Vortex Systems in Wakes1

Publisher Summary This chapter focuses on the assessment of the basic content of many theoretical and experimental papers that have been written about wakes for the range of Reynolds number up to about 2,500. It is in this range that systematic arrangements of vortices have been observed in the wakes of many different types of obstacle. Two theoretical investigations that stand out in this field are Karman's and Foppl's. As the Reynolds number is increased, the streamlines broaden out and form a closed region behind the obstacle; the inflow along the axis increases in magnitude. To compensate for this inflow, there is an outflow within the closed region, in the neighborhood of the bounding streamlines. This circulatory motion constitutes a vortex line pair. In every experimental arrangement in which the characteristics of the vortex pair have been investigated, there appear to be some critical Reynolds number in the region of which the vortices start to move away. The mechanism leading to the formation of vortices and to the motion subsequent to their discharge has been explained qualitatively in terms of boundary layers and vortex layers. At very small values of the Reynolds number, the two-dimensional vortex layers, originally on opposite sides of the body, join downstream. The three-dimensional wake behind bodies, in the range of Reynolds number in which there is a periodic discharge of vorticity, is in some ways similar to the two-dimensional one. It is clear from what has been written that a great deal remains to be investigated before a mathematically satisfactory description can be given of the phenomena of vortex systems in wakes.

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