Transcript Foil Shape
Human Movement in a Fluid Medium Biomechanics and ergonomics II Foil Shape Foil: shape capable of generating lift in the presence of a fluid flow One way in which lift force may be created is for the shape of the moving body to resemble that of a foil Foil Shape The difference in the velocity of flow on the curved side of the foil and opposed to the flat side of the foil creates a pressure difference in the fluid, in accordance with a relationship derived by the Bernoulli. Bernoulli principle: “an expression of the inverse relationship between relative velocity and relative pressure in a fluid flow” Lift force generated by a foil shape is directed from the region of relative high pressure on the flat side of the foil toward the region of relative low pressure on the curved side of the foil Foil Shape Factors effecting the magnitude of lift on foil shape: Relative Fluid velocity of fluid and foil density and surface area of flat side of foil (direct relation) coefficient of lift, which indicates a body’s ability to generate lift based on its shape. Foil Shape human hand resembles a foil shape when viewed from lateral side. When a swimmer slices a hand through the water, it generates lift force directed perpendicular to the palm. The lift force generated by rapid sculling motions enables elite synchronized swimmers to support their bodies in an inverted position with both legs extended completely out of the water. Foil Shape -Angle of attack “Angle between the longitudinal axis of a body and the direction of the fluid flow” semifoil shapes of projectiles generate some lift force when oriented at appropriate angles with respect to the direction of the fluid flow (e.g discus, javelin, football,boomerang, and Frisbee) Foil Shape -Angle of attack It is an important factor in launching a lift-producing projectile for maximum range (horizontal displacement) A positive angle of attack is necessary to generate a lift force. -Angle of attack- -Angle of attack- Airplanes that assume too steep an ascent can stall and lose altitude until pilots reduce the attack angle of the wings to enable lift Foil Shape -lift/drag ratio “the magnitude of the lift force divided by the magnitude of the total drag force acting on a body at a given time” To maximize the flight distance of projectile, increase lift and reduce drag Form drag is minimum at 0ᵒ, a poor angle for lift generation. The largest lift/drag ratio for a discus traveling at a relative velocity of 24 m/s is generated at an angle of attack of 10° Foil Shape In aerodynamics the optimum lift/drag ratio can be achieved with an attack between 4-15 degrees and for swimming it is between 30 and 50 degrees Foil Shape During the first part of the flight, ski jumpers should assume a small angle of attack to minimize drag. During the latter part of the flight, they should increase attack angle up to that of maximum lift. Magnus Effect Magnus force “Lift force created by spin” Magnus effect “Deviation in the trajectory of a spinning object toward the direction of spin, resulting form the magnus force” Magnus effect-Examples A ball projected with spin follows a trajectory that curves in the direction of the spin. The trajectory of a ball thrown with sidespin follows a regular curve due to the Magnus effect Magnus effect-Examples A banana shot in soccer results from imparting sidespin to the ball. Magnus effect-Examples The loft on a golf club is designed to produce backspin on the ball. A properly hit ball rises because of the Magnus effect. Summary Foil shape Magnus effect