Need help finding interface shape in a micro-fluidic channel

daenerys · March 6, 2018, 10:53

I'm trying to conduct a multi-phase analysis of a water-glycerol flow inside a micro-channel with a groove infused with silicon oil, as follows (water in blue, oil in red):

geometry.jpg

geometry2.jpg

The expectation is that the oil will drain from the groove due to shear forces with the water phase, until some final length of the oil will stay wetted inside the groove indefinitely, due to an adverse pressure gradient, with some streamwise curvature in the water-oil interface (see pic below).
This interface is what I'm looking to find in this simulation.

curvature.jpg

However, I have not managed to replicate these results in a simulation.
I use:
2D planer, transient time solver, with gravity in the negative y direction.
VOF model with implicit body force (I have tried to couple the Level Set function as well, with no success).
Phase interaction of 0.029 [N/m] with Wall Adhesion turned on.
Operating pressure was chosen to be that of water-glycerol, at some position in the channel consisting of only water.
Specified operating density was chosen to be that of water-glycerol.
Oil was chosen to be the primary phase, and water the secondary phase.
Velocity Inlet BC for the mixture, with a velocity magnitude of 0.00034 [m/s] and for the secondary phase (water) with a volume fraction of 1.
Pressure Outlet BC for the secondary phase, with a backflow volume fraction of 0.
All other borders are chosen to be wall, with the appropriate contact angle of water-glycerol on the edges of the channel, and the appropriate contact angle of the oil on the edges of the groove.
Pressure Velocity Coupling - PISO.
Gradient - Least Sqaures Cell Based.
Pressure - PRESTO.
Momentum - Second Order Upwind.
Volume Fraction - First Order Upwind.
Transient Formulation - First Order Implicit with non-iterative time advancement.
Solution is Initialized from Inlet, with gauge pressure 0 [Pa], x velocity of 0.00034 [m/s], y velocity of 0 [m/s], and water volume fraction of 1.
Then Adapt -> Region -> Input coordinates of the oil initial location -> Mark, and finally Patch -> water -> volume fraction -> hexahedron-r0 -> value 0 -> Patch.
I run the simulation with a time step size of 1e^-06.

Any tips on what I'd need to change for the results to make sense?
Many thanks!

daenerys · March 7, 2018, 06:44

The main problem that the results are turning out to be rubbish, as I'm seeing it, is that the water-glycerol velocity in the channel eventually subsides.
When I plot the velocity magnitude contours after running the simulation for a little while, it seems like there's no velocity in the channel.
How do I make sure there'll be a constant water flow inside the channel, all throughout the simulation?

March 6, 2018, 10:53	Need help finding interface shape in a micro-fluidic channel	#1
daenerys New Member Lily Join Date: Aug 2014 Posts: 27 Rep Power: 12	I'm trying to conduct a multi-phase analysis of a water-glycerol flow inside a micro-channel with a groove infused with silicon oil, as follows (water in blue, oil in red): geometry.jpg geometry2.jpg The expectation is that the oil will drain from the groove due to shear forces with the water phase, until some final length of the oil will stay wetted inside the groove indefinitely, due to an adverse pressure gradient, with some streamwise curvature in the water-oil interface (see pic below). This interface is what I'm looking to find in this simulation. curvature.jpg However, I have not managed to replicate these results in a simulation. I use: 2D planer, transient time solver, with gravity in the negative y direction. VOF model with implicit body force (I have tried to couple the Level Set function as well, with no success). Phase interaction of 0.029 [N/m] with Wall Adhesion turned on. Operating pressure was chosen to be that of water-glycerol, at some position in the channel consisting of only water. Specified operating density was chosen to be that of water-glycerol. Oil was chosen to be the primary phase, and water the secondary phase. Velocity Inlet BC for the mixture, with a velocity magnitude of 0.00034 [m/s] and for the secondary phase (water) with a volume fraction of 1. Pressure Outlet BC for the secondary phase, with a backflow volume fraction of 0. All other borders are chosen to be wall, with the appropriate contact angle of water-glycerol on the edges of the channel, and the appropriate contact angle of the oil on the edges of the groove. Pressure Velocity Coupling - PISO. Gradient - Least Sqaures Cell Based. Pressure - PRESTO. Momentum - Second Order Upwind. Volume Fraction - First Order Upwind. Transient Formulation - First Order Implicit with non-iterative time advancement. Solution is Initialized from Inlet, with gauge pressure 0 [Pa], x velocity of 0.00034 [m/s], y velocity of 0 [m/s], and water volume fraction of 1. Then Adapt -> Region -> Input coordinates of the oil initial location -> Mark, and finally Patch -> water -> volume fraction -> hexahedron-r0 -> value 0 -> Patch. I run the simulation with a time step size of 1e^-06. Any tips on what I'd need to change for the results to make sense? Many thanks!

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March 7, 2018, 06:44		#2
daenerys New Member Lily Join Date: Aug 2014 Posts: 27 Rep Power: 12	The main problem that the results are turning out to be rubbish, as I'm seeing it, is that the water-glycerol velocity in the channel eventually subsides. When I plot the velocity magnitude contours after running the simulation for a little while, it seems like there's no velocity in the channel. How do I make sure there'll be a constant water flow inside the channel, all throughout the simulation?