wind tunnel correction

7/30/2019 Wind Tunnel Correction

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Experimental Aerodynamics

Lecture 3:

Wind Tunnel Corrections

G. Dimitriadis

Experimental AerodynamicsExperimental Aerodynamics


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Wind tunnel corrections

! Wind tunnels cannot recreate flowfieldsidentical to the original.

! Wind tunnel flows are different to theoriginal due to several factors.

! These differences necessitate correctionsto be applied to measurements obtained inwind tunnels.

! The type and number of correctionsdepends on whether the simulated flow is2D or 3D.


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2D Wind tunnel corrections! In 2D experiments, the model is

constrained between flat plates or walls toforce the flowfield to be nearly 2D.

! There are several sources of inaccuracyfor 2D experiments in wind tunnels.

! The most important are:! Buoyancy! Solid blockage

!Wake blockage! Streamline curvature


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Buoyancy

! Wind tunnel buoyancy is caused by the factthat the boundary layer grows on the walls ofthe working section.

!Boundary layer growth is equivalent to acontraction of the working section area.

! The flow is accelerated, causing a drop in staticpressure.

! Therefore, models with a big frontal area arepushed backwards.

! Buoyancy artificially increases the drag


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Longitudinal pressure gradient! The longitudinal pressure drop is nearly

linear.

! The longitudinal pressure gradient is givenby:! Where k=0.016-0.040 is a factor that must

be measured for a given wind tunnel.! h is the height of the working section.

dp

dl= !k

"V2

2h


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Buoyancy correction

! The buoyancy correction depends on thelongitudinal pressure gradient and the volumeof the body, i.e.

! Where !DB

is the correction to be applied to themeasured drag force and V

Bthe body volume.

! More accurate corrections have been estimatedfor airfoils, Rankine ovals, ellipses and othersimple shapes.

!DB = dpdl

VB


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Solid blockage

! The presence of a model in the workingsection reduces the area through which

the air can flow.

! The air velocity is increased over themodel.

! This effect is called solid blockage.! The effect can be corrected by increasing

the effective wind tunnel airspeed.


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Solid blockage correction! The correction to the airspeed for a circular

cylinder is given by:

! Where Vu

is the uncorrected airspeed and !sb

is

given by:

! For a more general shape,

! WhereK1=0.74 for a horizontal model and 0.52 fora vertical model. Sis the working section area.

!V = "sbV

u

!sb

="

2

3

R

2

h2

!sb

=

K1VB

S3 / 2


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Wake blockage

Velocity < V

Velocity < V

Velocity > V

hV

The airspeed in the wake must be lower than V. In a

closed duct this means that the airspeed outside the

wake must be larger than Vfor a constant mass flowrate.

The wake blockage effect can also be corrected using

an increment in the effective airspeed.


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Wake blockage correction

! The airspeed correction is given by!

Where!

wb is given by:

! Where c is the models length (or wingchord) and

cdu is the uncorrected 2D dragcoefficient.

!V = "wbV

u

!wb

=

c

2hcdu


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Streamline curvature

! The wind tunnel ceiling and floor artificiallystraighten the curvature of the flow

streamlines around the model.

! The model appears to have more camberthan it really has, i.e. it has too much lift.

! This effect requires corrections to angle ofattack, lift coefficient and momentcoefficient


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Streamline curvature

corrections! The angle of attack becomes:! The lift coefficient becomes:! The moment coefficient around the half

chord becomes:

! Where !=!sb+!wband

! =!u+57.3"

2#clu+ 4c

m1/ 2u( )

cl= c

lu1!"! 2#( )

cm1/ 2

= cm1/ 2u

1! 2"( ) +#c

l

4

! ="

2

48

c

h

#$%

&'(2


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Total corrections

! The total corrected airspeed is:!

The total corrected dynamic pressure:

! The total corrected Reynolds number:! The total corrected drag (zero-lift):

V =Vu1+ !( )

q = qu1+ 2!( )

Re = Reu1+ !( )

cd0

= cd0u

1! 3"sb! 2"

wb( )

wind tunnel correction

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