Getting to know turbulence - Turbulence in the Atmosphere - page 27

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Getting to know turbulence
2.1 Average and instantaneous properties contrasted
Figure 2.1 is a famous snapshot of a turbulent wake, the region downstream of a
body in a moving fluid. The instantaneously thin, irregular boundary between the
turbulent and nonturbulent flow is continuously deformed by the turbulent eddies,
so that under averaging it becomes a broad, smooth transition region. Figure 2.2
illustrates this same feature at the top of the atmospheric boundary layer.
We have had access to instantaneous turbulence fields through remote sensing
and numerical simulation only since the 1970s. Perhaps that is why our descriptive
terms for turbulence tend to refer to its statistical properties, not its instantaneous
ones. For example,
• Homogeneous turbulence has spatially uniform statistical properties (with the excep-
tion of mean pressure). A turbulent flow can be homogeneous in zero, one, two, or
three directions. A sphere wake is an example of the first. The turbulent boundary
layer near the leading edge of a flat plate, or downstream of a change in surface con-
ditions, can be homogeneous in one direction (the lateral) but is inhomogeneous in
the wall-normal and streamwise directions. The turbulent boundary layer over a uni-
form surface can be homogeneous in two directions, those in the plane parallel to
the surface, but is necessarily inhomogeneous in the normal direction. The grid tur-
bulence produced by a grating of bars spanning the cross section of a wind tunnel is
homogeneous in the cross-stream plane but inhomogeneous in the streamwise direction
because it decays as it goes downstream (Chapter 5) . A carefully tailored homogeneous
shear flow in a wind tunnel ( Tavoularis and Corrsin , 1981 ) is, to a good approximation,
homogeneous in all three directions.
• Steady (also called stationary ) turbulence - turbulence in a laboratory flow
driven by a constant-speed blower, for example - has statistics that are independent of
time. Turbulence in the atmospheric boundary layer can be steady for up to a few hours,
but near sunrise and sunset in clear weather it is made unsteady (nonstationary) by the
changing surface energy budget (Chapter 9) .
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