Lift (force) - Lift Forces On Bluff Bodies

Lift Forces On Bluff Bodies

Further information: Vortex shedding and Vortex-induced vibration

The flow around bluff bodies – i.e. without a streamlined shape, or stalling airfoils – may also generate lift, besides a strong drag force. This lift may be steady, or it may oscillate due to vortex shedding. Interaction of the object's flexibility with the vortex shedding may enhance the effects of fluctuating lift and cause vortex-induced vibrations. For instance, the flow around a circular cylinder generates a Kármán vortex street: vortices being shed in an alternating fashion from each side of the cylinder. The oscillatory nature of the flow is reflected in the fluctuating lift force on the cylinder, whereas the mean lift force is negligible. The lift force frequency is characterised by the dimensionless Strouhal number, which depends (among others) on the Reynolds number of the flow.

For a flexible structure, this oscillatory lift force may induce vortex-induced vibrations. Under certain conditions – for instance resonance or strong spanwise correlation of the lift force – the resulting motion of the structure due to the lift fluctuations may be strongly enhanced. Such vibrations may pose problems, even collapse, in man-made tall structures like for instance industrial chimneys, if not properly taken care of in the design.

Read more about this topic:  Lift (force)

Famous quotes containing the words lift, forces, bluff and/or bodies:

    “If in madness of delusion, anyone shall lift his parricidal hand against this blessed union ... the arms of thousands will be raised to save it, and the curse of millions will fall upon the head which may have plotted its destruction.”
    —Andrew Jackson (1767–1845)

    “We know now that Government by organized money is just as dangerous as Government by organized mob.... [The organized moneyed people] are unanimous in their hate for me and I welcome their hatred.... I should like to have it said of my second administration that these forces met their master.”
    —Franklin D. Roosevelt (1882–1945)

    “When people are taken out of their depths they lose their heads, no matter how charming a bluff they may put up.”
    —F. Scott Fitzgerald (1896–1940)

    “At the round earth’s imagined corners, blow
    Your trumpets, angels, and arise, arise
    From death, you numberless infinities
    Of souls, and to your scattered bodies go,”
    —John Donne (1572–1631)