Experimental Exploration of Quasi-2D Turbulence Clamped by Boundary Layers

Many flows in nature and technical applications are quasi-two-dimensional.This means that one dimension is significantly shorter than the other two.Examples include atmospheric and oceanic circulation, flow between the fins of a cooler, and protoplanetary discs.The two-dimensionality leads to an inverse energy cascade, pumping energy from small scales towards larger ones.This effect, predicted by Kraichnan, leads to the formation of a large-scale energy condensate.We performed a simple experimental study of decaying grid turbulence clamped by boundary layers.We observed that the large-scale condensate appears when the layer thickness is comparable to the mesh parameter of the grid.In contrast to an ideal inviscid case, the condensate weakens with thinner layers.We explain this as the effect of boundary layers, which dissipate energy at large scales and, on the other hand, pump small-scale structures (e.g., hairpin vortices) into the main flow.