Applications

PIV Applications

1. Swirling flows, both reacting and non-reacting, occur in a wide range of applications such as gas turbine combustors used for aero and marine applications, burners, chemical processing plants, rotary kilns and spray dryers. Swirling jets are often used for flame stabilization in combustion chambers. The application of swirl to injected air and/or fuel has strong favorable effects on the flow field.

The effect of degree of swirl on confined flows was experimentally investigated for medium and high degrees of swirl (S = 0.52 and S = 1.01). Two test conditions were analyzed; i.e., the pressure drop across the swirler was maintained as 0.1 bar and 0.05 bar. The central region in the mean vorticity field resembles a solid body rotation and outside this, there is a region of negative vorticity for the swirler of vane angle 30°.

The instantaneous vorticity patches are localized at the centre, and these patches merge. On the other hand, the mean vorticity field at the near downstream of the swirler of vane angle 48°, has two circular bands of positive and negative vorticity which are distributed concentrically. The strength of the inner zone of vorticity (anticlockwise) field is observed to be greater than of the outer zone of vorticity (clockwise) field. For high degree of swirl, the mean vorticity contours expanded rapidly and the instantaneous vorticity patches are delocalized from the centre of the swirler. This was due to the high jet divergence caused by the high degree of swirl flow and also due to the inlet velocity at the upstream of the swirler for this case. The precessing vortex core was observed for the swirler of vane angle 30° at a longer axial distance downstream of the swirler exit as compared to the swirler of vane angle 48°.

High of swirl aids better level of mixing compared to medium degree of swirl because in high degree of swirl, micro-level mixing is achieved between fuel and air due to the delocalized vorticity patches.

The effect of flare geometry on swirl flow characteristics was experimentally investigated by comparing the flow through a swirler mounted on a flare with and without through holes, with a base line configuration of a sudden expansion geometry.

Two test conditions were analyzed; i.e., the pressure drop across the swirler is maintained as 0.1 bar and 0.05 bar. For unperforated sudden expansion case, the CTRZ has a jagged shape due to a highly non-uniform inlet velocity profile; however, for the unperforated smooth expansion case, the shape of the CTRZ is smoother. In the case of perforated flare, the holes in the flare also play an important role in making CTRZ further smoother than unperforated smooth expansion case. The flare leads to the generation of more vorticity patches and keeps them at the centre. The holes present on the flare were contributing to more vorticity patches at the centre than in the unperforated smooth expansion case. The vorticity patches are concentrated at the centre due to the confining effect of the secondary corner recirculation zone on the swirl flow. The width of the CTRZ was more for the perforated smooth expansion case as compared to the unperforated smooth and sudden expansion cases.

Perforated smooth expansion is better than the other two cases. Here the flare and the holes in the flare help in getting a smoother CTRZ which is important in producing higher values of vorticity at the centre. Although the perforations are provided in the flare of real combustor hardware for purpose of thermal management, it is observed here that they promote better fuel-air mixing due to the presence of the greater number of vorticity patches concentrated in the core.

1. The laser used in the present study is a Nd-YAG twin laser system (Big Sky Laser of Quantel, France Inc., make). The operating wavelength is 532 nm, with energy of 30 mJ per pulse. The repetition rate of a Nd-YAG laser is typically 10 – 20 Hz,

2. An oil seeder generator implementing the Laskin nozzles

3. A full frame interline transfer digital CCD cameras (SensiCam, PCO Imaging, Germany Inc., make with resolution 1024 x 1280 Pixels and PixelFly model, PCO Imaging, Germany Inc., make with resolution of 1360 ´ 1024 pixels.

4. The laser and the camera are synchronized with a sequencer (SequencerV8.0 HardSoft, Germany Inc., make). Seco2004, has been used as an interface program to generate the TTL pulses from the sequencer in a required sequence.

