Department of Physics and Theoretical Physics, Machine VP
Faculty of Science
Co-Investigators Russell R Boyce, John Sandeman, Neil J Mudford and John Milthorpe
Department of Physics and Theoretical Physics, Faculty of Science and
Department of Aerospace and Mechanical Engineering, Australian Defence Force Academy
Non-equilibrium Re-entry Flows (CFD Validation)
There is currently a great world wide interest in the development of a new generation of planes which are suborbital to orbital, the so called space planes. Their flight regime extends to regions where the flows cause molecular vibration and dissociation and lead to regions of non-equilibrium during these processes, which can be reproduced in the ANU T3 shock tunnel. As with all wind tunnels however, this facility can only partly simulate the free flight behaviour and it is vitally important to translate the wind tunnel data to the free flight regime. One of the most profitable ways of achieving this is to test the computational fluid dynamic codes which are currently under development by using them to simulate the shock tunnel flows. Once the code can reproduce these model flows there can be reasonable confidence that they may then be used to predict the free flight behaviour. This requires firstly obtaining as much data as possible from each tunnel run, and secondly running and developing the codes with the experimental input data to test their simulation capability. Obviously this is best done on a local machine where the whole program can be integrated.
Deutsche Aerospace (DASA) of Germany have been developing a family of coupled Euler/boundary layer CFD codes on a Fujitsu VP2200. These codes calculate hypersonic flows over blunt bodies, including the effects of nonequilibrium chemical reactions, and DASA have supplied them to us. The aim of this project is to attempt to validate those codes by means of comparison of their predictions with experimental data.
What are the basic questions addressed?
What are the important flow parameters to be measured to provide the most critical check of the CFD code? Do the physics, chemistry and numerical algorithms incorporated in the code successfully predict the experimental measurements of quantities such as bow shock standoff distance, wall heat transfer, shock layer temperatures and chemical species concentrations? If not, do the discrepancies arise from numerical inaccuracies in the code, or in bad assumptions or incorrect physics and chemistry incorporated? Do they arise from peculiarities associated with the experiments that are not accounted for by the code? Or do they arise from an insufficient knowledge of the nature of the free stream flow produced? With the range of flow conditions and measurements possible with the available experimental facilities, and with the results of the subsequent CFD/experiment comparisons, over what part of the flight domain can the code be said to be validated and hence confidently applied?
What are the results to date and the future of the work?
In the two previous years of this project, comparative studies of experiments and 2-D codes (particularly a nonequilibrium chemistry nitrogen code) were performed successfully validating the codes for a large number of flow parameters, flow conditions and test gases.
The period from December 1993 to June 1994 consisted of 3D nonequilibrium air calculations on the VP at the flow conditions investigated in the tunnel, and the generation of theoretical interferograms from the code. Several experimental interferograms have been recorded for various angles of a high enthalpy airflow over the test model at 15 degrees angle of attack, and since the theoretical interferograms generated are heavily dependent on the species concentrations calculated in the shock layer, which are in turn sensitive to nonequilibrium effects, such comparisons of the CFD code to experimental data will constitute a good validation experiment. This was done, and the theoretical interferograms compared reasonably well with the experimental ones for each of the seven different viewing angles. Thus confidence can be placed in the validity of the 3D density computations for nonequilibrium flows.
The work above for the perfect gas and nonequilibrium chemistry interferogram work were presented at two American Institute of Aeronautics & Astronautics (AIAA) conferences, in Reno, Nevada in January 1994, and in Colorado Springs, Colorado in June 1994.
In addition, theoretical temperature distributions in the shock layer on a cylinder in a Mach 3.5 perfect gas were computed and compared (very favourably) with Planar Laser Induced Fluorescence rotational temperature images obtained experimentally by the Principal Investigator while on sabbatical at Stanford University. These results were presented at the 1995 AIAA Reno conference.
