Figure 1
shows the PIV system’s operating sequence.
Flow Visualization is essential for exploring, examining, and understanding fluid behavior and can be both qualitative and quantitative. Thermochromic Liquid Crystals, TLCs, are a substance that changes color when applied to a heated surface. Each color corresponds to a unique surface temperature. Particle Image Velocimetry, PIV, is a system that measures airflow velocity by tracking particle displacement with respect to time. In this study, TLCs and PIV are two methods used to visualize airflow over selected airfoils. A wind tunnel was modified by attaching a liquid crystal surface perpendicular to the airflow direction through the tunnel. The TLCs are used to visualize the surface flow patterns. The surface was heated at discrete points using a pin-finned heat sink mounted behind the surface. Teardrop shaped thermal wakes indicate the flow direction across the surface. The PIV system is then used to record flow patterns that occur away from and perpendicular to the surface. This presentation will highlight the results obtained from these two visualization methods, which together provide a quantitative and complete picture of the flow field around an airfoil.
Introduction:
Juncture flow significantly affects numerous engineering applications. It is used to model and examine fluid flow and erosion patterns along riverbeds and their junctures with bridge piers, which is a great cause of catastrophic bridge failure. Juncture flow is also used to examine airflow behavior around aircraft, specifically in the region between the wing and fuselage. At supersonic speeds, airflow patterns become turbulent and complex. Juncture flow observations help reduce this chaos and complexity so that designs could be developed to resist mechanical failure in aircraft when enduring great turbulence and high speeds. Other important junctures in mechanical applications include conning tower-body junctures on submarines and blade-rotor junctures in turbomachinery.
In addition to aerospace applications, juncture flow plays a significant role in biomedical engineering applications. Three-dimensional flow visualization techniques are necessary to examine blood flow patterns through blood vessels.
Over 500 years ago, Leonardo da Vinci began examining fluid motion. This interest is evident in his renderings of eddying motion in a fluid. Since then, more advanced qualitative flow visualization techniques have been developed, which use smoke, strings, oil, and hydrogen bubbles.
The purpose of this project is to develop and compare two flow visualization methods, Liquid Crystal Thermography and Particle Image Velocimetry, for airflow across surfaces as well as at the junctures between these surfaces and airfoils or obstructions. The Liquid Crystal System will provide qualitative information on the surface flow field and the PIV system will provide both quantitative results away from and at the wall or flat surface.
The PIV system is a great way to acquire qualitative images, which can then be analyzed quantitatively. It can be used to examine various dimensions of a flow field. TSI Incorporated’s PIV system was used for this project.
Liquid crystals were previously used solely in heat transfer applications, however, recently, Batchelder, & Moffat (1998) began using them for flow visualization. A unit with a liquid crystal surface is attached to the side of a wind tunnel. A flexible heater is used to heat the base of pin-finned heat sink. The heat sink’s fins are placed against the liquid crystal surface, so that when heated, discrete, colored points appear across the surface. The thermal wakes around these points indicate airflow direction. Lussier and Anderson (1999) used a similar technique for a flow visualization method.
Overall, this report presents the Liquid Crystal Thermography and Particle Image Velocimetry study’s results. The remaining parts of the report contain background information on materials and equipment used in the experiment, procedural descriptions as well as a general summary and references. The Background section in this report reviews the use and brief history of PIV and TLCs and also presents considerations for the heat sink and encasement design. Computer-aided drawings of these designs, the heat sink, balsa encasement pieces, and Plexiglas wind tunnel attachment, are contained in Appendix A. The subsequent section describes the design and construction of a new test plate. A section that describes the test facility and equipment follows this.
Background:
Particle Image Velocimetry (PIV) System:
Flow visualization techniques have traditionally been strictly qualitative. Particle Image Velocimetry systems are much more accurate than these traditional methods. For this project, a PIV system from TSI Incorporated is used to measure a particle’s velocity by measuring its displacement over a time interval that is designated using a pulsed laser. This laser illuminates the airflow particles. A digital camera records the images. An instant later, a second image is acquired. Thus, two parameters, particle displacement between the two acquired images and the time interval between the moments when each image is acquired, are used to determine particle velocity, its displacement distance divided by its displacement time. Vectors are generated to demonstrate these determined displacement patterns. Furthermore, velocity data acquired from these images can be smoothed for analysis. Flow properties such as vorticity and turbulence can be analyzed and shown using PIV. It performs this analysis for several points, simultaneously. PIV can be used to solve design problems and perform research in combustion, aircraft design, turbulent channel flow, weather pattern simulation, erosion effects, turbomachinery,as well as natural and forced convection. A schematic of this image acquisition and analysis process is discussed later in the PIV Technique section of this report.
