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This work deals with the development of the direct reconstruction method (DRM) and its application to the volume integration of the discontinuous Galerkin (DG) method on multi-dimensional high-order mixed-curved meshes. The conventional quadrature-based DG methods require the humongous computational cost on high-order curved elements due to their non-linear shape functions. To overcome this issue, the flux function is directly reconstructed in the physical domain using nodal polynomials on a target space in a quadrature-free manner. Regarding the target space and distribution of the nodal points, DRM has two variations: the brute force points (BFP) and shape function points (SFP) methods. In both methods, one nodal point corresponds to one nodal basis function of the target space. The DRM-BFP method uses a set of points that empirically minimizes a condition number of the generalized Vandermonde matrix. In the DRM-SFP method, the conventional nodal points are used to span an enlarged target space of the flux function. It requires a larger number of reconstruction points than DRM-BFP but offers easy extendability to the higher-degree polynomial space and a better de-aliasing effect. A robust way to compute orthonormal polynomials is provided to achieve lower round-off errors. The proposed methods are validated by the 2-D/3-D Navier-Stokes equations on high-order mixed-curved meshes. The numerical results confirm that the DRM volume integration greatly reduces the computational cost and memory overhead of the conventional quadrature-based DG methods on high-order curved meshes while maintaining an optimal order-of-accuracy as well as resolving the flow physics accurately. • As a quadrature-free method, the DRM framework is proposed and applied to the volume integration of DG methods for the Navier-Stokes equations. • According to the target space and interpolation nodes, DRM-BFP and DRM-SFP are developed on multi-dimensional higher-order mixed-curved mesh. • Through numerical analyses and computations, efficiency and accuracy of DRM are extensively validated on higher-order mixed curved mesh. [ABSTRACT FROM AUTHOR]

Abstract The present paper deals with a new improvement of hierarchical multi-dimensional limiting process for resolving the subcell distribution of high-order methods on two-dimensional mixed meshes. From previous studies, the multi-dimensional limiting process (MLP) was hierarchically extended to the discontinuous Galerkin (DG) method and the flux reconstruction/correction procedure via reconstruction (FR/CPR) method on simplex meshes. It was reported that the hierarchical MLP (h MLP) shows several remarkable characteristics such as the preservation of the formal order-of-accuracy in smooth region and a sharp capturing of discontinuities in an efficient and accurate manner. At the same time, it was also surfaced that such characteristics are valid only on simplex meshes, and numerical Gibbs–Wilbraham oscillations are concealed in subcell distribution in the form of high-order polynomial modes. Subcell Gibbs–Wilbraham oscillations become potentially unstable near discontinuities and adversely affect numerical solutions in the sense of cell-averaged solutions as well as subcell distributions. In order to overcome the two issues, the behavior of the h MLP on mixed meshes is mathematically examined, and the simplex-decomposed P1 -projected MLP condition and smooth extrema detector are derived. Secondly, a troubled-boundary detector is designed by analyzing the behavior of computed solutions across boundary-edges. Finally, h MLP_BD is proposed by combining the simplex-decomposed P1 -projected MLP condition and smooth extrema detector with the troubled-boundary detector. Through extensive numerical tests, it is confirmed that the h MLP_BD scheme successfully eliminates subcell oscillations and provides reliable subcell distributions on two-dimensional triangular grids as well as mixed grids, while preserving the expected order-of-accuracy in smooth region. Highlights • Subcell monotonicity and its extension on 2-D mixed meshes have been realized. • Novel troubled-boundary detector is designed to capture subcell G–W oscillations. • h MLP_BD limiting is validated by extensive test cases on 2-D mixed meshes. [ABSTRACT FROM AUTHOR]

