Aerodynamic development of a full-scale truck presents a challenge for experimental testing due to the scale of the vehicle relative to existing wind-tunnel test facilities. Numerical simulation is becoming more prevalent for assessing design changes and improving vehicle aerodynamic drag. In this process, the cumulative effects of small design changes are needed. Furthermore, the drag must be considered at different crosswind yaw angles in order to properly represent typical driving conditions. It is well-known that the aerodynamics of heavy trucks are characterized by a highly transient wake flow that causes large fluctuations in base pressure, and therefore in the drag coefficient. These fluctuations are often dependant of the crosswind yaw angle. The transient wake flow presents a challenge for effectively using simulation tools to predict the drag effects of small design changes, which may have some influence on the wake flow and base pressure. In this study, long transient simulations are used to assess the variation in wake flow leading to drag fluctuations. This information is used to estimate the confidence interval for the drag coefficient prediction as a function of time-averaging period for a detailed Scania truck at 5° yaw angle and a generic truck shape at 3 different yaw angles. Analysis of the force and pressure time signals, as well as transient flow visualization and spectral analysis are used to identify the source of unsteadiness in the wake flow regions, and the impact of the unsteadiness on the surface pressure distribution. This study provides useful information on the range of certainty of drag values for future truck aerodynamics simulations, and facilitates using simulation to isolate the effects of small design changes from the inherent variability in the base pressure drag. Finally, improved understanding of the transient wake effects on the drag distribution leads to an adapted approach for using reduced time simulations for design optimization of the tractor unit.
Effects of unsteadiness on heavy truck aerodynamics
2012
13 Seiten, 12 Bilder, 1 Tabelle, 9 Quellen
Conference paper
English
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