why prism meshing near wall?

avinashjagdale · February 8, 2014, 00:53

Hello,

I have read at many places that hexahedral and prism elements resolve boundary layer better that unstructured tetrahedral elements.

Why is it so?

t.teschner · February 8, 2014, 08:29

because hexa and prism elements can be easily forced to have a constant distant away from the wall / next element due to their geometry. unstructured (tetra / tri) elements are much harder to control in terms of height so if you want to resolve a wall bounded flow which is turbulent, you need to resolve the boundary layer and hence you have to account for the y+ values. there are two regions, one where you resolve the boundary layer (i.e. first cell has a height of y+ ~= 1) and one where you have to use wall functions (depending on turbulence model and author, it could be anywhere between y+=5-30 or even higher values). with unstructured elements you are constantly changing your cell hight and thus your y+ value which, depending on your approach might give you wrong results, for example, you want to use wall functions but locally you have a very fine mesh (say an airfoil at the leading edge) and hence you get very small elements where you suddenly have y+=1. but since you are using wall functions you will get wrong results in that region and, in this particular example, this error would propagate downstream and giving you poor results there (for example separation point, pressure distribution etc.)

commercial software codes sometimes have an hybrid approach where the y+ value is calculated locally and depending on the region you are in, it will apply wall functions or not

avinashjagdale · February 9, 2014, 02:02

Thank you Tom. I'll get back to you in case of any further doubts.

avinashjagdale · February 17, 2014, 07:06

in a general case(not limited to boundary)
"hexahedral meshes will give more accurate solutions, especially if the grid lines are aligned with the flow"

does this statement apply to turbulent flow as well?
because,
In a turbulent flow due to random motion of eddies, there is no specific direction of flow, at a point,, unlike laminar flow, where a direction is well defined.

February 8, 2014, 00:53	why prism meshing near wall?	#1
avinashjagdale New Member avinashjagdale Join Date: Feb 2014 Location: pune,India Posts: 26 Rep Power: 12	Hello, I have read at many places that hexahedral and prism elements resolve boundary layer better that unstructured tetrahedral elements. Why is it so?

February 8, 2014, 08:29		#2
t.teschner Senior Member Tom-Robin Teschner Join Date: Dec 2011 Location: Cranfield, UK Posts: 211 Rep Power: 17	because hexa and prism elements can be easily forced to have a constant distant away from the wall / next element due to their geometry. unstructured (tetra / tri) elements are much harder to control in terms of height so if you want to resolve a wall bounded flow which is turbulent, you need to resolve the boundary layer and hence you have to account for the y+ values. there are two regions, one where you resolve the boundary layer (i.e. first cell has a height of y+ ~= 1) and one where you have to use wall functions (depending on turbulence model and author, it could be anywhere between y+=5-30 or even higher values). with unstructured elements you are constantly changing your cell hight and thus your y+ value which, depending on your approach might give you wrong results, for example, you want to use wall functions but locally you have a very fine mesh (say an airfoil at the leading edge) and hence you get very small elements where you suddenly have y+=1. but since you are using wall functions you will get wrong results in that region and, in this particular example, this error would propagate downstream and giving you poor results there (for example separation point, pressure distribution etc.) commercial software codes sometimes have an hybrid approach where the y+ value is calculated locally and depending on the region you are in, it will apply wall functions or not leflix, fportela, AshwaniAssam and 1 others like this.

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February 9, 2014, 02:02		#3
avinashjagdale New Member avinashjagdale Join Date: Feb 2014 Location: pune,India Posts: 26 Rep Power: 12	Thank you Tom. I'll get back to you in case of any further doubts.

February 17, 2014, 07:06		#4
avinashjagdale New Member avinashjagdale Join Date: Feb 2014 Location: pune,India Posts: 26 Rep Power: 12	in a general case(not limited to boundary) "hexahedral meshes will give more accurate solutions, especially if the grid lines are aligned with the flow" does this statement apply to turbulent flow as well? because, In a turbulent flow due to random motion of eddies, there is no specific direction of flow, at a point,, unlike laminar flow, where a direction is well defined.