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  • 51.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mamidala, Santhosh B.
    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.
    Mårtensson, H.
    Wallin, F.
    An experimental setup for idealised studies on transition to turbulence on a generic compressor outlet guide vane2018In: Proceedings of the ASME Turbo Expo, ASME Press, 2018, Vol. Volume 2A-2018, 2018Conference paper (Refereed)
    Abstract [en]

    The understanding of flow phenomena in turbomachinery has come far with respect to three-dimensional flow patterns and pressure distributions. Much is due to improved measurements and a continuously evolving fidelity in computational fluid dynamics (CFD). Turbulence and transition in boundary layers are two classical areas where improvements in modeling are desired and where experimental validation is required. Apart from this, fundamental improvements in efficiency can be obtained by developing experimental resources where technologies affecting transition can be studied. The reduction in friction drag can be considerable if the transition to turbulence can be delayed. An experimental setup in an idealized configuration has been designed and built with the objective to study transition on a very large-scale guide vane profile at low speed. The purpose of the rig is to enable high quality fundamental studies of technologies to delay transition, but also to see how effects of manufacturing or other constraints may affect the boundary layer. In the present paper we report the first validation of the experimental setup, by comparing the first test results to CFD calculations performed during the rig design, i.e. no post-calculations with experimental data as input to the simulations have been done yet. The pressure distribution is in line with the design intent, which is a good indicator that the tunnel design is suitable for the intended purpose. At last we report some velocity measurements performed in the wake and we calculate the total drag based on the wake velocity deficit for various Reynolds numbers and with and without turbulence tripping tape. We illustrate that a two dimensional tripping around 7% of the chord from the leading edge can increase the total drag by 50% with respect to the reference case without tripping tape.

  • 52.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Shahinfar, Shahab
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Talamelli, A.
    Transition to turbulence delay using miniature vortex generators – AFRODITE –2014In: Springer Proceedings in Physics, Springer, 2014, p. 71-74Conference paper (Refereed)
    Abstract [en]

    A laminar boundary layer has a relatively low skin-friction drag coefficient (cf) with respect to a turbulent one, and for increasing Reynolds number the difference in cf rapidly increases, and the difference can easily amount to an order of magnitude in many industrial applications. This explains why there is a tremendous interest in being able to delay transition to turbulence, particularly by means of a passive mechanism, which has the advantage of accomplishing the control without adding any extra energy into the system. Moreover, a passive, control does not have to rely on typically complicated sensitive electronics in sensor-actuator systems. Within the AFRODITE project [3] we now present the first experimental results where we are able to show that miniature vortex generators (MVGs) are really coveted devices in obtaining transition delay.

  • 53.
    Fransson, Jens H. M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Talamelli, A.
    Brandt, L.
    Cossu, C.
    Stabilisation of Tollmien-Schlichting waves by streaky base flow2003Conference paper (Other academic)
  • 54.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, A.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Cossu, C.
    Delaying transition to turbulence by a passive mechanism2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, no 6Article in journal (Refereed)
    Abstract [en]

    Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence.

  • 55.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, A.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, C.
    Experimental analysis of transition delay by means of roughness elements2006In: Collection of Technical Papers: 36th AIAA Fluid Dynamics Conference, 2006, p. 1464-1478Conference paper (Refereed)
    Abstract [en]

    An experimental activity aimed at the development of a passive method for viscous drag reduction in an aerodynamic body is presented. The reduction is achieved by delaying the laminar-turbulent transition of the boundary layer. The research is motivated by previous linear stability analysis where it was demonstrated that stable streaks in a boundary layer are capable to stabilize the growth of Tollmien-Schlichting waves. In this paper the results of a series of experiments on a flat plate are presented. Firstly, it is 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. Secondly, it is demonstrated that the generated streaks are really effective in reducing the growth of the Tollmien-Schlichting waves as predicted by the theory. Finally, it is shown that using this passive control technique it is possible to sensibly delay transition to turbulence.

  • 56.
    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.

