Laninar-turbulent boundary layer transition influenced by free stream turbulence

Sammanfattning: Laminar-turbulent transition in a flat plate boundary layerhas been considered, with a special emphasis on the influencefrom free stream turbulence (FST)First, experiments on linear stability ofTollmien-Schlichting (TS) waves in a Blasius boundary layer wasstudied. This is a well-known example from hydrodynamicstability theory, which has hitherto suffered from unresolveddiscrepancies between experiments and linear theory. Thepresent results showed good agreement with the theory, and itwas also shown that inappropriate leading edge conditions mayhave affected earlier experimental studies.In a series of experiments the interest has been focussed ontransition at moderate levels of free stream turbulence. Theresponse of a laminar boundary layer subjected to a free streamturbulence level of 1.5% formed the basis for the study, andthe general characteristics of FST-induced disturbances weredetermined. It was shown that the FST gives rise tonon-stationary streaky structures inside the laminar boundarylayer, which result in streamwise fluctuations with rms-valuesof the order of 10% before transition starts. Moreover, it wasshown that the introduction of TS-waves together with the largeamplitude streaks could lead to a considerable enhancement ofthe transition process.Since the random character of FST-induced boundary layerdisturbances can effectively obscure important details in theflow, a model experiment simulating the process through whichfree stream eddies transform into streaky structures inside theboundary layer was studied. This was done by means of alocalized free stream disturbance introduced upstream of theleading edge. The importance of streamwise vorticity in thereceptivity process was clearly indicated. It was also shownthat localized streaky structures, which decayed in amplitudeduring the downstream development, could exhibit a strongamplification if a TS-wave was generated simultaneously. It issuggested that similar non-linear interactions may occur in aboundary layer subjected to moderate levels of free streamturbulence.Besides the experimental work, a study related to transitionprediction at high levels of free stream turbulence is includedin the thesis. The possibility to use differential ReynoldsStress Transport Models for transition prediction isconsidered, and some aspects concerning the sensitivity to freestream boundary conditions are discussed.Keywords:Laminar-turbulent transition, hot-wiremeasurements, Tollmien-Schlichting waves, free streamturbulence, receptivity, localized disturbances, nonlinearinteraction, transition modelling