We model the detailed time-evolution of Discrete Absorption Components ( DACs ) observed in P Cygni profiles of the Si iv \lambda 1400 resonance doublet lines of the fast-rotating supergiant HD 64760 ( B0.5 Ib ) .
We adopt the common assumption that the DACs are caused by Co-rotating Interaction Regions ( CIRs ) in the stellar wind .
We perform 3D radiative transfer calculations with hydrodynamic models of the stellar wind that incorporate these large-scale density- and velocity-structures .
We develop the 3D transfer code Wind3D to investigate the physical properties of CIRs with detailed fits to the DAC shape and morphology .

The CIRs are caused by irregularities on the stellar surface that change the radiative force in the stellar wind .
In our hydrodynamic model we approximate these irregularities by circular symmetric spots on the stellar surface .
We use the Zeus3D code to model the stellar wind and the CIRs , limited to the equatorial plane .
We compute a large grid of hydrodynamic models and dynamic spectra for the different spot parameters ( brightness , opening angle and velocity ) .
We demonstrate important effects of these input parameters on the structured wind models that determine the detailed DAC evolution .

We constrain the properties of large-scale wind structures with detailed fits to DACs observed in HD 64760 .
A model with two spots of unequal brightness and size on opposite sides of the equator , with opening angles of 20 \arcdeg \pm 5 \arcdeg and 30 \arcdeg \pm 5 \arcdeg diameter , and that are 20 \pm 5 % and 8 \pm 5 % brighter than the stellar surface , respectively , provides the best fit to the observed DACs .
The recurrence time of the DACs compared to the estimated rotational period corresponds to spot velocities that are 5 times slower than the rotational velocity .

The mass-loss rate of the structured wind model for HD 64760 does not exceed the rate of the spherically symmetric smooth wind model by more than 1 % .
The fact that DACs are observed in a large number of hot stars constrains the clumping that can be present in their winds , as substantial amounts of clumping would tend to destroy the CIRs .