| 501. |
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Flow, Turbulence and Combustion : Special Issue: Jets, Wakes, and Separated Flows
- 2019
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Ingår i: Flow, Turbulence and Combustion. - : Springer Netherlands. - 1386-6184 .- 1573-1987.
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Samlingsverk (redaktörskap) (refereegranskat)abstract
- 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.
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| 502. |
- Forslund, Anders, 1982, et al.
(författare)
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Minimizing Weld Variation Effects Using Permutation Genetic Algorithms and Virtual Locator Trimming
- 2018
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Ingår i: Proceedings of the ASME 2017 International Mechanical Engineering Congress and Exposition. ; 2
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Konferensbidrag (refereegranskat)abstract
- The mass production paradigm strives for uniformity, and for assembly operations to be identical for each individual product. To accommodate geometric variation between individual parts in such a process, tolerances are introduced into the design. However, for certain assembly operations this method can yield suboptimal quality. For instance, in welded assemblies, geometric variation in ingoing parts can significantly impair quality. When parts misalign in interfaces, excessive clamping force must be applied, resulting in additional residual stresses in the welded assemblies. This problem may not always be cost-effective to address simply by tightening tolerances. Therefore, under new paradigm of mass customization, the manufacturing approach can be adapted on an individual level. Since parts in welded assemblies are not easily disassembled and reused, interchangeability is not a relevant concern. This recognition means that each welded assembly can be adapted individually for the specific idiosyncrasies of ingoing parts. This paper focuses on two specific mass customization techniques; permutation genetic algorithms to assemble nominally identical parts, and virtual locator trimming. Based on these techniques, a six-step method is proposed, aimed at minimizing thing effects of geometric variation. The six steps are nominal reference point optimization, permutation GA configuration optimization, virtual locator trimming, clamping, welding simulation, and fatigue life evaluation. A case study is presented which focuses on one specific product; the turbine rear structure of a commercial turbofan engine. Using this simulation approach, the effects of using permutation genetic algorithms and virtual locator trimming to reduce variation are evaluated. The results show that both methods significantly reduce seam variation. However, virtual locator trimming is far more effective in the test case presented, since it virtually eliminates seam variation. This can be attributed to the orthogonality in fixturing. Seam variation is linked to weldability, which in turn has significant impact on estimated fatigue life. These results underscore the potential of virtual trimming and genetic algorithms in manufacturing, as a means both to reduce cost and increase functional quality.
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| 503. |
- Forslund, Anders, 1982, et al.
(författare)
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Robust lifecycle optimization of turbine components using simulation platforms
- 2012
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Ingår i: Proceedings of the 28th Congress of the International Council of the Aeronautical Sciences, ICAS 2012. - 9781622767540 ; 4, s. 2593-2604
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Konferensbidrag (refereegranskat)abstract
- n early phases of turbofan engine component design, simulation is favored since it reduces the need for expensive physical testing. However, deterministic simulations for model validation do not consider uncertainty at all. Uncertainties can be classified into three types: aleatory uncertainties, epistemic uncertainties, and error. In this paper, we investigate the potential of a multidisciplinary simulation platform to address these uncertainties and errors for a given test case. We place specific focus on the geometry assurance of a given turbofan component - the Turbine Rear Structure (TRS). Simulations are generally performed based on nominal geometries, materials and loads. However, when a product is mass-produced, each realization of the product design will deviate from the nominal geometry. By generating CAD models from scanned 3D-data of manufactured parts and running them through the simulation platform, the effect that geometric variation has on aerodynamic and structural performance can be investigated. Further, by moving the reference points in a virtual assembly process, we can, to some extent, suppress the effects that this variation has on aerodynamic and structural performance. From a technical point of view, the suggested approach means a significantly improved ability to numerically simulate and optimize robustness of component designs with functionality criteria from principally different disciplines. From an industrial application point of view, the suggested approach provides a tool for including part variation in the early design face, rather than being treated downstream in the development process.
