# Create and merge data array
**URL:** https://discourse.paraview.org/t/create-and-merge-data-array/8513
**Category:** ParaView Support
**Created:** [November 29, 2021, 4:18pm UTC](https://discourse.paraview.org/t/create-and-merge-data-array/8513 "2021-11-29T16:18:28Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@fdekerme](https://discourse.paraview.org/u/fdekerme)
#### Post date: [November 29, 2021, 4:18pm UTC](https://discourse.paraview.org/t/create-and-merge-data-array/8513/1 "2021-11-29T16:18:28Z")
Hello,
(Paraview 5.10.0-RC1 on Windows10)
I’m trying to use the LagrangianParticleTrancker, and I’m wondering how to set the “Flow Density” and “Flow Dynamic Viscosity” parameters. In my simulation (in SimVascular), these are fixed values respectively worth
Blood Viscosity = 0.04 g/cmxs2
Blood Density = 1.06 g/cm3
However, they are not present in my “all\_results\_00\*” file. So I would like to create and add them there.
The problem is the following:
1. how to create a point data array with a constant value on the whole volume?
2. how to merge this point data array to my main file “all\_results\_00\*”" in order to be able to set up the Lagrangian tracker.
I tried to define these values (fixed on the whole volume) with the “Calculator”, and then to use the “Group Dataset” filter to add these values to “all\_results\_00\*”. However, both values are displayed as “partial” in the created DataSet.
Do I have to use a Python Programable Filter to do this (as I saw on some topic)?
Thanks !
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [November 30, 2021, 9:05am UTC](https://discourse.paraview.org/t/create-and-merge-data-array/8513/2 "2021-11-30T09:05:31Z")
> [@fdekerme](#):
>
> how to create a point data array with a constant value on the whole volume?
Calculator
> how to merge this point data array to my main file “all\_results\_00\*”" in order to be able to set up the Lagrangian tracker.
File → SaveData on the calculator, but not needed in your case.
> I tried to define these values (fixed on the whole volume) with the “Calculator”, and then to use the “Group Dataset” filter to add these values to “all\_results\_00\*”. However, both values are displayed as “partial” in the created DataSet.
Do not use Group Dataset, Create a Calculator on the first calculator, like this:
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### Author:  [@fdekerme](https://discourse.paraview.org/u/fdekerme)
#### Post date: [November 30, 2021, 2:14pm UTC](https://discourse.paraview.org/t/create-and-merge-data-array/8513/3 "2021-11-30T14:14:06Z")
Thanks for the answer! Indeed the “Calculator2” created contains both the Data Array “Density” and “Viscosity” AND the Data Array of “all\_results\_00\*”.I can therefore set “Calcultor2” as “Flow input” of the “Lagrangian Particle Tracker” to have access to the 2 constants “Density” and “Viscosity”.
However, I realize that the Lagrangian tracking implemented in Paraview calculates the complete trajectories of the particles for each time of my “Time Serie”.I thought the calculation was done like with the “Particle Tracker” filter, where the position of the particles is updated at each time step (which is what I am trying to do)
I think I will have to program a “Python Programmable Filter” to adapt the “Lagrangian Particle Tracker” to my situation.
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [November 30, 2021, 2:33pm UTC](https://discourse.paraview.org/t/create-and-merge-data-array/8513/4 "2021-11-30T14:33:12Z")
Yes, the LPT has an approach similar to the StreamTracer, not to the ParticleTracer, it does not fit your usecase.
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### Author:  [@fdekerme](https://discourse.paraview.org/u/fdekerme)
#### Post date: [December 1, 2021, 8:29am UTC](https://discourse.paraview.org/t/create-and-merge-data-array/8513/5 "2021-12-01T08:29:57Z")
For those who want to know more about the difference between LPT, Stream Tracer and Particle Tracker : [Difference between ParticleTracer and LangrarianParticleTracker - ParaView Support - ParaView](https://discourse.paraview.org/t/difference-between-particletracer-and-langrarianparticletracker/5181)