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C-CARE March 11, 2004 04:04

low turbulent reynold number
 
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

Glenn Horrocks March 11, 2004 17:01

Re: low turbulent reynold number
 
Hi C-Care,

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

Regards, Glenn

C-care March 12, 2004 02:58

Re: low turbulent reynold number
 
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


matej March 12, 2004 05:15

Re: low turbulent reynold number
 
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

C-CARE March 12, 2004 07:36

Re: low turbulent reynold number
 
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.



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