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| 504. |
- Giacomini, Enrico
(författare)
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Investigating Aerodynamic Challenges for Rotorcraft Airfoil in the Martian Athmosphere
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the past decade, there has been a considerable increase in space exploration efforts, driving the need for new methods to examine planets and other celestial bodies. The current trend involves designing spacecraft capable of surveying surfaces from elevated positions, with drones proving to be more suited for the task. . The focus of space missions has primarily been on exploring Mars, as evidenced by the pioneering flight of the Ingenuity helicopter in 2021. The Martian environment poses significant aerodynamic challenges due to its thin atmosphere and low pressure, complicating drone flight. The generation of lift is problematic owing to the scant atmosphere and the restricted dimensions required for space missions, resulting in low-chord Reynolds number flows. Despite the reduction in skin friction drag due to lower viscosity, the decrease in airfoil efficiency is significantly compromised, with only a partial counterbalance by the reduced gravitational pull. Two main challenges must be addressed: low chord-based Reynolds number flows and Martian dust. The former results in the formation of Laminar Separation Bubbles (LSB), severely impairing the aerodynamic efficiency of the airfoil. Concurrently, the accumulation of dust particles on the airfoil’s surface significantly affects its performance, altering its geometry and surface roughness. Thus, it is crucial to accurately determine the presence and location of both separation bubbles and particle deposition to predict performance degradation. \\This thesis presents a comprehensive survey on drones for planetary exploration and an analysis conducted on a cambered plate with 6$\%$ camber and 1$\%$ thickness, ideal for the types of flows considered. The studies are carried out for Reynolds number flows, namely 20,000 and 50,000, to observe the effects of rotor and airfoil dimensions. The computational study is performed using ANSYS Fluent, utilising a two-dimensional CFD model with a C-type mesh and the gamma-Re ($\gamma-Re_{\theta}$) transition model, which aids in capturing the behaviour of these flow regimes. Additionally, for the dust study, two phases are created: a primary phase, the atmosphere, and a secondary phase, the dust particles. The volume fraction of particles is assumed to be small enough to imply that the primary phase influences the secondary, but not vice versa (one-way coupling). To assess particle adhesion, a deposition model has been developed to check for the deposition of dust particles, working in conjunction with the Discrete Phase Modelling (DPM), which simulates the trajectory of particles within the control volume. The deposition model comprises a particle transport model, which accounts for the forces acting on the particles, and a particle-wall interaction model, which determines the particles' rebound or adhesion. The results are presented and discussed at the end of the thesis, along with a brief discussion of future studies focusing on alternative assumptions for dust modelling.
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| 505. |
- Gojon, Romain, et al.
(författare)
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Antisymmetric Oscillation Modes in Rectangular Screeching Jets
- 2019
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Ingår i: AIAA Journal. - 0001-1452 .- 1533-385X. ; 57:8, s. 3422-3441
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 506. |
- Gojon, Romain, et al.
(författare)
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Impact of an Adjacent Surface on a Rectangular Overexpanded Supersonic Jet
- 2023
-
Ingår i: Flow Turbulence and Combustion. - : Springer Nature. - 1386-6184 .- 1573-1987.
-
Tidskriftsartikel (refereegranskat)abstract
- Flow and acoustic fields of a rectangular over-expanded supersonic jet interacting with an adjacent parallel plate are investigated using compressible Large Eddy Simulations (LES). The jet exits from a converging diverging rectangular nozzle of aspect ratio 2 with a design Mach number 1.5. Four distances (0 to 3 equivalent diameters) between the plate and the adjacent lip of the rectangular jet in the minor axis plane are studied. The geometry of the nozzle, the positions of the plate, and the exit conditions are identical to the ones of an experimental study. Snapshots and mean velocity fields are presented. Good agreement with the PIV experimental measurements is obtained. Previously, the corresponding free jet has been found to undergo a strong flapping motion in the minor axis plane due to screech. Here, it is shown that the intensity of the screech increases for certain distances from the plate and decreases for others, as compared to the corresponding free jet. Two points space-time cross correlations of the pressure along the jet’s shear-layers show, in two cases, an amplification of the aeroacoustic feedback mechanism leading to screech noise in the jet shear-layer closer to the plate. This amplification is due to acoustic waves impinging on the plate, and generating propagating waves back towards the jet, thus exciting the shear-layer at the screech frequency, around the tenth shock cell. Moreover, when the jet develops as a wall jet on the plate, the screech frequency and its associated flapping motion is canceled but a symmetrical oscillation of the jet at a lower frequency becomes dominant and radiates in the near acoustic field. This oscillation mode, as the ones associated with the screech tones for the other cases studied, can be explained by the use of a vortex sheet model of the ideally expanded equivalent planar jet.
