Both roughness length $z_0$ and zero plane displacement $d$ seemed to be defined as the height above the ground at which wind speed theoretically becomes zero. But wind speed is also supposed to go to zero at $d+z_0$. What is the difference between the two, and how should they actually be defined?
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1$\begingroup$ It would be useful if you included a reference for the equivalent definition of $z_0$ and $d$. $\endgroup$– milancurcicCommented Apr 8, 2015 at 17:57
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2 Answers
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First some definitions:
$z_0$: Roughness length is defined as the height at which the mean velocity is zero due to substrate roughness. Real walls/ground are not smooth and often have varying degrees of roughness, this parameter (which is determined empirically) accounts for that effect.
$d$: Zero Plane displacement is defined as the height at which the mean velocity is zero due to large obstacles such as buildings/canopy.
The two parameters are not the same because they describe the effects of two fundamentally different processes. $d$ can be anywhere from $6$ to $20$ times larger than $z_0$.
The basis for most turbulence modeling is the eddy viscosity model:
$$-\overline{u'w'} = \nu_t \frac{\partial U}{\partial z}$$
where $\nu_t$ is the eddy viscosity. Employing some scaling arguments with basis on the Prandtl mixing length model and integrating one arrives at the logarithmic law of the wall:
$${U} = \frac{u^*}{\kappa} \ln\, \frac{z}{z_0}$$
Your equation is
$${U} = \frac{u^*}{\kappa} \ln\, \frac{z-d}{z_0}$$ which is the law of the wall with $d = 0$ because it applies to flat plates. It is easy to see then, that by subtracting $d$ from $z$ the effect is to reduce $U$ at that height, which makes sense because large obstacles remove energy from the mean flow and slow it down.
Note that if there are no large obstacles then $d \approx 0$, but $z_0$ is still larger than zero.
answered Apr 8, 2015 at 18:30
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$\begingroup$ Did you intend one of your equations to include d? $\endgroup$ Commented Apr 8, 2015 at 22:51
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Let me add to the previous answer: In mixing length theory the mixing length is often defined as the friction velocity ${u_*}$ divided by the vertical shear $\frac{dU}{dz}$. Both of these quantities can be measured more or less directly at different heights. In the absence of buildings/canopy the mixing length is to a first approximation proportional to ${z}$.
The displacement height ${d}$ is given by the height at which the mixing length approaches zero. The roughness length ${z_0}$ is, however, still needed to match the wind speed profile.
