FDS

[aminala...@gmail.com]

Hi FDS ussers !

i am new user of fds.i have a simple case of natural convection (2D) : it's a square cavity with two vertical walls (one is hot and the other is cold) and the horizontal walls are adiabatic.

my question is : what shall i do to obtain nusselt number  as output( file.dat)  ??

  

&HEAD CHID='Convection4', TITLE='convection'/

&TIME T_END=50./

&MISC DNS=.TRUE., RADIATION=.FALSE., SURF_DEFAULT='adiabatic_wall'/

&MESH ID='mesh', IJK=100,1,100, XB=0.0,0.143,0.0,0.143,0.0,0.143/

&INIT TEMPERATURE=20.0, XB=0.0,0.143,0.0,0.143,0.0,0.143/

&SURF ID='adiabatic_wall',

      COLOR='WHITE',

      ADIABATIC=.TRUE./

&SURF ID='hot_wall',

      COLOR='RED',

      TMP_FRONT=21.5/

&SURF ID='cold_wall',

      COLOR='BLUE',

      TMP_FRONT=19.5/

&VENT SURF_ID='hot_wall', XB=0.0,0.0,0.0,0.143,0.0,143/ Mesh Vent: mesh1 [XMIN]

&VENT SURF_ID='cold_wall', XB=0.143,0.143,0.0,0.143,0.0,0.143/ Mesh Vent: mesh1 [XMAX]

&SLCF QUANTITY='TEMPERATURE',PBY=0.075/

&SLCF PBY=0.075,QUANTITY='VELOCITY',VECTOR=.TRUE./

&BNDF QUANTITY='NET HEAT FLUX'/

&TAIL /

[dr_jfloyd]

FDS does not have an output for the Nusselt number. You will need to output the conductivity and gas temperatures at the wall and compute the Nusselt number yourself.

[aminala...@gmail.com]

thakns  you dr_jfloyd!

[Randy McDermott]

There is also a verification test series under development you could use as an example:

https://github.com/firemodels/fds/blob/master/Verification/Heat_Transfer/rayleigh_1.fds

I suggest using QUANTITY='CONVECTIVE HEAT FLUX', STATISTICS='SURFACE INTEGRAL' for both walls, make sure they come to a steady state (they should be the same), then compute the Nu for the cavity using this heat flux.

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[aminala...@gmail.com]

for my problem k(thermal conductivity ratio)=K(solid)/K(fluid) = 1 , for the fluid i don't have a problem because fds use the K(fluid) on ambient temperature (t =20°c) but for the walls how can i introduce their conductivity ??

[Randy McDermott]

Search on MATL (for material) in the FDS user guide.

On Tue, Oct 18, 2016 at 1:12 PM, <aminala...@gmail.com> wrote:

for my problem k(thermal conductivity ratio)=K(solid)/K(fluid) = 1 , for the fluid i don't have a problem because fds use the K(fluid) on ambient temperature (t =20°c) but for the walls how can i introduce their conductivity ??

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[dr_jfloyd]

You have defined a fixed temperature wall. The conductivity of the wall has no meaning in this case as FDS will not be computing heat transfer into the wall to determine the wall surface temperature (you have fixed it to a constant value). k in this case is the k of the gas in the cavity. Since you have not defined a gas, you will have the default FDS background species. k can be found in the .out file. If you want to control the value of k and keep it constant  you will need to define your own background species.

[aminala...@gmail.com]

ok, now i want to add the radiation in my fds code but i have to use the non-gray gas ( i have to consider the fluid (air) as non gray gas so his emessivity = 0).

i try this code but when i go on .out file i found that the emissivity of air =0.007 so fds considers it as a gray gas.

&HEAD CHID='Convection4', TITLE='convection/

&TIME T_END=5./

&MISC DNS=.TRUE., RADIATION=.TRUE., SURF_DEFAULT='adiabatic_wall'/

&RADI KAPPA0 = 0.0/

&MESH ID='mesh', IJK=100,1,100, XB=0.0,0.143,0.0,0.143,0.0,0.143/

&INIT TEMPERATURE=20.0, XB=0.0,0.143,0.0,0.143,0.0,0.143/

&SURF ID='adiabatic_wall',

      COLOR='WHITE',

      ADIABATIC=.TRUE./

&SURF ID='hot_wall',

      COLOR='RED',

      TMP_FRONT=21.5,

      TAU_T=0.0,

      EMISSIVITY=1.0/

&SURF ID='cold_wall',

      COLOR='BLUE',

      TMP_FRONT=19.5,

      TAU_T=0.0,

      EMISSIVITY=1.0/

[dr_jfloyd]

Per the Users Guide the default air species contains CO2 and water vapor. You can set those to be zero on &MISC.

