Air Flow and Turbulence in Boundary Layers

Shortly after arriving at NBS in 1929 as an employeein the Electrical Division, Galen B. Schubauer enrolledin a graduate study program at Johns Hopkins Univer-sity in Baltimore, as did about two dozen of his newlyhired colleagues. These studies required a commute toBaltimore several times per week. The difficult trip wasmade more efficient by cramming six to eight studentsinto a 1930s sedan, which made the trip much morearduous. During his graduate studies, Schubauer’sinterest began to focus on aerodynamics, and onboundary layer phenomena in particular. Consequently,he transferred to the Aerodynamics Section of NBS,then headed by Hugh L. Dryden, where he began anillustrious career of research on this topic.

His 1935 paper, Air Flow in a Separating BoundaryLayer [1], provided great insight into separation (orstall) of a laminar boundary layer developing over anairfoil with the cross-section of an elliptic cylinder.The purpose of the work was to test an approximatesolution to the applicable flow equations advanced byK. Pohlhausen. Schubauer’s experimental resultsshowed that Pohlhausen’s solution produced goodagreement in the forward portion of the surface, but itbegan to fail near the separation point on the surface andcould therefore not be used to predict the location ofthe separation point. The next paper, The Effect ofTurbulence on the Drag of Flat Plates [2], concerned theeffects of free-stream turbulence on four objects: a flatplate, a thin circular disk, a vane anemometer, and aPitot static tube. The results indicated that there is noappreciable effect of turbulence on the vane anemo-meter and the Pitot static tube, but there is a small effecton the drag of a flat plate and on the pressure differencebetween the front and rear of a disk. Furthermore, theeffect of turbulence was found to be independent of theair speed or Reynolds Number.

The hot-wire anemometer had emerged as the funda-mental instrument for measuring boundary layerturbulence as well as boundary layer velocity distribu-tions. This instrument was known to be sensitive to otherparameters which could cause errors in the turbulenceand velocity measurements if proper corrections werenot made. Water vapor (humidity) in the air stream wasrecognized to be such a possible extraneous parameter.In his next paper [3], Schubauer quantified this effect byverifying recent results published by W. Paeschke which

showed that the effect was to increase the hot-wire heatloss at higher humidities, giving a fractional change ofabout 2 % for an increase in relative humidity from25 % to 70 %. It was concluded that the phenomenoncould be explained by the effect of humidity on thethermal conductivity of air.

During the next decade, Schubauer continued tofocus his attention on the stability of the laminarboundary layer. He was particularly interested in aninstability theory which had been under developmentover a 40-year period and which was published in themid-1930s by two German investigators, W. Tollmienand H. Schlichting, who were working independently.This theory postulated that a small disturbanceintroduced into a laminar boundary layer would lead totransition of the laminar layer to a turbulent boundarylayer, depending primarily upon the frequency of thedisturbance and the longitudinal (in the flow direction)Reynolds Number. For certain values of the governingparameters the disturbance would be amplified andtransition would occur, but for other parameter valuesthe disturbance would be attenuated (or damped)and would not cause transition. The regions wheredisturbances are amplified or damped are separated bya curve of “neutral stability” as shown in Fig. 1.This curve is sometimes referred to as a “Tollmien-Schlichting noodle curve” because of its peculiar shape.

Working during the mid-1930s on his own time andwith available equipment, Schubauer began to investi-gate the T-S theory experimentally. His jury-riggedexperimental setup was substantially less than ideal, buthe was able to acquire enough data to convince himselfthat the T-S theory was basically valid. However,Dryden argued that the relatively crude experimentalsetup employed by Schubauer could not possibly allowmeasurement of the T-S phenomenon. There were toomany extraneous sources of disturbances in the flow,and the signal-to-noise ratio of the available hot-wireinstrumentation was insufficient. Substantial improve-ment of the experimental equipment would be requiredto provide necessary control of all the parameters andmake high quality measurements. Because of higherpriorities, Dryden would neither allow any of theAerodynamic Section’s current financial resources to beused for this work nor assist in soliciting additionalfunds for the research.

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As time passed, Schubauer persisted and continued towork nights and weekends with available equipment torefine his experimental setup. Finally, in the late 1930s,he was able to accumulate enough data to convinceDryden that he was, in fact, able to measure and verifythe results of disturbances of known frequenciesinjected into a laminar boundary layer and that theresults consistently fell very close to the T-S theory’spredictions.

In the late 1930s, Dryden actively helped Schubauerin securing funding to pursue this work. As World WarII approached, its value to the war effort was recognizedand it was given a Confidential classification. Now theproblem was no longer funding, but securing thematerials and equipment necessary to conduct a well-controlled experiment in a war-time economy. How-ever, as the war progressed, advances in the field ofelectronics were of great help in producing componentswith which hot-wire instrumentation of higher signal-to-noise ratio could be built, and ways were found toreduce the wind tunnel free-stream turbulence levelsbelow the required level of about 0.1 % of flow rate.

