1,721,047 research outputs found
A sliding characteristic interface condition for direct numerical simulations
No full textA characteristic interface condition serves as the basis for a novel sliding grid method, with a view to solving the compressible Navier-Stokes equations on block-structured grids that are delimited by boundary conditions in motion relative to each other. This requires that the convective and source terms of the equations in characteristic form be transformed to the reference frame of the neighbouring block, and interpolated. The method facilitates accurate interpolation at the interface, because the characteristic interface condition requires only a single layer of halo nodes. When a homogeneous direction is present, only 1-D interpolation is required, and schemes that might otherwise be too costly become affordable. The treatment also enjoys the same advantages as fixed characteristic interfaces do in relation to tolerance of grid discontinuities at block interfaces. The implementation and parallelisation of this method in a simulation code is described, and accuracy and performance demonstrated on a selection of test cases
Effect of the injector geometry on a sonic jet into a supersonic crossflow
No full textLarge-eddy simulation of a sonic injection from circular and elliptic injectors into a supersonic crossflow has been performed. The effects of injector geometry on various fundamental mechanisms dictating the intricate flow phenomena including shock/jet interaction, jet shear layer vortices and their evolution, jet penetration properties and the relevant turbulence behaviors have been studied systematically. As a jet issuing transversely into a supersonic crossflow, salient three-dimensional shock and vortical structures, such as bow, separation and barrel shocks, Mach disk, horseshoe vortex, jet shear layer vortices and vortex pairs, are induced. The shock structures exhibit considerable deformations in the circular injection, while their fluctuation becomes smaller in the elliptic injection. The jet shear layer vortices are generated at the jet periphery and their evolution characteristics are analyzed through tracing the centroid of these coherent structures. It is found that the jet from the elliptic injector spreads rapidly in the spanwise direction but suffers a reduction in the transverse penetration compared to the circular injection case. The turbulent fluctuations are amplified because of the jet/crossflow interaction. The vertical Reynolds normal stress is enhanced in the downstream of the jet because of the upwash velocity induced by the counter-rotating vortex pair.<br/
The Extrusion Wear of Steel Materials
No full text本論文使用Falex試驗機，以中碳鋼S45C與模具鋼SKD11為材料進行銷盤試驗。實驗內容主要在了解材料在高速下磨耗機制以及磨耗現象的觀察，並使用不同硬度的盤，探討盤的硬度對磨耗情形的影響。
實驗結果顯示，由於摩擦熱的影響，材料的硬度會降低，使得銷盤接觸面的材料會發生連續擠出的現象，隨著速度及負載的提升，擠出現象會變的更加顯著，磨耗率也跟著增加，當盤的硬度較低時，在較低速及較低負載下產生的磨耗率較低，在速度與負載增加到某一程度後，由於接觸溫度較低，氧化物不易生成，磨耗的情形會變得嚴重。A Falex machine was used to perform the pin-on-disk wear tests. Two kinds of steel which included medium carbon steel and die steel were used. The experiment was to understand the wear mechanism at higher sliding speed, and to observe the mechanism of wear. Besides, The disks of different hardness were used to study the influence of hardness of disk on wear mechanism.
