Véronique Roig

Wake attenuation in large Reynolds number dispersed two-phase flows

The dynamics of high Reynolds number-dispersed two-phase flow strongly depends on the wakes generated behind the moving bodies that constitute the dispersed phase. The length of these wakes is considerably reduced compared with those developing behind isolated bodies. In this paper, this wake attenuation is studied from several complementary experimental investigations with the aim of determining how it depends on the body Reynolds number and the volume fraction α. It is first shown that the wakes inside a homogeneous swarm of rising bubbles decay exponentially with a characteristic length that scales as the ratio of the bubble diameter d to the drag coefficient Cd, and surprisingly does not depend on α for 10−2≤α≤10−1. The attenuation of the wakes in a fixed array of spheres randomly distributed in space (α=2×10−2) is observed to be stronger than that of the wake of an isolated sphere in a turbulent incident flow, but similar to that of bubbles within a homogeneous swarm. It thus appears that the wakes in dispersed two-phase flows are controlled by multi-body interactions, which cause a much faster decay than turbulent fluctuations having the same energy and integral length scale. Decomposition of velocity fluctuations into a contribution related to temporal variations and that associated to the random character of the body positions is proposed as a perspective for studying the mechanisms responsible for multi-body interactions.

Attenuation of the wake of a sphere in an intense incident turbulence with large length scales

We report an investigation of the wake of a sphere immersed in a uniform turbulent flow for sphere Reynolds numbers ranging from 100 to 1000. An original experimental setup has been designed to generate a uniform flow convecting an isotropic turbulence. At variance with previous works, the integral length scale of the turbulence is of the same order as the sphere diameter and the turbulence intensity is large. In consequence, the most intense turbulent eddies are capable of influencing the flow in the close vicinity of the sphere. Except in the attached region downstream of the sphere where the perturbation of the mean velocity is larger than the standard deviation of the incident turbulence, the flow is controlled by the incident turbulence. The distortion of the turbulence while the flow goes round the sphere leads to an increase in the longitudinal fluctuation and a decrease in the transversal one. The attenuation of the transversal fluctuations is still significant at 30 radii downstream of the sphere whereas the longitudinal fluctuations relax more rapidly toward the incident value. The more striking result however concerns the evolution of the mean velocity defect with the distance x from the sphere. It decays as x−2 and scales with the standard deviation of the incident turbulence instead of scaling with the mean incident velocity.

Effect of turbulence on the downstream velocity deficit of a rigid sphere

The effect of external turbulence completely modifies how the velocity deficit behind a rigid sphere decays with distance, changing it from an x−1 (or x−2/3 depending on the Reynolds number characterising the flow past the sphere) to x−2 decay, where x is the distance downstream. The wake width is observed to grow linearly with x. This striking change has been reported by three recent studies. We develop a new model to explain these studies which combines a stochastic model for passive wake spreading in a turbulent flow with an integral constraint based on momentum flux conservation. These results also show that, in ambient flow with an integral scale L and intensity It, a new flow regime will ultimately emerge at a distance L/It downstream, where turbulence disperses the velocity deficit with a new x−1 law.

Self-excited oscillations in buoyant confined bubbly mixing layers

In this experimental investigation we analyze the nature of the primary instability of a bubbly mixing layer. We consider upward flows that develop in a vertical channel of finite dimensions, with bubbles injected on one side of the mixing layer at the inlet. The induced buoyancy effect generates longitudinal accelerations, which are at the origin of self-excited large-scale oscillations under certain conditions. We provide experimental evidence that these oscillations are global modes, and we develop a simple model able to predict the conditions of appearance of these global modes. The model includes as essential mechanisms the buoyancy and a transverse mass transfer due to liquid miscibility in the mixing layer.