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On the Importance of Turbulence Modelling of the Flow after a 90 Pipe Bend
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0002-6603-0099
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0001-7330-6965
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The flow in a free jet after a 90 pipe bend has been investigated by Large Eddy Simulations (LES) and Reynolds Averaged Navier-Stokes (RANS) simulations. The numerical results for the mean velocity profiles, flow fields, and power spectral densities have been compared to experimental data. The results show that LES has been able to predict the mean components of the velocity field and in resolving dynamic motions such as vortex switching. LES without an explicit SGS model (termed as ILES in the following) has been found to work well with flows of this type as long as the grid resolution is sufficiently fine. Different Sub-Grid-Scale (SGS) models have also been used. LES with different SGS models result in very similar results when a fine enough grid is used. The ILES approach also gives reasonably accurate results for the mean values even for coarser grids.

National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-151395OAI: oai:DiVA.org:kth-151395DiVA: diva2:748452
Note

QS 2014

Available from: 2014-09-19 Created: 2014-09-19 Last updated: 2014-09-19Bibliographically approved
In thesis
1. Large Eddy Simulations of Complex Flows in IC-Engine's Exhaust Manifold and Turbine
Open this publication in new window or tab >>Large Eddy Simulations of Complex Flows in IC-Engine's Exhaust Manifold and Turbine
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis deals with the flow in pipe bends and radial turbines geometries that are commonly found in an Internal Combustion Engine (ICE). The development phase of internal combustion engines relies more and more on simulations as an important complement to experiments. This is partly because of the reduction in development cost and the shortening of the development time. This is one of the reasons for the need of more accurate and predictive simulations. By using more complex computational methods the accuracy and predictive capabilities are increased. The disadvantage of using more sophisticated tools is that the computational time is increasing, making such tools less attractive for standard design purposes. Hence, one of the goals of the work has been to contribute to assess and improve the predictive capability of the simpler methods used by the industry.

By comparing results from experiments, Reynolds Averaged Navier-Stokes (RANS) computations, and Large Eddy Simulations (LES) the accuracy of the different computational methods can be established. The advantages of using LES over RANS for the flows under consideration stems from the unsteadiness of the flow in the engine manifold. When such unsteadiness overlaps the natural turbulence the model lacks a rational foundation. The thesis considers the effect of the cyclic flow on the chosen numerical models. The LES calculations have proven to be able to predict the mean field and the fluctuations very well when compared to the experimental data. Also the effects of pulsatile exhaust flow on the performance of the turbine of a turbocharging system is assessed. Both steady and pulsating inlet conditions are considered for the turbine case, where the latter is a more realistic representation of the real flow situation inside the exhaust manifold and turbine. The results have been analysed using different methods: single point Fast Fourier Transforms (FFT), probe line means and statistics, area and volume based Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD).

Abstract [sv]

Denna avhandling behandlar flödet i rörkrökar och radiella turbiner som vanligtvis återfinns i en förbränningsmotor. Utvecklingsfasen av förbränningsmotorer bygger mer och mer på att simuleringar är ett viktigt komplement till experiment. Detta beror delvis på minskade utvecklingskostnader men även på kortare utevklningstider. Detta är en av anledningarna till att man behöver mer exakta och prediktiva simuleringsmetoder. Genom att använda mer komplexa beräkningsmetoder så kan både nogrannheten och prediktiviteten öka. Nackdelen med att använda mer sofistikerade metoder är att beräkningstiden ökar, vilket medför att sådana verktyg är mindre attraktiva för standardiserade design ändamål. Härav, ett av målen med projektet har varit att bidra med att bedöma och förbättra de enklare metodernas prediktionsförmåga som används utav industrin.

Genom att jämföra resultat från experiment, Reynolds Averaged Navier-Stokes (RANS) och Large Eddy Simulations (LES) så kan nogrannheten hos de olika simuleringsmetoderna fastställas. Fördelarna med att använda LES istället för RANS när det gäller de undersökta flödena kommer ifrån det instationära flödet i grenröret. När denna instationäritet överlappar den naturligt förekommande turbulensen så saknar modellen en rationell grund. Denna avhandling behandlar effekten av de cykliska flöderna på de valda numeriska modellerna. LES beräkningarna har bevisats kunna förutsäga medelfältet och fluktuationerna väldigt väl när man jämför med experimentell data. Effekterna som den pulserande avgasströmning har på turboladdarens turbin prestanda har också kunnat fastställas. Både konstant och pulserande inlopps randvillkor har används för turbinfallet, där det senare är ett mer realistiskt representation av den riktiga strömningsbilden innuti avgasgrenröret och turbinen. Resultaten har analyserats på flera olika sätt: snabba Fourier transformer (FFT) i enskilda punkter, medelvärden och statistik på problinjer, area och volumsbaserade metoder så som Proper Orthogonal Decomposition (POD) samt Dynamic Mode Decomposition (DMD).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. viii, 68 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:20
Keyword
Large Eddy Simulations, Reynolds Averaged Navier-Stokes, Turbocharger, Turbine, Curved Pipes, Pulsatile Flow, Proper Orthogonal Decomposition, Large Eddy Simulations, Reynolds Averaged Navier-Stokes, Turboladdare, Turbin, Rörkrök, Pulserande Flöde, Proper Orthogonal Decomposition
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-151399 (URN)978-91-7595-270-1 (ISBN)
Public defence
2014-10-03, D1, Lindstedtsvägen 17, 5tr, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20140919

Available from: 2014-09-19 Created: 2014-09-19 Last updated: 2014-09-19Bibliographically approved

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