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1.
A method for visualizing streaklines in hypervelocity flows has been developed. The method uses the high temperatures produced in hypervelocity flows to ablate small amounts of sodium deposited onto a wire stretched across the flow and to broaden the lines in the sodium spectrum. By using a dye laser, tuned to a wavelength close to one of the sodium D-lines, as the light source in shadowgraph or Schlieren visualization, streaklines seeded with sodium become visible through absorption and/or enhanced refractivity. The technique has been used to investigate the stability of the shear layer produced by the curved bow shock on a cylindrically blunted wedge. The results suggest that the shear layer is unstable, exhibiting structures with a wavelength that is comparable to half the nose radius of the body.  相似文献   

2.
 In this paper, a new method of measuring scalar behavior in bulk aqueous fluid flows is presented. Using a simple ratiometric scheme, laser induced fluorescence from organic dyes can be normalized so that direct measurements of a scalar in the flow are possible. The technique dual emission laser induced fluorescence (DELIF) relies on normalizing the fluorescence emission intensity of one dye with the fluorescence emission intensity of a second dye. Since each dye fluoresces at a different wavelength, one can optically separate the emission of each dye. This paper contains an overview of the basic ratiometric technique for pH and temperature measurements as well as the spectral properties of nine water soluble dyes. It also covers the three most significant sources of error in DELIF applications. To demonstrate the technique, steady state turbulent jet mixing and temperature fields in a thermal plume were quantified. The accuracy was camera limited at under 3% of the fluorescence ratio which corresponds to 0.1 pH units or 1.8 °C. Received 7 June 1996/Accepted 17 June 1997  相似文献   

3.
Turbulent flow over a sinusoidal solid wavy surface was investigated by a direct numerical simulation using a spectral element technique. The train of waves has an amplitude to wavelength ratio of 0.05. For the flow conditions (Re=hU b/2ν= 3460) considered, adverse pressure gradients were large enough to cause flow separation. Numerical results compare favorably with those of Hudson's (1993) measurements. Instantaneous flow fields show a large variation of the flow pattern in the spanwise direction in the separated bubble at a given time. A surprising result is the discovery of occasional velocity bursts which originate in the separated region and extend over large distances away from the wavy wall. Turbulence in this region is very different from that near a flat wall in that it is associated with a shear layer which is formed by flow separation. Received 17 April 1996 and accepted 19 November 1997  相似文献   

4.
In shock tube experiments the profiles of light absorption in oxygen are obtained for the wavelength interval 200–260 nm over the temperature range 4000–10800 K. Using these data, the vibrational temperature profiles are measured for oxygen molecules behind the shock front. The method of determination of the vibrational temperature of oxygen is based on comparing absorption measurements and detailed absorption spectrum calculations for oxygen in the Schumann-Runge system.  相似文献   

5.
 The mixing characteristics of a round, turbulent, unsteady jet were studied experimentally. A gravity-driven flow was created by releasing dyed fluid from a vertical tube into a large water tank. The jet velocity increased from zero to a maximum and then decreased continuously such that each run lasted about  s. The jet dilution was examined by an optical absorption technique that measured the line integral of concentration across the jet diameter. These measurements revealed that the portion of the unsteady jet corresponding to the deceleration phase dilutes more than the steady jet. The molecular scale mixing, as deduced from an acid-base neutralization reaction, corroborated the finding that the jet mixes in a shorter distance than the steady jet. Received: 22 August 1996/Accepted: 4 February 1997  相似文献   

6.
 We describe the capabilities of coherent high resolution radar to observe remotely the effects of an upwelling subsurface flow on the water surface. This observation is possible because the radar radiation backscatters very strongly from surface features with dimensions similar to its wavelength, in this case X-band at 0.03 m. This technique provides imaging capability with relatively high spatial resolution (∼0.3 m) and fast time sampling (∼0.006 s) over a large surface area. The processed data reveal both the line-of-sight velocity spectrum of moving water surface features, and their water surface radar backscatter cross-section. We believe that the surface features are generated by subsurface vortices oriented normal to the surface. The vortices are advected with the bulk flow of the jet. Our radar observations of the down-stream flow from a submerged waterjet that is directed parallel to the surface are consistent with those previously measured by laser velocimetry. Received: 25 February 1994/Accepted: 16 May 1996  相似文献   

7.
李悦立  李栋  杨永 《力学学报》2011,43(1):45-54
基于前缘粗糙元阵列的后掠机翼被动层流控制技术的关键在于控制波长的选择. 从控制 波与流场相互作用的物理机制出发, 提出了快速波假设. 使用Mack方法求解三维不可压缩 Orr-Sommerfeld方程, 研究了无限展长后掠翼的最佳控制波长的选择问题. 对于Saric的实验状态采用定点计算方法验证了8mm为控制波长, 使用自适应搜索方法得到 了西北工业大学模型在不同状态下的最佳控制波长, 并通过风洞试验实现了转捩的推迟. 提出了丝网印刷添加粗糙元阵列技术与混合溶剂升华法, 丰富了实验手段.  相似文献   

8.
《力学快报》2023,13(3):100437
As a kind of classical low-frequency sound-absorbing material, the microperforated plate (MPP) has been widely used. Here, we inspired by the sound absorption mechanism of the MPP, a spiral metasurface (SM) is designed and the analytical solution of acoustic impedance and sound absorption coefficient are obtained. The relationship between the sound absorption properties of the MPP and the SM with their own structures is systematically studied, and the analytical solutions are used to optimise the structure. It is concluded that the MPP and the SM of the same thickness achieve effective absorption in the frequency range between 390-900 Hz and 1920-4266 Hz, with a total thickness less than 1/6 of the wavelength. Meanwhile, the numerical calculation shows that the MPP and SM can match well with the background medium in the effective rang. Our study provides new insights into the design methods of sound-absorbing materials and is potentially suitable for many acoustic engineering applications.  相似文献   

9.
Temperature and frequency dependences of the cross section of absorption of an external alternating electromagnetic field by a small metal particle was evaluated using the kinetic equation for the electron distribution function. It is assumed that the particle radius R satisfies the inequality lee ≪ R ≪ λ, where lee is the electron mean free path and λ is the electromagnetic radiation wavelength. A general expression for the absorption cross section is obtained for an arbitrary ratio between the linear particle size and the skin layer thickness. The effect of particle temperature variation under the action of the external alternating electromagnetic field on the absorption cross section was studied. It is proved that for R ≪ λ and R ≪ lee, the absorption cross section should be calculated using the quantum mechanical principles and considering the electron temperature equal to zero. Under these conditions, it is not correct to introduce the electron distribution function and the absorption cross section is a rather complicated function of the frequency of the external field.  相似文献   

10.
The present paper describes a new phase-stepping algorithm for the analysis of isochromatics and isoclinics using retarders with arbitrary retardation. A retarder used in the proposed method is not necessarily a quarter-wave plate specified for the wavelength of the light used. Not only the isochromatic and isoclinic parameters but also the retardation of the input and output retarders are determined simultaneously by the proposed phase-stepping method. Thus, accurate analysis can be performed by the proposed method even if accurate quarter-wave plates are not used. In addition, any wavelength of visible light can be used in a single polariscope without requiring matching the wavelength of the quarter-wave plate. Thus, a multi-wavelength technique is easily combined with the proposed method for accuracy improvement, phase-unwrapping, or correction of ambiguity.  相似文献   

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