| 511. |
- Golliard, Thomas, et al.
(författare)
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Swirling flow effects on highly-heated aerospike nozzle jets
- 2024
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Ingår i: 30th AIAA/CEAS Aeroacoustics Conference, 2024. - : American Institute of Aeronautics and Astronautics (AIAA). - 9781624107207
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Konferensbidrag (refereegranskat)abstract
- In the present study, the flow and acoustic characteristics of an aerospike nozzle supersonic jet at a Nozzle Pressure Ratio (NPR) = 3, three Temperature Ratios (TR) = 1, 3, 7 and two Swirl Numbers S = 0.10, 0.20 are presented. Implicit Large Eddy Simulations (ILES) are deployed to simulate the aerospike nozzle flow. The far-field aeroacoustic signature is computed based on the Ffowcs Williams-Hawkings (FWH) equation. In the vicinity of the aerospike bluff body, a shock-cell structure is formed for all the configurations. The shock strength and length as well as the pressure fluctuations are primarily affected by the TR in that jet region. The supersonic flow reattaches further downstream towards the aerospike bluff body as the TR increases at a fixed Swirl Number. This influences in particular the flapping motion of the annular shock-cell structure. The latter is characterized by power spectral density of the radial velocity at well-chosen monitoring points in that region. Subsequently, two-point cross-correlations in the annular jet shear layer are computed to detect azimuthal jet modes. The azimuthal jet excitation increases in amplitude with increasing Swirl Number S, leading to high Sound Pressure Levels at the Strouhal numbers observed. Downstream of the aerospike bluff body, a short circular shock-cell structure is observed at Swirl Number S = 0.10 for higher TR while the jet remains annular in the cold case. At S = 0.20, fewer shock cells are formed downstream of the aerospike bluff body. The shortening of the shock-cell structure leads to screech elimination at both Swirl Numbers. Further crosscorrelations for the axial velocity in the jet shear layers show supersonic convection velocities at Temperature Ratios (TR) = 3 and 7 for both Swirl Numbers which confirms the presence of Mach waves observed in the near-field snapshots. The Mach wave radiation features a slight helical propagation pattern in contrast with the baseline case without swirling motion. Furthermore, a skewness larger than 0.4 and a positive kurtosis of the pressure signals along the Mach wave radiation lines indicate crackle noise at TR7. The far-field spectra computed with the Ffowcs Williams-Hawkings equation display mixing noise only for S = 0.10. In the cold cases, high SPL are detected in agreement with the Broadband Shock-Associated Noise (BBSAN) central frequencies which were computed using the annular and circular shock-cell length. Additionally, high SPL are obtained due to Mach wave propagation, at the Strouhal numbers of the azimuthal modes and of radial motion of the annular shock-cell structure.
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| 512. |
- Gullbrekken, Lars, et al.
(författare)
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Wind pressure coefficients for roof ventilation purposes
- 2018
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105. ; 175, s. 144-152
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Tidskriftsartikel (refereegranskat)abstract
- Wind pressure coefficients (c p ) are important inputs for analytical calculations of wind load. The aim of this research is to investigate wind pressure coefficients on a test house located in Norway in order to pave the way for improved analysis of wind-driven roofing ventilation. The large-scale test measurements show that the wind pressure coefficient along the eaves of the house varies with different wind approach angles. Assuming wind-driven air flow through the air cavity beneath the roofing, an average Δc p ¯ value of 0.7 is derived for practical engineering purposes. The results from the study are applicable for single or two-storey houses with pitched roofs at different roof angles.
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| 513. |
- Hafsteinsson, Haukur, 1984, et al.
(författare)
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Noise Control of Supersonic Jet with Steady and Flapping Fluidic Injection
- 2015
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Ingår i: AIAA Journal. - 1533-385X .- 0001-1452. ; 53:11, s. 3251-3272
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulation is used to investigate steady-state mass flow injection into a supersonic jet stream with and without flapping motion of the microjets. The results are validated with particle image velocimetry and acoustic measurements. The effect of microjet penetration on the far-field acoustics is studied by altering the number of injectors, the cross-sectional area of each injector, and the injection mass flow. The injectors are evenly distributed around the nozzle exit. The injection angle is 90 deg relative to the main jet flow. This research is a continuation of a previous large-eddy simulation study of pulsed injection that showed that the unsteady injection-induced pressure pulses in the jet caused increased tonal noise for far-field observers at low angles. Flapping jet injection was shown to minimize the creation of the pressure pulses, except for high-amplitude flapping angles and high injection mass flows, where the injections divert out of the shear layer and introduce periodic superposition of the double shock-cell structure. Furthermore, the flapping injection did not show improved noise reduction compared with the steady injection, which is essentially promising because steady injection proves to be a more practical solution for implementation in real jet engine applications.
