Sawan Suman Sinha

The effect of wall treatment on the performance of k-ε model in incompressible, turbulent, separated flows with and without heat transfer has been evaluated in this study. We have simulated two benchmark cases: (i) flow past a circular cylinder at Re = 3900, and (ii) flow past a heated square cylinder at Re = 21 400 using the open source CFD package: OpenFOAM. We have compared three variants of the k-ε model, namely, Launder-Sharma k-ε model (Yap corrected) (LSKEY), Lam–Bremhorst k-ε model (Yap corrected) (LBKEY) and two-layer k-ε model (TLKE) along with the available experimental and direct numerical simulation (DNS) data. Comparisons are made in terms of the models’ capability to predict the mean flow variables, surface integral quantities, and heat transfer characteristics at different wake locations. On the basis of the presented study, we conclude that LSKEY performs better than the other models in predicting the wake and surface flow and heat transfer parameters. Further our comparisons show that, while LSKEY and LBKEY require comparable clock time per flow-through cycle, the computational time needed by TLKE is almost twice as compared to LBKEY or LSKEY. These results call for more attention from the CFD community onto the LSKEY model, in particular, so that it can be incorporated in various other flow fields, especially the scale resolving methodologies like the partially-averaged Navier-Stokes (PANS) equations, wherein a superior wall treatment along with a shorter computational time could be of immense advantage. In authors’ opinion, these benefits of the LSKEY model have largely been overlooked, perhaps because of a biased preference to the TLKE model, which enjoys the default presence in popular commercial CFD packages.

Purpose: In recent years, the partially averaged Navier–Stokes (PANS) methodology has earned acceptability as a viable scale-resolving bridging method of turbulence. To further enhance its capabilities, especially for simulating separated flows past bluff bodies, this paper aims to combine PANS with a non-linear eddy viscosity model (NLEVM). Design/methodology/approach: The authors first extract a PANS closure model using the Shih’s quadratic eddy viscosity closure model [originally proposed for Reynolds-averaged Navier–Stokes (RANS) paradigm (Shih et al., 1993)]. Subsequently, they perform an extensive evaluation of the combination (PANS + NLEVM). Findings: The NLEVM + PANS combination shows promising result in terms of reduction of the anisotropy tensor when the filter parameter (f_k) is reduced. Further, the influence of PANS filter parameter f on the magnitude and orientation of the non-linear part of the stress tensor is closely scrutinized. Evaluation of the NLEVM + PANS combination is subsequently performed for flow past a square cylinder at Reynolds number of 22,000. The results show that for the same level of reduction in f_k, the PANS + NLEVM methodology releases significantly more scales of motion and unsteadiness as compared to the traditional linear eddy viscosity model (LEVM) of Boussinesq (PANS + LEVM). The authors further demonstrate that with this enhanced ability the NLEVM + PANS combination shows much-improved predictions of almost all the mean quantities compared to those observed in simulations using LEVM + PANS. Research limitations/implications: Based on these results, the authors propose the NLEVM + PANS combination as a more potent methodology for reliable prediction of highly separated flow fields. Originality/value: Combination of a quadratic eddy viscosity closure model with PANS framework for simulating flow past bluff bodies.

In this work, we investigate the influence of compressibility on vorticity-strain rate dynamics. Well-resolved direct numerical simulations of compressible homogeneous isotropic turbulence performed over a cubical domain of 1024³ are employed for this study. To clearly identify the influence of compressibility on the time-dependent dynamics (rather than on the one-time flow field), we employ a well-validated Lagrangian particle tracker. The tracker is used to obtain time correlations between the instantaneous vorticity vector and the strain-rate eigenvector system of an appropriately chosen reference time. In this work, compressibility is parameterized in terms of both global (turbulent Mach number) and local parameters (normalized dilatation-rate and flow field topology). Our investigations reveal that the local dilatation rate significantly influences these statistics. In turn, this observed influence of the dilatation rate is predominantly associated with rotation dominated topologies (unstable-focus-compressing, stable-focus-stretching). We find that an enhanced dilatation rate (in both contracting and expanding fluid elements) significantly enhances the tendency of the vorticity vector to align with the largest eigenvector of the strain-rate. Further, in fluid particles where the vorticity vector is maximally misaligned (perpendicular) at the reference time, vorticity does show a substantial tendency to align with the intermediate eigenvector as well. The authors make an attempt to provide physical explanations of these observations (in terms of moment of inertia and angular momentum) by performing detailed calculations following tetrads {approach of Chertkov et al. [“Lagrangian tetrad dynamics and the phenomenology of turbulence,” Phys. Fluids 11(8), 2394–2410 (1999)] and Xu et al. [“The pirouette effect in turbulent flows,” Nat. Phys. 7(9), 709–712 (2011)]} in a compressible flow field.