Flow visualization (original) (raw)
Continuous coloured smoke-wire technique for flow visualisation
1988
A continuous colored smoke-wire technique has been developed for flow visualization in wind tunnels. The improved contrast of colored smoke sheet facilitates flow visualization. In the present technique, a vertical smoke-wire is used for introducing controlled sheets of smoke streaklines during a wind tunnel run. Regulated drops of a mixture of paraffin oil with colored dye are allowed to fall
Comparative analysis of experimental and numerical flow visualization methods
FME Transactions, 2006
The results of flow visualization around various models in MTI's wind and water tunnels are presented in this paper. They were obtained by different techniques: holographic interferometry, surface oil emulsion, dye and air bubbles. Besides experimental results, the flow simulations by Fluent for the same experimental conditions are presented too. The obtained comparative analyses shows that it is helpful to use both methods, experimental and numerical, in complex flows investigations
Some principles of flow visualization techniques in wind tunnels
Nowadays, there are various ways to study gas and fluid flows in wind tunnels and are still an important field of scientific research. The objectives of the research intended to present the basic principle of flow visualization techniques in wind tunnels and to simulate, visualize, observe, and measure how the flow around the object.
Development and application of streakline visualization in hypervelocity flows
Experiments in Fluids, 2002
In the traditional smoke-wire setup, the streak lines consist of smoke generated by oil that vaporizes as it runs down a heated wire placed upstream of a model in a wind tunnel. Provided that the wind tunnel flow has sufficiently Abstract 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 wave length close to one of the sodium D-lines, as the light source in shadowgraph or Schlieren visualization, streaklines seeded with sodium become visible through ab sorption 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.
Flow visualization techniques in wind tunnels–optical methods (Part II)
Scientific Technical Review, 2007
An attempt is made to describe and review the most widely used methods for flow visualization. The first part described the basis and applications of different visualization methods for subsonic and supersonic flow in wind and water tunnels. This part concentrates on optical methods (shadow, schlieren and interferometry) and their application in compressibe flow visualization. Almost all presented photos have been made in the laboratories of the VTI.
Visualization and picture processing of turbulent flow
Experiments in Fluids, 1984
The tracer method was used to visualize the threedimensional structure of turbulent open-channel flow. A horizontal cross-section of the flow was illuminated by light passing through a thin slit. The illuminated cross-section was shifted upward, and at the time, successive pictures of flow patterns were taken. The picture-taking system was then shifted in downstream direction to follow the flow structures. The pictures obtained were processed by computer. Various kinds of physical properties of the flow were quantitatively evaluated and displayed as graphical outputs. These results contribute to the elucidation of the threedimensional structure of turbulent open-channel flows.
Visualization of Near-Surface Flow Patterns for Air-Water Gas Transfer
2018
Near-surface flow patterns and their influence on local transport processes are investigated in a horizontal plane with visualization techniques. For this, a new method was developed that produces tracer particles with high effective Schmidt number (Sc = O(10 6)) by chemical reaction directly at the water surface. While different chemical systems were tried, best results were achieved using the precipitate AgCl, formed by AgNO 3 in the water body and a controlled influx of HCl gas. Trace amounts of precipitate are made visible by scattering laser light. Using the new method, the influence of wind induced turbulences at the water interface can be investigated with previously unachieved sensitivity. Illumination of the wavy water surface leads to shadowing and lensing effects that become apparent in the image data and cannot fully be compensated for with image processing. The results are qualitatively compared to data from active thermography (heat, Pr ≈ 7) and boundary layer imaging (gas, Sc ≈ 600), showing a close resemblance of the flow pattern. With higher Schmidt number of the tracer, the structure becomes more pronounced and higher sensitivity to surface convergence is observed. Experiments with glass spheres (d = 70 µm-2 mm, Sc → ∞) were conducted for further comparison. Tracking of individual particles allows for a precise measurements of the surface velocity and investigations of the local momentum transfer. Zusammenfassung Strömungsmuster nahe der Wasseroberfläche und ihre Auswirkungen auf lokale Transportprozesse werden mit Visualisierungsmethoden in horizontaler Ebene untersucht. Dafür wurde eine neue Technik entwickelt, die Tracerpartikel mit hoher effektiver Schmidtzahl (Sc = O(10 6)) direkt an der Wasseroberfläche durch chemische Reaktionen erzeugt. Unter verschiedenen getesteten chemischen Systemen ist eine Ausfällung von AgCl, erreicht durch in Wasser gelöstes AgNO 3 und kontrollierte Einfuhr von HCl in den Luftraum, am besten geeignet. Bereits geringe Teilchenkonzentrationen können durch die Streuung von Laserlicht sichtbar gemacht werden. Mit der neuen Methode kann der Einfluss von windinduzierten Turbulenzen an der Oberfläche mit bisher unerreichter Empfindlichkeit nachgewiesen und untersucht werden. Für höhere Windbedingungen führen Abschattungen und Reflektionen der Beleuchtung an der Wasseroberfläche zu Störungen, welche auch mit aufwändigen Bildverarbeitungsmethoden nicht hinreichend behoben werden können. Die Resultate werden qualitativ mit Daten aus der Thermographie (Wärme, Pr ≈ 7) sowie der Grenzschichtdickenvisualisierung (Gas, Sc ≈ 600) verglichen, wobei eine starke Ähnlichkeit der Strömungsmuster beobachtet wird. Mit ansteigender Schmidtzahl der Tracer wird eine klarere Struktur und größere Empfindlichkeit für Oberflächenkonvergenz beobachtet. Zum weiteren Vergleich wurden Experimente mit Glaskugeln (d = 70 µm-2 mm, Sc → ∞) durchgeführt. Tracking der einzelnen Partikel erlaubt eine präzise Bestimmung der Oberflächengeschwindigkeit und eine Vermessung des lokalen Impulstransfers. vii Chapter 1 Chapter 2 Theory Investigations of gas transfer at the water surface are under the influence of multifarious processes. The aim of this chapter is to provide an understandable mathematical background for the natural phenomenons and laboratory observations investigated in this thesis. Basic quantities and conceptual models for the description of air-sea gas transfer are introduced. Special emphasis is on the importance of the Schmidt number as well as the relevance and interpretation of chemical reactions at the interface. To conclude, an overview is given on the turbulent flow patterns at the water surface that are crucial for transport mechanisms and are closely investigated within this work.
