Michael Gloor

Climate change is affecting the severity and frequency of natural catastrophes. Around the world, these risks manifest in different ways: more local flooding, torrential rain, prolonged drought, severe wildfires and other extreme weather events. Urban sprawl and population growth in areas of high exposure, such as along the coast and on the fringes of forestland, are putting many more people and assets in harm’s way. It is therefore vital that we adapt to the mounting risks and make our… Mehr anzeigen

Climate change is affecting the severity and frequency of natural catastrophes. Around the world, these risks manifest in different ways: more local flooding, torrential rain, prolonged drought, severe wildfires and other extreme weather events. Urban sprawl and population growth in areas of high exposure, such as along the coast and on the fringes of forestland, are putting many more people and assets in harm’s way. It is therefore vital that we adapt to the mounting risks and make our communities more resilient.

But adaptation alone will not be enough. If unmitigated, climate change could trigger environmental tipping points, which would permanently alter the climate, irreversibly damage our planet and threaten many more livelihoods. This worst case scenario is becoming ever more likely as greenhouse gas emissions continue to reach record levels. To hit a target of a “net-zero” emission economy by mid-century will require drastic changes to land use, energy production, consumption, industrial processes, construction, transport systems and the development of cities.

Today climate change is a manageable risk for re/insurers. However, the rising threat is alarming. In response, the industry needs to improve risk models to better assess climate hazards: the mandate is to ensure development of the capabilities to be able to underwrite natural catastrophe risks in the future. Re/insurers can also play a key role in advancing the transition to a low-carbon economy by providing solutions to manage risks associated with large scale investments in new technology and innovation.
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Laminar-turbulent transition and noise radiation of a parametrized set of subsonic coaxial jetflows with a hot primary (core) stream are investigated numerically by Large-Eddy Simulation(LES) and direct noise computation. This study extends our previous research on local linear stability of heated coaxial jetflows by analyzing the nonlinear evolution of initially laminar flows disturbed by a superposition of small-amplitude unstable eigenmodes. First, a baseline configuration is studied to shed… Mehr anzeigen

Laminar-turbulent transition and noise radiation of a parametrized set of subsonic coaxial jetflows with a hot primary (core) stream are investigated numerically by Large-Eddy Simulation(LES) and direct noise computation. This study extends our previous research on local linear stability of heated coaxial jetflows by analyzing the nonlinear evolution of initially laminar flows disturbed by a superposition of small-amplitude unstable eigenmodes. First, a baseline configuration is studied to shed light on the flow dynamics of coaxial jetflows. Subsequently, LESs are performed for a range of Mach and Reynolds numbers to systematically analyze the influences of the temperature and the velocity ratios between the primary and the secondary (bypass) stream. The results provide a basis for a detailed analysis of fundamental flow-acoustic phenomena in the considered heated coaxial jetflows. Weniger anzeigen

This study presents a decomposition of the temporal growth rate ω_i which characterises the evolution of wave-like disturbances in linear stability theory for compressible flows. The decomposition is based on the disturbance energy balance by Chu (Acta Mech., vol. 1 (3), 1965, pp. 215–234) and provides terms for production, dissipation and flux of energy as components of ω_i. The inclusion of flux terms makes our formulation applicable to unconfined flows and flows with permeable or vibrating… Mehr anzeigen

This study presents a decomposition of the temporal growth rate ω_i which characterises the evolution of wave-like disturbances in linear stability theory for compressible flows. The decomposition is based on the disturbance energy balance by Chu (Acta Mech., vol. 1 (3), 1965, pp. 215–234) and provides terms for production, dissipation and flux of energy as components of ω_i. The inclusion of flux terms makes our formulation applicable to unconfined flows and flows with permeable or vibrating boundaries. The decomposition sheds light on the fundamental mechanisms determining temporal growth or decay of disturbances. The additional insights gained by the proposed approach are demonstrated by an investigation of two model flows, namely compressible Couette flow and a plane compressible jet. Weniger anzeigen

This study investigates the inviscid, linear spatio-temporal stability of heated, compressible, and incompressible coaxial jet flows. The influence of the temperature ratio and the velocity ratio between the core jet and the bypass stream on the transition from convectively to absolutely unstable flows is studied numerically. The investigation shows that for coaxial jets, absolute instability can occur for considerably lower core-stream temperatures than for single jets. The reason for this… Mehr anzeigen

This study investigates the inviscid, linear spatio-temporal stability of heated, compressible, and incompressible coaxial jet flows. The influence of the temperature ratio and the velocity ratio between the core jet and the bypass stream on the transition from convectively to absolutely unstable flows is studied numerically. The investigation shows that for coaxial jets, absolute instability can occur for considerably lower core-stream temperatures than for single jets. The reason for this modified stability character is the appearance of an additional unstable mode as a result of the outer velocity shear layer between the bypass stream and the ambient flow. The presence of two shear layers enables the interaction between otherwise free waves to give rise to new instabilities. When the bypass-stream velocity is increased, the classical absolute mode known from single jets (inner mode) is first stabilized and then destabilized for high bypass-stream velocities, whereas the outer mode reaches maximum spatio-temporal growth rates when the core-stream velocity is approximately equal to twice the bypass-stream velocity. Additionally, it is demonstrated that the spatio-temporal character of the modes is very sensitive to the shear-layer thickness and to the distance separating the two layers. Increasing the Mach number strongly dampens the onset of an absolute instability for both modes. Weniger anzeigen

Large-eddy simulations of heated coaxial jet flows were performed during this investigation. The paper provides a comprehensive study of the laminar-to-turbulent transition and the noise generation mechanisms for different velocity ratios and temperature ratios between the primary (core) and secondary (bypass) streams. Flow phenomena that are relevant for jet-engine exhaust noise are discussed in detail.

This study investigates the linear instability properties of laminar coaxial jet flows. It sheds light on the mechanisms that are responsible for the transition of initially laminar to fully turbulent jets.