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Structure of Turbulent Shear Flows: a New Look VOL. 14, NO. 10, OCTOBER 1976 AIAA JOURNAL 1349 Dryden Research Lecture Structure of Turbulent Shear Flows: A New Look Anatol Roshko California Institute of Technology, Pasadena, Calif Introduction large eddies were made by Townsend and his students. 2•3 In HE problem of turbulent now continues to be an out­ recent years it has become increasingly evident that turbulent T standing one in technology and in physics. Of the 1.ine shear nows do contain structures or eddies whose descripuon Dryden research lectures so far, four have been on some is more deterministic than had been thought, possessing in­ aspect of the turbulence problem. At meetings such as this one dentifiable characteristics, existing for significant lifetimes, the turbulence problem is always the subject of some sessions and producing recognizable and important events. More ac­ and lurks in the background of many others; for example, curate descriptions of their properties, how they fit into the separated now, combustion, jet noise, chemical lasers, at­ complete description of a 1Urbulent flow, to what extent are mospheric problems, etc. It is continually the subject of con­ they central to its development, and how they can be recon­ ferences, workshops and reviews. In his time Hugh Dryden ciled with the apparent chaos and disorder, are problems wrote several reviews of turbulent now. ln reading some of which are becoming of interest to an increasing number of researchers. lt is the purpose of this lecture to describe some them again~ one statement t particularly relevant to the present lecture caught my attention: "-it is necessary to of these new developments. The discussion will draw largely separate the random processes from the nonrandom on experiences from our own laboratory; it is not intended to be a complete survey. Other discussions of these ideas can be processes. It is not yet fully clear what the random elements 4 8 are in turbulent now." Neither is it fully clear what the found in various recent publications. nonrandom, orderly elements are, but some of them are beginning to be recogniz~d and described. The Turbulent Mixing Layer Generally the picture one has had of turbulence is of chaos Our own ideas about turbulent now structure were and disorder, implicit in the name. Although it was known drastically altered by an investigation of plane turbulent that organized motion could exist, superimposed on the mixing layers which we undertook, 9•10 initially to study the ef­ background of "turbulence," for example, vortex shedding fects of nonuniform density. Figure I is a schematic diagram 7 from a circular cylinder up to Reynolds numbers of 10 , such of this class of flows, in which the uniform streams on either examples were regarded as special cases closely tied to their side of the mixing layer are characterized by their velocities particular geometric origins and not characteristic of "well­ U1 and U1 , etc. What is amactive ab"ut this configuration is developed" turbulence. It was known that large structures are that, of all the flows in the catalog of basic turbulent shear important in the development of turbulent shear flows and nows, it allows the effects of variation of various parameters that these ought to possess some definable features. But even such as velocity ratio U1 JU1 , density ratiop1 /p 1, Mach num­ when the concept of a characteristic "big eddy" was explored, bers, etc. to be explored in the simplest way (this remark ap­ it was usually in the context of a statistical quantity. The plies to the concept, not to the actual realization in the earliest and most decisive auempts to define the form of such laboratory!). The panicular case of homogeneous flow Or. Roshko is known for his research work in soeral areas of gas dynamics and nuld mechanics. He hiti made conlributions to problems or separated now; bluff-body aerod~namics; shock-wa,·e bounda11-la)er interactions: structure of turbulent wakes and rni\ing hl)ers; shock-tube technolog). He Is coauthor with H.W. Liepmann of "Elements or Gas Dynamics,·• which has been widely used a a textbook during the past •~enty) ears. He "'as born in Canada, obtained his undergraduate education at the University of Alberia, from "'hich he received the B.Sc. degru in Engineering Ph) sics in 1945. He received the M.S. degree from Caltech in 1947 and lhe Ph.D. in 1952. His academic career Includes two years of teaching at the Unh·ersity or Alberta and man) )ears at Caltech. where he is Professor or Aeronautic . He is a Fellow of AIAA, the CaRlldian Aeronautics and Space Institute, and the American Academ) or Arts and Sciences; a member of the American Ph)sk1ll Societ) and the Arctic Institute of North America. He helped organize the Wind Engineering Research Council and is a member of its E.