CFD Investigations on Drag Reduction due to Cavitator Heads on Supercavitating Bodies

From the incompressible Bernoulli’s energy conservation equation it can be shown, that the static pressure decreases along a streamline with increasing velocity. If the static pressure drops below the saturation vapour pressure of the water, a phase transition from the liquid phase to the gas phase occurs and a cavitation bubble is formed. This usually happens at the tip of the diving body and can be controlled with a properly designed cavitator head. In this cavitation bubble, the friction drag of the body is significantly reduced. But on the other hand, the pressure drag can be increased due to the larger flow detachment at the afterbody due to the supercavitating bubble.

Thus, the cavitation bubble has to be carefully designed, so that only a thin air film is at the body skin reducing the friction drag but having no significant impact on the pressure drag. This can be carefully designed by using a cavitator, which is placed at the tip of the body.

In the present study, a supercavitating elliptical body was investigated, using the commercial CFD Solver Star‑CCM+. For the the study the Reynolds Averaged Navier-Stokes Equation was solved using the k-ω turbulence model. The cavitation model that was used is the Schnerr-Sauer model.  The shape of the elliptical bodies was designed according to the empirical formula of Reichardt, which is almost an ellipse where the ratio of the semi-axes of the quasi-ellipse is given as a function of the cavitation number. The present studies include investigations without a cavitator at the tip and with a flat, round and conical cavitator head design at the tip. For the conical cavitator head design, different cone angles were investigated. The effect of the size of the cavitator heads was also investigated.. For this, different ratios of the cavitator head to the body diameter were simulated. Because the occurrence of cavitation depends on the static pressure, also the influence of the increasing hydrostatic pressure with increasing water depth was investigated. Furthermore, the different designs were simulated for various Reynolds numbers.

For all studies, the drag coefficient was evaluated. For better understanding the drag coefficient is split into two parts, the friction drag coefficient and the pressure drag coefficient. The results are shown in dependency of the dimensionless Reynolds number and the cavitation number. In the simulations, the Reynolds number was changed by increasing the velocity and the cavitation number was changed by increasing the water depth.