Rotating Turbulence

Rotating turbulence is of great importance in engineering and geophysics. The most significant application in the first case is the development and the design of turbo-machinery. Here one has to take into account the detailed properties of the turbulent fluids, which pass through the device and are rotated (e.g., by the motion of the turbine blades). A detailed understanding of rotational effects on flow characteristics is essential for an advanced layout of these machines. Second, the whole field of geophysics is crucially determined by planetary rotation, which influences both atmospheric and oceanic flows and affects global climate as well as short-term weather forecasting. Understanding the fundamental processes in these fluid layer forms the basis for a detailed analysis of complex phenomena such as the development of climate anomalies like El Nino, the formation of hurricanes and tidal waves, the spreading of pollutants, and the oceanic circulation of nutrients.

Many subgrid-scale (SGS) models have been used to simulate rotating turbulence. However, there still exists a need for a better understanding of SGS models, both because anisotropic characteristics may influence large-eddy simulation (LES) modeling, and because comparative studies of model performance in rotating turbulence are inadequate. At least two issues should be considered

• Algebraic eddy viscosity models predict global dissipation fairly accurately. Through these models, small scales drain kinetic energy from large scales. However, kinetic energy transfer from small to large scales is known to occur, especially in anisotropic turbulence. In rotating flows, although the rotation term (the Coriolis term) does not explicitly show up in the kinetic energy equation, rotation has an immediate effect on kinetic energy transfer and weakens the fundamental property of the energy cascade. Eddy viscosity models may have difficulty capturing this process.

• Most traditional SGS models are based on the assumption that the modeled small scale turbulence is nearly homogeneous and isotropic. In rotating turbulence, coherent structures (e.g., two-dimensionality, and cyclonic dominance) are affected by interactions between resonant velocity modes and between near-resonant modes. Our recent studies found that the different micro-length scales occur in different directions and that this property can influence LES modeling.