Everyone enjoys mythology, and whether it manifests within a person as part of their belief system as to create a reality or it is disregarded as a comical vestige; they persist in society mostly due to the prior. I like to eat a healthy serving of Occam’s razor with a side of Physical Law and a sprinkle of the Scientific Method. Based on that, you won’t find it hard to guess my view on mythology and the supernatural. However, very few myths were created as such, and most were considered fact when they were fresh ideas. They remain as a colourful painting of past events and beliefs, and the world would be a boring place if every new thought was correct the first time it was spoken aloud.
Still, there are myths with neither a current foothold in reality nor a place for them in the future. I want to talk about three myths with the staying power of the Rolling Stones. Funny story; for years I wondered what the first name was of the lead singer, “McJagger,” but that’s neither here nor there. I’ve spoken of these myths before, but they’re so important as to require regular lecturing. They are as follows:
“Flow makes it go.” No it doesn’t. Flow is the time constituent of hydraulic energy which is only created when there is first pressure. However, there is only one factor in our entire vast universe able to create motion, and that is force. Force, as a result of pressure applied to a piston in a cylinder, for example, is what creates motion. After force creates motion, flow is enabled. Cosford’s Law: Force makes it go. Flow is the rate in which pressure is created.
“Pressure is resistance to flow.” No it isn’t. If the resistance created the pressure, then everything in a hydraulic system would move backwards. Hydraulics isn’t immune to the Second Law of Thermodynamics, and entropy cannot decrease any more than it can decrease in a perpetual motion machine. Stated differently, fluid can only move from an area of high pressure to an area of lower pressure. Should downstream pressure ever be higher than pressure at the pump, fluid would start to travel back to the pump. Pressure existing at a load or restriction is merely the result of Newton’s Third Law of Motion, stating that every action has an equal and opposite reaction.
“Hydraulic fluid is incompressible.” No it isn’t. Although we are taught a liquid doesn’t compress as a gas does, this is just not factual. Although fluid doesn’t compress at a rate inversely proportional to pressure as a gas does, it actually compresses half percent per thousand PSI. The consequence, especially in high volume, high pressure systems, is huge amounts of dense, stored energy. This energy can be released violently if it is not controlled, which is why decompression valves exist. The compressibility of matter is also referred to as bulk modulus, and besides being a safety and reliability concern, it also affects performance. In a high response system, compressibility leads to sponginess of motion which must be factored in motion control simulations before a machine is manufactured.