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  • 1.
    Alfredsson, P. Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kurian, Thomas
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, A.
    Rüedi, Jean-Daniel
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The diagnostic plot: a new way to appraise turbulent boundary-layer data2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 609-612Conference paper (Refereed)
  • 2.
    Fransson, Jens H. M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Talamelli, Alessandro
    KTH, Superseded Departments, Mechanics.
    Cossu, C.
    Experimental and theoretical investigation of the nonmodal growth of steady streaks in a flat plate boundary layer2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, p. 3627-3638Article in journal (Refereed)
    Abstract [en]

    An experimental and theoretical investigation aimed at describing the nonmodal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable nonmodal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale.

  • 3.
    Fransson, Jens H M
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, Carlo
    Experimental study of the stabilization of Tollmien-Schlichting waves by finite amplitude streaks2006In: Sixth IUTAM Symposium on Laminar-Turbulent Transition / [ed] Govindarajan, R, DORDRECHT: SPRINGER , 2006, Vol. 78, p. 299-304Conference paper (Refereed)
    Abstract [en]

    The stabilization of Tollmien-Schlichting (TS) waves in a spanwise modulated Blasius boundary layer has been experimentally verified in the MTL wind tunnel at KTH. The alternating high and low speed streaks were created by regularly spaced cylindrical roughness elements mounted on the flat plate. It is shown that the larger the streak amplitude the larger is the damping effect of the TS-waves, which is in agreement with recent theoretical work.

  • 4.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Shahinfar, Shahab
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Advanced Fluid Research On Drag reduction In Turbulence Experiments2011Conference paper (Refereed)
    Abstract [en]

    A hot topic in today's debate on global warming is drag reduction in aeronautics. The most bene cial concept for drag reduction is to maintain the major portion of the airfoil laminar. Estimations show that the potential drag reduction can be as much as 15%, which would give a signi cant reduction of NOx and CO emissions in the atmosphere considering that the number of aircraft take os, only in the EU, is over 19 million per year. An important element for successful ow control, which can lead to a reduced aerodynamic drag, is enhanced physical understanding of the transition to turbulence process.

  • 5.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Base flow modulations for skin-friction drag reduction2011In: Journal of Physics: Conference Series, 2011Conference paper (Refereed)
    Abstract [en]

    Recent experimental investigations have shown that spanwise modulations of thebase flow may delay transition to turbulence. In this paper we explore the possibility togenerate streaks of much larger amplitude than previously reported by using a row of miniaturevortex generators (MVGs). Here, we present the first boundary layer experiment where streakamplitudes exceeding 30% have been produced without having any secondary instability actingon them. Furthermore, the induced drag due to the streaky base flow is quantified and it isdemonstrated that the streaks can be reinforced by placing a second array of MVGs downstreamof the first one. In this way it is possible to make the control more persistent in the downstreamdirection. We conclude that the specially designed set of MVGs, as a boundary layer modulator,is a promising candidate for successfully setting up robust and persistent streamwise streaks,which is a prerequisite for a successful flow control.

  • 6.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, C.
    Transition to turbulence delay by means of a passive mechanism2009In: Proceedings of CEAS/KATNET II Conference on Key Aerodynamic Technology, 2009Conference paper (Refereed)
  • 7.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, Carlo
    LadHyX, CNRS Ecole Polytechnique.
    Delaying transition to turbulence by a passive mechanism2006In: EFMC6, The 6th Euromech Fluid Mechanics Conference, 2006, p. 360-Conference paper (Refereed)
  • 8.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI).
    Talamelli, Alessandro
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, Carlo
    Transition delay by means of a passive mechanism2005Conference paper (Other academic)
  • 9.
    Fransson, Jens H.M
    et al.
    KTH, Superseded Departments, Mechanics.
    Brandt, L
    Talamelli, Alessandro
    Cossu, Carlo
    On streamwise streaks generated by roughness elements in the boundary layeron a flat plate.2003In: 17th AIMETA Congress of Theoreticaland Applied Mechanics, 2003Conference paper (Other academic)
  • 10.
    Fransson, Jens
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the generation of steady streamwise streaks in flat-plate boundary layers2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 698, p. 211-234Article in journal (Refereed)
    Abstract [en]

