Computers & Fluids最新文献

Shock Wave Boundary Layer Interactions (SWBLI) occur due to the convergence of a shock wave and a viscous boundary layer. The resulting separation bubble is a major setback for the performance of high-speed air intakes. This paper presents a numerical investigation into the potential of a Pressure Feedback Technique (PFT) to alleviate shock-induced flow separation within a Scramjet engine intake operating at Mach 4. The PFT is a novel self-sustaining flow control technique that combines simultaneous suction and injection. The two main output parameters used to support the hypothesis of the present study are the size of the separation bubble as well as the total pressure recovery at the isolator outlet. The most prominent observation of this study is that with the installation of the PF tubes, the total pressure recovery at the exit is noted to rise. The effectiveness of the pressure feedback technique in reducing the intensity of the separation bubbles is found to depend on the diameter of the PFT, pitch-to-diameter ratio, and PFT tube design.

Langmuir turbulence consists of Langmuir circulation (LC) generated at the surface of rivers, lakes, bays, and oceans by the interaction between the wind-driven shear and surface gravity waves. In homogeneous shallow water, LC can extend to the bottom of the water column and interact with the bottom boundary layer. Large-eddy simulation (LES) of LC in shallow water was performed with the finite volume method and various forms of subgrid-scale (SGS) model characterized by different near-wall treatments of the SGS eddy viscosity. The wave forcing relative to wind forcing in the LES was set following the field measurements of full-depth LC during the presence of LC engulfing a water column 15 m in depth in the coastal ocean, reported in the literature. It is found that the SGS model can greatly impact the structure of LC in the lower half of the water column. Results are evaluated in terms of (1) the Langmuir turbulence velocity statistics and (2) the lateral (crosswind) length scale and overall cell structure of LC. LES with an eddy viscosity with velocity scale in terms of $S$ and $\Omega$ (where $S$ is the norm of the strain rate tensor and $\Omega$ is the norm of the vorticity tensor) and a Van Driest wall damping function (referred to as the S-Omega model) is found to provide best agreement with pseudo-spectral LES in terms of the lateral length scale and overall cell structure of LC. Two other SGS models, namely the dynamic Smagorinsky model and the wall-adapting local-eddy viscosity model are found to provide less agreement with pseudo-spectral LES, for example, as they lead to less coherent bottom convergence of the cells and weaker associated upward transport of slow downwind moving fluid. Finally, LES with the S-Omega SGS model is also found to lead to good agreement with physical measurements of LC in the coastal ocean in terms of Langmuir turbulence decay during periods of surface heating.

The high temperatures within the shock layer of an extremely hypersonic flow led to the excitation of internal energy modes, accompanied by the emission of thermal radiation. However, simulation of three-dimensional hypersonic flows tightly coupled with radiation effects so far remains a significant challenge due to the complexity of the problem and the high computational cost, and most of the previously available numerical simulations on hypersonic flows coupled with the radiation effects are limited to one-dimensional or axisymmetric cases. To enhance the efficiency for simulating two dimensional hypersonic flows tightly coupled with radiation effects, an efficient p-DSMC method was proposed in our previous work. In this study, we mainly focus on the extension of the $p$-DSMC method to three-dimensional hypersonic flows with radiation effects. Furthermore, to validate the effectiveness of the $p$-DSMC method, the hypersonic flow with $\text{Ma}=40$ at the altitude $H=80$ km over the FIRE-II was simulated by the $p$-DSMC method and the conventional Navier–Stokes equation based CFD scheme, and a satisfying agreement of the wall radiation heat flux results, particularly in the vicinity of the stagnation point, computed by these two different methods can be observed, which indicates that the $p$-DSMC method can be a reliable tool for three-dimensional hypersonic flows with radiation effects. Additionally, the radiation effects for the FIRE-II flying with $\text{Ma}=41.24$ at the altitude $H=86$ km with different angles of attack and radius were numerically investigated with the $p$-DSMC method and the numerical results show that, as the angle of attack increases, the convective heat approaches the edge of the FIRE-II model and the location of the maximum radiative heat moves away from the center of the FIRE-II model, but remains close to the stagnation point.

Downbursts can cause severe winds near ground level, potentially damaging buildings and structures. A particular problem is that downburst-induced wind action is not considered in the design stage as it is not included in the building codes. This paper provides an in-depth characterization of a downburst flow field including its vortical structures in both space and time. The analysis is based on Large Eddy Simulations (LES) to reproduce dedicated experiments of a vertical downburst carried out in the test chamber of the WindEEE Dome laboratory. The trajectory of the radial velocity maxima is evaluated, which indicates that the height of the maximum velocity increases with the traveled distance after having produced the strongest wind gusts. The spatial evolution of the convective velocity of the primary vortex across the test chamber is evaluated and three regions are distinguished: the speed-up (up to r/D = 1.25), the slow-down (1.25 $<$ r/D $<$ 2.29) and the deflection region (r/D $>$ 2.29). The analysis indicates that trailing ring vortices produce higher outflow velocities than the primary vortex after a sufficient time span, causing the radial locations between 0.8 and 1.8 r/D to be continuously exposed to strong gusts.

