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 Drag force and drag coefficient Cd 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: The front surface (projection perpendicular to the flow) if the object is not profiled. The maximum area projected on the ground if the object is shaped 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 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) . roughness (K) In hydrodynamics Naval sometimes used a method the method of dual models to determine the viscous resistance.   With MECAFLUX   Detail of the interface calculation of drag and lift in MECAFLUX Exemple External flow drag and lift