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Alfredsson, P. HenrikORCID iD iconorcid.org/0000-0002-1146-3241
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Publications (10 of 166) Show all publications
Kato, K., Alfredsson, P. H. & Lingwood, R. (2019). Boundary-layer transition over a rotating broad cone. Physical Review Fluids, 4(7), Article ID 071902.
Open this publication in new window or tab >>Boundary-layer transition over a rotating broad cone
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 7, article id 071902Article in journal (Refereed) Published
Abstract [en]

The route to turbulence in the boundary layer on a rotating broad cone is investigated using hot-wire anemometry measuring the azimuthal velocity. The stationary fundamental mode is triggered by 24 deterministic small roughness elements distributed evenly at a specific distance from the cone apex. The stationary vortices, having a wave number of 24, correspond to the fundamental mode and these are initially the dominant disturbance-energy carrying structures. This mode is found to saturate and is followed by rapid growth of the nonstationary primary mode as well as the stationary and nonstationary first harmonics, leading to transition to turbulence. The amplitudes of these are plotted in a way to highlight the continued growth after saturation of the fundamental stationary mode.

Place, publisher, year, edition, pages
American Physical Society, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-262558 (URN)10.1103/PhysRevFluids.4.071902 (DOI)000478049100001 ()2-s2.0-85070306680 (Scopus ID)
Note

QC 20191017

Available from: 2019-10-17 Created: 2019-10-17 Last updated: 2019-10-17Bibliographically approved
Vester, A. K., Nishio, Y. & Alfredsson, P. H. (2019). Investigating swirl and tumble using two prototype inlet port designs by means of multi-planar PIV. International Journal of Heat and Fluid Flow, 75, 61-76
Open this publication in new window or tab >>Investigating swirl and tumble using two prototype inlet port designs by means of multi-planar PIV
2019 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 75, p. 61-76Article in journal (Refereed) Published
Abstract [en]

Flow structures created during the intake stroke of an engine were investigated by means of multi-planar particle image velocimetry (PIV). A unique water-analogue engine model has been developed, where all essential parts and parameters, such as the cylinder head, valve timing, piston geometry and motion, etc. can easily be modified. Two cylinder heads with geometrically different inlet ports were investigated and experiments were performed with both moving and fixed valves. Three-dimensional visualisations of the flow field, mode decomposition through proper orthogonal decomposition, circulation as well as classical statistics were obtained and evaluated in order to gain an understanding of the flow structures, i.e. tumble and swirl, created by the two cylinder heads. It was clearly shown that one of the cylinder heads created a strong swirling motion in the cylinder. Three different fixed valve positions were investigated and the fully opened valve gave the strongest large-scale structures, whereas with smaller openings a larger amount of the kinetic energy was converted into small-scale turbulence. Results showed a more organised and stable flow field consisting of a well-defined swirl motion occupying the whole cylinder at the end of the intake stroke when the valves were fixed at the highest position. The moving valve case gave results similar to the fully open case but with slightly higher turbulence. Cycle-to-cycle variations were found to be less pronounced for these two cases as compared to the smaller fixed valve lifts. The second cylinder head showed a flow field that was more turbulent and much less coherent. Statistical analysis showed that this had a direct effect on cyclic variations in the flow where this head showed more profound variations.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2019
Keywords
Tumble, Swirl, Stereoscopic particle image velocimetry, Proper orthogonal decomposition, 3D3C reconstruction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-245149 (URN)10.1016/j.ijheatfluidflow.2018.11.009 (DOI)000458467700005 ()2-s2.0-85057489938 (Scopus ID)
Note

QC 20190313

Available from: 2019-03-13 Created: 2019-03-13 Last updated: 2019-05-20Bibliographically approved
Kawata, T. & Alfredsson, P. H. (2019). Scale interactions in turbulent rotating planar Couette flow: insight through the Reynolds stress transport. Journal of Fluid Mechanics, 879, 255-295, Article ID PII S0022112019006682.
Open this publication in new window or tab >>Scale interactions in turbulent rotating planar Couette flow: insight through the Reynolds stress transport
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 879, p. 255-295, article id PII S0022112019006682Article in journal (Refereed) Published
Abstract [en]

