A study of unsteady laminar boundary layer flow on a flat plate using a smoke-wire/silhoutte flow visualization technique

A smoke-wire/silhoutte flow visualization technique has been developed for the study of unsteady boundary layer flow. In this technique, a thin wire (diameter 100 μm) coated with liquid paraffin is heated ohmically to produce fine smoke streaks as tracer particles. The flow visualization of the streaklines is based on photographs of the perfect shadow of the tracer particles. In this method, the unobstructed light rays that impinge on the front projection material are reflected back along their originating path with 98% efficiency. The light rays that go through the smoke are scattered by the smoke particles and hence do not return along their originating path. Since the backscattered light rays have a very small intensity relative to the unobstructed light rays, the resulting photograph is a high-contrast print with the smoke streaks visible as well-defined black images on an otherwise white background. Unsteady boundary layer flow due to transverse plate vibration has been investigated qualitatively to show the applicability of this new cinematographic method to the visualization of small-scale structures.