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Experimental Study on Turbulent Boundary-Layer Flows with Wall

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Experimental Study on Turbulent Boundary-Layer Flows with Wall Experimental study on turbulent boundary-layer flows with wall transpiration by Marco Ferro October 2017 Technical Reports Royal Institute of Technology Department of Mechanics SE-100 44 Stockholm, Sweden Akademisk avhandling som med tillst˚andav Kungliga Tekniska H¨ogskolan i Stockholm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie doktorsexamen fredag den 24 November 2017 kl 10:15 i Kollegiesalen, Kungliga Tekniska H¨ogskolan, Brinellv¨agen 8, Stockholm. TRITA-MEK 2017:13 ISSN 0348-467X ISRN KTH/MEK/TR-17/13-SE ISBN 978-91-7729-556-3 c Marco Ferro 2017 Universitetsservice US{AB, Stockholm 2017 Experimental study on turbulent boundary-layer flows with wall transpiration Marco Ferro Linn´eFLOW Centre, KTH Mechanics, Royal Institute of Technology SE-100 44 Stockholm, Sweden Abstract 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 be- haviour 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 consid- erable 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 > 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. iii 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 10 000 / Reθ / 36 000. 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. Key words: Turbulent boundary layer, boundary-layer suction, boundary-layer blowing, wall-bounded turbulent flows, self-sustained turbulence. iv Experimentell studie av turbulenta gr¨ansskikt med v¨aggenomstr¨omning Marco Ferro Linn´eFLOW Centre, KTH Mekanik, Kungliga Tekniska H¨ogskolan SE-100 44 Stockholm, Sverige Sammanfattning Genom att anv¨anda sig av genomstr¨ommande ytor, med sugning eller bl˚asning, kan man relativt enkelt och effektivt p˚averka ett gr¨ansskikts tillst˚and.Genom sin potential att p˚averka olika str¨omningsfysikaliska fenomen s˚asom att senarel¨agga b˚adeavl¨osning och omslaget fr˚anlamin¨ar till turbulent str¨omning (genom sugning) eller som att exempelvis minska luftmotst˚andet i turbulenta gr¨ansskikt och ge kyleffekt (genom bl˚asning),s˚ahar ett otaligt antal studier genomf¨orts p˚a omr˚adetde senaste decennierna. Trots detta s˚a ¨ar den grundl¨aggande f¨orst˚aelsen bristf¨allig f¨or de str¨omningsfenomen som intr¨affar i turbulenta gr¨ansskikt ¨over genomstr¨ommande ytor. Det r˚aderstora meningsskiljaktigheter om de mest element¨ara str¨omningskvantiteterna, s˚asommedelhastigheten, n¨ar sugning och bl˚asningtill¨ampas ¨aven i det mest f¨orenklade gr¨ansskiktsfallet n¨amligen det som utvecklar sig ¨over en plan platta utan tryckgradient. F¨or att ta fram nya experimentella data p˚agr¨ansskikt med sugning och bl˚asninggenom ytan s˚ahar vi designat en ny experimentell uppst¨allning samt tagit den i bruk. Den genomstr¨ommande ytan sp¨anner ¨over hela bredden av vindtunnelns m¨atstr¨acka (1:2 m) och ¨ar 6:5 m l˚angi str¨omningsriktningen och ¨ar d¨armed betydligt l¨angre ¨an vad som anv¨ants i tidigare studier. Detta g¨or det m¨ojligt att b¨attre utforska gr¨ansskiktet som utvecklas ¨over ytan i str¨omningsriktningen. Kvaliteten p˚aden experimentella uppst¨allningen och valda m¨atprocedurerna har verifierats genom omfattande tester, som ¨aven inkluderar benchmarking mot tidigare resultat p˚aturbulenta gr¨ansskikt utan tryckgradient eller bl˚asning/sugningoch p˚alamin¨ara asymptotiska sugningsgr¨ansskikt. De experimentella resultaten p˚aturbulenta gr¨ansskikt med sugning bekr¨aftar f¨or f¨orsta g˚angen att det ¨ar m¨ojligt att experimentellt s¨atta upp