Flow regimes in a single dimple on the channel surface

Abstract

The boundaries of the domains of existence of flow regimes past single dimples made as spherical segments on a flat plate are determined with the use of available experimental results. Regimes of a diffuser-confuser flow, a horseshoe vortex, and a tornado-like vortex in the dimple are considered. Neither a horseshoe vortex nor a tornado-like vortex is observed in dimples with the relative depth smaller than 0.1. Transformations from the diffuser-confuser flow regime to the horseshoe vortex regime and from the horseshoe vortex flow to the tornado-like vortex flow are found to depend not only on the Reynolds number, but also on the relative depth of the spherical segment. Dependences for determining the boundaries of the regime existence domains are proposed, and parameters at which the experimental results can be generalized are given.Key words: dimple, vortex structures, horseshoe and tornado-like vortices, enhancement of heat transfer. Introduction.A large amount of experimental data on specific features of the flow structure and heat transfer in the flow past surfaces with spherical cavities (dimples) has been accumulated. There are some summary papers where numerous aspects of vortex formation in dimples under the action of a large number of factors were considered [1-6]. It was found in the first experiments that the increase in heat transfer in the flow past a surface with dimples made as spherical segments is accompanied by an insignificant relative increase in hydraulic resistance [7]. Large-scale dynamic vortex structures observed in a wide range of flow regimes are formed inside the dimples and in their vicinity. Despite a large number of publications dealing with using surfaces with dimples, however, some disputable issues have not been resolved (hypothesis of the tornado-like mechanism of heat-transfer enhancement [7,8], role of dimples in flows with high turbulence levels [9, 10], and determining the boundary between the domains of existence of separated and non-separated flows past spherical cavities [11,12]). This is explained by the complexity of the phenomenon, which is affected not only by the flow velocity, but also by the dimple size, shapes of its edges, character and thickness of the boundary layer at the dimple entrance, and channel height.Here, we make an attempt to determine the boundaries of the domains of existence of flow regimes in the dimple, depending on the external flow velocity and on the geometric characteristics of the dimple, by means of analyzing available experimental data. Similar studies were performed previously (see, e.g., [11,13]), but the previous results can now be refined because a large amount of new data appeared.Regimes of the Flow Past Dimples. The following flow regimes past a surface with a dimple can be identified. As low velocities, the flow has a diffuser-confuser character. The streamlines passing close to the dimple become curved. As the flow velocity is increased, the flow becomes separated near the leading edge of the dimple, forming a recirculation zone,...

Mechanics of drag reduction by shallow dimples in channel flow

“…Regions of flow separation have commonly been observed at the upstream edges of dimples and their size can vary with Reynolds number. 3,6,8 Increasing Reynolds number often has the effect of reducing the size of the separated flow region in dimples. 8,11 This reduction in the size of the flow separation zone with increasing Reynolds number also occurs in other types of flows such as flows over backward facing steps at sufficiently high Reynolds numbers 40,41 and for boundary layer separation bubbles at lower Reynolds numbers.…”

“…Most significant of these parameters is the dimple depth, often non-dimensionalized by the dimple diameter. The effect of the dimple depth to diameter ratio has been well studied both experimentally [1][2][3][4][5] and numerically. 6,7 Flow visualization 1,8 and numerical studies 6 show that dimples with depth to diameter ratios greater than 10% result in the generation of vertical and streamwise vortices.…”

“…Sharp edged dimples encourage flow separation as the flow enters the dimple depression, further increasing mixing and heat transfer. 3,8 The majority of these studies involve dimples in an internal flow environment such as in a channel or pipe. Numerous empirical relations have been proposed relating practically useful parameters such as Nusselt numbers and friction factors with the dimple depth to diameter ratio, Reynolds number, inlet turbulence intensity, channel height, and even the channel aspect ratio.…”

Flow Control of a Rotating Cylinder by the Use of a Structured Surface: A Visualization of Flow Structures

“…The flow structures for h/d < 0.1 are considered to be independent of Re d . 10 For a relative depth h/d > 0.1 and a small Reynolds number Re d the flow structure can remain in the diffuser -confuser shape depicted in figure 5(a). With an increasing Reynolds number, a horseshoe j H vortex is formed inside the dimple.…”

Numerical simulation of the turbulent air flow in the narrow channel with a heated wall and a spherical dimple placed on it for vortex heat transfer enhancement depending on the dimple depth

“…Thus, the problem of designing a surface (relief) rational in terms of heat transfer and hydraulic resistance is multiparameter in character. Interest to flow regime maps is shown by estimators and experimentalists [34][35][36][37][38][39]. (3) One of the remarkable numerical modeling results is associated with changing the separated flow structure to the symmetric monotornado-like one when increasing the depth of the spherical dimple that causes heat transfer to enhance spasmodically inside the dimple and in its wake.…”