Section: New Results

Applications for specific flow problems

Modelling cavitating flow around underwater missiles

Participants : Fabien Petitpas, Richard Saurel, B.K. Ahn, S. Ko.

The diffuse interface model of Saurel et al. [9] is used for the computation of compressible cavitating flows around underwater missiles. Such systems use gas injection and natural cavitation to reduce drag effects. Consequently material interfaces appear, separating liquid and gas. These interfaces may have a really complex dynamics such that only a few formulations are able to predict their evolution. Contrarily to front tracking or interface reconstruction method the interfaces are computed as diffused numerical zones, that are captured in a routinely manner, as is done usually with gas dynamics solvers for shocks and contact discontinuity. With the present approach, a single set of partial differential equations is solved everywhere, with a single numerical scheme. This leads to very efficient solvers. The algorithm derived in Saurel et al. [43] is used to compute cavitation pockets around solid bodies. It is first validated against experiments done in cavitation tunnel at CNU. Then it is used to compute flows around high speed underwater systems (Shkval-like missile). Performance data are then computed showing method ability to predict forces acting on the system [20] .

Propagation of a planar shock wave through a two-phase gas-liquid medium

Participants : Alain Chauvin, Georges Jourdan, Éric Daniel, Lazhar Houas, R Tosello.

We conducted a series of shock tube experiments to study the influence of a cloud of water droplets on the propagation of a planar shock wave. In a vertically oriented shock tube, the cloud of droplets was released downwards into the air at atmospheric pressure while the shock wave propagated upwards. Two shock wave Mach numbers, 1.3 and 1.5, and three different heights of clouds, 150 mm, 400 mm, and 700 mm, were tested with an air-water volume fraction and a droplet diameter fixed at 1.2 % and 500 $\mu$m, respectively. From high-speed visualization and pressure measurements, we analyzed the effect of water clouds on the propagation of the shock wave. It was shown that the pressure histories recorded in the two-phase gas-liquid mixture are different from those previously obtained in the gas-solid case. This different behavior is attributed to the process of atomization of the droplets, which is absent in the gas-solid medium. Finally, it was observed that the shock wave attenuation was dependent on the exchange surface crossed by the shock combined with the breakup criterion [12] .