Why Does Air Resistance Increase with Velocity in Freefall?

[danago]

Hey. Just a random though. Why does the force of air resistance increase for say...a freefalling body..as its velocity increases? Is it because it encounters more air particles to which it must overcome?

Thanks,
Dan.

 

[quasar987]

The collisions btw the body and air molecules happen at higher speed, so more momentum is transferred to the falling body as a result of each collisions. ==> dp/dt is greater.

 

[andrevdh]

As the object falls through the air the turbulent wake behind it gets stronger. This turbulence causes a decrease in pressure behind it - called pressure drag. This drag can be reduced by causing the turbulence to form further behind the object - like airfoils, or increasing the amount of micro turbulence around the object - the dimples in a golf ball, which results in a decreases in the size of the turbulent region behind the object.

 

[pavadrin]

okay, yes this isn't my post but i was also wondering about the same situation. I am not quite sure about what has been explained, and was wondering if it could be explained from a basic point onwards...many thanks,
Pavardin

 

[andrevdh]

The reason why modern cars are made so sleek today is to reduce the turbulence behind them. This turbulence decreases the speed and increases the consumption of the car. The turbulence creates a region of low pressure behind the car - commonly called a slip stream, where bicyclers like to get into for a free ride. This also explains why a ping pong ball balances vertically in an air stream. The low pressure region behind it "cancels" it's weight.

 

[Hawknc]

As the object falls through the air the turbulent wake behind it gets stronger. This turbulence causes a decrease in pressure behind it - called pressure drag. This drag can be reduced by causing the turbulence to form further behind the object - like airfoils, or increasing the amount of micro turbulence around the object - the dimples in a golf ball, which results in a decreases in the size of the turbulent region behind the object.

Technically speaking, drag still increases with velocity even if there is no separation, due to boundary layer effects (or friction due to air having viscosity). What you just described - the pressure difference across the body - is known as form (or pressure) drag. Skin friction drag also plays a major part, though.

 

[andrevdh]

The viscosity of air is very low so very little energy will be lost due to the air flowing over the object. With a golf ball the friction is deliberately increased so that micro turbulence is generated around it as it moves through the air. In spite of this increased friction the ball travels much further than it otherwise would. Which means that the turbulence plays a much larger roll in the drag than the other effects.

 

[Hawknc]

You're correct for the specific example of a golf ball - the dimples serve to trip the boundary layer to turbulent behaviour, which causes separation farther back than laminar behaviour would, which in turn results in lower profile drag. Skin friction plays a much more important role in slender objects, such as aerofoils.

 

[andrevdh]

Does skin friction also create micro turbulence (a term a coined in order to distinguish it from the larger turbulent structures behind the object) and thereby stealing kinetic energy from the object? What other forms of drag slows an object moving through a fluid down?

 

[Hawknc]

Skin friction is a result of "micro" turbulence, or turbulence within the boundary layer. The transition from laminar to turbulent can be caused by reaching the critical Reynolds number, a change in geometry (often at the "tip of the hump" in an object) or surface roughness. In fact, turbulent BLs have higher friction than laminar BLs. There's actually an interesting discussion on laminar and turbulent flow going on in the Mechanical and Aerospace Engineering forum.

As for the types of drag - given the two types of forces acting on an object in a viscous fluid (pressure which acts perpendicular to an object's surface, and shear stress which acts parallel to an object's surface), form drag and skin friction drag are the two primary types. There are other definitions mostly specific to aircraft, such as interference drag, induced drag, cooling drag and wave drag. However, profile drag (which is the sum of the form and skin friction drag forces) is the main force that applies to most objects in a viscous fluid.