Reynolds Stresses in Fluent

AeroCat · June 5, 2010, 11:09

Hello,

I am working the Re=3900 2-D cylinder problem for a turbulence class in graduate school. We were to use the k-e and k-w turbulence models and plot uu Reynolds stress profiles to compare to Kravchenko's paper.

How do I back out the Reynolds stress components I need from the turbulent quantities given by these two models? I imagine I just use the Bouss Approx, and write a UDF to plot them, but I've never used the UDF's.

Can someone give me some idiot-proof step by step instructions on how to do this?

Thanks!

quattro123 · January 10, 2011, 14:31

i have the same question right now

andre9 · February 1, 2012, 09:50

I'm facing the same problem right now.

dynamics · February 1, 2012, 12:57

You have to use Bous. Approx.
Use the Define->Custom Field Functions to calculate

delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz

then calculate strain rate tensor S_uu
S_uu=dx-velocity-dx - 1 / 3 * dukxk

Then use
-u'u'=2*viscosity-turb*S_uu/density - 2/3*turb-kinetic-energy

Far · September 15, 2013, 07:29

Quote:

Originally Posted by dynamics

You have to use Bous. Approx.
Use the Define->Custom Field Functions to calculate

delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz

then calculate strain rate tensor S_uu
S_uu=dx-velocity-dx - 1 / 3 * dukxk

Then use
-u'u'=2*viscosity-turb*S_uu/density - 2/3*turb-kinetic-energy

What is dukxk ?

maphd · April 14, 2014, 16:26

Quote:

Originally Posted by dynamics

You have to use Bous. Approx.
Use the Define->Custom Field Functions to calculate

delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz

then calculate strain rate tensor S_uu
S_uu=dx-velocity-dx - 1 / 3 * dukxk

Then use
-u'u'=2*viscosity-turb*S_uu/density - 2/3*turb-kinetic-energy

Hi Dynamix,
I need to define Reynolds stresses in fluent with k-e model. I also could not understand the term dukxk and you define in the beginning something called delu but you do not use it anywhere. could you explain what is that one also?
Thank you very much, I really appreciate it.

MiloslavDohnal · December 1, 2015, 10:16

Hello everyone

Yesterday, I found this thread and spent almost whole day by searching for answer with some, let's say, proof or explanation. I found that the problem raises from the transport equation of k in two equation models; and it's one of the problems in turbulence modeling. In general, we have more unknowns to solve than available equations; and Boussinesq aproximation is used to enclose transport equation of k in one- or two- equation models.

And now, back to problem:
let's have the Boussinesq aproximation

$-\rho\overline{u^{'}_{i}u^{'}_{j}}=\mu_t\left(\frac{\partial\overline{u_i}}{\partial{x_j}}+\frac{\partial\overline{u_j}}{\partial{x_i}}-\frac{2}{3}\frac{\partial\overline{u_k}}{\partial{x_k}}\delta_{ij}\right)-\frac{2}{3}\rho k\delta_{ij}$

where

$\delta_{ij}= \begin{cases} 1 & \text{if } i=j \\ 0 & \text{if } i\neq j \end{cases}$ is Kronecker delta

and also have those two vectors:
position vector

$\vec{x}=(x;y;z)$

and velocity vector

$\vec{u}=(u;v;w)$

if the flow is incompressible, then

$-\frac{2}{3}\frac{\partial\overline{u_k}}{\partial{x_k}}\delta_{ij}=0$

Now, it only depands what kind of Reynolds stress you want. For instance, if you want to know normal stress $\overline{u^{'}u^{'}}$ simply put

and

; then, substitute first element in $\vec{u}$ and $\vec{x}$ to the Boussinesq aproximation and do the math. You will obtain:

$-\overline{u^{'}u^{'}}=\frac{\mu_t}{\rho}\left(\frac{\partial\overline{u}}{\partial{x}}+\frac{\partial\overline{u}}{\partial{x}}\right)-\frac{2}{3}k= \nu_t\left(\frac{\partial\overline{u}}{\partial{x}}+\frac{\partial\overline{u}}{\partial{x}}\right)-\frac{2}{3}k$

And if you want to know shear stress $\overline{u^{'}v^{'}}$ , just put

and

and you will get:

$-\overline{u^{'}v^{'}}=\frac{\mu_t}{\rho}\left(\frac{\partial\overline{u}}{\partial{y}}+\frac{\partial\overline{v}}{\partial{x}}\right)=\nu_t\left(\frac{\partial\overline{u}}{\partial{y}}+\frac{\partial\overline{v}}{\partial{x}}\right)$

Apply same principle for any other Reynolds stress you want to know and just simply put this to Fluent using Custom Field Functions

Reference
Wilcox, D. C., 2006, Turbulence Modeling for CFD, 3rd ed., Dcw Industries, Incorporated.

