A Theoretical Approach towards the Self-Correcting Open Jet Wind Tunnel 2014-01-0579

Open jet wind tunnels are normally tuned to measure “correct” results without any modifications to the raw data. This is an important difference to closed wall wind tunnels, which usually require wind tunnel corrections. The tuning of open jet facilities is typically done experimentally using pilot tunnels and adding final adjustments in the commissioning phase of the full scale tunnel. This approach lacked theoretical background in the past. There is still a common belief outside the small group of people designing and using open jet wind tunnels, that - similar to closed wind tunnels, which generally measure too high aerodynamic forces and moments without correction - open jet wind tunnels measure coefficient too low compared to the real world. The paper will try to show that there is a solid physical foundation underlying the experimental approach and that the expectation to receive self-correcting behavior can be supported by theoretical models.

During the past years an improved understanding of test section interference in open jet wind tunnels has been developed. In addition there is a growing number of new automotive open jet wind tunnels, making use of several specific advantages of this design for development work in the car industry. Therefore, in this paper an attempt is made to develop design criteria for open jet wind tunnels to systematically approach a self-correcting behavior of the test section, based on classical theory for the prediction of test section interference.

In order to achieve a balance between different blockage effects which is independent (or only weakly dependent) of the characteristics of the test object, it is important to set up the balance in an appropriate way. Test section interference effects should be balanced against each other in a way that parameters increasing the interference act on the two counterparts in a similar magnitude. For example, effects related to solid blockage should be balanced against each other and gradient effects should be treated in a second group. The approach of this paper results in design charts for the distance between the nozzle and the model and design strategies to cope with the influence of the collector. Finally, an ideal open jet wind tunnel with reliably self-correcting behavior is outlined.