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Numerical simulations of massively separated turbulent flows
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

It is well known that most fluid flows observed in nature or encountered in engineering applications are turbulent and involve separation. Fluid flows in turbines, diffusers and channels with sudden expansions are among the widely observed areas where separation substantially alters the flow field and gives rise to complex flow dynamics. Such types of flows are referred to as internal flows since they are confined within solid surfaces and predominantly involve the generation or utilization of mechanical power. However, there is also a vast variety of engineering applications where the fluid flows past solid structures, such as the flow of air around an airplane or that of water around a submarine. These are called external flows and as in the former case the downstream evolution of the flow field is crucially influenced by separation.

The present doctoral thesis addresses both internal and external separated flows by means of direct numerical simulations of the incompressible Navier-Stokes equations. For internal flows, the wall-driven flow in a onesided expansion channel and the pressure-driven flow in a plane channel with a single thin-plate obstruction have been studied in the fully developed turbulent state. Since such geometrical configurations involve spatially developing turbulent flows, proper inflow conditions are to be employed in order to provide a realistic fully turbulent flow at the input. For this purpose, a newly developed technique has been used in order to mimic an infinitely long channel section upstream of the expansion and the obstruction, respectively. With this approach, we are able to gather accurate mean flow and turbulence statistics throughout each flow domain and to explore in detail the instantaneous flow topology in the separated shear layers, recirculation regions as well as the recovery zones.

For external flows, on the other hand, the flow past a prolate spheroid has been studied. Here, a wide range of Reynolds numbers is taken into consideration. Based on the characteristics of the vortical structures in the wake, the flow past a prolate spheroid is classified as laminar (steady or unsteady), transitional or turbulent. In each flow regime, the characteristic features of the flow are investigated by means of detailed frequency analysis, instantaneous vortex topology and three-dimensional flow visualizations.

Place, publisher, year, edition, pages
NTNU, 2010.
Series
Doctoral theses at NTNU, ISSN 1503-8181 ; 2011:5
Keyword [en]
Direct numerical simulations, incompressible flows, separation, turbulent inflow conditions, wall-bounded flows, sudden expansion flows, obstructed flows, bluff body flows, prolate spheroid
Identifiers
URN: urn:nbn:no:ntnu:diva-12217ISBN: 978-82-471-2494-9 (printed ver.)ISBN: 978-82-471-2495-6 (electronic ver.)OAI: oai:DiVA.org:ntnu-12217DiVA: diva2:403131
Public defence
2011-01-28, 00:00
Available from: 2011-03-11 Created: 2011-03-10 Last updated: 2011-03-11Bibliographically approved
List of papers
1. Inflow conditions for inhomogeneous turbulent flows
Open this publication in new window or tab >>Inflow conditions for inhomogeneous turbulent flows
2009 (English)In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 60, no 2, 227-235 p.Article in journal (Refereed) Published
Abstract [en]

A cost-effective method to generate inflow conditions for direct numerical simulations of wall-bounded flows is presented. The method recycles a finite-length time series of instantaneous velocity planes extracted from a precursor simulation and has earlier proved efficient for free shear layers. Now a spatially developing plane channel flow is considered. Different durations, of the time series are tested and compared. Excellent agreement with fully developed channel flow statistics is observed when, equals or exceeds the large-eddy turnover time scale. The present results are more realistic than those obtained with. synthetic turbulence generation and at the same time substantially cheaper than running an auxiliary simulation in parallel. Copyright (C) 2008 John Wiley & Sons, Ltd.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2009
Keyword
wall-bounded flow, turbulence, inhomogeneous flow, DNS, inflow BC, CFD
Identifiers
urn:nbn:no:ntnu:diva-11024 (URN)10.1002/fld.1884 (DOI)000265344700005 ()
Available from: 2010-10-04 Created: 2010-10-01 Last updated: 2011-03-11Bibliographically approved
2. Massive separation of turbulent Couette flow in a one-sided expansion channel
Open this publication in new window or tab >>Massive separation of turbulent Couette flow in a one-sided expansion channel
2010 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 31, no 3, 274-283 p.Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulation has been performed to study wall-driven flow over a backward-facing step at Reynolds number Re = 5200 based on the step height h and the upper-wall velocity U-w. The flow configuration consisted of a step with height equal to that of the upstream channel yielding an expansion ratio 2:1. Instantaneous enstrophy contours revealed the formation of Kelvin-Helmholtz instabilities downstream of the step. Intense velocity and vorticity fluctuations were generated in the shear-layer formed between the bulk flow and the massive recirculation zone in the lee of the step. Extraordinarily high turbulence levels persisted in the center region even 7.5h downstream of the step, i.e. where the separated shear-layer reattached to the wall. A fully redeveloped Couette flow cannot be reached in the downstream part of the channel due to the principle of mass conservation. The local wall pressure coefficient gave evidence of an adverse pressure gradient in the recovery region, where a Couette-Poiseuille flow type prevailed. (C) 2010 Elsevier Inc. All rights reserved.

