Abstract
We have developed a relatively simple cryostat which allows us to image turbulent flows in superfluid helium at temperatures below 2 K, using frozen H2 particles. We analyze the statistics of the velocities of these solid tracers, which follow the turbulent flow generated by oscillating bodies. We have also studied one of the oscillators working in air at room temperature, and traced the flow with solid talcum particles for comparison. Images were recorded by a digital camera at 240 frames per second, while frequencies of the oscillators are between 20 to 45 Hz. The flow is characterized by a modified Reynolds number Reδ based on the viscous penetration depth δ. Software in a dedicated particle tracking velocimetry code allows us to compute the trajectories and velocities of tens of thousands of particles. We have obtained the number of particles for equally spaced intervals of the velocity modulus. For the oscillators in the superfluid, the probability of finding particles at higher velocities has an exponential decay. Within our resolution the statistics in the superfluid for oscillating objects with sharp borders is largely independent of Reδ, while the logarithmic decay at low velocities seems faster than for high velocities for rounded objects. On the other hand, for data taken in air the result is closer to a classical Gaussian distribution of velocities.
Highlights
Quantum Turbulence studies have a long history[1, 2] and the subject is far from being exhausted [3]
Experimental Results The images of tracer particles have to be processed and analyzed with appropriate statistical tools and we have written a particle image velocimetry code based on available MATLAB modules[25], leaving aside the non trivial question of what is really traced by the tracer particles
If sharp edges are present, the cancellation is much less, so it is possible that the observed differences in the pdf may be related to the presence of sharp edges in the beam, and the sphere attached by a beam, and the more rounded geometry of the sphere attached by a wire
Summary
Quantum Turbulence studies have a long history[1, 2] and the subject is far from being exhausted [3]. We have been able to film H2 solid particles flowing around oscillating objects of different shapes, which generate turbulence when submerged in superfluid helium. By analyzing video images of the particle motion, we can obtain the statistics of the particle velocities and we report the results in the present paper.
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