Prediction Of Near-wall Turbulence And Application In Large Eddy Simulation

The high shear and strong anisotropy in the near-wall region of wall-bounded turbulent flows make it difficult to carry out large eddy simulation(LES).The vast amount of computational cost to fully resolve the near-wall region becomes a bottleneck for the engineering application of LES in high-Reynolds number wall turbulence.In the present work,the direct numerical simulations of the minimal flow units are employed to study the statistical properties of the near-wall small-scale turbulence.Based on the minimal flow unit,the predictive model of near-wall turbulence is built and the near-wall treatment of large eddy simulation is further discussed.Direct numerical simulations(DNS)are carried out for the minimal flow units at wall-friction Reynolds numbers from 1000 up to 10000.It is found that,in the near-wall region,the mean velocity profiles scaled by wall friction velocity in the minimal flow units agree well with those in the full-sized channel;the intensities of the velocity fluctuations show good agreement with those of the full-sized channel at scales smaller than the sizes of the corresponding minimal flow units.Turbulence statistics at different Reynolds numbers scale well in wall units,showing Reynolds number independence.This indicates that the minimal flow units can represent the universal properites of the near-wall small-scale turbulence in the full-sized channel.Using the DNS data of the full-sized channel turbulence at friction Reynolds number 2000,the near-wall universal signals spared from the influence of the outer large-scale motions are extracted and compared with the velocity fluctuations in the minimal flow units.Good similarities between the two are displayed.Combined with the superposition and modulation effects of the outer large-scale motions on the near-wall turbulence,the predictive model of the three velocity components within the near-wall region is built based on the minimal flow unit.The predicted turbulence intensities and the joint probability density functions of velocity fluctuations show good agreement with the DNS results of the full-sized channel turbulence.Furthermore,the near-wall treatment in large eddy simulation is explored.The accurate wall shear stresses as well as the velocity fluctuations provided by the minimal flow units are used as the boundary conditions for LES.In addition,the accurate Reynolds stress yielded by the minimal flow units is adopted to constrain the subgrid-scale stress.These treatments can largely reduce the near-wall computational cost in LES with good prediction of the flow in the outer region.