.. ARIA SIMPAC platform documentation mapdata service file, created by simpac@aria.fr 01/2021. =============================== Dispersion modellings in SIMPAC =============================== .. Note:: In this section, you may find the list of the possible dispersion modellings. ------------------------------------------------------------------------ Further information on dispersion modelling by PSPRAY may be found on the :download:`theoretical training material ` (part 1) and :download:`here ` (part 2). .. Important:: PSPRAY is a lagrangian three-dimensional-dispersion-model. It computes the pollutant plumes's trajectories from the wind fields calculated by PSWIFT. The dispersion is modeled through particles emitted at each time step. PSPRAY is a stochastic transport and diffusion model, meaning that a part of the particules's displacement is random. The emitted particules are *pseudo* particles and they each carry a part of the emitted mass. In the following formula, :math:`\overline{X}_0(t)` stands for the *position of a particle at a time* :math:`t`, :math:`\overline{U}\left(\overline{X}_0(t),t\right)`, the *local component of average wind (PSWIFT)* and :math:`U'\left(\overline{X}_0(t),t\right)` the *local component of the turbulence*. .. Admonition:: Equation of PSPRAY lagrangian model *The equation of transport may be written:* .. math:: \overline{X}_0(t+\mathrm{d} t)= \overline{X}_0(t) + \overline{U}\left(\overline{X}_0(t),t\right)\mathrm{d} t+ U'\left(\overline{X}_0(t),t\right)\mathrm{d} t *The inverse dispersion model means:* .. math:: \overline{X}_0(t-\mathrm{d} t)= \overline{X}_0(t) - \overline{U}\left(\overline{X}_0(t),t\right)\mathrm{d} t- U'\left(\overline{X}_0(t),t\right)\mathrm{d} t ------------------------------------------------------------------------ **On the SIMPAC platform, particles are emitted every ten seconds.** *The calculation time of a lagrangian model increases linearly with the number of particles in the calculation domain*. The number of emitted particles by time step relies on the configuration. *For the SIMPAC platform, the number of emitted particles is:* - 1 per source and per time step of emission for the **trajectory configuration** (in direct or reverse mode). Such simulations allow an indication of the direction or origin of the plume. The calculation is too noisy to make an estimation of the concentrations possible. The dispersed species is a tracer, and the the emitted flow is fixed at 1kg/h (1mcg/m3 for inverse mode). - 100 for all sources and per time step of emissions for **unitary configuration** (available only in direct mode). Such simulations allow an indication of the transport and dispersion of the plume. The calculation allows an estimation of the pollutant concentrations relative to unitary sources. The user has the choice of the dispersed species, but the emitted flow is fixed at 1kg/h. - 600 for all sources and per time step of emissions for the **cartographic configuration** (for direct or reverse mode). Such simulations allow an indication of the transport and dispersion of the plume and the calculation of pollutant concentrations with respect to real sources. The user has the choice of the species and quantities emitted. ------------------------------------------------------------------------ **On the SIMPAC platform, particles are emitted at times and locations determined beforehand by the user.** - In **direct mode**, the user gives information about the geometric features (the position (X, Y), the ground-height, the source's diameter) and about the physical features of the point-source, namely the pollutants' ejection speed, the source's temperature, and the emissions' dates. The user indicates the emitted quantity of a pollutants' outflow (kg/h). - In **direct mode**, the particles can also be emitted from a surface source. The description of the emissions' polygon is realized with the computer mouse through the interface (represented on the picture below). The ground-height, the pollutants' ejection speed, the emissions' dates are given by the user. The user indicates the emitted quantity of a pollutants’ outflow (kg/h). .. figure:: ./images/Emission'spolygon.png :align: center :width: 80ex Creation of an emission's polygon through the SIMPAC platform .. Admonition:: Caution 1 - A source for the dispersion model emits at least one particle per time step (:math:`dt_{min} = 10 s`). - The initial location of the particle is randomly selected within the source. - A source defined through the SIMPAC's interface is not necessarily understood as a source for the dispersion model. *The surface sources are automatically partioned into triangles or projected on the grid.