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Cheun, B

The thesis is concerned mainly with the experimental investigation of the near wake region of very simple two-dimensional square-edged bodies. Particular attention was given to the effect of upstream conditions upon the characteristics of the shear layer bounding the recirculating region, namely its thickness, its position and point of reattachment, its growth rate and the stresses within it. The first part deals with the very simple geometry of a backward facing step and examines the way in which the shear layer is affected by boundary layer thickness at the step edge. A thicker layer is associated with a shear layer that is thicker not only initially but throughout, although the peak stresses are rather lower. The second part deals with the case of a two-dimensional square-edged block set on a baseboard in a boundary layer of thickness equal to several times the block height. With a sufficiently long block, flow reattaches on the top, whereas, with a shorter block, there is no top reattachment. The most attention was given to the case with no top reattachment, streamwise length being equal to half the block height in this instance. It was found that, with a high turbulence intensity, such as is encountered in a 'rough' boundary layer, the position of the separated shear layer is much lower than in the case where upstream turbulence intensity is no more than a few percent, whether due to grid turbulence or the existence of a thick 'smooth' boundary layer. This in turn, affects other aspects of the situation, such as reattachment position and pressure distribution. For both bodies, the shear layer can usefully be compared with that of an axisymmetrical jet. Observations on the jet provided a basis for this comparison, but, more, served for the development and calibration of the pulsed-wire anemometer as used in the slant position and for a comparison with the conventional crossed-wire anemometer. This novel use of the instrument established its value for the measurement of shear stresses in highly turbulent zones. Though the present study reveals a number of important features of the near wake region of the square-edged bodies studied, the complex flow phenomena near reattachment zone merit further study

Separated shear layers behind two-dimensional square-edged bodies

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