  Recently, Portelli et al (2003) have semi-numerically obtained a functional
form of the probability distribution of fluctuations in the total energy flow
in a model for fluid turbulence. This follows earlier work suggesting that
fluctuations in the total magnetization in the 2D X-Y model for a ferromagnet
also follow this distribution. Here, starting from the scaling ansatz that is
the basis of the turbulence model we analytically derive the functional form of
this distribution and find its single control parameter that depends upon the
scaling exponents and system size of the model. Our analysis allows us to
identify this explicitly with that of the X-Y model, and suggest a possible
generalization.