    A study on the generation and development of high-amplitude steady streamwise streaks in a flat-plate boundary layer is presented. High-amplitude streamwise streaks are naturally present in many bypass transition scenarios, where they play a fundamental role in the breakdown to turbulence process. On the other hand, recent experiments and numerical simulations have shown that stable laminar streamwise streaks of alternating low and high speed are also capable of stabilizing the growth of Tollmien-Schlichting waves as well as localized disturbances and to delay transition. The larger the streak amplitude is, for a prescribed spanwise periodicity of the streaks, the stronger is the stabilizing mechanism. Previous experiments have shown that streaks of amplitudes up to 12 % of the free stream velocity can be generated by means of cylindrical roughness elements. Here we explore the possibility of generating streaks of much larger amplitude by using a row of miniature vortex generators (MVGs) similar to those used in the past to delay or even prevent boundary layer separation. In particular, we present a boundary layer experiment where streak amplitudes exceeding 30 % have been produced without having any secondary instability acting on them. Furthermore, the associated drag with the streaky base flow is quantified, and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one. In this way it is possible to make the control more persistent in the downstream direction. It must be pointed out that the use of MVGs opens also the possibility to set up a control method that acts twofold in the sense that both transition and separation are delayed or even prevented.

  • 11.
    S. Sattarzadeh, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Consecutive turbulence transition delay with reinforced passive control2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, no 6, p. 061001-Article in journal (Refereed)
    Abstract [en]

    Miniature vortex generators (MVGs) are able to delay the transition to turbulence in a flat plate boundary layer if properly designed. Unfortunately, the natural recovery of the modulated laminar base flow in the streamwise direction is of exponential space scale and hence the passive laminar control fades away fairly rapidly. Here we show that by placing a second array of MVGs downstream of the first one it is possible to nourish the counter-rotating streamwise vortices responsible for the modulation, which results in a prolonged streamwise extent of the control. With this control strategy it is possible to delay the transition to turbulence, consecutively, by reinforcing the control effect and with the ultimate implication of obtaining a net skin-friction drag reduction of at least 65%.

  • 12.
    Sattarzadeh, Sohrab Shirvan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Shahinfar, Shahab
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fallenius, Bengt
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Transition delay by means by means of base flow modulations2011Conference paper (Other academic)
  • 13.
    Sattarzadeh, Sohrab Shirvan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Shahinfar, Shahab
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fallenius, Bengt
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Transition delay by means of base flow modulations2011Conference paper (Other academic)
    Abstract [en]

    Recent experimental investigations have shown that spanwise modulations of the base flow may delay transition to turbulence.\footnote{Fransson et al. 2006 {\emph{Phys. Rev. Lett.}} {\bf{96}}, 064501.} In this study we explore the possibility to generate streaks of much larger amplitude than previously reported by using a row of miniature vortex generators (MVGs). Here, we present the first boundary layer experiment where streak amplitudes exceeding 30\% have been produced without having any secondary instability acting on them. Furthermore, the induced skin-friction drag due to the streaky base flow is quantified and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one. In this way it is possible to make the control more persistent in the downstream direction. We conclude that the specially designed set of MVGs, as a boundary layer modulator, is a promising candidate for successfully setting up robust and persistent streamwise streaks, which is a prerequisite for a successful flow control. This work is carried out within the AFRODITE programme funded by ERC.

