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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.
    Bagheri, Shervin
    et al.
    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.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Research on the interaction between streamwise streaks and Tollmien-Schlichting waves at KTH2007In: ERCOFTAC Bulletin, ISSN 2518-0991, Vol. 74, p. 37-43Article in journal (Refereed)
    Abstract [en]

    This paper summarises the experimental and numericalinvestigations on how two different types of disturbancesmay, in a positive way, interact in a flat plateboundary-layer flow. The project, which mainly hasbeen centered at KTH1, has been performed in collaborationwith colleagues from University of Bologna2and LadHyX CNRS Ecole Polytechnique3, duringthe last years. The main phenomena — the stabilisingeffect of streamwise boundary-layer streakson Tollmien-Schlichting waves (and other exponentialdisturbances) — have been captured both in experiments[1, 2] and with different numerical approachessuch as direct numerical simulations [3], parabolicstability equation calculations [5] and large-eddy simulations[6]. We will here briefly review the methodsand the main results of these studies, and discuss howthey correlate with each other. For related referencesoutside KTH the interested reader is referred to thejournal publications in the reference list.

  • 3. Bailey, S. C. C.
    et al.
    Hultmark, M.
    Monty, J. P.
    Alfredsson, Per Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Chong, M. S.
    Duncan, R. D.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hutchins, N.
    Marusic, I.
    McKeon, B. J.
    Nagib, H. M.
    Örlü, Ramis
    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.
    Smits, A. J.
    Vinuesa, R.
    Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 715, p. 642-670Article in journal (Refereed)
    Abstract [en]

    This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from Reθ = 11 100 to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here.

  • 4. Camarri, S.
    et al.
    Fallenius, Bengt E. G.
    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.
    Stability and sensitivity analysis of experimental flow fields measured past a porous cylinder2011Conference paper (Other academic)
  • 5. Camarri, S.
    et al.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Talamelli, A.
    Numerical investigation of the AFRODITE transition control strategy2014In: Springer Proceedings in Physics, Springer, 2014, p. 65-69Conference paper (Refereed)
    Abstract [en]

    The experiments carried out within the AFRODITE[2] project are aimed at exploring the effectiveness of properly shaped velocity miniature vortex generators in delaying transition to turbulence in a boundary layer. The present work details the direct numerical simulation setup designed to support and reproduce the AFRODITE experiments and provide results showing that the proposed DNS is in good agreement with the experiments. The results of the DNS also show that even a minimal delay of the transition point results in an overall gain in terms of drag when MVGs are installed on the plate.

  • 6. Camarri, S.
    et al.
    Trip, Renzo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Investigation of passive control of the wake past a thick plate by stability and sensitivity analysis of experimental data2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 828, p. 753-778Article in journal (Refereed)
    Abstract [en]

    In this paper we propose a strategy, entirely relying on available experimental data, to estimate the effect of a small control rod on the frequency of vortex shedding in the wake past a thick perforated plate. The considered values of the flow Reynolds number range between Re similar or equal to 6.6 x 10(3) and Re = 5.3 x 10(4). By means of particle image velocimetry, an experimental database consisting of instantaneous flow fields is collected for different values of suction through the body surface. The strategy proposed here is based on classical stability and sensitivity analysis applied to mean flow fields and on the formulation of an original ad hoc model for the mean flow. The mean flow model is obtained by calibrating the closure of the Reynolds averaged Navier-Stokes equations on the basis of the available experimental data through an optimisation algorithm. As a result, it is shown that the predicted control map agrees reasonably well with the equivalent one measured experimentally. Moreover, it is shown that even when turbulence effects are neglected, the stability analysis applied to the mean flow fields provides a reasonable estimation of the vortex shedding frequency, confirming what is known in the literature and extending it up to Re = 5.3 x 10(4). It is also shown that, when turbulence is taken into account in the stability analysis using the same closure that is calibrated for the corresponding mean flow model, the prediction of the vortex shedding frequency is systematically improved.

