  The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric
surveys, since its identification as a "rotating pinwheel nebulae", where
infrared images display a spiral dust lane revolving with a 1.4 year
periodicity. WR 98a hosts a WC9+OB star, and the presence of dust is puzzling
given the extreme luminosities of Wolf-Rayet stars. We present 3D hydrodynamic
models for WR 98a, where dust creation and redistribution are self-consistently
incorporated. Our grid-adaptive simulations resolve details in the wind
collision region at scales below one percent of the orbital separation (~4 AU),
while simulating up to 1300 AU. We cover several orbital periods under
conditions where the gas component alone behaves adiabatic, or is subject to
effective radiative cooling. In the adiabatic case, mixing between stellar
winds is effective in a well-defined spiral pattern, where optimal conditions
for dust creation are met. When radiative cooling is incorporated, the
interaction gets dominated by thermal instabilities along the wind collision
region, and dust concentrates in clumps and filaments in a volume-filling
fashion, so WR 98a must obey close to adiabatic evolutions to demonstrate the
rotating pinwheel structure. We mimic Keck, ALMA or future E-ELT observations
and confront photometric long-term monitoring. We predict an asymmetry in the
dust distribution between leading and trailing edge of the spiral, show that
ALMA and E-ELT would be able to detect fine-structure in the spiral indicative
of Kelvin-Helmholtz development, and confirm the variation in photometry due to
the orientation. Historic Keck images are reproduced, but their resolution is
insufficient to detect the details we predict.
