# X-Y plot of Yplus in Fluent 5.3

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 March 22, 2000, 11:48 X-Y plot of Yplus in Fluent 5.3 #1 Luo Guest   Posts: n/a Dear all, Using Fluent 5.3, I am doing a simulation of ventilation in a 2D room(Width x Height=9m x 3m). The turbulence model is standard k-e with conventional wall function. In the Fluent User Manual(P9-59), it is said that " A Yplus value close to lower bound (Yplus~30) is most desirable". But when I want to check the Yplus value by creating a ISO surface of first grid plane(a line in my case, say Y=0.08m) and then use X-Y plot(X--room width, Y--Yplus), I always get zero value on all the grid points, while on the Yplus contour display, by right click on the near wall layer(first grid layer), I can see from the FLUENT console window that my first mesh girds are mainly between 30~60(the Yplus value at some grid points are less than 30 but I cannot get all of them between 30~60). Can anyone tell me where have I done wrong? If the Yplus values at some points are out of the lower or upper bound, is it OK? Thank you very much in advance. SP

 March 22, 2000, 13:25 Re: X-Y plot of Yplus in Fluent 5.3 #2 allan thomson Guest   Posts: n/a If your Y+ values are around 30 to 60 then I would say that your ok. I don't know why you can't plot the Y+ values on an XY plot though. You can refine the mesh if the values of Y+ are a lot greater than 60 if you think its a really big deal. It's quite easy in fluent to do so. i.e. from the main menu choose: adapt, Y+/Y*, then set your min/max Y+ values, adapt and the mesh is refined. You then need to start the solution again. cheers at

 March 22, 2000, 13:47 Re: X-Y plot of Yplus in Fluent 5.3 #3 Luo Guest   Posts: n/a Hi Thomson, Thank you very much for your kind help. One more question: if I got my Yplus between 30~60, is it still necessary to test mesh dependance by refining the mesh(if I refine it the Yplus will be out of 30~60)? Because in Fluent User Manual it is said "using a excessive fine mesh near walls should be avoided because the wall functions cease to be valid in the viscous sublayer."(PP. 9-58) As a test, I have used a very fine mesh in the near wall regions and I got a flow pattern much like that of a laminar flow(i.e., much like that when specifying the flow is laminar although the Re based on the inlet height is about 5000!). Anyone has any idea about this and my X-Y plot problem? Much thanx in advance! Cheers! SP

 March 22, 2000, 14:14 Re: X-Y plot of Yplus in Fluent 5.3 #4 allan thomson Guest   Posts: n/a It is important to keep the first node from the wall outside the laminar sub-layer region in the boundary layer. The wall functions were developed to avoid the need to do this. If 30 < Y+ < 60 in my model I would tend to say that was ok unless I had data to compare it (i.e. skin friction heat transfer) with and if there was a significant difference then I would go for a low re turbulence model. Just a thought about your XY plot are you trying to plot cell or vertex data. You cannot plot Y+ as node values Allan

 March 22, 2000, 14:33 Re: X-Y plot of Yplus in Fluent 5.3 #5 Luo Guest   Posts: n/a Hi Allan, Thank you again for your quick response. Yes, the X-Y plot is a plot of vertex data because Fluent doesn't allow a X-Y plot of node data. I just wonder why I get the zero plot while on the Yplus contour they are non-zero? Where have I done wrong? I adapted the mesh as you have said, the result is not very much different to that of the previous result, so I should say it's a mesh independent result? Anyway it's impossible to get all the Yplus at the first grid points near wall between 30~60, so it's okay? Best regards, Shengping ahussai1 likes this.

 March 23, 2000, 12:24 Re: X-Y plot of Yplus in Fluent 5.3 #6 chris Guest   Posts: n/a y+ should be between 30 and 300. y+ too small is wrong and often causes convergence-problems. Try xy-plot, nodal-values off. chris

