OSCILLATIONS DURING AIR FLOW NEAR DENTS ON A FLAT SURFACE

The purpose of the work is to evaluate the intensification of heat transfer using various types of recesses when air flows around heat transfer surfaces with possible minimization of pressure losses. The creation of compact devices for space and the armed forces requires solving similar problems. One of the proposed methods is the use of recesses on streamlined heat transfer surfaces. This paper presents the results of experimental studies of flow around surfaces with depressions. Two types of recesses were considered: cylindrical and hemispherical segments. The influence of flow velocity on the formation of vortex structures when flowing around surfaces with depressions was studied. It was shown that at low speeds the flow has a diffuser and confuser character. As the speed increases at the inlet edge of the notch, the flow was been interrupted and a horseshoe-shaped vortex was formed in the notch. In this case, the boundary layer above the depression periodically collapses, and its lower end jumps from one half of the depression to the other, breaks off and turns into a Karman trace. With a further increase in speed, one of the ends of the horseshoe-shaped vortex rises above the depression, turning into a columnar one. The nature of the flow over the depression was been analyzed. It was been noted that part of the shear layer is deflected towards the excavation and hits its rear (aft) wall. The location of the impact is the source of sound vibrations. It was shown, that when the frequency of any harmonic of these oscillations coincides with the frequency of shear layers vortex structures, these two phenomena begin to reinforce each other. Flow fluctuations near and inside the excavation could be divided into classes. Hydrodynamic vibrations arising due to the instability of the shear layer. Oscillations resulting from the interaction of standing pressure waves in the depression and acoustic waves emitted from places where the surface profile changes sharply. Oscillations, when a sharp change in the shape of the surface causes the formation of vortices. It was shown, that the sound pressure caused by the presence of depressions decreases with increasing boundary layer thickness.