A Kinematic Description of the Key Flow Characteristics in an Array of Finite Height Rotating Cylinders

Abstract

Experimental data are presented for large arrays of rotating, finiteheight cylinders to study the dependence of the threedimensional (3D) mean flows on the geometric and rotational configurations of the array. Two geometric configurations, each with two rotational configurations, were examined at a nominal Reynolds number of 600 and nominal tipspeed ratios of 0, 2, and 4. It was found that the rotation of the cylinders drives the formation of streamwise and transverse flow patterns between cylinders and that net time–space averaged transverse and vertical flows exist within the developed flow region of the array. This net vertical mean flow provides an additional mechanism for the exchange of momentum between the flow within the array and the flow above it, independent from the turbulent exchange mechanisms which are also observed to increase by almost a factor of three in a rotating array. As an array of rotating cylinders may provide insight into the flow kinematics of an array of vertical axis wind turbines (VAWTs), this planform momentum flux (both mean and turbulent) is of particular interest, as it has the potential to increase the energy resource available to turbines far downstream of the leading edge of the array. In the present study, the streamwise momentum flux into the array could be increased for the rotatingelement arrays by up to a factor of 5.7 compared to the stationaryelement arrays, while the streamwise flow frontally averaged over the elements could be increased by up to a factor of four in the rotatingelement arrays compared to stationaryelement arrays.