Decay of grid turbulence in superfluid helium-4: Mesh dependence

Temporal decay of grid turbulence is experimentally studied in superfluid He-4 in a large square channel. The second sound attenuation method is used to measure the turbulent vortex line density (L) with a phase locked tracking technique to minimize frequency shift effects induced by temperature fluctuations. Two different grids (0.8 mm and 3.0 mm mesh) are pulled to generate turbulence. Different power laws for decaying behavior are predicted by a theory. According to this theory, L should decay as t(-11/10) when the length scale of energy containing eddies grows from the grid mesh size to the size of the channel. At later time, after the energy containing eddy size becomes comparable to the channel, L should follow t(-3/2). Our recent experimental data exhibit evidence for t(-11/10) during the early time and t (-2) instead of t(-3/2) for later time. Moreover, a consistent bump/plateau feature is prominent between the two decay regimes for smaller (0.8 mm) grid mesh holes but absent with a grid mesh hole of 3.0 mm This implies that in the large channel different types of turbulence are generated, depending on mesh hole size (mesh Reynolds number) compared to channel Reynolds number.