CFD post-processing

The main focus of the first post-setup simulations on EV23 was to identify important areas to validate at track before aero design work began. The intention of such validation is to increase the accuracy of the CFD setup in areas likely to have the greatest influence on performance of a side floor/diffuser. There's not much point spending both real time and compute time making small adjustments to a design to gain 5% downforce when the accuracy of the results isn't known. 

With no aerodynamic devices or even sidepods, the flow field around EV23 is relatively simple with most of the notable features being bluff wake structures behind the wheels, driver, and lower rear chassis/driveline area. Our Kiel probes capture total pressure, so this was the main metric of interest to display and sample values from for off-body flow. While total pressure magnitude is required for correlation to experimental results, we typically used the coefficient form (normalised by the freestream value) for CFD visualisations as they enable direct comparison between cases at different speeds, and are more appropriate for cornering cases where the freestream value is dependent on the radial coordinate. 

Before going into detail, I'll point out some interesting differences between the two scenarios:

• The upper wake of the outboard front wheel is much larger in the cornering case, and persists downstream as an almost completely separate structure

• Steering angle in the cornering case has a big influence on tyre squirt; stronger on the side closer to the centre of the corner, and weaker on the other

• The mid wake of the inboard front wheel gets sucked in behind the tyre much more aggressively in the cornering case, powering up the rotating upper wake structure 

• The opposite happens on the outboard front wheel, resulting in almost no rotation of the upper wake structure

• I'm slightly dubious of how large the squirt and lower wake structures are for the front wheels, particularly in the straight airflow scenario

The front wheel wake is of more interest to accurately model than the rear wheels or driver/headrest wake, as it directly impacts the area where side-floors would be mounted and so will have the most influence on their simulated performance. Some cross sections behind the front wheel are shown below for the straight airflow scenario. 

The intention for the straight airflow scenario was to have a rake set up at two cross section planes. This would add the additional aspect of being able to compare not just pressure magnitudes, but also the dissipation and any translation or rotation of the wake as it moves downsteam. We also planned for one rake for the representative cornering case. 

The criteria for selecting the two straight airflow cross sections were as follows:

• Must be a sufficient distance apart in Z such that any translation or rotation are able to noticeably transform the wake "cloud" between the two planes

• Forward-most plane must be close to the front tyre to allow the measurement of near-zero total pressures, but still rearward enough that distinct flow structures have formed

• Must be in locations that allow easy, stable mounting for a rake 

The last two sub-figures in the image above show our preferred two candidates, which are shown again below with more context.

These positions sit conveniently at the middle and rear chassis nodes of the side impact structure, which will allow easy mounting. Rake design and mounting will be covered in the next post. 

Next we decided on sampling locations for each rake. The construction method for the rake was still unknown at this point, but was likely to be made of either sheet metal, or 3D-printed plastic. While plastic printing would allow complete freedom of design, we limited ourselves to positioning the majority of probes in a grid layout in case we were forced to go with a sheet metal design if the plastic ended up too flexible. Looking back, this was a strange decision as I can't really imagine a scenario where anything less than 2mm sheet metal would be more rigid than a well-designed PLA structure. 

We have twenty sensors to use. Two of these are dedicated to the pitot-static and yaw probes, and one as a spare to provide a temperature correction, leaving seventeen for the rakes. We wanted to have some probes sampling inside the wake to make sure we got some total pressure magnitudes, while the rest were dedicated to tracking the perimeter, many of which could return freestream values giving useful positional information but no magnitudes to correlate. By looking at differences in predicted shape between the two planes, we got an idea of which parts of the wake were likely to be more sensitive, and positioned probes more densely in these areas. 

Ground clearance (with a margin for bumps and roll) limited how low probes could be placed, but the simulated shapes suggested there wasn't much detail that close to the ground. 

The same was then done for the cornering case, where probe placement was a compromise based on both inboard and outboard wake shapes. The tight corners and resulting influence on airflow around the car necessitated a different probe placment/rake design for the corner case to the straight case to get the most out of 17 sensors. If we had a few dozen sensors to work with then a simple dense grid would have probably sufficed for all cases. 

By this point we had made the decision to use a 3D-printed plastic construction for the rakes, which opened up the potential for 3D surface design. While the Kiel probes we designed exhibited angle insensitivity, it was still preferable to align the probes to the expected local velocity vector as some vectors were up to 20 degrees from the freestream direction. Flow alignment of the rake structure itself will also minimise blockage effects.