THE IMPORTANCE OF FROUDE AND REYNOLDS NUMBERS IN MODELLING DOWNWIND SAILS

摘要

This paper investigates the scaling problems associated with testing models of downwind sails in a wind tunnel. Besides the usual problems of scaling Reynolds number correctly, a Froude parameter is identified which measures the ratio of sail weight per unit area to the dynamic pressure. Specific experiments were devised to analyse separately the effects of each non-dimensional number. In order to isolate Reynolds number effects the sail shape was held constant by constructing rigid sails from sheet metal. To alter the Froude number at fixed Reynolds number, point load weights were added to fabric spinnakers and gennakers. The results indicated significant Reynolds number effects at model scale up to a value of about 7 x 10~5. However large changes in Froude number made no appreciable changes in the drag coefficient for downwind sails, even though changes in sail shape were observable. Measurements on spinnakers and gennakers trimmed for maximumthrust at a fixed angle of attack reinforced these conclusions.