Instability Of Three-Dimensional Boundary Layers And Modeling Bypass Transition

Boundary layer transition prediction and control is one of the mostambitions topics of research activities in aerodynamics. There are differenttransition physics between three-dimensional boundary layer andtwo-dimensional boundary layer flow. Based on the flow instability theory, thesecondary instability and control of three-dimensional boundary layer isinvestigated. And a new four-equation transition mode is developed by usinglocal variables for modern CFD soft.The crossflow instability of a three-dimensional boundary layer is a mainfactor affecting the laminar transition. The three-dimensional boundary layerflow distorted by the saturated crossflow vortex is very sensitive to the highfrequency disturbances, which foreshadows that the laminar transition willhappen. In this thesis, the crossflow instability of the incompressible flow over aswept wing is investigated by solving nonlinear parabolized stability equations(NPSE), and then the Floquet theory is applied to study the dependence of thesecondary and high-frequency instabilities of saturated steady crossflow vortexon curvature, chord Reynolds number and angle of swept (AOS). Thecomputational results show that the curvature in the present case has nosignificant effect on the secondary instabilities. The effect of the angle of sweptat35o,45oand55odegrees, respectively, is also studied in the framework of thesecondary instability theory. Larger angles of swept tend to decrease thespanwise spacing of the crossflow vortices, which correspondingly helps thestimulation of 'z' mode and the 'y' mode. The secondary instability of thesubsonic flow over a swept column is investigated with nonlinear parabolizedstability equations (NPSE) too. The Floquet theory is then applied to analyze theinfluence of localized steady wall suction and blowing on the secondaryinstability of crossflow vortex. The results show that suction can significantlysuppress the growth of the crossflow mode as well as the secondary instabilitymodes but blowing is in opposition with suction. The nature transition will be bypassed and a rapid transition process occurswhen free-stream turbulence intensity exceeds1%of the free-stream velocity.Prediction of bypass transition is of fundamental importance to the whole fluidmechanics community, for example, past turbine blades, flap configuration andhypersonic forebody. This paper presents a four-equation eddy-viscosityturbulence transition model for bypass transition prediction of two-dimensionalboundary layer based on the two-equation SST turbulence model and on thelaminar kinetic energy concept. A transport equation for the non-turbulentviscosity is designed to predict the development of the laminar kinetic energy inthe boundary layer flow that has been observed in experiments. The turbulencebreakdown process is then captured with an intermittency transport equation inthe transitional region. The performance of this new transition model isvalidated in the experimental cases of T3AM, T3A and T3B. Results show thatthis new transition model can reach good agreement in predicting bypasstransition, and is compatible with modern CFD software by using localvariables.