  • 7. Camarri, Simone
    et al.
    Fallenius, Bengt E. G.
    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.
    Stability analysis of experimental flow fields behind a porous cylinder for the investigation of the large-scale wake vortices2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 715, p. 499-536Article in journal (Refereed)
    Abstract [en]

    When the linear stability analysis is applied to the time-averaged flow past a circular cylinder after the primary instability of the wake, a nearly marginally stable global mode is predicted with a frequency in time equal to that of the saturated vortex shedding. This behaviour has recently been shown to hold up to Reynolds number Re = 600 by direct numerical simulations. In the present work we verify that the global stability analysis provides reasonable estimation also when applied to experimental velocity fields measured in the wake past a porous circular cylinder at Re similar or equal to 3.5 x 10(3). Different intensities of continuous suction and blowing through the entire surface of the cylinder are considered. The global direct and adjoint stability modes, derived from the experimental data, are used to sort the random instantaneous snapshots of the velocity field in phase. The proposed method is remarkable, sorting the snapshots in phase with respect to the vortex shedding, allowing phase-averaged velocity fields to be extracted from the experimental database. The phase-averaged flow fields are analysed in order to study the effect of the transpiration on the kinematical characteristics of the large-scale wake vortices.

  • 8. Camarri, Simone
    et al.
    Fallenius, Bengt E. G.
    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.
    Stability analysis of experimental flow fields behind aporous cylinder for the investigation of the large-scale wake vorticesReport (Other academic)
  • 9. Camarri, Simone
    et al.
    Trip, Renzo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Investigation of passive control of the wake past a thick plate by stability and sensitivity analysis of experimental dataManuscript (preprint) (Other academic)
  • 10. Cossu, C.
    et al.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, A.
    Using non-normality for passive laminar flow control2008In: PROGRESS IN INDUSTRIAL MATHEMATICS AT ECMI 2006, 2008, p. 139-145Conference paper (Other academic)
  • 11.
    Downs, R. S. , I I I
    et al.
    KTH.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Martensson, H.
    Miniature vortex generators for flow control in falkner-skan boundary layers2017In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 55, no 2, p. 352-364Article in journal (Refereed)
    Abstract [en]

    Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress the growth of unsteady disturbances that would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works demonstrated the versatility of these miniature vortex generators in zero-pressure-gradient boundary layers. In this work, mean-flow disturbances developing from miniature vortex generators in adverse and favorable pressure-gradient boundary layers are measured systemically to assess the possibility of extending miniature vortex generator-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values, and an empirical scaling is found based on existing results. The effect of streaks on transition in an adverse pressure-gradient boundary layer is also tested, and moderate increases to laminar flow extents are observed.

  • 12.
    Downs, Robert S. , I I I
    et al.
    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.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mårtensson, H.
    Evaluation of miniature vortex generators for flow control in Falkner-Skan boundary layers2016In: 54th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics, 2016Conference paper (Refereed)
    Abstract [en]

    Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress growth of unsteady disturbances which would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works have demonstrated the versatility of these miniature vortex generators (MVGs) in zero pressure gradient boundary layers. In this work, mean-flow disturbances developing from MVGs in adverse and favorable pressure gradient boundary layers are measured systemically to assess the possibility of extending MVG-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values and an empirical scaling is found in congruence with existing results. The effect of streaks on transition in an adverse pressure gradient boundary layer is also tested and moderate increases to laminar flow extents are observed.

  • 13.
    Downs, Robert S., III
    et al.
    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.
    Tollmien-Schlichting wave growth over spanwise-periodic surface patterns2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 754, p. 39-74Article in journal (Refereed)
    Abstract [en]

    A novel type of surface roughness is deployed in a zero-pressure-gradient boundary layer with the goal of delaying the onset of laminar-to-turbulent transition for drag reduction purposes. This proof-of-concept experiment relies on forcing phase-triggered Tollmien-Schlichting (TS) waves across a range of initial amplitudes to produce amplified boundary-layer disturbances in a controlled and repeatable manner. Building on earlier work demonstrating attenuation of forced disturbances and delay of transition with spanwise arrays of discrete roughness and miniature vortex generators (MVGs), the present work seeks a roughness shape which might find success in a wider range of flows. Toward that end, streamwise-elongated humps are regularly spaced in the spanwise direction to form a wavy wall. By direct modulation of the mean flow, growth rates of the forced disturbances are increased or decreased, depending on the roughness configuration. Boundary-layer velocity measurements with hot-wire probes have been performed in a parametric study of the effects of roughness-field geometry and forcing amplitude on TS-wave growth and transition. The roughness field proves detrimental to passive flow control efforts in some configurations, while a reduction in the TS-wave amplitudes compared with the smooth-wall reference case is observed at other conditions. Substantial delays in the onset of transition are demonstrated when TS waves are forced with large amplitudes.

