129 



The wind tunnel has a 2.44 m by 2.44 m closed- jet 

 test section, followed by a 7.16 m by 7.16 m open- 

 jet test section. The length of the open-jet sec- 

 tion is 6.40 m. The maximum air speed which can 

 be achieved is 61 m/sec; in the present experiments, 

 the velocity of the wind tunnel was held constant 

 at 30.48 m/sec. The measured ambient turbulence 

 level in the open-jet test section without the model 

 in place was 0.1 percent. Integration of the mea- 

 sured noise spectrum levels in the open-jet test 

 section, over the frequency range of to 10,000 Hz, 

 indicated that the typical background acoustic 

 noise at 30.48 m/sec was around 93 db re 0.0002 

 dyn/cm . These levels of ambient turbulence and 

 acoustic noise were considered low enough so as not 

 to unfavorably affect the measurements of boundary- 

 layer characteristics. 



Two axisymmetric convex afterbodies without 

 stern separation were used for the present experi- 

 mental investigation. Their afterbody length/ 

 diameter ratios (L^/D) were 4.308 and 2.247. The 

 detailed offsets for Models 1 and 2 are given in 

 Tables 1 and 2. Each afterbody was connected to a 

 parallel middle body of length L,. and an existing 

 streamlined forebody with a bow-entrance length 

 diameter ratio (L^/D) of 1.82. The total length 

 of each model (L) is fixed at a constant value of 

 3.066 m. The diameter of the parallel middle body 

 '""^max' ^^ 27.94 cm. The common forebody and a 

 portion of the parallel were constructed of wood. 



The afterbody and the remaining portions of the 

 parallel middle body were constructed of molded 

 fiberglass; specified profile tolerances were held 

 to less than to.4 mm, all imperfections were re- 

 moved, meridians were faired, and the fiberglass 

 was polished to a 0.64-micron rms surface finish. 

 The tail ends of the afterbody were shaped to ac- 

 commodate the hub of an existing propeller. This 

 modification caused a considerable change of body 

 curvature in the region of X/L ^ 0.96. However, 

 as will be seen later, the thicknesses of the 

 boundary layer in this region are much larger than 

 the local radii of the body. This deficiency does 

 not cause serious degradation of boundary-layer 

 flow at that point. 



The model was supported by two streamlined struts 

 separated by roughly one-third of the model length. 

 The upstream strut had a 15 cm chord and the down- 

 stream strut a 3 cm chord. The disturbances gener- 

 ated by the supporting struts were within the region 

 below the horizontal centerplane. Prior to the 

 experiments, pressure taps and Preston tubes were 

 used to check the axisymmetric characteristics of 

 the stern flow at X/L = 0.90, 0.95, and 0.98. The 

 circumferential variations of pressure and surface 

 shear stress on the upper half of the two after- 

 bodies at these three locations were within two 

 percent. All the final measurements were made in 

 each body's vertical centerplane along the upper 

 meridian where there was little extraneous effect 



TABLE 2 - Offsets for Model 2 



X/L 



Y/L 



Y/R 



Y/L 



Y/R 



X/L 



Y/L 



Y/R 



