A Novel Technique to Determine Free-Stream Velocity from Ground-Based Anemometric Measurements During Track Tests

A novel method was developed to predict the free-stream velocity experienced by a traveling vehicle based on track-side anemometric measurements. The end objective of this research was to enhance the reliability of the prediction of free-stream conditions in order to improve the accuracy of aerodynamic drag coefficient (C_D) assessments from track tests of surface vehicles. Although the technique was applied to heavy-duty vehicles in the present work, it is equally applicable to any vehicle type. The proposed method is based on Taylor’s hypothesis, a principle applied in fluid mechanics to convert temporal signals into the spatial domain. It considers that the turbulent wind velocity fluctuations measured at one point are due to the "passage of an unchanging pattern of turbulent motion over the point". The method is applied to predict the wind velocity that the vehicle will experience as it encounters a wind pattern detected earlier by an anemometer located upwind. The proposed technique has the potential to enhance the accuracy of free-stream velocity predictions aimed at calibrating a vehicle-mounted anemometer. A sensitivity analysis highlights the errors on the predicted free-stream speed and yaw angle that result from errors in the evaluation of the terrestrial wind speed and direction. Moreover, considering that a surface vehicle travels through the atmospheric boundary layer, it is demonstrated that using track-based anemometers to calibrate a vehicle-mounted anemometer installed at a different height leads to incoherency and can cause substantial errors in the evaluation of free-stream conditions. It is recommended that all ground-based anemometers be installed at the same reference height relative to local ground, as the vehicle-mounted anemometer.