The results from wind tunnels are more accurate the larger the car model. Most F1 tunnels used to use half scale models, now new state of the art tunnels use full scale cars. This is actually more economical in one way - expensive and accurate scale models do not have to be made - but the tunnels are large and expensive to build and their fans consume a great deal of power.

Wind tunnels are large because of an effect called choking. If the model occupies more than about 5% of the working area of the tunnel the flow close to the tunnel walls is affected and the results are inaccurate. Therefore if a car has a frontal area of 2m² the tunnel needs to have a working area cross section of 40m² or perhaps 8m x 5m.
A recirculating type tunnel of this size would be very expensive indeed.

An important consideration when operating a wind tunnel is that the airflow must travel at the same speed across the working part of the tunnel. This requires careful measurement and adjustment of guide vanes within the tunnel, while the tunnel is being set up. Such measurements need to be made periodically to ensure that the tunnel operates consistently.

If the model was placed on a stationary floor the flow patterns around it would be distorted by the development of a boundary layer, which would affect the results obtained. In order to avoid this the section of floor beneath the model can move in the direction of the airflow, its speed is then controlled to match that of the air. With no relative movement no boundary layer develops.

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