Non-linear acceleration in swim cap for data consistency(help)

[jeeloong]

Dear Experts!

I am currently try to measure the drag of an buoyant object. I did 10 replicates, in general terms 10 tests all with the same set-up and same release mechanisms.

To start off with I will explain my project: Drag reduction in swim cap
First of all I got one accelerometer with buoyant object. This buoyant object is shown in picture. Which I will measure the drag associated with this object (drag coefficient Cd). To do so I get a set of data acceleration versus time in X direction. Then I numerically integrate it to get velocity and again to get time.

The issue I am having is I couldn't get the data to overlap or consistent otherwise it would be pointless to say that ok the hydrophobic or hydrophilic solution that being used for the swim cap is actually reducing the drag.

It would be great if someone could explain the actual way of processing the data. Or if it is an unavoidable issue. It really hard for this problem in my report.

Thanks.

 

[tycoon515]

It's okay if the graphs do not overlap. Variation in the data is inevitable. What you want to look for is a common trend that all trials seem to follow. If you predicted that the hydrophobic/hydrophillic effect reduced the coefficient of drag, then you would expect force of fluid drag on the object to decrease in time, which means the net force acting on the object increases in time, and therefore its acceleration increases (i.e. it undergoes positive jerk). To observe this, look at the derivative of your a vs t graph and see when it's positive or simply look at the slope of the a vs t graph at various points. Is there an interval where the a vs t graph is increasing? If so, what does this say about how the coefficient of drag of the object changes in time?