Term
What is the rule for determining the pressure in a fluid flow exiting to the atmosphere? 

Definition
EXIT RULE: Subsonic flow always exits at ambient (atmospheric) pressure. 


Term
What is the Mach number and how is it used? 

Definition
Mach number is the ratio of the speed of a moving fluid to the speed of sound in the fluid.
 It helps determine if flow can be treated as incompressible
 low Mach # (<.3) usually represents fluid @incompressible flow



Term
Why is pressure typically a positive quantity? Discuss gas and liquid. 

Definition
Pressure typically ranges from 0infinity because we cannot have tension in a gas/fluid.
 Gas molecules are too far apart on average for the intermolecular fores to support the tensile stress and maintain cohesion
 liquid should in theory be able to support a tensile stress becase of its strong molecular forces. However, liquid boils as the pressure on liquid decreases to the vapor pressurevapor pockets prevent liquid from being in tension



Term
What formula is used to estimate the power required by a pump? 

Definition


Term
What is the Knudsen number and how is it used? 

Definition
molecular mean free path = λ
some representative physical length = d
 helps determine the best method of description (continuum vs. noncontinuum)
 if Kn# is close to 1: λ is close to length and we cannot use continuum method: must use Molecular Model



Term
What are the two types of forces in fluid mechanics? Describe. 

Definition
Body: long range & volumetric by nature.
 Acts ON the element's mass (ie EXTERNAL)
Surface: Short range & interfacial areal by nature.
 Exerted by pressure or shear through physical contact between element and its surroundings



Term
What is fully developed flow? 

Definition
 The flow is the same at every location in the pipe
 Velocity field does not change in flow direction



Term
What parameter measures the compressibility of a liquid? 

Definition
Bulk Modulus measures a substance's resistance to uniform compression
Δpressure
Δvolume 


Term
Why do we use a stress tensor instead of the stress vector? 

Definition
Stress Vector (Σ) can be used to calculate surface Force, but not to describe the state of stress in fluid, because it depends on orientation.
There are an infinite # of possible surface orientations for one point on the interior of a fluid.
Therefore Σ is not unique to a point in fluid.
Stress Tensor (σ) is orientation free so can uniquely describe a point in a fluid.



Term
How does the value of sea level atmosphere pressure relate to the total quantity of air in the earth's atmosphere? 

Definition
Sea level atmospheric pressure represents the weight of the column of air directly above a m^2 of Earth's surface.
The total weight of air in the Earth's atmosphere would be the sea level atmospheric pressure * Surface Area of the Earth. 


Term
What is the internal energy of a gas? How is it related to the kinetic energy? 

Definition
Internal E is primarily of function of T, mostly concerned with ΔU.
Kinetic E = mv^2
Higher U>Higher T>Higher Molecular V>Higher KE 


Term
What is the contact angle and what does it tell us? Sketch 

Definition
Helps us to find the net surface tension acing on the contact line because we don't know the surface tension between air and the surface of a solid
If Θ>90: not wetted
If Θ<90: wetted by liquid
If Θ=0: perfectly wetted 


Term
How do laminar and turbulent flows differ? 

Definition
Laminar: smooth flow, paths of each particle never interfere with each other. Velocity of fluid is constant at all points
Turbulent: irregular flow that is characterized by roughness and tiny whirlpool regions 


Term
How does the state of stress in a fluid at rest differ from that found in a fluid in motion? 

Definition
AT REST: Fluid has no shear stresses (only normal)
Σ=pn + 0
IN MOTION: Fluid exerts normal (pressure) and tangential stress on a surface
Σ=σn + τ 


Term
What is the defining characteristic of a Newtonian fluid? Name two such fluids. 

Definition
Fluid that obeys Newton's law of viscosity: τ=μdγ/dt
(shear stress is proportional to shear strain rate)
stress vs. strain rate = linear relationship w/ constant=μ
example: water & air 


Term
What is the inertial force? 

Definition
When Newton's 2nd Law is written in its inertial form
Σforces  d/dt mv=0
the product "d/dt mv" is called the inertial force because it has units of F.
It is in the nonaccelerating inertial reference frame 


Term
What does the "pressure/density" term in the energy equation represent? 

