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  • 1.
    Bodin, Olle
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Analysis of transonic shock/boundary-layer interaction by spectral methodsArtikel i tidskrift (Övrigt vetenskapligt)
  • 2.
    Bodin, Olle
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Wang, Yue
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Analysis of the flow structures in the exhaust manifold of a heavy duty engineArtikel i tidskrift (Övrigt vetenskapligt)
  • 3.
    Bodin, Olle
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wang, Yue
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    LES of the Exhaust Flow in a Heavy-Duty Engine2014Ingår i: Oil & gas science and technology, ISSN 1294-4475, E-ISSN 1953-8189, Vol. 69, nr 1, s. 177-188Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flow in the exhaust port and the exhaust manifold of a heavy-duty Diesel engine has been studied using the Large Eddy Simulation approach. Some of the flow characteristics in these components are: flow unsteadiness and separation combined with significant geometry-induced secondary flow motion. Detailed analysis of these features may add understanding which can be used to decrease the flow losses and increase the eciency of downstream components such as turbochargers and EGR coolers. Few LES studies of the flow in these components have been conducted in the past and this, together with the complexity of the flow are the motivations for this work. This paper shows that in the exhaust port, even global parameters like total pressure losses are handled better by LES than RANS. Flow structures of the type that afect both turbine performance and EGR cooler efficiency are generated in the manifold and these are found to vary significantly during the exhaust pulse. This paper also clearly illustrates the need to make coupled simulations in order to handle the complicated boundary conditions of these gas exchange components.

  • 4.
    Ceci, Alessandro
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Gojon, Romain
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. ISAE-SUPAERO, Toulouse, France.
    Mihaescu, Mihai
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage2018Ingår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Flow, Turbulence and Combustion, ISSN 1386-6184, s. 1-13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The combustion noise in aero-engines is known to originate from two different sources. First, the unsteady heat release in the combustion chamber generates the direct combustion noise. Second, hot and cold spots of air generated by the combustion process are convected and accelerated by the turbine stages and give rise to the so-called indirect combustion noise. The present work targets, by using a numerical approach, the generation mechanism of indirect combustion noise for a simplified geometry of a turbine stator passage. Periodic temperature fluctuations are imposed at the inlet, permitting to simulate hot and cold packets of air coming from the unsteady combustion. Three-dimensional Large Eddy Simulation (LES) calculations are conducted for transonic operating conditions to evaluate the blade acoustic response to the forced temperature perturbations at the inlet plane. Transonic conditions are characterized by trailing edge expansion waves and shocks. It is notably shown that their movement can be excited if disturbances with a particular frequency are injected in the domain.

  • 5.
    Ceci, Alessandro
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Gojon, Romain
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage2019Ingår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 102, nr 2, s. 299-311Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The combustion noise in aero-engines is known to originate from two different sources. First, the unsteady heat release in the combustion chamber generates the direct combustion noise. Second, hot and cold spots of air generated by the combustion process are convected and accelerated by the turbine stages and give rise to the so-called indirect combustion noise. The present work targets, by using a numerical approach, the generation mechanism of indirect combustion noise for a simplified geometry of a turbine stator passage. Periodic temperature fluctuations are imposed at the inlet, permitting to simulate hot and cold packets of air coming from the unsteady combustion. Three-dimensional Large Eddy Simulation (LES) calculations are conducted for transonic operating conditions to evaluate the blade acoustic response to the forced temperature perturbations at the inlet plane. Transonic conditions are characterized by trailing edge expansion waves and shocks. It is notably shown that their movement can be excited if disturbances with a particular frequency are injected in the domain.

  • 6.
    Chandramouli, Sathyanarayanan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Gojon, Romain
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fridh, Jens
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Kraft- och värmeteknologi. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical characterization of entropy noise with a density based solver2017Ingår i: 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, KTH Royal Institute of Technology, 2017Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this work, dbnsTurbFoam, a new coupled density based solver, written in the framework of FOAM-EXTEND, is considered. The solver is first assessed on two canonical compressible flow scenarios, namely the Sod's shock tube and the ONERA S8 transonic channel. Results are compared with analytical formulations and experiments, respectively. 2-D Unsteady Reynolds Averaged Navier-Stokes simulations and 3-D Large Eddy Simulations of the flow within the passages of a geometrically simplified High Pressure Turbine Nozzle Guide Vane are then performed. Results are compared against experimental data in order to justify the geometrical simplifications made. Finally, the case of a sinusoidal temperature forcing at the inlet is considered in order to study the phenomenon of indirect combustion noise. Notably, the impact of the forcing on the vortex shedding dynamics and on the losses is discussed.

  • 7.
    Chen, Song
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Gojon, Romain
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    High-Temperature Effects on Aerodynamic and Acoustic Characteristics of a Rectangular Supersonic Jet2018Ingår i: AIAA/CEAS Aeroacoustics Conference, AIAA AVIATION Forum, 2018 / [ed] AIAA, 2018, artikel-id 3303Konferensbidrag (Refereegranskat)
    Abstract [en]

    Implicit large-eddy simulations (LES) are performed in this work to study the flow field and acous-tic characteristics of a rectangular supersonic jet. The focus is to investigate the high-temperatureeffects, i.e. when the jet total temperature is as high as 2100 K. Four cases with a jet temperatureratio(TR) of 1.0, 2.0, 4.0 and 7.0 are investigated. The rectangular nozzle selected for this study hasan aspect ratio of 2. The jets are overexpanded, with a series of shock cells in the jet core region.An artificial dissipation mechanism is used to damp the numerical oscillation and to represent theeffect of small-scale turbulence. The temperature-dependent thermal properties of air within thehigh-temperature regime are also considered by using the chemical equilibrium assumption. Thenumerical results show that the high temperature significantly increases the jet velocity and acousticMach number, although the jet Mach number is maintained roughly the same. Meanwhile, the lengthof the jet core region of the hot jet (TR = 7.0) is found to be reduced by around 30 %, compared tothe cold jet. The convection velocity and acoustic convection Mach number in the shear layer are alsoobserved to be increased when the jet temperature is high. The elevated acoustic convection Machnumber directly leads to a strong Mach wave radiation, and the crackle noise component has beenidentified by the pressure skewness and kurtosis factors. The Strouhal number of the screech tone isfound to be decreased slightly, and good agreements between the numerical results and the theoreticalanalysis are observed. Moreover, the sound pressure levels (SPL) associated with turbulent mixing,screech, Mach wave radiation, and Broadband shock associated noise are all found to be amplified indifferent levels for the hot jets. In the far field, the SPL is strongly increased by the high-temperatureeffect. Higher SPL is notably observed in the Mach wave radiation directions.