In turbulent planar Couette flow under anticyclonic spanwise system rotation, large-scale roll-cell structures arise due to a Coriolis-force-induced instability. The structures are superimposed on smaller-scale turbulence, and with increasing angular velocity (Omega(z)) such roll cells dominate the flow field and small-scale turbulence is instead suppressed in a certain rotation number range 0 < Ro less than or similar to 0.1 (Ro = 2 Omega(z)h/U-w, where h is the channel half-width, U-w the wall velocity). At low rotation numbers around Ro approximate to 0.02 both large-scale roll cells and smaller-scale turbulence coexist. In the present study, we investigate interaction between these structures through a scale-by-scale analysis of the Reynolds stress transport. We show that at low rotation numbers Ro approximate to 0.01 the turbulence productions by the mean flow gradient and the Coriolis force occur at different scales and thereby the turbulent energy distribution over a wide range of scales is maintained. On the other hand at higher rotation numbers Ro greater than or similar to 0.05, a zero-absolute-vorticity state is established and production of small scales from the mean shear disappears although large-scale turbulence production is maintained through the Coriolis force. At high enough Reynolds numbers, where scale separation between the near-wall structures and the roll cells is relatively distinct, transition between these different Ro regimes is found to occur rather abruptly around Ro approximate to 0.02, resulting in a non-monotonic behaviour of the wall shear stress as a function of Ro. It is also shown that at such an intermediate rotation number the roll cells interact with smaller scales by moving near-wall structures towards the core region of the channel, by which the Reynolds stress is transported from relatively small scales near the wall towards larger scales in the channel centre. Such Reynolds stress transport by scale interaction becomes increasingly significant as the Reynolds number increases, and results in a reversed mean velocity gradient at the channel centre at high enough Reynolds numbers.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
rotating turbulence, turbulent boundary layers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-261930 (URN)10.1017/jfm.2019.668 (DOI)000487791200001 ()
Note

QC 20191015

Available from: 2019-10-15 Created: 2019-10-15 Last updated: 2019-10-15Bibliographically approved
Vernet, J. A., Örlü, R. & Alfredsson, P. H. (2018). Flow separation control by dielectric barrier discharge plasma actuation via pulsed momentum injection. AIP Advances, 8(7), Article ID 075229.
Open this publication in new window or tab >>Flow separation control by dielectric barrier discharge plasma actuation via pulsed momentum injection
2018 (English)In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 8, no 7, article id 075229Article in journal (Refereed) Published
Abstract [en]

Control of a turbulent boundary layer separating on a half-cylinder mounted on a flat plate has been investigated using a Dielectric Barrier Discharge (DBD) plasma actuator placed along the apex of a cylinder. The main focus of the study has been to evaluate if the control ability of the actuator can be improved through pulsed actuation compared to its steady counterpart. Investigations of the electric wind induced by the DBD plasma actuator in still air, when mounted on the flat plate, revealed that while the steady actuation produces an electric wind similar to a wall jet, the pulsed actuation creates a train of co-rotating vortices. The vortices are the result of a starting vortex produced by the actuator at each actuation pulse. A parametric study showed a dependence of the size, shape and propagation velocity of the vortices on the pulse frequency and duty cycle. With the actuator mounted along the apex of the cylinder, Particle Image Velocimetry measurements of the uncontrolled and controlled flow with a free-stream velocity of 5 m/s showed a clear reduction of the recirculation region downstream the cylinder when using plasma actuation. An even higher control effect could be achieved with pulsed actuation compared to the steady actuation. Phase-locked measurements of the unsteady actuation showed that pulsed actuation periodically shifted the flow separation location resulting in the propagation of vortical structures in the recirculation region. The size of the vortical structures showed a dependence on the pulsed actuation timing parameters.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2018
Keywords
Turbulent-Boundary-Layer, Vortex Generators
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-233426 (URN)10.1063/1.5037770 (DOI)000440602300090 ()2-s2.0-85050998450 (Scopus ID)
Funder
Swedish Energy Agency, 34186-1Swedish Foundation for Strategic Research
Note

QC 20180821

Available from: 2018-08-21 Created: 2018-08-21 Last updated: 2018-08-21Bibliographically approved
Kawata, T. & Alfredsson, P. H. (2018). Inverse Interscale Transport of the Reynolds Shear Stress in Plane Couette Turbulence. Physical Review Letters, 120(24), Article ID 244501.
Open this publication in new window or tab >>Inverse Interscale Transport of the Reynolds Shear Stress in Plane Couette Turbulence
2018 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, no 24, article id 244501Article in journal (Refereed) Published
Abstract [en]

Interscale interaction between small-scale structures near the wall and large-scale structures away from the wall plays an increasingly important role with increasing Reynolds number in wall-bounded turbulence. While the top-down influence from the large-to small-scale structures is well known, it has been unclear whether the small scales near the wall also affect the large scales away from the wall. In this Letter we show that the small-scale near-wall structures indeed play a role to maintain the large-scale structures away from the wall, by showing that the Reynolds shear stress is transferred from small to large scales throughout the channel. This is in contrast to the turbulent kinetic energy transport which is from large to small scales. Such an "inverse" interscale transport of the Reynolds shear stress eventually supports the turbulent energy production at large scales.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-231709 (URN)10.1103/PhysRevLett.120.244501 (DOI)000434937000013 ()2-s2.0-85048624827 (Scopus ID)
Note

QC 20180821

Available from: 2018-08-21 Created: 2018-08-21 Last updated: 2018-11-13Bibliographically approved
Alfredsson, P. H. & Örlü, R. (2018). Large-Eddy BreakUp Devices - a 40 Years Perspective from a Stockholm Horizon. Paper presented at European Drag Reduction and Flow Control Meeting (EDRFCM), APR 03-06, 2017, Monte Porzio Catone, Italy. Flow Turbulence and Combustion, 100(4), 877-888
Open this publication in new window or tab >>Large-Eddy BreakUp Devices - a 40 Years Perspective from a Stockholm Horizon
2018 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, no 4, p. 877-888Article in journal (Refereed) Published
Abstract [en]

In the beginning of the 1980's Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Turbulence, Drag reduction, Drag measurement, Skin friction, Momentum-loss calculation
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-230479 (URN)10.1007/s10494-018-9908-4 (DOI)000433113900002 ()2-s2.0-85047490005 (Scopus ID)
Conference
European Drag Reduction and Flow Control Meeting (EDRFCM), APR 03-06, 2017, Monte Porzio Catone, Italy
Note

QC 20180613

Available from: 2018-06-13 Created: 2018-06-13 Last updated: 2018-06-13Bibliographically approved
Vernet, J. A., Örlü, R., Söderblom, D., Elofsson, P. & Alfredsson, P. H. (2018). Plasma Streamwise Vortex Generators for Flow Separation Control on Trucks A Proof-of-concept Experiment. Paper presented at European Drag Reduction and Flow Control Meeting (EDRFCM), APR 03-06, 2017, Monte Porzio Catone, Italy. Flow Turbulence and Combustion, 100(4), 1101-1109
Open this publication in new window or tab >>Plasma Streamwise Vortex Generators for Flow Separation Control on Trucks A Proof-of-concept Experiment
Show others...
2018 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, no 4, p. 1101-1109Article in journal (Refereed) Published
Abstract [en]

An experimental study of the effect of Dielectric Barrier Discharge plasma actuators on the flow separation on the A-pillar of a modern truck under cross-wind conditions has been carried out. The experiments were done in a wind tunnel with a 1:6 scale model of a tractor-trailer combination. The actuators were used as vortex generators positioned on the A-pillar on the leeward side of the tractor and the drag force was measured with a wind-tunnel balance. The results show that the effect at the largest yaw angle (9 degrees) can give a drag reduction of about 20% and that it results in a net power reduction. At lower yaw angles the reduction was smaller. The present results were obtained at a lower Reynolds number and a lower speed than for real driving conditions so it is still not yet confirmed if a similar positive result can be obtained in full scale.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Vehicle aerodynamics, Separation control, Plasma actuation
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-230480 (URN)10.1007/s10494-018-9891-9 (DOI)000433113900013 ()2-s2.0-85041896980 (Scopus ID)
Conference
European Drag Reduction and Flow Control Meeting (EDRFCM), APR 03-06, 2017, Monte Porzio Catone, Italy
Note

QC 20180613

Available from: 2018-06-13 Created: 2018-06-13 Last updated: 2018-06-13Bibliographically approved
Appelquist, E., Schlatter, P., Alfredsson, P. H. & Lingwood, R. (2018). Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations. European journal of mechanics. B, Fluids, 70, 6-18
Open this publication in new window or tab >>Turbulence in the rotating-disk boundary layer investigated through direct numerical simulations
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 70, p. 6-18Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations (DNS) are reported for the turbulent rotating-disk boundary layer for the first time. Two turbulent simulations are presented with overlapping small and large Reynolds numbers, where the largest corresponds to a momentum-loss Reynolds number of almost 2000. Simulation data are compared with experimental data from the same flow case reported by Imayama et al. (2014), and also a comparison is made with a numerical simulation of a two-dimensional turbulent boundary layer (2DTBL) over a flat plate reported by Schlatter and Örlü (2010). The agreement of the turbulent statistics between experiments and simulations is in general very good, as well as the findings of a missing wake region and a lower shape factor compared to the 2DTBL. The simulations also show rms-levels in the inner region similar to the 2DTBL. The simulations validate Imayama et al.’s results showing that the rotating-disk turbulent boundary layer in the near-wall region contains shorter streamwise (azimuthal) wavelengths than the 2DTBL, probably due to the outward inclination of the low-speed streaks. Moreover, all velocity components are available from the simulations, and hence the local flow angle, Reynolds stresses and all terms in the turbulent kinetic energy equation are also discussed. However there are in general no large differences compared to the 2DTBL, hence the three-dimensional effects seem to have only a small influence on the turbulence.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Near-wall turbulence, Rotation, Turbulence statistics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-227538 (URN)10.1016/j.euromechflu.2018.01.008 (DOI)000432105000002 ()2-s2.0-85042080446 (Scopus ID)
Funder
Swedish e‐Science Research CenterSwedish Research Council
Note

QC 20180517

Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-05-31Bibliographically approved
Vester, A. K., Nishio, Y. & Alfredsson, P. H. (2018). Unravelling tumble and swirl in a unique water-analogue engine model. Journal of Visualization, 21(4), 557-568
Open this publication in new window or tab >>Unravelling tumble and swirl in a unique water-analogue engine model
2018 (English)In: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975, Vol. 21, no 4, p. 557-568Article in journal (Refereed) Published
Abstract [en]

The in-cylinder flow prior to combustion is considered to be one of the most important aspects controlling the combustion process in an engine. More specifically, the large-scale structures present in the cylinder, so-called tumble and swirl, before compression are believed to play a major role into the mixing and combustion processes. Their development during the intake stroke and their final strength depend mainly (but not only) on the inlet port design. In the present study, the turbulent large-scale structures during the intake stroke are investigated in a unique water-analogue engine where inlet ports and valve timings can easily be configured and tested. The flow field in the cylinder volume is reconstructed through multi-planar stereoscopic particle image velocimetry measurements which reveal a wealth of vortical structures during the stroke's various phases. The aim of the present paper is to present and show results from a unique setup which can serve as a test bench for optimisation of inlet port designs to obtain a desired vortical pattern in the cylinder after the intake stroke is finished. This setup can simulate the intake stroke in a much more realistic way as compared to a through-flow setup with a fixed valve lift.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
In-cylinder flow, Tumble, Swirl, Stereoscopic particle image velocimetry
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-232753 (URN)10.1007/s12650-018-0485-3 (DOI)000438859000004 ()2-s2.0-85043715388 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20180803

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-06Bibliographically approved
Ford, C. L., Winroth, P. M. & Alfredsson, P. H. (2018). Vortex-meter design: The influence of shedding-body geometry on shedding characteristics. Flow Measurement and Instrumentation, 59, 88-102
Open this publication in new window or tab >>Vortex-meter design: The influence of shedding-body geometry on shedding characteristics
2018 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 59, p. 88-102Article in journal (Refereed) Published
Abstract [en]

The periodic vortex shedding from bluff bodies may be used in flow metering applications. However, because the bluff-body is highly confined (typically in a pipe) the shed vortices may interact with the pipe wall; causing an undesirable non-linear behaviour. An experimental investigation has been conducted; examining the vortex-shedding characteristics of highly confined bluff-bodies in pipe flow, at high Reynolds number (ReD=4.4×104 to 4.4×105). The bluff-bodies were comprised of a forebody and tail; both of which affected the primary-shedding characteristics. The shedders typically produced two unsteady modes: Mode-I was associated with the vortex shedding and mode-II resulted from a separation of the pipe-wall boundary layer. The mode-I behaviour allowed two classes of shedder to be defined: long-tails and short-tails. Modes I and II interacted, particularly for long-tailed geometries. When the length-scale of mode-II exceeded 0.8κ (where κ is the physical scale of the primary shedding vortex), mode-II disrupted mode-I, as the mode-frequency ratio (fII/fI) approached an integer value. The coupling of modes I and II caused mode-I to deviate from its preferred Strouhal number. When the deviation exceeded 25–30%, mode-I locked on to the mode-II frequency. This did not happen for short-tailed geometries, as the length-scale of mode-I was always dominant. Mode-coupling for short-tails occurred only when the mode frequencies were equal. 

Place, publisher, year, edition, pages
Elsevier Ltd, 2018
Keywords
Bluff-body, Compressible, Frequency-characteristic, Splitter-plate, Vortex-meter, Vortex-shedding, Boundary layer flow, Boundary layers, Geometry, Reynolds number, Vortex shedding, Bluff body, Coupling of modes, Experimental investigations, Frequency characteristic, High Reynolds number, Nonlinear behaviours, Splitter plates, Vortex flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-223133 (URN)10.1016/j.flowmeasinst.2017.12.004 (DOI)000428102800012 ()2-s2.0-85039164249 (Scopus ID)
Note

QC 20190423

Available from: 2018-03-27 Created: 2018-03-27 Last updated: 2019-04-23Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-1146-3241

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