PIV boosts wind tunnel efficiency in F1

In recent years, efficiency of development and research has become increasingly important in a Formula One team. Whereas previously teams ran dual wind tunnels night and day, running hours have now been restricted to reduce costs and allow for closer competition.

At the same time, it has become clear as crystal that aerodynamics are now the most important part of an F1 car. Major regulation changes back in 2009 showed how teams could gain an impressive advantage only by exploiting aerodynamic loopholes in the regulations. Such feats can however only be achieved when engineers work closely together to find every little tenth of a second in a car. They would however fail to succeed without the right equipment, and a windtunnel has been indispensible since the 80's.

To achieve better efficiency, the former Toyota F1 team realised that it would be extremely helpful to be able to measure and visualise flow fields quickly from wind tunnel tests. As such, early in 2007 the team installed a permanent Particle Image Velocimetry (PIV) system in one of their tunnels after deciding on the system a year earlier. Before that time, Toyota, nowadays TMG, used the system from the third party, but owning a system allowed for much more flexibility in when and how to use the measurements.

TMG's second windtunnel was later installed with the same permanent system to further improve efficiency. In fact Toyota used the system intensively for their 2008 and 2009 contenders, cars that unsurprisingly showed improved performance compared to earlier years. TMG is adamant that PIV had a major role in this uprise as it allowed them to accurately measure the front wheel's wake, something that was pivotal for a car's performance in 2009.

Appliance in a windtunnel

Before the advent of PIV, engineers would create simple vehicle models and then study the flow on the surface of the model. This was inefficient because they could only see the flow on the car's surface. They didn't have the means to produce a 2D plane, for example, or accurately observe and record the critical wake of the wheels.

To overcome these weaknesses, a smoke probe was brought into the windtunnel to "see" the air flow. The whole problem in this erea though is how to visualise flow without introducing something new into that flow that could potentially compromise the results. The appliance of PIV posed an excellent solution to this problem.

As an optical flow visualisation method, PIV allows to visualize the flow field almost exactly as it appears, without influencing the very flow field that engineers are seeking to measure.

Toyota's PIV system involves filling the tunnel with a fog or mist spread by a seeding generator. The company is using Di-Ethyl-Hexyl-Sebacat (DEHS) for its visualization particles 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 that make up the fog simply float, making this PIV method as non-intrusive as current technology will allow.

Once the DEHS particles are evenly spread in the wintunnel, the camera is positioned at a 90-degree angle to the plane of the flow field that needs to be studied. Next, engineers illuminate the part that needs to be visualized with a high-powered laser, creating a 2D plane. Simultaneously, a series of two-set photos are taken in extremely rapid intervals. Equipped with this sort of ultra-slow-motion digital imaging, engineers can easily measure the direction of the flow field and the rate of flow.

Running a test

In addition to the design of the particular system as implemented at TMG, which allows for extremely accurate measurements, 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.

With a start up time of one hour to come from nothing to a fully active PIV system, TMG can run any measurement whenever they see fit. In fact, if the operators want to measure the front wheel wake, they don't even need to position the camera manually, as TMG has a camera attached to the ceiling of the tunnel to measure this critical area. For any other run, wind tunnel operators have to position the camera manually before starting the run. Even then, a standard set of common positions is already defined, making the camera installation quick and repeatable.

For most of the measurements (e.g. X-sections, Y-sections, Z-sections, underfloor measurements), specific optics are permanently installed in the windtunnel to get the laser sheet in the correct position. All the laser optics are mounted on traversing units, to be able to move the laser sheet to the measurement positions. This again makes the set-up of the measurements quick and repeatable.

A complete PIV test is a procedure that is easy for the engineers and quick to complete:

• Before a PIV test can run, special attention is required to prepare the model. Parts hit by the laser are specially prepared to reduce reflections.
• The camera position is calibrated first, then the laser can be correctly positioned based on this, with reference measurements taken. During the time the laser is active, lights are out inside the tunnel while an infra red camera allows for the engineers to keep an eye on the model during testing. At the same time, a 'laser curtain' closes off the windows between the control room and the test section.
• The seeding generator is turned on while the tunnel starts running at low speeds. During this time, final checks are performed to ensure an accurate measurement. One of the most important aspect here is to check whether the DEHS particles are evenly spread in the air.
• When everything is deemed ready for the test, the wind tunnel speeds up to the required testing speed and images are recorded with one of the cameras.
• At this time, it also easy to measure flow characteristics when the car is in a different state. Without requiring to go through all previous steps, the engineers can adjust the ride height of the car, change windtunnel speed or change the front wing flap angle from within the control room thanks to the model being rigidly attached to the movable windtunnel pylon. At TMG, the data system supports up to 512 measurement points within the tunnel model.
• All data collection results in 300 datasets, with each dataset containing two images taken 10-20 microseconds apart. After post-processing, this results in a complete 2D field of vectors.

When the test has completed, the specific properties of DEHS come to help, as the gas completely evaporates after several hours, without leaving any trace on the model or the windtunnel internals. Hence, regular wind tunnel testing can just continue, without requiring any cleanup whatsoever.

Understanding the results

During the 2009 Formula One season, engineers wanted to study the wake behind the front wheels of a vehicle. This is a critical part of the flow; if this isn't perfectly calibrated, the performance of the entire vehicle could be severely compromised. After being presented with the problem, the Toyota Motorsport team realized they needed to look at options for adding or modifying various front-end parts like an under-nose turning vane or modify the front wing to create or influence an outwash, which pushes the flow from underneath the nose of the car, forcing the wake of the car as outboard as possible.

After gathering the raw data from the PIV measurements of the 'separation point' on the front tires, engineers plotted the velocity magnitude, or vorticity, with vectors based on the average data. The result of the CFD computations was then imported into Tecplot software to create the same picture as with the PIV data. Engineers then compared the data sets to determine if their CFD methods are within acceptable accuracy ranges. Whenever necessary, the engineers tweaked the CFD process to get it closer to the wind tunnel results.

In the case of the separation point on the front tires, initial tests showed that the separation point was late and too far back from the tires. The engineers altered the CFD methodology based on these observations, imported the new results into Tecplot, created another picture and compared it with the PIV results to evaluate their progress. The process was iterated until they arrived at a design that placed the separation point at an optimal position on the tires.

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

Nowadays though, with TMG out of Formula One, clients are assured of precise measurement, and are provided with a set of images of the average of the 300 image pairs for in-plane velocity magnitude, both velocity components and the vorticity level. In addition, a set of videos is supplied showing the instantaneous results and exports to allow customers to import the data into the software of their choice.

Thanks to Toyota Motorsport