law of the wall, horizontal velocity near the wall with mixing length model

In fluid dynamics, the law of the wall (also known as the logarithmic law of the wall) states that the average velocity of a turbulent flow at a certain point is proportional to the logarithm of the distance from that point to the "wall", or the boundary of the fluid region. This law of the wall was first published in 1930 by Hungarian-American mathematician, aerospace engineer, and physicist Theodore von Kármán.^[1] It is only technically applicable to parts of the flow that are close to the wall (<20% of the height of the flow), though it is a good approximation for the entire velocity profile of natural streams.^[2]

General logarithmic formulation

The logarithmic law of the wall is a self similar solution for the mean velocity parallel to the wall, and is valid for flows at high Reynolds numbers — in an overlap region with approximately constant shear stress and far enough from the wall for (direct) viscous effects to be negligible:^[3]

${\displaystyle u^{+}={\frac {1}{\kappa }}\ln \,y^{+}+C^{+},}$   with   ${\displaystyle y^{+}={\frac {y\,u_{\tau }}{\nu }},}$   ${\displaystyle u_{\tau }={\sqrt {\frac {\tau _{w}}{\rho }}}}$   and   ${\displaystyle u^{+}={\frac {u}{u_{\tau }}}}$

where

${\displaystyle y^{+}}$	is the wall coordinate: the distance y to the wall, made dimensionless with the friction velocity uτ and kinematic viscosity ν,
${\displaystyle u^{+}}$	is the dimensionless velocity: the velocity u parallel to the wall as a function of y (distance from the wall), divided by the friction velocity uτ,
${\displaystyle \tau _{w}}$	is the wall shear stress,
${\displaystyle \rho }$	is the fluid density,
${\displaystyle u_{\tau }}$	is called the friction velocity or shear velocity,
${\displaystyle \kappa }$	is the Von Kármán constant,
${\displaystyle C^{+}}$	is a constant, and
${\displaystyle \ln }$	is the natural logarithm.

From experiments, the von Kármán constant is found to be ${\displaystyle \kappa \approx 0.41}$ and ${\displaystyle C^{+}\approx 5.0}$ for a smooth wall.^[3]

With dimensions, the logarithmic law of the wall can be written as:^[4]

${\displaystyle {u}={\frac {u_{\tau }}{\kappa }}\ln \,{\frac {y}{y_{0}}}\ }$

where y₀ is the distance from the boundary at which the idealized velocity given by the law of the wall goes to zero. This is necessarily nonzero because the turbulent velocity profile defined by the law of the wall does not apply to the laminar sublayer. The distance from the wall at which it reaches zero is determined by comparing the thickness of the laminar sublayer with the roughness of the surface over which it is flowing. For a near-wall laminar sublayer of thickness ${\displaystyle \delta _{\nu }}$ and a characteristic roughness length-scale ${\displaystyle k_{s}}$,^[2]

${\displaystyle k_{s}<\delta _{\nu }\,}$	: hydraulically smooth flow,
${\displaystyle k_{s}\approx \delta _{\nu }\,}$	: transitional flow,
${\displaystyle k_{s}>\delta _{\nu }\,}$	: hydraulically rough flow.

Intuitively, this means that if the roughness elements are hidden within the laminar sublayer, they have a much different effect on the turbulent law of the wall velocity profile than if they are sticking out into the main part of the flow.

This is also often more formally formulated in terms of a boundary Reynolds number, ${\displaystyle Re_{w}}$, where

${\displaystyle Re_{w}={\frac {u_{\tau }k_{s}}{\nu }}.\ }$

The flow is hydraulically smooth for ${\displaystyle Re_{w}<3}$, hydraulically rough for ${\displaystyle Re_{w}>100}$, and transitional for intermediate values.^[2]

Values for ${\displaystyle y_{0}}$ are given by:^[2][5]

${\displaystyle y_{0}={\frac {\nu }{9u_{\tau }}}}$	for hydraulically smooth flow
${\displaystyle y_{0}={\frac {k_{s}}{30}}}$	for hydraulically rough flow.

Intermediate values are generally given by the empirically derived Nikuradse diagram,^[2] though analytical methods for solving for this range have also been proposed.^[6]

For channels with a granular boundary, such as natural river systems,

${\displaystyle k_{s}\approx 3.5D_{84},\ }$

where ${\displaystyle D_{84}}$ is the average diameter of the 84th largest percentile of the grains of the bed material.^[7]

Power law solutions

Works by Barenblatt and others have shown that besides the logarithmic law of the wall — the limit for infinite Reynolds numbers — there exist power-law solutions, which are dependent on the Reynolds number.^[8][9] In 1996, Cipra submitted experimental evidence in support of these power-law descriptions.^[10] This evidence itself has not been fully accepted by other experts.^[11] In 2001, Oberlack claimed to have derived both the logarithmic law of the wall, as well as power laws, directly from the Reynolds-averaged Navier–Stokes equations, exploiting the symmetries in a Lie group approach.^[3][12] However, in 2014, Frewer et al.^[13] refuted these results.

For scalars

For scalars (most notably temperature), the self-similar logarithmic law of the wall has been theorized (first formulated by B. A. Kader^[14]) and observed in experimental and computational studies.^{[15][16][17][18]} In many cases, extensions to the original law of the wall formulation (usually through integral transformations) are generally needed to account for compressibility, variable-property and real fluid effects.

Near the wall

Below the region where the law of the wall is applicable, there are other estimations for friction velocity.^[19]

Notes

References

• Chanson, H. (2009), Applied Hydrodynamics: An Introduction to Ideal and Real Fluid Flows, CRC Press, Taylor & Francis Group, Leiden, The Netherlands, 478 pages, ISBN 978-0-415-49271-3
• Schlichting, Hermann; Gersten, K. (2000), Boundary-layer Theory (8th revised ed.), Springer, ISBN 3-540-66270-7

Further reading

• Buschmann, Matthias H.; Gad-el-Hak, Mohamed (2009), "Evidence of nonlogarithmic behavior of turbulent channel and pipe flow", AIAA Journal, 47 (3): 535, Bibcode:2009AIAAJ..47..535B, doi:10.2514/1.37032

External links

• Definition from ScienceWorld
• Formula on CFD Online
• Y+ estimator