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Flow facility design and experimental studies of wall-bounded turbulent shear-flows


Respondent	Huvudhandledare	Bihandledare	Datum
Björn Lindgren	Arne Johansson		2002-12-17

Opponent
Hassan Nagib, KTH, Mekanik

Betygsnämd
Per Elofsson, KTH, Mekanik Rolf Karlsson, Vattenfall Utveckling AB Lennart Löfdahl, CTH

Abstract

The present thesis spans a range of topics within the area of turbulent flows, ranging from design of flow facilities to evaluation of scaling laws and turbulence modeling aspects through use of experimental data. A new wind-tunnel has been designed, constructed and evaluated at the Dept. of Mechanics, KTH. Special attention was directed to the design of turning vanes that not only turn the flow but also allow for a large expansion without separation in the corners. The investigation of the flow quality confirmed that the concept of expanding corners is feasible and may be successfully incorporated into low turbulence wind-tunnels. The flow quality in the MTL wind-tunnel at the Dept. of Mechanics, KTH, was also investigated confirming that it still is very good. The results are in general comparable to those measured when the tunnel was new, with the exception of the temperature variation that has decreased by a factor of 4 due to an improved cooling system. Experimental data from high Reynolds number zero pressure-gradient turbulent layers have been investigated. These studies have primarily focused on scaling laws with e.g. confirmation of an exponential velocity defect law in a region, about half the size of the boundary layer thickness, located outside the logarithmic overlap region. The streamwise velocity probability density functions in the overlap region was found to be self-similar when scaled with the local rms value. Flow structures in the near-wall and buffer regions were studied and e.g. the near-wall streak spacing was confirmed to be about 100 viscous length units although the relative influence of the near-wall streaks on the flow was found to decrease with increasing Reynolds number. The separated flow in an asymmetric plane diffuser was determined using PIV and LDV. All three velocity components were measured in a plane along the centerline of the diffuser. Results for mean velocities, turbulence intensities and turbulence kinetic energy are presented, as well as for streamlines and back-flow coefficient describing the separated region. Instantaneous velocity fields are also presented demonstrating the highly fluctuating flow. Results for the above mentioned velocity quantities, together with the production of turbulence kinetic energy and the second anisotropy invariant are also compared to data from simulations based on the k-omega formulation with an EARSM model. The simulation data were found to severely underestimate the size of the separation bubble.
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