Abstract
In this
experiment we measured the drag in a circular cylinder by direct measurement
and by a wake survey method at a specific Reynold number. We took several
readings using the Pitot tube to find the velocity and with certain relations
we plot those with MS excel to gain the values of the drag coefficient either
by the slope for the direct measurement and the area under the curve for the
wake measurement.
Introduction
In fluid dynamics, drag (sometimes
called air resistance, a type of friction, or fluid resistance,
another type of friction or fluid friction) refers
to forces acting opposite to the
relative motion of any object moving with respect to a surrounding fluid. This
can exist between two fluid layers (or surfaces) or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of
velocity, drag forces depend on velocity.
Types of drag
- 1. Parasite drag (skin friction drag) due to shear stress
- 2. Pressure drag : due to pressure distribution and normal stress (form drag)
- 3. Profile drag : drag due to skin friction drag and pressure drag
- 4. Induced drag : drag due to lift
- 5. Wave drag : drag due to shock waves
- 6. Interference drag : drag due to cross flow
In other
references the skin friction drag and the pressure drag and the interference
drag are called Parasite drag.
(Found in Wikipedia)
The total drag is the combination of
drag due to lift and zero lift drag which affects the velocity of the airplane
for determining the gliding speed, maximum range, maximum (L/D).
Discussion
The drag
coefficient measured in the direct measurements is 0.626984 which is very close to the value
measured in the last experiment (Cd=0.63554)
Error = 1.3%
(negligible)
The drag
coefficient measured in the wake measurements is 0.05461
which is far from the value measured in the
last experiment (Cd=0.63554)
Error= 91.4%
(very huge)
Conclusion
The direct
method is more accurate the wake measurement method and it is very close to the
pressure distribution method.
The wake method
is very sensitive which define the large error in addition to the human error
and
calculations.
calculations.
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