# Alternative boundary conditions **URL:** \ **Category:** General\ **Created:** [18 August 2022 15:02 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675 "2022-08-18T15:02:01Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![fei](https://avatars.discourse-cdn.com/v4/letter/f/59ef9b/32.png) [@fei](https://pyfr.discourse.group/u/fei)\ **Post date:** [18 August 2022 15:02 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675/1 "2022-08-18T15:02:02Z")
When I apply the boundary conditions, I want to change the values on the wall which constructed the slip boundary condition. I need to write another function “BaseAdvectionBCInters”, but I don’t understand how to calculate **\_scal\_lhs** , **\_mag\_pnorm\_lhs** , **\_norm\_pnorm\_lhs** in the “BaseAdvectionBCInters”. Then I can use my method to calculate the corresponding value. And is there anyone use pyfr to simulate reacting flow? Whether there are relevant papers. ![{5UGX(SJW3A%BSMXOUW3~Y](https://global.discourse-cdn.com/free1/uploads/pyfr/original/1X/293dbc5bb90dcd8310a3f59aad2be6eee12e22c4.png) ![FZXO1SPZ$_N4${0X6AWZ0R](https://global.discourse-cdn.com/free1/uploads/pyfr/original/1X/60d8fd69cc52a6f7d64e0859997540b149eebe0f.png) ![9TREC)ORILZ`LSRV(17AP4](https://global.discourse-cdn.com/free1/uploads/pyfr/original/1X/bbb8a7cedd22d8e9fe08736e0fb4822dc1dbbabb.png) ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [18 August 2022 17:58 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675/2 "2022-08-18T17:58:31Z")
It sounds like you want to create your own BC type. Then you can write the BC kernel function to produce any results you like. A good reference point is the implementation of the existing conditions. Regards, Freddie. ---
**Author:** ![fei](https://avatars.discourse-cdn.com/v4/letter/f/59ef9b/32.png) [@fei](https://pyfr.discourse.group/u/fei)\ **Post date:** [19 August 2022 00:46 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675/4 "2022-08-19T00:46:16Z")
Thanks for your replay. And is there anyone use pyfr to simulate reacting flow? Whether there are relevant papers. ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [19 August 2022 12:48 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675/5 "2022-08-19T12:48:55Z")
Not that I am aware of. This would require a different system of equations. Regards, Freddie. ---
**Author:** ![fei](https://avatars.discourse-cdn.com/v4/letter/f/59ef9b/32.png) [@fei](https://pyfr.discourse.group/u/fei)\ **Post date:** [21 August 2022 14:48 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675/6 "2022-08-21T14:48:27Z")
Whether the right-hand view matrix is the divergence of the flux at the right hand side of Eq. (2.18)? and the left-hand view matrix is the divergence of the flux at the left hand side of Eq. (2.18)? If so, for the BC interface, why only have LHS state? And what’s the mean of **\_scal\_lhs** , **\_mag\_pnorm\_lhs** , **\_norm\_pnorm\_lhs**? Thanks a lot. ![image](https://global.discourse-cdn.com/free1/uploads/pyfr/original/1X/f2ba72f69cef8437fa5edde525c9df4741f8debb.png) ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [21 August 2022 19:47 UTC](https://pyfr.discourse.group/t/alternative-boundary-conditions/675/7 "2022-08-21T19:47:06Z")
BC surfaces also have RHS states, however, they are a function of the LHS state. The definition of this function determines the properties of the BC. Those variables are what make it possible to access the solution, physical normal magnitude, and unit normals, inside of the BC kernels. Regards, Freddie.