Quasi-streamwise Vortices Research Articles

Simulating turbulent flow over thin element and flat valley V-shaped riblets

The time-dependent flow over riblets of various shapes is calculated using a control volume finite element simulation technique and assuming a model of the near-wall viscous flow region of a turbulent boundary layer. The cross-sectional shapes considered include thin element blade riblets and saw-toothed riblets (but with flat valleys), which are similar to the more common continuous V-shaped riblets. Turbulence statistics derived from the calculations include skewness and flatness factors. It is concluded that to achieve maximum drag reduction, riblets not only need to ensure a thicker than usual viscous laminar-like flow in their grooves, but also the transient momentum exchange must be minimized between this inner region and the larger-scale outer motions associated with quasistreamwise vortices.

Direct numerical simulation of near-interface turbulence in coupled gas-liquid flow

Turbulence structures near the interface between two flowing fluids have been resolved by direct numerical simulation. As a first step the interface has been kept flat, corresponding closely to the recent gas-liquid flow experiments of Rashidi and Banerjee [Phys. Fluids A 2, 1827 (1990)], with the fluids coupled through continuity of velocity and shear stress boundary conditions. For density ratios between the fluids typical of air and water, the turbulence characteristics on the gas side are quite similar to that in wall regions. The liquid side shows larger velocity fluctuations close to the interface and ejections originate closer to the interface. The mean velocity distribution, turbulence intensities, Reynolds stress and various other statistical measures are significantly altered compared to those in the wall region of channel flows. Quasi-streamwise vortices form in the areas between high and low shear stress on both sides of the interface. At any given instant, about a fifth of these appear to be coupled across the interface. Whether the others are, but the coupling is too weak for the detection technique used, or were coupled previously remains an open question. In any case, sweeps usually occur on the high shear stress side of these vortices and ejections on the low shear stress side. Significant coupling exists across the interface with over 60% of the Reynolds stress in the region close to the interface being associated with coupled events –the main coupling coming through gas ejection-liquid ejection events over low shear stress regions, with a lesser but significant number of gas sweep-liquid sweep events over high shear stress regions.

Mechanism of Self-Sustenance and Regeneration of Quasi-Streamwise Vortices of Near-Wall Turbulence.

Quasi-streamwise vortices observed in near-wall turbulence arranged in a typical zigzag pattern in the streamwise direction are investigated by analyzing the flow field of data bases obtained by both high-resolution LES and numerical integration of the parabolized Navier-Stokes equation for channel flow, focusing attention on the mechanism of self-sustenance and regeneration. It has been revealed that quasi-streamwise vortices are inherently self-sustaining. Namely, they achieve their streamwise vorticity by stretching the spanwise vortices of mean flow by means of self induced velocity fields. The analysis of the regeneration mechanism also shows that the quasi-streamwise vortices have inherent potential to regenerate themselves using their own velocity fields. The approximate quasi-three-dimensional simulation based on the parabolized Navier-Stokes equation confirms the conclusions of the analysis of the LES data base.

Subgrid-scale energy transfer and near-wall turbulence structure

Conditional averages of the velocity field, subgrid-scale (SGS) stresses and SGS dissipation are calculated using the velocity fields obtained from the DNS of plane channel flow. The detection criteria isolate the coherent turbulent structures that contribute most strongly to the energy transfer between the large, resolved scales and the subgrid, unresolved, ones. Separate averages are computed for forward and backward scatter. The interscale energy transfer is found to be strongly correlated with the presence of the turbulent structures typical of wall-bounded flows: quasi-streamwise and hairpin vortices, sweeps and ejections. In the buffer layer, strong SGS dissipation is observed near lifted shear layers; the forward scatter is associated with ejections, the backscatter with sweeps. Both backward and forward scatter occur in close proximity to longitudinal vortices that form a very shallow angle to the wall. Further away from the solid boundary, in the logarithmic region and beyond, both forward and backward energy transfer are associated prevalently with ejections. Eddy viscosity models do not predict the three-dimensional structure of these events adequately, while scale-similar models reproduce the correlation between the large-scale coherent structures and the SGS events more accurately.

