4:10 pm Presentation
“Three-Dimensional Measurements of Flow Structure and Turbulence around a pair of Cubic Roughness Elements Embedded in the Inner Part of a Turbulent Channel Flow”
Presented by JIAN GAO (Adviser: Prof. Katz)
The 3D flow and turbulence around a pair of roughness cubes embedded in the inner part of a turbulent channel flow (Ret=2500) are measured using microscopic tomographic holography accelerated using GPU-based algorithms. The cube height, a=1 mm, is 90 wall units. The cubes decrease the near-wall velocity as far as 3a upstream. Horseshoe vortices form at the front surface, and propagate asymmetrically around the cubes, generating secondary vortices along the side surfaces. Each cube and its near wake are engulfed by a vortical canopy dominated by wall-normal vorticity along the sides and spanwise vorticity above the cube. Merging of the “tip vortices” developing along the cube upper edge, horseshoe and secondary vortices occurs at downstream locations, whose distance to the cube decrease with the spacing. They form a large streamwise vortex behind each cube, which rotates in the same direction as the inner leg of the horseshoe vortex. With decreasing spacing, the flow accelerates faster between cubes, but also decelerates faster in the near wake. Streamwise velocity fluctuations of 30% of the freestream velocity and negative Reynolds shear stress develop near the front upper corner of the cube. The turbulence remains high around the entire surface and near wake.
4:35 pm Presentation
“Input-Output Approach to Characterizing the Structure Interaction in Turbulent Boundary Layers”
Presented by IGAL GLUZMAN (Adviser: Prof. Gayme)
In this talk, I will present our recent efforts to support an experimental study performed at the University of Notre Dame. In the experiments, active flow control is used to characterize interactions between outer layer large-scale and near-wall small-scale structures in turbulent boundary layers. In our work, we extend input-output based analysis to investigate the flow response to different types of actuation. We focus on the frequencies and structures, which result in the largest amplification of the actuation input for the experimental setup in order to validate the model, which will be used as part of the design of future experiments. Our analysis accurately predicts the frequencies that produce the strongest modulating effect of large-scale structures on the turbulent structure in the near-wall region. In addition, we observe a qualitative agreement in the large-scale structures with our model, in which the response is obtained to pulse train forcing with controlled frequency and duty-cycle. These results demonstrate the promise of the approach in guiding actuation location and strategies for these experimental studies. Ongoing work aims to further understand the full potential of the model in reproducing the quantitative behavior of flow features.