  • 14. Schreyer, A. -M
    et al.
    Würz, W.
    Krämer, E.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental flow studies on separation and reattachment in the vicinity of sharp,wedge shaped leading edges at low Reynolds numbers2010In: New Results In Numerical And Experimental Fluid Mechanics VII, Springer Berlin/Heidelberg, 2010, p. 273-280Conference paper (Refereed)
    Abstract [en]

    The flow field around sharp leading edges in subsonic, low Reynolds number flow is investigated experimentally. Separation occurs already at small angles of attack. Under certain conditions reattachment takes place and a separation bubble develops. As this influences the lift and drag of an airfoil decisively, the conditions leading to reattachment and the formation of a laminar boundary layer are of special interest. The influences of the Reynolds number, angle of attack and leading edge apex angle on the boundary layer properties are studied by means of hot-wire anemometry, flow visualisation techniques and measurements of the pressure distribution over the leading edge. For a small parameter range, laminar separation followed by laminar reattachment is observed.

  • 15.
    Segalini, Antonio
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Preliminary studies on acoustic excitation in axisymmetric transitional jet flowsManuscript (preprint) (Other academic)
  • 16.
    Shahinfar, Shahab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    S. Sattarzadeh, Sohrab
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Scaling of streamwise boundary layer streaks and their ability to reduce skin-friction drag2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 733, p. 1-32Article in journal (Refereed)
    Abstract [en]

    Spanwise arrays of miniature vortex generators (MVGs)are used to generate energetic transient disturbance growth, which is able to modulate the boundary layer flow with steady and stable streak amplitudes up to 32% of the free-stream velocity. This type of modulation has previously been shown to act in a stabilizing manner on modal disturbance growth described by classical instability theory. In anattempt to reproduce a more realistic flow configuration, in the present experimental setup, Tollmien-Schlichting (TS) waves are generated upstream of the MVG array, allowingfor a complete interaction of the incoming wave with the array. Fifteen new MVG configurations are investigated and the stabilizing effect on the TS waves is quantified. We show that the streak amplitude definition is very importantwhen trying to relate it to the stabilization, since it may completely bypass information on the mean streamwise velocity gradient in the spanwise direction, which is an essential ingredient of the observed stabilization. Here, we use an integral-based streak amplitude definition along with a streak amplitude scaling relation based on empiricism,which takes the spanwise periodicity of the streaks into account. The results show that, applying the integral definition, the optimal streak amplitude for attenuating TS wave disturbance growth is around 30% of the free-stream velocity, which corresponds to ̃20% in the conventional definition when keeping the spanwise wavelength constant. The experiments also show that the disturbance energy level, based on the full velocity signal, is significantly reduced in the controlled case, and that the onset of transition may be inhibited altogether throughout the measured region in the presence of an MVG array.

  • 17.
    Shahinfar, Shahab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Revival of Classical Vortex Generators Now for Transition Delay2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 7, p. 074501-Article in journal (Refereed)
    Abstract [en]

    Classical vortex generators, known for their efficiency in delaying or even inhibiting boundary layer separation, are here shown to be coveted devices for transition to turbulence delay. The present devices are miniature with respect to classical vortex generators but are tremendously powerful in modulating the laminar boundary layer in the direction orthogonal to the base flow and parallel to the surface. The modulation generates an additional term in the perturbation energy equation, which counteracts the wall-normal production term and, hence, stabilizes the flow. Our experimental results show that these devices are really effective in delaying transition, but we also reveal their Achilles' heel.

  • 18.
    Talamelli, A.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malizia, F.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Cimarelli, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Temperature effects in hot-wire measurements on higher-order moments in wall turbulence2016In: Springer Proceedings in Physics, Springer, 2016, p. 185-189Conference paper (Refereed)
    Abstract [en]

    The effect of temperature fluctuations—as they are e.g. encountered in non-isothermal flows—on the mean, variance and spectra in wall-bounded turbulent flows when utilising hot-wire anemometry has recently been documented. The present work extends these efforts on the effect of temperature fluctuations on the skewness and flatness factors as well as assesses the performance of commonly employed temperature compensations.