5. Group: Combustion Flow Diagnostics Laboratory, Department of Aerospace Engineering, IIT Madras.

6. Supervisors: Dr. R.I. Sujith and Dr. S.R. Chakravarthy

7. Student: Arun Raj E, MS Research Student (2004-2006)

Downloads of Raw PIV datas

" 30degree swirler-20mmdownstream-0.1bar "

" 30degree swirler-30mmdownstream-0.1bar "

" 30degree swirler-40mmdownstream-0.1bar "

"30degree swirler-50mmdownstream-0.1bar "

"30degree swirler-100mmdownstream-0.1bar "

"30degree swirler-0 to 100 mm downstream of the swirler-0.1bar "

"48degree swirler-20 mm downstream of the swirler-0.1bar "

"48degree swirler-30 mm downstream of the swirler-0.1bar "

"48degree swirler-40 mm downstream of the swirler-0.1bar "

"48degree swirler-50 mm downstream of the swirler-0.1bar "

"48degree swirler-100 mm downstream of the swirler-0.1bar "

2. Flow Past An Impulsively Started Oscillating Elliptical Cylinder

Over the past two decades, the advancement of non-intrusive experimental techniques and DNS solvers has given scientists the impetus to undertake the study of unsteady aerodynamic problems with newfound resolve. Peculiar to this class of flows is the topic of oscillating aerofoils. Traditionally one which has been applied to flutter analysis, gust response and regarded as a possible avenue of alternative thrust production, this field has incited renewed interest due to its applications to aircraft maneuverability and relevance to insect flight.

The focus of the current study is on the flow past an impulsively started elliptical cylinder subjected to a sinusoidal oscillation at a nominal Reynolds number of 1000. Such a configuration is aimed at mimicking the twisting motion of an insect wing, one of the basic elements of insect flight. Whilst attempts have been made at producing dynamically scaled models to study the motion of such creatures in their entirety, here our emphasis is on examining the artefacts of wing twisting in isolation. Particle Image Velocimetry (PIV) is employed to study the effects of the reduced frequency and angular amplitude on the flow. For the purpose of comparison, the flow past an elliptical cylinder at fixed incidences is also investigated.

Fig. Flow Field of an Oscillating Elliptic Wing (DPIV) (Chng, Lim, et al. 15 AFMC Re=1000, k=0.5)

A preliminary study of an impulsively started elliptical cylinder in oscillatory motion has been conducted. The results show that the ratio of the aerofoil tip velocity to that of the freestream is the main parameter which governs the flow. When this ratio is small, as in the reduced frequency case of k=0.1, the flow continues to display characteristics reminiscent of an aerofoil in static stall, albeit with a delay in the separation at the leading edge during the pitch up motion. This brings to mind the classical, linearized theory of oscillating aerofoils which predicts that the solutions of an aerofoil conducting small amplitude oscillations are merely functions of the static cases with a certain phase difference.

When this ratio is large, the flow is dominated by the pitching motions of the aerofoil and characterized by the formation of a reverse Von Karman vortex street. Although the reduced frequency provides a reasonable estimate of this ratio, it is proposed that a more complete representation should include the angular amplitude of the oscillation, as well as the location of the pitching axis.

Reference paper: T.L. Chng, T. T. Lim, J. Soria, K. B. Lua and K. S. Yeo, Flow Past An Impulsively Started Oscillating Elliptical Cylinder, 15th Australasian Fluid Mechanics Conference, 13-17 December 2004.

1. A Nd:YAG laser capable of generating 2x300mJ pulses of 5ns duration is used as the illumination source, and produces a uniform light sheet of 2mm thickness.

2. Polyamide particles with a mean diameter of 20µm are used to seed the flow.

3. A full frame interline transfer digital CCD cameras ( PixelFly model, PCO Imaging, Germany Inc., make with resolution of 1360 x 1024 pixels).

4. Group: Fluid Mechanics Laboratory, Department of Mechanical Engineeirng, NUS, Singapore.

5. Supervisors: Dr. T. T. Lim

6. Student: T.L. Chng, Master Student

3. Flow investigation using the classical PIV

Particle Image Velocimetry (PIV) is a very common and useful tool to investigate the flow phenomena in models of blood vessels, heart valves or artificial organs. A 2D high-resolution PIV system has been developed, setup and successfully tested. The system contains a high-speed video camera (512 × 480 pixel at 250 fps, 512 × 240 pixel at 500 fps) for the image capture and a continuous laser light source (? = 682 nm, 75 mW) with an adapted light sheet optics for the illumination of the flow. After recording the images are stored to the hard disk drive of a PC and can be analyzed using the cross correlation technique. The software used for the analysis is Davis by LaVision, Göttingen. The system is easy to set up since no synchronisation between the light source and the camera is necessary. The system is very useful for hydromechanics, where the velocities are small compared to aerodynamics.

