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- Tofique, Muhammad Waqas, 1986-, et al.
(författare)
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Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branches
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents the development of the distributed dislocation dipole technique (DDDT) for the analysis of crack surface closure of crack cases involving kinks and branches. Crack cases in which closure occurs are analyzed by reformulating the Bueckner's principle taking the contact stresses at the contacting portions of the crack surfaces into account. Stress intensity factors corresponding to opening and sliding mode of deformation at the crack tips are computed. Three test cases involving kinked and/or branched cracks with at least one of the crack segments undergoing crack surface closure when subjected to remote tensile loading are analyzed. The results obtained from the DDDT are compared to those obtained from the Finite Element Method (FEM) analysis of the same crack cases. This comparison shows that the computation of stress intensity factors for the crack cases involving crack surface closure are less acurate compared to fully open crack cases. However, the stress intensity factors are still computed to an accuracy of within 2 percent if the Jacobi polynomial expansions of at least the sixth order are used to represent the crack surface opening and sliding displacements. Higher order Jacobi polynomials lead to increased accuracy.
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| 4. |
- Kleine, Vitor G., 1986-, et al.
(författare)
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Simulating airplane aerodynamics with body forces: an actuator line method for non-planar wings
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Two configurations typical of fixed-wing aircraft are simulated with the actuator line method (ALM): a wing with winglets and a T-tail. The ALM is extensively used in rotor simulations, modeling the presence of blades by body forces, calculated from airfoil data and the relative flow velocity. This method has not been used to simulate airplane aerodynamics, despite its advantage of allowing courser grids. This may be credited to the failure of the uncorrected ALM to accurately predict forces near the tip of wings, even for simple configurations. The conception of the vortex-based smearing correction showed promising results, suggesting such failures are part of the past. For the non-planar configurations studied in this work, differences between the ALM with the original smearing correction and a non-linear lifting line method (LL) are observed near the intersection of surfaces, because the circulation generated in the numerical domain differs from the calculated corrected circulation. A vorticity magnitude correction is proposed, which improves the agreement between ALM and LL. This second-order correction resolves the ambiguity in the velocity used to define the lift force. The good results indicate that the improved ALM can be used for airplane aerodynamics, with an accuracy similar to the LL.
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- Blomberg, Lars, et al.
(författare)
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Atrid-2 : An advanced auroral microprobe
- 1999
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Astri-2 is an advanced auroral microprobe with dual primary mission objectives; to do high-quality in situ measurements of the physical processes behind the aurora, and to demonstrate the usefulness of microspacecraft as advanced research tools. Mission success will open up entirely new possibilities to carry out low-budget multipoint measurements in near-Earth space. This long-desired kind of in situ measurements are the next major step forward in experimental space physics. Astrid-2 has platform dimensions of 45×45×30 cm, a total mass of just below 30 kg, and carries scientific instruments for measuring local electric and magnetic fields, plasma density and density fluctuations, ions and electrons, as well as photometers for remote imaging of auroral emissions. Attitude determination is provided by a high-precision star imager. Some 250 Mbytes' worth of scientific data will be received each day at the two ground stations. Astrid-2 will be launched as a piggy-back on a Russian Kosmos-3M launcher into an 83 deg inclination circular orbit at 1000 km altitude. Nodal regression will give complete coverage of all local time sectors every 3.5 months. © 1999 Elsevier B.V. All rights reserved.
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- Grönstedt, Tomas, 1970
(författare)
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Effektivare motorer, flyget och miljön
- 2018
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Flyget står inför stora mijöutmaningar. I presentationen ges fyra punkter där stora miljövinster kan åstadkommas och detta backas med teknik och åtgärder som kan stödja framstegen.
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| 7. |
- Grönstedt, Tomas, 1970
(författare)
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In response to the public consultation on Clean Aviation Partnership: A short comment on ultra-efficient gas turbines and sustainable aviation fuels
- 2020
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This two-page comment on the Clean Aviation SRIA, contains output from two Horizon 2020 projects, ULTIMATE (ultra-efficient engines) and ENABLEH2 (hydrogen aircraft). The report has also been updated by a coordinated effort on collecting strategic inputs from Chalmers University of Technology. Some output from efforts on road-mapping conducted together with leading aero engine manufacturers is also supporting this comment. The comment concentrates on giving input on the two key thrusts “3. Ultra-efficient Aircraft and Gas Turbines” and “4. Sustainable Aviation Fuels enabled Aircraft”. However, for all four thrust areas, it is noted that as aircraft design complexity increases the need for dynamic modelling, energy management and optimal control increases.
