result output (mass flow rate) & conservation of mass

alpharays786

Hi all,

 

 

I ran a simulation of air flow through ducting. It has one inlet (mass flow rate) and 4 outlets.

I want to measure the mass flow rate at each outlets.

Because of no mass flow rate as selection in the AcuFieldView, velocity magnitude is used to multiple with outlet's area and air density.

 

mass flow rate = density x area x velocity

 

However, mass flow rate calculated did not follow conservation of mass unless I choose directional velocity (x or y or z-velocity) which normal to outlet's surface. And, directional velocity used could lead to less accurate of mass flow rate if outlet surface is not normal to x/y/z axis.

 

Is there any method to calculate mass flow rate directly or if AcuFieldView has add-on extension for this?

 

 

Thanks

ydigit

The easiest way to monitor mass flux is using AcuProbe.

Surface Output > 'surface name' > mass_flux

 

You could also create a UDF in AcuProbe

value= Get( 'Surface Output - Outlet - mass_flux' ) + Get( 'Surface Output - Inlet - mass_flux' )

 

Note the sign is 'plus' in the above snippet, due to sign conventions.

ydigit

Another way is to use AcuOut from AcuConsole.

See screenshot.

<?xml version="1.0" encoding="UTF-8"?>

alpharays786

Both method I used (AcuProbe and AcuOut) and same results are given.

However, summation of all 4 outlets' mass flux are not same to inlet mass flux.

What could be wrong? AcuOut generation as per below.

 

 

acuTrans:                 Surface name = 13.INLET_DEF

acuTrans:                   Surface ID =          4

acuTrans: Integrated surface out. vars = mass_flux

acuTrans:  Variable                         Min                                      Max                         Ave

acuTrans: mass_flux                      -9.375619e-002        -9.023599e-002         -9.373978e-002

acuTrans:           Translate to stats = integrated surface output

 

acuTrans:                 Surface name = LH_Side_Outlet

acuTrans:                   Surface ID =         44

acuTrans: Integrated surface out. vars = mass_flux

acuTrans:  Variable                         Min                                 Max                                  Ave

acuTrans: mass_flux                      -1.780949e-006              2.019959e-006                -2.131666e-008

acuTrans:           Translate to stats = integrated surface output

 

acuTrans:                 Surface name = LH_center_Outlet

acuTrans:                   Surface ID =         45

acuTrans: Integrated surface out. vars = mass_flux

acuTrans:  Variable                         Min                              Max                              Ave

acuTrans: mass_flux                      -3.680867e-005           2.147409e-005           -6.251384e-007

acuTrans:           Translate to stats = integrated surface output

 

acuTrans:                 Surface name = RH_Side_Outlet

acuTrans:                   Surface ID =         46

acuTrans: Integrated surface out. vars = mass_flux

acuTrans:  Variable                         Min                                      Max                                 Ave

acuTrans: mass_flux                      -4.935872e-006                  3.599640e-006               -7.451954e-009

acuTrans:           Translate to stats = integrated surface output

 

acuTrans:                 Surface name = RH_center_Outlet

acuTrans:                   Surface ID =         47

acuTrans: Integrated surface out. vars = mass_flux

acuTrans:  Variable                         Min                                  Max                                      Ave

acuTrans: mass_flux                      -4.333608e-005              1.985709e-005                    -6.562757e-007

acuTrans:        Total CPU/Elapse time = 1.373000e+000 1.512000e+000 Sec

acuTrans:           Total Memory Usage = 6.011719e+000 Mbytes

ydigit

Looking at the low mass flux values at outlets, it seems that there is some surface that is blocking the flow in the domain. Some surface that ought to have no boundary condition is probably assigned a wall BC.

 

Could you upload you .inp and .Log files? Also a screenshot of the domain with contour plot of velocity.

alpharays786

Hi, I cannot share the whole content of INP and Log file.. As shown below, a surface with velocity plot is intentionally add at ducting outlet in order to use to get the value of mass flow rate. However, i don't think it block the air flow because it has no BC setup.

 

 

 

INP file as per below, all outlets doesn't have BC setting..

