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Finding nusselt number in ansys fluent for natural convection.

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Old   November 19, 2019, 13:17
Default Finding nusselt number in ansys fluent for natural convection.
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sujeet toppo
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Hey i am trying to solve natural convection. I have to find nusselt number in a closed square cavity with two adiabatic wall and one heated and other one is cooled wall. My problem is as i want to compare nusselt number from other cfd analysis or thesis paper. How can i get a nusselt number to compare from them.
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Old   November 19, 2019, 13:29
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Have you tried a tutorial or a video on youtube?


The Nusselt number is straightforward to obtain after you've solved a case. There's also different ways to obtain it: strictly using Fluent only, or CFD-post.
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Old   November 19, 2019, 13:37
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Yes i have tried it. I also get nusselt number bt im confused how to compare it from other paper as other paper specify a particular nusselt number to a particular rayleigh number. But my simulation shows many different nusselt number.
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Old   November 19, 2019, 13:43
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Can u suggest some ways to find the nusselt number??? As im confused many youtube video just shows the simulation part. How to get nusselt number for natural convection video s are not there. So please help me.
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Old   November 19, 2019, 14:39
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Step 1. Do the simulation.
Step 2. Make a contour plot of Nusselt number
Step 3. What else do you need?

The Nusselt number is available as a field variable. Like, what more do you need help with? Where are you stuck?

You can also take an average using a report definition and so on, if that's what you are looking for.

Do you know how to get the velocity, pressure, temperature and everything else? Nusselt number is the same.
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Old   November 20, 2019, 11:32
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Hey thanks for your help. I have another question to change the rayleigh number it is writen that to change the effective rayleigh number change the acceleration gravitational. Is it good approach ??? Please reply soon.
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Old   November 20, 2019, 11:53
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I don't really get what makes any effective Rayleigh number any different than any other Rayleigh number... But you need to change something. Gravitational acceleration is one way. Changing the geometry is another. There's certainly isn't a wrong way to change Rayleigh number.
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Old   November 20, 2019, 12:06
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Ok thank you for your help i will ask you if i get stuck in the simulation.
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Old   November 20, 2019, 13:13
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Quote:
Originally Posted by LuckyTran View Post
Step 1. Do the simulation.
Step 2. Make a contour plot of Nusselt number
Step 3. What else do you need?

The Nusselt number is available as a field variable. Like, what more do you need help with? Where are you stuck?

Just because it is available as a field variable does not mean it's definition is universal. What temperature difference does Fluent look at when it computes this value? Wall temperature is one, the other is what? Adjacent node temperature? Inlet Temperature? Average fluid temperature? Node based? Volume based? Fluid Temperature some distance away? etc. Some of these will be very mesh dependent. You will have to develop your own Nusselt number correlation, or figure out how Fluent calculates this variable if you want to compare with other sources.

For reference temperature in a flowing pipe, I use massFlowAve(T)@ a plane normal to the flow, and intersecting that point, for each point along the wall. See this websites Wiki >> FAQs >> ANSYS >> calculating HTC for more information. https://www.cfd-online.com/Wiki/Ansy...ficient_in_CFX
You could also use average values for the entire domain and surface. Depends on how others you wish to compare to have defined Nusselt in their work.
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Old   November 22, 2019, 08:23
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Ok if i have to change the fluid medium from air to something else like co2, n2 etc. I just have to select the medium from drop down menu of fluid particle or i have to do something else also??
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Old   November 22, 2019, 11:01
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Create your new medium in Materials and give it all the right properties. Or you could just edit the ones for the default "air" and rename it. You don't actually need to create another material unless you have multiple mediums.

Then go to cell zone conditions, choose your cell zone, click edit. And select your new material in the material name drop down.

Nusselt number is a field and you should have a bunch of Nusselt numbers (just like you have a bunch of velocities). That's normal. If you need help with getting a particular Nusselt number, then you should explain which one it is you want. And be very specific. I need help getting "a Nusselt number" could be anything.

Whatever paper or thesis you are reading, read it very very carefully and make sure you understand what it is you are looking at. Read read read. Nusselt number is just a fancy heat flux. You could just replace it with heat flux. But is it a spatiotemporal spanwise streamwise averaged Nusselt number computed from the vertex weighted averge when Mercury is in retrograde? Or does it come from a random number generator? No one here can tell you, it comes from reading whatever it is you are reading.
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Old   November 22, 2019, 13:40
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It average nusselt number and i have got one and compare it from the reference and it is somewhere near that. Thank you for the rply for other medium refrence its great help for me. 😊
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Old   November 27, 2019, 07:07
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Hey anyone know how to get the stream function in cfd post result??? It is present in solution under velocity contour. But im unable to find it in cfd post.
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