Turbulent wake behind side-by-side flat plates: computational study of interference effects

Abstract

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, $g=0.5d$ and $1.0d$ , were considered, both at a Reynolds number of 1000 based on the plate width $d$ and the inflow velocity. For gap ratio $g/d=0.5$ , 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 $g/d=1.0$ , 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 $1d$ and $2d$ , respectively. The wake bursts into turbulence some $5d$ – $10d$ downstream. The second simulation resulted in an asymmetric wake configuration similar to the asymmetric wake found for the narrow gap $0.5d$ , 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.

Perforation effects on the wake dynamics of normal flat plates

“…A recent DNS study of wakes by Dadmarzi et al. (2018) observed that initial conditions can alter significantly the final converged solution and thereby recreate many distinct states of flow reported independently in the literature. In the current study, therefore, all the simulations are started with the same initial conditions (a quiescent state).…”

“…2014; Hemmati, Wood & Martinuzzi 2016; Dadmarzi et al. 2018). The length of the plate, , is chosen based on a domain verification study (see Appendix B and also Singh & Narasimhamurthy 2018, 2021), where is varied from to .…”

“…This is reasonable since one can expect the jets to behave in an isolated fashion when the pitch separation c between them is sufficiently large compared to the hole dimension h). On the contrary, if the pitch c/h or the gap is sufficiently small, then the adjacent shear layers can experience proximity interference effects leading to engulfment or meandering oscillations and instabilities (Dadmarzi et al 2018). Villermaux & Hopfinger (1994) have reported a low-frequency (longitudinal) oscillation of the jet merging distance in their experimental study of co-flowing jets from a perforated body.…”

“…corresponding to h/d = 0, 0.2, 0.3, 0.35, 0.4, 0.45 and 0.5, respectively). Note that the present plate thickness, 0.02d, is same as that used in other numerical studies and is classified as 'thin' in the literature (Narasimhamurthy, Andersson & Pettersen 2008;Narasimhamurthy & Andersson 2009;Afgan et al 2013;Choi & Yang 2014;Tian et al 2014;Hemmati, Wood & Martinuzzi 2016;Dadmarzi et al 2018). The length of the plate, L y = 6d, is chosen based on a domain verification study (see Appendix B and also Singh & Narasimhamurthy 2018, 2021, where L y is varied from 1d to 12d.…”

A new phenomenon in interference effect on catamaran dynamic response

“…Wan and Yeung [65] addressed the cross-flow induced interference effect in a potential flow model and found out that in opposition to the wave resistance dominating at medium Fr, the drag induced by cross flow is dominant at high Fr. This cross flow may generate a wake with robust vortex shedding whose strength and configuration strongly correlate with the gap between two neighboring bodies that was studied by Dadmarzi, Narasimhamurthy, Andersson and Pettersen [66] using direct numerical simulation. Also longitudinal bilge vortex originating from the cross flow significantly affects the interference phenomena at high Fr and therefore CFD analysis would offer a comprehensive interference study of the flow field details around the catamaran [25].…”