Licentiatseminarium
Simulations of turbulent boundary layers with suction and pressure gradients

Respondent 
Huvudhandledare 
Bihandledare 
Datum 
Alexandra Bobke 
Philipp Schlatter 
Ramis Örlü 
20160512 

Opponent 
Ayse Gungor, Istanbul Technical University


Betygsnämd 


AbstractThe focus of the present licentiate thesis is on the effect of suction and pressure gradients on turbulent boundarylayer flows, which are investigated separately through performing numerical simulations. The first part aims at assessing history and development effects on adverse pressuregradient (APG) turbulent boundary layers (TBL). A suitable setup was developed to study nearequilibrium conditions for a boundary layer developing on a flat plate by setting the freestream velocity at the top of the domain following a power law. The computational box size and the correct definition of the topboundary condition were systematically tested. Wellresolved large eddy simulations were performed to keep computational costs low. By varying the freestream velocity distribution parameters, e.g. powerlaw exponent and virtual origin, pressure gradients of different strength and development were obtained. The magnitude of the pressure gradient is quantified in terms of the Clauser pressuregradient parameter ?. The effect of the APG is closely related to its streamwise development, hence, TBLs with nonconstant and constant ? were investigated. The effect was manifested in the mean flow through a much more pronounced wake region and in the Reynolds stresses through the existence of an outer peak. The terms of the turbulent kinetic energy budgets indicate the influence of the APG on the distribution of the transfer mechanism across the boundary layer. Stronger and more energetic structures were identified in boundary layers with relatively stronger pressure gradients in their development history. Due to the difficulty of determining the boundarylayer thickness in flows with strong pressure gradients or over a curved surface, a new method based on the diagnosticplot concept was introduced to obtain a robust estimation of the edge of a turbulent boundary layer. In the second part, largeeddy simulations were performed on temporally developing turbulent asymptotic suction boundary layers (TASBLs). Findings from previous studies about the effect of suction could be confirmed, e.g. the reduction of the fluctuation levels and Reynolds shear stresses. Furthermore, the importance of the size of the computational domain and the time development were investigated. Both parameters were found to have a large impact on the results even on loworder statistics. While the mean velocity profile collapses in the inner layer irrespective of box size and development time, a wake region occurs for too small box sizes or early development time and vanishes once sufficiently large domains and/or integration times are chosen. The asymptotic state is charactersized by surprisingly thick boundary layers even for moderate Reynolds numbers Re (based on freestream velocity and laminar displacement thickness); for instance, Re = 333 gives rise to a friction Reynolds number Re? = 2000. Similarly, the flow gives rise to very large structures in the outer region. These findings have important ramifications for experiments, since very large facilities are required to reach the asymptotic state even for low Reynolds numbers.
[Nerladdning
()]