Near-Wall Bursting Phenomena and Quasi-Streamwise Vortex Models.

Near-wall bursting phenomena have been detected both by the VITA technique and by a quadrant analysis. In addition to these detection schemes, visual burst detection has been performed in order to characterize the difference between VITA events and Q2 (second quadrant) events. It is shown that the spanwise yaw angle of visualized low-speed streaks for VITA events is greater than for Q2 events. Based on the experimental findings and the Euler equation, quasi-streamwise vortex models are proposed, and the results are compared with the experiments. Fluctuating velocity signals computed from the models are in good agreement with the experimental data for VITA events. Contour plots of the calculated axial velocity, spanwise vorticity and instantaneous Reynolds shear stress are presented. The generation processes of shear layer structures and Q2 and Q4 motions are demonstrated.

Effects of mean flow three dimensionality on turbulent boundary-layer structure

Several experimental studies examining the near-wall structure of three-dimensional turbulent boundary layers are reviewed. The reduction of the Reynolds shear stress by the mean flow three dimensionality is a common feature of all of these flows. The near-wall region (y + < 100) is found to be dominated by quasistreamwise vortices and low- and high-speed streaks similar to two-dimensional boundary layers. However, the development of the vortices and their production of shear stress is modified by the presence of mean flow skewing across the near-wall zone. The implications of these findings are discussed.

Coherent structures and riblets

This work deals with the effect of the riblets on the coherent structures near the wall. The emphasis is put on the genesis of the quasi-streamwise vortices in the presence of the riblets. The quasi-streamwise vortices regenerate by the tilting of wall normal vorticity induced by prevailing structures. This requires a mechanism which leads to a temporal streamwise dependence near the elongated flow structures and to a subsequent formation of new wall normal vorticity. It is suggested here that the action of existing quasi-streamwise vortices on the sidewalls of wall normal vorticity may create a local, streamwise dependent spanwise velocity and therefore, a secondary wall normal vorticity field. A preliminary analysis of the set-up and the time and space development of this secondary three-dimensional flow associated with the regeneration mechanism, is given. An attempt is made, in order to explain the drag reduction performed by the riblets through an intermittent model, based on the protrusion height. Logical estimates of the amount of drag reduction are obtained. The differences between the mechanism suggested here and those based on forced control experiments are also discussed.

Turbulent boundary layer over a smooth wall with widely separated transverse square cavities

Laser-Doppler velocimetry (LDV) measurements and flow visualizations are used to study a turbulent boundary layer over a smooth wall with transverse square cavities at two values of the momentum thickness Reynolds number (Rθ=400 and 1300). The cavities are spaced 20 element widths apart in the streamwise direction. Flow visualizations reveal a significant communication between the cavities and the overlying shear layer, with frequent inflows and ejections of fluid to and from cavities. There is evidence to suggest that quasi-streamwise near-wall vortices are responsible for the ejections of fluid out of the cavities. The wall shear stress, which is measured accurately, increases sharply immediately downstream of the cavity. This increase is followed by a sudden decrease and a slower return to the smooth wall value. Integration of the wall shear stress in the streamwise direction indicates that there is an increase in drag of 3.4% at bothRθ.

On Regeneration of Quasi-Streamwise Vortices of Near Wall-Turbulence.

Regeneration of quasi-streamwise eddies is investigated using a data base obtained by LES of high resolution for a channel flow. It has been revealed that quasi-streamwise eddies are regenerated both in the upstream of the upstream end of a mother eddy and also in the downstream of the tail of a mother eddy. In both cases, parent and young eddies have vorticities of opposite sign and constitute a chain of eddies in the flow direction which is one of the typical coherent structures in the near-wall region. It has also been revealed that the main contribution to the generation of young eddies at the infant stage is the tilting of wall-normal vortices, and that to maintaining matured eddies is stretching.

