Numerical investigation of wake flow characteristics of an eighteen-wheel truck with and without rear momentum injection and cross flow

Flow characteristics around and down stream of an eighteen-wheel tractor-trailer vehicle are numerically investigated using a COMPACT-3D Computational Fluid Dynamics (CFD) software package using a k-ϵ turbulence model. The investigations were performed at two free stream mean velocity of 10 and 30 m/sec and various uniform rear momentum injection and cross flow conditions. Results include three components of the mean velocity, turbulent kinetic energy and mean pressure upstream, around and downstream of the vehicle. Results show that at a free stream velocity of 10 m/sec., a strong uniform rear air momentum injection with mean velocity of 5 m/s or higher can induce a back pressure recovery, resulting in an overall drag reduction of the tractor-trailer unit. At this speed, the pressure recovery is still substantial, even when a strong cross flow with a mean velocity of 5 m/sec is present. The presence of the rear momentum injection also results in reduction in the regions of flow reversal and high turbulent kinetic energy behind the trailer.; When free stream velocity is increased to 30 m/sec, which corresponds to 108 km/hr or 67 miles/h highway speed of the tractor-trailer unit, a strong rear uniform momentum injection is not very effective in inducing a back pressure recovery. In this case, the presence of the cross flow further reduces the back pressure and the flow axial momentum, resulting in an increase in the overall drag of the vehicle.