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Pad Correction Estimation around 5 Belt Wind Tunnel Wheel Belts Using Pressure Tap Measurement and Mathematical Pressure Distribution Model
Abstract:
5 belt wind tunnels are the most common facility to conduct the experimental aerodynamics development for production cars. Among aerodynamic properties, usually drag is the most important development target, but lift force and its front/rear balance is also important for vehicle dynamics. Related to the lift measurement, it is known that the “pad correction”, the correction in the lift measurement values for the undesirable aerodynamic force acting on wheel belt surface around the tire contact patch, must be accounted.
Due to the pad correction measurement difficulties, it is common to simply subtract a fixed amount of lift values from measured lift force. However, this method is obviously not perfect as the pad corrections are different for differing vehicle body shapes, aerodynamic configurations, tire sizes and shapes. As an alternative, CFD is a possible solution to estimate pad correction, but it is still not practical to calculate all cases of the development due to the calculation cost and time.
One existing wind tunnel test measurement procedure is to utilize special arrangements in the car restraint apparatus. But this method requires a preparation test apart from the actual aerodynamic test run, which is not ideal in terms of wind tunnel test productivity.
In this paper, a new method to estimate the pad correction for 5 belt wind tunnel is proposed, introducing a newly developed mathematical model of pressure distribution on the belt surface. With this model, it is possible to estimate pad correction values without any other preparation test.
In order to develop this method, CFD analysis with tire rotation condition was made for 10 cases/6 vehicles. For the validation of CFD itself, these CFD results were compared with the pressure distribution data measured in a single belt wind tunnel.
The pressure distribution model was created ensuring its mathematically generated distribution can reproduce 10 cases of CFD results with a unified expression, by changing its 68 parameters. Also, these parameters were reduced to 7, in order to enable parameters estimation by multiple linear regression (MLR) from 42 pressure tap point data around the 5 belt wind tunnel wheel belt.
For the verification of the method, CFD results were used. The pad correction values were estimated from 42 pressure tap point data taken from the CFD results, and compared with directly calculated pad correction values from the same CFD results.
The verification of the estimation accuracy showed within error variation widths of 0.007 ~ 0.008 (widths between maximum and minimum errors) of pad correction for CLfront or CLrear correction values. Also, the error variation widths for the same car were 0.001 ~ 0.003 for CLfront pad correction, which is enough to predict its configuration difference trend.