[aminala...@gmail.com]

can you show me how shall i do because i try some ways but without result , !!!!

[Glenn Forney]

sorry, I don't understand your question.  If you are having trouble running FDS suggest you read the FDS user's guide

On Thu, Oct 20, 2016 at 4:41 PM, <aminala...@gmail.com> wrote:

can you show me how shall i do because i try some ways but without result , !!!!

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Glenn Forney

[aminala...@gmail.com]

how to draw the isotherms and streamlines on fds

[aminala...@gmail.com]

how to draw the iotherms and streamlines on fds

Le mardi 11 octobre 2016 12:42:05 UTC+1, aminala...@gmail.com a écrit :

[Randy McDermott]

For isotherms, you can either output an isosurface with ISOF (see user guide) or you can toggle "c" on a slice (SLCF) of temperature.  However, to generate high-quality isocontour images for a paper, I have found you need to parse the SLCF with something like Matlab (use slread.m in Utlities/Matlab/scripts).  Then you can get a vector image.

I don't think you can get streamlines, but streaklines can be obtained if you use tracer particles.  There are a couple different ways to do this.  First, you can add particles with MASSLESS=T to the FDS input file.  Or, you can release particles in Smokeview via Dialogs > Data > Particle Tracking...  I am less familiar with the second option, so you may need to play with it a bit.

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[aminala...@gmail.com]

i have a squre cavity and for my problem the bondry conditions are :

(Th=21.5°C) from X=0 at z=0 to 0.143 

(Tc=19.5°C) from X=0.143 at Z=0 to 0.143 

BUT when  i use fds2aci for getting values of temperature i find that

X                     Z           TEMP

0                      0             21.3°C

0.143                0            19.6°C

???????

[aminala...@gmail.com]

hi users, i have  problem with Mesh Stretching , when i run the simulation the process stops and the message of " numerical instability " is displayed.

&HEAD CHID='cas3'/

&TIME T_END=5.0/

&DUMP RENDER_FILE='cas3.ge1', DT_PL3D=60.0, DT_RESTART=300.0, WRITE_XYZ=.TRUE./

&MISC POROUS_FLOOR=.FALSE., DNS=.TRUE., SUPPRESSION=.FALSE., RADIATION=.FALSE./

&MESH ID='MESH', COLOR='GRAY 40', IJK=100,1,100, XB=0.0,0.1,0.0,0.1,0.0,0.1/

&TRNX CC=0.04, PC=0.02, MESH_NUMBER=1/ MESH

&TRNX CC=0.06, PC=0.08, MESH_NUMBER=1/ MESH

&TRNZ CC=0.04, PC=0.02, MESH_NUMBER=1/ MESH

&TRNZ CC=0.06, PC=0.08, MESH_NUMBER=1/ MESH

&INIT TEMPERATURE=20.0, XB=0.0,0.1,0.0,0.1,0.0,0.1/ Init Region

&SURF ID='hot',

      RGB=255,0,51,

      TMP_FRONT=35.0/

&SURF ID='cold',

      RGB=102,204,255,

      TMP_FRONT=5.0/

&SURF ID='ADIABATIC',

      COLOR='GRAY 80',

      ADIABATIC=.TRUE./

&VENT SURF_ID='hot', XB=0.0,0.0,0.0,0.1,0.0,0.1/ Vent

&VENT SURF_ID='cold', XB=0.1,0.1,0.0,0.1,0.0,0.1/ Vent

&VENT SURF_ID='ADIABATIC', XB=0.0,0.1,0.0,0.1,0.0,0.0, RGB=204,0,255/ Vent1

&VENT SURF_ID='ADIABATIC', XB=0.0,0.1,0.0,0.1,0.1,0.1, RGB=204,0,255/ Vent2

&BNDF QUANTITY='WALL TEMPERATURE'/

&SLCF QUANTITY='TEMPERATURE', VECTOR=.TRUE., PBY=0.05/

&TAIL /

[dr_jfloyd]

