This thesis explores gamma-ray bursts (GRBs), and more specifically the prompt emission phase, which is the first ∼ 10 seconds of gamma-rays. GRBs come from the launching of a relativistic jet in connection with core-collapse supernovae or compact object (neutron star or black hole) mergers. The relativistic jet accelerates and eventually most of the energy is kinetic, and that energy is then somehow converted into internal energy that is emitted in gamma-rays, and is what we observe. The mechanism responsible for this conversion in the prompt phase is not fully understood and this thesis deals with one possible such mechanism, radiation-mediated shocks (RMSs). Such shocks occurring below the photosphere alters the photon spectral energy distribution, which is then released at the photosphere towards an observer. An analogue model of RMSs, called the Kompaneets RMS approximation (KRA) is discussed and then later applied in Paper I & II. In Paper I we generalize a method to measure the bulk outflow Lorentz factor based on the properties of photospheric emission and the evolution of the photon energy distribution in the jet. We find that depending on the quality of the data, either a value or an upper limit can be found for the Lorentz factor. In Paper II we do a time-resolved spectral analysis of GRB 211211A, a GRB with a broad spectrum containing two breaks, one in the tens of keV and one around a few MeV. Using the method presented in Paper I we find typical Lorentz factor values of ∼ 300. From the Lorentz factors and the KRA model we find the time evolution of the RMS parameters, here a strong shock occurring at moderate optical depths. We also show that the KRA model can fit these broad spectra with two breaks very well.

 Venovenous extracorporeal membrane oxygenation (ECMO) is a life saving therapy for critically ill patients with refractory respiratory failure. To support gas exchange, blood is drained from the patient via a cannula and circulated outside the body through a membrane lung, after which the blood is returned to the patient. The exposure of blood to non-physiological conditions triggers blood damage and thrombus formation in the circuit. While cannula placement induces different blood flow dynamics, and thus distinct complications, the decision of cannula configuration often lands on the preferences of the medical center.

From medical imaging, a patient-averaged model was derived and cannulae were inserted in femoro-femoral (FF) configuration. Previously studied configurations (femoro-jugular and jugulo-femoral) were re-simulated in the updated geometry. Through large eddy simulations (LES), a direct comparison of configurations could be made. The focus was specifically on clinically relevant metrics for oxygenation performance, thrombus formation and blood damage. FF generated more pronounced negative pressures in the inferior vana cava, associated with a risk of vessel collapse. Wall shear stresses, linked to thrombosis and plaque formation, exceeded recommended limits even at low ECMO flow rates. Furthering our understanding of induced flow dynamics from cannulation by such means may provide insights on optimal cannulation strategies, aiding clinicians in making informed decisions.

Exact modeling of vascular walls and blood is challenging due to the complexity and variability of the cardiovascular system, which has led to a common modeling simplification being treating walls and heart chambers as rigid. While this assumption facilitates computational modeling, it is important to assess its validity. A dynamic model of the right atrium (RA) was thus created using mesh morphing, mirroring known motion of the atrial wall. Through LES simulations, hemodynamic metrics were compared to a rigid counterpart model, establishing a sensitivity assessment of the rigid wall assumption for RA modeling. The rigid model underestimated fluid activity in the auricle, leading to an underestimation in wall shear stress and an overestimation of blood residence time and stagnation in this region. These results provide guidance on the validity of the rigid wall assumption, to increase our understanding of which sensitivities are important to consider for specific modeling applications. 

The next generation of wireless communication systems envisions seamless global coverage through the integration of Non-Terrestrial Networks (NTNs) with terrestrial infrastructures. Unlike terrestrial Base Stations (BSs), NTN platforms such as Rotary Wing Drones (RWDs), Fixed Wing Drones (FWDs), High-Altitude Platforms (HAPs), and Low Earth Orbit (LEO) satellites introduce challenges due to platform mobility, power limitations, and architectural constraints. These factors directly affect the service time—or equivalently, the availability—of communication links.

This thesis analyzes the design factors and architectural trade-offs that govern service availability across diverse NTN platforms. For Aerial Base Stations (ABSs), a unified framework is developed to evaluate power consumption and energy harvesting. The results show that RWDs sustain service for only 5–60 minutes with negligible solar harvesting benefits, whereas FWDs and HAPs extend operation to several hours and days, respectively. A network dimensioning study quantifies the number of ABSs and backup batteries required for continuous coverage, highlighting deployment constraints of energy-limited aerial systems.

For satellite-based NTNs, a digital twin framework is introduced to model end-to-end handover delays under realistic 3rd Generation Partnership Project (3GPP)-compliant assumptions. The results show that placing the gNB on-board reduces cumulative Conditional Handover (CHO) delay by approximately 25–30% relative to Split 7.2x, at the expense of 55–70% higher on-board computation. Constellation design strongly impacts availability: increasing satellite density beyond a threshold yields diminishing availability due to more frequent handovers. A medium-density, low-altitude constellation exhibits 11 minutes of daily downtime, increasing to 13–16 minutes when densified, whereas a sparser, higher-altitude constellation achieves only 5–7 minutes. The commonly cited 99.9% availability target for LEO is shown to be impractical; a maximum of approximately 99.2% is achievable, with functional split choices further reducing availability (e.g., from 99% to 98.5% when moving from gNB onboard to Split 7.2x).

Overall, this thesis provides a unified perspective on service time as a fundamental performance metric across NTN platforms—whether constrained by energy limitations in aerial systems or by handover dynamics in LEO satellite constellations—offering practical insights to guide the design and optimization of NTN.