Daniel Edgington-Mitchell

The underexpanded jet Mach disk and its associated shear layer

High resolution planar particle image velocimetry is used to measure turbulent quantities in the region downstream of the Mach disk in an axisymmetric underexpanded jet issuing from a convergent nozzle. The internal annular shear layer generated by the slip line emanating from the triple point is shown to persist across multiple shock cells downstream. A triple decomposition based on Proper Orthogonal Decomposition shows that the external helical structure associated with the screech tone generated by the jet exerts a strong influence on velocity fluctuations in the initial region of the annular shear layer. This influence manifests as the external vortices producing oscillatory motion of the Mach disk, and thus a forcing of the internal annular shear layer. The internal shear layer is characterized by a number of azimuthal modes of varying wavenumber and type, including both helical and axisymmetric modes. Finally, the possibility of a previously hypothesized recirculation region behind the Mach disk is investigated, with no evidence found to support its existence.

Staging behaviour in screeching elliptical jets

The staging behaviour of jet screech in an elliptical nozzle with with an aspect ratio AR = 2.0 is examined across a range of pressure ratios. Acoustic measurements and high resolution imaging are used to assess the staging behaviour. Based on the frequency of the dominant screech tone, the jet staging behaviour can be separated into five distinct modes over the range of pressures studied. A statistical analysis of the schlieren images suggests that the first mode is varicose, the next three modes are flapping instabilities, and the final mode is helical in nature. A comparison of characteristic length scales in the jet is undertaken to explain the mode switches. A shift from the classical “weakest link” feedback model to an acoustic waveguide model, associated with a shift in characteristic length scale of the jet, is used to explain the discontinuous jumps in screech tone frequency. The results indicate that the different acoustic feedback mechanisms need not necessarily be associated with a particular instability mode shape.

Multimodal instability in the weakly underexpanded elliptic jet

Particle image velocimetry and schlieren imaging are used to study the interrelationship between the hydrodynamic and acoustic fields of a screeching underexpanded elliptical jet. An elliptical jet with an aspect ratio equal to two is studied at a nozzle pressure ratio of 2.2. Two-point correlation analysis of the schlieren image sets suggests that the flow is dominated by a flapping mode about the major axis plane, however, a proper orthogonal decomposition of the transverse velocity field suggests that there is also a helical mode that becomes dominant further downstream. The flapping mode appears to grow, saturate, and decay more rapidly and is associated with the production of discrete acoustic tones in the form of jet screech, whereas no discrete tone sound production is observed from the helical mode. It is suggested that the lack of tonal generation by the helical mode is linked to its slower growth; it does not produce coherent fluctuations in shear layer vorticity in the vicinity of the jet shock...

Novel method for investigating broadband velocity fluctuations in axisymmetric screeching jets

The noise spectrum of screeching jets consists of broadband and discrete components referred to as turbulent mixing noise and screech tones, respectively. The screech tones make the application of ...

Legumain is activated in macrophages during pancreatitis

Pancreatitis is an inflammatory disease of the pancreas characterized by dysregulated activity of digestive enzymes, necrosis, immune infiltration, and pain. Repeated incidence of pancreatitis is an important risk factor for pancreatic cancer. Legumain, a lysosomal cysteine protease, has been linked to inflammatory diseases such as atherosclerosis, stroke, and cancer. Until now, legumain activation has not been studied during pancreatitis. We used a fluorescently quenched activity-based probe to assess legumain activation during caerulein-induced pancreatitis in mice. We detected activated legumain by ex vivo imaging, confocal microscopy, and gel electrophoresis. Compared with healthy controls, legumain activity in the pancreas of caerulein-treated mice was increased in a time-dependent manner. Legumain was localized to CD68(+) macrophages and was not active in pancreatic acinar cells. Using a small-molecule inhibitor of legumain, we found that this protease is not essential for the initiation of pancreatitis. However, it may serve as a biomarker of disease, since patients with chronic pancreatitis show strongly increased legumain expression in macrophages. Moreover, the occurrence of legumain-expressing macrophages in regions of acinar-to-ductal metaplasia suggests that this protease may influence reprogramming events that lead to inflammation-induced pancreatic cancer.

