Some Developments in Boundary Layer Research in the Past Thirty Years

Abstract

SummaryBoundary layer theory is the cornerstone of our knowledge of the flow of air and other fluids of small viscosity under circumstances of interest in many engineering applications, especially also in aeronautics. Many complex problems in aerodynamics, as for instance the problem of skin friction, which was theoretically attacked very early by F. W. Lanchester, have been clarified by studying the flow within the boundary layer and its effects on the general flow around the body.Research work on boundary layers, as started by Prandtl in 1904. was for the first twenty years—up to Prandtl's Wilbur Wright Memorial Lecture to the Royal Aeronautical Society in 1927—almost entirely restricted to Prandtl's Institute at Göttingen. But since about 1930 boundary layer theory has been generally accepted, and in the past thirty years there has been an almost exponential rise of the number of contributors to its further development.The author tries to trace certain lines, along which this important branch of modern fluid dynamics has developed in the past thirty years. In this connection the following topics are treated to some extent:I.Transition from laminar to turbulent flow.II.Boundary layer control for high lift and low drag of aerofoils.III.Aerodynamic heating at high speed (high Mach numbers).IV.Boundary layer efff cts on swept wings and on rotating bodies.I. The theoretical investigations of the problem of transition start from Reynolds' and Lord Rayleigh's hypothesis of the instability of laminar flow. After many unsuccessful attempts Tollmien, 1930, finally succeeded in calculating the critical Reynolds number for the boundary layer on a flat plate. More than ten years later Tollmien's stability theory was completely confirmed by very careful experiments of Dryden and his co-workers.II. After many wind tunnel experiments the investigations of boundary layer control for high lift of aerofoils led to the construction of two aeroplanes with boundary layer suction at the aerodynamische versuchsanstalt gottingen, in 1938, which were quite successful. later on, also, a considerable reduction of the skin friction of aerofoils was obtained with the advent of the laminar flow aerofoil.III. In flow at high Mach numbers the velocity boundary layer is accompanied by a thermal boundary layer which is caused by frictional heating. The large increase in the temperature of a solid surface in a high speed stream which can be calculated from boundary layer theory only, poses a serious problem to aeronautical engineers (“thermal barrier”).IV. The aerodynamic characteristics of swept wings and Delta wings are largely governed by the behaviour of their boundary layer. Some of the draw-backs of such wing plan forms can be remedied by boundary layer control, as for instance by a “boundary layer fence.” For turbo-machines the influence of the centrifugal forces on the boundary layer plays an important role for their aerodynamic coefficients.