Flow in a pipe with Hagen-Poiseuille-Correlation

- May 29, 2019 at 9:48 am
  
  Isapu
  Subscriber
  
  Hello Everyone,
  
  I'm working with Ansys Fluent and want to simulate a simple case of laminar fluid flow in a pipe.I would like to receive the pressure drop along the pipe length.
  
  I set up the following properties: - density=1500 [kg/m^3]
  
                                                     - dyn. viscosity=2000 [Pa s]
  
                                                     - length=0,03 [m]
  
                                                     - radius=6,3801*10^-3 [m]
  
                                                     - mass-flow=0,01 [kg/s]
  
  I assigned the entry as mass-flow-inlet and the exit as pressure-outlet with a value of 0 Pa for Gauge Pressure.
  
  The pressure drop through the length in the simulation was 1421070 Pa. I checked the result by a manual calculation with Hagen-Poiseuille-Correlation and had a completely different outcome (614738 Pa).
  
  Does it mean to use Hagen-Poiseuille is not correct?
  
  Kind regards,
  
  Isabella
- May 29, 2019 at 12:43 pm
  
  Karthik Remella
  Administrator
  
  Hello Isabella,
  
  Has your model fully converged? Can you please share some plots of your residuals and monitors?
  
  What does your mesh look like?
  
  Also, does your flow become fully developed? Hagen-Poiseuille equation is applicable for fully-developed flow. Please check the entrance length in your case. If your flow is not fully developed, I suggest you use the fully-developed velocity profile at your inlet, rather than a mass flow rate or velocity inlet condition.
  
  I hope this helps.
  
  Best Regards,
  
  Karthik
- May 31, 2019 at 2:00 pm
  
  Isapu
  Subscriber
  
  Thank you for your reply!
  
  " alt="" width="456" height="463">
  
  Element No.: 781500
  
  Element size: 2E-4 [m]
  
  Type: Hex-Mesh
  
  It is confusing! After changing the mesh (element size), I get other results for the pressure drop. I can't imagine what's wrong here.
  
  The pictures below show the velocity vectors. How can I set the fully-developed velocity profile? Is there any tool for at boundary conditions?
- May 31, 2019 at 3:50 pm
  
  DrAmine
  Ansys Employee
  
  Actually by my carrying out a periodic flow you will get the laminar parabolic profile. Just make inlet and outlet translatory periodic provide flow rate and run it till deep convergence.
- May 31, 2019 at 4:48 pm
  
  DrAmine
  Ansys Employee
  
  Or elongate the pipe wuth at least 0.p5*ReYnolds*Diameter to get it fully developed without periodic