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Preface to “evaluation of data on simple turbulent reacting flows”
Prag gneroyCombust.Sci. 1986,Vol. 12. pp. 253-255.
0360 1285/86I0.00 + .50 Copyright 0 1986 Pergamon Journals Ltd.
Printed in Great Britain. All rights reaerved.
PREFACE TO "EVALUATION OF DATA ON SIMPLE TURBULENT REACTING FLOWS" WARREN C. STRAHLE
School of Aerospace Engineering, Georoia Institute of Technology, Atlanta, GA 30332, U.S.A.
During the past decade and one-half there has been a largely expanded base of knowledge developed in turbulent reacting flows. This has come about through development of advanced experimental methods and increased computational power. The area has always been an important one since virtually all combustion driven power extraction devices operate with a turbulent working fluid. During the summer of 1983 several members of the technical community were expressing concern over a feeling of chaos in the relationship of theory and experiment in this field. Experiments appeared to be diverse in purpose and several analytical models of different types had been developed with little comparison between methods. Given the prior methodology of a computation/experiment consolidation, 1- 3 the Air Force Office of Scientific Research (Dr Leonard H. Caveny was then the primary motivator) desired to conduct a program along the lines of the Stanford conference 2 of 1968, but on turbulent reacting flows as opposed to turbulent boundary layerg The papers to follow in this issue are a direct result of the effort that ensued, and they are extracted from a more detailed work 4 available in report form from the author or AFOSR. This author chaired the effort and was given immense help in initial organization from the following people: Professor Stephen J. Kline, Stanford University Professor Brian J. Cantwell, Stanford University Professor Craig T. Bowman, Stanford University Professor Howard W. Emmons, Harvard University Dr Dan L. Hartley, Sandia National Laboratories Professor Stanford S. Penner, University of California at La Jolla. The initial plan that was envisioned was that there would be a two-phase effort. The first would consist of establishment of a well-documented data base in turbulent reacting flows which would be encoded for a second-phase effort to consist of a computational test of various methods. For reasons given later, the effort stopped at the end of the first phase, and the papers in this issue report on the results of that phase. An Organization Committee was founded to carry out the data-base analysis. It consisted of the following people: 253
Dr Michael C. Drake, General Electric Professor Gerard M. Faeth, University of Michigan Professor Frederick C. Gouldin, Cornell University Dr Sheridan C. Johnston, Sandia National Laboratories Professor Wolfgang Kollmann, University of California at Davis Professor Spyridon G. Lekoudis, Georgia Institute of Technology Professor Paul A. Libby, University of California at La Jolla Dr Geoffrey J. Sturgess, United Technologies Corporation Professor G. S. Samuelson, University of California at Irvine Professor J. H. Whitelaw, Imperial College of Science and Technology Dr Edward J. Mularz, Army Research Officc This committee determined the categories of data bases to be examined, and, together with other people identified as authors in the papers to follow, actually selected appropriate data bases as adequate for some computational tests The Committee's decision was to limit the categories of turbulent reacting flows to be considered. First of all, consideration was limited to only those flows that could be analytically treated by parabolic methods. Elliptic flows were eliminated from consideration. The following four data classes were identified for further scrutiny: Variable-density nonreacting flows Fast reaction nonpremixed flows Slow reaction nonpremixed flows Premixed flows that could be parabolically treated. The charge to the data-base analysers, chosen from the Committee, was to (a) seek flows that were suitable for computational test, (b) identify, if possible, the accuracy of the data and (c) identify gaps in the data. It was established that the following items would be desirable in a data base which would be ideally suited for a computational test:
1. Measurement of a vector, scalar and some turbulence quantity
254
W.C. STRAHLE
2. Measurement at many streamwise and crossstream stations 3. Sufficiently high Reynolds number to guarantee turbulence 4. Measurement of some macroscopic variables such as flame length 5. Interpretability of measurement in terms of a Favre or conventional quantity 6. High measurement accuracy or at least an accuracy extimate 7. Large density differences in the case of variable density nonreacting flows 8. Confidence in the parabolic treatment 9. Measurement of initial conditions and adequate mean pressure gradient specification 10. Minimal intrusiveness of measurement I 1. Fully turbulent flow everywhere in the computational domain. It was concluded early in the effort, however, that no available data bases would meet these criteria. Some were found sufficiently close to warrant further scrutiny. However, because of the pessimism at the time, it was decided to delay any efforts at creating a computer-based data