Hello,
I am doing a simulation to estimate the pressure drop through a pipe (length = 25 cm, diameter = 9.5 mm, fluid = water, velocity at the inlet = 1.5 m/s).
I chose to run a fluid dynamics analysis with an incompressible fluid and a steadystate flow with a komega SST turbulent model.
The meshing operation I am using is a tetrahedral meshing with prismatic viscous layers.
In order to analyse the influence of the total layers thickness and the influence of the wall roughness on the pressure drop, I performed two different studies.
1st study
In order to test different reasonable layer thickness, I used the following equation:
layer thickness = ly_plussqrt(74)*Re^(13/14)
where l is the length scale (here the pipe diameter), y_plus lies between 30 and 300 and Re is the Reynolds number
I ran the simulation with y_plus = 30, 50, 100, 150, 200 and 300.
I did not activate the wall roughness in the simulation parameters.
The pressure drops I got are:
y_plus = 30; delta_p = 6.33 mmHg
y_plus = 50; delta_p = 6.89 mmHg
y_plus = 100; delta_p = 7.78 mmHg
y_plus = 150; delta_p = 7.89 mmHg
y_plus = 200; delta_p = 8.40 mmHg
y_plus = 300; delta_p = 8.58 mmHg
I am wondering how can I know which y+ is the optimal one and why delta_p rises in this way with the increase of y+.
2nd study
In order to analyse the influence of the wall roughness on the pressure drop, I used the same model than previously with y_plus = 30, and I activated the wall roughness.
The roughness constant is 0.5 and the different roughness height tested are:
0.0000013; 0.00001; 0.0001; 0.001.
The pressure drops I got are:
roughness height = 0.0000013; delta_p = 6.26 mmHg
roughness height = 0.00001; delta_p = 6.32 mmHg
roughness height = 0.0001; delta_p = 6.33 mmHg
roughness height = 0.001; delta_p = 6.30 mmHg
It seems that there is not a significant influence of the roughness height on the pressure drop. Moreover, the pressure drop value obtained without activating wall roughness is the same than with roughness height = 0.0001.
I have two questions:

When we don’t activate the wall roughness, does simscale set a standard value of roughness height and roughness constant? If yes, what are these values?

Is there an order of magnitude of roughness height for which I am supposed to detect an influence of roughness height on pressure drop?
I hope I am clear. If not, do not hesitate to ask me further details.
Thank you.