# Comparing residuals of momentum and energy equation

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 November 25, 2017, 14:12 Comparing residuals of momentum and energy equation #1 New Member   S Karmakar Join Date: Nov 2017 Location: Kolkata Posts: 3 Rep Power: 6 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.

 November 25, 2017, 19:09 #2 Senior Member   Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 287 Rep Power: 16 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 ?

 November 25, 2017, 21:29 #3 New Member   S Karmakar Join Date: Nov 2017 Location: Kolkata Posts: 3 Rep Power: 6 Incompressible ideal gas.. constant temperature basebody.. rest part are coupled.

 November 25, 2017, 21:51 #4 Senior Member   Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 287 Rep Power: 16 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.

November 25, 2017, 22:03
#5
New Member

S Karmakar
Join Date: Nov 2017
Location: Kolkata
Posts: 3
Rep Power: 6
Quote:
 Originally Posted by juliom 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...