  • 8.
    Chen, Song
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Nozzle Pressure Ratio Effects on Aerodynamics and Acoustics of a Highly-Heated Rectangular Supersonic Jet2019Ingår i: 25th AIAA/CEAS Aeroacoustics Conference, 2019, s. 16-, artikel-id AIAA 2019-2753Konferensbidrag (Refereegranskat)
    Abstract [en]

    Implicit large-eddy simulations (LES) are performed in this work to study the flow-field and acoustic characteristics of a highly-heated rectangular supersonic jet. The focus is on the nozzle pressure ratio(NPR) effects. Three NPRs are investigated including 3.0, 3.67, and 4.0, which correspond to the nozzle over-expansion, perfect-expansion, and under-expansion conditions respectively. The current hot jet has a nozzle temperature ratio (NTR) of 7.0, corresponding to a total temperature of around 2100K. The rectangular nozzle has an aspect ratio of 2.0 and has been extensively tested at the Universityof Cincinnati. An in-house CFD code with an artificial dissipation mechanism is used to perform the large-scale implicit LES computations. By studying the pressure contours, density gradients and dilatation, it is found that the three-dimensional jet shock/expansion wave structure changes signifi-cantly when the jet NPR increases from an over-expanded to under-expanded condition. The length of the laminar shear layer right outside the nozzle is extended to the downstream before transitioning to be turbulent. The distance between the nozzle lip and the first shock cell is doubled while the total number of shock cells keeps the same, which results in a longer jet potential core. The increase of nozzle NPR also provides about an 11% increase in jet velocity and a 25% increase in shear layer convection Mach number, which leads to a stronger Mach wave radiation noise component in the acoustic fields. Pressure spectra in the near field reveal that screech only exists in the over-expansion case and the broadband shock-associated noise is enhanced in the perfect and under expansion cases.The far-field acoustics at 40Deq is characterized by about 4 dB increase of the overall sound pressure level in the Mach wave radiation direction and about 2 – 3 dB increase in all other directions. The far-field pressure spectra also confirm that the screech noise component vanishes when the nozzle NPR is increased to perfect- and under-expanded conditions.

  • 9. de Luzan, Charles Farbos
    et al.
    Chen, Jie
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Khosla, Sid M.
    Gutmark, Ephraim
    Computational study of false vocal folds effects on unsteady airflows through static models of the human larynx2015Ingår i: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 48, nr 7, s. 1248-1257Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Compressible large eddy simulation is employed to numerically investigate the laryngeal flow. Symmetric static models of the human larynx with a divergent glottis are considered, with the presence of false vocal folds (FVFs). The compressible study agrees well with that of the incompressible study. Due to the high enough Reynolds number, the flow is unsteady and develops asymmetric states downstream of the glottis. The glottal jet curvature decreases with the presence of FVFs or the ventricular folds. The gap between the FVFs stretches the flow structure and reduces the jet curvature. The presence of FVFs has a significant effect on the laryngeal flow resistance. The intra-glottal vortex structures are formed on the divergent wall of the glottis, immediately downstream of the separation point. The vortices are then convected downstream and characterized by a significant negative static pressure. The FVFs are a main factor in the generation of stronger vortices, and thus on the closure of the TVFs. The direct link between the FVFs geometry and the motion of the TVFs, and by extension to the voice production, is of interest for medical applications as well as future research works. The presence of the FVFs also changes the dominant frequencies in the velocity and pressure spectra.

  • 10.
    Dragan, Valeriu
    et al.
    KTH.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    On the stability influence of trimmed vaneless diffusers in turbocharger applications2018Ingår i: Proceedings of the ASME Turbo Expo, ASME Press, 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    One key aspect of centrifugal compressor performance is the stability range, in particular at lower than optimal mass flows. This paper focuses on the stability improvement of a turbocharger-relevant compressor stage equipped with a trimmed vaneless diffuser, incorporated into its interior volute. Evidence in the literature suggests that such a diffuser might increase the stall margin, especially for the high speed lines. As is the case with all supercharged internal combustion engines, this would lead to improved overall efficiency, operational flexibility and lower emissions. Using advanced RANS turbulence modeling, a batch of test cases was setup in order to assess the feasibility of the concept as well as its potential drawbacks. Spalart-Allmaras and k-Ω SST models, both with rotation and curvature corrections, were benchmarked against the baseline experimental data. The new diffuser was compared with conventional diffuser pinching and shelving and the fully trimmed embodiment was found to be superior to all others. Stability was assessed through a number of user defined parameters, tailored to reveal the signs of flow instability even under steady state assumptions. In addition to monitoring the mass flow imbalance, individual loading of blade passages and radial force magnitude and orientation were also monitored. Stability was found to be increased with the degree of diffuser trimming, alas this came at an ever increasing pressure loss. Most entropy was found to be generated across the volute as the flow velocity was significantly higher than the baseline. Another aspect was the static pressure distortion across the outlet of the vaneless diffuser, which paradoxically is linked with higher stability. Hence, in order to maximize both stability and mitigate pressure losses, further optimization on the scroll area distribution will have to be carried out after considering the new scroll inlet angle and speed circumferential distributions.

  • 11.
    Duwig, Christophe
    et al.
    Energy Sciences, Lund University.
    Gherman, Bogdan
    Energy Sciences, Lund University.
    Mihaescu, Mihai
    Energy Sciences, Lund University.
    Salewski, Mirko
    Energy Sciences, Lund University.
    Fuchs, Laszlo
    Energy Sciences, Lund University.
    Numerical Study of Thermo-acoustic Waves Generation by a Swirling Flame Using a New Approach Based on large eddy simulation2005Ingår i: Volume 2: Turbo Expo 2005: Power for Land, Sea, and Air (GT2005), 2005, s. 67-75Konferensbidrag (Refereegranskat)
    Abstract [en]

    The new challenge of the Gas Turbine industry is to develop new technologies for meeting electricity demand growth and reducing harmful emissions. Thus, a better understanding of the combustion phenomenon and an improvement in simulation capabilities are needed. In this paper, we present a new technique that is computationally efficient, for capturing the thermo-acoustic waves in low Mach number combustors. The idea is to utilize the fact that the acoustic related pressure fluctuations are small as compared to the dynamic pressure. Semi-compressible LES of reacting flow is performed and while the pressure perturbations related to the acoustic-wave propagation are handled separately. The equations are solved individually while allowing interaction between the two systems. The technique has been applied to the study of the generation and propagation of thermo-acoustic waves in a combustion chamber. A sensitivity analysis is presented and most important instability modes are identified.

  • 12.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Analysis of 3 Dimensional Turbine Flow by using Mode Decomposition Techniques2014Ingår i: Proceedings of the ASME Turbo Expo, 2014, s. GT2014-26963-Konferensbidrag (Refereegranskat)
    Abstract [en]

     Today one of the most popular ways of lowering the fuel consumption and emissions of the Internal Combustion Engine (ICE) is by downsizing the engine. Downsizing means that the swept volumes of the cylinders are decreased; this lowers the frictional and thermal losses. By combining the downsizing with a well matched turbocharger system the performance is preserved while the advantages are retained. Since more and more of the development work is being performed by simulations there is an increasing need for more accurate methods. These methods are more complex and require more resources than the simpler, faster and more robust models used today. In this study Large Eddy Simulations (LES) of the unsteady flow in a radial turbine designed for a gasoline ICE has been performed and analysed. The flow inside the turbine is highly 3 dimensional, pulsating and characterized by secondary flow motions and high curvatures. All these are reasons for which the method of choice should be LES. LES is able to resolve a large range of scales and capture the flow dynamics. The considered case concerns a non-pulsating flow condition but with engine like mass flow and temperature. Post-processing tools based on Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are used to analyse the large amount of LES based flow data. The POD method is used to investigate the energy content of the dominant, large structures present in the flow. The DMD method on the other hand is used to reveal the flow structures responsible for specific frequencies found in the flow field. Preliminary data show a fair agreement between experimental data and LES results in terms of predicting the turbine performance parameters.

  • 13.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Analysis of Secondary Flow Induced by a 90 Bend in a Pipe Using Mode Decomposition Techniques2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this study unsteady simulations of the flow through and after a 90 pipe bend has been performed by Large Eddy Simulations (LES). In the passenger car engine there is an abundance of pipes and pipe bends. Since pipes and bends are often situated upstream of important engine components the flow in these needs to be well predicted. This entails that there is a need for accurate pipe flow simulations in order to ensure that the inflow conditions, to the e.g. cylinders or turbocharger, are as close to the experimental values as possible. The flow field is further studied by the use of Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). It has been found that there is a low frequency oscillation in the strength of the alternately dominant dean vortex at the exit of the pipe bend. This phenomenon is analysed and the mechanism for it is discussed.

