I assume you meant "insignificant".
Yep. My mistake.
Kinda, sorta, but not really.
It is not about being under "ideal conditions". Laws in physics take into account the conditions, whether ideal or not. That is exactly why I said the above are NOT laws - simply because they do not take into account all the variables.
No human law can take into account all conditions. Even Newton's laws of motion, the basis for practically all applied physics, fail once you go extreme enough in velocity or scale.
Laws merely encode observations of a relationship between multiple variables or properties, which can be verified for similar cases under the same conditions of the observation.
A law is not required to cover everything, it only need to be correct and testable to what it specifically observes. Darcy's law is provable for -controlled- porous media flow, but it's useless by itself in modeling -say- contaminated water flow because you also have law of conservation of mass in play. The latter itself fails, in its classical form, to cater for effects such as nuclear decay. Newton's law of viscosity holds for its class of fluids, but try applying it to paint. Gravitation law does work (barring above limitations), but once you're inside an atmosphere, you'll have to subtract drag/buoyancy to get any meaningful force. etc, etc...
I admit that "ideal conditions" was not the best term to use. What I meant that laws show relationship between two or more variables
with everything else being equal. Although they may indeed have limited ranges of physical properties within which they hold (referring again to laws of motion).
For example, what is the "ideal condition" for a computer case fan? 70°F @ 50% humidity and 0mph wind? Says who? What about barometric pressure? Dust in the air? Input voltage? Blade design? Duct/vent characteristics? Positive/negative case pressure?
Affinity laws imply that fluid parameters (density, pressure, temps, etc) are the same between the two cases it observes. And this is ok, because those params are independent from the observed ones and vice versa, within the ranges you'd use a pump or a fan on, of course. You can have different discharges at the same temperature and density, you can have different rotational speeds without -significantly- affecting flow temperature, and so on.
Again, those "Laws" Shrek noted are just basic formulas to "estimate" needs for the field engineer.
How a law came to be and how it's used are two different things. I agree that using affinity laws like this gives only rough estimates, but that doesn't deny that it is a sound law.
Engineers are idiots; we will abuse anything to get a quick estimate, even if it wasn't meant to be used that way. Don't hold these poor formulae accountable for our sins...