  • 14. Elvsén, Per Åke
    et al.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Sandberg, M.
    Vortical structures generated by a time varying jet flow in a ventilated enclosure2006Conference paper (Refereed)
  • 15.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sandberg, Mats
    Sattari, Amir
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental study on the effect of pulsating inflowto a closed volume2011Report (Other academic)
  • 16.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Trip, Renzo
    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.
    A new test-section for wind tunnel studies on wake instability and its control2009Report (Other academic)
  • 17.
    Fallenius, Bengt E. G.
    et al.
    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.
    Experimental investigation of the influence of inlet conditions to a bluff body wake2010Conference paper (Other academic)
    Abstract [en]

    Wind tunnel experiments have been performed in a bluff body wake with varying inlet conditions in order to enhance the physical understanding of the wake flow instability, which may lead to successful flow control and in turn reduced aerodynamic drag. The geometry consists of a rectangular-based forebody with permeable surfaces, an elliptic leading edge and a blunt trailing edge. Length, width and base height of the forebody is 2.3, 0.5 and 0.04 meters, respectively. Applying continuous suction or blowing, of different levels, through the permeable surfaces along the forebody, varies the wall-normal trailing edge velocity profile in a systematic way and hence the inlet condition to the wake. The streamwise velocity component has been measured both throughout the boundary layer and in the wake behind the body using hot-wire anemometry. High-speed stereo PIV has been used in the wake in order to collect statistics of vortical structures in the wake. The influence of boundary layer parameters on the wake flow characteristics, such as vortex shedding frequency and base pressure, will be presented.

  • 18.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental investigation of the wake instability behind a rectangular-based forebody2010Conference paper (Other academic)
  • 19.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experiments on a bluff body wake with varying inlet conditions2011Conference paper (Other academic)
  • 20.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sattari, A.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sandberg, Mats
    University of Gävle.
    Experimental study on the effect of pulsating inflow to an enclosure for improved mixing2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, p. 108-119Article in journal (Refereed)
    Abstract [en]

    Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ= 0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.

  • 21.
    Fallenius, Bengt
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics.
    On the vortex generation behind a passive V-shaped mixer in a pipe flow2008In: Proceedings of the 14th AIAA/CEAS Aeroacoustics Conference 2008, 2008Conference paper (Refereed)
  • 22.
    Fallenius, Bengt
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Vortex analysis in the near wake behind a porous cylinder subject to blowing or suction2008Conference paper (Refereed)
  • 23.
    Fallenius, Bengt
    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.
    Vortex analysis in the wake of a porous cylinder subject to continuous suction or blowing2008Report (Other academic)
  • 24.
    Fallenius, Bengt
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Renzo, T.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Measurements in a bluff body wake with variable inlet conditions2011Conference paper (Other academic)
  • 25.
    Ferro, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Downs, Robert S., III
    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.
    Stagnation Line Adjustment in Flat-Plate Experiments via Test-Section Venting2015In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 53, no 4, p. 1112-1116Article in journal (Refereed)
  • 26.
    Ferro, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fallenius, Bengt
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    On the scaling of turbulent asymptotic suction boundary layers2017In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, 2017, Vol. 2Conference paper (Refereed)
    Abstract [en]

    An analysis of turbulent suction boundary layers is carried out on the basis of new experimental data. The streamwise extent of the suction region of the present experimental apparatus is significantly longer than previous studies, allowing us to better investigate the development of boundary layers with wall suction. We show that it is possible to experimentally realize a turbulent asymptotic state where the boundary layer becomes independent of the streamwise direction and of the initial condition, so that the suction rate constitutes the only control parameter. Turbulent asymptotic suction boundary layers appear to be characterized by a mean velocity with a long logarithmic region, with a slope independent of the suction rate if outer scaling is adopted. In addition to the mean-velocity scaling of turbulent asymptotic suction boundary layers, the suction rate threshold for self-sustained turbulence is also investigated. 

