CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

low turbulent reynold number

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   March 11, 2004, 04:04
Default low turbulent reynold number
  #1
C-CARE
Guest
 
Posts: n/a
hi I get this message " **** NOTICE **** The turbulent Reynolds number is less than 1.00E+01 at 1.07 % of the nodes. Minimum value = 4.26E+00 at node [11,8,44]. The turbulent Reynolds number is defined as (turbulent viscosity)/(molecular viscosity * C_mu). The turbulent viscosity = 3.81E-04 at this node and is probably too low. Check that: 1) boundary conditions for k and epsilon are reasonable,

2) the grid is scaled correctly,

3) a consistent set of units are being used. If no errors are found, then please note that the application of the k - epsilon model in regions with very low turbulent Reynolds number may result in reduced accuracy. To suppress this notice set parameter "RTRBMN" to be less than the minimum turbulent Reynolds number given above." after simulation my rotating machine have been completed. Could you explain the method to check errors from number one to three?

I change to k-w but still receive this note.

Regards C-Care
  Reply With Quote

Old   March 11, 2004, 17:01
Default Re: low turbulent reynold number
  #2
Glenn Horrocks
Guest
 
Posts: n/a
Hi C-Care,

Can you describe what you are simulating, and what fluid properties you are using?

Regards, Glenn
  Reply With Quote

Old   March 12, 2004, 02:58
Default Re: low turbulent reynold number
  #3
C-care
Guest
 
Posts: n/a
Dear sir I am simulating axial flow pump without diffuser by CFX Tascflow; tip diameter 100 mm,number of blade=4, design head 1.8 m flow rate 18 kg/s speed 252 rad/s, design meridional velocity 3 m/s. VOF cavitation model have been used. Fluid properties ; working_fluid = Water @ STP (SI) working_vapor = Steam @ 400K & 1bar (SI) working_gas = Air @ STP (SI)

This is copy from out file; __________________ VERBATIM "PRM" FILE ECHO ___________________

cavitation_model = T transient = F SST_TRANSITION_MODEL = F grav@(0.0, 0.0, 0.0) tref = 0.0 high_speed_model = F !%save_library_properties = F scalar_diff_eq_visc = F arot@(0.000000,0.000000,0.000000) brot@(0.000000,0.000000,1.000000) omega = -252 cavitation_model_type = 2 cavitation_t_free_stream = 300.0 cmod1 = 50.0 cmod2 = 0.01 bubble_radius = 1.0E-6 relax_vsrco = 0.20 LIQ_MIX_DENSITY_F = 0.000000 LIQ_MIX_VISC_F = 0.000000 NC_MASS_FRACTION = 5e-007 gam1 = 0.0 density1 = 998.200012 density_liquid = density1 viscosity1 = 0.000993 cond1 = 0.597000 cv1 = 4182.000000 cp1 = 4182.000000 zmol1 = 18.016001 eqst1 = F lmcf1 = T gam2 = 0.0 density2 = 0.555000 density_vapor = density2 viscosity2 = 0.000013 cond2 = 0.026100 cv2 = 1552.000000 cp2 = 2014.000000 zmol2 = 18.016001 eqst2 = F lmcf2 = T gam3 = 0.0 density3 = 1.164000 viscosity3 = 0.000018 cond3 = 0.026100 cv3 = 716.500000 cp3 = 1003.500000 zmol3 = 28.966000 eqst3 = F lmcf3 = F phi3 = F !%working_fluid = Water @ STP (SI) vapor_pressure = 3531.000000 !%working_vapor = Steam @ 400K & 1bar (SI) !%working_gas = Air @ STP (SI) BCINFO = t POFF = 101325 DTIME = 1.0e-3 KNTIME = 10000 TIMESTEP_CHOICE = 1 ERTIME = 5e-4 TURBULENCE_MODEL = 2 TWO_EQUATION_MODEL = 3 ZONAL_KW_MODEL = 2 FIXED_WALL_DISTANCE_MODEL = T equation_of_state = T

________________ END VERBATIM "PRM" FILE ECHO _________________

------ B.C. Attribute Summary ------------------------------------------------

The following grid/flow attributes have been specified:

Flow field solution required.

Flow is incompressible.

Energy field is known.

Flow does not require real gases.

Flow is turbulent.

All turbulent walls use the SAME wall treatment.

Turbulent wall treatment: log-law.

Flow is non-reacting.

Flow includes additional scalar transport eqn's.

Scalar# 1: LIQUID

Scalar# 2: VAPOR

Flow does not include Lagrangian tracking.

Finite Volume Radiation Model NOT active.

Non-participating thermal radiation not active.

Diffusion model for radiation not active.

The domain is rotating.

Moving walls exist.

Overlap boundary condition attachment permitted.

Do not read in profile boundary file (PRO).

Transient boundary conditions required.

Internal objects exist.

Flow does not involve wet steam.

Regards C-CARE

  Reply With Quote

Old   March 12, 2004, 05:15
Default Re: low turbulent reynold number
  #4
matej
Guest
 
Posts: n/a
Hi,

As I looked to your first post - you have got Re<10 at 1% of a nodes. How many nodes you have? Have you looked where those nodes are? (following the info on the position of the lowest Re)

For what phase is the Re so low? Have a look at the contours of the Re number.

The error only informs you, that the flow is laminar at 1% of the domain while you apply the turbulent flow model. It could be convergence issue, it could be problem of mesh quality, it also could be physical or not important for the results you are seeking.

matej
  Reply With Quote

Old   March 12, 2004, 07:36
Default Re: low turbulent reynold number
  #5
C-CARE
Guest
 
Posts: n/a
Thank you for your advice. The mesh uses an O-grid around the blade and a H-grid in the blade passage. There is a single structured block with 61 node in the flow direction, 36 from leading to trailing edge and 46 from hub to shroud.

  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
low reynolds number models in Fluent doug Main CFD Forum 6 August 4, 2012 14:39
mesh file for flow over a circular cylinder Ardalan Main CFD Forum 6 April 17, 2010 23:40
not convergence in high angle of attack in low reynold number nuimlabib Main CFD Forum 1 October 2, 2009 14:10
About low Re number turbulent flows gorka Main CFD Forum 13 April 2, 2003 05:19
a low Re number model for impinging jets mahesh prakash Main CFD Forum 14 September 3, 1999 16:40


All times are GMT -4. The time now is 08:50.