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Alfredsson, P. HenrikORCID iD iconorcid.org/0000-0002-1146-3241
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Publications (10 of 151) Show all publications
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, 852-879 p.Article 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
Keyword
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
Rabault, J., Vernet, J. A., Lindgren, B. & Alfredsson, P. H. (2016). A study using PIV of the intake flow in a diesel engine cylinder. Paper presented at 16th Triennial Conference on Modelling Fluid Flow (CMFF), SEP 01-04, 2015, Budapest Univ Technol & Econ, Budapest, HUNGARY. International Journal of Heat and Fluid Flow, 62, 56-67.
Open this publication in new window or tab >>A study using PIV of the intake flow in a diesel engine cylinder
2016 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 62, 56-67 p.Article in journal (Refereed) Published
Abstract [en]

The admission flow generated by a parallel valve diesel engine cylinder head was investigated by planar and stereoscopic Particle Image Velocimetry in a steady flow test bench through measurements in the swirl and tumble planes. By combining several sets of measurements a full three-dimensional, three component reconstruction of the flow was made. The flow out of the valves forms a jet which collides with the cylinder wall before flowing down along the wall. Despite the fact that there is no piston a recirculation bubble is formed in the tumble plane. This is due to the entrainment of gas into the jet which needs to be replaced and thereby sets up a counter flow. In the swirl plane complex jet-dominated vortex structures are detected close to the cylinder top. Moving away from the cylinder top, a counter-rotating vortex-pair structure is observed from which a single coherent swirling structure develops further down the cylinder. Some clear differences are observed between the flow at high and moderate valve lifts, which correspond to a distinct change in the swirl intensity. By introducing a strong swirling motion the flow is stabilized which can be seen by tracking the instantaneous position of the swirl centre. For high swirl the variation of the position of the swirl centre decreases substantially. (C) 2016 Elsevier Inc. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Internal combustion engine, Swirling flow, Admission stroke
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-201256 (URN)10.1016/j.ijheatfluidflow.2016.06.020 (DOI)000391901800007 ()2-s2.0-84994850830 (Scopus ID)
Conference
16th Triennial Conference on Modelling Fluid Flow (CMFF), SEP 01-04, 2015, Budapest Univ Technol & Econ, Budapest, HUNGARY
Note

QC 20170215

Available from: 2017-02-15 Created: 2017-02-15 Last updated: 2017-11-29Bibliographically approved
Örlü, R. & Alfredsson, P. H. (2016). BINORMAL COOLING ERRORS IN SINGLE HOT-WIRE MEASUREMENTS. JOURNAL OF THEORETICAL AND APPLIED MECHANICS, 54(1), 305-310.
Open this publication in new window or tab >>BINORMAL COOLING ERRORS IN SINGLE HOT-WIRE MEASUREMENTS
2016 (English)In: JOURNAL OF THEORETICAL AND APPLIED MECHANICS, ISSN 1429-2955, Vol. 54, no 1, 305-310 p.Article in journal (Refereed) Published
Abstract [en]

In single-wire hot-wire measurements, velocity fluctuations acting normal to the hot-wire and its prongs will cause additional heat transfer known as binormal cooling. With respect to wall turbulence, the influence of this additional cooling is well-studied for crossed wires, while it is commonly ignored in single hot-wire measurements. The latter view is challenged in the recent work by Drozdz and Elsner (2014) that claims significant errors in variance measurements when using single-wire probes in turbulent boundary layers. This short communication revisits these claims and quantifies binormal cooling errors through an expansion of the effective-velocity concept and utilisation of direct numerical simulation data. Results support the common habit that binormal cooling errors can safely be ignored in single hot-wire measurements.

Keyword
hot-wire anemometry, measurement errors, wall turbulence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-183345 (URN)10.15632/jtam-pl.54.1.305 (DOI)000369506300026 ()2-s2.0-84955059366 (Scopus ID)
Note

QC 20160307

Available from: 2016-03-07 Created: 2016-03-07 Last updated: 2016-03-19Bibliographically approved
Imayama, S., Alfredsson, H. & Lingwood, R. J. (2016). Experimental study of rotating-disk boundary-layer flow with surface roughness. Journal of Fluid Mechanics, 786.
Open this publication in new window or tab >>Experimental study of rotating-disk boundary-layer flow with surface roughness
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 786Article in journal (Refereed) Published
Abstract [en]

