Session: Flow Visualization and Regular Poster Session

Paper Number: 158121

158121 - Effects of Reynolds Number on Separated Flow Around an Elongated Rectangular Cylinder 

Abstract: 

Introduction: Flow separation and reattachment around rectangular cylinders with sharp leading edges have received significant research attention due to their widespread application in diverse environmental and engineering applications. Most of these investigations have focused on the effects of Reynolds number (, where  is the freestream velocity and  is the kinematic viscosity) and streamwise aspect ratio (, where  and  are the streamwise length and height of the cylinder, respectively) on the mean flow as well as turbulence and drag characteristics. While short cylinders (AR < 3) as well as long cylinders (AR > 7) of rectangular cross-section have been examined in detail, research on separated flow around rectangular cylinders with intermediate aspect ratios is limited. In this paper, we investigate the effects of Reynolds number on the mean flow and turbulent statistics of separated flows induced by a rectangular cylinder with a fixed aspect ratio of AR = 6 using a high-resolution particle image velocimetry. The thickness and streamwise length of the rectangular cylinder are 20 mm and 120 mm, respectively, and seven Reynolds numbers (Re = 1000, 2000, 3000, 5000, 7500, 10000, 13200) were examined.

Experimental Procedure: The experiments were performed in an open recirculating water channel. The water depth in the channel was kept constant at  mm. The different Reynolds numbers were achieved by varying the freestream velocity from 0.05 m/s to 0.66 m/s. Silver coated hollow glass spheres with diameter 10 µm and specific gravity 1.4 were used to seed the flow.  The seeding particles were illuminated by a diode pumped dual-cavity dual-head high-speed Neodymium-doped yttrium lithium fluoride laser that emits a green light of wavelength 527 mm at a maximum pulse energy of 30 mJ/pulse. Two high speed complementary metal oxide semiconductor cameras of resolution 2560 pixel × 1600 pixel were positioned side-by-side to capture the particle images. The dimensions of the field of view of each camera were 144 mm ´ 90 mm in the streamwise and vertical direction, respectively, and an overlap of 15 mm in the streamwise direction was ensured between the two fields of view.  For each test case, 9000 statistically independent image pairs were collected at a sampling frequency of 3 Hz. Data acquisition, image post-processing and vector calculations were performed using a commercial software (DaVis version 10.2.1) provided by LaVision Inc.

Preliminary Results: Analyses of the mean flow demonstrate that the separated shear layer from the leading edge reattached on the cylinder regardless of Reynolds number. This is followed by a development of a boundary layer and a subsequent separation at the trailing edge and the formation of a wake region behind the cylinder.  The mean flow topology and reattachment length on the cylinder show a clear dependence on Reynolds number. For instance, the reattachment length decreases from  = 5.63 at Re = 1000 to a minimum value of 3.67 at Re = 3000. This was followed by a gradual rise with increasing Reynolds number until Re = 10 000, at which point it plateaus at around 4.2. The wake recirculation length, on the other hand, is nearly independent of Reynolds number. The levels of Reynolds stresses and turbulent kinetic energy also change with Reynolds number. The results will be analyzed to evaluate the effects of Reynolds number on the large-scale energetic structures using two-point auto-correlation coefficients and proper orthogonal decomposition.

Presenting Author: Erin Van Veen University of Manitoba

Presenting Author Biography: