Puneet Varma (Editor)

Roshko number

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In fluid mechanics, the Roshko number (Ro) is a dimensionless number describing oscillating flow mechanisms. It is named after the American Professor of Aeronautics Anatol Roshko. It is defined as

R o = f L 2 ν = S t R e R o = 0.212 R e 4.5 valid over [ 50 <= Re < 200] R o = 0.212 R e 2.7 valid over [200 <= Re < 2000] S t = f L U , R e = U L ν


  • St is the dimensionless Strouhal number;
  • Re is the Reynolds number;
  • U is mean stream velocity;
  • f is the frequency of vortex shedding;
  • L is the characteristic length (for example hydraulic diameter);
  • ν is the kinematic viscosity of the fluid.
  • Note: Linear relation between Ro# and Re# from Roshko's Caltech Thesis (figures 9-10); tested in air [over range Re=50 to Re=2000]
  • The kinematic viscosity of air at 15 °C is 1.48×105 m2/s. at 20 °C, the kinematic viscosity is 1.525×105 m2/s. at 25 °C, the kinematic viscosity is 1.57×105 m2/s.


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