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Download OH 5: Fluid Dynamics
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Fluid Dynamics All athletic events take place in a fluid environment water (swimming), air (cycling), both (water polo) Drag and lift forces are perpendicular to each other, produce different effects, and are affected by different factors Flow velocity – Fluid Drag Forces Two types: Both skin friction and profile drag are proportional to relative flow velocity, cross-sectional area, shape of object, smoothness of surface, and density of liquid Skin Friction Drag Caused by the fluid tending to rub (shear) along the surface of the body The layer of fluid next to the skin sticks to the body; however, the next layer is towed along and therefore slides relative to the innermost layer Laminar flow The smooth, layered, flow pattern of a fluid around an object with no disturbance Profile Drag The main form of drag in skiing, cycling, running , all projectiles events, and swimming Characterized by turbulent flow in which the pressure on the leading surface of a body is greater than the pressure on the trailing surface Turbulent flow This causes a large, turbulent, low-pressure zone to form behind the object The region of low pressure increases the amount of work done on the object Fluid Lift Forces Always directed perpendicular to the flow velocity More air flows over the upper curved surface than the lower flat surface, such that the difference in velocity across the surfaces results in a pressure difference between the two sides The external force resulting from the pressure difference is perpendicular to the direction of flow velocity, and can change the motion of the object Bernoulli’s principle – Angle of Attack Refers to the tilt of an object relativeto the flow velocity Function of the shape of an object and the flow velocity If the angle of attack increases too much, it approaches a critical maximum angle (stall angle), beyond which the lift force decreases as the drag force becomes dominant The Magnus Effect The changes in flight path are always perpendicular to the flow velocity of the projectile Why a Curveball Curves (topspin, under spin, sidespin) Body Balance and Stability Control Generally depends upon the location of the centre of mass and how stable that centre of mass is The stick figure model is useful Equilibrium The state of a system that is not experiencing any change in its direction or speed Static Equilibrium: Dynamic Equilibrium: Balance Process whereby the body’s state of equilibrium is controlled for a given purpose If the line of gravity passes through some part of the body’s base of support, the body will be balanced Stability A measure of the difficulty with which equilibrium can be disturbed There is a trade-off between maximizing stability and acquiring speed off a mark Increasing Stability in Static Equilibrium Increase the inertia of the body Increase the distance between the point where a vertical line from the centre of mass intersects the base of support Increasing Stability in Dynamic Equilibrium Enlarge the body’s base of support in the direction of the external horizontal forces Adopt a starting position in which the centre of pressure is close to the edge of the base of support whenever a quick acceleration is important When slowing down or reversing directions in running, lean backwards but keep normal frictional forces high to prevent slipping