The intermittently turbulent region of the boundary layer

Abstract

The intermittently turbulent region 01 the boundary layer There is increasing evidence that fully developed turbulent shear flows exhibit a velo city structure that is more coherent than generally expected. It is possible to detect an ordered, relatively coherent structure that moves randomly in space and time and has characteristic lifetimes and length scales which are, in general, much larger than the dissipation scale of the flow. Perhaps the most dramatic evidence of the existence of this type of quasi-ordered motion in the boundary layer is found in the work of KIM, KLINE, and REYNOLDS [Ref. 1], in which such a structure is detected near the sublayer by measurement of instantaneous velo city distributions by means of the hydrogen-bubble technique. These measurements showed the repeated appearance of this characteristic structure at random intervals of time. It is clear that without a set of "instantaneous" velo city profiles, conventional time averaging of the velocities would have overlooked such a phenomenon. The interest, of course, is not in the phenomenon itself, but on any light that it may cast on the production of turbulent energy by the shear flow, in this case through a possible instability of the basic flow. The phenomenon to which we address ourselves is the structure associated with the motion of the interface between the turbulent and non-turbulent fluid at the outer edge of the turbulent boundary layer, although similar phenomena occur in turbulent jets, wakes, and free shear flows. It is known that for these phenomena conventional time averaging techniques yield little evidence of such structure, since they weigh equally contributions from the turbulent and non-turbulent regions of the flow. However, investigators such as KOVASZNAY and KIBENS [Ref. 2], and the authors, have demonstrated the existence of such a structure and made detailed quantitative measurements of its properties. The question that is relevant in this region of the flow is the relationship between this detectable structure and the entrainment process of non-turbulent fluid. For the purpose of analyzing such phenomena, a facility for the digital processing of turbulent boundary -Iayer data was esta;blished at the University of Southern California. The following is a description of the initial measurements processed by this facility, some interpretation of the results, and abrief account of the techniques used. Conditional sampling and detector functions There are two basic requirements that must be met before the probing of randomly occurring coherent structures can proceed. i) The first of these is the ability to measure vclocities at several points in the flow field simualtaneously. This necessitates the use of an array of velo city sensors (hot-wire anemo-meters) with the size of the array of the order of thc characteristic scale of the structure. This technique was in fact successfully used by KOVASZNAY, KOMODA, and VASUDEVA [Ref. 3] in their study of the formation of turbulent spots during transition.