8 results
Role of flow reversals in transition to turbulence and relaminarization of pulsatile flows
- Joan Gomez, Huidan Yu, Yiannis Andreopoulos
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- Journal:
- Journal of Fluid Mechanics / Volume 917 / 25 June 2021
- Published online by Cambridge University Press:
- 26 April 2021, A27
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The instability and transition to turbulence and its evolution in pulsatile flows, which involve reverse flows and unsteady flow separations, is the primary focus of this experimental work. A piston driven by a programmable DC servo motor was used to set-up a water flow system and provide the pulsation characteristics. Time-resolved particle image velocimetry data were acquired in a refractive index matching set-up by using a continuous wave laser and a high-frame-rate digital camera. The position of the piston was continuously recorded by a laser proximity sensor. Five different experiments were carried out with Reynolds numbers in the range of 535–4825 and Womersley numbers from 11.91 to 23.82. The non-stationarity of the data was addressed by incorporating trend removal methods involving low- and high-pass filtering of the data, and using empirical mode decomposition together with the relevant Hilbert–Huang transform to determine the intrinsic mode functions. This latter method is more appropriate for nonlinear and non-stationary cases, for which traditional analysis involving classical Fourier decomposition is not directly applicable. It was found that transition to turbulence is a spontaneous event covering the whole near-wall region. The instantaneous vorticity profiles show the development of a large-scale ring-like attached wall vortical layer (WVL) with smaller vortices of higher frequencies than the pulsation frequency superimposed, which point to a shear layer Kelvin–Helmholtz (K–H) type of instability. Inflectional instability leads to flow separation and the formation of a major roll-up structure with the K–H vortices superimposed. This structure breaks down in the azimuthal direction into smaller turbulence patches with vortical content, which appears to be the prevailing structural content of the flow at each investigated Reynolds number (Re). At higher Re numbers, the strength and extent of the vortices are larger and substantial disturbances appear in the free stream region of the flow, which are typical of pipe flows at transitional Re numbers. Turbulence appears to be produced at the locations of maximum or minimum vorticity within the attached WVL, in the ridges between the K–H vortices around the separated WVL and the upstream side of the secondary vortex where the flow impinges on the wall. This wall turbulence breaks away into the middle section of the pipe, at approximately $Re \ge 2200$, by strong eruptions of the K–H vortices.
Transient force generation during impulsive rotation of wall-mounted panels
- Alexis Pierides, Amir Elzawawy, Yiannis Andreopoulos
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- Journal:
- Journal of Fluid Mechanics / Volume 721 / 25 April 2013
- Published online by Cambridge University Press:
- 13 March 2013, pp. 403-437
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Square and triangular shape actuator panels mounted on the wall of a wind tunnel beneath an air flow have been impulsively rotated with an angular velocity between 3 and $26~\mathrm{rad} ~{\mathrm{s} }^{- 1} $. A custom-designed balance was used to measure the time-dependent lift and drag forces during the deployment of the actuator, the position of which was monitored by a digital encoder. The measured forces have been compensated for inertia effects which are significant. The results indicated that all lift and drag force coefficients during the transient deployment are different than the corresponding coefficients under stationary conditions at the same deployment angle. It was found that these dynamic effects are augmented with increasing velocity ratio $\mathit{Str}$. The square actuator was found to have better aerodynamic performance than the triangular ones. Additional experiments within different boundary layers reveal that the generated unsteady forces on the moving panels are affected by the characteristics of the incoming boundary layers. The results showed that the thinner the boundary layer is the higher the forces are. Time-resolved flow visualization studies indicated that during the deployment of the panel the upstream turbulent boundary layer structures and the free stream fluid are decelerated and squeezed in the longitudinal direction as they approach the moving plate. A very thin and highly sheared wall layer develops over the moving panel, it generates a substantial amount of vorticity and it subsequently separates from the three edges of the panel to form a large-scale ring-like vortical structure which is responsible for the transient augmentation of the aerodynamic forces. This structure consists of wrapped around separated shear layers which contain pockets of compressed eddies and free stream fluid originated in the upstream incoming boundary layer and free stream. A horseshoe vortex starts to form over the moving plate and during the final stages of deployment it has been moved upstream while the incoming boundary layer turbulent structures are pushed and diverted upwards.
