In this paper the empty tunnel pressure gradient created by the use of boundary-layer suction and its effect on measured drag have been investigated. By using excess suction - more suction than necessary to remove the original boundary layer - it was possible to show experimentally, that the major part of the drag increase associated with boundary-layer suction is created by unintended gradient effects. Only a minor part of the drag increase is due to higher flow velocities at the tower parts of the vehicle, or in ether words, due to the improved ground simulation. These findings are qualitatively supported by CFD calculations, where - by using artificial boundary conditions - the effects of the pressure gradient and of the improved ground simulation can be separated. The unintended effect of the pressure gradient was nearly an order of magnitude higher than the effect of improved ground simulation in the CFD cases. Experiments with a variation of the distance between model and suction area further support the findings given. The effect of boundary-layer suction on lift was studied as well. In this case the influence of the improved ground simulation is bigger than the unintended effect of - in this case - flow angularity due to boundary-layer suction. The vertical pressure gradient induced by suction seems to be not of practical importance in the case with suction only in front of the vehicle. To improve the understanding of the effects of boundary-layer suction, an analytical model of the suction system was developed using potential flow theory to predict the flow properties. By using the concept of horizontal buoyancy the drag increase due to boundary-layer suction can be predicted with reasonable accuracy using the pressure gradient obtained experimentally or from the potential flow model. As was demonstrated in the paper, the use of boundary-layer suction induces unintended pressure gradients and flow angularities, which can significantly affect the measured results in terms of drag and lift. Therefore boundary-layer suction can no longer be generally recommended for the improvement of ground simulation as best practice for boundary layer control. Tangential blowing or long moving belts should be used instead or in combination with suction systems. In presence of ground simulation the empty tunnel gradient should still meet the same requirements as without. On the other hand, the use of boundary-layer suction is a clear advantage compared to the situation without any boundary layer control. As can be seen in this paper, ground simulation effects can be highly non-linear, whereas the gradient effects are fairly linear and predictable. Therefore a suction system is still a significant improvement of the ground simulation, although a tangential-blowing device probably creates less interference effects at the model. To compensate the interference effect on drag, the horizontal buoyancy correction has proven to be a useful tool.
Gradient effects on drag due to boundary-layer suction in automotive wind tunnels
Grenzschichtabsaugung in Fahrzeug-Windkanälen und Einfluß von Druckgradienten auf den Strömungswiderstand
2003
17 Seiten, 16 Bilder, 4 Tabellen, 19 Quellen
Aufsatz (Konferenz)
Englisch
Gradient Effects on Drag Due to Boundary-Layer Suction in Automotive Wind Tunnels
SAE Technical Papers | 2003
|Gradient effects on drag due to boundary-layer suction in automotive wind tunnels
Kraftfahrwesen | 2003
|2003-01-0655 Gradient Effects on Drag Due to Boundary-Layer Suction in Automotive Wind Tunnels
British Library Conference Proceedings | 2003
|Optimum Drag Balance for Boundary-Layer Suction
Online Contents | 1996
|Optimum drag balance for boundary-layer suction
AIAA | 1996
|