Wall transpiration, in the form of wall-normal suction or blowing through a permeable wall, is a relatively simple and effective technique to control the behaviour of a boundary layer. For its potential applications for laminar-turbulent transition and separation delay (suction) or for turbulent drag reduction and thermal protection (blowing), wall transpiration has over the past decades been the topic of a significant amount of studies. However, as far as the turbulent regime is concerned, fundamental understanding of the phenomena occurring in the boundary layer in presence of wall transpiration is limited and considerable disagreements persist even on the description of basic quantities, such as the mean streamwise velocity, for the rather simplified case of flat-plate boundary-layer flows without pressure gradients. In order to provide new experimental data on suction and blowing boundary layers, an experimental apparatus was designed and brought into operation. The perforated region spans the whole 1.2 m of the test-section width and with its streamwise extent of 6.5 m is significantly longer than previous studies, allowing for a better investigation of the spatial development of the boundary layer. The quality of the experimental setup and measurement procedures was verified with extensive testing, including benchmarking against previous results on a canonical zero-pressure-gradient turbulent boundary layer (ZPG TBL) and on a laminar asymptotic suction boundary layer. The present experimental results on ZPG turbulent suction boundary layers show that it is possible to experimentally realize a turbulent asymptotic suction boundary layer (TASBL) where the boundary layer mean-velocity profile becomes independent of the streamwise location, so that the suction rate constitutes the only control parameter. TASBLs show a mean-velocity profile with a large logarithmic region and without the existence of a clear wake region. If outer scaling is adopted, using the free-stream velocity and the boundary layer thickness (δ99) as characteristic velocity and length scale respectively, the logarithmic region is described by a slope Ao=0.064 and an intercept Bo=0.994, independently from the suction rate (Γ). Relaminarization of an initially turbulent boundary layer is observed for Γ&gt;3.70×10−3. Wall suction is responsible for a strong damping of the velocity fluctuations, with a decrease of the near-wall peak of the velocity-variance profile ranging from 50% to 65% when compared to a canonical ZPG TBL at comparable Reτ. This decrease in the turbulent activity appears to be explained by an increased stability of the near-wall streaks. Measurements on ZPG blowing boundary layers were conducted for blowing rates ranging between 0.1% and 0.37% of the free-stream velocity and cover the range of momentum thickness Reynolds number 10000&lt;Reθ&lt;36000. Wall-normal blowing strongly modifies the shape of the boundary-layer mean-velocity profile. As the blowing rate is increased, the clear logarithmic region characterizing the canonical ZPG TBLs gradually disappears. A good overlap among the mean velocity-defect profiles of the canonical ZPG TBLs and of the blowing boundary layers for all the Re number and blowing rates considered is obtained when normalization with the Zagarola-Smits velocity scale is adopted. Wall blowing enhances the intensity of the velocity fluctuations, especially in the outer region. At sufficiently high blowing rates and Reynolds number, the outer peak in the streamwise-velocity fluctuations surpasses in magnitude the near-wall peak, which eventually disappears. / Genom att använda sig av genomströmmande ytor, med sugning eller blåsning, kan man relativt enkelt och effektivt påverka ett gränsskikts tillstånd. Genom sin potential att påverka olika strömningsfysikaliska fenomen så som att senarelägga både avlösning och omslaget från laminär till turbulent strömning (genom sugning) eller som att exempelvis minska luftmotståndet i turbulenta gränsskikt och ge kyleffekt (genom blåsning), så har ett otaligt antal studier genomförts på området de senaste decennierna. Trots detta så är den grundläggande förståelsen bristfällig för de strömningsfenomen som inträffar i turbulenta gränsskikt över genomströmmande ytor. Det råder stora meningsskiljaktigheter om de mest elementära strömningskvantiteterna, såsom medelhastigheten, när sugning och blåsning tillämpas även i det mest förenklade gränsskiktsfallet nämligen det som utvecklar sig över en plan platta utan tryckgradient. För att ta fram nya experimentella data på gränsskikt med sugning och blåsning genom ytan så har vi designat en ny experimentell uppställning samt tagit den i bruk.Den genomströmmande ytan spänner över hela bredden av vindtunnelns mätsträcka (1.2 m) och är 6.5 m lång i strömningsriktningen och är därmed betydligt längre än vad som använts i tidigare studier. Detta gör det möjligt att bättre utforska gränsskiktet som utvecklas över ytan i strömningsriktningen. Kvaliteten på den experimentella uppställningen och valda mätprocedurerna har verifierats genom omfattande tester, som även inkluderar benchmarking mot tidigare resultat på turbulenta gränsskikt utan tryckgradient eller blåsning/sugning och på laminära asymptotiska sugningsgränsskikt. De experimentella resultaten på turbulenta gränsskikt med sugning bekräftar för första gången att det är möjligt att experimentellt sätta upp ett turbulent asymptotiskt sugningsgränsskikt där gränsskiktets medelhastighetsprofil blir oberoende av strömningsriktningen och där sugningshastigheten utgör den enda kontrollparametern. Det turbulenta asymptotiska sugningsgränsskiktet visar sig ha en medelhastighetsprofil normalt mot ytan med en lång logaritmisk region och utan förekomsten av en yttre vakregion. Om man använder yttre skalning av medelhastigheten, med friströmshastigheten och gränsskiktstjockleken som karaktäristisk hastighet respektive längdskala, så kan det logaritmiska området beskrivas med en lutning på Ao=0.064 och ett korsande värde med y-axeln på Bo=0.994, som är oberoende av sugningshastigheten. Om sugningshasigheten normaliserad med friströmshastigheten överskrider värdet 3.70x10^-3 så återgår det ursprungligen turbulenta gränsskiktet till att vara laminärt. Sugningen genom väggen dämpar hastighetsfluktuationerna i gränsskiktet med upp till 50-60% vid direkt jämförelse av det inre toppvärdet i ett turbulent gränsskikt utan sugning och vid jämförbart Reynolds tal. Denna minskning av turbulent aktivitet verkar härstamma från en ökad stabilitet av hastighetsstråken närmast ytan. Mätningar på turbulenta gränsskikt med blåsning har genomförts för blåsningshastigheter mellan 0.1 och 0.37% av friströmshastigheten och täcker Reynoldstalområdet (10-36)x10^3, med Reynolds tal baserat på rörelsemängds-tjockleken. Vid blåsning genom ytan får man en stark modifiering av formen på hastighetesfördelningen genom gränsskiktet. När blåsningshastigheten ökar så kommer till slut den logaritmiska regionen av medelhastigheten, karaktäristisk för turbulent gränsskikt utan blåsning, att gradvis försvinna. God överens-stämmelse av medelhastighetsprofiler mellan turbulenta gränsskikt med och utan blåsning erhålls för alla Reynoldstal och blåsningshastigheter när profilerna normaliseras med Zagarola-Smits hastighetsskala. Blåsning vid väggen ökar intensiteten av hastighetsfluktuationerna, speciellt i den yttre regionen av gränsskiktet. Vid riktigt höga blåsningshastigheter och Reynoldstal så kommer den yttre toppen av hastighetsfluktuationer i gränsskiktet att överskrida den inre toppen, som i sig gradvis försvinner. / <p>QC 20171101</p>

This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli &amp; Gavarini (Flow, Turbul. &amp; Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson &amp; Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A  number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent  buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations. / QC 20100820