- **OpenFOAM**
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- - **low Mach compressible flows**
(*https://www.cfd-online.com/Forums/openfoam/94282-low-mach-compressible-flows.html*)

low Mach compressible flowsDear all openfoam users,
Any body suggest which solver is suitable for The numerical simulation of low Mach compressible flows. Is there any preconditioning method available in openfoam. Thanking you, |

Dear venkataramana,
I am using successfully the rhoPisoFoam class of solvers for Low Mach compressible flows. Which kind of application are you interested in ? Do you really have a weakly compressible flow or a variable density (combustion) flow to solve ? My best regards, Franco |

Dear
Franco,I have one question, Through rhopisoFoam or some other is it possible to solve for density = constant. Regards, |

Dear
Franco,thanks for your reply, I am interested in weakly compressible flows I have some questions, 1) there are many solvers for compressible flows, how to identify which solver is suitable for weakly compressible flows 2)Particularly in openFoam is there any solver (compressible flow solver) to solve incompressible flows (density = 0). But we have icoFoam, simpleFoam for incompressible flows. Through rhopisoFoam or some other is it possible to solve for density = 0. any comments or suggestions are welcome. Regards, |

Dear Venkataramana,
sorry for my late answer. I was offline during the weekend. About the first question: - what I know is that for weakly compressible flows you need a procedure that allows to go behind the limit of stability established by the Courant number, computed with the largest signal velocity that arise in your system. In the case of weakly compressible flows this is usually the u+c velocity, where u is the flow velocity and c the speed of sound. This can be achieved in several ways: preconditioning, artificial compressibility and projection are probably the most common approaches. I recognize the Piso algorithm to belong to the last class of approaches. Therefore, in practice, you need to recognize if in the solver you have chosen (the beauty of OpenFOAM is especially that, thanks to the wonderful cpp coding, you can immediately realize in the main code the general algorithm implemented), the iterative procedure corresponding to the projection algorithm is present. - the second question is unclear to me. density = 0 is equivalent to vacuum conditions. This is not the meaning of incompressible flows, that mathematically correspond to ensure $\div \rho = 0$, being \rho the density and \div the divergence operator . Kind regards, Franco |

Dear
Franco,thanks for your reply, I am sorry about the second question it was my mistake here density = constant question number 2) Particularly in openFoam is there any solver (compressible flow solver) to solve incompressible flows (density = constant or Mach Number approaching 0 ). But we have icoFoam, simpleFoam for incompressible flows. Through rhopisoFoam or some other is it possible to solve for density = constant. Regards, |

one more question here
out of these approaches "preconditioning, artificial compressibility and projection " in openFoam,is there any solver adopting the above approaches? Regards, |

Dear Venkataramana,
the rhoPisoFoam solver, as well as many others suitable for combustion cases (that is my first subject of research work), is able to deal with cases where rho can be costant . It is a projection methos in the sense that a vector field of known divergence is sought at every time step. This leads to the elliptic equation for the pressure that give you the possibility to know how pressure disturbances propagate in the whole field at each time step. Several other details need to be specified to say if you are considering only really incompressible flow or weakly compressible flows. I do not have a so deep knowledge of all the OpenFOAM solvers to say you how many solvers exists for weakly compressible flows. Combustion solvers are usually suitable for low Mach flows, even to the limit of rho=const. However I do not know of any solver adopting a preconditioning approach. I have a vague recollection of a post in the forum about artificial compressibility method. Maybe a search in the forum could help you. Kind regards, Franco |

Dear Franco,
Thank you for your kind information, as per my knowledge The projection method is an effective means of numerically solving time-dependent incompressible fluid-flow problems. It was originally introduced by Alexandre Chorin in 1967 and independently by Roger Temam as an efficient means of solving the incompressible Navier-Stokes equations. The key advantage of the projection method is that the computations of the velocity and the pressure fields are decoupled.How it is applicable for compressible flows, any modification we need to do in the solver, and rhopisoFoam developed developed for compressible flows regards, |

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