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- Altintas, Atilla, 1979, et al.
(författare)
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A new approximation to modulation-effect analysis based on empirical mode decomposition
- 2019
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:2
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Tidskriftsartikel (refereegranskat)abstract
- The modulation effect, namely, the amplification or attenuation of near-wall small-scale (SS) structures by outer large-scale (LS) structures, is one of two commonly accepted ways that outer LS turbulent fluctuations can influence near-wall ones. Mode decomposition based on filtering is widely used to analyze the modulation effect. In the present study, a new approximation is proposed based on empirical mode decomposition (EMD) to investigate the aforementioned amplitude modulation effect. Both methods are used, and their results are compared for two-point and single-point analyses. It has been shown that the LS and SS signals that are decomposed by filtering and EMD follow identical paths. Despite the similarities of the signals, the suggested method exhibits a slightly higher correlation coefficient R compared to the method based on filtering for the two-point analysis. For the one-point analysis, however, the suggested method gives a rational correlation coefficient for the one-point analysis compared to the two-point analysis, while the existing method seems far from giving a rational correlation coefficient value, which is too low compared to that of the two-point analysis. The suggested method is relevant to many recent studies that questioned the reliability of calculating the correlation coefficient with the existing method. The variation of R for identical signals extends the discussion of the correlation-coefficient calculations to the very first process, namely, obtaining LS and SS data from the original signal.
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| 2. |
- Altintas, Atilla, 1979, et al.
(författare)
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Direct numerical simulation of drag reduction by spanwise oscillating dielectric barrier discharge plasma force
- 2020
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 32:7
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Tidskriftsartikel (refereegranskat)abstract
- DBD (dielectric barrier discharge) plasma actuators have in recent years become increasingly attractive in studies of flow control due to their light structures and easy implementation, but the design of a series of actuators enabling drag reduction depends on many parameters (e.g., the length of the actuator, the space between actuators, and voltage applied) and remains a significant issue to address. In this study, velocities created by the DBD plasma actuators in stagnant flow obtained by the numerical model are compared with experimental results. Then, a DNS study is carried on, and spanwise oscillated DBD plasma actuators are examined to obtain a drag reduction in a fully developed turbulent channel flow. This study connects the conventional spanwise oscillated force in drag reduction studies with DBD plasma actuators. While the former is one of the most successful applications for the drag reduction, the latter is a most promising tool with its light and feasible structure.
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| 4. |
- Arvidson, Sebastian, 1981, et al.
(författare)
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Grey-area mitigation using commutation terms at the interfaces in hybrid RANS-LES modeling
- 2018
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Ingår i: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Cham : Springer International Publishing. - 1612-2909 .- 1860-0824. ; 137, s. 113-124
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Tidskriftsartikel (refereegranskat)abstract
- © 2018, Springer International Publishing AG. With the aim to mitigate the grey area at the RANS-LES interface, the effect of commutation terms is investigated. Simulations of fully developed channel flow and spatially developing boundary layer flow are presented using the commutation terms at the RANS-LES interfaces. The commutation terms are added as source terms in the k, ω and momentum equations of a zonal RANS-LES model. It is concluded that as an inlet in embedded LES of the developing boundary layer flow, the use of the proposed commutation terms are needed for the LES simulated flow to accurately predict the skin friction. However, it is demonstrated for flows where the RANS-LES interface aligns with the mean flow direction that the effect of the proposed interface methodology is weak.
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| 5. |
- Arvidson, Sebastian, 1981, et al.
(författare)
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Hybrid RANS-LES Modeling Using a Low-Reynolds-Number k−ω Based Model
- 2014
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Ingår i: 52nd AIAA Aerospace Sciences Meeting - AIAA Science and Technology Forum and Exposition, SciTech 2014, National Harbor, United States, 13-17 January 2014. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102561
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Konferensbidrag (refereegranskat)abstract
- Hybrid RANS-LES modeling is proposed using a Low-Reynold-Number (LRN) k − ω model. The model is demonstrated in a zonal RANS-LES approach and in an embedded LES approach. The model is calibrated and evaluated using Decaying Homogeneous Isotropic Turbulence (DHIT), turbulent channel flow and turbulent flow over a hump. The effect of different LES length scales on log-layer mismatch and turbulence resolving capability is demonstrated using the proposed model. Interface conditions are proposed in the embedded LES approach in order to reduce the grey area zone in the LES domain downstream of the RANS region. To further improve the development of turbulence resolving flow in the LES region downstream of the interface, anisotropic turbulent velocity fluctuations from synthetic turbulence are added. The hybrid RANS-LES modeling approaches that are presented, using the LRN k − ω based model, show that predictions of turbulence resolving flows are in reasonable agreement with experimental data and DNS data. Moreover, the choice of the LES length scale using the proposed model is shown to be of great importance in reducing the log-layer mismatch.
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| 6. |
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| 7. |
- Arvidson, Sebastian, 1981, et al.
(författare)
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Interface methods for grey-area mitigation in turbulence-resolving hybrid RANS-LES
- 2018
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X. ; 73:Oct., s. 236-257
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Tidskriftsartikel (refereegranskat)abstract
- © 2018 A grey area mitigation method is proposed for hybrid RANS-LES modeling. The proposed methodology is evaluated using a hybrid RANS-LES method based on a Low-Reynolds-Number k−ω model applied to channel flow, boundary layer flow and a spatially developing mixing layer flow. Emphasis is put on the use of commutation terms at the RANS-LES interfaces in the transport equations for the turbulent kinetic energy, the specific dissipation rate and the momentum equation in order to rapidly reduce the turbulent viscosity across a RANS-to-LES interface and to stimulate the development of resolved turbulent fluctuations. The proposed methodologies are applied at both wall-normal (and inlet) and wall-parallel RANS-LES interfaces. The proposed methodology gives a rapid reduction of the turbulent viscosity at the wall-normal RANS-LES interface from its RANS level to its LES level. Moreover, the proposed methodology contributes to a substantially more rapid establishment of the turbulence-resolving LES flow downstream of the wall-normal RANS-LES interface than if no grey-area mitigation method is applied. However, the proposed methodology has a weaker effect at wall-parallel RANS-LES interfaces, due to a stronger entrainment of LES contents into the near-wall RANS region, than at the wall-normal RANS-LES interfaces. Good agreement with experimental data is obtained with the proposed interface method for the evaluated flow cases. The most obvious grey area mitigation effect is given in the simulated mixing layer flow. Turbulent velocity fluctuations are efficiently established with the commutation term in the momentum equation at the RANS-LES interface in this flow as well as a rapid reduction of the turbulent viscosity due to the commutation terms in the k and ω equations, which gives an almost negligible delay in the development of the resolved turbulence.
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