Everything about Fluid Friction totally explained
In
fluid dynamics,
drag (sometimes called
resistance) is the force that resists the movement of a
solid object through a
fluid (a
liquid or
gas). Drag is made up of
friction forces, which act parallel to the object's surface plus
pressure forces, which act in a direction perpendicular to the object's surface.
For a solid object moving through a fluid, the drag is the sum of all the
aerodynamic or
hydrodynamic forces in the direction of the movement. Forces perpendicular to this direction are considered
lift. It therefore acts to oppose the motion of the object, and in a powered vehicle it's overcome by
thrust.
In
astrodynamics, depending on the situation,
atmospheric drag can be regarded as inefficiency requiring expense of additional energy during
launch of the space object or as a bonus simplifying return from orbit.
Types of drag are generally divided into three categories:
parasitic drag,
lift-induced drag, and
wave drag. Parasitic drag includes
form drag,
skin friction, and
interference drag. Lift-induced drag is only relevant when
wings or a
lifting body are present, and is therefore usually discussed either in the aviation perspective of drag, or in the design of either semi-planing or
planing hulls.
Wave drag occurs when a solid object is moving through a fluid at or near the
speed of sound in that fluid. The overall drag of an object is characterized by a
dimensionless number called the
drag coefficient, and is calculated using the
drag equation. Assuming a constant drag coefficient, drag will vary as the square of
velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity. The standard equation for drag is one half the coefficient of drag multiplied by the
fluid density, the
cross sectional area of the specified item, and the square of the velocity.
Wind resistance or
air resistance is a layman's term used to describe drag. Its use is often vague, and is usually used in a relative sense (
for example, A
badminton shuttlecock has more
wind resistance than a
squash ball).
Stokes' drag
The equation for
viscous resistance or
linear drag is appropriate for small objects or particles moving through a fluid at relatively slow speeds where there's no turbulence (for example low
Reynolds number,
). In this case, the force of drag is approximately proportional to velocity, but opposite in direction.
(External Link
) The equation for viscous resistance is:
» :
~ 0.3 m/s and so on. Actual terminal velocity for very small objects (pollen, etc) is even smaller due to the viscosity of air.
Terminal velocity is higher for larger creatures, and thus more deadly. A creature such as a mouse falling at its terminal velocity is much more likely to survive impact with the ground than a human falling at its terminal velocity. A small animal such as a
cricket impacting at its terminal velocity will probably be unharmed. This explains why small animals can fall from a large height and not be harmed.
Further Information
Get more info on 'Fluid Friction'.
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