Wake vortex evolution of square cylinder with a slot synthetic jet positioned at the rear surface

Abstract

Wake vortex evolution of a square cylinder with a slot synthetic jet issuing from the cylinder’s rear surface has been experimentally investigated using the time-resolved particle image velocimetry technique. The Reynolds number based on the side length of the square cylinder is $Re=836$. The excitation frequency normalized by the natural shedding frequency $f_{e}/f_{0}$ varies from 0 to 6 at the dimensionless stroke length $L_{0}/w=72.6$. The distributions of the time-averaged Reynolds stresses present significant differences as the excitation frequency increases. With control, the mean streamwise velocity deficit of the wake recovers more quickly in comparison with the natural case, and the vertical velocity fluctuation intensity becomes weaker. Moreover, a drag reduction can be achieved for the control cases, especially, for $f_{e}/f_{0}=4$ and $f_{e}/f_{0}=6$, a thrust instead of drag reduction can be obtained. The profiles of the mean streamwise velocity tend to have jet-like distributions. The wake vortex dynamics and its evolution with the excitation frequency are revealed. (i) For the low excitation frequency cases ($f_{e}/f_{0}=0.5$, 1, 2), no significant changes in the dominant frequency and the spanwise vortex structures are observed in comparison with the natural case. (ii) For the moderate excitation frequency case ($f_{e}/f_{0}=3$), the wake vortex shedding frequency is locked on half of the control frequency. In this case, the shear layer is divided into two parts by the synthetic jet vortex, and the wake vortices with smaller scales still shed asymmetrically and appear closer to the square cylinder. (iii) For the high excitation frequency case ($f_{e}/f_{0}=6$), the flow is governed by the synthetic jet. As a result of strong perturbations of the synthetic jet, the wake vortex shedding becomes symmetric with the shedding frequency consistent with the control frequency. And the separation is suppressed effectively. The different control effects of the slot synthetic jet on a square cylinder and a circular cylinder are also compared in detail. Generally speaking, the circular cylinder is easier to be controlled due to its non-fixed separation points.

The interactions of rectangular synthetic jets with a laminar cross-flow

“…2015; Qu et al . 2017). The FMD can decompose the flow field into a train of modes corresponding to the coherent structures with a specific frequency based on discrete Fourier transformation (DFT) of the data at each sampling point.…”

Effect of excitation frequency on flow characteristics around a square cylinder with a synthetic jet positioned at front surface

“…2005; Qu et al. 2017). The time coefficients of the first two POD modes, which tend to distribute around a circle in the normalized plane, are thus used as the phase indicator for the quasi-periodic flow in our present study.…”

“…The definitions of parameters associated with the synthetic jet in above equations make reference to the previous works (Smith & Glezer 1998; Amitay & Cannelle 2006; Feng & Wang 2010; Qu et al. 2017).…”

“…2017), jet (Kim, Yang & Eom 2003; Akansu & Firat 2010 a ; Saha & Shrivastava 2015), synthetic jet (Akansu & Firat 2010 b ; Qu et al. 2017) and blowing/suction (Sohankar, Khodadadi & Rangraz 2015; Sohankar & Najafi 2018). Compared with passive control, active control is much easier to change the control parameters according to the external flow conditions, and it can be more efficient and energy-saving if sensors are introduced to establish a feedback system.…”

Numerical Investigation of Multi-SDBD Plasma Actuators for Controlling Fluctuating Wind Load on Building Roofs

“…Wang et al [6] used two small affiliated rotating cylinders to reduce the drag of a circular cylinder by suppressing the vortex shedding. Qu et al [7] found that a slot synthetic jet located at the rear surface of a square cylinder was used to achieve a drag reduction and shorten the wake vortex formation length. However, these active strategies require complex moving parts or mechanical configuration.…”