u/U = sin( /2); = y
description
Transcript of u/U = sin( /2); = y
u/U = sin(/2); = y/
Given U and viscosity table 9.2
Sketch (x), *(x), w(x)
LAMINAR
TABLE
9.2
Sketch (x), *(x), w(x)
u/U = sin(/2); = y/
…..
u/U = sin(/2); = y/
WHICH
HAS
MORE
FD
?
A
VS
B
A
B
U
Which plate experiences the most drag?
LAMINAR / dp/dx = 0Incompressible, steady, 2-D…
A
B
U Which plate experiences the most drag?
FD = wdA A = bL
Cf = w/( ½ U2) = 0.730/Rex
FD = 0.73b(LU3)1/2
FD(A) = 0.73(4L)(LU3)1/2
FD(B) = 0.73(L)(4LU3)1/2
FD(A)/FD(B) = 2
b=4L
b=L
L=4L
L=L
A
U
How Does Drag Change if 2U?
b
L
A
U How Does Drag Change if 2U?
FD = wdA A = bL
Cf = w/( ½ U2) = 0.730/Rex
FD = 0.73b(LU3)1/2
If U goes to 2U, FD goes to (8)1/2
b
L
(*/ = 0.344) Table 9.2
(*/ = 0.344)
0.344
y /
u / USinusoidal, parabolic, cubic look similar to Blasius solution.
u/Ue = 2(y/) – (y/)2
u/Umax = (y/)1/7
I give you Rex and x, how do you get w(x)? DL?
Laminar boundary-layer over flat plate, zero pressure gradient
1.8 m
0.9 mU = 3.2 m/s
u/U = sin[(/2)(/y)]
Drag = ?using Table 9.2
FD = wdA = U2 d/dx bdx = U2 bL
FD = wdA = U2 d/dx bdx = U2 bL
x=L
u/y|Wall = /y U[(2y/) – (y/)2]y=0
u/y|Wall = U2/
PIPE: u/y|Wall = Ucl2/R
Fully developed laminar flow
u/y = /y {y/}1/7 = 1/7 {y/}-6/7
u/y|y=0 =
So how get
wall
for turbulent velocity profile
u/U0 = (y/)1/7
?
u/Uo = (y/)1/7
w = 0.0332 (V)2[/(RV)]1/4 Eq(8.39)
V/Uc/l = (2n2)/[(n+1)(2n+1)] Eq(8.24)
w = 0.0233 (U0)2[/(Uo)]1/4
If Re transition occurs at 500,000 what is the skin friction drag on fin?
FD = CD (½ Uo2A)
Laminar Flow – starting at x=0
CD = 1.33/ReL1/2
Turbulent Flow – starting at x=0
CD = 0.0742/Re1/5 {u/U = (y/)1/7}for 5x105<ReL<107
CD = 0.455 /(log ReL)2.58
For ReL < 109
Calculating transition location, x:Assume xtr occurs at 500,000
Rex = Ux/ = 500,000
ReL = UL/ = 1.54 x 107
Rex/ReL = xtr/L = 500,000 / 1.54 x 107
xtr = 0.0325 L = 0.0325 x 1.65 = 0.0536 m
ReL = UL/ = 1.54 x 107
Turbulent Flow – starting at x=0
CD = 0.455 /(log ReL)2.58
L = 1.65m
xtr = 0.054m
FLAT PLATE dp/dx = 0
LAMINAR:From Theory
CD = 1.33 / ReL1/2
Eq. 9.33
TURBULENT:From Experiment
5x105< ReL<107
xtransition = 0CD = 0.0742/ReL
1/5
ReL<109
xtransition = 0CD = 0.455/(logReL)2.58
For Re transition of 500,000 and 5x105 < ReL < 109
CD = 0.455/(log ReL)2.58 – 1610/ReL (Eq. 9.37b) CD = 0.00281 – 0.000105 = 0.0027
~ 4% less
FD = CD2LH(1/2)U2 = 91.6 N2 sides so 2 x Area
Smooth flat plate, dp/dx = 0; flow parallel to plate
CD ~ 0.0028
CD = FD/( ½ Uo2); Cf(x) = w(x) /(1/2 Uo
2)
CD = FD/( ½ Uo2); Cf(x) = w(x) /(1/2 Uo
2)
Where does transition occur for these data?
These data include rough plates?
CD = FD/( ½ Uo2); Cf(x) = w(x) /(1/2 Uo
2)
ReL
breath
Plastic plate falling in water, find terminal velocity -
Plastic plate falling in water, find terminal velocity -
Sum of forces = 0
DRAGBOUYANCY
WEIGHT
+ z
- z
FD = CD ½ U2A
Given of water
and plastic
Assume: turbulent flow, transition at leading edge, doesn’t flutter, 5 x 105 < ReL < 107
Then CD = 0.0742/Reh1/5
Plastic plate falling in water, find terminal velocity -
Sum of forces = 0Drag Force = f (U)
DRAGBOUYANCY
WEIGHT
+ z
- z
Net Force = 0 = FB + FD - W
FB – W = (plastic - water) g (hLt)FD = CD ½ U2A = [0.0742/Reh
1/5] ½ water U2AReh = Uh/
Solving for U get 11.0 ft /sec
Check Re number: 1.86 x 106
5 x 105 < ReL < 107
THE END
THE
END
Extra Examples
U1 = Uo = 80 ft/sec1= 0.8 in.
U2 = ?P2 – P1 = ?2= 1.2 in.
