Default HRRPUV value in Smokeview and related questions

[Leo Dee]

Dear FDS/Smokeview developers:

As the request from Kevin McGrattan and A/P Fleischmann, this issue
has been shifted to this Discussion Group.

I’m a Fire engineering student in University of Canterbury. Currently
I’m doing a project of comparing FDS modeled flame with experimental
flame. When I use 3D SMOKE showing the HRRPUV variable to define the
contour of flame in Smokeview, I noticed that there is a default
HRRPUV threshold value given without an explanation.

The default HRRPUV value varies with the grid cell sizes defined. For
example, when grid cell size is 0.1m x0.1mx0.1m, the HRRPUV cut-off
default value is 200kW/m3. When, grid cell size is 0.05mx0.05mx0.05m,
the HRRPUV cut-off default value is 400kW/m3. When grid cell size is
0.02mx0.02mx0.02m, the HRRPUV cut-off default value is 1000kW/m3.
Meanwhile, the sizes of these flames look similar visually in each
example.

I have emailed Kevin McGrattan and Dr. Glenn Forney about this default
value. Dr. Glenn Forney informed me that this value is computed by
FDS and passed to Smokeview. Kevin McGrattan considered this issue
be searchable, and suggested that this question be posted to this
Discussion Group.

I also mentioned about this question in Issue 1043 at Issue Tracker
section. Mcgratta commented the value of HRRPUV cut-off is 20/dx, and
there is no particular reason for this choice, other than to capture
the grid effect, and this will change in FDS version 5.5.

My question now:
How this default HRRPUV cut off value is affected by Grid cell size?
What is the computational method going to be used in FDS version 5.5
to capture this ‘grid effect’?
What are the future possible strategies to define the contour of flame
in FDS or result a better flame visual representation?

Regards
Di Liu

[Kevin]

There is a limit on the amount of energy released in a grid cell. By
default, that limit is 200/dx kW/m3. In this case, dx is the size of
the grid cell, and 200 is an empirical value related to the amount of
energy released per unit area of flame sheet. This is just an upper
bound. The amount of heat released in a cell is a function of the fuel
and oxygen concentration, plus the "mixing time" that is computed from
the turbulent diffusivity. See discussion in the FDS Technical
Reference Guide, Vol 1, about the eddy dissipation concept. The HRRPUV
cut-off in Smokeview is based on the same principle. If you think of a
sheet of flame cutting through a grid cell, the amount of energy
released in the cell is HRRPUA_SHEET*dx^2/dx^3. As the grid gets more
and more coarse, the "flame" gets smeared out, and the HRRPUV
decreases accordingly. So we need to adjust the "cut-off" in Smokeview
to account for this.

[Leo Dee]

There are some changes have been made in FDS version 5.5.0 to adjust
this "cut-off" in Smokeview.

Grid cell size HRRPUV Cut-off value
0.6m 33.3 kW/m3
0.4m 50 kW/m3
0.2m 100 kW/m3
0.1m 200 kW/m3
0.05m 200 kW/m3
0.02m 200 kW/m3

It seems that when 20/dx < 200kW/m3, HRRPUV cut-off value equals 20/
dx; when 20/dx> 200kW/m3, HRRPUV cut-off value stays at 200kW/m3. Am I
right?

On Mar 25, 12:52 am, Kevin <mcgra...@gmail.com> wrote:
> There is a limit on the amount of energy released in a grid cell. By
> default, that limit is 200/dx kW/m3. In this case, dx is the size of
> the grid cell, and 200 is an empirical value related to the amount of
> energy released per unit area of flame sheet. This is just an upper
> bound. The amount of heat released in a cell is a function of the fuel
> and oxygen concentration, plus the "mixing time" that is computed from
> the turbulent diffusivity. See discussion in the FDS Technical
> Reference Guide, Vol 1, about the eddy dissipation concept. TheHRRPUV
> cut-off in Smokeview is based on the same principle. If you think of a
> sheet of flame cutting through a grid cell, the amount of energy
> released in the cell is HRRPUA_SHEET*dx^2/dx^3. As the grid gets more
> and more coarse, the "flame" gets smeared out, and theHRRPUV
> decreases accordingly. So we need to adjust the "cut-off" in Smokeview
> to account for this.
>
> On Mar 24, 12:31 am, Leo Dee <deec...@gmail.com> wrote:
>
>
>
> > Dear FDS/Smokeview developers:
>
> > As the request from Kevin McGrattan and A/P Fleischmann, this issue
> > has been shifted to this Discussion Group.
>
> > I’m a Fire engineering student in University of Canterbury.  Currently
> > I’m doing a project of comparing FDS modeled flame with experimental

> > flame.  When I use 3D SMOKE showing theHRRPUVvariable to define the

> > contour of flame in Smokeview, I noticed that there is a default

> >HRRPUVthreshold value given without an explanation.
>
> > The defaultHRRPUVvalue varies with the grid cell sizes defined.  For

> > example, when grid cell size is 0.1m x0.1mx0.1m, theHRRPUVcut-off
> > default value is 200kW/m3. When, grid cell size is 0.05mx0.05mx0.05m,

> > theHRRPUVcut-off default value is 400kW/m3. When grid cell size is
> > 0.02mx0.02mx0.02m, theHRRPUVcut-off default value is 1000kW/m3.

> > Meanwhile, the sizes of these flames look similar visually in each
> > example.
>
> > I have emailed Kevin McGrattan and Dr. Glenn Forney about this default
> > value. Dr. Glenn Forney  informed me that this value is computed by
> > FDS and passed to Smokeview.   Kevin McGrattan considered this issue
> > be searchable, and suggested that this question be posted to this
> > Discussion Group.
>
> > I also mentioned about this question in Issue 1043 at Issue Tracker

> > section. Mcgratta commented the value ofHRRPUVcut-off is 20/dx, and

> > there is no particular reason for this choice, other than to capture
> > the grid effect, and this will change in FDS version 5.5.
>
> > My question now:

> > How this defaultHRRPUVcut off value is affected by Grid cell size?

> > What is the computational method going to be used in FDS version 5.5
> > to capture this ‘grid effect’?
> > What are the future possible strategies to define the contour of flame
> > in FDS or result a better flame visual representation?
>
> > Regards

> > Di Liu- Hide quoted text -
>
> - Show quoted text -

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