Vagesh D Narasimhamurthy

We have considered the three-dimensional wake behind a cross formed by two intersecting flat plates using direct numerical simulations. The Reynolds number based on the uniform inflow velocity U0 and the plate width d was 1000. The vortex shedding in the wake was totally suppressed in a 4d wide intersection region and this gave rise to a massive zone of recirculating flow. Quasi two-dimensional vortex shedding with a primary frequency 0.165 U0/d occurred behind the outer branches more than 7d from the intersection. The wake behind the outer branches of the crossing plates closely resembled the wake behind a single flat plate. However, the wake flow in an intermediate region (located between the intersection region and the outer branches) was affected by persistent secondary flows. Further, shear-layer (K-H) instabilities have been observed in this region. The mean wake structure revealed the formation of four symmetrically positioned pairs of swirling vortices close to the intersection corner next to the plate's edges.

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.

The wake behind T-shaped intersecting flat plates has been studied by direct numerical simulations and compared with the wake behind intersecting plates forming a cross. The Reynolds number based on the uniform inflow velocity and the plate width d was 1000. Similar to the cross-plate the vortex shedding was suppressed in a 4d wide intersection region with a substantial base suction pressure reduction. Shear-layer (K-H) instabilities have been observed and its characteristic frequency obtained. In contrast to the cross-plate, a main feature of the mean wake structure behind the T-plate is the formation of two symmetrically positioned swirling vortices close to the internal corners of the T. This was examined by considering pressure contours and the turbulent production terms of mean streamwise vorticity. In spite of some similarities, major features of the wake behind the T-plate turned out to be distinctly different from the wake behind a cross-plate configuration.

The effect of coupled transverse and in-line motion of an elastically mounted rigid circular cylinder, subjected to vortex induced vibrations (VIV), is predicted using a reduced-order model. The model comprises of coupled wake and structural oscillators, where the nonlinearities in the fluid damping and forcing terms of the structural oscillator are retained. The classical van der Pol equation is used to model the wake oscillator. The unknown model constants are tuned to fit to experimental data. The influence of these tuning constants on the model performance are identified. The nonlinear contributions are shown to be insignificant in predicting the VIV characteristics associated with the transverse (y-only) oscillations of the cylinder at low Re. Surprisingly, the nonlinear terms were found to play a key role in predicting the two degree-of-freedom (2 DoF) motion of the cylinder. The model results for the cylinder with mass ratios in the low and moderate ranges are in good agreement with the experiments.

This paper reports occurrence of vortex shedding behind bluff-bodies in gas explosions, methods to suppress them using passive flow control techniques, and their overall impact on explosion overpressures. The pressure-time histories from a series of explosion tests, using an initially quiescent propane-air mixture in a vented channel of dimensions 1.5 m × 0.28 m × 0.3 m, are presented. Selected high-speed video frames visualizing the flame propagation are also presented. Three different bluff-obstruction scenarios are considered: 1) a reference case with a single smooth circular cylinder of diameter D = 0.0157 m, 2) a single cylinder identical to that in the reference case, mounted with a splitter plate of varying length from 5.13D to 0.26D, width 17.8D and thickness 0.06D, and 3) a single helically wired cylinder with wire diameter 0.1D and pitch 4D or 8D. All circular cylinders had a length of 17.8D and were mounted normal to the direction of the flow, spanning the channel cross-section 0.5 m downstream of the ignition point. The obstructions were inserted in the rig using a unique experimental setup. The peak overpressure generated by the explosion is of main interest. Both vortex shedding suppression techniques 2) and 3) yielded significant reduction in maximum overpressures when compared to the reference cylinder case 1). While all splitter plate configurations successfully reduced the maximum explosion overpressure, the splitter plates with length 1.02D and 0.51D were the most efficient, with an average reduction in overpressure of 32 ± 3%. The helical steel wire configurations also had a significant effect, with 25 ± 3% and 20 ± 3% reduction in the maximum overpressure for pitch 4D and 8D, respectively. The high-speed video visualization further buttressed the quantitative findings in the pressure measurements and clearly showed vortex shedding suppression. The current observations imply that the contribution from vortex shedding, i.e. apart from turbulence effects, to the overpressure generation in gas explosions is significant. The modelling community must consider this while preparing their simulators.

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.

The complex wake behind two side-by-side flat plates placed normal to the inflow direction has been explored in a direct numerical simulation study. Two gaps, and , were considered, both at a Reynolds number of 1000 based on the plate width and the inflow velocity. For gap ratio , the biased gap flow resulted in an asymmetric flow configuration consisting of a narrow wake with strong vortex shedding and a wide wake with no periodic near-wake shedding. Shear-layer transition vortices were observed in the wide wake, with characteristic frequency 0.6. For , two simulations were performed, started from a symmetric and an asymmetric initial flow field. A symmetric configuration of Kármán vortices resulted from the first simulation. Surprisingly, however, two different three-dimensional instability features were observed simultaneously along the span of the upper and lower plates. The spanwise wavelengths of these secondary streamwise vortices, formed in the braid regions of the primary Kármán vortices, were approximately and , respectively. The wake bursts into turbulence some - downstream. The second simulation resulted in an asymmetric wake configuration similar to the asymmetric wake found for the narrow gap , with the appearance of shear-layer instabilities in the wide wake. The analogy between a plane mixing layer and the separated shear layer in the wide wake was examined. The shear-layer frequencies obtained were in close agreement with the frequency of the most amplified wave based on linear stability analysis of a plane mixing layer.

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.