低马赫数射流火焰多维直接数值模拟

Direct Numerical Simulations of Low Mach Number Plane Jet Flames

应用高精度类谱紧致Padé格式,配合特征边界(NSCBC)技术,采用方程分裂的投射算法,并通过可变系数Poisson方程多重网格求解,使得散度约束条件获得满足,克服了模拟中虚假热膨胀问题,发展了针对低马赫数射流火焰的直接模拟程序.对2种典型的射流火焰问题进行了较为详细的计算,表明在零初始速度条件下,射流火焰具有Kelvin-Helmholtz涡旋结构,但与可变密度射流相比,后涡旋区域大尺度运动衰减较快.在平行流动初始条件下,射流火焰具有一致的射流不稳定控制频率,同时射流火焰本身具有绝对不稳定性特征.

A method is proposed to solve a low-Mach number jet flame. It is dissipation-free, the spurious heat release is found to be greatly controlled and thus avoids violation of the conservation of kinetic energy. The key ingredient is that the solver of the governing equations is built based on a projection algorithm and a Poisson equation with variable coefficient for the hydrodynamic pressure is solved by using a multigTid technique. Accurate boundary conditions NSCBC（navier-stokes characteristics boundary condition） suggested by Poinsot compatible with modem non-dissipative high order spectral-like finite difference schemes developed by Lele was adopted, and the free jet flames were studied numerically using two kinds of initial conditions. Two characteristic Kelvin Helmholtz vortices are observed under the zero initial velocity condition. This is consistent with the results of variable-density jets in recent literature, it is shown, however, that behind large vortical structtaes attenuate more quickly than that of variable-density jets. Under longitudinal initial velocity conditions, the spectra analyses demonstrate that the jet preferred mode is dominant in jet flames. Meanwhile, it verifies jet flames as absolutely unstable flow.