Experimental Comparison of Turbulent Statistics in Superfluid ⁴He Coflow and Counterflow

The flow in a quantum turbulent field is formed from normal and superfluid components. The two components are known to interact with each other, hence the flow is complex. In this presentation, we discuss the difference between ⁴He coflow and thermal counterflow flow with a focus on turbulent universality in small scales. We consider that while both components move in the same direction in the coflow, the thermal counter flow moves in the opposite direction, resulting in differences in turbulence statistic. Experimental images were taken of (i) thermal counterflow with a heat source, and (ii) coflow in a grid generated wind tunnel turbulence (Maximum grid Re = 6.46 ×10⁴, RMS = 18.0 %). Small particles were added to visualize the flow. Lagrangian velocity was calculated for these data by applying the 2D PTV method. When analyzing these velocity data, we must keep in mind that the velocity can be affected by the particle size. Frequency spectra were calculated based on the time variation of the tracked particle velocities together with the wave number spectrum. We also calculated the longitudinal second-order structure function f (r) ≡ 〈δv_‖²(r)〉 , where δv_‖ is the radial component of the relative velocity in nearest particle pairs at the initial image. From this result, we attempt to discuss whether the Kolmogorov’s -5/3 law is observed or not in the spectrum.