 March 24, 2000, 05:42 Re: X-Y plot of Yplus in Fluent 5.3 #8 Luo Guest   Posts: n/a Dear Chris and Dr. Kim, Thank you for your postings. I still have some questions about Y+ and Y*, could you explain it a little more further, Dr. Kim? In Fluent User Manual, Page 9-49, after defining U* and y*, it is said:" The logarithmic law for mean velocity is known to be valid for y*>30~60. In FLUNET, the log-law is employed when y*>11.225. ... It should be noted that, in FLUENT, the law-of-the-wall for mean velocity and temperature are based on the wall unit, y*, rather than Y+. (!!!) These quantities are approximately equal in equilibrum turbulent boundary layers." But then, on Page 9-58, you said: "It is known that the log-law is valid for Y+>30~60. ... A Y+ value close to the lower bound (Y+~30)is most desirable". So what I should use as a criterion: Y* or Y+? Although you said "Note that Y+ and Y* have comparable values when the first cell is placed in the log-layer."(PP9-58), I must first decide whether my first cell is placed in the log-layer with one of the above two Ys!! And you said that in Fluent the log-law is based on Y*, not Y+, but after that in the guidelines(9.8) you always use Y+ as the criterion, what should I use? On the other hand, in my project, the flow is ventilation in rooms, the Re is about 5000 based on the inlet wall jet height(U~0.5m/s). The problem is not that I cannot make all the Y+ below 60, but that I cannot make all of them above 30! If I want to make all of them above 30(say by mesh adaption with Y+ or Y*), I get only two mesh points in the spanwise of the jet, it's not a good resolution, yes? What should I do? Thank you all for your feedbacks in advance. Regards, Shengping

 March 24, 2000, 06:32 Re: X-Y plot of Yplus in Fluent 5.3 #9 Jonas Larsson Guest   Posts: n/a If you have no problem to resolve the walls well then why don't you use a low-Re model or a two-layer model with y+<1 everywhere? Then you don't need to worry about resolving some walls too well.

 March 24, 2000, 07:09 Re: X-Y plot of Yplus in Fluent 5.3 #10 Luo Guest   Posts: n/a Hi Jonas, The problem is: one of my friend developed a inhouse code for the calculation of ventilation in rooms, he utilises the standard K-e model with standard wall function, we want to compare the results with different codes. But I cannot get all the Y+ above 30, otherwise I should use a very croase grid near wall, it's evidently not appropriate. Is it not appropriate that I insist on using standard k-e model with standard wall function approach for this problem? In addition, when working with wall function, I should use Y+(=30~60) or Y*(=30~60) as criterion for the first mesh point? Best regards, Shengping

 March 24, 2000, 07:58 Re: X-Y plot of Yplus in Fluent 5.3 #11 Jonas Larsson Guest   Posts: n/a Y+ is most appropriate, but it shouldn't make much difference, Y+ and Y* should be fairly equal in the near-wall region where you are looking - if they are very different you have problems and then you can not expect the wall-laws to work well! Fluent, as Sung-Eun said, uses Y* just because it is more expensive to compute Y+.

 March 24, 2000, 08:08 Re: X-Y plot of Yplus in Fluent 5.3 #12 Luo Guest   Posts: n/a Thank you, Jonas! I can hardly make Y+ and Y* comparable, that means that it's not appropriate to use the wall function approach? Really the flow in my problem is too slow. Regards!

