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  • 1.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Downs, Robert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Skin-friction drag reduction using the method of spanwise mean velocity gradient2015In: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, TSFP-9 , 2015Conference paper (Refereed)
    Abstract [en]

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

  • 2.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Shahinfar, Shahab
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Advanced Fluid Research On Drag reduction In Turbulence Experiments2011Conference paper (Refereed)
    Abstract [en]

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

  • 3.
    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.

  • 4.
    Kalpakli Vester, Athanasia
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Sattarzadeh, Sohrab Shirvan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Combined hot-wire and PIV measurements of a swirling turbulent flow at the exit of a 90° pipe bend2015In: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975Article in journal (Refereed)
    Abstract [en]

    Measurements of turbulent swirling flow by means of hot-wire anemometry and stereoscopic particle image velocimetry were performed, 0.67 pipe diameters downstream a 90° pipe bend. The flow for a wide range of swirl numbers up to (Formula presented.), based on the angular velocity of the pipe wall and bulk velocity, was investigated and compared to the non-swirling case. The limitations and advantages of using a single hot-wire probe in a highly complex flow field are investigated and discussed. The present paper makes available a unique database for a kind of flow that has been neglected in literature and which is believed to be useful for validation purposes for the computational fluid dynamics community.

  • 5.
    S. Sattarzadeh, Sohrab
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Comparison between different passive vortex generators and their stabilizing effect on Tollmien-Schlichting wavesManuscript (preprint) (Other academic)
    Abstract [en]

    An experimental investigation is performed on the streaky boundary layers generated by three types of vortex generators; namely cylindrical roughness elements, and miniature vortex generators (MVG) with triangular and rectangular blades. The stability of the generated streaks and the amplitude of the spanwise modulations are compared together with their efficiency to stabilize Tollmien--Schlichting (TS) waves, where the TS waves are generated upstream of the vortex generators. It is shown that for constant geometrical and boundary layer parameters the amplitude of the streaks generated by rectangular MVGs is considerably higher compared to triangular MVGs and roughness elements, even though a higher critical roughness-height Reynolds numbers is obtained for MVGs with triangular blades. In the investigated streaky boundary layers similar TS wave amplitude attenuation rates are obtained for rectangular and triangular MVGs at branch~II of the neutral stability curve whereas a higher TS wave amplitude is obtained for roughness elements compared to the two-dimensional reference boundary layer.

  • 6.
    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\%.

  • 7.
    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. 

  • 8.
    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%.

  • 9.
    Sattarzadeh Shirvan, Sohrab
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Attenuation of boundary layer disturbances by means of streamwise vortices2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The stability of the boundary layer developed on a flat plate is studied in the presence of streamwise streaks where disturbances of different types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at flow control by means of passively damping the energy growth of the disturbances in the streaky boundary layer and furthermore delay the onset of the transition in the streamwise direction.

    The streamwise streaks are introduced to the two-dimensional Falkner-Skan boundary layer by means of a spanwise-periodic array of miniature vortex generators (MVGs), mounted on the flat plate, that generate pairs of counterrotating vortices elongated in the streamwise direction. The spanwise modulation of the boundary layer into regions of high and low speed streaks is hence obtained passively since the MVG blades utilize the existing momentum in the shear flow to set up the streamwise vortices with no energy input to the boundary layer. The disturbances are introduced as planar Tollmien-Schlichting (TS) waves as well as three dimensional oblique and pair of oblique waves. In an attempt to obtain a more realistic configuration, the disturbance slot is moved upstream of the MVG array as opposed to the earlier studies. It is shown that the passive control method is capable of stabilizing the disturbance waves in the linear regime for a wide range of frequencies albeit an initial receptivity of the disturbance amplitudes is observed immediately downstream of the MVG array. The control method is proven to be extended in the non-linear regime of the disturbances where transition to turbulence delay is obtained. This results in significant drag reduction when comparing the skin friction drag of a laminar boundary layer to a turbulent one. In addition, a parameter study on a wide range of MVG configurations is performed in order to investigate the transient growth of the streaks as well as the damping effects. A universal scaling of the streak amplitudes is found based on empiricism where an integral amplitude definition is proposed for the streaks. The damping effect is found to be optimized for an integral streak amplitude of 30% of the free-stream velocity which takes into account the periodic wavelength of the streaky base flow. Moreover, it is shown that the passive control method can be extended in the streamwise direction by placing a second array of the MVGs in an already streaky boundary layer which results in further delay of the transition to turbulence location.

  • 10.
    Sattarzadeh Shirvan, Sohrab
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Boundary layer streaks as a novel laminar flow control method2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    A novel laminar flow control based on generation of spanwise mean velocity gradients (SVG) in a flat plate boundary layer is investigated where disturbances of different types are introduced in the wall-bounded shear layer. The experimental investigations are aimed at; (i) generating stable and steady streamwise streaks in the boundary layer which set up spanwise gradients in the mean flow, and (ii) attenuating disturbance energy growth in the streaky boundary layers and hence delaying the onset of turbulence transition.

