Understanding the impact of aircraft speed heterogeneity on air traffic operation is crucial for airspace design and air traffic flow management. Speed heterogeneity (or aircraft mix) is recognized as a causal factor for complexity in air traffic operations, through qualitative or statistical analysis. Quantitative metrics on how it affects current and future Trajectory-Based Operation (TBO) is lacking, however. In this paper, we present an in-depth investigation of the impact of speed heterogeneity, by defining air traffic robustness at microscopic (aircraft pair) and macroscopic (air traffic flow) levels in nominal situations, derived mathematically and validated through fast-time computer simulations. A human-in-the-loop study follows investigating six 4D en route operation scenarios, where operators were instructed to resolve a large disturbance in a sector, using a novel interface. Results confirm the negative impact of speed heterogeneity on air traffic controller performance in terms of flow efficiency and workload. The mechanism of such impact is substantiated through analyzing several speed-based robustness metrics. Although the simulated traffic scenarios have similar baseline robustness, those with mixed speeds lead to significantly lower robustness and operational performance. This emphasizes the need to incorporate speed heterogeneity in robustness evaluations of air traffic control in current and future TBO environments.

Future Air Traffic Management concepts will require air traffic controllers to move from a tactical to a strategic way of operation. This paper evaluates two novel concepts which support controllers to perform four-dimensional trajectory management in a contingency situation. The Travel Space Representation and Time-Space Diagram are both solution-space based displays where automation calculates all possible actions in real-time. All decision-making is still to be done by the operator but is greatly facilitated by this automation, as it shows all possible actions at a glance. An experiment is described which evaluated the performance of novice controllers in managing a sector where suddenly a bad weather cell emerged, requiring them to re-route traffic in space and time. Results show that the display concepts work well and support operators even in complex situations with a heterogeneous mix of aircraft types and speeds. Performance and workload indicators become worse for the higher-density, higher-heterogeneity situations.

Two fluid model simulations based on our recently introduced kinetic theory of granular flow (KTGF) for rough spheres and rough walls, are validated for the first time for full three-dimensional (3D) bubbling fluidized beds. The validation is performed by comparing with experimental data from Magnetic Particle Tracking and more detailed Discrete Particle Model simulations. The effect of adding a third dimension is investigated by comparing pseudo-2D and full 3D bubbling fluidized beds containing inelastic rough particles. Spatial distributions of key hydrodynamic data as well as energy balances in the fluidized bed are compared. In the pseudo-2D bed, on comparison with the KTGF derived by Jenkins and Zhang, we find that the present KTGF improves the prediction of bed hydrodynamics. In the full 3D bed, particles are more homogeneously distributed in comparison with the pseudo-2D bed due to a decrease of the frictional effect from the front and back walls. The new model results are in good agreement with experimental data and discrete particle simulations for the time-averaged bed hydrodynamics.