  • 57.
    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)
  • 58.
    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)
  • 59.
    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)
  • 60.
    Fransson, Jens H.M.
    KTH, Superseded Departments, Mechanics.
    Leading edge design process using a commercial flow solver2004In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 37, no 6, p. 929-932Article in journal (Refereed)
    Abstract [en]

    A design process of an asymmetric leading edge, for laminar stability measurements in a flat plate boundary layer, is reported. The purpose is to minimize the leading edge pressure gradient region, which affects the stability characteristics of the flow. Finally, the design success is verified by wind tunnel testing.

  • 61.
    Fransson, Jens H.M
    KTH, Superseded Departments, Mechanics.
    PIV–measurements in the wake of a cylinder subject to continuous suction or blowing.Manuscript (preprint) (Other academic)
  • 62.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P.H.
    KTH, Superseded Departments, Mechanics.
    On the disturbance growth in an asymptotic suction boundary layer2003In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 482, p. 51-90Article in journal (Refereed)
    Abstract [en]

    An experimental and theoretical study on the effect of boundary layer suction on the laminar-turbulent transition process has been carried out. In the study an asymptotic suction boundary layer was established in a wind tunnel with a free-stream velocity of 5.0 m s(-1). Wall-normal suction (suction velocity 1.44 cm s(-1)) was applied over a large area and the boundary layer was nearly constant over a length of 1800 mm. Measurements were made both with and without suction so comparisons between the two cases could easily be made. Measurements of the development of the mean velocity distribution showed good agreement with theory. The Reynolds number based on the displacement thickness for the suction boundary layer was 347. Experiments on both the development of forced Tollmien-Schlichting (TS) waves and boundary layer disturbances introduced by free-stream turbulence were carried out. Spatial linear stability calculations for TS-waves, where the wall-normal velocity component is accounted for, were carried out for comparison with the experiments. This comparison shows satisfactory agreement even though the stability of the asymptotic suction profile is somewhat overpredicted by the theory. Free-stream turbulence (FST) was generated by three different grids, giving turbulence intensities at the leading edge of the plate between 1.4% and 4.0%. The FST induces disturbances in the boundary layer and it was shown that for the present suction rate the disturbance level inside the boundary layer is constant and becomes proportional to the FST intensity. In all cases transition was prevented when suction was applied whereas without suction the two highest levels of grid turbulence gave rise to transition. Despite a twofold reduction in the boundary layer thickness in the suction case compared to the no suction case the spanwise scale of the streaky structures was almost constant.

  • 63.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P.H.
    KTH, Superseded Departments, Mechanics.
    On the hydrodynamic stability of channel flow with cross flow2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 2, p. 436-441Article in journal (Refereed)
    Abstract [en]

    We study plane channel flow, with a homogeneous cross flow through porous walls, mainly with respect to the stability to two-dimensional wave disturbances. Since the stability of a shear flow depends both on the velocity distribution and the Reynolds number we partly investigated this flow under the conditions that the flow Reynolds number was constant. The flow exhibits some interesting and unexpected stability characteristics. The effect of the cross flow was for certain parameter regions stabilizing and for others destabilizing. The latter result is in contrast to previous studies.

  • 64.
    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)
  • 65.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Corbett, P
    Optimal linear growth in the asymptotic suction boundary layer2003In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 22, no 3, p. 259-270Article in journal (Refereed)
    Abstract [en]

    A variational technique in the temporal framework is used to study initial configurations of disturbance velocity which maximize perturbation kinetic energy in the asymptotic suction boundary layer (ASBL). These optimal perturbations (OP) excite significant and remarkably persistent transient growth, on the order of that which occurs in the Blasius boundary layer (BBL). In contrast, classical modal analysis of the ASBL predicts a critical Reynolds number two orders of magnitude larger than that for the BBL. As in other two-dimensional boundary layer flows, disturbances undergoing maximum amplification are infinitely elongated in the direction of the flow and take the form of streamwise-oriented vortices which induce strong variations in the streamwise perturbation velocity (streaks). The Reynolds number dependence of the maximum growth, and the best choice of scaling for the spanwise wavenumber of the perturbation causing it, are elucidated. There is good agreement between the streak resulting from OP and disturbances measured in experiments in which the asymptotic suction boundary layer is subject to free stream turbulence (FST). This agreement is shown to improve as the level of FST increases.