Between July 1994 and January 1995, considerable numbers of calculations have been performed using all the codes (axisymmetric, 3D, perfect gas, equilibrium chemistry, nonequilibrium chemistry) at revised experimental conditions for inclusion in the PhD thesis of one of the investigators, Mr Russell Boyce. The results have in general improved as a result for all of the work described above.
The future of the work lies with employing all of the codes in support of the developing Laser Induced Fluorescence and Degenerate Four Wave Mixing diagnostic tools on the T3 shock tunnel. These diagnostics will be used to generate data which in turn can be compared with the CFD codes. In addition to this work, much previously unused experimental data exist, over a wide range of flow conditions, with which validation of the other codes can continue.
What computational techniques are used and why is a supercomputer required?
The complete calculation of a flow field involves solving the 3D Navier-Stokes equations. However, this requires far too much CPU time and memory. The codes use a much cheaper but quite accurate method, in which the flow field is divided into its viscous part (the boundary layer on the body) and its inviscid part (the region between the boundary layer and the shock). The Euler equations (the inviscid form of the N-S equations) are solved in the inviscid region using a split-matrix algorithm with Runge-Kutta time stepping, starting from an initial guess for the shock shape and inviscid flow field and using a bow-shock fitting approach. The second-order boundary layer equations (the viscous high Reynolds number second-order approximation to the N-S equations) are solved in the boundary layer using a finite-difference space-marching method. Both calculations are iteratively coupled together, the output of one used as the input for the other. Good results are obtained with only one iteration. Different versions of the code compute 2D/axisymmetric or 3D flowfields, for perfect gas, equilibrium chemistry or nonequilibrium chemistry.
The complexity of the flow fields, especially those exhibiting non-equilibrium chemical effects, and the mathematical nature of the governing equations means that enormous computing speed and memory are required in order to obtain accurate and economically viable (for the code's use as an engineering tool) results. For example, it is estimated that a converged inviscid solution for the 3-d nonequilibrium air code would take approximately 4 months of CPU time if run on the local Sun 4/280.
Rotational and vibrational temperature measurements using CARS in a hypervelocity shock layer flow and comparisons with CFD calculations, R R Boyce, D R N Pulford, A F P Houwing, and Ch Mundt, Shock Waves Journal (1995), submitted.
CFD Validation using multiple interferometric views of 3D shock layer flows over a blunt body, R R Boyce, J W Morton, A F P Houwing, Ch Mundt and D J Bone, AIAA paper 94-0282, 32nd Aerospace Sciences Exhibit, Reno, January 1994.
Experimental validation of a CFD code using multiple interferometric views of 3D shock layer flows, R R Boyce, A F P Houwing, Ch.Mundt, J W Morton and D J Bone (1995), accepted for publication in the Journal of Spacecraft and Rockets.
CFD Validation of real gas air flows over a blunt body using multiple interferometric views, R R. Boyce, A F P. Houwing, Ch. Mundt and J W. Morton, AIAA paper 94-2599, 18th Aerospace Ground Testing Conference, Colorado Springs, June 1994.
Experimental validation of a CFD code for nonequilibrium real gas air using multiple interferometric views of 3D shock layer flows, R R Boyce, A F P Houwing, Ch. Mundt and J W Morton (1995), Journal of Spacecraft and Rockets, to be submitted.
PLIF thermometry in a high temperature shock layer flow over a cylinder in a supersonic jet, A F P Houwing, J L Palmer, R R Boyce, M C Thurber, S D Wehe and R K Hanson, AIAA Paper No. 95-0515, 33rd Aerospace Sciences Exhibit, Reno, January 1995.
Computational Flow Imaging and Experimental Comparisons for 3D Shock Layer Flows over a Blunt Body, R R Boyce, A F P Houwing, J W Morton and Ch Mundt (1995), Second Pacific International Conference on Aerospace Science and Technology - Sixth Australian Aeronautical Conference, Melbourne, Australia, March 1995