Large eddy simulation of serration effects on an ultra-high-bypass-ratio engine exhaust jet

Serrated jet nozzles are considered to be an efficient and practical passive control approach for jet noise. However, some fundamental mechanisms of serration effects on jet noise are not fully understood, especially in terms of the sound source. In this paper, a high-fidelity simulation framework using large-eddy simulation (LES) is demonstrated to predict near-field turbulence and far-field acoustics from an ultra-high-bypass-ratio engine with round and serrated nozzles. Far-field sound is predicted using Ffowcs Willams–Hawkings (FWH) integration. The results show that the serrated nozzle increases mixing near the nozzle and hence the turbulence decay rate, reducing the turbulence level downstream. The serrations shift the energy from the low frequencies to the high frequencies and decrease overall sound pressure levels by about 3 dB over the low-frequency range. Sound sources are analysed based on fourth-order space–time correlations. There are six major source components ($R_{1111}$, $R_{2222}$, $R_{3333}$, $R_{1313}$, $R_{1212}$, and $R_{2323}$) inside the jet shear layers. The serrations are able to reduce the amplitude of these source terms, causing them to decay rapidly to a level below the round nozzle jet within 2D downstream of the nozzle.     

自由旋涡气动窗口全流场的数值模拟

建立自由旋涡气动窗口全流场仿真模型,对大密封压比气动窗口的全流场展开数值研究,得到自由旋涡气动窗口的流场结构,发现大密封压比气动窗口形成的自由旋涡射流在光束输出通道内无明显的波系结构.根据模拟结果对自由旋涡气动窗口的性能进行优化,对自由旋涡喷管上壁面型线进行二次粘性修正.优化自由旋涡射流场后,激光器输出光束通道内压力分布稳定上升;增加扩压器外端壁吹气1.19MPa、内端壁吹气1.68MPa时,自由旋涡射流总能提高,气动窗口密封压力从37.5torr降低至6torr.该研究结果对自由旋涡气动窗口技术的发展具有参考意义.     

Particle image velocimetry investigation on mixing enhancement of non-circular sharp edge nozzles

An experimental study using Particle Image Velocimetry (PIV) on free jets issuing from different orifice plate (OP) nozzles is reported. Mean velocity, turbulence intensity and higher order profiles relevant for large and small scale mixing are considered in the near field and interaction zone (0 < X/D < 20). This is done to determine mixing enhancement due to rectangular, squared, elliptic and triangular nozzles in comparison to circular nozzle results in two orthogonal planes. The effect of Reynolds number on the differences among the nozzle shapes is also considered by performing measurements just after laminar–turbulent transition (Re = 8000) and in the fully turbulent regime (Re = 35,000). The results at low Reynolds number show two classes of jets, i.e. at one side, those closer to axial-symmetric conditions, as circular, square and triangular jets, whereas on the other side those with elongated nozzles as rectangular and elliptic. The reason for the different behavior of the latter is connected to the phenomenon of axis-switching which allows a rearrangement of turbulence over the different velocity components and directions. However, for the highest Reynolds number investigated, all nozzles show similar behavior especially in the jet far field (X/D > 10), thus suggesting a significant Reynolds number dependence of the results.     

Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.     

Jet impingement onto a cylindrical cavity: consideration of annular nozzle cone angles,and cavity diameter

In the present study, gas jet emerging from an annular nozzle and impinging onto a cylindrical cavity is considered. The geometric configuration of the nozzle is varied in the simulations. Air is used as impinging gas while stainless steel is considered as workpiece material. Reynolds turbulence model is accommodated to account for the turbulence. A numerical scheme employing a control volume approach is used to simulate the flow field. Heat transfer characteristic and shear stress distribution around the cavity are computed. It is found that outer cone angle of the annular nozzle influences the heat transfer rates from the cavity wall. The flow structure around the cavity changes significantly with increasing cavity diameter. Moreover, increasing cavity depth results in stagnation zone moving into the cavity.     

