Eva Kwoll

Thanks to GLOMAR and a research grant through the Zentrale Forschungsförderung, I was able to conduct a set of flume experiments in the Environmental Fluid and Sediment Dynamics Laboratory situated at the Simon Fraser University in Canada. While most of my PhD research focusses on quantifying the interaction of flow and bedforms in the field from shipborne surveys, this visit offered me the unique opportunity to examine the effect of particular parameters in high resolution in the controlled environment of the laboratory. Under the supervision of Dr. Jeremy Venditti, I built three sets of dunes with the same height and length, the only varying parameter being the angle of the leeward dune side (10, 20 and 30°). Studies have shown that the presence of dunes influences the flow in the leeside: behind high-angle dunes flow reversal is present while behind low-angle dunes flow deceleration has been observed. It is still not known when this transition in flow behavior occurs and how it influences the turbulence generated above the dunes. Turbulence plays a crucial role in suspending and transporting sediment and therefore I set out to look at the problem systematically.

I built the dunes out of styrofoam and covered the top with sand, to create a natural roughness. I had the opportunity to use two types of instruments to quantify the flow structure over the dunes. The first was a Laser Doppler Velocimeter (LDV) yielding 3D velocity data at frequencies of about 500 Hz at a point in the water column. By means of a traverse it follows a user-defined path to measure at positions along an entire dune and across the water column. With these data I plan to quantify characteristic differences in the mean flow field above the dunes particularly over the leeward side of the dunes. I also deployed Particle Image Velocimetry (PIV). A high-speed camera takes pictures of the flow after neutrally buoyant particles have been added. Analyzing a sequence of images, single particles can then be traced and the evolution of eddies in the dune leeward side and along the dune examined. This combined with longer time series of the LDV will enable determination of the frequency and magnitude of large turbulent eddies - usually referred to as coherent flow structures – depending on the leeward side angle.

I really enjoyed my time in Canada; I came home with lots of data and lots of ideas to publish. The working group was great and during the 6 months I got insights into education and research at a Canadian University. Last but not least, the British Columbia countryside is simply beautiful and Vancouver an exciting city to live in.