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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.
Churchill equation [24] (1977) is the only equation that can be evaluated for very slow flow (Reynolds number < 1), but the Cheng (2008), [25] and Bellos et al. (2018) [8] equations also return an approximately correct value for friction factor in the laminar flow region (Reynolds number < 2300). All of the others are for transitional and ...
In aerodynamics, aerodynamic drag, also known as air resistance, is the fluid drag force that acts on any moving solid body in the direction of the air's freestream flow. [ 22 ] From the body's perspective (near-field approach), the drag results from forces due to pressure distributions over the body surface, symbolized D p r {\displaystyle D ...
This function shares the same values for its term in common with the Kármán–Prandtl resistance equation, plus one parameter 0.305 or 0.34 to fit the asymptotic behavior for R ∗ → ∞ along with one further parameter, 11, to govern the transition from smooth to rough flow. It is exhibited in Figure 3.
The equation is precise – it simply provides the definition of (drag coefficient), which varies with the Reynolds number and is found by experiment. Of particular importance is the u 2 {\displaystyle u^{2}} dependence on flow velocity, meaning that fluid drag increases with the square of flow velocity.
In air, the same theory can be used to explain why small water droplets (or ice crystals) can remain suspended in air (as clouds) until they grow to a critical size and start falling as rain (or snow and hail). [6] Similar use of the equation can be made in the settling of fine particles in water or other fluids. [citation needed]
The equations ignore air resistance, which has a dramatic effect on objects falling an appreciable distance in air, causing them to quickly approach a terminal velocity. The effect of air resistance varies enormously depending on the size and geometry of the falling object—for example, the equations are hopelessly wrong for a feather, which ...
The Kantrowitz limit therefore acts a "speed limit" - for a given ratio of tube area and pod area, there is a maximum speed that the pod can travel before flow around the pod chokes and air resistance sharply increases. [5] In order to break through the speed limit set by the Kantrowitz limit, there are two possible approaches.