The R.A.E. 4-ft x 3-ft experimental low-turbulence wind tunnel. Part IV. Further turbulence measurements

Abstract

Further measurements of turbulence in the working section are given with 2 and 3 screens in the bulge. The extended region of high intensity turbulence near the walls of the working section, which was observed with 9 screens in the bulge, disappeared when the number of screens was reduced from 9 to 2 or 3. The longitudinal component of turbulence is approximately independent of the nmnber of screens ; the lateral component does not change, if the number of screens is reduced from 9 to 3, but increases by a factor 2.5 to 3, if the number of screens is further reduced from 3 to 2. In order to explain the origin of the turbulence in the working section, further turbulence measurements have been made at the end of the second diffuser, before the rapid expansion and in the bulge. The intensities of turbulence are about 12 per cent of mean speed at the end of the second diffuser and drop to about 4 to 6 per cent before the rapid expansion. However, this turbulence seems to be reduced by the screens in the rapid expansion and in the bulge below the level of disturbances set up by inhomogeneities of the last screen. These disturbances are the origin of the lateral components of turbulence in the working section. The extended region of high intensity turbulence near the walls of the working section is connected with the existence of a return flow in the bulge, but several possible explanations exist as to how this produces the region of high intensity turbulence in the working section.