    The streamwise streaks generated by four different methods are investigated, which are spanwise arrays of triangular/rectangular miniature vortex generators (MVGs) and roughness elements, non-linear pair of oblique waves, and spanwise-periodic finite discrete suction. For all the investigated methods the boundary layer is modulated into regions of high- and low speed streaks through formation of pairs of counter-rotating streamwise vortices. For the streaky boundary layers generated by the MVGs a parameter study on a wide range of MVG configurations is performed in order to investigate the transient growth of the streaks. A general scaling of the streak amplitudes is found based on empiricism where an integral amplitude definition is proposed for the streaks.

    The disturbances are introduced as single- and broad band frequency twodimensional Tollmien–Schlichting (TS) waves, and three-dimensional single and a pair of oblique waves. In an attempt to obtain a more realistic configuration compared to previous investigations the disturbances are introduced upstream of the location were streaks are generated. It is shown that the SVG method is efficient in attenuating the growth of disturbance amplitudes in the linear regime for a wide range of frequencies although the disturbances have an initial amplitude response to the generation of the streaks. The attenuation rate of the disturbance amplitude is found to be optimized for an integral streak amplitude of 30% of the free-stream velocity which takes into account the periodic wavelength of the streaky base flow.

    The stabilizing effect of the streamwise streaks can be extended to the nonlinear regime of disturbances which in turn results in transition to turbulence delay. This results in significant drag reduction when comparing the skin friction coefficient of a laminar- to a turbulent boundary layer. It is also shown that consecutive turbulence transition delay can be obtained by reinforcing the streaky boundary layer in the streamwise direction. For the streaky boundary layer generated by pair of oblique waves their forcing frequency sets the upper limit for the frequency of disturbances beyond which the control fails.

  • 11.
    Sattarzadeh Shirvan, Sohrab
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Comparison between dierent passive vortex generators and their stabilizing effect on Tollmien-Schlichting wavesManuscript (preprint) (Other academic)
    Abstract [en]

    An experimental investigation is performed on the streaky boundary layers generated by three types of vortex generators; namely cylindrical roughness elements, and miniature vortex generators (MVG) with triangular and rectangular blades. The stability of the generated streaks and the amplitude of the spanwise modulations are compared together with their efficiency to stabilize Tollmien--Schlichting (TS) waves, where the TS waves are generated upstream of the vortex generators. It is shown that for constant geometrical and boundary layer parameters the amplitude of the streaks generated by rectangular MVGs is considerably higher compared to triangular MVGs and roughness elements, even though a higher critical roughness-height Reynolds numbers is obtained for MVGs with triangular blades. In the investigated streaky boundary layers similar TS wave amplitude attenuation rates are obtained for rectangular and triangular MVGs at branch~II of the neutral stability curve whereas a higher TS wave amplitude is obtained for roughness elements compared to the two-dimensional reference boundary layer.

  • 12.
    Sattarzadeh Shirvan, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden..
    Ferro, Marco
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden..
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden..
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. KTH Mech, Linne Flow Ctr, SE-10044 Stockholm, Sweden..
    Revisiting the Near-Wall Scaling of the Streamwise Variance in Turbulent Pipe Flows2014In: PROGRESS IN TURBULENCE V / [ed] Talamelli, A Oberlack, M Peinke, J, SPRINGER INT PUBLISHING AG , 2014, p. 113-119Conference paper (Refereed)
    Abstract [en]

    Apparent contradictory results regarding the Reynolds number scaling of the near-wall peak of the variance distribution in turbulent pipe flows are discussed. Inconsistencies in the conclusions from the Princeton SuperPipe published between 2010-2012 are highlighted and new experimental evidence in the Karman number range 500-2500 is supplemented. The new results support the view that the inner-scaled peak amplitude increases with Reynolds number as for channel and turbulent boundary layer flows, and in agreement with trends observed in recent direct numerical simulations and other experiments.

  • 13.
    Sattarzadeh Shirvan, Sohrab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kalpakli, Athanasia
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Hot-Wire Calibration at Low Velocities: Revisiting the Vortex Shedding Method2013In: Advances in Mechanical Engineering, ISSN 1687-8132, E-ISSN 1687-8140, Vol. 2013, p. 241726-Article in journal (Refereed)
    Abstract [en]

    The necessity to calibrate hot-wire probes against a known velocity causes problems at low velocities, due to the inherent inaccuracy of pressure transducers at low differential pressures. The vortex shedding calibration method is in this respect a recommended technique to obtain calibration data at low velocities, due to its simplicity and accuracy. However, it has mainly been applied in a low and narrow Reynolds number range known as the laminar vortex shedding regime. Here, on the other hand, we propose to utilize the irregular vortex shedding regime and show where the probe needs to be placed with respect to the cylinder in order to obtain unambiguous calibration data.

  • 14.
    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.

  • 15.
    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.

  • 16.
    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%.

  • 17.
    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%.

  • 18.
    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)
  • 19.
    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.

  • 20.
    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.

  • 21.
    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.

  • 22.
    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.

1 - 22 of 22
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