A MODIFIED POINT VORTEX MODEL FOR DESCRIBING THE ROLL-UP OF TIP VORTICES

A modified point-vortex method is presented which describes the roll-up process of vortex sheets to concentrated tip vortices. To prevent chaotic motions in the vortex core, an amalgamation process is introduced where the vortex core is modelled by a modified Lamb vortex. The free parameters of the vortex model are adapted at each amalgamation step by conserving the energy and the second moment of vorticity. The method is applied to the roll-up of a vortex sheet with an elliptically distributed circulation. It shows the growing of the vortex core and the reduction of the maximum tangential velocity during the roll-up. The last effect has been confirmed by LDA measurements. The high sensitivity of the results to variations in the energy of the flow field reveals the importance of conserving this parameter.