  • 19.
    Talamelli, Alessandro
    et al.
    Università di Bologna, Italy .
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    High amplitude steady streaks in flat plate boundary layers2010In: 40th AIAA Fluid Dynamics Conference, American Institute of Aeronautics and Astronautics, 2010, p. 2010-4291-Conference paper (Refereed)
    Abstract [en]

    This experimental activity is part of a large research program aimed at developing a passive control method for viscous drag reduction of aerodynamic bodies. The drag reduction, accomplished by delaying the laminar-turbulent transition of the boundary layer, is obtained by generating stable laminar streamwise streaks inside the boundary layer, which are known to stabilize the growth of Tollmien-Schlichting waves as well as localized disturbances. A first series of experiments have shown that by means of suitable roughness elements it is possible to generate stable steady streaks of amplitudes up to 12% of the free stream velocity. In this paper we explore the possibility to generate streaks of much larger amplitude by using a row of Miniature Vortex Generators (MVGs). Experiments have been performed showing that amplitudes up to 30% can be obtained without having any secondary instability acting on the streaks. The induced drag due to the streaky base flow is quantified and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one.

  • 20.
    Talamelli, Alessandro
    et al.
    II Facoltà di Ingegneria, Università di Bologna.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    High amplitude streaks in boundary layers: a new passive mechanism for transition delay2011Conference paper (Refereed)
  • 21.
    Talamelli, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, Carlo
    Stabilization of Tollmien-Schlichting waves by means of roughness generated streaks2005Conference paper (Refereed)
  • 22.
    Talamelli, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Buresti, Guido
    A note on the effect of the separation wall in the initial mixing of coaxial jets2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 3, p. 1483-Article in journal (Refereed)
    Abstract [en]

    Experiments are carried out to characterize the mixing field of two coaxial jet configurations, having, respectively, a thick and a sharp inner duct wall. The influence of the separation wall thickness on the initial development of the "mixing transition'' of the two streams is analysed as a function of the jet velocity ratio through measurements with hot-and cold-wire anemometry and by using temperature as a passive scalar. To study the mixing at the near-exit region, a new thickness measure based on the mean scalar concentration is introduced. It is shown that the presence of a sufficiently thick wall significantly increases the interpenetration between the two streams, effectively enhancing the mixing process. However, this is observed only for nearly unitary velocity ratios, which correspond to the existence of a regular alternate vortex shedding from the two sides of the inner duct wall. Conversely, for small and large velocity ratios, the difference in mixing between the two geometries greatly decreases and becomes of the order of the experimental accuracy.

  • 23.
    Talamelli, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Correcting hot-wire spatial resolution effects in third- and fourth-order velocity moments in wall-bounded turbulence2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 4, p. 1496-Article in journal (Refereed)
    Abstract [en]

    Spatial averaging, resulting from the finite size of a hot-wire probe, significantly affects the accuracy of velocity measurements in turbulent flows close to walls. Here, we extend the theoretical model, introduced in Segalini et al. (Meas Sci Technol 22: 104508, 2011) quantifying the effect of a linear spatial filter of hot-wire probes on the mean and the variance of the streamwise velocity in turbulent wall-bounded flows, to describe the effect of the spatial filtering on the third-and fourth-order moments of the same velocity component. The model, based on the three-(four) point velocity-correlation function for the third-(fourth-) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse microscale for the second-order moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. The model shows good accuracy except for the third-order moment in the region where a zero-crossing of the third-order function is observed and where the equations become ill-conditioned. An "a posteriori" correction procedure, based on the developed model, to correct the measured third-and fourth-order velocity moments is also presented. This procedure, based on combining the measured data by two single hot-wire sensors with different wire lengths, is a natural extension of the one introduced by Segalini et al. (Exp Fluids 51:693-700, 2011) to evaluate both the turbulence intensity and the transverse Taylor microscale in turbulent flows. The technique is validated against spatially averaged simulation data showing a good capacity to correct the actual profiles over the entire height of the boundary layer except, as expected, for the third-order moment in the region where the latter exhibits a zero-crossing. Moreover, the proposed method has been tested on experimental data from turbulent pipe flow experiments.

