Thursday, December 13, 2012

Inside wind tunnels

In Formula 1 world there are three types of testing validation for new parts: CFD, on the track (free practices or tests) and Wind Tunnel installations.
Lot has been said about the latter, so I'm not going into details about rolling floors or boundary layer suction, but instead I'm going to focus on what and how specifically is being measured in a Wind tunnel and why is that of paramount importance for F1 teams.

One of them has recently been all over the specialized news feeds, namely Ferrari, with their Wind Tunnel struggles, failing to produce correct correlation between tested parts and what's actually being measured on the track as results.
Various sources have been suggesting that Ferrari F1 Team will shut the tunnel down and make re-calibration, system checks, improvements, even calling external consultants, presumably from aerospace industry to check what's wrong. Just how much of all these is true is unknown to the general public, but talks about new wind tunnel construction have also appeared. It is perhaps worth saying that this is large investment and its rough estimated cost is about 40 to 50 million US dollars, besides it takes a lot of time for pure construction and realization.
UPDATE [20.Dec.2012]: Stefano Domenicali confirmed that Ferrari's wind tunnel will be undergoing major overhaul, will remain closed until August and Ferrari's 2013 car will be developed exclusively in TMG facility - see details below.

Apart from the economic outlook, teams are allowed to test with scaled model, up to 60% of the real size, and use speed of up to 50 m/s, which is 180 km/h max or 111.8 mp/h if you prefer imperial units.

Since the model parts do not represent the real world car size, Reynolds number is, for example, used to make coefficient correlation between the 60% model parts and 100% model. However, it is important what the source data shows.

Below you may find a sample spreadsheet with totally bogus and unrelated data, which is there to simply demonstrate the various parameters being measured in a wind tunnel session. Time spent inside the installation is also precious, as teams are having certain time frames allowed.

 Let's break down the data sheet into details:
  1. Cell B holds the value for Lift, in our case there is downforce generated, hence the negative sign. It's usually being measured in Nm, but it may vary in different wind tunnels implementation (Kilograms are often seen, too). 
  2. Next we have Drag, which is pretty obvious.
  3. L:D is lift-to-drag ratio - one of the most used measures for aerodynamic efficiency - sure, you can add more downforce via Angle of attack settings, but this comes at the expense of added drag, too, so balance is important. 
  4. Drag power - this is the consumed horse power, the power required to overcome aerodynamic drag.
    Note: this is a velocity-sensitive parameter, and the formula is simple:
    DRAG HP = ( Drag coefficient (0.9 or 1 for F1 car) * Frontal Area in square meters (assume 1.35) * current velocity (200 km/h = 55 m/s) cubed ) / air density (1225 kg/m3) = ~ 124 HP to overcome aero drag. The same formula gives about 380 HP at 250 km/h, etc.
  5. The next three columns, FY, FX and FZ are moments, measured as distribution of aerodynamic forces around Center of gravity, aligned with the three axes:
    Copyright: F1 Framework
  6. Next, in columns I and J we have vertical loads on the front and the rear, which is very important parameter measured from suspension point of view. While measures here differ, these are usually Newton meters, in this sample sheet the values are in kilograms.
These are some of the most common parameters that teams are measuring in a wind tunnel. Apart from those numbers, more can be crunched as a mathematical representation of the measurements taken, namely calculating coefficients. All of those numbers and their values are carrying certain amount of knowledge in regards to how your car will supposedly behave and that's why it is very important that the output is correct, as this decides what configuration and parts to put on the car.


The next chunk of details goes directly into the heart of Toyota Motorsport GmbH wind tunnel facility in Cologne. F1 fans have heard a lot about it, especially when it comes to Ferrari, but first, I, with "no wax", like to thank their staff for being gracious enough to release those images and data for the public and being patient enough to explain everything. Below you may find some exclusive images from the tunnel.

Outside look of the tunnel's diffuser section
The instrumentation of this state-of-art facility consists of two wind tunnels : one full size and one 60% scales.
Both wind tunnels use a continuous steel belt rolling road which has a maximum speed of 70 m/s. What can really describe those installations is accuracy and high quality - 512 pressure measurement channels are available. The tunnels are also equipped with Particle Image Velocimetry system (PIV) - an optical method of flow visualization used to obtain local flow velocities. The PIV system is usually used to validate CFD predictions and thus better calibrate the setup.
One of the advantages of such PIV system is that its preparation time is almost zero, due to the seeding technology used. That means that the local air is filled with tracer particles and there's no cleanup needed after that. During the process of measurement, cameras are used to obtain images and samples, which are then processed by computer software.
A laser is used to illuminate the flow field around the car’s front wheel to take particle image velocimetry (PIV) measurements (see below).

One of the main advantages of this method, just as well as the PSP method, is that it's non-intrusive, as opposed to the traditional pressure taps.
"Many of today’s motorsports cars are based on existing, commercially-available cars." Frank Michaux, a researcher at TMG, reveals.
"If researchers can identify a way to reduce drag on a motorsports car, it’s reasonable to assume that this information also may apply to future versions of a normal road car." Frank continues: "We need to deliver data quickly. If you see that you are not capturing the flow correctly, then you need to adjust your CFD methodology until you get it right".

After gathering the raw data from the PIV measurements of the “separation point” on the front tires, engineers post-process the data using Tecplot software, which allowes them to see and measure the exact position of separation. Each of Toyota’s PIV measurements consist of 300 datasets, with each dataset containing two images taken 10–20 microseconds apart.

The Tecplot software package itself is widely recognized and used throughout the industry.


During the 2009 season, F1 engineers wanted to carefully study the flow wake behind the front wheels. Since this is critical part of overall car performance, the goal was to push this flow as outboard as possible. Engineers have realized that they need precise back-to-back correlation, and here's a direct comparison between CFG and PIV images: 

In order to get to those precise points, the CFD methodology has to be optimized, and here's a real case:

The next thing about Toyota Motorsport GmbH is its Continuous Motion System.

In CMS mode, a user-defined programme of ride height, yaw, roll, steer and individual pre-load changes provides continuous motion on a predefined trajectory while the HSDA system is continuously acquiring data at high frequency.
This allows realistic road or track analysis and increasing the amount of useful data from each individual test compared to standard motion and acquisition systems.
The typical "Wind-On" time is reduced by 70 to 100%  with CMS, which allows more time to be dedicated to, for example, changing parts. This Wind-On time is extremely precious for F1 teams, which we all know run under certain restrictions when it comes to wind tunnel time. 

Below you may find some exclusive images from inside the tunnel (full sizes available upon click): 
Very good representation of scaled model compared to a man

Toyota TS030 early prototype version

The control and monitoring room

Testing preparation - Toyota TF110
Front wing flap angle at 27 degrees
Surveillance and monitoring  
Glad you have reached this point - I assume you were very interested or your scroll button was stuck :)
As usual, comments and questions are welcome.