| 1. |
- Arvidson, Sebastian, 1981, et al.
(författare)
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EFFECT OF LES LENGTH SCALE AND NUMERICAL SCHEME IN HYBRID RANS-LES OF FREE SHEAR LAYER FLOWS
- 2022
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Ingår i: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022. ; 4, s. 2419-2437
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Konferensbidrag (refereegranskat)abstract
- The effect of LES length scales, treatment of near-wall functions in the LES domain and numerical discretization schemes has been evaluated in SA-DDES simulations of free shear layer flows. In a fundamental low Mach number free shear layer flow it has been shown that the choice of LES length scale, how the near-wall functions are treated in LES mode and the numerical scheme is of vital importance for an accurate prediction of the mixing and the resolved turbulent Reynolds stresses. In simulations of a transonic flow over the generic M219 cavity it was observed that the choice of LES length scale is not as evident as in the fundamental free shear layer. However, it was clear that vorticity based LES length scales improve the prediction of the overall sound pressure levels, the Rossiter modes and the mean flow field compared to Δmax used in the original formulation of SA-DDES.
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| 2. |
- Hallgren, Christoffer, et al.
(författare)
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Classification and properties of non-idealized coastal wind profiles - an observational study
- 2022
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Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
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Tidskriftsartikel (refereegranskat)abstract
- Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.
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| 3. |
- Nilsson, Charlotte, 1985-, et al.
(författare)
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Phosphorus speciation in sewage sludge from three municipal wastewater treatment plants in Sweden and their ashes after incineration
- 2022
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Ingår i: Waste Management & Research. - : Sage Publications. - 0734-242X .- 1096-3669. ; 40:8, s. 1267-1276
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Tidskriftsartikel (refereegranskat)abstract
- Given the high efficiency in phosphorus removal at municipal wastewater treatment plants (MMWWTP), sewage sludge constitutes a promising resource for phosphorus (P) recovery. Sewage sludge is, however, a complex matrix and its direct use as fertiliser is limited by its content of metals/metalloids and organic pollutants. In order to increase its usability as a potential resource of P, there is a need for increased knowledge on phosphorus speciation in these matrices. The sludge composition is highly influenced by local conditions (i.e. wastewater composition and treatment method), and it is therefore important to study sludge from several MMWWTPs. In this study, three different protocols for sequential extraction were utilised to investigate the chemical speciation of phosphorus in sludge from three different MMWWTP sludges in Sweden, as well as in corresponding ashes following incineration. The results showed that the total amounts of phosphorus ranged from 26 to 32 mg g-1 sludge (dry weight), of which 79-94% was inorganically bound (IP). In the sludge, 21-30% of the IP was associated with calcium (Ca-P), which is the preferred species for fertiliser production. Following incineration, this fraction increased to 54-56%, mainly due to transformation of iron-associated phosphorus (Fe-P), while aluminium-associated species of phosphorus (Al-P) remained unaltered. The results from this study confirm that incineration is a suitable treatment for sewage sludge in terms of potential phosphorus recovery.
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| 4. |
- Nilsson, Stefan, 1985
(författare)
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Advanced Fluid-Structure Interaction Modelling and Simulation for Aerospace Applications
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical fluid-structure interaction (FSI) methods for the prediction of aeroelastic phenomena are important within aerospace. The continuous development of computer technologies has enabled the use of more advanced FSI methods. The use of advanced methods has the potential to provide more accurate predictions. It also enables simulation of applications for which engineers traditionally have relied upon wind tunnel testing and flight testing, and still do to a large extent. Hence, the use of more advanced FSI methods would limit the need for wind tunnel testing and flight testing, and in extension reduce the lead time and cost of aircraft development. High Reynolds number flows, involving separated flow, are very challenging to simulate. Hybrid Reynolds-averaged Navier-Stokes (RANS)-large-eddy simulation (LES) techniques provide the possibility to simulate such flows for industrial purposes. Hybrid RANS-LES methods are employed in this thesis for two applications which require turbulence-resolving techniques. First, the effects of elastic walls on the aeroacoustics in transonic cavity flow are investigated. The prediction of structural vibrations is also important since vibrations may endanger the structural integrity, additionally, vibrations may negatively affect other apparatuses. The features of cavity flow appear in weapon bays and landing gear bays in an aircraft. In a deep cavity, the flow constitutes of broadband and tonal noise, referred to as Rossiter modes. The cavity structure is simulated with a modal-based approach and with a non-modal approach where the equation of motion is solved for all degrees-of-freedom of a reduced order finite element model. The results evince that the aeroacoustic field is altered by the elastic walls. For the investigated case, the energy of the 4th Rossiter mode is depleted and a strong tone is induced at a frequency below the 4th Rossiter mode, which is absent in the rigid cavity; these observations are made with both the structural simulation methods. However, with the non-modal approach, a second strong tone is induced at a frequency above the 4th Rossiter frequency. The second investigated application is the aeroelastic prediction of a wing at Mach numbers ranging from subsonic to supersonic speeds. The viscous effects become significant at transonic speeds and may provoke shock induced flow separation. It is shown that the viscous effects play an important role under such circumstances and that both static and dynamic structural responses differ significantly depending on whether hybrid RANS-LES or unsteady RANS is employed for the flow simulation.
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| 5. |
- Nilsson, Stefan, 1985, et al.
(författare)
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Conjunction of Aeroelasticity and Aeroacoustics in Transonic Cavity Flow
- 2022
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Ingår i: Proceedings of the International Forum of Aeroelasticity and Structural Dynamics 2022, IFASD 2022.
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Konferensbidrag (refereegranskat)abstract
- The effects of elastic cavity walls on noise generation at transonic speed are investigated for the generic M219 cavity. The flow is simulated with Spalart-Allmaras (SA) improved delayed detached-eddy simulation (IDDES) in combination with a wall function. The structural analysis software, which is strongly coupled with the flow solver, uses a modal formulation. The first 50 structural normal mode shapes are included in the simulation, spanning frequencies 468–2280 Hz. Results are compared with those from a reference simulation with rigid cavity walls. Spectral analysis of pressure fluctuations at microphones above the cavity evinces a distinct tone at 816 Hz, which is absent in the reference simulation. Moreover, the power of the 4th Rossiter mode at 852 Hz is depleted, implying a significant energy transfer from the fluid to the structure. Spectral proper orthogonal decomposition (SPOD) is employed for analyses of cavity wall pressure fluctuations and wall displacements. The SPOD mode energy spectra show results consistent with the spectra at the microphones, regarding the tone at 816 Hz and the depletion of the energy at the 4th Rossiter mode. In addition, the SPOD mode energy spectra show energy spikes at additional frequencies, which coincide with structural eigenfrequencies
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| 6. |
- Nilsson, Stefan, 1985, et al.
(författare)
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Effects of Aeroelastic Walls on the Aeroacoustics in Transonic Cavity Flow
- 2022
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Ingår i: Aerospace. - : MDPI AG. - 2226-4310. ; 9:11
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Tidskriftsartikel (refereegranskat)abstract
- The effects of elastic cavity walls on noise generation at transonic speed are investigated for the generic M219 cavity. The flow is simulated with the Spalart–Allmaras (SA) improved delayed detached-eddy simulation (IDDES) turbulence model in combination with a wall function. The structural analysis software uses a modal formulation. The first 50 structural normal mode shapes are included in the simulation, spanning frequencies of 468–2280 Hz. Results are compared with those from a reference simulation with rigid cavity walls. A spectral analysis of pressure fluctuations from a microphone array above the cavity evinces a distinct tone at 816 Hz, which is absent in the reference simulation. Furthermore, the power of the 4th Rossiter mode at 852 Hz is depleted, implying a significant energy transfer from the fluid to the structure. Spectral proper orthogonal decomposition (SPOD) is employed for analyses of cavity wall pressure fluctuations and wall displacements. The SPOD mode energy spectra show results consistent with the spectra of the microphone array with respect to the tone at 816 Hz and the depletion of the energy at the 4th Rossiter mode. Furthermore, the SPOD mode energy spectra show energy spikes at additional frequencies, which coincide with structural eigenfrequencies.
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| 7. |
- Nilsson, Stefan, 1985, et al.
(författare)
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Effects of Viscosity and Density on the Aeroelasticity of the ONERA M6 Wing from Subsonic to Supersonic Speeds
- 2022
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Ingår i: AIAA AVIATION 2022 Forum. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- The effects of fluid viscosity and density on the aeroelasticity of the ONERA M6 wing over a wide range of free-stream Mach numbers, spanning 0.6–1.1, based on viscous and inviscid flow assumptions are studied. Both static and dynamic responses of the wing are examined. We employ a hybrid Reynolds-Averaged Navier-Stokes (RANS) - Large-Eddy Simulation (LES) method for viscous flow, namely Spalart-Allmaras Delayed Detached-Eddy Simulation (SA-DDES). The inviscid flow solver uses the Euler equations. A few selected cases are also analysed using Unsteady RANS (URANS). The flow solvers are strongly coupled to a structural analysis software, which uses a modal formulation. The structural responses are analysed using a constant free-stream density for all Mach numbers. In addition, higher densities are used for all Mach numbers in order to find the critical dynamic pressure where flutter is obtained. A substantial difference in the aeroelastic responses is found for Mach numbers ranging 0.8750–0.9500, when comparing viscous and inviscid flow simulations. Furthermore, it is shown that viscosity is of minor importance at subsonic and supersonic speeds. At Mach number 0.8395 it is shown that DDES provides close to identical structural responses as URANS. The largest difference of the predicted flutter boundaries is found in the transonic region. The flutter boundary for viscous flow stands out at Mach 0.9250 where it is 53 per cent higher than the flutter boundary predicted by inviscid flow. Simulations with URANS showed that it would predict a lower flutter boundary at Mach 0.9250, compared to SA-DDES.
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