The phenomena associated with the entrainment of free-stream turbulence （FST） into boundary-layer flows are relevant for a number of subjects. It has been be- lieved that the continuous spectra of the Orr-Sommerfeld...The phenomena associated with the entrainment of free-stream turbulence （FST） into boundary-layer flows are relevant for a number of subjects. It has been be- lieved that the continuous spectra of the Orr-Sommerfeld （O-S）/Squire equations describe the entrainment process, and thus they are used to specify the inlet condition in simulation of bypass transition. However, Dong and Wu （Dong, M. and Wu, X. On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances. Journal of Fluid Mechanics, 732, 616-659 （2013）） pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism. They further proposed a large-Reynolds-number asymptotic approach, and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance, for which the response concentrates in the edge layer. In this paper, the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers. It is found that the numerical results can be accurately predicted by the asymptotic solution, implying that the latter is adequate for moderate Reynolds numbers. By introducing a series of such solutions as the inflow perturbations, the bypass transition is investigated via the direct numerical simulation （DNS）. The transition processes, including the evolution of streaks, the amplification of secondary-instability modes, and the emergence of turbulent spots, agree with the experimental observations.展开更多
Transition prediction is of great importance for the design of long distance flying vehicles. It starts from the problem of receptivity, i.e., how external disturbances trigger instability waves in the boundary layer....Transition prediction is of great importance for the design of long distance flying vehicles. It starts from the problem of receptivity, i.e., how external disturbances trigger instability waves in the boundary layer. For super/hypersonic boundary layers, the external disturbances first interact with the shock ahead of the flying vehicles before entering the boundary layer. Since direct numerical simulation （DNS） is the only available tool for its comprehensive and detailed investigation, an important problem arises whether the numerical scheme, especially the shock-capturing method, can faithfully reproduce the interaction of the external disturbances with the shock, which is so far unknown. This paper is aimed to provide the answer. The interaction of weak disturbances with an oblique shock is investigated, which has a known theoretical solution. Numerical simulation using the shock-capturing method is conducted, and results are compared with those given by theoretical analysis, which shows that the adopted numerical method can faithfully reproduce the interaction of weak external disturbances with the shock.展开更多
We compare simulated galaxy distributions in the cold ΛCDM and warm ΛWDM dark matter models. The ΛWDM model adds one parameter to the ΛCDM model, namely the cut-off wavenumber kfs of linear den...We compare simulated galaxy distributions in the cold ΛCDM and warm ΛWDM dark matter models. The ΛWDM model adds one parameter to the ΛCDM model, namely the cut-off wavenumber kfs of linear density perturbations. The challenge is to measure kfs. This study focuses on “smoothing lengths” π/kfs in the range from 12 Mpc to 1 Mpc. The simulations reveal two distinct galaxy populations at any given redshift z: hierarchical galaxies that form bottom up starting at the transition mas?Mfs, and stripped down galaxies that lose mass to neighboring galaxies during their formation, are near larger galaxies, often have filamentary distributions, and seldom fill voids. We compare simulations with observations, and present four independent measurements of kfs, and the mass mh of dark matter particles, based on the redshift of first galaxies, galaxy mass distributions, and rotation curves of spiral galaxies.展开更多
The laminar-turbulent transition has always been a hot topic of fluid mechanics. Receptivity is the initial stage and plays a crucial role in the entire transition process. The previous studies of receptivity focus on...The laminar-turbulent transition has always been a hot topic of fluid mechanics. Receptivity is the initial stage and plays a crucial role in the entire transition process. The previous studies of receptivity focus on external disturbances such as sound waves and vortices in the free stream, whereas those on the leading-edge receptivity to the three-dimensional free-stream turbulence (FST), which is more general in the nature, are rarely reported. In consideration of this, this work is devoted to investigating the receptivity process of three-dimensional Tollmien-Schlichting (T-S) wave packets excited by the three-dimensional FST in a flat-plate boundary layer numerically. The relations between the leading-edge receptivity and the turbulence intensity are established, and the influence of the FST directions on the propagation directions and group velocities of the excited T-S wave packets is studied. Moreover, the leading-edge receptivity to the anisotropic FST is also studied. This parametric investigation can contribute to the prediction of laminar-turbulent transition.展开更多
The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally in- vestigated at low and high free-stream turbulence intensity conditions. Measurements have been carded o...The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally in- vestigated at low and high free-stream turbulence intensity conditions. Measurements have been carded out in order to analyze the boundary layer transition and separation processes at a low Reynolds nttmber, under both steady and unsteady inflows. Static pressure distributions along the blade surfaces as well as total pressure distri- butions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions. Particle. Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields. The flow field has been surveyed in two orthogonal planes （a blade-to-blade plane and a wall-parallel one）. These measurements allow the identification of the Kelvin-Helmholtz large scale cohe- rent structures shed as a consequence of the boundary layer laminar separation under steady inflow, as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks. A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state, thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.展开更多
网格生成所需人力资源和工作时间在整个流场数值模拟周期中占比较高,是计算流体力学应用软件的瓶颈。大量文献提供了结构网格质量特性对差分格式计算结果有影响的算例,但是很少给出正交性和光滑性影响误差的机理分析。分析了MUSCL和WEN...网格生成所需人力资源和工作时间在整个流场数值模拟周期中占比较高,是计算流体力学应用软件的瓶颈。大量文献提供了结构网格质量特性对差分格式计算结果有影响的算例,但是很少给出正交性和光滑性影响误差的机理分析。分析了MUSCL和WENO差分格式在非均匀网格上产生的几何诱导误差,发现正交性和误差之间并无直接关联,有影响的几何参数是相邻网格的偏转角。理论推导证明,误差主要源自控制方程和差分格式,提高网格质量特性能够明显减小几何诱导误差,但是不能完全消除。近年来作者致力于改进算法,先后提出离散等价方程及其等价离散准则(discrete equivalence equation and its discrete rule,DEER)和非结构有限差分法。通过对自由流保持和线性流保持等算例的模拟,利用改进的算法在质量较差的网格上也能得到较好的计算结果。展开更多
基金Project supported by the National Natural Science Foundation of China(Nos.11472189 and11332007)
文摘The phenomena associated with the entrainment of free-stream turbulence （FST） into boundary-layer flows are relevant for a number of subjects. It has been be- lieved that the continuous spectra of the Orr-Sommerfeld （O-S）/Squire equations describe the entrainment process, and thus they are used to specify the inlet condition in simulation of bypass transition. However, Dong and Wu （Dong, M. and Wu, X. On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances. Journal of Fluid Mechanics, 732, 616-659 （2013）） pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism. They further proposed a large-Reynolds-number asymptotic approach, and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance, for which the response concentrates in the edge layer. In this paper, the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers. It is found that the numerical results can be accurately predicted by the asymptotic solution, implying that the latter is adequate for moderate Reynolds numbers. By introducing a series of such solutions as the inflow perturbations, the bypass transition is investigated via the direct numerical simulation （DNS）. The transition processes, including the evolution of streaks, the amplification of secondary-instability modes, and the emergence of turbulent spots, agree with the experimental observations.
