P. Paranthoën

Application of multiple exposure digital in-line holography to particle tracking in a Bénard–von Kármán vortex flow

Digital in-line holography is applied to studying the trajectories of individual water droplets in airflow. In order to track the particles, multiple exposure holography is performed using a modulated laser diode emitting at the wavelength of 635 nm and a lens-less CCD camera. This method leads to an accuracy better than 100 µm on the axial location. A study of the signal-to-noise ratio of such holograms shows that the number of exposures must be limited. Preliminary tests of this method are carried out in a Benard–von Karman street first characterized by laser Doppler velocimetry and hot wire anemometry. An example of a trajectory of a water droplet obtained in this flow at Reynolds number Re = 63 and Strouhal number St = 0.13 shows that digital holography is a promising method to extract the trajectories of droplets in laminar or turbulent flows.

Mesures de température dans les écoulements turbulents

The paper reports on the use of cool wire thermometers and thermocouples for temperature measurements

Anisotropy of a thermal field at dissipative scales in the case of small-scale injection

The anisotropy of a thermal field at the level of dissipation has been studied experimentally and by means of modeling, downstream of a heated line source placed, successively, in a turbulent boundary layer and a turbulent plane jet. This situation represents anisotropic small-scale injection of a passive scalar in a turbulent medium. All three instantaneous temperature gradients have been measured. In the central region of the thermal sheet, experimental data reveal a high degree of anisotropy of temperature dissipation near the line source and return to isotropy further downstream. Comparison of measurements with modeling allows interpreting the data and estimating the return-to-isotropy time scale.

Fluctuations de température et flux de chaleur en aval d'une source linéaire placée dans une couche limite turbulente

Abstract The purpose of the present communication is to present and discuss some recent experimental results concerning the dispersion downstream of a heated line source located, in a turbulent boundary layer, successively at two distances from an adiabatic wall. Information on the mean and fluctuating temperature fields and associated heat fluxes are presented and analysed by testing closure assumptions for model dispersion at first and second orders. This model, which only requires the use of temporal velocity scales of the flow, leads to a rather good agreement with measurements.

Dynamic behaviour of cold wires in heated airflows (300 < T < 600 K)

A theoretical and experimental investigation of cold wire frequency response is presented for the case of heated airflows (T < 600 K). Experiments using an external heating technique have been carried out for sensors in different flow situations. Particular attention has been paid to the influence of temperature on cold wire transfer function characteristics (time-constant and plateau level).

On the role of vorticity in the microstructure of a passive scalar field

Analysis of experimental and modeling results on second-order moments of temperature derivatives downstream of a heated line source suggests a prevailing effect of vorticity in the destruction of anisotropy at this level. The influence of vorticity on small-scale anisotropy is also confirmed in the quite different case of a forced passive scalar in isotropic turbulence; assuming that vorticity promotes reorientation of large instantaneous scalar gradients through rotation of scalar fronts enables us to explain the trend of the scalar gradient skewness with increasing Schmidt number which is observed in recent numerical simulations.

The Heated Bénard–Kármán Street: A Review of the Effective Reynolds Number Concept

This chapter focuses on the wake flow behind a heated circular cylinder in the laminar vortex shedding regime where the Benard–Karman street appears. This flow is of fundamental importance both from the viewpoint of the hydrodynamic stability theory and engineering applications. Even in absence of buoyancy forces, this wake flow is more complicated than in the isothermal case due to temperature differences generated within the fluid leading to variations of its physical properties. In this situation, experiments showed that heat is never a passive contaminant. Due to the respective thermal dependence of the kinematic viscosity, heating the cylinder stabilizes the flow in air while it destabilizes the flow in water. This phenomenon led to the definition of an effective Reynolds number that is associated to an effective temperature. Value of the effective temperature is shown to depend on the nature of the fluid. In air, global and local flow similarities between wake flows of the same “effective” Reynolds numbers are pointed out, underlying the physical significance of this concept.

Experimental Study of the Benard-Karman Instability Downwind of a Heated Cylinder

The periodic pattern of the vortex street downstream of a circular cylinder at low Reynolds numbers (30 1000), Freymuth and Uberoi [4], LaRue and Libby [5], Matsamura and Antonia [6], Xenopoulos et al. [7]. Over the low Reynolds numbers range mentioned above, corresponding investigations have not yet widely been achieved and experimental results are sparse, Eckert and Soehngen [8], Vilimpoc et al. [9].