Continuity U1A1eff = U2A2eff
A1eff = (h -2 *1 ) (h -2 *1 ) A2eff = (h -2 *2 ) (h -2 *2 )
*1
*1
* = 0.8 in *1
u/U = y/ =
Continuity U1A1eff = U2A2eff
A1eff = (h -2 *1 ) (h -2 *1 ) A2eff = (h -2 *2 ) (h -2 *2 )
*2
*2
*2 = 1.2 in *2
First a bit of mathematic legerdemain before attempting Ex. 9.3 ~
= ?
L = 1.8 m
U = 3.2 m/s
= ?; * = ?; w = ?
b = 0.9m
Laminar and assume u/U = sin(/2); = y/
*/ = 1o (1-u/U) d
/ = 1o u/U(1-u/U) d
w = U2 d/dx = u/y0
w = u/y0 = (U/)d(u/U)/d(y/)0
w = (U/)dsin(/2)/d0
w = (U/)(/2)cos(/2)0
w = (U/)(/2) = U/(2)
u/U = sin(/2); = y/
*/ = 1o (1-u/U) d
u/U = sin(/2); = y/
*/ = 1o (1 – sin(/2) )d
*/ = + (2/)cos (/2)from 1 to 0
*/ = 1 - 2/ = 0.363* = 0.363
/ = 1o u/U(1-u/U) d
= 0.137
Finding */ = 1o (1-u/U) d
/ = 1o u/U(1-u/U) d
w = U2 d/dx
w = U2 d/dxw = u/y0
…..
u/U = sin(/2); = y/
u/U = sin(/2); = y/
Fdrag = 0L wbdx = 0
L U2(d/dx)bdx = 0
L U2bd = U2b0L
Fdrag = U2bL
(whew!)
Calculating drag on a flat plate, zero pressure gradient – turbulent flow
*
*u/y blows up at y = 0
Can’t usewall = du/dy y=0*
Turbulent Flow
Tripped at leading edge so turbulent flow everywhere on plate
= 0.424 Rex-1/5x
w = 0.0243 U2 (/(U))1/4
w = 0.0243 U2 (/(U 0.424 Rex-1/5x ))1/4
= 0.424 Rex-1/5x
w = 0.0301 U2 Rex-1/5
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2
u/U = (y / )1/6
u/U = (y / )1/7
u/U = (y / )1/8
u/U
y/
Re increases, n increases, wall shear stress increases, boundary layer increases, viscous sublayer decrease
LAMINAR BOUNDARY LAYER AT SEPARATION
Given: u/U = a + b + c2 + d 3
What are boundary conditions?
Given: u/U = a + b + c2 + d 3
= y/
= 0; u = 0; a = 0= 0; u/y = 0; b = 0= ; u =U; 1 = c + d= ; u/y = 0; 2c + 3d = 02(1-d) + 3d = 2 + d = 0 so d = -2 and c = 3
Separatingu/y = 0
u/U = 3 2 -2 3
dp/dx = 0
Separating Flowu/U = 3 2 -2 3
dp/dx > 0
U
mg = 120 kgU = 6 m/s
FD = CD ½ U2A
Sum of forces = 0,pick A so U = 6 m/s
mg = 120 kgU = 6 m/s
FD = CD ½ U2A
Sum of forces = 0,pick A so U = 6 m/s
CD = 1.42 for open hemisphereFD = CD ½ U2A = W = mg
A = d2/4d2 = [mg]/[CD U2 /8]
d = [(8/)(120kg)(9.8 m/s2) (1/1.42)(1/1.23 kg/m3)(1/6m/s)2
d = 6.9 m
FD = CD ½ U2A
FD = ma = m(dU/dt) = m(dU/dx)(dx/dt)
CD ½ U2A = m (dU/dx) U
CD ½ U2A = m (dU/dx) UdU/U = CD A dx /(2m)
Integrating from x=0
to x=x gives:ln Uf – lnUi = CD Ax /(2m)CD = 2m ln {Uf/Ui} /[ Ax]
CD = 0.299
Find POWER = F U for this conditionAnd then see if that is enough to win bet.
air
8.33 m/s
2.78 m/s
FD = CD ½ U2AA = 23.4 ft2
CD = 0.5FR = 0.015 x 4500 lbf
FD = FR to find U where aerodynamic force = frictional force
Total Power = FDU + FRU
FD [CD ½ U2A] = FR[.015xW](0.5)½ 0.00238slug/ft3U2(23.4ft2)
= 0.015 x 4500 lbf
U2 = 67.5/0.0139
U = 69.7 ft/s = 47.5 mph
Where does aerodynamic force = frictional force ?
Average of 44 sport cars is 0 43, not much better than the 0.47 of ’37 the Lincoln Zephyr
(245 km/hr)
A model of a river boat is to be tested at 1:13.5 scale. The boat is designed to travel at 9mph in fresh water at 10o C. (1)Estimate the distance from the bow on the full-scaleship where transition occurs. (2) Where should transition be stimulated on model?
What equations are relevant to this problem?
What approach should we take?
A model of a river boat is to be tested at 1:13.5 scale. The boat is designed to travel at 9mph in fresh water at 10o C.Estimate the distance from the bow where transition occurs.Where should transition be stimulated on model?
Things we know: Re = UL/ ReL transitions ~ 5 x 105 (experimentally determined for flat plate and dP/dx = 0)
Want same fraction of L to be turbulent for both cases: [xtr/L]model = [xtr/L]boat so ration of xtr = ratio of boat lengths
= 13.5
?
Three unknowns, A, B, and C- will need three boundary conditions. What are they?
1 m
U = 2.7 m/s
cd
(5) Two-Dimensional(6) Incompressible assumed assumptions
cd
u(x,y)/U = y/ = (y)
1
0
cd
FDrag
cd
d c