Hierarchical multi-dimensional limiting process (MLP) is improved and extended for flux reconstruction or correction procedure via reconstruction (FR/CPR) on unstructured grids. MLP was originally developed in finite volume method (FVM) and it provides an accurate, robust and efficient oscillation-control mechanism in multiple dimensions for linear reconstruction. This limiting philosophy can be hierarchically extended into higher-order Pn approximation or reconstruction. The resulting algorithm is referred to as the hierarchical MLP and facilitates detailed capture of flow structures while maintaining formal order-of-accuracy in a smooth region and providing accurate non-oscillatory solutions across a discontinuous region. This algorithm was developed within modal DG framework, but it can also be formulated into a nodal framework, most notably the FR/CPR framework. Troubled-cells are detected by applying the MLP concept, and the final accuracy is determined by a projection procedure and the hierarchical MLP limiting step. Extensive numerical analyses and computations, ranging from two-dimensional to three-dimensional fluid systems, have demonstrated that the proposed limiting approach yields outstanding performances in capturing compressible inviscid and viscous flow features. [ABSTRACT FROM AUTHOR]

Notwithstanding the effectiveness of ventilated cavitation as a safe way to accelerate and achieve natural supercavitation, detailed physics of ventilated cavitating flows and hydrodynamic effects on underwater vehicles have been rarely explored. The present work deals with computational investigation on ventilated supercavitating flows around a high-speed underwater vehicle and its hydrodynamic characteristics. The homogeneous mixture model is adopted for describing the cavitating flow field composed of water, water vapor, and a ventilated gas. After extensively validating the flow solver with experiments of ventilated cavitating flows, we carry out a series of computations of supercavitating flows around a high-speed underwater vehicle with four control fins, and investigate the flow physics and the vehicle's hydrodynamic characteristics. The computations are performed under different freestream velocities, ventilation rates, and angles of attack. Furthermore, the unsteady effects of turbulence are examined by comparing the computed results based on unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation and Detached Eddy Simulation (DES). The computational results show that the cavitating flows cause substantial nonlinear characteristics in cavity shapes and hydrodynamic forces such as lift, drag, and pitching moment, due to the presence of natural and ventilated cavitations and their interaction. This can be beneficially exploited in understanding and predicting ventilated supercavitating flows around a high-speed underwater vehicle, and establishing an accurate dynamic model of the vehicle's motion. • Computations of ventilated cavitating flows around a vehicle are presented. • All the results exhibit non-linear hydrodynamic characteristics of the vehicle. • As a cavity passes through the fins, noticeable drag and lift changes are observed. • As the AOA increases, the pitching moment changes from nose-down to nose-up. • URANS and DES computations are compared for a specific flow condition. [ABSTRACT FROM AUTHOR]

• A new cavitation model is proposed by adopting physical bubble growth rates. • Empiricism inherent in the bubble number density is substantially reduced. • Smooth temperature recovery in cryogenic cavitation is well captured. • Existing characteristics under isothermal conditions are kept. Cavitation in thermosensitive fluids is accompanied by a noticeable temperature drop inside a cavity. Motivated by the fact that most popular cavitation models have been developed without considering thermodynamic effects, the present study proposes physics-based corrections to existing cavitation models. Two primary corrections are adopting the physical bubble growth rate and separating bubble number density in the vaporization and condensation terms. The bubble growth rate in the cavitation model is chosen as the limiting factor between the inertia-controlled rate and heat diffusion-controlled rate from the bubble growth theories. To implement the theoretical growth rate of the thermal regime into the computational framework based on the homogeneous mixture model, the bubble time and liquid temperature are newly modeled. Furthermore, the bubble number density, which is often set uniformly throughout the whole computational domain, is separated for the vaporization and condensation processes. The vaporization bubble number density is further modeled by experimental data to reduce the empiricism inherent in the user-defined coefficients. The relation between the remaining model coefficients and characteristic temperature drop (Δ T *) is developed and it is verified to be effective in various types of fluids and working conditions. The newly proposed model is validated by cryogenic cavitating tests and applied to the computations of the 3-D cryogenic cavitating flows around a turbopump inducer. Unlike the sudden temperature recovery by the previous inertial rate-based models, the proposed model predicts a gradual temperature recovery inside a non-isothermal cavity, which is consistent with the experimental observations. It is also confirmed that the new model chooses the physical inertial rate under isothermal conditions, and thus, the model can be used regardless of the flow conditions. [ABSTRACT FROM AUTHOR]