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| 514. |
- Hahn, Berthold, et al.
(författare)
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Recommended practices for wind farm data collection and reliability assessment for O&M optimization
- 2017
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Ingår i: Energy Procedia. - : Elsevier BV. - 1876-6102. ; 137, s. 358-365
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Konferensbidrag (refereegranskat)abstract
- The paper provides a brief overview of the aims and main results of IEA Wind Task 33. IEA Wind Task 33 was an expert working group with a focus on data collection and reliability assessment for O & M optimization of wind turbines. The working group started in 2012 and finalized the work in 2016. The complete results of IEA Wind Task 33 are described in the expert group report on recommended practices for "Wind farm data collection and reliability assessment for O & M optimization" which will be published by IEA Wind in 2017. This paper briefly presents the background of the work, the recommended process to identify necessary data, and appropriate taxonomies structuring and harmonizing the collected entries. Finally, the paper summarizes the key findings and recommendations from the IEA Wind Task 33 work.
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| 515. |
- Hammer, Steffen, et al.
(författare)
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IDENTIFICATION OF CRITICAL ACOUSTIC MODES FOR AN OPEN BOX CAVITY WITH ADJUSTABLE DEPTH
- 2022
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Ingår i: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022. - : International Council of the Aeronautical Sciences. ; , s. 2910-2922
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Konferensbidrag (refereegranskat)abstract
- Aerospace engineers are facing increased challenges in designing the next generation of aero engines. Engines need to be lighter, compact, more powerful and consume less fuel. On the other hand the designs are supposed to be more reliable than before. Modern compressors in those engines are playing a big role in fulfilling parts of these requirements by increased aerodynamic efficiency. Additionally, increasing requirements on compactness forcing new designs that may alter the acoustic resonance picture significantly. All these design adjustments face additional risks of component damage or fatigue. Air bleed systems are necessary parts in the compressor section of an aero engine to allow a wider operational range. However, these bleed systems can additionally introduce acoustic resonance due to the nature of air flow over an opening. The bleed design needs to be adapted to avoid this kind of resonance as much as possible. This paper is a continuation of an experimental investigation of cavity resonance for an open box cavity. The operating range covers the typical flow speeds in aero engine low pressure compressors. The geometry of a compressor bleed system is simplified to allow the use of theoretical prediction models. The previous results of detected acoustic resonances are screened for critical modes in terms of amplitude. This screening identified vulnerable operating points at higher Mach numbers in the range of 0.65 to 0.8 for all cavity length to depth ratios between 4 and 0.5. The test data show that an interaction of Rossiter modes with other feedback mechanisms can significantly amplify the acoustic resonance.
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| 516. |
- He, Kan, et al.
(författare)
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Influence of the rounded rear edge on wake bi-stability of a notchback bluff body
- 2021
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:11
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Tidskriftsartikel (refereegranskat)abstract
- The wake bi-stability behind notchback Ahmed bodies is investigated by performing wind tunnel experiments and large eddy simulations (LESs). The focus of this study is on the suppression of bi-stable wakes achieved by rounding the roof's trailing edge of the body. The suppression effect is found to depend on the Reynolds number (R e). The wake behind a sharp edge remains bi-stable for all tested R e. However, for a rounded edge with small radius, wake bi-stability at R e = 0.5 × 10 5 and wake symmetrization with 0.75 × 10 5 ≤ R e ≤ 1.5 × 10 5 are observed. Increasing R e with R e ≥ 1.75 × 10 5, the wake returns to the bi-stable state. Particularly, with R e ≥ 2 × 10 5, a stable asymmetric wake state with no switches is observed for long periods. Performing LES confirms the expected wake asymmetry at R e = 0.5 × 10 5 and symmetry at R e = 1 × 10 5 for the case of rounded edge with a small radius. Besides, another wake symmetry is observed for the rounded edge with a large radius at R e = 0.5 × 10 5. For the two wake symmetries shown in the LES results, the symmetrization is attributed to wake suppression in the notchback region, forcing the flow separation from the rear roof to attach to the slant on both sides of the body.
