[CFD] What is the difference between y+ and y*?

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  • Опубликовано: 21 авг 2024

Комментарии • 122

  • @awos127
    @awos127 4 года назад +242

    Everyone saying thanks, meanwhile I just hit Y and missed enter xd

  • @noahzhang892
    @noahzhang892 4 года назад +22

    Finally, there is someone doing this kind of in-depth explanations of those parameters we use in the simulation. Well done!

  • @thekoseng
    @thekoseng Год назад +3

    i was searching for youtube but accidently typed y+, but nice video 👍

  • @hydrokbar2019
    @hydrokbar2019 2 года назад +2

    thanks a lot dear dr. aiden. it is impossible to find these materials as easy as here. I appreciate

  • @rohanverma4145
    @rohanverma4145 5 лет назад +6

    Hello sir, thank you for uploading this. Its very informative because I was always confused in this topic. Please keep posting the vedio like this on CFD.

    • @fluidmechanics101
      @fluidmechanics101  5 лет назад +5

      Thanks Rohan! Yes, i want to keep posting CFD videos, particularly on the confusing topics. I have struggled with them for a long time and want to share what i have found to help everyone out :)

  • @mdfaiazkhaled1347
    @mdfaiazkhaled1347 2 года назад +1

    Once again. A brilliant and articulate lecture.

  • @sreekar443
    @sreekar443 5 лет назад +2

    You upload exactly when I'm searching for it. Great explanation mate

    • @fluidmechanics101
      @fluidmechanics101  5 лет назад +5

      Fantastic! Im trying to upload all the videos that seem to be missing from RUclips

  • @user-ty6dg2zj7b
    @user-ty6dg2zj7b 4 года назад +3

    Very explanatory video helping me a lot while getting through CFD literatures. Thanks a lot!

  • @megadronGR
    @megadronGR 5 лет назад +15

    man u rock, keep it up

  • @MultiMartini23
    @MultiMartini23 3 года назад +3

    I really love your lectures, it is a great help in understanding CFD codes/models

  • @riccardocolombo7108
    @riccardocolombo7108 4 года назад +2

    I love your contents, many thanks from PoliMi ! 🇮🇹

  • @teddya2193
    @teddya2193 7 месяцев назад

    Thank you for the explanation. Very helpful for me who is learning CFD.

  • @medusillaycalamar
    @medusillaycalamar 4 года назад +2

    Thank you man =) Great video! Precise and concise

  • @prasisgod
    @prasisgod 4 года назад +1

    Brilliant upload. Exactly what I was looking for!

  • @nightwalker4397
    @nightwalker4397 4 года назад +2

    Thanks for the video, very clear explanation indeed.

  • @TahsinEngin-Akademi
    @TahsinEngin-Akademi 5 лет назад +3

    Excellent. Keep doing great job.

  • @aksamija
    @aksamija 3 года назад +1

    excellent video as always

  • @93papai
    @93papai 2 года назад +1

    Awesome explanation!

  • @nikhilnick8886
    @nikhilnick8886 5 лет назад +1

    Excellent. you are a saviour

  • @nhhtzc
    @nhhtzc 5 лет назад +3

    Great Job, Thanks for sharing

  • @khalilalani2707
    @khalilalani2707 2 года назад +1

    Great work . keep on please .

  • @guilhermemillerferreiradas1902
    @guilhermemillerferreiradas1902 4 года назад +1

    Thanks Bro, excellent video!

  • @josematiasarbeloromero3954
    @josematiasarbeloromero3954 7 месяцев назад +1

    Hi, great videos. I find your explanations exceptional. Just one question if I may. Is "yp" the wall normal distance for the first grid point, or is it half this distance? Best reards

    • @fluidmechanics101
      @fluidmechanics101  7 месяцев назад

      yP is the distance to the cell centroid. yH = 2yP is the height of the cell

  • @manelsouguir8126
    @manelsouguir8126 4 года назад +1

    Thanks for sharing the information

  • @guanguan7658
    @guanguan7658 Год назад +1

    Many mannnnny Thanks!

  • @CesarLopez-qt9mo
    @CesarLopez-qt9mo 5 лет назад

    Hello, thank you very mucha again. What an amazing explanation Dr Aidan.

  • @mossab7384
    @mossab7384 3 года назад +1

    Thank you so much,great job.

  • @shahryarhabibi7187
    @shahryarhabibi7187 2 года назад +2

    Thank You.

