Coherent flow structures upstream a circular cylinder near a plane wall

There has been a significant recent interest in using autonomous vehicles to study phenomena in the ocean including temperature and salinity profiles, contaminant transport, and biological processes such as plankton assemblages and algae blooms. It is challenging to deploy mobile robots with bounded actuation and limited communication in a highly nonlinear, time varying, and stochastic flow field such as the ocean. Despite natural stochasticity and arbitrary time dependence, geophysical flows do admit persistent transport controlling features known as Lagrangian coherent structures. These structures, which behave similarly to stable and unstable manifolds in time invariant dynamical systems, serve to characterize transport phenomena and have been shown to have many useful properties for ocean sampling and efficient navigation. In this work, a strategy for tracking manifolds of 2D conservative vector fields is described and the application of this strategy to track coherent structures in general time dependent flows is presented. Computer simulations of the tracking strategy on a variety of analytical and measured flow fields are presented, and it is shown that, in many cases, the strategy is capable of successfully tracking coherent structures in simulation. In addition, the development of an experimental testbed consisting of a fleet of small autonomous surface vessels and three flow tanks is described. Preliminary experimental results related to tracking coherent structures using this flow testbed are presented. The overall results of this effort have indicated that the tracking strategy, although designed for time invariant flows, can be used to track coherent structures in certain time dependent flows quite reliably.

Based on the large eddy simulation method, this study performed the three-dimensional transient numerical analysis of the near-wall flow field of the spiral flow in a circular pipe and applied the sub-grid model of the kinetic energy transport. The low-speed bands, streamwise vortices and hairpin vortices of the spiral flow in the near-wall region of the circular pipe are determined using the Q criterion. The ejection and sweeping of coherent structures are identified using the velocity vector of the near-wall region; moreover, the two methods of creating the hairpin vortices are established by the image time series. The results demonstrate that the development directions of the near-wall bands, streamwise vortices and hairpin vortices of the spiral flow in the circular pipe develop along the path of the spiral line. The average spanwise period of the low-speed bands in the near-wall region is approximately 120 wall units, the length is more than 900 wall units and the height is not more than 40 wall units. The separation distance of the streamwise vortices is about 119 wall units. It has a certain angle with the wall (approximately 22°). The average burst period of a hairpin vortices is less than 0.015 s.

In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded–unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

Large Eddy Simulations (LES) are performed to investigate the coherent structures in flows past a single and an array of tandem plates. Lagrangian coherent structures (LCS) are used to investigate the nonlinear vortex dynamics of flow past a single plate. The Finite-Time Lyapunov Exponent (FTLE) is calculated using the velocity data obtained from Large Eddy Simulations (LES). All simulations are conducted at Reynolds number of 50,000. LCS for a single plate is presented in this study to elucidate and comprehend highly turbulent flow interactions in these flows. The LCS is compared against most commonly used Eulerian structures which are contours of the vorticity and the Q-criterion. The FTLE fields reveal much clearer turbulent structures compared to the Eulerian structures. FTLE better describes the evolution of larger scale eddies. The Q-criterion of flows past an array of plates is also presented.

