I am studying the supersonic flutter characterisitcs of rectangular panels. I am new to this domain and have certain fundamental questions that I am unable to answer. I am employing the Piston theory for determining the flutter boundary, i.e. the flutter speed and the corresponding frequency. I have seen several works in which the v-g (velocity-damping) and v-f (velocity-frequency) graphs are plotted and the critical velocity is determined from the v-g curve corresponding to the point where the damping corresponding to a particular mode crosses from negative to positive region. The velocity (is it correct to say free-stream velocity?) is varied from 0 to a particular velocity of interest.

If I am correct, these v-g and v-f curves are plotted for a particular Mach number (M>1 for supersonic) . If this is the case, I am unable to comprehend why the velocity is varied from zero to a higher value? In most of the papers that I have read, M is defined as the free stream Mach number, M=(free-stream velocity/sonic speed). Is the velocity being varied (from vmin=0 to v) while keeping M (>1) constant?

Any help in clearing these misconceptions would be greatly appericiated.

Thanks