see: fORMULA drag force
The drag is the strength (paralell to fluid velocity) of resistance, fluid exerts on an object when the fluid or the object is moving one relative to another
The drag force can be calculated from:
The coefficient called Cd , ( The Cd is the drag coefficient, is determined experimentally Wind tunnels)
The fluid velocity,
The fluid density,
the surface reference depending on the subject studied:
with this formula:
p is the density fluid Kg/m3
The S surface Reference m²
v the relative velocity of the fluid * m / s
* whether the fluid, the object or both moving, we take into account the speed of one relative to another .
Cd(drag cofficient) and Cl (lift coefficient) are measured experimentally are accessible to all in databases or The value of Cd an cl is always given as a function of Reynolds number which includes the kinematic viscosity of fluid.
Methods of mathematical determinations or Cd Cl exist and achieve accuracies interesting . The coefficients of drag and lift are often represented by graphs called Polar
( In its profile editor Mecaflux integrates XFOIL to determine lift and drag of profiles that you can not find the polar wind )
The force drag expressed in Newtons is the one that opposes or slows the parachute movement.
The Cd allows us to extrapolate resistance to progress to a series of geometrically similar profiles.
When the drag is studied in water we talk about viscous resistance Rv and the formula for calculating the drag coefficient is related to Reynolds number Re and take the name of coefficient of viscous resistance Cv
Rv = Cv. (Re. K / L) .
In hydrodynamics Naval sometimes used a method the method of dual models to determine the viscous resistance.
hydrodynamic, ship displacement resistance
Detail of the interface calculation of drag and lift in MECAFLUX
Exemple External flow drag and lift