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.