Pharos UniversityME 253 Fluid Mechanics 2
Revision for Mid-Term Exam
Dr. A. Shibl
Streamlines
•A Streamline is a curve that iseverywhere tangent to theinstantaneous local velocityvector.
•Consider an arc length
• must be parallel to the localvelocity vector
•Geometric arguments results inthe equation for a streamline
Kinematics of Fluid Flow
Stream Function forTwo-DimensionalIncompressible Flow
•Two-Dimensional Flow
Stream Function
Stream Function forTwo-DimensionalIncompressible Flow
•Cylindrical Coordinates
Stream Function (r,)
Is this a possible flow field
Given the y-component Find the X- Component of the velocity,
Determine the vorticity of flow field described byIs this flow irrotational?
Momentum Equation
•Newtonian Fluid: Navier–Stokes Equations
Example exact solutionPoiseuille Flow
Example exact solutionFully Developed Couette Flow
•For the given geometry and BC’s, calculate the velocity andpressure fields, and estimate the shear force per unit areaacting on the bottom plate
•Step 1: Geometry, dimensions, and properties
Fully Developed Couette Flow
•Step 2: Assumptions and BC’s
–Assumptions
1.Plates are infinite in x and z
2.Flow is steady, /t = 0
3.Parallel flow, V=0
4.Incompressible, Newtonian, laminar, constant properties
5.No pressure gradient
6.2D, W=0, /z = 0
7.Gravity acts in the -z direction,
–Boundary conditions
1.Bottom plate (y=0) : u=0, v=0, w=0
2.Top plate (y=h) : u=V, v=0, w=0
Fully Developed Couette Flow
•Step 3: Simplify
3
6
Note: these numbers referto the assumptions on theprevious slide
This means the flow is “fully developed”or not changing in the direction of flow
Continuity
X-momentum
2
Cont.
3
6
5
7
Cont.
6
Fully Developed Couette Flow
•Step 3: Simplify, cont.
Y-momentum
2,3
3
3
3,6
7
3
3
3
Z-momentum
2,6
6
6
6
7
6
6
6
Fully Developed Couette Flow
•Step 4: Integrate
Z-momentum
X-momentum
integrate
integrate
integrate
Fully Developed Couette Flow
•Step 5: Apply BC’s
–y=0, u=0=C1(0) + C2 C2 = 0
–y=h, u=V=C1h C1 = V/h
–This gives
–For pressure, no explicit BC, therefore C3 can remain anarbitrary constant (recall only P appears in NSE).
•Let p = p0 at z = 0 (C3 renamed p0)
1.Hydrostatic pressure
2.Pressure acts independently of flow
Fully Developed Couette Flow
•Step 6: Verify solution by back-substituting intodifferential equations
–Given the solution (u,v,w)=(Vy/h, 0, 0)
–Continuity is satisfied
0 + 0 + 0 = 0
–X-momentum is satisfied
Fully Developed Couette Flow
•Finally, calculate shear force on bottom plate
Shear force per unit area acting on the wall
Note that w is equal and opposite to the
shear stress acting on the fluid yx(Newton’s third law).
Momentum Equation
•Special Case: Euler’s Equation
20
Inviscid Flow for Steady incompressibleInviscid Flow for Steady incompressible
•For steady incompressible flow, the equation reduces to
where = constant.
•Integrate from a reference at along any streamline =C :
21
Two-Dimensional Potential Flows Two-Dimensional Potential Flows
•Therefore, there exists a stream function such that
in the Cartesian coordinate and
in the cylindrical coordinate
Potential Flow
23
Two-Dimensional Potential Flows
•The potential function and the stream function are conjugatepair of an analytical function in complex variable analysis.
•The constant potential line and the constant streamline areorthogonal, i.e.,
and
to imply that .
Stream and Potential Functions
•If a stream function exists for the velocity fieldu = a(x2 -- y2) & v = - 2axy & w = 0
Find it, plot it, and interpret it.
•If a velocity potential exists for this velocity field.Find it, and plot it.
Summary
•Elementary Potential Flow Solutions
26
Uniform Stream
U∞y
U∞x
Source/Sink
m
mln(r)
Vortex
-Kln(r)
K
