WIND TUNNEL AND MODEL 

 The experimental investigation was conducted in the DTNSRDC Anechoic Wind Tunnel 

 Facility. The wind tunnel has a closed Jet test section that is 8 ft (2.4 m) square 

 and 13.75 ft (4.19 m) long. The corners have fillets which are carried through the 

 contraction. The test section is followed by an acoustically-lined large chamber 

 23.5 ft (7.16 m) long. It was found previously, by Huang et al., that the ambient 



free-stream turbulence levels, IVu' /U jx 100, are 0.075, 0.090, 0.100 and from 



0.12 to 0.15 for free-stream velocities, U , of 24.4, 30.5, 38.1, and 45.7 m/s, 



o 



respectively. Integration of the measured noise spectrum levels in the test section 



from 10 to 10,000 Hz indicated that the typical background acoustic noise levels at 



2 

 30.5 m/s were about 93 dB re 0.0002 dyne/cm (0.0002 Pa). These levels of ambient 



turbulence and acoustic noise were considered low enough so as not to unfavorably 



affect the measurement of boundary- layer characteristics. The maximiom air speed that 



can be achieved is 200 ft/sec (61 m/s); in the present experiments the wind tunnel 



velocity was held constant at 100 ft/sec (30.48 m/s). 



A simple three-dimensional body, having a 3:1 elliptic transverse cross section 

 with a bow entrance length of 6.23 ft (1.897 m) , was used for the present experi- 

 mental investigation. The total model length is 10.06 ft (3.07 m) with a maximum 

 major axis of 1.588 ft (0.48 m) and a maximum minor axis of 6.35 in. (16.12 cm). 

 A schematic of the three-dimensional afterbody with the 3:1 elliptic cross section 

 is shown in Figure 1. The major and minor elliptic axes are shown in Figure 1 as 

 a and b, respectively. The model is shown in the anechoic wind tunnel facility in 

 Figure 2. The support struts shown in the figure are not the struts used for this 

 experiment. Model offsets are presented in Table 1. 



The model was supported by two streamlined struts separated by one-third of the 

 model length. The struts are 0.5-in. (1.27-cm) thick with a 1.5-in. (3.81-cm) chord 

 upstream and 2. 25- in. (5.72-cm) thick with a 6.0-in. (15.24-cm) chord downstream. 

 The model is designed to rotate 90 degrees radially about a center axis to permit 

 vertical traversing normal to the surface pressure taps (see section on 

 Instrumentation) . The disturbances generated by the supporting struts were within 

 the region below the horizontal centerplane. Therefore, all of the experimental 

 data were taken above the model on the vertical centerplane along the upper meridian 



