Owen Williams, PhD
Gas Dynamics Laboratory
Princeton University
Thursday, November 6, 2014, 2pm
801 22nd Street NW, Phillips Hall 736
Washington, DC 20052
Hosted by: Dr. Michael Plesniak ([email protected])
Abstract: The thermally stable atmospheric surface layer is characteristic of nocturnal or polar conditions. At present, theoretical treatments of such flows have been found to be inaccurate, significantly limiting our ability to predict surface heat fluxes, freezing/melting rates of polar ice, and dispersion of pollutants under such conditions. Experiments were conducted within a specially modified wind tunnel using particle image velocimetry (PIV). Smooth and rough surfaces were investigated to give a greater correspondence to the atmospheric surface layer.
Interestingly, the turbulent stresses were found to scale with the wall shear stress for low levels of stability, prior to the collapse of steady turbulence at a critical stratification, separating weakly and strongly stable regimes. Changes in profile shape correlate with the local stratification profile, and as a result, the collapse of near-wall turbulence is not intrinsic to the strongly stable regime as had been suggested by previous studies. The critical bulk stratification is sensitive to surface roughness and potentially Reynolds number, and not constant as previously thought. Stable stratification has only a small effect on turbulent structure for weak stability, with its influence increasing toward the collapse of turbulence. Hairpin vortices, which contribute to the majority of turbulent production, are reduced in strength and angle at high stratification, leading to fewer and weaker productive motions and hence to the collapse of turbulence.
Biographical Sketch: Owen Williams’ research interests include turbulence, heat transfer and flows with strong density gradients. He obtained his Master’s degree at Imperial College, London, investigating the diurnal cycle and power generation potential of solar updraft towers, for which he was awarded the E-On prize for energy. More recently, he obtained his PhD in Mechanical and Aerospace Engineering at Princeton University, where he experimentally examined two contrasting flows: the stable atmospheric surface layer, applicable to our understanding of polar regions, and hypersonic boundary layers. Whereas the former involves strong buoyancy effects, the latter can involve significant fluctuations in pressure and density, leading to very different scaling theories.