The floor boundary layer that builds up from the contraction section of the tunnel is a consideration for automotive testing and can be corrected / removed using a Boundary Layer Removal System (BLRS).  The BLRS draws air from the front of the test section to improve the quality of airflow beneath an automotive model.

A porous plate can be installed in the test section floor, ahead of the Turntable, with a chamber below the plate and ductwork connected to a BLRS fan (or two fans).  The fan(s) can have a variable speed drive to adjust the amount of suction and this will allow the system to be varied to achieve optimum conditions.  Extracted air from the BLRS can be routed above the test section and directed back into the circuit in the roof of the test section diffuser.  The re-entry can be configured for minimal flow disturbance and the breather gap at the rear of the test section will balance the flow in the circuit.