Flow in an artificial heart assist system

Analyzed was the flow in a blood chamber of a new heart assist system. A movie of three images shows the diastole (inflow), the flow between diastole and systole and systole (outflow).

Development of a new measurement technique for the investigation of the wall flow

A new method for the spatial and temporal assessment of the wall shear stress is currently being developed at the Biofluid Mechanics Lab. This method can be considered a special development of the classical PIV. It permits to look selectively at the flow close to the wall. The selection is made by using a fluid, which does not permit the light to penetrate deeply into the flow. The transparent flow model is illuminated by a monochromatic diffuse light source. Due to the limited penetration depth of the light only the particles moving close to the wall are lighted. Within the illuminated layer, the particles appear more or less bright, depending of their distance dp to the wall. A gray value analysis with a special image processing program permits to determine this distance, which is necessary for the calculation of the wall shear stress.

The method was tested to assess the laminar flow in a rectangular U-Duct with a backward-facing step at Re = 50. The following image shows the velocity field immediately after the step at a distance dp = 0.2 mm from the wall duct.

Flow at the wall in a rectangular U-Duct with a backward-facing step.

Group: Biofluid Mechanics Lab, Department of Cardiovascular Surgery, Charité - Universitätsmedizin Berlin.

4. Two-Phase Particle Image Velocimetry (PIV)

One of the most challenging problems in the area of dilute two-phase flows is trying to predict and understand the interaction of particles with a turbulent carrier fluid. Although much work has been done in this area through the use of numerical simulation, there are many unanswered questions that require experimental observation.

Along these lines, instrumentation needs to be developed which will allow detailed measurements of the spatially resolved, instantaneous velocity field of both phases, as well as the concentration of the dispersed phase. Previous work has been done which provides a statistical single-point description, and conditional averaging of coherent structures has allowed for a view into the structure of well-organized flows, but a method for examining the interaction a the smaller scales of the flow remains is still in its infancy.

Research at our two-phase laboratory is currently working on extending single-phase Particle Image Velocimetry techniques to measure quantities important to understanding dilute two-phase coupling within turbulent flows. These techniques utilize high-speed imaging, scanning light sheets and advanced image processing for discrimination and quantification of the separate phases. The current method we have developed uses image pairs from a single camera and a median filter to separate the information of the two phases. Once the images have been separated, standard cross-correlation PIV methods are used to extract the carrier phase motion, and particle tracking methods are used to determine the location and displacements of the dispersed phase. The information is then recombined to provide a simultaneous, spatially resolved description of the coupled particle/fluid motion.

This method works well in two-phase flows where there is a distinct size difference within the image between the dispersed phase and the tracers particles used to track the carrier fluid motion. Details on the method and its validation can be found in the article:

Kiger, K. T. and Pan, C., PIV technique for simultaneous measurement of dilute two-phase flows, Journal of Fluids Engineering, 122(4), pp 811-818, 2000.

Experimental Setup

The experimental test cases for the technique all examine heavyparticle sedimentation in the turbulent wake of a cylinder. A vertical,recirculating water channel with a 100 mm by 100mm square cross-section and a maximum velocity of 100 mm/s is used to conduct the tests. The images are acquired 25 mm downstream of a 12 mm diameter cylinder, at a Reynolds number of 840 based on the cylinder diameter. The test section is illuminated by a high speed pulsed Nd:YAG laser (wavelength~532 nm) with a pulse intensity of 15 mJ focused to a sheet of width dz;0.8 mm. The image area is approximately 27mm x 27 mm, and recorded using a Kodak Megaplus ES1.0 camera (1008 x 1018 pixel array; 9 mm pixel spacing) with a 200 mm focal length lens ( f#=8) and a time separation of 3 ms between image pairs. Hollow silver-coated glass spheres with an average diameter of 15 mm, and a specific gravity around 1.5 are seeded as PIV tracer particles in the flow.

Group: Associate Professor Ken Kiger, Department of Mechanical Engineering, University of Maryland.

5. Universal Time Scale for Vortex Ring Formation

The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using Digital Particle Image Velocimetry (DPIV) for a wide range of piston stroke to diameter (L/D) ratios.

