The Experts below are selected from a list of 7725 Experts worldwide ranked by ideXlab platform
Xiaodong Ren - One of the best experts on this subject based on the ideXlab platform.
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Friction power analysis and improvement for a tilting-pad journal bearing considering air entrainment
Applied Thermal Engineering, 2018Co-Authors: Aoshuang Ding, Xiaodong RenAbstract:Abstract This paper analyses the effects of air on oil–air distribution and energy characteristics of a tilting-pad journal bearing via computational fluid dynamics. With a gaseous cavitation model, air entrainment from an Outlet Boundary is analysed for the TPJB at a 3000 rpm rotation speed under a 180 kN load. The simulated bearing friction power is consistent with the experimental data, which indicates that the air entrainment from the Outlet Boundary is near the actual working conditions. According to the analyses, the shear stress of the loaded area is mainly influenced by velocity gradient in the normal direction to the rotor-side wall, whereas the shear stress of the unloaded area is mainly influenced by air volume fraction. With air cavitation and entrainment, the air volume fraction increases and affects the viscosity of a mixture considerably, thereby eventually influencing the shear stress on the rotor-side wall and bearing friction power. On the basis of this relationship, an improvement, which closes two oil inlet holes of the unloaded area whilst increasing the oil-supplied pressure of the loaded area, is proposed to remarkably decrease the bearing friction power whilst keeping load capacity. Simulation results validate the effectiveness and feasibility of the improvement.
Aoshuang Ding - One of the best experts on this subject based on the ideXlab platform.
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Numerical Investigation for Characteristics and Oil-Air Distributions of Oil Film in a Tilting-Pad Journal Bearing
Volume 7B: Structures and Dynamics, 2018Co-Authors: Aoshuang Ding, Yaobing XiaoAbstract:This paper analyzes the effects of air in the oil film of a tilting-pad journal bearing on oil-air distributions and characteristics. With a gaseous cavitation model and shear stress transport (SST) model with low-Re correction included, the air backflow from the Outlet Boundary is analyzed in numerical simulations of a titling-pad journal bearing at 3000 rpm rotation speed and under 180 kN load. The simulated bearing load, pressure and mechanical loss are in good accordance with the experimental data, indicating that the simulation results of the air backflow from the Outlet Boundary can catch the hydrodynamic characteristics accurately. Based on the analyses of simulated air volume fraction and shear stress, the shear stress of the high-pressure loaded area is mainly influenced by the velocity gradient in the normal direction to the rotor-side wall, not the air backflow and gaseous cavitation. In the unloaded area, the gaseous cavitation occurs around the center part, following the gaseous cavitation mechanisms. The backflow air flows into the low-pressure unloaded area from the Outlet Boundary and has a clear interval with the air from the gaseous cavitation. The air volume fraction increases with these two air sources and affects the mixture viscosity significantly, eventually influencing the shear stress on the rotor-side wall and bearing mechanical loss.
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Friction power analysis and improvement for a tilting-pad journal bearing considering air entrainment
Applied Thermal Engineering, 2018Co-Authors: Aoshuang Ding, Xiaodong RenAbstract:Abstract This paper analyses the effects of air on oil–air distribution and energy characteristics of a tilting-pad journal bearing via computational fluid dynamics. With a gaseous cavitation model, air entrainment from an Outlet Boundary is analysed for the TPJB at a 3000 rpm rotation speed under a 180 kN load. The simulated bearing friction power is consistent with the experimental data, which indicates that the air entrainment from the Outlet Boundary is near the actual working conditions. According to the analyses, the shear stress of the loaded area is mainly influenced by velocity gradient in the normal direction to the rotor-side wall, whereas the shear stress of the unloaded area is mainly influenced by air volume fraction. With air cavitation and entrainment, the air volume fraction increases and affects the viscosity of a mixture considerably, thereby eventually influencing the shear stress on the rotor-side wall and bearing friction power. On the basis of this relationship, an improvement, which closes two oil inlet holes of the unloaded area whilst increasing the oil-supplied pressure of the loaded area, is proposed to remarkably decrease the bearing friction power whilst keeping load capacity. Simulation results validate the effectiveness and feasibility of the improvement.
Ali Cemal Benim - One of the best experts on this subject based on the ideXlab platform.
