lecture # 07: laminar and turbulent flowshuhui/teaching/2014sx/class-notes/aere344... ·...
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Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Dr. Hui Hu
Department of Aerospace EngineeringIowa State University
Ames, Iowa 50011, U.S.A
Lecture # 07: Laminar and Turbulent Flows
AerE 344 Lecture Notes
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Laminar Flows and Turbulence Flows
• Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. Inflow dynamics laminar flow is a flow regime characterized by high momentum diffusion, low momentum convention, pressure and velocity almost independent from time. It is the opposite of turbulent flow.
– In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough."
• In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum conversation, and rapid variation of pressure and velocity in space and time.
– Flow that is not turbulent is called laminar flow
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Turbulent Flows in a Pipe
VD
Re
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Characterization of Turbulent Flows
'';' wwwvvvuuu
Tt
t
Tt
t
Tt
t
dttzyxwT
wdttzyxvT
vdttzyxuT
u0
0
0
0
0
0
),,,(1;),,,(1;),,,(1
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Turbulence intensities
0'0';0' wvu
0)'(0)'(;0)'(1)'( 2222
0
wvdtuT
uTt
t
o
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Turbulent Shear Stress
yu
lam
Laminar flows:
''vuyu
turblam
Turbulent flows: ''vuturb
A.laminar flow b.turbulent flow
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Laminar Flows and Turbulence Flows
AU
DCd2
21
DU
Re
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Flow Around A Sphere with laminar and Turbulence Boundary Layer
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!X/D
Y/D
-101234
-1
0
1
2
U m/s: -10 -5 0 5 10 15 20 25 30 35 40
X/D
Y/D
-101234
-1
0
1
2 U m/s: -10 -5 0 5 10 15 20 25 30 35 40
X/D
Y/D
-101234
-1
0
1
2 U m/s: -10 -5 0 5 10 15 20 25 30 35 40
Smooth ball Rough ball Golf ball
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1.0
-2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
smooth-ballrough-ballgolf-ball
Distance (X/D)
Cen
terli
ne V
eloc
ity (U
/U)
Laminar Flows and Turbulence Flows
Re=100,000
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Automobile Aerodynamics
Mercedes Boxfish Vortex generator above a Mitsubishi rear window
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CONVENTIONAL AIRFOILS and LAMINAR FLOW AIRFOILS
• Laminar flow airfoils are usually thinner than the conventional airfoil.
• The leading edge is more pointed and its upper and lower surfaces are nearly symmetrical.
• The major and most important difference between the two types of airfoil is this, the thickest part of a laminar wing occurs at 50% chord while in the conventional design the thickest part is at 25% chord.
• Drag is considerably reduced since the laminar airfoil takes less energy to slide through the air.
• Extensive laminar flow is usually only experienced over a very small range of angles-of-attack, on the order of 4 to 6 degrees.
• Once you break out of that optimal angle range, the drag increases by as much as 40% depending on the airfoil
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Flow Separation on an Airfoil
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Aerodynamic Performance of An Airfoil
X/C *100
Y/C
*100
-20 0 20 40 60 80 100 120
-40
-20
0
20
40
60
vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
shadow region
GA(W)-1 airfoil
25 m/s
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 2 4 6 8 10 12 14 16 18 20
CL=2Experimental data
Angle of Attack (degrees)
Lift
Coe
ffici
ent,
Cl
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0 2 4 6 8 10 12 14 16 18 20
Experimental data
Angle of Attack (degrees)
Dra
g C
oeffi
cien
t, C
d
X/C *100
Y/C
*100
-40 -20 0 20 40 60 80 100 120 140
-60
-40
-20
0
20
40
60
vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
shadow region
GA(W)-1 airfoil
25 m/s
Airfoil stall
Airfoil stall
Before stall
After stallcV
DCd2
21
cV
LCl2
21
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Flow Separation and Transition on Low-Reynolds-number Airfoils
• Low-Reynolds-number airfoil (with Re<500,000) aerodynamics is important for both military and civilian applications, such as propellers, sailplanes, ultra-light man-carrying/man-powered aircraft, high-altitude vehicles, wind turbines, unmanned aerial vehicles (UAVs) and Micro-Air-Vehicles (MAVs).
• Since laminar boundary layers are unable towithstand any significant adverse pressure gradient, laminar flow separation is usually found on low-Reynolds-number airfoils. Post-separation behavior of the laminar boundary layers would affect the aerodynamic performances of the low-Reynolds-number airfoils significantly
• Separation bubbles are usually found to formon the upper surfaces of low-Reynolds-number airfoils . Separation bubble would burstsuddenly to cause airfoil stall at high AOA when the adverse pressure gradient becoming too big.