Chapter 5 - Cross-layer Optimized Video Streaming over Wireless Multi-hop Mesh Networks
- from Part II - 802.11 Quality of Service
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- By Yiannis Andreopoulos, Queen Mary University of London, Nicholas Mastronarde, University of California, Los Angeles, Mihaela van der Schaar, University of California, Los Angeles
- Edited by Benny Bing, Georgia Institute of Technology
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- Book:
- Emerging Technologies in Wireless LANs
- Published online:
- 10 December 2009
- Print publication:
- 05 November 2007, pp 105-130
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Summary
The proliferation of wireless multi-hop communication infrastructures in office or residential environments depends on their ability to support a variety of emerging applications requiring real-time video transmission between stations located across the network. We propose an integrated cross-layer optimization algorithm aimed at maximizing the decoded video quality of delay-constrained streaming in a multi-hop wireless mesh network that supports quality-of-service (QoS). The key principle of our algorithm lays in the synergistic optimization of different control parameters at each node of the multi-hop network, across the protocol layers - application, network, medium access control (MAC) and physical (PHY) layers, as well as end-to-end, across the various nodes. To drive this optimization, we assume an overlay network infrastructure, which is able to convey information on the conditions of each link. Various scenarios that perform the integrated optimization using different levels (“horizons”) of information about the network status are examined. The differences between several optimization scenarios in terms of decoded video quality and required streaming complexity are quantified. Our results demonstrate the merits and the need for cross-layer optimization in order to provide an efficient solution for real-time video transmission using existing protocols and infrastructures. In addition, they provide important insights for future protocol and system design targeted at enhanced video streaming support across wireless mesh networks.
Introduction
Wireless mesh networks are built based on a mixture of fixed and mobile nodes interconnected via wireless links to form a multi-hop ad-hoc network.
Velocity and vorticity in weakly compressible isotropic turbulence under longitudinal expansive straining
- SAVVAS XANTHOS, MINWEI GONG, YIANNIS ANDREOPOULOS
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- Journal:
- Journal of Fluid Mechanics / Volume 584 / 10 August 2007
- Published online by Cambridge University Press:
- 25 July 2007, pp. 301-335
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The response of homogeneous and isotropic turbulence to streamwise straining action provided by planar expansion waves has been studied experimentally in the CCNY shock tube research facility at several Reynolds numbers. The reflection of a propagating shock wave at the open endwall of the shock tube generated an expansion fan travelling upstream and interacting with the induced flow behind the incident shock wave which has gone through a turbulence generating grid.
A custom-made hot-wire vorticity probe was designed and developed capable of measuring the time-dependent highly fluctuating three-dimensional velocity and vorticity vectors, and associated total temperature, in non-isothermal and inhomogeneous flows with reasonable spatial and temporal resolution. These measurements allowed the computations of the vorticity stretching/tilting terms, vorticity generation through dilatation terms, full dissipation rate of kinetic energy term and full rate-of-strain tensor. The longitudinal size of the straining zone was substantial so that measurements within it were possible. The flow accelerated from a Mach number of 0.23 to about 0.56, a value which is more than twice the initial one.
Although the average value of the applied straining was only between S11 = 130 s−1 and S11 = 240 s−1 and the gradient Mach number was no more than 0.226, the amplitude of fluctuations of the strain rate S11 were of the order of 4000 s−1 before the application of straining and were reduced by about 2.5 times downstream of the interaction. This characteristic of high-amplitude bursts and the intermittent behaviour of the flow play a significant role in the dynamics of turbulence.
One of the most remarkable features of the suppression of turbulence is that this process peaks shortly after the application of the straining where the pressure gradient is substantial. It was also found that the total enthalpy variation follows very closely the temporal gradient of pressure within the straining region and peaks at the same location as the pressure gradient.
Attenuation of longitudinal velocity fluctuations has been observed in all experiments. It appears that this attenuation depends strongly on the characteristics of the incoming turbulence for a given straining strength and flow Mach number. The present results clearly show that in most of the cases, attenuation occurs at large times or distances from the turbulence generating grids where length scales of the incoming flow are high and turbulence intensities are low. Thus, large eddies with low-velocity fluctuations are affected the most by the interaction with the expansion waves. Spectral analysis has indicated that attenuation of fluctuations is not the same across all wavenumbers of the spectrum. The magnitude of attenuation appears to be higher in cases of finer mesh grids.
