Stably-Curved Mixing Layer
Experiments by Castro and Bradshaw
Description
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As part of a general investigation of complex turbulent flows, extensive a one-point measurements
have been made of the turbulence structure of the mixing layer bounding a normally impinging plane jet with
an irrotational core. The ratio of shear-layer thickness to streamline radius of curvature reaches a maximum
of about 0.2, the sense of the curvature being stabilizing. downstream of the impingement region tile shear
layer returns asymptotically to being a classical plane mixing layer. After the start of a region of
stabilizing curvature, subsequent changes being slow, but they did not make enough measurements to extract
an energy balance. We decided that one of the most useful flows for developing and testing calculation
methods would be the strongest possible perturbation of an initially self-preserving shear layer with a
subsequent return to the same self-preserving state: both end states would be clearly defined and, with
sufficiently detailed measurements, the effect of curvature history would be assessable.
The configuration chosen for the present experiments is shown in figure 1; the maximum value of
V/x
in the central region of the mixing layer is about
-0.2U/y
(using x, y axes aligned with the local direction of the shear
layer) so that the fairly thin shear layer' limit is exceeded for a short streamwise distance only. The
flow can be thought of as half a two-dimensional impinging jet, with a potential core; the 'floor' replaces
the plane of symmetry. It was chosen as the only obvious case of a monotonic shear layer (i.e. one with mean
shear of the same sign everywhere) which could be strongly perturbed by a short region of large curvature
without the occurrence of a change of species (e.g. a change from a free jet to a wall jet or from a
boundary layer to a separated shear layer). Two disadvantages of the present configuration are that only a
stabilizing sense of streamline curvature can be obtained and that the mixing layer merges with the boundary
layer on the impingement surface before relaxation to the self-preserving state is complete. The second
disadvantage is minor, since we documented the self-preserving plane mixing layer in a companion experiment;
the first is outweighed by the clarity with which (stabilizing) curvature effects are demonstrated by this
flow. and by the practical importance of impinging jet flows.
The measurements presented here were intended to document the transport equations for turbulent energy
and shear stress as fully as possible, and include the triple products that effect turbulent transport of
shear stress and turbulent energy. We have not measured any quantities involving pressure fluctuations, and
our deductions of energy dissipation rates from frequency spectra may not be absolutely reliable although
they should be adequate for comparative purposes. The unmeasured pressure-strain 'redistribution' term
in the shear-stress transport equation can be deduced as the difference in the measured terms if transport
by pressure fluctuations can be neglected: this neglect seems to be justified except near
the high velocity edge of the curved shear layer. So far, only one-point measurements have been made,
but the results emphasize the need for correlation measurements to provide information about eddy length
scales.
Measurement Details
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Previous and Reference Numerical Solutions
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References
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