| 1. |
- Ahn, Myeonghwan, et al.
(författare)
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A numerical study on near-field pressure fluctuations of symmetrical and anti-symmetrical flapping modes of twin-jet using a high-resolution shock-capturing scheme
- 2021
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Ingår i: Aerospace Science and Technology. - : Elsevier. - 1270-9638 .- 1626-3219. ; , s. 107147-107147
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Tidskriftsartikel (refereegranskat)abstract
- Screeching supersonic jets appears at off-design operating conditions and is perceived as an intense tonal noise. In a twin nozzle configuration, mutual interactions between the two jet plumes may occur with various coupling modes developing depending on the operating conditions and lateral distance between the jets. The investigation of the detailed flow behaviors and near-field pressure fluctuations with relevance to the twin jets system, the analysis of the developed instabilities, will enhance understanding of fundamental features associated with jets located close to each other.In the present study, the single jet is considered first to assess the large eddy simulation (LES) approach used and the near-field pressure fluctuation predictions. Based on the validated solver, twin jets are simulated. Two different twin-nozzle configurations having different separation distance or nozzle-to-nozzle centerline spacing are scrutinized for the same Mach number of 1.358. Notably, the twin jets are screeching by the coupling mode for both set-ups; however, the case of closer inter-nozzle distance presents a symmetrical dominant flapping mode, while the other case shows an anti-symmetrical flapping mode. The strength of the pressure fluctuation at the fundamental frequency changes depending on the location of the observer point (upstream or downstream) and the reference plane (twin-jet and normal to the twin-jet plane). The screech tones of the two cases, observable in the upstream region, are significantly different in the normal to the twin-jet plane direction because of the phase difference of fluctuating pressure. However, the first harmonic component remains strong, regardless of the flapping mode. It is also observed that, at the fundamental frequency, the amplitude of the pressure fluctuation at downstream locations is found to be strong in the normal to the twin-jet plane when the symmetrical flapping mode occurs. This feature is also observed in the twin-jet plane in the case of the opposite mode. By analyzing the developed vertical structures and performing correlation analyses of pressure fluctuations along jet shear layers, the periodicity of the flow in the downstream region with relevance to the fundamental frequency is revealed.
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| 2. |
- Ahn, MyeongHwan, et al.
(författare)
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Effects of Temperature on the Characteristics of Twin Square Jets by Large Eddy Simulations
- 2022
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Ingår i: AIAA Science and Technology Forum and Exposition, AIAA SciTech Forum 2022. - Reston, Virginia : American Institute of Aeronautics and Astronautics (AIAA).
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Konferensbidrag (refereegranskat)abstract
- In this study, we investigate the effects of temperature on the aerodynamic and aeroacoustics characteristics of twin square jets. Implicit Large Eddy Simulations (ILES) are performed for twin jets with a fixed nozzle pressure ratio (NPR) of 3.0 and temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0. A second-order central scheme is used to resolve acoustic waves, and an artificial dissipation model is applied to capture shock waves and to suppress non-physical oscillations. In addition, the variation of a specific heat ratio as function of temperature is considered under the chemical equilibrium assumption. The numerical results show that the length of potential core is reduced with the increase of temperature due to the enhanced mixing in jet shear layers which can be estimated by turbulent kinetic energy (TKE). Meanwhile, the fluctuations of the transverse velocity show different trends between the cases within the corresponding potential core length, which can be associated with the screeching phenomena of the twin-jet. As temperature increases, the convection Mach number in the jet shear layers is also increased so that the Mach wave is generated for TR of 2.0, 4.0, and 7.0. However, a crackle noise is only observed for TR of 4.0 and 7.0, whose generation is identified by the skewness and kurtosis factors. Relatively low temperature jets (TR of 1.0 and 2.0) are screeching so that peaks are observed in the spectra obtained upstream. On the other hand, broadband component is gradually increased when the jets are heated, and the largest increase is observed at the location exposed to the Mach wave radiation.
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| 3. |
- Ahn, Myeonghwan, et al.
(författare)
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Flow and Near-field Pressure Fluctuations of Twin Square Jets
- 2021
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Ingår i: AIAA Propulsion and Energy Forum, 2021. - Reston, Virginia : American Institute of Aeronautics and Astronautics Inc, AIAA.
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Konferensbidrag (refereegranskat)abstract
- We aim to investigate the aerodynamic and acoustics characteristics of a twin square jet using an implicit Large Eddy Simulation (ILES). A screeching cold jet condition, a nozzle pressure ratio (NPR) of 3.0, is considered to simulate a coupled twin-jet. A second-order central scheme with a modified version of Jameson’s artificial dissipation is adopted to damp numerical oscillations and to mimic the effect of small-scale turbulence without an explicit subgrid-scale (SGS) model. Numerical results show that the overall trends of time-averaged streamwise velocity profiles are similar to the experimental data, with the largest differences observed at locations associated with the presence of the shock-cell structures. A detailed investigation of the flow fluctuations in jet shear layers is performed. The amplitude of the velocity fluctuations is highly dependent on the location of the shear layers with respect to the twin-jet configuration (upper, lateral, or inner). The coupling mode of twin jets associated with the screech tone is determined as a symmetrical flapping mode be a two-points spacetime cross-correlation analysis. The overall trends of near-field pressure fluctuation spectra by LES agree well with the experimental results in both upstream and downstream regions. Near-field pressure fluctuation spectra by ILES agree well with the experimentally obtained spectra at different locations in the nozzle exit plane as well as at several downstream locations in the near-field acoustic region. The highest screech tone is observed at the inter-nozzle region where superposition of in-phase waves and standing waves are found. Fourier phase and amplitude fields at the fundamental frequency also confirm the symmetrical flapping mode of the twin jets by showing in-phase relations of hydrodynamic/acoustic waves and noise directivities.