  • 27.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Advanced Fluid Research On Drag reduction In Turbulence Experiments2011Conference paper (Other academic)
  • 28.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Advanced Fluid Research On Drag reduction In Turbulence Experiments: AFRODITE2011Conference paper (Other academic)
    Abstract [en]

    A hot topic in today's debate on global warming is drag reduction in aeronautics. The most beneficial 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 significant reduction of NOx and CO emissions in the atmosphere considering that the number of aircraft take offs, only in the EU, is over 19 million per year. In previous tuned wind tunnel measurements it has been shown that roughness elements can be used to sensibly delay transition to turbulence$\footnote{Fransson et al. 2006 {\emph{Phys. Rev. Lett.}} {\bf{96}}, 064501.}$. The result is revolutionary, since the common belief has been that surface roughness causes earlier transition and in turn increases the drag, and is a proof of concept of the passive control method per se. The beauty with a passive control technique is that no external energy has to be added to the flow system in order to perform the control, instead one uses the existing energy in the flow. Within the research programme AFRODITE, funded by ERC, we will take this passive control method to the next level by making it twofold, more persistent and more robust.

  • 29.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Aktiv grid för generering av friströmsturbulens1999Conference paper (Other academic)
  • 30.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Control of vortex shedding behind a cylinder by means of blowing and suction2003Conference paper (Refereed)
  • 31.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental analysis of transition delay by means of roughness elements2005Conference paper (Other academic)
  • 32.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental investigation of turbulent patch evolution in spatially steady boundary layers2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 149-150Conference paper (Refereed)
  • 33.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental study of turbulent spot evolution in the ASBL2008Conference paper (Refereed)
  • 34.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Flow control of boundary lagers and wakes2003Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Both experimental and theoretical studies have beenconsidered on flat plate boundary layers as well as on wakesbehind porous cylinders. The main thread in this work iscontrol, which is applied passively and actively on boundarylayers in order to inhibit or postpone transition toturbulence; and actively through the cylinder surface in orderto effect the wakecharacteristics.

    An experimental set-up for the generation of the asymptoticsuction boundary layer (ASBL) has been constructed. This studyis the first, ever, that report a boundary layer flow ofconstant boundary layer thickness over a distance of 2 metres.Experimental measurements in the evolution region, from theBlasius boundary layer (BBL) to the ASBL, as well as in theASBL are in excellent agreement with boundary layer analysis.The stability of the ASBL has experimentally been tested, bothto Tollmien-Schlichting waves as well as to free streamturbulence (FST), for relatively low Reynolds numbers (Re). For the former disturbances good agreement is foundfor the streamwise amplitude profiles and the phase velocitywhen compared with linear spatial stability theory. However,the energy decay factor predicted by theory is slightlyoverestimated compared to the experimental findings. The latterdisturbances are known to engender streamwise elongated regionsof high and low speeds of fluid, denoted streaks, in a BBL.This type of spanwise structures have been shown to appear inthe ASBL as well, with the same spanwise wavelength as in theBBL, despite the fact that the boundary layer thickness issubstantially reduced in the ASBL case. The spanwise wavenumberof the optimal perturbation in the ASBL has been calculated andis β = 0.53, when normalized with the displacementthickness. The spanwise scale of the streaks decreases withincreasing turbulence intensity (Tu) and approaches the scale given by optimalperturbation theory. This has been shown for the BBL case aswell.

    The initial energy growth of FST induced disturbances hasexperimentally been found to grow linearly as Tu2Rexin the BBL, the transitional Reynolds numberto vary as Tu-2, and the intermittency function to have a relativelywell-defined distribution, valid for all Tu.

