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1.
S.-T. Ding  G. Li 《实验传热》2013,26(3):266-284
Abstract

Flow through a blade trailing edge passage has been experimentally investigated in this article. This rectangular passage is divided into two channels by either a straight or a 120° wavy partition. Fluid in the first channel was injected into the second channel via 14 openings (holes) distributed uniformly and in the crests and troughs for the wavy configuration. Particle image velocimetry and a charge coupled device camera were used in experiment, with Reynolds number varying from 1,800 to 6,700. Experimental results show that the wavy configuration produces an oscillation flow in the second channel. The cause for the oscillation may due to the 120° wavy partition causing shear-layer instabilities and the formation of spanwise eddies by the wavy board. Pictures from the charge coupled device camera revealed strong mixing in the trailing edge, and this mixing could enhance heat transfer, which was confirmed in previous research. The amplitude of oscillation depends on Reynolds number and hole positions. It was observed that the injection flow was not symmetric to the openings, and it was skewed to the direction of the first channel main flow. The ratio of flow rate through the outlet and the bleeding holes was from 1:1 to 1:3.  相似文献   
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3.
依据现有的叶片尾迹宽度计算公式,计算了一空调室外机风机叶片的尾迹宽度,然后,以此尾迹宽度为参考基准,设计了两组不同形状和大小的锯齿形叶片尾缘,制作并试验研究了锯齿形尾缘对风机气动噪声的影响规律。结果表明,锯齿尾缘有明显降噪效果,正弦形锯齿较正三角形更好。锯齿尾缘通过降低宽频噪声降噪,而对离散噪声影响很小。采用与叶片尾迹...  相似文献   
4.
尾缘凹陷轴流风轮内流特性分析与降噪研究   总被引:1,自引:0,他引:1  
为减少空调室外机用低压轴流风轮叶片的颤振,改善音质,降低噪音和电机功率。本文设计了尾缘凹陷新型三叶轴流风轮,同时除对前缘局部加厚处理外,对叶片其他区域整体减薄。文中采用Navier-Stokes方程对新风轮和原风轮的内流及气动特性进行了三维数值模拟,同时对两者的流量压力曲线和噪音频谱进行了实验研究。研究表明:对轴流风轮叶片尾缘进行凹陷设计,对前缘以外的区域减薄设计,能够减轻风轮重量,降低电机负荷同时减弱转子尾迹;对叶轮前缘进行局部加厚处理可以减小叶尖颤振,降低叶片旋转频率峰值噪音,从整体上降低噪音,改善音质。  相似文献   
5.
Vortex shedding in the wake of two-dimensional bluff bodies is usually accompanied by three dimensional instabilities. These instabilities result in streamwise and vertical vorticity components which occur at a certain spanwise wavelength. The spanwise wavelength of the instabilities (λZ) depends on several parameters, including profile geometry and Reynolds number. The objective of the present work is to study the three dimensional wake instabilities for a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, and to manipulate these instabilities to control the aerodynamic forces. Results of numerical simulations of flow around the body at Re(d) = 400, 600, and 1000, as well as planar Laser Induced Fluorescence (LIF) flow visualizations at Re(d) = 600 and 1000 are analyzed to determine the wake vorticity structure and λZ. Based on the findings of these analyses, an active flow control mechanism for attenuation of the fluctuating aerodynamic forces on the body is proposed. The flow control mechanism is comprised of a series of trailing edge injection ports distributed across the span, with a spacing equal to λZ. Injection of a secondary flow leads to amplification of the three dimensional instabilities and disorganization of the von Kármán vortex street. Numerical simulations indicate that the flow control mechanism can attenuate the fluctuating aerodynamic forces at lower Reynolds numbers (Re(d) = 400 and 600) where λZ is constant in time. However, the control mechanism loses its effectiveness at Re(d) = 1000, due to the temporal variations of λZ.  相似文献   
6.
The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function (Cω(t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of Cω(t) is found to be responsible for the ultrashort T2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of Cω(t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (FO(t)) and hydrogen (FH(t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate.  相似文献   
7.
During the mixing of viscous incompressible flows with different velocities, in the vicinity of a trailing edge an interaction region with a three-layer structure is formed, similar to that in the case of symmetric shedding with equal velocities. The boundary layers developing on the upper and lower sides of the airfoil form a viscous mixing layer, or vortex sheet, which separates the flows downstream of the trailing edge. The boundary value problem corresponding to the flow in the viscous sublayer in the vicinity of the trailing edge of a flat plate is solved for high Reynolds numbers using an efficient numerical method for solving the equations of asymptotic interaction theory.  相似文献   
8.
In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal pi rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor (15)N nuclei I=N (D) and S=N(A) in RNA Watson-Crick base pairs with homonuclear scalar couplings J(IS)=(2h)J(N(D), N(A)) across the hydrogen bonds.  相似文献   
9.
本文采用二维Ffowcs Williams&Hawkings(FW-H)方程对平行剪切层远声场辐射特性进行了研究。近流场时间精确数据通过计算气动声学(Computational Aeroacoustics,CAA)技术数值模拟获得,声远场信息则通过FW-H方程对近流场内的可穿透积分面进行积分获得。该方法首先采用具有解析解的涡/尾缘干涉问题进行了校核,进一步采用CAA/FW-H匹配技术对二维平行剪切层声辐射问题进行了预测,计算结果表明,积分解与计算域内的CAA数值解吻合较好。  相似文献   
10.
徐力平 《力学学报》1991,23(4):393-399
本文提出一个无粘的底部压力模型。证明了在给定反压的情况下,超音出口的涡轮叶栅的叶片尾缘的底压以及叶型损失可由该模型确定。与无粘的流场计算相结合,可用该模型计算二维叶栅的底压。对模型的参数研究表明:具有正曲率的喉道下游叶背型面将有利于减少损失;而出口为音速时流动则最为不利。  相似文献   
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