Definition
Ep=M*p/ρ=pV
P/ρ represents the perunitmass form of pressurepotential energy
ability of a fluid to perform mechanical work at high pressure 


Term
Explain the continuum hypotheis 

Definition
underlying molecular structure of a fluid is ignored>deliberate ommission in mathematical model
considers macroscopic properties at a point in the fluid as averages instead of individual molecules
assumes all properties are continuous flow
mean free path<<physical length
(almost always works in liquid b/c distance b/w molecules is small) 


Term
What is the inertial reference frame and why is the concept needed? 

Definition
Nonaccelerating frame that makes forces evident to an observer 


Term
What is the essential difference between a fluid and a solid? 

Definition
the response to shear stress which is tangential Force/Area
FLUID: Resists but cannot prevent deformation due to shear. continues to deform as long as shear is applied.
SOLID: Can prevent deformation. Quickly returns to rest and retains its new shape 


Term
What is the difference between an inviscid fluid and inviscid flow? 

Definition
Inviscid Fluid: means the fluid has viscosity=0
(no fluids technically exist)
Inviscid Flow: While fluid may have viscosity, the viscous effects are negligable (ex. boundary layer) 


Term
What is the state of stress in a fluid at rest? 

Definition
No shear stresses acting on a fluid at rest
states of stress is influenced by body forces
purley compressive forces and characterized by hydrostatic pressure distribution 


Term
What is a boundary layer? 

Definition
effect of viscosity
layer around plate
velocity changes smoothly and continuously from 0 on the plate to free stream value.
Thickness increases down the length from the leading edge
flow changes from laminar to turbulent
close to plate: fluid does not move 


Term
If you know the stress tensor, how do you get the stress vector? 

Definition
Σ=n·σ
take the dot product of the normal vector for plane and the stress tensor 


Term
Sketch and explain the concept of an outward normal on both closed and open surfaces 

Definition
n denotes the orientation of a surface element
n points at the fluid or the source of the force
plays a key role in description of surface forces
CLOSED: always points away from the interieor
OPEN: points from surface toward agent responsible for force 


Term
Draw a pitotstatic tube and describe its operation and outputs 

Definition
Pitottube can give Nose Pressure (Pn) or measures velocity in a fluid
Connected to a manometer 


Term
How is the concept of incompressible fluid different from the concept of incompressible flow? 

Definition
INCOMPRESSIBLE FLUIDS: density of the fluid is constant (Impossible to assume)
INCOMPRESSIBLE FLOW: density of a fluid particle doesnt change its density as it moves 


Term
Difference between mechanical and thermodynamic pressure? 

Definition
Mechanical: Represented and measured as p=F/A (almost always measurable)
Thermodynamic: macroscopic model describing molecules colliding. Can only be measured in fluid is @ equilibrium 


Term
When would the second coefficient of viscosity or bulk viscosity be significant? 

Definition
Signifcant when fluids with complex molecula structure undergo compression/expansion
Responsible for a viscous loss of energy when it occus rapidly
ie: interior of a shock wave: ultrasound 


Term
What is a buoyancy force? 

Definition
Net vertical force felt by an object immersed in a fluid at rest due to hydrostatic pressure 


Term
Explain the concept of steady flow 

Definition
Velocity field is independent of time 


Term
What is the key difference between liquid and gas? 

Definition
Response to compressive stress
Fixed amount of liquid occupies the same volume under most conditions>difficult to compress (large force=small dV)
Distance b/w molecules of gas is greater and variable so a small force=large dV 


Term
What is a barometer and what does it measure? 

Definition
A device flled with mercury or water that is used to measure atmospheric pressure.
used by meterologist to forecast shortterm changes in weather



Term
What is the reynolds number and how is it used? 

Definition
Re=ρVL/mu
The ration between inertial forces and viscous forces
measures the importance of inertial forces
Tells if the flow is laminar or turbulent
high reynolds #: dominance and turbular flow
low reynolds #: are laminar flows (viscous) 


Term
What three thrings affect the value of the surface tension coefficient? 

Definition
1) chemical composition of surface
2) temperature
3) charge on surface 


Term

Definition
compounds/acting agents that lower the surface tension of a liquid/interfacial tension
ex detergents, wetting agents, pulmonary surfactants



Term
What is a manometer and how does it work? 

Definition
liquid (water/mercury usually) filled tube that measures the pressure differene by the height difference of the liquid 

