Instantaneous three-dimensional flow visualization of a supersonic mixing layer

An imaging technique capable of time-resolved, three-dimensional visualizations of compressible flows is described and applied to a supersonic mixing layer. The three-dimensional planar imaging system uses a custom high-speed camera to acquire 10 successive planar images through the mixing layer at a rate of 10⁷ frames per second. Mixed fluid in the layer is visualized by Mie scattering of a laser light sheet from condensed alcohol droplets. After collection, the planar images are corrected for distortions and stacked to form data volumes. Comparative visualizations at low and moderate convective Mach numbers (M _c = 0.43 and 0.62) are used to examine the effects of compressibility on large-scale structure in mixing layers. The visualizations graphically reveal the shift from two-dimensional spanwise rollers to three-dimensional structure, such as oblique and V-shaped bands, with increasing compressibility. Additionally, direct comparison between the high- and low-speed edges of the mixing layer shows the high-speed interface to be smoother than its lowspeed counterpart.The authors would like to acknowledge Dr. Michael Miller and Dr. Jerry Seitzman for many helpful discussions, and William Urban for assistance in performing the experiments. This work is supported by the Air Force Office of Scientific Research, Aerospace Sciences Directorate, J. Tishkoff technical monitor.     

A cross-correlation technique for velocity field extraction from particulate visualization

A rapid time series of photographs of the horizontal cross-sections of several y ⁺ locations were taken of a turbulent open-channel water flow with Re _d = 3,900. A pair of photographic images were obtained with a time difference of 1.3 v/u ² at each y ⁺ locations. The pictures were digitized into 8 bit data with a spatial resolution of 2.5 viscous scales. Instead of identifying discrete particles, a variable interval spatial correlation technique was used to extract the velocity components. With this technique, two-dimensional spatial cross-correlations of the illumination intensities were taken between a pair of picture images. The correlations were taken over small areas and the peak of the correlation coefficients were used to obtain the convection velocity yielding the u and w components of velocity. Some statistical properties were calculated and are shown to be comparable with previous data. Spatial correlations of the velocity components revealed some unique characteristics related to the structure of turbulence.     

Instantaneous visualization of surface flows

An experimental system is described for visualizing the surface flow of a wing, using an oil smoke tracer technique. The method leads to the determination of the instantaneous velocity direction at the output of surface injectors. A preliminary investigation is made on a flat plate to optimize the conditions of oil smoke injection. Then, the visualization is performed on the upperside of a sweptback wing in the vicinity of the reattachment of the vortex flow. This visualization technique can be applied to other types of wall flows — separated or not — around various bodies.List of symbols b wing span - c _n normal (to leading edge) chord - c _r streamwise (or root) chord - d diameter of the injectors - distance from the apex along the leading edge - relative distance from the apex along the leading edge ( = /C _d) - sweep angle - e injector geometric parameter (e = d/l) - angle of attack - K injection parameter - l length of the injectors - v kinematic viscosity - P _t, P_s total and static pressure of the flow - P _inj injection pressure - P _r reduced pressure (P _r = (P_inj – P_t)/(P_t – P_s)) - Re flow Reynolds number (Re = V ·c _n/v) - Re _i injector Reynolds number (Re = V ·d/v) - s curvilinear distance along c _d - s relative curvilinear distance along c _d(s = s/c _d) - V infinite upstream flow velocity     

A two-dimensional lagrangian technique for flow field measurement under high dynamic pressure

In this paper, a two-dimensional (2-D) Langrangian technique for flow field measurement under high dynamic pressure is presented, which included a set of experimental device and 2-D Lagrange composite manganin-constantan ring gages. With this kind of gage, the histories of pressure and radial displacement can be measured simultaneously at different Lagrange positions in an axisymmetric shock loading flow field. The technique has some advantages over the 1-D one, such as, simplified loading device, continuously adjustable output pressure, no restriction on sample length and the availability for the study of lateral rarefaction in shock propogation. As a preliminary application, the processes of 2-D shock initiation and attenuation in compacted TNT are measured.     

