Lift Forces On Bluff Bodies
Further information: Vortex shedding and Vortex-induced vibrationThe 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:
“For every man that Bolingbroke hath pressed
To lift shrewd steel against our golden crown,
God for his Richard hath in heavenly pay
A glorious angel. Then if angels fight,
Weak men must fall; for heaven still guards the right.”
—William Shakespeare (1564–1616)
“In literary circles, the men of trust and consideration, bookmakers, editors, university deans and professors, bishops, too, were by no means men of the largest literary talent, but usually of a low and ordinary intellectuality, with a sort of mercantile activity and working talent. Indifferent hacks and mediocrities tower, by pushing their forces to a lucrative point, or by working power, over multitudes of superior men, in Old as in New England.”
—Ralph Waldo Emerson (1803–1882)
“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)
“In time, after a dozen years of centering their lives around the games boys play with one another, the boys’ bodies change and that changes everything else. But the memories are not erased of that safest time in the lives of men, when their prime concern was playing games with guys who just wanted to be their friendly competitors. Life never again gets so simple.”
—Frank Pittman (20th century)