Measurements Near Bluff Bodies in Turbulent Boundary Layers Intended to Simulate Atmospheric Surface Layers

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Report Number: AFOSR-TR-74-0964
Author(s): Tan-atichat, Jimmy, Nagib, Hassan M.
Corporate Author(s): Mechanics and Mechanical and Aersopace Engineering Department, Illinois Institute of Technology
Laboratory: Air Force Office of Scientific Research
Corporate Report Number: IIT Fluids & Heat Transfer Report R74-2
Date of Publication: 1974-05
Pages: 187
Contract: F44620-29-C-0022
DoD Task:
Identifier: AD0782090

The relatively new counter-jet technique is shown to be suitable for producing thick turbulent boundary layers which may be used to simulate neutral atmospheric surface layers in wind tunnels of moderate length. The increased thickness is achieved in the "I.I.T. Environmental Wind Tunnel" by providing large momentum defects at the wall through upstream-oriented, spanwise-discrete wall jets, with changeable jet velocities and controllable jet angles. This technique permits rapid alteration of reproducible boundary layers from outside the tunnel while the experi­ments are in progress. It is demonstrated how various mean velocity profiles (which can be represented by a wide range of power law exponents) and turbulence intensity distri­butions of the boundary layer are obtained at the same streamwise position using different settings of the counter­jet parameters and different types of artificial surface roughness. The transverse uniformity of these layers is also documented. Selected measurements of the flowfield near a bluff body for two wind directions in three different layers are compared in order to examine the sensitivity of measured effects to changes in the characteristics of the turbulent layers. While small changes are observed when results obtained in the two thick boundary layers are com­pared, large differences are noted between these and those obtained from tests in the thinner "natural" boundary layer of the tunnel. It is in this boundary layer (thickness of boundary layer equal to approximately half of the model height) that the effect of the wind direction with respect to the bluff body is most evident. The data consistently demonstrate that the higher turbulence level within the two thicker layers increases the spreading and decay rates of the wake of the model. Influence of the wind direction on the flowfield on top of the bluff body is much stronger than that due to changes in the boundary layers. Similar sensitivity to the orientation of the bluff body is observed downstream of the model at low elevations.

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