NACA-TN-4018

Abstract

This report deals with the problem of the influence of free-stream turbulence on the transfer of heat from a cylinder in forced convection at very low Mach numbers but at large Reynolds numbers. In particular, an attempt is made to determine whether the sole influence of turbulence is to shift the point of laminar separation in subcritical flow, or the point of transition in supercritical flow, and thus effect a change in the rate of heat transfer. It is shown that this is not the case and that varying the free-stream turbulence affects local rates of heat transfer. <br>The results are presented in the form of curves of&#160;Nu against Re and St against Re (where Nu is Nusselt number, Re is Reynolds number, and St is Stanton number, all based on mean properties); each curve has been plotted for a constant value of turbulence intensity, the temperature effects having been eliminated by the use of integral mean values of the thermodynamic properties of the fluid over the boundary layer. The experimental results unmistakably demonstrate that in the subcritical range the Nusselt number is not independent of the intensity of turbulence. <br>An attempt to correlate the variation of the Nusselt number at constant Reynolds and Prandtl numbers with the Taylor parameter A does not lead to a useful result. Thus, the intensity of turbulence seems to be the primary parameter, at least in the small range of scale values L = 0.162 to 0.574 centimeter covered. <br>This paper presents a survey of related analytical and experimental work and shows that the present tentative conclusions find ample support in previous investigations. It is also pointed out that an oscillation in the free stream has a different effect on the velocity profile and on the temperature profile in the boundary -layer which may cause departures from Reynolds analogy, inasmuch as the latter is proved for steady flow only. Hence, it is thought that the Reynolds analogy is a limiting law for zero turbulence intensity.