Photographic evidence of the formation and growth of vorticity behind plates accelerated from rest in still air

Abstract

The shedding of vorticity from the edges of a plate, which is accelerated normal to itself from rest in still air, is investigated experimentally by means of a new flow-visualization technique using spark-lighted shadowgraphs and heated air, benzene or other vapours to introduce sufficient changes in density along the surface of discontinuity.The photographs show small-scale undulations and the formation of a number of centres of vorticity along the well known big-scale vortex sheets which roll up along their edges.

Starting flow and structures of the starting vortex behind bluff bodies with sharp edges

“…8 a and b. Similar small vortex structures were observed by Pierce (1961), and by Pullin and Perry (1980) who discussed whether these vortex like structures were really small vortices or the result of a vibrating system. As the tracers were introduced far upstream of these small vortices, the time history of the tracers due to disturbances made the visualized pictures difficult to analyze.…”

“…These quantities were measured from the m o t i o n picture films. In order to correlate the test results of various model sizes a n d various flow accelerations, the measured results were " Pierce (1961) model (the width of the flat plate, the diameter of the circular disk or the diameter of the hollow hemi-sphere), a is the acceleration of the on-coming flow, t* is a characteristic time equal to the time that a fluid particle started from zero velocity travels a distance L/2 with constant acceleration a. The non-dimensional growth rate of the spiral starting vortex, ddv/df, and the translational speed of the spiral vortex center, d~v/df, are approximately constant during the starting stage, and are almost invariant for various model sizes and accelerations.…”

“…There are small variations of these values for different model shapes as shown in Table 1. The test results of Pierce (1961) are included for comparison. In Pierce's experiment, the model was accelerated at a very high value, about 1,540 m/s 2, which is 18,000-64,000 times the flow acceleration used in the present experiment.…”

“…Therefore most investigations on the starting vortex were carried out by using visualization methods. Pierce (1961) investigated the starting vortex behind a square plate using shadowgraphy. Didden (1979) used dye filaments in water to study the starting vortex rings.…”

Comparison of viscous and inviscid numerical simulations of the start-up vortex issuing from a semi-infinite flat plate

“…Nevertheless, in the same work, he proposed numerical solutions of regular and well defined start-up vortex spirals obtained by solving the Birkhoff-Rott equation written in similarity variables and without an explicit presence of the time variable. Moreover, the stability of this class of flows was not clarified by experiments that, performed by different authors, proposed different conclusions, Pierce [5], Pullin and Perry [6]. Luchini, among others, studied this phenomenon by computing the inviscid vortex shedding from a semi-infinite wedge [7] and from an airfoil [8].…”