You can(’t) leave your head on. Headless Horseman leverages Simcenter for more sustainable horror

By Simon Fischer and Stephan Gross

Story: Simon Fischer; Simulations/Visuals: Stephan Groß

Leuven/Sleepy Hollow – It scares the heck out of me. For a moment, I couldn’t resist and looked straight at the place where his head should be. It’s already almost 40 degrees Celsius and climbing in the huge hall and I couldn’t tell if it was pure fear or the sheer heat that was making me sweat.

I have interviewed many customers in my marketing career, and admittedly I’m always a bit nervous upfront, but this one… it’s just pure horror. I have the usual cheat sheets in my – more than usual – shivering hands, with all the usual questions lined up:

What’s your name? Which company do you work for? What’s your role? Which challenges are your industry facing? And your company? And you as an engineer? What role does simulation play to resolve this? Etc. etc.

But honestly, all I could think about while our camera team gets set up is:

Don’t lose your freakin’ head!

We are in our testing facilities in Leuven. Opposite me sits a man on a comparatively tiny chair (well it feels normal size for me though), 2 meters 20 tall, dressed up like a Hessian soldier from the 1790s. His axe is leaning on the side of the chair, the blank metal almost blinding me as the camera spots flash over it. To ease my stress, I let my view wander into the background. Next to our Aeroacoustics testing facilities the Simrod demonstrator vehicles are lined up. It is a welcome distraction to think about the engineering beauty in them. Temporary relief from the sinister man. That is until my view hits the horse parked right next to the Simrods. Equally dressed up like his master in heavy armor, and with flaming eyes, that beast is somehow even more ominous.

Up until that point, somebody not knowing the truth probably would just be thinking that this is all a Halloween prank. And so the engineers passing by the hallway smile at the scene and laugh out loud as they see the horse and the strange man in front of the camera.

But it’s not exactly fun for me. The disturbing reality is that this soldier sitting in front of me has no head on his shoulders. This is not a prank or a talented makeup artist’s best trickery. Rather he holds his head, looking like the most freaking pumpkin one can imagine under his arm. With flaming eyes that seem to burn you the moment you keep staring into them for a second too long…

“All right! Ready” says our cameraman of the day. “Heads-up. Let’s get rolling.”

“Don’t lose your head,” I keep telling myself, “don’t lose your freakin’ head”. And off we go…

Let’s get rolling Mr. Headless Horseman

Ehm all right Mr. – or is it Sir? – Headless Horseman. That’s your name right?”

“Mr. is just fine. And with or without head… I don’t care” he goes (or better the pumpkin head in his hand goes) and the body next to it chuckles so heavily that I almost fall from my chair.

“Ok then, Mr. Headless Horseman, can you briefly introduce yourself, the company your work for, and what you are doing there?”

The interview starts rolling and as we dive into his story I start to relax a bit. If you can forget for a moment that this guy literally lost his head in a war 200+ years ago, you can seriously get excited about the amazing engineering success story he is about to tell…

“You know, I have been in this business for more than two hundred years now. I’ve scared generations of people. But these days!? It’s not the same anymore. Our industry is at a crossroads. When I started in 1790 nobody cared about the environment. I was just riding around scaring folks making my cut if you know what I mean.

“I think I do (but thanks for the reminder)”

“Even with the industrial revolution, things remained rather straightforward. People remained frightened by us, and kept us alive with their tales, but never did they question our environmental impacts. Nobody in 1900 went: oh yeah that headless horseman, how can he even dare to ride that horse. Has he not thought about his carbon footprint. And this ignorance continued for another century. In the 1970s it was such an oblivious time that one scary figure after the other popped off the ground! People made movies about us. Many movies. It was an amazing time. We were heroes. But then over the last decades, many things changed… I tell you this the Horror business is undergoing a serious era of disruption.”

The green disruption of horror

What changed?”

