Drag forces and flow structure around and inside a square array of emerged cylinders in an open channel

Eddy resolving simulations are used to describe the mean flow and turbulence structure inside and around a square array of width D containing emerged (i.e., cylinder extending up, or above, the free surface), rigid cylinders of width d . The array is placed at the centre of a long open channel of width W . The study describes how the structure of the wake and shear layers forming along the lateral sides of the array, the wake spectral content and shedding modes, the bleeding velocity, the lengths of the steady and total wake regions, the coherence of the horseshoe vortices forming around the frontal face of the array, the bed shear stresses and the sediment entrainment capacity of the flow inside and outside the array change with the main geometrical parameters defining the array (i.e. the solid volume fraction, SVF, the non-dimensional frontal area per unit volume of the array, italic aD aD aD , the shape of the cylinders and d divided by upper D d / D $d/D$ ) and the blockage ratio, upper W divided by upper D W / D $W/D$ . The effects of varying the same geometrical parameters on the total non-dimensional streamwise drag force acting on the array and on the streamwise distribution of the drag force inside the array are quantified. The mean flow and turbulence inside the array are analysed based on the streamwise distributions of the double-averaged (i.e. over time, D and the flow depth, H ) streamwise velocity, upper U overbar overbar U ¯ ¯ Ū , and turbulent kinetic energy. Contrary to wakes generated behind circular arrays of emerged cylinders, several streamwise-oriented vortices are generated inside the wake of square arrays for a limited range of italic aD aD aD . These vortices increase flow three-dimensionality and vertical transport behind square arrays. Despite the relatively large values of the array and channel Reynolds numbers (i.e. Re D = 40 000, Re H = 14 000), the mean (time and spanwise-averaged) cylinder drag coefficient, upper C prime Subscript d C d ′ Cd^