Engineering in Extreme Environments

Craig White, Kostas Kontis

The research focuses on gas dynamics in extreme environments, typically where the Navier-Stokes-Fourier equations break down and recourse must be made to the Boltzmann equation to capture the physics of the gas flows accurately. These types of gas flows are found in many important engineering applications, such as hypersonic flows, micro- and nano-scale devices, porous media, plume-surface interactions, and vacuum systems. The main research topics are:

  • Hypersonic flows: Many hypersonic flows take place at high altitudes where the atmospheric density is greatly reduced. An example is atmospheric re-entry from low Earth orbit, where Mach numbers greater than 25 are experienced, resulting in the air being compressed to very high temperatures and pressures in the shock wave. These temperatures are high enough to ionise the atmospheric gases and produce plasma. The continuum assumption and local thermodynamic equilibrium break down and methods that provide a solution to the Boltzmann equation must be used. One such method is direct simulation Monte Carlo (DSMC), a stochastic particle-based numerical method that returns a solution to the Boltzmann equation. Many physical models can be implemented in this technique, such as chemical reactions, making it a popular and powerful numerical technique in the hypersonic flow research domain. The research is led by Dr Craig White, who is the lead developer of the dsmcFoam code, which is implemented within the framework of the C++ fluid dynamics toolbox OpenFOAM. 

  • Micro/nano-scale flows: At atmospheric pressure, a gas flow can become rarefied due to the length scales of the system becoming small enough. Non-equilibrium gas flows at the micro- and nano-scales present non-intuitive results compared to those of macroscopic continuum fluids. Rarefied gas flows result in velocity slip and temperature jump at solid boundaries, the Knudsen paradox in mass flow rates, anti-Fourier heat transfer, etc. The applications include the cooling of micro-chip devices, which is of ever-increasing importance as the features decrease in size and increase in performance, resulting in more heat being produced at decreasing length scales. Flows through porous media can also become a micro-/nano-scale flow with enhanced mass flow rates due to the Klinkenberg effect through the individual pores. The figure below shows pressure contours from a DSMC simulation of rarefied gas flow through a porous sandstone material.                                
  • Plume-Surface Interactions: In space applications, the plume from reverse-thrusters or the main-thruster of a landing module can present various risks to the lander itself. The plume surface interaction (PSI) can be characterised through various mechanisms, such as cratering of the regolith, erosion, and ejecta dynamics. During the erosion and ejecta processes, the rocket exhaust plume will fluidise granules on the extraterrestrial surface, and the entrained particles can interact with the landing module, changing its stability characteristics. It was reported by the Apollo astronauts that the entrained dust deteriorated the view from optical windows and reduced the efficiency of solar panels and thermal protection systems. In addition, the regolith material has been found to attach to surfaces, forming a dust coating layer on thermal radiators, space suits, and astronauts, which interfered with their normal operation. With the increased desire to create lunar bases, understanding PSI physics has an important role in protecting the landing module itself and facilities around the landing site. We have numerical and experimental capabilities for investigating PSI problems including the large-scale vacuum chamber facility, in which supersonic nozzles can be fired at regolith simulants to investigate crater formation and regolith transport using a variety of advanced flow visualisation and diagnostic techniques. The figure below shows the dispersion of regolith material due to a supersonic plume impingement obtained from the rarefiedMultiphaseFoam solver.

  • Quantum effects: Quantum effects are important in various aspects of rarefied flows. To obtain the correct thermophysical properties of a gas, it is essential to model the internal energy modes with an acceptable degree of accuracy. At high temperatures, such as those experienced in shock waves in hypersonic flows, this can mean that the vibrational energy must be modelled using a quantised anharmonic oscillator model. For some gases, particularly light species such as hydrogen, the rotational energy mode should be quantised, even at room temperature. To recover the correct thermophysical properties, it will also be necessary to ensure that the rotational, vibrational, and electronic energy modes are coupled. Additionally, the scattering angles resulting from gas-gas collisions must be derived from ab-initio potentials to correctly account for the quantum effects with a reduction in temperature. For hydrogen gas, these quantum effects can be important even at room temperature.

  • Vacuum systems: Due to the reduced gas density, accounting for flow rarefaction is essential to accurately model gas flows in vacuum environments. One key engineering technology where operation under vacuum conditions is a key enabling technology is the photolithography machines that produce the latest semiconductor chips for TSMC, Samsung, Apple, and Intel. Owing to its favourable thermal and optical properties, hydrogen gas at low pressures is used and the quantum effects discussed above can also become an important aspect for the modelling and simulation.