|
[Sponsors] |
![]() |
![]() |
#1 |
Member
Nguyen Trong Hiep
Join Date: Aug 2018
Posts: 48
Rep Power: 8 ![]() |
Hi, i have some problem with flow in pipe
My case is an ideal gas flow in pipe (0.4 m diameter and 2m length). 150m/s velocity inlet and pressure outlet with 0 gage pressure. i used k-esilon model. The problem is pressure drop from 670 pa (inlet) to 0pa at outlet and velocity increase from 145 at inlet and 153 at outlet. can anyone explain that. |
|
![]() |
![]() |
![]() |
![]() |
#2 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,426
Rep Power: 49 ![]() ![]() |
Several possible reasons, in no particular order:
1) Conservation of mass. As pressure drops towards the end of the pipe, velocity has to increase in order to transport the same amount of mass. 2) Transient phenomena with compressible effects. If there is a pressure wave within the pipe, you can have different mass flow rates at inlet and outlet. This does not violate conservation of mass. The mass difference is just added or subtracted to the mass in the pipe 3) Viscosity. As the flow travels through the pipe, a velocity profile develops with higher velocity in the center. Since you don't state how you post-processed velocities, it could max values. 4) Invalid solution. With 150m/s as a boundary condition you should get 150m/s at this boundary, not 145m/s. Edit: or which part of the result was unexpected? |
|
![]() |
![]() |
![]() |
![]() |
#3 | |
Member
Nguyen Trong Hiep
Join Date: Aug 2018
Posts: 48
Rep Power: 8 ![]() |
Quote:
3) the boundary layer <5% and all the velocity at all point in outlet face is the same (1% diffirent). 4) this must have pressure at inlet, so the fluid can move from inlet to outlet. But fluent will ignored when flow is subsonic. So how i can fix the velocity at inlet. In my case, i think a part of dynamic pressure come to static pressure at inlet and convert all to dynamic pressure at outlet. i dont know what should i do. ref of equation:https://www.engineeringtoolbox.com/p...pes-d_852.html |
||
![]() |
![]() |
![]() |
![]() |
#4 | ||
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,426
Rep Power: 49 ![]() ![]() |
Quote:
Other than that, the formula you are referring to is most likely based on at least 2 assumptions: fully developed flow and fully developed turbulence. Maybe even more. Your CFD model would have to account for this. Just two examples: Velocity at the inlet of your simulation will likely be a uniform profile, leading to much higher shear stress than in a fully developed flow. This in turn leads to higher pressure drop. And you would have to choose turbulence quantities at the inlet very carefully to match a fully developed turbulent flow. And even then, the formula you are comparing to seems to be empirical. So some discrepancy is to be expected, even when being very careful with the CFD setup. Based on your observation that the discrepancy increases with higher Mach numbers, one could conclude that the formula you are comparing to is based on a third assumption: low Mach number or small pressure differences. The note a little further down confirms this suspicion: Quote:
Last edited by flotus1; August 5, 2019 at 09:19. |
|||
![]() |
![]() |
![]() |
![]() |
#5 | |
Member
Nguyen Trong Hiep
Join Date: Aug 2018
Posts: 48
Rep Power: 8 ![]() |
Quote:
|
||
![]() |
![]() |
![]() |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
Discrepancy between the measurements (pressure drop; VOF) | blackemperor | FLUENT | 2 | March 6, 2016 03:40 |
Constant velocity of the material | Sas | CFX | 15 | July 13, 2010 08:56 |
Terrible Mistake In Fluid Dynamics History | Abhi | Main CFD Forum | 12 | July 8, 2002 09:11 |
Total pressure in real gas (compressible flow) | Bart Prast | Main CFD Forum | 3 | November 14, 2000 10:44 |
Hydrostatic pressure in 2-phase flow modeling (long) | DS & HB | Main CFD Forum | 0 | January 8, 2000 15:00 |