As a result of the Confidential classification placedon the work, Schubauer and his colleagues were pre-vented from publishing anything about it until afterWorld War II, except as an advance National AdvisoryCommittee for Aeronautics (NACA) ConfidentialReport in 1943. By 1947, after Dryden had become thehead of NACA and Schubauer had become chief of theNBS Aerodynamics Section, the work was published inthe open literature in the Journal of Research of theNational Bureau of Standards [4]. The paper was

co-authored by H. K. Skramstad, who was now chief ofthe NBS Guided Missiles Section and who contributedimprovements to the hot-wire instrumentation and otherelectronic components of the test apparatus. The resultswere also published in other media, and additionalpublications by Schubauer and his colleagues about thissame time described associated research work.

The T-S theory and its experimental verification bySchubauer and Skramstad contributed greatly to thefundamental understanding of the transition of fluidboundary layers from laminar to turbulent flow. This hasled to extensive work in boundary layer control resultingin a myriad of designs for controlling transition to makelifting surfaces, internal flows, and submerged vehiclesmore efficient. The value of the work has been recog-nized to the extent that it was nominated for the NobelPrize in Physics.

Galen Brandt Schubauer was born in Mechanicsburg,PA. He graduated from Pennsylvania State Universityand received a master’s degree in physics from theCalifornia Institute of Technology and a doctorate inphysics from Johns Hopkins University. His honorsincluded the Fluid Dynamics Prize of the AmericanPhysical Society, which he received in 1988 for seminalresearch findings that concerned the design of windtunnels and other aspects of research. In 1944 he wasthe recipient of the Washington Academy of Sciencesaward for achievement in engineering sciences, and in1947 he shared the Sylvanus Albert Reed Award of theInstitute of Aeronautical Sciences. He was a fellow ofthe American Physical Society, the WashingtonAcademy of Sciences, and the American Institute of

Fig. 1. Zone of Amplification Enclosed by Neutral Curve, According to Schlichting.(Adapted from Figure 13 of [4])

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Aeronautics and Astronautics. He was a member ofthe National Academy of Engineering and of thePhilosophical Society of Washington.

Upon his retirement from NBS in 1968, Allen V.Astin, then Director of NBS, wrote “Few men haveserved longer at the National Bureau of Standards thanGalen B. Schubauer, and probably none with morededication. His reputation as an international authorityin fluid mechanics, particularly in aerodynamics,reflected credit on the man and, secondarily, on NBSitself. Certainly the papers of Dr. Schubauer are ofinterest to his friends, to this institution, and to the scien-tific community. They cover a period of time in whichscience, technology, and the world itself were com-pletely transformed; yet his last papers are as relevant ashis earliest. . . . ”

A bound volume [5] containing all of Schubauer’sscientific publications that could be found (totaling 25)was prepared by colleagues and given to the NBSlibrary. In the preface to this volume, P. S. Klebanoffwrote “On April 30, 1968, Dr. Galen Brandt Schubauerretired from the National Bureau of Standards. Hisassociation with the Bureau was a most fruitful andillustrious one, spanning a period of almost 38 years. . . .His research on boundary layers, flow instability andturbulence not only provided new insight into theseimportant problems, but also provided for new avenuesof approach both theoretical and experimental. Hiswork contributed to the prestige of the National Bureauof Standards and in large measure was responsible forthe stature of the Aerodynamics Section in the scientificcommunity. Many of his publications are widely citedand are regarded as classical references in the field. . . ”

Klebanoff continued, “It is perhaps a comment onman’s nature that the occasion of one’s retirement is oneof the few times that it is considered appropriate toexpress respect and affection. . . . ”

Prepared by Norman E. Mease and James E. Potzick.

Bibliography

[1] Galen B. Schubauer, Air Flow in a Separating Laminar BoundaryLayer, NACA Report No. 527, National Advisory Committee forAeronautics (1935).

[2] Galen B. Schubauer and Hugh L. Dryden, The Effect of Turbu-lence on the Drag of Flat Plates, NACA Report No. 546, Na-tional Advisory Committee for Aeronautics (1935).

[3] Galen B. Schubauer, Effect of Humidity on Hot-Wire Anemome-try, J. Res. Natl. Bur. Stand. 15, 575-578 (1935).

[4] Galen B. Schubauer and Harold K. Skramstad, LaminarBoundary-Layer Oscillations and Transition on a Flat Plate,J. Res. Natl. Bur. Stand. 38, 251-292 (1947).

[5] G. B. Schubauer, Collected Papers, 1933–1968. Availablethrough the library of the National Institute of Standards andTechnology.

Fig. 2. Galen B. Schubauer.

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