In fluent users guid 28.1 it is stated that for 2d fluent calulate for unit depth and for 2d axisymtric it calculates for 2pi radians

i have pipe of .2 dia and 8 m lenght with inlet velocity 1 m/s, viscosity 0.002 and density 1 kg/m3(these are assumed values to compare analytical and fluetn result) with reynodles number 100

for 2d i am modeling it as rectangle of 0.2 * 8 and for axisymtric it is 0.1 * 8

in the case of axisymtric the results are similar for pressure drop but for 2d case it is half.

wat should i make changes to 2d model so taht i get same result as 3d or axisymtric

2d pipe flow? It's an axisymmetric problem, why would you run a 2d pipe? Even if you enter a depth value in the reference values, this doesn't play into any of the flowfield calculations. This is only used as a reference value when calculating flow rates, forces, etc... When you make a 2d problem, it assumes that the profile goes into and out of the screen for an infinite distance, so setting up this 2d problem would be the same as flow between infinite plates, not like a pipe.

You might be able to fake a problem setup to get the same results for pressure drop, but faking the problem setup = faking the results.

Good luck, Jason