Measuring momentum flux in FDS

[Vijayananda Vivek Devananda]

Dear friends,

I am trying to measure momentum due to the passage of a mixture of gases through a window. I am able to measure the mass flux through the area using AREA_INTEGRAL. However, if I am getting a momentum value integrated over an area, what velocity should I multiply it with to get the rate of momentum change or the force?  Thank you.

[Vijayananda Vivek Devananda]

I understood that if I need to find momentum coming out through an area in FDS, I need to integrate the momentum flux P" Kg/m-s2 at a point over an area. I am wondering if such a facility is available in FDS.  

[Randy McDermott]

That specific output is ambiguous.  You could generate those units in several ways.  Presumably, you mean rho*u*u, which is equivalent in FDS to a multiplication of the QUANTITIES 'TOTAL MASS FLUX' and 'U-VELOCITY'.  You could also look at the QUANTITY 'F_X', which contains all the momentum terms.  You would need to read the FDS Tech Guide to see exactly what this represents... it is basically the right hand side of the momentum equation, but it has units of m/s^2, because the RHS has been divided through by density.  Look at the user guide Table 22.4 for a list of other possible "gas phase output quantities".

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[Vijayananda Vivek Devananda]

Thank you so much, Dr. McDermott. 

I have one doubt about what you wrote above. TOTAL_MASS_FLUX is an area-integrated quantity with units kg/s. However, U_VELOCITY is found at a point, right? So how can these two be multiplied together? So shouldn't the momentum flux be found first and integrated over the area? 

Also, if I need to multiply two output quantities can I make FDS report that as a new quantity? or should I work on the XLS files FDS generates? Thanks

Vijay

[Randy McDermott]

No, total mass flux is rho*u [=] kg/(m^2*s).

You can multiply DEVC outputs using control functions, see FDS user guide 18.2, Advanced Control Functions.

Or... you can output both to a DEVC file and multiply them yourself with Excel or Python.

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[Vijayananda Vivek Devananda]

Dear Dr. McDermott,

as shown below, if the total mass flux is integrated over an area, its unit will not have m^-2 right? Please guide me. 

[Randy McDermott]

There is no momentum flux you can just apply the area integral to and get what you want.  You will have to do pointwise multiplication of rho*u (total mass flux in the x direction) with u (velocity in x).  You will have to create an array of such devices and then do the integration manually.

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[Vijayananda Vivek Devananda]

Thank you so much Dr. McDermott for the clarification. 

[Vijayananda Vivek Devananda]

Dear Dr. McDermott,

I found that TOTAL MASS FLUX X should be given a SPEC_ID if used at a point (XYZ=........) or it should be given SPATIAL_STATISTIC (such as an area integral), which means it's not possible to get total mass flux x for all species at one point right? Thanks 

[Randy McDermott]

Sorry, I was not thinking.  "Total" here means advective plus diffusive.  You are better off just using DENSITY and U-VELOCITY (or 'CELL U')and constructing rho*u*u that way.  Note that the density is stored at cell centers and the velocity is staggered (unless you use 'CELL U').

I am not sure what you are trying to compare.  This will not give you all the momentum, just the advection term.

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[Vijayananda Vivek Devananda]

Dear Dr. McDermott, 

I am trying to find out the momentum or the force caused by the hot gases that escape during the backdraft. or in other words what damage the backdraft causes. 

What if I use TOTAL MASS FLUX for each gas combined with the U_VEL? I will get the total x momentum for that gas, right? I can do that for all species and sum them up. 

[Randy McDermott]

You could sum all the SPEC_ID just using MASS FLUX (not TOTAL) and combine that with U-VELOCITY.  But I would use DENSITY and (CELL U)^2.

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