Change search
Refine search result
123 101 - 134 of 134
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 101.
    Segajini, A.
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dahlberg, J. -Å
    Gust structure and generation in canopy flows2011In: European Wind Energy Conference and Exhibition 2011: EWEC 2011, 2011, p. 217-221Conference paper (Refereed)
    Abstract [en]

    Due to an increased demand to place wind turbines in forest areas it is of importance to know the details of the wind flow field over forest canopies. In this study we compare the flow characteristics of an atmospheric boundary layer over a forest canopy and a wind tunnel canopy model. Both the mean velocity distribution and the turbulence intensities were measured for both cases and the friction velocity was determined from the Reynolds shear stress above the edge of the canopy. The comparison between the two data sets is reasonably good, especially close to the canopy edge. The analysis of the time series of both measurement sets has highlighted that the turbulent canopy flow (in both wind tunnel and atmospheric data sets) is composed, beside the high frequency turbulence, by some flow structures detectable only through phase averages approaches.

  • 102.
    Segalini, Antonio
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    An experimental analysis of canopy flows2011In: 13th European Turbulence Conference, ETC13: Geophysical and Magnetohydrodynamic Turbulence, Institute of Physics Publishing (IOPP), 2011, Vol. 318, article id 072018Conference paper (Refereed)
    Abstract [en]

    An analysis of forest canopy flows with a wind tunnel model at high Reynoldsnumber is presented and discussed. Measured mean velocity and Reynolds stress profiles agreewith observations made in real canopies with no sensible Reynolds number dependence, addingconfidence to the results obtained with the present setup. The analysis of power densityspectra of the three velocity components and of the shear stress co-spectra is reported withnew coordinate scalings able to improve the collapse of the spectra compared to standardnormalizations. This scaling is mostly based on the respective integral time scale and a simple fitis proposed to estimate such a quantity in real canopies. From the analysis of joint probabilitydensity functions, three different regions have been localized where a change in the coherentstructure behavior is supposed to take place, similarly to what happens in rough wall turbulentboundary layers.

  • 103.
    Segalini, Antonio
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Scaling Laws in Canopy Flows: A Wind-Tunnel Analysis2013In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 148, no 2, p. 269-283Article in journal (Refereed)
    Abstract [en]

    An analysis of velocity statistics and spectra measured above a wind-tunnel forest model is reported. Several measurement stations downstream of the forest edge have been investigated and it is observed that, while the mean velocity profile adjusts quickly to the new canopy boundary condition, the turbulence lags behind and shows a continuous penetration towards the free stream along the canopy model. The statistical profiles illustrate this growth and do not collapse when plotted as a function of the vertical coordinate. However, when the statistics are plotted as function of the local mean velocity (normalized with a characteristic velocity scale), they do collapse, independently of the streamwise position and freestream velocity. A new scaling for the spectra of all three velocity components is proposed based on the velocity variance and integral time scale. This normalization improves the collapse of the spectra compared to existing scalings adopted in atmospheric measurements, and allows the determination of a universal function that provides the velocity spectrum. Furthermore, a comparison of the proposed scaling laws for two different canopy densities is shown, demonstrating that the vertical velocity variance is the most sensible statistical quantity to the characteristics of the canopy roughness.

  • 104.
    Segalini, Antonio
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent Structures in Canopy Flows2014In: WIND ENERGY: IMPACT OF TURBULENCE, Springer Berlin/Heidelberg, 2014, p. 85-91Conference paper (Refereed)
    Abstract [en]

    A wind-tunnel experiment has been performed by means of a new forest canopy model. Hot-wire anemometry and Particle Image Velocimetry have been used to describe the flow from the statistical point of view together with a conditional sampling approach based on the Variable Interval Time Average method. The analysis demonstrated qualitative agreement between both measurement techniques, regardless of the canopy density that affects the structure intensity but not its topology.

