of both rigid and membrane wings under the MAV flight condition. To understand membrane wing performance, the fluid and structure interaction is of critical importance. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble, and tip vortex, as well as structural dynamics in response to the surrounding flow field are discussed. Based on the computational capabilities for treating moving boundary problems, an automated wing shape optimization technique is also developed. Salient features of the flexible-wing-based MAV, including the vehicle concept, flexible wing design, novel fabrication methods, aerodynamic assessment, and flight data analysis are highlighted.

To gain insight into the aerodynamics of flexible wing-based micro air vehicles (MAVs), we study the threedimensional interaction between a membrane wing and its surrounding fluid flow. A nonlinear membrane structural solver and a Navier‐Stokes flow solver are coupled through the moving boundary technique and time synchronization. Under the chord Reynolds number of 9 × × 10 4 , the membrane exhibits self-initiated vibrations in accordance with its material properties and the surrounding fluid flow. The vortical flow structure, its effect on the aerodynamic parameters, and the implications of the membrane deformation on the effective angle of attack and flow structure are discussed. Nomenclature C D = drag coefficient CL = lift coefficient c = chord length c p = pressure coefficient D = drag Fpx = form drag Fpy = lift caused by pressure force Fτ x = drag caused by friction L = lift U = freestream speed u = chordwise velocity v =v ertical velocity x = chordwise distance from the leading edge Z = half-wing span z = spanwise distance from the root α = angle of attack

The approach of proper orthogonal decomposition (POD) has been extensively adopted for fluid dynamics in fixed geometries. This technique is examined here for fluid flow with moving boundaries; in the context of cavitating and phase change flows, and fluid–membrane interaction. The purpose is to assess the capability of POD in extracting the salient features and offering a compact representation to the CFD solutions associated with boundary movement. The cavitating flow simulations are investigated to distill the effect of turbulence modeling, between the Launder–Spalding and a filter-based turbulence models. The lower-order eigenmodes of the flow field, for both turbulence models, show different flow structures and global parameters between higher and lower cavitation numbers. The effect of multi-timescales produced by the filter-based turbulence model is discerned by POD analysis. For 3-D, membrane wing flows, very few POD modes seem sufficient for accurate representation of the velocity field. However, reduced-order analysis of the aerodynamic performance, which is strongly dictated by pressure, may be coarsened by moving membrane dynamics. The flow with fusion is further considered for its solid–liquid phase front propagation. While few modes can sufficiently construct the flow field for the later interval of the flow, a larger number of POD modes are required to provide the flow scales for the initial part of the phase change process.

A posteriori error estimators are fundamental tools for providing confidence in the numerical computation of PDEs. To date, the main theories of a posteriori estimators have been developed largely in the finite element framework, for either linear elliptic operators or non-linear PDEs in the absence of disparate length scales. On the other hand, there is a strong interest in using grid refinement combined with Richardson extrapolation to produce CFD solutions with improved accuracy and, therefore, a posteriori error estimates. But in practice, the effective order of a numerical method often depends on space location and is not uniform, rendering the Richardson extrapolation method unreliable. We have recently introduced (Garbey, 13th International Conference on Domain Decomposition, Barcelona, 2002; 379–386; Garbey and Shyy, J. Comput. Phys. 2003; 186:1–23) a new method which estimates the order of convergence of a computation as the solution of a least square minimization problem on the residual. This method, called least square extrapolation, introduces a framework facilitating multi-level extrapolation, improves accuracy and provides a posteriori error estimate. This method can accommodate different grid arrangements. The goal of this paper is to investigate the power and limits of this method via incompressible Navier Stokes flow computations. Copyright © 2005 John Wiley & Sons, Ltd.

Turbulent cavitating flow computations need to address both cavitation and turbulence modelling issues. A recently developed interfacial dynamics-based cavitation model (IDCM) incorporates the interfacial transport into the computational modelling of cavitation dynamics. For time-dependent flows, it is known that the engineering turbulence closure such as the original k–e model often over-predicts the eddy viscosity values reducing the unsteadiness. A recently proposed filter-based modification has shown that it can effectively modulate the eddy viscosity, rendering better simulation capabilities for time-dependent flow computations in term of the unsteady characteristics. In the present study, the IDCM along with the filter-based k–e turbulence model is adopted to simulate 2-D cavitating flows over the Clark-Y airfoil. The chord Reynolds number is Re=7.0 × 105. Two angles-of-attack of 5 and 8° associated with several cavitation numbers covering different flow regimes are conducted. The simulation results are assessed with the experimental data including lift, drag and velocity profiles. The interplay between cavitation and turbulence models reveals substantial differences in time-dependent flow results even though the time-averaged characteristics are similar. Copyright © 2005 John Wiley & Sons, Ltd.

