Ground proximity simulation addresses a number of phenomena experienced by objects that move on or near the ground. The presence of the ground has an important impact on the behavior of the flow over the object and consequently the aerodynamic forces. Dependent on the distance from the ground, the air circulation generated around the object is influenced. Whereas in reality the object moves in relation to the air and the ground, this is reversed in the wind tunnel where the wind moves in relation to the object and the floor. Consequently, a boundary layer is developed along the tunnel floor (and the other tunnel walls), which is not representative for the real situation. For a good simulation of the effects of ground proximity, the following experimental aspects are therefore of prime importance:
- removal of the boundary layer on the tunnel floor and
- movement of the tunnel floor in relation to the object
To minimize the effects of the floor boundary layer, DNW uses a ground plane mounted on the tunnel floor in combination with an integrated scoop system. The movement of the ground can be simulated by a moving belt system that is integrated in the ground plane. Moving belt systems are also often referred to as rolling road systems.
Floor boundary layer
In ground simulation testing, the effect of the floor boundary layer must be eliminated to the extent that it does not interfere with the flow around the test object. The thickness of the boundary layer in the wind tunnel varies and depends on the location and the wind speed; the layer is thicker for lower wind speeds and grows with distance.
One of the techniques used to reduce the effect of the boundary layer is the use of a ground plane below the object. The ground plane is mounted onto the tunnel floor at an elevation higher than the thickness of the tunnel wall boundary layer and is used in combination with an integrated special scoop system. This system scoops away the boundary layer that was built-up on the tunnel floor upstream of the ground plane in order to avoid a change in flow direction at the stagnation point of the ground plane. A new boundary layer will start growing on the ground plane, but it will be significantly thinner than the tunnel wall boundary layer, and therefore its effect on the measurement results will be much weaker.
The thickness of the tunnel floor boundary layer can also be reduced by tangential blowing. This is done by means of high pressure air blown along the floor of the tunnel or along the ground plane, such that the momentum loss in the boundary layer in the area close to the test object is compensated.
Moving belt ground plane
The moving belt ground plane is a technique specially adapted for use in aeronautical testing by DNW. The plane consists of a belt that can move at the speed of the air in the tunnel. The moving belt is fully integrated in the DNW ground plane/scoop system. DNW’s integration of the moving belt ground plane is rather unique and has resulted in a belt system that can move synchronously with the air at and up to high wind speeds of 80 m/s. The belt stays flat at all operational conditions, enabling a perfect simulation of the conditions encountered by an aircraft above the runway.
The moving belt ground plane technique is of special importance in conducting tests on the landing and take-off of aircraft. The gliding, the powered and unpowered conditions and the thrust reversal conditions can all be tested above the moving belt simulation of the runway.
The moving belt is also used for measurements of aerodynamic effects experienced by ground vehicles. See also this publication.