Rotating-disk boundary-layer flow is known to be locally absolutely unstable at R> 507 as shown by Lingwood (J. Fluid Mech., vol. 299, 1995, pp. 17-33) and, for the clean-disk condition, experimental observations show that the onset of transition is highly reproducible at that Reynolds number. However, experiments also show convectively unstable stationary vortices due to cross-flow instability triggered by unavoidable surface roughness of the disk. We show that if the surface is sufficiently rough, laminar turbulent transition can occur via a convectively unstable route ahead of the onset of absolute instability. In the present work we compare the laminar turbulent transition processes with and without artificial surface roughnesses. The differences are clearly captured in the spectra of velocity time series. With the artificial surface roughness elements, the stationary-disturbance component is dominant in the spectra, whereas both stationary and travelling components are represented in spectra for the clean-disk condition. The wall-normal profile of the disturbance velocity for the travelling mode observed for a clean disk is in excellent agreement with the critical absolute instability eigenfunction from local theory; the wall-normal stationary-disturbance profile, by contrast, is distinct and the experimentally measured profile matches the stationary convective instability eigenfunction. The results from the clean-disk condition are compared with theoretical studies of global behaviours in spatially developing flow and found to be in good qualitative agreement. The details of stationary disturbances are also discussed and it is shown that the radial growth rate is in excellent agreement with linear stability theory. Finally, large stationary structures in the breakdown region are described.

Place, publisher, year, edition, pages
Cambridge University Press, 2016
Keyword
absolute/convective instability, nonlinear instability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-180207 (URN)10.1017/jfm.2015.634 (DOI)000366298600008 ()2-s2.0-84948390863 (Scopus ID)
Note

QC 20160120

Available from: 2016-01-20 Created: 2016-01-08 Last updated: 2017-11-30Bibliographically approved
Kawata, T. & Alfredsson, P. H. (2016). Experiments in rotating plane Couette flow - momentum transport by coherent roll-cell structure and zero-absolute-vorticity state. Journal of Fluid Mechanics, 791, 191-213.
Open this publication in new window or tab >>Experiments in rotating plane Couette flow - momentum transport by coherent roll-cell structure and zero-absolute-vorticity state
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 791, 191-213 p.Article in journal (Refereed) Published
Abstract [en]

In spanwise rotating plane Couette flow (RPCF) a secondary flow dominated by three-dimensional roll-cell structures develops. At high enough rotation rates the flow exhibits a state of zero absolute vorticity at the centre of the channel, as described by Suryadi et al. (Phys. Rev. E, vol. 89, 2014, 033003). They suggested that the zero-absolute-vorticity state is caused by the secondary flow motion of the coherent roll-cell structure induced by the Coriolis force. In the present study we focus on the momentum transport caused by the roll-cell structure of laminar RPCF in order to further understand how the zero-absolute-vorticity state is maintained by the coherent roll cells. The flow is studied through stereoscopic particle image velocimetry measurements, which allow both the Reynolds shear stress and the wall shear stress to be quantified and used as measures of the momentum transport across the channel. Various types of roll-cell structures at different system rotation rates and the momentum transport induced by them are investigated, and the processes in which the momentum is transported in the wall-normal direction are discussed based on a displaced-particle argument as well as the production of the Reynolds stresses. It is shown that the wall-normal fluid motion driven by secondary flow of the roll-cell structure induces two different effects on the mean flow which conflict each other, the momentum transport in the wall-normal direction and the Coriolis acceleration, and the zero-absolute-vorticity state is a stable state where these two effects cancel each other.

Place, publisher, year, edition, pages
Cambridge University Press, 2016
Keyword
nonlinear instability, rotating flows
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-184028 (URN)10.1017/jfm.2016.57 (DOI)000371068900016 ()2-s2.0-84958987267 (Scopus ID)
Note

QC 20160324

Available from: 2016-03-24 Created: 2016-03-22 Last updated: 2017-11-30Bibliographically approved
Örlü, R., Segalini, A., Klewicki, J. & Alfredsson, P. H. (2016). Generalization of the diagnostic plot to higher-order moments in turbulent boundary layers. In: Springer Proceedings in Physics: . Paper presented at 5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015 (pp. 333-338). Springer.
Open this publication in new window or tab >>Generalization of the diagnostic plot to higher-order moments in turbulent boundary layers
2016 (English)In: Springer Proceedings in Physics, Springer, 2016, 333-338 p.Conference paper, Published paper (Refereed)
Abstract [en]

The present work extends the diagnostic plot concept for the streamwise turbulence intensity in wall-bounded turbulent flows [Alfredsson and Örlü, Eur. J. Mech. B/Fluids 42, 403 (2010)], and generalizes it for higher-order (even and odd) moments, thereby providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layer data up to a friction Reynolds number of 20000 are employed and demonstrate the feasibility of the diagnostic plot to scale data throughout the logarithmic and wake regions. © Springer International Publishing Switzerland 2016.

