Ch. 4: Fluids Flashcards
defn: fluids
characterized by their ability to flow and conform to the shapes of their containers (liquids and gases)
defn: solids
do not flow, are rigid enough to retain a shape independent of their containers
aka: tangential forces
shear forces (only withstood by solids)
defn + scalar or vector: density
ratio of mass to volume
scalar quantity –> has no direction
what is a milliliter equivalent to?
a cubic centimeter
how many liters are in a cubic meter?
1000!
defn + unit: specific gravity
the density of a fluid as compared to that of pure water at 1 atm and 4 deg C
unitless
what can specific gravity be used as?
a tool for determining if an object will sink or folat in water
defn + SI unit + scalar v. vector: pressure
a ratio of the force per unit area
SI unit: pascal (Pa)
scalar, magnitude but no direction
what are 3 commonly used units of pressure other than Pa?
mmHg
torr
atm
why is pressure a scalar and not a vector?
place a surface inside a closed container filled with gas
the individual molecules will exert pressure that is the same at all points within the container
pressure applies in all directions at any point
is atmospheric pressure the same everywhere in the world?
no! it changes with altitude
defn + aka: absolute pressure
aka: hydrostatic pressure
the total pressure that is exerted on an object that is submerged in a fluid
defn + aka: incident pressure
aka: ambient pressure
the pressure at the surface of a fluid
defn: gauge pressure
the difference between the absolute pressure inside the tire and the atmospheric pressure outside the tire
AKA
the amount of pressure in a closed space above and beyond atmospheric pressure
defn: hydrostatics
the study of fluids at rest and the forces and pressures associated with standing fluids
defn: incompressible fluid
fluids with volumes that cannot be reduced by any significant degree through application of pressure
defn: Pascal’s principle
for incompressible fluids, a change in pressure will be transmitted undiminished to every portion of the fluid and to the walls of the containing vessel
defn: hydraulic systems
systems that take advantage of the near-incompressibility of liquids to generate mechanical advantage
how do hydraulic systems generate output force?
by magnifying an input force by a factor equal to the ratio of the cross-sectional area of the larger piston to that of the smaller piston
are the volumes of fluid displaced by piston 1 and piston 2 equal or different in a hydraulic system?
equal!
defn + layman’s: Archimedes’ principle
a body wholly or partially immersed in a fluid will be buoyed upwards by a force equal to the weight of the fluid that it displaces
in layman’s terms: any object placed in a fluid will cause a volume of fluid to be displaced equal to the volume of the object that is submerged
how can we visualize the buoyant force?
it is the force of the liquid trying to return to the space from which it was displaced, thus trying to push the object up and out of the water
is the buoyant force due to the liquid or the object?
the liquid!
will an object float no matter what it is made of and its mass if its average density is less than or equal to the density off the fluid into which it is placed?
yes!
talk about objects being submerged in relation to the specific gravity of water
if we express the object’s specific gravity as a percent, this directly indicates the percent of the object’s volume that is submerged (when the fluid is pure water)
object SG < or = 1 –> object floats
object SG > 1 –> object will sink
object SG = 1 –> 100% of the object will be submerged but it will not sink
effect: surface tension
causes the liquid to form a thin but strong layer like a “skin” at the liquid’s surface
defn: cohesion
the attractive force that a molecule of liquid feels toward other molecules of the same liquid
the force behind surface tension
explain the process of surface tension in terms of the intermolecular forces between the separate molecules of liquid water and how it differs for molecules below the surface and molecules on the surface
molecules below the surface: there are attractive intermolecular forces coming from all sides (they balance out)
molecules on the surface: molecules only have these strong attractive forces from the molecules below them –> pulls the surface of the liquid toward the center –> establishes tension in the plane of the surface of the water
when there is an indentation on the surface (like a bug’s foot), then the cohesion can lead to a net upward force
defn: adhesion
the attractive force that a molecule of liquid feels toward the molecules of some other substance
defn: meniscus
+ why does it form
a curved surface in which the liquid “crawls” up the side of the container a small amount
forms when the adhesive forces are greater than the cohesive forces
defn + why does it form: backwards (convex) meniscus
meniscus with the liquid level higher in the middle than at the edges
occurs when the cohesive forces are greater than the adhesive forces
defn: fluid dynamics
the study of fluids in motion
defn: viscosity
the resistance of a fluid to flow
defn: viscous drag
a nonconservative force that is analogous to air resistance
increases with increased viscosity of a fluid
defn: inviscid
ideal fluids with no viscosity
defn: laminar flow
smooth, orderly
modeled as layers of fluid that flow parallel to each other (the layers will not necessarily have the same linear speed)
defn: Poiseuille’s law
a way to calculate the rate of laminar flow through a pipe or confined space
what is the relationship between the radius and pressure gradient of laminar flow through a pipe or confined space as described by Poiseuille’s law
what does this translate to tangibly
inverse exponential to the fourth power
even a very slight change in the radius of the tube has a significant effect on the pressure gradient assuming a constant flow rate
defn: turbulent flow
rough, disorderly
causes the formation of eddies
defn: eddy
swirls of fluid of varying sizes occuring typically on the downstream side of an obstacle
when can turbulence arise in unobstructed fluid flow?
when the speed of the fluid exceeds a certain critical speed which depends on the physical properties of the fluid
defn: boundary layer
the thin layer of fluid adjacent to the wall where laminar flow occurs even when the rest of the fluid has complex flow patterns due to the critical speed of the fluid being exceeded
defn: Reynolds number
a dimensionless constant that depends on factors such as the size, shape, and surface roughness of any objects within the fluid
defn: streamlines
indicate the pathways followed by tiny fluid elements as they move
what is the relationship between the velocity vector of a fluid particle and the streamline?
the velocity vector of a fluid particle will always be tangential to the streamline at any point
do streamlines cross each other?
no, never!
defn + char for closed system: flow rate
volume per unit time
constant, independent of changes in cross-sectional area
defn: linear speed
a measure of the linear displacement of fluid particles in a given amount of time
does linear speed change relative to cross-sectional area?
yes!
impact: continuity equation
tells us that fluids will flow more quickly through narrow passages and more slowly through wider ones
what are the two principles of conservation that lead to Bernoulli’s equation?
- continuity equation arises from the conservation of mass of fluids –> for a constant flow rate, there is an inverse relationship between the linear speed of the fluid and the cross-sectional area of the tube: fluids have higher speeds through narrower tubes
- fluids that have low viscosity and demonstrate laminar flow can also be approximated to be conservative systems
defn: dynamic pressure
the pressure associated with the movement of a fluid (essentially the kinetic energy of the fluid divided by the volume)
defn: term pgh of Bernoulli;s equation
the pressure associated with the mass of fluid sitting above some position
defn: energy density
a ratio of energy per cubic meter
essentially pressure
defn: static pressure
essentially absolute pressure
defn: Bernoulli’s equation
+ defn in terms of energy conservation
the sum of the static pressure and the dynamic pressure will be constant within a closed container for an incompressible fluid not experiencing viscous drag
energy conservation: more energy dedicated toward fluid movement means less energy dedicated toward static fluid pressure (and less movement means more static pressure)
defn: pitot tube
specialized measurement devices that determine the speed of fluid flow by determining the difference between the static and dynamic pressure of the fluid at given points along the tube
defn: Venturi effect
as the tube narrows, the linear speed increases
thus, the pressure exerted on the walls decreases, causing the column above the tube to have a lower height at this point
describe the circulatory system in terms of fluid dynamics
it is a closed loop that has a nonconstant flow rate
defn: pulse
the feeling and measurement of the nonconstant flow rate of the circulatory system
