# Flow through a 90°pipe bend (2)

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November 6, 2016, 00:07
Flow through a 90°pipe bend (2)
#1
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Hi,

Can somebody explain me why when I impose parabolic velocity profile at inlet of pipe flow, there is a separation region of flow near the inside of the bend whereas when I impose a turbulent flow velocity profil (power-law velocity profile) the separation region appears at the end of the bend only (90°).

Is it because with a parabolic velocity profil, the thickness of the shear zones is more important than with a power-law velocity profil and therefore the velocity that arrives near the inside of bend is smaller so the centripetal forces is not high enough in this location to balance radial pressure gradient ?
Attached Images
 ParabolicVelocityProfile.PNG (38.4 KB, 17 views) PowerVelocityProfile.PNG (36.7 KB, 15 views)

 November 6, 2016, 05:45 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,984 Rep Power: 107 Is this simulation turbulent or laminar? What Re number?

November 6, 2016, 09:27
#3
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The two simulations are turbulent with Re = 2*10^6

Quote:
 Originally Posted by ghorrocks Is this simulation turbulent or laminar? What Re number?

 November 6, 2016, 18:32 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,984 Rep Power: 107 In that case you will probably find you have different turbulence profiles associated with the different velocity profiles. Turbulence affects separations, and so I suspect that is why the different profiles have different separations.

November 6, 2016, 18:59
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Yes, you have right, the evolution of turbulence quantities is not the same although they have been imposed with the same values at inlet.
Is the evolution different because they depend on the flow field ?

The adverse pressure gradient at the outlet of the bend, near the inner wall, can also contribute to the separation, isn't it ?

Quote:
 Originally Posted by ghorrocks In that case you will probably find you have different turbulence profiles associated with the different velocity profiles. Turbulence affects separations, and so I suspect that is why the different profiles have different separations.

 November 6, 2016, 19:26 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,984 Rep Power: 107 In simple terms: Turbulence is generated by shear stresses and dissipated by viscosity. So when you change the shear stresses you change the production/dissipation balance the turbulence level changes. Yes, adverse pressure gradients also contribute to turbulence (although it might not be modelled very accurately in the turbulence model).

November 21, 2016, 22:07
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Thank you for your response, it's a little more clear now.

But, is it trivial for you that turbulence intensity and turbulence length increase at the end of the bend ?

Should we not talk about coherent structure in a bend ?
Quote:
 Originally Posted by ghorrocks In simple terms: Turbulence is generated by shear stresses and dissipated by viscosity. So when you change the shear stresses you change the production/dissipation balance the turbulence level changes. Yes, adverse pressure gradients also contribute to turbulence (although it might not be modelled very accurately in the turbulence model).

 November 21, 2016, 22:15 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,984 Rep Power: 107 The points I have mentioned are just general characteristics of turbulence. For exactly what factors are contributing in this specific configuration will require some research and experimentation. I do not have the time to do this for you, so I will leave this more detailed study up to you. It is an interesting case so it is worth investigating it a bit more deeply if you can.

November 27, 2016, 19:13
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Yes of course it's my work. I am just wondering if my question was trivial, apparently it's not.

One other point that I would like a clarification, is it right to say that the intensity of secondary flow after the bend decreases because of viscous dissipation ?

Thank you.

Quote:
 Originally Posted by ghorrocks The points I have mentioned are just general characteristics of turbulence. For exactly what factors are contributing in this specific configuration will require some research and experimentation. I do not have the time to do this for you, so I will leave this more detailed study up to you. It is an interesting case so it is worth investigating it a bit more deeply if you can.

 November 27, 2016, 19:18 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,984 Rep Power: 107 I am not an expert on turbulence but I would think attributing the change intensity of the secondary flow to viscous dissipation is far too simplistic. There is a lot more going on than just viscous dissipation.

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