Crossflow-vortex-induced laminar breakdown in a three-dimensional flat-plate
boundary-layer flow is investigated in detail by means of spatial direct numerical
simulations. The base flow is generic for an infinite swept wing, with decreasing
favourable chordwise pressure gradient. First, the downstream growth and nonlinear
saturation states initiated by a crossflow-vortex-mode packet as well as by single
crossflow-vortex modes with various spanwise wavenumbers are simulated. Second,
the secondary instability of the flow induced by the saturated crossflow vortices is
scrutinized, clearly indicating the convective nature of the secondary instability and
strengthening knowledge of the conditions for its onset. Emphasis is on the effect of
crossflow-vortex-mode packets and of the spanwise vortex spacing on the secondary
stability properties of the saturation states. Saturated uniform crossflow vortices initiated
by single crossflow-vortex modes turn out to be less unstable than vortices
initiated by a packet of vortex modes, and closely spaced saturated vortices are
even stable. Third, we investigate the transition control strategy of upstream flow
deformation by appropriate steady nonlinear vortex modes as applied in wind tunnel
experiments at the Arizona State University. A significant transition delay is shown
in the base flow considered here, and the underlying mechanisms are specified.
The selective catalytic reduction (SCR) based on ureawater-solution is an effective technique to reduce nitrogen oxides (NO x ) emitted from diesel engines. A 3D numerical computer model of the injection of urea-water-solution and their interaction with the exhaust gas flow and exhaust tubing is developed to evaluate different configurations during the development process of such a DeNOxsystem. The model accounts for all relevant processes appearing from the injection point to the entrance of the SCR-catalyst:• momentum interaction between gas phase and droplets• evaporation and thermolysis of droplets• hydrolysis of isocyanic acid in gas phase• heat transfer between wall and droplets• spray/wall-interaction• two-component wall film including interaction with gas phase and exhaust tubeThe single modeling steps are verified with visualizations, patternator measurements, phase-doppler-anemometer results and temperature measurements. CFD simulations of a SCR DeNOx-system are compared to experimental data to determine the decomposition parameters for ureawater-solution droplets. Numerical results for an injection including all processes addressed above are discussed.
The laminar breakdown induced by purely travelling crossflow vortices in a three-dimensional flat-plate boundary-layer flow is investigated in detail by means of spatial direct numerical simulations. The base flow considered is generic for an infinite swept wing, with decreasing favourable chordwise pressure gradient and a sweep angle of $45^\circ$. First, the primary downstream growth and nonlinear saturation state of a single crossflow wave are simulated. Secondly, background disturbance pulses are added, and the subsequent mechanisms triggering transition to turbulence in this scenario are identified and analysed in detail. The saturated travelling crossflow vortex is found to give rise to a co-travelling secondary instability not unlike the instability in the much investigated steady crossflow-vortex case, but with characteristic differences. An analysis method with a spanwise Galilean transformation to travel with the primary wave and a consequently adapted timewise/spanwise Fourier decomposition of the disturbance flow enables unambiguous isolation of the various secondary disturbance modes. The resulting flow structures and their dynamics in physical space are visualized.
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