CFD Online URL
[Sponsors]
Home > Wiki > A roughness-dependent model

A roughness-dependent model

From CFD-Wiki

(Difference between revisions)
Jump to: navigation, search
(Algebraic model for the turbulent kinetic energy)
(The mean velocity profile)
 
(3 intermediate revisions not shown)
Line 6: Line 6:
where:  
where:  
<math> C_{\mu} = 0.09 </math>
<math> C_{\mu} = 0.09 </math>
 +
 +
[http://www.cfd-online.com/Wiki/Standard_k-epsilon_model <math>k</math>-<math>\epsilon</math> model]
==One-equation eddy viscosity model==
==One-equation eddy viscosity model==
Line 12: Line 14:
\nu _t  = k^{{1 \over 2}}  l  
\nu _t  = k^{{1 \over 2}}  l  
</math></td><td width="5%">(2)</td></tr></table>
</math></td><td width="5%">(2)</td></tr></table>
 +
 +
[http://www.cfd-online.com/Wiki/Prandtl%27s_one-equation_model One-equation model]
==Algebraic eddy viscosity model==
==Algebraic eddy viscosity model==
Line 54: Line 58:
==The mean velocity profile==
==The mean velocity profile==
-
In local equilibrium region, we are able to find the mean velocity <math>u</math> profile from the mixing length <math>l_m</math> and the turbulent kinetic energy <math>k</math> by:  
+
For local equilibrium, we are able to find the mean velocity profile <math>u</math> from the turbulent kinetic energy <math>k</math> (equation 4) and the mixing length <math>l_m</math> (equation 5), by:  
-
 
+
<table width="70%"><tr><td>
<table width="70%"><tr><td>
<math>  
<math>  
{{d u} \over {d y}}  = C_{\mu}^{1 \over 4} {{k^{1 \over 2}} \over {l_m}}  
{{d u} \over {d y}}  = C_{\mu}^{1 \over 4} {{k^{1 \over 2}} \over {l_m}}  
</math></td><td width="5%">(8)</td></tr></table>  
</math></td><td width="5%">(8)</td></tr></table>  
-
With equations (4) and (5), we obtain [[#References|[Absi (2006)]]]:
+
 
 +
Figure (1) shows that the velocity profile obtained from equations (8), (4) and (5) (solid line) is more accurate than the logarithmic velocity profile (dash-dotted line).
[[Image:fig7a.jpg]]
[[Image:fig7a.jpg]]
[[Image:fig7b.jpg]]
[[Image:fig7b.jpg]]
-
Fig. Vertical distribution of mean flow velocity.  
+
'''Figure 1''', Vertical distribution of mean flow velocity.  
<math>A = {{h} \over {c_1}}</math>; <math>c_1 = 1</math>;  
<math>A = {{h} \over {c_1}}</math>; <math>c_1 = 1</math>;  
-
Dash-dotted line: logarithmic profile; solid line: obtained from equation (8); symbols: experimental data (Sukhodolov et al). a) profile 2: <math>y_0 = 0.062 cm</math>; <math>h = 145 cm</math>; <math>u_\tau = 3.82 cm/s</math>. b) profile 4: <math>y_0 = 0.113 cm</math>; <math>h = 164.5 cm</math>; <math>u_\tau = 3.97 cm/s</math> ; values of <math>y_0 , h, u_\tau</math>  are from  [[#References|[Sukhodolov et al. (1998)]]].
+
Dash-dotted line: logarithmic profile; solid line: obtained from equation (8); symbols: experimental data ([[#References|[Sukhodolov ''et al.'' (1998)]]]). a) profile 2: <math>y_0 = 0.062 cm</math>; <math>h = 145 cm</math>; <math>u_\tau = 3.82 cm/s</math>. b) profile 4: <math>y_0 = 0.113 cm</math>; <math>h = 164.5 cm</math>; <math>u_\tau = 3.97 cm/s</math> ; (values of <math>y_0 , h, u_\tau</math>  are from  [[#References|[Sukhodolov ''et al.'' (1998)]]]); Figure from [[#References|[Absi (2006)]]].
== References ==
== References ==
-
* {{reference-paper|author=Absi, R. |year=2006|title=[http://www.jstage.jst.go.jp/article/jscejb/62/4/62_437/_article A roughness and time dependent mixing length equation]|rest=''Journal of Hydraulic, Coastal and Environmental Engineering, (Doboku Gakkai Ronbunshuu B),'' Japan Society of Civil Engineers, Vol. 62, No. 4, pp.437-446}}
+
* {{reference-paper|author=Absi, R. |year=2006|title=[http://www.jstage.jst.go.jp/article/jscejb/62/4/62_437/_article A roughness and time dependent mixing length equation]|rest=''Journal of Hydraulic, Coastal and Environmental Engineering, (Doboku Gakkai Ronbunshuu B),'' Japan Society of Civil Engineers, Vol. '''62''', No. 4, pp.437-446}}
* {{reference-paper|author=Nezu, I. and Nakagawa, H. |year=1993|title=Turbulence in open-channel flows|rest=A.A. Balkema, Ed. Rotterdam, The Netherlands}}
* {{reference-paper|author=Nezu, I. and Nakagawa, H. |year=1993|title=Turbulence in open-channel flows|rest=A.A. Balkema, Ed. Rotterdam, The Netherlands}}
-
* {{reference-paper|author=Sukhodolov A., Thiele M. and Bungartz H. |year=1998|title=Turbulence structure in a river reach with sand bed|rest=''Water Resour. Res.'', 34, pp. 1317-1334}}
+
* {{reference-paper|author=Sukhodolov, A., Thiele, M. and Bungartz, H. |year=1998|title=Turbulence structure in a river reach with sand bed|rest=''Water Resour. Res.'', Vol. '''34''', pp. 1317-1334}}