VISUALIZATION OF STRATIFIED AND ROTATING FLOWS FINE STRUCTURE
Visualization by sensitive optic methods namely schlieren and shadow as well as electrolytic precipitation and dyeing are widely used for stratified flows. Conditions of experiment completeness and adequacy are discussed. Bright colour schlieren images of the flow are produced using 'natural rainbow method' exploring refraction and dispersion of a light in the fluid. Wide view field and a high spatial resolution of schlieren instruments allow observing simultaneously large and small scale flow components. General features of substances transportation in stratified and vortex flows are marked. Streaky structures in flows are identified and their transformation into vortex system is registered. Anisotropic transport of a dye in a compound vortex contacting a free surface is registered.
Optical methods in wind tunnel flow visualization
FME Transactions, 2006
Optical methods in wind tunnel flow visualization This paper reports the application of optical methods: shadow, schlieren and holographic interferometry in wind tunnel flow visualization. Some examples obtained in the MTI wind tunnels are presented. The comparative advantages of holographic method in regard to shadow and schlieren method for quantitative flow field test are analyzed.
A computer-aided visualization method for flow analysis
Flow Measurement and Instrumentation, 2014
This paper presents a non-contact method for velocity field calculation from a series of images containing illuminated planar layer of fluid with a pollutant mixed in. Velocity field is calculated using a model similar to optical flow based on the advectiondiffusion equation. The model which was also implemented into our software, ADMflow, is evaluated on different sets of synthetic images. Calculated velocity fields are in a good agreement with the true velocity fields, mostly deviating by less than 10% in magnitude and 1° in flow direction.
Measurements of Turbulent Flows
Springer eBooks, 2007
An understanding of turbulence, with its complex spatio-temporal structure, remains elusive. Ideally, an instrument to study the dynamics of turbulent flows would be capable of resolving the smallest relevant spatial and temporal scales in three dimensions. This dissertation details the development of a new optical measurement technique, three-dimensional, long-distance, high-speed micro particle image velocimetry (3-D LHµPIV), and its application to two types of turbulent flow. This technique uses three cameras to simultaneously locally measure all three components of the velocity and all nine velocity gradients. Bibliography vi LIST OF FIGURES 2.1 a) Three laser sheets illuminate three faces of a cubical volume within the flow. Each camera sees a two-dimensional particle field on one of the three faces. The fields of view for the three cameras are offset from the vertex of the cube. Particles within the cube and on the remaining three faces are not visualized. b) After being broken into subsections and digitally analyzed, each face yields an array of twodimensional velocity vectors to be fit to a three-dimensional model.. 2.2 Sample camera image of 1 µm polystyrene spheres suspended in the flow. The particles are illuminated by a laser sheet 0.018 cm thick in a plane perpendicular to the viewing angle. The resolution of the image is 240×210 pixels, corresponding to a region of flow 0.11×0.10 cm. Approximately 450 particles are visible in this image.. .. .. . 2.3 Cylindrical converging lenses are used to create laser sheets. A beam passed through the lens narrows to a thin waist before expanding again.
The 8th International Symposium on Flow Visualization
Journal of Visualization, 1999
The measurement principle behind Surface Stress Sensitive Films (S 3 F) is based on the transformation of the measured deformation of an elastic media into surface loads (pressure and shear stresses). According to Gook's Law, the 3D deformation of the surface of an elastic film is a function of the applied tangential and normal tensions. Such displacements can be measured using several different 3D displacement techniques. The results presented in the paper demonstrate that a stable S 3 F can be created having a shift modulus between 30-50 Pa. Such highly sensitive elastic films provide the possibility of conducting surface stress field visualization with a resolution up to 0.1 Pa. S 3 F's are chemically inert and have small dielectric and optical losses. These features permit their use in liquid and various gaseous environments. Their properties are linear over a wide range of deformation (up to 10%-15% of the film thickness) and can be adjusted by changing the chemical composition. This paper describes the measurement methodology and data-reduction algorithms currently used with the S 3 F technique. Experimental results obtained with an S 3 F optimized for low speed measurements, i.e., 5-50m/s (air flow) and 0.1-0.5m/s (water flow), are presented and analyzed. A comparative analysis of the S 3 F and PSP techniques for a low speed air flow is presented..
