No abstract

No abstract

The objective of this study is to evaluate the capability of the partially averaged Navier–Stokes (PANS) method in a moderately high Reynolds number (ReD 1.4×105) turbulent flow past a circular cylinder. PANS is a bridging closure model purported for use at any level of resolution ranging from Reynolds-averaged Navier–Stokes to direct numerical simulations. The closure model is sensitive to the length-scale cut-off via the ratios of unresolved-to-total kinetic energy (fk) and unresolved-to-total dissipation (fε). Several simulations are performed to study the effect of the cut-off length-scale on computed closure model results. The results from various resolutions are compared against experimental data, large eddy simulation, and detached eddy simulation solutions. The quantities examined include coefficient of drag (Cd), Strouhal number (St), and coefficient of pressure distribution (Cp) along with the mean flow statistics and flow structures. Based on the computed results for flow past circular cylinder presented in this paper and analytical attributes of the closure model, it is reasonable to conclude that the PANS bridging method is a theoretically sound and computationally viable variable resolution approach for practical flow computations.

The partially averaged Navier–Stokes (PANS) approach has emerged as a viable scale-resolving bridging method over the last decade. Conventional PANS method, based on the linear eddy viscosity closure, overcomes the scale-resolving inadequacies of Reynolds-averaging but still suffers from limitations arising from linear constitutive modeling of turbulent stresses. Linear PANS has been evaluated in a variety of complex flow fields, including the benchmark case of flow around a sphere. In this work, the authors assess the potential of nonlinear eddy viscosity closure and further extend the evaluation of nonlinear closure in predicting thermal characteristics (besides hydrodynamics) of flow past a sphere. The presented evaluation has been performed on the basis of various surface-related and wake-related quantities. Our results are compared against available experimental and direct numerical simulation (DNS)/large eddy simulation studies. Our study shows that for the same value of the filter-control parameters, nonlinear PANS performs significantly better than linear PANS.

Bridging methods for turbulence simulation are adaptive eddy-viscosity schemes based on the accuracy-on-demand paradigm, much like hybrid approaches. The object is to resolve more scales of motion than Reynolds-averaged Navier-Stokes (RANS) by suitably reducing the eddy viscosity as a function of grid spacing. Currently, there are two proposals for extending the RANS two-equation model with Boussinesq constitutive relation to bridging methods. In the first approach, it is suggested that the coefficient in the Boussinesq relation be reduced and the transport equations for the length and velocity scales left unaltered for achieving the desired level of viscosity reduction. The second approach suggests that the viscosity reduction is better achieved by suitably modifying the transport equations rather than altering the Boussinesq coefficient. In this paper, we compare the merits of the two methods in two important benchmark cases: flows past circular cylinder and backward facing step. Three types of evaluations are performed. First, we verify if the requisite viscosity reduction is indeed achieved in the calculations. Then, we compare some crucial qualitative aspects of the solutions from the two methods. Finally, we evaluate the two solutions against experimental data for the same level of intended viscosity reduction.

No abstract

No abstract

This paper summarises the results from a blind-prediction study for models developed for estimating the consequences of vented hydrogen deflagrations. The work is part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA). The scenarios selected for the blind-prediction entailed vented explosions with homogeneous hydrogen-air mixtures in a 20-foot ISO container. The test program included two configurations and six experiments, i.e. three repeated tests for each scenario. The comparison between experimental results and model predictions reveals reasonable agreement for some of the models, and significant discrepancies for others. It is foreseen that the first blind-prediction study in the HySEA project will motivate developers to improve their models, and to update guidelines for users of the models.