Latest revision as of 12:47, 22 June 2007

Contents

Two-equation k-\epsilon eddy viscosity model

 
\nu _t  = C_{\mu} {{k^2 } \over \epsilon }
(1)

where:  C_{\mu} = 0.09

k-\epsilon model

One-equation eddy viscosity model

 
\nu _t  = k^{{1 \over 2}}  l 
(2)

One-equation model

Algebraic eddy viscosity model

 
\nu _t(y)  = {C_{\mu}}^{{1 \over 4}} l_m(y) k^{{1 \over 2}}(y) 
(3)

l_m is the mixing length.

Algebraic model for the turbulent kinetic energy


k^{{1 \over 2}}(y) = {1 \over {C_{\mu}}^{{1 \over 4}}}  u_\tau  e^{\frac{-y}{A}} 
(4)

u_\tau is the shear velocity and A a model parameter.

For steady open channel flows in local equilibrium, where the energy production is balanced by the dissipation, from the modeled k-equation [Nezu and Nakagawa (1993)] obtained a similar semi-theoretical equation.

Algebraic model for the mixing length

For local equilibrium, an extension of von Kármán’s similarity hypothesis allows to write, with equation (4) [Absi (2006)]:


l_m(y) = \kappa \left( A - \left(A - y_0\right) e^{\frac{-(y-y_0)}{A}} \right)
(5)

\kappa = 0.4, y_0 is the hydrodynamic roughness. For a smooth wall (y_0 = 0):

 
l_m(y) = \kappa  A  \left( 1 - e^{\frac{-y}{A}} \right)  
(6)

the algebraic eddy viscosity model is therefore

 
\nu _t(y)  = \kappa \left( A - \left(A - y_0\right) e^{\frac{-(y-y_0)}{A}} \right)
 u_\tau  e^{\frac{-y}{A}}  
(7)


The mean velocity profile

For local equilibrium, we are able to find the mean velocity profile u from the turbulent kinetic energy k (equation 4) and the mixing length l_m (equation 5), by:

 
{{d u} \over {d y}}  = C_{\mu}^{1 \over 4} {{k^{1 \over 2}} \over {l_m}} 
(8)

Figure (1) shows that the velocity profile obtained from equations (8), (4) and (5) (solid line) is more accurate than the logarithmic velocity profile (dash-dotted line).

Fig7a.jpg Fig7b.jpg

Figure 1, Vertical distribution of mean flow velocity. A = {{h} \over {c_1}}; c_1 = 1; Dash-dotted line: logarithmic profile; solid line: obtained from equation (8); symbols: experimental data ([Sukhodolov et al. (1998)]). a) profile 2: y_0 = 0.062 cm; h = 145 cm; u_\tau = 3.82 cm/s. b) profile 4: y_0 = 0.113 cm; h = 164.5 cm; u_\tau = 3.97 cm/s ; (values of y_0 , h, u_\tau are from [Sukhodolov et al. (1998)]); Figure from [Absi (2006)].

References

  • Nezu, I. and Nakagawa, H. (1993), "Turbulence in open-channel flows", A.A. Balkema, Ed. Rotterdam, The Netherlands.
  • Sukhodolov, A., Thiele, M. and Bungartz, H. (1998), "Turbulence structure in a river reach with sand bed", Water Resour. Res., Vol. 34, pp. 1317-1334.


My wiki