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Outflow (slip) coefficient γ is defined for the decreased energy transmission:

 

 

The cu-difference is called slip velocity.

 

The smaller the outflow coefficient, the higher the deviation of flow compared to the direction given by blade.

 

Von Backstroem developed an empirical equation for the estimation of the outflow coefficient assuming one single relative eddy in the rotor.

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Here sol is the solidity defined by:

 

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The limiting radius ratio εLim = 0.5, the radius ratio ε = Max(r1/r2, εLim). The constant F0 = 5.