LIMITATIONS OF THE PRESSURE DISTRIBUTION CALCULATION 



The major limitation of the method used to calculate the pressure distribution 

 was the inability to account for the local irregularities in the section shape in a 

 manner compatible with real-fluid effects. The extreme fluctuations in the pressure 

 distributions appeared to overpredict the influence of the section irregularities. 



This problem may be partially due to the numerical representation of the section 



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 shape used during the pressure distribution computation. The technique of Brockett 



is a conformal mapping procedure using trigonometric interpolation polynomials to 

 describe the section shape. A trigonometric series is fitted analytically to the 

 offsets at the prescribed chordwise points. By requiring the series to pass through 

 the given points, unfair offsets will produce sinusoidal variations in the section 

 shape between points. The conformal mapping procedure then calculates the potential 

 pressure distribution of the series-described section shape. Sharp pressure fluc- 

 tuations may result from the relatively large variations in geometry between pre- 

 scribed offset points. 



Figure 38 shows a comparison between the section geometry created from the 

 trigonometric series representation and the measured offsets interpolated to a 

 cosine distribution from the measured geometry. Correlation can be seen between the 

 pressure fluctuations and the irregularities in the section shape. In Figure 38c, 

 the series representation of the offsets are quite close to the measured values, 

 while Figures 38a and 38b produce distinct deviations causing changes in section 

 slope at the points where pressure is calculated. The variations produced by the 

 series representation in these cases is considered small compared to the size of the 

 irregularity of the section. Therefore, it is believed that the potential flow 

 pressure distribution calculation is reasonable from a mathematical standpoint. The 

 details of the pressure distribution about each irregularity, in some cases, is only 

 approximated by the series representation of the geometry, but since the measured 

 irregularity is not sufficiently defined, a more exact pressure distribution is 

 inappropriate . 



The boundary layer must also be considered when interpreting the calculated 

 pressure distributions of the measured sections. Small irregularities in section 

 geometry, producing fluctuations in the potential-flow pressure distribution, will 



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