Hot-film measurements in a fully developed channel flow have been made in an attempt to gain more insight into the process of Reynolds stress production. The background for this effort is the observation of a certain sequence of events (deceleration, ejection and sweep) in the wall region of turbulent flows by Corino (1965) and Corino & Brodkey (1969). The instantaneous product signal uv was classified according to the sign of its components u and v, and these classified portions were then averaged to obtain their contributions to the Reynolds stress $-\rho\overline{uv} $. The signal was classified into four categories; the two main ones were that with u negative and v positive, which can be associated with the ejection-type motion of Corino & Brodkey (1969), and that with u positive and v negative, associated with the sweep-type motion. It was found that over the wall region investigated, 3·5 [les ] y [les ] 100, these two types of motion give rise to a stress considerably greater than the total Reynolds stress. Two other types of motion, (i) u negative, v negative, corresponding to low-speed fluid deflected towards the wall, and (ii) u positive, v positive, corresponding to high-speed fluid reflected outwards from the wall, were found to account for the ‘excess’ stress produced by the first two categories, which give contributions of opposite sign.The autocorrelations of the classified portions of uv were obtained to determine the relative time scales of these four types of motion. The positive stress producing motions (u < 0, v > 0 and u > 0, v < 0) were found to have significantly larger time scales than the negative stress producing motions (u < 0, v < 0 and u > 0, v > 0). It was further surmised that turbulent energy dissipation is associated with the Reynolds stress producing motions, since these result in localized shear regions in which the dissipation is several orders of magnitude greater than the average dissipation at the wall.

The objective of this study is to investigate for turbulent flow the fluid motions very near a solid boundary, and to create a physical picture which relates these motions to turbulence generation and transport processes. An experimental technique was developed which permitted detailed observations of the regions very near a pipe wall, including the viscous sublayer, without requiring the introduction of any injection or measuring device into the flow. This technique involved suspending solid particles of colloidal size in a liquid, and photographing their motions with a high-speed motion picture camera moving with the flow. To provide greater detail, the field of view was magnified.Fluid motions were observed to change in character with distance from the wall. The sublayer was continuously disturbed by small-scale velocity fluctuations of low magnitude and periodically disturbed by fluid elements which penetrated into the region from positions further removed from the wall. From a thin region adjacent to the sublayer, fluid elements were periodically ejected outward toward the centreline. Often there was associated with these events a zone of high shear at the interface between the mean flow and the decelerated region that gave rise to the ejected element. When the ejected element entered this shear zone, it interacted with the mean flow and created intense, chaotic velocity fluctuations. These ejections and resulting fluctuations were the most important feature of the wall region, and are believed to be a factor in the generation and maintenance of turbulence.

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A three-dimensional Particle Tracking Velocimetry (3-D PTV) technique has been developed to provide timeresolved, three-dimensional velocity field measurements throughout a finite volume. This technique offers many advantages for fundamental research in turbulence and applied research in areas such as mixing and combustion. The data acquired in 3-D PTV is a time sequence of stereo images of flow tracer particles suspended in the fluid. In this paper, the implementation of the technique is discussed in detail, as well as the results of an extensive statistical investigation of the performance of the algorithms. The technique has been optimized to allow fully automatic processing of long sequences of image pairs in a computationally efficient manner, hereby providing a viable, practical tool for the study of complex flows. List of symbols x, y, zParticle position u, v, w Particle velocity 1 Introduction A compelling goal of research into the fundamentals of turbulence is to understand the mechanistic aspects of production and dissipation. It is believed that vorticity is a key element in the turbulence cycle and must be measured to provide a basis for that understanding. Because the vorticity vector is defined in terms of spatial derivatives of the velocity field, its measurement is extremely challenging. One measurement method is the direct measurement of the rotation velocity of particles in the flow; however, this technique [Frish and Webb (1981) adaptable to full-field measurements. Simultaneous multi-point measurements using laser Doppler anemometry (LDA) or other probe techniques would be uneconomical [Vukoslavcevic et al. (1991) ]. Two-dimensional components of vorticity over a plane can be measured using a multipoint technique known as LIPA (laser induced photochemical anemometry) [Falco and Nocera (199z)]. Extension of LIPA to three-dimensional velocity vector measurement over a limited volume appears feasible. The most direct and least complex approach, however, appears to be the measurement of the velocity vectors by particle tracking velocimetry (PTV) from stereoscopic image pairs obtained over the full flow field. Because of the large amount of data that must be acquired and processed, a fully automated and time efficient system is necessary. Once perfected, such a system can be used for practical research problems. Indeed, we are currently investigating two important processes, one being the dynamics of fluid motions in the cylinder of an internal combustion engine during the intake stroke, which is helping to understand the relation between engine design and combustion [Kent et al. (1989)]. The other is the detailed complexities of the flow in an impeller mixing vessel (with applications to biotechnology and chemical manufacturing processes). On a more fundamental basis, we plan to use 3-D PTV for studies on the turbulence mechanism and the role of coherent structures in mixing and combustion processes. It is clear that a robust, accurate, and fully automated data extraction technique is...

