# Basic Governing Differential Equations

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Basic Governing Differential Equations
CEE 331 April 16, 2017

Overview Continuity Equation Navier-Stokes Equation
(a bit of vector notation...) Examples (all laminar flow) Flow between stationary parallel horizontal plates Flow between inclined parallel plates Pipe flow (Hagen Poiseuille)

Conservation of Mass in Differential Equation Form
Mass flux out of differential volume Rate of change of mass in differential volume Mass flux into differential volume

Continuity Equation Mass flux out of differential volume
Higher order term out in Rate of mass decrease 1-d continuity equation

Continuity Equation u, v, w are velocities in x, y, and z directions
3-d continuity equation u, v, w are velocities in x, y, and z directions Vector notation If density is constant... or in vector notation True everywhere! (contrast with CV equations!)

Continuity Illustrated
What must be happening? x

Navier-Stokes Equations
Derived by Claude-Louis-Marie Navier in 1827 General Equation of Fluid Motion Based on conservation of ___________ with forces… ____________ ___________________ U.S. National Academy of Sciences has made the full solution of the Navier-Stokes Equations a top priority momentum Gravity Pressure Shear

Navier-Stokes Equations
h is vertical (positive up) Inertial forces [N/m3], a is Lagrangian acceleration Is acceleration zero when dV/dt = 0? Pressure gradient (not due to change in elevation) Shear stress gradient What is in a static fluid? _____ Zero!

Dx Over what time did this change of velocity occur (for a particle of fluid)? Why no term?

Notation: Total Derivative Eulerian Perspective
Total derivative (chain rule) Material or substantial derivative Lagrangian acceleration N-S

Application of Navier-Stokes Equations
The equations are nonlinear partial differential equations No full analytical solution exists The equations can be solved for several simple flow conditions Numerical solutions to Navier-Stokes equations are increasingly being used to describe complex flows.

Navier-Stokes Equations: A Simple Case
No acceleration and no velocity gradients xyz could have any orientation 1 Let y be vertical upward For constant g

Infinite Horizontal Plates: Laminar Flow
Derive the equation for the laminar, steady, uniform flow between infinite horizontal parallel plates. y x x 1 y Hydrostatic in y z

Infinite Horizontal Plates: Laminar Flow
Pressure gradient in x balanced by shear gradient in y No a so forces must balance! Now we must find A and B… Boundary Conditions

Infinite Horizontal Plates: Boundary Conditions
No slip condition a t u u = 0 at y = 0 and y = a let be___________ negative What can we learn about t?

Laminar Flow Between Parallel Plates
U q a u y x No fluid particles are accelerating Write the x-component

Flow between Parallel Plates
u is only a function of y h and p are only functions of x General equation describing laminar flow between parallel plates

Flow Between Parallel Plates: Integration
U q a u y x

Boundary Conditions u = 0 at y = 0 u = U at y = a Boundary conditions

Discharge Discharge per unit width!

Example: Oil Skimmer g = 8430 N/m3 m = 1x10-2 Ns/m2
An oil skimmer uses a 5 m wide x 6 m long moving belt above a fixed platform (q=30º) to skim oil off of rivers (T=10 ºC). The belt travels at 3 m/s. The distance between the belt and the fixed platform is 2 mm. The belt discharges into an open container on the ship. The fluid is actually a mixture of oil and water. To simplify the analysis, assume crude oil dominates. Find the discharge and the power required to move the belt. h g = 8430 N/m3 l m = 1x10-2 Ns/m2 30º

Example: Oil Skimmer (per unit width) Q = 0.0027 m2/s
dominates Q = m2/s (per unit width) In direction of belt Q = m2/s (5 m) = m3/s

Example: Oil Skimmer Power Requirements
How do we get the power requirement? ___________________________ What is the force acting on the belt? Remember the equation for shear? _____________ Evaluate at y = a. Power = Force x Velocity [N·m/s] Shear force (t · L · W) t = m(du/dy)

Example: Oil Skimmer Power Requirements
(shear by belt on fluid) FV = 3.46 kW

Example: Oil Skimmer Where did the Power Go?
Where did the energy input from the belt go? Lifting the oil (potential energy) Heating the oil (thermal energy) Dh = 3 m

Example : Oil Skimmer Was it Really Laminar Flow?
We assumed that the flow was laminar (based on the small flow dimension of 2 mm) We need to check our assumption!!!! m3/s = 1.36 m/s 0.002 m* 5 m 1.36 m/s Laminar

Example: No flow Find the velocity of a vertical belt that is 5 mm from a stationary surface that will result in no flow of glycerin at 20°C (m = 0.62 Ns/m2 and g =12300 N/m3)

Laminar Flow through Circular Tubes
Different geometry, same equation development (see Munson, et al. p 367) Apply equation of motion to cylindrical sleeve (use cylindrical coordinates)

Laminar Flow through Circular Tubes: Equations
R is radius of the tube Max velocity when r = 0 Velocity distribution is paraboloid of revolution therefore _____________ _____________ average velocity (V) is 1/2 vmax Q = VA = VpR2

Laminar Flow through Circular Tubes: Diagram
Shear Velocity Laminar flow Shear at the wall True for Laminar or Turbulent flow

The Hagen-Poiseuille Equation
cv pipe flow Constant cross section h or z Laminar pipe flow equations From Navier-Stokes CV equations!

Example: Laminar Flow (Team work)
Calculate the discharge of 20ºC water through a long vertical section of 0.5 mm ID hypodermic tube. The inlet and outlet pressures are both atmospheric. You may neglect minor losses. What is the total shear force? What assumption did you make? (Check your assumption!)

Summary Navier-Stokes Equations and the Continuity Equation describe complex flow including turbulence The Navier-Stokes Equations can be solved analytically for several simple flows Numerical solutions are required to describe turbulent flows

Glycerin

Example: Hypodermic Tubing Flow
= weight!

Euler’s Equation Along a Streamline
Inviscid flow (frictionless) x along a streamline Velocity normal to streamline is zero v = u = velocity in x direction

We’ve assumed: frictionless and along a streamline
Euler’s Equation We’ve assumed: frictionless and along a streamline Steady x is the only independent variable (Multiplying by dx converts from a force balance equation to an energy equation) Euler’s equation along a streamline

Bernoulli Equation Integrate for constant density Bernoulli Equation
Euler’s equation Integrate for constant density Bernoulli Equation The Bernoulli Equation is a statement of the conservation of ____________________ p.e. k.e. Mechanical Energy

Hydrostatic Normal to Streamlines?
x, u along streamline y, v perpendicular to streamline (v = 0)

Laminar Flow between Parallel Plates
h a dy U l u y q dl q

Equation of Motion: Force Balance
+ pressure - - shear + gravity + q = acceleration l

Equation of Motion h But q l Laminar flow assumption!

Limiting cases y U x u q Motion of plate Pressure gradient
Hydrostatic pressure Linear velocity distribution Both plates stationary Parabolic velocity distribution

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