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-   -   Why: Point velocity always monitored as 0.0? (http://www.cfd-online.com/Forums/fluent/43601-why-point-velocity-always-monitored-0-0-a.html)

 Tanhn February 4, 2007 19:01

Why: Point velocity always monitored as 0.0?

I want to monitor the point velocity at my axisymmetry 2D model; however, the axial velocity is always plotted as ZERO. In fact, when I checked the axial velocity value at 0.5m. It has a non-zero value.

----------------------------------------------- The steps taken are:

(1) Surface->Point(x0=0.5,y0=0.0)-> name: p0.5m

(2) Solve-> monitors-> surface->monitor 1/every time step/define...:

Report type: mass-weighted average

Report of: Velocity/axial velocity

Surface: p0.5m

(3) then, seclect OK

(4) iterate...

 Jason February 5, 2007 10:07

Re: Why: Point velocity always monitored as 0.0?

If I had to guess, I'd think it's the mass-weighted average (mass flow through a point is 0 even if the velocity has a value because the area of a point is 0)... you're looking at a point, so use the vertex average.

Hope this helps, and good luck, Jason

 Tanhn February 5, 2007 10:39

Re: Why: Point velocity always monitored as 0.0?

Hi,Jason,

It works when using the vertex average.Thanks.

Although this problem is solved. However, I stilll do not understand how to choose the REPORT TYPE for different variables concerned(such as temperature, pressure,volume fraction,velocity,...).

Afterall, there are a number of options under the REPORT TYPE item. So, could you give me a general indication for performing a choice properly?

 Jason February 6, 2007 10:47

Re: Why: Point velocity always monitored as 0.0?

It depends on what you're looking for. For a point, there is only one vertex, so max, min and average are the same. Both a point and a line have zero area, therefore any of the report types having to do with area will be zero (mass report types, because mdot=rho*A*v, if A=0, mdot=0).

If you're looking at the average across a surface, then you have to pick from vertex, facet, area, and mass flow report types.

Vertex averaging interpolates the flow field solution to each vertex on the surface you're interested in, then sums all of the values for the vertices you're interested in and divides by the number of vertices:

<indent>Vertex_avg = sum(Value)/(number of vertices)

Facet averaging is the same thing, but interpolates and sums across the facet centers instead. (I don't think Facet averaging works along a line, but it may interpolate to the edge centers. If someone's tried it, can you comment?)

Area weighted averaging goes a step further from facet averaging and takes into account the area of the facet:

Area_weight_avg = sum(Facet_Area * Value)/sum(Facet_Area)

This is helpful because typically your cell faces aren't of a constant area and you don't want tiny cells counting the same as large cells. Example is flow through a pipe with a BL mesh. You don't want the cell next to the wall which is extremely tiny and has a velocity of close to zero to count as much as the large cell at the centerline. This also explains why it doesn't work for a point or a line, because when it multiplies Facet_Area by Value it always gets zero.

Flow weighted averaging is similar, but uses the flow through a facet instead of the area of the facet:

Flow_weight_avg = sum(Facet_Flow * Value)/sum(Facet_Flow)

Choosing between the different averaging techniques depends on what you're looking for and what you're comparing it to. If you're comparing it to area averaged data, then you should be using area averaged calculations. If you're comparing it to mass flow weighted values, then you should be comparing it to mass flow weighted calculations.

Hope this helps, and good luck, Jason

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