 March 24, 2000, 10:36 Re: X-Y plot of Yplus in Fluent 5.3 #13 Sung-Eun Kim Guest   Posts: n/a My apology for the confusion. Let me see if I clarify: o FLUENT codes (both FLUENT 4.5 and FLUENT 5) does not use y+ (u_tau y/nu) at all for any purpose whatsoever, except the postprocessing (y+ at wall adjacent cells). o FLUENT User's Guide refers to y+ only because most of the near-wall data and correlations have been catalogued in terms of y+, and because y+ is still the most widely used wall unit in the modeling community. o The demarcation of the whole inner layer into "linear" (laminar) layer and the log-lauer with y* = 11.225 as a cut-off is a pure numerical artifact and is adopted in FLUENT (and many other commercail codes) solely for implementation's sake, i.e., a provision to prevent anomalous results when Y*. In reality, there is no such sharp division. There's always a buffer layer. o The difference between y* and y+ is that y* adopts u* = sqrt(sqrt(C_mu)*k) as turbulent velocity scale, whereas y+ uses u_tau(= sqrt(tau_w/rho)) as turbulent velocity scale. y* recovers y+ in the log region of equilibrium boundary layers and fully developed pipe/ducts flows, where u*/u_tau tends toward 1.0 (This is the definition of equilibrium layer!). In the viscosity-dominated near-wall region, say y+ = 1.0, u*/u_tau is much smaller than 1.0 and consequently y* will be much smaller than y+, inasmuch as the turbulent kinetic energy is typically very small (correlation data show k/u_tau**2 = 0.05 y+**2). o You might wonder then why FLUENT uses y*, not y+, causing troubles and confusing people who are get used to the universal law of the wall. The reason is that the y* based law of the wall performs sigificantly better than the universal law of the wall, expecially for non-equilibrium flows (strong pressure gradients, separated flows, rapidly changing wall-bounded flows with strong streamline curvature, etc.). One quick example. Consider a backward facing step where the upstream cold flow reattaches to the bottom wall being heated. You would expect that the heat transfer will be maximum at or near the reattachment point where the cold flow first hits the hot wall. If you use u_tau as turbulent velocity scale and use the temperature law of the wall based on u_tau frequently shown in the textbokks, you end up predicting a "minimum", instead of maximum) heat trnasfer coefficient at "reattachment" point. For your room flows, I suggest you use one of the low-Re k-epsilon models offered in FLUENT 5. You can activate the low-Re model using the "define/models/viscous/low-re" and "define/models/viscous/ low-re-ke-index" text commands. The model indices are; Abid(0), Lam-Bremhorst(1), Launder and Sharma (2), Yang & Shih (3), Abe-Kondo-nagano(4), Chang-Hsieh-Chen(5). You can use two layer model. But I'm a bit concerned that your Reynolds number (I'm not quite sure what length scale is sued for the Reynolds number) seems to be very low and the whole domain is in Re_y<200. If it's the case, the two layer model redices to the one-equation model of Wofstein. I think you'd better model epsilon equation.

 March 24, 2000, 11:37 Re: X-Y plot of Yplus in Fluent 5.3 #14 Luo Guest   Posts: n/a Dear Dr. Kim, Thank you for your kind help. I will try your LRN models and two-layer model. Fortunately, I have some experimental data for this case, so I can compare the performance of different models. The Re I mentioned is based on the jet height, it's a 2D jet in a 2D room. Best regards! Shengping

 March 24, 2000, 18:55 Re: X-Y plot of Yplus in Fluent 5.3 #16 Jonas Larsson Guest   Posts: n/a Yea, you could say that, if Y+ and Y* are very different that probably means that your grid points are not in the inner log-layer (y+ ~ 30-200). It could also mean that you have a boundary layer that is very far from equilibrium. In any case, this indicates that wall-functions won't work well. Use a low-Re model instead that can be integrated down to the wall.

 March 25, 2000, 06:29 Re: X-Y plot of Yplus in Fluent 5.3 #17 Luo Guest   Posts: n/a Dear Dr. Kim, Thank you for your kind help. I will try your LRN models and two-layer model. Fortunately, I have some experimental data for this case, so I can compare the performance of different models. The Re I mentioned is based on the jet height, it's a 2D jet in a 2D room. Best regards! Shengping

 March 25, 2000, 11:26 Re: X-Y plot of Yplus in Fluent 5.3,some solutions and information #18 Luo Guest   Posts: n/a Dear Dr. Chien, Just want to tell you that I apreciate very much your nice suggestions and kind help, I will follow the steps you suggested to "probe" this problem to get some "feelings" about turbulence modeling, but I just have experimental data for Re=5000. Regards, Shengping

 March 30, 2000, 01:33 Re: X-Y plot of Yplus in Fluent 5.3 #19 Michael Soso Guest   Posts: n/a In order to solve your plotting problem, deselect the node values flag in the top left corner of the panel. I think that you may be trying to plot averaged y+ values which does not work and so plots everything on the 0 axis.

 March 30, 2000, 11:51 Re: X-Y plot of Yplus in Fluent 5.3 #20 Volker Pawlik Guest   Posts: n/a Hello Dr.Kim, I too have a problem including a jet but a round one with a similar Re of about 5000 related to the nozzle diameter. The jet enters into a mixing duct where more than 75% of the domain lies within the zone Re-t<200 due to the entrained ambient air. At the moment I am using the 2 layer approach at the wall together with the realizable-k-eps model which the User's Guide advises to use in the case of round jets. My question is whether you can evaluate which error is worse the one depending on the "exorbitant" use of the 1-equation model or the one related to the use of a not "whole" appropriate low-Re-k-eps model.? Thanks in advance Volker

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