  • @haoli6258
    @haoli6258 4 года назад +1

    good job, dude. it helps alot

  • @adampiechna4962
    @adampiechna4962 3 года назад +1

    Well explained.

  • @niki4291
    @niki4291 8 месяцев назад

    I always end up here when I want to type "y" in my search bar to get to youtube but mistype and end up typing "y+"

  • @JammySparks
    @JammySparks 4 года назад +3

    Hi Aiden, thank you very much for these videos, I am very new to all of this and they are greatly helping. I'm a little confused by this one though, it sounds like Y* has all of the advantages that Y+ has but none of the negatives, so unless I am missing something here, why does Y+ still exist? It feels as if it should have been replaced by Y* entirely.

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +3

      Yes, you are right. Y+ is historic and doesnt really have much use anymore as CFD codes use y* almost entirely. However, they are both available as quantities you can look at (try plotting them in your post processor and have a look). I would still recommend using y+ for your reports though, as many senior academics /professors / old cfd users arent aware of y* and might get a bit confused unless they are turbulence modellers ...

  • @rpapa
    @rpapa 3 года назад +1

    Really good. Thanks a lot.......

  • @akbarravan5604
    @akbarravan5604 16 дней назад

    Hello Dr. Aidan. Thanks for these great contents.
    I completely understood how y+ is being calculated. I justed wanted to ask why we used the the log-law profile for calculating tau_w in Equation 6, andthen deriving Equation 7? In low-Re simulations, the first cell is not in the log-region. Is the explained procedure only works for high-Re simulations?
    Thanks in advance

  • @nickdavis4049
    @nickdavis4049 3 года назад +1

    Thank you!

  • @AeroDZ
    @AeroDZ 4 года назад +1

    Nice Video Bro , I like It

  • @bakrybsata3114
    @bakrybsata3114 4 года назад +2

    Thank you sooo much :)

  • @anupamdas8277
    @anupamdas8277 3 года назад +1

    Dr Aiden, can you please make a video explaining Two-Layer Model for Enhanced Wall Treatment..........

  • @manelsouguir8126
    @manelsouguir8126 4 года назад +1

    keep on !

  • @dhaneshnarwade6374
    @dhaneshnarwade6374 3 года назад +2

    Please tell me more easily. about y+ I kinda didn't exactly understand

    • @fluidmechanics101
      @fluidmechanics101  3 года назад +3

      The simple explanation is: run your CFD calculation and plot y+ in your post processor. You want y+ to be less than 5 (1 if possible) over all the walls. If it isnt less than 5, go back to your mesh and make the cells smaller. This will reduce y+. Check again and keep repeating until y+ is less than 5. Y* is slightly different but you dont need to worry about it as they are basically the same 👍

  • @KuldeepToliames
    @KuldeepToliames 2 года назад

    Hello Aidan,
    I have a doubt regarding the implementation of the equation-10 in the slide where y_plus has to be computed iteratively. So the variable y_p in equation-10 refers to the distance of cell centre to the near wall (let's assume there is only one wall distance to account for simplicity). So for each cell, there has to be performed iterations to calculate y_plus of each cell, is that what I understood is correct? Can you please let me know?
    Secondly, for the implementation of say Van Driest damping function in a Smagorinsky SGS model, the formulation itself contains y_plus to calculate the parameter 'D' (which is used as nu_sgs limiter). So a commerical package like say FLUENT (which is y_star based) will compute y_plus or y_star?

  • @minhn1994
    @minhn1994 5 лет назад +1

    Hello, this is a wonderful video, but I have a quick question, regarding a point made at 10:10. How can y+ and y* (assuming you're in an equilibrium turbulent boundary layer) end up being the same, when tau_w, as a measure of shear stress, is a value that is independent of wherever your y is, whereas turbulent kinetic energy k_p is going to necessarily change depending on your distance from the wall?

    • @fluidmechanics101
      @fluidmechanics101  5 лет назад

      Hi Minh, i will have to get back to you on this one. I think this is covered in the Launder and Spalding paper in their justification for using y* but i cant quite remember, since it has been a long time since i looked at it. In your CFD simulations, you can always just have a look at the solution for y+ and y* yourself and see that they are very similar.

  • @Lea-jm6op
    @Lea-jm6op 3 года назад +1

    thanks men

  • @milansekularac6196
    @milansekularac6196 4 года назад +1

    I am wondering, unless I am missing something in my reasoning, regarding the y* approach - when your first wall adjacent cell is IN the viscous sublayer, what is the physical meaning of kp there (as there shouldn't be any turbulence that near a wall) ? Thanks for the excellent videos, wonderful.

