Abstract
The study of flows with a high degree of turbulence in boundary layers, near-wall jets, gas curtains, separated flows behind various obstacles, as well as during combustion is of great importance for increasing energy efficiency of the flow around various elements in the ducts of gas-dynamic installations. This paper gives some general characteristics of experimental work on the study of friction and heat transfer on a smooth surface, in near-wall jets, and gas curtains under conditions of increased free-stream turbulence. Taking into account the significant effect of high external turbulence on dynamics and heat transfer of separated flows, a similar effect on the flow behind various obstacles is analyzed. First of all, the classical cases of flow separation behind a single backward-facing step and a rib are considered. Then, more complex cases of the flow around a rib oriented at different angles to the flow are analyzed, as well as a system of ribs and a transverse trench with straight and inclined walls in a turbulent flow around them. The features of separated flow in a turbulized stream around a cylinder, leading to an increase in the width of the vortex wake, frequency of vortex separation, and increase in the average heat transfer coefficient are analyzed. The experimental results of the author are compared with data of other researchers. The structure of separated flow at high turbulence—characteristic dimensions of the separation region, parameters of the mixing layer, and pressure distribution—are compared with the conditions of low-turbulent flow. Much attention is paid to thermal characteristics: temperature profiles across the shear layer, temperature distributions over the surface, and local and average heat transfer coefficients. It is shown that external turbulence has a much stronger effect on the separated flow than on the boundary layer on a flat surface. For separated flows, its intensifying effect on heat transfer is more pronounced behind a rib than behind a step. The factor of heat transfer intensification by external turbulence is most pronounced in the transverse cavity and in the system of ribs.
Highlights
In the literature, there are many publications dealing with the study of laminar and turbulent boundary layers under conditions of increased turbulence of the incoming flow in the channels, for example, the monograph [1,2] and papers [3,4,5,6,7,8,9,10,11,12,13,14]
This conclusion is confirmed in the experiments of [15,16], where it was shown that the thickness of the thermal layer under conditions of external turbulence can change very strongly, while the dynamic characteristics remain almost unchanged
It is interesting that in some publications, it was noted for laminar boundary layers that external turbulence in the transitional regime, instead of the Tollmien–Schlichting instability characteristic of natural turbulence, causes the Kelvin–Helmholtz instability, which leads to the so-called upper bypass transfer [6,17]
Summary
There are many publications dealing with the study of laminar and turbulent boundary layers under conditions of increased turbulence of the incoming flow in the channels, for example, the monograph [1,2] and papers [3,4,5,6,7,8,9,10,11,12,13,14]. To generalize the experimental data on friction and heat transfer coefficients, in addition to the generally accepted Prandtl and Reynolds similarity numbers, Dyban and Epik [2] proposed to use the effective Reynolds number of turbulence at the external border of the boundary layer Reff = νeff /ν, where νeff is the effective kinematic viscosity at the beginning of laminar-turbulent transition. This parameter is quite physically justified, since it characterizes the development of velocity pulsations in the boundary layer. From a practical point of view, the use of the proposed parameter is not always possible
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