Influence of gap-ratio on flow dynamics and heat transfer for a square cylinder approaching a moving wall in turbulent regime

摘要

Flow past a square cylinder near a moving wall is investigated for 0.1 ≤G/D (gap-ratio) ≤ 1.0 at a constant Reynolds number of 22,000 using large eddy simulations. Influence of the gap-ratio on various flow features has been studied. It is observed that for G/D ≤ 0.3, a regular Karman vortex shedding from the cylinder gets suppressed which causes a large change in the flow dynamics. Unlike the case of a stationary wall, no ground vortex is formed in the present case. For G/D = 1.0, turbulence is strong in the near wake region and for low G/D values (≤ 0.3) it gets weaker and shifts downstream. With a decrease in G/D, an early reattachment of the lower shear-layer with the bottom surface of the cylinder occurs while the upper shear-layer is deflected and K-H instability occurs away from the top surface. The so-called 'extreme events' which contain high frequency fluctuations are observed for G/D = 1.0 and these disappear for low G/D values. Large coherent structures are observed for high G/D (≥ 0.5) while for low G/D uniformly distributed small structures are formed. Evolution of enstrophy has been examined for G/D = 0.1 and the role of the strain-rate tensor in the enstrophy production has been investigated. The lift coefficients are negative for all G/D values while drag coefficients decrease with a decrement in G/D and are nearly constant for G/D ≤ 0.3. Due to the formation of large coherent structures, power spectra for high G/D follows -5/3 law over a large range of frequency. Unlike the case of a laminar flow, Nusselt number on the cylinder surface monotonically decreases with a decrease in G/D.