encoding process until after further work.,,,.. At a final Committee meeting in December, 1984 several flows had been identified which could be used for a computational test, to varying degrees of completeness and certainty. The Committee had to reach, however, to some data bases that were not yet complete in their documentation in the published literature. Moreover, the evaluation of the data was in two cases partially carried out by workers who were closely allied to the original data-taking process. There were, however, sufficient independent checks by nonallied workers that this was not believed to be a problem. The primary decision at the final meeting was to recommend that a computational effort n o t be initiated at this time. This decision was unanimous but not applauded. There were several reasons for this opinion, and some of them were independent of the data bases' quality and were linked to an opinion of what the computors could do. Most theories or models of turbulent reacting flows are applicationspecific and cannot be readily used for flows of different character or chemistry from those for which they were developed. This precludes asking the computor community to calculate several mandatory flows which may cross technical lines (¢g. a premixed flow and a diffusion flame). Indeed, for many flows, even though of relative simplicity, the calculation requires a research effort of considerable magnitude. Acknowledging, however, that a computational effort for individual flows might be of use, the decision to abort a large community-wide computational effort finally laid at the quality of the data bases. Here there are some problems with some of the flows in (a) completeness, (b) low Reynolds number, (c) speci-
fication of the initial and boundary conditions, (d) containment of a laminar-turbulence transition, (e) uncertainty as to the accuracy of measurement and (f) uncertainty in the type of weighing (averaging) of the measurement. Moreover, there is some uncertainty in some of the flows whether or not a parabolic treatment would be adequate, and it is certain in some of the flows that buoyancy would have to be considered. In short, the Committee's opinion was that the computation of each flow was a subject of research, not routine computation, and that calculation of these flows was best handled on an individual basis where the uncertainties could be systematically explored. This is not an indictment of the turbulent reacting flows experimental community. Most of the work reviewed was never intended to act as a data base to test models and computation accuracy; they were often intended to test specific physical hypotheses or provide exploratory information. Indeed, the generation of such data bases is a relatively new activity for the community. The Committee, however, was looking for a breadth of information on each flow which was often absent because the data were generated for purposes other than computational test The fact that some flows were rejected from consideration for the purpose at hand is therefore not intended as a judgement concerning the quality of the work. Some may question if it was appropriate of the Committee to emphasize relatively simple turbulent flows involving chemical reaction without the complication of complex geometries, radiative transfer and multiple phases. These complications enter significantly in practical applications, and the Committee was well aware that a parallel effort of application of models to these situations was being carried out by industrial and government organizations. It is important to recall that in a simpler but related field, namely in the phenomenology of turbulent flows with constant fluid properties, there is currently much discussion and controversy concerning the new sophisticated methods applicable to such flows. There are some who believe that such methods should develop in an evolutionary manner, through simple toward complex flows, so that ultimately the flows of more practical interest can be treated with soundly based approaches. Others are impatient with this view and consider that use of the new methods is justified by their ability to attack practical problems even though many details of the analysis are uncertain. Moreover, when applied to entirely new situations in the absence of experimental data the results are suspect In the view of this Committee, the added complexities of turbulent combustion, in particular the presence of significant variation in density, leading to the possibility of new transport and turbulence production mechanisms, suggest that the conservative perspective of the first group should be adopted. It is hoped that in due
P reface course the evolving predictive methods assessed and improved on the basis of the experimental data emphasized here, and expected to be forthcoming in the near future, will lead to soundly-based methods of direct use to the designer. REFERENCES
1. ANOn. Free turbulent shear flows, NASA SP-321 {1972j.
255
2. KLINE, S. J., MORKOVIN, M. J. and MOFFAT, Computation of turbulent boundary layers, 1968 AFOSR-IPStanlord Conlt,rence (1969). 3. KLINt~, S. J.. CANTWla,L, B. J. and LILLI~Y, G. M., Comparison of computation and experiment, The 1980 81 AFOSR-HTTM-STANFORD Conference on Compho: Turhuh,m Flows (1981).
