Measurement of Pressure Distribution, Drag, Lift , and Velocity for an Airfoil

Measurement of Pressure Distribution, Drag, Lift , and Velocity for an Airfoil

PurposeTest designMeasurement system and ProceduresUncertainty Analysis

PurposeExamine the surface pressure distribution on a

Clark-Y airfoilCompute the lift and drag forces acting on the

airfoilSpecify the flow Reynolds numberCompare the results with benchmark dataUncertainty analysis for

Pressure coefficientLift coefficient

Test Design

Facility consists of: Closed circuit vertical wind tunnel. Airfoil Temperature sensor Pitot tube Load cell Pressure transducer Hot wire anemometer Automated data acquisition system

Test Design (contd.)Airfoil (=airplane surface: as wing) is placed

in test section of a wind tunnel with free-stream velocity of 15 m/s. This airfoil is exposed to: Forces acting normal to free stream = Lift Forces acting parallel to free stream =

Drag Only two dimensional airfoils are considered:Top of Airfoil: The velocity of the flow is greater than the

free-stream. The pressure is negativeUnderside of Airfoil: Velocity of the flow is less than the the free-

stream. The pressure is positiveThis pressure distribution contribute to the lift

Measurement systems

Instrumentation Protractor – angle of attack Resistance temperature detectors

(RTD) Pitot static probe – velocity Scanning valve – scans pressure

ports Pressure transducer (Validyne) Digital Voltmeter (DVM) Load cell – lift and drag force

Airfoil Model

Pitot Tube(Free

Stream)

Pressure Taps

Bundle o ftubes

Digita li/o

A/DBoards

SerialCom m .(C O M 1)

Softw are- Surface Pressure- Velocity- W T C ontro l

PC

ScanivalvePosition

Circu it (S PC)

RTD

M etrabyteM 2521Signal

Cond itioner

ScanivalveSignal

Cond itioner(S SC)

ScanivalveController

(S C)

Scanivalve

PressureTransducer(Validyne)

Digita lVo ltim eter

(D VM )

PressureInput

Measurement systemsMeasurement systems

Hot-wire anemometry• Correlates velocity and voltage• Survey of velocity in wake

region is preformed• Lower velocities are found

directly downstream of airfoil• Higher velocities are located far

above and below airfoil• Apply momentum flux equations

to calculate drag force

AOA, and Pressure taps positions

Data reduction

In this experiment, the lift force, L on the Airfoil will be determined by integration of the measured pressure distribution over the Airfoil’s surface. The figure shows a typical pressure distribution on an Airfoil and its projection .

Data reduction

Calculation of lift and drag forces The lift force L is determined by integration of the

measured pressure distribution over the airfoil’s surface.

It is expressed in a dimensionless form by the pressure coefficient Cp where, pi = surface pressure measured, = P pressure in the free-stream

The lift force is also measured using the load cell and data acquisition system directly.

U = free-stream velocity, r = air density ( temperature),

pstagnation = stagnation pressure measured at the tip of the pitot tube, L = Lift force, b = airfoil span, c = airfoil chord

cU

dspp

C sL

2

21

sin

2

21

U

ppC ip

ppU stagnation2

bcU

LCL 2

2

dsppLs

sin

bcU

DcbUDCD 2

2,,,,

Calibration of load cell

-0.5 -0.4 -0.3 -0.2 -0.1 0

0

0.5

1

1.5

2

f(x) = − 3.97809750155233 x − 0.0791748292766911R² = 0.999247913596543

Lift

Fzavg

Linear (Fzavg)

Volts

Ma

ss

mass (kg) Volts

0 -0.021

0.295 -0.1525

0.415 -0.203

0.765 -0.3565

1.31 -0.5935

1.635 -0.7385

Calibration program

Program output

Curve fitting method

Data acquisition

Setting up the initial motor speed Visualization of wind tunnel conditions

Data acquisition (contd.)

Data needed: Observation point list Sampling Rate Settling Time Length of each Sample Angle of attack

Airfoil pressure visualization

Calculation of lift force

Program to measure lift force in volts

Calculation of drag force

Program to measure velocity in volts

Uncertainty analysis

Uncertainty analysisPressure coefficient Lift coefficient

),,( UppfC ip

222CpCpCp PBU

2)(

2)(

2

1

22

ppippii

j

iiCp BBB

2_

2

Upp

C

i

pppi

MSP CpCp 2

),,,,( cUppfC iil

222CLCLCL PBU

2)(

2)(

2

1

22

ppippii

j

iiCL BBB

MSP CLCL 2

Benchmark data

0 10 20 30 40 50 60 70 80 90 100

-4

-3

-2

-1

0

1

2

Benchmark data for pressure coefficient for AOA = 13

AOA ...

x/c

Co

eff

cie

nt

of

pre

ss

ure

(C

p)

a) Distribution of the pressure coefficients for a = 0, 6, 13, 16 and Re = 300,000; , Benchmark data

Benchmark data for pressure coefficient for AOA = 0

-4

-3

-2

-1

0

1

2

0 20 40 60 80 100

x/c

Co

eff

cie

nt

of

pre

ss

ure

(C

p)

AOA = 0

Benchmark data for pressure coefficient for AOA = 6

-4

-3

-2

-1

0

1

2

0 20 40 60 80 100

x/c

Co

eff

cie

nt

of

pre

ss

ure

(C

p)

AOA = 6

Benchmark data for pressure coefficient for AOA = 16

-4

-3

-2

-1

0

1

2

0 20 40 60 80 100

x/c

Co

effc

ien

t of p

ress

ure

(Cp

)

AOA = 16

Benchmark data continued

Reference data for CL

Reference data for CD

Benchmark data for lift coefficient

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20 25 30 35

Angle of attack (AOA)

Lif

t c

oe

ffic

ien

t (C

l)

Benchmark data for drag coefficient

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 5 10 15 20 25 30 35

Angle of attack (AOA)

Dra

g c

oef

ficie

nt (

Cd

)

Velocity ProfileAOA 0, 6, 13, 20

0

10

20

30

40

50

60

6 8 10 12 14 16 18

Velocity m/s

Po

siti

on

cm AOA 20

AOA 13

AOA 6

AOA 0

Velocity profiles