# About Non-dimensionalization

**URL:** <https://pyfr.discourse.group/t/about-non-dimensionalization/601>\
**Category:** General\
**Created:** [9 May 2022 14:46 UTC](https://pyfr.discourse.group/t/about-non-dimensionalization/601 "2022-05-09T14:46:52Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Zhenyang](https://avatars.discourse-cdn.com/v4/letter/z/ec9cab/32.png) [@Zhenyang](https://pyfr.discourse.group/u/Zhenyang)\
**Post date:** [9 May 2022 14:46 UTC](https://pyfr.discourse.group/t/about-non-dimensionalization/601/1 "2022-05-09T14:46:52Z")

</div>

Hi all,

I have this question for quite a long time. I want to know more about how the code deal with non-dimensional values. The non-dimensional compressible NS should have a Reynolds number appear in the viscous term. But the compressible solver only asks for viscosity as an input. So how do you calculate other reference values that Reynolds number needs. And viscosity should be a function of temperature as well. So my guessing is everything is based on initial condition I set?

Best wishes,  
Zhenyang

---

<div class="post-metadata">

**Author:** ![WillT](https://avatars.discourse-cdn.com/v4/letter/w/bc79bd/32.png) [@WillT](https://pyfr.discourse.group/u/WillT)\
**Post date:** [9 May 2022 17:35 UTC](https://pyfr.discourse.group/t/about-non-dimensionalization/601/2 "2022-05-09T17:35:09Z")

</div>

This might be a good topic to start with, if you haven’t already read it. [Transonic 2D Cylinder flow NaN](https://pyfr.discourse.group/t/transonic-2d-cylinder-flow-nan/538)

---

<div class="post-metadata">

**Author:** ![Zhenyang](https://avatars.discourse-cdn.com/v4/letter/z/ec9cab/32.png) [@Zhenyang](https://pyfr.discourse.group/u/Zhenyang)\
**Post date:** [10 May 2022 08:56 UTC](https://pyfr.discourse.group/t/about-non-dimensionalization/601/3 "2022-05-10T08:56:59Z")

</div>

Hi Will,

Thanks for reply. I still have question about it. So PyFR doesn’t solve non-dimensional equations, it treats any input as dimensional? As long as I can write done my relationship to get desired Mach number and Reynolds number, it will take those input variables rho,u,v,w,p as reference from initial condition? Am I right in this way?

---

<div class="post-metadata">

**Author:** ![WillT](https://avatars.discourse-cdn.com/v4/letter/w/bc79bd/32.png) [@WillT](https://pyfr.discourse.group/u/WillT)\
**Post date:** [11 May 2022 09:37 UTC](https://pyfr.discourse.group/t/about-non-dimensionalization/601/4 "2022-05-11T09:37:35Z")

</div>

PyFR doesn’t care about dimensions at all. So it neither uses reference density, temperature, and pressure to non-dimensionalise things nor does it output results in set dimensions. It is up to the user to interpret the output as either m/s, inches/hours or light years/sol.