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Vortical patterns in turbulent flow downstream a 90° curved pipe at high Womersley numbers
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-8127-8124
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-1663-3553
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-1146-3241
2013 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, 692-699 p.Article in journal (Refereed) Published
Abstract [en]

The present experimental work focuses on highly pulsatile, i.e. inertia dominated, turbulent flow downstream a curved pipe and aims at investigating the vortical characteristics of such a flow. The flow parameters (Dean and Womersley number) investigated are of the same order as those met in the internal combustion engine environment. The technique employed is time-resolved stereoscopic particle image velocimetry at different cross-sections downstream the pipe bend. These measurements allow the large-scale structures that are formed to be analyzed by means of proper orthogonal decomposition. The flow field changes drastically during a pulsatile cycle, varying from a uniform flow direction across the pipe section from the inside to the outside of the bend to vortical patterns consisting of two counter-rotating cells. This study characterizes and describes pulsatile curved pipe flow at Womersley numbers much higher than previously reported in the literature. Furthermore, the oscillatory behaviour of the Dean cells for the steady flow - the so-called 'swirl switching' - is investigated for different downstream stations from the bend exit and it is shown that this motion does not appear in the immediate vicinity of the bend, but only further downstream.

Place, publisher, year, edition, pages
2013. Vol. 44, 692-699 p.
Keyword [en]
Curved pipe, Proper orthogonal decomposition, Pulsatile flow, Stereoscopic particle image velocimetry, Turbulent flow, Counter-rotating cells, Curved pipes, Flow downstream, Flow parameters, Large-scale structure, Proper orthogonal decompositions, Womersley numbers, Flow visualization, Internal combustion engines, Principal component analysis, Velocimeters, Velocity measurement
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-139863DOI: 10.1016/j.ijheatfluidflow.2013.09.008ISI: 000329594600061Scopus ID: 2-s2.0-84888431227OAI: oai:DiVA.org:kth-139863DiVA: diva2:690097
Funder
Swedish Energy Agency
Note

QC 20140122

Available from: 2014-01-22 Created: 2014-01-15 Last updated: 2017-12-06Bibliographically approved
In thesis
1. 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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