lecture # 06: boundary layer flows and drag · title: microsoft powerpoint - lecture-06-notes...
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Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Dr. HDr. Hui Huui Hu
Department of Aerospace EngineeringDepartment of Aerospace EngineeringIowa State University Iowa State University
Ames, Iowa 50011, U.S.AAmes, Iowa 50011, U.S.A
Lecture # 06: Boundary Layer Flows and DragLecture # 06: Boundary Layer Flows and Drag
AerEAerE 311L & AerE343L Lecture Notes311L & AerE343L Lecture Notes
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Some of the Comments and Suggestions from Students Some of the Comments and Suggestions from Students
•• ……To let the TA to explain the equipment usage manuals on To let the TA to explain the equipment usage manuals on TuesdayTuesday’’s class in the lab weekss class in the lab weeks……
•• ……Need more time for lab reports...Need more time for lab reports...•• ……To do the same lab alternative week...To do the same lab alternative week...•• ……To rotate the orders the groups to the lab firstTo rotate the orders the groups to the lab first……•• ……To ask students spend more time on preTo ask students spend more time on pre--lab assignments. lab assignments.
30min TA explanation is still too long30min TA explanation is still too long……
•• ……To put class notes online before class...To put class notes online before class...
•• ……If one group is waiting to do experiment, they can watch the If one group is waiting to do experiment, they can watch the other groups to get better idea of what is going onother groups to get better idea of what is going on……
•• ……Need to finish the lab on time in order to attend next classNeed to finish the lab on time in order to attend next class……..
•• ……What score did we get on the lab report What score did we get on the lab report #1?#1? ……
•• No. of students to chose No. of students to chose ““smaller groupsmaller group””: : 5454•• No. of students to chose No. of students to chose ““larger grouplarger group””: : 88•• No. of students to chose No. of students to chose ““does not matterdoes not matter””: : 33
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
yy
xx
Lab 3: Airfoil Wake Measurements and Hotwire Anemometer CalibratLab 3: Airfoil Wake Measurements and Hotwire Anemometer Calibrationion
∫
∫∑∑
∫∫
∫∑
∞∞
∞
∞∞
∞
∞∞
−=⇒
−=−=⇒
−=⇒
≈=
−+−=
+−=
22
2
22
2
;
21
)])(1()([
)]1)(()([
)( Since
)(
)ˆ(
dAU
yUU
yUUD
dAU
yUU
yUUDF
DF
pyppp
dAypdApD
dAnpDF
X
X
up
up
CSXX
ρ
ρ
∫
∫
∞∞
∞
∞∞
∞
∞
−=⇒
−==
2
2
22
2
2
)])(1()([221
)])(1()([
21
dyU
yUU
yUC
C
CU
dAU
yUU
yUU
CU
DC
D
D
ρ
ρ
ρ
•• Compared with the drag coefficients obtained based on airfoil sCompared with the drag coefficients obtained based on airfoil surface urface pressure measurements (lab#02) at the same angles of attack!pressure measurements (lab#02) at the same angles of attack!
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
yy
xx80 mm80 mm
Pressure rake with 41 total pressure probesPressure rake with 41 total pressure probes(the distance between the probes d=2mm)(the distance between the probes d=2mm)
Lab 3: Airfoil Wake Measurements and Hotwire Anemometer CalibratLab 3: Airfoil Wake Measurements and Hotwire Anemometer Calibration ion
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Lab 3: Airfoil Wake Measurements and Hotwire Anemometer CalibratLab 3: Airfoil Wake Measurements and Hotwire Anemometer Calibrationion
CTA hotwire probeCTA hotwire probe
Flow Field
Current flow through wire
V
),(2ww
w TVqRidt
dTmc &−=
•• ConstantConstant--temperature anemometrytemperature anemometry
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Hotwire Anemometer CalibrationHotwire 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 voltaTo quantify the relationship between the flow velocity and voltage output from ge output from the CTA probethe CTA probe
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
ReynoldsReynolds’’ experimentexperiment
μρDU
=Re
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Laminar Flows and Turbulence Flows Laminar Flows and Turbulence Flows
•• Laminar flow, sometimes known as streamline flow, Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no occurs when a fluid flows in parallel layers, with no disruption between the layers. Inflow dynamics disruption between the layers. Inflow dynamics laminar flow is a flow regime characterized by high laminar flow is a flow regime characterized by high momentum diffusion, low momentum convention, momentum diffusion, low momentum convention, pressure and velocity almost independent from time. pressure and velocity almost independent from time. It is the opposite of turbulent flow. It is the opposite of turbulent flow.
–– In nonscientific terms laminar flow is In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough.""smooth," while turbulent flow is "rough."
•• In fluid dynamics, turbulence or turbulent flow is a In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic fluid regime characterized by chaotic, stochastic property changes. This includes low momentum property changes. This includes low momentum diffusion, high momentum conversation, and rapid diffusion, high momentum conversation, and rapid variation of pressure and velocity in space and time. variation of pressure and velocity in space and time.
–– Flow that is not turbulent is called laminar flowFlow that is not turbulent is called laminar flow
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Turbulent Flows in a Pipe
μρVD
=Re
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Characterization of Turbulent FlowsCharacterization of Turbulent Flows
'';' wwwvvvuuu +=+=+=
∫∫∫+++
===Tt
t
Tt
t
Tt
t
dttzyxwT
wdttzyxvT
vdttzyxuT
u0
0
0
0
0
0
),,,(1;),,,(1;),,,(1
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Turbulence intensitiesTurbulence intensities
0'0';0' === wvu
0)'(0)'(;0)'(1)'( 2222
0
>>>= ∫+
wvdtuT
uTt
t
o
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Turbulent Shear StressTurbulent Shear Stress
yu
lam ∂∂
= μτLaminar flows:
''vuyu
turblam ρμτττ −∂∂
=+=Turbulent flows: ''vuturb ρτ −=
A.laminar flowA.laminar flow b.turbulent flowb.turbulent flow
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Laminar Flows and Turbulence Flows
AU
DCd2
21
∞
=ρ
μρDU
=Re
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Automobile aerodynamics
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Quantification of Boundary Layer FlowQuantification of Boundary Layer Flow
yy
∞==
Uu 99.0,yat δ
Displacement thickness:Displacement thickness:
Momentum thickness:Momentum thickness:
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Boundary Layer TheoryBoundary Layer Theory
X
X
X
Xf
X
X
X
X
C
XU
Re664.0Re72.1*
Re0.5Re328.1
Re
=
=
=
=
= ∞
θ
δ
δ
μρ
0≈∂∂yp
XXYY BlasiusBlasius solution for solution for
laminar boundary layer:laminar boundary layer:
wallw y
U∂∂
= μτ
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Boundary Layer TheoryBoundary Layer Theory
5/1
5/1
)(Re37.0
)(Re074.0
Re
X
Xf
X
X
C
XU
=
=
= ∞
δ
μρ
0≈∂∂yp
XXYY
Turbulent boundary layer:Turbulent boundary layer:
wallw y
U∂∂
= μτ
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Boundary Layer FlowsBoundary Layer Flows
wallw y
U∂∂
= μτ
XXYY
Which one will induce Which one will induce moremore drag? drag? Laminar boundary layer? Laminar boundary layer? Turbulent boundary layer? Turbulent boundary layer?
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
Boundary Layer FlowsBoundary Layer Flows
wallw y
U∂∂
= μτ
Which one will induce more drag? Which one will induce more drag? Laminar boundary layer? Turbulent boundary Laminar boundary layer? Turbulent boundary layer? layer?
Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!
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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