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Hydrodynamics

Hydrodynamics, general

When an object is immersed in a fluid stream, there is the phenomena of friction and turbulence.

• when the fluid is air, the study of these phenomena is the aerodynamics.

• when the fluid is water, the study of these phenomena is the Hydrodynamics .

The study of hydrodynamic resistance ( Rh ) does not currently have comprehensive theory which would apply an equation to meet the different cases accurately. We therefore divided the phenomena following parameters:

• If the object is completely immersed (Drag force) its coefficient will be studied by hydrodynamic cumulation its frictional resistance to its strength of form. The combination of these twoforces is named viscous resistance or drag . viscous resistance ( Rv (Newtons) ) or drag force, Whose coefficient of viscous resistance ( Cv or Cd if drag has been measured in wind tunnel ) Is related to Reynolds number ( Re ) And roughness ( K ) On the length of the hull ( L )

• Rv= Cv . (Re . K/L)

• Drag force = Cd .r.s.(v²/2) the Cd is given for Reynolds numbers

Total coefficient of hydrodynamic resistance ( Ch = Rh / Displacement) For an object moving in deep water without surface effect is defined as:

• Ch = Rv= Cv.(Re.K/L)

• If the object moving near the surface   Hydrodynamics of ships viscous resistance or drag be accompanied by a resistance due to the formation of surface waves, wave resistance ( Rw ), Whose coefficient of resistance to the wave ( Cw) is bound to Froude number ( Fr )

• Rw= Cw.(Fr)

• Cw =Rw/(1/2)r.s.v²(r Water density, S surface in contact with water, V velocity.

Total coefficient of hydrodynamic resistance ( Ch = Rh / Displacement) For an object moving near the surface is defined as:

• If this object is a type hovercraft forces of hydrodynamic resistance will be significantly diminished .The force and power of the propellers used to create the lift must be evaluate( propeller calculation software ) .

Different types of approaches to assess the hydrodynamic coefficient (Ch):

• Despite advances in computer modeling, testing models are still essential. The effects of friction measured experimentally in basin are themselves subject to problems of scaling resolved through laws of similarity . (see Froude )

scale model tests in basin

• Another technique for evaluating hydrodynamic behavior is to compare data measured on a series of standard forms and extrapolate the consequences of changing forms. .

 la série de Taileur la série de NPL la série de Nordstrom les 63 séries la série de SSPA les 64 séries

The naval hydrodynamics is studying the hydrodynamic resistance force of ships