Turbulent channel flow over hydrophobic surface. Even though I changed the contact angle, the result

bekir4260bekir4260 Member Posts: 1

Dear friends,

In my experimental work: I coated the inner surface of the cylindrical pipe with 3 different hydrophobic materials.

I measured 3 different contact angles. (contact angles 110, 124, 138)

As the contact angle increased, the friction on the inner surface of the tube decreased.

As a result: Pressure losses between inlet and outlet are reduced and efficiency is increased.

I'm trying to model my experimental work with fluent.

I will model a steady and turbulent pipe flow.

I used the VOF model.

Although I used 3 different contact angles on the same model during the solution, they all give the same result.

Why is the result not different?

Why is the result the same even though the contact angle values are different?

thank you.

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Answers

  • DrAmineDrAmine GermanyPosts: 7,890Forum Coordinator

    Please insert the pictures in the text. Ansys employees do not download attachements.

  • DrAmineDrAmine GermanyPosts: 7,890Forum Coordinator

    I do not understand what do you expect here? Do you have a multiphase case? Are you injecting anything? How are you using the contact angle information to affect the total pressure drop?

  • bekir4260bekir4260 Posts: 12Member

    I am interested in the plumbing business.

    I am looking for ways to reduce the pressure loss in the installation.

    I coated the inner surface of the pipe piece with hydrophilic structure with hydrophobic material.

    hydrophobic material prevents water from sticking to the surface.

    friction on the inner surface of the pipe is reduced.

    no slip condition.

    I tested it on 3 different surfaces with 3 different hydroobic materials.

    Friction and pressure slip were the least in the pipe with the highest contact angle.

    now i want to model it with fluent.

    The only place where I can enter the contact angle in the fluent program is the VOF model.

    In fact, only water is flowing.

  • RobRob UKPosts: 11,730Forum Coordinator

    Contact angle is set between phases. If there's no meniscus the value is meaningless.

  • bekir4260bekir4260 Posts: 12Member
    edited November 5

    (If there is no meniscus, the value is meaningless.) what did you mean? I do not understand.

    hydrophobic surface means:

    the surface is in nano-sized rough structure.

    air is trapped in the roughness during the flow of water, so the water does not stick to the surface and the surface does not hold water.

    In fact, only water flows on the surface.

    Air is trapped in the spaces between the water and the surface.

    I am attaching the hydrophobic surface and velocity profile photo.


  • RobRob UKPosts: 11,730Forum Coordinator

    Correct. However, if you read the Fluent manual you'll find we measure contact in the droplet. If the surface is fully wetted there is no free surface to attach the angle to. You'll also see the section explaining how the VOF model works.

  • bekir4260bekir4260 Posts: 12Member

    I understand.

    So how can I model?

    can you help me please

  • bekir4260bekir4260 Posts: 12Member

    The diameter of the pipe is 0.3m and the length is 10m.

    contact angle measured at the surface: 110 degrees , 132 degrees , 148 degrees

    I will calculate the pressure loss due to friction in the pipe.

    The flow is continuous and turbulent.

  • bekir4260bekir4260 Posts: 12Member

    input speed 0.30404m/s

  • RobRob UKPosts: 11,730Forum Coordinator

    Best approach is probably to alter the wall shear rates in the boundary condition panel.

  • bekir4260bekir4260 Posts: 12Member

    What should I change in the boundary conditions?

  • RobRob UKPosts: 11,730Forum Coordinator

    Wall conditions, you'll need to alter the wall Shear Conditions, that may require a UDF or expression. And, no, we don't have the expression, you'll need to figure that out.

  • DrAmineDrAmine GermanyPosts: 7,890Forum Coordinator

    You will rather need to code a wall shear stress to incorporate the effects on the momentum equation instead of using multiphase model for applications where only one phase is present (Navier Slip implementation).

  • bekir4260bekir4260 Posts: 12Member

    There is only one phase.

    If I do as you say, there are contact angles and efficiency values that I have obtained experimentally.

    How do I enter these values into the fluent program?

  • bekir4260bekir4260 Posts: 12Member

    Are there any ready made udf codes?

  • JSMacdonaldJSMacdonald Posts: 2Member

    I think you've got a really interesting problem but I don't think it's as straightforward to solve as you'd like it to be.


    I found this paper which has some pertinent aspects for your problem. https://journals.sagepub.com/doi/full/10.1177/1475090217740470

    Specifically, it shows development of a user defined function for "β - the sliding coefficient, which depends on the wall surface properties", used to calculate shear rate.


    Also from the discussion section

    "There is no straightforward correlation between the CA and the slip length. However, as an example, Ou et al.23 reached a slip length of 20 μm with a surface having CA ≈ 160°. Therefore, it can be expected that the above-mentioned fabrication methods with CA > 160° are capable of producing slip lengths greater than 20 μm."

    It sounds to me like there is not an established way to link shear rate with 3-phase contact angle (or surface energy, which would be the more appropriate parameter here).


    As I don't work for Ansys I can also say that I think your experimental conclusions sound dodgy

    "As the contact angle increased, the friction on the inner surface of the tube decreased." Why is this?

    "air is trapped in the roughness during the flow of water, so the water does not stick to the surface and the surface does not hold water." This is for the superhydrophobic regime, however at the contact angles you mention you are in a fully wetted regime.


    It would be good to get your thoughts, good luck with the model

  • bekir4260bekir4260 Posts: 12Member
    edited November 15

    (It sounds to me like there is not an established way to link shear rate with 3-phase contact angle (or surface energy, which would be the more appropriate parameter here).

    There are many articles about the relationship between the contact angle and the sliding speed on the surface.

    I found many articles in the literature search, but there is no flow modeling on the hydrophobic surface in the FLUENTT program.

    I am writing my e-mail address. bekir.dogan@gop.edu.tr

    Please I would be very happy if you could help.


    If you know the FLUENTT program well, we can work on an article together.

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