  • 66.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Konieczny, P
    Alfredsson, Per Henrik
    KTH, Superseded Departments, Mechanics.
    Flow around a porous cylinder subject to continuous suction or blowing2004In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 19, no 8, p. 1031-1048Article in journal (Refereed)
    Abstract [en]

    In the present experimental investigation the surface pressure distribution, vortex shedding frequency, and the wake flow behind a porous circular cylinder are studied when continuous suction or blowing is applied through the cylinder walls. It is found that even moderate levels of suction/blowing (less than or similar to 5% of the oncoming streamwise velocity) have a large impact on the flow around the cylinder. Suction delays separation contributing to a narrower wake width, and a corresponding reduction of drag, whereas blowing shows the opposite behaviour. Both uniform suction and blowing display unexpected flow features which are analysed in detail. Suction shows a decrease of the turbulence intensity throughout the whole wake when compared with the natural case, whilst blowing only shows an effect up to five diameters downstream of the cylinder. The drag on the cylinder is shown to increase linearly with the blowing rate, whereas for suction there is a drastic decrease at a specific suction rate. This is shown to be an effect of the separation point moving towards the rear part of the cylinder, similar to what happens when transition to turbulence occurs in the boundary layer on a solid cylinder. The suction/blowing rate can empirically be represented by an effective Reynolds number for the solid cylinder, and an analytical expression for this Reynolds number representation is proposed and verified. Flow visualizations expose the complexity of the flow field in the near wake of the cylinder, and image averaging enables the retrieval of quantitative information, such as the vortex formation length.

  • 67.
    Fransson, Jens H.M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Matsubara, Masaharu
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P.H.
    Transition induced by free-stream turbulence2005In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 527, p. 1-25Article in journal (Refereed)
    Abstract [en]

    Free-stream turbulence (FST) is perhaps the most important source inducing by-pass transition in boundary layer flows. The present study describes the initial energy growth of streamwise-oriented disturbances in the boundary layer originating from the presence of FST with intensities between 1.4% and 6.7%, but the study is mainly focused on the modelling of the transition zone. For this study three passive and one active turbulence-gene rating grids were used. The active grid was used in order to vary the turbulence intensity (Tu(2)) without changing the setup in the test section. It is shown that the initial disturbance energy in the boundary layer is proportional to Tu(2). The experiments also show that the energy grows in linear proportion to the Reynolds number based on the downstream distance. Furthermore the transitional Reynolds number is shown to be inversely proportional to Tu(2) for the whole range of Tu studied. The intermittency in the transitional zone was determined and it was shown that the intermittency function has a universal shape if the downstream distance is scaled with the length of the transition zone. The Reynolds number based on this transition zone length was found to increase linearly with the transition Reynolds number; however it was also noted that this non-dimensional length has a minimum value. With these results we were able to formulate an expression for the spot production rate which has a better physical base than previous models.

  • 68.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Westin, K. Johan A.
    KTH, Superseded Departments, Mechanics.
    Errors in hot-wire X-probe measurements induced by unsteady velocity gradients2002In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 32, no 3, p. 413-415Article in journal (Refereed)
    Abstract [en]

    Errors in hot-wire X-probe measurements due to unsteady velocity gradients are investigated by a comparison of hot-wire and laser Doppler velocimetry (LDV) measurements. The studied flow case is a laminar boundary layer subjected to high levels of free-stream turbulence, and the hot-wire data shows a local maximum in the wall-normal fluctuation velocity inside the boundary layer. The observed maximum is in agreement with existing hot-wire data, but in conflict with the present LDV measurements as well as existing results from numerical simulations. An explanation for the measurement error is suggested in the paper.

  • 69.
    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.

  • 70.
    Karlsson, Mikael
    et al.
    Swenox AB.
    Holmberg, Andreas
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Fallenius, Bengt
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental determination of the aero-acoustic properties of an in-duct flexible plate2008In: 14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), 2008Conference paper (Refereed)
    Abstract [en]

    The potential reduction of the aeroacoustic noise generated by an in duct plate, byallowing it to be flexible, is studied experimentally. The test object is a triangular plateinserted at an angle in a circular flow duct. Results are given for the active and passiveacoustic properties. In addition the flow field and the vibrations of the plate arecharacterized. It is found that an appropriately yielding plate reduces the flow generatednoise while keeping the mean flow field unaffected.

