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Direct Numerical Simulation of Boundary Layer Transition with Free-stream Turbulence

Time: Fri 2022-02-11 14.00

Location: D3

Video link: Hybrid on Zoom

Subject area: Engineering Mechanics

Doctoral student: Ðurović , Engineering Mechanics

Opponent: Prof. Tamer Zaki (Johns Hopkins University)

Supervisor: Dan Henningson, Ardeshir Hanifi, Philipp Schlatter

Abstract:
This thesis considers the generation and influence of free-stream turbulence to
boundary layer transition on both flat and curved bodies in the flow. Various
flow configurations such as flow around the flat plate with a sharp leading edge
and low-pressure turbine blades are considered. This study aims at contributing
to a better understanding of stability characteristics and different transition
mechanisms in such flows, which are of great interest for fundamental and
industrial applications.
In the first part of the thesis, we study the effects of the free-stream
turbulence characteristic length scales and intensity on the transition in an
incompressible 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 turbulence
upstream of the leading edge is designed to reproduce the characteristics of the
grid-generated turbulence in the wind tunnel experiments. Various combinations
of integral length scales are simulated. To ensure the quality of the data, classical
turbulence statistics and integral quantities are carefully evaluated, showing
close agreement with the corresponding experimental data.
In the second part, we study both the effect of the free-stream turbulence
level and the effect of the wake on the low-pressure turbine blades. The
homogeneous and isotropic free-stream turbulence is prescribed at the inlet as
a superposition of Fourier modes with a random phase shift. In the second
stage of the study, cylinders moving in front of the leading edge of the turbine
are included to model the effect of the wake coming from the upstream blade.
That is done using the tool NekNek which simultaneously runs two different
simulations that communicate with each other at each time-step through a
specific boundary condition.
We also analysed laminar/turbulent regions in the boundary layer flow for
both cases mentioned earlier. To achieve this, we proposed a topology-based
method based on extracting the extrema of the flow data. The goal was to
propose a method to reduce the subjective choices to a minimum and provide
efficient results regardless of the chosen flow case.

Page responsible:Ardeshir Hanifi
Belongs to: FLOW
Last changed: Feb 09, 2022