  This paper develops a simple model of the inertial range of turbulent flow,
based on a cascade of vortical filaments. A binary branching structure is
proposed, involving the splitting of filaments at each step into pairs of
daughter filaments with differing properties, in effect two distinct
simultaneous cascades. Neither of these cascades has the Richardson-Kolmogorov
exponent of 1/3. This bimodal structure is also different from bifractal models
as vorticity volume is conserved. If cascades are assumed to be initiated
continuously and throughout space we obtain a model of the inertial range of
stationary turbulence. We impose the constraint associated with Kolmogorov's
four-fifths law and then adjust the splitting to achieve good agreement with
the observed structure exponents $\zeta_p$. The presence of two elements to the
cascade is responsible for the nonlinear dependence of $\zeta_p$ upon $p$.
  A single cascade provides a model for the initial-value problem of the
Navier--Stokes equations in the limit of vanishing viscosity. To simulate this
limit we let the cascade continue indefinitely, energy removal occurring in the
limit. We are thus able to compute the decay of energy in the model.
