New Model of Decelerating Bluff-Body Drag

A new model of the drag force generated by a freely decelerating bluff body is presented. The model is based, mainly, on the premise that the wake of an object accelerating downwind in a moving fluid is identical to that of the same object decelerating in the fluid at rest. After arguing for the drag of a wind drifter to depend only on a power function of its speed relative to the wind, a Galilean transformation is used to provide a formula for decelerating body drag of the form FD~V(beta). The value of exponent (beta)is dependent on the amount of external force applied to the body, as well as on its initial and final drag coefficients and its initial speed. By implication, this exponent depends on the specific history of the motion. Applications to powered and unpowered vehicles trailing a parachute or any other high-drag devices are presented and discussed. The model is also shown to compare favorably with parachute-test drop data.