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FLUIDS MIDTERM 1
Short Essay Questions
37
Aerospace Engineering
09/18/2012

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 0-infinity 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 pressure--vapor pockets prevent liquid from being in tension
Term
 What formula is used to estimate the power required by a pump?
Definition
 Power=Q*Δp
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. non-continuum) 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 non-accelerating 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 per-unit-mass form of pressure-potential 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<
Term
 What is the inertial reference frame and why is the concept needed?
Definition
 Non-accelerating 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 pitot-static tube and describe its operation and outputs
Definition
 Pitot-tube 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 short-term 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
 What is a surfactant?
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
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