A Wind Tunnel Study on the Aeromechanics of Dual-Rotor Wind Turbines

In the present study, we report our recent efforts to develop a novel dual-rotor wind turbine (DRWT) concept to improve aerodynamic efficiency of isolated turbines as well as wind farms. The DRWT concept employs a secondary, smaller, co-axial rotor with two objectives: (1) mitigate losses incurred in the root region of the main rotor by using an aerodynamically optimized secondary rotor, and (2) mitigate wake losses in DRWT wind farms through rapid mixing of turbine wake. Mixing rate of DRWT wake will be enhanced by (a) increasing radial shear in wind velocity in wakes, and (b) using dynamic interaction between primary and secondary rotor tip vortices. Velocity shear in turbine wake are tailored (by varying secondary rotor loading) to amplify mixing during conditions when wake/array losses are dominant. The increased power capacity due to the secondary rotor can also be availed to extract energy at wind speeds below the current cut-in speeds, in comparison to conventional single-rotor wind turbine (SRWT) design. For a DRWT system, the two rotors sited on the same turbine tower can be set to rotate either in the same direction (i.e., co-rotation DRWT design) or at opposite directions (i.e., counter-rotating DRWT design). It should be noted that a counter-rotating rotor concept (i.e., the rotors rotate at opposite directions) has been widely used in marine (e.g., counter-rotating propellers used by Mark 46 torpedo) and aerospace (e.g., Soviet Ka-32 helicopter with coaxial counter-rotating rotors) applications to increase aerodynamic efficiency of the systems. The recent work Ozbay et al. (2015) reveal that, with the two rotors in counter-rotating configuration (i.e., counter-rotating DRWT design), the downwind rotor could benefit from the disturbed wake flow of the upwind rotor (i.e., with significant tangential velocity component or swirling velocity component in the upwind rotor wake). As a result, the downwind rotor could harvest the additional kinetic energy associated with the swirling velocity of the wake flow. With this in mind, the effects of relative rotation direction of the two rotors on the aeromechanics performances of DRWTs (i.e., co-rotation DRWT design vs. counter-rotating DRWT design) and the turbulent mixing process in the DRWT wakes are also evaluated in the present study. The experimental study was performed in a large-scale Aerodynamics/Atmospheric Boundary Layer (AABL) Wind Tunnel located at the Aerospace Engineering Department of Iowa State University. Scaled DRWT and SRWT models were placed in a typical Atmospheric Boundary Layer (ABL) wind under neutral stability conditions. In addition to measuring the power outputs of the DRWT and SRWT systems, static and dynamic wind loads acting on the test models were also investigated to assess the effects of the secondary, smaller, co-axial rotor in either counter-rotating (rotors rotate at opposite directions) or co-rotating (rotors rotate at same direction) configuration on the power production performance and the resultant dynamic wind loads (both aerodynamic forces and bending moments) acting on the DRWT models. Furthermore, a high-resolution stereoscopic Particle Image Velocimetry (Stereo-PIV) system was also used to make both "free-run" and "phase-locked" measurements to quantify the transient behavior (i.e., formation, shedding and breakdown) of unsteady wake vortices and the flow characteristics behind the DRWT and SRWT models. The detailed flow field measurements were correlated with the power output data and dynamic wind loading measurements to elucidate underlying physics for higher total power yield and better durability of wind turbines operating in turbulent non-homogenous atmospheric boundary layer (ABL) winds.