Published Book or Work by:
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Structure Identification Within A Transitioning Swept-Wing Boundary Layer
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| Published by Clarkson University |
| May, 1996 |
| Extensive measurements are made in a
transitioning swept-wing boundary layer using hot-
film, hot-wire, and cross-wire anemometry. The
crossflow-dominated flow contains stationary
vorticies that breakdown near mid-chord. The most
amplified vortex wavelength is forced by the use of
artificial roughness elements near the leading edge.
Two-component velocity and spanwise surface-
stress correlation measurements are made at two
constant chord locations, before and after
transition. Surface shear stresses are also measured
through the entire transition region.
Correlation techniques are used to identify
stationary structures in the laminar regime and
coherent structures in the turbulent regime. Basic
techniques include observation of the spatial
correlations and the spatially distributed auto-
spectra. The primary and secondary instability
mechanisms are identified in the spectra in all
measured fields. The primary mechanism is seen to
grow, cause transition and produce large-scale
turbulence. The secondary mechanism grows
through the entire transition region and produces
the small-scale turbulence.
Advanced techniques use linear stochastic
estimation (LSE) and proper orthogonal
decomposition (POD) to identify the spatio-
temporal evolutions of structures in the boundary
layer. LSE is used to estimate the instantaneous
velocity fields using temporal data from just spatial
locations and the spatial correlations. Reference
locations are selected using the maximum RMS
values to provide the best available estimates. POD
is used to objectively determine modes
characteristic of the measured flow based on
energy. The stationary vorticies are identified in the
first laminar modes of each velocity component and
shear component. Experimental evidence suggests
that neighboring vorticies interact and produce
large coherent structures with spanwise periodicity
at double the stationary vortex wavelength. An
objective transition region detection method is
developed using streamwise spatial POD solutions
which isolate the growth of the primary and
secondary instability mechanisms in the first and
second modes, respectively. Temporal evolutions of
the dominant POD modes in all measured fields are
calculated. These scalar POD coefficients contain
the integrated characteristics of the entire field,
greatly reducing the amount of data required to
characterize the instantaneous field. These modes
may then be used to train future flow control
algorithms based on neural networks. |
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