Characteristics of Shallow Turbulent Near Wakes at Low Reynolds Numbers

The present study deals with the noninvasive measurement of both velocity and concentration in the near region of shallow turbulent wakes using a laser-Doppler anemometer and a video-imaging technique. A 40 mm wide flat plate placed normal to the flow is used as the wake generator. The flow depths considered in the present study are small compared to the width of the channel and the generated wakes are categorized as shallow. Tests were conducted at two depths of flow $(h=20$ and 40 mm) and the boundary layer thickness of the approaching flow is comparable to the depth. The Reynolds number of the flow based on the approaching freestream boundary layer momentum thickness varies from 180 to 400, while, the Reynolds number based on the test body width was maintained nearly constant (≈4000). Measurements were carried out at three axial stations (2.5, 5, and 10 plate widths) downstream of the bluff body. At each axial station, the velocity measurements were carried at distances of $h/4,$$h/2,$ and $3h/4$ from the channel bottom and spanning the cross section of the wake. Appropriate length and velocity scales are identified to characterize the wake. The axial variation of the shallow wake half-width based on both the lateral velocity profile and the lateral concentration profile is also obtained. A shear parameter is introduced to analyze the relative effects of transverse shear and bed friction. [S0098-2202(00)01802-2]