Study of turbulent spots in the flat plate boundary layer

Abstract

The present work has something of the character of conditional sampling in that only fully developed turbulence, In .the form of turbulent spots, was examined.

The spots were initiated by injecting a small amount of air through a hole in the flat plate on which a laminar boundary layer was formed. A typical hot wire trace, resulting from the passage of a turbulent spot, is shown in Fig. (1.1). The trace is similar to that shown by Schubauer and Klebanoff although the methods by which the spots were produced were different; A PDP8 digital computer was used to control experiments and record data.

Many turbulent spots were observed at each chosen position in the boundary layer and mean flow velocities and turbulent intensities were computed from the digitised data. Crossed constant temperature hot wires gave streamwise and spanwise mean velocity components and the streamwise fluctuation component. The normal mean velocity was computed using the equation of continuity.

Diagrams and contour maps showing distributions of streamwise, spanwise and normal mean velocity, streamwise root mean square fluctuation and streamwise, spanwise and normal mean vorticity components at selected positions downstream are presented.

The on-line facility of the wind tunnel was used to óontrol experiments and collect digitised data. The shape and velocity of propagation of the turbulent spot were determined and the streamwise and spanwise componentsof mean velocity and the streamwise turbulent component were measured. The mean velocity component normal to the flat plate was derived using the equation of continuity and the measured mean velocity components. Contour maps of the mean velocity components and the turbulent fluctuations were drawn using a computer controlled graph plotter. From these maps it is deduced that the mean structure of the turbulent spots consists of three spanwise vortices on top of each other. It is proposed that the action of the vortices maintains the high level of turbulence in the turbulent spot, by entraining fluid from the free stream.

Turbulent bursting events were also observed and are discussed in the light of reports by other workers.