# The direction of fixedgradient at boundary patch

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 August 27, 2014, 21:15 The direction of fixedgradient at boundary patch #1 Member   Kan Join Date: Feb 2014 Location: Australia Posts: 48 Rep Power: 3 Hi, Can anybody tell me the direction of fixedgradient at boundary interface, I understand that the normal vector at boundary interface points outward, does that mean the direction of fixedgradient also points outward? Thank you Kan

August 28, 2014, 01:53
#2
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Kan
Join Date: Feb 2014
Location: Australia
Posts: 48
Rep Power: 3
Quote:
 Originally Posted by nwpukaka Hi, Can anybody tell me the direction of fixedgradient at boundary interface, I understand that the normal vector at boundary interface points outward, does that mean the direction of fixedgradient also points outward? Thank you Kan
To make more clear, I have tried to simulation a simple heat conduction problem, just 2 dimensional, the north boundary is fixed value, while fixedgradient for south boundary.
I thought the definition of fixedgradient should be (T_c-T_i)/(p_c-p_i), subscript c means cell centre, while i for boundary patch, p_c is larger than p_i at south boundary patch, when I set a negative fixedGradient, then I thought I should get T_c < T_i, but what I actually get is T_c > T_i. could the definition of my fixedgradient is wrong, can anyone clarify the definition of fixedgradient?

Thank you
Kan

August 28, 2014, 02:51
#3
Member

Kan
Join Date: Feb 2014
Location: Australia
Posts: 48
Rep Power: 3
Quote:
 Originally Posted by nwpukaka To make more clear, I have tried to simulation a simple heat conduction problem, just 2 dimensional, the north boundary is fixed value, while fixedgradient for south boundary. I thought the definition of fixedgradient should be (T_c-T_i)/(p_c-p_i), subscript c means cell centre, while i for boundary patch, p_c is larger than p_i at south boundary patch, when I set a negative fixedGradient, then I thought I should get T_c < T_i, but what I actually get is T_c > T_i. could the definition of my fixedgradient is wrong, can anyone clarify the definition of fixedgradient? Thank you Kan
I've found the definition of fixedGradient from http://www.foamcfd.org/Nabla/guides/...Guidese11.html
That solved my question

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