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Boundary conditions in natural ventilation of a room with open windows

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Old   January 31, 2022, 23:31
Default Boundary conditions in natural ventilation of a room with open windows
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I have a very simple scenario where I'm simulating natural ventilation within a room with some constant heat load (e.g. a light bulb in the middle of the room) and calm wind conditions. In my setup, I have a rectangular room, with different sized squares (representing open windows) scattered on the walls. I am after the temperature (and maybe also velocity) distribution within the space.

My model setup is steady-state, incompressible ideal gas (or maybe Boussinesq) to account for buoyancy effects, zero heat flux on the walls, with a ke or kw turbulence model, and I may or may not turn on a radiation model.

I am wondering what boundary conditions I should be using for the windows since I don't know which of my windows will act as inlets or outlets. Since I am assuming calm external conditions, can I just set all the windows to a pressure outlet with 0 gauge pressure (i.e. atmospheric conditions)? Since there is no velocity inlet, is this setup well-defined? If not, what boundary conditions should I be using? Would using an outflow BC instead make a difference?

If it helps, I am using ANSYS Fluent.
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Old   February 1, 2022, 04:48
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A stagnation pressure inlet set to atmospheric conditions is the way to go. In Fluent, this is simply the pressure inlet. Set the total pressure and temperature and you're all set.


Pressure outlet can also work but you still need to set the backflow properties in the case that the outlet is actually an inlet. The reason you should not use a pressure outlet is because it fixes the static properties of the flow, which is not physically correct with what occurs in reality when you open a window.
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Old   February 1, 2022, 06:09
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Quote:
Originally Posted by LuckyTran View Post
A stagnation pressure inlet set to atmospheric conditions is the way to go. In Fluent, this is simply the pressure inlet. Set the total pressure and temperature and you're all set.


Pressure outlet can also work but you still need to set the backflow properties in the case that the outlet is actually an inlet. The reason you should not use a pressure outlet is because it fixes the static properties of the flow, which is not physically correct with what occurs in reality when you open a window.
Thank you for your help. Would a pressure inlet allow reverse flow in the event that the window is an exit?

Also, how do I know the total pressure and temperature? The total pressure is the sum of the static and dynamic pressure, but I don't know the dynamic pressure at the openings. Since the flow is incompressible, the total temperature is just equal to the static temperature, and I don't know what the temperature will be at the window.

Please let me know if my understanding of it is completely wrong.
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Old   February 1, 2022, 06:23
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Actually, I do know the external ambient temperature, so I suppose I can set that to be the total temperature. But I still don't know how to get the total pressure.
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Old   February 1, 2022, 08:33
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A pressure inlet still allows outflow.


The total pressure at the window is equal to the ambient pressure for the same reason that the total temperature at the window is equal to the ambient temperature.


And none of this has anything to do with the flow being compressible or incompressible.
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Old   February 2, 2022, 07:51
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Quote:
Originally Posted by LuckyTran View Post
A pressure inlet still allows outflow.


The total pressure at the window is equal to the ambient pressure for the same reason that the total temperature at the window is equal to the ambient temperature.


And none of this has anything to do with the flow being compressible or incompressible.
I'm still slightly confused as to how we can set the total pressure window if we don't know what the dynamic pressure will be at there (since total = static + dynamic). Is it because the pressure at the window is arbitrary? I.e. it doesn't matter what we set the total pressure to be, since the pressure within the flowfield is just being calculated relative to this total pressure? Or is it because the assumption of zero external wind means the dynamic pressure is zero there (even though flow is entering/exiting with non-zero velocity)?
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Old   February 2, 2022, 10:46
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You need either a static pressure or a total pressure as a boundary condition. The question is, which one do you actually know? Unless you put a sensor at the window boundary, you'll never know what the static pressure is there. So not only do you not know what is the dynamic pressure there, you actually don't know the static pressure either (because you haven't measured it). The same goes for temperature.

There's two things going on:

1) The velocity at ambient is zero because ambient is ambient. So if you stick a barometer into an ambient and try to measure any pressure, you simultaneously measure both static and total pressure because they are the same. Similarly, if you measure the temperature of ambient, you measure simultaneously both the static and total temperature because they are the same.

So, if you tell me that you simply have or know the ambient pressure and temperature then (to me) there is no ambiguity as to which pressure and which temperature that corresponds to because static and total properties are the same for an ambient.

