Abstract:
The main scope of this PhD thesis is the analysis of unsteady laminar and transitional
suddenly expanded flows. For this reason Implicit Large Eddy Simulation
(ILES) approach was used in combination with high order, high resolution numerical
methods. The numerical methods examined are a 2nd order Monotonic Upwind
Scheme for Scalar Conservation Laws (MUSCL) with Van Leer limiter, a high order
(3rd) interpolation and a 5th order Weighted Essentially Non-Oscillatory scheme
(WENO).
First the numerical data for three low (steady state) Reynolds numbers and for
two unsteady ( in the form of primary frequencies) were compared to the experimental
data and were found in good agreement. A grid convergence study was
undertaken for two Reynolds numbers demonstrating grid convergence and justifying
the selection of the grid. The three numerical methods were evaluated for
two Reynolds numbers showing good agreement for Reynolds number 412 and discrepancies
at Reynolds number 900 between MUSCL and WENO with the MUSCL
demonstrating a very dissipative behavior.
The physical behavior of the flow in a wide range of Reynolds numbers were
examined. For this range the flow behavior changed from steady to unsteady laminar
and finally exhibiting localized transition to turbulence. The behavior of the main
recirculation areas was described and the vortex shedding that occur there and how
this change with the Reynolds number. The flow was observed to change from
an unsteady quasi three dimensional flow at Reynolds number 412 to an increased
transitional state with three dimensional vortical structures at Reynolds number
550.
Kinetic energy spectra were calculated for the aforementioned range of Reynolds
numbers. The primary frequencies are increasing with Reynolds number as expected.
The slopes that were calculated for the inertial subrange revealed a trend. As the
Reynolds number is increasing the slopes are decreasing approaching the value given
by Kolmogorov -5/3.
