redo fluid lab
tiwoo1407
copia_de_compiled_report.pdf
Table of Contents
Abstract……………………………………..2
A: Nomenclature…………...……....2
Introduction………………………….……...3
Method……………………………………....4
Results……………………………………....5
Discussion.………………………………….5
Appendices………………………………….7
A: Calculations…………...…….…...7
B: Data…….…………,……………...8
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MENG 382 LABORATORY #2 PRESSURE AND FORCE
Abstract:
This experiment is guided by measuring the action of pressure and force over the back
portion of a golf ball in a controlled environment which creates an ideal flow of air, while a Pitot
Rake is reading the total pressure behind the ball. Its purpose is to use the pressures that were
collected to calculate the Net force, that in some cases is called Drag force, the experiment was
conducted with a Smooth ball and a Dimpled Ball to evaluate which one have smaller forces
acting. Integrating the polynomial equation of a trendline (pressure VS radius) a volume was
found and it’s equal to the total Net force. After all the data was compiled and analyzed the
Dimpled Ball offered a significantly lower drag force (91.5% smaller) , coming in at
0.03079[N] versus the 0.3639[N] in the smooth ball .
Nomenclature:
V = Velocity (m/s)
A = Area (m2) P ressure = Total pressure (Pa) P static = Static Pressure (Pa) F = Net Force (N) Vsolid = Solid Volume (N) p = Desinsity of Air (kg/m3)
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Introduction:
In this lab was used a wind tunnel to create a pressure difference on a golf ball. Having
hollow tube system with in the airstream, line it up called as a Pitot Rake system. Having it so
that the flow, then enter through the open end of the tube, also having a closed off downstream at
the end of the tube making a Pitot tube. With this the Pitot tube system it will measure the
pressure inside getting i.e. the Stagnation Pressure. The Static Pressure that the fluid had while it
was moving makes a Dynamic Pressure. Which is related to the kinetic energy (KE) of the
moving fluid which is then converted to additional Static Pressures. Why? It is brought to zero
velocity. As a prediction it was expected that we would see loss in energy. This is within the
pressure of the flow. This is ill propionate balance, the pressure acting upstream of the flow with
that downstream. The ball creates a net force in the direction of the flow. This effect we
commonly refer to as drag on the ball. Being that the Static Pressure of the fluid is what affects
the forces acting on the ball when it is moving through a fluid the air. To see how this works we
used the connection amongst Static Pressure and Total Pressure is given as
There are two different golf balls used in this experiment. The first of the two had its
normal dimples covering the surface of the sphere. Though the second ball had the dimples
smoothly filled in. All of this information from the tube devices got uploaded as text on the
computer. The data is used to calculate the net Drag force acting on the two different balls. This
data was the recorded values for total pressure from the back of the ball. Calculating the
freestream velocity there was a decrease in the pressure immediately behind the ball due to the
viscous losses.
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Method
1. We placed two different golf balls inside the wind tunnel.
2. One of the golf balls has normal dimples, and the other one the dimples have all been
smoothly filled.
3. Then, we turned the on the device and used the computer to measure the total pressure value
behind the ball, and record the freestream velocity.
4. We noticed that the pressure went down immediately behind the ball due to the viscous losses.
5. After that, we took the data from the computer program and used that data to calculate the net
Drag force acting on the two different golf balls.
6. As shown in the results, we converted the pressure values to Static Pressure in our calculations
for the golf balls determined by (P total = P static + ½*p*V^2)
7. The diameter of the golf balls and the distance between the pitot tubes were given, so we could
calculate the plot the pressure vs perpendicular distance from the ball.
8. With the pressure plotted, we fit a trendline in Excel and used the resulting equation to
integrate a solid of revolution.Conceptually, we integrated pressure times length with respect to
length, yeilding pressure times length squared, or pressure times volume, which is equal to force.
9. The solid of revolution gave the area under the curve, so we subtracted it from the cylinder
that enclosed it, this difference was our calculated force due to drag.
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Results:
Initial Estimate F D: 0.146 N Difference: 0.3331
F D Smooth Ball: 0.3639 N % Difference: 91.5%
F D Dimpled Ball: 0.03079
Discussion of Results:
For our results, we first calculated the static pressure value using the total (stagnation)
pressure and velocity using P total = P static + 0.5(ρ)v2 for each data point. We then averaged the data
points with regard to the radius of the center of the ball. With these average values we used
Excel to fit a curve in order to determine an equation for pressure with respect to radius,
assuming that the effect should be radially symmetrical. We integrated this function with respect
to radius, computing the total as a solid of revolution, as we currently had P(L) and the integral
would yield P(L2) or P(A), knowing P(A) = F. Since the only source of force came from the
fluid flow in the chamber, we deduced that this pressure force was equal to the drag force of the
ball. The easiest way to visualize this concept is that as the ball forces its way through the
moving fluid it creates a low pressure region behind the direction of travel and this pressure
difference exerts a force, much in the same way the buoyancy force act in opposition to the force
of gravity. After analyzing all our data, we found that the dimpled ball generated a significantly
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smaller pressure gradient, resulting in a calculated drag force of only 0.0177 N versus the 0.3639
N found in the smooth ball, as illustrated below. These results seemed to agree at least
reasonably well with our expectations; using a simplified drag equation F = ( )CDA v2 we21 ρ
estimated the value at 0.146 N. If these calculated results are in fact correct, even if only in
relative magnitude, the drag reduction due to dimpling is impressive to say the least. Although
we can confirm that our answer is reasonable, we have very low confidence in its accuracy. This
lack of confidence stems from the observed variations in the measurement equipment,
particularly the pitot tubes that determined our stagnation pressure. In several case we removed
values that were clear statistical outlier, adding in interpolated values to replace them where
necessary. In order to attempt to mitigate this for the smooth ball the aggregated all the results
from the different lab groups in our section, hoping that the average results would be less volatile
than just a few data points.
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