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101.
102.
The flow characteristics and the structure of highly buoyant jet of low density fluid issuing into a stagnant surrounding of high density fluid is studied by scanning stereo PIV combined with proper orthogonal decomposition (POD) analysis. The experiment is carried out at Froude number of 0.3 and Reynolds number of 200, which satisfies the inflow condition due to the unstable density gradient near the nozzle exit. An increase in the maximum mean velocity occurs and the vertical velocity fluctuation is highly amplified near the nozzle exit, which suggests the influence of inflow due to the unstable density gradient. The POD analysis indicates that the vertical velocity fluctuation is the major source of fluctuating energy contributing to the development of the highly buoyant jet. The examination of the POD modes show that the longitudinal structure of the vertical velocity fluctuation is generated along the jet axis having the opposite sign of velocity fluctuation on both sides of the jet axis. The vertical scale of the POD mode decreases with increasing the mode number and results in the frequent appearance of cross-flow across the buoyant jet. The reconstruction flow from the POD modes indicates that the vortex structure is caused by the highly sheared layer between the upward and downward velocity and the inflow is induced by the vortex structure. The magnitude of the vortex structure seems to be weakened with an increase in the distance from the nozzle and the buoyant jet approaches to an asymptotic state in the further downstream. 相似文献
103.
合成射流对失速状态下翼型大分离流动控制的试验研究 总被引:1,自引:0,他引:1
为研究低速状态合成射流在抑制翼型气流分离和推迟失速方面的控制机理, 开展了NACA0021 翼型失速特性射流控制的风洞试验研究. 通过系统性的模型测力、翼型瞬态流场粒子图像测速和边界层速度测定的对比试验, 深入探索了合成射流各参数对翼型失速特性控制效果的影响规律. 试验结果表明射流偏角在翼型升力和失速迎角控制方面的效果对射流动量系数较为敏感: 当动量系数较大时, 近切向射流的控制效果更好. 射流动量系数为0.033 时, 偏角30°的射流使得翼型升力系数峰值提高23.56%, 失速迎角增大5°; 而动量系数较小时, 偏角较大的射流能够获得最佳控制效果. 射流动量系数为0.0026 时, 法向射流对翼型最大升力系数控制效果最好(提高9.2%). 相似文献
104.
105.
106.
Temporal evolution of flame stretch due to turbulence and the hydrodynamic instability 总被引:1,自引:0,他引:1
A.M. Steinberg J.F. Driscoll S.L. Ceccio 《Proceedings of the Combustion Institute》2009,32(2):1713-1721
The temporal evolution of the strain rate on a turbulent premixed flame was measured experimentally using cinema-stereoscopic particle image velocimetry. Turbulence strains a flame due to velocity gradients associated both directly with the turbulence and those caused by the hydrodynamic instability, which are initiated by the turbulence. The development of flame wrinkles caused by both of these mechanisms was observed. Wrinkles generated by the turbulence formed around vortical structures, which passed through the flame and were attenuated. After the turbulent structures had passed, the hydrodynamic instability flow pattern developed and caused additional strain. The hydrodynamic instability also caused the growth of small flame front perturbations into large wrinkles. In the moderately turbulent flame investigated, it was found that the evolution of the strain rate caused by turbulence–flame interactions followed a common pattern involving three temporal regimes. In the first, the turbulence exerted extensive (positive) strain on the flame, creating a wrinkle that had negative curvature (concave towards the reactants). This was followed by a transition period, leading into the third regime in which the flow pattern and strain rate were dominated by the hydrodynamic instability mechanism. It was also found that the magnitudes of the strain rate in the first and third regimes were similar. Hence, the hydrodynamic instability mechanism caused significant strain on a flame and should be included in turbulent combustion models. 相似文献
107.
Matthew D. Pinchak Vincent G. Shaw Ephraim J. Gutmark 《Proceedings of the Combustion Institute》2019,37(4):5163-5171
The dynamics of a premixed ethylene-air jet injected transverse to a vitiated cross-flow were investigated using high-repetition rate particle image velocimetry (PIV). Both non-reacting and reacting jets were found to be characterized by a dominant frequency associated with the jet wake vortex system. For the isothermal jet, increasing the momentum flux ratio (J) has only a slight effect on the frequency of the oscillation but significantly increases its magnitude. The reacting jet was found to exhibit different behavior, with a monotonic increase in the dominant frequency with J. The jet equivalence ratio (?j) was found to have little effect on the rate of wake vortex shedding but affects the overall magnitude of the oscillation. Comparison with data reported in the literature suggests the relationship between the wake Strouhal number (Stw) and J is fuel dependent. Application of a vortex detection algorithm shows a stark difference in the location of the wake vortices under non-reacting and reacting conditions. Under isothermal conditions, the vortices are found close to the jet centerline and dissipate relatively quickly. Reaction confines the vortices to a narrow shear layer until a farther distance downstream and the vortices convect through the entire area of interest. Additionally, the vortex circulation strength was found to increase with J. Proper orthogonal decomposition (POD) analysis of the non-reacting and reacting jets demonstrates the dominance of the wake vortex structures in the oscillating flow fields. In both cases, the temporal information extracted from the most energetic modes is identical to the dominant frequencies measured in the flow fields. The primary effect of heat release is to reduce the overall amount of coherence and to delay the appearance of elevated coherence levels until a larger streamwise distance from the jet exit. 相似文献
108.
