Non-dimensional number effects on downwind sail modelling

Abstract

This Thesis investigates the importance of scale when modelling downwind sails in a wind tunnel. If the performance characteristics obtained from a model yacht are to be scaled up to full size then certain non-dimensional numbers must remain constant to ensure accurate results. The non-dimensional numbers that were considered to be most important are the Reynolds number, terrain roughness ratio and the sail weight/pressure ratio, so each of these variables were analysed to determine their aerodynamic effects on downwind sails. Specific experiments were devised to analyse each non-dimensional number. To study the Reynolds number effects the sail shape had to remain constant, so rigid aluminium sails were constructed and tested. To accurately alter the weight/pressure ratio without changing the Reynolds number, point load weights were added to fabric spinnakers and gennakers. The Tamaki open jet wind tunnel was used for all the experiments and it was found necessary to improve the boundary layer flow wind structure in order to obtain the required results. The results did show signs of Reynolds number effects, i.e. the sail's force coefficients decreased as the Reynolds number increased. When the Reynolds number value of 6x 105 was reached, the force coefficients remained fairly constant which may have indicated the end of the boundary layer transition period. It was also found that the reliability of the measurements decreased with wind speed. Therefore wind speeds below 2.2 m/s were not used. The results from the weight/pressure ratio tests showed no appreciable changes in the drag coefficient for downwind sails, even though a change in sail shape occurred. However the tests used to study the weight/pressure ratio did reinforce the existence of Reynolds number effects.