  • 71. Khan, M.
    et al.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Gävle, Sweden.
    Sandberg, M.
    On the wake pressure footprint and its potential application for wake flow analysis2013In: European Wind Energy Conference and Exhibition, EWEC 2013: Volume 2, European Wind Energy Association, 2013, p. 939-945Conference paper (Refereed)
    Abstract [en]

    This paper introduces a new wake analysis technique behind wind turbines, called pressure footprint (p-f) method, as a simple technique for wind tunnel experiments as well as for field measurements. It is based on the assumption that the pressure at ground is related to the total pressure at the hub height, which in turn can be correlated to the velocity deficit of the wake. The p-f method requires that the static pressure can be measured on the ground and for this purpose we here use a pressure plate with 400 pressure taps. A single wind turbine model was positioned in the middle of the plate and the pressures were measured using a scani-valve. The static and total pressures at hub height were also measured. The effect on the wake pressure footprint when varying the hub height was studied, and by an appropriate definition of the applied pressure coefficient the variation of the footprint size vanishes, which is an important first step in relating the footprint area to the velocity deficit in the wake. We also show the interaction of two wind turbine models, located on the same centre line, with varying reciprocal streamwise distance. With the current method the relative size between the upstream and the downstream pressure footprint remains constant with a factor of two. Next, we will investigate the correlation between the pressure footprint and the velocity deficit in the wake by performing particle image velocimetry measurements in the same setup.

  • 72.
    Kurian, Thomas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Grid-generated turbulence revisited2009In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 41, no 2, p. 021403-Article in journal (Refereed)
    Abstract [en]

    In this study we characterize the turbulence, by means of energy spectra, characteristic turbulence length scales, energy dissipation, kinetic energy decay rate etc., behind a set of grids with the feature of having roughly the same solidity but different mesh and bar widths. This is one way of being able to vary the turbulence characteristic length scales while keeping the same turbulence intensity, which is usually a difficult task for experimentalists. Measurements are performed by using, on the one hand, traditional hot-wire x-probes oriented in both directions giving information about all three directional velocity components and, on the other hand, small single-wire probes for faster frequency response. Independent procedures to calculate some quantities are summarized and performed in the present paper and compared with correlation functions based on homogeneous isotropic turbulence as well as semi-empirical relations. For grid-generated turbulence, which often erroneously is described as isotropic (actually a rare condition), relations derived based on isotropic turbulence are frequently used. Here, we show that dissipation rates and length scales may be inaccurate by as much as 50% or more when compared with valid anisotropic relations. The paper ends with a comparison of the turbulence characteristics between the zero pressure gradient case and a favorable pressure gradient case with a small degree of cross flow. With the pressure gradient, a reduction of the integral and Taylor length scales of about 20% and 30%, respectively, is reported for a large mesh width, whereas no change is observed for a small one.

  • 73. Kurian, Thomas
    et al.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Effect of surface roughness and FST on crossflow instability2008Conference paper (Refereed)
  • 74.
    Kurian, Thomas
    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.
    New results on grid-generated turbulence2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 921-921Conference paper (Refereed)
  • 75.
    Kurian, Thomas
    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.
    Receptivity of crossflow instabilities to free-stream turbulence and surface roughnessArticle in journal (Other academic)
  • 76. Kurian, Thomas
    et al.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Transient growth in the asymptotic suction boundary layer2007Conference paper (Other academic)
  • 77.
    Kurian, Thomas
    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), Centres, Linné Flow Center, FLOW.
    Transient growth in the asymptotic suction boundary layer2011In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 51, no 3, p. 771-784Article in journal (Refereed)
    Abstract [en]

