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.

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.