A flow visualization technique for low-speed wind-tunnel studies

The paper describes a flow visualization technique (direct injection method) used in low-speed wind-tunnel studies. Mixture of titanium tetrachloride and carbon tetrachloride is used to produce closely spaced parallel streaklines of white smoke of long duration. The technique is described in detail and representative photographs of various flow patterns are presented.     

Instantaneous concentration profiles of oxygen accompanying absorption in a stratified flow

Absorption of oxygen from air to water is studied for a stratified flow. Instantaneous concentration profiles in the liquid, close to the interface, are measured by utilizing oxygen quenching of the fluorescence from pyrenebutyric acid. When waves are present very large temporal variations of the concentration profile and of the mass transfer rate are observed. These results seem to rule out the notion that turbulence in the liquid is controlling mass transfer.This work is being supported by the National Science Foundation under NSF CIS 92-09877 and by the U.S. Department of Energy under DEF Go2-86-ER13556. The idea behind this work evolved from discussions with Professor G. Weber of the Department of Biochemistry, the University of Illinois, Urbana.     

Instantaneous density measurement in two-dimensional gas flow by high speed differential interferometry

The aim of this paper is to present an experimental set-up using a Wollaston prism differential interferometer producing up to twenty successive short exposure white light interferograms at a high framing rate. It is shown that, through optical component calibration, the interferograms can be analysed to yield the instantaneous density field. This method has been successfully tested in the two-dimensional unsteady flow generated by the interaction of a mixing layer and a cavity.List of symbols h height of the downstream edge of the cavity - H height of backward facing step - M Mach number - t time - t time interval between two successive frames - N frequency - double-prism median plane - birefringence angle - _p pressure fluctuation - C _p pressure coefficient - biprism abscissa corresponding to any colour - ₀ biprism reference abscissa corresponding to background colour - _y deviation of light rays - R radius of curvature of spherical mirror - L virtual distance from the middle of the test section to the spherical mirror - E optical thickness - E _e optical thickness corresponding to background colour - d _E difference of optical thickness - d _x abscissa difference - gas density - ₀ stagnation gas density - _e gas density of background colour     

Alignment and application of the conical shadowgraph flow visualization technique

An optical bench study has been carried out to assess the effects of both beam alignment and nonconical disturbances in the application of conical shadowgraphy for flow visualization. Conical and quasiconical plastic test models were immersed in a refractive index matching fluid and then examined by shadowgraphy with a conical light beam. The results show that problems of interpretation may arise due to both axial and transverse beam misalignment. Among these difficulties, axial misalignment with positive vertex displacement is the least serious. Also, the effect of a particular nonconical disturbance field was found to introduce fewer difficulties of interpretation than that of beam misalignment.     

Instantaneous flow field above the free end of finite-height cylinders and prisms

The flow above the free ends of surface-mounted finite-height circular cylinders and square prisms was studied experimentally using particle image velocimetry (PIV). Cylinders and prisms with aspect ratios of AR = 9, 7, 5, and 3 were tested at a Reynolds number of Re = 4.2 × 10⁴. The bodies were mounted normal to a ground plane and were partially immersed in a turbulent zero-pressure-gradient boundary layer, where the boundary layer thickness relative to the body width was δ/D = 1.6. PIV measurements were made above the free ends of the bodies in a vertical plane aligned with the flow centreline. The present PIV results provide insight into the effects of aspect ratio and body shape on the instantaneous flow field. The recirculation zone under the separated shear layer is larger for the square prism of AR = 3 compared to the more slender prism of AR = 9. Also, for a square prism with low aspect ratio (AR = 3), the influence of the reverse flow over the free end surface becomes more significant compared to that for a higher aspect ratio (AR = 9). For the circular cylinder, a cross-stream vortex forms within the recirculation zone. As the aspect ratio of the cylinder decreases, the reattachment point of the separated flow on the free end surface moves closer to the trailing edge. For both the square prism and circular cylinder cases, the instantaneous velocity vector field and associated in-plane vorticity field revealed small-scale structures mostly generated by the separated shear layer.     