_____L/D =2________________________L/D=14______

The results indicate that the flow field generated by large L/D consists of a leading vortex ring followed by a trailing jet. The vorticity field of the formed leading vortex ring is disconnected from that of the trailing jet. On the other hand, flow fields generated by small stroke ratios show only a single vortex ring. The transition between these two distinct states is observed to occur at a stroke ratio of approximately 4, which, in this paper, is referred to as the "formation number". In all cases, the maximum circulation that a vortex ring can attain during its formation is reached at this non-dimensional time or "formation number". The universality of this number was tested by generating vortex rings with different jet exit diameters and boundaries, as well as with various non-impulsive piston velocities. It is shown that the "formation number" lies in the range of 3.6 - 4.5 for a broad range of flow conditions. An explanation is provided for the existence of the "formation number" based on the Kelvin-Benjamin variational principle for steady axis-touching vortex rings. It is shown that based on the measured impulse, circulation and energy of the observed vortex rings, the Kelvin-Benjamin principle correctly predicts the range of observed "formation numbers".

Group: Wind and Sea, Gharib Research Group, California Insititute of Technology

6. “Top Secret” Technology To Help U.S. Swimmers Trim Times at Beijing Olympics.

Milliseconds can mean the difference between triumph and defeat in the world of Olympic sports, leading more trainers and athletes to look toward technology as a tool to get an edge on the competition. A fluids mechanics professor at Rensselaer Polytechnic Institute in Troy, N.Y., is using experimental flow measurement techniques to help American swimmers sharpen their strokes, shave seconds from their lap times, and race toward a gold medal in Beijing this summer. Professor Timothy Wei, head of Rensselaer’s Department of Mechanical, Aerospace, and Nuclear Engineering and acting dean of the university’s School of Engineering, helped develop top-secret, state-of-the-art equipment and mathematical techniques that USA Swimming coaches have been using to help train Olympians. “This is the real thing,” Wei said. “We have the physical system, we’re taking flow measurements of actual swimmers, and we’re getting more information than anyone has ever had before about swimming and how the swimmer interacts with the water. And so far, these techniques have contributed to some very significant improvements in the lap times of Olympic swimmers.” In years past, swimming coaches have used computer modeling and simulation to hone the techniques of athletes. But Wei developed state-of-the-art water flow diagnostic technologies, modifying and combining force measurement tools developed for aerospace research with a video-based flow measurement technique known as Digital Particle Image Velocimetry (DPIV), in order to create a robust training tool that reports the performance of a swimmer in real-time. “This project moved the swimming world beyond the observational into scientific fact,” said USA Swimming Coach Sean Hutchison. “The knowledge gained gave me the foundation for which every technical stroke change in preparation for the Beijing Olympics was based.”

The secret, Wei said, is in understanding how the water moves. The new system incorporates highly sophisticated mathematics with stop-motion video technology to identify key vortices, pinpoint the movement of the water, and compute how much energy the swimmer exerts. “You have to know the flow,” Wei said. “To see how a swimmer’s motion affects the flow, you need to know how much force the swimmer is producing, and how that force impacts the water.” “Swimming research has strived to understand water flow around a swimmer for decades because how a swimmer’s body moves the surrounding water is everything,” said USA Swimming’s Biomechanics Manager Russell Mark. “The ability to measure flow and forces in a natural and unimpeded environment hasn’t been available until recently, and Dr. Wei’s technology and methods presented USA Swimming with a unique opportunity that United States swimmers and coaches could learn a lot from.” Wei has been working with USA Swimming for several years, but the idea and design of the new flow measurement tool really took shape in 2007. Most of the preliminary tests were conducted in October 2007, and the coaches and swimmers have spent the past several months incorporating what they have learned into their training regimes. For any swimmer, it takes time to make adjustments to their strokes and practice new techniques, Wei said.

