REYNOLDS STRESS STRUCI-URE IN TURBULENT BOUNDARY LAYER 31 3
the u and u signals throughout the boundary layer. This leads one to speculate that the turbulence in the inner part of the turbulent boundary layer
may be considered as a LL universal motion ” plus an “ irrelevant motion ” as
suggested by Townsend (1957, 1961). The universal motion m a y be considered as random Occurrence (both temporally and spatially) of bursts,
which is controlled by the outer flow, plw the ensuing more d i b return
flow, which may be related to the sweep events. The irrelevant motion may
be considered as the accumulation of the remnants of what has happened
upstream. The contribution to u7? from the latter would be small.
From the measurements of sampled Reynolds stress (UU) near the wall
using the sampling criteria that the velocity u,, at the edge of the viscous
sublayer is low and decreasing, it is found that there are burst events producing large contributions to the Reynolds stress that are convected at speeds
lower than the local mean speed. In addition the line in the x-y plane on
which the peak values of ( u v ) occur, at no time delay, travels outward from
the wall at an angle of 16-H)”. This may be explained by the convection past
the measuring station of a certain deterministic pattern, for example, the
hairpin vorticity model proposed by Willmarth and Tu (1967). This model
was suggested and is consistent with the numerous space-time correlation
measurements reported by Willmarth and Wooldridge (1962, 1963) and Tu
and Willmarth (1966). As a matter of fact, this pattern of vorticity, if
imagined to evolve to a larger scale, may also be used to describc the time
sequence of the instantaneous velocity profiles near the wall as observed by
Kim et al. (1968) (see Fig. 4.13 in their report) and could produce intermittent turbulent bulges at the outer edge of the boundary layer. This would
provide the interaction between the inner and outer regions of the boundary
layer that is implied by the scaling of the mean time between bursts with the
outer flow variables. Since a large part of the Reynolds stress near the wall is
produced during the times when our sampling procedure indicates that
bursts occur, it is likely that a model like that of Willmarth and Tu (1967)
may determine the flow structure near the wall and may well be a part of the
universal motion mentioned above.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support of the Fluid Dynamics Branch of the Office
of Naval Research and the Engineering Division of the National Science Foundation.
REFERENCES
Blackwelder. R. F.. and Kaplan. R . E. (1971). Intermittent structure in turbulent boundary
layer. A G A K D (‘onf: Proc. 93, 5.
I’orino, E. R.. and Brodkey, R. S. (1969). . I . Fluid Mcrh. 37, 1.
Grass, A. J. (1971). J . Fluid Much. 50. 233.
the u and u signals throughout the boundary layer. This leads one to speculate that the turbulence in the inner part of the turbulent boundary layer
may be considered as a LL universal motion ” plus an “ irrelevant motion ” as
suggested by Townsend (1957, 1961). The universal motion m a y be considered as random Occurrence (both temporally and spatially) of bursts,
which is controlled by the outer flow, plw the ensuing more d i b return
flow, which may be related to the sweep events. The irrelevant motion may
be considered as the accumulation of the remnants of what has happened
upstream. The contribution to u7? from the latter would be small.
From the measurements of sampled Reynolds stress (UU) near the wall
using the sampling criteria that the velocity u,, at the edge of the viscous
sublayer is low and decreasing, it is found that there are burst events producing large contributions to the Reynolds stress that are convected at speeds
lower than the local mean speed. In addition the line in the x-y plane on
which the peak values of ( u v ) occur, at no time delay, travels outward from
the wall at an angle of 16-H)”. This may be explained by the convection past
the measuring station of a certain deterministic pattern, for example, the
hairpin vorticity model proposed by Willmarth and Tu (1967). This model
was suggested and is consistent with the numerous space-time correlation
measurements reported by Willmarth and Wooldridge (1962, 1963) and Tu
and Willmarth (1966). As a matter of fact, this pattern of vorticity, if
imagined to evolve to a larger scale, may also be used to describc the time
sequence of the instantaneous velocity profiles near the wall as observed by
Kim et al. (1968) (see Fig. 4.13 in their report) and could produce intermittent turbulent bulges at the outer edge of the boundary layer. This would
provide the interaction between the inner and outer regions of the boundary
layer that is implied by the scaling of the mean time between bursts with the
outer flow variables. Since a large part of the Reynolds stress near the wall is
produced during the times when our sampling procedure indicates that
bursts occur, it is likely that a model like that of Willmarth and Tu (1967)
may determine the flow structure near the wall and may well be a part of the
universal motion mentioned above.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support of the Fluid Dynamics Branch of the Office
of Naval Research and the Engineering Division of the National Science Foundation.
REFERENCES
Blackwelder. R. F.. and Kaplan. R . E. (1971). Intermittent structure in turbulent boundary
layer. A G A K D (‘onf: Proc. 93, 5.
I’orino, E. R.. and Brodkey, R. S. (1969). . I . Fluid Mcrh. 37, 1.
Grass, A. J. (1971). J . Fluid Much. 50. 233.