Vagesh D Narasimhamurthy

This study assesses different turbulence modeling approaches for simulation of two-phase coaxial annular swirling jet flows. The problem selected from literature involves an analytical inlet profile for an annular liquid sheet sandwiched between two coaxial annular gaseous jets. The liquid-gas interface is resolved using the volume-of-fluid (VOF) model with continuum surface force approximation. 3D unsteady Reynolds averaged Navier-Stokes simulations using up to 8.4 million grid cells and 64 HPC cores are conducted using the Fluent 17.2 software to obtain transient multiphase CFD data for this problem. Different turbulence models explored include the k-epsilon RNG with swirl modification, the Reynolds stress model (RSM), and RSM with scale adaptive simulations (RSM-SAS). Comparisons with the direct numerical results from literature suggest that the scale-adaptive simulation using RSM-SAS approach better predicts the onset of instability, liquid jet column collapse, jet mixing, vortex breakup, and the overall characteristics of this flow.

Several model validation studies on gas dispersion scenarios have been conducted in the past on the Reynolds averaged Navier Stokes (RANS) based eddy viscosity turbulence models. However, many of these studies are based on a limited number of validation cases involving simple geometries and conformal mesh. In the area of safety engineering, the application of RANS-based CFD for consequence analysis is a widely used methodology. Best practice on use of CFD in this context, as the document developed in the COST Action 732 (Franke et al., 2007), focus primarily on validation and verification aspects as well as simulation setup and definition of input data. Guidelines on turbulence models also exist, among which the ERCOFTAC CFD Best Practice Guidelines, and the works of Meroney et al. (2016) and Mcbride et al. (2001). However, there is no unique recommended model for dispersion simulations. The objective of the present study is to assess the three well-known RANS eddy viscosity models, namely, Standard k-ε, Re-Normalization group (RNG) k-ε and Realizable k-ε, in a representative range of gas dispersion cases by comparing models’ behavior with experimental data. The current validation cases include dense CO2 release in a cross-wind, impinging hydrogen jet, and a dense chlorine jet release in an industrial site. All the simulations were conducted using the commercial CFD code FLACS. Turbulence models were assessed based on the ability to reproduce experimental concentrations, required computational-time and numerical-stability. Overall, Standard k-ε and RNG k-ε models were found to be reasonably good in all cases. Nevertheless, Realizable k-ε model shows promise in yielding good results in cases involving complex-geometries and dense-phase gas-releases. These results may also be explained with the interplay between the Porosity/Distributed Resistance subgrid models used in FLACS and turbulence models.

Parallel computations of flow past a perforated plate of porosity 25% at Reynolds number 250 (based on plate width, d and inflow velocity, Uo) is carried out. The effect of aspect ratio is studied with different span-wise lengths of the domain (1d, 3d and 6d). Present results revealed that an aspect ratio of 6d is required to capture the transient wake dynamics. It was found that statistical quantities stemming from aspect ratio 3d and 6d cases agree with each other, though the dynamical behavior of the wake is very different. The signature period doubling effects associated with short constrained domains were visible in the 1d and 3d aspect ratio cases. Enforcing periodic boundary condition along the short span-wise domains may thus adversely affect the flow.