v Velocity, l Characteristic length,
ν co-efficient of kinematic viscosity
In order to compare models in different scales and media, an identical Reynold's number has to be achieved, with which geometric similarity can be guaranteed. The variable l describes a characterictic length of the body to be compared, v is the velocity of flow. The co-efficient of kinematic viscosity is a material constant. It amounts to 0.14*10-4 for air and 0.01*10-4 for water.
In order to better describe the process, an example is used. The air resistance of a passenger car of length 4.2 m is to be investigated at a velocity of 30 m/s using a model. The Reynold's number here amounts to 9*105. The model scaled downed to 1:3 should produce the same Reynold's number both in air as well as water. The models have a length of 1.4 m, with the co-efficients of kinematic viscosities for air and water known. The 1:3 model should be subjected to a flow velocity of 90 m/s in air and 6.4 m/s in water.