One highlight of working on the project was when Mark arranged for Wei to attend the 2007 and 2008 U.S. Summer Nationals and be on deck with the swimmers. “How often does a researcher get to do something like this?” said Wei, whose young son and daughter also swim competitively. “It’s been a journey into a world that someone like me would have never before gotten the privilege to see first-hand.” Wei began his research career as an aeronautical and mechanical engineer, including hydrodynamics research for the U.S. Navy. But lately he has expanded into bio-related research, such as working with a vascular surgeon to study effects of flow over endothelial cells, and partnering with a neurosurgeon to understand the mechanisms behind hydrocephalus, or excess fluid in the brain. As a young researcher, Wei dreamed of measuring flow around swimming whales, but the idea never progressed to fruition. Recently, however, in the midst of his work with USA Swimming, Wei worked with marine biologists Frank Fish and Terrie Williams to measure the flow around swimming bottlenose dolphins at the University of California, Santa Cruz. Wei said he’s confident that the United States will have a strong showing in swimming at the 2008 Olympics in Beijing, and that he’s already thinking of ways to improve his technology to be even more effective when training swimmers to compete in the 2012 London Olympics. “It’s been a wonderful, unique experience,” he said. “It’s everyone’s dream to make a difference, and I’m excited to keep helping the team for as long as they need me.” Wei is also currently working with the U.S. Olympic skeleton team and looking at new flow measurement techniques to help shave precious milliseconds off downhill times. You can see an interview with Tim Wei, and some videos of the swimmers in action:

Group: Professor Timothy Wei, Rensselaer Polytechnic Institute in Troy, N.Y., Head of Department of Mechanical, Aerospace, and Nuclear Engineering and acting dean of the School of Engineering.

7. Measurement of Flow-Induced Vibration of Hard Disk Drives.

Unknown to many users of hard disk drives, fluid mechanics actually has got a lot to do with the drives. Researchers in the field has known for long that the flow induced vibration in the drives is one of the key factors that limit the drives' achievable storage capacity. Due to the high rotational speed of the spindle motors, the airflow in the drivers is highly turbulent and it poses a great challenge to reserachers trying to predict the behaviour of such flow. Numerous numerical schemes have been adopted for the analysis of such flow, for example, RANS (Reynolds Averaged Navier-Strokes Equations) and LES (Large Eddy Simulation). Such schemes, however, remain to be validated by extensive experimental measurements.

Researchers from the Data Storage Institute (DSI) of A*STAR have illustrated some experiments to to characterize the flow induced vibration in computer hard drives using LDA (Laser Doppler Anemometer) and PIV (Particle Image Velocimetry).

Group: Dr Ong Eng Hong (Manager)/ Dr Zhang Qide (Research Scientist) / Dr Yip Teck Hong (Senior Research Fellow), Mechatronics and Recording Channel Divison, DSI, ASTAR, Singapore.

8. High-speed cameras image flow successfully.

by K. Eisele, Z. Zhang, F. Hirt, and N. Perschke

Fast imaging techniques promise low-cost flexible flow visualization with continuous-wave rather than pulsed lasers. The evolution of flow visualization and laser-based particle image velocimetry (LPIV) has led to development of a variety of different hardware and software systems. Several laser anemometry manufacturers offer commercially available LPIV products, while many specialized systems have been developed by researchers at universities, institutes, and industrial laboratories. 1,2 Most of these LPIV systems are based on visible-wavelength pulsed or chopped continuous wave (CW) lasers. For pulsed output, Nd:YAG lasers with double-pulse capability or with double cavities are typical, although, recently, pulsed copper-vapor lasers and laser diodes have also been used.3 The advantage of pulsed lasers is their high pulse energy with very short pulse duration. In the case of CW output, argon-ion lasers are most common and operate in conjunction with electro-optic modulators or mechanical choppers.4 In CW-laser-based systems, though, the argon-ion laser can also be used as a light source for laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The laser light is controlled in such a manner that direct real-time vectorization is possible.5

For widespread use in an industrial environment, a more flexible and efficient approach to particle image velocimetry is required that does not use an expensive high-power laser. We have developed a system based on an ultrahigh-speed gated video camera from Stanford Computer Optics (Palo Alto, CA) in conjunction with a sheet of light from a CW argon-ion laser. The flexible high-speed shutter of the light-amplifying video camera provides the equivalent of the double or multiple laser pulses characteristic of a more-typical LPIV system. The camera and its shutter are computer controlled so the camera can be synchronized with an external event such as a signal from a rotating or oscillating machine. This synchronization means measurements of the phase-averaged-velocity field are simplified because only one device has to be synchronized with an external event. Computer control of the shutter also enables rapid optimization of the various parameters for particle image velocimetry. Both the external synchronization and the fast parameter optimization are important features for efficient industrial use of the system. Also important is that the hardware operates independently of the particle image velocimetry software, which means the system can be applied to both particle tracking and correlation techniques.