The discontinuous Galerkin methods demonstrated to be well suited for scale resolving simulations of complex configurations, characterized by different fluid dynamics phenomena, such as transition, separation, shock/boundary layer interaction. Unfortunately, solvers based on these methods cannot yet reach the computational efficiency of well-established standard solvers, e.g., based on finite volume methods. To reduce the computational cost and the memory footprint, while not spoiling the accuracy, different approaches were investigated, which act on the spatial or the temporal discretization, and on the linear algebra. The above approaches have been extensively investigated, but rarely considering their mutual interactions. In this work, a $p$-adaptation algorithm is coupled with a time step-size adaptation algorithm to mitigate robustness issues arising after each adaptation cycle, probably related to the projection of the old solution on the new polynomial orders distribution. Moreover, this strategy is able to control the transient phase before each adaptation cycle, and can handle also the presence of initial steady solution, when the mesh is too coarse and the polynomial degree has not yet compensated for the lack of spatial accuracy. The effectiveness and accuracy of the proposed algorithm are demonstrated on transonic viscous flows with shock-wave/boundary-layer interaction.

In this paper, a discrete unified gas kinetic scheme (DUGKS) with a sparse grid method applied in velocity space (DUGKS-SG) is proposed for simulating rarefied gas flows. The DUGKS-SG decomposes the computationally demanding problem into smaller and independent subproblems, thereby reducing the computational costs and exhibiting good parallelism. Several numerical tests, including the two-dimensional Riemann problem and the lid-driven microcavity flow, have been conducted to validate the performance of the DUGKS-SG. Comparisons with the original DUGKS and the Direct Simulation Monte Carlo (DSMC) method demonstrate that DUGKS-SG can provide satisfactory results with improved efficiency. Specifically, a maximum speedup of 9.486 for a 2D case with 7 CPU cores and 13.035 for a 3D case with 8 CPU cores can be achieved. These results suggest that the proposed DUGKS-SG can serve as an efficient numerical method for rarefied gas flow simulations.

The periodic hills simulation case is a well-established benchmark for computational fluid dynamics solvers due to its complex features derived from the separation of a turbulent flow from a curved surface. We study the case with the open-source implicit large-eddy simulation (ILES) software Lethe. Lethe solves the incompressible Navier–Stokes equations by applying a stabilized continuous finite element discretization. The results are validated by comparison to experimental and computational data available in the literature for Re = 5600. We study the effect of the time step, averaging time, and global mesh refinement. The ILES approach shows good accuracy for average velocities and Reynolds stresses using less degrees of freedom than the reference numerical solution. The time step has a greater effect on the accuracy when using coarser meshes, while for fine meshes the results are rapidly time-step independent when using an implicit time-stepping approach. A good prediction of the reattachment point is obtained with several meshes and this value approaches the experimental benchmark value as the mesh is refined. We also run simulations at Reynolds equal to $10600$ and $37000$ and observe promising results for the ILES approach.

This paper brings a numerical analysis of the TRT modeling for the Lattice-Boltzmann method when solving flow for dilatant and pseudoplastic power-law fluids. Firstly, the method was reviewed to describe the required simulation parameters and the numerical methodology. Secondly, a mathematical procedure was performed to identify the characteristic relaxation frequency as a function of both flow and fluid properties and to work as a guide parameter for LBM operation. Then, a simple shearing Poiseuille flow was employed so its characteristic shear rate could be calculated as a function of fluid properties, given the flow was characterized by the Reynolds and Mach numbers. For this test case, convergence was then explored for a broad range of parameters, and its non-monotonic dependency on the Mach number for a given convergence criterion was shown. Then, stability maps were constructed based on the characteristic relaxation frequency, which showed a strong dependency between consistency and flow index so the simulation could converge. This was explored against the results from the converged tests, which pointed out the usefulness of the characteristic relaxation frequency in predicting stable solutions. Finally, quantitatively, it was shown that for this power-law fluid flow, the $\left|\right|L_2\left|\right|$ relative error depends on the Mach number to the power of $2(2-n)$ being now a function of the flow index, extending the previously reported dependency of the Mach number to the power of 2 for plane-Poiseuille flow.

The potential of sequential Data Assimilation (DA) techniques to improve the numerical accuracy of Large Eddy Simulation (LES) performed on coarse grid is assessed. Specifically, this paper evaluates the performance of the Multigrid Ensemble Kalman Filter (MGEnKF) method, recently introduced by Moldovan, Lehnasch, Cordier and Meldi (Journal of Computational Physics, 2021). The international benchmark referred to as BARC (Benchmark of the Aerodynamics of a Rectangular 5:1 Cylinder) is chosen as test configuration, as it includes several complex flow dynamics encountered in turbulence studies. The results for the statistical moments of the velocity and pressure flow field show that the data-driven techniques employed are able to significantly improve the predictive features of the solver for reduced grid resolution. In addition, it was observed that, despite the sparse and asymmetric distribution of observation in the data-driven process, the DA augmented LES exhibits symmetric statistics and a significantly improved accuracy also far from the observation zone.

This paper presents the development of tangent linear and adjoint-over-tangent meshfree solvers to accurately compute the longitudinal static stability derivative and its shape sensitivities. These solvers are constructed using algorithmic differentiation techniques. The meshfree solver is based on the Least Squares Kinetic Upwind Method (LSKUM) for two-dimensional inviscid compressible flows. To obtain smooth shapes that make incremental changes in the stability derivative using gradient algorithms, shape sensitivities are smoothed using a two-step procedure. In the first step, sensitivities are smoothed using the Sobolev gradient smoothing. Later, the sensitivities are further smoothed using the Savitzky–Golay filter to get shapes with smooth curvature variation. The LSKUM primal, tangent, and adjoint-over-tangent solvers are applied to the test case of a subsonic flow over the MS0313 airfoil. Numerical results have shown that the adjoint-over-tangent solver computes the shape sensitivities very accurately and matches up to machine precision with the values obtained from the tangent-over-tangent solver. Perturbed airfoil shapes that increase or decrease the stability derivative are presented.