Fluids Flashcards
Fluids
are able to flow and conform to shapes of their containers; ie: gases, liquids
What is the equation for density?
p = m/V
Units: kg/m^3 or g/mL or g/cm^3
What is the density of water?
1 g/cm^3 = 1000 kg/m^3
What is the equation for an objects weight when you have density?
Fg = pVg
What is the equation for specific gravity and why is it useful?
SG = p/ 1 g/cm^3
It is a useful tool for determining if an object will sink or float in water. If the SG > 1, then the object will sink. If the SG < 1, then the object will float
What is the equation for pressure?
P = F/A P = pressure F = magnitude of the normal force vector A = area Units = Pascal (Pa) = 1 N/m^2, mmHg, torr, atm
What are the conversions between the four units of pressure?
1.013 x 10^5 Pa = 760 mmHg = 760 torr = 1 atm
What is absolute/hydrostatic pressure?
the total pressure that is exerted on an object that is submerged in a fluid
What is the equation for absolute pressure?
P = P0 + pgz P = absolute pressure P0 = incident/ambient pressure (surface pressure) p = density of fluid g = a due to gravity z = depth of the object
What is gauge pressure?
the difference between the absolute pressure inside an object and the atmospheric pressure outside
What is the equation for gauge pressure?
Pgauge = P - Patm = (P0 +pgz) - Patm
Pgauge =
P = absolute pressure
Unless the surface pressure is equal to atm pressure, then Pgauge = pgz
Hydrostatics
the study of fluids at rest and the forces and pressure associated with standing fluids
Pascal’s Principle
a pressure applied to an incompressible fluid will be distributed undiminished throughout the entire volume of the fluid
Hydraulic system
Operate on pascal’s principle to generate mechanical advantage
What equation would you use for a two piston hydraulic system utilizing pascal’s principle?
P = F1/A1 = F2/A2
What is Archimedes’ Principle?
states that a body wholly or partially immersed in a fluid will be buoyed upwards by a force equal to the eight of the fluid it displaces
What is the equation for buoyacy?
Fbouy = pfluidVfluiddisplacedgravity = pfluidVfluidsubmergedgravity
What is surface tension?
causes the liquid to form a thin but strong layer like a “skin” at the liquid’s surface
What is cohesion?
the attractive force that a molecule of liquid feels toward other molecules of the same liquid
What is adhesion?
the attractive force that a molecule of the liquid feels toward the molecules of some other substance
Fluid Dynamics
study of fluids in motion
What is viscosity?
the resistance of a fluid to flow (n)
What is viscous drag?
a nonconservative force that is analogous to air resistance
What is the units for viscosity?
Pas = (Ns/m^2)
What kinds of flow can a fluid have?
Laminar flow - smooth and orderly
Turbulent flow - rough and disorderly
How do you calculate laminar flow?
Using Poiseuille's Law: Q = pi*r^4*deltaP/8*m*L Q = flow rate (volume flowing per time) r = radius of the tube deltaP = pressure gradient n (eta) = viscosity of the fluid L = length of pipe
What is the relationship between the pressure gradient and the radius of a tube in laminar flow?
it is inverse exponential to the fourth power
slight changes in radius have significant effects on the pressure gradient
What are eddies?
swirls of fluid of varying sizes occurring typically on the downstream side of an obstacle
What is critical speed?
a speed of flow that if exceeded, the flow turns into turbulent flow. critical speed depends on the physical properties of the fluid and the diameter of the tube
What is the boundary layer?
a thin layer of fluid adjacent to the wall that is in laminar flow when the rest of the fluid has passed the critical speed in turbulent flow
What is the equation for critical speed?
v(subc) = N(subR)*n/pD v(subc) = critical speed N(subR) = Reynolds number (will be given) n = viscosity of fluid p = density D = diameter of the tube
What are streamlines?
indicate pathways followed by tiny fluid elements (some times called fluid particles) as they move
Flow rate vs linear speed
Flow rate is constant (for a closed system) - volume per time while linear speed changes relative to cross sectional area and is not constant. Linear speed is a measure of the linear displacement of fluid particles in a given amount of time
What is the equation for flow rate (aka the continuity equation)?
Q = v1/A1 = v2/A2
Q = flow rate
v1 and v2 = linear speeds of the fluid at points 1 and 2
A1 and A2 = cross-sectional areas at these points
What is Bernoulli’s equation?
P1 +1/2pv1^2 + pgh1 = P2 +1/2pv2^2 + pgh2 P = absolute pressure of the fluid p = density of the fluid v = linear speed g = gravity h = height of the fluid above some datum
What is energy density and how does it relate to pressure?
Energy per cubic meter which is equal to pressure. higher pressures have higher energy densities
Venturi flow meter
an hourglass shaped tube on it’s side with two tubes sticking straight up, one on one side and another in the middle part. When fluid flows through, it has to increase it’s linear speed in the narrow middle and pressure decreases so the liquid in the vertical tube decreases relative to the other tube
What is the Venturi effect?
with a lower absolute pressure, the column of fluid sticking up from the venturi tube will be lower at the middle vertical tube than the other vertical tube