Abstract: On the basis of the Eu’s generalized hydrodynamic (GH) theories for diatomic single species gas and monatomic multi-species gas, an axisymmetric GH computational model for multi-species gas containing monatomic and diatomic molecules is developed for the numerical simulation of hypersonic rarefied gas flows. The multi-species GH computational model includes monatomic and diatomic species of , NO, O, N. The mass diffusion flux of the gas mixture is included in the GH constitutive relation. In addition, the physical relationship between the mass diffusion and heat fluxes is added to the evolution equation set. The multi-species GH theory includes the rotational nonequilibrium effect of diatomic molecules by introducing excess normal stress associated with the bulk viscosity. An efficient multi-species GH numerical solver for axisymmetric rarefied flows is then developed by adopting various numerical techniques, such as an adequate nonlinear equation solver for the GH constitutive relation, an accurate flux splitting scheme, multi-grid convergence acceleration and slip-wall boundary conditions. For validation, the proposed computational model is applied to hypersonic rarefied flows over a space shuttle nose, a sphere and a reentry body as well as 1D shock structure. By comparing the results of the multi-species GH model with those of the Navier–Stokes equation and the DSMC, the accuracy and physical consistency of the GH computational model are critically examined. [Copyright &y& Elsevier]

Abstract: A numerical study of separation control has been made to investigate aerodynamic characteristics of NACA23012 airfoil with synthetic jets. Computed results demonstrated that stall characteristics and control surface performance could be substantially improved by resizing separation vortices. The maximum lift was obtained when the separation point coincides with the synthetic jet location and the non-dimensional frequency is about 1. In addition, separation control effect was proportional to the peak velocity of the synthetic jet. It was observed that the actual flow control mechanism and flow structure is fundamentally different depending on the range of synthetic jet frequency. For low frequency range, small vortices due to synthetic jet penetrated to the large leading edge separation vortex, and as a result, the size of the leading edge vortex was remarkably reduced. For high frequency range, however, small vortex did not grow up enough to penetrate into the leading edge separation vortex. Instead, synthetic jet firmly attached the local flow and influenced the circulation of the virtual airfoil shape which is the combined shape of the main airfoil with the separation vortex. As a way to reduce the jet peak velocity, performance of a multi-array synthetic jet was investigated. Moreover, a high frequency multi-location synthetic jet was exploited to efficiently eliminate the unstable flow structure which was observed in low frequency range. Finally, by changing the phase angle in multi-location synthetic jets, highly controlled flow characteristics could be obtained with multi-array/multi-location synthetic jets. This shows efficiency of the current approach in separation control using synthetic jet. [Copyright &y& Elsevier]

Abstract: The present paper deals with an efficient and accurate limiting strategy for multi-dimensional compressible flows. The multi-dimensional limiting process (MLP) which was successfully proposed in two-dimensional case [K.H. Kim, C. Kim, Accurate, efficient and monotonic numerical methods for multi-dimensional compressible flows. Part II: Multi-dimensional limiting process, J. Comput. Phys. 208 (2) (2005) 570–615] is modified and refined for three-dimensional application. For computational efficiency and easy implementation, the formulation of MLP is newly derived and extended to three-dimensional case without assuming local gradient. Through various test cases and comparisons, it is observed that the newly developed MLP is quite effective in controlling numerical oscillation in multi-dimensional flows including both continuous and discontinuous regions. In addition, compared to conventional TVD approach, MLP combined with improved flux functions does provide remarkable increase in accuracy, convergence and robustness in steady and unsteady three-dimensional compressible flows. [Copyright &y& Elsevier]

