An Experimental Study of the Generic Conventional Model (GCM) in the NASA Ames 7-by-10-Foot Wind Tunnel

Abstract

The 1/8-scale Generic Conventional Model (GCM) was studied experimentally in the NASA Ames 7- b 10-Ft Wind Tunnel. The model was designed for validation of computational fluid dynamics (CFD) for a realistic model of a conventional tractor-trailer vehicle with the engine in front of the cab. The model was simplified so that mirrors, exhaust stacks, flow through engine compartment, and other details were not defined on the model. The gap between the tractor and trailer could be varied from 40 to 80 inches full scale. The model was tested with and without wheels. Side and roof extenders were tested that enclosed the gap from 30 to 60 percent. Two trailer configurations (conventional and lowboy) were tested. Aerodynamic boattail plates were also tested on the back of the trailer for both configurations. A simplified configuration was also tested were the gap between the tractor and trailer was filled in. Experimental measurements included the body-axis forces and moments on the total vehicle, the body-axis drag and yawing moment of the tractor, surface pressures on the vehicle, unsteady pressures on the rear of the tractor and the front and rear of the trailer, oil-film interferometry to measure skin friction, and 3-D particle image velocimetry (PIV) in the gap and behind the trailer. For the basic model, a strong hysteresis effect was observed on the aerodynamic forces and moments with the drag changing over 35% in the loop at yaw angles greater than 10°. The wind-averaged drag coefficients for the model without and with side extenders were 0.594 and 0.437, respectively. Besides reducing the drag, the side extenders also eliminated the aerodynamic hysteresis. PIV measurements in the gap between the tractor and trailer indicate the strength of the vortical flow in the gap was significantly weaker with the side extenders installed. In the lowboy trailer configuration with side extenders, the wind-averaged drag coefficient was 0.376. For the conventional and lowboy trailer configurations with side extenders, the wind-averaged drag coefficients were 0.397 and 0.309, respectively. The methodology for calculation of the force and pressure coefficients is included to facilitate comparison between computation and experiment.