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Most of the research and development that goes into sculpting and refining the aerodynamic
shapes of the various parts of F1 cars is made in the wind tunnel and in sophisticated
computer models called computational flow dynamics or CFD models.
Make no mistake, this is where the majority of the hard work that goes into designing
and testing aero parts like wings and diffusers takes place.
But when the car hits the track there’s a big opportunity for some final aero checking
and verification. One of the ways teams can do this is with the use of flow visualisation
paint, or Flow Viz. You’ve probably seen Flow Vis paint used
on the cars in practice or testing sessions. The most eye catching use recently was when
Williams absolutely covered their car in various colour blocks of Flow Viz to try and solve
some of the mysteries of their misbehaving chassis.
More typically, though, Flow Viz is seen used sparingly, precisely and in specific areas
of the cars. You might see it on the front wing, over diffusers, bargeboards, etc.
So what is Flow Viz paint? How does it work? And how does it help engineers and analysts
understand their cars better? The main idea of Flow Viz paint is to show
the airflow patterns that manifest along the bodywork of the car as it drives at speed.
These flows are normally invisible because… well, have you seen air lately? Exactly. No,
you haven’t. Flow Viz is a pretty simple product. It’s
just a powdered dye suspended in paraffin or kerosene or a similarly light oil. By suspended,
I mean the dye sits happily in amongst the oil once stirred but will eventually sink
and separate from the oil with time. The oil is just a medium to keep the dye flowing and
free. The useful thing about this mixture is that
it doesn’t dry particularly quickly when it’s all globbed together so you can splosh
it liberally over the areas you want and the paint will continue to run and flow as you
drive the car at speed through the air. A lot of normal paints would dry or stick
around the areas you painted them and not do anything particularly useful.
The flow of the air around the car as it drives will push the oil - and therefore the dye
- along the surface of the bodywork all free and flowing. But paraffin evaporates fairly
readily and this will leave the dye behind, stuck to the bodywork in all the flowing shapes
its made in the airflow. The aerodynamicists can then take photos of
the car once it’s parked back up in the garage and check to see if the paint patterns
show the results they’d been expecting. You’ll often see very brightly coloured
dyes that stick out against the bodywork so they can be easily seen when photographed
by the team. But the dye itself may also be fluorescent, meaning it will show up much
more strongly under UV lighting. So if you see Flow Viz that seems a little faint and
unclear, the chances are the paint will look a LOT more striking back in the garage under
the right lights. Now - importantly - the point of Floz Viz
paint isn’t really to gain much quantifiable or precise data about the aerodynamics. Instead,
it’s a fairly quick and dirty method to double check that the computer models and
wind tunnel tests corroborate with real-life running.
Teams still can’t fully replicate the many eccentricities of the real world from inside
their own HQs (though Lord knows they try), so a full scale run on a real track is the
final, definitive test for that new front wing or whatever that they’ve been developing
behind the scenes. Did it behave as expected? Does it differ
from the old spec front wing in the way they intended? Or is it giving some strange flow
behaviour that you need to analyse and double-check? This is the benefit of flow viz in the real
world, though it has of course been used in wind tunnels for many, many years - way before
it showed its head in testing and practice. Now, there may be extreme cases, like Williams
this years, where teams just want to evaluate the whole package and record lines over the
whole chassis. You can do that, if you want, but you really shouldn’t ever be at that
point with a car halfway through a season. OK, so you’ve slapped a load of flow viz
paint on the part of the car you want to evaluate. Your driver has done a lap or two and brought
the car back to the garage. How do you interpret the colourful mess left
all over the carbon fibre? Well, firstly - as outsiders, we’re never
sure exactly what the aerodynamicists are looking for in any given tests, particularly
when testing one part against another. They may be looking for any number of characteristics
so we can’t speak to the result of any particular evaluation.
That being said, here’s what happens to Flow Viz paint under various, important circumstances.
Firstly, we can see clearly if there is a strong, clean, smooth air flow along the bodywork,
as opposed to a dirty and turbulent flow. Just using a wing profile here as an example
(as it’s easy to visualise): smooth air flow will run nicely along the surface in
good, coherent lines all running together from beginning to end.
You’ll see this in the Flow Viz pattern too. Smooth, coherent lines just like someone
took a nice, quality paintbrush and drew a clean stroke across the bodywork. Satisfying
to look at. Smooth airflow is desirable because it’s
predictable and stick to surfaces. It also continues to be easily manipulated by further
aero devices downstream as long as it continues to stay coherent and flowing.
Turbulent airflow is where the air gets chaotic and starts to misbehave in twists and curls
and acting in an unpredictable, blustery way. This is often undesirable, particularly along
the bodywork, which is where Flow Viz makes its measurements.
Turbulent flow will throw the paint into chaotic splotches. You may see pools of paint gathering
together in seemingly random patterns, distorted waves of colour and other messy patterns.
Just generally, paint gathering in anything but those nice smooth lines we just saw is
an influence of turbulence. Another thing to look for is where the air
flow is attached to and detached from the bodywork.
For the most part aerodynamicists design the shapes of wings and other body parts (particularly
aero devices) to keep the air flow attached to their surfaces.
This means the air sticks to and follows the surface closely.
At high speeds it’s easy for the air to fly away from the surface if that surface
changes direction suddenly and this is called airflow detachment.
Detached airflow is mostly not desired as it greatly reduces downforce and increases
the risk of turbulent further down the body. But whatever kind of attachment pattern you’re
looking for, you can spot it with Flow Viz, for the most part.
If the airflow is attached it will carry and spread the dye along the body of the surface.
A nice, strong, continuous attached flow will show itself by trailing a lot of dye along
the area of the attached flow. If the air tends to detach from the surface
it won’t be able to drag the Flow Viz paint and you’ll see gaps on the bodywork with
a distinct absence of paint. There is a limitation here, however. Often
airflow detachments will occur at higher speeds and if the paint has already been allowed
to flow through the lower speeds as the car accelerates up to top speeds it may be very
difficult to spot that this is happening from Flow Viz alone.
In this shot of the Red Bull rear wing we can see some detachment from the underside
here among otherwise strong, attached flows. Here is looks like there’s some detachment
though it may just be an unpainted area. This show of the McLaren diffuser shows where
the airflow sticks and detaches to the underbody shape. Here, of course, some detachment may
be part of the design. Again, we don’t know what tests are being
run so we can’t overstate what we’re looking at but we can take a lot of interest in the
areas being studied. For example, Sauber here are clearly evaluating
how the air flow disrupted by the front wing continues to behave all the way down the side
of the car. Here the McLaren is checking how the air front
the front wing flows through the bargeboards and turning vanes.
Here it looks like McLaren are specifically evaluating the efficiencies of the airflows
on the left and right sides of their car separately, to have a look at the differences on a specific
track. Flow Viz paint is a striking but limited method
for confirming aerodynamic expectations on track. Teams will often get a lot more data
from their wind tunnels and these pitot tubes structures but Flow Viz continues to be a
quick and easy way to sign everything off.
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