超声速中等欠膨胀冲击射流的涡声关系研究

使用大涡模拟方法对冲击面为平面的超声速中等欠膨胀冲击射流进行了数值模拟.利用三阶迎风和四阶对称紧致格式对无量纲化轴对称可压缩滤波N-S方程进行空间离散,时间上推进采用的是三阶精度的TVD型Rugge-Kutta法. 通过与经典的冲击射流实验比较,证实了程序的可靠性. 数值模拟得到了流场中不同尺度的涡结构和激波结构,观察到了上行声波和反射波以及流场中不同位置的声源,分析了冲击区剪切层附近区域的压强和涡旋转强度变化的频率、冲击平板上的压强变化频率以及射流剪切层中不同位置的涡合并出现的频率,发现冲击区剪切层附近区域的压强和涡强度变化以及射流剪切层中的涡合并现象和离散频率的冲击单音有重要关联.      

Large-eddy simulation of wing tip vortex in the near field

Large-eddy simulation with filtered-structure-function subgrid model and implicit large-eddy simulation (ILES without explicit subgrid model) using high-order accuracy and high resolution compact scheme have been performed on the tip vortex shedding from a rectangular half-wing with a NACA 0012 airfoil section and a rounded wing tip. The formation of the tip vortex and its initial development in the boundary layer and the near field wake are investigated and analysed in detail. The physics, why the tip vortex, which is originally turbulent in the boundary layer, is re-laminarised and becomes stable and laminar rapidly after shedding in the near field, is revealed by this simulation. The computation also shows the widely used second-order subgrid model is not consistent to six-order compact scheme and would degenerate the six-order LES results to second-order. Therefore, high-order schemes, grid refinement and six-order subgrid models are critical to LES approaches.     

Turbulent Structure of a Gas-Liquid Impinging Jet

The turbulent structure of a submerged axisymmetric impinging jet containing small gas bubbles is studied experimentally under conditions of periodic external excitation. On the basis of measuring the surface-friction pulsatory component in the jet impinging on an obstacle, the effect of the suppression of large-scale eddies at large gas volume fractions is registered. The conditions of resonant growth of coherent structures and the suppression of wide-band turbulence are determined for both the single-phase and the two-phase impinging jet. An analysis of the development of different pulsatory friction components in the impinging-jet gradient region is presented.     

激波与涡对相互作用的实验研究

在方截面激波管中进行了平面运动激波和涡对的二维相互作用实验,研究了激波与同向涡对、激波与反向涡对相互作用的非定常过程．根据实验照片,分析讨论了作用过程中激波的变形,二次激波和三波点的形成、演变,激波与激波的相互作用,以及旋涡结构的变化等．实验表明,激波通过涡核时,激波发生剧烈变形,旋涡强度增大,涡核形状改变．     

一种三阶有限体积法及其在欠膨胀射流激波结构数值模拟中的应用

以喷管出口欠膨胀射流为研究对象,在Lagrange坐标系下建立欠膨胀射流二维积分形式的流动方程。通过在单元交接面处进行三阶ENO(essentially nonoscillatory)格式插值,构造得到一种适用于求解该方程的三阶ENO有限体积法。采用该格式对一维Sod激波管算例和喷管出口欠膨胀射流进行数值计算。计算结果表明,该方法具有高精度、基本无振荡的特点,能很好地捕捉包含激波、滑移线以及三波交点等复杂流场波系结构。计算得到的波系结构中马赫盘的位置与实验结果吻合很好,相对误差小于1.1%。     

激波在平板上诱导的旋涡强度的测量

运动激波通过两个等攻角平板后诱导出两个同向旋转的旋涡,这两个旋涡在随当地气流向下游运动的同时,绕涡核连线中心旋转。本文通过测量涡对的转动角度速度,获得了每个旋涡的强度。实验结果表明,由此测得的旋涡强度不同用于小攻角平板起动涡公式计算了起动涡强度。     

Oscillatory behavior of supersonic impinging jet flows

Oscillatory flows of a choked underexpanded supersonic impinging jet issuing from a convergent nozzle have been computed using the axisymmetric unsteady Navier--Stokes system. This paper focuses on the oscillatory flow features associated with the variation of the nozzle-to-plate distance and nozzle pressure ratio. Frequencies of the surface pressure oscillation and flow structural changes from computational results have been analyzed. Staging behavior of the oscillation frequency has been observed for both cases of nozzle-to-plate distance variation and pressure ratio variation. However, the staging behavior for each case exhibits different features. These two distinct staging behaviors of the oscillation frequency are found to correlate well if the frequency and the distance are normalized by the length of the shock cell. It is further found that the staging behavior is strongly correlated with the change of the pressure wave pattern in the jet shear layer, but not with the shock cell structure. Communicated by K. Takayama PACS 02.60.Cb; 47.40.−x; 47.40.Nm; 47.35.+I; 47.15.−x     

