Characteristics of the Flow Past a Wall-Mounted Finite-Length Square Cylinder at Low Reynolds Number With Varying Boundary Layer Thickness

Abstract

Direct numerical simulation (DNS) is performed to investigate the modes of shedding of the wake of a wall-mounted finite-length square cylinder with an aspect ratio (AR) of 7 for six different boundary layer thicknesses (0.0–0.30) at a Reynolds number of 250. For all the cases of wall boundary layer considered in this study, two modes of shedding, namely, anti-symmetric and symmetric modes of shedding, were found to coexist in the cylinder wake with symmetric one occurring intermittently for smaller time duration. The phase-averaged flow field revealed that the symmetric modes of shedding occur only during instances when the near wake experiences the maximum strength of upwash/downwash flow. The boundary layer thickness seems to have a significant effect on the area of dominance of both downwash and upwash flow in instantaneous and time-averaged flow field. It is observed that the near-wake topology and the total drag force acting on the cylinder are significantly affected by the bottom-wall boundary layer thickness. The overall drag coefficient is found to decrease with thickening of the wall boundary layer thickness.

The effect of aspect ratio on the wake structure of finite wall-mounted square cylinders

“…This phenomenon is known as intermittent shedding and has been observed in previous studies (Wang & Zhou 2009) and is also analogous to the amplitude modulation of the acoustic signal observed by Porteous et al (2017). Wang & Zhou (2009) and, more recently, Behera & Saha (2019) relate this intermittent change in amplitude to the shedding alternating between anti-symmetric (high amplitude) and symmetric (low amplitude) modes. There is also spanwise variation in the strength of the shedding, which is strong near the junction but becomes slightly weaker towards the free end, as is more obviously shown for L/W = 4.0.…”

“…(2017). Wang & Zhou (2009) and, more recently, Behera & Saha (2019) relate this intermittent change in amplitude to the shedding alternating between anti-symmetric (high amplitude) and symmetric (low amplitude) modes. There is also spanwise variation in the strength of the shedding, which is strong near the junction but becomes slightly weaker towards the free end, as is more obviously shown for .…”

“…From the size of the computational domain shown in figure 2, the gap between the cylinder's top surface and the top boundary is 2L for the cylinder the highest aspect ratio (L/W = 18.6). Behera & Saha (2019) performed numerical simulations of the flow past a finite wall-mounted square cylinder, where they examined three different gap heights of 1.3L, 1.7L and 2.1L, and found that the flow field predicted in the three domains is almost identical. To confirm the domain independence in the current study, a simulation was run using a computational domain with a gap height of L. Figure 3 compares the time-averaged pressure coefficient on the back face (or 'base', facing downstream) obtained using the domains with 2L and L. Unless stated, the pressure data herein are presented as a pressure coefficient,…”

The Steady Wake of a Wall-Mounted Rectangular Prism with a Large-Depth-Ratio at Low Reynolds Numbers

“…They reported the simultaneous occurrence of the two modes of anti-symmetric and symmetric wake shedding processes regardless of the boundary thickness. Behera et al (2019) [21] also mentioned that occurrence of the symmetric mode of shedding coincides with the strongest values of upwash and downwash flow. Zhou et al (2013) [22] investigated the flow features of a two-dimensional rectangular prism with depth ratio of 5 in non-shear and shear incoming flow at Reynolds number of Re = 2.2 × 10 4 .…”

“…Although these were limited to higher Reynolds numbers, similar behavior is intuitively expected for lower Re flows. Behera et al (2019) [21] investigated the effect of changing the boundary layer thickness on the flowfield of a square cylinder with an aspect ratio of 7 at Reynolds number of 250. They reported the simultaneous occurrence of the two modes of anti-symmetric and symmetric wake shedding processes regardless of the boundary thickness.…”