Hydraulics/Hydrology Seminar Series
Examining the Importance of Stratification and Unsteadiness in Law-of-the-wall Velocity and Turbulence Scaling
Tuesday, March 28, 2017
We use a high-resolution set of ADCP measurements from the deep (55m) hypolimnetic waters of Lake Michigan to investigate the validity and limitations of using law-of-the-wall (LOW) scaling to approximate velocity and turbulence structure within the bottom boundary layer (BBL), a topic of interest for researchers who hope to model physical and biological processes that rely on estimates of bottom-stress and roughness. In spite of the weak currents (U1m=3 cm/s), logarithmic velocity structure is found within the bottom 1m of flow over 90% of the time, with 1m drag coefficient and roughness values of 0.0044 and 0.0015m, respectively. Despite this, drag coefficients show strong speed dependence, with large standard errors at low speeds (U50<2cm/s). Log-linear modifications to the LOW are used to explain this phenomenon, with the increased scatter in drag coefficients largely attributed to seiche-scale unsteadiness (dU50/dt=±10-5 m/s2). While the effects of stratification (=10-6 rad2/s2) are deemed unimportant to near-bed flow structure, its effects on flow outside the BBL are appreciable and well-resolved by log-linear LOW modifications. Measured turbulence characteristics (e.g. dissipation, diffusivity) are elevated above LOW estimates and only approach expected LOW approximations at the highest flow speeds, with vertical structures that suggest an unsteadiness-related scale limitation.
David grew up in Indiana near Lake Michigan and graduated with his Bachelor’s degree from Purdue in 2014. As a Ph.D student in Civil Engineering, he is currently researching Great Lakes physical processes, with an emphasis on bottom boundary layer dynamics. David is most interested in the role that these processes play on the interaction between biota and their environments.