
[Sponsors] 
December 13, 2007, 06:01 
log law for wind velocity profile

#1 
Guest
Posts: n/a

Can someone help please, I'm confused...
I'm trying to create a simple spreadsheet to give wind speeds at a range of heights, from one input wind speed at a reference height. I want to do it using the log law, where u is a function of friction velocity. Friction velocity is fucntion of the wall shear stress. And the wall shear stress is a function of du/dy. I don't understand how to use this du/dy? I have 1 value for flow velocity (u) at height (y). I know du/dy is not simply u/y, but don't understand what else I can do! Would greatly appreciate any help. Thank you. 

December 13, 2007, 07:11 
Re: log law for wind velocity profile

#2 
Guest
Posts: n/a

how many u velocity you have?


December 13, 2007, 10:04 
Re: log law for wind velocity profile

#3 
Guest
Posts: n/a

Since u = 0 at the wall (where y = 0) then to a first approximation du/dy ~ u/y, where these values are taken very close to the wall. Depending on how far you are away from the wall this approximation will either be a good representation of du/dy, or a really bad approximation of du/dy. If you are assuming turbulent flow, and your reference velocity is in what would be the freestream, then you would be better off using a powerlaw profile (see Schlichting or some other basic fluid mechanics reference). Is there a reason why you want to use the loglaw profile?


December 14, 2007, 06:00 
Re: log law for wind velocity profile

#4 
Guest
Posts: n/a

You need something extra: the rougness on the floor/bottom. In the atmosferic boundary layer world it's often called 'z0' and it corresponds to a kind of aerodynamic rougness length (it can be converted to Schlichtings sand rougness: z0=ks*exp(kappa*B)). For a meteo mast on a plain grass field the 'z0' is 3 cm (standard WMO).
Take the loglaw equation: u+ = 1/kappa ln( (z+zd)/z0 ) with u+ = u/u* (u* the friction velocity) zd = displacement thickness, in case of a meteo mast set it to zero (it doesnot matter much in that case; contrary to the influence of kappa + 0.42) your measurement is at z = 10 m, you have a velocity u, assume a value for z0 (3cm) => u* follows => whole profile is known. there is no reason to use a powerlaw profile. 

December 14, 2007, 08:15 
Re: log law for wind velocity profile

#5 
Guest
Posts: n/a

Thanks ag. Ok, good explanation. The reason I want to use the log law is because I have read in a few sources that it is more accurate than the power law, for example from wikipedia, the "alpha component of the power law is an empirically derived value which depends on the stability of the atmosphere...in places where trees or structures impede the nearsurface wind, the use of a constant 1/7 value for alpha may yield quite erronous estimates, and the log wind profile is preferred...even under neutral stability conditions, an exponent of 0.11 is more appropriate"
It is for a feasibility study for a wind turbine project, where I have a figures for the wind speed at 10m, 25m, and 45m AGL, from the BWEA. I would just use these to create a curve, but I'm pretty sure that they are just using the power law to give values for 2 out of the 3. Greatly looking forward to your replies! 

December 14, 2007, 08:20 
Re: log law for wind velocity profile

#6 
Guest
Posts: n/a

Hi Hendrik
ah, but u* = sqrt(wall shear stress / density), where the wall shear stress is a function of du/dy, hence du/dy is my problem? Thank you for the time so far...please see my reply to Ag as the reason for doing this...Cheers 

December 14, 2007, 10:31 
Re: log law for wind velocity profile

#7 
Guest
Posts: n/a

Are you able to use something like this?
http://i78.photobucket.com/albums/j9...ityProfile.jpg You will need to know the roughness height (zo) as was mentioned earlier. I believe the reference to Stull, 2000 is titled meteorology for scientists and engineers http://www.amazon.com/MeteorologySc.../dp/0534372147 

December 14, 2007, 15:16 
Re: log law for wind velocity profile

#8 
Guest
Posts: n/a

Yes and no... you need to start the curve/profile somehow, call it 'z0', du/dy or wall shear stress; they are linked together. A rough(er) wall has a higher shear stress, lower du/dy and a higher 'z0'. In the z0form of the loglaw you use the friction velocity u*, that's all. (ignoring the displacement thickness).
So you have data on 10, 25 and 45 m; a large empty field (grass, water) in front. Start curve fitting and find the best 'z0'. You're done. (if you cannot estimate the surrounding of a mast, then the only option you have is to work out the rms of the fluctuating velocities; it's a measure of the roughness (in front!) of the mast. then you assume that the shear is constant in the lower part of the boundary layer etc... estimating 'z0' is easier) 

January 10, 2016, 23:12 
Numerical modeling of the atmospheric boundary layer with the account of its stratifi

#9 
New Member
adel kuptsoff
Join Date: Feb 2012
Posts: 3
Rep Power: 5 
A computational model for a horizontally uniform atmospheric boundary layer accounting for various options of atmospheric stability is developed on the basis of MoninObukhov’s similarity theory. Numerical modeling of the atmospheric boundary layer is carried out using the Fluent package. For a turbulent closure, the standard k–ε turbulence model with modified turbulence closure constants and an additional source term in the equation for k are applied. Cases of neutral, stable and unstable stratification are considered. The computational model allows preserving horizontal uniformity of vertical profiles of velocity, pressure, density, temperature and atmospheric turbulent viscosity which are set as boundary conditions at the domain’s inlet. The obtained model can be applied to modeling of propagation of hazardous gas clouds in the atmosphere over the terrestrial surfaces including surfaces filled with buildings.
http://www.fundamentalresearch.ru/ru/article/view?id=35083 UDF velocity profile 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Problem with assigned inlet velocity profile as a boundary condition  Ozgur_  FLUENT  5  August 25, 2015 04:58 
UDF error  parabolic velocity profile  3D turbine  Zaqie  Fluent UDF and Scheme Programming  8  May 11, 2014 08:34 
FSI Pipe uniform velocity profile inlet  Absy  Main CFD Forum  0  April 6, 2010 03:01 
LES NearWall Velocity Profile  Alex May  FLUENT  2  June 12, 2008 12:36 
What is 3D turbulent velocity profile ?  conan  FLUENT  0  June 9, 2008 06:44 