Low-Reynolds-number effects on near-wall turbulence

Direct numerical simulations of a fully developed turbulent channel flow for two relatively small values of the Reynolds number are used to examine its influence on various turbulence quantities in the near-wall region. The limiting wall behaviour of these quantities indicates important increases in the r.m.s. value of the wall pressure fluctuations and its derivatives, the r.m.s. streamwise vorticity and in the average energy dissipation rate and the Reynolds shear stress. If the normalization is based on the wall shear stress and the kinematic viscosity, these changes are shown to be consistent with an increase in strength – but not the average diameter or average location – of the quasi-streamwise vortices in the buffer region. Evidence of this strengthening is provided by the increased sum of the stretching terms for the meansquare streamwise vorticity. It is also shown that a normalization based on Kolmogorov velocity and lengthscales, defined at the wall, is more appropriate in the near-wall region than scaling on the wall shear stress and kinematic viscosity.

Funnel-shaped vortical structures in wall turbulence

The structure of the turbulent boundary layer in a horizontal open channel was investigated experimentally by means of laser Doppler anemometry (LDA) and by flow visualization synchronized with the LDA. These experiments indicate that the dominant structures in the wall region are large scale streamwise vortices which originate at the wall and grow and expand into the outer flow region. The shape of the vortices is that of an expanding spiral, wound around a funnel which is laid sideways in the direction of flow. Most of the observations of wall turbulence phenomena made over the years, such as quasistreamwise vortices, ejections, and sweeps seem to be part of these funnel-shaped vortices.

Streaky Structures in a Turbulent Square-Duct Flow. Near-Wall Coherent Structures Associated with Low-Speed Streaks.

Streaky structures have been studied in a fully developed turbulent square-duct flow by using combined dye flow visualization-LDV velocity measurement techniques. Velocity fluctuations were characterized both by topological properties of low-speed streaks, such as coalescing and branching, and by the transverse location λ of the visually identified low-speed streaks. Near-wall coherent structures are discussed for each topological type of low-speed streak in terms of the dependence of the most probable values of the velocity fluctuations on λ. It is shown that without coalescing and branching, a spanwise symmetric pair of counter-rotating quasi-streamwise vortices is, statistically speaking, associated with one low-speed streak. However, upstream of the coalescing point and downstream of the branching point of low-speed streaks, an asymmetric single quasi-streamwise vortex may be associated with one low-speed streak.

Near-Wall Bursting Phenomena and Quasi-Streamwise Vortex Models.

Near-wall bursting phenomena have been detected both by the VITA technique and by quadrant analysis. In addition to these detection schemes, visual burst detection has been performed in order to characterize the difference between VITA events and Q2 (second quadrant)events. It is shown that the yaw angle of visualized low-speed streaks for VITA events is greater than for Q2 events. Based on the experimental results and Euler equation, quasi-streamwise vortex models are proposed, and the results are compared with the experiments. Fluctuating velocity signals computed from the models are in good agreement with the experimental data for VITA events. Contour plots of the calculated streamwise velocity, spanwise vorticity and instantaneous Reynolds shear stress are presented. The generation processes of shear layer structures and Q2 and Q4 motions are shown.

Vortex dynamics and the production of Reynolds stress

The physical mechanisms by which the Reynolds shear stress is produced from dynamically evolving vortical structures in the wall region of a direct numerical simulation of turbulent channel flow are explored. The complete set of quasistreamwise vortices are systematically located and tracked through the flow by the locus of the points of intersection of their centres of rotation with the (y, z) numerical grid planes. This approach assures positive identification of vortices of widely differing strengths, including those whose amplitude changes significantly in time. The process of vortex regeneration, and the means by which vortices grow, distort and interact over time are noted. Ensembles of particle paths arriving on fixed planes in the flow are used to represent the physical processes of displacement and acceleration transport (Bernard & Handler 1990a) from which the Reynolds stress is produced. By interweaving the most dynamically significant of the particle paths with the evolving vortical structures, the dynamical role of the vortices in producing Reynolds stress is exposed. This is found to include ejections of low-speed fluid particles by convecting structures and the acceleration and deceleration of fluid particles in the cores of vortices. Sweep dominated Reynolds stress close to the wall appears to be a manifestation of the regeneration process by which new vortices are created in the flow.