Flow Analysis with Spectrophotometric and Luminometric Detection
2012
Home-made components were dominant in the early development of flow analysis, as can be seen in Fig. 2.2 showing the prototype airsegmented flow analyser assembly. In the initial development of flow injection analysis, excellent results were obtained with very simple instruments, and devices were often assembled with pieces originally designed for children's toys [1,2] (see also Fig. 2.5). Nowadays, there are several instrument manufacturers [3e6], and the use of commercial equipment for high-throughput analysis is a reality. Novel, dedicated components for specific tasks are still being proposed and the use of computer-controlled instrumentation is increasing. A flow system comprises three units: fluid propulsion, sample handling and detection and the components necessary to assemble and integrate these units, e.g., transmission lines and connectors. This chapter describes all of these units and components.
Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2003
A visualization study and Quantitative velocity measurements have been performed in Taylor-Couette flow with a medium-gap (ç= 0.356), over a large range of Taylor numbers (2.1x10 4 < Ta < 1.1x10 11), with the outer cylinder fixed and the inner cylinder rotating about its axis. Quantitative velocity measurements were carried out using the PHANTOMM flow tagging technique. Two techniques were used for visualization study: the PHANTOMM technique that allowed flow structure visualization from small to moderate Taylor numbers, and the Particle Streak Imaging, PSI (the flow was seeded with neutrally buoyant polystyrene micro spheres) that permitted the flow structure visualization from moderate to high Taylor numbers. The results illustrate the expected three-dimensional features of flow and presence of Taylor cells at low Taylor numbers. Our study examined the interplay between small and large scales present in the flow as well as showed the gradual transition to turbulence with increasing Taylor numbers. Taylor cells were found for Taylor numbers less than 1.13 x 10 10. At low Taylor numbers, the flow in the cells appeared to be a rotational laminar flow with a high degree of coherence. At higher Taylor numbers, the cells aspect became more irregular, and the flow inside them became turbulent. The Görtler instability developed inside Taylor cells and close to the inner cylinder wall. At the highest Taylor numbers, turbulence increased up to the point where no Taylor cells could be detected. For the flow in our study, at Ta = 1.13 x 10 10 , the homogenization by turbulence spread across the gap, and the flow structure sharply changed its pattern as a toroidal vortex in helical motion developed in a thin layer on the inner cylinder wall. Instantaneous velocities, average velocities, angularmomentum ratio and spectral density function were computed for all ranges of Taylor numbers in the range studied. These quantitative results show the same conclusions as the ones presented by the visualization study.
A laboratory method for the visualization and quantification of hyporheic flow paths and velocities
Canadian Journal of Civil Engineering
Hyporheic flow, the flow of water through the permeable material immediately surrounding a river, is important for nutrient cycling, dissolved oxygen transport, and contaminant transport. In addition, there is recent concern regarding the role of hyporheic flow on the contamination of rivers following oil spills. To better understand hyporheic flow paths and velocities, it is important to measure hyporheic flow at high spatial and temporal resolution. A practical method to measure hyporheic flow in a laboratory flume based on dye injection, digital images, and moment analysis was developed. An experiment conducted using a single gravel bar demonstrated good agreement between observations and estimates based on image processing. The measured hyporheic flow field showed upstream and downstream flow that discharged downstream of the bar top, the presence of a flow divide and flow stagnation, and hyporheic flow velocities indicative of turbulent flow for which Darcy’s law is not applica...
Flow Visualization Around Airfoils
2015
This work presents visualization of low Reynolds number flow around airfoils, NACA0012 and NACA23012, using Hele-Shaw apparatus. The study focuses on the variation of stream surface contour as it moves past airfoils at different angles of attack and pressure heads. For each airfoil, at the stalling angle, the genesis of a bubble has been captured. The bubble size evolution indicates gradual growth for symmetric airfoil, whereas higher growth is associated with asymmetric case with increasing angle of attack as well as Reynolds number. Considering the formation of bubble as a benchmark, the existing procedure may be effectively utilized for stall characterization of airfoils, where laminar boundary layer prevails.
On flow visualization using reflective flakes
Journal of Fluid Mechanics, 1985
An analysis of flow visualization using small reflective flakes is introduced. This rational analysis is based on a stochastic treatment of Jeffery's (1922) solution for the motion of ellipsoidal particles in a viscous fluid, wherein thin flakes tend to align with stream surfaces. The predicted light fields are confirmed by examples of parallel flows, the flow over a rotating disk, and the spinup from rest in a cylindrical cavity. The Tollmien–Schlichting wave packet trailing a turbulent spot is taken as an example to discuss the suitability of the technique for visualizing small-amplitude waves. Attenuation of light through a suspension is described.