Human beings are extremely interested in the observation of nature, as this was and still is of utmost importance for their survival.

Transcription

1 Historical Background Human beings are extremely interested in the observation of nature, as this was and still is of utmost importance for their survival. ( 1

2 Historical Background (Cont.) Leonardo de Vinci ( ) sketched various flow fields over objects in a flowing stream. ( 2

3 Historical Background (Cont.) A great step forward in the investigation of flows was made after it was possible to replace such passive observations of nature by experiments carefully planned to extract information about the flow utilizing visualization techniques. Ludwig Prandtlin 1904 in front of his tunnel, driving the flow manually by rotating a blade wheel. 3

4 Historical Background (Cont.) The flow is visualized by distributing a suspension of mica particles on the surface of the water. Ludwig Prandtlstudied the structures of the flow in steady as well as unsteady flow (at the onset of flow) with this arrangement. Being able to change a number of parameters of the experiment (model, angle of incidence, flow velocity, steady unsteady flow) Prandtlgained insight into many basic features of unsteady flow phenomena. However, at that time only a qualitative description of the flow field was possible. No quantitative data about flow velocity, etc., could be achieved. 4

5 Particle Image Velocimetry: From the past to the present The development of particle image velocimetryduring the past 20 years is characterized by the fact that analog recording and evaluation techniques have been replaced by digital techniques. 1977, Laser Speckle, originally developed in solid mechanics, was adopted in fluid flow measurements. 1983, A doctoral student working at v. Karman Institute, Belgium, Meynart, was the leading practitioner of this method (Laser Speckle Velocimetry, LSV). 1984, The first explicit recognition of the importance of particle images was made in two short, contemporaneous papers by Pickering and Halliwell, and Adrian. The image plane would contain images of individual particles. The name particle image velocimetry(piv) was proposed to distinguish this mode of operation from the laser speckle mode , Willertand Gharib, and Westerweel, published results promoting the advantage of using digital cameras. 1998, Santiago et al., came up with micro-piv , a great leap in calculating algorithms. 5

6 PIV: An Art of Photography Velocity=ds/dt As long as you know how to take a picture!! 6

7 Flow Visualization ( Courtesy of NASA Glenn Research Center 7

8 PIV system Optical assembly: Double pulsed laser Light sheet generator (Lens set) 8

9 Flow apparatus: PIV system (Cont.) Window tunnel or flow chamber Fog generator 9

10 Data acquisition system: PIV system (Cont.) Camera Synchronizer PC 10

11 Procedure of Data Processing Adrian, R.J., Exp. Fluids,

12 Principle of PIV (1) Before going into the details of the PIV technique, some general aspects have to be discussed : 1. Non-intrusive velocity measurement In contrast to techniques for the measurement of flow velocities employing probes such as pressure tubes or hot wires, the PIV technique being an optical technique works non-intrusively. 2. Indirect velocity measurement the PIV technique measures the velocity of a fluid element indirectly by means of the measurement of the velocity of tracer particles within the flow, which in most applications have been added to the flow before the experiment starts. 3. Whole field technique PIV is a technique which allows to record images of large parts of flow fields in a variety of applications in gaseous and liquid media and to extract the velocity information out of these images.the spatial resolution of PIV is large, whereas the temporal resolution (frame rate of recording PIV images) is limited due to current technological restrictions. 12

13 Principle of PIV (2) 4. Velocity lag The need to employ tracer particles for the measurement of the flow velocity requires us to check carefully for each experiment whether the particles will faithfully follow the motion of the fluid elements, at least to that extent required by the objectives of the investigations. Small particles will follow the flow better. 5. Illumination For applications in gas flows a high power light source for illumination of the tiny tracer particles is required in order to well expose the photographic film or the video sensor by scattered light. 6. Duration of illumination pulse 7. Time delay between illumination pulses The time delay between the illumination pulses must be long enough to be able to determine the displacement between the images of the tracer particles with sufficient resolution and short enough to avoid particles with an out-ofplane velocity component leaving the light sheet. 13

