The Investigation Of The Effect Of Blade Geometry On Tip Vortex Cavitation Inception And Its Mechanism

Tip vortex cavitation of propeller generally appears firstly when ship speed increasingand a speed, at which any type of cavitation incepts is defined as cavitation inception speed.Once cavitation appears the noise dramatically increases and in some cases it may inducestrong stern vibration. Inception speed of a surface navy ship is one of the importantparameters characterizing the stealthiness of the ship. So delaying tip vortex cavitationinception is a necessary measure to enhance the stealthiness of a ship. Designing a tipunloading propeller has become a classical way to delay tip vortex cavitation. However it mayreduce propeller efficiency or it is not enough in nowadays. Therefore, investigating the bladeshape, e.g. the form of skew, rake and thickness distribution closing to tip for delaying the tipvortex cavitation is interesting to propeller designers.The aim of this research is to find a method for predicting tip vortex inception, as muchas possible to understand some insight of the tip vortex and the pressure inside the core, andfinally to investigate the influence of blade shape on the tip vortex and its cavitationinception.Therefore present study covers following aspects:For easier to capture the location of tip vortex and understanding the tip vortex structurein details, a3D twisted foil was designed. Its geometry including the contour, swept, anhedral,thickness distribution and spanwise load distribution was similar to propeller’s blade contour,skew, rake, thickness distribution and radial loading distribution respectively. LDVmeasurements for the foil were carried out in CSSRC’s large cavitation tunnel to get thespacial location of the tip vortex cavitation. The velocity distributions in the region of tipvortex on a number of cross sections were measured by LDV. The test data were thefoundation for the validation of the numerical analysis.RANS method was adopted to simulate the tip vortex flow around the foil and thepropeller. A grid scheme with boundary layer grid, together with a local refined grid in the tipvortex can capture the local tip vortex and leading edge vortex near the tip region. The formation of the tip vortex and the vortical flow characteristics influenced by the geometryparameters were analyzed. Based on the CFD results, a simplified vortex model was appliedto calculate the pressure distributions in the vortex core. Based on the correlation analysisbetween the calculated and the measured inception condition, a correlation factor ‘K’ has beenformulated from the data of the twisted foil. This factor has also been applied in the propellercase. The approach can be used for analyzing the effectiveness of the measures to delay tipvortex cavitation inception and providing a relative comparison.Present research, the influences from the variation of thickness in tip region, skew andrake form have been analyzed separately for the foil. The mechanism of the delay of tipvortex inception was studied. The variations of the geometry can defer the leading edgevortex rolling into the local tip vortex and weaken the cross flow at the tip. It can help toreduce the tip vortex strength and delay inception of cavitation.Based on the investigation on the3D twisted foil, the geometry of a reference propellerwas modified that including the modification of the skew form, rake form, and the thicknessat tip region. It intends to delay the inception of tip vortex cavitation. Model tests have beencarried out for the reference propeller and the new propeller. The experimental results validatethat the tip vortex cavitation was delayed. The trend of calculation results were reflected inthe test results. But the calculation results had to be modified with an empirical correlationcoefficient. So there is a correlation problem remaining.The research indicates a direction of the way for delaying tip vortex cavitation inceptionby geometry variations. According to analysis of the numerical calculation result, theeffectiveness of delay can be evaluated at propeller design stage. The presented variations ofgeometry are very promising in engineering application.