Environmental Engineering Reference
In-Depth Information
2. Brief Outline of the Models
MicroSpray is part of the model system MSS (Tinarelli et al., 2007; Moussafir et al.,
2004) that comprises MicroSwift (it is an analytically modified mass consistent
interpolator over complex terrain, able to derive a diagnostic turbulence inside the
flow zones modified by obstacles) and MicroSpray (it is a Lagrangian Particle
Dispersion model derived from SPRAY, able to account for the presence of
obstacles). The new MicroSpray version is oriented to deal with dense and light
gas dispersion in urban environment and industrial sites. It includes new algorithms
to simulate dense and light gas dispersion in the following cases: plume with initial
momentum in any direction, plume without initial momentum and plume spread at
the ground due to gravity.
The Mercure model (Carissimo et al., 1997) includes: 3-D flow simulation,
influence of terrain and obstacles, multiple fluids and full non-hydrostatic formu-
lation. Mercure solves the classic Navier-Stokes equations system with adaptations
for multiple fluids and for passive scalar tracer variables. A conservation relation
for thermodynamic energy (enthalpy or virtual potential temperature) is optionally
solved. Solving the thermal energy equation implies that thermal buoyancy (or
dense) effects are included in the solution. Turbulence closure is by means of
supplementary equations for the conservation of turbulent kinetic energy and dis-
sipation using the k-e model. Important aspects of the Mercure setup for this study
include: (1) ideal gas equation of state, (2) Boussinesq approximation is used,
implying that density variations only affect the flow through buoyancy (or dense)
terms, (3) gravity is the only retained volume force (Coriolis effects are ignored),
(4) thermal forcing due to radiative flux divergence is negligible.
3. Intercomparison Mercure/MSS
Simulations with Mercure and MSS have been performed in several different flow
conditions. Here we propose a case of a high horizontal momentum jet, characterized
by 50 m/s of gas speed at the source, released in neutral stability conditions. The
wind velocity of the environment flow is 5 m/s at a height of 10 m The emission
stack is 10 m high. A regular obstacle (Lx = 26.2 m, Ly = 23.3 m and H = 47 m),
whose base centre was located 50 m downwind the source, was included in the
domain. In order to verify that the two models behave similarly in a base case,
both Mercure and MSS have firstly considered a continuous emission without
dense gas effects. Then the two models simulated an emission two times denser
than air.
Figure 1
shows the iso-surface of 0.01 kg/kg for the mixing ratio of con-
centration obtained for the neutral gas by the two models.
Figure 2
shows instead
the comparison of the dense emission case.
