Design of tractor-trailer add-on drag reduction devices using CFD

Simplified truck models have been used for a bluff body external flow investigation in order to reduce aerodynamic drag. At low speed, the tire rolling resistance dominates the total tractive resistance. However, at speeds higher than 90 km/hr, aerodynamic drag starts exceeding tire rolling resistance. Thus, aerodynamic drag is an important issue, especially at normal freeway speed. Since the 1960s, fuel savings by drag reduction of various add-on devices has been studied on commercial vehicles. The focus of most of these efforts was on add-on devices such as boat-tail flaps, roof/side extenders, and side skirts. The effects of those devices can reduce the drag by 18.3% and save 4225 gallons of fuel for one tractor-trailer per year.;In the current investigation, new add-on devices in the form of humps and curved boat-tail flaps are investigated for base-drag reduction using unsteady computational fluid dynamic simulations. These devices are installed on the trailer top, bottom, and sides and at the end of the tractor-trailer near the base of the trailer. The shape and placement of these devices help to direct the flow from the top, bottom, and sides of the trailer to the rear to reduce the size and influence of the main vortex structure in the near-wake region and raise the base pressure. In this investigation, the Ansys Fluent Navier-Stokes code is used to perform time-averaged, unsteady simulations of validation configurations and then tractor-trailer-like configurations with add-on devices. The results from these simulations show the effect of numerical variables, including computational grid density, on solution accuracy and the drag-reduction possible from add-on devices.;Prior to investigation of designs, the solution procedure has been validated against experimental data with a 2D square cylinder model. The drag was predicted with different mesh densities to determine the grid sensitivity. The flow physics and predicted drag from these validation simulations are in good agreement with the existing experimental data. A 3D Ahmed body model with zero slant-angle has been being used to simulate a real tractor-trailer in the current research. The flow speed is 60 m/s, which corresponds to the Reynolds number of 4,300,000. This Reynolds number is within one order of magnitude of a real tractor-trailer Reynolds number. The unsteady and time-averaged results from 3D simulations are compared with existing experimental data. In addition, results from a number of simulations in which add-on devices in the form of humps and curved boat-tail flaps are shown demonstrating drag reduction up to 50.9%.