Limited-view optical tomography technique and its application to temperature field measurement in flow of hot air

In this paper, a limited-view optical tomography technique is presented, which contains an orthographic holography system, an image processing system and the simultaneous algebraic reconstruction technique (SART). Using this technique, the temperature field of a cross section in the flow of hot air from a nozzle is measured. The measured results are satisfactory. The project supported by the National Natural Science Foundation and the State Science and Technology Commission of China     

Instantaneous planar pressure determination from PIV in turbulent flow

This paper deals with the determination of instantaneous planar pressure fields from velocity data obtained by particle image velocimetry (PIV) in turbulent flow. The operating principles of pressure determination using a Eulerian or a Lagrangian approach are described together with theoretical considerations on its expected performance. These considerations are verified by a performance assessment on a synthetic flow field. Based on these results, guidelines regarding the temporal and spatial resolution required are proposed. The interrogation window size needs to be 5 times smaller than the flow structures and the acquisition frequency needs to be 10 times higher than the corresponding flow frequency (e.g. Eulerian time scales for the Eulerian approach). To further assess the experimental viability of the pressure evaluation methods, stereoscopic PIV and tomographic PIV experiments on a square cylinder flow (Re _D  = 9,500) were performed, employing surface pressure data for validation. The experimental results were found to support the proposed guidelines.     

An improved smoke-wire flow visualization technique using capacitor as power source

An improved smoke-wire flow visualization technique using a large capacitor as the power source was proposed. Electric current discharged from capacitors was used to heat a fine metal wire suspended in the flow field. The oil droplets attached to the wire were vaporized and smoke filaments followed the flow motions. A digital camera was used to record the images of the smoke filaments. The actions of discharging and camera shutter were triggered by signals from a microcontroller to ensure the accurate timing. Clear images of the streaklines were captured at a freestream velocity up to 12.9 m/s, much higher than the limits of the existing techniques.     

A three-dimensional visualization technique applied to flow around a delta wing

The visualization of flows in two dimensions by using planar laser light sheets is a commonly used technique. We extend this technique to three dimensions by rapidly scanning the laser light sheet to obtain a set of slices of the flow around a full span delta wing. The leading edge vortices, which are marked with smoke, are unburst by tangential blowing around the leading edges at angles of attack in excess of 25°. Since the measurement period is on the order of the smallest convective time scale, we obtain a virtually instantaneous set of planar cross sections of the flow. Software based on the marching cubes algorithm is used to stack the slices and reconstruct a three-dimensional surface of the smoke-seeded fluid. This surface, which corresponds to the vortices, clearly shows the qualitative effects of blowing on the delta wing flow.     

Visualization and measurement of helicopter rotor flow with swept back tip shapes at hover flight using the “flow visualization gun” time line technique

The Flow Visualization Gun (FVG), a novel time line visualization technique, has been used to investigate the flow field of a helicopter rotor with swept back tip shape in hover flight condition. After introducing the FVG-technique, the paper presents some visualization photographs of the rotor blade tip vortices and the rotor downwash. Using orthogonal sets of flow photographs and digital image analysis, the 3-dimensional time line displacement within the flow and the tip vortex structure are determined. The data of 16 time line experiments are interpolated in space and time to obtain velocity data on an evenly spaced 3-dimensional grid. Vorticity contour plots of the flow field show the complex arrangement of the tip vortices of the blade itself and of the preceding blade and some additional vorticity in the direct wake which may form a secondary vortex. Understanding and control of this vorticity distribution is important for the design of new efficient tip shapes. While the FVG technique has been used so far for qualitative investigation of complex flow patterns at local velocities of up to 20 m/s, a good comparison to laser velocimetry data validates the technique as an interesting tool for both qualitative and quantitative investigation. This research was supported by a “Poste Rouge” grant by the Centre National de la Recherche Scientifique (C.N.R.S.). The authors would like to thank all members of the IRPHE for their help concerning the mechanical and electronical set up of the experiments.     