To meet the increasingly stringent noise regulation, aircraft manufacturers are searching for solutions to jet noise. This, which constitutes a significant amount of the total noise emitted by civil aircrafts, is generated by the mixing processes between the exhaust flow leaving the engine and the atmosphere. A detailed understanding of such mixing process is of paramount importance to identify mechanisms responsible for noise production, i.e. the aeroacoustic source, and, ultimately, to develop noise control strategies. This thesis proposes an unprecedented experimental-based approach to visualize and measure the aeroacoustic sources in jet flows. Time-resolved tomographic particle image velocimetry (TR-TOMO PIV) is employed to obtain time-dependent three-dimensional (3D) measurements of the turbulent flow patterns, while the instantaneous aeroacoustic source is explored using Powell’s aeroacoustic analogy. TR-TOMO PIV experiments are conducted in a tailored jet facility. Measurements are performed both on jets issued through circular and 6-chevron nozzles, with the latter configuration that is nowadays used in jet engines to reduce acoustic emissions. The attention is placed upon the 3D organization and dynamics of flow transition, where coherent flow structures play a role in the generation of noise. The full 3D approach enables unambiguous descriptions of the vortex topology, while the temporal resolution allows addressing the growth and development of the coherent flow structures along with their mutual interaction. In the circular jet, the characteristic pulsatile motion of vortex ring shedding and pairing is accompanied by the growth of azimuthal instabilities and the formation of streamwise vortices leading to the breakdown of the vortex rings. In the chevron jet, instead, the axisymmetric ring-like coherence is replaced by streamwise flow structures whose decay is accompanied by the formation of C-shaped structures. The relation between coherent structures and the instantaneous acoustic source is investigated recalling Powell’s aeroacoustic analogy, with the acoustic source that is identified by the second time derivative of the Lamb vector. The spatio-temporal evolution of the source is mapped and is compared to that of the vortices, to detect flow events involved in the acoustic generation. In the circular jet, most pronounced source activity is observed during the vortex-ring breakdown, whereas, in the chevron configuration, is associated with the process of streamwise vortex decay and C structure formation. Performing unbiased acoustic predictions of the jet noise with TR-TOMO PIV measurements is a challenging task due to the constraints on the extent of the instantaneous measurement domain and on the required spatial and temporal resolutions. To meet this challenge, the thesis finally proposes a strategy to perform far-field acoustic predictions by direct evaluation of Powell’s analogy using TR-TOMO PIV data.

The technique of large-eddy simulation (LES) was used to study the influence and the resulting flow mechanisms of the active flow control applied to a two-dimensional vehicle geometry. Duplication of a previous experimental study performed with LES was used to verify the numerical approach. This was followed with an exploration of the influence of flow actuation on the near-wake flow and resulting aerodynamic forces. Not only was good agreement with the previous experimental study obtained, but new knowledge was gained in the form of complex interaction of the actuation with the coherent flow structures. The resulting time-averaged flow shows a strong influence of the extension of the actuation slits and the lateral solid walls on the near-wake flow structures and thereby on the resulting drag.

This study experimentally investigates unsteady flow behavior and coherent structures in T-junctions with a closed branch, employing time-resolved refractive index matched particle image velocimetry (TR-RIM-PIV). The effects of three closed branch depths (L/D= 1, 2, and 4) on flow characteristics were analyzed for both closed straight and side branches at a Reynolds number of 10 000. High-resolution spatiotemporal flow features were captured using TR-RIM-PIV, while proper orthogonal decomposition revealed the energetic flow structures. Phase-dependent variations of these structures were examined to establish correlations between flow dynamics and noise generation. In the closed straight branch configuration (model 1), large-scale vortex structures emerged due to flow separation at both the leading edge and the opposite wall. The downstream shedding of the leading-edge vortex was driven by Kelvin–Helmholtz instability. For L/D= 1, the lateral jet intermittently impacted the rear wall, intensifying pressure fluctuations and converting hydrodynamic energy into acoustic energy. As the cavity depth increased, vortex–wall interactions weakened, resulting in one, two, and three recirculation zones for L/D= 1, 2, and 4, respectively, with decreasing intensity and size. In the closed side branch configuration (model 2), the turning jet adhered to the wall due to the Coanda effect. At the downstream corner, the jet split into two oppositely phased components, forming double-wavepacket structures that intermittently shed and propagated downstream. For L/D= 1, a single clockwise recirculation zone formed. For L/D= 2 and 4, deeper jet penetration produced additional counterclockwise and clockwise recirculation zones.

