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Alfredsson, P. HenrikORCID iD iconorcid.org/0000-0002-1146-3241
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Publications (10 of 160) Show all publications
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
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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

Export Date: 13 February 2018; Article; CODEN: FMEIE; Correspondence Address: Ford, C.L.; KTHSweden; email: cford@mech.kth.se. QC 20180327

Available from: 2018-03-27 Created: 2018-03-27 Last updated: 2018-04-11Bibliographically approved
Vernet, J. A., Örlü, R. & Alfredsson, P. H. (2017). Flow separation control behind a cylindrical bump using dielectric-barrier-discharge vortex generator plasma actuators. Journal of Fluid Mechanics, 835, 852-879
Open this publication in new window or tab >>Flow separation control behind a cylindrical bump using dielectric-barrier-discharge vortex generator plasma actuators
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 835, p. 852-879Article in journal (Refereed) Published
Abstract [en]

Dielectric-barrier-discharge plasma actuators are arranged to produce counter-rotating streamwise vortices to control flow separation on a cylindrical bump on a flat plate that is approached by a turbulent boundary layer. The control was tested for different free-stream velocities and actuation driving voltages. The recirculation area downstream of the bump was reduced by the actuation for velocities up to 15 m s(-1) at the highest voltage achievable of the present set-up. However, the flow shows a bi-modality, the nominal two-dimensional wake flow is shown to consist of large-scale streamwise vortices, which are energised by the actuation until a phenomenon of lock-on of these vortices occurs at sufficiently high driving voltages. The wavelength of the actuation is half that of the large-scale vortices. The lock-on shifts sometimes, i.e. the large streamwise vortices centre switch spanwise location, explaining the bi-modality in the flow. The details of the bi-modality are further investigated by conditional averaging and proper orthogonal decomposition.

Place, publisher, year, edition, pages
Cambridge University Press, 2017
Keywords
flow control, separated flows, wakes/jets
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-220486 (URN)10.1017/jfm.2017.773 (DOI)000416945300004 ()
Funder
Swedish Energy Agency, 34186-1
Note

QC 20171222

Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2017-12-22Bibliographically approved
Ikeya, Y., Örlü, R., Fukagata, K. & Alfredsson, P. H. (2017). Towards a theoretical model of heat transfer for hot-wire anemometry close to solid walls. Paper presented at 11th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements (ETMM), SEP 21-23, 2016, Palermo, Italy. International Journal of Heat and Fluid Flow, 68, 248-256
Open this publication in new window or tab >>Towards a theoretical model of heat transfer for hot-wire anemometry close to solid walls
2017 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 68, p. 248-256Article in journal (Refereed) Published
Abstract [en]

Hot-wire anemometry readings where the sensor is close to a solid wall become erroneous due to additional heat losses to the wall. Here we examine this effect by means of experiments and numerical simulations. Measurements in both quiescent air as well as laminar and turbulent boundary layers confirmed the influences of parameters such as wall conductivity, overheat ratio and probe dimensions on the hot-wire output voltage. Compared to previous studies, the focus lies not only on the streamwise mean velocity, but also on its fluctuations. The accompanying two-dimensional steady numerical simulation allowed a qualitative discussion of the problem and furthermore mapped the temperature field around the wire for different wall materials. Based on these experimental and numerical results, a theoretical model of the heat transfer from a heated wire close to a solid wall is proposed that accounts for the contributions from both convection and conduction.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Hot-wire anemometry, Wall turbulence, Heat transfer
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-221391 (URN)10.1016/j.ijheatfluidflow.2017.09.002 (DOI)000418973400021 ()2-s2.0-85029514300 (Scopus ID)
Conference
11th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements (ETMM), SEP 21-23, 2016, Palermo, Italy
Note

QC 20180117

Available from: 2018-01-17 Created: 2018-01-17 Last updated: 2018-01-17Bibliographically approved
Appelquist, E., Schlatter, P., Alfredsson, P. H. & Lingwood, R. J. (2017). Transition to turbulence in the rotating-disk boundary-layer flow with stationary vortices. Journal of Fluid Mechanics, 836, 43-71
Open this publication in new window or tab >>Transition to turbulence in the rotating-disk boundary-layer flow with stationary vortices
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 836, p. 43-71Article in journal (Refereed) Published
Abstract [en]

This paper proposes a resolution to the conundrum of the roles of convective and absolute instability in transition of the rotating-disk boundary layer. It also draws some comparison with swept-wing flows. Direct numerical simulations based on the incompressible Navier-Stokes equations of the flow over the surface of a rotating disk with modelled roughness elements are presented. The rotating-disk flow has been of particular interest for stability and transition research since the work by Lingwood (J.FluidMech., vol.299, 1995, pp.17-33) where an absolute instability was found. Here stationary disturbances develop from roughness elements on the disk and are followed from the linear stage, growing to saturation and finally transitioning to turbulence. Several simulations are presented with varying disturbance amplitudes. The lowest amplitude corresponds approximately to the experiment by Imayama etal. (J.FluidMech., vol.745, 2014a, pp.132-163). For all cases, the primary instability was found to be convectively unstable, and secondary modes were found to be triggered spontaneously while the flow was developing. The secondary modes further stayed within the domain, and an explanation for this is a proposed globally unstable secondary instability. For the low-amplitude roughness cases, the disturbances propagate beyond the threshold for secondary global instability before becoming turbulent, and for the high-amplitude roughness cases the transition scenario gives a turbulent flow directly at the critical Reynolds number for the secondary global instability. These results correspond to the theory of Pier (J.EngngMaths, vol.57, 2007, pp.237-251) predicting a secondary absolute instability. In our simulations, high temporal frequencies were found to grow with a large amplification rate where the secondary global instability occurred. For smaller radial positions, low-frequency secondary instabilities were observed, tripped by the global instability.

Place, publisher, year, edition, pages
Cambridge University Press, 2017
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-220598 (URN)10.1017/jfm.2017.771 (DOI)000417902300001 ()2-s2.0-85038587829 (Scopus ID)
Note

QC 20180116

Available from: 2018-01-16 Created: 2018-01-16 Last updated: 2018-01-16Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-1146-3241

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