Earlier measurements of the contribution of four distinct classes of motions, i.e. (u < 0, v > 0), (u > 0, v < 0), (u < 0, v < 0) and (u > 0, v > 0), to the Reynolds stress $-\rho \overline{uv}$ in the wall region of a bounded turbulent shear flow have been extended. These classes were obtained by truncating the u and v signals about zero. Various statistical properties of the truncated streamwise and normal velocity components u and v and of their product uv have been determined in an attempt to characterize quantitatively the motions in this flow. Average values and probability density distributions both of the truncated and untruncated signals have been taken.

A study was conducted in which analytical, computational, and experimental measurements combined with analysis were made to characterize the local energy dissipation rate in a variety of conditions, vessels, and geometries that animal cells would encounter in typical bioprocessing situations. With no gas-liquid interfaces present, as expected, the local energy dissipation rate is typically orders of magnitude lower than what has been experimentally demonstrated to catastrophically damage typically used, suspended animal cells. However, local energy dissipation rates shown to remove animal cells from microcarriers are achievable under some normal operating conditions and geometries. Whether local energy dissipation rates created under typical operating conditions can have nonlethal effects is still an open question and currently under investigation. Whether the sensitivity of other, nontypical, suspended animal cells such as cells obtained directly from tissue (primary cells) and clusters of cells, such as islets, are more sensitive than the typically used cells is also still under investigation.

A visual study of a turbulent boundary-layer flow was conducted by photographing the motions of small tracer particles using a stereoscopic medium-speed camera system moving with the flow. In some experiments, dye injection at the leading edge of the flat plate helped to delineate the outer edge of the boundary layer. The technique allowed the three-dimensional aspects of the flow to be studied in some detail, and in particular allowed axial vortex motions in the wall region to be identified.The flow was found to exhibit three characteristic regions which can be roughly divided into the wall and outer regions of the boundary layer and an irrotational region, unmarked by dye, outside the instantaneous edge of the boundary layer. Briefly, the outer region of the boundary layer was dominated by transverse vortex motions that formed as a result of an interaction between low-speed and high-speed (sweep) fluid elements in that region. The present results clearly show that bulges in the edge of the boundary layer are associated with transverse vortex motions. In addition, the transverse vortex motions appear to induce massive inflows of fluid from the irrotational region deep into the outer region of the boundary layer. The outer edge of the boundary layer thus becomes further contorted, contributing to the intermittency of the region. Furthermore, the outer-region motions give rise to the conditions necessary for the dominant wall-region activity of ejections and axial vortex motions. It is not the energetic wall-region ejections that move to the outer region and give rise to the contorted edge of the boundary layer as has been suggested by others.The wall-region axial vortex motions were intense and lasted for a time short compared with the lifetime of outer-region transverse vortex motions. The present results strongly suggest that wall-region vortex motions are a result of interaction between the incoming higher-speed fluid from the outer region of the boundary layer and the outflowing low-speed wall-region fluid. This is in direct contrast to all models that suggest that axial vortex pairs in the wall region are the factor that gives rise to the outflow of low-speed fluid trapped between.Although all the elements necessary to make up a horseshoe vortex structure riding along the wall were present, such a composite was not observed. However, this could be visualized as a possible model to represent the ensemble average of the flow.Finally, the massive inflows from the irrotational region were observed to precede the appearance of low- and high-speed fluid elements in the boundary layer, thus completing the deterministic cycle of individual coherent events.

It is now well established that coherent structures exist in turbulent shear flows. It should be possible to recognize these in the turbulence signals and to program a computer to extract and ensemble average the corresponding portions of the signals in order to obtain the characteristics of the structures. In this work only the u-signal patterns are recognized, using several simple criteria; simultaneously, however, the v or w signals as well as uv or uw are also processed. It is found that simple signal shapes describe the turbulence structures on the average. The u-signal pattern consists of a gradual deceleration from a local maximum followed by a strong acceleration. This pattern is found in over 65% of the total sample in the region of high Reynolds-stress production. The v signal is found to be approximately 180° out of phase with the u signal. These signal shapes can be easily associated with the coherent structures that have been observed visually. Their details have been enhanced by quadrant truncating. These results are compared with randomly generated signals processed by the same method.