CFX Results
 

I am new in computational fluid dynamics modeling.... i want to apply it in environmental cases.... anyway i have some general questions and if you can reply me i would be very grateful....

I simulated 3 stacks from a factory and i want to know if the output results can be taken in the evaluation with time...e.g concentration= f(hour) and if yes how?

I put only 1 wind speed as initial condition and i want to know the plume values or the concentration values that i take as output would be for that value? e.g if the wind speed is the average of 48 hours the concentration would also be average for those 48 hours?

Thank you for your time...if someone know i want to discuss it and maybe help me

This is not my area of expertise but:

I assume you are running a multispecies run (air and smoke, or something like that)?

I also assume you want just the concentration (perhaps kg_smoke/(kg_smoke+kg_air) for just the plume portion and not the entire domain.

I also assume you are doing this in the post processor, and not monitoring it in real time.

I think the first thing you need to do is define what you mean by "plume". In my simple assesment you could, in the post processor, you can create an isovolume on smoke mass fraction (or however else you want to define your plumne with). Set the limits to an appropriate value determined by you (I have no idea what the accepted normal "plume" concentration is).

Once you have the plume defined, you can assess mass_fraction, volume_fraction, (or whatever you want to determine your concentreation on) on the plume portion only.

i want my results to be compered with another air pollution model. My outputs in ansys are in kg/m3 of smoke... by plume i mean the emissions coming out of the stacks that form a certain shape.... what is coming out of a factory stack and how it is dispersed generally you can call it plume.....

I was wondering since i did not use the input of time in ansys (dont know if it has that option) my results-outputs would be constant? average since my initial conditions are also average values? or i dont know what....

Thank you again for your time

Yes I know what a plume is, but CFX doesnt. You need to define plume parameters on which the CFD model can work on. There are many definitions that can be used to define "plume". There can be a "thermal
plume, a "gas" plume, or whateever physics you are looking at. And each of those defined plumes might or might not have the same shape/volume (most certainly the shape isnt fixed but is part of the solution). That is for you to decide and put into the model in order to get the answers you want. If you are comparing to liturature, use what they used.

From what you are saying, it sounds like you ran this model as a steady state run. If you want time statistics, you need to run a transient. Unless, of course, the plume isnt transient in nature, but just provides a constant mdot into the air domain. In which case, steady state is fine. Your concentrations will be fixed (just use the appropriate values that you are interested in).

Another important point for this type of modelling is that you need a realistic atmospheric boundary layer to get the velocity distribution, turbulent mixing and possibly other parameters about right for your model to be accurate. If you use simple constant velocity boundary conditions your results will be miles off.

Atmospheric boundary layer modelling is a whole branch of fluid mechanics in itself and is quite complex. And of course, it changes day to day with the weather - so you need to decide whether you are modelling a windy day, a calm day, or some other weather for that day.

Its clear now thank you very much....but i also have a minor question and if you can answer me i would be grateful.
If i put one value of inlet for temperature and define the atmospheric boundary layer... the temperature for the air inside the domain should be ambient or in the same value as it is the temperature of the inlet?

Thank you very much in advance for your help

That depends on what you are modelling inside the domain, of course.....

I think it worthwhile to head down to the local university library and do some research into atmospheric boundary layers.

its more complicated than that i know what a boundary layer is...my field is atmospheric pollution.... but i dont know the model i am learning it now i use only gaussian models and dynamic models not noumerical based on navier stokes model that has so many inputs..... anyway... i think i will do both and see what happens.... thank you again for your time