fluids - nc state university · pseudo-plastic fluids: shear-thinning fluids, often solutions of...
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Fluids Models
“Water is fluid, soft and yielding. But water will wear away rock, which is rigid andcannot yield. As a rule, whatever is fluid, soft and yielding will overcome whateveris rigid and hard. This is another paradox: what is soft is strong” Lao-Tzu
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Fluid Phenomena
Definition (Fluid): Any liquid or gas that cannot sustain a shearing force when atrest and that undergoes a continuous change in shape when subjected to such astress. Compressed fluids exert an outward pressure that is perpendicular to thewalls of their containers. A perfect fluid lacks viscosity, but real fluids do not.
Comparison: A solid deforms only until the external and internal forces arebalanced.
Chimera: Granular materials (e.g., sand)
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Research Topics: Improved Aeronautic Designs
Aircraft Design: Computational fluid dynamics (CFD) codes widely used in thedesign of the Boeing 787.
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Research Topics: Improved Aeronautic Designs
Windmill Design: Improved airfoils yield substantially improved efficiency.
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Research Topics: Biomedical Systems
Bloodflow: Diagnose and treat circulatory ailments; e.g., clots
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Research Topics: Improved Hydroelectric Designs
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Research Topics: Traffic Modeling and Control
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Newton’s Law of Viscosity
Deformation Rate: Rate of change in distance between two neighboring pointsmoving with fluid divided by the distance between the points. That is, “change inlength per unit length per unit time.”
Strain: In solids, strain is the change in length per unit length.
Note: Deformation rate thus often referred to as strain rate.
Shear Stress: Stress applied parallel or tangential to face of material; e.g., slidedeck of cards.
Viscosity: Measure of the resistance of a fluid that is being deformed by a shear ortensional stress. The study of viscosity is termed rheology.
• High viscosity: honey, magma
• Low viscosity: air, superfluids
• Units:
• E.g., Lubricating oils (~100 centipoise), water (~1 centipoise), air (~10e3-centipoise)
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Newton’s Law of Viscosity
Newton’s Law of Viscosity: Consider a moving plate separated from fixed plate byfluid. For a “Newtonian fluid”, the force required to move the plate is proportional tothe velocity and area and inversely proportional to distance between the plates.
Relation:
Experiments: Couette Viscometer
Note:
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Non-Newtonian Fluids
Bingham Fluids: exhibit yield stress;e.g., paint, ketchup
Bingham
Newtonian
Pseudo-plastic
Dilatant
Newtonian Fluids: e.g., water, oils,glycerin, air and other gases at low tomoderate shear rates
Pseudo-Plastic Fluids: shear-thinningfluids, often solutions of large,polymeric molecules in solution withsmaller molecules; e.g., styling gel
Dilatant Fluids: shear-thickening fluidswhich have increased viscosity athigher rates; e.g., uncooked paste ofcornstarch and water, coupling fluidsused in 2-wheel/4-wheel drive vehicles
Time-Dependent Viscosity: viscositydecreases with duration of stress;e.g., honey under certain conditions
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Model Development: Shear Stresses
Stress Orientation:
Stress Tensor:
Sign Convention: Stress acting on coordinate plane having a positive outwardnormal normal is positive if the stress itself also acts in the positive direction.It is also considered positive if it acts in a negative direction on a surface withnegative outward normal.
Note: Fluid at rest experiencesonly pressure which is a normalforce that acts opposite to theoutward normal.
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Conservation of MomentumMomentum Balance: x-direction
Momentum: x-component
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Conservation of Momentum
Momentum: y-component
Momentum: z-component
Note: Combination with the continuity equation yields
Because stress is a tensor, this is not a simple divergence.
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Strain Rate Behavior
Goal: Generalize Newton’s law of viscosity for 3-D flows. As a prelude, we firstdiscuss the possible kinematics of a fluid element.