  • 14.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Effects of inlet geometry on turbine performanceManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    In this study comparisons have been performed between gas-stand experiments,Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations, and LargeEddy Simulations (LES) of the ow through a radial turbine of a turbochargerdesigned for an internal combustion engine. The long term goal for the projectis to improve the prediction capabilities for the simpler computational modelsused by industry in the research and development of new products. At thepresent stage the eects of using simplied geometries and methods for turbineperformance predictions are assessed. Additionally, data obtained from URANScalculations is compared against experimental data and against unsteady LESresults. The comparisons are made in order to evaluate the employed methodologies,to know when to use simplied models with reasonable accuracy, as wellas to justify the use of more advanced methods when the models are inadequate.It was found out that the LES results are closer to the gas-stand experimentsthan the URANS predictions are.

  • 15.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Effects of Pulsation Frequency and Pulse Shape on Turbine PerformanceManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

      The current paper studies the pulsating flow in the exhaust manifold and turbine of a passenger car engine. The study focuses on three engine RPMs and two different pulse shapes, one normal shape and one shorter DEP like shape. By simulating different valve strategies one can investigate how to maximize the available exhaust flow energy to the turbine. The simulations have been performed with the Large Eddy Simulation (LES) method with boundary conditions received from GT-Power simulations. The study focuses on kinetic energy and turbine torque analysis of the incoming flow and of the flow in the turbine wheel region. It is found that the pulse shape is affecting the wheel torque significantly and that the kinetic energy entering the wheel region is different depending on which cylinder fired.

  • 16.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Exhaust flow pulsation effect on radial turbine performance2015Ingår i: 11th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2015, European Conference on Turbomachinery (ETC) , 2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    In the current vehicle manufacturing world the chase for a better fuel economy and better driveability is in high gear. One way of doing so is to investigate and optimize the turbocharger. In this paper the flow in a radial turbine of a passenger car turbocharger has been analysed by Large Eddy Simulations. The current simulations have investigated the effects of changing the inlet pulse frequency and inlet pulse shape on turbine's performance parameters (e.g. efficiency, shaft power, pressure distributions). Three different engine speeds and two different pulse shapes were chosen to be compared and analysed. With the total mass flow per pulse being constant for all cases there is a clear dependence of both pulse shape and frequency when it comes to e.g. wheel momentum. Higher frequency increases the peak momentum more than the minimum level. A reduction in pulse duration also increases peak momentum more than the minimum.

  • 17.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Numerical investigations of a free turbulent jet after a 90° pipe bendManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    In this study the ow out of pipe after a 90pipe bend has been investigated byperforming Large Eddy Simulations. Comparisons of velocity proles, ow eldsand power spectral densities have been performed between numerical simulationsand experiments. Pipe bends exists in many industrial applications especiallyin the internal combustion engine. Performing accurate and predictive pipe owsimulations is becoming more important when more of the development work isbeing performed by simulations. The long term goal of this project is to helpwith development of simpler models that are faster to run but give the sameaccuracy. During this study it was found that the method used works well bothin predicting mean values and in resolving dynamic motions such as vortexswitching. LES calculations without a SGS model has been proven to work wellwith ows of this type as long as the grid resolution is suciently ne.

  • 18.
    Fjällman, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    On the Importance of Turbulence Modelling of the Flow after a 90 Pipe BendManuskript (preprint) (Övrigt vetenskapligt)
    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.

  • 19. Gherman, B. G.
    et al.
    Malael, I.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Porumbel, I.
    Jet pump optimization through Reynolds averaged: Navier-Stokes simulation analysis2015Ingår i: 22nd AIAA Computational Fluid Dynamics Conference, American Institute of Aeronautics and Astronautics, 2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    The paper presents the aerodynamic analysis of an air jet pump by means of Reynolds Averaged Navier-Stokes (RANS) simulations. A baseline configuration, reproducing an existing jet pump is first analysed from the perspective of overall mean compressible flow behaviour, turbulence production and mixing, and efficiency performance. Several constructive solutions are proposed in order to achieve enhanced mixing and efficiency performance. All the cases are investigated for the same operating condition of interest. As a result of the numerical analysis, two solutions are selected as potential improved constructive solutions in terms of mixing and efficiency performance. It is intended to complement the study by further experimental measurements for the baseline set-up and for the two selected configurations, in order to assess their actual performance and to validate the numerical data.

  • 20.
    Gojon, Romain
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Baier, Florian
    Univ. of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet2017Ingår i: Proceedings of the 55th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics, 2017, s. 19-, artikel-id AIAA2017-0002Konferensbidrag (Refereegranskat)
    Abstract [en]

    In the first part of the paper, a modified artificial dissipation mechanism permitting to perform Large-Eddy Simulations of highly compressible flows is proposed. This dissipation mechanism is evaluated using one linear 2-D test case and two non-linear 2-D test cases. In the second part, the flow and acoustic near-field of rectangular supersonic jets are explored using compressible LES based on this modified artificial dissipation mechanism. At the exit of a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5, the jets are overexpanded. Four simulations with four different temperature ratios ranging from 1 to 3 are performed in order to characterize the effect of the temperature on the aerodynamic and aeroacoustic fields of the jets. The geometry of the nozzle and the exit conditions are chosen in order to match those in the experimental study conducted at the University of Cincinnati. It is shown that the total number of cells in the shock cell structure decreases with the increase of the temperature ratio. However, the temperature does not influence the size of the first shock cell and the linear decrease of the shock cell size in the downstream direction. The Overall Sound Pressure Levels are then plotted along the minor and major axis. It is seen that the intensity of the screech feedback mechanism increases with the Temperature Ratio. Moreover, for JetTR2.5 and JetTR3, the strong flapping motion of the jet along the minor axis due to the screech feedback mechanism seems to yield to an asymmetric organization of the Mach wave radiation. The convection velocity of the turbulent structures in the jet shear layers along the minor axis is then studied. Once normalized by the jet exit velocity, the convection velocity is shown to decrease with the jet temperature ratio. In the last part of the paper, the near- and far-field acoustic are studied. In the near-field, screech tones which frequencies are consistent with both experimental data and a theoretical model are observed. In the far-field, four acoustic components typical of non-ideally supersonic jets are observed, namely the screech noise, the broadband shock-associated noise, the mixing noise and the Mach wave noise. Their directivities and frequencies are in agreement with experimental results and models.

  • 21.
    Gojon, Romain
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Bogey, Christophe
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Large Eddy Simulation of Highly Compressible Jets with Tripped Boundary Layers2019Ingår i: Direct and Large-Eddy Simulation XI. ERCOFTAC Series. / [ed] Salvetti M., Armenio V., Fröhlich J., Geurts B., Kuerten H., Springer, 2019, vol. 25, s. 333-339Kapitel i bok, del av antologi (Refereegranskat)
    Abstract [en]

    In high-speed aircraft, supersonic jets used for propulsion can lead to very intense aerodynamically generated acoustic noise. Thus, there is a need to study the aerodynamic and aeroacoustic properties of highly compressible jets. In previous studies (Gojon et al, Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet, 2017, [1], Gojon et al, On the response of a rectangular supersonic jet to a near-field located parallel flat plate, 2017, [2]), several simulations of supersonic jets have been conducted. Unfortunately, the turbulence intensity at the nozzle exit was dependent on the internal geometry of the nozzle and could not be tuned. This is a pity given that, as shown experimentally (Zaman, AIAA J, 50(8):1784–1795, 2012, [3]) and numerically (Bogey et al, J Fluid Mech, 701:352–385, 2012, [4], Brés et al, Nozzle wall modeling in unstructured large eddy simulations for hot supersonic jet predictions, 2013, [5]) for subsonic and supersonic jets, the boundary layer state of the jet affects the jet flow and noise.

