Advection of a Coherent Structure in a Turbulent Boundary Layer

Abstract

Recently, a new extraction method, which is available for scrutiny of a coherent structure in turbulent shear flows, has been developed with introducing initial disturbances and applying an ensemble-average technique. The coherent structure extracted in a turbulent boundary layer forms into a streak elongating in the streamwise direction, and drifts downstream. As an initial disturbance, a periodical sinusoidal disturbance generated by a round speaker was introduced into the turbulent boundary layer from the wall in order to excite the linear coherent structure. Utilizing periodicity of the initial disturbance, ensemble averages were applied to the instantaneous wall pressure measured by a MEMS microphone array installed inside the wall. In the ensemble-averaged pressure distribution, high and low regions alternate in the streamwise direction and their phase shifts downstream. The advection velocity evaluated from this phase shift is proportional to the friction velocity. The proportionality constant of 14 expresses the advection velocity in the wall unit, indicating the universality of the wall scale can apply not only to the mean velocity profile but also to the dynamical fluid motions near the wall.