Farfield BCs for compressible flows

celestial · July 22, 2021, 10:37

Hi everybody

I want to avoid an ill-posed problem - either under or over specified. What are permissible combinations of primitive variables like pressure, density, temperature and velocity in the far field boundary for

1) an external flow (a domain containing an airplane wing for example)

2) an internal flow (a pipe with inlet and outlet as the far field boundaries)

Best regards

celestial · July 23, 2021, 14:08

I could have added that using various combinations of static and total variables (variables like pressure temperature) seems to be the norm in compressible CFD applications. But it would be important to be able to derive these combinations from the well-known theory of characteristic variables in order to ensure we are working with a well-posed problem.

Thank you all

dlahaye · July 24, 2021, 06:13

Possibly it helps to look into Chapter 16 of the book by Moukalled, Manga and Darwish, The finite volume method in computational fluid dynamics, Springer 2015 , http://www.springer.com/gp/book/9783319168739.

Other possible relevant information follows

* general information on boundary conditions: https://www.openfoam.com/documentati...onditions.html

* difference between imposed and calculated boundary conditions. Imposed boundary conditions remain fixed throughout computations. Calculated boundary conditions maybe be updated throughout computations. Imposed fixed (as opposed to calculated) conditions on density proved to be essential to reach convergence in our cases;

* valuable discussiion updateCoeff() and evaluate(): updateCoeffs() and evaluate()

* use of “value” in some boundary conditions: Clarification on k and omega settings

* wave transmissive boundary conditions: from the users guide https://www.openfoam.com/documentati...8H_source.html or https://openfoamwiki.net/index.php/H...dary_condition (confusing to read)

celestial · July 28, 2021, 23:27

Thank you so much, Domenico, for having provided 6 links. Unfortunately, my university does not own the book by Moukalled so that I will have to wait at least a week for the inter library loan before reading Chapter 16. I know of CFD books by Hirsch and Laney that show the matrix transformations between primitive, conservative and characteristic variables but I have yet to find a way to derive a proper static and total variable combination in the far field.
I will keep everybody posted whenever I am successful.

Marc

July 22, 2021, 10:37	Farfield BCs for compressible flows	#1
celestial Member Join Date: Mar 2019 Posts: 86 Rep Power: 7	Hi everybody I want to avoid an ill-posed problem - either under or over specified. What are permissible combinations of primitive variables like pressure, density, temperature and velocity in the far field boundary for 1) an external flow (a domain containing an airplane wing for example) 2) an internal flow (a pipe with inlet and outlet as the far field boundaries) Best regards

July 24, 2021, 06:13		#3
dlahaye Senior Member Domenico Lahaye Join Date: Dec 2013 Posts: 722 Blog Entries: 1 Rep Power: 17	Possibly it helps to look into Chapter 16 of the book by Moukalled, Manga and Darwish, The finite volume method in computational fluid dynamics, Springer 2015 , http://www.springer.com/gp/book/9783319168739. Other possible relevant information follows * general information on boundary conditions: https://www.openfoam.com/documentati...onditions.html * difference between imposed and calculated boundary conditions. Imposed boundary conditions remain fixed throughout computations. Calculated boundary conditions maybe be updated throughout computations. Imposed fixed (as opposed to calculated) conditions on density proved to be essential to reach convergence in our cases; * valuable discussiion updateCoeff() and evaluate(): updateCoeffs() and evaluate() * use of “value” in some boundary conditions: Clarification on k and omega settings * wave transmissive boundary conditions: from the users guide https://www.openfoam.com/documentati...8H_source.html or https://openfoamwiki.net/index.php/H...dary_condition (confusing to read) Malkocoglu likes this.

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July 23, 2021, 14:08		#2
celestial Member Join Date: Mar 2019 Posts: 86 Rep Power: 7	I could have added that using various combinations of static and total variables (variables like pressure temperature) seems to be the norm in compressible CFD applications. But it would be important to be able to derive these combinations from the well-known theory of characteristic variables in order to ensure we are working with a well-posed problem. Thank you all

July 28, 2021, 23:27		#4
celestial Member Join Date: Mar 2019 Posts: 86 Rep Power: 7	Thank you so much, Domenico, for having provided 6 links. Unfortunately, my university does not own the book by Moukalled so that I will have to wait at least a week for the inter library loan before reading Chapter 16. I know of CFD books by Hirsch and Laney that show the matrix transformations between primitive, conservative and characteristic variables but I have yet to find a way to derive a proper static and total variable combination in the far field. I will keep everybody posted whenever I am successful. Marc