  • 22.
    Gojon, Romain
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. Univ Toulouse, DAEP, ISAE SUPAERO, 10 Ave Edouard Belin, F-31400 Toulouse, France.
    Bogey, Christophe
    École Centrale de Lyon, 69134 Ecully Cedex, France.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Oscillation Modes in Screeching Jets2018Ingår i: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, nr 7, s. 2918-2924Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nonideally expanded supersonic jets generate three basic noise components, namely, the turbulent mixing noise, the broadband shock-associated noise, and the screech noise. The mixing noise, obtained for both subsonic and supersonic jets, is most intense in the downstream direction; and it occurs at Strouhal numbers of around 0.15. The broadband shock-associated noise is radiated mainly in the radial direction, and it has a central frequency varying with the emission angle. The screech noise consists of tones measured in the upstream direction. These tones are due to an aeroacoustic feedback mechanism establishing between turbulent structures propagating downstream and acoustic waves propagating upstream.

  • 23.
    Gojon, Romain
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Gutmark, E.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    On the response of a rectangular supersonic jet to a near-field located parallel flat plate2017Ingår i: 23rd AIAA/CEAS Aeroacoustics Conference, 2017, American Institute of Aeronautics and Astronautics, 2017Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this paper, the flow and acoustic fields of a rectangular over-expanded supersonic jet interacting with a parallel plate are investigated using compressible LES. The jet exits from a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5. Four simulations with four different distances between the lower inner lip of the minor axis of the rectangular jet and the plate ranging from 0 to 3 equivalent diameters are performed. The geometry of the nozzle, the positions of the plate, and the exit conditions are chosen in order to match those in an experimental study conducted at the University of Cincinnati. Snapshots and mean velocity fields are first presented. A good agreement with the PIV experimental measurements is found. The Overall Sound Pressure Levels are then plotted along the minor and major axis. In a previous paper, the corresponding free jet has been found to undergo a strong flapping motion along the minor axis due to the screech feedback mechanism. In the present study, it is seen that the intensity of the screech feedback mechanism increases for some distances and decreases for some others compared to the one in the corresponding free jet. A study of the jets shear-layers is then proposed first by looking at two points space-time cross correlation of the axial velocity. The convection of the turbulent is thus studied. Then, two points space-time cross correlation of the pressure along the jets shear-layers are proposed and an amplification of the aeroacoustic feedback mechanism leading to screech noise is observed in the lower jet shear-layers for two cases. It is also observed that the screech feedback mechanism establishes mainly between the nozzle lips and the end of the tenth shock cell. The acoustic loading on the plate is finally studied. As pointed out in a previous study, the flapping motion of the jet at the screech frequency seems to yield to an asymmetric organization of the Mach wave radiation also at the screech frequency. Those organized Mach waves impinge in the plate and propagate back to the jet, exciting the shear-layer at the screech frequency. This will amplify the screech mechanism in the lower jet shear-layer. However, this amplification happens only for some nozzle-to-plate distances. Indeed, the screech mechanism leads to the formation of a standing wave pattern in terms of pressure loading at the screech frequency on the plate. There are regions with high amplitude, meaning the acoustic loading is organized mainly at the screech frequency, and regions with low amplitude, which means the acoustic loading is not organized mainly at the screech frequency. The amplification then depends on the location of the standing wave compared to the overall acoustic loading on the plate. If a region of high amplitude of the standing wave pattern coincide with the region of maximal acoustic loading, there is amplification of the screech mechanism.

  • 24.
    Gojon, Romain
    et al.
    ISAE-SUPAERO, University of Toulouse, Toulouse, France.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Antisymmetric Oscillation Modes in Rectangular Screeching Jets2019Ingår i: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 57, nr 8, s. 3422-3441Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, the origin and the properties of the oscillation modes in screeching non-ideally expanded rectangular jets are investigated using compressible implicit LES of rectangular supersonic jets. At the exit of a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5, the jets are under- and over-expanded. Seven simulations with four different temperature ratios ranging from 1 to 3 and two different nozzle pressure ratios are performed. The geometry of the nozzle and the exit conditions are chosen such that to match the experimental study conducted at the University of Cincinnati. First, the over-expanded jets are studied. It is shown that the total number of shock cells decreases with increased temperature ratio. However, the temperature does not influence the size of the first shock cell and the linear decrease of the shock cell size in the downstream direction. The spreading of the jet is observed to be higher along the minor axis plane than along the major axis plane. The intensity of the screech noise increases with the temperature ratio in the present study although the opposite is observed in the experiments. Moreover, for jet temperature ratios of 2.5 and 3, the strong flapping motion of the jet along the minor axis plane due to the screech feedback mechanism yields to an antisymmetric organization of the Mach wave radiation. Thereafter, the near- and far-field acoustic are studied. In the near-field, screech tones are captured, whose frequencies are consistent with both experimental data and theoretical models. In the far-field, four acoustic components typical of non-ideally expanded supersonic jets are observed, namely the screech noise, the broadband shock-associated noise, the mixing noise and the Mach wave noise. Their directivities and frequencies are in agreement with experimental results and models. The mechanism of the screech noise generation is studied by using a Fourier decomposition of the pressure field. For the four over-expanded jets, a flapping motion along the diagonal or along the minor axis plane of the jet is noted. Finally, the hypothesis that the acoustic waves completing the feedback loop in these jets are linked to the upstream-propagating acoustic wave modes of the equivalent ideally expanded jets is tested. Using a jet vortex sheet model to describe the dispersion relations of these modes, it is found that this hypothesis allows us to explain the antisymmetric jet oscillation observed at the screech frequencies. Based on frequency-wavenumber decomposition of the pressure fluctuations in the jets, it is shown that at the screech frequencies, acoustic waves propagating in the upstream direction at the ambient speed of sound exist also in the jet flow, additionally to the acoustic waves propagating outside of the jet. These acoustic waves belong to the neutral acoustic wave modes of the equivalent ideally expanded jet. These results support the idea that a vortex sheet model of the corresponding 2-D planar ideally expanded jet is capable of predicting the wave modes of a non-ideally expanded rectangular supersonic jet. They also suggest that these waves are involved in the feedback part of the screech mechanism; explaining why, for the simulated screeching rectangular jets, the associated oscillation mode is antisymmetric.

  • 25.
    Gutmark, Ephraim
    et al.
    Aerospace Engineering, University of Cincinnati.
    Toshihiko, ShakouchiMie University, Tsu, Japan.Mihaescu, MihaiKTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Flow, Turbulence and Combustion: Special Issue: Jets, Wakes, and Separated Flows2019Proceedings (redaktörskap) (Refereegranskat)
    Abstract [en]

    The Sixth International Conference on Jets, Wakes, and Separated Flows (ICJWSF17), was held at the University of Cincinnati, Ohio, USA in October 9–12, 2017. This special issue contains 14 papers selected from a total of 80 papers presented during the ICJWSF17 Conference. The conference goal was to give scientists, engineers and students from the US and other countries an opportunity to discuss research on jets, wakes, and separated flows, topics that are of significant importance and interest in the field of fluid mechanics, with impact on diverse fields of engineering such as aeroacoustics, combustion, heat transfer, multi-phase flows, and turbomachinery. The conference aims to promote exchange of information on advances in the field, and to spawn collaborative research between the participants. This special issue of Flow Turbulence and Combustion is the first to include selected papers from this conference. The papers were selected and reviewed on the basis of their quality as judged by the reviewers in relation to the aims and scope of the Journal.

