Date of Award

4-26-2024

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Elongated bluff bodies;Near-wall submerged bluff bodies;Sharp-edged Bluff bodies;Square cylinder;Varying streamwise length;Wake characteristics

Supervisor

Ghassan Nasif

Supervisor

Ram Balachandar

Abstract

This thesis aims to investigate how varying the streamwise length affects the wake characteristics of sharp-edged bluff bodies in the presence of an underbody gap using large eddy simulation (LES). To investigate this, three models with the same width (W) and height (h), varying only in their streamwise lengths (L) were employed. The bodies had different streamwise elongation ratios (L/h) corresponding to L/h = 1, 2 and 3, respectively. The different streamwise lengths correspond to bodies for which the mean separated shear layers from the leading edges are shed directly into the wake (L/h =1), reattach at the top surface of the body (L/h =3) and an intermediate case (L/h = 2). The underbody gap (C) between the bottom face of the body and the wall was fixed at 0.14h for the three cases. A fully developed turbulent boundary layer approach flow was specified at the inlet and the turbulent boundary layer thickness was 3.6h. By varying the streamwise length while maintaining a small underbody gap, the findings of this study demonstrate a notable impact on the mean flow structure around the body and within the wake. The mean flow in the streamwise, spanwise and cross-sectional planes as well as the Reynolds stresses are presented to elucidate the effect of varying the streamwise length. Premultiplied frequency spectra of velocity fluctuations were utilized to examine the oscillating properties of the wake. Additionally, the fluid structures were presented using λ_2 criterion to further elucidate on how the different streamwise lengths influenced the behaviour of these structures. The mean flow field revealed no visible flow reattachment on the top and side surfaces for case L/h = 1, while flow reattachment was observed to occur for cases L/h = 2 and 3, respectively. Further investigations on the instantaneous images for L/h = 2 showed intermittent flow reattachment on the top surface of the body. Interestingly, the mean flow in the streamwise plane also revealed two large and distinct recirculation bubbles in the wake for L/h = 1 and 3 and three recirculation bubbles for L/h = 2. The results also showed that the shear layer initiated by the top leading edge displayed more extensive regions of higher Reynolds stresses compared to the shear layers emanating from the gap. A single dominant vortex shedding frequency was observed for L/h = 1 and 3, whereas dual vortex shedding frequencies were exhibited by the L/h = 2 case. The emergence of trailing vortices was observed to be more prominent as the streamwise length reduced. Interestingly, the presence of a fairly weak horseshoe vortex which occurred intermittently was captured close to the bed for the three cases.

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