ABSTRACTLarge‐scale flow structures (LSFS) in the streamwise direction are important features of gravel‐bed river flows, because they may contribute to sediment transport and gas exchange. In the present study, these structures are detected using Huang's empirical mode decomposition and reconstructed with phase‐averaging techniques based on a Hilbert transform of the velocity signal. The analysis is based on the fluctuating component of 15 quasi‐instantaneous velocity profiles measured with a three‐dimensional (3D) acoustic Doppler velocity profiler (ADVP) in an armoured gravel‐bed river with a low relative submergence of 2.9 (ratio between flow depth and bed grain diameter). LSFS were identified in most of the measured profiles and consistently showed similar features. We were able to characterize the geometry of these large‐scale coherent structures: the front has a vertical linear shift in the time domain and a vertical profile corresponding to a first quarter moon with the apex situated at z/h ≈ 0.4. In the vertical, the front scales with flow depth h, and in the streamwise direction, LSFS scale with three to seven times the mean flow depth. On the bed, the effect of LSFS is a periodic non‐linear variation of the friction velocity on average between 0.90 and 1.10 times the mean value. A model for the friction velocity cycle resulting from LSFS oscillation is presented. Copyright © 2014 John Wiley & Sons, Ltd.

Revisiting the resolution requirements for turbulence simulations in nuclear heat transfer

Fluctuation characteristics induced by energetic coherent structures in air-core vortex: The most complex vortex in the tidal power station intake system

Breaking waves transport momentum, gas, and sediment vertically. Recent particle image velocimetry (PIV) measurements have elucidated the coherent vortex and turbulence structures under breaking waves. However, clarifying three-dimensional fluid motion under breaking waves with PIV remains challenging. Although particle methods have performed wave-breaking simulations, they have never been validated through velocity distributions for breaking waves above movable sediment beds. This study simulated the sediment transport process under plunging waves using a 3D Lagrangian fluid-particle mixture model. The investigation focuses on the first wave plunging for a fundamental investigation. Our simulation shows the instantaneous velocity distributions at different wave phases, which agree with the PIV measurement. A physically-based turbulence extraction method is introduced: a numerical filter based on the coherence between the water elevation and horizontal velocity. Visualized three-dimensional vortex and turbulence structures are consistent with previous studies. The transport equation of the kinetic turbulence energy investigates the turbulence budget near the bed surface and emphasizes the negative contribution of the fluid-particle interaction. Our findings show that a contribution of the pressure gradient to offshore sediment transport. The pressure gradient is difficult to correlate to the water surface gradient and is derived from the large vortex induced by the plunging wave.

This study experimentally investigates the wake dynamics of a two-dimensional square cylinder with dual rigid splitter plates (DRSP) of varying spanwise widths (W/D = 2–∞) at Re = 1.2 × 105. Splitter plates are used for wake stabilization but most studies vary the length and consider the spanwise extent to be infinite, so the three-dimensional effects of a finite-width DRSP—and the resulting edge-induced flow structures are not well understood. This study uses time-resolved particle image velocimetry (TR-PIV) to measure the instantaneous velocity field in the near-wake region and proper orthogonal decomposition (POD), spectral analysis, and vorticity-based flow visualization. The mean streamwise velocity fields feature a deepening of the wake velocity deficit as W/D increases, and there is an increase in the lateral extent. Narrower plates promote a more rapid wake recovery, but wider configurations (W/D ≥ 8) create a deeper and more persistent velocity deficit. The distributions of streamwise and spanwise velocity fluctuations are progressively reduced as W/D increases to a limit if W/D = ∞, at which point, large-scale wake motions are most effectively damped. Turbulent kinetic energy features a similar monotonic attenuation, which shows that there is progressive inhibition of shear–layer interactions as lateral confinement increases. POD analysis shows that there is a consistent modal structure for all cases: Modes 1 and 2 form a classical antisymmetric vortex-shedding pair, and higher-order modes feature lower-energy, less coherent flow structures in the wake. The shape of the mode remains qualitatively similar, but the associated modal energies decrease as W/D increases, which shows that global instability decreases. Spectral analysis of the POD temporal coefficients shows that distinct second and third harmonics (2f1 and 3f1) are present for the W/D = 2 configuration. For intermediate DRSP widths (W/D = 4–8), these harmonic components do not occur, but there is an additional dominant frequency (f2). As the DRSP width increases (W/D = 10 and ∞), the spectra exhibit broadened frequency peaks, which show that there is a loss of discrete harmonic content in the wake dynamics. Instantaneous vorticity fields show that the wake structure evolves continuously and there is no abrupt transition: narrow DRSP widths permit sustained shear–layer communication, but increasing DRSP width progressively leads the wake toward the limiting behavior of an idealized infinite DRSP configuration (W/D = ∞). This is characterized by increased symmetry and the formation of larger, more coherent vortex structures associated with elongated shear–layer development. Increasing W/D progressively weakens the temporal organization of vortex shedding and delays the development of strong shear–layer interactions. As a result, wake unsteadiness is redistributed rather than uniformly suppressed, leading to a continuous and quantifiable transition from strongly interacting near wakes toward a state characterized by weakened shear–layer interaction and an asymptotic wake response at sufficiently large W/D.

