  A quantitative understanding of the dominant mechanisms that govern the
generation and decay of the counter-rotating vortex pair (CVP) produced by
yawed wind turbines is needed to fully realize the potential of yawing for wind
farm power maximization and regulation. Observations from large eddy
simulations (LES) of yawed wind turbines in the turbulent atmospheric boundary
layer and concepts from the airplane trailing vortex literature inform a model
for the shed vorticity and circulation. The model is formed through analytical
integration of simplified forms of the vorticity transport equation. Based on
an eddy viscosity approach, it uses the boundary layer friction velocity as the
velocity scale and the width of the vorticity distribution itself as the length
scale. As with the widely used Jensen model for wake deficit evolution in wind
farms, our analytical expressions do not require costly numerical integration
of differential equations. The predicted downstream decay of maximum vorticity
and total circulation agree well with LES results. We also show that the
vorticity length scale grows linearly with downstream distance and find several
power laws for the decay of maximum vorticity. These results support the notion
that the decay of the CVP is dominated by gradual cancellation of the vorticity
at the line of symmetry of the wake through cross-diffusion.