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +3

      Correct. Physically there will be no turbulence near the wall as the fluctuations are damped by viscosity. However, the CFD code will compute a turbulent kinetic energy value in every cell in the mesh (even though the value is small). This small value is used to compute y* (which will also be small as we are in the viscous sub-layer). 😊

    • @milansekularac6196
      @milansekularac6196 4 года назад

      @@fluidmechanics101 yes, after watch ing your other video, it is my understanding u_tau is ised to compute kp, when the wall adjacent cell is in the viscous sublayer? Thanks. 🙏

  • @thisboyislocobeats
    @thisboyislocobeats 4 месяца назад

    🔥🎆

  • @gabrieljiang4653
    @gabrieljiang4653 4 года назад +1

    Hi, I read from a reference that in viscous sublayer nu_t is in proportion to y+^3 and it is respected in low Re models like Launder-Sharma and Chien (while in Lam-Bremhost it is actually y+^4) ... Why there is a such difference?

  • @linanikolaidou9201
    @linanikolaidou9201 5 лет назад +1

    Great explanation, along with the rest of your wall treatment videos, things are a lot more clear now, thanks! Is it not possible then to use nutUWallFunction for separated flows? In addition are the above equally applied in RANS and LES? Thank you!

    • @fluidmechanics101
      @fluidmechanics101  5 лет назад +1

      Yep, you could use nutUwallFunction for separated flows. Just make sure your y+ < 5 (preferably < 1) if you want the right answer .... im not clear on the LES treatment yet, let me get back to you on that!

    • @idiosinkrazijske.rutine
      @idiosinkrazijske.rutine 4 года назад

      For LES look for an implementation of the Werner&Wengle wall functions.

  • @Yiqingzhang-vn8cr
    @Yiqingzhang-vn8cr 6 месяцев назад

    I would like to ask you, how to calculate the characteristic length of open channel flow?Y+ needs to be calculated when drawing the grid, so the value of the feature length needs to be known. I want to know whether the feature length of open channel flow is calculated by hydraulic radius or hydraulic diameter? Can you give specific formulas for reference?

  • @anatolykrasnow
    @anatolykrasnow 8 месяцев назад

    Check your formula (1). If you use dimensions for the parameters on the right side you will get that Y+ has dimension of meters! So, it is not dimensionless!!!

  • @aqumy
    @aqumy 3 года назад +2

    I hit Y and + in URL XD

  • @kampfmuffin3535
    @kampfmuffin3535 4 года назад +1

    totally did not miss that enter button ._.

  • @mgmktaha6043
    @mgmktaha6043 4 года назад +1

    Excellent sir. If l have pipe or duct , the Re number based on diameter or Dh ,which correlation can I use to estimate the friction coefficient to get the shear stress

    • @fluidmechanics101
      @fluidmechanics101  4 года назад

      The flat plate correlation should be fine. Remember it is an estimate after all 😄

    • @mgmktaha6043
      @mgmktaha6043 4 года назад

      Fluid Mechanics 101 sorry sir I don’t understand what you mean . But if I have pipe or duct The turbulent flow will be exist when the Re number more than 2300 and the friction coefficient correlation differ For pipe to flat plate. I mean can I use the friction coefficient for the pipe instead of flat plate to determine it and then use it to determine the shear stress , the friction velocity near the wall and y plus

  • @SuperJaylad
    @SuperJaylad 4 года назад +1

    What turbulence model should be used if you do not have sufficient computational power to resolve the viscous sub-layer at wall boundaries?

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +1

      I would probably go with k epsilon with wall functions (make sure your y+ is between 30 and 200). What is your flow scenario? Is it external aerodynamics or internal heat transfer, multiphase?

    • @SuperJaylad
      @SuperJaylad 4 года назад

      @@fluidmechanics101 I have one scenario where the flow is model over a hill for external flow, then a second one with internal heat transfer. Standard k-e with non-equilbrium wall functions was my thought for the external flow due to the curved surface. However I was unsure about the internal?

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +1

      Yea k epsilon should also be fine for internal flow unless you have any seperated regions/diffusing sections. It is not the best for separated flow 👍

    • @SuperJaylad
      @SuperJaylad 4 года назад +1

      @@fluidmechanics101 Great, thanks for your input. Fantastic channel, keep up the good work! (Y)

  • @sachintom5606
    @sachintom5606 4 года назад +1

    Hii Sir...Nice explanation.....
    Sir I have a doubt about plotting uplus vs yplus at any radial direction in Fluent. I seen uplus can be determined by u/ut....where ut can be determined from wall shear stress..What about yplus?Is it same as turbulent reynolds number defined in fluent? Can I make the plot b/w uplus and Ret.....which will be same as uplus vs yplus....
    Pls help me on this....

