Numerical investigation of diffusion effect of non-condensable gas on cavitating flows around isolated hydrofoil based on mass transfer model with its advection

It is known that non-condensable gas (NCG) sometimes plays a non-negligible role in cavitating flows. In general, NCG can be present in a liquid in the following two forms, either dissolved in a liquid or trapped in free nuclei. Currently, the effect of the dissolved NCG on a cavitation process, especially for the diffusion effect, has usually been neglected in most numerical cavitation models because the diffusion is generally slow compared to the flow convection. Nevertheless, the diffusion effect should be paid attention to in some special cases such as the cavitating performance of pumps operated at off-design conditions, especially in the case of low flow rates. To achieve a breakthrough in predicting cavitating flows, our group have proposed a simple mass transfer model across liquid-gas interface based on a well-known homogeneous cavitation model by Sauer and Schnerr (SS model), called as SS-NCG model, and also coupled an additional advection-diffusion equation of the NCG concentration for a two-dimensional computational fluid dynamics simulation around a hydrofoil. In the present study, a three-dimensional delayed detached eddy simulation coupled with the cavitation model has been employed to investigate the detailed influence of NCG on cavitation. By comparing three typical cases, i.e., no dissolved oxygen (SS model), low and high amounts of dissolved oxygen cases (SS-NCG model), the unsteady and statistical features of the NCG influences are indicated. The results show that the presence of NCG will enhance the cavitation degree and may lead to the occurrence of gaseous cavitation. From the perspective of unsteady characteristics, the cavitation process is stabilized and damped by NCG. The NCG concentration in cavitating regions is statistically lower than that in the bulk liquids, and the concentration difference is increasing in the case with a higher value of NCG content. In addition, it is found that the mechanism of the cavitation process considering NCG and not considering it is quite different. With the presence of NCG, cavitation can be regarded as an interaction between the varying pressures inside and outside the bubbles, where the NCG mainly affects the cavitation dynamics through its modulation of the local compressibility inside the bubbles. In contrast, the cavitation process without NCG is only dominated by the local static pressure.