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The Connector, the newsletter for CFD Mesh Generation from Pointwise

January / February 2012

Pointwise Aids Flight Experiment, Ground-Based Research for SCRAMSPACE International Access-to-Space Project

By Russell R. Boyce, Sandy C. Tirtey, Melrose Brown, Hideaki Ogawa, Centre for Hypersonics, The University of Queensland (Australia)

Access-to-space capability is necessary for the deployment of space-based systems for communications, remote sensing, space science, and so on. Safe, economical and ecologically responsible access to space is a major challenge for all nations due to the dependence of the global economy on assured and secure access to space-based services. The most promising way to meet this challenge is to extend aeronautical technology to hypersonic vehicles powered, at least partially, by air breathing supersonic combustion ramjet engines (scramjets).

The key advantage of the scramjet over current rocket technology is that they are calculated to be between three and eight times more efficient than a solid rocket, which is about half as efficient as a liquid hydrogen rocket. Scramjets can be combined with rockets to produce a more fuel-efficient hybrid launch system with more mass available for payload and/or redundant systems and health monitoring capability, thus leading to greater reliability and more economical insertion of payloads into orbit. The cost of placing a satellite into space is extremely high – typically of the order of $10-15 million per metric ton. Trajectory and systems studies indicate that with scramjets replacing the middle stage of a three-stage access-to-space rocket system, the overall payload mass can be increased by around 50 percent. This represents significant cost reductions per kilogram, in addition to reliability improvements and possible partial reusability.

Australian hypersonics has developed a 20-year road map to scramjet-based access-to-space systems. SCRAMSPACE (Scramjet-based Access-to-Space Systems) is an Australian Space Research Program funded project that represents the first phase of this road map. SCRAMSPACE is centered around an affordable, expertise-building flight at the scramjet entry point to the access-to-space Mach range. The flight will address scramjet performance, materials, and instrumentation, supported by ground-based performance and vehicle control developments in this range. Future phases of the road map will progressively incorporate scramjet technologies, currently being developed in Australian hypersonics laboratories, into flight experiments of increasing speed and sophistication. Ultimately, scramjet and rocket technologies will be brought together to demonstrate a prototype hybrid rocket / scramjet access-to-space system. By that stage, we expect that the developments will be led by an Australian space industry, in partnership with key international players.

SCRAMSPACE is a three-year, $14 million international project led by The University of Queensland (UQ), with consortium partners including University of New South Wales (UNSW), University of Adelaide (UA), University of Southern Queensland (USQ), Defence Science and Technology Organisation (DSTO), BAE Systems, Teakle Composites, AIMTEK, Japanese Aerospace Exploration Agency (JAXA), Italian Aerospace Research Center (CIRA), German Aerospace Center (DLR), University of Minnesota (UMN), and the Australian Youth Aerospace Association (AYAA).

depiction of SCRAMSPACE vehicle in orbit

Figure 1: The SCRAMSPACE I flight experiment +

SCRAMSPACE I will measure, in flight, the reduction in drag obtained when H2 fuel is switched on in a simple axisymmetric inlet-fuelled shock-induced combustion scramjet flowpath. The flowpath of interest is based on a triple-cone-inlet, six-porthole-injector, cylindrical-combustion chamber that has been tested in both UQ's T4 shock tunnel in Australia and in the large HEG shock tunnel at the German Aerospace Center (DLR) in Germany.

Pointwise software – both the Pointwise and Gridgen software suites – is playing an important role in this activity, both in the flight experiment and in the ground-based research that supports it. The activities can be divided into three broad area: numerical reconstruction of the shock tunnel experiments of the turbulent combusting scramjet flowpath and prediction of the flight combustion phenomena; multi-disciplinary design optimization of the scramjet thrust nozzle; and analysis of the external aerodynamics for the assessment of stability of the flight vehicle. In each case, the computational fluid dynamics (CFD) code that has been used is CFD++ from Metacomp Technologies. The simulations have been performed on two large parallel clusters under the umbrella of the Australian National Computational Infrastructure National Facility, typically using 128 or 256 cpus per simulation.