Place, publisher, year, edition, pages
Springer, 2016
Keyword
Boundary layer flow, Boundary layers, Probability density function, Probability distributions, Reynolds number, Turbulence, Turbulent flow, Wakes, General description, Higher order moments, Higher-order, Probability density distribution, Stream-wise velocities, Turbulence intensity, Turbulent boundary layers, Wall-bounded turbulent flows, Atmospheric thermodynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-194624 (URN)10.1007/978-3-319-30602-5_42 (DOI)000387431400042 ()2-s2.0-84979052108 (Scopus ID)9783319306001 (ISBN)
Conference
5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015
Note

Correspondence Address: Örlü, R.; Linné FLOW Centre, KTH MechanicsSweden; email: ramis@mech.kth.se. QC 20161101

Available from: 2016-11-01 Created: 2016-10-31 Last updated: 2016-12-22Bibliographically approved
Örlü, R., Segalini, A., Klewicki, J. & Alfredsson, P. H. (2016). High-order generalisation of the diagnostic scaling for turbulent boundary layers. Journal of turbulence, 17(7), 664-677.
Open this publication in new window or tab >>High-order generalisation of the diagnostic scaling for turbulent boundary layers
2016 (English)In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 17, no 7, 664-677 p.Article in journal (Refereed) Published
Abstract [en]

The diagnostic scaling concept, introduced for the streamwise turbulence intensity in wall-bounded turbulent flows (Alfredsson, Segalini and Örlü, Phys. Fluids 2011;23:041702), is here extended and generalised not only for the higher even-order central statistical moments, but also for the odd moments and thereby the probability density distribution of the streamwise velocity fluctuations. Turbulent boundary layer data up to a friction Reynolds number of 60,000 are employed and demonstrate the feasibility of the diagnostic scaling for the data throughout the logarithmic and wake regions. A comparison with the generalised logarithmic law for even-order moments by Meneveau and Marusic (J. Fluid Mech. 2013;719:R1) based on the attached-eddy hypothesis, is reported. The diagnostic plot provides an apparent Reynolds-number-independent scaling of the data, and is exploited to reveal the functional dependencies of the constants needed in the attached-eddy-based model. In particular, the invariance of the lowest order diagnostic scaling poses an intriguing incompatibility with the asymptotic constancy of the Townsend–Perry constant.

Place, publisher, year, edition, pages
Taylor & Francis, 2016
Keyword
turbulent boundary layers, Turbulent flows
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-187183 (URN)10.1080/14685248.2016.1169282 (DOI)000380171000003 ()2-s2.0-84964329594 (Scopus ID)
External cooperation:
Note

QC 20160830

Available from: 2016-05-20 Created: 2016-05-18 Last updated: 2017-11-30Bibliographically approved
Appelquist, E., Imayama, S., Alfredsson, H., Schlatter, P. & Lingwood, R. (2016). Linear disturbances in the rotating-disk flow: A comparison between results from simulations, experiments and theory. European journal of mechanics. B, Fluids, 55, 170-181.
Open this publication in new window or tab >>Linear disturbances in the rotating-disk flow: A comparison between results from simulations, experiments and theory
Show others...
2016 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 55, 170-181 p.Article in journal (Refereed) Published
Abstract [en]