2) In the absence of work and heat transfer, the total pressure and total temperature between two points is the same for any flow (whether or not it is incompressible or compressible). So if I can draw a line between ambient and the window (and I can), then the total pressure and total pressure at the window is equal to the total pressure and total temperature at ambient. So, unlike the static pressure and temperature, I actually do know that the total pressure and temperature at the window is equal to the ambient pressure and temperature. This holds until you install a fan at the window or there is an elephant in the way.


A bonus! Regardless of how much flow comes in/out through the window, the total pressure and temperature is the same ambient pressure and temperature. But the static pressure and temperature will change with flow and you have the measure these every time. You can generalize this to ambients with non-zero velocities (i.e. freestreams or any upstream conditions) and come to the same conclusions. And these reasonings are why every CFD solver asks for total pressure and total temperature at a pressure inlet.
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Old   February 7, 2022, 20:53
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Quote:
Originally Posted by LuckyTran View Post
You need either a static pressure or a total pressure as a boundary condition. The question is, which one do you actually know? Unless you put a sensor at the window boundary, you'll never know what the static pressure is there. So not only do you not know what is the dynamic pressure there, you actually don't know the static pressure either (because you haven't measured it). The same goes for temperature.

There's two things going on:

1) The velocity at ambient is zero because ambient is ambient. So if you stick a barometer into an ambient and try to measure any pressure, you simultaneously measure both static and total pressure because they are the same. Similarly, if you measure the temperature of ambient, you measure simultaneously both the static and total temperature because they are the same.

So, if you tell me that you simply have or know the ambient pressure and temperature then (to me) there is no ambiguity as to which pressure and which temperature that corresponds to because static and total properties are the same for an ambient.

2) In the absence of work and heat transfer, the total pressure and total temperature between two points is the same for any flow (whether or not it is incompressible or compressible). So if I can draw a line between ambient and the window (and I can), then the total pressure and total pressure at the window is equal to the total pressure and total temperature at ambient. So, unlike the static pressure and temperature, I actually do know that the total pressure and temperature at the window is equal to the ambient pressure and temperature. This holds until you install a fan at the window or there is an elephant in the way.


A bonus! Regardless of how much flow comes in/out through the window, the total pressure and temperature is the same ambient pressure and temperature. But the static pressure and temperature will change with flow and you have the measure these every time. You can generalize this to ambients with non-zero velocities (i.e. freestreams or any upstream conditions) and come to the same conclusions. And these reasonings are why every CFD solver asks for total pressure and total temperature at a pressure inlet.
Thanks for your detailed response, that definitely helps. I did have one final question: is the total pressure an absolute or a gauge pressure? I noticed that in Fluent, it does not say gauge, so I am assuming it is absolute?
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Old   February 8, 2022, 18:09
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All pressures in Fluent are gauge
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Old   February 17, 2022, 06:55
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Quote:
Originally Posted by LuckyTran View Post
All pressures in Fluent are gauge
I'm getting some strange results depending on how I set up the pressure inlets. See attached image, which shows an elevation (side) view of the room. The blue and red surfaces are described below. The green dotted surface is a porous screen, which represents a set of louvres. Since Fluent does not let me set a porous jump condition at an inlet, I had to extrude the domain slightly further out, which is what you see in the image.

Case 1: Blue and red surfaces set to pressure inlet with zero total pressure.
Case 2: Blue surface set to pressure inlet with zero total pressure. Red surfaces set to walls.

I get very different results for the temperature distribution within the room between the 2 cases. In particular, temperatures are several degrees higher in Case 1 than in Case 2. Once the flow is past the porous jump, it leads to ambient conditions via those blue/red surfaces, so I don't understand why it matters how I model the region beyond the porous screen, since everything beyond the porous screen just leads to ambient conditions anyway (hope I'm making sense here). If the two scenarios are distinctly different, could you help me understand which one is correct as well as why they're different?

If it helps, I found that for Case 1, a lot of the flow was just being diverted vertically straight down by entering from the upper red surface and exiting through the lower red surface. Not sure whether this matters or not.
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Old   February 17, 2022, 08:42
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"blyatman"
I don't have an answer for your question, but I just wanted to say I love your username.
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Old   February 28, 2022, 07:00
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I don't have an answer for your question, but I just wanted to say I love your username.
Lol thanks!
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