MANTENIMIENTO DE AERÓDROMOS

ment point

The terrain module which constitutes Case I-II was quite large. The terrain was made out of Styrofoam, and was mounted on a wooden plate. As time passed, the materials changed in different ways. As a result, the underside of the wooden plate became slightly curved. It was attempted to force the large module into the original shape by fixing it to the floor in the wind tunnel with large screws. This was only partly successful. As the main focus in the flow above such terrain often is at the region near the crest, this part was mounted as horizontal as possible. Hence, the largest deviations from the original shape and the theoretical terrain model was at the end of the plateaus in Case I and II. This can have affected the flow by introducing slightly increased vertical velocities above the plateau, but the effect is considered to be unimportant compared to the other and much more significant effects of the terrain in this study.

In general the positioning of the probe was a challenge, as the probe was mounted on a traversing system in the centre of the test section, with few references. When the probe was positioned above the measurement point (seeing the xy-plane), a level could be used to confirm that the probe was normal to the xy-plane. The laser beam was then also seen at the surface, making it easy to find the correct position. When mounting the probe at the side of the measurement point (seeing the xz-plane), no reference was present except the walls in the tunnel. Different methods had to be

B.3. Positioning of terrain model and measurement point

applied to find the correct position above ground, dependent on the topography near the measured position. In some locations, the position of the measurement volume in the vertical direction had to be adjusted by the help of equipment like a folding rule. In some other positions, with the probe tilted towards the ground, the zero-level was set at the point of maximum reflections from the ground and then adjusted to the first measuring height by moving the traverse. The surface also often had some unevenness, complicating the determination of the correct vertical positioning of the measurement volume. After this positioning was done, the tilt of the probe in the vertical plane could be measured using a level. The required tilt in the horizontal plane used in a few measurement positions was difficult to quantify.

The LDA probe was used both with and without an expander. The size and weight increased significantly when adding the expander, hence altering both the natural frequency and the drag of the probe. The probe was always shaking to some extent during measurements, despite that the fixing device was strengthened with a supporting rack pushed against the surface in most of the measured positions. The supporting rack and the mounting of the probe are shown in Figure B.1. The shaking varied with the size of the probe, the flow in the region of measurements and whether or not is was possible to utilize the supporting rack. The positioning of the probe was done with zero velocity in the tunnel, so due to the probe drag there was always a probability that the measurement volume was slightly displaced when the tunnel started.

Altogether this gives a positioning uncertainty of the measurement point in the vertical direction of approximately ±1 mm. The corresponding horizontal uncer- tainty in the direction of the main flow is approximately ±2 mm. These positioning errors will have the largest impact near the ground and in regions with separated flow, where the spatial variation of the flow field is largest. The uncertainty in the spanwise direction is about ±1 mm. This is not of importance in Case I-IV, which are approximately two-dimensional.

The LDA probe was mounted on a traversing mechanism driven by stepper motors. This was used for moving the probe vertical during measurements of each profile. The accuracy of this traverse was estimated to be better than 0.1 mm. This is quite small, and considered to be included in the estimates given above for the positioning of the measurement point.

It is normally desired to make the LDA measurement volume as small as pos- sible to have a good spatial resolution. A small measurement volume is especially important in regions with large mean velocity gradients. Errors due to the size of the measurement volume in the current study were considered negligible compared to other uncertainties, such as the positioning of the measurement volume.

The model scale was chosen to be quite large just to reduce the error related to the measurement positions. It is important to notice that an error of 2 mm in model scale corresponds to only 2 m in full-scale, which is a relatively small distance in the field of atmospheric flows.

Appendix B. Quality of data in more detail

Figure B.1: Fixing device for the LDA equipment. This photo illustrates the probe with an expander, mounted on the traverse and with an additional supporting rack. The terrain is Case VIII.