Flow Visualization of Internal Waves and Wakes of a Streamlined Body in a Stratified Fluid

The wake and internal waves of a moving three dimensional (3D) airfoil body in a stratified fluid has been investigated in a large stratified tank with a finite depth using movies of shadowgraphs of the flow fields. Typical Reynolds and Froude numbers of the flow varied between 10^3 and 10^4, and 0.3 and 2 respectively. The flows are generated often by towing the body in a uniformly stratified flow, while limited cases are carried out with body stationary and the channel was in recirculating mode. For some experiments the density profile had a stepped like shape. The wake flow is often consisted of internal waves including random and coherent ones. Distortion of density fields was also observed ahead and above the body in cases where the Froude number was subcritical. Results show that as the Froude number (Fr=U/Nh, where U is the body relative velocity, N is buoyancy frequency and h is the thickness of the body) is increased, the flow undergoes from a subcritical narrow wake (for Fr 1) to an internal waves dominated flow (for Fr~1) and then to a hydraulic jump with a turbulent wake with some mixing (for Fr 1). Typical wavelength of the exited internal waves is increased with Fr, as the theory predicts. The wake of the flow for Fr 1.4 appeared to collapse and some internal waves emission from it could be observed. Usually two types of internal waves, namely random small scale and large scale, more regular waves are observed.