Fundamentals of Flow Visualization

By:

Nathan Andersen

For:

ME 3663: Fluid Mechanics

University of Texas at San AntonioCollege of Engineering

San Antonio, Texas

Instructor:

Clark M. Butler P.E.Mechanical Engineering Department

Table of Contents

Abstract ………………………………..…………………………..………………..…………… Page 2

Fundamentals of Flow Visualization …………………..………………………..…… Page 2

A Brief History of Flow Visualization …………………………………………………. Page 2

Introduction to Flow Visualization Terminology ………………………..……… Page 2

Surface Flow Visualization ………………………………………………….....………… Page 3

Free-Surface Flow Visualization ………………………………………………………… Page 5

Optical Flow Visualization ………………………………………………….…………….. Page 6

Computational Flow Visualization ………………………………………….………… Page 8

Conclusion ………………………………………………………………………………..……… Page 8

References ……………………………………………………………………………..………… Page 9

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Abstract

Fluid flow plays a role in numerous areas of engineering. Flow visualization is an important tool which can provide an overall picture of the flow field. This paper presents an overview of flow visualization including a brief history and descriptions of surface, free surface, optical, and computational flow visualization techniques.

Fundamentals of Flow Visualization

Flow visualization is an experimental means of studying the dynamic behavior of fluids as they flow around a body and its surfaces. Flow visualization is a necessity in the field of fluid mechanics because insight into physical processes is facilitated by the establishment of patterns produced by or related to the process. Because most fluids are transparent their motion is invisible to inspection by the naked eye. Flow visualization includes the techniques that are used to make the fluid motion recognizable.

A Brief History of Flow Visualization

The first records of flow visualization came from Leonardo Da Vinci in the 1400’s. Da Vinci sketched images of flows that he visualized using wood shavings and fine particles. In the 1800’s Osborne Reynolds created a device to visualize the transition between flow regimes in a pipe. He is also the namesake of the non-dimensionless constant Reynolds number. In the late 1800’s Ernst Mach developed shadowgraphy, the first technique for visualizing supersonic flows. In the early 20th century Ludwig Prandtl, the father of modern aerodynamics, made monumental progress in the field of flow visualization by produced films that show the motion of aluminum particles on the surface of water flowing around objects piercing the water. Prandtl’s films helped him develop the boundary layer theory and no-slip condition. With the advent of the computer came new, powerful techniques for analyzing and simulating fluid flows. In the 1970’s several groups began working on particle image velocimetry, a computational method of analyzing films like the Prandtl films. Following Moore’s law, the computational power of computers has grown exponentially since the 1970’s. Today computational fluid dynamic simulations output terabytes amounts of data, and it has become a difficult task for experimenters to locate, extract, and analyze flow features of interest from the massive datasets. Because the brain can process large amounts of visual information very quickly an ideal way of treating these large data sets is to visualize them on a computer by creating two and three dimensional plots. Advances in flow visualization implementation and automated analysis ensure that flow visualization will continue to be a valuable tool in the future.

Introduction to Flow Visualization Terminology

A steady flow is a flow that does not change with time. For steady flows streamlines, pathlines, and streaklines are coincident.

A vorticity is a location in a fluid flow where the velocity field curls, giving the magnitude and direction of angular velocity for each particle in field.

A streamline is a curve that is everywhere tangent to the instantaneous local velocity vector. Streamlines indicate the instantaneous direction of fluid motion. A streamtube is a bundle of streamlines. By definition a fluid element cannot cross a streamline. Streamlines can only be visualized

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directly in steady flows. Streamlines and tubes are instantaneous quantities and may vary considerably from one instant to another.

A pathline is the path traveled by an individual particle over time. A pathline is the easiest to understand because it coincides with our Lagrangian education. Pathlines as well as streaklines have a time-dependent history associated with them.

A streakline is the locus of fluid particles that have passed sequentially through a prescribed point in the flow. Streaklines are the most common flow generated in physical experiments. Streaklines have a time-dependent history associated with them. The difference between pathlines and streaklines is that a streamline is an instantaneous snapshot of the time-integrated flow pattern, whereas a pathline is a time-exposed flow path of an individual particle.

A timeline is a line connecting set of adjacent fluid particles that were marked at the same previous instant in time. Timelines are curves that are displaced with time. Timelines are useful when the uniformity of a flow (or lack thereof) is of interest. Where timelines intersect the surface of an object the no-slip condition anchors the timeline to that location.

Surface Flow Visualization

Surface flow visualization is realized by applying strings, liquids, or coatings to the surface of an object. It reveals the flow streamlines at the limit as the surface is approached. Surface flow visualization can also reveal separation points and boundary layer regime transition points.

In surface oil flow visualization oil treated with a dye is applied in small dabs at some point upstream of the location of interest. Different colored dyes can be used to aid in the visualization of flows. Oil surface visualization will indicate points of flow separation since the oil cannot pass through the separation point. They can also indicate the transition between turbulent and laminar when special oils are applied because the variation in skin friction between laminar and turbulent allows the oil downstream of the transition point to be swept away. Oil flow visualization is primarily used in wind tunnel tests. Some skill and experience is required to properly place the oil dabs.

In this flow visualization created by NASA tufts and oil are used to visualize the flow around and through this engine inlet.

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The emitted fluid technique involves secreting a special chemical from small diameter holes on the surface. The fluid is secreted and allowed to evaporate while at operating temperature. This method can be used to visualize streamlines and areas of separated flows.

Another surface flow visualization technique is to apply tufts, small lengths of string that are frayed at the end, to various points on the surface. If tufts are applied to the entire surface they can indicate regions of cross flow, reverse flow, or flow separation when photographed. When recorded by video they can also indicate regions of unsteady flow. Tufts are relatively cheap and easy to install with little experience.

Flow visualization using temperature sensitive paints and pressure sensitive paint utilizes a material that is painted onto the surface that will react to temperature and pressure respectively. TSPs can be used to indicate the transition between laminar and turbulent flow because convective heat transfer is much higher in turbulent flow than laminar. TSPs will also provide information about the surface temperature distribution. PSPs provide information about the pressures applied to the surface. PSPs are primarily used with transonic and supersonic flows. PSPs can provide information about shocks and pressure gradients.

This figure shows comparison of the pressure gradient results between PSP (right side) and CFD (left side).

Infrared imaging utilizes an infrared temperature sensor that is used to locate the boundary layer flow regime transition. The turbulent regime has higher heat convection and the surface downstream of the transition will be warmer.

Subliming chemicals can be used to indicate the boundary layer transition. In the turbulent region of flow higher heat convection and larger shear forces at the surface cause the chemical to sublime faster than the laminar region leaving no chemicals in the turbulent region.

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Free-Surface Flow Visualization

Free-Surface flow visualization techniques are primarily used to investigate the properties of a flow outside of the boundary layer. Free-Surface flow experiments are typically conducted in wind or water tunnels as well as in operation conditions. Free-Surface flows can be used to realize vortical flow, flow separation, and boundary layers.

Smoke visualization is a tracer method in which the model is set up in a wind tunnel and dense white smoke is emitted from small pipes in front of the model. These streaklines can then be photographed and uploaded to a computer to be analyzed. Smoke visualization can be used up to flow speeds in the supersonic region. Developments in smoke deployment and lighting ensure that smoke flow visualization will continue to see use. However this method can be costly and the setup is time consuming.

Dye visualization with dyes in liquids is analogous to smoke visualization in air. A model is submerged in a water flow, and dye is injected from nozzles suspended in front of the model or surface. Dyes should have similar densities, high contrast, and stable diffusion conditions. When testing in a closed circuit flow the water will become increasingly clouded as time progresses. An interesting solution to this problem is the creation of a temporary dye by using laser light to induce electrolytic and photochemical reactions along a specific line in the fluid flow. After the fluid flows through the laser a photochemical reaction takes place and the reacted fluid is blue and just a second or two later the fluid returns to being transparent.

The figure above shows dye visualization in a water tunnel. The different colored lines represent various streaklines.

Tracer particle visualization is used in conjunction with analytical methods to achieve flow visualization. Tracer particles can be gas, liquids, or solids they typically range in size from 0.1 to 20 microns. Tracer particles should be light, not corrosive, nor toxic, and they should reflect light well. The particles are imaged at specific intervals and analyzed using Particle Image Velocimetry. PIV systems with advanced lighting and imaging technology can be designed to give real time instantaneous velocity in a cross section. PIV can be utilized in up to three dimensions by using stereoscopic imaging. Stereoscopic imaging uses two images from different angles to measure the velocity vector in three dimensions.

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The figure above shows several sequential images of an airfoil that is impulsively moved from right to left. Theses snapshots are from a video of a PIV analysis of one of Prandtl’s original flow films.

