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Shock Unsteadiness and Shock Induced Separation at Transonic Flow Over a Bump
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
2008 (English)In: 38th AIAA Fluid Dynamics Conference and Exhibit, 2008Conference paper, Published paper (Refereed)
Abstract [en]

Numerical simulations of internal, transonic flow has been carried out using Large Eddy Simulation. The motion of the unsteady shock and separation has been studied by using Large Eddy Simulations for steady and unsteady boundary conditions. The shock position is highly sensitive to small changes in boundary conditions in the transonic flow range. The shock position and the extent of the separated flow behind it exhibit hysteretic behavior. For the steady-state inflow conditions one observes the presence of certain modes. One of these can be related to the acoustics of the channel, where as another can be related to the shear-layer instability associated with the separation bubble.

Place, publisher, year, edition, pages
2008.
Keyword [en]
Boundary conditions, Bubbles (in fluids), Fluid dynamics, Hysteresis, Large eddy simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-49128Scopus ID: 2-s2.0-78049513303ISBN: 9781563479427 (print)OAI: oai:DiVA.org:kth-49128DiVA: diva2:459280
Conference
38th Fluid Dynamics Conference and Exhibit, 23 - 26 June 2008, Seattle, Washington
Projects
CICERO
Note

QC 20111129

Available from: 2011-11-25 Created: 2011-11-25 Last updated: 2014-10-10Bibliographically approved
In thesis
1. Simulations of compressible flows associatedwith internal combustion engines
Open this publication in new window or tab >>Simulations of compressible flows associatedwith internal combustion engines
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Vehicles with internal combustion (IC) engines fueled by hydrocarbon compoundshave been used for more than 100 years for ground transportation.During these years and in particular the last decade, the environmental aspectsof IC engines have become a major political and research topic. Followingthis interest, the emissions of pollutants such as NOx, CO2 and unburnedhydrocarbons (UHC) from IC engines have been reduced considerably.Yet, there is still a clear need and possibility to improve engine efficiencywhile further reducing emissions of pollutants. The maximum efficiency ofIC engines used in passenger cars is no more than 40% and considerably lessthan that under part load conditions. One way to improve engine efficiencyis to utilize the energy of the exhaust gases to turbocharge the engine. Whileturbocharging is by no means a new concept, its design and integration intothe gas exchange system has been of low priority in the power train designprocess. One expects that the rapidly increasing interest in efficient passengercar engines would mean that the use of turbo technology will become morewidespread.The flow in the IC-engine intake manifold determines the flow in the cylinderprior and during the combustion. Similarly, the flow in the exhaust manifolddetermines the flow into the turbine, and thereby the efficiency of theturbocharging system.In order to reduce NOx emissions, exhaust gas recirculation (EGR) is used.As this process transport exhaust gases into the cylinder, its efficiency is dependenton the gas exchange system in general. The losses in the gas exchangesystem are also an issue related to engine efficiency. These aspects have beenaddressed up to now rather superficially. One has been interested in globalaspects (e.g. pressure drop, turbine efficiency) under steady state conditions.In this thesis, the flow in the exhaust port and close to the valve as wellas in the exhaust manifold is studied. Since the flow in the port can be transonic,we study first the numerical modeling of such a flow in a more simplegeometry, namely a bump placed in a wind tunnel. Large-Eddy Simulationsof internal transonic flow have been carried out. The results show that transonicflow in general is very sensitive to small disturbances in the boundaryconditions. Flow in the wind tunnel case is always highly unsteady in the transonicflow regime with self excited shock oscillations and associated with that 

also unsteady boundary-layer separation. The interaction between separationzone and shock dynamics was carried out by one-, and two-point correlationsas well as dynamic mode decomposition (DMD). A clear connection betweenseparation bubble dynamics and shock oscillation was found. To investigatesensitivity to periodic disturbances the outlet pressure in the wind tunnel casewas varied periodically at rather low amplitude. These low amplitude oscillationscaused hysteretic behavior in the mean shock position and appearance ofshocks of widely different patterns.The study of a model exhaust port shows that at realistic pressure ratios,the flow is transonic in the exhaust port. Furthermore, two pairs of vortexstructures are created downstream of the valve plate by the wake behind thevalve stem and by inertial forces and the pressure gradient in the port. Thesestructures dissipate rather quickly. The impact of these structures and thechoking effect caused by the shock on realistic IC engine performance remainsto be studied in the future.The flow in a heavy-duty exhaust manifold was studied under steady andengine-like boundary conditions. At all conditions, significantly unsteady flowis generated in the manifold and at the inlets to the turbine and EGR cooler.The inflow to the turbine is dominated by a combination of the blow-downpulse coming from one cylinder, and the scavenging pulse from another at thefiring frequency.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. x, 87 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:17
Keyword
LES, Gas exhange, exhaust port, exhaust manifold, POD, DMD, exhaust, LES, Gasväxling, avgasport, avgasgrenrör, POD, DMD
National Category
Fluid Mechanics and Acoustics Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-114261 (URN)978-91-7501-623-8 (ISBN)
Public defence
2013-02-08, F3, Linstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Projects
CCGEx
Note

QC 20130117

Available from: 2013-01-17 Created: 2013-01-15 Last updated: 2013-01-17Bibliographically approved

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Scopushttp://www.aiaa.org/content.cfm?pageid=406&keyword=AIAA%202008-4174

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