    All papers published in this Special Issue were subjected to the same rigorous peer- review process as normal submissions to the Journal. It was therefore independent from the review of the conference submissions, which was used only in order to select the authors that were invited to submit a paper for this volume. The Guest Editors are grateful for the opportunity to publish this volume, which makes a substantial contribution to advancing the field of fluid mechanics.

  • 26.
    Jyothishkumar, V
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Semlitsch, Bernhard
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical Flow Analysis in a Centrifugal Compressor near Surge Condition2013Ingår i: 43rd AIAA Fluid Dynamics Conference and Exhibit, 2013 / [ed] AIAA, AIAA , 2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    This numerical study presents data relevant to the flow characteristics inside of a centrifugal compressor, at design and near-surge conditions. The main objectives were to characterize the flow structures and the associated instabilities near the stall point (prior to surge) and to contrast the obtained results against data acquired for a design operation condition. Generally, the operational range of compressors is limited at low mass flow rates by development of instabilities, e.g. stall and rotating stall. Such conditions lead to breakdown of the operability of the compressor, with flow reversal in the wheel passage. This results in large mass flow variations and pressure fluctuations within the compressor, lowering the compressor efficiency and pressure ratio. Large vibratory stresses are induced in the blade under such off-design operating conditions, affecting the blade life duration. Compressor stall and rotating stall are frequently regarded as “precursors” to the more damaging surge instability.  The flow fields under design and off-design operating conditions are calculated using the Large Eddy Simulation (LES) approach.  The complete geometry (360 degree) of the compressor is considered during analysis. It includes the ported shroud, the compressor wheel, the vaneless diffuser, the volute, and the exit pipe. The computationally expensive transient sliding mesh technique is used in order to capture the interaction between the wheel, the flow, and the stationary components of the compressor. The LES data are validated against available experimental measurements obtained under the same operating conditions (i.e. at design and off-design). The calculated frequency spectra when the compressor operated near-by the surge line indicated the presence of the rotating stall.

  • 27.
    Kerres, Bertrand
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.). Competence Center for Gas Exchange.
    Cronhjort, Andreas
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. Competence Center for Gas Exchange.
    A Comparison of On-Engine Surge Detection Algorithms Using Knock AccelerometersManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    On-engine surge detection could help in reducing the safety margintowards surge, thus allowing higher boosting pressures and ultimatelylow-end torque. In this paper, experimental data from a truckturbocharger compressor mounted on the engine is investigated. Ashort period of compressor surge is provoked through a sudden, largedrop in engine load. The compressor housing is equipped with knockaccelerometers. Different signal treatments are evaluated for theirsuitability with respect to on-engine surge detection: the signal rootmean square, the power spectral density in the surge frequency band,the recently proposed Hurst exponent, and a closely related conceptoptimized to detect changes in the underlying scaling behavior of thesignal. For validation purposes, a visual observation of the air filtervibrations are also used to diagnose surge. The four signal treatmentsare compared with respect to their reliability as surge indicator andthe time delay between surge onset and indication. Results show thatthe signal power in the surge frequency band has reasonably goodproperties as surge indicator. The normal Hurst exponent isproblematic, since periodic vibrations from engine firing dominatethe scaling behavior. Root mean square and the above mentionedscaling exponent do not measure vibrations caused by surge directly,but rather the reduction in housing vibrations due to the engine loaddrop; nevertheless, it was found to be possible to design an indicatorthat gives good results based on the change in scaling behavior.

  • 28.
    Kerres, Bertrand
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.). KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Cronhjort, Andreas
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.). KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Experimental investigation of upstream installation effects on the turbocharger compressor map2016Ingår i: The 12th International Conference on Turbochargers and Turbocharging, London, UK, 17-18 May, 2016, 2016Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper experimentally investigates the effects of an upstream bended pipe on the compressor speedline slopes and surge line. Different orientation angles for the incoming bended pipe relative to the compressor scroll are investigated. The tests were carried out on a cold gas stand on a passenger car sized turbocharger. A bended pipe upstream of the compressor leads to an increase of the surge margin. This effect does not depend on the orientation of the bend. Comparisons with a straight inlet with artificially generated pressure losses indicate that the increase in operating range is an effect of the pressure losses generated in the bend.

  • 29. Kerres, Bertrand
    et al.
    Cronhjort, Andreas
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.), Förbränningsmotorteknik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Stenlåås, Ola
    A comparison of on-engine surge detection algorithms using knock accelerometers2017Ingår i: SAE Technical Papers, 2017Konferensbidrag (Refereegranskat)
    Abstract [en]

    On-engine surge detection could help in reducing the safety margintowards surge, thus allowing higher boost pressures and ultimatelylow-end torque. In this paper, experimental data from a truckturbocharger compressor mounted on the engine is investigated. Ashort period of compressor surge is provoked through a sudden, largedrop in engine load. The compressor housing is equipped with knockaccelerometers. Different signal treatments are evaluated for theirsuitability with respect to on-engine surge detection: the signal rootmean square, the power spectral density in the surge frequency band,the recently proposed Hurst exponent, and a closely related conceptoptimized to detect changes in the underlying scaling behavior of thesignal. For validation purposes, a judgement by the test cell operatorby visual observation of the air filter vibrations and audible noises, aswell as inlet temperature increase, are also used to diagnose surge.The four signal treatments are compared with respect to theirreliability as surge indicator and the time delay between surge onsetand indication. Results show that the signal power in the surgefrequency band has reasonably good properties as surge indicator.The normal Hurst exponent is problematic, since periodic vibrationsfrom engine firing dominate the scaling behavior. Root mean squareand the above mentioned scaling exponent do not measure vibrationscaused by surge directly, but rather the reduction in housingvibrations due to the engine load drop. Nevertheless, it was found tobe possible to design an indicator that gives good results based on thechange in scaling behavior.

  • 30.
    Kerres, Bertrand
    et al.
    KTH, Skolan för industriell teknik och management (ITM).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Gancedo, Matthieu
    Univ. of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Optimal Pressure based Detection of Compressor Instabilities using the Hurst Exponent2017Ingår i: SAE International Journal of Engines, ISSN 1946-3936, E-ISSN 1946-3944, Vol. 10, nr 4, s. 1917-1926Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The compressor surge line of automotive turbochargers can limit the low-end torque of an engine. In order to determine how close the compressor operates to its surge limit, the Hurst exponent of the pressure signal has recently been proposed as a criterion. The Hurst exponent quantifies the fractal properties of a time series and its long-term memory. This paper evaluates the outcome of applying Hurst exponent based criterion on time-resolved pressure signals, measured simultaneously at different locations in the compression system. Experiments were performed using a truck-sized turbocharger on a cold gas stand at the University of Cincinnati. The pressure sensors were flush-mounted at different circumferential positions at the inlet of the compressor, in the diffuser and volute, as well as downstream of the compressor. Results show that the previously identified threshold value distinguishing between surge and stable operation when the analysis was carried out for a different and smaller compressor can be used also for this much larger compressor. The investigation concerning the sensor locations reveals that pressure sensors at the outlet or shortly upstream the volute tongue give the clearest distinction between fully stable operation and operation close to the surge line. Further investigations show that as currently implemented, the criterion would need a minimum sampling duration of 500 ms and sampling frequency of 512 Hz. An extended algorithm based on distinguishing between a mono- and multifractal pressure signal is shown to have potential as an early warning indicator.