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +1

      Turbulence Reynolds number is not the same as yPlius. To make the plot normal to the wall you need to plot u vs y along a line normal to the wall, then plot wall shear stress vs y along the same line. You will notice that the wall shear stress is zero along the line except the first value. This is the value you should use to calculate utau, then uplus and then yplus for all the values along the line (you will need to do this yourself either in excel or matlab). I hope this helps

    • @sachintom5606
      @sachintom5606 4 года назад +1

      @@fluidmechanics101 Thanks sir...How to plot theta plus along the radial direction?In theta plus expression (Tw-Tp) term appears..how to take it?We need to fix Tw as wall node value and give local Tp values along radial nodes?

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +1

      Yes, you are correct. This is a bit tricky to do manually so you might want to set up some scripts to do it. I usually use paraview as the python scripting is really good

  • @achyuth_rj
    @achyuth_rj 3 года назад

    Thanks for a great video.
    In calculating y+, what values of density and dynamic viscosity do we use?
    I believe we do not use free-stream values of the fluid for these; so does density and dynamic viscosity vary (just like y) as we calculate y+ ?

    • @fluidmechanics101
      @fluidmechanics101  3 года назад

      This depends on what type of flow you have. If your flow is incompressible and single phase Newtonian then density and dynamic viscosity are constant everywhere. If not, then take a best estimate for what you think density and dynamic viscosity should be near the wall. Remember that this is just an estimate for y+, you will have to go back and update the mesh anyway, so best to just go with your best guess here for density and dynamic viscosity

    • @achyuth_rj
      @achyuth_rj 3 года назад +1

      I see, thanks Aiden. That makes sense. Keep up the great work!

  • @yadabamahanand1197
    @yadabamahanand1197 4 года назад +1

    sir, how can I plot u+ vs y+ for fully developed turbulent flow through a rectangular duct. please tell me the procedures.

    • @fluidmechanics101
      @fluidmechanics101  4 года назад +8

      1. Calculate the wall shear stress.
      2. Calculate the friction velocity (square root of the wall shear stress)
      3. Plot a line normal to the wall
      4. Extract the velocity along a line
      5. U+ = U / friction velocity
      6. Y+ = y * friction velocity / kinematic viscosity
      👍

  • @kshitijbhosale8364
    @kshitijbhosale8364 4 года назад

    Sir, what is C in the formula of y*? What is that quantity?

  • @archdawe
    @archdawe 3 года назад +1

    Good afternoon, my y+ and y* are very different around the reattachment point of a separated flow (y+=5 and y*=21), is this to be expected as a result of the confused velocities at the wall? Or have I done something wrong? Thanks

    • @fluidmechanics101
      @fluidmechanics101  3 года назад

      Did you calculate y+ and y* yourself or did the CFD code calculate it? You may also need to be careful with the wall shear stress you are using in y+ formula. Recall that wall shear stress is a vector with 3 components and you need a scalar value to calculate y+. This could be the cause of your error ... Also, if you are a CFX user there is a 'solver y+' and 'y+' which give different values. Might be worth looking in the use manual to check what version you are using

    • @archdawe
      @archdawe 3 года назад

      @@fluidmechanics101 I got the values from Fluent post-processing so hopefully no human error there... I'm doing the classic backward facing step for some context, y* and y+ on the top wall are pretty similar but it seems they are not in agreement at all along the bottom wall. I guess my main question is should I be concerned/is this unexpected? Thanks

    • @fluidmechanics101
      @fluidmechanics101  3 года назад +1

      Hmmm if you are doing backwards facing step, you probably want to refine your mesh. Regardless of whether we look at y+ or y*, you should be aiming for around 1. The reattachment point is likely to be very sensitive to the wall normal and spanwise resolution so I would probably refine your mesh and try again? 2D backwards step is pretty quick to run so would probably be worth remeshing. I also doubt that y+ and y* will give good agreement around the reattachment point as the wall shear stress tends to zero at the inflexion point. Y* is probably more reliable

    • @archdawe
      @archdawe 3 года назад

      @@fluidmechanics101 Okay great thanks, will give that a go!
      Thanks again for the help Aidan

  • @MuhammadSaad-uk6xd
    @MuhammadSaad-uk6xd 3 года назад

    Hello Dr. Aiden
    Excellent content and extremely helpful. I have one question what value should be used for density and dynamic viscosity for y plus. I am simulating a Heat exchanger and properties are defined piecewise linearly so I don't understand what value should I use for y plus calculations or put in reference values in fluent.

