Particle Image Velocimetry (PIV)


Particle Image Velocimetry mesurement on colibri bird in flight

Aero development and research has become more and more important in a Formula One, especially after wind tunnel running hours has been restricted to reduce costs and allow for closer competition. Exploiting aerodynamic loopholes in the regulations became more and more important. Such feats can however only be achieved when engineers work closely together to find every little tenth of a second in a car. But for something like that you will need right equipment, and a wind tunnel has been indispensible since the 80's.
To achieve better efficiency, F1 teams realized that it would be extremely helpful to be able to measure and visualize flow fields quickly from wind tunnel tests. Newly developed Particle Image Velocimetry (PIV) system allowed technician to visualize air flow without introducing smoke stream in very sensitive air stream in front of testing subject. Smoke probe was efficient, but the whole problem in this area is how to visualize flow without introducing something new (smoke probe) into that flow that could potentially compromise the results. The appliance of PIV posed an excellent solution to this problem.

Particle Image Velocimetry system setup

It will take wind tunnel about one hour of a start up time to come from stand still to a fully active PIV system. PIV system start up involves slowly filling the wind tunnel with a fog or mist of some kind, spreaded by a fog or mist generator. The fog generator is turned on while the tunnel starts running at low speeds. The most used chemical for fog generation is Di-Ethyl-Hexyl-Sebacat (DEHS) mainly because it is non-toxic, is liquid at room temperature and has nearly the same density as air. When the air flows through the tunnel, the small particles of DEHS that make up the fog simply float, making this PIV method as non-intrusive as possible. Fog generator is located in opposite side of the tunnel or inside honeycomb panels. Uniform spread of fog particles is of critical importance. Speed of the air is slowly increased to full or required speed. During this time, final checks are performed to ensure an accurate measurement.
Particle Image Velocimetry cameraOnce the DEHS particles are evenly spread in the win tunnel, the camera is positioned at a 90-degree angle to the air flow field that needs to be tested. Next, engineers illuminate the part that needs to be visualized with a high-powered laser, creating a 2D plane. In the same time, a series of two-set photos are taken in extremely rapid intervals of two images taken 10-20 microseconds apart. Equipped with this sort of ultra-slow-motion digital imaging, engineers can easily view and measure the direction of the flow field and the rate of flow. PIV as a system also does not prevent wind tunnel engineers to run other tests at the same time as the PIV measurement is running.

Before a PIV test can run, special attention is required to prepare the model. Surface of the parts hit by the laser are specially prepared to reduce reflections. Position of the cameras must be calibrated first, and then the laser can be correctly positioned based on camera positions. For testing same subject (e.g. same Formula 1 car), cameras are permanently installed in the wind tunnel to get the laser sheet in the correct position. All the laser optics is mounted on traversing units, to be able to move the laser sheet to the measurement positions. This makes the set-up of the measurements quick and repeatable.
During the time the laser is active, lights are out inside the tunnel while an infra red Particle Image Velocimetry on helicoptercamera allows for the engineers to keep an eye on the model during testing. When everything is deemed ready for the test, the wind tunnel speeds up to the required testing speed, laser is fired up and images are recorded. At this time, it also easy to measure flow characteristics when the car is in a different configurations. Without requiring to go through all previous steps, the engineers can adjust the ride height of the car, change wind tunnel speed or change the front wing flap angle from the control room thanks to the model being rigidly attached to the movable wind tunnel pylon.
When the test has completed, DEHS gas completely evaporates after several hours, without leaving any trace on the model or the wind tunnel internals. Hence, regular wind tunnel testing can just continue, without requiring any cleanup whatsoever.
Particle Image Velocimetry calculationAfter gathering the raw data from the PIV measurements engineers can plot the velocity magnitude and vorticity of certain area, and then compare the data sets to determine if their CFD methods are within acceptable accuracy ranges. The result of the CFD computations was then imported into CFD software to create the same picture as with the PIV data. They can alter the CFD methodology based on these observations, import the new results into software, created another CFD pictures and compare it with the PIV results to evaluate their progress. The process can be repeated until they arrive at an optimal design. Whenever necessary, the engineers can tweak the CFD process to get it closer to the wind tunnel results.

In its F1 days, PIV proved vital to improve the correlation between modeled airflow simulation through CFD and real world testing in the wind tunnel or on a track. It allows for engineers to rely on the CFD results to validate or improve the design of a component.

Particle Image VelocimetryParticle Image Velocimetry

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Some useful links:

- f1technical.net, a great site with a lot of technical information’s and explanations. Site is updated daily with news from F1 word.

 - autosport.com, This site is a legend. A bible for racing lovers. News from all around the word. Unfortunately, to get access to all news, interviews and to open the site completely you should be subscribed to Autosport magazine. Anyway, great read.

James Allen on F1
- JA.F1 site (or blog) ovned by ITV Sport’s lead commentator on Formula 1 James Allen

Joe Saward blog
- joesaward is the Joe Saward official blog about Formula 1 world. Joe is an journalist, who write primarily about politics in and around motorsport, specifically on the FIA Formula 1 World Championship

Vital F1
 - vitalf1.com/ is another great site for Motor Sports fan’s like me. Site is relatively new, but great fun, with great discussion forum, Formula 1 news and forum.

 GP update
- f1.gpupdate.net, Site with fresh news from Formula 1

Planet F1
 - planetf1, another site with many different articles, news and statistics. Biased toward British teams, but anyway good read.

Gurney flap
 - gurneyflap.com, Great history site. You can learn a lot from this site. Pictures, cars and many many more. Great.

4ormula1 is a database of Formula 1 history and statistics of drivers, teams, grand prix, and all results since 1950

Racecar engineering
-Racecar Engineering, an online magazine with a lot to learn from, a lot of technical information’s and explanations

 - fia.com, La Fédération Internationale de l'Automobile, representing the interests of motoring organisations and motor car users. Head organisation and ruler in auto sport.

 - wikipedia.org, I don’t believe that I have to tell you anything about this site. It’s not about Formula 1 technology, but you can learn a lot about that too.

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 - carbibles.com, a great site for normal car users. Here you can find explanations of almost everything about your car and how it works. Technical reviews and explanations of some in-car gadgets.

Dare To Be Different
- Daretobedifferent.org Susie Wolff and UK governing body of UK motorsport have joined forces to launch Dare To Be Different, a high-profile new initiative which is about increasing female participation, not just on the track but in all aspects of the sport.