    The wake behind a porous cylinder subject to continuoussuction or blowing has been studied, where amongst other thingsthe Strouhal number (St) has been shown to increase strongly with suction,namely, up to 50% for a suction rate of 2.5% of the free streamvelocity. In contrast, blowing shows a decrease ofStof around 25% for a blowing rate of 5% of the freestream velocity in the considered Reynolds number range.

    Keywords:Laminar-turbulent transition, asymptoticsuction boundary layer, free stream turbulence,Tollmien-Schlichting wave, stability, flow control, cylinderwake.

  • 35.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Free-stream turbulence and its influence on boundary-layer transition2017In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, International Symposium on Turbulence and Shear Flow Phenomena, TSFP10 , 2017, Vol. 2Conference paper (Refereed)
    Abstract [en]

    Free-stream turbulence (FST) gives, undoubtedly, rise to the most complicated boundary-layer transition-toturbulence scenario. The reason for the complexity is that the boundary layer thickness grows with the downstream distance at the same time as the turbulence intensity (Tu) of the FST decays and the FST characteristic length scales grow. The FST is present everywhere in the free stream, but changes characteristics with the downstream distance. This implies that the actual forcing by the FST on the boundary layer changes gradually, which makes it an intricate receptivity problem. Today, we cannot honestly say that we are capable to accurately predict the transition location subject to FST in the simplest boundary layer flow, namely the one developing over a flat plat under zero-pressure gradient condition. Based on a set of original experimental data, consisting of 42 unique FST conditions, we here report on a semiempirical transition prediction model, which takes into account both the integral length scale and the turbulence velocity fluctuation at the leading edge. We show that the Tu, used in all existing models, is not the leading variable. Instead, our data show that the necessary ingredients in a successful transition prediction model includes, firstly, a FST Reynolds number (Refst) as leading variable, secondly, an FST parameter being the integral length scale Reynolds number (ReL) which further accounts for the effect of different length scales and, thirdly, a scale-matching model between the FST and the boundary layer. However, the importance of Tu can still be realized, since it constitutes the quotient of the two Reynolds numbers, namely Tu = Refst=ReL, even though Tu does not explicitly appear in the model.

  • 36.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Friströmsturbulensens effekt på ett asymptotiskt sugningsgränsskikt2001Conference paper (Other academic)
  • 37.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Investigations of the asymptotic suction boundary layer2001Licentiate thesis, comprehensive summary (Other scientific)
  • 38.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Transition to Turbulence Delay Using a Passive Flow Control Strategy2015In: Procedia IUTAM, Elsevier, 2015, p. 385-393Conference paper (Refereed)
    Abstract [en]

    Since the mid-50s, until the first proof-of-concept experiment of transition delay using circular roughness elements in 2006, there was a strong consensus within the research community that roughness elements in general promoted the transition to turbulence process. From a series of wind tunnel experiments miniature vortex generators have emerged as being a coveted passive device for transition delay and hence skin-friction drag reduction. These devices are miniature with respect to classical vortex generators typically used for separation control and fall under the appealing category passive flow control device, since it uses the existing energy in the flow and hence no energy has to be added to the control system in order to accomplish the control. The underlying physical mechanism is attributed to an additional term in the perturbation energy equation, when the boundary layer is modulated in the direction orthogonal to the base flow by the control devices, which counteracts the wall-normal production term and, hence, stabilizes the flow. The present paper briefly reviews the current state-of-art of passive flow control using physical devices for transition delay with the implication of reducing skin-friction drag. This passive control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.

  • 39.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Turbulent spot evolution in spatially invariant boundary layers2010In: PHYSICAL REVIEW E, ISSN 1539-3755, Vol. 81, no 3Article in journal (Refereed)
    Abstract [en]

    A demanding task, for successful fluid dynamic design in many industrial applications, is being able to predict the transition to turbulence location in boundary layer flows. The focus of the present experimental study is on the late stage of transition scenarios where turbulent spots are borne. We report on a natural stabilizing mechanism on the growth rate of turbulent spots, which takes place in a specific bypass transition scenario, and show that there is a palpable history effect of the origin of the turbulent spot on the streamwise evolution. Furthermore, experimental evidence on Reynolds number effects on the spot evolution in boundary layers is put forward. This has been made possible by setting up an idealized experiment, which usually only is considered as a schoolbook example.