    In the present experimental setup, the transient disturbance growth in a spatially invariant boundary layer flow, i.e., the asymptotic suction boundary layer (ASBL), has been investigated. The choice of the ASBL brings along several advantages compared with an ordinary spatially growing boundary layer. A unique feature of the ASBL is that the Reynolds number (Re) can be varied without changing the boundary layer thickness, which in turn allows for parameter variations not possible to carry out in traditional boundary layer flows. A spanwise array of discrete surface roughness elements was mounted on the surface to trigger modes with different spanwise wavenumbers (beta). It is concluded that for each mode there exists a threshold roughness Reynolds number (Re (k) ), below which no significant transient growth is present. The experimental data suggests that this threshold Re (k) is both a function of beta and Re. An interesting result is that the energy growth curves respond differently to a change in Re (k) when caused by a change in roughness height k, implying that Re remains constant, compared with a change in the free-stream velocity U(infinity), which also affects the Re. The scaling of the energy rowth curves both in level and the downstream direction is treated and appropriate scalings are found. The result shows a complex non-linear receptivity mechanism. Optimal perturbation theory, which has failed to predict the energy evolution in growing boundary layers, is tested for the ASBL and shows that it may satisfactorily predict the evolution of all transiently growing modes that are triggered by the roughness elements.

  • 78.
    Kurian, Thomas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Boundary layer receptivity to free-stream turbulence and surface roughness over a swept flat plate2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 3, p. 034107-Article in journal (Refereed)
    Abstract [en]

    An experimental study of the receptivity of disturbances and their subsequent development into a three-dimensional boundary layer has been carried out. The three-dimensional boundary layer was set up using a flat plate with a swept leading edge and a pressure gradient using a displacement body at the ceiling of the test section. Low level free-stream turbulence was generated with five different screens and was shown to generate traveling crossflow modes for all but the lowest turbulence level, i.e., for Tu > 0.2%, where instead a stationary crossflow disturbance dominated. Stationary crossflow disturbances were triggered by small cylindrical roughness elements arranged in an array. For high enough roughness Reynolds number (Re-k) stationary disturbances growing exponentially were seen and their amplitude seems to scale with Re-k(2.3).

  • 79.
    Kurian, Thomas
    et al.
    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), Centres, Linné Flow Center, FLOW.
    Evolution of traveling crossflow modes over a swept flat plate2009Conference paper (Refereed)
    Abstract [en]

    An experimental investigation has been carried out to examine the growth of traveling crossflow instabilities over a swept flat plate mimicking the Falkner-Skan-Cooke boundary layer. Different turbulence generating grids were placed upstream of the leading edge to vary incoming parameters. Hot-wire measurements were taken for one component of velocity and compared with linear PSE analysis. These showed a decrease in the growth rate for increasing turbulence intensity, which was most likely cause by nonlinear effects. Streamwise correlation measurements were also taken. All the cases except one triggered the same spanwise integral length scale inside the boundary layer. Receptivity coefficients are needed to do the PSE calculations and to see the need for nonlinear PSE.

  • 80.
    Lögdberg, Ola
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Streamwise evolution of longitudinal vortices in a turbulent boundary layer2009In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 623, p. 27-58Article in journal (Refereed)
    Abstract [en]

     In this experimental study both smoke visualisation and three component hotwire measurements have been performed in order to characterize the streamwise evolution of longitudinal counter-rotating vortices in a turbulent boundary layer. The vortices were generated by means of vortex generators (VGs) in different configurations. Both single pairs and arrays in a natural setting as well as in yaw have been considered. Moreover three different vortex blade heights h, with the spacing d and the distance to the neighbouring vortex pair D for the array con guration, were studied keeping the same d / h and D / h ratios. It is shown that the vortex core paths scale with h in the streamwise direction and with D and h in the spanwise and wall-normal directions, respectively. A new peculiar "hooklike" vortex core motion, seen in the cross-ow plane, has been identi ed in the far region, starting around 200h and 50h for the pair and the array con guration, respectively. This behaviour is explained in the paper. Furthermore the experimental data indicate that the vortex paths asymptote to a prescribed location in the cross-ow plane, which rst was stated as a hypothesis and later veri ed. This observation goes against previously reported numerical results based on inviscid theory. An account for the important viscous e ects is taken in a pseudo-viscous vortex model which is able to capture the streamwise core evolution throughout the measurement region down to 450h. Finally, the e ect of yawing is reported, and it is shown that spanwiseaveraged quantities such as the shape factor and the circulation are hardly perceptible. However, the evolution of the vortex cores are di erent both between the pair and the array con guration and in the natural setting versus the case with yaw. From a general point of view the present paper reports on fundamental results concerning the vortex evolution in a fully developed turbulent boundary layer.

