In this work, we consider the effect of a small-scale helical driving force on fluid with a stable temperature gradient with Reynolds number. At first glance, this system does not have any instability. However, we show that a large scale vortex instability appears in the fluid despite its stable stratification. In a non-linear mode this instability becomes saturated and gives a large number of stationary spiral vortex structures. Among these structures there is a stationary helical soliton and a kink of the new type. The theory is built on the rigorous asymptotical method of multi-scale development.

In this paper, we find a new large scale instability which appears in obliquely rotating flow with the small scale turbulence, generated by external force with small Reynolds number. The external force has no helicity. The theory is based on the rigorous method of multi-scale asymptotic expansion. Nonlinear equations for instability are obtained in the third order of the perturbation theory. In this article, we explain in detail the nonlinear stage of the instability and we find the nonlinear periodic vortices and the vortex kinks of Beltrami type.

С использованием асимптотического метода многих масштабов построена нелинейная теория возникновения крупномасштабных структур в стратифицированной проводящей среде при наличии мелкомасштабных осцилляций поля скорости и магнитных полей. Такие стационарные мелкомасштабные осцилляции поддерживаются малыми внешними источниками при малых числах Рейнольдса. Получена нелинейная система уравнений, описывающая эволюцию крупномасштабных структур поля скорости и магнитных полей. Линейная стадия эволюции приводит к известной неустойчивости. Рассмотрены стационарные крупномасштабные структуры, возникающие при стабилизации линейной неустойчивости.Ключевые слова: стратифицированная проводящая среда, нелинейная система уравнений, неустойчивость, крупномасштабные структуры, метод многих масштабов

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In this work we consider the effect of a small-scale helical driving force on fluid with a stable temperature gradient with Reynolds number Re ≪ 1. At first glance, this system does not appear to have any instability. However, we show that large-scale vortex instability appears in fluid despite its stable stratification. In the non-linear mode, this instability gets saturated and gives a large number of stationary spiral vortex structures. Among these structures there is a stationary helical soliton and a kink of a new type. The theory is built on the rigorous asymptotical method of multi-scale development.