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Three dimensional numerical analysis of hemodynamic of stenosed artery considering realistic Outlet Boundary conditions
Computer methods and programs in biomedicine, 2019Co-Authors: Arindam Bit, Ali Cemal Benim, Adel Alblawi, Himadri Chattopadhyay, Qurratul Ain Quais, Mohammad Rahimi-gorjiAbstract:Abstract Background and objective Mortality rate increases globally among which one third is due to diseased blood vessels. Due to late diagnoses of the disease in vessels (severe stenoses), qualitative and rapid assessment becomes difficult. Earlier assessment of stenoses can lead to formulation of effective treatment protocol. It is often found that proliferation of secondary stenoses at downstream of a stenosed vessel depends on the degree of severity of primary stenoses. Numerical investigation of flow dynamics of blood in such condition helps in prediction of distributed field of secondarystenoses. This investigation also requires consideration of rigorous Boundary conditions at inlet and Outlet of defined flow domain. Methods Patient-specific geometry of aortic arch with stenoses in descending aorta was considered for numerical estimation of biofluid dynamics. Boundary conditionsat inlet and Outlet were extracted from time-resolved pulsed Doppler Ultrasound imaging at appropriate sections of the vessel. Womersley inlet flux was considered. Flow parameters like wall shear stress, oscillatory shear index, etc. were evaluated at upper and lower aortic arch of the vessel at different combinations of Boundary conditions at inlet and four Outlets respectively. Results Effect of Outlet Boundary conditions were acknowledged for the progression of secondary stenoses. Severity of primary stenoses was found influencing the progression of secondary stenoses. It was found that the Outlets Left Subclavian Artery and Left Common Carotid Artery greatly influence the flow dynamic structure within the stenosed aortic arch. Simultaneously, lower wall of aortic-arch had shown more affinity for secondary stenoses progression. Conclusion Aortic arch is a vital anatomical region of circulatory system which is vulnerable to progression of secondary stenoses in presence of primary stenoses in ascending or descending aorta. It also drives the author to speculate the influence of anurysm in descending aorta on this landmark of aortic arch.
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validation of loss coefficient based Outlet Boundary conditions for simulating aortic flow
Journal of Mechanics in Medicine and Biology, 2016Co-Authors: Ali Cemal Benim, Alexander Assmann, Payam Akhyari, Artur Lichtenberg, Franz JoosAbstract:Flow in a polyurethane model of a human aorta, driven by a heart-lung machine, is analyzed experimentally and computationally for antegrade and retrograde perfusion. The purpose of the analysis is the validation of the previously proposed loss-coefficient-based Outlet Boundary condition for aortic branches. This model is claimed to be commonly applicable to different perfusion modes of the aorta, unlike the alternative straightforward constant-pressure Outlet Boundary condition. First, the antegrade perfusion is analyzed computationally and experimentally. This step delivers the loss-coefficients that are to be used in any other perfusion mode of the aorta. Subsequently, a retrograde perfusion is applied to the same aorta, where the flow rates at the Outlets of the aortic branches are measured and predicted by applying the loss-coefficient-based Outlet Boundary conditions. A very good agreement of the predictions with the measurements is observed. The predictions delivered by the standard constant-pressure Outlet Boundary condition are observed, on the contrary, to be highly in error. Thus, the advocated loss-coefficient-based Outlet Boundary condition is experimentally validated. It is shown that it is applicable to different perfusion modes with a quite good accuracy, which is much higher compared to the straightforward constant-pressure Outlet Boundary condition.
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simulation of blood flow in human aorta with emphasis on Outlet Boundary conditions
Applied Mathematical Modelling, 2011Co-Authors: Ali Cemal Benim, A Nahavandi, A Assmann, D Schubert, P Feindt, S H SuhAbstract:Blood flow in human aorta and its major branches is analyzed by computational fluid dynamics, for physiologic and extracorporeal circulation, the latter being the main focus. Mainly, a steady-state analysis is applied corresponding to extracorporeal circulation conditions. For physiologic circulation, pulsatile flow is also investigated. Distensibility of aorta walls is neglected. Blood is modeled as Newtonian fluid. The SST model is employed for turbulence in all cases, for a coherent treatment of the flows exhibiting Reynolds numbers encompassing the transitional regime. For modeling Outlet Boundary conditions, a simple model based on the prescription of loss coefficients is proposed, which is believed to be more favorable than some more straightforward techniques such as the prescription of an Outlet pressure. For physiologic circulation, it is observed that the time-averaged velocity field of pulsatile flow does not show remarkable differences to steady-state results. For extracorporeal circulation, two cases, namely an antegrade and a retrograde perfusion are investigated. Flow patterns observed for the physiologic circulation and the extracorporeal circulation techniques show considerable differences. For extracorporeal circulation, much larger wall shear stress values are predicted. This indicates that mobilization of arteriosclerotic plaques needs to be considered as a very important issue for the extracorporeal circulation.