\ecuthe Board. Or. Roshko has been a consultant to government laboratories and various com­ panies, panicularly the McDonnell-Douglas Corporation. During 1961~2 he was Liaison cientist with the U .S. Office or Na,aJ Research in London and in 1969 was an NSF Visiting Scientist in India. Receh·ed Jan. 22, 1976: pre~nted a~ Paper 76-78 at the AIAA 141h Aerospace Sciences Meeting, Washington D.C .. Jan. 26-28, 1976; revision recei"ed Jul}' 6, 1976. The author is mdeb1ed for "aluable discussions and use of material to many colleagues and students. Their names (except L. Bernal) are indicated by asteri~ks in the list of references. Most of the material in thi~ lecture i~ ba<>ed on research conducted in collaboration with G. L. BroY.n and supported by the Office of a val Research under i1s Fluid Oynamio Program and Project SQUID. Jndex categories: Je1s, Wakes. and Viscid Flow Interactions; Boundary Layers and Convective Heat Transfer-Turbulent. 1350 A. ROSHKO AIAA JOURNAL ~ti~U2•1'2 U2 l'a fig. I Plane mi"ing la)er bet"een t"o streams with 'l'elocilies u / andU1 ,densitiesp1 andp1 • Fig. 2 Mlxing la, er bet'teen helium and nitrogen U 1 ; u, = O.J8: P1IP1=l:p,L1Llp.1=l.l, 0.6and O.Jx ! OJ, respective!), from top to bottom. (Lis lhe width of lhe picture.) <P =p ) at low Mach number, along with other in­ 1 1 Fig. 3 Mhing la,er bet"een nicrogen and a helium -argon mi,ture of compressible nows such as wakes, jets and boundary layers, 5 the same density. p 1 U1L/ p. 1 = 8.5 X !0 • from lop to bottom: frame had long been a pan of the classical literature of turbulent numbers I. 6, 9, 13, and 19 of a sequence. shear flow. No hint of other than a classical "turbulent" pic­ ture existed. It was therefore rather astonishing when shadow pictures of nows. In fact, they are che same flows, they are turbulent, but the flow revealed the presence of well-defined large structures the existence of the organized structures had not previously (Fig. 2) which have the appearance of breaking waves or been recognized. Still another example obtained in our 15 rollers or vortices. In this example, the Reynolds number laboratory, in which che vortex structures were observed varies from a loy, value at which fine-scale turbulence is with dye visualizations, was a mixing layer in a water channel 6 largely absent to a higher value at which it is present at Reynolds number of 3 x 10 . throughout the now,superimpOSed On the large-scale lWO­ We emphasize these points because typical initial reaction dimensiona( motion. What is significant is that the m~sured to these piccures (including our own) has tended 10 be one of mean ~ropenies of the flow , the velocity and density profiles, skepticism as to whether the now is really turbulent, whecher spreading rate, etc. are the same for all three cases. The mean Reyn()lds number is high enough, etc. Although these nows flow is controlled by the large, organized structures which, it had been studied for many years and the presence of the may be seen, are noi affected by the small-scale turbulence ap­ coherent structures not suspected, once the structures are pearing at the higher values of Reynolds number. known to be there it is rather easy to find them! It is also in­ That the large-scale organized phenomena are not peculiar teresting that they were being recognized in other in­ to the large-density difference is shown by Fig. 3, which is a vestigations 5·16 carried on at about the same time as our 9 sequence of shadowgraphs of a flow in which the density is own; as mentioned earlier, there has been an increasing u~iform . Optical visualization was made possible by using awareness over many years of the possibility of organized different gases, of the same densiLy, on the 1wo sides of the structures in turbulent shear flows. mixing layer: nitrogen on the high-speed side and a mixture of Given that they do exist, many questions occur: how can helium and argon on the low-speed side. (The imaging method they be described; how are they formed; how long do they used to obtain the pictures in a high-speed framing camera did exist; what do they do? Some aspects of these are discussed in not resolve the fine-scale structure.) The Reynolds number is the following paragraphs. 8.5 x 10s , comparable to the highest values in previous, well­ known investigations of turbulent mixing layers. 11- 11 Thus Coherence and Lifelimes these flows, with organized large structure, correspond in The vonex-like structures on the photographs in Figs.
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