References of "Ramjatan, Sahadeo"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailBlackout analysis of Mars entry missions
Ramjatan, Sahadeo; Lani, Andrea; Boccelli, Stefan et al

in Journal of Fluid Mechanics (2020)

A new methodology to accurately and efficiently examine the radio frequency blackout phenomenon during the hypersonic reentry process is introduced and validated. The current state-of-the-art ... [more ▼]

A new methodology to accurately and efficiently examine the radio frequency blackout phenomenon during the hypersonic reentry process is introduced and validated. The current state-of-the-art thermochemical modelling of CO2 flows is reviewed and one-dimensional stagnation line studies are performed in order to determine a suitable chemical mechanism for the electron density modelling. Hypersonic computational fluid dynamics (CFD) simulations are performed with a simplified chemical model including only neutral species, in order to calculate the flow field surrounding the ExoMars Schiapparelli module in flight conditions. A novel decoupled CFD approach is then applied where the calculation of the electron density is performed separately using a computationally inexpensive Lagrangian approach. Subsequently, a ray tracing algorithm is applied in order to model the propagation of electromagnetic waves in the wake flow past the ExoMars vehicle accounting for collisions between electrons and gas particles. The numerical results of the proposed novel approach for blackout analysis consisting of CFD, Lagrangian and ray tracing algorithms are in good agreement with the flight data. [less ▲]

Detailed reference viewed: 55 (6 UL)
Full Text
Peer Reviewed
See detailBlackout analysis of small cone-shaped reentry vehicles
Ramjatan, Sahadeo; Magin, Thierry; Scholz, Thorsten et al

in Journal of Thermophysics and Heat Transfer (2017), 31(2), 269--282

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free ... [more ▼]

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free electrons and ions. This production of free electrons and ions creates a plasma or ionized flowfield around the vehicle that is known to degrade the quality of radio-wave signal propagation, leading to a loss of communication or "blackout." This study involves performing hypersonic computational fluid dynamics simulations in the commercial software CFD++ with a bluntnosed cone geometry at different flight conditions to predict how and when ground communication can be achieved to aid in the design of an alert transmitter, which is an extension of aircraft collision-avoidance system technology. Computational results show that a small blunt-nosed cone geometry has decreased ionization regions as the cone angle decreases due to a shifting of the reaction zone further downstream. As a result, higher freestream velocities have less of an impact in determining the location for an antenna, and communicating along the stagnation line is seen to be independent of cone angle. [less ▲]

Detailed reference viewed: 2 (0 UL)
See detailBlackout analysis of small reentry vehicles
Ramjatan, Sahadeo; Magin, Thierry E.; Scholz, Thorsten et al

in Proceedings of the 53rd AIAA Aerospace Sciences Meeting (2015)

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free ... [more ▼]

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free electrons and ions. This production of free electrons and ions creates a plasma or ionized flow field around the vehicle that is known to degrade the quality of radio-wave signal propagation. As a result, radio frequency waves can be attenuated and/or reflected by electrons in the plasma leading to a loss of communication or "Black-Out." The vehicle subsequently loses all communication including GPS signals, data telemetry, and can travel for hundreds of miles during this loss of communication. For example, Apollo 13 experienced a blackout phase of 6 minutes and the Mars pathfinder experienced a 30-second radio blackout. One of the most promising approaches in reducing the blackout phase for vehicles is aerodynamic shaping. It is known that small, slender reentry vehicles approximately 20 cm in diameter will have less gas ionization on the surface resulting in a smaller blackout period. Thus, a primary goal of this project is to investigate the flow field of various geometries at different flight conditions to improve the design of airborne collision avoidance system (ACAS) technology. Hypersonic CFD simulations are conducted in the commercial software CFD++ where Dunn and Kang's reaction file is implemented to model the chemical non-equilibrium phenomenon of the flow-field. It is found that as the cone angle decreases, an increase in the electron density occurs further downstream. Due to the shift of the reaction zone towards the aft position for smaller cone angles, higher mach numbers are seen to have less of an impact in determining the location of an antenna. The 15° geometry is seen to be best in terms of communicating through the blackout period. Results are in good agreement with literature. [less ▲]

Detailed reference viewed: 14 (3 UL)