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Improvement in Vehicle Motion Performance by Suppression of Aerodynamic Load Fluctuations
Abstract:
This study focuses on fluctuations in the aerodynamic load acting on a hatchback car model under steady-state conditions, which can lead to degeneration of vehicle motion performance due to excitation of vehicle vibrations. Large eddy simulations were first conducted on a vehicle model based on a production hatchback car with and without additional aerodynamic devices that had received good subjective assessments by drivers. The numerical results showed that the magnitudes of the lateral load fluctuations were larger without the devices at Strouhal numbers less than approximately 0.1, where surface pressure fluctuations indicated a negative correlation between the two sides of the rear end, which could give rise to yawing and rolling vibrations.
Based on the numerical results, wind-tunnel tests were performed with a 28%-scale hatchback car model. An antisymmetric flow structure with weak periodicity became apparent from simultaneous measurements of the streamwise velocity fluctuations in the near-wake region and the time-dependent yawing moment. Delta-winglet vortex generators (VGs), which have been widely adopted as separation control devices, mounted on each side of the rear end provided a marked reduction in lateral load fluctuations without a pronounced effect on the time-averaged longitudinal load, although the negative correlation was still prominent.
The delta VGs were then applied to an actual hatchback car and resulted in higher subjective ratings during a track test. This is a promising result that supports the hypothesis that suppression of aerodynamic load fluctuations improves vehicle motion performance.