  • 105.
    Segalini, Antonio
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Dahlberg, J.-Å.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Gust structure and generation in canopy flows2011In: Proceedings of Wake Conference, 2011Conference paper (Refereed)
  • 106.
    Shahinfar, Shahab
    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.
    Effect of free-stream turbulence characteristics on boundary layer transition2011Conference paper (Refereed)
    Abstract [en]

    The present measurement campaign on the free-stream turbulence inducedboundary layer transition scenario has provided a unique set of experimental data. This newset of data has the potential to enhance the understanding of the eect of the free-streamturbulence characteristic length scales on the transition location and not only the turbulenceintensity, which has been the focus in most previous studies. Recent investigations where theturbulence intensity has been kept essentially constant, while the integral length scale has beenchanged, show that the transition location is advanced for increasing length scale. The presentdata conrms previous results for low turbulence intensities, but shows the opposite behavior forhigh turbulence intensities, i.e. that the transition location is advanced for decreasing integrallength scales. Important to underline here is that the integral length scale has a relatively smallinuence on the transition location as compared to the turbulence intensity and data analysesare now directed towards enhanced understanding of how the dierent parts of the incomingenergy spectrum aects the energy growth inside the boundary layer.

  • 107.
    Shahinfar, Shahab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Free-stream turbulence boundary layer transition2011Conference paper (Other academic)
  • 108.
    Shahinfar, Shahab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Natural by-pass boundary layer transition2011Conference paper (Other academic)
    Abstract [en]

    The present measurement campaign on the free-stream turbulence induced boundary layer transition scenario has provided a unique set of experimental data, with potential to enhance the understanding of the effect of the free-stream turbulence characteristic length scales on the transition location and not only the turbulence intensity, which has been the focus in most previous studies. Recent investigations where the turbulence intensity has been kept essentially constant, while the integral length scale has been changed, show that the transition location is advanced for increasing length scale. However, the present data show that the integral length scale has a relatively small influence on the transition location as compared to the turbulence intensity and data analyses are now directed towards enhanced understanding of how the different parts of the incoming energy spectrum affects the energy growth inside the boundary layer.

  • 109.
    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.
    On transitional boundary layers caused by free-stream turbulenceManuscript (preprint) (Other academic)
    Abstract [en]

    The present measurement campaign on the free-stream turbulence (FST) induced boundary layer transition scenario has provided a unique set of experimental data, with potential to enhance the understanding of the effect of the free-stream turbulence characteristic length scales on the transition location and not only the turbulence intensity, which has been the focus in most previous studies. Recent investigations where the turbulence intensity has been kept essentially constant, while the integral length scale has been changed, show that the transition location is advanced for increasing length scale. The present data confirm this result for low Tu-levels, but show the opposite effect for higher Tu-level, i:e. a delay in the transition location for longer integral length scales

  • 110.
    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.

  • 111.
    Shahinfar, Shahab
    et al.
    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.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Passive boundary layer control of oblique distrubances by finite-amplitude streaks2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 749, p. 1-36Article in journal (Refereed)
    Abstract [en]

    Recent experimental results on the attenuation of two-dimensional Tollmien-Schlichting wave (TSW) disturbances by means of passive miniature vortex generators (MVGs) have shed new light on the possibility of delaying transition to turbulence and hence accomplishing skin-friction drag reduction. A recurrent concern has been whether this passive flow control strategy would work for other types of disturbances than plane TSWs in an experimental configuration where the incoming disturbance is allowed to fully interact with the MVG array. In the present experimental investigation we show that not only TSW disturbances are attenuated, but also three-dimensional single oblique wave (SOW) and pair of oblique waves (POW) disturbances are quenched in the presence of MVGs, and that transition delay can be obtained successfully. For the SOW disturbance an unusual interaction between the wave and the MVGs occurs, leading to a split of the wave with one part travelling with a 'mirrored' phase angle with respect to the spanwise direction on one side of the MVG centreline. This gives rise to 3-vortices on the centreline, which force a low-speed streak on the centreline, strong enough to overcome the high-speed streak generated by the MVGs themselves. Both these streaky boundary layers seem to act stabilizing on unsteady perturbations. The challenge in a passive control method making use of a non-modal type of disturbances to attenuate modal disturbances lies in generating stable streamwise streaks which do not themselves break down to turbulence.

  • 112.
    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.