Internal Flow – Concepts and Applications. By E. M. GREITZER, C. S. TAN & M. B. GRAF. Cambridge University Press, 2004. 707 pp. ISBN 0 521 34393 3. £ 70.00
- YIANNIS ANDREOPOULOS
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- Journal:
- Journal of Fluid Mechanics / Volume 533 / 25 June 2005
- Published online by Cambridge University Press:
- 15 June 2005, pp. 408-409
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Studies of interactions of a propagating shock wave with decaying grid turbulence: velocity and vorticity fields
- JUAN H. AGUI, GEORGE BRIASSULIS, YIANNIS ANDREOPOULOS
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- Journal:
- Journal of Fluid Mechanics / Volume 524 / 10 February 2005
- Published online by Cambridge University Press:
- 09 February 2005, pp. 143-195
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The unsteady interaction of a moving shock wave with nearly homogeneous and isotropic decaying compressible turbulence has been studied experimentally in a large-scale shock tube facility. Rectangular grids of various mesh sizes were used to generate turbulence with Reynolds numbers based on Taylor's microscale ranging from 260 to 1300. The interaction has been investigated by measuring the three-dimensional velocity and vorticity vectors, the full velocity gradient and rate-of-strain tensors with instrumentation of high temporal and spatial resolution. This allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. The time-dependent signals of enstrophy, vortex stretching/tilting vector and dilatational stretching vector were found to exhibit a rather strong intermittent behaviour which is characterized by high-amplitude bursts with values up to 8 times their r.m.s. within periods of less violent and longer lived events. Several of these bursts are evident in all the signals, suggesting the existence of a dynamical flow phenomenon as a common cause. Fluctuations of all velocity gradients in the longitudinal direction are amplified significantly downstream of the interaction. Fluctuations of the velocity gradients in the lateral directions show no change or a minor reduction through the interaction. Root mean square values of the lateral vorticity components indicate a 25% amplification on average, which appears to be very weakly dependent on the shock strength. The transmission of the longitudinal vorticity fluctuations through the shock appears to be less affected by the interaction than the fluctuations of the lateral components. Non-dissipative vortex tubes and irrotational dissipative motions are more intense in the region downstream of the shock. There is also a significant increase in the number of events with intense rotational and dissipative motions. Integral length scales and Taylor's microscales were reduced after the interaction with the shock in all investigated flow cases. The integral length scales in the lateral direction increase at low Mach numbers and decrease during strong interactions. It appears that in the weakest of the present interactions, turbulent eddies are compressed drastically in the longitudinal direction while their extent in the normal direction remains relatively the same. As the shock strength increases the lateral integral length scales increase while the longitudinal ones decrease. At the strongest interaction of the present flow cases turbulent eddies are compressed in both directions. However, even at the highest Mach number the issue is more complicated since amplification of the lateral scales has been observed in flows with fine grids. Thus the outcome of the interaction strongly depends on the initial conditions.
An experimental study of the dissipative and vortical motion in turbulent boundary layers
- YIANNIS ANDREOPOULOS, ANANT HONKAN
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- Journal:
- Journal of Fluid Mechanics / Volume 439 / 25 July 2001
- Published online by Cambridge University Press:
- 23 July 2001, pp. 131-163
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The experimental data of Honkan & Andreopoulos (1997a) have been further analysed and some new statistical results obtained. In the present work, particular emphasis is given to the time-dependent behaviour of the kinematic shear stress, vorticity, enstrophy, dissipation rate, vorticity stretching and several of the matrix invariants of the velocity-gradient tensor, strain-rate tensor and rotation-rate tensor. The invariants are linked with terms appearing in the transport equations of enstrophy and dissipation rate. Indicative of the existence of extremely high fluctuations is that all r.m.s. values are considerably larger than the corresponding mean values. All invariants exhibit a very strong intermittent behaviour, which is characterized by large amplitude of bursts, which may be of the order of 10 times the r.m.s. values. A substantial qualitative agreement is found between the present experimentally obtained statistical properties of the invariants and those obtained from direct numerical simulation data. Patterns with high rates of turbulent kinetic energy dissipation and high enstrophy suggest the existence of strong shear layers in the near-wall region. In many instances, locally high values of the invariants are also associated with peaks in the shear stress. Conditional analysis provides some evidence of the existence of sequences of several vortices during strong vortical activities, with an average frequency of appearance four times higher than the frequency of appearance of hairpin vortices.
Vorticity, strain-rate and dissipation characteristics in the near-wall region of turbulent boundary layers
- ANANT HONKAN, YIANNIS ANDREOPOULOS
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- Journal:
- Journal of Fluid Mechanics / Volume 350 / 10 November 1997
- Published online by Cambridge University Press:
- 10 November 1997, pp. 29-96
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Experimental results are presented that reveal the structure of a two-dimensional turbulent boundary layer which has been investigated by measuring the time-dependent vorticity flux at the wall, vorticity vector, strain-rate tensor and dissipation-rate tensor in the near-wall region with spatial resolution of the order of 7 Kolmogorov viscous length scales. Considerations of the structure function of velocity and pressure, which constitute vorticity flux and vorticity, indicated that, in the limit of vanishing distance, the maximum attainable content of these quantities which corresponds to unrestricted resolution, is determined by Taylor's microscale. They also indicated that most of the contributions to vorticity or vorticity flux come from the uncorrelated part of the two signals involved. The measurements allowed the computation of all components of the vorticity stretching vector, which indicates the rate of change of vorticity on a Lagrangian reference frame if viscous effects are negligible, and several matrix invariants of the velocity gradient or strain-rate tensor and terms appearing in the transport equations of vorticity, strain rate and their squared fluctuations. The orientation of vorticity revealed several preferential directions. During bursts or sweeps vorticity is inclined at 35° to the longitudinal direction. It was also found that there is high probability of the vorticity vector aligning with the direction of the intermediate extensive strain corresponding to the middle eigenvector of the strain-rate matrix. The results of the joint probability distributions of the vorticity vector orientation angles showed that these angles may be related to those of hairpin vortex structures. All invariants considered exhibit a very strong intermittent behaviour which is characterized by large-amplitude bursts which may be of the order of 10 r.m.s. values. Small-scale motions dominated by high rates of turbulent kinetic energy dissipation and high enstrophy density are of particular interest. It appears that the fluctuating strain field dominates the fluctuations of pressure more than enstrophy. Local high values of the invariants are also often associated with peaks in the shear stress.