Learning Forum

[peppers]

I use the parabolic velocity profile with the expression formula and it causes reversed flow when using the outlet 'outflow' boundary condition of 100% area, whereas when I don't use parabolic I immediately enter a constant velocity at the velocity-inlet and no reversed flow occurs. why is this happening and how to solve it? I have also extended the pipe at the inlet and outlet but it hasn't worked

[Rob]

Don't use outflow, it's an obsolete option and really should be removed from the GUI. Pressure outlet is the correct choice. 

Best guess with the reverse flow is you got a negative in the wrong place in the Expression or UDF. If you initialise from the inlet which way do the velocity vectors point? 

[peppers]

 

I’ve used a pressure-outlet but reversed flow still occurs.

then what do I change is this correct? i expect the flow to move to the positive x-axis but due to reversed flow, the flow is inlet to the negative x-axis

this is the expression of the parabolic formulation that I use
umax = my velocity 11.96m/s
radius =  sqrt(x**2+y**2)
Radius = area (0.03m)

uprofile = umax*(1-(radius/Radius)**2)

and this is the geometry that I analyzed where convection occurs in the middle of the box in the air flow

[Rob]

 

If you plot uprofile in Excel what are you seeing? If you plot velocity on the inlet surface (vectors or velocity component) which way is it going? 

The image didn't load when I was commenting. What surfaces have you got in the box region - the yellow/grey chequer-boarding suggests two surfaces in very close proximity and yellow is typically a symmetry boundary. 

 

[peppers]

The uprofile plot is in the form of a parabolic curve.

forgive me for not understanding your point. what does the velocity plot on the inlet surface look like? i expect the flow to move to the positive x-axis but the result is reversed flow, the flow is inlet to the negative x-axis

[Rob]

My point is it's possible you added a negative in somewhere by accident. The other check is position: is the inlet centre at y = z = 0 ?   Please post the code, if it's an easy fix I'll tell you.