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In fluid dynamics, the drag coefficient (commonly denoted as: , or ) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water.
Wave drag, sometimes referred to as compressibility drag, is drag that is created when a body moves in a compressible fluid and at the speed that is close to the speed of sound in that fluid. In aerodynamics, wave drag consists of multiple components depending on the speed regime of the flight.
The force on an object that resists its motion through a fluid is called drag. When the fluid is a gas like air, it is called aerodynamic drag or air resistance . When the fluid is a liquid like water it is called hydrodynamic drag, but never "water resistance".
For most large objects such as cyclists, cars, and baseballs not moving too slowly, the magnitude of the drag force F D F D is proportional to the square of the speed of the object. We can write this relationship mathematically as F D ∝ v 2 .
Table lists some typical drag coefficients for a variety of objects. Notice that the drag coefficient is a dimensionless quantity. At highway speeds, over 50% of the power of a car is used to overcome air drag. The most fuel-efficient cruising speed is about 70–80 km/h (about 45–50 mi/h).
Air friction, or air drag, is an example of fluid friction. Unlike the standard model of surface friction, such friction forces are velocity dependent. The velocity dependence may be very complicated, and only special cases can be treated analytically.
Drag. Aerodynamic drag, sometimes called wind resistance, is an effect which a vehicle must overcome more and more as speed increases. Drag is a form of friction, though some of the air's effect on the vehicle is due to a low pressure zone behind the vehicle, and a high pressure zone in front.