The objective of this paper is to investigate the transient conical bubble structure (CBS) and acoustic flow structure in ultrasonic field. In the experiment, the high-speed video and particle image velocimetry (PIV) techniques are used to measure the acoustic cavitation patterns, as well as the flow velocity and vorticity fields. Results are presented for a high power ultrasound with a frequency of 18 kHz, and the range of the input power is from 50 W to 250 W. The results of the experiment show the input power significantly affects the structures of CBS, with the increase of input power, the cavity region of CBS and the velocity of bubbles increase evidently. For the transient motion of bubbles on radiating surface, two different types could be classified, namely the formation, aggregation and coalescence of cavitation bubbles, and the aggregation, shrink, expansion and collapse of bubble cluster. Furthermore, the thickness of turbulent boundary layer near the sonotrode region is found to be much thicker, and the turbulent intensities are much higher for relatively higher input power. The vorticity distribution is prominently affected by the spatial position and input power. 相似文献
109.
来流湍流干扰噪声在风力机叶片气动总噪声级中占有重要地位.选取圆柱/翼型干涉模型从实验和数值两方面研究此类干涉发声现象.实验中通过对翼型表面非定常载荷的测量,重点研究了圆柱位置和翼型攻角的影响,选取的翼型包括两个NACA系列翼型(NACA0012和NACA0018)和两个风力机翼型(s809和s825),同时利用PIV(particle image velocimetry)技术对低攻角状态下翼型的前缘流场进行了研究.实验结果表明翼型表面非定常压力与圆柱涡脱落存在一定相关性.与此同时采用非定常Reynolds平均(URANS)方法对圆柱/NACA0012翼型的干涉流场进行了非定常数值模拟,并将得到的翼型表面压力频谱与实验结果进行了对比. 相似文献
110.
Yi Wu Hangfeng Wang Zhaohui Liu Jing Li Liqi Zhang Chuguang Zheng 《Acta Mechanica Sinica》2006,22(2):99-108
Particle-laden flows in a horizontal channel were investigated by means of a two-phase particle image velocimetry (PIV) technique.
Experiments were performed at a Reynolds number of 6 826 and the flow is seeded with polythene beads of two sizes, 60 μm and 110 μm. One was slightly smaller than and the other was larger than the Kolmogorov length scale. The particle loadings were relatively
low, with mass loading ratio ranging from 5×10−4 to 4×10−2 and volume fractions from 6×10−7 to 4.8×10−5, respectively. The results show that the presence of particles can dramatically modify the turbulence even under the lowest
mass loading ratio of 5×10−4. The mean flow is attenuated and decreased with increasing particle size and mass loading. The turbulence intensities are
enhanced in all the cases concerned. With the increase of the mass loading, the intensities vary in a complicated manner in
the case of small particles, indicating complicated particle-turbulence interactions; whereas they increase monotonously in
the case of large particles. The particle velocities and concentrations are also given. The particles lag behind the fluid
in the center region but lead in the wall region, and this trend is more prominent for the large particles. The streamwise
particle fluctuations are larger than the gas fluctuations for both sizes of particles, however their varying trend with the
mass loadings is not so clear. The wall-normal fluctuations increase with increasing mass loadings. They are smaller in the
60 μm particle case but larger in the 110 μm particle case than those of the gas phase. It seems that the small particles follow the fluid motion to certain extent while
the larger particles are more likely dominated by their own inertia. Finally, remarkable non-uniform distributions of particle
concentration are observed, especially for the large particles. The inertia of particles is proved to be very important for
the turbulence modification and particles behaviors and thus should be considered in horizontal channels.
The project supported by the National Natural Science Foundation of China (50276021), and Program for New Century Excellent
Talents in University, Ministry of Education (NCET-04-0708) The English text was polished by Yunming Chen. 相似文献