Derivation of Forces on a Sail using Pressure and Shape Measurements at Full-Scale
Examensarbete för masterexamen
Aerodynamic forces are usually computed numerically or measured in a wind tunnel. These forces may be used to predict the performance of a yacht at full-scale. The aim of this project was to develop a new method to measure the forces and moments directly at full-scale. The theory was that by simultaneously measuring the shape of the sails and the pressures around them the force normal to the surface could be derived by summation of the product of the pressure, unit normal vector and the relevant area of the discretised sail shape. A code was written for the interpolations of the discrete shape and pressure measurements to the full sail shapes and pressures over them. The shape was then discretised and the pressure at the centre of each cell was assumed constant over it. The results were validated through testing at model-scale. The calculated forces and moments were compared directly to the results obtained from a force balance on which the model yacht was mounted. The results were also used in a VPP to assess the accuracy when predicting heel angle and boat speed. The first test was conducted on a set of semi-rigid sails with surface mounted pressure taps. A study of the pressure distributions found using a VLM was used to optimise the pressure tap locations. The results were encouraging; however it appeared that the interference on the flow caused by the taps resulted in large under-predictions of the forces and moments. This lead to a second round of model testing using sails with internal pressure taps leaving the sail surfaces smooth. The results showed good agreement, although there was still a significant under-prediction of the drive force. Full-scale testing was carried out using a purpose built pressure measuring system. Although fullscale testing is extremely difficult due to the unsteady environment, good results were obtained where clear trends in the boat speed and heel angle were also seen in the calculated drive force and heeling moment.
Transport , Farkostteknik , Strömningsmekanik , Transport , Vehicle Engineering , Fluid mechanics