  • 113. Siconolfi, L.
    et al.
    Camarri, S.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Boundary layer stabilization using free-stream vortice2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 764, p. R2-Article in journal (Refereed)
    Abstract [en]

    In this numerical investigation we explore the possibility of applying free-stream vortices as a passive flow control method for delaying the transition to turbulence. The work is motivated by previous experimental studies demonstrating that stable streamwise boundary layer (BL) streaks can attenuate both two-and three-dimensional disturbances inside the BL, leading to transition delay, with the implication of reducing skin-friction drag. To date, successful control has been obtained using physical BL modulators mounted on the surface in order to generate stable streaks. However, surface mounted BL modulators are doomed to failure when the BL is subject to free-stream turbulence (FST), since a destructive interaction between the two is inevitable. In order to tackle free-stream disturbances, such as FST, a smooth surface is desired, which has motivated us to seek new methods to induce streamwise streaks inside the BL. A first step, in a systematic order, is taken in the present paper to prove the control idea of generating free-stream vortices for the attenuation of ordinary Tollmien-Schlichting waves inside the BL. In this proof-of-concept study we show that, by applying a spanwise array of counter-rotating free-stream vortices, inducing streamwise BL streaks further downstream, it is possible to alter the BL stability characteristics to such a degree that transition delay may be accomplished. For the demonstration we use direct numerical simulations along with stability analysis.

  • 114. Siconolfi, L.
    et al.
    Camarri, S.
    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.
    Stability analysis of boundary layers controlled by miniature vortex generators2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 784, p. 596-618Article in journal (Refereed)
    Abstract [en]

    It is currently known that Tollmien-Schlichting (TS) waves can be attenuated by the introduction of spanwise mean velocity gradients in an otherwise two-dimensional boundary layer (BL). The stabilizing effect, associated with an extra turbulence production term, is strong enough to obtain a delay in transition to turbulence induced by TS waves, with the implication of reducing skin-friction drag. Miniature vortex generators (MVGs), mounted in an array, have successfully been used to obtain velocity modulations by the generation of alternating high-and low-speed streaks in the spanwise direction to control the BL. Experimentally, an initial amplification of the TS waves has been reported, which takes place in the near-wake region of the MVG array. The higher the streak amplitude, the stronger the downstream stabilizing effect becomes, but with the drawback of experiencing an even stronger initial amplification. This can lead to a sub-critical transitional Reynolds number, which would not only mean that the control has failed but, even worse, also lead to an advancement of the transition location. Here, direct numerical simulations and a local spatial stability analysis have been performed in order to reach a deeper understanding of this behaviour. The results agree well with experiments and we propose an explanation of the described behavior in terms of stability properties of the controlled BL. This important knowledge can be used in future designs of BL modulators, which can lead to improved stability of the control and to an extended region of laminar flow.

  • 115. Talamelli, A.
    et al.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Brandt, L.
    Cossu, C.
    Experimental analysis of teh evolution of steady disturbances in a flat plate boundary layer2003Conference paper (Refereed)
  • 116. Talamelli, A.
    et al.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Brandt, L.
    Cossu, C.
    Experimental analysis of the evolution of steady disturbances in a flat plate boundary layer2003Conference paper (Refereed)
  • 117.
    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.

  • 118.
    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)
  • 119.
    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)
  • 120. Talamelli, Alessandro
    et al.
    Persiani, Franco
    Fransson, Jens H. M.
    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.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    Rueedi, Jean-Daniel
    Sreenivasan, Katepalli R.
    Monkewitz, Peter A.
    CICLoPE-a response to the need for high Reynolds number experiments2009In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 41, no 2Article in journal (Refereed)
    Abstract [en]

    Although the equations governing turbulent flow of fluids are well known, understanding the overwhelming richness of flow phenomena, especially in high Reynolds number turbulent flows, remains one of the grand challenges in physics and engineering. High Reynolds number turbulence is ubiquitous in aerospace engineering, ground transportation systems, flow machinery, energy production (from gas turbines to wind and water turbines), as well as in nature, e.g. various processes occurring in the planetary boundary layer. High Reynolds number turbulence is not easily obtained in the laboratory, since in order to have good spatial resolution for measurements, the size of the facility itself has to be large. In this paper, we discuss limitations of various existing facilities and propose a new facility that will allow good spatial resolution even at high Reynolds number. The work is carried out in the framework of the Center for International Cooperation in Long Pipe Experiments (CICLoPE), an international collaboration that many in the turbulence community have shown an interest to participate in.