基金supported by the National Natural Science Foundation of China(Nos.11472188 and11332007)the National Key Research and Development Program of China(No.2016YFA0401200)
文摘Transition prediction is of great importance for the design of long distance flying vehicles. It starts from the problem of receptivity, i.e., how external disturbances trigger instability waves in the boundary layer. For super/hypersonic boundary layers, the external disturbances first interact with the shock ahead of the flying vehicles before entering the boundary layer. Since direct numerical simulation （DNS） is the only available tool for its comprehensive and detailed investigation, an important problem arises whether the numerical scheme, especially the shock-capturing method, can faithfully reproduce the interaction of the external disturbances with the shock, which is so far unknown. This paper is aimed to provide the answer. The interaction of weak disturbances with an oblique shock is investigated, which has a known theoretical solution. Numerical simulation using the shock-capturing method is conducted, and results are compared with those given by theoretical analysis, which shows that the adopted numerical method can faithfully reproduce the interaction of weak external disturbances with the shock.
文摘We compare simulated galaxy distributions in the cold ΛCDM and warm ΛWDM dark matter models. The ΛWDM model adds one parameter to the ΛCDM model, namely the cut-off wavenumber kfs of linear density perturbations. The challenge is to measure kfs. This study focuses on “smoothing lengths” π/kfs in the range from 12 Mpc to 1 Mpc. The simulations reveal two distinct galaxy populations at any given redshift z: hierarchical galaxies that form bottom up starting at the transition mas?Mfs, and stripped down galaxies that lose mass to neighboring galaxies during their formation, are near larger galaxies, often have filamentary distributions, and seldom fill voids. We compare simulations with observations, and present four independent measurements of kfs, and the mass mh of dark matter particles, based on the redshift of first galaxies, galaxy mass distributions, and rotation curves of spiral galaxies.
基金supported by the National Natural Science Foundation of China(Nos.11472139 and11802143)the Natural Science Foundation of Jiangsu Province of China(No.BK20180781)
文摘The laminar-turbulent transition has always been a hot topic of fluid mechanics. Receptivity is the initial stage and plays a crucial role in the entire transition process. The previous studies of receptivity focus on external disturbances such as sound waves and vortices in the free stream, whereas those on the leading-edge receptivity to the three-dimensional free-stream turbulence (FST), which is more general in the nature, are rarely reported. In consideration of this, this work is devoted to investigating the receptivity process of three-dimensional Tollmien-Schlichting (T-S) wave packets excited by the three-dimensional FST in a flat-plate boundary layer numerically. The relations between the leading-edge receptivity and the turbulence intensity are established, and the influence of the FST directions on the propagation directions and group velocities of the excited T-S wave packets is studied. Moreover, the leading-edge receptivity to the anisotropic FST is also studied. This parametric investigation can contribute to the prediction of laminar-turbulent transition.
文摘The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally in- vestigated at low and high free-stream turbulence intensity conditions. Measurements have been carded out in order to analyze the boundary layer transition and separation processes at a low Reynolds nttmber, under both steady and unsteady inflows. Static pressure distributions along the blade surfaces as well as total pressure distri- butions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions. Particle. Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields. The flow field has been surveyed in two orthogonal planes （a blade-to-blade plane and a wall-parallel one）. These measurements allow the identification of the Kelvin-Helmholtz large scale cohe- rent structures shed as a consequence of the boundary layer laminar separation under steady inflow, as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks. A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state, thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.
文摘网格生成所需人力资源和工作时间在整个流场数值模拟周期中占比较高,是计算流体力学应用软件的瓶颈。大量文献提供了结构网格质量特性对差分格式计算结果有影响的算例,但是很少给出正交性和光滑性影响误差的机理分析。分析了MUSCL和WENO差分格式在非均匀网格上产生的几何诱导误差,发现正交性和误差之间并无直接关联,有影响的几何参数是相邻网格的偏转角。理论推导证明,误差主要源自控制方程和差分格式,提高网格质量特性能够明显减小几何诱导误差,但是不能完全消除。近年来作者致力于改进算法,先后提出离散等价方程及其等价离散准则(discrete equivalence equation and its discrete rule,DEER)和非结构有限差分法。通过对自由流保持和线性流保持等算例的模拟,利用改进的算法在质量较差的网格上也能得到较好的计算结果。
