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SUBSONIC WIND TUNNEL
YEDİTEPE UNIVERSITY
DEPARTMENT OF MECHANICAL ENGINEERING
1
Subsonic Wind Tunnel
1. Objective:
 To determine the drag force and drag coefficient for various objects.
2. Equipment:
 Five objects
o Hemisphere, convex to air flow direction
o Hemisphere, concave to air flow direction
o A circular disc
o Streamlined shape
 Pitot tube
 Pressure transducer
3. Theory:
3.1 The Drag Force and Drag Coefficient:
Drag is defined as the force component, parallel to the relative approach velocity, exerted on the body
by the moving fluid and is defined as follows,
1
FD  C D    A  V 2
2
The drag coefficient, CD, is defined as
2  FD
CD 
  A V 2
where,
A
the projected area of the body on a plane normal to the flow [m2]

Specific gravity of the fluid (in this experiment, air) [kg/m3]
the fluid velocity [m/s]
V
The projected area of the body can be defined as,

A  D2
4
where D is the diameter of the object [m], (use 64 mm for each model)
In general, drag coefficients are plotted versus Reynolds number (Re) which is defined as,
VD
Re 

where  is the kinematic viscosity of the fluid (in this experiment, air)
3.2 Velocity Measurement
Pitot tubes can be used to measure the air velocity by using the pressure difference between
two taps on its surfaces. The basic structure of a pitot tube is shown below.
2
The pressure corresponds to the stagnation point is called total pressure PT  , and the pressure
corresponds to the static taps is called static pressure PS  , and the relation between these two
pressures is given as,
V2
PT  Ps  
2
By using a differential pressure transducer, the pressure difference, ( PT  Ps ), can be
measured and the fluid velocity can be calculated as follows.
V  2( PT  Ps ) / ρ
4. Procedure
4.1 The Drag Force and Drag Coefficient:
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Open the wind tunnel by sliding.
Mount the model in the drag balance (axis-symmetric flow is desired, thus pay
attention to the pins, situated at the base of the support rod of each model)
Close the tunnel.
Adjust the drag counterbalance weight if necessary.
Turn on the fan with the desired frequency.
Check the air velocity from the inclined manometer.
Measure the drag force by using the drag balance in [N] (the balance arm parallel to
the air flow).
Turn off the fan.
Replace the model with another one.
Do the same procedure.
Fill the Table.
3
4.2 Velocity Measurement:
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Locate the Pitot tube inside the tunnel.
Mount the taps to the manometer.
Turn on the fan.
Travel along the section of the tunnel in order to get an average velocity.
Do not shake the Pitot tube, otherwise it gives wrong values.
5. Analysis and Discussion
1. Give a sample calculation of the drag coefficient and the air velocity.
2. Show the variations of the CD (in y axis) - Re (in x axis) for each model in the same
graph.
3. Check the CD values from literature, and compare them with your findings.
4. DO NOT CHEAT!
4
MODEL
V (volt)
(Pt - Ps ) [Pa]
V (m/s)
Re
Sphere
Hemisphere, Convex
Hemisphere, Concave
Circular Disc
Streamlined Shape
Air Foil
Air Foil
5
FD(N)
CD
FL (N)