ett turbulent asymptotiskt sugningsgr¨ansskikt d¨ar gr¨ansskiktets medelhastighetsprofil blir oberoende av str¨omningsriktningen och d¨ar sugningshastigheten utg¨or den enda kontrollparametern. Det turbulenta asymptotiska sugningsgr¨ansskiktet visar sig ha en medelhastighetsprofil normalt mot ytan med en l˚anglogaritmisk region och utan f¨orekomsten av en yttre vakregion. Om man anv¨ander yttre skalning av medelhastigheten, med fristr¨omshastigheten och gr¨ansskiktstjockleken som karakt¨aristisk hastighet respektive l¨angdskala, s˚akan det logaritmiska omr˚adet beskrivas med en lutning p˚a Ao = 0:064 och ett korsande v¨arde med y-axeln p˚a Bo = 0:994, som ¨ar oberoende av sugningshastigheten. Om sugningshasigheten normaliserad med fristr¨omshastigheten ¨overskrider v¨ardet 3:70×10−3 s˚a˚aterg˚ar det ursprungligen turbulenta gr¨ansskiktet till att vara lamin¨art. Sugningen genom v¨aggen d¨ampar hastighetsfluktuationerna i gr¨ansskiktet med upp till v 50 − 60% vid direkt j¨amf¨orelse av det inre toppv¨ardet i ett turbulent gr¨ansskikt utan sugning och vid j¨amf¨orbart Reynolds tal. Denna minskning av turbulent aktivitet verkar h¨arstamma fr˚anen ¨okad stabilitet av hastighetsstr˚aken n¨armast ytan. M¨atningar p˚aturbulenta gr¨ansskikt med bl˚asninghar genomf¨orts f¨or bl˚asningshastighetermellan 0:1 och 0:37% av fristr¨omshastigheten och t¨acker Reynoldstalomr˚adet(10−36)×103, med Reynolds tal baserat p˚ar¨orelsem¨angds- tjockleken. Vid bl˚asninggenom ytan f˚arman en stark modifiering av formen p˚a hastighetesf¨ordelningen genom gr¨ansskiktet. N¨ar bl˚asningshastigheten ¨okar s˚a kommer till slut den logaritmiska regionen av medelhastigheten, karakt¨aristisk f¨or turbulent gr¨ansskikt utan bl˚asning,att gradvis f¨orsvinna. God ¨overens- st¨ammelse av medelhastighetsprofiler mellan turbulenta gr¨ansskikt med och utan bl˚asningerh˚allsf¨or alla Reynoldstal och bl˚asningshastighetern¨ar profil- erna normaliseras med Zagarola-Smits hastighetsskala. Bl˚asning vid v¨aggen ¨okar intensiteten av hastighetsfluktuationerna, speciellt i den yttre regionen av gr¨ansskiktet. Vid riktigt h¨oga bl˚asningshastigheteroch Reynoldstal s˚akommer den yttre toppen av hastighetsfluktuationer i gr¨ansskiktet att ¨overskrida den inre toppen, som i sig gradvis f¨orsvinner. Nyckelord: Turbulent gr¨ansskikt, gr¨ansskiktssugning, gr¨ansskiktsbl˚asning, v¨aggbundna turbulenta fl¨oden, sj¨alv-f¨ors¨orjande turbulens. vi Other publications The following paper, although related, is not included in this thesis. Marco Ferro, Robert S. Downs III & Jens H. M. Fransson, 2015. Stagnation line adjustment in flat-plate experiments via test-section venting. AIAA Journal 53 (4), pp. 1112{1116. Conferences Part of the work in this thesis has been presented at the following international conferences. The presenting author is underlined. Marco Ferro, Robert S. Downs III, Bengt E. G. Fallenius & Jens H. M. Fransson. On the development of turbulent boundary layer with wall suction. 68th Annual Meeting of the APS Division of Fluid Mechanics. Boston, 2015. Marco Ferro, Bengt E. G. Fallenius & Jens H. M. Fransson. On the turbulent boundary layer with wall suction. 7th iTi Conference in Turbulence. Bertinoro, 2016. DOI: 10.1007/978-3-319-57934-4 6. Marco Ferro, Bengt E. G. Fallenius & Jens H. M. Fransson. On the scaling of turbulent asymptotic suction boundary layers. 10th international symposium on Turbulence and Shear Flow Phenomena (TSFP10). Chicago, 2017. vii Contents Abstract iii Sammanfattning v Introduction 1 Chapter 1. Basic concepts and nomenclature 3 1.1.
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