  • 121.
    Trip, Renzo
    et al.
    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.
    An experimental study on the combined effect of boundary layer modification and a splitter plate on the wake characteristicsManuscript (preprint) (Other academic)
  • 122.
    Trip, Renzo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    An experimental study on the relation between the wake inlet conditions and the near wake topology2016In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, p. 133-138Conference paper (Refereed)
    Abstract [en]

    A study on the relation between the wake inlet conditions and the wake characteristics of a bluff body by means of Particle Image Velocimetry is presented. The wake inlet condition, being a laminar boundary layer at the trailing edge of the body, are varied by means of wall-suction. Measurements are carried out at Reh = 6.7 × 103 based on the body thickness h. The induced radius of curvature of the streamlines is shown to be a promising parameter in explaining the increase in base drag and decrease in vortex shedding frequency associated with a thinner boundary layer.

  • 123.
    Trip, Renzo
    et al.
    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.
    An experimental study on the wake behind a rectangular forebody with a laminar and turbulent suction boundary layerManuscript (preprint) (Other academic)
  • 124.
    Trip, Renzo
    et al.
    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.
    An experimental study on the wake of a rectangular forebody with suction applied over the surfacesManuscript (preprint) (Other academic)
  • 125.
    Trip, Renzo
    et al.
    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.
    On the effect of boundary layer modification on the near wake topology of a rectangular forebodyManuscript (preprint) (Other academic)
  • 126.
    Trip, Renzo
    et al.
    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.
    Bluff body boundary-layer modification and its effect on the near-wake topology2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 9, article id 095105Article in journal (Refereed)
  • 127.
    Trip, Renzo
    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.
    Boundary layer modification by means of wall suction and the effect on the wake behind a rectangular forebody2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 12, p. 125105-Article in journal (Refereed)
    Abstract [en]

    The wake characteristics of a two-dimensional rectangular forebody with a smooth leading edge and a blunt trailing edge are investigated. Wall suction is applied along the forebody in order to modify the developing boundary layer. An initially laminar boundary layer subject to suction yields an asymptotic suction boundary layer at the trailing edge of the body, whereas a high enough suction coefficient relaminarizes an initially turbulent boundary layer. The critical suction velocity required to achieve this significant modification of the boundary layer properties is typically in the order of 1% of the free-stream velocity, where the critical suction coefficient depends on the Reynolds number. We show that a thinner boundary layer induces a higher vortex shedding frequency and a lower base pressure. Furthermore, the boundary layer state, laminar or turbulent, has a significant influence on the wake. For example, the Strouhal number based on the effective body thickness is being reduced by 25% from laminar to turbulent inlet conditions.

  • 128. Tsuji, Y.
    et al.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Shear effect on pressure and particle acceleration in high-Reynolds number turbulence2008In: IUTAM SYMPOSIUM ON COMPUTATIONAL PHYSICS AND NEW PERSPECTIVES IN TURBULENCE, 2008, p. 11-14Conference paper (Refereed)
    Abstract [en]

    Pressure fluctuations are an important ingredient in turbulence, however the variation of the pressure fluctuations inside a turbulent boundary layer is not well known and its generation not fully understood. One reason for this is the difficulty inherent in measuring the fluctuating pressure and, consequently, a lack of published experimental results of this quantity. In the present study, the new methods and improved measurement techniques are developed. Fluctuating pressure has been measured in turbulent boundary layer up to Rθ ≃ 20000 and in mixing layer up to Rλ ≃ O(10 3). In this report, the shear effect on the Kolmogorov’s -7/3 powerlaw scaling of pressure spectrum is discussed and the preliminary measurement of particle acceleration is reported.

  • 129. Tsuji, Y.
    et al.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Pressure statistics and their scaling in high-Reynolds-number turbulent boundary layers2007In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 585, p. 1-40Article in journal (Refereed)
    Abstract [en]

    Pressure fluctuations are an important ingredient in turbulence, e.g. in the pressure strain terms which redistribute turbulence among the different fluctuating velocity components. The variation of the pressure fluctuations inside a turbulent boundary layer has hitherto been out of reach of experimental determination. The mechanisms of non-local pressure-related coupling between the different regions of the boundary layer have therefore remained poorly understood. One reason for this is the difficulty inherent in measuring the fluctuating pressure. We have developed a new technique to measure pressure fluctuations. In the present study, both mean and fluctuating pressure, wall pressure, and streamwise velocity have been measured simultaneously in turbulent boundary layers up to Reynolds numbers based on the momentum thickness R-theta similar or equal to 20 000. Results on mean and fluctuation distributions, spectra, Reynolds number dependence, and correlation functions are reported. Also, an attempt is made to test, for the first time, the existence of Kolmogorov's -7/3 power-law scaling of the pressure spectrum in the limit of high Reynolds numbers in a turbulent boundary layer.

