Accurate CFD Simulations & 'Not Straight-Line Testing'

Over the past decade, many development programs have increasingly moved directly from CFD to production, bypassing traditional wind tunnel testing. This shift isn’t just about reducing costs, advances in computing power now allow for highly detailed models and the ability to test across multiple flow conditions, ensuring the production of accurate CFD simulations.

In a world that increasingly bypasses a layer of physical testing, it becomes essential to create designs that are robust to changes in on-coming flow. Moreover, we must recognize that ‘real world’ conditions do not always align with our CFD models.

Thankfully, there are often some simple changes that we can make to our testing methodologies to encourage robust engineering and accurate CFD simulations. It is why, for example, we encourage testing with a small amount of yaw on straight-line car simulations.

Not quite straight-line testing

Adding a small amount of yaw to straight-line flow conditions to reflect real-world conditions in CFD

Race cars will reach their highest speeds along the straight sections of the track. However, the maximum speed that can be obtained on a given straight will be determined by a number of factors such as the engine power, gear ratios and, importantly for us, the aerodynamic drag – essentially, how much the car resists being moved through the air.

Consequently, aerodynamicists simulate their vehicles in straight-line conditions, seeking design improvements that will reduce drag.

In this setup, we typically see:

A lower, flatter ride height compared to corner conditions. The increased aerodynamic load at higher speeds tends to push the car towards the ground.
No body roll or steer of the wheels.
The wind coming straight onto the vehicle, i.e. no ‘yaw’.

RANS approach

Although there has been a move towards running transient CFD simulations (where we model flow oscillating with time), a popular method remains the Reynolds Averaged Navier-Stokes (RANS) approach where a ‘steady-state’ or time-averaged approximation of the flow field is calculated. Due to RANS solvers offering good accuracy at much reduced calculation times.

computer showing consistency in bramble CFD software

In a RANS simulation with a straightline (no yaw) flow field and a symmetrical car, we’ll end up with a symmetrical flow field. That is to say, the flow on the left side of the car will be a mirror of that on the right. As a result, we can precisely tune our aerodynamic designs to the on-coming flow. Safe in the knowledge that what we do on the left side of the car will work on the right.

The problem with this approach is that the car will rarely encounter a true straight-line (symmetrical) flow. For instance, there might be a crosswind affecting the vehicle’s dynamics, or the car could be drifting across the track. Additionally, it may be overtaking another vehicle, which further complicates the flow conditions. Regardless of the reason, our design will ultimately not experience the exact conditions for which it was optimized. Consequently, we need to consider these real-world factors in our CFD simulations to ensure our designs perform effectively under various scenarios.

The reality

Consider this simplified example. We’re looking down on two vanes, one being on the left of the car and one on the right. These vanes need to be a mirror of each other so they are symmetrical left to right on the car. We want to minimise drag in our straightline flow field. So the simplest thing to do is make them thin and aligned to the flow.

Create accurate cfd simulations reflecting real-world conditions by changing yaw

However, in reality, our vanes will experience a small amount of yaw. Being thin, the flow easily separates from their leading edges creating an unwanted loss in performance. The gains achieved in CFD doesn’t carry through to the real world and our hard work is rubbished!

adding small amount of yaw to reflect real-world conditions and create accurate cfd simulations

The solution

In closing, if we had started development with a small amount of yaw, we would have introduced some asymmetry into the flow. With the left-hand vane seeing air coming from inboard to outboard, but the right vane seeing it coming outboard to inboard. To handle this, we would have perhaps made the vanes thicker with a more rounded leading edge.

developing real-world conditions in CFD by adding yaw

A small change to the model setup has us closer to creating real-world conditions in CFD and nudged us into making a more robust design. Something that is more likely to work when transferred to the real world.

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