Wind Tunnel Testing
Aerodynamic development is done almost entirely experimentally, guided by empiricism based on the experience. The primary list facility is the wind tunnel. Wind tunnels, in the broadest sense, try to simulate the aerodynamic conditions that a vehicle would experience in its operating environment. However, the aerodynamics generates in a wind tunnel is not identical, but merely equivalent (to some degree) to the aerodynamic experienced “on-road”. For example, in a wind tunnel, both the far-field boundary and the ground boundary are represented in some convenient and yet useful manner.
Wind Tunnel Testing Procedure
Likewise, the test-section flow velocity (around a stationary vehicle) attempts to re-create the relative motion between a moving vehicle and the surrounding (stagnant) environment. Where differences exist between wind tunnel and road, it is important to know how they influence the results that rate generated by wind tunnel testing. This has been extensively studied for many years and continues to be an area of enduring interest. It has resulted in many useful ways of dealing with the lack of their replication of on-road conditions.
History of Wind Tunnel Testing
Historically, the wind tunnel testing of automobiles started with small-scale models. Scales like 1:4 or 1:5 were preferred in Europe, the somewhat larger 3/8 scale in the U.S. The advantages of small-scale testing are that the models are cheaper than full- scale ones, are easy to handle, and can be quickly modified. Furthermore, one small wind tunnels are needed, and these are more generally available and can be rented at moderate cost.
For two reasons, small-scale testing was eventually only rarely used. At first, this was because test results from partial-scale models very often did not reproduce full-scale values with the accuracy needed. This deficiency was partly due to a lack of geometric similarity in the models, and partly to the unpredictable effects of Reynolds number.
Wind Tunnel Testing of Vehicles
However, geometric similarity is not a fundamental problem but rather a matter of the skill and care of the model maker. Also a Reynold-number gap on the order of two can be bridged by artificially increasing the turbulence level of the wind-tunnel. Consequently, small-scale testing has again come into favour and is used by some car manufacturers with great success.
The second and even stronger objection to small-scale testing is non-aerodynamic in nature. Vehicle exterior design is done in full-scale, because shapes in small scale cannot be adequately assessed aesthetically. Therefore, a full scale model always exists, and if it is built on a realistic chassis, such as the one from the preceding model year, it can also be used as the wind-tunnel model.
The aerodynamic forces and moments experienced by a vehicle in its operating condition are often of crucial interest to aerodynamicists. Force and moment balances have served as the most direct tool for determining these qualities. Much progress has been made in minimizing their sources of error. Moreover, the motivation to understand the causes of dynamic loads and to create more opportunities for the diagnosis of flow conditions has led to significant advances in the kinds of tools that are available.
Wind Tunnel Testing Software
These tools, many of them derived from computer-based devices and methods, have helped shed light on many previously unknown flow phenomena, both local and global. The plethora of these tools ranges from simple subjective flow visualization to very complex quantitative measurement systems. They have been used in applications for both external and internal flow situations.