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2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Boundary layer Equations Contents:
– Boundary Layer Equations;– Boundary Layer Separation;– Effect of londitudinal pressure gradient on boundary layer
evolution– Blasius Solution– Integral parameters: Displacement thickness and momentum
thickness
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Laminar Thin Boundary Layer Equations (<<x) over flat plate
Steady flow, constant and . Streamlines slightly divergent0 yp
dxdpxp e
2
2
2
21yu
xu
xp
yuv
xuu
2D Navier-Stokes Equations along x direction:
Compared with 2
2
yu
dxdpe
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Laminar Thin Boundary Layer Equations (<<x) over flat plate
2
21yu
dxdp
yuv
xuu e
Laminar thin boundary layer equations (<<x) for flat plates
pe external pressure, can be calculated with Bernoulli’s Equation as there are no viscous effects outside the Boundary Layer
Note 1. The plate is considered flat if is lower then the local curvature radius
Note 2. At the separation point, the BD grows a lot and is no longer thin
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
to adverse pressure gradientsTurbulent flows (fuller profiles)- more resistant to
adverse pressure gradients
Boundary Layer Separation
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Boundary Layer Sepaation
Longitudinal and intense adverse pressure gradient does not cause separation
=> there’s not viscous forces
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Bidimensional (2D) Thin Boundary Layer (<<x) Equations to flat plates:
2
2
yu
yuv
xuu
0
yv
xu
Boundary Condition: y=0 u=v=0y=∞ u=U
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius hypothesis: with fUu
The introdution of η corresponds to recognize that the nondimension velocity profile is stabilized.
nxAy
A and n are unknowns
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Remark: eyx
Ay n
xny
xnA
x n
1
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Procedure: oUsing current function:
yu
x
v
o Remark:
yydd
AxUf
Axu
nn
F
dfAxU
n
o Replace u/U=f(η) e at the boundary layer equation, choose n such that the resulting equation does not depend on x and A in order to simplify the equation.
.
xv
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
o FUy
u
results:
Fx
nUxu o
o Fx
UAyu
n
o F
xUA
yu
n
2
2
2
2
o FxnFnxAU
xv nn
11
FAxU
n
From:
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
We will obtain:
02
12
FFA
UnxFn
2
2
yu
yuv
xuu
00, xu
00, xv
Uxu ,
Boundary Conditions:
o Making n=1/2 and the equation comes:UA
02 FFF xUy
with
00 FU 00 F 00 FF 00 F
UFU 1F
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Graphical Solution:
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Displacement thickness:
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
0
* 1 dyuUUd
0
1 dyuUUd
0
udyUU d
U
0
dyuU
Ideal Fluid flow rate
Real Flow rate
Déficit of flow rate due to velocity reduction at BD
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Displacement thickness :
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
0
1 dyuUUd
0
1 dyuUUd
0
udyUU d
Ideal Fluid flow rate
Real Flow rate
Déficit of flow rate due to velocity reduction at BD
dUq
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Displacement thickness :
0
1 dyuUUd
0
1 dyuUUd
0
1 udyUd
Initial deviation of BD
δ
Deviation of outer streamlines
Section where the streamline become part of boundary layer
δdq/U LC
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Blasius Solution for displacement thickness:
x
d
x Re72,1
δ dq/U LC
Ux
x Recom
344,0dou
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Momentum thickness:
0
2
1 udyuUUm
0
2
1 udyuUUm
dU mdUdyu
2
0
2
mUudyUdyu
2
00
2
0
2
0
2 dyuudyUU m
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Momentum flow rate through a section of BD:
mdqm UUUdyuqx
222
0
2
Momentum flow rate of uniform
profile
UU
Reduction due to deficit of flow
rate
dUU
Reduction due to deficit
momentum flow rate at BD
mUU
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Longitudinal momentum balance between the leading edge and a cross section at x:
xxqmxqm xx
qqD
0
dU 2 mU d 2mU 2
δ d-d LCx
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
Blasius Solution to momentum thickness:
x
m
x Re664,0
Ux
x Rewith
133.0mor
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Laminar Boundary Layer Equations Contents:
– Thin Boundary Layer Equations with Zero Pressure Gradient;
– Boundary Layer Separation;– Effect of longitudinal pressure gradient on the evolution of
Boundary Layer– Blasius Solution– Local Reynolds Number and Global Reynolds Number– Integral Parameters: displacement thickness and momentum
thickness
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Recommended study elements:– Sabersky – Fluid Flow: 8.3, 8.4– White – Fluid Mechanics: 7.4 (sem método de Thwaites)
Blasius Solution to Laminar Boundary Layer Equation over a flat plate with dpe/dx=0
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
=1,2 kg/m3, =1,810-5 Pa.s(Rex)c =106.
Exercise
L=2m
U=2m/s 0dxdpe
Large plate with neglectable thickness, lenght L=2m. Parallel and non-disturbed air flow. (=1,2 kg/m3, =1,810-5 Pa.s) with U=2 m/s. Zero pressure gradient over the flat plate. Transition to turbulent at Rex=106.
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
a) Find boundary layer thickness at sections S1 and S2, at distance x1=0,75 m and x2=1,5 m of the leading edge
Exercise
mU
x cxc 5,72
2,1108,110Re5
6
Find xc:
xx Re5
Laminar Boundary layer at x1 and x2 – We can apply Blasius Solution
=1,2 kg/m3, =1,810-5 Pa.s(Rex)c =106.
L=2m
U=2m/s 0dxdpe
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Exercise
xx Re5
Laminar Boundary layer at x1 and x2 – We can apply Blasius Solution
mx 75,01 5
51 10105,1
75,02Re
x m0119,010
575,051
mx 5,12 52 102Re x
m0168,02
a) Find boundary layer thickness at sections S1 and S2, at distance x1=0,75 m and x2=1,5 m of the leading edge
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Exercise
b) Check that it is a thin boundary layer.
=1,2 kg/m3, =1,810-5 Pa.s(Rex)c =106.
L=2m
U=2m/s 0dxdpe
A: Thin Blayer if /x<<1: 0159,075,0
0119,0
1
x
0112,05,1
0168,0
2
xWhy/x at 2 is lower than
/x at 1?
y=(x)
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Exercise
d) Find the value of y1 at x1 of the streamline passing through the coordinates x2=1,5 and y2=.
=1,2 kg/m3, =1,810-5 Pa.s(Rex)c =106.
L=2m
U=2m/s 0dxdpe
Streamline
x2=1,5mx1=0,75m
y1=?
A: We have the same flow rate between the streamline and the plate at both cross sectionsFlow rate through a cross section of BD: dUq
Flow rate through section 2: 22 dUq
Flow rate through section 1: 11 yU 1dU 1
1
11
001
yy
udyudyudyq
1221 ddy
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Exercise
d) Find the value of y1 at x1 of the streamline passing through the coordinates x2=1,5 and y2=.
=1,2 kg/m3, =1,810-5 Pa.s(Rex)c =106.
L=2m
U=2m/s 0dxdpe
Linha de corrente
x2=1,5mx1=0,75m
y1=?
A: We have the same flow rate between the streamline and the plate at both cross sections
1221 ddy Laminar BD: 344,0d
0,0168m 0,0058m 0,0041m y1=0,0151m
2004 Mecânica dos Fluidos II Prof. António Sarmento - DEM/IST
Exercise
e) Find the force per unit leght between sections S1 and S2.