Particles for tracing turbulent liquid helium

We address the problem of making quantitative measurements of local flow velocities in turbulent liquid helium, using tracer particles. We survey and evaluate presently available particles and previous work to establish the need to develop new particles for the purpose. We present the first practical solution for visualizing fluid motions using a suspension of solid hydrogen particles with diameters of about 2 lm. The hydrogen particles can be used to study flows with Taylor-microscale Reynolds numbers between 85 and 775. The particles can be used equally well with the PIV, LDV, or particle-tracking techniques. List of symbols A Hamaker’s constant d particle diameter g acceleration of gravity kB Boltzmann’s constant L characteristic size of the large eddies N number of particles n number of particles per unit volume T temperature U characteristic velocity of the large eddies u local and instantaneous fluid velocity Vp total volume of the particles Vf total volume of the fluid Vo volume of laser sheet that is visible to the camera v particle velocity Dq density difference between particle and fluid e mean energy dissipation rate U volume fraction of particles in the fluid l dynamic viscosity of the fluid m kinematic viscosity of the fluid qf mass density of the fluid qp mass density of the particle