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Comparing residuals of momentum and energy equation

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Old   November 25, 2017, 14:12
Default Comparing residuals of momentum and energy equation
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S Karmakar
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I am actually quite curious to know , when the iteration of residual to attain convergence is carried out, it is often observed the residual for energy equation is much much lower that residual of momentum , x and y component velocity in CONVECTION.

Although if we closely observed the differential equation of momentum i.e Navier-Stokes and of energy is quite analogous considering viscous dissipation in energy equation to be neglected.

For my project case where momentum residual reaches in order of 1e-03 , my energy residual is in order of 1e-11.

Kindly helped​ with this query.
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Old   November 25, 2017, 19:09
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Julio Mendez
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Is your process compressible or incompressible ? Is your analysis isothermal or does it have any type of heat flux ? What are the boundary conditions for your problem ?
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Old   November 25, 2017, 21:29
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Incompressible ideal gas.. constant temperature basebody.. rest part are coupled.
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Old   November 25, 2017, 21:51
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What do you mean by coupled? the Energy equation is always coupled to the mometum equation, regardless of the BC. If your problem is isothermal, then you can decoupled (neglect) energy and the solution will be the same since it does not play an important role in the problem, thus the residual is very small. The residual becomes small when the correction is small. Thus, if you have little or no much things affecting energy the residual won't change.
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Old   November 25, 2017, 22:03
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Quote:
Originally Posted by juliom View Post
What do you mean by coupled? the Energy equation is always coupled to the mometum equation, regardless of the BC. If your problem is isothermal, then you can decoupled (neglect) energy and the solution will be the same since it does not play an important role in the problem, thus the residual is very small. The residual becomes small when the correction is small. Thus, if you have little or no much things affecting energy the residual won't change.
Yours answer has a point.. but take a case.. of horizontal surface at constant temperature subjected to enclosure containing air resulting in natural convection. will u still say due to constant temperature the convection part of energy equation tends to zero? My problem is a fin arrangement is tested on a constant base temperature in an enclosure in laminar flow condition. the fin base is subjected to a constant temperature...
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