* - *One triangle source* :math:`\Longleftrightarrow` *one triangle for the dispersion model* - *One rectangle source* :math:`\Longleftrightarrow` *two triangles for the dispersion model* - *One polygon source* :math:`\Longleftrightarrow` *as many sources as there are grid cells covering the surface for the dispersion model* - For a polygon source: - A polygon source with an area smaller than the mesh will be spread over the mesh. - The projection on the grid of a polygon means a larger number of point sources, and therefore a longer computation time. In the example below, the polygon is described in 4532 sources. .. figure:: ./images/ImageSlide5.png :align: center :width: 80ex Example of a triangle source (left), of a rectangle source (center) and of a polygon source (right) - For a surface ground fire, the interface assesses the absolute power and the emitted pollutants. To keep the SIMPAC platform easy to use, the burning material is pre-established according to the kind of fire. The pollutants' list is determined regarding the kind of fire, and the flow emitted regarding the information given on the fire, meaning the burning material mass and whether the fire is well or poorly ventilated. .. Admonition:: The assumptions for the calculation are the following: .. math:: \begin{array}{l} 100\% C \Longrightarrow CO + CO_2 \\ 100\% Cl \Longrightarrow H Cl\\ 100\% SO_2 \Longrightarrow SO_2 \end{array} *Following the recommendation of the INERIS, three types of fire can be modelized.* The burning speed is derived from the literature `[1] `_ . **The duration of the fire is chosen by the user and is not limited by the amount of material burned.** - **Burning buildings** : ranges made of 30% wood, 50% of products, 15% of P.E and 5% of burning PVC. :math:`CO_2`, :math:`CO` and :math:`HCl` are the emitted pollutants. The burning speed were obtained through the literature (*An introduction to fire dynamic*, 2nd Dougal Drusdale). - **Pneumatical burning** : the composition of the tires are derived from the literature `[2] `_ and the emitted particles are :math:`CO_2`, :math:`CO`, and :math:`SO_2`. The burning speed were obtained through the literature `[3] `_. - **Fuel oil burning**, containing 34% of petrol, 33% fuel oil and 33% of diesel fuel. These values were arbitrarily chosen. The elemental composition of each component is taken from the literature `[4] `_ . :math:`CO_2` and :math:`CO` are the emitted pollutants. *In the case of a poorly ventilated fire, the combustion's area corresponds to 10% of the entire area, and the burning speed is approximately a factor of 2.5 lower than if well ventilated.* :math:`CO_2` emissions compared to :math:`CO` emissions decrease from :math:`\frac{[CO_2]}{[CO]}=15.7\mathrm{kg.kg}^{-1}` to :math:`5\mathrm{kg.kg}^{-1}`, i.e. :math:`\frac{[CO_2]}{[CO]}=10\mathrm{mol.mol}^{-1}` to :math:`3.2\mathrm{kg.kg}^{-1}`. .. Admonition:: Caution 2 In a case of a poorly ventilated fire, the decrease of the thermal power reduces the final plume rise and explains the stronger ground track on the following example. .. figure:: ./images/ImageSlide13.png :align: center :width: 80ex Example of a properly ventilated fire (on the left) and of a poorly ventilated fire (on the right) *Usually, the fire outbreak, that is to say before the walls go down, corresponds to the poorly ventilated case.* *In a properly ventilated fire, one molecule of CO is emitted for 10 molecules of CO2.* *The rate is of one third for a poorly ventilated fire. The reduction of the fire area and of the burning rate explains the reduction of the thermal power and of the burning flow.* - In the **reverse mode**, the initial position of the Lagrangian particles represents that of a receptor. Only its position ( X, Y) and the ground height are given. The user notices the pollutants' quantity by a concentration expressed in mcg/m3. .. Important:: For assessing the concentrations, the virtual particles are projected on the horizontal calculation grid, and on a two-meter elevation tape. The averages of the fifteen-minute-ground-concentration are displayed. The concentrations are emitted by all the pollutants's sources. The estimation of the source's location (cartographic configuration in reverse mode) only works for now with one pollutant. There are as many retro-plumes as receptors. The assessment of the source's location is obtained by counting the overlaps of the retro-plumes. The most likely source's location is where the highest number of retro-plumes are situated, at the same place and at the same time.