The interaction between the separated shear layer emanating from the leading edge and the near wake region of bluff bodies is investigated using rectangular cylinders in a uniform flow. The height of the rectangular cylinders (h) was kept constant while the streamwise extents (L) were varied, resulting in streamwise aspect ratios (AR = L/h) of 1, 3, 4 and 5. The Reynolds number based on cylinder height and free-stream velocity was 15900. The cylinders spanned the entire width of the water channel ensuring a two-dimensional flow at the mid-span of the test section. To resolve the various time and length scales of this highly unsteady flow, time-resolved particle image velocimetry was used with two 4MPx cameras positioned side by side. A total number of 96000 images were captured, resulting in a sampling time of 120 s and 2120 eddy shedding cycles.The topological characteristics and spatial interactions between the shear layer emanating from the leading edge and the near wake region are analyzed in terms of the mean spanwise vorticity, turbulence statistics and turbulent transport. Interestingly, the results show that there is mean reattachment over the cylinder in AR = 3, 4 and 5. Analysis of the instantaneous flow fields indicate that flow reattaches 38%, 65%, 80% and 88% of the time in the AR = 1, 3, 4 and 5 cases, respectively. Consequently, the size of the secondary recirculation region near the leading edge reduces with reducing AR. The shear layer emanating from the leading edge exhibits identical topologies in the tested AR cases. Vorticity is rapidly generated at the leading edge and diffuses over the cylinder. However, the diffusion of vorticity is differentiated at the trailing edge where the elevated regions of vorticity are observed to increase in magnitude with reducing AR. The magnitudes of the Reynolds stresses in AR = 1 are an order higher than that in AR = 5, while the vertical location of the peak magnitudes are identical in all cases. Regions of positive Reynolds shear stress implying counter gradient diffusion are observed at the leading edge, up to 0.3h in all tested cases. Above the cylinder, the third-order moments switch signs at the highest elevation of the mean separating streamline. This suggests that turbulence motions tend to transport kinetic energy outwards above the mean recirculating region, but inwards within the mean separation bubble.In the full paper, turbulent transport of Reynolds normal stresses, probability density functions, vortex stretching analysis, signed swirling strength and two-point correlations are used to further investigate the structural coherency between the shear layer emanating from the leading edge and the near wake region.

Interactions Between the Shear Layer Emanating From Rectangular Cylinders and the Near Wake Region

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.