Aerodynamic drag of transiting objects by large-scale tomographic-PIV

Experiments are conducted that obtain the aerodynamic drag of a sphere towed within a rectangular duct from PIV. The drag force is obtained invoking the time-average momentum equation within a control volume in a frame of reference moving with the object. The sphere with 0.1 m diameter is towed at velocity of 1.5 m/s, corresponding to Re = 10,000. The tomographic PIV measurements are conducted at 500 Hz in a volume of approximately 3 x 40 x 40 cubic centimeters. The large-scale measurement is attained making use of neutrally buoyant Helium-filled soap bubbles of approximately 0.3 mm diameter as air-flow tracers, preluding to a potential upscale of the technique. The measured drag depends upon three terms, namely the flow momentum, the pressure and the velocity fluctuations. These individual terms vary largely at different distances behind the sphere, while the sum attains a relatively constant value. More than two diameters behind the object the drag only varies by about 1%, yielding a practical criterion for the drag evaluation of bluff objects with this technique.