Slip or Non-slip conditions?

nsi · March 7, 2018, 19:53

Dear CFD fellows!

Can I impose non-slip conditions at wall in an Euler equations solver (meaning, compressible and inviscid flow), or I MUST use slip conditions?

If latter, how do I impose slip conditions? I know for normal velocity, we have

vn = 0;

and for other primitive variables, we have

drho/dn = 0; dE/dn = 0 (dp/dn = 0);

but we still need two more.

nsi

piu58 · March 8, 2018, 00:56

It depends of what you try to simulate.

If thee are real walls, you need to use a non slip b.c.

If you have simply the borders at the region you want to simulate, but in reality the area/volume is more extended, use slip b.c.

FMDenaro · March 8, 2018, 04:11

Euler equations are first order, therefore only one BC.s can be prescribed to be the problem well posed. If you would set no-slip BC.s how do you ensure the body is not-permeable?

selig5576 · March 13, 2018, 12:17

Maybe this is obvious, but I would like to note that in implementing no-slip BC, it is not the same as in incompressible NS. For incompressible NS (assuming a collocated grid) you can literally set $\mathbf{u} \cdot \mathbf{n} = 0$ . In my experience with compressible Euler equations you must use interpolation with a layer of ghost cells otherwise you run into problems. For the left BC: $\frac{u_{1} + u_{-1}}{2} = u_{wall} \Rightarrow u_{1} = 2 u_{wall} - u_{-1}.$ In my experience (which could be due to programming errors) simplying setting $u_{1} = 0,$ is not sufficient since you have to take into account the other primitive variables.

nsi · March 13, 2018, 15:54

Quote:

Originally Posted by selig5576

Maybe this is obvious, but I would like to note that in implementing no-slip BC, it is not the same as in incompressible NS. For incompressible NS (assuming a collocated grid) you can literally set $\mathbf{u} \cdot \mathbf{n} = 0$ . In my experience with compressible Euler equations you must use interpolation with a layer of ghost cells otherwise you run into problems. For the left BC: $\frac{u_{1} + u_{-1}}{2} = u_{wall} \Rightarrow u_{1} = 2 u_{wall} - u_{-1}.$ In my experience (which could be due to programming errors) simplying setting $u_{1} = 0,$ is not sufficient since you have to take into account the other primitive variables.

Thanks! What if it is not a left/right/top/bottom boundary, but an immersed body?

selig5576 · March 13, 2018, 23:09

Quote:

Originally Posted by nsi

Thanks! What if it is not a left/right/top/bottom boundary, but an immersed body?

Unfortunately, I am not experienced in the immersed boundary method. I would look at the CFD book by Anderson or Versteeg. They might discuss type of situation. With that said, if I recall correctly Versteeg deals with more incompressible flow numerics.

March 7, 2018, 19:53	Slip or Non-slip conditions?	#1
nsi New Member Join Date: Oct 2017 Posts: 7 Rep Power: 8	Dear CFD fellows! Can I impose non-slip conditions at wall in an Euler equations solver (meaning, compressible and inviscid flow), or I MUST use slip conditions? If latter, how do I impose slip conditions? I know for normal velocity, we have vn = 0; and for other primitive variables, we have drho/dn = 0; dE/dn = 0 (dp/dn = 0); but we still need two more. nsi

March 8, 2018, 00:56		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	It depends of what you try to simulate. If thee are real walls, you need to use a non slip b.c. If you have simply the borders at the region you want to simulate, but in reality the area/volume is more extended, use slip b.c. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

March 8, 2018, 04:11		#3
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	Euler equations are first order, therefore only one BC.s can be prescribed to be the problem well posed. If you would set no-slip BC.s how do you ensure the body is not-permeable? selig5576 likes this.

March 13, 2018, 12:17	Boundary conditions for Euler equations	#4
selig5576 Senior Member Selig Join Date: Jul 2016 Posts: 213 Rep Power: 10	Maybe this is obvious, but I would like to note that in implementing no-slip BC, it is not the same as in incompressible NS. For incompressible NS (assuming a collocated grid) you can literally set $\mathbf{u} \cdot \mathbf{n} = 0$ . In my experience with compressible Euler equations you must use interpolation with a layer of ghost cells otherwise you run into problems. For the left BC: $\frac{u_{1} + u_{-1}}{2} = u_{wall} \Rightarrow u_{1} = 2 u_{wall} - u_{-1}.$ In my experience (which could be due to programming errors) simplying setting $u_{1} = 0,$ is not sufficient since you have to take into account the other primitive variables.

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