  • 40.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Turbulent spot evolution in the ASBL2008In: Bull. Am. Phys. Soc., 2008Conference paper (Other academic)
  • 41.
    Fransson, Jens H. M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P. H.
    Free stream turbulence induced disturbances in a uniform suction boundary layer2001Conference paper (Refereed)
  • 42.
    Fransson, Jens H. M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P. H.
    Free stream turbulence induced disturbances in a uniform suction boundary layer2000Conference paper (Other academic)
  • 43.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Talamelli, A.
    Cossu, C.
    Experimental study of the stabilization of Tollmien-Schlichting waves by finite amplitude streaks2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 5Article in journal (Refereed)
    Abstract [en]

    It has recently been found by using temporal and spatial numerical simulations that steady optimal streaks of moderate amplitude, i.e., sufficiently large but not exceeding the critical amplitude for the inflectional instability, are able to reduce the growth of Tollmien-Schlichting (TS) waves up to their complete suppression. This investigation aims at experimentally verifying this stabilizing effect by generating stable and symmetric, close to sinusoidal, streaks of moderate amplitudes (similar to 12% of the free-stream velocity) by means of a spanwise array of cylindrical roughness elements. The three-dimensional (3D) streaky base flow is then subjected to a secondary instability generated through a spanwise slot in the plate by means of regulated blowing and suction. In this study the stabilizing role of the streaks on TS waves is unambiguously confirmed and by increasing the height of the roughness elements, thus inducing larger amplitude streaks, we are also able to show that the stabilizing action on the TS waves increases with the streak amplitude. These results are the first to confirm the numerical predictions reported in earlier works. The full cross-stream plane has been measured at different downstream positions allowing a complete evaluation and comparison of the different amplitude measures used in previous experimental works. Furthermore, theoretical impulse response analysis and stability calculations are applied to the present experimental streaky base flow enabling a qualitative comparison of the 3D modulated TS wave distribution.

  • 44.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, A.
    Cossu, C.
    Transition delay by roughness elements2006Conference paper (Refereed)
  • 45.
    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.

  • 46.
    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.

  • 47.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Downs, Robert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Skin-friction drag reduction using the method of spanwise mean velocity gradient2015In: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, TSFP-9 , 2015Conference paper (Refereed)
    Abstract [en]

    Over the last decade wind tunnel experiments and numerical simulations have shown that steady spanwise mean velocity gradients are able to attenuate the growth of different types of boundary layer disturbances if introduced in a controlled way. In this paper some different techniques to setup the spanwise mean velocity variations are discussed and their stabilizing effect leading to transition delay are quantified. This control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.

  • 48.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wake flow manipulation by means of blowing and suction2008In: Proc. XXII ICTAM, 25-29 Aug., Adelaide, Australia, 2008, p. 128-Conference paper (Refereed)
  • 49.
    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.

  • 50.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hutchins, N.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Chong, M.
    Turbulence Measurements with hot-wires in high Reynolds number boundary layers2009Conference paper (Other academic)
    Abstract [en]

    During the last decade there has been a renewed interest in the scaling of turbulent boundary layers, especially with regard to the mean and fluctuation velocity distributions. Recently the ICET team carried out velocity measurements in three different wind tunnels (at KTH, Univ. Melbourne and IIT) for overlapping Reynolds numbers in the range 11,000<Reθ<70,000. The use of different facilities enables measurements at similar Reynolds numbers, but with different free stream velocities (due to different development length for the boundary layer in the different wind tunnels). A number of different hot-wire probes and anemometers were used. In addition, accurate and independent skin friction measurements using oil film interferometry have been made to determine the friction velocity (uτ), which is essential for accurate scaling of the data. The peak value of the near wall rms of the streamwise velocity was found to increase with Reynolds number, when scaled with uτ. On the other hand, the skewness and flatness of the streamwise velocity are found to exhibit similarity in the near wall region if measured with sufficiently small (in viscous units) hot-wire probes, indicating a similarity of the probability density distributions independent of Reynolds number. The measurements also provide time series that are used to evaluate the scaling of spectra and other time-domain quantities.

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