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
• 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.

Image

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.