Velocity field and surfactant concentration measurement techniques for free-surface flows

 Noninvasive measurement techniques were developed to obtain the surfactant concentration and the velocity field of a vortex pair interacting with a contaminated free surface. The optical method of second-harmonic generation (SHG) was utilized to determine the time-varying concentration of a surfactant at a point on the surface, and the established technique of digital particle image velocimetry (DPIV) was used for the measurement of the velocity field. The evolution of the initially uniform, insoluble monolayer along with the vorticity field are described. One of the present limitations is the temporal resolution of the concentration measurements which is of order 5 Hz. The extension of the SHG technique to high Reynolds number, turbulent flows is discussed. Received: 24 January 1996 / Accepted: 17 July 1996     

Surface oil flow technique and liquid crystal thermography for flow visualization in impulse,wind tunnels

This paper describes flow visualization techniques employing surface oil flow and liquid crystal thermography suitable for use in impulse wind tunnels. High spatial resolution photographs of oil flow patterns and liquid crystal thermograms have been obtained within test times ranging from 7 to 500 ms and have been shown to be very useful for revealing the detailed features of 3-D separated flow. The results from oil flow patterns, liquid crystal thermograms, schlieren photographs and heat flux measurements are shown to be in good agreement.     

Residence time measurement of an isothermal combustor flow field

Residence times of combustors have commonly been used to help understand NO_x emissions and flame blowout. Both the time mean velocity and turbulence fields are important to the residence time, but determining the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three-dimensional nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using a time-resolved planar laser-induced fluorescence (PLIF) technique, but a second camera for particle image velocimetry (PIV) was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures. Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after injection is stopped: this equivalent to the sum of the convection time and the half-life values. The technique was applied to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core, where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NO_x production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales.     

The round jet in a uniform counterflow: flow visualization and mean concentration measurements

The structure and concentration field of a round water jet in a uniform counterflow is investigated using planar laser-induced fluorescence for jet to counterflow velocity ratios between 1.6 and 10.0. At low jet- to counterflow velocity ratios, the flow appears to be quite stable, with a nearly constant downstream extent and regular vortex shedding. As the velocity ratio increases, the flow becomes more unstable, with significant fluctuations along both the downstream and radial directions. The flow is quite sensitive to directional perturbations, with even slight inclinations of the jet with respect to the counterflow resulting in regular asymmetric vortex shedding and a decreased mean downstream extent. Data on the mean and maximum downstream extents, as well as the maximum radial extent of the flow at various velocity ratios, are shown. A simple model is introduced to explain the dependence of the normalized mean downstream extent of the flow upon the velocity ratio. In addition, mean concentration data are presented. The results suggest that there exists a core region of jet-like dilution in the flow.We wish to thank Dipl.-Ing. Olaf König, whose engineer's thesis laid the foundations for this work; Peter Schergun, who built much of the flow facility, and Dr. Dietrich Bechert, for his many valuable suggestions. MY's stay in Berlin was sponsored by the National Science Foundation Program for Research at Foreign Centers, contract number INT9202425, and an Alexander von Humboldt Foundation Research Fellowship.     

CPT磁力仪暗态耦合磁场测量技术

相干粒子数俘获(CPT)磁力仪是通过精确追踪光与原子相互作用下获得的透射信号来计算磁场强度的。针对由于透射信号的方向性衰减特性而造成的系统测量盲区的问题,提出了一种暗态耦合测量方案,通过使用特定频率的调制光场来实现多个暗态的同时激发和耦合测量,基于原子密度矩阵法建立了原子五能级系统模型并进行数值仿真研究,分析了该方案的控制条件和特点。实验结果证明,使用暗态耦合测量技术获得的透射信号幅值可以达到相同条件下单暗态测量时信号幅值的两倍左右,能够有效消除测量盲区,提高透射信号的信噪比,从而进一步提高CPT磁力仪的测量灵敏度。