| 1. |
- Fransson, Jens H. M., 1973-, et al.
(författare)
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Advanced Fluid Research On Drag reduction In Turbulence Experiments
- 2011
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Fransson, Jens H. M., 1973-, et al.
(författare)
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Skin-friction drag reduction using the method of spanwise mean velocity gradient
- 2015
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Ingår i: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015. - : TSFP-9. - 9780000000002
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 3. |
- Fransson, Jens H. M., et al.
(författare)
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Transition to turbulence delay using miniature vortex generators – AFRODITE –
- 2014
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Ingår i: Springer Proceedings in Physics. - Cham : Springer. - 9783319018591 ; , s. 71-74
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Kalpakli Vester, Athanasia, et al.
(författare)
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Combined hot-wire and PIV measurements of a swirling turbulent flow at the exit of a 90° pipe bend
- 2015
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Ingår i: Journal of Visualization. - : Springer. - 1343-8875 .- 1875-8975.
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Sattarzadeh, Sohrab Shirvan, et al.
(författare)
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Mastering nonlinear flow dynamics for laminar flow control
- 2016
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Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 2470-0045. ; 94:2
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Tidskriftsartikel (refereegranskat)abstract
- 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%.
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| 6. |
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| 7. |
- Sattarzadeh, Sohrab Shirvan, et al.
(författare)
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Transition delay by means of base flow modulations
- 2011
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Sattarzadeh Shirvan, Sohrab, 1986-
(författare)
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Attenuation of boundary layer disturbances by means of streamwise vortices
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
- Sattarzadeh Shirvan, Sohrab, 1986-
(författare)
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Boundary layer streaks as a novel laminar flow control method
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 10. |
- Sattarzadeh Shirvan, Sohrab
(författare)
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Comparison between dierent passive vortex generators and their stabilizing effect on Tollmien-Schlichting waves
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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