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The compact form factor of nanosatellites or even smaller satellites makes them predestined for distributed systems such as formations, constellations or large swarms. However, when it comes to orbit insertion of multiple satellites, these ride share payloads have constrains in the deployment parameters such as sequence, direction, velocity and time interval. Especially for formation flight missions without propulsion, where the satellites should minimize their relative distance drift either passively or by atmospheric drag control, the initial ejection parameters must find a proper trade-off between collision probability and relative drift. Hence, this article covers short-term (first orbit) collision analysis, long-term (30 days) drift analysis and atmospheric drag control strategy for long-term distance control of multiple satellites. The collision analysis considers various orbit deployment parameters such as insertion direction and tolerance, orbital elements of insertion and time span. To cover the parameter space, a Monte Carlo simulation was conducted to identify the impact of these parameters. It showed that for collision probability the major factor is the time span between two ejections and the precision of the deployment vector. For long-term drift analysis, orbit perturbation such as atmosphere and J2 terms are considered. The result showed that for drift minimizing, minimizing the along-track variation is more substantial than reducing the time span between ejections. Additionally, a drag control strategy to reduce the relative drift of the satellites is described. The results have been applied on the S-NET mission, which consists of four nanosatellites with the task to keep their relative distance within 400 km to perform intersatellite communication experiments. The flight results for orbital drift show equal or better performance (0.1–0.7 km/day) compared to the worst-case simulation scenario, implying that orbit perturbation was chosen correctly and all orbit injection tolerances were within specified range. The drag control maneuver showed good matching to the flight results as well with a deviation for the maneuver time of approximately 10%. [ABSTRACT FROM AUTHOR]

The aim of this work is to control the flow around ground vehicles by active or/and passive strategies. The active control is achieved by steady, pulsed or closed-loop jets located at the backof the simplified car model. The passive control is performed using porous layers between the solid body and the fluid in order to modify the shear forces. The two previous control methods can be coupled to improve the drag reduction.

Experimental and numerical studies were carried out to evaluate the effect of vortex generator on a small cylindrical protrusion at Mach number 2.0. The experiments were performed using the supersonic blow down wind tunnel on different heights of cylindrical protrusion with vortex generator placed ahead of them. The upstream and downstream flow around the cylindrical protrusion is influenced by vortex generator as is observed using both visualization and pressure measurement techniques. Numerical studies using three dimensional steady implicit formulations with standard k-ω turbulence model was performed. Results obtained through the present computation are compared with the experimental results at Mach 2.0. Good agreements between computation and experimental results have been achieved. The results indicate that the aerodynamic drag acting on cylindrical protrusion can be reduced by adopting vortex generator. [ABSTRACT FROM AUTHOR]

Flow over circular cylinders covered with circumferential riblets of triangular shape with spanwise coverage ratios from 0 (bare cylinder) to 1 (fully covered) is investigated by large eddy simulation at three subcritical Reynolds numbers (2000, 3000 and 3900 respectively). The bare cylinder is considered as a reference case and for validation purpose. Detailed flow structures and boundary layer characteristics are described. For the fully covered cylinder, it is found that flows on the two sides of each riblet interact, forming a structure with stronger turbulent fluctuations and leading to a significantly shorter recirculation length compared to the other cases. The turbulent fluctuations are attenuated to various degrees for the partly covered cylinders. Two semicircular-shaped flow zones with distinct spanwise flows are observed around the riblets adjacent to the bare sections. Among the three partly covered cases under consideration (with spanwise coverage ratios of 0.25, 0.5 and 0.75 respectively), the half-covered cylinder has the largest semicircular-shaped flow zones, associated with the lowest turbulence in the flow field. A zigzag flow separation line is observed on all sections covered with riblets. The variations of the flow behaviours result in different characteristics of the hydrodynamic forces acting on the various cylinders. It is revealed that the lift fluctuation is the lowest for the half-covered cylinder and the highest for the fully covered cylinder. An examination of the sectional hydrodynamic forces further reveals that the sectional drag is higher at the riblet tip than that in the valley. • Spanwise coverage ratio of riblets has significant effects on the vortical structures around the cylinders. • The longest recirculation length is observed behind the half-covered cylinder, whereas the shortest recirculation length is observed behind the fully covered cylinder. • The half-covered cylinder has the lowest spanwise averaged fluctuating lift among all the cases. • A zigzag-shaped flow separation line is observed on the cylinders covered with riblets.. [ABSTRACT FROM AUTHOR]

Micro-Suction technique(MST) is proposed by the development of micro-blowing technique(MBT). Current experiments and numerical studies are mostly focusing on MBT, micro-suction flow control in turbulent boundary layer remains unstudied. Thus, based on 2D-RAE2822 supercritical airfoil, numerical research on MST and MB/ST compounding flow control have been carried out. And Micro-Porous Wall Model(MPWM) is used to represent the real effects that the blowing and suction of the micro-porous wall has on the surface flow.The numerical results of micro-suction indicate that micro-suction control is capable of inducing a lower pressure zone in front of the micro-porous zone, and a higher pressure zone in the rear end. Given that micro-porous zone is placed on the particular section of airfoil, where the pressure drag direction is backward parallel to free-stream direction, the reduction of the pressure induced by micro-suction control can result in the decrease of pressure drag. Finally, combining the previous study on the MBT, a compounding configuration of micro-blowing/suction flow control on RAE2822 airfoil is proposed. The compounding configuration can take the advantage of both micro-blowing and micro-suction, over 20% increase of the lift and up to 15% decrease of the drag can be realized. Therefore, the compounding flowing control of micro-blowing/suction shows a great potential for the flow control on aircraft. [ABSTRACT FROM AUTHOR]

Abstract: A vortex generated behind a simplified vehicle induces a pressure force at the back wall that contributes to a significant part of the drag coefficient. This pressure force depends on two parameters: the distance of the vortex to the wall and its amplitude or its circulation. Therefore there are two ways to reduce the drag coefficient: pushing the vortices away from the wall and changing their amplitude or their dynamics. Both analytical studies and numerical simulations show that these two actions decrease the pressure force and consequently reduce the drag coefficient. The first action is achieved by an active control procedure using pulsed jets and the second action is achieved by a passive control procedure using porous layers that change the vortex shedding. The best drag coefficient reduction is obtained by coupling the two procedures. [Copyright &y& Elsevier]

The aim of this work is to control the flow around ground vehicles by active or/and passive strategies. The active control is achieved by steady, pulsed or closed-loop jets located at the backof the simplified car model. The passive control is performed using porous layers between the solid body and the fluid in order to modify the shear forces. The two previous control methods can be coupled to improve the drag reduction. [ABSTRACT FROM AUTHOR]