Modelling of Transient On-Road Conditions in a Closed Test-Section Wind Tunnel with Active Flaps

Abstract

There is a high interest to improve the static approach of wind tunnel testing by simulating realistic transient flow conditions. To determine these conditions, different driving maneuvers (driving behind, passing) were conducted in previous experiments on a runway. The transient, incoming flow and the vehicle’s surface pressure were measured. To create the realistic inflow conditions in a wind tunnel, a new system of four symmetric airfoils with active flaps was developed for the Side-Wind Facility Göttingen (SWG), a closed loop, closed test-section facility with a moving belt and suction system. The airfoils heights are half the test section’s height and the trailing third of their chords are active flaps. The reduced inertia - resulting from only one third of the airfoil moving - allows for individual high-speed, rotational movements of the flap at with up to 50 Hz and angles of up to ±20°. Time-resolved velocity component measurements in the empty test section were conducted with an array of eleven 5-hole probes connected to an ESP64HD pressure transducer system for various flap configurations. These measurements agreed well with the turbulence intensities, length scales and transient yaw angles from the on-road measurements. Measurements with a 1:4 scaled VW Golf VII model were conducted with four internal, piezo-electric force transducers and 128 surface pressure taps with the aforementioned ESP64HD pressure system. They showed that influences of different transient inflow conditions from the on-road experiments are observable in transient forces and surface-pressure. Increasing fluctuations of drag coefficient in the range of ±2% were found when simulating the actual on-road signal in the wind-tunnel of closing the gap during the passing maneuver. Up to 3.5% were observed while actually passing the other vehicle was simulated. The authors thank the members of The Research Association of Automotive Technology (FAT) for their support of the project.