In gas bubble visualization tracer bubbles are injected into the flow or generated by electrolysis. In water these gas bubbles should have a specific gravity less than 1. When air is the flowing fluid the gas should have a similar specific gravity to air. This method limited to higher velocities and laminar flows.

Natural condensation flow visualization can sometimes be used to visualize areas of vortical flow, expansion, shock waves, attached flows, and separated flows. These flow visualizations usually occur on aircraft in flight. At high velocities and low pressures the relative local humidity rises and can cause vapor in the air to condense and become visible.

Due to the unreliability of natural condensation smoke generators are employed mainly to visualize flow vortex flow. Smoke is emitted upstream near the origin of the vortex. The smoke is entrained in the vortex permitting a view of the vortex path.

Optical Flow Visualization

Optical flow visualization utilizes the change in refractive index with respect to density to visualize the flow. Optical flow visualizations are direct flow visualization methods, because the image creation does not disturb the flow.

Schlieren flow visualization is used to image in-homogenous zones in a fluid flow. The refractive index of most gases is a function of density. Light is deflected by refractive index gradient; this light is then focused using lenses and compared to un-deflected light on a viewing screen. The image produced is an intensity representation of the expansions and compressions that characterize the flow. The Schlieren technique can be used to visualize shock waves.

Shadowgraph flow visualization is similar to the Schlieren method. Both methods produce an intensity representation of the flow density. Shadowgraphy produces a mere shadow of the flow non-uniformities. As the distance between the invisible disturbance and the imaging screen increases the

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accuracy of the shadowgram decreases. That is to say the image is out of focus, unlike the Schlieren method which produces a focused image. The advantage of the shadowgraph comes with its simplistic setup. Shadowgraphy is used in supersonic flows to visualize shocks, as well as in natural convection studies.

This figure is Ernst Mach’s shadowgram of a bullet in supersonic flight.

Interferometry is based on the principles of interference. Interferometry uses a beam of monochromatic light that is split into two beams. One beam, the test beam, passes through the test section and the other beam, the reference beam, passes through a compensating section. The beams are then recombined and their superposition produces an interference pattern. The light ray will deflect vertically an amount dependent on the refractive index gradient. Interferometry optical and mechanical equipment requires a high degree of perfection. This necessity for precision makes this method increasingly expensive the larger the test section.

This figure shows the typical results of the Mach-Zehnder interferometry. Large phase shifts are seen around the center of the candle flame visualization (right).

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Computational Flow Visualization

Computational flow visualization is a visualization method that is created from data sets recorded by computational fluid dynamic simulations. As CFD datasets grow it is increasingly important that engineers have variety of flow visualization tools to carry out their analysis. CFD produces large amounts of data containing the properties of fluid flow in Eulerian form. Computational flow visualization transforms this data from Eulerian form into various forms of Lagrangian data depending on the phenomenon of interest.

The figure on the left shows a computational visualization of the pressure gradients in the fluid flow. The figure to the right shows a computational visualization of the airflow over a boat.

Conclusion

“A picture is worth a thousand words” is a phrase that also applies fluid dynamics. Flow visualization encompasses all the techniques used to transform a fluid flow into images that can be interpreted both qualitatively and quantitatively. With the trend in computational power of computers and modern advances in lighting and image recording, the future of fluid dynamics promises increasingly automated analysis of flow visualization and extraction of quantitative and qualitative information.

References

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1. Flow Visualisation Techniques in Wind Tunnels. Slavica Ristić, PhD

2. Leonardo’s Vision of Flow Visualization. M. Gharbi, D. Kremers, M.M. Kooches, M. Kemp

3. Note on the History of the Reynolds Number. N. Rott, Annual Reviews Fluid Mechanics

4. Surface Measurement Techniques Temperature and Pressure Sensitive Paints. J. Sullivan, T. Liu

5. Fluid Mechanics Fundamentals and Applications. Y. Cengel, J. Cimbala

6. http://www.holomap.com/hpiv.htm

7. http://www.velocimetry.net/stereo_principles.htm

8. http://www.nasa.gov/centers/dryden/pdf/88145main_H-1524.pdf

9. http://ftp.rta.nato.int/public//PubFulltext/RTO/EN/RTO-EN-006///EN-006-05.pdf

10. http://www.grc.nasa.gov/WWW/k-12/airplane/tunvoil.html

11. http://pixel.ecn.purdue.edu:8080/projects/ITRweb/

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