The purpose of this study is to develop a numerical method for turbulent flows at low Mach number range. To account for the weak compressibility, we modify the time marching method of the usual incompressible scheme which is based on the elliptic equation for pressure. To deal with high Reynolds number flows appropriately, we propose a new one-equation subgrid scale model (SGS). In this model, the concept of coherent structure function model is introduced to treat the energy transfer from grid scale to SGS portion of turbulence kinetic energy. The feature of our one-equation SGS model is that the energy production rate of SGS kinetic energy is calculated without the dynamic procedure, and any parameters such as the friction velocity on the wall or distance from the wall are not necessary. The new one-equation SGS model is incorporated into the weakly compressible scheme to treat a small density variation. Computational examinations have been conducted for fully developed turbulent flows in a plane channel and turbulent flows around NACA0012 airfoil at low Mach number, and have shown satisfactory results. We show that the consideration of the density variation even in low Mach number flows is essential to reproduce the pressure fluctuation and vortical structure appropriately.
Three-dimensional homogeneous isotropic turbulence simulations are conducted using moment-based lattice Boltzmann method and the large-eddy simulation based lattice Boltzmann method. for two Taylor's microscale Reynolds number, Reλ = 225 and Reλ = 314. On comparing the results obtained from these two methods, we found that the two methods produced favorable agreement in the statistical results. However, the time-dependant results are of different characteristic after the enstrophy value reached its peak. The moment-based lattice Boltzmann method can be concluded as one form of implicit large-eddy simulation. The method is also more efficient in terms of memory usage compared to the traditional large-eddy simulation lattice Boltzmann method.
An investigation was carried out into diffusion control for a circular jet using a concentric dielectric barrier discharge plasma actuator, focusing on the effects of electrode size, driving frequency and applied voltage. The actuator was found to induce a flow that was first directed toward the center of the jet, and that was then ejected along the jet central axis. The plasma-induced flow caused an increase in the velocity of the jet, allowing control of velocity fluctuations and the three-dimensional collapse of the jet structure. Increasing the size of the electrode led to an increase in both the volume flow rate and the width of the main jet. In contrast, a smaller electrode caused enhanced contraction of the jet. The results indicated that the actuator provided an effective means of controlling diffusion in the jet.
In this research, unsteady particle image velocimetry (PIV) data are reconstructed / analyzed by a proper orthogonal decomposition (POD) with a cross validation approach. At unsteady PIV flow measurements we have interests, it is sometimes difficult to gather the PIV velocity vector in its entire flow field, mainly due to the lack of particles and/or low (even) contrasting density in the flow field. A POD-based gappy data reconstruction approach is applied to recover the gappy PIV data. The appropriate numbers of iterations as well as considering POD modes are determined by the cross validation approach. The PIV measurements are performed for the wake of a rectangular cylinder as well as for a self-developed flapping wing object. Especially in the case of the flapping wing object, the PIV measurement is performed at a hovering condition in which it is difficult to scatter the particles appropriately in its entire flow field. It results in the failures of PIV measurement at some regions of flow field, and then those incomplete PIV data are reconstructed by the developed approach. The flow/vortex structures can be discussed reasonably well by using the recovered PIV data.