Learning Forum

[VSK1989]

Dear Team Ansys,

 

I would like to calculate the reference values for fluent simulation for the following geometry.

 

Please let me know which dimension to be taken as length .

 

[Amine Ben Hadj Ali]

Which dimensionless number do you want to calculate?

[VSK1989]

Hi I would like to calculate Coefficient of drag .

Thank you

[Amine Ben Hadj Ali]

Reference length is not used in the calculation of drag coefficient but reference density velocity and reference area.

[ytama6430]

hey, good day. i also have the same problem with analysis of drag coefficient in 2D. i use the reference value and put in the velocity,area, and length there. the velocity was derived from turbulent Re number ( i use 20,000). Compared to the table on Yunus Cengel Fluid mechanic we always got the wrong number of Cd. what might go wrong with my setting? 

 

[ytama6430]

 here is my reference value setting

[Rob]

How good is the convergence, and how far off the value you're expecting are you?

[ytama6430]

the convergence is good enough i think, it already has steady result at 200 iteration. my result differ around 0.5 than the values on the Cengel book, and the problem goes on, first it's only the positioning of the object that matters  with Cd, but now i realize that changing meshing would change the Cd too. is it the Boundary Layer properties that i am dealing with (something with y plus)? my model  just a 2d, can anything go wrong with 2d modelling?

 

[Rob]

Lots can go wrong in any model if you're not sure what you're doing (we all learnt at some point). 

Look at the domain size (read the guidelines on wind tunnels re blockage factor), then look at the mesh. For the mesh you need to consider the y+ (as you've done) but also the near object mesh and wake. This is an easy model to set up (physics) but not easy to mesh and the flow is a lot more complex than novices (and project setters) realise.