14 Principle of PIV (3) 8. Distribution of tracer particles in the flow A homogeneous distribution of medium density is desired for high quality PIV recordings in order to obtain optimal evaluation. 9. Density of tracer particle images The three modes of particle image density: 10. Number of illuminations per recording For both photographic and digital techniques, we have to distinguish whether it is possible to store images of the tracer particles on different frames for each illumination or whether all particle images due to the different illuminations are stored on a single frame. 14

15 Principle of PIV (4) 11. Number of components of the velocity vector Only two (in plane) components of the velocity vector can be determined in standard two-component PIV (2C-PIV). Methods are available to extract the third component of the velocity vector as well, which would be labeled 3C-PIV. 12. Temporal resolution Most PIV systems allow to record with high spatial resolution, but at relative low frame rates. However, the recent development of high-speed lasers and cameras allows time resolved measurements of most liquid and low-speed aerodynamic flows. 13. Spatial resolution 15

16 Major Technologies and Milestones of PIV (1) 1. Reliable high power sources for applications in air The use of double oscillator Nd:YAGlasersallowedfor the first time the illumination of a plane in the flow with laser pulses of the same, constant energy at any time delay between the two pulses as required by the experiment at repetition rates of the order of 10Hz. 2. Ambiguity removal Especially with photographic recordings it was not possible in most cases to store the images of the tracer particles due to first and second illumination on two different recordings. The most widely used technique was image shifting. 3. Generation and distribution of tracer particles in the flow The development of powerful aerosol generators and the know-how to distribute tracer particles of a well defined size within the flow homogeneously improved the particle image density and the quality of the PIV recordings considerably. 16

17 Major Technologies and Milestones of PIV (2) 4. Computer Hardware The improvement of computer hardware with respect to processor speed and larger memory still continues according to Moore s law. Difficult only a decade ago, the handling and processing of numerous mega-pixel sized PIV images has become trivial on today s personal computers. Processing of these images is possible in fractions of a second. 5. Improved peak detection 6. High-repetition rate laser The introduction of kilo Hertz CMOS cameras has generated a need for laser light sources that can operate at frame rates of 1000 s of frames per second. As a consequence thereof, diode pumped lasers (Nd:YAGor Nd:YLF) were adapted to high-speed PIV. 17

18 Major Technologies and Milestones of PIV (3) 7. Cross-correlation and high-speed cameras Today progressive scan video cameras allow users to store a pair of images of the tracer particles on separate frames for each illumination with interframingtimes of less than 1 μs. This feature immediately solves the problem of ambiguity removal even for high-speed flows. Another very recent technical improvement for PIV applications was the development of CMOS sensors with the active pixel sensor (APS) technology in which, in addition to the photodiode, a readout amplifier is incorporated into each pixel. 8. Micro-PIV recording 18

19 Major Technologies and Milestones of PIV (4) 9. Adaptive evaluation algorithms Standard evaluation algorithms provide reliable velocity vectors. However, their accuracy is limited due to the loss of particle image pairs in complex flow regions. As a straight forward approach to tackle this problem, the second interrogation window can be displaced with respect to the window in the first image or different window sizes can be used in combination with a direct correlation scheme. 10. Theoretical understanding of PIV At the beginning of the development of PIV the understanding of the technique was a more intuitive one. Progress was often made just by trial and error. In the past few years the theoretical understanding of the basic principles of the PIV technique has been improved considerably. Such theoretical considerations as well as simulations of the recording and evaluation process give useful information on many parameters important for the layout of an experiment utilizing PIV. 19

20 Oceanography: Applications of PIV Algae floating on the surface of water serve as flow markers for elementary particle image velocimetry. 20

21 Applications of PIV (Cont.) Meteorology: Global Climate Tornado arstoday.html gallery/tornado.php 21

22 Applications of PIV (Cont.) Aerodynamics: What the hummingbird has done is take the body and most of the limitations of the bird, but tweaked it a little and used some of the aerodynamic tricks (Purcell s Scallop Theorem) of an insect to gain a hovering ability. 22

23 Applications of PIV (Cont.) Manufacturing: Streamline to reduce Drags. Less Drags behind a vehicle. 23

24 Applications of PIV (Cont.) Biomedical engineering: In-vitro study of wall shear stress on endothelial cells Haidekker et al., J. Biol. Eng.,