On the three-dimensional study of flows over a fence in a duct

Laminar flow over a fence of height ratios, h/H = 0.5 and 0.25, in a duct w ith various aspect ratios, W/h, is numerically solved by three-dimensional simulations. Velocity profiles, reattachment length, and secondary recircul ation length at the symmetrical plane are obtained and compared with measur ements of Tropea and Gackstatter [1]. Numerical results show that the three -dimensional simulations fit the measured data well. Flow distributions alo ng the span-wise direction reveal that the main stream motions are nearly t wo-dimensional, while the flow motions in reversed flow are quite three-dim ensional. Particle tracers in the main stream track nearly two-dimensional motions. In the reversed flow, particle tracers seeded near the side-wall s piral along the span-wise direction in an increasingly larger loops. As app roaching to the symmetry plane, the spiraling particles are lifted up and t ravel downstream. Symmetry-plane flows tend to become two-dimensional for l arger aspect ratio. With a duct width to height aspect ratio, W/h, greater than 72, the three-dimensional flow yields a nearly two-dimensional flow at the symmetry plane. The effect of the height ratio h/H is further investig ated. Reduction of the height ratio from 0.5 to 0.25 leads to the vanishing of the secondary reversed flow on the roof wall, and consequently, the thr ee-dimensional flow characteristic has weakened.