Viscous Effects: Angular deformation and dilatation
Translation
Rotation
Dilatation
Shear Strain
Rigid body motion
Deformation motion
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Strain Rate Behavior
Translation:
Rotation:
Relation to Vorticity:
Similarly:
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Strain Rate Behavior
Shear Strain:
Dilatation: Increased volume
Incompressible Fluid:
• e.g., water, air at supersonic speeds
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Strain Rate Relations
Velocity Gradient Tensor:
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Constitutive Relations
Stress-Strain Rate Relations:
Note: In component form, this yields
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Navier Stokes Equations
Strategy: Employ stress relations in momentum equations
E.g., First component
Note: These equations are often simplified through various assumptions
• e.g.,
• Incompressible fluids:
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Euler and Burgers’ Equations
Euler Equations: Used when viscosity is negligible (inviscid fluids)
Inviscid Burgers’ Equation: Nonlinear equation illustrates formation of shocks
Burgers’ Equation: Combines nonlinear behavior and dissipation. Provides asimplified model for analysis and testing numerical methods
Conservation Form
Quasilinear Form
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Advection Equation
Advection Equation: Constant wave speed c
Characteristics: Satisfy
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Advection Equation
Advection Equation: Variable wave speed c(x)
Characteristic Equations:
Nonconstant Solution: Solution to differential equation
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Inviscid Burgers EquationBurgers Equation:
Characteristic Equations: Solutions:
Example:
Question: How do we uniquelyspecify solution after t=1?
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Rankine-Hugoniot Jump Condition
Conservation Relation: (Differential form)
Conservation Relation: (Integral form)
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Rankine-Hugoniot Jump Condition
Note:
Shock Speed:
Previous Example:
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Entropy Condition
Example: Consider the inviscid Burgers equation with initial data
Entropy Condition: Require Definition: A curve that satisfies
is termed a shock. Resultingsolutions u are termed generalizedsolutions.
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Traffic Flow ExampleModel:
Characteristic Speed:
Shock Speed:
Entropy Condition:
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Traffic Flow ExampleCase 1: Initial density
Vehicle Trajectories
Characteristics
Case 2:
Characteristics
Vehicle Trajectories
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Numerical Methods for Burgers EquationConservation Form:
First-Order Differencing:
Note: You will often get an incorrect shock speed with the quasilinear approximation
General Conservation Relation:
Note: More comprehensive methods need to be employed if f’(u) switches sign.
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Poiseuille Flows
Parallel Plates:
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Hagen-Poiseuille Flows
Pipe Flow: Analogous development
Notation:
Note:
Hagen-Poiseuille Equation:
Resistance to flowbetween two points
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Conservation of EnergyNote: Must include energy balance if heat or work affects the flow.
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Conservation of Energy
Definition: The energy of a body is defined as the capacity of the body to do work.The units of energy are the same as those of work.
Definition: The kinetic energy is that due to its motion.
Definition: The internal energy is that stored within the substance and is dueto the activity and spacing of molecules.
Definition: The heat or heat transfer is defined as thermal energy in transitiondue to a temperature difference between two points.
Principle:Increase in energy (internaland kinetic) of unit mass
Rate at which body forces(e.g., gravity, EM) do workon the body
Rate which surface work isdone on the body
Rate at which heat istransferred to the body
Integrate in Time:
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Conservation of EnergyNotation:
Energy Balance: Flow in x-direction
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Conservation of Energy
Note:
Energy Relation: Flow in x-direction
Surface work done on bodyHeattransferred tobody
Note: Can simplify usingmomentum equation
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Conservation of Energy
Energy Relation:
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Conservation of Energy
Energy Relation: Differential form
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Temperature Relation
Note:
Fourier’s Law:
Assumptions: Negligible shear stresses
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Bernoulli’s Principle
Assumptions: Steady,1-D flow
Energy Relation:
Conservation of Mass:
Combination yields
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Bernoulli’s Principle
Reformulation in terms of change per unit mass yields
Assumption: Incompressible flow
Zero Work Case:
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Bernoulli’s PrincipleCommon Form:
Applications:
• Lift from an airfoil (be careful of analysis)
• Carburetor design (venturi creates region oflow pressure that draws in fuel and mixes it
• Pitot tube used to determine airspeed of anaircraft
• Allows sail-craft to move faster than the wind!
Venturi meter
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Bernoulli’s Principle
Easy Experiments:
• Blow between two ping-pong ballssuspended from strings
• Collapse a paper house
• Suspend a balloon or ping-pong ballover a stream of air
• Blow air out of a funnel
• Other cool activities :)
Applications:
• Noncontact grippers (e.g., for solar cellswhich are easily contaminated)
• Windcube: Generator employing shroud thatgenerates electricity with wind speeds as lowas 5 mph
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Bernoulli’s PrincipleApplications:
• Curve ball