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Transcript

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/{0.5U2A}, 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