Relation between the turbulent intensity on the crest of a ridge and wind direction relative to the ridge line–Part 2: Wind tunnel experiments

This paper is one of the series describing the relation between the turbulence on the crest of ridges and wind direction / ridge orientation angle. In the present paper, we studied the flow over reduced model of the eastern ridge of Mt. Hyonosen in the wind tunnel. The reduced model experiments were compared with the field measurements which were analyzed in the paper of Part 1.

The experiments were carried out in the atmospheric boundary-layer wind tunnel of our laboratory. The length, width and height of the working section are 3 m × 0.3 m × 0.3 m. A neutral boundary layer was simulated in the wind tunnel. The generated boundary layer was 80 mm high at the reference location (upwind of the model ridge) and had a power low vertical wind profile with p = 0.24. The value of turbulent intensity in the longitudinal direction is 0.17 and at the reference point of 5 mm high above the towel (20 m in full scale). The surface roughness parameter was 0.3 mm at the reference location. The upstream wind speed outside the boundary layer was set at 5.4 m/s. The Reynolds Number Re was 29,000.

The reduced model ridge for the experiment ranged between 700 to 794 m altitudes. The model scale of the eastern ridge of Mt. Hyonosen was 1:4430. The beads were glued and had a nearest-neighbour separation of 3 mm.

Results of experiments are as follows:

The directional profile of the turbulent intensity was opposite to that of the average wind speed. That is, the turbulent intensity was larger for winds blowing parallel to the ridge line as compared to those blowing perpendicular to it. While the average wind speed was larger for winds blowing perpendicular to the ridge line as compared to those blowing parallel to it, which was the same results as discussed before. This is because, when wind blows perpendicular to the ridge line, the airflow converges along the upwind slope. The wind tunnel experiments agreed with the field measurements in that, the turbulence was larger for winds parallel to the ridge line as compared to that perpendicular to it. The variation in turbulent intensity in the longitudinal direction with respect to the wind direction decreased with an increase in the height.