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tracer experiment for age of air

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Old   October 21, 2003, 11:34
Default tracer experiment for age of air
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Tad
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Hey all

I am a cFX4.3 user and would like to determine the local mean age of air at different parts of a ventilated room. Is there anyone who is experienced with this, please lend me a hand, since I am new to do this study. ohhh, the tracer I am using is CO2.

thanks a lot

Tad
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Old   October 26, 2003, 23:55
Default Re: tracer experiment for age of air
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Jeff
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Tad,

If you need the "age" of any volume of air, you can set a user scalar to simply calculate it's own age.

You want d(USRD_age)/dt = 1.0, so that, when integrated, the age scalar will equal time. Since the scalar equations are actually:

d(density*phi)/dt, you'll want to set a source term equal to the fluid density. Your integrated scalar value will then be the "age" of the fluid.

d(rho*USRD_age)

--------------- + CONV. = rho

dt

Turn off diffusion by setting an extremely small diffusivity (i.e. age doesn't diffuse because there is younger/older fluid in the vicinity) or turn diffusion off completely. If it's a turbulent flow, you'll have to overwrite the turbulent diffusivity in USRDIF with a very small value. Leave convection on, however, as younger/older fluid will be convected into and out of each cell and mixed to give a "mean" age.

Note, this all depends on what you set the "initial" age to be. Make sure you set the initial guess for your USRD_age scalar appropriately.

Also, use CCCT (bounded third order) spatial discretization, and Crank Nicholson (second order) temporal discretization to minimize numerical diffusion in the solution (as well as the finest grid you can stand).

Hope this helps, Jeff
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