Statistical theory of turbulence-II

The methods described in Part I have been used by Mr. L. F. G. Simmons, of the National Physical Laboratory, to find experimentally the correlation between the turbulent components of velocityu 0 andu y at two points distanty apart in a direction transverse to the stream. The measurements were made at mean speed U = 25 feet per second in a wind tunnel behind a honeycomb with 0·9-inch square mesh. The results are shown in fig. 1 where the ordinates are Ry = ¯¯u ouy / ¯¯u 2 and the abscissae are the corresponding values ofy . It will be seen that the Ry curve is apparently rounded at the top and that Ry falls to 0·08 aty = 0·38 inches. No measurements were made beyond this point, but extrapolation seems to show that Ry = 0 wheny is about 0·5 inches,i.e ., wheny is slightly greater than 1/2M. Integrating the curve of fig. 1 the value ofl 2 =∫ 0 0.5" Ry = 0·175 inch, so thatl 2 /M = 0·175/0·9 = 0·195.