Laminar and turbulent flowWhen objects move through air or water slowly they do not disturb the flow a great deal and the air or water will flow in a smooth, parallel manner around the object = laminar flow.

Human movement through air and water

When objects move through air or water rapidly they disturb the flow to the point that layers next to the surface of the object get mixed together = turbulent flow

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This is upwards force acting on a body within water.

The buoyant force = weight of fluid displaced by an object.

The greater density a fluid has, the more buoyant objects will be when placed in this medium.

When buoyancy force and weight are equal an object will float, but when weight of an object is greater than the buoyancy force, it will sink.

Buoyancy

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Is created when an object moves through a fluid or when fluid flows around a stationary object.

In air the drag force is known as air resistanceIn water the drag force is known as hydrodynamic resistance

Drag force

Forces will always act from the area of high pressure to an area of low pressure.

Streamlining decreases turbulence created at the back of an object and thusreduces drag.

Bernoulli’s Principle explains how as the velocity of fluid increases, the pressure decreases. This causes an area of turbulence behind the object where pressure is less than in front of the object.

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Drag is affected by the following factors:

• the shape of an object and how streamlined it is• the density of the fluid (air or water)• the velocity of the object as it moves through

the fluid• the cross sectional area of the object in the

direction of the motion.

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Lift force acts perpendicular to the flow of the fluid and is affected by:

• the velocity of the fluid• the density of the fluid• the size, shape and position of the

object• the foil shape• angle of object relative to direction of

flow• the Magnus effect• uneven surface coatings.

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A foil has a curved and a flat surface which causes fluid to flow faster over the curved surface than the flat one, leading to pressure differences.

Lift force is generated from the area of high pressure to the area of low pressure.

This is the angle between the long axis of the object and the direction of air / water flow.A positive angle of attack is required to create lift force

Angle of attack

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Lift can be created by spin (topspin, backspin and sidespin) and the deviation towards the direction of spin is knows as the Magnus effect. A lift force will act from the area of high pressure to the area of low pressure.

The Magnus effect

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The Magnus effect – Top Spin

Due to the air resistance formed at the top of the ball, the air is slowed down causing an area of high pressure above the ball. Below the ball however, the air moves in the same direction as the spin, allowing for greater velocity and lower pressure.As all forces follow a pressure gradient, the force line must move from above the ball, to beneath the ball, causing it to drop.

Due to the air resistance formed at the bottom of the ball, the air is slowed down causing an area of high pressure below the ball. Above the ball however, the air moves in the same direction as the spin, allowing for greater velocity and lower pressure.As all forces follow a pressure gradient, the force line must move from below the ball, to above the ball, causing it to rise.

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• Air will travel across a smooth surface quicker than over a rough surface.

• Turbulent air from the rough surface is at lower pressure than air flowing over the smoother surface which is at higher pressures.

• This causes a pressure difference, and lift force is towards the side with lower pressure.

Surface differences

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