THE RESPONSE OF PROPULSORS 

 TO TURBULENCE 



Maurice Sevik 



Ordnance Research Laboratory, the Pennsylvania State University 



University Park, Pennsylvania 



. ABSTRACT ■ ■ - - 



The response of a propulsor to random velocity fluctuations has been 

 analyzed. As a special case the theory has been used to predict the 

 force fluctuations on a propeller of low solidity having blades of high 

 aspect ratio and operating in a homogeneous, isotropic turbulence 

 field. The response depends critically on two functions which involve 

 the ratios of the propeller diameter and of the chord of the blades to a 

 characteristic length scale of the energy-containing eddies. The power 

 spectrum peaks at the origin and drops off rapidly with increasing 

 frequency. Experiments performed in a water tunnel with a free- 

 streann propeller placed downstream of grids of various mesh sizes 

 indicate good agreement between theoretical predictions and experi- 

 mental results. . ... 



INTRODUCTION 



It is well-known that the fluid- dynamic forces on the blades of turbo machines 

 are unsteady. These unsteady forces may cau^e a number of undesirable effects 

 such as fatigue failures, high vibration levels, or objectionable acoustic radia- 

 tion. 



In many applications the spectrum of the time-dependent forces exhibit 

 strong lines which generally correspond to harmonics of the blade frequency. 

 These lines are created by effects which are periodic over one revolution of the 

 machine such as mutual interactions between rows of blades or spatial nonuni- 

 formities in the inflow velocity field. Kemp and Sears (1,2) were the first to 

 contribute to our understanding of the fundamental unsteady flow phenomena 

 which occur in compressors and turbines by analyzing the aerodynamic inter- 

 ference between rows of blades in relative motion. In the naval field Lewis (3) 

 pioneered theoretical and experimental investigations as early as 1936 in con- 

 nection with unsteady propeller forces which often cause severe vibrations in 

 ships. Recently Tsakonas (4) developed a lifting surface theory which predicts 

 the time-dependent forces acting on marine propellers possessing numerous, 

 low- aspect-ratio blades. Good agreement between Tsakonas' predictions and 

 test results were observed by Boswell (5). 



The studies mentioned, as well as numerous other investigations which have 

 been published from time to time, are all concerned with nonuniformities in the 



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