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Home » AS & A2 LEVEL (A-Level) Revision » Physics » Force & Motion » Drag & Terminal Velocity »
Streamlines & Turbulence
Streamlines & Turbulence
To be as fuel-efficient as possible, a car, train or aircraft needs to be designed to
minimise the resistive forces acting on it. To investigate the resistive or drag
forces due to movement through the air, a car body is placed in a wind tunnel.
Here the car body is held stationary and the air moves around it; vapour trails show the air flow over the car
body.
The diagram below shows the air flow at a low speed over a car body. The flow is said to be laminar or
streamlined.
In a wind tunnel the air moves over a stationary object such as a car body to model the movement of the
car through the air.
This is a valid model, as it is the relative speed of the air and the object that determines the pattern of flow.
In laminar flow:
the air moves in layers, with the layer of air next to the car body being stationary and the velocity of the
layers increasing away from the car body
particles passing the same point do so at the same velocity, so the flow is regular
the drag force is caused by the resistance of the air to layers sliding past each other
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2. 10/6/13 Streamlines & Turbulence | a2-level-level-revision, physics, force-motion, drag-terminal-velocity, streamlines-turbulence | Revision World
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more viscous air has a greater resistance to relative motion and exerts a bigger drag force
the drag force is proportional to the speed of the car relative to the air.
Viscosity is a measure of the resistance of a fluid to flowing. Syrup and tar are viscous fluids; hydrogen has
a very low viscosity.
As the speed of the air passing over the car body is increased, the flow pattern changes from laminar to
turbulent. This is shown in the diagram above.
The changeover from laminar flow to turbulent occurs at a speed known as the critical velocity. Turbulent
flow causes much more drag than laminar flow.
In turbulent flow:
the air flow is disordered and irregular
the drag force depends on the density of the air and not the viscosity
the drag force is proportional to the (speed)2 of the car relative to the air.
Related resources
Terminal Velocity
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