Measuring shear layer growth rates in aeroacoustically forced axisymmetric supersonic jets

Underexpanded jets at a single pressure ratio and Reynolds number are studied in their free and impinging conditions. The influence of different kinds of aeroacoustic self-forcing is studied through Proper Orthogonal Decomposition and Dynamic Mode Decomposition analysis of high resolution Particle Image Velocimetry data. The free jet is primarily characterized by the presence of a strong helical instability, though a weaker axisymmetric instability is also present. The impinging jet shear layer is dominated by a fast-growing axisymmetric instability, with a weaker helical instability also evident. Ultra-high-speed schlieren imaging reveals a much more rapid instability growth close to the nozzle for the impinging jet case. Analysis of the streamwise growth rates via DMD suggests that the shear layer instability grows more than twice as fast in the near-nozzle region for the impinging jet. Further downstream the growth rate in the free jet case is higher.

Sound production by shock leakage in supersonic jet screech

Damon Honnery is Deputy Dean Operations in the Faculty of Engineering, a Professor in the Department of Mechanical and Aerospace Engineering, and he jointly directs the Laboratory for Turbulence Research in Aerospace and Combustion. He obtained his undergraduate degree in mechanical engineering from the University of Sydney in 1985. Following this he was employed as a research fellow at the University of Sydney during which he obtained a MEngSc in 1987. In 1987 he was awarded a cadet research scientist position in the Aeronautical Research Laboratory (ARL-DSTO, now DSTG) during which he obtained his PhD in gas turbine related research from the University of Sydney in 1992. This was followed by an appointment as a research scientist at ARL in the Propulsion Branch where he undertook research on soot formation in gas turbines systems. In 1993 he took up a lectureship at Monash University at the Caulfield Campus, he then moved to the Clayton Campus in 1999 where he established the Monash Aerospace Engineering Degree. With Julio Soria he jointly established the Laboratory for Turbulence Research in Aerospace and Combustion in 1999. In the Department of Mechanical and Aerospace Engineering he has been Aerospace Course Director, Director of Undergraduate Affairs, Director of Research, and Deputy Head. In the Faculty of Engineering he has played a leading role in the development of the Monash Makerspace and was Interim Joint Director of the Woodside-Monash Energy Partnership. His research interests range from spray systems, particle flows, pollutant formation, renewable energy and climate change mitigation.

Impingement Tones and Associated Shock Instabilities in Supersonic Plug Nozzle Flows

An investigation into a supersonic impinging jet emitted from a truncated plug nozzle is presented for a nozzle pressure ratio of 3.4. High-resolution schlieren and acoustic measurements verify that the jet behavior close to the nozzle is significantly different from round jets. Impingement tones are not observed for plate distances less than 3.2 nozzle diameters from the throat. Sufficiently far downstream from the nozzle, the jet shock structure is similar to that of a round jet. Ultrahigh-speed schlieren is used to track the motion of the standoff shock located above the impingement plate. The shock-oscillation frequency matches the impingement-tone frequency within statistical uncertainty. The average shock standoff distance and oscillation amplitude are related to the relative position of the impingement plate with respect to the shock structure in the equivalent freejet. Shock splitting is directly observed for certain plate distances.

Interaction of a Supersonic Underexpanded Jet with a Flat Plate

High-spatial resolution measurements of the velocity field of a supersonic jet impinging on a flat surface were conducted. To study the effect of the boundary condition, two nozzle outer geometries were investigated. The experimental results presented here are for a nozzle pressure ratio of NPR = 3.0 and a nozzle to plate spacing of \(Z/D = 3.5\). The flow fields of the corresponding cases show that although the general features of the flow are similar, the ambient air entrainment into the shear layer is different. This change in entrainment influences the rest of the jet structure.