Counterflow-induced decoupling in superfluid turbulence

In mechanically driven superfluid turbulence, the mean velocities of the normal- and superfluid components are known to coincide: U-n = U-s. Numerous laboratory, numerical, and analytical studies showed that under these conditions, the mutual friction between the normal- and superfluid velocity components also couples their fluctuations: u(n)'(r, t) approximate to u(s)'(r, t), almost at all scales. We show that this is not the case in thermally driven superfluid turbulence; here the counterflow velocity U-ns equivalent to U-n -U-s not equal 0. We suggest a simple analytic model for the cross-correlation function < u(n)'(r, t) center dot u(s)'(r', t)> and its dependence on U-ns. We demonstrate that u(n)'(r, t) and u(s)' (r, t) are decoupled almost in the entire range of separations |r - r'| between the energy-containing scale and intervortex distance.