  • 130.
    Tsuji, Yoshiyuki
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Nagoya University, Japan .
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Pressure statistics in high-Reynolds number turbulent boundary layer2005In: 4th International Symposium on Turbulence and Shear Flow Phenomena, 2005, p. 27-32Conference paper (Refereed)
    Abstract [en]

    Static pressure fluctuations are an important ingredient in turbulence, e.g. in the pressure strain terms which redistribute turbulence among the different fluctuating velocity components. In the present study, both mean and fluctuating static pressure and wall pressure have been measured simultaneously in turbulent boundary layers. The pressure inside the boundary layer is measured with a specially designed probe. Results on mean and fluctuation distributions, spectra, Reynolds number dependence, and correlation functions are reported.

  • 131. Yoshioka, S.
    et al.
    Fransson, Jens H. M.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P. H.
    Evolution of disturbances in boundary layers with wall suction2003Conference paper (Refereed)
  • 132. Yoshioka, S
    et al.
    Fransson, Jens H.M.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P. Henrik
    KTH, Superseded Departments, Mechanics.
    Free stream turbulence induced disturbances in boundary layers with wall suction2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, p. 3530-3539Article in journal (Refereed)
    Abstract [en]

    An experimental investigation of free stream turbulence (FST) induced disturbances in asymptotic suction boundary layers (ASBL) has been performed. In the present study four different suction rates are used and the highest is 0.40% of the free stream velocity, together with three different FST levels (Tu=1.6, 2.0, and 2.3%). A turbulence generating grid of the active type is used and offers the possibility to vary the Tu-level while the scales of the turbulence remain almost constant. It is known that FST induces elongated disturbances consisting of high and low velocity regions, usually denoted streaky structures, into the boundary layer. The experiments show that wall suction suppresses the disturbance growth and may significantly delay or inhibit the break-down to turbulence. Two-point correlation measurements in the spanwise direction show that the averaged streak spacing decreases with increasing FST-level, whereas the spanwise scale in the ASBL is more or less constant if scaled with the free stream velocity and viscosity. This is in contrast to what is observed in a Blasius boundary layer where streaks develop and adapt their spanwise scale close to the boundary layer thickness.

  • 133.
    Örlu, Ramis
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On near wall measurements of wall bounded flows-The necessity of an accurate determination of the wall position2010In: Progress in Aerospace Sciences, ISSN 0376-0421, E-ISSN 1873-1724, Vol. 46, no 8, p. 353-387Article, review/survey (Refereed)
    Abstract [en]

    The present review paper is an account on the experimental determination of the wall position relative to the probe in wall-bounded turbulent flow studies. A thorough review on common measurement techniques as well as correction methods reveals, that there are a number of pitfalls, that-when not accounted for-can lead to wrong conclusions about the wall position and thereby also on the near-wall behaviour of mean and turbulence quantities. Employing the state-of-the-art databases from direct numerical simulations of wall-bounded turbulent flows various indirect methods have been tested and assessed in terms of their robustness and accuracy. It is also demonstrated that accurate measurements reaching the viscous sublayer are necessary in order to ensure a correctly deduced wall position, and dependent quantities as for instance the near-wall scaling of mean (e.g. Reynolds number dependence of the buffer region or the log law constants) and turbulence (e.g. the near-wall peak location of Reynolds stresses) quantities. In experiments using hot-wires near the wall it is well known that heat conduction between the hot-wire and the wall gives errors and mean velocity data from the viscous sublayer can usually not be used to determine the wall position. In this paper we introduce a new method which takes advantage of the similarity of the probability density distributions (pdf) in the near wall region. By using the high velocity data of the pdf, which is shown not to be affected by heat conduction, the heat conduction problem can be circumvented. Extensive appendices are included, describing the history and present knowledge about the scaling of the mean velocity in the near wall and overlap regions in wall bounded turbulent flows.

  • 134.
    Örlü, Ramis
    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. 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.
    On imperfect hot-wire resolution issues and their effect on mean quantities2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 605-608Conference paper (Refereed)
123 101 - 134 of 134
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf