Computational and Empirical Investigation of Propeller Tip Vortex Cavitation Noise

In this study,non-cavitating and cavitating flow around the benchmark DTMB 4119 model propeller are solved using both viscous and potential based solvers.Cavitating and non-cavitating propeller radiated noises are then predicted by using a hybrid method in which RANS(Reynolds-averaged Navier-Stokes)and FWH(Ffowcs Williams Hawkings)equations are solved together in open water conditions.Sheet cavitation on the propeller blades is modelled by using a VOF(Volume of Fiuld)method equipped with Schnerr-Sauer cavitation model.Nevertheless,tip vortex cavitation noise is estimated by using two different semi-empirical techniques,namely Tip Vortex Index(TVI,based on potential flow theory)and Tip Vortex Contribution(TVC).As the reference distance between noise source and receiver is not defined in open water case for TVI technique,one of the outputs of this study is to propose a reference distance for TVI technique by coupling two semi-empirical techniques and ITTC distance normalization.At the defined distance,the starting point of the tip vortex cavitation is determined for different advance ratios and cavitation numbers using potential flow solver.Also,it is examined that whether the hybrid method and potential flow solver give the same noise results at the inception point of tip vortex cavitation.Results show that TVI method based on potential flow theory is reliable and can practically be used to replace the hybrid method(RANS with FWH approach)when tip vortex cavitation starts.

Numerical Prediction of Total Resistance Using Full Similarity Technique

Model tests are often conducted by researchers in a real or a numerical towing tank to calculate residuary resistance of a ship with the aid of Froude similarity. Common ITTC-1957 formula is usually employed to calculate frictional resistance. As computer technologies develop over time, CFD tools are used for calculating total resistance of a ship at full scale without establishing any dynamic similarities. In this paper, both Froude and Reynolds similarities are numerically implemented to four different model scales by using virtual fluids. The total resistance at different Fr numbers calculated by the numerical study is validated against the experimental data of DTMB 5512 (L=3.048 m) model hull. The results show that establishing Froude and Reynolds similarities together in numerical simulation is possible in principle. To determine whether it has advantages for prediction of full-scale ship total resistance by employing this method, it is also examined the model scale with the same number of elements and Reynolds number of the full-scale ship. Results show that numerical calculation of total resistance for a full-scale ship in a model scale by defining virtual fluids has only slight advantages on the prediction of residuary resistance. Additionally, no advantage in the calculation of frictional resistance is observed.