Research Spotlight - Solar Racing Aerodynamics with Robin Hodda

By Daniel Pavlich

ANU is full of exciting research that is often at the very cutting edge of its respective field. Unfortunately, if you aren’t directly involved or know someone that is, it is highly likely that you won’t hear anything about it. That stops now.

ANU Solar Racing is a completely student-run team which designs, researches, funds and builds a completely solar-powered race car to compete in the biennial World Solar Challenge. This challenges teams from all over the globe to race their solar cars 3022 km on public roads from Darwin to Adelaide.

Robin Hodda, the head of aerodynamics for ANU’s Solar Racing team, explains how to harness the powerful forces of nature to design, build and race a solar car from Darwin to Adelaide.

The process of trying to work out which of those three would be best was incredibly stressful but taught me about how to deal with complexity, uncertainty and how to structure decisions to be as robust as possible. We had to consider the placement of wheels, battery, driver, solar cells and other major components so that the chassis was as streamlined as possible while still stable and lightweight. After drawing some mock-ups of the layouts, we concluded that a monohull would either be too narrow to be stable enough, or so wide that any aerodynamic advantage would be lost.

This left a decision between catamaran and trimaran, and ultimately, we found the reduction in weight and complexity of having one less wheel in a trimaran was beneficial. Hence, ‘Spirit’, the trimaran was born.

The Importance of Aerodynamics

Aerodynamics is the study of the movement or flow of air around shapes. It’s perhaps most obviously useful in planes, allowing a hundred-tonne aircraft to take flight. However, the application of aerodynamics is incredibly broad; it is used in air conditioners, cars, turbines in power plants, rockets and skyscraper design.

For solar cars, there is only so much solar power that can be collected, so using that power most efficiently helps maintain a higher speed than the competition. For reference, solar cars can go up to 90km/hr using less power than a toaster! The power that is required to push through the air increases with the cube of the car’s speed. This means that to go twice as fast, it requires 8x the power, making the car’s aerodynamics the single largest contributor to performance!

Whilst there is a mathematical description of air movement (the Navier-Stokes equations), it is basically impossible to completely solve when applied to anything in the real world. Therefore, my job becomes a game of leveraging carefully developed empirical theory and combining it with physical tests and simulations.

The team is made up of a business branch, an operations branch and a technical branch. It’s a diverse group of students with people from all 7 of the academic colleges at ANU, all with the same passion for solar racing. Solar racing is all about automotive efficiency taken to the extreme in every facet. Pushing the envelope on solar cell efficiency, electronic control, tires, motors, weight, and most of all: aerodynamics.

I originally joined the team part way through my first semester at ANU in 2019 to revamp the team website. As the year went on and the 2019 race became ever closer, I was drafted in as an extra set of hands to help finish the car build, which I adored. I was fortunate enough to go with the team to the 2019 World Solar Challenge and was completely won over. Despite not having any formal background in it, I joined the mechanical team in 2020 and began design work on the new car. As aerodynamic drag on the car is the largest consumer of power, a major challenge is to reduce the drag as much as possible to make the car go faster.

The process of designing and developing the chassis (the structural frame of the car) had challenges at every step, so initially we looked back at all the solar cars from the last 30 years. People have tried all sorts of wacky things. It became clear to us that there were three general layouts that have been successful: a trimaran with two wheels at the front and a pod at the rear for the driver; a catamaran, with the driver on one side and four wheels, or a monohull where the driver and wheels are all in the same shape.

One of the key challenges we are still working on is finding a good balance between the accuracy of the simulation, which is tied to how fine the mesh is, and the simulation length. The finer the mesh is, the longer it takes to run. The simulations we run range from a couple of hours to a week, but it is possible for this to become months in search of even higher fidelity. Understanding this balance to try and maximise the useful information gained in a limited amount of time has led to a lot of research into the tradeoffs associated with shorter simulations.

Increasing the mesh density in key places is vital (up to a point) to help capture the finer details of airflow in the areas that have the most impact on aerodynamics. However, there are some positions where there won’t be many changes, in which case you can get away with larger cells to reduce the simulation time.

Overall CFD is incredibly useful for the team because it allows us to understand the airflow around the solar car and test designs we have come up with to see if they provide any improvement without actually having to build them. For example, refining the wing design to minimise lift given its proximity to the ground, which is vital to create enough downforce to stop the car taking off!

The solar car team is always on the lookout for people who want to get involved in any arm of the organisation. Their main recruiting windows are at the beginning of each semester, but follow them on social media to keep up with how the team performs in October this year!