A pulsar wind is a relativistic outflow dominated by Poynting energy at its base .
Based on the standard ideal magnetohydrodynamic ( MHD ) model of pulsar wind nebulae ( PWNe ) with the ordered magnetic field , the observed slow expansion v _ { PWN } \ll c requires the wind to be dominated by kinetic energy at the upstream of its termination shock , which conflicts with the pulsar wind theory ( \sigma -problem ) .
In this paper , we extend the standard model of PWNe by phenomenologically taking into account conversion of the ordered to turbulent magnetic field and dissipation of the turbulent magnetic field .
Disordering of the magnetic structure is inferred from the recent three-dimensional relativistic ideal MHD simulations , while magnetic dissipation is a non-ideal MHD effect requiring a finite resistivity .
We apply this model to the Crab Nebula and find that the conversion effect is important for the flow deceleration , while the dissipation effect is not .
Even for Poynting-dominated pulsar wind , we obtain the Crab Nebula ’ s v _ { PWN } by adopting a finite conversion time-scale of \sim 0.3 yr .
Magnetic dissipation primarily affects the synchrotron radiation properties .
Any values of the pulsar wind magnetization \sigma _ { w } are allowed within the present model of the PWN dynamics alone , and even a small termination shock radius of \ll 0.1 pc reproduces the observed dynamical features of the Crab Nebula .
In order to establish a high- \sigma _ { w } model of PWNe , it is important to extend the present model by taking into account the broadband spectrum and its spacial profiles .