LES of Flow Through and Around a Finite Patch of Thin Plates

Large eddy simulations (LESs) are performed for turbulent flow through and around a porous patch of thin vertical plates at a plate Reynolds number of Re-p=5,800. The plates are arranged in a staggered pattern, presenting an elliptical planform and mimicking streamwise-oriented blades of emergent vegetation. The immersed boundary method is employed to explicitly resolve the interaction between flow and plates. Three flow cases, each with a different number of plates within the same planform area, that is, different patch density, are studied. The Reynolds number based on freestream velocity and plate length is the same in all cases. Inspection of the distribution of velocity and vorticity in the horizontal plane reveals that downstream plates are significantly impacted by the wakes from upstream plates. It is therefore proposed that the plates can be divided into two groups based on the local flow characteristics, which are a function of position within the patch: a free group and a wake group. This classification is subsequently used in the quantitative analysis of boundary layer development and drag force at plate scale. The thickness and character of the simulated boundary layers on the plates differ significantly from predictions based on analytical or empirical relationships, which is due to wake effects and the finite length of the plates. The simulations demonstrate the so-called sheltering effect; that is, the drag forces acting on downstream plates (in the wake group) are significantly lower than those acting on upstream plates, a result of the lower approach flow speed. Although the front-area-to-lateral-area ratio of the plates is low (1/40), pressure drag is observed to be larger than friction drag for each plate. The ratio of pressure drag to the total drag at patch scale shows only very little dependence on the plate density of the patch.