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

A roughness-dependent model

From CFD-Wiki

Jump to: navigation, search

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