FDS User's Guide

4.2 Common Error Statements

Numerical Instability Errors: It is possible that during an FDS calculation the flow velocity at some location in the domain can increase due to numerical error causing the time step size to decrease to a point where logic in the code decides that the results are unphysical and stops the calculation with an error message in the file CHID.out. In these cases, FDS ends by dumping out one final Plot3D file giving you a hint as to where the error is occurring within the computational domain. Usually, a numerical instability can be identified by fictitiously large velocity vectors emanating from a small region within the domain. Common causes of such instabilities are mesh cells that have an aspect ratio larger than 2 to 1, high speed flow through a small opening, a sudden change in the heat release rate, or any number of sudden changes to the flow field. There are various ways to solve the problem, depending on the situation.

Try to diagnose and fix the problem before reporting it. It is difficult for anyone but the originator of the input file to diagnose the problem.

Your grid stretching results in aspect ratios of close to 6 to 1.

[Amina Laouti]

Thnks for your answer dr_jfloyd but what do you mean by aspect ratio?? I dont understand where is my mistake?

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[dr_jfloyd]

Take the ratio of two sides of a grid cell. With your grid stretching, you have cells where the length/width is large.

[aminala...@gmail.com]

hi users, i have some important questions for which i need answers quickly please:

1- what is the function of this instruction "DT_RESTART" ?

2- is it NORMAL that a simulation of only natural convection in square cavity (L=0.1) ,differentialy heated (Th=35°C , Tc= 5°C ), with uniform mesh (100,1,100) is so long (of the order of 30min )?

3- is it possible to put a convergence criteria in stead of T_END??

4- is it possible to use a function for Mesh Stretching different from those cited in the fds user( lineair, polynomial)???

these were my questions, answer me i am waiting please!! thanks a lot.

[dr_jfloyd]

1 - search the User's Guide for DT_RESTART

2 - 0.1 m with 100 cells is 1 mm grid cells.  Your time step is going to be very small and having the calculation take a long time would be expected.

3 - No. FDS does not have a steady-state mode of operation

4 - No.  Consider using multiple meshes rather than stretching.

[aminala...@gmail.com]

hi, thanks you for ansewring.

1- if i put DT_RESTART=0 , will this influence my temperature  value??

2- I put several plan for SLCF ( PBY=0 ,PBY=0.1) , but when i run fds2asci to extract the temperature values, there is just the results for one plan!! so which plan was considerated by fds??

[dr_jfloyd]

1. restart files are simply FDS dumps of the current contents of memory to disk at the end of each DT_RESTART time interval. It has no impact on the calculation itself. DT_RESTART = 0 means that you would be writing restart files every time step. This would greatly increase your runtime as writing large files to disk takes a long time.

2. The domain you previously referred to is a mesh that is 100 x 1 x 100, if that is the case here, then there is only 1 y-plane in the calculation.

[aminala...@gmail.com]

hi FDS users, i want to check if i use the good source fds code of radiation (modul rad) , i found it on https://github.com/firemodels/fds/blob/master/Source/radi.f90  ???

what about these reals and integers , what each one represent     REAL(EB) :: THETAUP,THETALOW,PHIUP,PHILOW,F_THETA,PLANCK_C2,KSI,LT,RCRHO,YY,YY2,BBF,AP0,AMEAN,RADIANCE,TRANSMISSIVITY,X_N2

                                                                                               INTEGER  :: N,I,J,K,IPC,IZERO,NN,NI,II,JJ,IIM,JJM,IBND,NS,NS2,NRA,NSB,RADCAL_TEMP(16)=0,RCT_SKIP=-1,OR_IN,I1,I2??

please is there any documentation about code source of FDS or how to add or change some instructions on  FDS source codes?? please Help me

[Randy McDermott]

If you are asking whether you are allowed to use it, yes, it is open source.

The only documentation is in the manuals on our website.  The code has limited comments.  It can take some time to get familiar with it.  But this type of question is not going to get you very far.  If you have a specific question, we can help.

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