Universal Characteristics Of Small-scale Motions In Near-wall Turbulence And Application In Wall Modeled Large Eddy Simulation

For wall turbulent flows(such as fully developed channel flow or plate boundary layer flow),the physical mechanism of near wall turbulence is complex due to inhomogeneity and anisotropy,which is also exceedingly difficult in experimental measurement or numerical simulation.In this paper,the direct numerical simulation data in the shear Reynolds number range 180-5200 are used to study the near-wall small-scale fluctuation properties.In the range of shear Reynolds number 1000 to 5200,the near-wall small-scale turbulence statistical characteristics are independent on Reynolds number.In addition,in the range of shear Reynolds number 180 to 600,it is found that there exist Reynolds number effect in near wall small-scale turbulence,and then the critical Reynolds number is given for the change of statistical properties of near wall turbulence.Finally,the outer input signal provided by coarse grid wall modeled large eddy simulation is used to predict the near-wall full-scale turbulence statistics.Using the improved near wall prediction model(Baars et al,2016),the near wall turbulence velocity is decomposed into large-scale and small-scale fluctuation signals.Similarly,the wall shear stress is decomposed by using the wall shear stress prediction model(Marusic et al,2013).By changing the vertical height of the external input signal in the above prediction model,Reynolds number-independent small-scale signals of near wall turbulence and wall shear stress statistics are obtained in the range of shear Reynolds number 1000 to 5200.While,in the range of shear Reynolds number 180 to 600,the decomposed of small-scale velocity fluctuation signal and wall shear stress fluctuation signal still exist Reynolds number effect,and the statistics increase with Reynolds number.It is considered to be the low Reynolds number effect of small-scale signal which is caused by the gradual increase of vortex intensity with Reynolds number.At the same time,the critical Reynolds number is given for the turning point of near-wall small-scale statictiscal properties.It provides a lower limit of the applicable range of Reynolds number for the prediction model.Finally,it is found that the small-scale part of the external large-scale footprint can be well scaled by the viscous scale.Wall modeled large eddy simulation with Lagrangian scale dependent subgrid model and equilibrium wall model can yield promising prediction in the outer region y/?>0.1,consistented with DNS results,such as mean velocity,turbulence intensity and premultiplied energy spectra.In another words,it provides an outstanding example and demonstrate the capability of wall modeled large eddy simulation to resolve outer large scale motions.According to the attached eddy hypothesis,the near-wall spatialtemporal velocity signal is modified as the input signal for the prediction model,and predict the near-wall turbulence statistics reasonably.