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
-0.10-0.05
00.050.100.15
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
X/C
Y/C
-3 .0
-2 .5
-2 .0
-1 .5
-1 .0
-0 .5
0
0 .5
1 .0
1 .50 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 .0
A O A = 06 degA O A = 08 degA O A = 09 degA O A = 10 degA O A = 11 degA O A = 12 degA O A = 14 deg
X / CC
P
Separation point
Turbulence transition
Reattachment point
Surface Pressure Coefficient distributions (Re=68,000)
Typical surface pressure distribution when a laminar separation bubble is formed (Russell, 1979)
7 .5
8 .0
8 .5
9 .0
9 .5
1 0 .0
1 0 .5
1 1 .0
1 1 .5
1 2 .0
0 0 .0 5 0 .10 0 .1 5 0 .20 0 .2 5 0 .30 0 .3 5 0 .40 0 .4 5 0 .50
T ra n s itio n R e a tta ch m e n tS e p a ra tio n
X /C
AO
A (d
egre
e)
GA (W)-1 airfoil(also labeled as NASA LS(1)-0417 )
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Laminar Separation Bubble on a Low-Reynolds-number Airfoil
PIV measurement results at AOA = 10 deg, Re=68,000
X/C*100
Y/C
*100
-5 0 5 10 15 20 25 30 35
0
5
10
-18.0 -14.0 -10.0 -6.0 -2.0 2.0 6.0 10.0 14.0 18.0
10 m/sGA (W)-1 airfoilSpawisevorticity
X/C*100
Y/C
*100
-5 0 5 10 15 20 25 30 35
0
5
10
U m/s: 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0
GA (W)-1 airfoilreattachmentseparation
X/C*100
Y/C
*100
18 20 22 24 26 28 30 32 343
4
5
6
7
8
9
10
11
U m/s: 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00
10 m/sReattachment
X/C*100
Y/C
*100
18 20 22 24 26 28 30 32 343
4
5
6
7
8
9
10
11
-25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0
SpanwiseVorticity(1/s * 103)
10 m/s
Instantaneous flow field Ensemble-averaged flow field
(Hu et al., ASME Journal of Fluid Engineering, 2008)
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Stall Hysteresis Phenomena
• Stall hysteresis, a phenomenon where stall inception and stall recovery do not occur at the same angle of attack, has been found to be relatively common in low-Reynolds-number airfoils.
• When stall hysteresis occurs, the coefficients of lift, drag, and moment of the airfoil are found to be multiple-valued rather than single-valued functions of the angle of attack.
• Stall hysteresis is of practical importance because it produces widely different values of lift coefficient and lift-to-drag ratio for a given airfoil at a given angle of attack. It could also affect the recovery from stall and/or spin flight conditions.
Increasing AOA
decreasing AOA
AOA – Angle of Attack
Lift
coef
ficie
nt
Lift coefficient curve of a “typical” airfoil Lift coefficient curve with stall hysteresisAOA – Angle of Attack
Increasing AOA
decreasing AOA
Lift
coef
ficie
nt
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Measured airfoil lift and drag coefficient profiles
-0.2
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
-4 -2 0 2 4 6 8 10 12 14 16 18 20
AOA increasingAOA decreasing
Angle of Attack (degree)
Lift
Coe
ffici
ent,
Cl
Hysteresis loop
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
-4 -2 0 2 4 6 8 10 12 14 16 18 20
AOA increasingAOA decreasing
Angle of Attack (degree)
Dra
g C
oeffi
cien
t, C
d
Hysteresisloop
• The hysteresis loop was found to be clockwise in the lift coefficient profiles, and counter-clockwise in the drag coefficient profiles.
• The aerodynamic hysteresis resulted in significant variations of lift coefficient, Cl, and lift-to-drag ratio, l/d,for the airfoil at a given angle of attack.
• The lift coefficient and lift-to-drag ratio at AOA = 14.0 degrees were found to be Cl = 1.33 and l/d = 23.5when the angle is at the increasing angle branch of the hysteresis loop.
