an improved method and device for the reduction of aerodynamic drag and for improved performance of ground vehicles by increasing the pressure on the base area of a bluff - base ground vehicle or vehicle component by conditioning the undercarriage flow and controlling the discharge of the undercarriage flow into the trailing wake of a ground vehicle . the subject invention provides an improved method and device for generating a reduction in the drag force on a bluff - base object moving through air . the apparatus consists of aerodynamically shaped surfaces that are attached to the undercarriage of a vehicle between the base area and the aft most set of wheels . the surfaces are shaped and positioned such that they may be combined with the vehicle lower surface and the ground to form a nozzle with the smallest opening located at the aft end of the vehicle and the largest opening located forward of the smallest opening . the largest opening has a width that is equivalent to the width of the vehicle and has a height that is approximately equal to the distance between the vehicle lower surface and the ground . the smallest opening is located at the base of the vehicle and has an area and shaped opening to maximize the discharge of the undercarriage flow into the wake . the surfaces that comprise the invention are aerodynamically shaped to minimize separation of the flow passing through the device and on the outer surface of the device and at the same time , to increase the momentum for the flow passing through the device and to entraining additional air externally to the device . the reduction in drag force results from the increase in pressure acting on the base area of the vehicle . the increase in base pressure is a result of the increase in wake energy and reduction in the unsteady wake flow due to the addition of the high momentum undercarriage flow into the wake . the objects and advantages also extend to other applications in which an object or vehicle is moving through either a gas or fluid .