# Initial Values for Turbulence Coefficients **URL:** \ **Category:** Fluid Flow / CFD\ **Created:** [June 3, 2016, 9:19pm UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950 "2016-06-03T21:19:17Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 1, 2018, 9:26am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/21 "2018-09-01T09:26:55Z")
The kinematic molecular viscosity of air can be taken as 1.5e-5 m^2/s for a wide range of applications. You can refer to this table [Dry Air - Thermodynamic and Physical Properties](https://www.engineeringtoolbox.com/dry-air-properties-d_973.html). Turbulent viscosity ratio = turbulent viscosity / molecular viscosity. For incompressible flow, OpenFOAM uses the kinematic version of viscosity. In this case you have kinematic viscosity ratio (nut/nu) = kinematic turbulent viscosity / kinematic molecular viscosity = kinematic turbulent viscosity / 1.5e-5. The kinematic turbulent viscosity (nut) is calculated by the turbulence model. With each pair of k and omege values, you will get a unique nut. ---
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 1, 2018, 9:33am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/22 "2018-09-01T09:33:20Z")
I should have explained this better. The table on [Dry Air - Thermodynamic and Physical Properties](https://www.engineeringtoolbox.com/dry-air-properties-d_973.html) contains molecular/laminar viscosity only. The molecular viscosity is a fluid property independent of the turbulence models. The turbulent viscosity is the result of turbulence modelling. ---
**Author:** ![DaleKramer](https://www.simscale.com/forum/user_avatar/www.simscale.com/dalekramer/32/20201_2.png) [@DaleKramer](https://www.simscale.com/forum/u/DaleKramer)\ **Post date:** [September 1, 2018, 9:36am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/23 "2018-09-01T09:36:18Z")
No problem! > [@dylan](#): > > The turbulent viscosity is the result of turbulence modelling. So how can I calculate or find the value of this so that I know I am within the 0.1-2 range? ---
**Author:** ![jousefm](https://www.simscale.com/forum/user_avatar/www.simscale.com/jousefm/32/34908_2.png) [@jousefm](https://www.simscale.com/forum/u/jousefm)\ **Post date:** [September 1, 2018, 9:40am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/24 "2018-09-01T09:40:38Z")
Hi @dylan and good to see you back! The SA 1 equation turbulence model is adapted for flows around an airfoil and as you know boussinesq approximations to not behave super accurate (whatever you define as accurate in this case), but a contracting section is a good example as the eddies loose their identity along the way. And standard k-epsilon is by definition not realizable (need a separate post to explain what that means) so one usually uses realizable k-epsilon but fully developed pipe flow for instance work pretty well with standard k-epsilon. k-omega SST does a pretty good job and there is no need for SA model, what do you think? Best, Jousef ---
**Author:** ![DaleKramer](https://www.simscale.com/forum/user_avatar/www.simscale.com/dalekramer/32/20201_2.png) [@DaleKramer](https://www.simscale.com/forum/u/DaleKramer)\ **Post date:** [September 1, 2018, 9:43am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/25 "2018-09-01T09:43:05Z")
Wow, Sydney, Florida and Germany all at the same time 😃😃😃 ---
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 1, 2018, 9:47am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/26 "2018-09-01T09:47:15Z")
From [Spalart and Rumsey](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070035069.pdf), you have ![image](https://www.simscale.com/forum/uploads/default/original/3X/7/9/791653c954379cf93156c0535750ea62bbc1105c.png) Calculate k and omega using a length scale of your choice. From this link [Turbulence free-stream boundary conditions -- CFD-Wiki, the free CFD reference](https://www.cfd-online.com/Wiki/Turbulence_free-stream_boundary_conditions), you have ![image](https://www.simscale.com/forum/uploads/default/original/3X/5/5/558533c24ff6f34f439d0fa6cfbfa0de7e39d9b8.png) where, ![image](https://www.simscale.com/forum/uploads/default/original/3X/f/a/fa50e3e00f1eeba8aceadfa8dacc855085c17ff3.png) is the turbulent viscosity ratio; ![image](https://www.simscale.com/forum/uploads/default/original/3X/6/5/65be00b456218aaa9613bd07c2eb2052da4b022d.png) is air density; ![image](https://www.simscale.com/forum/uploads/default/original/3X/8/8/88e54e6742b3bdc3052834573cea301fc424e00b.png) is dynamic molecular viscosity of air (not kinematic). ---
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 1, 2018, 9:52am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/27 "2018-09-01T09:52:18Z")
@jousefm I exclusively use SpalartAllmaras, RealizableKE, and SST as RANS. In fact, I find SpalartAllmaras very useful for external aerodynamics with mild separation, such as a plane or a wing. I think @DaleKramer’s problem can be very well solved by SA, because I have done similar projects. ---
**Author:** ![DaleKramer](https://www.simscale.com/forum/user_avatar/www.simscale.com/dalekramer/32/20201_2.png) [@DaleKramer](https://www.simscale.com/forum/u/DaleKramer)\ **Post date:** [September 1, 2018, 9:52am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/28 "2018-09-01T09:52:31Z")
Aha, I finally see it, I can just back it out from known values. And this means I really need to go to bed now … Thanks. Dale ---
**Author:** ![jousefm](https://www.simscale.com/forum/user_avatar/www.simscale.com/jousefm/32/34908_2.png) [@jousefm](https://www.simscale.com/forum/u/jousefm)\ **Post date:** [September 1, 2018, 9:59am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/29 "2018-09-01T09:59:18Z")
Hi @dylan, I totally agree here but as SA is not implemented on our platform we have to work with another turbulence model. Best, Jousef ---