According to the experimental results, the hardness of materials were decreased under the influence of frictional heat. In addition, the pin would be continuously extruded from the contact interface. The phenomenon of extrusion became more remarkable with increasing normal load and sliding speed, the wear rate increased correspondingly. When a pin slid against a soft disk, the wear rate was low at lower load and lower sliding speed. Owing to a lower contact temperature, the formation of oxides became difficult, which resulted in severe wear.第一章 緒 論……………………………………………………………1
1-1實驗動機……………………………………………………1
1-2相關理論……………………………………………………2
1-2-1真實接觸面積………………………………………2
1-2-2摩擦理論……………………………………………2
1-3磨耗機構……………………………………………………3
1-3-1黏著磨耗……………………………………………3
1-3-2刮磨磨耗……………………………………………4
1-3-3氧化磨耗……………………………………………4
1-3-4微動磨耗……………………………………………5
1-3-5剝層磨耗……………………………………………5
1-4文獻回顧……………………………………………………6
1-5論文目錄簡介………………………………………………9
第二章 實驗設備與方法………………………………………………10
2-1實驗規劃……………………………………………………10
2-2試片處理……………………………………………………10
2-3實驗設備……………………………………………………11
2-4實驗步驟……………………………………………………11
2-5氧化層剖面觀察……………………………………………13
2-6慢速壓縮試驗………………………………………………14
第三章 數據量測與分析方法…………………………………………15
3-1負載…………………………………………………………15
3-2速度…………………………………………………………15
3-3摩擦力與摩擦係數…………………………………………15
3-4溫度…………………………………………………………16
3-4-1外觀接觸溫度………………………………………16
3-4-2真實接觸溫度………………………………………17
3-5磨耗率………………………………………………………17
第四章 結果與討論……………………………………………………19
4-1實驗現象觀察………………………………………………19
4-2摩擦面觀察…………………………………………………21
4-3氧化層剖面…………………………………………………22
4-4分析與討論…………………………………………………23
4-4-1外觀接觸溫度……………………………………………23
4-4-2真實接觸溫度………………………………………24
4-4-3磨耗率………………………………………………24
4-4-4摩擦係數……………………………………………26
第五章 結論與建議……………………………………………………28
5-1結論…………………………………………………………28
5-2建議…………………………………………………………29
參考文獻…………………………………………………………………3
Numerical investigation of a jet from a blunt body opposing a supersonic flow
Full text linkNumerical investigation of a sonic jet from a blunt body opposing a supersonic flow with a free stream Mach number M1 D 2:5 was carried out using large-eddy simulation for two total pressure ratios of the jet to the free stream, i.e. P D 0:816 and 1.633. Results have been validated carefully against experimental data. Various fundamental mechanisms dictating the flow phenomena, including shock/jet interaction, shock/shear-layer interaction, turbulent shear-layer evolution and coherent structures, have been studied systematically. Based on the analysis of the flow structures and features, two typical flow states, i.e. unstable and stable states corresponding to the two values of P, are identified and the behaviours relevant to the flow states are discussed. Small-scale vortical structures mainly occur in the jet column, and largescale vortices develop gradually in a recirculation region when the jet terminates through a Mach disk and reverses its orientation as a conical free shear layer. The turbulent fluctuations are enhanced by the rapid deviation of the shear layer and the interaction with shock waves. Moreover, the coherent structures of the flow motion are analysed using the proper orthogonal decomposition technique. It is found that the dominant mode in the cross-section plane exhibits an antisymmetric character for the unstable state and an axisymmetric one for the stable state, while statistical analysis of unsteady loads indicates that the side loads can be seen as a rotating vector uniformly distributed in the azimuthal direction. Further, we clarify a feedback mechanism whereby the unsteady motion is sustained by the upstream-propagating disturbance to the Mach disk through the recirculation subsonic region and downstream propagation in the conical shear layer. Feedback models are then proposed which can reasonably well predict the dominant frequencies of the two flow states. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the opposing jet/supersonic flow interaction
Numerical investigation of the compressible flow past an aerofoil
No full textNumerical investigation of the compressible flow past an 18% thick circular-arc aerofoil was carried out using detached-eddy simulation for a free-stream Mach number M? = 0.76 and a Reynolds number Re = 1.1 × 107. Results have been validated carefully against experimental data. Various fundamental mechanisms dictating the intricate flow phenomena, including moving shock wave behaviours, turbulent boundary layer characteristics, kinematics of coherent structures and dynamical processes in flow evolution, have been studied systematically. A feedback model is developed to predict the self-sustained shock wave motions repeated alternately along the upper and lower surfaces of the aerofoil, which is a key issue associated with the complex flow phenomena. Based on the moving shock wave characteristics, three typical flow regimes are classified as attached boundary layer, moving shock wave/turbulent boundary layer interaction and intermittent