shear flow. Figure 2.25 shows the Strouhal number based on centerhne velocity for L/D = 34, j8 = 

 0.032, and Re = 4000. This figure confirms the existence of two cells at the boundaries and shows a 

 tendency toward a cellular vortex pattern over the central section of the stationary cylinder. Regions 

 with a similar frequency can be identified over a limited distance, but it is not clear that cells with well- 

 defined boundaries exist. When the data from Fig. 2.25 are presented as Strouhal numbers based on 

 the local velocity, the data are grouped around Strouhal numbers of 0.2 to 0.21 (41). The data indi- 

 cates that some cellular structure occurs but that no clear cell boundaries can be identified in many 

 regions. 



Two smoke visualization photographs of the wake of the same cylinder at a Reynolds number 

 slightly below 2000 are shown in Fig. 2.26. The region observed is roughly from the centerline to L = 

 8D below the centerline where no clear indication of a cell structure was evident from frequency meas- 

 urements in the wake. The smoke was illuminated by a strobe light that was synchronized with the 

 shedding frequency, and an exposure time long enough to cover about six or seven shedding cycles was 

 used. The periodic structure seen in the smoke pattern along the cylinder length is a result of smoke 

 introduction through equally spaced holes in the base of the cylinder. If a coherent cell shedding at the 

 strobe frequency for seven or eight cycles existed during the exposure time, the result would appear as 

 a banded system as seen in the top photograph. If a cell with a frequency different from the strobe fre- 

 quency was shedding or if no shedding occurred during the exposure time, the result would look like 

 the bottom photo. However, the photos in Fig. 2.26 were taken at two different times using the same 

 strobe frequency. These results suggest that cells of finite size, but of different frequencies and extent, 

 exist at different times in the wake (41). An inclined pattern of vortex shedding similar to that shown 

 in the top photograph was observed by Stansby (39) for a cylinder with L/D = 16, ^ = 0.025 and Re 

 = 3000. 



The available evidence through 1980 relevant to shear flow effects on bluff bodies has been sum- 

 marized in an NRL report (42). Additional detailed findings are given in the proceedings of recent 

 International Conferences on Wind Engineering (see references 43 and 44) and in several of the refer- 

 ences just cited. 



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