Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids

The drag coefficient, C_D of a solid sphere moving in fluid is known to be only a function of the Reynolds number, Re and diminishes rapidly at the drag crisis around Re sim 3 times 10^5. A Leidenfrost vapor layer on a hot sphere surface can trigger the onset of the drag crisis at lower Re. By using a range of high viscosity perfluorocarbon liquids, we show that the drag reduction effect, can occur over a wide range of Re, from as low as sim 600 to 10^5. The Navier slip model with a viscosity dependent slip length can fit the observed drag reduction and wake shape.