Particle velocity and Stokes number assumptions
Posted: June 17th, 2021, 2:01 pm
I have a few questions about some of the basic assumptions of the Langevin model used by HYSPLIT:
- The (virtual or Lagrangian) particle velocity is given by the sum of the mean fluid velocity (from the meteorological data) and the turbulent fluid velocity component (determined using the [discretized] turbulent fluid velocity Langevin equation). This assumes then that the (virtual or Lagrangian) particle velocity is given by the carrier fluid (i.e. air) velocity, yes? This is to say that there is no difference between particle velocity and instantaneous / total fluid velocity at the particle position.
- If this is the case, does this implicitly require that the Stokes number of the physical particles represented by the virtual or Lagrangian particles be very small so that particles respond sufficiently rapidly to the carrier fluid flow velocity so as to have equal velocity? If not, why not?
- If that is correct, then what is an approximate upper bound on the particle response time and the particle diameter for a typical solid particle density (e.g. 2500 kg/m^3) which could be modeled with the HYSPLIT Langevin model?