Pascal Dupont

Turbulent mixing of two immiscible fluids

The emulsification process in a static mixer HEV (high-efficiency vortex) in turbulent flow is investigated. This new type of mixer generates coherent large-scale structures, enhancing momentum transfer in the bulk flow and hence providing favorable conditions for phase dispersion. We present a study of the single-phase flow that details the flow structure, based on LDV measurements, giving access on the scales of turbulence. In addition, we discuss the liquid-liquid dispersion of oil in water obtained at the exit of the mixer/emulsifier. The generation of the dispersion is characterized by the Sauter diameter and described via a size-distribution function. We are interested in a local turbulence analysis, particularly the spatial structure of the turbulence and the turbulence spectra, which give information about the turbulent dissipation rate. Finally, we discuss the emulsifier efficiency and compare the HEV performance with existing devices.

Internal waves generated by a translating and oscillating sphere

Abstract At high Reynolds and Froude numbers, lee waves owing to the horizontal motion of a body in a stratified fluid are superseded by random waves generated by its wake. The origin of these waves lies in the buoyant collapse of the large-scale coherent structures of the wake, and can be modelled as a source moving at the velocity of the body and of strength oscillating at the frequency of vortex shedding. In the present paper two parallel studies of the associated wavefield are described. The first of these is theoretical and considers localized and extended models of the source, while the second is experimental and involves a vertically oscillating and horizontally translating sphere. Oscillation frequencies both smaller and larger than the Brunt-Vaisala frequency are considered, and reasonably good agreement between theory and experiment is obtained concerning, e.g. the shape of the surfaces of constant phase, the streamwise evolution of the wavelength, and the domain of existence of the waves. Calculations are then presented for a realistic turbulent wake, and comparison with available experimental results is performed.

Internal waves generated by the wake of Gaussian hills

Depending on their structure and dynamics, the wakes of various obstacles can generate different kinds of internal waves in stratified fluids. Experiments on waves emitted by three-dimensional Gaussian models towed uniformly in a linearly stratified fluid were carried out. Beyond a critical value Frc of the Froude number Fr, the developed wake was observed to radiate an internal wave field shorter than the lee waves of the hill. The emergence of such waves is correlated with the periodic shedding of three dimensional vortical structures at Fr>Frc. Measurements show that the wavelengths of these short waves are constant in space and time, and proportional to Fr. Their phase and group velocities are proportional to the coherent structure velocity which is estimated from velocity measurements inside the wake. All those spatio-temporal characteristics prove that these short waves are generated by the displacement of the coherent structure inside the wake.

Eddy Heat Transfer by Secondary Görtler Instability

Experimental measurements of flow and heat transfer in a concave surface boundary layer in the presence of streamwise counter-rotating Gortler vortices show conclusively that local surface heat-transfer rates can exceed that of the turbulent flat-plate boundary layer even in the absence of turbulence. We have observed unexpected heat-transfer behavior in a laminar boundary layer on a concave wall even at low nominal velocity, a configuration not studied in the literature: The heat-transfer enhancement is extremely high, well above that corresponding to a turbulent boundary layer on a flat plate. To quantify the effect of freestream velocity on heat-transfer intensification, two criteria are defined for the growth of the Gortler instability: P z for primary instability and P rms for the secondary instability. The evolution of these criteria along the concave surface boundary layer clearly shows that the secondary instability grows faster than the primary instability. Measurements show that beyond a certain distance the heat-transfer enhancement is basically correlated with P rms , so that the high heat-transfer intensification at low freestream velocities is due to the high growth rate of the secondary instability. The relative heat-transfer enhancement seems to be independent of the nominal velocity (global Reynolds number) and allows predicting the influence of the Gortler instabilities in a large variety of situations.

Turbulent Mixing of Two Immiscible Fluids

The global trend in chemical and manufacturing industries is towards improved energy efficiency, cleaner synthesis, reduced environmental impact and smaller, safer, multifunctional process plants. Such concerns are the driving force for the intensification of batch processes, which are being replaced with continuous high-intensity in-line mass- and heat-transfer equipment. In this context the process intensification (PI) approach, in which the fluid dynamics of the process is matched to the reaction in order to improve selectivity and minimize the byproducts, takes on particular importance.Copyright