Aerodynamics is closely associated with the design of a vehicle. Aerodynamic investigations can be undertaken on models in original size or models scaled down upto a ratio of 1:5. Air or water can be used as the flow medium.
Invisible flows over the vehicle body can be made more visible using water as the medium. Models of 1:4 or 1:5 are often used here. Using a wire introduced into the flowing water, tiny bubbles are formed as a result of electrolysis.These bubbles can be used to make the flow visible. In the figure, the flow reaches only upto the lower edge of the frame of the windshield, since only the flows around the passenger cell are of significance. Due to the physical characteristics of water, aerodynamic processes can be covered right from a flow velocity of 1 m/s.
By conducting investigations in the watertunnel on the E-Class, Mercedes Benz could optimise the underbody, the form of the boot cover and the rear windshield. Mini spoilers on the wheel housings are the result of these tests.
In order to stay as close to reality as possible, models both in original size as well as scaled-down models are investigates in the climate windtunnel.
Identical Flows
The mathematical description of flow processes is connected to the solution of complicated differential equations. A flow is completely described if the velocity vector and the thermodynamic quantities such as pressure, density and temperature are known at all points. Thus six quantities - three velocity components and three thermodynamic variables – have to be determined. Therefore the laws of conservation of mass, impulse and energy as well as the equations of state, which link the thermodynamic variables together, need to be considered. These are indicated in the form of partial differential equations, the solution of which requires the knowledge of initial and boundary conditions. Generally, this is not possible due to the complexity of mathematical solution. Therefore, flow processes are often experimentally determined.
In wind tunnels, aerodynamic investigations are usually executed on scaled-down models since the constructionof a 1:1 model is expensive. Using the theory of similarity, these test results of the scaled models will be transferred to the main model.
Similarity theory implies that two flows are similar if both geometrical and dynamic similarities are maintained. Geometric similarity implies that the length proportions of the model correspond to that of the real vehicle. Dynamic similarity requires that the dimensionless characteristic numbers are equal. This implies that in the windtunnel, the Reynold's Number and Mach Number must match.
The Reynolds number indicates the relation between inertia and frictional forces. If this number is very large, the friction forces can be neglected. Similarly with very small Reynolds numbers the forces of inertia can be neglected.
The Mach number (Ma) is a measure of the influence of compressibility on flow. It indicates the relation between the fluid flow rate and the speed of sound. As a thumb rule, it is considered that the compressibility of air may be neglected for flows up to Ma < 0.3.
In model tests, a compliance to the laws of similarity is often associated with difficulties. In order to achieve the same Reynolds number, when air is used as a flow medium, the incident flow velocity, with the same material values, must be scaled by the reciprocal value of the model scale. Since the model is smaller than the prototype, the incident flow velocity has to be much higher. Due to a large solid mass and also due to a high degree of curvature of the outer profile, high speeds occur at vehicles locally. However at very high speeds, the flow becomes incompressible and this affects the dynamic similarity.
 
 
Um die Fahrbahn möglichst perfekt darzustellen, wurde in der Vergangenheit das Spiegelbild-Verfahren angewandt. Dazu werden zwei gleiche Modelle spiegelbildlich, mit den Rädern zueinander aufgehängt. Die sich zwischen den Modellen bildende, ebene Stromfläche wird als feste Wand gedeutet. Die Annahme die Spiegelebene sei eine Symmetriebene der Strömung und könne die Fahrbahn voll ersetzen erwies sich als problematisch. Der Grund dafür ist die Wirbelbildung hinter dem Fahrzeug, die diese Symmetrieebene durchbricht und somit von den realen Bedingungen abweicht. Sie beeinflußt die Umströmung und damit den Modellwiderstand. Eine im hinteren Bereich zwischen den Modellen angebrachte Platte kann diese Wirbel teilweise unterbinden. Zwei weitere Nachteile begleiten das Verfahren. Es werden zwei Modelle und ein Windkanal doppelter Größe benötigt. Das Spiegelbild-Verfahren ist heute nicht mehr in Gebrauch.