Andrew Johnson

Unsteady Phenomena in Supersonic Nozzle Flow Separation

This work considers the instability of the jet plume from an overexpanded, shockcontaining convergent-divergent nozzle and attempts to correlate this instability to internal shock-induced separation phenomena. Time resolved wall pressure measurements and Pitot measurements are used as primary diagnostics. For the conditions of this study flow separation is asymmetric resulting in a large separation zone on one wall and a small separation zone on the other wall. Correlations of wall pressures indicate a low-frequency, piston-like shock motion without any resonant tones. Correlations of Pitot pressure with wall pressures indicate strong coherence of shear-layer instability with the shock motion. The likely source of the plume instability is the interaction of unsteady waves generated past the main separation shock with the shear layer of the large separation region.

Experimental and Numerical Study of Jet Mixing from a Shock-Containing Nozzle

The compressible jet plume emerging from a planar convergent-divergent nozzle containing a separation shock is investigated experimentally and numerically. The investigation encompasses exit-to-throat area ratios (Ae=At) from 1.0 to 1.8 and nozzle pressure ratios from 1.2 to 1.8. Experiments were conducted in a variable-geometry nozzle facility, and computations solved the Reynolds-averaged Navier-Stokes equations with several turbulence models. The computed mean velocity field outside the nozzle compares reasonably well with the experimental data. Among the different turbulence models tested, the two-equation shear stress transport model is found to provide the best agreement with the experiments. Jet mixing is governed by Ae=At and, to a lesser extent, by nozzle pressure ratios. Increasing Ae=At results in an increased growth rate and faster axial decay of the peak velocity. The experimental trends of jet mixing versus Ae=At and nozzle pressure ratios are captured well by the computations. Computations of turbulent kinetic energy show that, with increasing Ae=At , the peak turbulent kinetic energy in the plume rises and moves toward the nozzle exit. The significant increase of turbulent kinetic energy inside the nozzle is associated with asymmetric flow separation.

A tale of two seas: contrasting patterns of population structure in the small-spotted catshark across Europe

Elasmobranchs represent important components of marine ecosystems, but they can be vulnerable to overexploitation. This has driven investigations into the population genetic structure of large-bodied pelagic sharks, but relatively little is known of population structure in smaller demersal taxa, which are perhaps more representative of the biodiversity of the group. This study explores spatial population genetic structure of the small-spotted catshark (Scyliorhinus canicula), across European seas. The results show significant genetic differences among most of the Mediterranean sample collections, but no significant structure among Atlantic shelf areas. The data suggest the Mediterranean populations are likely to have persisted in a stable and structured environment during Pleistocene sea-level changes. Conversely, the Northeast Atlantic populations would have experienced major changes in habitat availability during glacial cycles, driving patterns of population reduction and expansion. The data also provide evidence of male-biased dispersal and female philopatry over large spatial scales, implying complex sex-determined differences in the behaviour of elasmobranchs. On the basis of this evidence, we suggest that patterns of connectivity are determined by trends of past habitat stability that provides opportunity for local adaptation in species exhibiting philopatric behaviour, implying that resilience of populations to fisheries and other stressors may differ across the range of species.

Aerodynamics of Wedge-Shaped Deflectors for Jet Noise Reduction

A wedge-shaped deflector placed in the fan exhaust of a turbofan engine can reduce jet noise in a range of azimuthal directions opposite the wedge. The aerodynamics of the fan flow deflector (FFD) wedge are investigated with pressure surveys and flow visualization. The flow field is found to differ substantially from that of a classical wedge-shaped cylinder. There is a significant force developed on the side of the FFD wedge, whereas the classical wedge develops nearly zero side force. The base pressure coefficient of the FFD wedge is substantially less negative than that of the classical wedge. Consequently, the FFD wedge develops about 75% less drag than one would have expected from 2D wedge data.

Body Force Model for the Aerodynamics of Inclined Perforated Surfaces

This is a joint experimental and computational research effort on the aerodynamics of perforated surfaces inclined to a freestream. The goal is to characterize the key parameters affecting the flowfield in order to construct a macroscopic computational model that obviates resolution of the detailed perforations. The model simulates the effects of the perforated surfaces by locally applying a body force term in the momentum equation. The study considers wedge-shaped deflectors used for jet noise reduction in turbofan engine nozzles in the form of flaps with varying angles and perforation shapes. The experiments measured the mean velocity field inside and in the wake of theflapswhile the numerical analysis comprised direct computations of simplifiedperforatedflaps and computations using the body force model. It is found that the effective porosity of the surface is affected by flow separation within the perforations. Theflowblockage of the separation regions is primarily a function of the angle to the freestreamand the ratio of the thickness to hole diameter. Accordingly, a formulation for the body force model is proposed that incorporates this finding. The resulting velocity field is in good agreement with the experiments and with the direct computations.

TerraWatchers, Crowdsourcing, and At-Risk World Heritage in the Middle East

Cultural resources, like natural resources, are finite. Every country has cultural resources that reflect local history and prehistory and are vital both economically for tourism and socially as a foundation of cultural identity. This material cultural record is part of world heritage, something recognized by UNESCO since 1972 (http://whc.unesco.org). As archaeological and historical sites occur in relatively restricted areas, they are a limited resource that should be cared for, curated, and preserved for local, national, and international communities. Like natural resources, cultural resources are sensitive to human and natural intervention. Unfortunately, over the past 4 years, more than any other region on the planet, archaeological heritage sites in the Eastern Mediterranean and Middle East have suffered destruction at an alarming rate. Political instability, war, extreme ideologies, economic downturns, and other factors have led to the wanton destruction of heritage sites. As part of the University of California Office of the President Catalyst Grant project “At-Risk Cultural Heritage and the Digital Humanities,” the TerraWatchers Crowdsourcing platform is being used to involve citizen scientists in monitoring archaeological site destruction in war-torn Syria and Iraq using satellite imagery. This chapter presents an overview of TerraWatchers, how it relies on the expertise of the American Schools of Oriental Research (ASOR) Cultural Heritage Initiatives to identify at-risk sites in the region, and how university students in a classroom setting were trained to participate in this digital cultural heritage experiment.

Shock Motion and Flow Instabilities in Supersonic Nozzle Flow Separation

This study evaluates the role of shock wave motion on the instability of the plume exiting an over-expanded, convergent-divergent nozzle. An array of wall pressure transducers was used to track the position of the shock in time, and a Pitot probe was used to obtain simultaneous measurement of the total pressure fluctuations at various points in the jet that emerges from the separation shock. Analysis of the shock motion revealed that the shock wave becomes more unstable as it becomes stronger, as evidenced by an increase in the range of motion and in the frequency of large-scale oscillations. For strong shocks, there is a substantial correlation between shock motion and total pressure fluctuation in the plume. Such correlation is absent for relatively weak shocks. The study indicates that shock motion affects the plume instability if the separation shock is very strong, and that other mechanisms govern the plume instability when the shock is relatively weak.