Thermophysics

The Air Force Research Laboratory (AFRL) Center of Excellence for Computational Sciences developed a new vibration-dissociation coupling suitable for hypersonic blunt body and nozzle flows. The model, based on the collisional approach, solves the vibrational master equations coupled to the fluid dynamic equations. The deviation of the quasisteady distribution from an equilibrium Boltzmann distribution is derived to model the effect of population depletion in the vibrational manifold due to dissociation. The model lowers Park’s dissociation rates and helps explain the restricted success of Park’s dissociation model in certain temperature ranges of hypersonic flow past a blunt body. Continuing efforts seek to reconcile the differences between the two divergent views of weak and strong vibrational bias mechanisms.

Cairo University laboratories and research facilities have been developing a simulation capability for predicting the flow regimes and heat transfer in air-conditioned spaces. The approach involves the coupling of thermo-turbulence equilibrium Navier-Stokes calculations with multidimensional, in-depth material thermal response calculations. The coupling takes into account heat transfer at the furniture inside the room (with associated numerical mesh adaptation). Measured and predicted flow patterns were in good agreement for full-scale rooms (7.23433 m). More recent work has concentrated on extending the capability into 3D flow situations in actual operating theaters, which are geometrically complex systems. Finally, the effect of scale on model size and the adequacy of the prediction capabilities were clearly demonstrated.

Modeling and simulation

The AFRL has undertaken the task of developing a unified framework for the design, analysis, and optimization of revolutionary aerospace vehicles by expanding the role of entropy and the second law of thermodynamics to modeling and simulating thermofluid processes. The new methodology, known as energy-based design, seeks to implement the second law as a fundamental design constraint to complement traditional methods based on energy conservation.

Incorporating the second law into the design of complex aerospace systems applies common metrics from the system level down to each component, thus precluding the possibility of nonphysical results while expanding the horizon of possibilities.

The University of Texas at Tyler, in conjunction with NASA-Johnson, is using computational fluid dynamics (CFD) to study and recreate the processing of carbon nanotubes at the laser ablation facility at NASA-Johnson. The university’s Dept. of Mechanical Engineering has been conducting CFD simulations to gain insight into the thermophysics of single-walled carbon nanotube formation.

In the laser ablation process, two coaxial lasers ablate a carbon and catalyst target in a background argon gas. The laser beams instantly vaporize the solid carbon, providing a "seed stock" of carbon in atomic or molecular configuration that forms a plume into the argon mixture. As the plume cools, the carbon molecules reassemble around the nickel or cobalt catalysts, forming elongated nanotubes. Current investigations are being conducted with simplified reacting carbon models to reduce the computational loading. Each nanotube, with one more carbon atom than the previous nanotube, is a different chemical species leading to potentially thousands of chemical species in the reacting chemistry model, which could overwhelm computational resources.

Aerospikes and Entropy Techniques

Recent research at the High Enthalpy Aerodynamics Laboratory at the Indian Institute of Science is focused on the measurement of heat transfer data and aerodynamic forces for large angled blunt cones at hypersonic Mach numbers. The aerodynamic heating problem in hypersonic vehicles is reduced by using blunt-nosed bodies, but the consequence of this is the large wave drag during the flight of the vehicle through the Earth’s atmosphere. Many strategies have been devised to tackle the wave drag problem, and the simplest yet most effective strategy of all is to have a forward-facing nose spike on a hypersonic vehicle. Experiments have indicated about 55% reduction in drag for a suitably chosen spike assembly at zero angle of attack. Appreciable reduction of heat transfer to the body due to the presence of aerospike has also been observed from the measured data. The experimental results are compared with the numerically computed ones, and the agreement between the two has been found to be good.

AFRL and the University of Manitoba in Canada have taken new steps in applying entropy-based techniques to thermofluid system analysis. Entropy serves as a key parameter in achieving the upper limits of performance and quality in aerospace technology. Also, entropy and exergy shed new light on various thermophysical processes. More specifically, the new advances have appeared in uncovering the somewhat esoteric role of the second law in computational methods. In some cases, new questions are addressed with potentially deeper implications than initially speculated. Is numerical stability ensured when the second law is satisfied locally? What are the mechanisms leading to computed entropy destruction, and how can they be offset? Can the second law identify the erroneous (non-physical) simulation results in the absence of experimental data? The studies indicate that the implications of these relationships appear to extend to well-quantified error bounds and better robustness metrics.

Missions and Milestones

The thermophysics-associated groups at JPL have participated in a number of missions that reached important milestones this year. The prime directive of the Genesis mission, launched on August 8, is to return solar-ejected particles from the Lagrange 1 point, where the gravities of the Earth and Sun are balanced. Galaxy Explorer (GALEX) reached a milestone with the shipping of the telescope and instrument to Orbital Sciences for inclusion on a Pegasus launch. A telescope-based instrument, GALEX will perform a two-year-long survey that will catalog and investigate deep-space objects.

Galileo, a mission to Jupiter, has performed its last flyby of Io, a volcanic moon. Thermophysics-related personnel at JPL have continued to monitor Galileo as part of Mission Operations Team activities. Cassini, also supported by JPL thermophysics groups, is thermally and mechanically healthy and is continuing along its mission toward encounter with Saturn in July 2004.

The Mars Exploration Rover 2003 project, NASA’s mission to send dual rovers to Mars, presented unique system-level thermal design challenges as well as an accelerated schedule. The initial flight system design relied on Mars Pathfinder heritage. For the thermal design, this included the mechanically pumped fluid loop that maintains rover temperatures during cruise to Mars.

The rover is significantly larger than Sojourner, which accompanied the Mars Path-finder Lander. It has three distinct internal thermal zones, which created the need for a heat switch to maintain the rechargeable battery zone within some relatively tight temperature limits during Mars surface operations. The heat switch development has culminated in fully flight-qualified hardware. To enable relatively long afternoon direct-to-Earth communication sessions, a miniature-loop heat pipe was used to shuttle power amplifier waste heat overboard. This heat pipe was ultimately removed from the design, but the development and qualification testing will be completed.

Nanotechnology

AFRL’s Propulsion Directorate, in conjunction with the University of Southern California, has developed a novel nano-Newton force balance, which has been used to measure steady-state forces as low as 80 nN (±30%). The force balance allows for gas flows to be introduced into a microdevice on the end of the force balance arm by using a liquid seal. As a result, direct connection to the balance by gas feed lines is avoided. This is critical for nano-Newton force measurements, because the feed lines can impart forces greater than those to be measured from a typical microdevice.

Researchers plan to investigate gas flows through a variety of microelectromechanical systems, including microthrusters. The symmetric nature of the balance allows very small differences between devices to be investigated, which can be important in determining the effectiveness and repeatability of some batch fabrication processes.