Dear Sirs,

For the design analysis of a high and slender vent gas scrubber, I've chosen the vortex shedding analysis to investigate a possible limited fatigue life. The vortex shedding analysis for tall vertical vessels is based on the non-dimensional Strouhal number, S = f.D/v [Bednar, p. 111].

Since I've run into design problems with this specific scrubber, I've performed the analysis in both English and metric units to check the consistency. More specific for this scrubber, the PVElite 4.2 output calculated a very limited service life, due to too high alternating stresses. I discovered that the calculation of the critical wind speed (based on the eigen frequencey f, weigthed diameter D, and Strouhal S =0.2) and the velocity pressure Qh remain both unchanged, i.e. both outcome are indentical (Vh = 117.85 m/s and 117.85 miles/hr, Qh = 41.88 N/m2 and 41.88 psf).

Therefore I've the following questions:
- What is the meaning of the constant 0.6818 in the PVElite output ?
- How is the weigthed diameter Davg of a vessel determined, used in the Strouhal equation ?
- If vortex shedding showed out to be critical for a vessel design (limited service life due to too high alternating stresses) what design variables do I have ?
- Do you know the excistence of a recommended engineering guideline for vortex breakers (spiralized rafter or beams on the OD of tall vertical columns and stacks ?

Kind regards,