# Initial/Boundary Conditions Setup

**URL:** <https://pyfr.discourse.group/t/initial-boundary-conditions-setup/1151>\
**Category:** Cases\
**Created:** [20 August 2024 19:42 UTC](https://pyfr.discourse.group/t/initial-boundary-conditions-setup/1151 "2024-08-20T19:42:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![pyfr\_starting\_man](https://avatars.discourse-cdn.com/v4/letter/p/d26b3c/32.png) [@pyfr\_starting\_man](https://pyfr.discourse.group/u/pyfr_starting_man)\
**Post date:** [20 August 2024 19:42 UTC](https://pyfr.discourse.group/t/initial-boundary-conditions-setup/1151/1 "2024-08-20T19:42:27Z")

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A few quick general questions:

When defining your initial and boundary conditions. How do you scale your parameters such that u = 1, etc? For example, in the test case for triangular airfoil, we have:  
[soln-bcs-inlet]  
type = char-riem-inv  
rho = 1  
u = 1  
v = 0  
w = 0  
p = 31.74603174603175

What exactly does this mean? If we were converting to metric, how would this apply? Also, how are you able to determine Mach number of flow if there is no temperature input?

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**Author:** ![tdzanic](https://avatars.discourse-cdn.com/v4/letter/t/f05b48/32.png) [@tdzanic](https://pyfr.discourse.group/u/tdzanic)\
**Post date:** [20 August 2024 20:15 UTC](https://pyfr.discourse.group/t/initial-boundary-conditions-setup/1151/2 "2024-08-20T20:15:32Z")

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We typically scale geometry characteristic length, density, and velocity to 1 and set the pressure/viscosity to obtain the correct Mach and Reynolds numbers. For Mach, `M = u/c = u/sqrt(gamma*P/rho)`, which gives us `P = rho * u^2/(gamma * M^2)` which equals `1/(gamma*M^2)` if density and velocity are 1. Similarly, `Re = rho * u * L / mu ` so `mu = rho * u * L / Re` which equals `1/Re` if your density, velocity, and geometry characteristic length are 1.