This paper presents the results of an experimental study to determine the effect of a row of vertical cylindrical rods on the structure of turbulent flow inside a symmetrical rectangular compound open channel. The coherent turbulent flow structure or bursting events and associated shear stresses play an important role in sediment entrainment and bed instability in an open channel. Therefore, in this study, three-dimensional bursting analysis is used to find the turbulent flow structure at the edge of a floodplain. The study used a model of a symmetrical rectangular compound channel in which ratio of the main channel width B to the floodplain width b was 1·5. A row of cylindrical wooden rods was installed on the floodplain bed at different distances d from the edge of the floodplain (d/b = 0, 0·1, 0·2, 0·3 and 0·4). Three-dimensional flow velocities were measured using an acoustic Doppler velocity meter (micro-ADV). For different experimental tests, the contribution probability of bursting events to the Reynolds shear stress was computed at different points of the flow depth. Minimum values for the contribution probability of shear stress were found for internal, external sweep and ejection events at the edge of floodplain for d/b equal to 0·1. Furthermore, the transition probability of internal, external sweep and external ejection events for d/b = 0·1 decreased at the intersection of the floodplain and main channel. Although scaling of the experimental results requires further study for application to wide rivers, these findings may be useful in practical applications for narrow rivers.

Flows behind obstacles (such as islands) are shallow if the transverse scale of the flow is much larger than water depth. Field, laboratory and numerical data show that the flow pattern in shallow wakes exhibits a complex eddy‐like motion. Experimental and theoretical analyses provide evidence for the presence of two‐dimensional coherent structures in shallow water flows and show that the development of shallow wakes is different from the wakes in deep water due to the following reasons: first, the development of three‐dimensional instabilities is prevented by limited water depth and second, bottom friction acts as a stabilizing mechanism for suppressing the transverse growth of perturbations. Several authors have used the linear and weakly nonlinear stability theory in order to understand when shallow flows become unstable. Two‐dimensional depth‐averaged Saint‐Venant equations are usually used for the analysis. One of the main assumptions in shallow water theory is the independence of the velocity distribution on the vertical coordinate. In many cases, however, this assumption may not be valid. This paper presents an attempt to evaluate the influence of the assumption on the results of linear stability analysis of shallow wake flows with bottom friction. Momentum correction coefficients β 1 and β 2 are used in order to take into account the non‐uniformity of the velocity distribution in the vertical direction. Linear stability calculations show that the stability boundary is quite sensitive to the variation of the parameters β 1 and β 2. The role of the linear and weakly nonlinear stability analysis on the formation of two‐dimensional coherent structures in shallow water flows is discussed.

Near-wall coherent structures in the turbulent channel flow of a dilute polymer solution