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Dean vortices in turbulent flows: rocking or rolling?
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).ORCID iD: 0000-0001-8127-8124
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).ORCID iD: 0000-0002-1663-3553
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).ORCID iD: 0000-0002-1146-3241
2012 (English)In: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975, Vol. 15, no 1, 37-38 p.Article in journal (Refereed) Published
Abstract [en]

Flows in pipe bends have been studied extensively over the last decades due to their occurrence both in the human respiratory and blood systems as well as in many technical applications. When a fluid flows through a pipe bend an adverse pressure gradient is generated forcing high velocity fluid towards the outer wall which is then replaced by low velocity fluid moving along the wall towards the inner side of the bend. The physical effect is that the high velocity fluid is experiencing a large centrifugal force, resulting in an unstable ‘‘stratification’’ making the high velocity fluid in the centre deflect outwards along the pipe bend, thereby forming two counter-rotating roll cells, so-called Dean vortices. While their behavior in laminar flows has been nicely visualized, the picture of their unsteady behavior in turbulent flows still remains rather blurry, and in fact ‘‘the questions, for example, whether the Dean vortices stay symmetric with respect to the geometric plane of symmetry or whether the strength of the Dean vortices varies with time are hardly addressed’’ (Rütten et al 2005). In the present study, stereoscopic particle image velocimetry has been employed to seize the unsteady behavior of the Dean vortices at the exit of a 90 degree pipe bend at a Reynolds number and Dean number of 34,000 and 19,000, respectively. While the time-averaged flow field shows two symmetrical roll cells, that can be observed both in the streamwise and cross stream velocities, as well as in the streamwise vorticity, the instantaneous snapshots reveal an unsteady behavior where the roll cells are pushing one another alternatively towards the lower or upper half of the pipe, in what could be described as a ‘‘rocking’’ motion of the high speed ‘‘stem’’ in between the roll cells. Hence, the real question is not whether ‘‘to be, or not to be’’ in regards to the instantaneous existence of the Dean vortices in turbulent flows, but rather why, when and how they roll (as their time-averaged counterpart) or rock between the states caught in the presented snapshots.

Place, publisher, year, edition, pages
Springer, 2012. Vol. 15, no 1, 37-38 p.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-83164DOI: 10.1007/s12650-011-0108-8ISI: 000302382600003Scopus ID: 2-s2.0-84862658176OAI: oai:DiVA.org:kth-83164DiVA: diva2:498765
Funder
StandUp
Note

QC 20120216

Available from: 2012-02-12 Created: 2012-02-12 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Experimental study of turbulent flows through pipe bends
Open this publication in new window or tab >>Experimental study of turbulent flows through pipe bends
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with turbulent flows in 90 degree curved pipes of circular cross-section. The flow cases investigated experimentally are turbulent flow with and without an additional motion, swirling or pulsating, superposed on the primary flow. The aim is to investigate these complex flows in detail both in terms of statistical quantities as well as vortical structures that are apparent when curvature is present. Such a flow field can contain strong secondary flow in a plane normal to the main flow direction as well as reverse flow.

The motivation of the study has mainly been the presence of highly pulsating turbulent flow through complex geometries, including sharp bends, in the gas exchange system of Internal Combustion Engines (ICE). On the other hand, the industrial relevance and importance of the other type of flows were not underestimated.

The geometry used was curved pipes of different curvature ratios, mounted at the exit of straight pipe sections which constituted the inflow conditions. Two experimental set ups have been used. In the first one, fully developed turbulent flow with a well defined inflow condition was fed into the pipe bend. A swirling motion could be applied in order to study the interaction between the swirl and the secondary flow induced by the bend itself. In the second set up a highly pulsating flow (up to 40 Hz) was achieved by rotating a valve located at a short distance upstream from the measurement site. In this case engine-like conditions were examined, where the turbulent flow into the bend is non-developed and the pipe bend is sharp. In addition to flow measurements, the effect of non-ideal flow conditions on the performance of a turbocharger was investigated.

Three different experimental techniques were employed to study the flow field. Time-resolved stereoscopic particle image velocimetry was used in order to visualize but also quantify the secondary motions at different downstream stations from the pipe bend while combined hot-/cold-wire anemometry was used for statistical analysis. Laser Doppler velocimetry was mainly employed for validation of the aforementioned experimental methods.

The three-dimensional flow field depicting varying vortical patterns has been captured under turbulent steady, swirling and pulsating flow conditions, for parameter values for which experimental evidence has been missing in literature.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2012. viii, 85 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:05
Keyword
Turbulent flow, swirl, pulsation, pipe bend, hot-wire anemometry, cold-wire anemometry, laser Doppler velocimetry, stereoscopic particle image velocimetry.
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-93316 (URN)978-91-7501-321-3 (ISBN)
Presentation
2012-05-03, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20120425Available from: 2012-04-25 Created: 2012-04-13 Last updated: 2012-04-25Bibliographically approved
2. Vortices in turbulent curved pipe flow-rocking, rolling and pulsating motions
Open this publication in new window or tab >>Vortices in turbulent curved pipe flow-rocking, rolling and pulsating motions
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is motivated by the necessity to understand the flow structure of turbulent flows in bends encountered in many technical applications such as heat exchangers, nuclear reactors and internal combustion engines. Flows in bends are characterised by strong secondary motions in terms of counter-rotating vortices (Dean cells) set up by a centrifugal instability. Specifically the thesis deals with turbulent flows in 90° curved pipes of circular cross-section with and without an additional motion, swirling or pulsatile, superposed on the primary flow.  The aim of the present thesis is to study these complex flows in detail by using time-resolved stereoscopic particle image velocimetry to obtain the three-dimensional velocity field, with complementary hot-wire anemometry and laser Doppler velocimetry measurements.

In order to analyse the vortical flow field proper orthogonal decomposition (POD) is used. The so called ``swirl-switching'' is identified and it is shown that the vortices instantaneously, ``rock'' between three states, viz. a pair of symmetric vortices or a dominant clockwise or counter-clockwise Dean cell. The most energetic mode exhibits a single cell spanning the whole cross-section and ``rolling'' (counter-)clockwise in time. However, when a honeycomb is mounted at the inlet of the bend, the Dean vortices break down and there is strong indication that the ``swirl-switching'' is hindered.

When a swirling motion is superimposed on the incoming flow, the Dean vortices show a tendency to merge into a single cell with increasing swirl intensity. POD analysis show vortices which closely resemble the Dean cells, indicating that these structures co-exist with the swirling motion. In highly pulsating turbulent flow at the exit of a curved pipe, the vortical pattern is diminished or even eliminated during the acceleration phase and then re-established during the deceleration.

In order to investigate the effect of pulsations and curvature on the performance of a turbocharger turbine, highly pulsating turbulent flow through a sharp bend is fed into the turbine. Time-resolved pressure and mass-flow rate measurements show that the hysteresis loop in the pressure-ratio-mass-flow plane, may differ significantly between straight and curved inlets, however the mean operating point is only slightly affected.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. x, 53 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:15
Keyword
Turbulence, curved pipes, swirling flow, pulsatile flow, time-resolved stereoscopic particle image velocimetry, hot-wire anemometry, proper orthogonal decomposition, turbocharger
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145311 (URN)978-91-7595-160-7 (ISBN)
Public defence
2014-06-13, E2, Lindstedtsvägen 3, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20140523

Available from: 2014-05-23 Created: 2014-05-15 Last updated: 2014-05-23Bibliographically approved

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Örlü, RamisAlfredsson, P. Henrik

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