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| 4. |
- Ahn, MyeongHwan, et al.
(författare)
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Large-eddy simulations of flow and aeroacoustics of twin square jets including turbulence tripping
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigate the flow and aeroacoustics of twin square (i.e., aspect ratio of 1.0) jets by implicit large-eddy simulations (LESs) under a nozzle pressure ratio of 3.0 and a temperature ratio of 1.0 conditions. A second-order central scheme coupled with a modified Jameson's artificial dissipation is used to resolve acoustics as well as to capture discontinuous solutions, e.g., shock waves. The flow boundary layer inside of the nozzle is tripped, using a small step in the convergent section of the nozzle. The time-averaged axial velocity and turbulent kinetic energy of LES with boundary layer tripping approaches better to particle image velocimetry experimental data than the LES without turbulence tripping case. A two-point space–time cross-correlation analysis suggests that the twin jets are screeching and are coupled to each other in a symmetrical flapping mode. Intense pressure fluctuations and standing waves are observed between the jets. Spectral proper orthogonal decomposition (SPOD) confirms the determined mode and the relevant wave propagation. The upstream propagating mode associated with the shock-cell structures is confined inside jets. Far-field noise obtained by solving Ffowcs Williams and Hawkings equation is in good agreement with the measured acoustic data. The symmetrical flapping mode of twin jets yields different levels of the screech tone depending on observation planes. The tonalities—the fundamental tone, second and third harmonics—appear clearly in the far-field, showing different contributions at angles corresponding to the directivities revealed by SPOD.
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| 5. |
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| 6. |
- Berg, Niclas, 1988-
(författare)
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Blood flow and cell transport in arteries and medical assist devices
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The cardiovascular system is responsible for transport of nutrients, oxygen, as well as the cells and molecules making up the immune system. Through the hemostatic system, the body maintains the integrity of the blood vessels, and prevents bleeding. The biochemical and physical processes governing the circulation interact, and take place at a large range of time and length scales - from those related to the individual cells up to the large scale flow structures. Dysfunctions of the heart or the circulatory system may have severe consequences. Cardiovascular diseases (CVD) is a heterogeneous group of diseases, responsible for about 50% of all death cases in the western world.Patients with severe but transient heart and/or lung disease may require the assistance of a heart-lung machine to bridge over the period required for the affected organ to recover. One such system is the Extracorporeal Membrane Oxygenator (ECMO) circuit, consting of a blood pump, a membrane oxygenator, cannulae and tubing system. While the therapy is life-saving, it is associated with relatively frequent thromboembolic (blood clotting and/or bleeding) events. Modeling of the flow in some components of the ECMO circuit was undertaken. The flow data was used together with models for platelet activation to assess the risk for thrombus formation. The results indicated locations of elevated risk of thrombosis in the centrifugal blood pump, the ECMO cannulae and the pipe connectors. The identified locations agreed well with clinical observations. The results lead to a direct recommendation to minimize the use of tube connectors. Further study of the sensitivity of the platelet activation models to uncertainties and errors was carried out. Some recommendations for improved modeling were proposed.Arteriosclerosis develops slowly over a long period of time (years or decades). It manifests initially at some common sites; arteries of certain sizes with relatively strong flow rate, as well as near artery bifurcations and locations of strong vessel curvature. The location specificity indicates that the blood flow plays a central role in the arteriosclerotic process. Being able to predict the future development of arteriosclerotic lesion and its location for an individual patient would imply that pre-emptive actions could be taken. This idea was the foundation of some of the numerical simulations in this thesis. A stenoted patient specific renal artery was considered, and was reconstructed to a non-pathological state by removing the stenosis using different segmentation methods. We could then evaluate if common stenosis markers based on functions of time-averages of the Wall Shear-Stress (WSS) could be use as predictive parameters. It was shown that these markers are not adequate as predictive tools. Furthermore, it was shown that the sensitivity to reconstruction technique was at least of the same order as the effect of the choice of blood rheology model. The rheology of blood was further studied through detailed simulations resolving the blood plasma flow and its interaction with the red blood cells (RBC) and the platelets. A hybrid Immersed boundary-Lattice Boltzmann method was applied, and the rheological data was compared to the Quemada model. It was found that the Quemada model could underpredict the effective viscosity by as much as 50%. The same methodology was applied to study the transport of RBCs and platelets, and the influence of RBC polydispersity. An increased degree of variability in RBC volume was found, under certain circumstances, to lead to an increase of the transport of platelets to the vessel wall (margination).
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| 7. |
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| 8. |
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| 9. |
- Bodin, Olle, et al.
(författare)
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LES of the Exhaust Flow in a Heavy-Duty Engine
- 2014
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Ingår i: Oil & gas science and technology. - : EDP Sciences. - 1294-4475 .- 1953-8189. ; 69:1, s. 177-188
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Bodin, Olle
(författare)
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Simulations of compressible flows associatedwith internal combustion engines
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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