  • 81.
    Matsubara, Masaharu
    et al.
    KTH, Superseded Departments, Mechanics.
    Bakchinov, A.A
    Fransson, Jens, H.M.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P.H.
    KTH, Superseded Departments, Mechanics.
    Growth and  breakdown of streaky structures in boundary layer transition induced by free stream turbulence1999In: Laminar-Turbulent Transition 2000: Proc. IUTAM symp. / [ed] Fasel, H.F & Saris, W.S, Springer , 1999, p. 371-376Conference paper (Other academic)
  • 82.
    Odemark, Ylva
    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.
    Dahlberg, J-Å
    Phase-locked hot-wire measurements on the breakdown of wind turbine tip vorticesManuscript (preprint) (Other academic)
  • 83.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    An experimental study of tip vortex instabilities and breakdown2011Conference paper (Other academic)
  • 84.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    An experimental study on tip vortex instabilities and breakdown2011Conference paper (Refereed)
  • 85.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Optimization and control of wind farms: Tip vortex breakdown and wake flow interaction2010Conference paper (Other academic)
  • 86.
    Odemark, Ylva
    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.
    The stability and development of tip and root vortices behind a model wind turbine2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 9, p. 1591-Article in journal (Refereed)
    Abstract [en]

    When designing new wind farms, one has to rely on models describing the flow field around and inside the farm, since direct numerical simulation is far too computationally expensive. In order to develop better models for power prediction of wind farms, knowledge about the flow field around turbines, the stability of the wakes and the interaction between them is essential. Since the conditions during field measurements are difficult to control, wind tunnel measurements play an important role when studying wakes behind wind turbines. Within the present work, an experimental methodology has been developed to study the evolution and stability of the tip vortices shed from the rotor blades of a small-scale turbine model. The stability of the tip vortices was studied by introducing a periodic disturbance to the flow, which is shown to have a clear effect on the development of the vortices. Prior to the vortex breakdown, clear signs of vortex pairing were also observed. A parameter study was performed by varying the amplitude and frequency of the forced disturbance, and the effect on the tip vortices was evaluated. This experiment is one of the first where the influence of a periodic disturbance on a wind turbine wake is studied, something that previously has been performed in a number of numerical studies.

  • 87.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wake evolution and trailing vortex instabilities2011Conference paper (Other academic)
    Abstract [en]

    The production losses and inhomogeneous loads of wind powerturbines placed in the wake of another turbine is a well-known problemwhen building new wind power farms, and a subject of intensive research.The present work aims at developing an increased understanding of thebehaviour of turbine wakes, with special regard to wake evolution andthe stability of the trailing vortices. Single point velocity measurementswith hot-wire anemometry were performed in the wake of a small-scalemodel turbine. The model was placed in the middle of the wind tunneltest section, outside the boundary layers from the wind tunnel walls.In order to study the stability of the wake and the trailing vortices,a disturbance was introduced at the end of the nacelle. This was ac-complished through two ori¯ces perpendicular to the main °ow, whichwere connected to a high-pressure tank and two fast-switching valves.Both varicose and sinusoidal modes of di®erent frequencies could be trig-gered. By also triggering the measurements on the blade passage, themeandering of the wake and the disturbance frequency, phase averagedresults could be computed. The results for di®erent frequencies as wellas studies of wake evolution will be presented.

