A study of the Navier-Stokes-alpha model for two-dimensional turbulence
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abstract
The Navier-Stokes- model of turbulence is a mollification of the Navier-Stokes equations, in which the vorticity is advected and stretched by a smoothed velocity field. The smoothing is performed by filtering the velocity field over spatial scales of size smaller than . This is achieved by convolution with a kernel associated with Green's function of the Helmholtz operator scaled by a parameter . The statistical properties of the smoothed velocity field are expected to match those of Navier-Stokes turbulence for scales larger than , thus providing a more computable model for those scales. For wavenumbers k such that k 1, corresponding to spatial scales smaller than , there are three candidate power laws for the energy spectrum, corresponding to three possible characteristic time scales in the model equations: one from the smoothed field, the second from the rough field and the third from a special combination of the two. In two dimensions, the second time scale may be understood to characterize the dynamics of the conserved enstrophy. We measure the scaling of the energy spectra from high-resolution simulations of the two-dimensional Navier-Stokes- model, in the limit as . The energy spectrum of the smoothed velocity field scales as k-7 in the direct enstrophy cascade regime, consistent with dynamics dominated by the timescale associated with the rough velocity field. We are thus able to deduce that the dynamics of the dominant cascading conserved quantity, namely the enstrophy of the rough velocity, governs the scaling of all derived statistical quantities.
