Yipeng Shi

Constrained subgrid-scale stress model for large eddy simulation

In this letter, we propose to impose physical constraints in the dynamic procedure of the dynamic subgrid-scale (SGS) stress model in large eddy simulation, and to calculate the SGS model coefficients using a constrained variation. Numerical simulations of forced and decaying isotropic turbulence demonstrate that the constrained dynamic mixed model predicts the energy evolution and the SGS energy dissipation well. The constrained SGS model also shows a strong correlation with the real stress and is able to capture the energy backscatter, manifesting a desirable feature of combining the advantages of dynamics Smagorinsky and mixed models.

Effect of shocklets on the velocity gradients in highly compressible isotropic turbulence

The effect of randomly generated shocklets on velocity gradients in a three-dimensional compressible isotropic turbulence was systematically studied. The forced flows obtained from high-resolution simulations had a turbulent Mach number of 1.0 and a Taylor microscale Reynolds number around 180. The shock detection algorithm developed by Samtaney et al. [“Direct numerical simulation of decaying compressible turbulence and shocklet statistics,” Phys. Fluids 13, 1415 (2001)] was applied to extract the shocklets. Using reference frames moving with the detected shocks, we obtained statistical properties of velocity and its gradients both upstream and downstream of the shocks. It was shown that the shocks induced flow modulation at a wide range of length scales, including the inertial subrange scales. The shocks intensified enstrophy in the shock regions and this enhanced enstrophy production was partially redistributed over various scales and dissipated by straining and viscous effects outside the shock region...

Constrained large-eddy simulation of separated flow in a channel with streamwise-periodic constrictions

Constrained large-eddy simulation (CLES) method has been recently developed by Chen and his colleagues for simulating attached and detached wall-bounded turbulent flows. In CLES, the whole domain is simulated using large-eddy simulation (LES) while a Reynolds stress constraint is enforced on the subgrid-scale (SGS) stress model for near wall regions. In this paper, CLES is used to simulate the separated flow in a channel with streamwise-periodic constrictions at Re = 10,595. The results of CLES are compared with those of Reynolds-averaged Navier-Stokes (RANS) method, LES, detached eddy simulation (DES) and previous LES results by Breuer et al. and Ziefle et al. Although a coarse grid is used, our results from the present LES, DES and CLES do not show large deviations from the reference results using much finer grid resolution. The comparison also shows that CLES performs the best among different turbulence models tested, demonstrating that the CLES provides an excellent alternative model for separated flows. Furthermore, the cross-comparisons among different CLES implementations have been carried out. Our simulation results are in favor of using the constraint from algebraic RANS model or solving the RANS model equations in the whole domain with a length scale modification according to the idea from DES.

Constrained large-eddy simulation of wall-bounded compressible turbulent flows

A constrained large-eddy simulation (CLES) approach is developed for wall-bounded compressible turbulent flows based on its incompressible analogue [Chen et al., “Reynolds-stress-constrained large-eddy simulation of wall-bounded turbulent flows,” J. Fluid Mech. 703, 1–28 (2012)]. In the new CLES approach, both the subgrid-scale (SGS) stress and the SGS heat flux are decomposed into an averaged part and a fluctuating part in the near-wall region with the mean SGS stress and heat flux constrained by prescribed Reynolds stress model and turbulent heat flux model, respectively. The Smagorinsky SGS models are employed to approximate the SGS stress and heat flux in the remaining region of the flow domain. The present CLES method is validated by simulating the compressible turbulent channel flows at various Reynolds numbers and Mach numbers. The mean velocity profiles, mean temperature profiles, and other statistical quantities and turbulent structures are obtained and well compared among the present approach, d...

Morphology of gas cavities on patterned hydrophobic surfaces under reduced pressure

Gas cavities trapped on structured hydrophobic surfaces play important roles in realizing functionalities such as superhydrophobicity, drag reduction, and surface cleaning. The morphology of the cavities exhibits strong dependence on system parameters which impact the performance of these surfaces. In this work, a complete theoretical analysis is presented to predict cavity morphological change under reduced liquid pressure, on a submerged hydrophobic surface patterned with cylindrical pores. Equilibrium solutions are derived for five different phases, namely, (I) pinned recession, (II) depinned recession, (III) Cassie-Baxter, (IV) expansion, and (V) coalescence; their stabilities are also analyzed. A phase map is developed outlining the different regimes with respect to the gas amount and liquid pressure. Importantly, phase (IV) exhibits a complex stability behavior that leads to two possible routes to coalescence, which lends two different mechanisms of cavitation. Accordingly, the threshold pressure fo...

Energy transfer, pressure tensor, and heating of kinetic plasma

Kinetic plasma turbulence cascade spans multiple scales ranging from macroscopic fluid flow to sub-electron scales. Mechanisms that dissipate large scale energy, terminate the inertial range cascade and convert kinetic energy into heat are hotly debated. Here we revisit these puzzles using fully kinetic simulation. By performing scale-dependent spatial filtering on the Vlasov equation, we extract information at prescribed scales and introduce several energy transfer functions. This approach allows highly inhomogeneous energy cascade to be quantified as it proceeds down to kinetic scales. The pressure work,

Interactions between inertial particles and shocklets in compressible turbulent flow

-\left( \boldsymbol{P} \cdot \nabla \right) \cdot \boldsymbol{u}

Statistics and structures of pressure and density in compressible isotropic turbulence

, can trigger a channel of the energy conversion between fluid flow and random motions, which is a collision-free generalization of the viscous dissipation in collisional fluid. Both the energy transfer and the pressure work are strongly correlated with velocity gradients.

Compressibility effect on coherent structures, energy transfer, and scaling in magnetohydrodynamic turbulence

Numerical simulations are conducted to investigate the dynamics of inertial particles being passively convected in a compressible homogeneous turbulence. Heavy and light particles exhibit very different types of non-uniform distributions due to their different behaviors near shocklets. Because of the relaxation nature of the Stokes drag, the heavy particles are decelerated mainly at downstream adjacent to the shocklets and form high-number-density clouds. The light particles are strongly decelerated by the added-mass effect and stay in the compression region for a relatively long time period. They cluster into thin filament structures near shocklets.

Constrained large-eddy simulation of laminar-turbulent transition in channel flow

We study statistics and structures of pressure and density in the presence of large-scale shock waves in a forced compressible isotropic turbulence using high-resolution numerical simulation. The spectra for pressure and density exhibit a −2 scaling over an operational definition of the inertial range. Both the numerical simulation and a heuristic PDF model reveal that the PDFs of pressure increment exhibit a −2 power law region for the separation in the operational definition of inertial range, quantitatively similar to the PDF of pressure gradient, which also displays a −2 power law region. Moreover, the statistical relation between density increment and pressure increment has been investigated through a shock-relation model. There is a positive correlation between the vorticity magnitude and pressure, which is different from the case of incompressible turbulence. We argue that this difference is due to large-scale shock waves, another type of intermittent structures in addition to vortex structures in incompressible turbulence.