The properties of a standard hydraulic jump depend critically on a Froude
number Fr defined as the ratio of the flow velocity to the gravity waves speed.
In the case of a horizontal circular jump, the question of the Froude number is
not well documented. Our experiments show that Fr measured just after the jump
is locked on a constant value that does not depend on flow rate Q, kinematic
viscosity {\nu} and surface tension {\gamma}. Combining this result to a
lubrication description of the outer flow yields, under appropriate conditions,
a new and simple law ruling the jump radius RJ : RJ(ln(RJR∞))3/8∼Q5/8ν−3/8, in excellent agreement
with our experimental data. This unexpected RJ result asks an unsolved question
to all available models.Comment: 5 pages, 3 figure