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| 507. |
- Gojon, Romain, et al.
(författare)
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Large Eddy Simulation of Highly Compressible Jets with Tripped Boundary Layers
- 2019. - vol. 25
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Ingår i: Direct and Large-Eddy Simulation XI. ERCOFTAC Series.. - Cham : Springer. - 9783030049140 - 9783030049157 ; , s. 333-339
-
Bokkapitel (refereegranskat)abstract
- 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.
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| 508. |
- Gojon, Romain, et al.
(författare)
-
Oscillation Modes in Screeching Jets
- 2018
-
Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics. - 0001-1452 .- 1533-385X. ; 56:7, s. 2918-2924
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 509. |
- Gojon, Romain, et al.
(författare)
-
Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet
- 2017
-
Ingår i: Proceedings of the 55th AIAA Aerospace Sciences Meeting. - Reston, Virginia : American Institute of Aeronautics and Astronautics. ; , s. 19-
-
Konferensbidrag (refereegranskat)abstract
- 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.
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| 510. |
- Golliard, Thomas, et al.
(författare)
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Computational Aeroacoustics for a Cold, Non-Ideally Expanded Aerospike Nozzle
- 2023
-
Ingår i: Proceedings of ASME Turbo Expo 2023. - : American Society of Mechanical Engineers (ASME).
-
Konferensbidrag (refereegranskat)abstract
- In supersonic aerospace applications, aerospike nozzles have been subject of growing interest. These devices lead to enhanced thrust performance compared to conventional nozzles due to continuous altitude adaption and improved thrust vector control. However, supersonic non-ideally expanded jets are known to generate high levels of noise. The aeroacoustic behaviour of circular and rectangular nozzles has been largely discussed whereas data on the aeroacoustic behaviour of aerospike nozzles is scarce. For further industrial development, the identification of the noise generation mechanisms in such configurations is necessary. This study sheds light on the main noise components of a cold jet exhausting an aerospike nozzle. Implicit Large Eddy Simulations (ILES) are deployed to simulate the flow of the cold aerospike at a Nozzle Pressure Ratio (NPR) = 3. For far-field acoustic computation, the Ffowcs-Williams Hawkings (FWH) equation is applied. A mesh sensitivity study is first performed. Then, the configuration is analyzed in terms of near-field instantaneous and time-averaged flow characteristics. It is of crucial interest to characterize the features of the shock-cell structures. The annular shock structure near the aerospike bluff body displays two non-attached shock-cells of length L/Dj ∼ 0.43. The annular jet is then reattaching and this reattachment leads to longer shock-cells of length L/Dj ∼ 0.77. Downstream of the bluff body, a second expansion process takes place and leads to the emergence of a circular shock-cell structure with a first shock-cell length of L/Dj ∼ 1.20. The interaction between the vortical flow structures in the shear layers and the shocks generates Broadband Shock-Associated Noise (BBSAN). In order to enhance understanding of the noise generation mechanism for this configuration, several analyses are performed. Two-point cross-correlations of data acquired in monitoring points located along axial lines in the circular shear layers are used for quantifying the upstream propagating waves associated to a strong tonal component at a Strouhal number St = 0.51. This strong tonal component is known as screech. It is generated by a feedback mechanism between the coherent fluid flow structures propagating downstream in the jet shear layer and the upstream propagating acoustic waves generated at the same frequency by vortex-shock interactions, waves that are interacting with the nozzle lip and excite shear layer instabilities at the frequency of screech. Power spectral density of the radial velocity at monitoring points in the annular jet structure displays three main peaks at St = 0.68, St = 1.21 and St = 2.59. These frequencies correspond to the oscillation modes of the annular shock-cell structure in radial direction. Furthermore, a vortex sheet model is adapted to predict the length of the annular shock-cells. A good agreement is reached between the analytically derived shock-cell length and the simulation results. The shock-cell length is used to predict the central frequency of BBSAN as a function of observation angles. The far-field spectra show mixing noise as well as Broadband Shock-Associated Noise, related to the interaction between the convected vortices in the shear layers and the shock-cell structure. High sound pressure levels (SPL) are detected in agreement with the BBSAN central frequencies which were computed using the annular and circular shock-cell length. Finally, high SPL are obtained at the radial oscillation frequencies for the annular shock-cell structure.
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