Characteristics of shallow turbulent near wakes at low Reynolds numbers

The present study deals with the noninvasive measurement of both velocity a nd concentration in the near region of shallow turbulent wakes using a lase r-Doppler anemometer and cc video-imaging technique. A 40 mm wide flat plat e 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 chan nel and the generated wakes are categorized as shallow. Tests were conducte d 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 he flow: based on the approaching freestream boundary layer momentum thick ness varies from 180 to 400, while, the Reynolds number based on the test b ody width was maintained nearly constant (approximate to 4000). Measurement s were carried out at three axial stations (2.5, 5, and 10 plate widths) do wnstream of the bluff body. Ar each axial station, the velocity measurement s were carried at distances of h/4, h/2, and 3h/4 from the channel bottom a nd 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 bused on both the lateral velocity profile and the lateral concentration profile is also obtained. A shear parameter is introd uced to analyze the relative effects of transverse shear and bed friction. [S0098-2202(00)01802-2].