A note on two-dimensional fence flows,with emphasis on wall constraint

An experimental investigation of the effects of wind tunnel blockage on the flow over two-dimensional surface mounted flat plates is reported. The blockage ratio was varied for a range of boundary layer thickness to fence height ratio ($\text{δ}\text{h}$) and wall friction to free stream velocity ratios ($\text{U}{}_{\mathrm{\tau }}\text{U}{}_1$). Measurements of fence surface pressures and the length of the downstream recirculation zone indicated that the effect of wall constraint was, in general, inseparable from the effects of the upstream flow characteristics. In contrast to the case of plates normal to a uniform free stream, blockage had a significant effect on the maximum front face pressures. This effect, like that on the base pressure, seemed to correlate with $\text{U}{}_{\mathrm{\tau }}\text{h}\text{ν}$ whereas the resultant variations of drag with blockage were dependent on $\text{δ}\text{h}$. Both the normalised length, $\text{R}{}_{\mathrm{<mtext>L</mtext>}}\text{h}$, of the recirculating region at zero blockage and its variation with blockage were also determined by $\text{δ}\text{h}$ only, but there was clearly no simple qualitative connection between $\text{R}{}_{\mathrm{<mtext>L</mtext>}}\text{h}$ and the base pressure, C_pp, as their often is for bluff bodies away from a boundary.Despite previous attempts in the literature to look for a correlation between fence drag and a single boundary layer parameter (Plate [1]; Good and Joubert [2]; Raju et al. [3], it is argued that any such correlation is probably marginal. The implications of these results for bluff body experiments of the ‘environmental’ kind are briefly discussed.