Keyword
Couette flow, Separation, DNS, Backward-facing step
Identifiers
urn:nbn:no:ntnu:diva-11021 (URN)10.1016/j.ijheatfluidflow.2010.01.008 (DOI)000279062700005 ()
Available from: 2010-10-04 Created: 2010-10-01 Last updated: 2011-03-11Bibliographically approved
3. Simulating turbulent Dean flow in Cartesian coordinates
Open this publication in new window or tab >>Simulating turbulent Dean flow in Cartesian coordinates
2009 (English)In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 60, no 3, 263-274 p.Article in journal (Refereed) Published
Abstract [en]

A simplified approach to simulate turbulent flows in curved channels is proposed. A set of governing equations of motion in Cartesian coordinates is derived from the full Navier-Stokes equations in cylindrical coordinates. Terms to first order in the dimensionless Curvature parameter are retained, whereas higher-order terms are neglected. The curvature terms are implemented in a conventional Navier-Stokes code using Cartesian coordinates. Direct numerical simulations (DNS) of turbulent flow in weakly Curved channels are performed. The pronounced asymmetries in the mean flow and the turbulence statistics observed in earlier DNS studies are faithfully reproduced by the present simplified Navier-Stokes model. It is particularly rewarding that also distinct pairs of counter-rotating streamwise-oriented vortices are embedded in the simulated flow field. Copyright (C) 2008 John Wiley & Sons, Ltd.

Keyword
channel flow, streamline curvature, roll cells, DNS
Identifiers
urn:nbn:no:ntnu:diva-12165 (URN)10.1002/fld.1886 (DOI)000265996700002 ()
Available from: 2011-03-03 Created: 2011-03-03 Last updated: 2011-03-11Bibliographically approved
4. Crossflow past a prolate spheroid at Reynolds number of 10 000
Open this publication in new window or tab >>Crossflow past a prolate spheroid at Reynolds number of 10 000
2010 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 659, 365-374 p.Article in journal (Refereed) Published
Abstract [en]

The flow field around a 6: 1 prolate spheroid has been investigated by means of direct numerical simulations. Contrary to earlier studies the major axis of the spheroid was oriented perpendicular to the oncoming flow. At the subcritical Reynolds number 10 000 the laminar boundary layer separated from the frontal side of the spheroid and formed an elliptical vortex sheet. The detached shear layer was unstable from its very inception and even the near-wake turned out to be turbulent. The Strouhal number associated with the large-scale shedding was 0.156, significantly below that of the wake of a sphere. A higher-frequency mode was associated with Kelvin-Helmholtz instabilities in the detached shear layer. The shape of the near-wake mirrored the shape of the spheroid. Some 10 minor diameters downstream, the major axis of the wake became aligned with the minor axis of the spheroid.

Keyword
bluff body, DNS, ellipsoid, turbulent flows, wakes
Identifiers
urn:nbn:no:ntnu:diva-12164 (URN)10.1017/S0022112010003216 (DOI)000282118200015 ()
Available from: 2011-03-03 Created: 2011-03-03 Last updated: 2011-03-11Bibliographically approved
5. Asymmetries in an obstructed turbulent channel flow
Open this publication in new window or tab >>Asymmetries in an obstructed turbulent channel flow
2010 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 9, 095103- p.Article in journal (Refereed) Published
Abstract [en]

The asymmetric flow pattern caused by a single thin-plate obstruction in a plane channel has been explored by means of direct numerical simulations. The blockage ratio was 1:2 and the bulk Reynolds number about 5700. In order to mimic an infinitely long channel section upstream of the obstruction, realistic dynamic inflow conditions were provided by a promising technique proposed by Barri ["Inflow conditions for inhomogeneous turbulent flows," Int. J. Numer. Methods Fluids 60, 227 (2009)]. The fluid downstream of the symmetric obstruction was sucked toward one side where a modestly long region of rather strong recirculating flow was observed. The weaker recirculation bubble formed at the opposite side was 17 times longer than the obstruction height and almost four times the size of the shorter bubble. The overall flow pattern turned out to be rather different from that observed in a similar study of channel flow subjected to periodically repeating obstructions by Makino ["Turbulent structures and statistics in turbulent channel flow with two-dimensional slits," Int. J. Heat Fluid Flow 29, 602 (2008)]. An anomalous variation of the pressure coefficient was observed with an excessively low pressure below the shorter of the bubbles. A locally high pressure occurred where the deflected jet flow impinges on the wall, whereas another pressure minimum could be associated with the flow acceleration caused by the severe blockage due to the major recirculation bubble. The turbulent fluctuations were suppressed due to the acceleration through the obstruction and high levels of streamwise velocity persisted far downstream. Exceptionally high turbulence levels were observed in the mixing-layers emanating from the two sides of the obstruction. The turbulence in these mixing-layers turned out to be qualitatively and quantitatively different on the two sides and exhibited distinctly different anisotropies. (C) 2010 American Institute of Physics. [doi:10.1063/1.3478974]

Keyword
bubbles, channel flow, jets, mixing, numerical analysis, pattern formation, turbulence, two-phase flow
Identifiers
urn:nbn:no:ntnu:diva-12163 (URN)10.1063/1.3478974 (DOI)000282437100034 ()
Available from: 2011-03-03 Created: 2011-03-03 Last updated: 2011-03-11Bibliographically approved
6. Wakes behind a prolate spheroid in crossflow
Open this publication in new window or tab >>Wakes behind a prolate spheroid in crossflow
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:no:ntnu:diva-12197 (URN)
Available from: 2011-03-09 Created: 2011-03-09 Last updated: 2011-03-11Bibliographically approved
7. DNS of backward-facing step flow with fully turbulent inflow
Open this publication in new window or tab >>DNS of backward-facing step flow with fully turbulent inflow
2009 (English)In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Wiley, 2009
Identifiers
urn:nbn:no:ntnu:diva-11031 (URN)10.1002/fld.2176 (DOI)
Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2011-03-11Bibliographically approved

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