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Effect of inlet and Outlet Boundary conditions on swirling flows
Computers & Fluids, 1997Co-Authors: J.l. Xia, Ali Cemal Benim, B.l. Smith, J. Schmidli, G. YadigarogluAbstract:Abstract Numerical simulations are conducted for both three-dimensional, turbulent flow in a multi-channel swirler and axisymmetric, isothermal, turbulent flow in combustion chambers using the standard κ−ϵ turbulence model. Calculations are first carried out for three-dimensional, isothermal and turbulent flow inside the swirler channels in order to derive the velocity profiles of both air and gas at the swirler Outlets, which are used as inlet Boundary conditions of the model combustor and can also be used in future studies for different combustors with the same type of swirler. In order to study the sensitivity of swirling flow inside the chamber to the inlet and Outlet Boundary conditions, different inlet velocity profiles and Outlet Boundary conditions are also employed. The results show that in the cases considered, the flow behaviour in the chamber is not very sensitive to the actual shape of the inlet velocity profiles provided the averages of the inlet axial, radial and azimuthal velocity components are separately preserved. Other conditions being equal, we find that the swirling flow performance in the combustor depends not only on the inlet swirl number, but also strongly on the relative magnitude of the radial velocity component at inlet and introduce a new dimensionless number N r , analogous to the swirl number, to measure the relative importance of this quantity. Outlet Boundary conditions have some influence near the Outlet, but nearly no effect further upstream for the cases investigated.
Anouck Girard - One of the best experts on this subject based on the ideXlab platform.
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ACC - Resilient Physics-Based Traffic Congestion Control
2020 American Control Conference (ACC), 2020Co-Authors: Hossein Rastgoftar, Anouck GirardAbstract:This paper offers a new physics-based approach to model and resiliently control traffic congestion. We model the traffic congestion as a mass conservation problem and provide discussions and proofs for the feasibility of the conservation- based traffic dynamics. The paper spatially discretizes the governing continuity equation by using a directed graph with the nodes classified as (i) interior nodes, (ii) Boundary inlet nodes, (iii) Boundary Outlet nodes, and (iv) anomalous nodes. At the interior nodes, the traffic dynamics is modeled as a probabilistic process. At the inlet Boundary nodes, the traffic inflow rates can be planned and controlled but they must satisfy certain equality and inequality constraints. This paper assumes that the traffic inflow and outflow rates are identical at the Outlet Boundary nodes, which in turn implies that Outlet Boundary nodes have no dynamics. Furthermore, the traffic coordination cannot be controlled at the anomalous nodes and they are modeled as disturbance sources. We apply the model predictive control approach to effectively control the traffic congestion through the inlet Boundary nodes in the presence of anomalies. The objective of the control problem is to assign the Boundary traffic inflow rate such that traffic density is uniformly distributed across the traffic nodes. The Boundary control inputs are assigned as the solution of a constrained quadratic programming problem.
Christian Oliver Paschereit - One of the best experts on this subject based on the ideXlab platform.
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Effect of inlet and Outlet Boundary conditions on rotating detonation combustion
Combustion and Flame, 2020Co-Authors: Richard Bluemner, Christian Oliver Paschereit, Myles D. Bohon, Ephraim GutmarkAbstract:Abstract In this work, the effect of different injector geometries and different Outlet restrictions on the operating modes of a hydrogen-air Rotating Detonation Combustor (RDC) is investigated. The different operating modes are identified based on pressure measurements in the combustor annulus and the reactant supply, combined with high-speed video from the aft end of the combustor. The pressure frequency spectra are analyzed to determine the global operating mode in terms of number, direction, and speed of waves. The results explore the ability of the RDC to establish rotating, counter-rotating, and longitudinal waves, as well as their superpositions. A good agreement between longitudinal modes and the acoustic resonance frequencies of the RDC annulus was found. Overall, operation was found to be highly injector and Outlet restriction dependent. Adding an Outlet restriction helped to suppress counter-rotating waves, which is a prerequisite to stabilize single detonation waves. However, it was also shown to prompt high frequency pulsed operation. Apart from the total reactant supply pressure, the relative strength of the injectors was identified as a key factor for stable RDC operation. Pressure feedback into the reactant supply was observed to be dependent on the reactant supply pressure and the RDC operating mode. The study further revealed the presence of transverse resonance modes in the fuel plenum, however these oscillations were weak relative to the injector pressures and do not appear to influence the combustion.