We are under massive pressure. People no longer accept that we wildly go around scaring people and not caring about the planet. Evil forces [says the creepy headless horseman warrior in front of me] have started to implement regulations for everything. And make no mistake- those folks don’t even shy away from the horror industry. Greenhouse gas emissions are regulated, even for my breed. And that has massive disruptive impacts. If we don’t adopt quickly…It’s a massacre!

“Can you give an example?”

Take Christine, proper V8, no big deal in 1978 but today? Bet she’s either electric in the meantime or out of business…

Then I talked to Leatherface the other day. Guess what, the man is forced to go all-electric now. I mean electric! Who is scared of an electric chainsaw? Pennywise just said to LF this is it. Now People think you are just a clown. Welcome to the club!

So next up they even went after me and my horse. Not green enough they said! I mean this is highly controversial. Depending on who you ask and how you look at it, a horse is either safe this planet or is the biggest climate killer ever.

Walking dead is not an option

For a moment I thought about getting rid of the horse and walking. I called up Freddy. But well, Freddy, he has this benefit of acting in a dream world so zero-emission mobility is not an issue to him anyways. Next, I contacted all those 80s and 90s slasher guys. They have this secret, you know: Jason, Michael, Ghostface, the guy who knew what they did last summer. No one was willing to tell me how they always seem to move super slowly and calmly when they chase after sporty youngsters in their prime, and yet they still catch them one by one. I have been doing this job for 200 years, but that is a serious mystery to me. So, I asked them about their secret but each one declined to reveal it.

“So walking dead was not an option?”

No, it was pretty clear to me that no horse is no option for me. I mean I have been a riding soldier all my life, not one of these shabby infantry slashers. I don’t intend to give up that status symbol.

So I pushed back on the evil authorities. Never will you get my horse. I would rather lose my head than give up my ride.

Well, that sounds like a plan, they went! We can live with that! (They didn’t really care whether I could)

You can’t leave your head on!

In desperation, I called up Dullahan. I don’t like that guy but I thought he must be in a similar situation and he might have a solution…and guess what he said!

“That stupid pumpkin…”

“You mean my head!?

“Yeah whatever. Get rid of it!”

“What? Are you kidding me?

“Dump it!”

No way. I am carrying this head…”


“…head, since I lost my real one. It’s part of my brand, it’s become part of my DNA.  I make almost all of my profit out of it.  Halloween party engagements, you know. It gives me this less scary -yet scary enough vibe!

“Get rid of it.”

Why would I?”


“Drag? What do you mean – drag?

“Aerodynamics! Ever heard of it?”

“Look, I am a Hessian soldier from 1790 why would I care about aerodynamics.

“Because -and correct me if I am wrong here- you want to continue to scare people.”


“See, and to keep doing so you need to go green. And as you don’t intend to reduce your horsepower, aerodynamics is your best option. And that means, get rid of that stupid pumpkin on your shoulders. Look, I do pretty well without a head and some studies prove that such a pumpkin head can have quite some aerodynamic impact! If you now hand that in, you reduce your drag quite a bit, become a little greener, and please those folks quite a bit. So maybe you’ll stay in business… “

“Then he showed me these ever-occurring Halloween jokes of a CFD simulation of pumpkin aerodynamics.

Seriously? Those „studies“! An isolated pumpkin. I mean, What does that have to do with reality?”

“Your call. He went. I decided to get rid of that head lately. It’s as simple as that: if you want to stay in business: You can’t leave your head on! Oh and looks ridiculous on you anyways!”

And then he hung up.

It took me quite some discipline to not start laughing out at that point. I mean two headless horsemen on the phone discussing aerodynamics of a pumpkin head. How stupid can a story be!? Luckily the man in front of me – whom I started to kind of like – continued in excitement so left me no time to laugh anyway …

How much greener can a Headless Horseman truly become without a fake pumpkin head?

