Segment V9.6: Oscillating Sign

(Related to Textbook Section 9.3.3 - Drag Coefficient Data and Examples)

Steady flow past a blunt object may produce an oscillating Karman vortex street wake behind the object. The shedding of vortices from the body exerts a periodic force on the body that, if the frequency is right, can cause the body to oscillate.

As shown in a computational fluid dynamics simulation, flow past a flat plate can produce a well-defined Karman vortex street. Similarly, wind blowing past a rectangular speed limit sign can excite the sign into significant twisting motion, provided the conditions (wind speed and direction, stiffness of the sign support, etc.) are correct. (CFD video courtesy of ITS Corp.)