A major challenge to the successful full-scale development of modem aerospace systems is to address competing objectives such as improved performance, reduced costs, and enhanced safety. Accurate, high-fidelity models are typically time consuming and computationally expensive. Furthermore, informed decisions should be made with an understanding of the impact (global sensitivity) of the design variables on the different objectives. In this context, the so-called surrogate-based approach for analysis and optimization can play a very valuable role. The surrogates are constructed using data drawn from high-fidelity models, and provide fast approximations of the objectives and constraints at new design points, thereby making sensitivity and optimization studies feasible. This paper provides a comprehensive discussion of the fundamental issues that arise in surrogate-based analysis and optimization (SBAO), highlighting concepts, methods, techniques, as well as practical implications. The issues addressed include the selection of the loss function and regularization criteria for constructing the surrogates, design of experiments, surrogate selection and construction, sensitivity analysis, convergence, and optimization. The multi-objective optimal design of a liquid rocket injector is presented to highlight the state of the art and to help guide future efforts.

The statistical characteristics of unsteady Reynolds-averaged Navier–Stokes (RANS) turbulence models, utilizing the eddy-viscosity concept, are investigated. Detailed assessment is made to the original Launder-Spalding k –ϵ model as well as a nonlinear modification utilizing multiple scales from both turbulent variables and the local mean strain rate. The purpose of the study is to shed light on the issues of unsteady RANS computations, not to recommend a “good” model. The models are tested for flow past a square cylinder, employing progressively refined resolutions. The comparison between the length scale imposed by the turbulence model and the length scale of the spatial resolution helps understand the fidelity of the simulations. If the eddy viscosity is low, the flow field can be numerically underresolved in RANS computations, and consequently the solutions exhibit noticeable sensitivity to numerics such as discretization schemes and grid distribution. Under such situations, in practical comp...

The interaction between a flexible structure and the surrounding fluid gives rise to a variety of phenomena with applications in many areas, such as, stability analysis of airplane wings, turbomachinery design, design of bridges, and the flow of blood through arteries. Studying these phenomena requires modeling of both fluid and structure. Many approaches in computational aeroelasticity seek to synthesize independent computational approaches for the aerodynamic and the structural dynamic subsystems. This strategy is known to be fraught with complications associated with the interaction between the two simulation modules. The task is to choosing the appropriate models for fluid and structure based on the application, and to develop an efficient interface to couple the two models. In the present article, we review the recent advancements in the field of fluid–structure interaction, with specific attention to aeroelastic applications. One of the key aspects to developing a robust coupled aeroelastic model is the presence of an efficient moving grid technique to account for structural deformations. Several such techniques are reviewed in this paper. Also, the time scales associated with fluid–structure interaction problems can be very different; hence, appropriate time stepping strategies and/or sub-cycling procedures within the individual field need to be devised. The flutter predictions performed on an AGARD 445.6 wing at different Mach numbers are selected to highlight the state-of-the-art computational and modeling issues.

The geometric conservation law (GCL) is an important concept for moving grid techniques because it directly regulates the treatments of the fluid flow and grid movement. With the grid movement at every time instant, the Jacobian, associated with the volume of each element in curvilinear co‐ordinates, needs to be updated in a conservative manner. In this study, alternative GCL schemes for evaluating the Jacobian have been investigated in the context of a pressure‐based Navier‐Stokes solver, utilizing moving grid and the first‐order implicit time stepping procedure as well as the PISO scheme. GCL‐based on first and second‐order, implicit as well as time‐averaged, time integration schemes were considered. Accuracy and conservative properties were tested on steady‐state, laminar flow inside a 2D channel and time dependent, turbulent flow around a 3D elastic wing; both treated with moving grid techniques. It seems that the formal order of accuracy is not a decisive indicator. Instead, the speed of grid movement and the interplay between the flow solver and the GCL treatments make a more noticeable impact.