Abstract: An optimal shape design approach is presented for a subsonic S-shaped intake geometry using aerodynamic sensitivity analysis. Sensitivity analysis is performed for the three-dimensional Navier–Stokes equations coupled with two-equation turbulence models using a discrete adjoint method. For code validation, the result of the flow solver is compared with experimental data and other computational results. Through the study on turbulence models and grid refinement, results from several turbulence models are compared and the minimal number of grid points yielding an accurate numerical solution is obtained. And, the adjoint sensitivity code is also validated by the comparison with complex step derivative results. To obtain a sufficient and flexible design space, NURBS equations are employed as a geometric representation and a new grid modification technique, Least Square NURBS Grid Approximation is applied. With the verified flow solver, sensitivity analysis code and geometric modification technique, the optimization of the S-shaped intake is carried out and the enhancement of overall intake performance is achieved. In addition, the off-design performance of a designed S-shaped intake is tested to confirm the robustness of the current design approach. As a result, the capability and efficiency of the present design tools are successfully demonstrated in three-dimensional high Reynolds subsonic intake design. [Copyright &y& Elsevier]

Abstract: The present papers deal with numerical methods toward the accurate and efficient computations of multi-dimensional steady/unsteady compressible flows. In Part I, a new spatial discretization technique is introduced to reduce excessive numerical dissipation in a non-flow-aligned grid system. Through the analysis of TVD limiters, a criterion is proposed to predict cell-interface states accurately both in smooth region and in discontinuous region. According to the criterion, a new way of re-evaluating the cell-interface convective flux in AUSM-type methods is developed. The resultant flux reduces numerical dissipation remarkably in multi-dimensional flows. Also, the monotonicity of AUSM-type methods is achieved by modifying the pressure splitting function directly based on the governing equations and the detection of sonic transition point with respect to a cell-interface. It is noted that the newly formulated AUSM-type flux for Multi-dimensional flows, named M-AUSMPW+, possesses many improved properties in term of accuracy, computational efficiency, monotonicity and grid independency. Through numerous test cases from contact and shock discontinuities, vortex flow, shock wave/boundary-layer interaction to viscous shock tube problems, M-AUSMPW+ proves to be efficient and about twice more accurate than conventional upwind schemes. The three-dimensional implementation of M-AUSMPW+ is expected to provide accuracy and efficiency improvement furthermore. [Copyright &y& Elsevier]

Abstract: Through the analysis of conventional TVD limiters, a new multi-dimensional limiting function is derived for an oscillation control in multi-dimensional flows. And, multi-dimensional limiting process (MLP) is developed with the multi-dimensional limiting function. The major advantage of MLP is to prevent oscillations across a multi-dimensional discontinuity, and it is readily compatible with more than third order spatial interpolation. Moreover, compared with other higher order interpolation schemes such as ENO type schemes, MLP shows a good convergence characteristic in a steady problem and it is very simple to be implemented. In the present paper, third and fifth order interpolation schemes with MLP, named MLP3 and MLP5, are developed and tested for several real applications. Through extensive numerous test cases including an oblique stationary contact discontinuity, an expansion fan, a vortex flow, a shock wave/vortex interaction and a viscous shock tube problem, it is verified that MLP combined with M-AUSMPW+ numerical flux substantially improves accuracy, efficiency and robustness both in continuous and discontinuous flows. By extending the current approach to three-dimensional flows, MLP is expected to reduce computational cost and enhance accuracy even further. [Copyright &y& Elsevier]

This paper deals with the development of an improved Roe scheme that is free from the shock instability and still preserves the accuracy and efficiency of the original Roe’s Flux Difference Splitting (FDS). Roe’s FDS is known to possess good accuracy but to suffer from the shock instability, such as the carbuncle phenomenon. As the first step towards a shock-stable scheme, Roe’s FDS is compared with the HLLE scheme to identify the source of the shock instability. Through a linear perturbation analysis on the odd–even decoupling problem, damping characteristic is examined and Mach number-based functions f and g are introduced to balance damping and feeding rates, which leads to a shock-stable Roe scheme. In order to satisfy the conservation of total enthalpy, which is crucial in predicting surface heat transfer rate in high-speed steady flows, an analysis of dissipation mechanism in the energy equation is carried out to find out the error source and to make the proposed scheme preserve total enthalpy. By modifying the maximum-minimum wave speed, the problem of expansion shock and numerical instability in the expansion region is also remedied without sacrificing the exact capturing of contact discontinuity. Various numerical tests concerned with the shock instability are performed to validate the robustness of the proposed scheme. Then, viscous flow test cases ranging from transonic to hypersonic regime are calculated to demonstrate the accuracy, robustness, and other essential features of the proposed scheme. [Copyright &y& Elsevier]

