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The drag equation may be derived to within a multiplicative constant by the method of dimensional analysis. If a moving fluid meets an object, it exerts a force on the object. Suppose that the fluid is a liquid, and the variables involved – under some conditions – are the: speed u, fluid density ρ, kinematic viscosity ν of the fluid,
Parasitic drag, or profile drag, is the sum of viscous pressure drag (form drag) and drag due to surface roughness (skin friction drag). Additionally, the presence of multiple bodies in relative proximity may incur so called interference drag , which is sometimes described as a component of parasitic drag.
Drag coefficients in fluids with Reynolds number approximately 10 4 [1] [2] Shapes are depicted with the same projected frontal area. 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.
Note the minus sign in the equation, the drag force points in the opposite direction to the relative velocity: drag opposes the motion. Stokes' law makes the following assumptions for the behavior of a particle in a fluid: Laminar flow; No inertial effects (zero Reynolds number) Spherical particles; Homogeneous (uniform in composition) material
The most typical case of air resistance, in case of Reynolds numbers above about 1000, is Newton drag with a drag force proportional to the speed squared, =. In air, which has a kinematic viscosity around 0.15 cm 2 /s, this means that the product of object speed and diameter must be more than about 0.015 m 2 /s.
The above lift equation neglects the skin friction forces, which are small compared to the pressure forces. By using the streamwise vector i parallel to the freestream in place of k in the integral, we obtain an expression for the pressure drag D p (which includes the pressure portion of the profile drag and, if the wing is three-dimensional ...
Where force (F) equals lift (L) for forces measured perpendicular to the airstream to determine C = C L or force (F) equals drag (D) for forces measured in line with the airstream to determine C = C D on a sail of area (A) and a given aspect ratio (length to average cord width).
The derivation of Stokes' law, which is used to calculate the drag force on small particles, assumes a no-slip condition which is no longer correct at high Knudsen numbers. The Cunningham slip correction factor allows predicting the drag force on a particle moving a fluid with Knudsen number between the continuum regime and free molecular flow.