Anders Lundbladh
Thesis title: Simulation of bypass transition to turbulence in wall bounded shear flows
Date: September 16, 1993
Faculty opponent: Dr. Robert Moser, NASA Ames Research Center
Evaluation Committee: Prof. H. Gustavsson LuTH, Prof. Arthur Rizzi KTH, TeknD
Mats Ramnefors Volvo Data.
Main Advisor ('huvudhandledare'): Prof. Arne Johansson
The scope of the study is the investigation of transition where either no linear, exponentially growing, instability exists or where it is not the primary cause of growth of the disturbance amplitude. The work is based on a large number of numerical simulations of growth and transition in flows along plane boundaries. Most of the thesis deals with disturbances which are localized in space, i.e., come from a source which is much smaller than the base flow. The results are discussed in the framework of linear transient growth which has recently caught considerable attention from a number of workers, and confirmations of various theoretical predictions in this area are given. Further two important non-linear mechanisms are discovered, a non-linear deformation creating streamwise streaks which gives considerably increased growth by interaction with the linear transient growth, and second a secondary instability on the streaks, that rapidly creates small scales and causes the breakdown of the flow into turbulence.
Anders is employed full-time at FFA as of October 1.
John Matsson
Thesis title: Channel flow instabilities induced by curvature and rotation
Date: December 17, 1993
Faculty opponent: Dr. H. Bippes, DLR, Göttingen
Evaluation Committee: Prof. E. Olsson, CTH, Prof. B. Sundén, LTH, TeknD H.
Tinoco, Vattenfall Utveckling AB.
Main Advisor ('huvudhandledare'): Prof. Henrik Alfredsson
The thesis deals with the effects of curvature and system rotation on the stability of incompressible viscous channel flow. Hot-wire measurements, flow visualizations, linear stability analysis as well as comparisons with numerical simulations have been used to investigate the different flow structures and their spatial development. For Reynolds numbers above the critical, measurements of the streamwise velocity field in a curved channel showed a primary instability in the form of streamwise counter-rotating vortices, known as Dean vortices, that developed downstream. At higher Reynolds numbers several different types of secondary instabilities were observed. When spanwise system rotation was introduced, linear stability analysis predicted that the critical Reynolds number could be increased four times for negative rotation, i.e. when the Coriolis force counteracts the centrifugal force. Both measurements and visualizations showed a complete cancellation of Dean vortices for certain negative values of rotation, in accordance with linear theory.
John starts a post-doc position in Trondheim in August 1994.
Xue-Song Bai
Thesis title: On the modeling of turbulent combustion at low Mach numbers
Date: April 12, 1994
Faculty opponent: Dr. Peter Lindstedt, Imperial College, London.
Evaluation Committee: Prof. Jerzy Chomiak, CTH, Prof. Bengt Sundén, LTH, TeknD Lars
Strömberg, Vattenfall, Norrköping.
Main Advisor ('huvudhandledare'): Prof. Laszlo Fuchs.
The thesis deals with the modeling of turbulent reacting flows in gas turbine combustion chambers and furnaces at low Mach numbers (less than 0.3). The study is based on the reduced chemistry of hydrocarbon combustions. Comparisons between experimental data and calculated results are used to investigate some sub-models for turbulent combustion.
Together with the two-equation 
model for modeling turbulent transport
fluxes, three different models are employed to handle the interaction between
turbulence and combustion. These are the presumed Probability Density Function
(PDF) approach based on the Shvab-Zel'dovich formulation for an infinitely
fast chemistry, the Eddy Dissipation Concept (EDC) model based on finite
reaction rates (one-, two- and four-step reactions), and the presumed PDF with a
laminar flamelet library. It has been shown that the use of a more detailed
chemistry yields not only more information about chemical species, but also gives
better temperature distributions. The difference between these three models is
small when the time scale of turbulent mixing is small. These three models are
also less sensitive to the choice of model parameters when the time scale of
turbulent mixing is small. The PDF together with the laminar flamelet model can
predict the effect of flame quenching due to the turbulent stretching of flames.