  • 88.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Khan, M.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of the inflow conditions on the performance and near wake structure of a model wind turbineManuscript (preprint) (Other academic)
  • 89.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Khan, M.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the wake flow interaction between two perforated discs with varying mutual distanceManuscript (preprint) (Other academic)
  • 90.
    S. Sattarzadeh, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Mastering non-linear flow dynamics for laminar flow controlManuscript (preprint) (Other academic)
    Abstract [en]

    A laminar flow control technique based on spanwise mean velocity gradients (SVG) has recently proven successful in delaying transition in boundary layers. Here we take advantage of a well-known non-linear effect, namely the interaction of two oblique waves at high amplitude, to produce spanwise mean velocity variations. Against common belief we are able to fully master the first stage of this non-linear interaction to generate steady and stable streamwise streaks, that in turn trigger the SVG method. Our experimental results show that the region of laminar flow can be extended with up to 230\%.

  • 91.
    S. Sattarzadeh, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Spanwise boundary layer modulations using finite discrete suction for transition delayManuscript (preprint) (Other academic)
    Abstract [en]

    Discrete suction is deployed in a flat plate boundary layer to create spanwise mean velocity gradients with the goal of delaying the onset of laminar-to-turbulent transition. It is shown that finite boundary layer suction through a set of holes in a spanwise-oriented array in the flat plate is successful in setting up steady and robust streamwise streaks in the boundary layer. Today, this type of three-dimensional  boundary layer is known to attenuate the growth of Tollmien--Schlichting (TS) waves and delay the transition to turbulence. The control technique is also tested successfully for natural transition where a broad frequency band of TS~waves are naturally induced in the boundary layer. 

  • 92.
    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%.

  • 93. Sattari, A.
    et al.
    Fallenius, Bengt
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sandberg, M.
    PIV Visualisation study in a two-dimensional room model with rapid time varying ventilation flow rates2011In: ROOMVENT, The 12th International Conference on Air Distribution in Rooms / [ed] Vojislav Novakovic, Sten Olaf Hanssen, Hans Martin Mathisen, 2011Conference paper (Refereed)
    Abstract [en]

    Optimal control of inlet jet flows is of wide applicative interest in order to enhance mixing and reduce stagnation in a ventilated room. The general approach in mechanical ventilation is to use a constant flow rate forced convection system providing the ventilation air. This type of ventilation may cause several problems such as draught, stagnation at certain occupied locations, and subsequently low ventilation efficiencies. An alternative to increase the ventilation quality that has been of interest in this study is to introduce flow variations, which is considered as a potential to reduce stagnation and increase efficiency of the ventilation. The study was conducted as a model experiment in a small-scale, two-dimensional (2-D) room model with dimensions 30200.9 cm3 with water as operating fluid. The size of the model made it possible to investigate the 2-D velocity vector field within the entire room using Particle Image Velocimetry (PIV) method and further consequent dynamical and statistical analyses have been done from the resulted PIV vector fields. The comparison between cases of constant flow rate and flow variations have been conducted for the cases of two set of base flow rates and for each one, the cases of constant flow rate and flow variations with frequencies of 0.3, 0.4 and 0.5 Hz, is considered. In this investigation we show that the calm region, with a large stagnation zone, without pulsating inflow condition becomes more active in the sense that the stagnation points are moved and that the small-scale structures are grown for increasing pulsation frequency.

  • 94.
    Sattarzadeh, Sohrab S.
    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.
    Experimental investigation on the steady and unsteady disturbances in a flat plate boundary layer2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 12, p. 124103-Article in journal (Refereed)
    Abstract [en]

    Recent experiments have shown that miniature vortex generators (MVGs) are coveted devices to stabilize unsteady disturbances in flat plate boundary layers and to delay the onset of turbulence by modulating the base flow in the spanwise direction. The spanwise modulation is a result from the non-modal transient growth of steady and spanwise periodic streamwise vortices being generated by the MVGs. The present experimental investigation aims at studying the transient growth of non-modal disturbances induced by a spanwise periodic array of MVGs and its stabilizing effect on non-linear unsteady disturbances in the boundary layer originating from planar Tollmien-Schlichting (TS) waves. Measurements consist of cross-stream planes at different downstream locations in the boundary layer and a spatio-temporal analysis of different modes of the disturbances is carried out. In the streaky boundary layer generated by the MVGs the fundamental spanwise mode, with the same wavelength as the MVG pairs in the array, and its first harmonic, both undergo transient growth whereas the higher harmonics decay immediately downstream of the array. In the unstable region formed in the wake of the MVG blades, i.e., just downstream of the array, a wide range of spanwise modes contributes to an initial growth in the energy of unsteady disturbances. Similar behavior is observed upstream of branch II position of the neutral stability curve where the unsteady disturbances undergo a second energy growth in the unstable region. It is shown that the spatial gradients of the base flow in the wall-normal and spanwise directions are contributing to the amplification and attenuation of the TS wave disturbances, respectively, in the streaky boundary layer.