4. STRAttLLW. C. and LI-KOUDIS,S. G., Evaluation of data on simple turbulent reacting flows, AFOSR TR.85-0880 (1985).
0360 1285/86I0.00 + .50 Copyright 0 1986 Pergamon Journals Ltd.
Printed in Great Britain. All rights reaerved.
PREFACE TO "EVALUATION OF DATA ON SIMPLE TURBULENT REACTING FLOWS" WARREN C. STRAHLE
School of Aerospace Engineering, Georoia Institute of Technology, Atlanta, GA 30332, U.S.A.
During the past decade and one-half there has been a largely expanded base of knowledge developed in turbulent reacting flows. This has come about through development of advanced experimental methods and increased computational power. The area has always been an important one since virtually all combustion driven power extraction devices operate with a turbulent working fluid. During the summer of 1983 several members of the technical community were expressing concern over a feeling of chaos in the relationship of theory and experiment in this field. Experiments appeared to be diverse in purpose and several analytical models of different types had been developed with little comparison between methods. Given the prior methodology of a computation/experiment consolidation, 1- 3 the Air Force Office of Scientific Research (Dr Leonard H. Caveny was then the primary motivator) desired to conduct a program along the lines of the Stanford conference 2 of 1968, but on turbulent reacting flows as opposed to turbulent boundary layerg The papers to follow in this issue are a direct result of the effort that ensued, and they are extracted from a more detailed work 4 available in report form from the author or AFOSR. This author chaired the effort and was given immense help in initial organization from the following people: Professor Stephen J. Kline, Stanford University Professor Brian J. Cantwell, Stanford University Professor Craig T. Bowman, Stanford University Professor Howard W. Emmons, Harvard University Dr Dan L. Hartley, Sandia National Laboratories Professor Stanford S. Penner, University of California at La Jolla. The initial plan that was envisioned was that there would be a two-phase effort. The first would consist of establishment of a well-documented data base in turbulent reacting flows which would be encoded for a second-phase effort to consist of a computational test of various methods. For reasons given later, the effort stopped at the end of the first phase, and the papers in this issue report on the results of that phase. An Organization Committee was founded to carry out the data-base analysis. It consisted of the following people: 253
Dr Michael C. Drake, General Electric Professor Gerard M. Faeth, University of Michigan Professor Frederick C. Gouldin, Cornell University Dr Sheridan C. Johnston, Sandia National Laboratories Professor Wolfgang Kollmann, University of California at Davis Professor Spyridon G. Lekoudis, Georgia Institute of Technology Professor Paul A. Libby, University of California at La Jolla Dr Geoffrey J. Sturgess, United Technologies Corporation Professor G. S. Samuelson, University of California at Irvine Professor J. H. Whitelaw, Imperial College of Science and Technology Dr Edward J. Mularz, Army Research Officc This committee determined the categories of data bases to be examined, and, together with other people identified as authors in the papers to follow, actually selected appropriate data bases as adequate for some computational tests The Committee's decision was to limit the categories of turbulent reacting flows to be considered. First of all, consideration was limited to only those flows that could be analytically treated by parabolic methods. Elliptic flows were eliminated from consideration. The following four data classes were identified for further scrutiny: Variable-density nonreacting flows Fast reaction nonpremixed flows Slow reaction nonpremixed flows Premixed flows that could be parabolically treated. The charge to the data-base analysers, chosen from the Committee, was to (a) seek flows that were suitable for computational test, (b) identify, if possible, the accuracy of the data and (c) identify gaps in the data. It was established that the following items would be desirable in a data base which would be ideally suited for a computational test:
1. Measurement of a vector, scalar and some turbulence quantity
254
W.C. STRAHLE
2. Measurement at many streamwise and crossstream stations 3. Sufficiently high Reynolds number to guarantee turbulence 4. Measurement of some macroscopic variables such as flame length 5. Interpretability of measurement in terms of a Favre or conventional quantity 6. High measurement accuracy or at least an accuracy extimate 7. Large density differences in the case of variable density nonreacting flows 8. Confidence in the parabolic treatment 9. Measurement of initial conditions and adequate mean pressure gradient specification 10. Minimal intrusiveness of measurement I 1. Fully turbulent flow everywhere in the computational domain. It was concluded early in the effort, however, that no available data bases would meet these criteria. Some were found sufficiently close to warrant further scrutiny. However, because of the pessimism at the time, it was decided to delay any efforts at creating a computer-based data encoding process until after further work.,,,.. At a final Committee meeting in