A boundary layer consists of a layer of “dead air” that is dragged along by a moving object. In a testing situation, the effects of this layer must be eliminated so that it does not interfere with the measurement results.

 

The height of the boundary layer in a testing situation varies, and depends on the size of the model and the wind speed. Its height can be measured by means of probes, either individually or as a collection of probes positioned in a rake.

 

Because the boundary layer is thicker for lower wind speeds, it plays a particularly important role in DNW’s Low-Speed tunnels in the Noordoostpolder and Braunschweig.

 

One of the techniques used to eliminate the effects of the boundary layer is a special scoop system. The scoop system trips the air and allows a new boundary layer to start only at the lip ofthe ground plane. Comparison tests have demonstrated that the effect of the wind behind the scoop edge remains within the overall measurement accuracy.

 

The effects of a boundary layer can also be reduced by means of tangential blowing. This is done by means of air being blown along the floor of the tunnel, which minimizes the boundary layer in the area closest to the test object.