Propeller Pressure Field in a Nonuniform Flow 



1.2 to 1.6 times greater than those calculated under the uniform inflow assump- 

 tion. The lower limit corresponds to the case of a moderately nonuniform flow 

 behind the hull of a twin-screw ship; the upper one corresponds to the case of a 

 significantly nonuniform wake of a single -screw ship. These values are in 

 agreement with the corresponding evaluations mentioned in Ref. 2. 



A special experiment was carried out (by B. A. Biskup) in the cavitation 

 tunnel to check the theoretically predicted effect of flow nonuniformity on the 

 propeller-induced pressures. The amplitudes of pulsating pressures were 

 measured with a three-bladed propeller operating both in the free stream and 

 behind a screen generating a significantly nonuniform velocity field. The pro- 

 peller geometry was: blade area ratio A/A^ = 0.5, pitch ratio h/d = 1.2, and 

 thickness ratio tg/D = 0.05. Measurements were made at a fixed radial distance 

 r = l.2Ro and various axial distances from -0.4Ro to o.4Ro. In the case of non- 

 uniform flow, pressures were measured at two angular positions y = o and 

 7 = 77/2, corresponding to the regions of maximum and minimum wake respec- 

 tively. The results are presented in Fig. 4 (pressure amplitudes having been 

 made dimensionless by division with p^/ttR^). The dashed lines in Fig. 4 cor- 

 respond to the calculated amplitudes of the first harmonic of the blade -frequency 

 pressure. Loading-associated pressures were computed according to the equa- 

 tion given in the present report, whereas pressures due to the thickness effect 

 were computed by means of a corresponding equation for a propeller operating 

 in uniform flow. 



o •xparlJMBtal ralnaa 



lifting -line theory 



lifting- surface Xhuvrj 



Fig. 4 - Effect of flow nonuniformity on 

 pressures generated by a propeller operat- 

 ing behind a screen: 1, maximum wake; 

 Z, minimum wake; 3, unifornn inflow 



It can be seen that the lifting-line representation of the propeller leads to 

 overestimated (as compared with the experimental data) values of pressure am- 

 plitudes for the case of both nonuniform and uniform flow. Actually if the 

 loading-associated pressure for the case of uniform flow is calculated according 



11 



