264
R. A. ANTONIA AND R. E. LUXTON
1968). On the meso-scale, the turbulence structure and diffusion properties
of an internal layer, if present, are likely to have a sipificant effect on the
diffusion and advection of material dischaqed into the atmosphere from a
flue or stack. On the synoptic scale, the active discussion of the paper by
Lettau (1959) at the Oxford conference clearly evidenced the importance of
the relation between surface drag and large scale atmospheric motions.
While measurements of mean field and time amaged turbuknce quantities have been made in an internal layer, it would Seem that the turbulence
structure has not yet bcen studied. To this end we have reanalyzed data
recorded in our 1968 laboratory experiments on the flow downstream of a
step change in surface roughness (Antonla and Luxton, 1971a, 1972, which
are from now on referred to as I and 11, respectively). Surface roughness
changes from smooth to rough (I), and, from rough to smooth (11) are
considered. The skewness and flatness factor of the streamwise (u) and
normal (w) velocity fluctuations and of their instantaneous product ( U W )
within the internal layer have been calculated. A significant difference in the
behaviour of these quantities is found near the internal layer/external layer
interface and it is suggested that this difference is related to an effective
new ” turbulence/” old ” turbulena (” signal/noise ”) ratio when the signals
are considered as being a random switching between two differtnt turbulent
flows, analogous with the swit+ing between turbulent and irrotational flow
at the free stream boundary of a layer. Propagation velocities associated
with the growth of the internal layer are found to be appreciably higher for
the smooth to rough change than for the rough to smooth case, in agreement
with the earlier finding that the internal layer grows faster in the first case.
When seeking scaling relationships for spectra, it is found that the internal
layer thickness is a significant length scale for most of the internal layer. We
attempt to relate this to the ‘*active” and “inactive” motion ideas of Townsend (1961) and Bradshaw (1967b).
2. EXPERIMENTAL CONDITIONS
A detailed description of the experimental equipment and techniques used
in the investigation may be found in I and 11. Briefly, for the smooth to
rough surface change, the tunnel fioor boundary layer was tripped about
0.3 m from the start of the working scctbn and then developed over a
further 2.14 m at which point tho surface roughness bagan. The rou&ness
consisted of a 2.44-111 long section with 3.2-mm square section bars placed
transversely across the floor of the tunnel at a streamwise spacing of
12.7 mm (roughness height to spacing ratio 1 : 4) with the crests of the
roughness elements aligned with the smooth surface. For the rough to
smooth case, the last 1.22 m of the roughness section was moved and
R. A. ANTONIA AND R. E. LUXTON
1968). On the meso-scale, the turbulence structure and diffusion properties
of an internal layer, if present, are likely to have a sipificant effect on the
diffusion and advection of material dischaqed into the atmosphere from a
flue or stack. On the synoptic scale, the active discussion of the paper by
Lettau (1959) at the Oxford conference clearly evidenced the importance of
the relation between surface drag and large scale atmospheric motions.
While measurements of mean field and time amaged turbuknce quantities have been made in an internal layer, it would Seem that the turbulence
structure has not yet bcen studied. To this end we have reanalyzed data
recorded in our 1968 laboratory experiments on the flow downstream of a
step change in surface roughness (Antonla and Luxton, 1971a, 1972, which
are from now on referred to as I and 11, respectively). Surface roughness
changes from smooth to rough (I), and, from rough to smooth (11) are
considered. The skewness and flatness factor of the streamwise (u) and
normal (w) velocity fluctuations and of their instantaneous product ( U W )
within the internal layer have been calculated. A significant difference in the
behaviour of these quantities is found near the internal layer/external layer
interface and it is suggested that this difference is related to an effective
new ” turbulence/” old ” turbulena (” signal/noise ”) ratio when the signals
are considered as being a random switching between two differtnt turbulent
flows, analogous with the swit+ing between turbulent and irrotational flow
at the free stream boundary of a layer. Propagation velocities associated
with the growth of the internal layer are found to be appreciably higher for
the smooth to rough change than for the rough to smooth case, in agreement
with the earlier finding that the internal layer grows faster in the first case.
When seeking scaling relationships for spectra, it is found that the internal
layer thickness is a significant length scale for most of the internal layer. We
attempt to relate this to the ‘*active” and “inactive” motion ideas of Townsend (1961) and Bradshaw (1967b).
2. EXPERIMENTAL CONDITIONS
A detailed description of the experimental equipment and techniques used
in the investigation may be found in I and 11. Briefly, for the smooth to
rough surface change, the tunnel fioor boundary layer was tripped about
0.3 m from the start of the working scctbn and then developed over a
further 2.14 m at which point tho surface roughness bagan. The rou&ness
consisted of a 2.44-111 long section with 3.2-mm square section bars placed
transversely across the floor of the tunnel at a streamwise spacing of
12.7 mm (roughness height to spacing ratio 1 : 4) with the crests of the
roughness elements aligned with the smooth surface. For the rough to
smooth case, the last 1.22 m of the roughness section was moved and