Propeller-Induced Velocity Field Due to Thickness and Loading Effects

Abstract

Blade thickness plays a dual role, contributing to the lifting characteristics of the blade because of its nonplanar form as well as to its nonlifting characteristics due to the generation of a symmetrical flow disturbance. However, since the so-called "nonplanar thickness" has been shown to have little effect on the blade pressure distribution and thus presumably to have a negligible effect on the velocity and pressure fields around an operating propeller, the present investigation is limited to the so-called "symmetrical flow disturbance thickness." The effect of this thickness on the oscillatory velocity field around the propeller is studied by means of the "thin body" approach, where the blade section is represented by a source-sink distribution of strength proportional to the slope of the blade thickness distribution. A numerical procedure is devised and adapted to the CDC-6600 high-speed digital computer for the evaluation of the thickness effect on the velocity field. The total propeller-induced velocity field is then obtained by adding the computed velocity components due to thickness, with proper phase, to the results due to propeller loading calculated by means of the lifting-surface theory. Sets of calculations performed for a 3-blade propeller operating in a 3-cycle screen-generated wake and for a 5-blade propeller operating in a realistic hull wake reveal that the effect of thickness in forming the components of the resultant velocity varies from moderate to large, depending on the magnitude of the thickness distribution, on the location of the field point, and on the intensity of the nonuniformity of the inflow field.