131 



in the comparison are the Douglas CS differential 

 boundary-layer method in conjunction with the dis- 

 placement body concept. The iteration procedures 

 for numerical computation are given by Huang et al. 

 (1976). In this investigation, the displacement 

 body concept for solving the interaction between 

 the thick stern boundary layer and potential flow 

 will be examined and an eddy-viscosity model will 

 be evaluated. 



suggest that the displacement body concept as used 

 by Huang et al. (1976) permits accurate computation 

 of the pressure distribution on the stern. 



The measured and computed distributions of local 

 shear stress, C^-, are compared in Figure 3. The 

 agreement between theory and measurement is 

 also very good for both afterbodies except for 

 x/L > 0.95 where the measured values of C are lower 

 than the computed values . 



Measured and Computed Pressure and Shear Stress 

 Distributions 



Significant improvement in the accuracy of measur- 

 ing surface pressure and shear stress have been made 

 by using a precision pressure transducer. The 

 present results are more reliable than the earlier 

 results of Huang et al. (1976) , although the dif- 

 ferences are small. 



The measured and computed values of the pressure 

 coefficient, Cp = 2 (p - Pq)/pUq , are compared in 

 Figure 1 for Afterbody 1 and in Figure 2 , for After- 

 body 2; p is the local static pressure, p is the 

 mass density of the fluid, Uq is the free-stream 

 velocity and Pq is the ambient pressure (the qui- 

 escent chamber static pressure of the open-jet sec- 

 tion) . The pressure coefficients computed on the 

 displacement body were carried radially back to the 

 hull surface and the radial distribution of pres- 

 sure at a given axial station was assumed to be a 

 constant between the hull surface and the fictitious 

 displacement surface. The maximum error in the 

 static pressure associated with this assumption is 

 less than tvvo percent of the dynamic pressure (next 

 section) . The agreement between theory and measure- 

 ment is excellent for both afterbodies. The results 



Measured and Computed Static Pressure Distribution 



The measured and computed static pressure coeffi- 

 cients for Afterbody 1 are compared in Figure 4 at 

 various locations across the stern boundary layer 

 and in Figure 5 for the near wake. Figures 6 and 7 

 show the comparisons for Afterbody 2. The off-body 

 option of the Douglas potential- flow computer code 

 was used to compute the static pressure distribu- 

 tions off the displacement body. As can be seen in 

 Figures 4 through 7 , the computed static pressure 

 distributions across the entire stern boundary layer 

 and near wake mostly agree well with the measured 

 static pressure distributions. The discrepancy 

 between the measured and computed values of Cp is 

 in general less than 0.01 which is about the accuracy 

 of the measurement. 



As will be seen later, both displacement bodies 

 are convex from the parallel middle body up to X/L 

 = 0.91 and become concave downstream from X/L > 

 0.91. However, the actual afterbodies are convex 

 all the way up to X/L = 0.96. The measured values 

 of Cp shown in Figures 4 through 7 increase with 

 radial distance for X/L < 0.91, indicating that 

 the mean streamlines are convex; and measured values 

 of C decrease with radial distance for X/L > 0.91, 



0.65 



0.70 



0.75 



0.80 



085 



0.90 



095 



10 



X/L 



FIGURE 1. Computed and measured stern pressure distribution on afterbody 1. 