 

# | Surface Output                                                       |

# +----------------------------------------------------------------------+

 

SURFACE_OUTPUT( 'LH_Side' ) {

    surfaces                            = Read( 'MESH.DIR/demist_05_tran.Air_volume.tet4.LH_Side.tri3.ebc' )

    shape                               = three_node_triangle

    element_set                         = 'Air_volume'

    integrated_output_frequency         = 1

    integrated_output_time_interval     = 0.0

    statistics_output_frequency         = 1

    statistics_output_time_interval     = 0.0

    nodal_output_frequency              = 0

    nodal_output_time_interval          = 0.0

    num_saved_states                    = 0

}

 

# +----------------------------------------------------------------------+

# | Surface Output                                                       |

# +----------------------------------------------------------------------+

 

SURFACE_OUTPUT( 'LH_center' ) {

    surfaces                            = Read( 'MESH.DIR/demist_05_tran.Air_volume.tet4.LH_center.tri3.ebc' )

    shape                               = three_node_triangle

    element_set                         = 'Air_volume'

    integrated_output_frequency         = 1

    integrated_output_time_interval     = 0.0

    statistics_output_frequency         = 1

    statistics_output_time_interval     = 0.0

    nodal_output_frequency              = 0

    nodal_output_time_interval          = 0.0

    num_saved_states                    = 0

}

 

# +----------------------------------------------------------------------+

# | Surface Output                                                       |

# +----------------------------------------------------------------------+

 

SURFACE_OUTPUT( 'RH_Side' ) {

    surfaces                            = Read( 'MESH.DIR/demist_05_tran.Air_volume.tet4.RH_Side.tri3.ebc' )

    shape                               = three_node_triangle

    element_set                         = 'Air_volume'

    integrated_output_frequency         = 1

    integrated_output_time_interval     = 0.0

    statistics_output_frequency         = 1

    statistics_output_time_interval     = 0.0

    nodal_output_frequency              = 0

    nodal_output_time_interval          = 0.0

    num_saved_states                    = 0

}

 

# +----------------------------------------------------------------------+

# | Surface Output                                                       |

# +----------------------------------------------------------------------+

 

SURFACE_OUTPUT( 'RH_center' ) {

    surfaces                            = Read( 'MESH.DIR/demist_05_tran.Air_volume.tet4.RH_center.tri3.ebc' )

    shape                               = three_node_triangle

    element_set                         = 'Air_volume'

    integrated_output_frequency         = 1

    integrated_output_time_interval     = 0.0

    statistics_output_frequency         = 1

    statistics_output_time_interval     = 0.0

    nodal_output_frequency              = 0

    nodal_output_time_interval          = 0.0

    num_saved_states                    = 0

}

 

 

 

LOG file for all outlets as per below:

 

acuPrep: Reading SURFACE_OUTPUT( 'LH_Side' )

acuPrep: Reading SURFACE_OUTPUT( 'LH_center' )

acuPrep: Reading SURFACE_OUTPUT( 'RH_Side' )

acuPrep: Reading SURFACE_OUTPUT( 'RH_center' )

 

acuPrep: Processing SURFACE_OUTPUT( 'LH_Side' )

acuPrep: Processing SURFACE_OUTPUT( 'LH_center' )

acuPrep: Processing SURFACE_OUTPUT( 'RH_Side' )

acuPrep: Processing SURFACE_OUTPUT( 'RH_center' )

----------------------------------------------------------------

acuPrep:                  Output data = Surface output

ydigit

It would be more helpful to see boundary conditions part of the .INP file. Surface output doesnt help.
I suspect some surface is blocking the flow. Otherwise you should relatively more outflow.

 

I guess with 'more reply options' you might be able to upload entire .INP file?

alpharays786

.inp file as per attached

Unable to find an attachment - read this blog

ydigit

Sorry cant make out much from this input file. AS I said above, I suspect something is blocking the flow. e-8 means almost nothing at outlet.

Check if there is any surface that needs to be assigned 'no boundary condition', i.e. interior face.

acupro

Each outlet should also have a Simple BC with type = outflow.  It seems that convergence would not be good at all if there is an inlet, but the boundaries intended for outlets have almost zero flow.  Also, when you look at 'statistics' for integrated output, you are seeing the min/max/average integrated values for that surface for the entire simulation.  You should probably be using just the last value (or average of some of the last values) for the run so all values of the intergrated quantities represent the same 'time' of the calculation.

 

The set '14.OUTLET' is set to type = outflow.  Maybe the boundary conditions and/or sets need to be reviewed.

 

It may be good to contact your local support to get more dedicated assistance with this question.