Water tunnel simulation study of the later stages of water entry of conical head bodies. Phase 2: Effect of the afterbody on steady state ventilated cavities

This report documents the second phase of the water tunnel studies that are intended to simulate the cavity running phase of water entry. The primary purpose of the investigation was to study the effect of afterbody arrangement on steady state ventilated cavities. The models utilized were 1.0 inch diameter conical nosed bodies with a 45 deg. apex angle. Three models were tested: one having a 1.0 inch diameter afterbody, one having a 0.5 inch diameter afterbody and the third having no afterbody. The cavitation number as a function of cavity length and ventilation air flow coefficient as a function of cavitation number were measured for all models. The pressure distribution was found for a wide range of cavity lengths for the model with the 1.0 inch diameter afterbody. Data for the ventilation air flow coefficient and pressure distribution are in good agreement with previous studies of similar models. An interesting result is that the model requiring the lowest ventilation air flowrate for a given cavitation number was the model with no afterbody. For most of the flow states investigated, the model with the 0.5 inch diameter afterbody required the highest flowrate. The transition between the reentrant jet and twin vortex regimes was studied briefly. At transition, the cavity would grow four or five times its original length for a slight increase in ventilation air flowrate. A cavity attrition test was conducted to simulate the decay of a cavity behind a missile.