[dev325]

To understand mesh refinement concepts I modelled a pipe (10 mm dia, 500 mm long) having three flow velocities (4, 6 and 8 m/s, water as fluid). Mesh refinement was carried out in the 2 mm thick layer by changing no of elements in the layer. Four runs were taken with no of elements 5, 7, 9 and 11. The mesh was generated in such a way that element thickness is smallest near the wall and gradually increases towards the centre. Figure shows the typical grid profile showing 11 elements in 2 mm thick layer.

Fig-1

The analysis was carried out using k-e model with scalable wall function. (As a rule, none of the defaults were changed). Inlet temperature for water was 300 K. Wall temperature was 373 K.
Values for y+ and wall heat transfer coefficient (hf) were noted at a distance of 400 mm from entry. Line averaging was carried out.
Inlet pressure was also noted (using ‘area averaged’ tool). Outlet pressure was set 0 Pa.

Figure shows variation in y+ as a function of number of elements (at a plane 400 mm from inlet).

Fig-2



Figures show variation in heat transfer coefficient (at a plane 400 mm from inlet) and inlet pressure as a function of number of elements.


Fig-3 and Fig-4


Doubts:-
- Can I consider that I have obtained mesh convergence with 5 elements? The two important parameters ‘heat transfer coefficient’ and ‘pressure drop’ remain practically unchanged.
- Should I be greedier and try to use even less than 5 elements say 3 or 4 in the 2 mm thick layer near wall?