The incompressible Navier-Stokes equations have an exact similarity solution for the flow over an infinite rotating disk giving a laminar boundary layer of constant thickness, also known as the von Kármán flow. It is well known now that there is an absolute instability of the boundary layer which is linked to transition to turbulence, but convective routes are also observed. It is these convective modes that we focus on here. A comparison of three different approaches to investigate the convective, so called Type-I, stationary crossflow instability is presented here. The three approaches consist of local linear stability analysis, direct numerical simulations (DNS) and experiments. The ’shooting method’ was used to compute the local linear stability whereas linear DNS was performed using a spectral-element method for a full annulus of the disk, a quarter and 1/32 of an annulus, each with one roughness element in the computational domain. These correspond to simulating one, four and 32 roughness elements on the full disk surface and in addition a case with randomly-distributed roughnesses was simulated on the full disk. Two different experimental configurations were used for the comparison: i) a clean-disk condition, i.e. unexcited boundary-layer flow; and ii) a rough-disk condition, where 32 roughness elements were placed on the disk surface to excite the Type-I stationary vortices. Comparisons between theory, DNS and experiments with respect to the structure of the stationary vortices are made. The results show excellent agreement between local linear stability analysis and both DNS and experiments for a fixed azimuthal wavenumber (32 roughnesses). This agreement clearly shows that the three approaches capture the same underlying physics of the setup, and lead to an accurate description of the flow. It also verifies the numerical simulations and shows the robustness of experimental measurements of the flow case. The effects of the azimuthal domain size in the DNS and superposition of multiple azimuthal wavenumbers in the DNS and experiments are discussed.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Direct numerical simulations, Hot-wire anemometry, Linear stability theory, Rotating-disk boundary layer
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-181460 (URN)10.1016/j.euromechflu.2015.09.010 (DOI)000367762900016 ()2-s2.0-84948457564 (Scopus ID)
Funder
Swedish Research Council, 2013-5786
Note

QC20160202

Available from: 2016-02-02 Created: 2016-02-02 Last updated: 2017-11-30Bibliographically approved
Pastuhoff, M., Tillmark, N. & Alfredsson, P. H. (2016). Measuring surface pressure on rotating compressor blades using pressure sensitive paint. Sensors, 16(3), Article ID 344.
Open this publication in new window or tab >>Measuring surface pressure on rotating compressor blades using pressure sensitive paint
2016 (English)In: Sensors, ISSN 1424-8220, E-ISSN 1424-8220, Vol. 16, no 3, 344Article in journal (Refereed) Published
Abstract [en]

Pressure sensitive paint (PSP) was used to measure pressure on the blades of a radial compressor with a 51 mm inlet diameter rotating at speeds up to 50 krpm using the so called lifetime method. A diode laser with a scanning-mirror system was used to illuminate the paint and the luminescent lifetime was registered using a photo multiplier. With the described technique the surface-pressure fields were acquired for eight points in the compressor map, useful for general understanding of the flow field and for CFD validation. The PSP was of so called fast type, which makes it possible to observe pressure variations with frequencies up to several kHz. Through frequency spectrum analysis we were able to detect the pulsating flow frequency when the compressor was driven to surge.

Place, publisher, year, edition, pages
MDPI AG, 2016
Keyword
Compressor, Pressure sensitive paint, PSP, Turbo charger
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-187304 (URN)10.3390/s16030344 (DOI)000373713600015 ()2-s2.0-84960470161 (Scopus ID)
Note

QC 20160519

Available from: 2016-05-19 Created: 2016-05-19 Last updated: 2017-11-30Bibliographically approved
Vernet, J. A., Orlu, R. & Alfredsson, P. H. (2016). Turbulent Boundary Layer Upstream, Over and Downstream a Cylindrical 2D Bump. In: PROGRESS IN TURBULENCE VI: . Paper presented at 6th ITI Conference on Turbulence, SEP 21-24, 2014, Bertinoro, ITALY (pp. 279-283). Springer.
Open this publication in new window or tab >>Turbulent Boundary Layer Upstream, Over and Downstream a Cylindrical 2D Bump
2016 (English)In: PROGRESS IN TURBULENCE VI, Springer, 2016, 279-283 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Springer, 2016
Series
Springer Proceedings in Physics, ISSN 0930-8989 ; 165
Identifiers
urn:nbn:se:kth:diva-196488 (URN)10.1007/978-3-319-29130-7_48 (DOI)000385788600048 ()2-s2.0-84967144633 (Scopus ID)978-3-319-29130-7; 978-3-319-29129-1 (ISBN)
Conference
6th ITI Conference on Turbulence, SEP 21-24, 2014, Bertinoro, ITALY
Note

QC 20161124

Available from: 2016-11-24 Created: 2016-11-14 Last updated: 2016-11-24Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-1146-3241

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