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SMS-491: Physical solutions of everyday problems in
aquatic sciences.
Lab 5, High Re # flows past objects.
Location: wave tank.
Today we will measure the force it takes to tow an object at a constant speed in water.
We will use a cylinder (2 orientations) and two disks.
We will tow each at 6-8 velocities and measure the force the strain the object applies on a
strain gauge. This strain is proportional to the drag force the object experience, and the
instrument measuring the strain provide an output that is proportional to force (provided
in pounds=mg).
For each measurements you will receive the velocity (in frequency units that are
proportional to velocity) and the force (in counts that can be translated to pounds).
In the lab:
Measure the dimensions of each body and its cross sectional area to the flow.
Input all data into worksheets.
As homework (using a computer worksheet):
1. Input all the data into the computer (provide units, and estimated uncertainties).
2. Calculate the Re=UL/, based on L=sqrt(A), =10^-6m2/sec and the velocity (U),
where A is the cross-sectional area in direction of motion.
3. Calculate the drag force (in Newtons).
4. Calculate the drag coefficient CD=FD/{0.5U2A}, where kg/m3.
5. Plot for each shape the Force as function of U and U2.
6. Plot CD vs. Re # on the same plot for all the different bodies (and orientation).
Answer the following question:
1. Does the force vary linearly with U? Does the force vary linearly with U2?
2. Do both disks have the same CD to Re# plot?
3. Which has a higher drag coefficient, a cylinder with its circular cross section facing the
flow or the disk of the same cross sectional area? Extra credit: why do you think this is
the case?
Submit homework as printout and/or file.
Boss and Jumars, 2003