  • 31.
    Kerres, Bertrand
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Optik och Fotonik, OFO.
    Nair, Vineeth
    KTH.
    Cronhjort, Andreas
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.), Förbränningsmotorteknik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Analysis of the Turbocharger Compressor Surge Margin Using a Hurst-Exponent-based Criterion2016Ingår i: SAE International Journal of Engines, ISSN 1946-3936, E-ISSN 1946-3944, Vol. 9, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Turbocharger compressors are limited in their operating range at low mass flows by compressor surge, thus restricting internal combustion engine operation at low engine speeds and high mean effective pressures. Since the exact location of the surge line in the compressor map depends on the whole gas exchange system, a safety margin towards surge must be provided. Accurate early surge detection could reduce this margin. During surge, the compressor outlet pressure fluctuates periodically. The Hurst exponent of the compressor outlet pressure is applied in this paper as an indicator to evaluate how close to the surge limit the compressor operates. It is a measure of the time-series memory that approaches zero for anti-persistence of the time series. That is, a Hurst exponent close to zero means a high statistical preference that a high value is followed by a low value, as during surge. Maps of a passenger-car sized turbocharger compressor with inlet geometries that result in different surge lines are measured on a cold gas stand. It is demonstrated that the Hurst exponent in fact decreases as the compressor moves towards surge, and that a constant value of the Hurst exponent can be used as a threshold for stable operation. Transient pressure signals of the compressor entering surge are analyzed in order to evaluate the time lag until surge can be detected using the Hurst exponent. Two surge cycles are usually needed to detect unstable operation. However, since the amplitude of these oscillations is relatively small for the first cycles, detection is possible before the oscillations grow into deep surge.

  • 32.
    Kerres, Bertrand
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.). KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Sanz, Sergio
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.). KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Sundström, Elias
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    A Comparison of Performance Predictions between 1D Models and Numerical Data for a Turbocharger Compressor2017Ingår i: Proceedings of 12th European Conference on Turbomachinery Fluid dynamics and Thermodynamics ETC12, April 3-7, Stockholm, Sweden, KTH Royal Institute of Technology, 2017Konferensbidrag (Refereegranskat)
    Abstract [en]

    Compressor performance prediction models, based on integral conservation of mass, momentum and energy with empirical loss terms, are important tools in early design stages. Two such models from literature are compared to numerical results for an automotive turbocharger radial compressor with a vaneless diffuser and a volute. Results show that these models are less accurate than fully three-dimensional numerical RANS CFD calculations at low impeller speeds and choke, but can compete at high impeller speeds. Of the two impeller models, one gives a more accurate prediction than the other. The diffuser and volute models investigated here show large differences to the CFD calculations at off-design conditions. A comparison of the impeller loss terms to CFD entropy increase indicates also possibilities for improvement in the impeller models.

  • 33.
    Lagerström, Robert
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Nätverk och systemteknik.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Critical Success Factors in E-Learning for Project-Based Courses2014Ingår i: EDULEARN14: 6th International Conference on Education and New Learning Technologies / [ed] Chova, LG; Martinez, AL; Torres, IC, 2014, s. 6125-6134Konferensbidrag (Refereegranskat)
  • 34.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Aerothermodynamics and Exergy Analysis in Radial Turbine With Heat Transfer2018Ingår i: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 140, nr 9, artikel-id 091007Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study was motivated by the difficulties to assess the aerothermodynamic effects of heat transfer on the performance of turbocharger turbine by only looking at the global performance parameters, and by the lack of efforts to quantify the physical mechanisms associated with heat transfer. In this study, we aimed to investigate the sensitivity of performance to heat loss, to quantify the aerothermodynamic mechanisms associated with heat transfer and to study the available energy utilization by a turbocharger turbine. Exergy analysis was performed based on the predicted three-dimensional flow field by detached eddy simulation (DES). Our study showed that at a specified mass flow rate, (1) pressure ratio drop is less sensitive to heat loss as compared to turbine power reduction, (2) turbine power drop due to heat loss is relatively insignificant as compared to the exergy lost via heat transfer and thermal irreversibilities, and (3) a single-stage turbine is not an effective machine to harvest all the available exhaust energy in the system.

  • 35.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Aerothermodynamics and exergy analysis of a turbocharger radial turbine integrated with exhaust manifold2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    Large temperature gradients are associated with automotive turbocharger and thus the turbine experiences significant heat loss. Currently, the investigation of aerothermodynamic effects as a result of heat loss in turbine is commonly done by looking at the global performance parameters, i.e. pressure ratio and efficiency. This study aims to investigate the aerothermodynamic effects of heat transfer on a radial turbine operating under engine-like pulsating flow condition by identifying and quantifying the loss mechanisms via an exergy-based method using Detached Eddy Simulation data. Major findings with this study are: 1) Although exergy lost via heat transfer and internal irreversibilities could be as much as the turbine power, the drop of turbine power is only 4% as compared to an adiabatic turbine;2) Only about 12% of the available exhaust energy is extracted by the investigated turbine.

  • 36.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Exergy analysis on turbocharger radial turbine with heat transfer2017Ingår i: 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, KTH Royal Institute of Technology, 2017Konferensbidrag (Refereegranskat)
    Abstract [en]

    Inconsistent results about heat transfer effects on performance and poor understanding of the aerothermodynamics loss mechanisms related to heat transfer in turbocharger turbine motivated this study. This study aimed to investigate the sensitivity of performance to heat loss and to quantify loss mechanisms associated with heat transfer in a turbine by using exergy analysis. A hybrid simulation methodology, i.e. Detached Eddy Simulation (DES) was used to compute the three-dimensional flow field of a turbine operating under hot gas stand continuous flow condition. Principal findings of this study were 1) Pressure ratio is less sensitive to heat loss as compared to turbine power, 2) Turbine power drop due to heat loss is relatively insignificant as compared to the exergy lost by heat transport and exergy destroyed by thermal irreversibilities, and 3) Assuming the most ideal isentropic gas expansion, more than 80% of the inflow exergy is unutilized in the investigated turbine system.

  • 37.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Influence of Upstream Exhaust Manifold on Pulsatile Turbocharger Turbine Performance2019Ingår i: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 141, nr 6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This research was primary motivated by limited efforts to understand the effects of secondary flow and flow unsteadiness on the heat transfer and the performance of a turbocharger turbine subjected to pulsatile flow. In this study, we aimed to investigate the influence of exhaust manifold on the flow physics and the performance of its downstream components, including the effects on heat transfer, under engine-like pulsatile flow conditions. Based on the predicted results by detached eddy simulation (DES), qualitative and quantitative flow fields analyses in the scroll and the rotor's inlet were performed, in addition to the quantification of turbine performance by using the flow exergy methodology. With the specified geometry configuration and exhaust valve strategy, our study showed that (1) the exhaust manifold influences the flow field and the heat transfer in the scroll significantly and (2) although the exhaust gas blow-down disturbs the relative flow angle at rotor inlet, the consequence on the turbine power is relatively small.

