Aerodynamic Characterisation of a Compact Car Driving Behind a Heavy Vehicle

Abstract

The transient incoming flow a compact car experiences whilst driving 10 m – 100 m behind a heavy vehicle on a runway has been characterised. The incoming flow was measured using a 2D array of 11 five-hole probes mounted 1 m in front of an operational, full-scale compact car. Additionally, 188 surface pressure taps were used to measure the effect of the incoming flow conditions on the compact car. The experiments were performed under ideal conditions on a 2.9 km long runway in Faßberg near the DLR in Trauen, Germany. The experiments showed that the heavy vehicle and the distance between the two vehicles have a significant impact on the flow characteristics: with decreasing distances between the vehicles the fluctuations increase from ±0.5 m/s to ±6 m/s and exhibit coherent dominant frequencies at 2.5 Hz in the spanwise direction that are attributed to the vortex shedding in the wake of the heavy vehicle. The turbulence intensities increase from 1% to 25% while the length scales decrease from 25 m to 0.25 m. The change in the incoming flow has a demonstrable influence on the surface pressure of the compact car. The pressure coefficient fluctuates by up to ±0.6 and cp, rms increases from 5% to 30%. The effect of the heavy vehicle on the pressure distribution indicates that the global forces acting on the compact car are influenced.