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, finite-height cylinders to study the dependence of the three-dimensional (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 tip-speed 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 rotating-element arrays by up to a factor of 5.7 compared to the stationary-element arrays, while the streamwise flow frontally averaged over the elements could be increased by up to a factor of four in the rotating-element arrays compared to stationary-element arrays.

Performance enhancement of downstream vertical-axis wind turbines

“…It has been shown in field experiments that the energy deficit in the wake of VAWTs recovers in only 4-6 D, 11 and numerical simulations have shown that the spatial extent of this recovery decreases with the increased rotational speed. 12 This rapid recovery is potentially aided by both an induced downward mean vertical flow behind each turbine, as observed in an analogous array of rotating cylinders, 13 and through the turbulent planform flux of energy into the array from above, an effect seen in HAWT arrays, 14 the wake of a single VAWT, 15 and VAWT arrays. 16 In this paper, we explore mean flow dynamics in VAWT arrays that complement the aforementioned physics.…”

Low order physical models of vertical axis wind turbines

“…In recent years, vertical axis wind turbine (VAWT) arrays have attracted significant attention as a potentially valuable and versatile approach to wind energy extraction. [1][2][3][4][5][6][7][8] Of particular interest has been the spatial and rotational configuration of the turbines in these arrays, which determines the nature of the interaction between the turbines and thus the overall power output of the array.…”

“…2,10 Although the 2D simulations have typically modeled the turbine geometry to high fidelity, they require an underlying assumption that the horizontal plane interactions between turbines in the array are sufficient to understand the performance of the arrays, without any contribution or influence from interactions with the freestream flow above the array or the bottom boundary layer. This assumption is called into question by the results of both field work 4,5 and laboratory experimental work, 7,8 which indicate the importance of the three-dimensional interactions, particularly through the top of the array. Underlying the 3D LES simulations are assumptions concerning the modeling of the turbine: as an actuator line model (or immersed boundary model), an actuator swept surface model, or an actuator "disk" (flat plate) model.…”

“…Similar to the numerical studies, they provide a controlled test environment, relatively easy reconfiguration of the arrays, and reasonable spatial coverage of (at least) 2 velocity component flow measurements. In the laboratory experiments reported to date, 7,8 however, multiple simplifications to the physical array were required in order to test the large arrays desired. These simplifications included reduced Reynolds number, externally driven elements (at a constant rotation rate throughout the array), and modeling the turbines as cylinders.…”

“…These simplifications included reduced Reynolds number, externally driven elements (at a constant rotation rate throughout the array), and modeling the turbines as cylinders. A full discussion of the implications of the simplifications was given in the supplementary materials of Craig et al, 7 from which it was concluded that while there were some strong similarities between the examined rotating cylinder arrays and VAWTs, additional work needed to be undertaken to further characterize similarities and differences. One key question identified was to what extent the increase in solidity of the array elements from the bladed turbine to a solid cylinder would influence the flow kinematics.…”