Ideal vs Real Fluids & Fluid Flow Video Lecture - Civil Engineering (CE)

hey guys so next video we're going to
start talking about fluids in motion or
fluid flow and the first thing we're
going to cover is the distinction
between real fluids and ideal fluids
let's check it out all right so the
motion of fluids or the flow of fluids
can get pretty complicated it's actually
an area that's still under active
research in physics but we're going to
simplify things by using a model called
ideal fluid so the idea is that fluids
are pretty complicated but if we
eliminate a ton of the complexities the
more complicated complexities of real
fluids you end up with something that's
called ideal fluids they don't exist but
they are a way to make things more
manageable cool and there's three things
you should know first you should know
that these fluids will always be idea
fluids will always be incompressible
that's a simplification in complex
compressible means constant density
remember density has to do with how
tightly packed molecules are and then
ideal fluids that's going to be constant
in real fluids molecules could get
tighter or less dense less tightly
packed depending on what's going on okay
so incompressible constant density that
simplifies things a bunch the second
thing you should know is that ideal
fluids are always going to have what's
called laminar flow in laminar flow just
means steady flow so if you look at
water going to a pipe if it's let's say
see-through pipe or something like that
you would see that there's just a
constant stream of water that looks very
clean and neat as opposed to real fluids
that could have what's called turbulent
flow turbulent flow or turbulence you
could have turbulence if the liquid is
moving or the fluid more generally is
moving too fast okay so imagine if water
is going way too fast and it goes
through a little a little clot type
thing here some sort of constriction
then it could be that the water starts
going all over the place and this is
turbulence and this is generally bad
news lucky for you you're probably not
going to see any turbulence questions
you may just have to know this
conceptually cool and the third
distinction between them and I'm going
to start over here is whether this
motion is going to have viscosity
whether the fluid is going to have
viscosity or not
okay so the defining characteristic this
is the most important of the three here
the defining characteristic of real
fluids is that they have what's called
viscous flow viscous flow in other words
the liquid has viscosity and viscosity
has to do with the thickness thickness
of the fluid so for example honey right
if you get a cup full honey and you turn
it like this it's going to move very
very slowly it's because there's a lot
of discuss 'ti ideal fluids have no
viscosity at all they have no resistance
viscosity is essentially fluid friction
it's essentially fluid friction and it
works very similar to air resistance or
a kinetic friction and that it slows it
down okay so real fluids could have have
viscosity and they could have a viscous
friction ideal fluids are always going
to have what's called non viscous not
very creative name non viscous flow in
other words no friction so that's the
big difference if you have an ideal
fluid it's going to flows it's going to
flow smoothly with no friction a real
fluid could have turbulence and it has
viscosity okay now lucky for you most
problems you see and maybe even all of
the problems you see will be about ideal
fluids in fact a lot of professors don't
even get into real fluids so if yours
doesn't your life is simpler we're going
to also assume ideal fluids unless
something says explicitly that this is a
real fluid or if they refer to viscosity
which is fluid resistance right so if
they say that there's some sort of
viscosity then that means there's
resistance what you mean it's a real
which now it means that you have a real
fluid in most real fluid problems you're
going to have viscosity because it's the
defining characteristic but you're not
going to have turbulence and you're also
not going to have compression of the
fluid okay so that you're really not
going to see this most of the time
they're not going to see this most of
the time but you are going to see this
quite a bit so you're going to have no
turbine
hence in no compression of the fluid
okay so really it's going to come down
to whether or not it has viscosity so
it's a quick control so you can know
some of the terminology and that's it
let's keep going