The TUFLOW Particle Tracking Module (PTM) allows the 2D or 3D simulation of discrete Lagrangian particles as they are transported by the flow field (or other drivers such as wind). Particle behavior such as settling, buoyancy, decay, sedimentation and re-suspension can all be simulated. The tracking of discrete particles can be used to output particle fate and age, which is useful for purposes of animal migration, search and rescue, environmental contaminants and oil spill modelling.
The PTM is invoked through the TUFLOW FV hydrodynamic engine which controls the overall simulation and supplies the hydrodynamic forcing to the particle transport module.
The tutorial will cover a number of aspects of the particle tracking model:
For the purposes of the particle tracking tutorial, we'll be using the hydrodynamic model created in Tutorial Module 5 (Estuary Application) and supplementing with additional information to represent the particles.
Setting up a Point Source of a Group of Particles
Creating a .FVPTM file
The first step in the TUFLOW FV particle tracking module is to generate an .fvptm file which defines the commands to define particle characteristics and processes. This .fvptm file is then referenced via the TUFLOW FV control file (.fvc).
- Use notepad++ to create a new file with extension .fvptm. Name the file PT_000.
- Add the following commands to define a group of particles representing microplastics which enter our model via a combined sewer overflow within our eastuarine reach and can flow out of the model at the downstream tidal boundary.
!PTM to asses fate of Combined Sewer Overflow Outflow ! use nodestring definition from HD model to allow particles to leave model domain. Open boundary nodestring == 2 !downstream tidal boundary !Time Commands !_________________________________________________________________________________________________________________________________ langrangian timestep == 60.!seconds eulerian timestep == 60. !seconds !Global Commands !_________________________________________________________________________________________________________________________________ Nscalar == 1 !Particle Group Commands</tt> !_________________________________________________________________________________________________________________________________ group == microplastics initial scalar mass == 1000 settling model == none horizontal dispersion model == constant horizontal dispersion parameters == 1.0 vertical dispersion model == HD model vertical dispersion parameters == 1.0 erosion model == none deposition model == none end group !_________________________________________________________________________________________________________________________________ !CSO seed particles== point, 159.09971, -31.39463, particle groups== microplastics timeseries file== ..\bc_dbase\CSO.csv timeseries header== Date,microplastics end seed !_________________________________________________________________________________________________________________________________ !Output Settings output dir== ..\results\ output== ptm_netcdf output groups== microplastics output interval== 1800. output parameters== uvw, depth, water_depth, uvw_water end output
With the Nscalar command the scalar mass constituents are defined. These can be thought of as separate mass ‘containers’ that reside on a particle.
With the Group settings we are specifying the characteristics of the particles. In this exercise the particles defined won't settle, deposit on the bed or be subject to erosion. We will use a constant horizontal dispersion model and use the hydrodynamic model to define the vertical dispersion.
With the Seed Particles settings we are specifying that the particles are added to the model at a point inflow at co-ordinate 159.09971, -31.39463. The particles will be added to the model based on a mass flux time series defined in an CSO.csv file which we define in a later step.
With the Output Settings settings, we're specifying the location and format of the outputted results as well as the output interval and type for our group of particles. These are Lagrangian outputs specific to the individual particles. Eulerian outputs, for example the concentration of particles within the water column, are specified in the TUFLOW FV Control File (.fvc)
- Once the above commands have been added, save and close the PT_000.fvptm file
Mass Flux Time Series File
Part of the commands added in the .fvptm, was to define the seed particles for the microplastics. In this instance, the particles to be tracked are added at a point within the model domain at 159.1269, -31.41123. There is also a reference to a csv file in the bc_dbase folder which is the mass flux time series file. This defines the temporal variation of the particle input into the model with the units in grams/second. This works in conjunction with the initial scalar mass value we defined in the PTM control file to determine the number of particles added to the model.
The number of particles per second is equal to:-
Time Series Mass (g/s)/Initial Scalar Mass (g)
- Create a csv file called CSO.csv within the bc_dbase folder.
- Add column headers ‘Date’ and ‘Plastics’ and populate with the following values:
Your CSV file should look like the below. This will introduce particles to our model at the beginning of the simulation for a period of 1 hour.
The time series mass flux value is 20g/s. The initial scalar mass defined in our particle group settings is 1000g. Therefore the number of particles released per second is:-
20/1000=0.02 particles per second. Over the 1 hour release period, this equates to 72 particles being released in total.
- Save and close the CSO.csv file.
Update the TUFLOW FV Control File
With the changes to the model, we now need to update our TUFLOW FV control file (.fvc) to read in the additional data which define our particle tracking module.
- Copy the existing HYD_002.fvc file and rename to PT_000.fvc.
- Add the following reference to the PT_000.fvc file in order to reference are TUFLOW FV particle tracking module control file.
!Particle Tracking Input !_________________________________________________________________________________________________________________________________ Particle Tracking Control File dir== PT_000.fvptm
- Also within the output commands add the PTM_1 results parameter to the netcdf output parameters.
output== netcdf output parameters== h,v,d, PTM_1 output interval== 3600 end output<
- Save and close the PT_000.fvc file