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Effects of Outlet Boundary Conditions on the Reacting Flow Field in a Swirl-Stabilized Burner at Dry and Humid Conditions
Journal of Engineering for Gas Turbines and Power, 2012Co-Authors: Steffen Terhaar, Bernhard C. Bobusch, Christian Oliver PaschereitAbstract:During the design and testing process of swirl-stabilized combustors, it is often impractical to maintain identical Outlet Boundary conditions. Furthermore, it is a common practice to intentionally change the acoustic Boundary conditions of the Outlet in order to suppress thermoacoustic instabilities. In the presented work the susceptibility of the reacting flow field to downstream perturbations is assessed by the application of an area contraction at the Outlet. Since combustion and fuel composition are shown to be important parameters for the influence of the Boundary conditions on the flow field, highly steam diluted flames are investigated in addition to dry flames at different equivalence ratios and degrees of swirl. The applied measurement techniques include particle image velocimetry, laser doppler velocimetry, and emission analysis. The results reveal a clear correlation of the susceptibility of the flow field to downstream perturbations to both the inlet swirl number and the amount of dilatation caused by the flame. The concept of an effective swirl number downstream of the flame is applied to the results and is proven to be the dominating parameter. A theoretical explanation for the influence of this parameter is provided by the usage of the well known theory of subcritical and supercritical swirling flows, where perturbations can propagate upstream solely in subcritical flows via standing waves. Knowledge of the flow state is of particular importance for the evaluation of combustion tests with differing exit Boundary conditions and the results emphasize the need for realistic exit Boundary conditions for numerical simulations.
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Effects of Outlet Boundary Conditions on the Reacting Flow Field in a Swirl-Stabilized Burner at Dry and Humid Conditions
Volume 2: Combustion Fuels and Emissions Parts A and B, 2012Co-Authors: Steffen Terhaar, Bernhard C. Bobusch, Christian Oliver PaschereitAbstract:During the design and testing process of swirl-stabilized combustors, it is often impractical to maintain identical Outlet Boundary conditions. Furthermore, it is a common practice to intentionally change the acoustic Boundary conditions of the Outlet in order to suppress thermoacoustic instabilities. In the presented work the susceptibility of the reacting flow field to down-stream perturbations is assessed by the application of an area contraction at the Outlet. Since combustion and fuel composition are shown to be important parameters for the influence of the Boundary conditions on the flow field, highly steam diluted flames are investigated in addition to dry flames, at different equivalence ratios and degrees of swirl. The applied measurement techniques include Particle Image Velocimetry, Laser Doppler Velocimetry, and emission analysis. The results reveal a clear correlation of the susceptibility of the flow field to downstream perturbations to both, the inlet swirl number and the amount of dilatation caused by the flame. The concept of an effective swirl number down-stream of the flame is applied to the results and is proven to be the dominating parameter. A theoretical explanation for the influence of this parameter is provided by the usage of the well known theory of subcritical and supercritical swirling flows, where perturbations can propagate upstream solely in subcritical flows via standing waves. Knowledge of the flow state is of particular importance for the evaluation of combustion tests with differing exit Boundary conditions and the results emphasize the need for realistic exit Boundary conditions for numerical simulations.Copyright © 2012 by ASME
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Planar Investigation of Outlet Boundary Conditions Effect on Isothermal Flow Fields of a Swirl-Stabilized Burner
Volume 2: Combustion Fuels and Emissions, 2009Co-Authors: Ahmed Emara, Arnaud Lacarelle, Christian Oliver PaschereitAbstract:The swirling flow velocity profiles can be strongly influenced by the Outlet conditions of the combustion chamber especially at subcritical flow conditions. The effect of such changes on the mean flow or coherent structures is still unclear. It is investigated in the present work in an industrial swirl inducing burner in cold flow conditions with help of PIV. Proper orthogonal decomposition (POD) as well as acoustic measurements were used to characterize the coherent structures shed from the burner mouth. The combustor length (670, and 2020mm) and the Outlet area contraction ratio (1, 0.56, 0.27, and 0.09) are varied. Major changes in the flow field are achieved when using a short combustor and the smallest contraction ratio. For this case, a central jet with streamwise velocity is added to the typical central recirculation zone. The POD analysis of the contraction ratios 1 and 0.09 for the long combustor shows that the first helical mode as well as Kelvin Helmholtz vortices are present with minor changes for both cases. At a contraction ratio of 0.09, some new structures at the jet location and near the combustor wall appear.Copyright © 2009 by ASME