June 5, 2010, 11:09	Reynolds Stresses in Fluent	#1
AeroCat New Member Join Date: Jun 2010 Posts: 9 Rep Power: 15	Hello, I am working the Re=3900 2-D cylinder problem for a turbulence class in graduate school. We were to use the k-e and k-w turbulence models and plot uu Reynolds stress profiles to compare to Kravchenko's paper. How do I back out the Reynolds stress components I need from the turbulent quantities given by these two models? I imagine I just use the Bouss Approx, and write a UDF to plot them, but I've never used the UDF's. Can someone give me some idiot-proof step by step instructions on how to do this? Thanks!

February 1, 2012, 09:50	Same doubt	#3
andre9 New Member Andre Novgorodcev Join Date: Jan 2012 Posts: 3 Rep Power: 14	I'm facing the same problem right now.

February 1, 2012, 12:57		#4
dynamics New Member Join Date: Mar 2011 Posts: 7 Rep Power: 15	You have to use Bous. Approx. Use the Define->Custom Field Functions to calculate delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz then calculate strain rate tensor S_uu S_uu=dx-velocity-dx - 1 / 3 * dukxk Then use -u'u'=2viscosity-turbS_uu/density - 2/3*turb-kinetic-energy maphd likes this.

December 1, 2015, 10:16	Boussinesq aproximation solves the problem	#7
MiloslavDohnal New Member Miloslav Dohnal Join Date: Jan 2014 Location: Brno, The Czech Republic Posts: 7 Rep Power: 12	Hello everyone Yesterday, I found this thread and spent almost whole day by searching for answer with some, let's say, proof or explanation. I found that the problem raises from the transport equation of k in two equation models; and it's one of the problems in turbulence modeling. In general, we have more unknowns to solve than available equations; and Boussinesq aproximation is used to enclose transport equation of k in one- or two- equation models. And now, back to problem: let's have the Boussinesq aproximation $-\rho\overline{u^{'}_{i}u^{'}_{j}}=\mu_t\left(\frac{\partial\overline{u_i}}{\partial{x_j}}+\frac{\partial\overline{u_j}}{\partial{x_i}}-\frac{2}{3}\frac{\partial\overline{u_k}}{\partial{x_k}}\delta_{ij}\right)-\frac{2}{3}\rho k\delta_{ij}$ where $\delta_{ij}= \begin{cases} 1 & \text{if } i=j \\ 0 & \text{if } i\neq j \end{cases}$ is Kronecker delta and also have those two vectors: position vector $\vec{x}=(x;y;z)$ and velocity vector $\vec{u}=(u;v;w)$ if the flow is incompressible, then $-\frac{2}{3}\frac{\partial\overline{u_k}}{\partial{x_k}}\delta_{ij}=0$ Now, it only depands what kind of Reynolds stress you want. For instance, if you want to know normal stress $\overline{u^{'}u^{'}}$ simply put and ; then, substitute first element in $\vec{u}$ and $\vec{x}$ to the Boussinesq aproximation and do the math. You will obtain: $-\overline{u^{'}u^{'}}=\frac{\mu_t}{\rho}\left(\frac{\partial\overline{u}}{\partial{x}}+\frac{\partial\overline{u}}{\partial{x}}\right)-\frac{2}{3}k= \nu_t\left(\frac{\partial\overline{u}}{\partial{x}}+\frac{\partial\overline{u}}{\partial{x}}\right)-\frac{2}{3}k$ And if you want to know shear stress $\overline{u^{'}v^{'}}$ , just put and and you will get: $-\overline{u^{'}v^{'}}=\frac{\mu_t}{\rho}\left(\frac{\partial\overline{u}}{\partial{y}}+\frac{\partial\overline{v}}{\partial{x}}\right)=\nu_t\left(\frac{\partial\overline{u}}{\partial{y}}+\frac{\partial\overline{v}}{\partial{x}}\right)$ Apply same principle for any other Reynolds stress you want to know and just simply put this to Fluent using Custom Field Functions Reference Wilcox, D. C., 2006, Turbulence Modeling for CFD, 3rd ed., Dcw Industries, Incorporated. masood135, Grigor Nikolov, srv537 and 5 others like this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Two questions on Fluent UDF	Steven	Fluent UDF and Scheme Programming	7	March 23, 2018 03:22
Obtaining Reynolds Number in FLUENT	Emmanuel	FLUENT	2	April 7, 2016 01:46
RMS velocity and reynolds stres with fluent	gina	FLUENT	1	May 20, 2013 04:05
Reynolds stresses without velocity correlations?	Phil	Main CFD Forum	1	October 2, 2006 04:29
Reynolds stresses and second law	JF	Main CFD Forum	6	June 2, 2005 22:57

January 10, 2011, 14:31		#2
quattro123 New Member Smith, John Join Date: Nov 2010 Posts: 8 Rep Power: 15	i have the same question right now