284
R. A. ANTONIA AND R. E. LUXTON
smooth to rough internal layer suggest an iaterrnittency ” of the interface
associated with “switching” between two different turbulence fields. One
has the impression of energetic smaller scale new” turbulence eating into
the tired larger scale “old ” turbulence of the original smooth wall boundary
layer. Such a picture suggests a relatively rapid propagation of the new
turbulence as found earlier and as indicated by our present discussion of
propagation velocity W,. Although the two turbulence fields seem to have
separate identities which could, with different techniques, be separated even
where they interact at the interface, one may still expect some large-scale
“stirring” of the internal layer by the external layer, as indicated by changes
in the scaled spectra at low frequencies.
In the rough to smooth change situation, the picture is not nearly as
clearly drawn but it is nevertheless similar in outline. The sharp, intermittently “switched” interface is no longer revcakd by the overall distributions
of flatness and skewness factors. This is probably because the relative intensities of the new ” and the -old turbulence are comparable even though
their scales are quite different. Flatness and Ekewness of band passed signals
could reveal the intermittent switching at the interface but this has not yet
been done. Temperature tagging” of one of the layers by low intensity
surface heating would also allow details of the switching to be determined as
well as some statistical details of each of the turbulence fields (by conditional
sampling). This information is needed if highcrsrder moments are to be
predicted.
The present experimental situation, while providing much useful information, is probably too complicated to understand fully, given our present
knowledge of turbulent shcar flows. A similar investigation using a change of
low intensity heat flux at the boundary, similar to that studied by Johnson
(1959), and conditional sampling techniques to study in detail the internal/
external layer interface would provide directly information about the nature
of diffusive processes in a simple turbulent boundary layer. But the atmospheric boundary layer is not simple so more wmpkx experiments are also
required, e.g., the interaction of an internal hycr with a density stratified
external layer would be of direct interest. From tBc isothermal rcsuits of this
paper it would appear that every effort should bc made to discharge smoke
into an external layer if one exists.
A major quation whicb arises from this work concerns the scale on which
the turbulent/turbulent intemction occurs at the bterfaoe. At the outer edge
of a turbulent boundary layer we have a twbulent/irrotational interface
which, presumably, involvas molecular scales in a viscous superlayer. Can
we conceive a comparable hietachy of scales in a turbuhmt/turbulent interaction? With the aid of conditional sampling the answer to this question
does not appear to be unattainable.
R. A. ANTONIA AND R. E. LUXTON
smooth to rough internal layer suggest an iaterrnittency ” of the interface
associated with “switching” between two different turbulence fields. One
has the impression of energetic smaller scale new” turbulence eating into
the tired larger scale “old ” turbulence of the original smooth wall boundary
layer. Such a picture suggests a relatively rapid propagation of the new
turbulence as found earlier and as indicated by our present discussion of
propagation velocity W,. Although the two turbulence fields seem to have
separate identities which could, with different techniques, be separated even
where they interact at the interface, one may still expect some large-scale
“stirring” of the internal layer by the external layer, as indicated by changes
in the scaled spectra at low frequencies.
In the rough to smooth change situation, the picture is not nearly as
clearly drawn but it is nevertheless similar in outline. The sharp, intermittently “switched” interface is no longer revcakd by the overall distributions
of flatness and skewness factors. This is probably because the relative intensities of the new ” and the -old turbulence are comparable even though
their scales are quite different. Flatness and Ekewness of band passed signals
could reveal the intermittent switching at the interface but this has not yet
been done. Temperature tagging” of one of the layers by low intensity
surface heating would also allow details of the switching to be determined as
well as some statistical details of each of the turbulence fields (by conditional
sampling). This information is needed if highcrsrder moments are to be
predicted.
The present experimental situation, while providing much useful information, is probably too complicated to understand fully, given our present
knowledge of turbulent shcar flows. A similar investigation using a change of
low intensity heat flux at the boundary, similar to that studied by Johnson
(1959), and conditional sampling techniques to study in detail the internal/
external layer interface would provide directly information about the nature
of diffusive processes in a simple turbulent boundary layer. But the atmospheric boundary layer is not simple so more wmpkx experiments are also
required, e.g., the interaction of an internal hycr with a density stratified
external layer would be of direct interest. From tBc isothermal rcsuits of this
paper it would appear that every effort should bc made to discharge smoke
into an external layer if one exists.
A major quation whicb arises from this work concerns the scale on which
the turbulent/turbulent intemction occurs at the bterfaoe. At the outer edge
of a turbulent boundary layer we have a twbulent/irrotational interface
which, presumably, involvas molecular scales in a viscous superlayer. Can
we conceive a comparable hietachy of scales in a turbuhmt/turbulent interaction? With the aid of conditional sampling the answer to this question
does not appear to be unattainable.