• The values were found to become Cl = 0.8 and l/d = 3.66 for the same AOA=14.0 degrees when the angle is at the deceasing angle branch of the hysteresis loop
GA(W)-1 airfoil, ReC = 160,000
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
8 9 10 11 12 13 14 15 16 17 18 19 20
AOA decreasing
AOA increasing
Angle of Attack (degree)
Lift
Coe
ffici
ent,
Cl
PIV Measurement results
X/C *100
Y/C
*100
-40 -20 0 20 40 60 80 100 120 140
-60
-40
-20
0
20
40
60
vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
shadow region
GA(W)-1 airfoil
25 m/s
X/C *100
Y/C
*100
-20 0 20 40 60 80 100 120
-40
-20
0
20
40
60
vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
shadow region
GA(W)-1 airfoil
25 m/s
X/C *100
Y/C
*100
-20 0 20 40 60 80 100 120
-40
-20
0
20
40
60
vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
shadow region
GA(W)-1 airfoil
25 m/s
separation bubble
X/C *100
Y/C
*100
-40 -20 0 20 40 60 80 100 120 140
-60
-40
-20
0
20
40
vort: -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5
shadow region
GA(W)-1 airfoil
25 m/s
(Hu, Yang, Igarashi, Journal of Aircraft, Vol. 44. No. 6 , 2007)
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
8 9 10 11 12 13 14 15 16 17 18 19 20
AOA decreasing
AOA increasing
Angle of Attack (degree)
Lift
Coe
ffici
ent,
Cl
Refined PIV Measurement Results
X/C *100
Y/C
*100
-5 0 5 10 15 20 25 30 35-10
-5
0
5
10
15
20
25
-18.0 -14.0 -10.0 -6.0 -2.0 2.0 6.0
GA(W)-1 airfoil 25 m/s
X/C *100
Y/C
*100
-5 0 5 10 15 20 25 30 35-10
-5
0
5
10
15
20
25
vort: -18.0 -14.0 -10.0 -6.0 -2.0 2.0 6.0
GA(W)-1 airfoil 25 m/s
X/C *100
Y/C
*100
-5 0 5 10 15 20 25 30 35
-5
0
5
10
15
20
vort: -18.0 -14.0 -10.0 -6.0 -2.0 2.0 6.0
GA(W)-1 airfoil25 m/s
X/C *100
Y/C
*100
-5 0 5 10 15 20 25 30 35
-10
-5
0
5
10
15
20
vort: -18.0 -14.0 -10.0 -6.0 -2.0 2.0 6.0
GA(W)-1 airfoil25 m/s
(Hu, Yang, Igarashi, Journal of Aircraft, Vol. 44. No. 6 , 2007)
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
y
x
Lab 6: Airfoil Wake Measurements and Hotwire Anemometer Calibration
22
2
22
2
;
21
)])(1()([
)]1)(()([
)( Since
)(
)ˆ(
dAU
yUU
yUUD
dAU
yUU
yUUDF
DF
pyppp
dAypdApD
dAnpDF
X
X
up
up
CSXX
2
2
22
2
2
)])(1()([221
)])(1()([
21
dyU
yUU
yUC
C
CU
dAU
yUU
yUU
CU
DC
D
D
• Compared with the drag coefficients obtained based on airfoil surface pressure measurements at the same angles of attack!
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
y
x80 mm
Pressure rake with 41 total pressure probes(the distance between the probes d=2mm)
Lab 3: Airfoil Wake Measurements and Hotwire Anemometer Calibration
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Lab 3: Airfoil Wake Measurements and Hotwire Anemometer Calibration
CTA hotwire probe
Flow Field
Current flow through wire
V
),(2ww
w TVqRidt
dTmc
• Constant-temperature anemometry
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Hotwire Anemometer Calibration
0
2
4
6
8
10
12
14
16
18
20
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2
Curve fittingExperimental data
y=a+bx+cx2+d*x3+e*x4 max dev:0.166, r2=1.00a=10.8, b=3.77, c=-26.6, d=13.2
voltage (V)
Flow
vel
ocity
(m/s
)
• To quantify the relationship between the flow velocity and voltage output from the CTA probe
Copyright © by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Required Measurement Results
Required Plots:
• Cp distribution in the wake (for each angle of attack) for the airfoil wake measurements
• Cd vs angle of attack (do your values look reasonable?) based on the airfoil wake
measurements
• Your hot wire anemometer calibration curve: Velocity versus voltage output of hotwire
anemometer (including a 4th order polynomial fit)
Please briefly describe the following details:
• How you calculated your drag—you should show your drag calculations
• How these drag calculations compared with the drag calculations you made in the previous
experiment.
• Reynolds number of tests and the incoming flow velocity