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clarkent August 24, 2013 07:39

Air flow over oiled surface
Hello everyone,

Does anyone have idea about how to create setup roughly for flow of air over a surface that has oil in micrometer thickness on it? The aim is to investigate pressure value and distribution over the surface under the conditions with and without oil. Should I define a specific shear stress on the related wall? Would use of a multiphase model work?


ghorrocks August 25, 2013 19:25

Are you talking about a micro-scale model (ie modelling the ripples and bumps on the oil film) or a macro scale model (replacing the oil film with some form of wall condition boundary)?

clarkent August 26, 2013 01:47

Actually I am only interested in the effect of oil presence under airflow to pressure, I am not interested in oil film behaviour. My understanding is that macro scale model would be enough. However, I could not find any related study in this subject and could not therefore foresee how important is micro scale modelling. If I adjust the wall boundary condition to represent the oil film, which parameters do you think I should add? For information, air velocity is about 3-4m/s and plate surface size is 25x25cm.

Thank you.

ghorrocks August 26, 2013 02:13

Are you interested in the film's effect on the air flow, or the motion of the oil film on the surface (I assume the film will get blown about on the surface).

CFX already has a wall film model used in modelling wall impingement of fuel sprays in internal combustion engines. The purpose of this model is to model the collection, motion and evaporation of fuel on walls as it contributes to fuel evaporation and therefore emissions and power. Is this of any relevance?

clarkent August 26, 2013 04:04

I am investigating what will be the pressure if air impinge on the oiled surface with 3-4m/s velocity and with an impingement duration of 0.001s. I have not decided how to assure 0.001s of impingement yet however let's focus on the first subject. I am not interested in film motion. For fuel on wall, the impinged fluid is fuel in my case however it is air. I am not sure if the film model is relevant to my problem but I am going to check it.
I think same as you that oil would get blown in a while but in reality the surface must stay slippery since oil cannot be dissolved in air. Do you think doing a single phase simulation and defining a specified shear stress on the wall of interest relative to velocity vectors to represent the slippage would be enough to get a rough solution?

ghorrocks August 26, 2013 06:10

Can you explain the geometry you are talking about? How big is the oil? How big is the duct? Also how is the air jet done? What velocity is the air jet?

clarkent August 26, 2013 07:03

1 Attachment(s)
Attached file is the properties of the geometry. Velocity range at the inlet of the duct will be around 3-4m/s.

ghorrocks August 26, 2013 07:14

I see. My guess is the effect of the oil on the surface is going to be quite small. Is this what you expect?

There is going to be a tiny effect due to the surface of the oil moving, which will be a slight difference to the stationary wall. And there will be a larger effect due to ripples forming on the oil.

So my guess is the most important effect is the rippling of the oil film surface (and even that effect will be very small). Is this correct? On second thoughts, your flow velocity is quite slow and rippling might not occur. There is going to be some form of Reliegh-like stability critereon defining when the oil film goes rippling - finding this stability critereon is probably important.... I will leave that up to you (sorry about the stream of random thoughts, I hope you can understand it).

And to answer your previous question - no, a wall shear stress does not sound physically valid to me. Can you explain why it would be?

clarkent August 26, 2013 14:24

If the velocity was high but in the limits of incompressible flow to create setup of the problem do you think the only option is wall film model then? I have no solid reason for wall shear stress, was just thinking loudly.

ghorrocks August 26, 2013 18:22

My point is that the effect of the oil film on the air flow is going to be tiny. Why are you interested in this tiny difference? Is this academic research or do you have an application for it?

clarkent August 27, 2013 06:39

It is related to part of an academic research but anyway thank you for your help.

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