Optical configuration

A high-energy pulsed laser is not required in our LPIV system because image capture is based on a video camera operating in conjunction with a CW light source. The main part of the camera is a 25-mm microchannel-plate (MCP) intensifier, which is precisely coupled to the charge-coupled-device CCD and controlled by a microprocessor. The MCP sensitivity can be adjusted between 50 and 500,000 ASA; minimum sensitivity is 1 microlux at normal video mode and increases to 2.5 nanolux with an 8-s integration time. The MCP allows shutter timing from 5 ns to several seconds. The camera operates in several modes. Free-running, it provides only black images if the shutter is closed, while single trigger mode--once per field--enables exposures to be taken only during the frame integration period of the CCD chip. This latter mode is most important for PIV measurements. The camera can also provide a master synchronization signal for experimental timing. In operation, the camera could be software-instructed to operate in field or frame mode. Although the CCD resolution is 752 lines by 582 vertical pixels, the resolution of the camera system is limited to 600 lines by the MCP. European- and US-standard video outputs are also available.
Two different applications serve to illustrate the flexibility of our approach. First, analysis of the unsteady flow field behind a prosthetic bileaflet heart valve has been carried out in a cardiac pulse duplication system in which the mean velocity was about 1 m/s. Second, the unsteady diffuser flow in a radial pump has been studied. The phase-averaged flow field was determined with particle tracking software; the mean velocity of the diffuser flow was 10 m/s.

Studying heart-valve flow

A pulse duplicator testbed that simulates the human heart cycle was used to examine the complex unsteady flow behind a bileaflet prosthetic heart valve (see photo on cover and p. 75). Cardiac output is simulated by a feedback-controlled piston-in-cylinder pump connected to the left heart ventricle, which consists of the aortic and the mitral test sections. Both sections are made from acrylic plastic, and a prosthetic heart valve is placed at the center of each. The contours of the vessels correspond to those of the average human heart.6 Measurement of the phase-averaged flow velocity is facilitated by a pulse generator coupled to the pump. The pulse generator produces an appropriate number of equally spaced pulses over a complete cardiac cycle so that flow images can be captured at specific times during one cycle. The unsteady velocity measurements are conducted in a tuned system, which is defined by the aortic pressure ratio, the flow rate, and the frequency.
A typical particle image at a large opening angle of the aortic position shows how the flow separates at the leading edge of the lower leaflet because of the high angle of attack (see Fig. 1). A large separation zone can be seen below the center line and is characterized by a very small flow speed with high unsteady fluctuations. Classical shear instabilities--which periodically produce large-scale vortices--occur at the trailing edges of both leaflets and at the boundaries of the separation zones. These vortices decay further downstream into small-scale turbulence. The phase-averaged velocity vector field is characterized by numerous critical local flow patterns and clearly shows the large reverse flow in the center. In addition, the three-dimensional nature of the flow field can be seen from the low upstream flow rate caused by the flow reversal in the center, which itself results from the flow separation occurring at the leaflet.

Investigating pump diffuser flow

A test setup representing one stage of a multiple-stage pump was designed in order to collect data on the steady and unsteady flow in a radial pump and its diffuser (see Fig. 2). The LPIV measurements obtained from this setup were used to evaluate numerical flow simulation methods and were compared with detailed LDA measurements in the diffuser.7 Particle tracking velocimetry (PTV) provided images of the velocity vectors in the pump. A comparison of instantaneous PTV particle traces with the phase-averaged velocity vectors indicates that the recirculation on the pressure side of the vane is unsteady. Each of the phase-averaged vectors was determined by averaging the vectors from 100 images. The spatial resolution of the PTV system was 0.2 mm/pixel, and the accuracy of a single velocity vector is between 1% and 3% in the center of the image. The periodic component of velocity was also determined from the PTV results by subtracting the time-averaged velocity from the phase-averaged velocities (see Fig. 3). The PTV-measured velocity field has a high spatial resolution, so that some details of the vortex structure of the blade wake in the periodic component can be observed. The images in Fig. 3 and similar images obtained at other impeller vane positions enable one to observe the passage of a blade wake through the diffuser channel. The flow entering the diffuser immediately behind the impeller blade is slower than the mean component and is forced by the subsequent jet-flow from the center of the impeller blade passage to be directed toward the diffuser suction surface. The subsequent convection of this flow through the diffuser channel can then be followed. With these vector fields the vorticity field in the x-y plane was calculated. The wake of the impeller blade produces regions with high positive and high negative vorticities, such that the strength of the positive and negative vorticity regions is the same. The vorticities are convected through the diffuser at the mean flow and disappear rapidly downstream of the diffuser throat because of the high turbulent flow. If the PTV velocity components are compared with the unsteady LDA results, one can see that the LDA results are part of the same complex flow field. A direct quantitative comparison of the LDA and PTV velocity components at three different impeller positions shows excellent agreement.
Experiments such as those described above demonstrate the effectiveness--in two different industrially relevant applications--of an LPIV system with a continuous sheet of laser light and a video camera. The camera works well in unsteady flows and in phase-averaging mode. The simplicity with which the various PIV parameters and the light intensifier can be optimized gives this technique a significant advantage over the more-traditional set-ups, especially in industrial research departments where different flow situations have to be analyzed in a very short time (see table on p. 82). This system is also less expensive than a general pulsed laser for PIV studies, which could lead to its use in new imaging studies such as resolving the flow fields in terms of local patterns. This would help improve the efficiency and reliability of products like turbomachinery, valves for power plants, and devices for mass and heat transfer.