I admittedly don’t like this guy, but maybe he had a point. And even if he was wrong, then proving that alone would feel so evil-good. So I figured I needed to get to bottom of this before losing my head:

How much greener can a Headless Horseman truly become without a fake pumpkin head?

Only science could answer! And aerodynamics. And drag.”

“So, I knew I had to do this thoroughly.

I started contacting wind tunnels. Gosh, those prices! I mean we have inflation, but seriously? And to add to the horror I need climatic test conditions as well (it can be pretty cold in Sleepy Hollow at times, and I intend to work more than just from the summer season to Halloween)…” 

“No, wind tunnel tests were out of the question. Too expensive. Back to the drawing board. I could already see that head of mine becoming pumpkin pie and me becoming another headless horseman with no signature look.  Like that soulless Irishman. I saw myself no longer getting the license to scare. Declared as dead as the internal combustion engine. Riding a unicycle. A clown like Pennywise, or… Leatherface. Either way, I saw the end end of my career.

Who would have thought that the key to sustainable horror lies in digital twins!

And then I stumbled over this Virtual Reality showcase with a CFD simulation of two cars at a huge convention… It showcased external aerodynamics of two variants of the same vehicle with and without windshield. Now, that is pretty close I thought and got in touch with the engineers behind that study. That’s when I got in touch with Siemens.”

So here was the solution at my fingertips! CFD! Computational Fluid Dynamics!

But not just some CFD. I knew I had to model the complexity to reveal the truth. I knew one can’t answer the capital question with just some simple aero of a pumpkin study. No, that horse, the pumpkin and I needed to go into a decent virtual wind tunnel. And I had to be fast because Halloween season was around the corner and I needed to go green beforehand. And indeed, with the decision for CFD things went very fast. I was impressed by how fast.

Leveraging the power of Simcenter STAR-CCM+, Siemens’s multiphysics CFD software, you guys quickly got me and my horse into the virtual wind tunnel, just swap a bit of CAD and off we went. In no time the engineers at Siemens had run the CFD simulations and came back with the results. I was impressed”

Thanks to Simcenter STAR-CCM+ we had a serious prediction of aerodynamics and related drag coefficients with and without the pumpkin head. Who would have thought that the key to sustainable horror lies in computational fluid dynamics?!

The Headless Horseman

Drag or drop? Aerodynamics of the Headless Horseman

Or not head?

The study by the Siemens engineers allowed us to precisely assess the effect of not wearing the pumpkin head. The Headless Horseman received insights that went beyond just a plain number: Thanks to the detailed CFD results the effects of the pumpkin head on the flow field could be understood and analyzed in detail.

Even for a rookie in aerodynamics like the Headless Horseman, the effect of the pumpkin on his head could be understood with the help of CFD. As the data reveals there is a noticeable reduction of the recirculation area in the wake of the Horseman, if he goes without top. And with the help of collaborative Virtual Reality in Simcenter STAR-CCM+ the engineers could even walk the bodyless pumpkin head through the CFD results.

Well, I am neither an engineer nor an expert in aerodynamics. But not only did the CFD engineers run the simulations quickly and come up with the quantitative assessment, thanks to collaborative VR in Simcenter STAR-CCM+ they also did an amazing job in explaining those results to me.

But the killer question remained: How much will it really help to drop that head? Was Dullaham right or did he just try to fool the competition? How much energy will the Headless Horseman truly save by no longer carrying that pumpkin head? And what will that mean for his carbon footprint?

The answer is 7.

Seven percent less drag force can be obtained by losing your head, Mr. Headless Horseman. That means 7% less drag-related energy losses. This is at 6 meters per second and translates into a drag coefficient reduction of 2.5%.

And while all the kids go “Trick or Treat” tonight, with 7% potential savings, the Headless Horseman with the pumpkin head will now need to answer this final question – for the sake of becoming a little greener:

“Drag or Drop!?”

Whatever he decided, watch your heads tonight everyone!

This article first appeared on the Siemens Digital Industries Software blog at