Bolund is a 12 m high, 130 m long and 75 m wide isolated hill situated to the North of RisøDTU in Roskilde Fjord, Denmark. It is surrounded by water in all directions except to the E, where a narrow isthmus leads to the mainland. The hill is characterized by a uniform roughness of 0.015 m and surrounded by water with a roughness length of 0.0003 m. An almost vertical escarpment in the prevailing W-SW sector ensures flow separation in the windward edge resulting in a complex flow field, quite challenging for flow models.
The masts are positioned along two lines: A and B. Two additional masts (M0 and M9) were installed to measure the incoming undisturbed flow for westerly and easterly winds respectively. Mast M9 is placed in the coastline, where the roughness length is again 0.015 m. The masts are equipped with 23 sonic (Metek USA 1-Basic) and 12 cup anemometers (Risø Wind Sensor P2546) at heights between 2 and 15 m.
The Bolund experiment comprises a measurement campaign of three months between 2007 and 2008 (Bechmann et al., 2009, Berg et al., 2011). During the measurement campaign the absolute water level was monitored, which covered the isthmus most of the time. The campaign was designed for W-SW winds where the fetch ranges 4 to 7 km, ensuring undisturbed velocity profiles over water. The E sector is more difficult to characterize due to a more the heterogeneous land cover. The prevailing stability regime was neutral to slightly stable conditions (1/L < 0.04). The data was averaged over 30 min periods.
A blind test experiment was conducted in 2009 consisting on the simulation of four wind direction cases (270º, 255º, 239º and 90º) with prescribed boundary conditions of neutral flow (Bechmann et al., 2011).
In total 49 different simulations were submitted, composed of 3 physical models, 9 linearized numerical models and 37 CFD models (5 LES, 7 RANS 1-equation and 25 RANS 2-equation). The physical models predicted reasonably well the mean velocity profiles but under-predicted the turbulent kinetic energy. Linear models produced the worse results as they were not capable of reproducing the flow around the steep escarpment. RANS models provided the best results although the spread of the simulations was quite big, indicating user dependencies especially regarding mesh generation. LES-based models had problems but presented promising results with regard to turbulence modelling in the flow separation area just after the escarpment. The average error in the simulation of the mean wind speeds over all the measurement locations was 13-17% in the top 10 best models. This error dropped to 4-10% if only the measurements at 5 m were considered.
Bechmann A., Berg J., Courtney M.S., Jørgensen H.E., Mann J. and Sørensen N.N., 2009, The Bolund Experiment: Overview and Background. Technical Report Risø-R1658(EN), Risø-DTU National Laboratory, Technical University of Denmark
Bechmann A., Sørensen N.N., Berg J., Mann J. and Réthoré P.-E., 2011, The Bolund Experiment, Part II: Blind Comparison of Microscale Flow Models, Boundary Layer Meteorol. 141: 245-271
Berg J., Mann J., Bechmann A., Courtney M.S. and Jørgensen H.E., 2011, The Bolund Experiment, Part I: Flow over a steep, three-dimensional hill, Boundary-Layer Meteorol. 140:1-25