December, 1984 several flows had been identified which could be used for a computational test, to varying degrees of completeness and certainty. The Committee had to reach, however, to some data bases that were not yet complete in their documentation in the published literature. Moreover, the evaluation of the data was in two cases partially carried out by workers who were closely allied to the original data-taking process. There were, however, sufficient independent checks by nonallied workers that this was not believed to be a problem. The primary decision at the final meeting was to recommend that a computational effort n o t be initiated at this time. This decision was unanimous but not applauded. There were several reasons for this opinion, and some of them were independent of the data bases' quality and were linked to an opinion of what the computors could do. Most theories or models of turbulent reacting flows are applicationspecific and cannot be readily used for flows of different character or chemistry from those for which they were developed. This precludes asking the computor community to calculate several mandatory flows which may cross technical lines (¢g. a premixed flow and a diffusion flame). Indeed, for many flows, even though of relative simplicity, the calculation requires a research effort of considerable magnitude. Acknowledging, however, that a computational effort for individual flows might be of use, the decision to abort a large community-wide computational effort finally laid at the quality of the data bases. Here there are some problems with some of the flows in (a) completeness, (b) low Reynolds number, (c) speci-
fication of the initial and boundary conditions, (d) containment of a laminar-turbulence transition, (e) uncertainty as to the accuracy of measurement and (f) uncertainty in the type of weighing (averaging) of the measurement. Moreover, there is some uncertainty in some of the flows whether or not a parabolic treatment would be adequate, and it is certain in some of the flows that buoyancy would have to be considered. In short, the Committee's opinion was that the computation of each flow was a subject of research, not routine computation, and that calculation of these flows was best handled on an individual basis where the uncertainties could be systematically explored. This is not an indictment of the turbulent reacting flows experimental community. Most of the work reviewed was never intended to act as a data base to test models and computation accuracy; they were often intended to test specific physical hypotheses or provide exploratory information. Indeed, the generation of such data bases is a relatively new activity for the community. The Committee, however, was looking for a breadth of information on each flow which was often absent because the data were generated for purposes other than computational test The fact that some flows were rejected from consideration for the purpose at hand is therefore not intended as a judgement concerning the quality of the work. Some may question if it was appropriate of the Committee to emphasize relatively simple turbulent flows involving chemical reaction without the complication of complex geometries, radiative transfer and multiple phases. These complications enter significantly in practical applications, and the Committee was well aware that a parallel effort of application of models to these situations was being carried out by industrial and government organizations. It is important to recall that in a simpler but related field, namely in the phenomenology of turbulent flows with constant fluid properties, there is currently much discussion and controversy concerning the new sophisticated methods applicable to such flows. There are some who believe that such methods should develop in an evolutionary manner, through simple toward complex flows, so that ultimately the flows of more practical interest can be treated with soundly based approaches. Others are impatient with this view and consider that use of the new methods is justified by their ability to attack practical problems even though many details of the analysis are uncertain. Moreover, when applied to entirely new situations in the absence of experimental data the results are suspect In the view of this Committee, the added complexities of turbulent combustion, in particular the presence of significant variation in density, leading to the possibility of new transport and turbulence production mechanisms, suggest that the conservative perspective of the first group should be adopted. It is hoped that in due
P reface course the evolving predictive methods assessed and improved on the basis of the experimental data emphasized here, and expected to be forthcoming in the near future, will lead to soundly-based methods of direct use to the designer. REFERENCES
1. ANOn. Free turbulent shear flows, NASA SP-321 {1972j.
255
2. KLINE, S. J., MORKOVIN, M. J. and MOFFAT, Computation of turbulent boundary layers, 1968 AFOSR-IPStanlord Conlt,rence (1969). 3. KLINt~, S. J.. CANTWla,L, B. J. and LILLI~Y, G. M., Comparison of computation and experiment, The 1980 81 AFOSR-HTTM-STANFORD Conference on Compho: Turhuh,m Flows (1981).
4. STRAttLLW. C. and LI-KOUDIS,S. G., Evaluation of data on simple turbulent reacting flows, AFOSR TR.85-0880 (1985).