
[Sponsors] 
October 3, 2001, 13:43 
mass & area average

#1 
Guest
Posts: n/a

hi,
in visualise when I do quantitative calculation, I see area average and mass average. what are these ,could someone please explain thanks henry 

October 3, 2001, 21:15 
Re: mass & area average

#2 
Guest
Posts: n/a

Hi Henry,
The area average of a scalar is calculated by integrating the scalar times the area divided by the total area over the region. A mass averaged quantity is obtained by integrating the scalar time mass flow divided by total mass flow over the region. area_ave = int( Phi*dA ) / A mass_ave = int( Phi*dm ) / M With an area average, the spacially dominant quantity will have the greatest influence on the result. A mass average is a bit more difficult to conceptualize. Mass averaging will return the quantity which is dominant in the flow. When considering which to use, think conservation. Area averaging pressure results in summing the forces and dividing by the total area, ie area averaged pressure is is the same as total force over total area. On the other hand, a flow quantity such as Total Pressure or Enthalpy is not spacially conserved. If you need an average of these, you must take the mass_average. A good example is to consider the flow downstream of a turbine blade; if you take the area average, the low total pressure of wake may have undue influence, since there is not much mass flow through it. If you take the mass average, the mean flow will dominate the result. Hope this clears things up. Regards, Robin 

January 10, 2014, 01:56 
Why area avg require direction then?

#3 
New Member
Bitte56
Join Date: Mar 2013
Location: India
Posts: 15
Rep Power: 5 
Hi Robin,
I read your post which cleared at least some doubts. But i wonder, why area avg quantity (pressure for instance) required us to give direction.? what does it signify.? 

January 10, 2014, 08:08 

#4 
Senior Member
Bharath kumar
Join Date: Apr 2009
Posts: 169
Rep Power: 9 
Give none instead of (x,y,or z) direction


April 19, 2014, 22:12 

#5 
New Member
Nandan
Join Date: Jul 2012
Posts: 12
Rep Power: 6 
Hi Robin
I don't quite understand the concept of mass flow averaging v/s area averaging. The math formulae are solid and I have no doubt about that. I'm trying to understand from a physical point of view, when to use what in any given situation. From your example, "When considering which to use, think conservation. Area averaging pressure results in summing the forces and dividing by the total area, ie area averaged pressure is is the same as total force over total area." What pressure am I area averaging? total pressure?? And could you give me another example that might shed some light on the mass flow averaging concept? Thanks 

April 20, 2014, 06:21 

#6 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100 
Robin has not posted on this forum for 5 years unfortunately.
But to answer your question  think about the physical thing you are doing. Pressure acts on a surface, total pressure only exists in the middle of a flow. At a stationary surface the total pressure and pressure are the same as there is no dynamic component. So if you are trying to get a representative pressure on an inlet then total pressure is probably better, but if you are trying to get a representative pressure over a surface then static pressure is the one to choose. 

April 20, 2014, 12:03 

#7 
New Member
Nandan
Join Date: Jul 2012
Posts: 12
Rep Power: 6 
Cool!!! Thanks Mr Horrocks, that helps me a lot ... now to see if I understood your concept correctly, since static pressure acts on a stationary surface, I would choose to area average it (mass flow averaging static pressure on the same stationary surface would give me the same value as area averaging it).
However, for calculating total pressure at the inlet/ outlet, mass averaging total pressure would give me relatively more accurate information. Area averaging total pressure at the inlet will not take into account the dynamic component. Let me know if I'm thinking correctly. 

April 20, 2014, 23:35 

#8 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100 
The is no flow across a surface in most cases so doing a massflow average on a surface is not meaningful (for most applications)
If I was trying to get a representative pressure over an inlet or outlet I would probably use an areaAve of static pressure. massflowAve is good to use for things like temperature, enthalpy and additional variables as you are then summing up the energy content (or additional variable content) over the boundary. 

September 30, 2016, 03:26 

#9 
Member
Brett
Join Date: May 2013
Posts: 65
Rep Power: 5 
Some great points on area vs mass flow weighted average. I would point out thought that the default in CFX turbo mode is to use massflowaveabs in calculating both total and static pressures:
1 [atm] + massFlowAveAbs(Total Pressure in Stn Frame)@inlet Inlet Granted this can be changed however I think some consideration should be given to this. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Area weighted or mass weighted average  SAM  Main CFD Forum  24  July 8, 2015 06:15 
Water subcooled boiling  Attesz  CFX  7  January 5, 2013 04:32 
Diffference between mass weighted average and Area  giogio  FLUENT  4  September 25, 2012 01:36 
Facet, Area and Mass Average in Fluent  Tushar  Main CFD Forum  0  February 4, 2009 05:37 
CFX Solver Memory Error  mike  CFX  1  March 19, 2008 08:22 