Ahmed body and Aerodynamics

Hello everyone! Welcome back to Week 2 on transit thoughts.

The week's been a mixed bag - the initial dopamine rush of starting this newsletter had begun to fade. I'll admit, it was a tough task to bring myself up to writing this. But hey, if I'm going to give up, shouldn't it be after at least six months?

Just kidding! I'm here to stay, and this week, we're diving into Road Car Aerodynamics.

Have you ever heard of the "Ahmed Body"? It’s a term often encountered in CFD simulations. When I first saw what it was, I found it surprisingly simple – a cuboid with rounded corners and a slant at the back. It made me wonder, why not just call it a special box? Why is it relevant at all? Let’s find out.

What is it?

The Ahmed body, introduced by Ahmed, is a simple car model used to understand basic flow structures around vehicles. Ground vehicles are considered bluff bodies that move close to the road surface. Ahmed’s paper [1] aimed to (a) identify the time-averaged flow structures in the wake of a basic vehicle shape, and (b) analyze how body geometry affects wake structure, pressure distribution, and drag, especially at the rear end.

To design this model, it was crucial that it generated strong 3D displacement flow at the front, uniform flow in the middle, and a large wake structure at the back. The figure below shows an example of an Ahmed body, which has a length: width: height ratio of 3.36: 1.37: 1.

Why is it used as a benchmark?

The Ahmed body is simple, hence it can be replicated easily, yet it allows accurate flow simulations relevant to automobiles [2]. It demonstrates how to calculate the turbulent flow field around a simple car-like geometry. Different rear slant angles create various wake regions at the rear, which mainly cause vehicle drag. This simplicity and effectiveness make the Ahmed body a benchmark for validating numerical models and designing new car models.

My own Simulation explained

I felt it would be apt on my part to actually perform an investigation myself and see how the structures turn out. I wanted this to be quick. Unfortunately, it was anything but. I had to use openFoam because it is an open source software. I realised I had lost touch when I was struggling to even perform the Spallart Almaras model on the simple Ahmed body mesh that I had. But in the end, I got it.

The tutorial suggested I run the case for 3000 seconds, but my system's humble specs no way! So, I settled for a 165 seconds simulation and came up with some satisfactory results. I am still a bit rusty when it comes to openFoam, so I could not plot the Coefficient of Pressure values (Cp) values and neither was I able to calculate the aerodynamic coefficients. Since I had to send this article out by today, I am just printing whatever I got. The images of the simulation are given below. I ran this with the SA model, as I mentioned and with an inlet velocity of 40 m/s. Here is what I obtained

Notice how the velocity drops as soon as it hits the front of the bluff body. I believe this is called Stagnation. We can see the formation of vortices in the rear end of the body. These type of structures causes drag in the vehicles and this is my understanding that it needs to be avoided. I am still reading up on this matter and in the future, I will surely back this up with certainty.

My experience simulating this? - it was a mixed bag. The simulation kept throwing Sigfpe errors - basically showing that there was an instance in the analysis where a quantity was getting divided by zero - which is not natural and which should not be happening. I switched the solvers from GAMG to PCG to solve the pressure and it seemed to do the trick. I only hope I encounter less of these errors because believe me, these drain the life out of you. Especially if you are just getting started. But whatever you do, don't stop - because this is a steep learning curve and it only gets better.

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References

[1] Some Salient Features of the Time -Averaged Ground Vehicle Wake

[2] CFD Simulation of Flow around External Vehicle: Ahmed Body

[3] Google Images