Measurement of spectra over the Bolund hill in wind tunnel

Abstract

We have determined the normal Reynolds stresses and spectra of the wind velocity over a 1:115 scale mock-up of the Bolund hill. The experiment was run in a neutral boundary layer wind tunnel using 3-component hot-wire velocimetry, 2-component particle image velocimetry, and a high-precision traversing system. Spectra have been determined at different points along transects at 2 and 5 m height above ground level. The experiment was run for 270 • wind direction and for two Reynolds numbers, Re h 1 = 4.25×10 4 and Re h 2 = 8.21×10 4 , based on the maximum height of the hill and the free wind speed at this height. Our results show how the normalized power spectral density S ii = fS ii ∕u 2 i changes over the hill. The analysis of the normalized streamwise spectrum at 2 m height, just after the escarpment, reveals that part of the energy is concentrated in the interval of normalized frequencies n h ≈ 0.01 − 0.02, which could be a signature of a weakened "flapping" phenomenon described in the literature for flows over forward facing steps. The departure of the spectra slope in the inertial subrange, from the value −5/3, was found to be correlated with the hill geometry. KEYWORDS atmospheric boundary layer, complex terrain, wind energy, wind tunnel simulation 1 INTRODUCTION The Bolund experiment, run by RISØ-DTU in 2007, is probably the most relevant test case of flow models oriented to wind energy analysis over highly complex terrains, in neutral conditions and nonaffected by Coriolis forces. 1,2 Bolund is a reference case to identify flow patterns and to validate numerical and physical models, due to the number and quality of sensors installed on the full-scale hill and the amount of numerical and physical analyses available such as literature. 1-6 Bolund is a small hill of about 130 m × 75 m × 12 m, surrounded by water with a long uniform fetch for most of the upstream directions of interest (see Figure 1-left). A large amount of periods with nearly neutral atmospheric conditions can be found in the full-scale database provided by RISØ-DTU. These characteristics convert Bolund into an ideal case for wind tunnel modeling without considering flow stratification effects.The escarpment facing westerly winds is one of the main geometric characteristics of Bolund (see Figure 1). The escarpment height varies slightly for the 200 • to 295 • wind direction interval, being roughly equal to the maximum height of the island, h = 11.73 m. Flow detachment at the escarpment has been observed in the full-scale experiment 1,7 and in wind tunnel tests. 4,8 Reproducing the flow characteristics in this region has revealed to be a challenge, particularly at lower heights, where the interaction between the turbulent inflow and the detachment dynamics is highly complex. Most of the numerical and physical models have failed to reproduce the high reduction in the mean wind speed and the high increment in the turbulent kinetic energy (TKE) found in the full-scale experiment in met mast M6 at 2 m height for 270 • wind direction, see Figure 2. ...

Effect of Topography Truncation on Experimental Simulation of Flow over Complex Terrain

“…According to spectra of measurement locations in the valley region, one of the distinct differences is the shift of the spectra peak toward the high-frequency range and a reduction in the low-frequency range in all cases. Strong flow separation in the valley region may account for this difference, and another peak at high-frequency might support this explanation, which is associated with a vortex-shedding process that was also measured in the wake region of an escarpment by Alvaro et al [37]. In the case of OT, owing to the strong perturbation from complete terrain, spectra at high frequency are higher than those in the other three cases.…”

“…To quantify the shift of energy for spectra in frequency, three normalized frequencies [37]: f l , f m, and f u , which are the cut-off frequencies of the different energy intervals, can be calculated from Equations ( 6) and (7):…”

Multi-scale modeling of a wind turbine wake in complex terrain

“…Met mast steep topography can also lead to high flow slope angles and high turbulence regions (even flow recirculation areas) [4,5,6], which may cause an increase in wind turbine loading and power variability if the wind turbine location is not carefully decided. The topology of the flow in these regions is affected not only by the topography but also by the stability conditions of the atmosphere [4].…”