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Vortices in turbulent curved pipe flow-rocking, rolling and pulsating motions
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-8127-8124
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 [en]
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: urn:nbn:se:kth:diva-145311ISBN: 978-91-7595-160-7 (print)OAI: oai:DiVA.org:kth-145311DiVA: diva2:717536
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
List of papers
1. High Womersley number pulsatile turbulent flow past a straight and bent pipe
Open this publication in new window or tab >>High Womersley number pulsatile turbulent flow past a straight and bent pipe
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145308 (URN)
Note

QS 2014

Available from: 2014-05-15 Created: 2014-05-15 Last updated: 2014-05-23Bibliographically approved
2. Dean vortices in turbulent flows: rocking or rolling?
Open this publication in new window or tab >>Dean vortices in turbulent flows: rocking or rolling?
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
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-83164 (URN)10.1007/s12650-011-0108-8 (DOI)000302382600003 ()2-s2.0-84862658176 (Scopus ID)
Funder
StandUp
Note

QC 20120216

Available from: 2012-02-12 Created: 2012-02-12 Last updated: 2017-12-07Bibliographically approved
3. Turbulent pipe flow downstream a 90 degrees pipe bend with and without superimposed swirl
Open this publication in new window or tab >>Turbulent pipe flow downstream a 90 degrees pipe bend with and without superimposed swirl
2013 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 41, 103-111 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, the turbulent flow downstream a 90 degrees pipe bend is investigated by means of stereoscopic particle image velocimetry. In particular, the three dimensional flow field at the exit of the curved pipe is documented for non-swirling and swirling flow conditions, with the latter being generated through a unique axially rotating pipe flow facility. The non-swirling flow was examined through snapshot proper orthogonal decomposition (POD) with the aim to reveal the unsteady behaviour of the Dean vortices under turbulent flow conditions, the so-called "swirl-switching" phenomenon. In respect to the swirling turbulent pipe flow, covering a wide range of swirl strengths, POD has been employed to study the effect of varying strength of swirl on the Dean vortices as well as the interplay of swirling motion and Dean cells. Furthermore, the visualised large scale structures in turbulent swirling flows through the bend are found to incline and tear up with increasing swirl intensity. The present time-resolved, three component, experimental velocity field data will provide a unique and useful database for future studies; in particular for the CFD community.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Turbulent flow, Stereoscopic particle image velocimetry, Dean vortices, Curved pipe, Vortex switching, Swirl, Proper orthogonal decomposition
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-124984 (URN)10.1016/j.ijheatfluidflow.2013.01.003 (DOI)000321078700011 ()2-s2.0-84878525295 (Scopus ID)
Conference
9th International Symposium on Engineering Turbulence Modelling and Measurements (ETMM), JUN 06-08, 2012, Thessaloniki, Greece
Note

QC 20130806

Available from: 2013-08-06 Created: 2013-08-02 Last updated: 2017-12-06Bibliographically approved
4. Turbulent pipe flow past a pipe bend: effects of upstream conditions and curvature ratio
Open this publication in new window or tab >>Turbulent pipe flow past a pipe bend: effects of upstream conditions and curvature ratio
2014 (English)Report (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145309 (URN)
Note

QC 20140523

Available from: 2014-05-15 Created: 2014-05-15 Last updated: 2014-05-23Bibliographically approved
5. Vortical patterns in turbulent flow downstream a 90° curved pipe at high Womersley numbers
Open this publication in new window or tab >>Vortical patterns in turbulent flow downstream a 90° curved pipe at high Womersley numbers
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.

Keyword
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:nbn:se:kth:diva-139863 (URN)10.1016/j.ijheatfluidflow.2013.09.008 (DOI)000329594600061 ()2-s2.0-84888431227 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20140122

Available from: 2014-01-22 Created: 2014-01-15 Last updated: 2017-12-06Bibliographically approved
6. Some observations of pulsating, curved pipe flow and its influence on turbine maps
Open this publication in new window or tab >>Some observations of pulsating, curved pipe flow and its influence on turbine maps
2014 (English)Report (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145310 (URN)
Note

QC 20140523

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

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