**Author:** ![DaleKramer](https://www.simscale.com/forum/user_avatar/www.simscale.com/dalekramer/32/20201_2.png) [@DaleKramer](https://www.simscale.com/forum/u/DaleKramer)\ **Post date:** [September 1, 2018, 3:32pm UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/30 "2018-09-01T15:32:11Z")
@dylan Just curious as to why you suggest calculating ut/u from the [cfd\_online.com](https://www.cfd-online.com/Wiki/Turbulence_free-stream_boundary_conditions) formula rather than from the [Spalart and Rumsey](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070035069.pdf) recommendation of 2e-7 \* Re ? There is a 20% difference in my case. And by the way, I did have to change my reference length from a fuselage length of 5m to a value of 2m to stay in your suggested ut/u range of 0.1~2. My ut/u value is now 1.89 (EDIT actual value is now 1.6182, see edit 2 posts down). Dale ---
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 2, 2018, 12:55am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/31 "2018-09-02T00:55:37Z")
Did you use the same length scale in Reynolds number? ---
**Author:** ![DaleKramer](https://www.simscale.com/forum/user_avatar/www.simscale.com/dalekramer/32/20201_2.png) [@DaleKramer](https://www.simscale.com/forum/u/DaleKramer)\ **Post date:** [September 2, 2018, 12:57am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/32 "2018-09-02T00:57:53Z")
Yes ![k-omegaCORRECTED-forumpost](https://www.simscale.com/forum/uploads/default/original/3X/a/1/a18baf989e02bc781f7351180622ebe30f80649c.jpeg) EDIT: corrected the value for u (dynamic viscosity) which is equal to SimScale Kinematic Viscosity \* Density This makes ut/u smaller than the original post… ---
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 3, 2018, 7:41am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/33 "2018-09-03T07:41:05Z")
The 2e-7\*Re is just an estimation. The actual nut calculated at inlet should return a nut/nu = 1.888 or much closer to 1.888. I don’t think a length scale of 2 or 3 will lead to much difference in Cd or Cl. It is safe to use what you have here. ---
**Author:** ![dylan](https://www.simscale.com/forum/user_avatar/www.simscale.com/dylan/32/2798_2.png) [@dylan](https://www.simscale.com/forum/u/dylan)\ **Post date:** [September 4, 2018, 9:49am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/34 "2018-09-04T09:49:15Z")
> [@DaleKramer](#): > > Turbulent viscosity ratio = nut/nu > > Eddy viscosity ratio = mut/mu > > and mut/mu \<\> nut/nu Turbulent viscosity and eddy viscosity are the same thing. However, nut = mut/rho, where nut is commonly used in incompressible flow, and mut in compressible flow. ---
**Author:** ![bdewar](https://www.simscale.com/forum/letter_avatar/bdewar/32/6_22734026db12803ebb3e059622ac3534.png) [@bdewar](https://www.simscale.com/forum/u/bdewar)\ **Post date:** [June 3, 2019, 4:44am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/35 "2019-06-03T04:44:48Z")
Hey guys. Could you help with this? I think it might be related. > **[Flow rate and phi not matching?? -- CFD Online Discussion Forums](https://www.cfd-online.com/Forums/openfoam-post-processing/217884-flow-rate-phi-not-matching.html#post735070)** > > Hi guys, I've got an interesting situation. I've got 2 inlet patches, each set to: Inlet1 { type flowRateInletVelocity; volumetricFlowRate constant 65; ---
**Author:** ![jousefm](https://www.simscale.com/forum/user_avatar/www.simscale.com/jousefm/32/34908_2.png) [@jousefm](https://www.simscale.com/forum/u/jousefm)\ **Post date:** [June 6, 2019, 7:06am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/36 "2019-06-06T07:06:18Z")
Hey @bdewar! Is this related to a problem you have with a project on our platform? Best, Jousef ---
**Author:** ![Fozan](https://www.simscale.com/forum/user_avatar/www.simscale.com/fozan/32/27368_2.png) [@Fozan](https://www.simscale.com/forum/u/Fozan)\ **Post date:** [September 15, 2020, 6:08pm UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/37 "2020-09-15T18:08:05Z")
Hi @DaleKramer, I hope you are doing well. I would like to know, is it okay if my ut/u ratio is equal to 0.1005 ? or Should I adjust any further ? My case is more or less similar, I am running a simulation of flying wing aircraft. Kind regards Mohammad Fozan ---
**Author:** ![Fozan](https://www.simscale.com/forum/user_avatar/www.simscale.com/fozan/32/27368_2.png) [@Fozan](https://www.simscale.com/forum/u/Fozan)\ **Post date:** [September 15, 2020, 6:14pm UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/38 "2020-09-15T18:14:21Z")
@dylan what’s’ your take on this ? ---
**Author:** ![vinulfernando](https://www.simscale.com/forum/letter_avatar/vinulfernando/32/6_22734026db12803ebb3e059622ac3534.png) [@vinulfernando](https://www.simscale.com/forum/u/vinulfernando)\ **Post date:** [October 15, 2020, 6:54am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/39 "2020-10-15T06:54:59Z")
here what’s C? Is it the speed of the sound? I need to find turbulent viscosity ratio for my SA model. Here my velocity is set 30 m/s. please help me! ---
**Author:** ![vinulfernando](https://www.simscale.com/forum/letter_avatar/vinulfernando/32/6_22734026db12803ebb3e059622ac3534.png) [@vinulfernando](https://www.simscale.com/forum/u/vinulfernando)\ **Post date:** [October 15, 2020, 6:56am UTC](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950/40 "2020-10-15T06:56:17Z")
what did u consider as C in his equations? [Previous page](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950.md?page=1) [Next page](https://www.simscale.com/forum/t/initial-values-for-turbulence-coefficients/25950.md?page=3)