平面液体层碎裂过程实验研究

小宽厚比喷嘴喷射出的平面水膜进入静止空气中,在不同气流流速环境下对水膜碎裂过程进行了实验研究。结果表明,静止空气中的水膜表面波呈现对称波形,射流的碎裂长度随雷诺数的增大而增大,喷射压力对射流碎裂长度没有直接影响。空气助力作用使平面射流表面波的上、下气液交界面出现相位差。水膜的碎裂长度随空气助力气流速度的增大而减小;空气助力对于低雷诺数水膜射流具有很强的促进碎裂作用,所以会极大地改善低雷诺数射流的一次雾化效果。随着水流雷诺数的提高,空气助力作用对水膜碎裂长度的影响大为减弱;即使在高速助力空气的作用下,水膜仍长期保持较稳定的射流流态,没有出现明显的水膜撕裂现象。说明在小宽厚比喷嘴的瑞利(Rayleigh)模式射流中,高雷诺数射流是水膜的稳定因素。与气液流速比、气流马赫数等无量纲参数相比,液体喷射的雷诺数是射流碎裂的主要影响因素。     

A combined analytical singularity subtraction method and compact implicit differencing scheme for aerodynamic mixing

In spite of the rapid advances in both scalar and parallel computational tools, the large number and breadth of variables involved in both design and inverse problems make the use of sophisticated fluid flow models impractical. With this restriction, it may be concluded that an important family of methods for mathematical/computational development are reduced or approximate models. In this study, a combined perturbation/numerical modeling methodology is developed. The numerical portion of the model uses a compact finite difference scheme, while analytical solutions are used to resolve singular behavior that is inherent to this flow. Solutions are presented to illustrate the efficiency of this methodology.     

Experimental evaluation of side-loads in LE-7A prototype engine nozzle

During development tests of the LE-7A prototype engine, severe side-loads were observed. The side-load peaks appeared only in certain limited conditions during start-up and shut-down transients. To investigate phenomena causing those severe side-loads observed in the LE-7A prototype engine nozzle, series of cold-flow tests and hot-firing tests as well as CFD analyses were conducted. As a result of the hot-firing tests, two different phenomena were found to cause severe side-loads in the LE-7A prototype engine nozzle. One was a restricted shock separation (RSS) flow structure and the other was a phenomenon termed “separation jump,” the rapid movement of the separation location in the vicinity of the step. A step was installed in the LE-7A prototype to supply film-cooling gas. Hot-firing test results showed that RSS can occur for a limited mixture ratio. Detailed flow structure of RSS on the nozzle surface was revealed by the cold-flow tests. Measured pressures and visualized images of cold-flow tests clarified the mechanism causing the separation jump. The key phenomenon ruling the separation jump was found to be the base flow behind the step. Based on the results of the present study, the latest LE-7A engine nozzle design has been changed to eliminate the severe side-load.

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Effect of Sudden Expansion on Entrainment and Spreading Rates of a Jet Issuing from Asymmetric Nozzles

Turbulent free jets issuing from five different nozzle geometries; smooth pipe, contracted circular, rectangular, triangular, and square, are experimentally investigated by using TSI 2-D laser Doppler velocimetry (LDV) to assess the effect of nozzle geometry and quarl (i.e. a cylindrical sudden expansion) on jet entrainment and spreading. The centerline mean velocity decay and the jet half-velocity width, which are indicators of jet entrainment and spreading rates, are determined for each nozzle’s flow configuration, i.e. with and without sudden expansion. Furthermore, turbulence quantities, such as the flow mean velocities and their mean fluctuating components, as well as Reynolds shear stresses, are all measured along the centerline plane of the jet to facilitate understanding the extent of the effect of nozzle’s geometry (i.e. nozzle’s orifice shape and sudden expansion) on jet’s entrainment and spreading. The main results show that the jet flow with the presence of sudden expansion exhibits higher rates of entrainment and spreading than without. In addition, these results reveal that sudden expansion exercises a greater effect on the asymmetric jet characteristics, especially for the triangular and rectangular nozzles compared to their axisymmetric counterparts (i.e. circular contracted nozzle).     

Flow of a dispersed phase in the Laval nozzle and in the test section of a two-phase hypersonic shock tunnel

An unsteady gas-particle flow in a hypersonic shock tunnel is studied numerically. The study is performed in the period from the instant when the diaphragm between the high-pressure and low-pressure chambers is opened until the end of the transition to a quasi-steady flow in the test section. The dispersed phase concentration is extremely low, and the collisions between the particles and their effect on the carrier gas flow are ignored. The particle size is varied. The time evolution of the particle concentration in the test section is obtained. Patterns of the quasi-steady flow of the dispersed phase in the throat of the Laval nozzle and the flow around a model (sphere) are presented. Particle concentration and particle velocity lag profiles at the test-section entrance are obtained. The particle-phase flow structure and the time needed for it to reach a quasi-steady regime are found to depend substantially on the particle size. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 5, pp. 102–113, September–October, 2008.