  • 38.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Mihaescu, Mihai
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Influence of upstream geometry on pulsatile turbocharger turbine performance2018Rapport (Övrigt vetenskapligt)
    Abstract [en]

    This research was primary motivated by limited efforts to understand the effects of secondary flow and flow unsteadiness on the heat transfer and the performance of a turbocharger turbine subjected to pulsatile flow. In this study, we aimed to investigate the influence of exhaust manifold on the flow physics and the performance of its downstream components, including the effects on heat transfer, under engine-like pulsatile flow conditions. Based on the predicted results by Detached Eddy Simulation (DES), qualitative and quantitative flow fields analyses in the scroll and the rotor’s inlet were performed, in addition to the quantification of turbine performance by using the flow exergy methodology. With the specified geometry configuration and exhaust valve strategy, our study showed that 1) The exhaust manifold influences the flow field and the heat transfer in the scroll significantly, and 2) Although the relative inflow angle at the rotor’s inlet is significantly affected by the initial exhaust gas blow down from the exhaust manifold, the consequence on the turbine power is relatively small.

  • 39.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Influence of upstream geometry on pulsatile turbocharger turbine performance2018Ingår i: Microturbines, Turbochargers, and Small Turbomachines; Steam Turbines, ASME Press, 2018, Vol. 8Konferensbidrag (Refereegranskat)
    Abstract [en]

    Unlike conventional turbomachinery, an automotive turbocharger's turbine is operated under unsteady hot and pulsatile flow due to the inherent nature of reciprocating engine. Although the turbine is integrated with exhaust manifold in real application, some experiments and numerical studies ignore its presence. In this study, we aimed to investigate the effects of upstream complex exhaust manifold on the prediction of pulse flow turbine performance via Detached Eddy Simulation (DES). Heat transfer was incorporated and the exergy based approach was used to quantify the heat transfer associated losses. Our primary results showed that under the investigated turbine stage, although the presence of exhaust manifold influences the prediction of heat transfer and internal irreversibilities in the scroll significantly, it does not significantly affect the prediction of turbine power, heat transfer and irreversibilities at the downstream components.

  • 40.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Wall Treatment Effects on the Heat Transfer in a Radial Turbine Turbocharger2016Ingår i: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, s. 439-447Konferensbidrag (Refereegranskat)
    Abstract [en]

    Contradicting results about heat transfer effects on the performance of turbine turbocharger motivated this study. It was aimed to assess the effects that the wall treatment in a numerical sense has on the performance of a radial turbine of automotive turbocharger operating under a continuous flow condition. Adiabatic and non-adiabatic conditions were analyzed by using Unsteady Reynolds Averaged Navier-Stokes (URANS), Large Eddy Simulations (LES) and Detached Eddy Simulations (DES) approaches. When considering heat transfer, heat transfer loss at various locations is highly dependent on the near-wall modelling approach employed. Development of thermal boundary layer in the upstream region of turbine affects how the gas is convected in the downstream components, such as the scroll and the rotor. As long as the deviation in predicting thermal boundary layer does not affect the prediction of gas temperature at the inlet and outlet of the rotor, the difference in turbine power prediction by different near-wall modelling approaches was found to be small.

  • 41.
    Lim, Shyang Maw
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Mihaescu, Mihai
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Dahlkild, Anders
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Aerothermodynamics and exergy analysis of a turbocharger radial turbine integrated with exhaust manifold2018Ingår i: Institution of Mechanical Engineers - 13th International Conference on Turbochargers and Turbocharging 2018, Institution of Mechanical Engineers , 2018, s. 459-471Konferensbidrag (Refereegranskat)
    Abstract [en]

    Large temperature gradients are associated with automotive turbocharger and thus the turbine experiences significant heat loss. Currently, the investigation of aerothermodynamic effects as a result of heat loss in turbine is commonly done by looking at the global performance parameters, i.e. pressure ratio and efficiency. This study aims to investigate the aerothermodynamic effects of heat transfer on a radial turbine operating under engine-like pulsating flow condition by identifying and quantifying the loss mechanisms via an exergy-based method using Detached Eddy Simulation data. Major findings with this study are: 1) Although exergy lost via heat transfer and internal irreversibilities could be as much as the turbine power, the drop of turbine power is only 4% as compared to an adiabatic turbine; 2) Only about 12% of the available exhaust energy is extracted by the investigated turbine.

  • 42.
    Majal, G. M.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Karlsson, M.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Katoshevski, D.
    Ben Gurion University of the Negev.
    Particle Number Reduction in Automotive Exhausts by Controlled Grouping2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    Particulate emissions from internal combustion engines is a well-known issue with direct implications on air quality and human health. Recently there is an increased concern about the high number of ultrafine particles emitted from modern engines. Here we explore a concept for grouping these particles, reducing their total number and shifting the relative size distribution towards fewer larger particles. Particles having a non-zero relaxation time may be manipulated to yield regions of high particle concentration, accommodating agglomeration, when introduced into an oscillating flow field. The oscillating flow field is given by simple periodic geometrical changes of the exhaust pipe itself. It is discussed how the shape of these geometrical changes and also the engine pulses effect the grouping behavior for different size particles, including when Brownian motion becomes relevant. Simulations are performed using a bespoke 1D-model sufficient for the basic parameter studies of the concept given here. 

  • 43.
    Malla, Bhupatindra
    et al.
    School of Aerospace Systems, University of Cincinnati.
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Kastner, Jeffrey
    School of Aerospace Systems, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Proper Orthogonal Decomposition on LES and PIV Data Sets from a Mach 0.9 Jet.2012Ingår i: 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition / [ed] AIAA, AIAA , 2012, s. 12-Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper contains the complementary analysis of the results obtained through computational and experimental methods using the technique of Proper Orthogonal Decomposition (POD). A detailed study of the experimentally obtained flow field data using the method of POD has already been accomplished. The experimental data consists of the 2D visualization of the velocity field obtained using Particle Image Velocimetry (PIV). The computational process is initialized with the simulation of the 3D flow field using a steady state RANS flow solver. RANS is used as a platform to proceed towards capturing the unsteadiness of the flow field which is accomplished using the Large Eddy Simulation (LES). A significant match in the characteristics of the mode shapes has been observed, in both the axial and radial components of the velocity field, however, there is difference in the distribution of the energies over the modes. The LES predicts the energies to be higher at the lowest modes and the energy drop off rate is higher. On the contrary, PIV predicts lower energies at the first few modes, in comparison to the LES results, however the energy drop off rate is smaller.

  • 44.
    Mihaescu, Mihai
    Energy Sciences, Lund University.
    Computational Aeroacoustics Based on Large Eddy Simulation and Acoustic Analogies2005Doktorsavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    The thesis presents a numerical method developed by the author and its applications for computing the generated sound by an unsteady flow field and its propagation. The full equations of motion for compressible and unsteady flows describe both flow field and sound generation and propagation. It is assumed that the flow variables can be decomposed into semi-compressible / incompressible components and inviscid, irrotational acoustic components. The present method is based on Large Eddy Simulation (LES) to compute the turbulent flow and an approach based on an inhomogeneous wave equation to compute the radiated acoustic field. In this way one can avoid the necessity for a very large computational effort associated with direct simulation of the near- and specially far- field sound generated by a turbulent flow. The governing equations are written in the form of a non-homogeneous wave equation for the acoustic fluctuation with acoustic sources on the right-hand side. The thesis includes the details of the coupling between the flow solver and the acoustic one, as well as the results for test cases employed to validate the numerical algorithm and the implemented boundary conditions. The method has been successfully applied to compute the near- and far- acoustic fields generated by various unsteady flows such as a round hot turbulent jet ejected from a pipe close to a solid boundary, coaxial turbulent non-isothermal jets (separate exhaust system), or the flow around a wind-turbine.

