Doctoral defense
Boundary layer streaks as a novel laminar flow control method
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| Defendant |
Main Advisor |
Extra Advisor |
Date |
| Sohrab S. Sattarzadeh |
Jens Fransson |
Alessandro Talamelli |
2016-02-26 |
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| Opponent |
| Edward White, Texas A&M University
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| Evaluation committee |
Per Elofsson, Scania
Jonathan Morrison, Imperial College London
Stefan Wallin, KTH
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AbstractA novel laminar ow control based on generation of spanwise mean velocity gradients (SVG) in a at plate boundary layer is investigated where disturbances of di erent types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at; (i) generating stable and steady streamwise streaks in the boundary layer which set up spanwise gradients in the mean ow, and (ii) attenuating disturbance energy growth in the streaky boundary layers and hence delaying the onset of turbulence transition. The streamwise streaks generated by four different methods are investigated, which are spanwise arrays of triangular/rectangular miniature vortex generators (MVGs) and roughness elements, non-linear pair of oblique waves, and spanwise-periodic finite discrete suction. For all the investigated methods the boundary layer is modulated into regions of high- and low speed streaks through formation of pairs of counter-rotating streamwise vortices. For the streaky boundary layers generated by the MVGs a parameter study on a wide range of MVG con figurations is performed in order to investigate the transient growth of the streaks. A general scaling of the streak amplitudes is found based on empiricism where an integral amplitude de finition is proposed for the streaks. The disturbances are introduced as single- and broad band frequency two-dimensional Tollmien-Schlichting (TS) waves, and three-dimensional single and a pair of oblique waves. In an attempt to obtain a more realistic con figuration compared to previous investigations the disturbances are introduced upstream of the location were streaks are generated. It is shown that the SVG method is efficient in attenuating the growth of disturbance amplitudes in the linear regime for a wide range of frequencies although the disturbances have an initial amplitude response to the generation of the streaks. The attenuation rate of the disturbance amplitude is found to be optimized for an integral streak amplitude of 30% of the free-stream velocity which takes into account the periodic wavelength of the streaky base ow. The stabilizing effect of the streamwise streaks can be extended to the nonlinear regime of disturbances which in turn results in transition to turbulence delay. This results in signifi cant drag reduction when comparing the skin friction coefficient of a laminar- to a turbulent boundary layer. It is also shown that consecutive turbulence transition delay can be obtained by reinforcing the streaky boundary layer in the streamwise direction. For the streaky boundary layer generated by pair of oblique waves their forcing frequency sets the upper limit for the frequency of disturbances beyond which the control fails. Descriptors: boundary layer stability, laminar to turbulent transition, laminar flow control, drag reduction, Tollmien-Schlichting waves, oblique waves, streaky boundary layers, miniature vortex generators, discrete suction
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