可压缩流向涡与反向运动激波相互作用的实验

对可压缩流向涡与反向运动激波相互作用的现象进行了实验研究．实验在９４ｍｍ×９４ｍｍ的方截面激波管中进行．在实验段上游安装了一个有限翼展平直机翼．当入射激波通过机翼后,波后２区气流在模型翼尖诱导出一条流向涡．入射激波在激波管端壁反射后,形成的反射激波在观察窗处和流向涡发生作用．实验中拍摄了激波与流向涡作用全过程的纹影照片,观察到了一些和定常激波与旋涡相互作用不同的现象,并与数值计算结果进行了初步比较     

数值研究激波与旋涡的相互作用

从非定常形式的Euler方程出发,数值模拟了运动激波与旋涡相互作用的非定常流动过程。为保证激波具有较高的分辨率,采用对称型TVD格式进行了数值计算。结果表明。这样可以有效地模拟流场中一些复杂的流动现象,如激波变形、激波分叉和三波点的形成,以及旋涡结构的变化过程等,并与已有的实验流动显示相符良好。同时,也是对TVD格式求解这类问题的一次初步尝试。     

半封闭狭缝湍流冲击射流的数值模拟

将Yakhot和Orszag提出的RNGk-ε模型推广应用于半封闭狭缝冲击射流场的数值模拟,以评价该模型对这种复杂湍流的预测能力.将计算得到的流场平均速度分布、湍流强度分布和流函数分布与标准k-ε模型的预测结果以及相应的实验数据进行了比较,结果表明:RNG k-ε模型的预测结果总体上要好于标准k-ε模型,但与实验值相比,所有预测结果都还存在不同程度的误差,尤其是近壁区和滞止点较远下游处的湍流强度分布.说明RNG模型虽然已在某些湍流的预测中取得了一定的成功,但要定量准确地预测冲击射流场,还必须针对其流动特征对模型加以改进.     

Interaction of a Streamwise Vortex with an Oblique Shock Wave

Interaction of a supersonic streamwise vortex with an oblique shock wave is considered. A mathematical model of the streamwise vortex is constructed. Three interaction regimes (weak, moderate, and strong) are found. It is shown numerically that vortex breakdown is possible in the case of strong interaction. The influence of the governing parameters on the interaction type is studied. It is shown that the main effect on the interaction type is exerted by the streamwise velocity and angle of the wedge forming the shock wave. The effect of splitting of the primary vortex on the shock wave in the case of moderate and strong interaction regimes is found.     

STUDY ON THE FUEL AIR MIXING INDUCED BY A SHOCK WAVE PROPAGATING INTO A H_2-AIR INTERFACE

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圆射流中拟序涡配对的数值模拟

用三维涡丝法结合涡丝的增加与合并技术,对不可压圆射流场,数值模拟了在扰动中加入次谐波后流场拟序涡的配对,配对情况与实验结果一致,通过对不同时刻涡量图的比较与分析,给出了次谐波初始强度对涡配对的影响,初始强度越大,涡配对出现越早,涡配对位置越往上游移动;同时还得到了基波与次谐波初始相位差和涡配对的关系,没有相位差时,次谐波增长最快,相位差增加时,涡配对的速度降低。说明控制次谐波的初始强度和基波与次谐波的相位差,能起到控制圆射流剪切层乃至控制整个射流场的目的。     

Application of underwater shock wave and laser-induced liquid jet to neurosurgery

Paper deals with applications of underwater shock waves to medicine. A historical development of underwater shock wave generation by using pulsed Ho:YAG laser beam irradiation in water is briefly described and an overview is given regarding potential applications of shock waves to neuro-surgery. The laser beam irradiation in a liquid-filled catheter produces water vapor bubble and shock waves intermittently produces micro-liquid jets in a controlled fashion from the exit of the catheter. Correlations between shock dynamics and bubble dynamics are emphasized. To optimize the jet motion, results of basic parametric studies are briefly presented. The liquid jet discharged from the catheter exit has an impulse high enough to clearly exhibit effectiveness for various medical purposes. In liquid jets we observed reasonably strong shock waves and hence invented a compact shock generator aiming to apply to microsurgery. We applied it to a rat's bone window and developed an effective method of brain protection against shock loading. The insertion of Gore-Tex® sheet is found to attenuate shock waves drastically even for very short stand off distance and its physical mechanism is clarified. The laser-induced liquid jet (LILJ) is successfully applied to soft tissue dissection. Animal experiments were performed and results of histological observations are presented in details. Results of animal experiments revealed that LILJ can sharply dissect soft tissue with a minimum amount of liquid consumption, while blood vessels larger than 0.2 mm in diameter are preserved. Shock waves and LILJ have a potential to be indispensable tools in neuro-surgery.This paper was based on work that was presented at the 2nd International Symposium on Interdisciplinary Shock Wave Research, Sendai, Japan, March 1–3, 2005.Communicated by K. Takayama PACS 42.62.Be, 47.40.-x, 42.62.-b