EFFECT OF VELOCITY RATIO ON VORTEX SHEDDING FROM A SHORT STACK

摘要

The influence of the jet-to-cross-flow velocity ratio, R, on vortex shedding from a cylindrical stack of aspect ratio AR = 9 was investigated using hot-wire anemometry. The cross-flow Reynolds number was Re_D = 2.3×10~4 and R was varied from 0 to 3. The stack was partially immersed in a flat-plate boundary layer, where the boundary layer thickness-to-height ratio at the location of the stack was δ/H = 0.5. In the downwash flow regime, when R < 0.7, a single Strouhal number (e.g., St = 0.167 at R = 0) was measured along the entire stack height. In the crosswind-dominated flow regime, when 0.7 ≤ R < 1.5, a higher Strouhal number was obtained (e.g., St = 0.185 at R = 1.0). In the transitional and jet-dominated flow regimes, a jump in Strouhal number (e.g., from St = 0.176 to 0.193 at R = 2.0), occurred within the flat-plate boundary layer. The power spectra showed that the shape and strength of the vortex shedding peak changed along the stack height. In general, the peak was more broad-banded near the base of the stack, became sharper and more distinct within the middle of the combined stack and jet wakes, and then reduced in strength in the jet wake and jet regions. In the transitional and jet-dominated flow regimes, the jump in Strouhal number was seen as a gradual change in dominance between two closely spaced peaks, with both peaks co-existing near the edge of the boundary layer.