We present a novel shock-capturing strategy for high-order methods including discontinuous Galerkin (DG) method with data-driven gain optimization. Inspired by the classical control theory, we utilize a proportional–integral–derivative (PID) controller for capturing shock waves with monotonic subcell distributions. The proposed closed-loop control system for shock-capturing, named shock-capturing PID controller (SPID), consists of two key elements: error estimation based on the multi-dimensional limiting strategies (h MLP and h MLP_BD) and shock stabilization using the Laplacian artificial viscosity (LAV). The two elements are combined in a complementary manner to maximize the advantages of limiting strategy and artificial viscosity while overcoming each weakness. First, based on the multi-dimensional limiting process (MLP) condition and the troubled-boundary detector, the estimated error gives a signal to the SPID how much flow variables stray out of monotonic shock profiles. Second, the SPID estimates the amount of artificial viscosity to stabilize the target shock wave and superimposes numerical diffusion to the governing equations in the form of LAV. Each control action of the SPID (i.e., proportional, integral, and derivative) has a distinct role in capturing and stabilizing shock waves. The proportional action determines a minimal amount of background artificial viscosity. The integral action reinforces a shock-stabilizing numerical diffusion where the artificial viscosity by the proportional action alone is insufficient. The derivative action damps out sudden rises of error and spurious oscillations. The SPID incorporates the gain parameters that regulate the impact of each control action, and each gain parameter is determined via a surrogate-based optimization approach utilizing artificial neural networks (ANN). The SPID with the data-driven gain parameters is verified and validated by conducting extensive numerical tests and by comparing the results to other shock-capturing methods (h MLP, h MLP_BD, and Laplacian artificial viscosity). The numerical results demonstrate the excellent performance of SPID in terms of capturing shock waves and stabilizing shock-induced oscillations. Moreover, the SPID successfully preserves unsteady turbulent eddies for large-eddy simulations (LES) of subsonic and supersonic flows and improves convergence characteristics of steady transonic/supersonic flows. • A new shock-capturing method using a PID controller is proposed for high-order methods. • It combines advantages of limiter and artificial viscosity, while overcoming each weakness. • Accurate and smooth subcell resolution is achieved for shock–vortex interactions. • Eddy vortices in subsonic and supersonic turbulent simulations are preserved. • Convergence property is noticeably improved on steady supersonic/transonic flows. [ABSTRACT FROM AUTHOR]

Details a study which described a dual-point design procedure for transonic wings using an inverse-design method. Comparison of the inverse design method with the direct numerical optimization method; Flow analysis; Target pressure optimization; Results and discussion.

• DRM has been extended to DG surface integration of Euler/Navier-Stokes equations. • Adaptive MGS process is designed for optimal physical bases of restricted space. • DRM has been verified to significantly reduce computing cost and memory overhead. As the second extension of the direct reconstruction method (DRM), DRM for the surface integration of the discontinuous Galerkin (DG) weak formulation is presented on multi-dimensional high-order mixed meshes. From the previous study, DRM was successfully extended to the volume integration of the DG method, and produced a significant reduction in both the computational cost and memory overhead, particularly for high-order solution approximations on 3-D high-order meshes. DRM has the potential to reduce the computational cost and memory overhead further by treating the surface integration term consistently. To realize this, we design a linear dependency detector and formulate an adaptive orthonormalization process from the modified Gram-Schmidt (MGS) process. DRM is then applied to the restricted function space of the surface integration in an adaptive and optimal manner. The resulting methods, namely the DRM surface integration with the brute force points (BFP) and shape function points (SFP) methods, perform very well on high-order mixed-curved meshes. In addition, the adaptive MGS process is robust and accurate in extracting linear faces/edges from a given set of mixed-curved meshes. Various benchmark tests from the compressible Euler and Navier-Stokes equations verify the impact of DRM for 3-D large-scale computations on mixed-curved meshes. As an example, the DRM framework applied to the 3-D surface and volume integrations with DG- P 3 approximation on P 3-mesh may yield O (10) × speed-up, compared to conventional quadrature-based methods. [ABSTRACT FROM AUTHOR]

