Session: 10.4.2 - Vortex Dynamics II

Paper Number: 158146

158146 - Spatiotemporal Dynamics of Separated Flows Around Elongated Rectangular Prism With Different Leading-Edge Separation Angles 

Abstract: 

Leading edge modification is an effective method for modification of flow separation, vortex shedding, and turbulent transport phenomena in flow around bluff bodies.  Understanding these mechanisms is critically important for designing strategies to mitigate unsteady aerodynamic loads, wind-induced vibration, and fatigue failure of structures such as bridges and buildings. The objective of this paper is to evaluate the effects of leading-edge separation angle on the spatiotemporal dynamics and turbulent characteristics in separated flow around an elongated bluff body with a chord-to-thickness ratio of 6 using time-resolved particle image velocimetry. To achieve this objective, experiments were performed for four different rectangular prisms with leading-edge separation angles (θ = 15°, 30°, 60°, and 90°). The base rectangular prism (θ = 90°) has a thickness of t = 20 mm and chord length of c = 120 mm. For the other three models, the base rectangular prism has extended forebodies of triangular cross-section with half interior angles of θ = 15°, 30°, and 60° at the leading edge. The blockage ratio and Reynolds number based on the thickness of the prism and freestream velocity (Re = U_∞t/𝜈) were maintained at 4.8% and 10,000, respectively.

The experiments were performed in a recirculating open water channel. The test section has dimensions of 6000 mm long, 600 mm wide, and h = 450 mm high. The water was seeded with silver-coated hollow glass spheres of mean particle diameter of 10 µm, and specific gravity of 1.4. The water depth was kept at h = 420 mm for all test cases. The incoming water velocity was kept constant at U_∞ = 0.5 m/s and the Froude number based on water depth (Fr = U_∞√(gh)) was maintained at 0.25 so that the effects of surface wave on the flow characteristics are negligible. The flow was illuminated with a diode pumped dual-cavity high-speed neodymium-doped yttrium lithium fluoride laser with maximum pulse energy of 30 mJ/pulse. Two high-speed complementary metal oxide semiconductor (CMOS) cameras with resolution 2560 pixels × 1600 pixels and pixel pitch of 10 µm were positioned side-by-side to image the flow field simultaneously. The sizes of the two fields of view, FOV₁ and FOV₂, are 90.0 mm × 143.9 mm and 88.0 mm × 140.9 mm, respectively. There was an overlap of 15 mm in the streamwise direction between FOV₁ and FOV₂. For each test case, 60,000 images were captured at a sampling frequency of 800 Hz. Data acquisition, image processing, and vector calculations were performed using a commercial software (DaVis version 10). A GPU multi-pass cross-correlation algorithm was used for velocity vector calculations. An initial interrogation area (IA) of 128 pixels × 128 pixels with 50% overlap followed by four final passes of 24 pixels × 24 pixels IA with 75% overlap was used.

The complete paper will provide a detailed literature review and a full description of the experimental procedure. The results will be discussed in terms of the mean flow, turbulent kinetic energy and its transport terms to understand the effects of leading-edge angle on the time averaged statistics, and turbulence transport. The spectra of the fluctuating velocities in the separated shear layer and wake region will be analyzed to evaluate the effects of leading-edge angle on the vortex shedding process, and analyses of two-point auto-correlation and integral length scales will be performed to evaluate the effects of leading-edge separation angle on the spatial coherence and large-scale structures in the separated shear layers.

Presenting Author: Bronwyn Rempel University of Manitoba

Presenting Author Biography: Bronwyn Rempel is a M.Sc. student at the University of Manitoba in the department of Mechanical Engineering.