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
where 
* 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.