  • 95.
    Sattarzadeh, Sohrab S.
    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.
    On the scaling of streamwise streaks and their efficiency to attenuate Tollmien-Schlichting waves2015In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 3, article id 58Article in journal (Refereed)
    Abstract [en]

    Streaky boundary layers generated by an array of miniature vortex generators (MVGs) mounted on a flat plate have recently shown to have a stabilizing effect on both two-and three-dimensional disturbances. An experimental study on the effect of the geometrical parameters of MVGs on the generated streamwise streaks in the flat plate boundary layer is carried out, and the corresponding stabilizing effect on Tollmien-Schlichting (TS) wave disturbances is quantified. The new experimental configurations have led to an improved empirical scaling law, which includes additional geometrical parameters of the MVGs compared to the previously reported relation. It is found that the MVG configuration can be optimized with respect to the attenuation of disturbances. In addition, the streamwise location of branch I of the neutral stability curve, with regard to the location of the MVG array, is found to be correlated with the initial receptivity of TS waves on the MVG array and the attenuation of the TS wave amplitude in the unstable region.

  • 96.
    Sattarzadeh, Sohrab S.
    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.
    Spanwise boundary layer modulations using finite discrete suction for transition delay2017In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 58, no 3, article id 14Article in journal (Refereed)
    Abstract [en]

    Discrete suction is deployed in a flat plate boundary layer to create spanwise mean velocity gradients (SVG) with the goal of delaying the onset of laminar-to-turbulent transition. It is shown that finite boundary layer suction through a set of holes in a spanwise oriented array in the flat plate is successful in setting up steady and robust streamwise streaks in the boundary layer. Today, the SVG method for transition control is known to attenuate the growth of Tollmien-Schlichting (TS) waves and delay the transition to turbulence. In this investigation, low-amplitude forced TS waves are attenuated with the implication of extending the laminar flow by at least 120% for a discrete suction of 0.8% of the free-stream velocity. The control technique is also tested successfully for natural transition, with a resulting transition delay of 30%.

  • 97.
    Sattarzadeh, Sohrab Shirvan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Mastering nonlinear flow dynamics for laminar flow control2016In: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, ISSN 2470-0045, Vol. 94, no 2, article id 021103Article in journal (Refereed)
    Abstract [en]

    A laminar flow control technique based on spanwise mean velocity gradients (SVGs) has recently proven successful in delaying transition in boundary layers. Here we take advantage of a well-known nonlinear effect, namely, the interaction of two oblique waves at high amplitude, to produce spanwise mean velocity variations. Against common belief we are able to fully master the first stage of this nonlinear interaction to generate steady and stable streamwise streaks, which in turn trigger the SVG method. Our experimental results show that the region of laminar flow can be extended by up to 230%.

  • 98.
    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)
  • 99.
    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.

  • 100.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. 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.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. 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.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent boundary layers up to Re-theta=2500 studied through simulation and experiment2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Physics of Fluids, Vol. 21, no 5, p. 051702-Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented up to Reynolds number Re-theta=2500, based on momentum thickness theta and free-stream velocity. For the first time direct comparisons of DNS and experiments of turbulent boundary layers at the same (computationally high and experimentally low) Re-theta are given, showing excellent agreement in skin friction, mean velocity, and turbulent fluctuations. These results allow for a substantial reduction of the uncertainty of boundary-layer data, and cross validate the numerical setup and experimental technique. The additional insight into the flow provided by DNS clearly shows large-scale turbulent structures, which scale in outer units growing with Re-theta, spanning the whole boundary-layer height.

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