boundary layer separation. The turbulent statistical quantities have been analysed in detail, and different behaviours are found in the three flow regimes. Some quantities, e.g. pressure-dilatation correlation and dilatational dissipation, have exhibited that the compressibility effect is enhanced because of the shock wave/boundary layer interaction. Further, the kinematics of coherent vortical structures and the dynamical processes in flow evolution are analysed. The speed of downstream-propagating pressure waves in the separated boundary layer is consistent with the convection speed of the coherent vortical structures. The multi-layer structures of the separated shear layer and the moving shock wave are reasonably captured using the instantaneous Lamb vector divergence and curl, and the underlying dynamical processes are clarified. In addition, the proper orthogonal decomposition analysis of the fluctuating pressure field illustrates that the dominated modes are associated with the moving shock waves and the separated shear layers in the trailing-edge region. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to this complex flow
Towards large-eddy simulation of turbulent flow in a centrifugal impeller
No full textLarge-eddy simulation (LES) using wall-adapting local eddy-viscosity (WALE) subgrid scale model has been applied towards elucidating the complex turbulent flow physics in a centrifugal impeller. Several canonical cases of increased complexity were analyzed to better understand the advantages and challenges of applying the LES framework to the aforementioned target problem. These include turbulent flow in a rotating channel, a straight and a curved duct. Results obtained with LES are compared in detail with two-equation eddy-viscosity Reynolds Averaged Navier-Stokes (RANS) turbulence models widely used in industry, as well as, for some of the canonical cases, with hybrid RANS/LES approaches such as the detached eddy simulation (DES) and scale-adaptive simulation (SAS). Finally, LES has been applied to turbulent flow in NASA CC3 centrifugal impeller with grids of increased resolution (up to 100 million computational cells per passage
Large-eddy simulation of the compressible flow past a wavy cylinder
No full textNumerical investigation of the compressible flow past a wavy cylinder was carried out using large-eddy simulation for a free-stream Mach number M? = 0.75 and a Reynolds number based on the mean diameter Re = 2 × 105. The flow past a corresponding circular cylinder was also calculated for comparison and validation against experimental data. Various fundamental mechanisms dictating the intricate flow phenomena, including drag reduction and fluctuating force suppression, shock and shocklet elimination, and three-dimensional separation and separated shear-layer instability, have been studied systematically. Because of the passive control of the flow over a wavy cylinder, the mean drag coefficient of the wavy cylinder is less than that of the circular cylinder with a drag reduction up to 26%, and the fluctuating force coefficients are significantly suppressed to be nearly zero. The vortical structures near the base region of the wavy cylinder are much less vigorous than those of the circular cylinder. The three-dimensional shear-layer shed from the wavy cylinder is more stable than that from the circular cylinder. The vortex roll up of the shear layer from the wavy cylinder is delayed to a further downstream location, leading to a higher-base-pressure distribution. The spanwise pressure gradient and the baroclinic effect play an important role in generating an oblique vortical perturbation at the separated shear layer, which may moderate the increase of the fluctuations at the shear layer and reduce the growth rate of the shear layer. The analysis of the convective Mach number indicates that the instability processes in the shear-layer evolution are derived from oblique modes and bi-dimensional instability modes and their competition. The two-layer structures of the shear layer are captured using the instantaneous Lamb vector divergence, and the underlying dynamical processes associated with the drag reduction are clarified. Moreover, some phenomena relevant to the compressible effect, such as shock waves, shocklets and shock/turbulence interaction, are analysed. It is found that the shocks and shocklets which exist in the circular cylinder flow are eliminated for the wavy cylinder flow and the wavy surface provides an effective way of shock control. As the shock/turbulence interaction is avoided, a significant drop of the turbulent fluctuations around the wavy cylinder occurs. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the passive control of the compressible flow past a wavy surface
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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