Inside a vehicle is both one of the best and worst places to release a plume of your own nauseating personal fumes. On one hand, being alone means you can liberally break wind without worrying about being shamed. On the other, being trapped in such a small space means escaping from the subsequent odor can be difficult. The solution is simple, though: Roll down them windows, fool.
But then again, since I have a deep-rooted appetite for divine knowledge, I genuinely want — no, I need — to know exactly how long the windows should remain down to release the entire stench cloud from my vehicle, depending on what speed I’m driving. So I reached out to the only person I know both capable of and willing to answer such a ludicrous question — Alex Klotz, an assistant professor in the department of physics and astronomy at California State University, Long Beach.
Before we dive into his response, I should mention that coming up with a conclusive answer to this question is almost certainly impossible. For starters, you’d need to consider the aerodynamic characteristics of the vehicle at hand, as well as things like the size of the side view mirrors and capabilities of the HVAC system. You’d also need to take into account environmental factors, like the presence of wind, as well as the temperature and volume of the fart, both which could affect how it moves about the small space. Needless to say, the individual circumstances of every fart in every car at every speed in every condition are extremely specific, and therefore, their exact dispersal rates are virtually incalculable.
Nevertheless, Klotz did his best to quell my demand for answers. “The main physical effect here is Bernoulli’s principle, in which a fluid that’s increasing in velocity experiences a corresponding drop in pressure,” he explains. Pressure is vital in this situation, since the high outside pressure should theoretically enter the vehicle to fill the lower inside pressure, pushing the fart outside in the process. “In this scenario, the air is speeding up as it goes over the car, and the pressure in the car remains the same, so there’s a pressure difference between the interior and exterior of the car. This pressure difference can be calculated and is proportional to the square of the velocity of air moving past the car (when you’re in the car, it seems like the air is moving past you).”
“Plugging in some numbers,” Klotz continues, “going 100 miles per hour gives a pressure difference of about one percent of the atmosphere, equivalent to the drop in pressure from the ground to the top of a 300-foot hill. This difference is smaller at lower speeds — about half as much at 70 miles per hour.” So while we’ve yet to come upon any specific numbers, you can see how the higher pressure difference at higher speeds can contribute to removing a fart from inside a car quicker.
Continuing the equation, Klotz says, “In principle, you could argue that the necessary time for airflow is inversely proportional to the pressure difference — based on Darcy’s law, which doesn’t quite apply here, but roll with it — so that you’d have to lower the window for four times as long at 40 miles per hour as you would at 80.”
Sounds like we’re getting somewhere! Oh, wait…
“However,” Klotz emphasizes, “that’s probably wrong. With a lot of things in fluid dynamics, which include airflow, you basically have to do a simulation (or an experiment) of the specific scenario. It’s hard to generalize from fundamental principles to specific scenarios. The closest thing I could find was a paper looking at airflow to cool Indian buses with the windows down and the air conditioning off — they built a tiny bus, filled it with dye, put it in flowing water and watched the dye get sucked out of the mini bus. They also compared that to simulations and an experiment with smoke on an actual bus. Sadly, I couldn’t find any literature about farts in a car.”
The researchers in this study also had trouble figuring things out, which poses even more problems for my purposes of coming to some kind of convincing answer. “The studies show that flow inside a bus with open windows is complex,” the research concludes.
Making things worse, Klotz mentions that rolling down the windows might not even help that much. “If the fart gas has spread through the air, it’s everywhere and flowing some of the air out won’t necessarily help,” he says. “It would only be beneficial if the odor plume itself got sucked out the window by the Bernoulli effect.” Which is honestly asking a lot of physics.
What I can share to at least provide some help to anyone who regularly farts in their car is this tidbit Rob Murdoch, a technical expert when it comes to vehicles, provided to Autotrader:
“All else being equal, a general rule is to open the driver’s window a couple of inches, and the rear passenger window half way. This causes a turbulent diagonal cross-shear of the air-mass through the cabin, meaning the offending smell will be drawn across and to the rear of the vehicle, and out the window expediently.”
So to review what we learned today, driving faster should theoretically speed up the removal of a fart from the interior of your vehicle, but also, there are still some questions in this world that even science can’t solve.