Underwater explosion (UNDEX) events involve complex physical phenomena and can be categorized into three stages: initial shock wave propagation, cavitation, and bubble pulsation. This study focuses on double UNDEX bubbles, elucidating their highly nonlinear interactions across these stages. Both synchronous and asynchronous explosion cases are examined, with a particular emphasis on assessing the influence of phase change. Employing the 3-D high-fidelity computational framework that incorporates the physics-based cavitation model and non-ideal equations of state for water and explosion gas properties, we conduct validations with field experimental data on single bubbles and extend our analysis to double bubbles. Our principal finding is the interaction between the cavity created by the shock wave and the bubble pulsations. Notably, we discern a significant phase change effect in asynchronous explosion, wherein the vapor cavity region delays the contraction of the previously-generated bubble. This delay is attributed to the low-density region created by the phase change, allowing the bubble to remain expanded for an extended period, thereby enhancing agreement with experimental data. To our knowledge, this study represents the pioneering effort to explore the critical role of phase change in accurately simulating the intricate interactions within double UNDEX scenarios. • Computational investigations on double UNDEX scenarios with thermodynamic cavitation process and non-ideal EOS. • Interactions between the cavity caused by the shock wave and the bubble pulsations are captured. • Phase change effect is more pronounced in asynchronous explosion. • Low-density cavity region delays the previously-generated bubble's contraction. • 3-D effect induced by the misalignment of gravity and bubble contraction direction. [ABSTRACT FROM AUTHOR]

We present the experimental design optimization of a flapping wing and the development of a non-circular gear based flapping mechanism to enhance the propulsive efficiency of a flapping wing micro air vehicle (FWMAV). The proposed flapping mechanism consists of a very simple structure, which excludes any linkages but utilizes non-circular gears to achieve a desired flapping amplitude and angular velocity. An optimal wing was obtained via a surrogate-based global optimization reflecting the noise generated from the experiments. Compared to the baseline wing, the optimal wing was improved more than three times in terms of propulsive efficiency. The analysis showed that one of wing geometric parameters, the non-dimensional radius of second moment of area r ˆ 2 , plays an essential role in designing an optimal wing. To achieve high efficiency, r ˆ 2 should decrease as the wing area increases. Simultaneously, r ˆ 2 determined other optimal values of geometry parameters such as aspect ratio and camber angle. Furthermore, r ˆ 2 was also closely related to an optimal vein position which not only generated a moderate wing angle of attack but also reduced the adverse effect of angle of attack fluctuation. Through the optimal wing and the proposed flapping mechanism, we could be able to achieve better efficiency and robust manufacturing compared with the previously reported tailless-FWMAVs. [ABSTRACT FROM AUTHOR]

Abstract: The present paper deals with an efficient and accurate limiting strategy for the multi-dimensional hyperbolic conservation laws on unstructured grids. The multi-dimensional limiting process (MLP) which has been successfully proposed on structured grids is extended to unstructured grids. The basic idea of the proposed limiting strategy is to control the distribution of both cell-centered and cell-vertex physical properties to mimic multi-dimensional nature of flow physics, which can be formulated to satisfy so called the MLP condition. The MLP condition can guarantee high-order spatial accuracy and improved convergence without yielding spurious oscillations. Starting from the MUSCL-type reconstruction on unstructured grids followed by the efficient implementation of the MLP condition, MLP slope limiters on unstructured meshes are obtained. Thanks to its superior limiting strategy and maximum principle satisfying characteristics, the newly developed MLP on unstructured grids is quite effective in controlling numerical oscillations as well as accurate in capturing multi-dimensional flow features. Numerous test cases are presented to validate the basic features of the proposed approach. [Copyright &y& Elsevier]

