104 research outputs found
Drag force on an oscillating object in quantum turbulence
This paper reports results of the computation of the drag force exerted on an
oscillating object in quantum turbulence in superfluid He. The drag force
is calculated on the basis of numerical simulations of quantum turbulent flow
about the object. The drag force is proportional to the square of the magnitude
of the oscillation velocity, which is similar to that in classical turbulence
at high Reynolds number. The drag coefficient is also calculated, and its value
is found to be of the same order as that observed in previous experiments. The
correspondence between quantum and classical turbulences is further clarified
by examining the turbulence created by oscillating objects.Comment: 7 pages, 5 figures, 1 tabl
Surface spin waves in 3He-A, a probe for vortex phenomena in narrow gaps
We report measurements on a new collective spin-wave mode trapped by the textural boundary layers of 3 He-A within a stack of thin Mylar plates. The surface mode was seen as a new peak in the cw NMR spectrum measured at H0=284 Oe. Rotation of the sample, with Ω orthogonal to the gaps, increased the spectral weight of the surface mode, indicating an increase in the textural boundary layers caused by a counterflow-induced transition. This phenomenon was used to study vortex creation and persistent currents.Peer reviewe
Crossover from hydrodynamic to acoustic drag on quartz tuning forks in normal and superfluid 4He
We present measurements of the drag forces on quartz tuning forks oscillating at low velocities in normal and superfluid 4He. We have investigated the dissipative drag over a wide range of frequencies, from 6.5 to 600 kHz, by using arrays of forks with varying prong lengths and by exciting the forks in their fundamental and first overtone modes. At low frequencies the behavior is dominated by laminar hydrodynamic drag, governed by the fluid viscosity. At higher frequencies acoustic drag is dominant and is described well by a three-dimensional model of sound emission
On cavitation in liquid helium in a flow due to a vibrating quartz fork
Cavitation in normal and superfluid liquid ⁴He at saturated vapor pressure and slightly elevated pressures
has been experimentally studied in a flow due to quartz forks vibrating at high amplitudes. Above the
temperature- and pressure-dependent critical velocity, heterogeneous cavitation is observed both visually
and electrically, as a breakdown of the resonance response of the fork.We compare our results with available
experimental and discuss them using existing theoretical models. In particular, we show that thermal effects
leading to local overheating of the vicinity of the fork have to be taken into account, especially in normal liquid
⁴He
Transition to superfluid turbulence governed by an intrinsic parameter
Hydrodynamic flow in both classical and quantum fluids can be either laminar
or turbulent. To describe the latter, vortices in turbulent flow are modelled
with stable vortex filaments. While this is an idealization in classical
fluids, vortices are real topologically stable quantized objects in
superfluids. Thus superfluid turbulence is thought to hold the key to new
understanding on turbulence in general. The fermion superfluid 3He offers
further possibilities owing to a large variation in its hydrodynamic
characteristics over the experimentally accessible temperatures. While studying
the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp
transition at 0.60Tc between two regimes, with regular behaviour at
high-temperatures and turbulence at low-temperatures. Unlike in classical
fluids, this transition is insensitive to velocity and occurs at a temperature
where the dissipative vortex damping drops below a critical limit. This
discovery resolves the conflict between existing high- and low-temperature
measurements in 3He-B: At high temperatures in rotating flow a vortex loop
injected into superflow has been observed to expand monotonically to a single
rectilinear vortex line, while at very low temperatures a tangled network of
quantized vortex lines can be generated in a quiescent bath with a vibrating
wire. The solution of this conflict reveals a new intrinsic criterion for the
existence of superfluid turbulence.Comment: Revtex file; 5 pages, 2 figure
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