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| 517. |
- Hoffman, Johan, et al.
(författare)
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New Theory of Flight
- 2016
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Ingår i: Journal of Mathematical Fluid Mechanics. - : Springer. - 1422-6928 .- 1422-6952. ; 18:2, s. 219-241
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Tidskriftsartikel (refereegranskat)abstract
- We present a new mathematical theory explaining the fluid mechanics of sub-sonic flight, which is fundamentally different from the existing boundary layer-circulation theory by Prandtl-Kutta-Zhukovsky formed 100 year ago. The new the-ory is based on our new resolution of d’Alembert’s paradox showing that slightlyviscous bluff body flow can be viewed as zero-drag/lift potential flow modified by3d rotational slip separation arising from a specific separation instability of po-tential flow, into turbulent flow with nonzero drag/lift. For a wing this separationmechanism maintains the large lift of potential flow generated at the leading edgeat the price of small drag, resulting in a lift to drag quotient of size15
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| 518. |
- Johansson, Anders, et al.
(författare)
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Aircraft Approach Noise Trials
- 2022
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Ingår i: Internoise 2022. - : The Institute of Noise Control Engineering of the USA, Inc..
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Konferensbidrag (refereegranskat)abstract
- This article presents the results from a series of aircraft approach trials that were conducted with the aim to investigate noise reduction procedures within the boundaries of a normal ILS approach. The significant decline in air traffic at Stockholm Arlanda, that occurred during the pandemic meant that the empty airspace and the availability of grounded aircrafts could be utilized to perform controlled flights - something that would have been difficult to achieve during normal traffic conditions. The approach trials were performed by two Airbus A321, which alternately carried out interrupted landing procedures starting 17 nautical miles (nm) from the runway threshold. During the trials, the aircraft speed and configuration (high lift devises and landing gear) were varied according to a predetermined schedule. To capture these variations, flight data (FDR) were recorded while the noise on ground was measured at positions approximately once every nautical mile along the flight track. Results suggest that speed and configuration recommendations can be effective to reduce noise, especially for the final 7 nautical miles of the flight track. However, whether a low speed is to be advocated during the entire approach is currently unclear.
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| 519. |
- Jonsson, Isak, 1990, et al.
(författare)
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Infrared Thermography Investigation of Heat Transfer on Outlet Guide Vanes in a Turbine Rear Structure
- 2020
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Ingår i: International Journal of Turbomachinery, Propulsion and Power. - : MDPI AG. - 2504-186X. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Aerothermal heat transfer measurements in fluid dynamics have a relatively high acceptance of uncertainty due to the intricate nature of the experiments. The large velocity and pressure gradients present in turbomachinery application add further complexity to the measurement procedure. Recent method and manufacturing development has addressed some of the primary sources of uncertainty in these heat transfer measurements. However, new methods have so far not been applied in a holistic approach for heat transfer studies. This gap is bridged in the present study where a cost-effective and highly accurate method for heat transfer measurements is implemented, utilising infrared thermography technique (IRT) for surface temperature measurement. Novel heat transfer results are obtained for the turbine rear sturcture (TRS), at engine representative conditions for three different outlet guide vane (OGV) blade loading and at Reynolds Number of 235000. In addition to that, an extensive description of the implementation and error mitigation is presented.
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| 520. |
- Joshi, N., et al.
(författare)
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MEMS Based Micro Aerial Vehicles
- 2016
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 757:1
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Konferensbidrag (refereegranskat)abstract
- Designing a flapping wing insect robot requires understanding of insect flight mechanisms, wing kinematics and aerodynamic forces. These subsystems are interconnected and their dependence on one another affects the overall performance. Additionally it requires an artificial muscle like actuator and transmission to power the wings. Several kinds of actuators and mechanisms are candidates for this application with their own strengths and weaknesses. This article provides an overview of the insect scaled flight mechanism along with discussion of various methods to achieve the Micro Aerial Vehicle (MAV) flight. Ongoing projects in Chalmers is aimed at developing a low cost and low manufacturing time MAV. The MAV design considerations and design specifications are mentioned. The wings are manufactured using 3D printed carbon fiber and are under experimental study. © Published under licence by IOP Publishing Ltd.
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