Reference paper: I. Suppliers of particle image velocimetry systems include TSI (St. Paul, MN), Dantec Technology (Skovlunde, Denmark), and Oxford Lasers Inc. (Oxford, UK).

II. J. C. Lin and D. Rockwell, Experiments in fluids 17, 110 (1994).

III. T. Uemura et al., "PTV measurements on a rotating system using semiconductor laser and CCD camera," Intl. Symp. on Appl. of Laser Techniques to Fluid Mechanics, July 1994 (Lisbon, Portugal).

IV. L. J. W. Graham and J. Soria, "A study of an inclined cylinder wake using digital particle image velocimetry," Intl. Symp. on Appl. of laser Techniques to Fluid Mechanics, July 1994 (Lisbon, Portugal).

V. B Ruck, Laser und Optoelektronik 26(5), 67 (1994; in German).

VI. F. Hirt, E. Jud, and Z. Zhang, "Investigation of the local flow topology in the vicinity of a prosthetic heart valve using particle image velocimetry," Intl. Symp. on Applications of Laser Anemometry to Fluid Mechanics, July 1994 (Lisbon, Portugal).

VII. K. Eisele, Z. Zhang, and F. Muggli, "Investigation of the unsteady diffuser flow in a radial pump," Intl. Symp. on Applications of Laser Anemometry to Fluid Mechanics, July 1994 (Lisbon, Portugal).

Laser-based particle image velocimetry system is used in conjunction with a pulse duplication system for flow studies in the vicinity of cardiac implants.

FIGURE 1. Unsteady flowfield behind prosthetic bileaflet heart valve with large opening angle (left) and phase-averaged velocity vector field in the symmetry plane at the aortic position (right) show the large reverse flow in the center. The three-dimensional nature of the flow field can be inferred from the low flow speed directed upstream due to the reversed flow. In this experiment the valve is placed at the aortic position and visualized at steady forward flow.

FIGURE 2. Single-stage testbed was built to determine steady and unsteady flow characteristics in a multiple-stage pump and its diffuser. The impeller, its shroud and the diffuser, vanes, and return channel plate are made of acrylic plastic. Impeller has a 350-mm diameter and rotates at 900 rpm. It has 7 blades and the diffuser has 12; blade passing time is about 10 ms. The impeller shaft is equipped with a rotating angle decoder that provides accurate trigger pulses for LDA and PIV measurements.

FIGURE 3. Subtracting the time-averaged velocity from the phase- averaged velocities provides the periodic component of the velocity vectors at the nominal flow rate used in the PTV experiment. Pattern of turbulence varies with position of impeller blade (black line).

Group: K. EISELE, Z. ZHANG, and F. HIRT are researchers at the Fluid Dynamics Laboratory, Sulzer Innotec, Winterthur, Switzerland. N. PERSCHKE is manager of the laser and analytic department at GM¥Laser and Analytic, Renens, Switzerland.

Courtesy: Laser Focus World, Volume: 32,Issue: 5,May 1996.

