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- Kehoe, Laura, et al.
(författare)
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Make EU trade with Brazil sustainable
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6438, s. 341-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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- Abreu, Leandra I., et al.
(författare)
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Wavepackets in turbulent flows around airfoils
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Motivated by the recent analysis by Sano et al. 2019, Phys. Rev. Fluids, vol. 4, p. 094602, of spanwise-coherent structures in the turbulent flow around airfoils and their connection to trailing-edge noise, we carry out a thorough characterisation of such structures in three simulation databases. We analyse two different numerical simulations of incompressible flow in turbulent regime, both at chord Reynolds number of 400,000: a large-eddy simulation for a NACA 0012 profile at zero angle of attack, and a direct numerical simulation for a NACA 4412 airfoil with an angle of attack of 5 degrees. Snapshots of the flow field were analysed using Spectral Proper Orthogonal Decomposition (SPOD), in order to extract the dominant coherent structures of the flow. Focus is given to the aforementioned spanwise-coherent fluctuations, which two-dimensional disturbances in the computational domain due to the use of periodic boundary conditions. The leading SPOD modes show that the most energetic coherent structures are wavepackets, extending over the whole turbulent boundary layers around the airfoils with significant amplitudes near the trailing-edge. Higher amplitudes are observed in the region of stronger adverse pressure gradient at the suction side of the NACA 4412 airfoil. To understand how such structures in the turbulent field can be modelled, the linear response of the flow using the singular value decomposition of the linearised resolvent operator was performed, using the mean field as a base flow and considering a locally parallel approximation. Such analysis shows that the leading SPOD modes can be associated to optimal, linearised flow responses, particularly for stations far from the trailing edge; the latter introduces a discontinuity in boundary conditions, and the locally parallel approximation becomes questionable. We then focus on evaluating the dependence of such wavepackets on the domain size, to ensure that these structures are not an artifact of the use of periodic boundary conditions in small computational boxes. To do so, we performed an incompressible LES of a zero-pressure gradient turbulent boundary layer (ZPGTBL), for three different spanwise sizes: Lz=32 δ*, Lz=64 δ* and Lz=128 δ*, where δ* is a reference displacement thickness in a region of developed turbulent flow, with Reynolds number matching the values in the airfoil simulations. The signature of such wavepackets is seen in non-premultiplied spanwise wavenumber spectra, which reaches, for the three domain sizes, a plateau for spanwise wavelengths going to infinity (or wavenumbers going to zero); this plateau is representative of the spanwise-coherent structures seen in the airfoil simulations. Similar SPOD and resolvent analyses were carried out for the zero spanwise wavenumber of the ZPGTBL, and the same coherent wavepackets were observed for the three domains, with very similar amplitudes. Such wavepackets were also accurately modelled using the optimal resolvent response. These results confirm that the spanwise-elongated structures are not domain-size dependent for the studied simulations, and are thus a feature of turbulent boundary layers.
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- Brito, Pedro P. C., et al.
(författare)
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Experimental control of Tollmien-Schlichting waves using pressure sensors and plasma actuators
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This manuscript shows the successful application of the inverse feed-forwardcontrol (IFFC) technique for the cancellation of Tollmien–Schlichting (TS)waves. Active wave cancellation of two-dimensional broadband TS disturbancesis performed with a single dielectric barrier discharge (DBD) plasma actuator.The measurements required for the IFFC are performed with microphones,instead of hot wires most used for this task, in order to reduce the spaceoccupied by the sensors and assess the suitability of simpler and cheaperdevices. The experiments are conducted in an open-circuit wind-tunnel witha NACA0008 wing profile. An attenuation of the TS-wave amplitude of oneorder of magnitude is achieved. Direct numerical simulations (DNS) are alsoperformed, and compared to the outcome of the experiments. The modeling ofboth actuator and sensors in the DNS is not based on data from the presentexperiments. The plasma actuator used is a mapping of the force field inFabbiane et al., J. Fluid Mech. 2015, to the NACA0008 wing profile, whereasthe microphones are modeled as pressure probes. Despite these modellingchoices, a remarkable level of agreement between the DNS and the experimentsis achieved. However, the control performance is better in the DNS, withattenuations of three orders of magnitude of TS-wave amplitude. Furtheranalysis of experiments and simulations shows that the limiting factor in theexperiments is the ambient low-frequency acoustic waves in the wind tunnel,which are sensed by the microphones, acting as noise in the analysis of TS-waveevolution and thus leading to lower coherences between sensors and actuators.This in turn leads to a suboptimal control kernel in the experiment.179
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