  • 45.
    Mihaescu, Mihai
    et al.
    Energy Sciences, Lund University.
    Fuchs, Laszlo
    Energy Sciences, Lund University.
    Sound Generated by an Unsteady Flow Field, Using a Hybrid Method2004Ingår i: Modelling Fluid Flow: The State of the Art, Springer-Verlag , 2004, 1, , s. 169-178s. 169-178Kapitel i bok, del av antologi (Refereegranskat)
    Abstract [en]

    The compressible Navier-Stokes equations describe both flow field as well as sound generation. The classical acoustic theory uses the hypothesis that the part of the flow field, which is the source of the acoustic field, has distinct scales so that the acoustic waves do not interfere with the flow. Thus, the velocity vector, the pressure and the density are split into semi-compressible components and an inviscid, irrotational acoustical components. The paper includes the details of the coupling between the turbulent flow (based on Large Eddy Simulation) and the acoustical part (based on Lighthill's acoustic analogy). The approach is applied to the acoustics of a non-isothermal jet.

  • 46.
    Mihaescu, Mihai
    et al.
    Aerospace Systems, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Modeling of the Flow and Acoustical Field due to a Single Jet with Chevrons2007Ingår i: 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 2007, s. 7372-7380Konferensbidrag (Refereegranskat)
    Abstract [en]

    The turbulent jets exhausting from jet engines are the main sources of aircraft noise. Finding new jet noise suppression methods and developing cheap and accurate jet noise prediction approaches was and is the major interest for many research groups. In the present paper the acoustic field generated by flow unsteadiness in an isothermal turbulent jet with forced mixers is determined using a hybrid approach. The computational domain includes in the first instance the near-field ”source” region and a small part of the acoustical region. The dynamics of the turbulent jet and its mixing with the surrounding air generate the acoustic sources that are calculated, on this restricted grid, by solving the flow field with Large Eddy Simulation (LES) approach. The sound radiated in the far-field is computed on a much larger domain by solving an inhomogeneous wave equation with acoustic source terms provided by near-field LES calculations. The flow and the acoustic data from the single jet case with chevrons are compared with those obtained from a circular jet, showing the acoustic benefit when chevrons are used on the nozzle.

  • 47.
    Mihaescu, Mihai
    et al.
    Aerospace Engineering, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Numerical Simulations of Jets Exhausting from Elliptic Nozzles without and with Conic Plug2008Ingår i: Proceedings of Int. Conf. on Jets, Wakes and Separated Flows, ICJWSF-2008, 2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Computational Fluid Dynamics was employed to model spatially developing turbulent jets issued from a low aspect ratio elliptic nozzle and from an elliptic plug nozzle. Results from Large Eddy Simulation and steady Reynolds-Averaged Navier-Stokes modeling of a benchmark elliptic jet exhausting from a 2:1 aspect ratio nozzle are compared. It was observed that only LES captured the axis-switching phenomenon. Further on, LES was used to calculate the flow exhausting from an elliptic plug nozzle. In this case the simulations predicted that a bifurcated jet is generated. The jet bifurcates just downstream of the center-body tip in the major-axis plane and the two potential core regions diverge as the jet spreads. The obtained data are presented in terms of instantaneous, time-averaged, and fluctuating quantities and are supported by experimental measurements.

  • 48.
    Mihaescu, Mihai
    et al.
    Aerospace Engineering, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Fuchs, Laszlo
    Energy Sciences, Lund University.
    Computational Aeroacoustics of the Coaxial Flow Exhaust System of a Gas Turbine Engine2007Ingår i: Volume 6: Turbo Expo 2007, Parts A and B, ASME , 2007, s. 1531-1539Konferensbidrag (Refereegranskat)
    Abstract [en]

    Jet engine noise is an environmental problem that needs to be addressed. Several methods to reduce the jet noise have been proposed in the last decades. The main issue is to find methods that reduce noise without causing considerable loss of thrust. Experimental and computational tools are mandatory in successfully reducingjet engine noise emissions. One of the challenging issues ofcomputing the jet engine noise is the presence of verylarge scales (associated with the wave length of the acousticwave) and at the same time also small scales that are responsible for the acoustical sources. In the field of Computational Aero-Acoustics (CAA) different hybrid approaches have been introduced to handle the different scales using problem specific models and methods.Here, a decomposition of flow variables is used that allowsseparation of flow and acoustic computations. Large Eddy Simulation approachis employed to compute the flow field and the acousticsources. An inhomogeneous wave equation is used to perform acousticcomputations. The paper investigates numerically the flow and the near-fieldacoustic data from a coaxial jet case with chevrons onthe core nozzle that are compared with those obtained froma baseline coaxial jet, showing the spatial character of the acoustic benefit when chevrons are used on the core nozzle.Comparisons in terms of sound pressure levels with experimental data performed with the same geometry show a good agreement.

  • 49.
    Mihaescu, Mihai
    et al.
    Aerospace Systems, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Khosla, Sid
    Otolaryngology, Head & Neck Surgery, University of Cincinnati-Medical Center.
    Scherer, Ronald
    Otolaryngology, Head & Neck Surgery, University of Cincinnati-Medical Center.
    Fuchs, Laszlo
    Energy Sciences, Lund University.
    Flow and Acoustics Simulations Based on LES and an Acoustic Analogy: an Application to Laryngeal Airflow2007Ingår i: 45th AIAA Aerospace Sciences Meeting 2007, 2007, s. 11141-11153Konferensbidrag (Refereegranskat)
    Abstract [en]

    The paper presents an efficient method for computational aeroacoustics applied to simulate the flow and the acoustics for a static laryngeal model considering the vocal folds widely open. The work analyses the whisper-like process defined as the sound generated by the turbulent glottal airflow in the absence of vocal fold vibration. A decomposition of the flow variables is used that allows separation of flow and acoustic computations. The approach consists in solving the low Mach number flow field by incompressible Large Eddy Simulation. This provides the possibility to compute the acoustic sources. The inhomogeneous wave equation derived from the fundamental equations of motion for a compressible fluid is employed to compute the acoustic field. The purpose of the study is to provide realistic numerical predictions of the flow patterns and the generated acoustic field in the glottis and the vocal tract regions.

  • 50.
    Mihaescu, Mihai
    et al.
    Aerospace Systems, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Szasz, Robert
    Energy Sciences, Lund University.
    Fuchs, Laszlo
    Energy Sciences, Lund University.
    Flow and Acoustics of a Coaxial Nozzle: a Sensitivity Study to the Inlet Boundary Conditions2006Ingår i: 44th AIAA Aerospace Sciences Meeting 2006, 2006, s. 7375-7393Konferensbidrag (Refereegranskat)
    Abstract [en]

    The jet noise generated by the high velocity hot stream exhausting from a jet engine represents a major component of the aircraft noise. Reductions in jet noise has been achieved by using high bypass turbo-fan engines. To solve numerically this problem the flow and acoustic fields generated by two coaxial jets are considered. Large Eddy Simulation (LES) is used to handle the non-isothermal turbulent flow field, while for the acoustic field a wave equation with source terms provided by the instantaneous LES is employed. The effects of the inlet boundary conditions on the flow and acoustics are assessed in this paper. A comparison of the computed mean flow field and the SPL with experimental data show excellent agreement at distances of over a few jet diameters downstream of the the jet exit plane. In the proximal part the agreement is less good. However, the discrepancy between the simulations and the experiments is less than the corresponding variations due to the uncertainties in the experimental boundary conditions.

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