  • 24.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fiorini, T.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Bellani, G.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Università di Bologna, Italy.
    Alfredsson, P Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Università di Bologna, Italy.
    Reynolds stress scaling in pipe flow turbulence-first results from CICLoPE2017In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, no 2089, article id 20160187Article in journal (Refereed)
    Abstract [en]

    This paper reports the first turbulence measurements performed in the Long Pipe Facility at the Center for International Cooperation in Long Pipe Experiments (CICLoPE). In particular, the Reynolds stress components obtained from a number of straight and boundary-layer-type single-wire and X-wire probes up to a friction Reynolds number of 3.8 x 10(4) are reported. In agreement with turbulent boundary-layer experiments as well as with results from the Superpipe, the present measurements show a clear logarithmic region in the streamwise variance profile, with a Townsend-Perry constant of A(2) approximate to 1.26. The wall-normal variance profile exhibits a Reynolds-number-independent plateau, while the spanwise component was found to obey a logarithmic scaling over a much wider wall-normal distance than the other two components, with a slope that is nearly half of that of the Townsend-Perry constant, i.e. A(2,w) approximate to A(2)/2. The present results therefore provide strong support for the scaling of the Reynolds stress tensor based on the attached-eddy hypothesis. Intriguingly, the wall-normal and spanwise components exhibit higher amplitudes than in previous studies, and therefore call for follow-up studies in CICLoPE, as well as other large-scale facilities. This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.

  • 25.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malizia, Fabio
    Cimarelli, Andrea
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Univ Bologna, Italy.
    The influence of temperature fluctuations on hot-wire measurements in wall-bounded turbulence2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 7, p. 1781-Article in journal (Refereed)
    Abstract [en]

    There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations-as they are, e. g. encountered in non-isothermal flows-on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces-despite a local mean temperature compensation of the velocity reading-significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction.

  • 26.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the passive control of the near-field of coaxial jets by means of vortex shedding2008In: Int. Conf. on Jets, Wakes and Separated Flows, ICJWSF-2, September 16-19, 2008, Technical University of Berlin, Germany, 2008, p. 1-16Conference paper (Refereed)
  • 27.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Passive control of mixing in a coaxial jet2008In: Proc. 7th Int. ERCOFTAC Symp. on Engineering Turbulence Modelling and Measurements (ETMM7), 2008, p. 450-455Conference paper (Refereed)
    Abstract [en]

    An experimental investigation regarding interacting shear layers in a coaxial jet geometry has been performed. The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (2006), who proposed that the wake behind the separation wall between the two stream of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, may provide 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 has been demonstrated.

  • 28.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulence Enhancement in Coaxial Jet Flows by Means of Vortex Shedding2009In: PROGRESS IN TURBULENCE III / [ed] Peinke, J.; Oberlack, M.; Talamelli, A., 2009, Vol. 131, p. 235-238Conference paper (Refereed)
    Abstract [en]

    Over the past decades a variety of passive and active flow control mechanisms have been tested and applied in a variety of canonical as well as applied flow cases. An example for the latter is the coaxial jet flow, which has mainly been investigated regarding the receptivity to active flow control strategies (see e.g. [1]), probably due to the multitude of parameters characterising the complex flow field [2]. Physical and numerical experiments (see e.g. [3] and [5]) have established that the vortical motion in coaxial jet flows is dominated by the vortices emerging from the outer shear layer. The frequency of these vortices is related to the Kelvin- Helmholtz instability as predicted by linear stability analysis for single jets. The vortices in the inner shear layer, on the other hand, are trapped in the spaces left free between two consecutive outer shear layer vortices, and are therefore sharing the frequencies of the most amplified modes of the outer shear layer and do not relate to the values one would expect from linear stability analysis. This fact has become known as the “locking phenomenon”, which describes the mutual interaction of both shear layers. Nevertheless it is believed that only the outer shear layer is able to significantly control the evolution of the inner shear layer [7], which may explain the focus of control strategies on the outer shear layer.

1 - 28 of 28
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