Helicity Effect on Turbulent Passive and Active Scalar Diffusivities
Turbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neit...
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IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/adc691 |
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| author | Axel Brandenburg Petri J. Käpylä Igor Rogachevskii Nobumitsu Yokoi |
| author_facet | Axel Brandenburg Petri J. Käpylä Igor Rogachevskii Nobumitsu Yokoi |
| author_sort | Axel Brandenburg |
| collection | DOAJ |
| description | Turbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neither the usual second-order correlation approximation nor the various τ approaches have been able to capture this. Here we show that the helicity effect on the turbulent passive scalar diffusivity works in the opposite sense and leads to an enhancement. We have also demonstrated that the correlation time of the turbulent velocity field increases with the kinetic helicity. This is a key point in the theoretical interpretation of the obtained numerical results. Simulations in which helicity is being produced self-consistently by stratified rotating turbulence resulted in a turbulent passive scalar diffusivity that was found to be decreasing with increasing rotation rate. |
| format | Article |
| id | doaj-art-b457b055dad54ccbb4c5cac3ebe1cb97 |
| institution | OA Journals |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj-art-b457b055dad54ccbb4c5cac3ebe1cb972025-08-20T02:28:54ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-0198418810.3847/1538-4357/adc691Helicity Effect on Turbulent Passive and Active Scalar DiffusivitiesAxel Brandenburg0https://orcid.org/0000-0002-7304-021XPetri J. Käpylä1https://orcid.org/0000-0001-9619-0053Igor Rogachevskii2https://orcid.org/0000-0001-7308-4768Nobumitsu Yokoi3https://orcid.org/0000-0002-5242-7634Nordita, KTH Royal Institute of Technology and Stockholm University , Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden ; brandenb@nordita.org; The Oskar Klein Centre, Department of Astronomy, Stockholm University , AlbaNova, SE-10691 Stockholm, Sweden; McWilliams Center for Cosmology & Department of Physics, Carnegie Mellon University , Pittsburgh, PA 15213, USA; School of Natural Sciences and Medicine, Ilia State University , 3-5 Cholokashvili Avenue, 0194 Tbilisi, GeorgiaInstitut für Sonnenphysik (KIS) , Georges-Köhler-Allee 401a, 79110 Freiburg im Breisgau, GermanyNordita, KTH Royal Institute of Technology and Stockholm University , Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden ; brandenb@nordita.org; Department of Mechanical Engineering, Ben-Gurion University of the Negev , P.O. Box 653, Beer-Sheva 84105, IsraelInstitute of Industrial Science, University of Tokyo , Komaba, Meguro, Tokyo 153-8505, JapanTurbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neither the usual second-order correlation approximation nor the various τ approaches have been able to capture this. Here we show that the helicity effect on the turbulent passive scalar diffusivity works in the opposite sense and leads to an enhancement. We have also demonstrated that the correlation time of the turbulent velocity field increases with the kinetic helicity. This is a key point in the theoretical interpretation of the obtained numerical results. Simulations in which helicity is being produced self-consistently by stratified rotating turbulence resulted in a turbulent passive scalar diffusivity that was found to be decreasing with increasing rotation rate.https://doi.org/10.3847/1538-4357/adc691Magnetic fieldsHydrodynamics |
| spellingShingle | Axel Brandenburg Petri J. Käpylä Igor Rogachevskii Nobumitsu Yokoi Helicity Effect on Turbulent Passive and Active Scalar Diffusivities The Astrophysical Journal Magnetic fields Hydrodynamics |
| title | Helicity Effect on Turbulent Passive and Active Scalar Diffusivities |
| title_full | Helicity Effect on Turbulent Passive and Active Scalar Diffusivities |
| title_fullStr | Helicity Effect on Turbulent Passive and Active Scalar Diffusivities |
| title_full_unstemmed | Helicity Effect on Turbulent Passive and Active Scalar Diffusivities |
| title_short | Helicity Effect on Turbulent Passive and Active Scalar Diffusivities |
| title_sort | helicity effect on turbulent passive and active scalar diffusivities |
| topic | Magnetic fields Hydrodynamics |
| url | https://doi.org/10.3847/1538-4357/adc691 |
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