We present global heat-transfer and local temperature measurements, in an asymmetric parallelepiped Rayleigh-Bénard cell, in which controlled square-studs roughnesses have been added. A global heat transfer enhancement arises when the thickness of the boundary layer matches the height of the roughnesses. The enhanced regime exhibits an increase of the heat transfer scaling. Local temperature measurements have been carried out in the range of parameters where the enhancement of the global heat transfer is observed. They show that the boundary layer at the top of the square-stub roughness is thinner than the boundary layer of a smooth plate, which accounts for most of the heat-transfer enhancement. We also report multistability at long time scales between two enhanced heat-transfer regimes. The flow structure of both regimes is imaged with background-oriented synthetic Schlieren and reveals intermittent bursts of coherent plumes.

Submitted to Physics of Fluids in 2017International audienceThe intermittency of turbulent superfluid helium is explored systematically in a steady wake flow from 1.28 K up to T > 2.18K using a local anemometer. This temperature range spans relative densities of superfluid from 96% down to 0%, allowing to test numerical predictions of enhancement or depletion of intermittency at intermediate superfluid fractions. Using the so-called extended self-similarity method, scaling exponents of structure functions have been calculated. No evidence of temperature dependence is found on these scaling exponents in the upper part of the inertial cascade, where turbulence is well developed and fully resolved by the probe. This result supports the picture of a profound analogy between classical and quantum turbulence in their inertial range, including the violation of self-similarities associated with inertial-range intermittency

Several Rayleigh-Bénard experiments in water are performed with smooth or rough boundaries. We present new thermal transfer measurements obtained with large roughness elements arranged in a square lattice. The data are compared to previous ones obtained with smaller elements in the same cell (Tisserand et al., 2011). Experiments in the same apparatus without roughness are fully presented, as reference results, to allow for comparison. In the rough case, several regimes of heat transfer are identified : one similar to the smooth case, an enhanced heat transfer one characterized by a modification of the Nusselt vs Rayleigh numbers relation, and a third part where the relation can be similar to a smooth one with a corrected prefactor.

Filamentary regions of high vorticity irregularly form and disappear in the turbulent flows of classical fluids. We report an experimental comparative study of these so-called "coherent structures" in a classical versus quantum fluid, using liquid helium with a superfluid fraction varied from 0% up to 83%. The low pressure core of the vorticity filaments is detected by pressure probes located on the sidewall of a 78-cm-diameter Von Kármán cell driven up to record turbulent intensity (R λ ∼ √ Re 10000 ). The statistics of occurrence, magnitude and relative distribution of the filaments in a classical fluid are found indistinguishable from their superfluid counterpart, namely the bundles of quantized vortex lines. This suggest that the internal structure of vortex filaments, as well as their dissipative properties have a negligible impact on their macroscopic dynamics, such as lifetime and intermittent properties.

Velocity measurements in turbulent superfluid helium between co-rotating propellers are reported. The parameters are chosen such that the flow is fully turbulent, and its dissipative scales are partly resolved by the velocity sensors. This allows for the first experimental comparison of spectra in quantum versus classical turbulence where dissipative scales are resolved. In some specific conditions, differences are observed, with an excess of energy at small scales in the quantum case compared to the classical one. This difference is consistent with the prediction of a pileup of superfluid kinetic energy at the bottom of the inertial cascade of turbulence due to a specific dissipation mechanism.

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We present the principle for a micro-sensor aimed at measuring local correlations of turbulent velocity and temperature. The operating principle is versatile and can be adapted for various types of flow. It is based on a micro-machined cantilever, on the tip of which a platinum resistor is patterned. The deflection of the cantilever yields an estimate for the local velocity, and the impedance of the platinum yields an estimate for the local temperature. The velocity measurement is tested in two turbulent jets: one with air at room temperature which allows us to compare with well-known calibrated reference anemometers, and another one in the GReC jet at CERN with cryogenic gaseous helium which allows a much larger range of resolved turbulent scales. The recording of temperature fluctuations is tested in the Barrel of Ilmenau which provides a controlled turbulent thermal flow in air. Measurements in the wake of a heated or cooled cylinder demonstrate the capability of the sensor to display the cross correlation between temperature and velocity correctly.

In this paper, we analyze the mean velocity profile and the Reynolds shear stress in a turbulent, inclined, heat pipe. We show that the simplest version of a mixing length model is unable to reproduce the evolution of the velocity profile shape with the inclination angle psi. An improvement of this model, taking into account some buoyancy effects, gives nice qualitative agreement with the observations. The agreement implies surprisingly a low value for the gradient Richardson number Ric above which the flow is laminar. The comparison with previous works shows however good agreement when some care is taken in the evaluation of the Richardson number Ri