Abstract
The objective of this investigation is to study possible means for reducing the base drag of a tractor-trailer. The experiments are conducted in the Dryden wind tunnel at the USC Ground Vehicle Aerodynamics Laboratory. A roughly 1/15 scale model resembling a trailer is utilized for the study. The model is fitted with a shaped nose-piece to ensure attached flow over the forward portion of the model. The model is equipped with a force balance to measure drag. In addition base pressures are measured, and hot-wire wake surveys are conducted downstream from the model base. The Reynolds numbers (based on the square-root of the model cross-sectional area), range from 0.1 × 106 to 0.4 × 106.
Drag reduction is effected by means of flaps attached along the edges of the model base, and inclined inward to decrease the size of the downstream wake. In addition, an oscillatory perturbation is applied at the flap origin in an attempt to maintain attached flow for larger angles of flap inclination.
The present study has found that a simple, passive base-flap deflection—no forcing whatsoever—produces significant drag saving. The maximum drag reduction is 0.06 – 0.08 at an angle of 9–10 degrees. The magnitude of the saving is in accord with both early and recent measurements in other laboratories.
The present results also show that oscillatory momentum addition has little effect on drag reduction unless the net oscillatory momentum flux coefficient is equal or greater than 0.1%. Increasing the oscillatory momentum perturbation to a coefficient value of 0.3% produces drag savings at angles greater than 9–10 degrees, but has very little effect upon the maximum saving at 9–10 degrees.
To further study this anomalous behavior, follow-on experiments are planned to investigate a larger range of forcing amplitudes, and a variety forcing-function duty cycles. In addition, Digital Particle Image Velocimetry will be used to capture the detailed flow-field in the vicinity of the flap.
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Hsu, TY., Hammache, M., Browand, F. (2004). Base Flaps and Oscillatory Perturbations to Decrease Base Drag. In: McCallen, R., Browand, F., Ross, J. (eds) The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains. Lecture Notes in Applied and Computational Mechanics, vol 19. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-44419-0_27
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DOI: https://doi.org/10.1007/978-3-540-44419-0_27
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