Study on characterization of initial flow of an elliptical jet in crossflow
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摘要:
为了研究流动控制中非圆射流与外围来流作用过程的流场特性，基于多组分可压缩Navier-Stokes方程，采用大涡模拟（LES）方法和高精度调谐中心差分（TCD）格式，对不同速度横流作用下的亚声速椭圆射流初期流动特性进行了数值研究。聚焦于射流喷出后主涡环形成、演化直至发生首次轴置换的初始发展阶段。数值结果清晰描述了横流速度对射流涡结构演化、穿透深度及混合效率的影响，得到了横流作用下非圆射流主涡环的三维流动形态演变机理，发现因横流绕射流表面运动时的切向速度有效抵消了非圆射流自诱导变形而出现的切向速度，抑制了迎流侧剪切层上肋状流向涡的生成，提升了迎流侧剪切层上周向涡管的稳定性。当横流绕过长轴两端剪切层时，内外侧切向速度差诱导长轴两端形成一对强反向流向涡对（CVP），并在射流后期发展过程中逐渐占据主导控制地位，最终导致射流截面形状转变为典型的反向涡对结构。此外，射流剪切层上反向流向涡对与背流侧涡环段的耦合作用，加剧了主涡环的破碎与失稳，使射流穿透深度降低，而混合效率得到提升。
Abstract:Based on the multicomponent compressible Navier-Stokes equations, the initial flow characteristics of a subsonic elliptical jet under different crossflow velocities were numerically investigated using the large-eddy simulation (LES) method and a high-precision tuned centered-difference (TCD) scheme. This study aimed to elucidate the initial flow characteristics during the interaction between the noncircular jet and the crossflow under flow control conditions. The initial development stage of the primary vortex ring, from its formation and evolution after jet ejection until the first occurrence of axis switching was analyzed. The numerical results clearly described the effects of crossflow velocity on the evolution of the jet vortex structures, penetration depth, and mixing efficiency, and revealed the evolution mechanism of the three-dimensional flow pattern of the primary vortex loop in the noncircular jet under crossflow. It was found that the tangential velocity of the crossflow moving around the jet surface effectively counteracted the self-induced tangential velocity arising from the deformation of the noncircular jet. This counteraction suppressed the generation of rib vortices in the windward shear layer and enhanced the stability of the Leading-edge vortex loops there. When the crossflow moved around the shear layer at both ends of the major axis, the tangential velocity difference between the inner and outer flow paths induced the formation of a strong pair of counter-rotating streamwise vortices (CVP) at these locations. This CVP became dominant in the later stages of jet development, eventually causing the jet cross-section to transform into the typical counter-rotating vortex pair structure.In addition, the coupling between the CVP on the jet shear layer and the vortex ring segment on the leeward side promoted the breakup and destabilization of the primary vortex loops. This process reduced the jet penetration depth while improving the mixing efficiency.
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Key words:
- crossflow /
- elliptical jet /
- axis-switching /
- penetration depth /
- mixing efficiency
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图 3 无横流情况下，在t=0.1 ms时不同网格总量下流场特征
Figure 3. Flow field characteristics at different grid resolutions without crossflow at t=0.1 ms
图 4 不同方位观察的椭圆涡环发展过程
Figure 4. Development of primary vortex loops viewed from different orientations
图 5 无横流情况下轴置换过程中横截面涡核处流向涡量
Figure 5. Distirbution of the streamwise vorticity near the vortex core without crossflow during the process of axis switching
图 6 Ma0=0.12时，不同时刻混合分数（f=0.8）与Q准则等值面显示的主涡环
Figure 6. Primary vortex loops of the elliptical jet shown by the mixture fraction （f=0.8） and the Q-criterion isosurfaces at different times for Ma0=0.12
图 7 Ma0=0.12时，轴置换过程中横截面涡核处流向涡量
Figure 7. Distirbution of the streamwise vorticity near the vortex core during the process of axis switching for Ma0=0.12
图 8 t=0.25 ms和Ma0=0.12时，不同方位展示的射流剪切层结构
Figure 8. Shear layer structures of jet exhibited in different orientations at t=0.25 ms and Ma0=0.12
图 9 t=0.14 ms时，不同横流速度作用下Q准则等值面显示的主涡环形态
Figure 9. Primary vortex loop morphology visualized by Q-criterion isosurfaces for different crossflow velocities at t=0.14 ms
图 10 主涡环几何特征参数随时间的变化曲线
Figure 10. Variations of geometric characteristic parameters of primary vortex loop with respect to time
图 11 不同来流马赫数作用下剪切层处涡结构
Figure 11. Vortex structure at the shear layer for different crossflow velocities
图 12 不同横流速度下椭圆射流穿透深度随时间的变化
Figure 12. Variation of penetration depth with respect to time for different crossflow velocities
图 13 不同横流速度下t=0.36 ms时流场混合效率变化
Figure 13. Variation of mixing efficiency of jet flow field with respect to crossflow velocity at t=0.36 ms
表 1 计算初始条件
Table 1. Simulated initial conditions
参数 来流 射流 γ 1.4 1.31 ν/10−5 （m2/s） 1.57 1.68 T/K 300 300 p/kPa p0=100 pj=100 
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表 2 计算工况
Table 2. Simulated cases
参数 工况1 工况2 工况3 工况4 工况5 Ma0 0 0.12 0.15 0.2 0.3 u0/（m/s） 0 41.7 52.1 69.4 104.2
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