9. MORPHOLOGY OF UNCONFINED AND CONFINED SWIRLING FLOWS UNDER NON-REACTING AND COMBUSTION CONDITIONS.

by Sean Stacey Archer and Professor Ashwani K. Gupta

Swirl is used in practically all types of combustion systems, including gas turbine combustion, furnaces and boilers. In combustion systems, the strong favorable effect of swirl to combustion air and/or fuel has been extensively used for flame stabilization, high heat release per unit volume, and clean efficient combustion. Flow and combustion characteristics of non-reacting and reacting swirl flowfields are characterized using a simulated Lean Direct Injection (LDI) method in a double concentric swirl burner. The LDI scheme had a large number of small size holes for fuel injection to provide rapid fuel mixing into the surrounding combustion air. The double concentric burner allowed examination of radial distribution of swirl in the burner (co- and counter-flow) under unconfined and confined conditions, both without and with combustion. The input thermal loading to the burner was held constant at 33 kW for all flames.

Particle image velocimetry (PIV), optical emission spectroscopy (OES), infrared (IR) thermometry, gas analysis and computer compensated microthermocouple measurements, were used for diagnostics. These diagnostics provided information on spatial and temporal distribution of flowfield, flame generated radicals, mean gas species concentration, and mean and rms temperatures compensated to high frequencies as well as the associated integral- and micro-thermal time scales, respectively.

Unconfined co-swirl flows had generally wider (except non-reacting) and longer internal recirculation zones, slower velocity decay, smaller reverse flow velocity, lower intensity of flame generated radicals, higher temperatures, and longer integral- and micro-thermal time scales as compared to its counter part. Confinement altered the global flame structure dramatically by rapid radial expansion of the flame to the combustor walls. It increased length and decreased width of the internal recirculation zone, delayed the velocity decay, increased temperatures, amplified intensity of flame generated radicals over a greater region, and enlarged turbulence levels. The vortical structures associated with the instantaneous flow for all cases revealed significant dynamical behavior in the flow as compared to the mean flow case. Infrared thermometry results supported the micro-thermocouple data on mean temperatures. The trend for NOx emissions was higher for the confined case in both co- and counter-swirl cases.

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10. Whole field flow measurements of attraction channel as entrance to fishways.

Migrating fish that swim upstream rivers for reproduction need to overcome obstructions like hydropower plants in their path to the spawning grounds. To guide the fish pass the obstruction fishways are often used. Since most of the water in regulated rivers flow through the power plant fish are often attracted to the turbine tailrace instead of the fishways. The efficiency of the fishways is often low due to inefficient attraction water.

In this report an attraction channel which uses a small fraction of the tailwater, or any free stream, to create the attraction water is studied. The channel is open and Ushaped. A local acceleration of the water is created by changing the cross sectional area in the downstream end of the channel. The flow through the channel is subcritical and the bump which accelerates the water also blocks the water flowing into the channel. To study how the water flows are affected of the blockage as the geometry is altered, a model of the channel is studied in lab scale using Particle Image Velocimetry (PIV). With PIV instant flow field can be studied in a plane of the flow.

The results show the water flow in and around the channel and how the flow pattern in the channel changes with water depth. Increased depth over the bump increases the downstream traceability of the acceleration. It is also shown that it is possible to obtain accelerations up to 50% downstream the attraction channel compared to the pure flow upstream.

Author: Green, Torbjörn M.
Source: Lulea University of Technology.
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11. Setup of Particle Image Velocimetry (PIV) in Hypersonic Flows.

by Moazzam Anwar

The accuracy of numerical methods in calculating the flow of backward facing steps in turbulent hypersonic flows is limited due to a lack of flow measurements. Such measurements are necessary to validate numerical techniques and turbulent models. The lack of measurements concerns especially quantitative data on the dynamics of large turbulent structures. One approach to solve this problem is to measure the global velocity field. Therefore Particle Image Velocimetry (PIV) has to be arranged for use in high speed flows. In this project PIV setup was installed for the use in hypersonic Ludwieg tube Braunschweig (HLB). After the setup of PIV system a generic hyperboloid flare configuration was examined.The influence of shocks on particle concentration can be identified in these measurements. On this model previous infrared thermography measurements and numerical calculations are available, therefore a comparison was made.

Author: Moazzam Anwar.
Source: Department of Mechanical Engineering, Blekinge Insititute of Technology, Karlskrona, Sweden.
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