Direct Numerical Simulations (DNS) of micro-turbulent flows

It is well known that the classic hydrodynamic Navier-Stokes model comes with certain limitations. One of them is the inaccurate description of fluids with internal microstructure, which is a general class of fluids that includes among others, suspensions, polymers, liquid crystals and blood. In this case, an appropriate model which is able to include geometry, deformation and motion of internal particles, is desirable. One of the best-described and well-established such theory is developed by Eringen the theory of micropolar fluids, which is effectively a simple generalization of the Navier-Stokes model. The term micropolar describes fluid with microstructure where additional degrees of freedom, their inter-rotation, have been assigned. Results of our study on micropolar flows revealed that low micropolar volume fractions lead to monotonic turbulence enhancement along with Reynolds number (Re) increment, while sufficiently high micropolar volume fractions lead to opposite results, turbulence intensity attenuation as Re increases. This behavior has been attributed to the micropolar force term on the flow, which enforces turbulence activity to move away from the wall and suppresses near-wall turbulent phenomena.The micropolar model has been also exploited for the first time by our team to study a typical case of environmental flows, the turbulent open channel flow, in combination with fully resolved Direct Numerical Simulations (DNS). In most cases, the present micropolar model achieves better predictions of the flow characteristics compared to existing models in the literature. The results of the micropolar case seem very encouraging, while the model is able to capture the usual flow characteristics of a particulate flow, giving in some cases better physical explanation of the underlying phenomena.