Log-law velocity profile in CFX-Pre

jm2011 · February 29, 2012, 02:57

Hi guys,

I would like to model wind flow over a terrain using a log-law velocity profile for my inlet. I have modelled a rectangular prism and assigned one of the faces to be my inlet. Unfortunately I'm not very good with the syntax used in expressions, so I am continuously facing error messages, which I'm not sure how to fix.

The main error is "Argument dimensions resolve to 'm'; expected '<dimensionless>" when I using the "Uz" expression for the inlet velocity:

ufric = 0.35
hz = z
kappa = 0.4
z0 = 0.0001
Uz = (ufric/kappa)*loge(hz/z0) [m s^-1]

Obviously hz is in meters and will change with the height of the inlet face, but CFX-Pre won't accept a non-dimensional value in the loge function...?

Also is it necessary to specify the min and max z values of the inlet face?

I appreciate your help!

Lance · February 29, 2012, 03:09

Yep, loge (or ln) wont accept an argument with dimensions, see cfx_ref.pdf at page 168 (for version 14). Just normalize it with /1[m] or make z0 = 0.0001 [m] and you should be fine. If you want your inlet expression to go all the way to the top (assuming z= 0 at the bottom) then you dont need to set any min/max values.

jm2011 · February 29, 2012, 03:54

Cool, thanks for that!

I tried doing that actually, but when I subsequently ran the simulation, it terminates with the following error:

"The problem was encountered in executing the expression for: |
| Uz |
| The complete expression is: |
| (Ufric/kappa)*loge(hz/z0[m]) [m s^-1] |
| The error occurs on sub-expression: |
| loge(hz/z0[m])"

...so it must still not like this expression somehow...

Lance · February 29, 2012, 04:00

Yes, when z = 0 it will result in loge(0) which is undefined, thats why you get an error. One thing you could try is loge((hz+eps)/z0[m]) where eps is a very small number. But be aware that it might give you the wrong solution close to the wall.

jm2011 · February 29, 2012, 04:16

Awesome, thanks Lance!

I understand that there is likely to be an error, but I assume it will be pretty small if you choose the added value to be very small.
In fact, I used the same value as for z0 because then at z=0, log(1)=0, which would correspond to reality where wind velocity is zero at the ground.
Perhaps this is still an acceptable method for a large domain.

February 29, 2012, 02:57	Log-law velocity profile in CFX-Pre	#1
jm2011 New Member Join Date: Dec 2011 Posts: 11 Rep Power: 14	Hi guys, I would like to model wind flow over a terrain using a log-law velocity profile for my inlet. I have modelled a rectangular prism and assigned one of the faces to be my inlet. Unfortunately I'm not very good with the syntax used in expressions, so I am continuously facing error messages, which I'm not sure how to fix. The main error is "Argument dimensions resolve to 'm'; expected '<dimensionless>" when I using the "Uz" expression for the inlet velocity: ufric = 0.35 hz = z kappa = 0.4 z0 = 0.0001 Uz = (ufric/kappa)*loge(hz/z0) [m s^-1] Obviously hz is in meters and will change with the height of the inlet face, but CFX-Pre won't accept a non-dimensional value in the loge function...? Also is it necessary to specify the min and max z values of the inlet face? I appreciate your help!

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Export & Import Velocity profile as Inlet	eRzBeNgEl	STAR-CCM+	6	March 26, 2012 05:16
Simulation with UDF for species mass fraction and velocity profile	virgy	Fluent UDF and Scheme Programming	8	February 7, 2012 04:30
Log Law Wind Profile for Urban analysis in Star-CCM+ 4	plainstyle	Siemens	1	June 18, 2010 10:46
Wind Velocity Profile in ANSYS CFX	diluom	CFX	7	December 10, 2009 22:50
log - velocity profile in unstructured grid	michael Rasmussen	CFX	7	December 15, 2003 19:59

February 29, 2012, 03:09		#2
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	Yep, loge (or ln) wont accept an argument with dimensions, see cfx_ref.pdf at page 168 (for version 14). Just normalize it with /1[m] or make z0 = 0.0001 [m] and you should be fine. If you want your inlet expression to go all the way to the top (assuming z= 0 at the bottom) then you dont need to set any min/max values.

February 29, 2012, 03:54		#3
jm2011 New Member Join Date: Dec 2011 Posts: 11 Rep Power: 14	Cool, thanks for that! I tried doing that actually, but when I subsequently ran the simulation, it terminates with the following error: "The problem was encountered in executing the expression for: \| \| Uz \| \| The complete expression is: \| \| (Ufric/kappa)*loge(hz/z0[m]) [m s^-1] \| \| The error occurs on sub-expression: \| \| loge(hz/z0[m])" ...so it must still not like this expression somehow...

February 29, 2012, 04:00		#4
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	Yes, when z = 0 it will result in loge(0) which is undefined, thats why you get an error. One thing you could try is loge((hz+eps)/z0[m]) where eps is a very small number. But be aware that it might give you the wrong solution close to the wall.

February 29, 2012, 04:16		#5
jm2011 New Member Join Date: Dec 2011 Posts: 11 Rep Power: 14	Awesome, thanks Lance! I understand that there is likely to be an error, but I assume it will be pretty small if you choose the added value to be very small. In fact, I used the same value as for z0 because then at z=0, log(1)=0, which would correspond to reality where wind velocity is zero at the ground. Perhaps this is still an acceptable method for a large domain.