Point Measurement Technique:
PIV generates velocity vectors between two points in a flow field by measuring particles’ displacements within a small region in the flow field. Images can then be adjusted to highlight certain flow field characteristics. Vectors can be sorted according to size and color to indicate speed differences at various points in the flow field. The system acquires two images, and then, within a small area in the flow field, identifies particle displacement by determining identical patterns between the two images, and thus drawing vectors between particles in their former and latter positions. This technique is much more accurate and visually appealing than traditional qualitative and quantitative flow visualization measurement methods. PIV can be used to measure a flow field’s properties such as velocity, mean velocity, velocity standard deviation, turbulence, and vorticity.
Vorticity:
Vorticity is the measure of particle rotation in a flow field, or degree of swirling. It is quite difficult to see without PIV, since it is found only by differentiating a flow field, which requires highly accurate point displacement measurements. Like velocity, vorticity measurements can be sorted by color to emphasize vortices’ rotational direction and magnitude within flow field.
PIV Technique:
The PIV system operates in three steps.
Figure 1
shows the PIV system’s operating sequence.
The PIV system’s laser and digital camera are first synchronized. The laser illuminates the airflow particles. In this project, fog vapor is used. The digital camera then photographs the airflow at two subsequent instances. These instances are denoted by images A and B, which are images of airflow patterns generated using a swirler. The researcher could adjust time intervals between images A and B. Images A and B are analyzed using Insight, the PIV system’s image processing software, which generates the image with the vectors. The vectors denote the particles’ displacements from image A to their respective locations in image B. This image is then analyzed using TECPLOT, another processing software package, and produces a colorful image with vectors, which shows both particle displacement and speed. Jean-Marc Gourlet acquired these PIV images of swirled airflow patterns.
Overall, the PIV system is used to generate more accurate velocity measurements over a relatively large flow field.
Acquiring flow field image:
Fog Fluid is used to create small particles suspended in airflow. Particles provide identification points. A laser, pulsed twice, is used to illuminate flow particles instantaneously. A digital camera used to acquire the flow image is placed perpendicular to the laser beam. The first laser pulse illuminates the flow particles and their positions are recorded onto an image acquired by the digital camera. The particles continue to move within the airflow. A second pulse illuminates the particles and a second image is acquired, recording the new positions of the particles. To obtain multiple images, the laser must be pulsed several additional times. The images are then analyzed using Insight, which determines velocity by identifying similar patterns in the flow field between the two images and then drawing vectors between common particles between the two images. The researcher can adjust the time interval between the two laser pulses and image acquisitions. The program considers rotational shifts when determining common patterns; turbulence often induces such rotational flow shift. Rotating mirrors are sometimes used to implement image shift to compensate for flow shifts.
Guidelines for Good PIV Results:
As many particles as possible visible within a flow field will generate more comparison points and thus more accurate results.
Analysis:
The two acquired images are visible to allow the researcher to examine the concentration of particles, their size, and background noise levels so that adjustments could be made to either minimize or maximize these characteristics.
Displaying the Vector Field:
The vectors’ arrow lengths demonstrate particle speed and direction. Using the PIV software, Insight, the researcher can edit the data and filter out certain vector magnitude values and enhance the image. Furthermore, the researcher can adjust the acceptable noise level to filter inaccurate vector measurements, or "bad vectors" in the results.
Conclusions:
PIV generates flow field information more accurately than any other flow visualization techniques. With the ability to capture an image instantaneously, PIV can be used to show various flow field characteristics such as vorticity and turbulence. PIV’s accuracy stems from its capability to subtract mean velocity from each individual velocity measurements, represented by a vector, to show differences in the flow fields.
Thermochromic Liquid Crystals:
Thermochromic Liquid Crystals were first created roughly 30 years ago. In Farina (3) a liquid crystal is described as "a thermodynamic phase that is between the pure solid and pure liquid phases of matter and exists in some organic compounds under certain conditions". They are a rather inexpensive way to demonstrate and measure heat flow across a surface and are commonly used in electronics thermal management, gas turbine heat transfer, boiling heat transfer, and fluid temperature measurement.