    • @fluidmechanics101
      @fluidmechanics101  3 года назад +1

      You will have to estimate! Maybe try something like the mean of the wall temperature and the free stream temperature. Then look up the properties from the polynomial. Remember, these are just estimates!

    • @MuhammadSaad-uk6xd
      @MuhammadSaad-uk6xd 3 года назад +1

      @@fluidmechanics101 Dr. Aiden thanks for feedback I will try different values. Sir I have few more questions, if you have some time can I ask them?
      Also keep continuing with the excellent work. I would recommend your channel to my peers as well.

    • @fluidmechanics101
      @fluidmechanics101  3 года назад

      Sure, ask away. It is always useful to put questions in the comments because other people can see the answers and benefit as well 😄

    • @MuhammadSaad-uk6xd
      @MuhammadSaad-uk6xd 3 года назад

      Thanks..
      1) i am running simulations with 1.2 million cells but as i refine mesh to 1.8 million and 2.6 million cells results deviate further away from theoretical calculations instead of improving for heat exchanger simulations. I don't understand why?
      2) I get accurate results for RNG at smaller mass flow rates but as I move towards higher mass flow rates results get worse and opposite happens for k epsilon realizable.

    • @fluidmechanics101
      @fluidmechanics101  3 года назад

      I think you need to be careful. Your results don't 'get worse' as you refine the mesh. The results get closer to a mesh independent solution, which may or may not match the experimental results. Once you have a 'mesh independent' solution, then you can start looking at reasons why your results may be different to the experiments. If you send me an email (FluidMechanics101@gmail.com) I can send you a copy of my 'CFD for professionals' course. It explains this idea in much more detail. I think it might be just what you are looking for!

  • @santiagogordillo1057
    @santiagogordillo1057 3 года назад

    Which program do you use to make the presentation? Most of my teachers used it, and i never knew which program they were using.

    • @fluidmechanics101
      @fluidmechanics101  3 года назад +1

      Latex is the programme, with the beamer package for presentations 👍 it is free and works on all operating systems (Linux, Mac, windows) so you should be able to get it

    • @santiagogordillo1057
      @santiagogordillo1057 3 года назад +1

      @@fluidmechanics101 tks a lot!

  • @heattransfer8003
    @heattransfer8003 3 года назад +1

    ı THOught they were the same thing damn

  • @fresh218
    @fresh218 4 года назад

    y+

  • @parthsavyasachi9348
    @parthsavyasachi9348 2 года назад

    Aidan good video but this is not how things go in commercial software world.
    Though you did good with the video given the fact that this is most confusing area of cfd. I intend to write a book on wall treatment just for this reason.
    In commercial codes we do y+ then using this and k we try to find u-star or u-tau in your video.
    This u-star is the backbone of everything that your video missed (you did very good though).
    Then we compute y-star using yplus and u-star. Then in the end shear stress is calculated. Will be used for y plus and ustar next iteration.
    All softwares do things differently and there is no fixed one way. Even fluent has many approaches to get that ustar and shear after yplus.

    • @fluidmechanics101
      @fluidmechanics101  2 года назад

      Hi Parth, thank you for your insight. This is a confusing topic for users, as we often don't know what the code is doing and have to try and work things out by guessing! Your comments are really useful.Thank you

    • @parthsavyasachi9348
      @parthsavyasachi9348 2 года назад

      @@fluidmechanics101 i wanted to reply but free speach platform youtube keep deleting my comment for no reasons.

    • @fluidmechanics101
      @fluidmechanics101  2 года назад

      That's a shame. You can always send me an email: fluidmechanics101@gmail.com

  • @kacpergrzesik8445
    @kacpergrzesik8445 3 года назад

    where are you from?

  •  4 года назад +1

    Fluid Mechanics Forces on Planar Surfaces Example
    ruclips.net/video/DY25-LD9wYY/видео.html

  • @monsterpro1174
    @monsterpro1174 4 года назад +2

    me ecris y+ google
    i yal ebna youtupe

  • @daiixs_2
    @daiixs_2 2 года назад +1

    xd

  • @EnterB_
    @EnterB_ 2 года назад