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| 8. |
- Kern, J. Simon, et al.
(författare)
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Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thin airfoil dynamic stall at moderate Reynolds numbers is typically linked to the sudden bursting of a small laminar separation bubble close to the leading edge. Given the strong sensitivity of laminar separation bubbles to external disturbances, the onset of dynamic stall on a NACA0009 airfoil section subject to different levels of low-amplitude freestream disturbances is investigated using direct numerical simulations. For turbulence intensities at the leading edge of Tu = 0.02 %, the flow is practically indistinguishable from clean inflow simulations in literature. For Tu = 0.05 %, the bursting process is found to be considerably less smooth and strong coherent vortex shedding from the laminar separation bubble is observed prior to the formation of the dynamic stall vortex. The non-linear simulations are complemented with a transient linear stability analysis of the time-dependent evolution of the laminar shear layer in the bursting separation bubble using a spatially localised formulation of the Optimally Time-Dependent (OTD) framework, with which the most unstable part of the instantaneous tangent space of the non-linear trajectory is tracked over time. The resulting modes reveal intermittent switching between two regimes. Rapid growth of the Kelvin--Helmholtz rolls on the separated shear layer and complicated secondary instabilities on the transitional part of the separation bubble. The appearance of the latter is linked to large spikes in the instantaneous growth rate within the linear subspace and more rapid transition in the non-linear baseflow. These intense growth spikes are well correlated with the subsequent shedding of energetic vortices from the laminar separation bubble.
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| 9. |
- Kern, J. Simon, et al.
(författare)
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Onset of absolute instability on a pitching airfoil
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A transient linear stability analysis of the time-dependent flow around a natural laminar flow airfoil undergoing small-amplitude pitching motion is performed using the Optimally Time-Dependent (OTD) framework. The most salient linear instabilities associated with the instantaneous basic state are computed and tracked over time. The resulting OTD modes reflect the variations in the basic state and can be used as predictors of its evolution including the formation of a laminar separation bubble, its gradual spanwise modulation via primary global instability leading to secondary instability and finally rapid breakdown to turbulence. The study confirms and expands upon earlier stability analyses of the same case based on the local properties of span-wise averaged velocity profiles in the bubble that predicted the onset of absolute instability soon followed by rapid breakdown of the separation bubble. The three-dimensional structure of the most unstable OTD mode is extracted which compares well with both the locally absolutely unstable mode and the evolution of the basic state itself.
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| 10. |
- Kern, J. Simon, et al.
(författare)
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Space-time Proper Orthogonal Decomposition of the onset of leading edge dynamic stall
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The increased availability of large scale computing hardware brings the analysis of fully three-dimensional non-autonomous flow cases within reach. In these flow scenarios, the simplifying assumption of temporal homogeneity is not applicable and with it many data-driven analysis techniques that rely on it. Within the well-established modal decomposition framework of Proper Orthogonal Decomposition (POD), we can treat time in the same way as the spatial dimensions and apply the method to statistical ensembles of non-autonomous flows in order to extract coherent structures in space and time from the resulting experimental or numerical data, leading to the space-time POD formulation. This extension of the existing method is demonstrated on the model problem of the complex Ginzburg--Landau equation, modified to include non-autonomous parameter variations. Subsequently, the space-time POD analysis is carried out on a numerical dataset of 25 realisations of the onset of leading edge dynamic stall on a NACA0009 airfoil section subject to low levels of background disturbances. The space-time POD, combined with extended POD, is used to extract the spatio-temporal structure of energetic wavetrains during the bursting of the laminar separation bubble close to the leading edge, which are found to be statistically relevant phenomena in the context of incipient dynamic stall. The potential of the space-time POD methodology to objectively extract coherent structures from ensembles of non-autonomous data is demonstrated.
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