Although several types are available, in this project, the sprayable, microencapsulated R25C15W TLCs were used to display locations of heated points on a polyester surface. Encapsulation in these 5-10 micron diameter capsules protects the material from contamination, ultra-violet radiation, and thus prolongs its utility. TLCs react almost instantaneously, providing rapid, precise, and reproducible temperature range measurements at relatively low cost. For this project, the liquid crystals used were suspended in a water-based material, which was sprayed onto the bar’s top surface using an artist’s airbrush.
The TLC notation stands for an activation temperature of 25oC and a 15oC temperature range. Below its event temperature, a liquid crystal is solid and transparent. Due to its chemical makeup, at its event temperature, however, it reflects a particular wavelength of visible light, which produces a particular color. As its temperature rises, its color will continue to change; each temperature point in its bandwidth corresponds to a unique color. When the temperature finally is greater than its clearing point temperature, or the upper limit of its bandwidth, it will become purely liquid and become clear again. Since this colorful progression occurs throughout temperature changes, it will occur whether the liquid crystals are being heated or cooled. Reflected colors typically range between red and blue, which correspond to long and short wavelengths, respectively. Since TLCs also reflect some "incident" light, by placing a heated TLC coated surface on a black background, only those colors particular to a specific temperature appear.
Using calibrated TLCs, temperature visualization is facilitated both qualitatively and quantitatively, unlike most solely qualitative methods.
Data obtained from indicated color changes as well as visual observations make liquid crystal thermography a simple temperature visualization method. This method is rather easy and inexpensive to execute and allows for high spatial resolution for surface images. Since the TLCs respond instantaneously to temperature fluctuations, they are ideal ways of quickly determining thermal patterns on a surface due to airflow rates or directions. They are also widely used in cooling models for electronics.
Methods using TLCs with narrow bandwidths, or those that respond to a slim temperature range, are accurate ways of determining temperature distribution on a surface on surfaces, which are ideally isothermal, or contain constant temperature values across the surface. Such TLCs with narrow bandwidths are excellent for models used with digital imaging software since they are highly responsive to slight temperature fluctuations. Wide-band TLCs however are used when a surface contains very large temperature differences.
The imaging software used in conjunction with liquid crystal thermography functions much like the human eye, which separates all colors into different amounts and combinations of red, green, and blue. Software and machines that mimic this method are not quite time efficient, since TLC imaging software must analyze colors at a great number of points within an indicated region. Recent developments, however, have generated methods of this image analysis using a similar method computationally efficiently.
Prior to using TLCs, they must be calibrated to use to determine color response characteristics properly and accurately; the corresponding color and temperature values must be known prior to beginning the experiment.
In addition to these benefits associated with TLCs, there are some sources of error that must be considered and avoided diligently. Several sources of error, such as improper TLC or surface preparation, improper TLC calibration, equipment error, and human error produce inaccurate temperature measurements that have make quantitative analysis much more difficult.
The digital image acquisition system for this project contains a test plate with a liquid crystal surface and a digital image processing system, which consists of a digital camera, code converter, microprocessor, display monitor, and printer. Figure 2 is a schematic of the system. The camera and code converter decompose the image into three colors, red, green, and blue. It then sends a signal to the microprocessor, where it is analyzed and displayed on the monitor. Since these values are sensitive to light value, intensity and direction, they can be used to characterize an image entirely. The liquid crystals must also be calibrated prior to experimental use. The test plate and calibration piece should be calibrated simultaneously to preserve consistency; slight nuances in light greatly affect the liquid crystals perceived color. Ideally, calibration should be done in an environment as dark as possible. Furthermore, the black paint below the applied liquid crystal surface minimizes the amount of light reflected off the specimen’s surface.
Figure 2 shows a schematic of the digital image acquisition system, courtesy of Brett Lussier (1999).
Image Acquisition:
The wind tunnel is run and set to a preferred velocity. The DC power supply is also turned on and then flexible heater’s input power level is also set. Once the liquid crystal surface begins reacting to the heat and airflow, images should then be acquired. If the surface becomes black, its temperature is too high. Images are then analyzed using software such as TempVIEW, which can be used both for calibrating and analyzing acquired images. TempVIEW analyzes color values and assigns temperature values to appropriate colors. Using Transform, another software package, these temperature values are then analyzed and plotted as a colorful temperature distribution. Figure 3 shows an image of a pin-fin heated liquid crystal surface acquired using this technique.
