F R 2 - 9 7

Home | Update | Latest Issue | Gallery | FR Profile | Datafiles | FR 2/97 WIND TUNNELS STILL PLAY VITAL ROLES IN RESEARCH by Heinrich Hemker Wind tunnels are still essential tools for aerodynamicists and aircraft designers. They have three different tasks: First, they help to reveal certain physical phenomena. Second is the generation of experimental data to validate numeric procedures and, third, is the finding of performance data in the development of new aircraft. The role of the wind tunnel has changed in the past few years due to the rapid development of numeric methods of calculation. Still, CFD calculations (computational fluid dynamics) have not rendered the tunnels superfluous. "Numeric computations allow a preliminary selection before going into the wind tunnel, such using the tunnel trials more concentrated in the development of aircraft", explains Dr. Günter Redeker, head of DLR's wind tunnel division. Wind tunnels are differentiated by the speed regions in which the models are tested. The renown DNW (German-Dutch Wind Tunnel) in Nordoostpolder and NASA's tunnel at the Ames Research Center in California, with 24 by 36 meters cross section the world's largest wind tunnel, are low speed tunnels. The second category comprises of the transonic wind tunnels. The European Transonic Windtunnel in Cologne, Germany, is one of the most efficient ones. With a cross-section of 2 by 2,4 meters the tunnel allows a realistic simulation of cruise conditions for modern transport aircraft. The USA has a similar equipment available. However, the so called NTF (national transonic facility) belonging to NASA at Langley is only accessible to US customers. DLR's wind tunnel in Göttingen, Germany, is covering the Mach number region from 0,5 to 2,2 and is provided with the worldwide largest adaptive test section for minimizing the so called wall interferences. The adaptive horizontal walls of the test section provide the ability to adapt the course of flow as much as possible to free flight conditions. Beyond the transonic wind tunnel category are the supersonic and hypersonic tunnels, most of which are designed as blown down tunnels. Unlike the standard tunnels, in which air is blown in a closed circle, super/hypersonic tunnels are connected to a pressure reservoir. Once the valve is opened the air is flowing through the test section into an accumulator or into the open. Even though the measuring times are in the region of only seconds, modern test methods allow the recording of all required data in this short period of time. A wind tunnel must fulfill two similarity parameters in order to generate results that can be transferred into real life. First is the Mach number, the relation between flying speed and the speed of sound. The tunnel model's Mach number must comply to the air stream in real life. The second similarity parameter is the so called Reynolds number which is decisive to determine the drag of an aircraft. The number should be the same for model and real airframe. Since the Reynolds number depends very much on the aircraft size and wind tunnel models are most often rather small, this goal is only difficult to achieve in reality. New measurement methods have been developed in the past few years to facilitate the evaluation of data gained in wind tunnel tests, the most modern development being the pressure sensitive paint. The model is covered with a special paint which changes its colour depending on the oxygen concentration. This allows the possibility to precisely make surface pressures visible. Currently there are approximately 50 larger wind tunnels in Europe, too many to be used to full capacity by the aerospace industry. Due to this fact, DLR is looking for an economical solution, for example by incorporating further tunnels into the DNW association. Furthermore, a special working group of the European AEREA group (Association of European Research Establishment of Aeronautics) is working on a solution. Xavier Bouis, Director of the French ONERA, is not excluding the future shutdown of wind tunnels. Of course, this is a highly sensitive topic since no country likes to lose such an highly valuable research instrument. The USA is one step ahead. Blair Gloss, within NASA responsible for wind tunnels, stated that the number of wind tunnels will decrease from 83 in 1993 to 58 in 1996. Assuming a similar overcapacity for Europe, the European research institute is facing a painful consolidation process. From page 56 of FLUG REVUE 2/97 Home | Update | Latest Issue | Gallery | FR Profile | Datafiles | FR 2/97 Copyright 1997 by Motor-Presse Stuttgart. All rights reserved. Last updated January 10, 1997