Noisy or stinky !

Today we are going to talk about a common phenomenon in life – “no stink, no stink”. Let’s look at this phenomenon from the perspective of fluid mechanics.


The composition of fart           


“Fart” is a mixture of a variety of substances, nitrogen, hydrogen, carbon dioxide, methane and oxygen account for more than 99% of the total, these gases are tasteless. The real odour comes from less than 1% ammonia, hydrogen sulfide, stench, volatile amine, volatile fatty acids and other pungent odour substances, although they are small but powerful, hydrogen sulfide is the chief culprit that makes fart stink.           

Cause Analysis 

Physiological interpretation of “stinky or noisy” is generally: when intestinal excretion increases, intestinal tract is not smooth, airflow will be relatively slow-release, its odour elements will increase (can be understood as farting, mostly stool time), so it is generally believed that fart will not stink, no stink.           

From the point of Reynolds number in fluid mechanics, this paper analyses that ” stinky or noisy “. Reynolds number is a measure of the ratio of inertial force to viscous force in fluid flow. It is defined as:           


Re= P *VD/ u


In the formula V is the velocity of fluid flow (according to scientific test, the average speed of fart ejection is 14 kilometres per hour); D is the geometric characteristic size of flow field (such as the diameter of anus); D is the density of fluid; and Mu is the viscosity of fluid. At the same speed, the same anus, the same density (due to the density of hydrogen sulfide and air almost the same, so its content has little effect on the overall density of the farts), the Reynolds number depends largely on the viscosity of the fluid.           

Because hydrogen sulfide is always mixed in the buttocks, most of them come from mucus. When the content of hydrogen sulfide increases, the viscosity mu increases, and Re = rhodium VD/mu, so as the content of hydrogen sulfide increases (that is, the odour of the buttocks increases), Re decreases.           

That is to say, the more smelly the fart, the smaller the Re will be, and the easier the gas will maintain laminar flow. The gas movement is smooth and orderly. The less stinky the fart is, the bigger the Re will be. The gas movement will change from laminar flow to turbulent flow (or turbulence). At this time, the internal movement of the gas is irregular and easy to produce a collision. Therefore, under the combined action of the fart, anus and air, it will produce vibration and make an indecent sound.           


How to keep farting silently?             


If you want to let the fart go off (that is, the value is smaller), you can properly reduce the ventilation speed (the same gas volume, expand the anal diameter can effectively reduce the gas velocity, so the increase of anal cross-section area will reduce the Reynolds number Re).


Therefore, keeping a calm state during the farting, slowing down slowly will effectively reduce noise; and standing or squatting is less noisy than sitting fart, which is why many people sit up and fend up the side of their hips when they want to fart, because this will temporarily expand the diameter of the anus, effectively reduce the number of Re and reduce the noise. These are just qualitative explanations of ” stinky or noisy “. As for the density (p) and viscosity (mu) of the fart, which is not controlled by a human, these two values are closely related to people’s food and physical condition. Of course, there will also be “sound and stink” and “no stink” phenomena in life.

One Response to “Noisy or stinky !”

  1. icrossing says:

    Funny topic but super interesting! I love having weird everyday things like this explained scientifically. I’m just a little confused by the part in the first sentence where it says “no stink, no stink.” Great job!