A series of computational studies on the enhancement of parallel supersonic-subsonic mixing wakes, where M1 = 1.78 and M2 = 0.30 at baseline, is conducted and compared with available experimental data. The mixing problem of this parallel flow is characterized by the interaction of “complex” turbulent shear flow with acoustic waves emitted from a wall-mounted cavity. The first aim of the present work is to show a direct comparison between numerical predictions and equivalent experimental data for the baseline case. Hence we make a comparison of eight independent turbulence models, which were selected from the most widely using turbulence models in use in the engineering field, consisting of six eddy-viscosity models and two Reynolds stress models. The Pitot pressure distribution data are in good agreement between computation and experiment, and the results show that Menter's SST model gives the best performance. Further, we investigate the effects of primary parameters such as the length-to-depth (L/D) ratio of the cavity, the position of the cavity, and the arrangement of the cavity at the given flow condition. Consequently, this methodology can contribute to finding an optimal guideline for the idea of mixing enhancement in the free turbulent wake using the acoustic wave produced by wall cavities. [ABSTRACT FROM AUTHOR]

An insect-type flapping wing micro aerial vehicle offers high aerodynamic efficiency and maneuverability in confined spaces. The complicated aerodynamic/structural behavior of flapping wing micro aerial vehicle, however, causes difficulties regarding the dynamic control and parametric design. This paper develops a moderately accurate numerical framework taking into account the passive motion of the main wings. Finite-element-based multibody dynamics and two-dimensional unsteady aerodynamics are combined to simulate the hover of a flapping wing micro aerial vehicle. In addition, flexible and rigid wings are compared through numerical simulation considering the flexibility. In terms of the average thrust, numerical simulation by fluid–structure interaction shows good agreements against the experimental results within 5% discrepancy. [ABSTRACT FROM AUTHOR]

During cavitation bubble pulsations, a phase change intensively occurs near the collapsing moment due to high pressure and temperature inside bubbles, accompanying distinctive flow features: the rate of evaporation and condensation significantly changes according to the phase change regime. To account for this non-isothermal effect, the high-fidelity computational framework incorporating the physics-based cavitation model and a new fluid property model based on artificial neural network is proposed. The key finding of this study is the interplay between the thermal and inertial effects during multiple pulsations. At the early stages of bubble contraction, the phase change is primarily driven by fluid inertia. However, as the bubble continues to compress, the thermal effect becomes dominant and controls the entire phase change region at each moment of collapse. It is observed that the isothermal model relying on the inertial bubble growth rate only, does not capture this transition of dominance and eventually fails to predict multiple pulsations. The physics-based cavitation model successfully captures the bubble pulsation beyond the second collapse. These findings highlight that explicit consideration of the non-isothermal effect is essential for problems with varying phase change regimes, and a phase change model reflecting this effect is vital for accurate computations. • High-fidelity multi-phase computational framework ACTFlow_MP is proposed. • New fluid property model based on artificial neural network is constructed for water. • The thermal effect turns out to be dominant at the moment of each bubble collapse. • The physics-based model successfully captured bubble pulsations beyond 2nd period. [ABSTRACT FROM AUTHOR]

This study investigates the unsteady aerodynamic characteristics of the cambered wings of a flapping-wing micro air vehicle (FW-MAV) in hover. A three-dimensional fluid–structure interaction solver is developed for a realistic modeling of large-deforming wing structure and geometry. Cross-validation is conducted against the experimental results obtained also in the present study to establish more accurate analyses of cambered wings. An investigation is carried out on the unsteady vortex structures around the wings caused by the passive twisting motion. A parametric study is then conducted to evaluate the aerodynamic performance with respect to the camber angle at three different flapping frequencies including normal operating conditions. The camber angles producing the largest thrust and highest propulsive efficiency are estimated at each flapping frequency, and their effects on aerodynamic performance are analyzed in terms of the stroke phase. The timing and magnitude of the passive twisting motion, which are dependent on the camber angle at the operating frequency, greatly affects the unsteady vortex structure. Consequently, the camber angle designed at the operating frequency plays a key role in enhancing the aerodynamic performance of FW-MAVs. [ABSTRACT FROM AUTHOR]