Commercial quartz oscillators of the tuning-fork type with a resonant frequency of ∼ 32 kHz have been investigated in helium liquids. The oscillators are found to have at best Q values in the range 10 5 -10 6 , when measured in vacuum below 1.5 K. However, the variability is large and for very low temperature operation the sensor has to be preselected. We explore their properties in the regime of linear viscous hydrodynamic response in normal and superfluid 3 He and 4 He, by comparing measurements to the hydrodynamic model of the sensor.

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We present measurements of the drag forces on quartz tuning forks oscillating at low velocities in normal and superfluid 4 He. 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.

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Flow due to a commercially available vibrating quartz fork is studied in gaseous helium, He I and He II, over a wide range of temperatures and pressures. On increasing the driving force applied to the fork, the drag changes in character from laminar (characterized by a linear drive vs velocity dependence) to turbulent (characterized by a quadratic drive vs velocity dependence). We characterize this transition by a critical Reynolds number Recrdelta=Ucrdelta/nu, where Ucr is the critical velocity, nu stands for the kinematic viscosity, delta=sqrt[2nu/omega] is the viscous penetration depth, and omega is the angular frequency of oscillations. We have experimentally verified that the corresponding scaling Ucr proportional, sqrt[nuomega] holds in a classical viscous fluid over two decades of nu.

We describe the first measurement on Andreev scattering of thermal excitations from a vortex configuration with known density, spatial extent, and orientations in 3 He-B superfluid. The heat flow from a blackbody radiator in equilibrium rotation at constant angular velocity is measured with two quartz tuning fork oscillators. One oscillator creates a controllable density of excitations at 0.2 Tc base temperature and the other records the thermal response. The results are compared to numerical calculations of ballistic propagation of thermal quasiparticles through a cluster of rectilinear vortices. We find good agreement which supports the current understanding of Andreev reflection.

When a hot body A is thermally connected to a cold body B, the textbook knowledge is that heat flows from A to B. Here, we describe the opposite case in which heat flows from a colder but constantly heated body B to a hotter but constantly cooled body A through a two-phase liquid-vapor system. Specifically, we provide experimental evidence that heat flows through liquid and vapor phases of cryogenic helium from the constantly heated, but cooler, bottom plate of a Rayleigh-Bénard convection cell to its hotter, but constantly cooled, top plate. The bottom plate is heated uniformly, and the top plate is cooled by heat exchange with liquid helium maintained at 4.2 K. Additionally, for certain experimental conditions, a rain of helium droplets is detected by small sensors placed in the cell at about one-half of its height.O ur cryogenic experiment takes place in a cryostat containing a Rayleigh-Bénard (RB) convection cell shown in Fig. 1. The cylindrical RB cell, 300 mm in both diameter and height, is designed to minimize the influence of its structure on the convective flow (1) and is capable of running at very high Rayleigh numbers up to 10 15 (2, 3). These studies used cryogenic helium gas as the working fluid and have been performed under nearly Oberbeck-Boussinesq conditions (all physical properties of working fluid assumed constant except its density varying linearly with temperature) (2), as well as for the case when non-OberbeckBoussinesq conditions cause asymmetry between the top and bottom boundary layers (3). Here, we report results of experiments on two-phase heat transport, using cryogenic helium vapor and normal liquid 4 He as working fluids with remarkable, well-known and in situ tunable properties (4, 5). We stress that the heat conductivity of the thin stainless-steel cylindrical wall and any possible parasitic heat input to the RB cell are negligibly small in this work.We start very near equilibrium conditions, with the RB cell filled one-half with normal liquid helium and one-half with helium vapor. The temperature of the cell is approximately that of the thermally connected liquid helium vessel (LHeV), as shown for time t < 0 in Fig. 2A, before the homogeneously distributed resistive heater in the bottom plate is turned on at t = 0. This condition closely corresponds to a point on the equilibrium saturated vapor curve (SVC), calculated based on the continuously monitored value of pressure, P, in the cell. Then we start heating the bottom plate with a constant input power into the resistive heater, and, using built-in germanium thermometers (1, 2), continuously monitor the temperatures T B and T T , of the highly conductive bottom and top copper plates, respectively, as well as the temperature readings T 1 . . . T 4 of four small Ge sensors (6) installed within the cell as shown in Fig. 1. The temperature of the upper plate is not controlled; it is only affected by heat exchange with the helium inside the cell and by the weak thermal link to the LHeV. For an isolated system consisting of liquid and...