Figure 3 show both the acquired image and processed, analyzed images, courtesy of Brett Lussier (1999).
The upper left image shows the heated liquid crystal surface and thermal wakes. The image below it is the same image however after having been processed and analyzed using a color and temperature distribution relation. This image was then enhanced; it is shown in the lower right.
Wind Tunnel and Plexiglas test chamber.
The wind tunnel used in this project’s air speed is adjustable. The removable Plexiglas test chamber contains an attachment piece, where the heat sink and encasement unit is attached. The chamber’s cross-section is roughly 13.75" x 6". Figure 4 shows the wind tunnel and test chamber.
Figure 4 shows the wind tunnel and test chamber.
The digital image acquisition system is beside the wind tunnel.
Another goal for this part of the project was to redesign and construct a new heat sink and encasement that could be fastened permanently to the side of the wind tunnel, unlike the previous design which was taped to its side. The researcher’s specific goals were to design a heat sink whose pin-fins were closer than the previous student’s design. Thus, the new heat sink was designed with one-third less space between each pin and therefore a much denser array of pin-fin points touching the liquid crystal surface. This would generate much more accurate flow field measurements and images.
The greatest priorities considered for this heat sink and encasement design were permanence, low weight, insulation, replaceable materials, and low cost. Initially, several options were considered for the heat sink and its encasement:
Procedures:
See Appendices A and B.
Summary:
Overall, flow visualization has been popular for centuries. It techniques have progressed from naked eye observations and penciled renderings to computer aided digitally acquired images. Not only does this project demonstrate the importance of the development of quantitative flow visualization techniques, in addition to previously used, solely qualitative methods, but it also aims to compare two relatively new, accurate techniques.
References:
1. Bjorkquist, Daniel C. & Fingerson, L.M. "Particle Image Velocimetry.",
2. Batchelder, K. A. & R. J. Moffat, "Surface Flow Visualization Using the Thermal Wakes of Small Heated Spots." Experiments in Fluids 25, Springer-Verlag, 1998. 104-107.
3. Farina, D. J., "Introduction to Liquid Crystal Thermography." Image Therm Engineering, Waltham, MA.
4. Lussier, Brett & Anderson, Ann, "Surface Flow Visualization and Heat Transfer Measurement Using Liquid Crystals." Union College, Schenectady, NY, 1999.
5. Post, Martiqua, "The Effect of Turbulence on Thermal Influence Factors Using Liquid Crystal Image Processing"
6. http://es.epa.gov/ncerqa_abstracts/sbir/97/pollpre1/bilanin.html
7. http://es.epa.gov/ncerqa_abstracts/sbir/97/pollpre1/bilanin.html
8. http://www.tsi.com/fluid/piv_cat/overview.htm
Appendices
(Pictures will be added in the near future)
Appendix A:
This section contains all computer-aided drawings for the heat sink, its encasement, and the Plexiglas attachment to the wind tunnel.
Appendix B:
This section contains descriptions for preparing and constructing the heat sink, heat sink test pieces using two different manufacturing techniques, balsa blocks for the heat sink’s encasement, Plexiglas attachment to the wind tunnel, test piece and encasement unit, liquid crystal surface, and finally running the tests using the two test pieces.
Appendix B:
Project Materials:
Block of 6061 Aluminum (enough to cut at least one 4" x 4" x .5" piece, and at least two 1" x 1" x .5" pieces), 250 .05" diameter and 2" length 6061 Aluminum pins, Balsa wood (enough to cut two BALSA SIZES HERE, etc), four (SIZE) screws, two 13.75" x 13" x .75" pieces of Plexiglas, one flexible heater, one 30 gram bottle water soluble black paint, one 30 gram bottle R25W5 thermochromic liquid crystals, TLC application set, artist’s airbrush, clear tape, heat gun, 5" x 5" polyester sheet.
Construction:
I. Heat Sink.
If not, apply liquid crystals again until surface color intensity is uniform.
To compare manufacturing methods such as Shrink Fitting and using Locktight, the researcher first obtained two 1" x 1" x .5" test pieces, each made by one of the two methods, and tested the pieces using the following testing method: