Direct Numerical Simulation of Boundary-layer Transition with Free-stream Turbulence
Abstract: This thesis considers the generation and influence of free-stream turbulence toboundary layer transition on both flat and curved bodies in the flow. Variousflow configurations such as flow around the flat plate with a sharp leading edgeand low-pressure turbine blades are considered. This study aims at contributingto a better understanding of stability characteristics and different transitionmechanisms in such flows, which are of great interest for fundamental andindustrial applications.In the first part of the thesis, we study the effects of the free-streamturbulence characteristic length scales and intensity on the transition in anincompressible flat-plate boundary layer through direct numerical simulations(DNS). Computations are performed using the spectral element code Nek5000.The numerical setup corresponds to the experimental investigations by Fransson & Shahinfar (2020). Numerically generated homogeneous isotropic turbulenceupstream of the leading edge is designed to reproduce the characteristics of thegrid-generated turbulence in the wind tunnel experiments. Various combinationsof integral length scales are simulated. To ensure the quality of the data, classicalturbulence statistics and integral quantities are carefully evaluated, showingclose agreement with the corresponding experimental data.In the second part, we study both the effect of the free-stream turbulencelevel and the effect of the wake on the low-pressure turbine blades. Thehomogeneous and isotropic free-stream turbulence is prescribed at the inlet asa superposition of Fourier modes with a random phase shift. In the secondstage of the study, cylinders moving in front of the leading edge of the turbineare included to model the effect of the wake coming from the upstream blade.That is done using the tool NekNek which simultaneously runs two differentsimulations that communicate with each other at each time-step through aspecific boundary condition.We also analysed laminar/turbulent regions in the boundary layer flow forboth cases mentioned earlier. To achieve this, we proposed a topology-basedmethod based on extracting the extrema of the flow data. The goal was topropose a method to reduce the subjective choices to a minimum and provideefficient results regardless of the chosen flow case.
CLICK HERE TO DOWNLOAD THE WHOLE DISSERTATION. (in PDF format)