麻省理工船舶海洋流体力学课本mit+marine+fluid[课程编号13_精品.pdf

13.021 – Marine Hydrodynamics, Fall 2004 Lecture 16 13.021 - Marine Hydrodynamics Lecture 16 4.1.4 Vortex Shedding and Vortex Induced Vibrations Consider a steady flow Uo over a bluff body with diameter D. F (lift) y U 0 D Fx (drag) We would expect the average forces to be: However, the measured oscillatory forces are: F F Fx Average Fx Average F t y F t y • The measured drag Fx is found to oscillate about a non-zero mean value with frequency 2f . • The measured lift Fy is found to oscillate about a zero mean value with frequency f . • f = ω/2π is the frequency of vortex shedding or Strouhal frequency. 1 Reason: Flow separation leads to vortex shedding. The vortices are shed in a staggered array, within an unsteady non-symmetric wake called von Karman Street. The frequency of vortex shedding is the Strouhal frequency and is a function of Uo , D, and ν .