Coefficient of drag

A Mobile moving in a Fluid weighing sudden on behalf of this fluid a distribution of pressure from which the resulting is opposed to its walk. The component of this resultant according to the direction of the mobile is called Traînée. The intensity of the force of trail can be expressed according to speed, of the form and the size of the mobile. The coefficient of drag $C_x$ is a Nombre without dimension characteristic of the shape of the mobile.

Coefficient of drag or $C_x$

The

C_x

indicates the coefficient of drag of a body, in mechanics of the fluids, Aérodynamique or Hydrodynamique.

It is a Nombre without dimension which makes it possible to quantify the force of resistance of a surface.

It is the relationship between the force of trail $F_x$ and the product of the dynamic Pression $q$ by the surface of reference $S$ of the mobile:

$C_x = \ frac {F_x} {Q \ times S},$

where the dynamic Pression is given by

q = \ frac {1} {2} \ cdot \ rho \ cdot v^2,

with

\ rho

density of the fluid and

v

= speed of the mobile.

The surface of reference is that of the Master-couple, greater section of the solid by a plan perpendicular to displacement, except in the case of the wings in aeronautics, for which one reports the forces to wing surface, projection of the wings on a level containing the airfoil chord, which makes it possible to compare profiles independently their thicknesses.

Smoothness

The

C_x

intervenes in the calculation of the smoothness (it is the report/ratio

\ frac {C_z} {C_x}

)

Values of the $C_x$

The total

C_x

of a mobile depends on the Coefficient of friction, of the form of the profile speed (“turbulent” profile, laminar Profil), of the induced Traînée by the bearing pressure.

The coefficient of friction depends on the Reynolds number and the degree of laminarity of surface.

in aerodynamics, the $C_x$ of a Aile is approximately 0,005 to 0,010 in flight, according to the Reynolds number and the laminarity;
in hydrodynamics, the $C_x$ of Foil (immersed foil) is approximately 0,03 with cruising speed (for a $C_z$ of 0,6 and a smoothness of 20).

Internal bonds

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Coefficient of drag