It is shown that radiative heat transfer plays an important role in combustion. The influence of turbulence on the radiative heat transfer is important. A ray tracing scheme is proposed for handling arbitrary geometries and computational cells. A Multi-Grid method with optional local grid refinements have been extended in this work in order to handle turbulent reacting flows. The computational efficiency and numerical accuracy are considerably improved.
The numerical simulations have also been used for predicting combustion efficiency, pollutant emissions and geometrical optimizations of a furnace.
Xue-Song will continue to work with Prof. Fuchs in Lund.
Elna Holmberg
Thesis title: Evolution of spherical flames in turbulence
Date: June 2, 1994
Faculty opponent: Prof. N. Peters, Inst. für Technische
Mechanik, RWTH, Aachen.
Evaluation Committee: Prof. Jerzy Chomiak, CTH, Prof. Ingemar Denbratt,
AB Volvo, Göteborg, Prof. Lars-Erik Eriksson, Volvo Flygmotor AB,
Trollhättan.
Advisor: Dr. Anthony Burden.
The thesis concerns the development of a spark-induced flame kernel in a turbulent premixed gas of fuel and air. Models have been developed which take the size of the flame kernel in comparison to the integral scale of the turbulence into account. Computations have been performed of the early development and subsequent steady propagation of the on average spherical flame kernel, as well as of the quenching process. The turbulent transport is modeled by a gradient diffusion expression which approximately describes the transport of heat and reactants caused by eddies which are small enough to interact with the flame kernel. The combustion process is assumed to take place in a single step and the rate of this process is modeled by three different expressions. One model is sensitive to the mean temperature, but does not take the influence of the turbulence into account. The second and the third models are based on the increase of flame surface area due to the turbulence. The total flame surface area and its distribution are prescribed in one of these expressions, while they are calculated by means of a transport equation in the other.
The computations are adaptive in time and space. The computed flame development is compared with experimental data, and good agreement is achieved for some of the models.
Elna will start a research associate position at Chalmers in July 1994.
Mårten Levenstam Thesis title: Thermocapillary convection in floatzones
Date: June 3, 1994
Faculty opponent: Prof. G M Homsy, Dept. of Chemical
Engineering, Stanford Univ.
Evaluation Committee: Prof. Erik Janzén IFM-FOA Linköpings Univ., Prof.
Claes Johnson CTH, Univ.lektor Jesper Oppelstrup KTH.
Main advisor ('huvudhandledare'): Docent Gustav Amberg
The
floatzone method is one of the most commonly used method for the
production of single crystals of semiconductor materials such as
Silicon. The main advantage of the method is that the crystal melt is
never in contact with a container during the growth. It is kept
in place by surface tension. For most semiconductor materials
the strength of the surface tension is dependent on the temperature. Thus
a temperature gradient will lead to a surface tension gradient and
this in turn will cause a convection in the melt; thermocapillary
convection. The fluid motion critically influences the efficiency of
the process, a crucial example of this is the redistribution of
impurities.
The flow in the floating zone is analysed with the aid of direct
numerical simulation with a higher order accurate Finite Element
Method. Different aspects of the convection is analysed. The
segregation, or distribution, of a dopant material is computed and
compared with recent space experiments. The prospect of using a laser
as a heat source is analysed.
In many experiments a highly unwanted form of segregation has been
observed. The dopant has been found to be concentrated in regular
bands in the finished crystal. These bands, striations, are
caused by an oscillatory convection in the melt. This phenomenon has
been studied as a stability problem and the numerical simulations
gives predictions for a critical Reynolds number for the onset of
time dependent motion. These critical Reynolds numbers are compared to
recent experiments. A simple analogy with the stability of thin vortex
rings is also made. This analogy explains the cause of the instability
and makes it possible to deduce the space structure of the bifurcated
solution.
Mårten will start a research associate position at Chalmers in September 1994.