On the dynamics of flow past a cylinder: Implications for CTD package motions and measurements

During the collection of ship-based observations, heaving of the vessel may lead to variation in the descent rate of a CTD package. This can result in the package being pulled upward through previously sampled water, leading to difficult to quantify errors due to the complex wake. To reduce this problem to one of manageable stature, we use the simple paradigm of two-dimensional flow past a cylinder. By using a tracer with a gradient along the flow, we quantify the effect of the cylinder on its distribution and the impact of postprocessing. At high Reynolds numbers, over 200, uniform translation leads to a small error in the tracer value. This error is likely negligible at the much higher Reynolds number of the ocean. When the flow is oscillated longitudinally, there are two main sources of error; attached vortices may propagate around the cylinder and/or shed vortices may translate into the path of the cylinder. Postprocessing by removing records from previous pressure levels removes much of the first error, due to it occurring as the package ascends. However, the second source of error is more difficult to remove, due to it occurring when the package is once again descending. In general, results indicate that long-period oscillations are preferable. While the magnitude of the errors are comparable to those from short-period oscillations, they are spread farther apart in time and space and the overall effect is to localize the errors in small regions of the final depth profile.

Details