Numerical Reconstruction of the Scramjet Flowpath

For these simulations, a hybrid mesh has been constructed that makes use of the symmetry planes in the scramjet flowpath at 0° angle of attack – the streamwise planes that bisect the injectors, and the streamwise planes midway between each injector pair. Figure 2 shows the computational domain that results, representing a 30° sector of the full flow domain. The mesh includes the blunt leading edge (and therefore the flow in the vicinity of the leading edge, both internal and external. This mesh contains 13.9M cells, clustered toward walls, compression corners and the leading edge for appropriate resolution of shocks and boundary layers.

schematic of the scramjet flowpath

Figure 2: The computational domain for the scramjet flowpath at 0° angle of attack +

Figure 3 shows the structured mesh in the region of one of the injectors, where the mesh has been mirrored across the injector symmetry plane for the purpose of illustration. The full mesh is a hybrid one due to the use of a tube of prisms in the vicinity of the central axis of symmetry that has been extruded in the streamwise direction from an unstructured spanwise plane. This was done to ease the complexity of constructing the structured mesh in the rest of the domain.

Figure 4 shows CFD pressure contours in the inlet and combustion chamber of the scramjet flowpath. Significant combustion-induced pressure rise in the combustion chamber has been observed experimentally and successfully replicated with the numerical simulations. For the purpose of illustration, the mesh and solution has been mirrored appropriately so that the full surface can be seen.

Simulations have also been performed at a variety of angles of attack. For these cases, internal symmetry is reduced, and a 180° sector, containing 85 million cells has been employed.

mesh near a fuel injector

Figure 3: The computational mesh in the vicinity of a fuel injector in the inlet +

mesh near a fuel injector

Figure 4: CFD pressure contours in the inlet and combustion chamber of the axisymmetric flowpath to be flow on SCRAMSPACE I +

Multi-Disciplinary Design Optimization

A thrust nozzle has been designed with the aid of multi-disciplinary design optimization, employing Gridgen as the mesh generator and CFD++ as the CFD engine, embedded in scripts that employ evolutionary algorithms to perform shape optimization. These algorithms are assisted by surrogate modeling of the CFD-predicted response of the thrust of the nozzle to changes in its shape. The optimizations were performed with axisymmetric simulations only, to reduce computational expense, and the inflow to the nozzle computational domain was provided by stream-thrust-averaging the fuel-on reactions-on flow properties at the combustor exit in the 3D simulations. It was found that while a fully contoured thrust nozzle produced greatest thrust, a conical approximation to that contour provided considerable design and manufacturing advantages at the minimal loss of thrust (less than 10 percent).

External Aerodynamics

external flowfield

Figure 5: Flow in and around the free-flyer at 2° angle of attack, fuel off +

A fully structured mesh has been constructed using Gridgen for the external flowfield around the SCRAMSPACE I vehicle, including the fins needed to stabilize the vehicle during its descent through the Earth's atmosphere. By combining the internal fuel-off and fuel-on CFD simulations with the external flow simulations, the full nose-to-tail structure of the flow can be determined (see Figure 5), and from this predictions of the lift and drag of the vehicle have been made at various altitudes.

The Flight

The development of the flight vehicle is currently under way, led by the flight experiment technical team at UQ, with input from University of New South Wales (UNSW), DSTO, DLR, CIRA, Teakle Composites and BAE Systems. The primary experiment on the flight has been described above – the axisymmetric inlet-fueled shock-induced combustion scramjet. Scramjet performance will be determined by comparing the change in acceleration (net drag) due to pulsing the fuel on and off. Test conditions will be Mach 8 over the altitude range 32-27 kilometers, and will be achieved on re-entry after launch by and exo-atmospheric separation from a two-stage sounding rocket selected on the basis of performance, cost, reliability and availability – lower stage S30, upper stage Improved Orion. The launch will be conducted by the DLR Mobile Rocket Base at the Woomera Test Range in the South Australian desert. Deployment takes place after the payload/rocket stack leaves the Earth's atmosphere. Progress on the development of the flight is excellent, with the Preliminary Design Review milestone having been recently passed. The launch is anticipated to take place in March 2013.

Note: Preliminary information about the Scramspace project was reported in the April 25/May 2 2011 issue of Aviation Week.


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