MECHANICS of FLUIDS LABORATORY - Mechanical Engineering

EXPERIMENT 10
DRAG FORCE DETERMINATION
An object placed in a uniform flow is acted
upon by various forces. The resultant of these
forces can be resolved into two force components,
parallel and perpendicular to the main flow
direction. The component acting parallel to the
flow is known as the drag force. It is a function of
a skin friction effect and an adverse pressure
gradient. The component perpendicular to the
flow direction is the lift force and is caused by a
pressure distribution which results in a lower
pressure acting over the top surface of the object
than at the bottom. If the object is symmetric
with respect to the flow direction, then the lift
force will be zero and only a drag force will exist.
Measurement of the drag force acting on an object
immersed in the uniform flow of a fluid is the
subject of this experiment.
Equipment
Subsonic Wind Tunnel
Objects
A description of a subsonic wind tunnel is
given in Experiment 9 and is shown schematically
in Figure 9.3. The fan at the end of the tunnel
draws in air at the inlet. An object is mounted on a
stand that is pre calibrated to read lift and drag
forces exerted by the fluid on the object. A
schematic of the test section is shown in Figure
10.1. The velocity of the flow at the test section is
also pre calibrated. The air velocity past the
object can be controlled by changing the angle of
the inlet vanes located within the fan housing.
Thus air velocity, lift force and drag force are
read directly from the tunnel instrumentation.
There are a number of objects that are
available for use in the wind tunnel. These
include a disk, a smooth surfaced sphere, a rough
surface sphere, a hemisphere facing upstream,
and a hemisphere facing downstream. For
whichever is assigned, measure drag on the object
as a function of velocity.
Data on drag vs velocity are usually graphed
in dimensionless terms. The drag force D f is
customarily expressed in terms of the drag
coefficient C D (a ratio of drag force to kinetic
energy):
in which ρ is the fluid density, V is the free
stream velocity, and A is the projected frontal
area of the object. Traditionally, the drag
coefficient is graphed as a function of the
Reynolds number, which is defined as
Re = VD
ν
where D is a characteristic length of the object
and ν is the kinematic viscosity of the fluid. For
each object assigned, graph drag coefficient vs
Reynolds number and compare your results to
those published in texts. Use log-log paper if
appropriate.
Questions
1. How does the mounting piece affect the
readings?
2. How do you plan to correct for its effect, if
necessary?
uniform flow
lift force
measurement
object
mounting stand
drag force
measurement
FIGURE 10.1. Schematic of an object mounted in
the test section of the wind tunnel.
D f
C D
=
ρV 2 A/2
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