Lecture # 06: Boundary Layer Flows and Drag · Title: Microsoft PowerPoint - Lecture-06-Notes Author: huhui Created Date: 2/17/2009 9:27:58 AM

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


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


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!


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


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


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


ReynoldsReynolds’’ experimentexperiment

μρDU

=Re


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


Turbulent Flows in a Pipe

μρVD

=Re


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


Turbulence intensitiesTurbulence intensities

0'0';0' === wvu

0)'(0)'(;0)'(1)'( 2222

0

>>>= ∫+

wvdtuT

uTt

t

o


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


Laminar Flows and Turbulence Flows

AU

DCd2

21

∞

=ρ

μρDU

=Re


Automobile aerodynamics


Quantification of Boundary Layer FlowQuantification of Boundary Layer Flow

yy

∞==

Uu 99.0,yat δ

Displacement thickness:Displacement thickness:

Momentum thickness:Momentum thickness:


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


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


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?


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?


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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Lecture # 06: Boundary Layer Flows and Drag · Title: Microsoft PowerPoint - Lecture-06-Notes Author: huhui Created Date: 2/17/2009 9:27:58 AM