Experimental study of turbulence using soap film tunnel

The results presented here represent experimental study of quasi-two-dimensional turbulence in gravity-driven soap-film flow, performed under the expert guidance of Dr. Peter Vorobieff. The study mainly focuses on the differences between the actual behavior of the flow and the theoretical predictions of two-dimensional turbulence. The use of Digital Particle Image Velocimetry (DPIV) has made it possible to acquire a previously unattainable quality of velocity field acquisition. For grid turbulence, the highly-resolved flow field data are analyzed statistically and presented in terms of velocity structure functions, as well as energy and enstrophy averages at different downstream positions. The results show a divergence from the theoretical predictions for the rate of decay of the energy and enstrophy averages which happen to be quantifiably greater than the forecasts of the two-dimensional turbulence theory. This increased decay is likely to be the manifestation of the extra dissipation mechanism present in soap-film flows and prominent on the larger scales---air drag. The structure function analysis confirms the notion. The second experiment involves flow instabilities like the Kelvin-Helmholtz instability in two-dimensional flows. It is caused by merging two flows driven at different velocities to generate a shear layer instability in the merged flow. The results show that the transition to turbulence in two-dimensional shear flows is different than that seen in three-dimensional systems. The presence of air drag also influences the evolution of the quasi-2D shear layer. The dissipation of flow structures larger than the 1 cm is greatly affected by the influence of air drag. The energy from such large flow features is drained rapidly due to the coupling with boundary layers in air near the soap film, resulting in an apparent "freeze" of the quasi-2D shear layer evolution. The third experiment describes the generation of forced shear layer with the aid of a motor driven conveyor belt. The described experimental setup is used to show that the air drag can be used to our advantage to pump the energy into the flow and sustain forced shear layer in the flow.