Fig 1.
Pressure changes caused by an operating wind turbine.
A low-pressure region forms over the blade suction surface (downwind side) and a high-pressure region forms over the blade pressure surface (upwind side of the blade) as a result of local flow accelerations. A region of low pressure is also created by the vortex that forms as air flows around the tip from the blade pressure side to the suction side. The tip-vortex propagates downstream in the direction of the wind as shown.
Fig 2.
Smoke visualization of a blade-tip vortex at the national aeronautics and space administration’s ames research center 40-by-80-ft wind tunnel.
The visible helical vortex indicates the region of low pressure caused by the blade-tip vortex. Note that this visualization was performed under low-turbulence conditions in a wind tunnel, allowing the vortex to advect downwind with the mean flow with its helical structure intact. In the atmosphere, where the turbulence intensity is typically much higher, the vortex structure is commonly unrecognizable within one rotor diameter downstream of the rotor plane. Photo by Lee Jay Fingersh, National Renewable Energy Laboratory, 55062.
Table 1.
Design and operating characteristics of the NREL 5-MW reference turbine [23].
Table 2.
NREL 5 MW turbine operating conditions considered.
Fig 3.
Computational mesh, which includes (a) the c-grid computational domain, (b) the near-blade mesh topology, and (c) the boundary layer mesh extrusion near the blade surface.
Fig 4.
Comparison of the blade surface pressure coefficient, Cp, for a NACA 0012 airfoil predicted in the current study using STAR-CCM+ compared with experimental results from Ladson et al. [39] and CFD results generated using CFL3D and a 897-by-257 computational grid [38].
All results are shown at a blade chord Reynolds number of 6 million.
Fig 5.
Blade vorticity distribution along the blade of the NREL 5-MW reference turbine calculated using OpenFAST.
Note that solid circles identify the maximum spanwise vorticity, which corresponds to the strength of the corresponding blade-tip vortex.
Fig 6.
Blade surface pressure for wind speeds of 5 m/s, 7.5 m/s, and 10 m/s.
(Left frames) Full-field pressure contours. (Right frames) Pressure contours showing regions of the pressure field where the pressure is lower than -2000 Pa and higher than 1000 Pa, with respect to atmospheric pressure.
Fig 7.
Bat flight paths by the 90% blade span location of the NREL 5 MW wind turbine operating at a wind speed of 10 m/s.
(Top) Flight paths for bats flying at 0 m/s with respect to the wind. (Middle) Flight paths for bats flying at ±5 m/s, with respect to the wind. (Bottom) Flight paths for bats flying at ±10 m/s, with respect to the wind. The pressure contours show the pressure variations caused by the blade that are below -2000 Pa and above 1000 Pa, with respect to atmospheric pressure. For scale, note that the bat flight paths are separated by 20 mm in the direction normal to the flight paths.
Fig 8.
Bat flight paths by the 90% blade span location of the NREL 5 MW reference turbine from the upwind and downwind direction when the turbine is operating at wind speeds of 5 m/s, 7.5 m/s, and 10 m/s.
Note that only the front section of the blade is shown and that the flight paths are for a bat flight speed of 10 m/s, with respect to the oncoming wind direction. For scale, note that the bat flight paths are separated by 20 mm in the direction normal to the flight paths.
Fig 9.
Pressure vs. time history for bat flight paths near the NREL 5 MW reference wind turbine blade at the 90% blade span location for wind speeds of 5 m/s, 7.5 m/s, and 10 m/s.
(Top) flight paths 100 mm from the blade surface and (bottom) flight paths <1 mm from the blade surface. Note that the pressure-time histories have been shifted to the minimum or maximum pressure that occurs at 0 s. Time before 0 seconds is when bats are approaching the peak low or high pressure, and time after 0 seconds is when bats are flying away from peak low or high pressure.
Table 3.
The minimum and maximum pressures bats could experience when flying near the 90% blade span location at a flight speed of 10 m/s and near the blade-tip vortex of the NREL 5 MW reference turbine, with respect to atmospheric pressure.
Fig 10.
(Top) Velocity caused by the blade-tip vortex of the NREL 5 MW reference turbine calculated using Eq 2. (Bottom) Pressure change caused by the tip-vortex calculated using Eq 4.
Fig 11.
Occurrence of lung hemorrhaging (top) and mortality (bottom) in rats and rabbits as a result of sudden exposure to low pressures.
The circles present the data from Junkui et al. [50] for rats and rabbits. The triangle indicates the largest magnitude low pressure our results indicate bats could be exposed to when flying near the NREL 5 MW wind turbine when the wind speed is 10 m/s. Note that there is no mortality or hemorrhaging data available for bats rapidly exposed to low pressures.
Fig 12.
50% mortality threshold levels for various mammals rapidly exposed to high pressures from Richmond et al. [51].
The triangle indicates the largest magnitude high pressure we predict bats could be exposed to when flying near the NREL 5 MW wind turbine when the wind speed is 10 m/s. Note that there is no mortality data available for bats rapidly exposed to high pressures.