Graduate Seminar in Fluid Mechanics

November 17, 2017 @ 3:00 pm – 4:00 pm
316 Hodson Hall

3:00 pm Presentation

 Aerodynamics of Ventilation in Termite Mounds”

 Presented by SHANTANU BAILOOR (Adviser: Prof. Mittal)

Fungus-cultivating termites collectively build massive, complex mounds which are much larger than the size of an individual termite and effectively use natural wind and solar energy, as well as the energy generated by the colony’s own metabolic activity to maintain the necessary condition for the colony survival. We seek to understand the aerodynamics of ventilation and thermoregulation of termite mounds through computational modeling. A simplified model accounting for key mound features, such as soil porosity and internal conduit network, is subjected to external draft conditions. The role of surface flow conditions in the generation of internal flow patterns and the ability of the mound to transport gases and heat from the nursery are examined. The understanding gained from our study could be used to guide sustainable bio-inspired passive HVAC system design, which could help optimize energy utilization in commercial and residential buildings.

3:25 pm Presentation

Strict Nonlinear Bounds on Transition Reynolds Number in High-Speed Boundary Layers

 Presented by REZA JAHANBAKHSHI (Adviser: Prof. Zaki)

Laminar-to-turbulence transition in high-speed flows has significant implications on drag and heat transfer. As a result of its sensitivity to the disturbance environment, small changes in the initial perturbation can lead to unpredictable changes in the transition mechanism and location. Previous computational studies, including direct numerical simulations, have generally started from assumptions grounded in linear theory, e.g. inflow perturbations that are particular or superposition of linear instability waves. Their predictions of transition are therefore dependent on choices informed only by linear theory, and hence are not reliable predictors of performance in realistic environments where transition is triggered by a different free-stream perturbation spectrum.  In this work, we present a new approach that circumvents this deficiency.  We seek strict nonlinear bounds on transition Reynolds number. An ensemble-based variational approach is adopted, where the objective is to compute the inflow disturbance that has a specified initial energy, whose evolution satisfies the non-linear Navier-Stokes equations, and which causes transition to turbulence as far upstream as possible.  Spectral decomposition of this perturbation highlights the essential elements to promote breakdown to turbulence, and the associated transition mechanism.  Most importantly, the present approach provides a strict, nonlinear, minimum bound on transition Reynolds number for a given level of disturbance energy and, as such, a benchmark for robust flow design.

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