Dominant Flow Structures Research Articles

Large-eddy simulation and 3D proper orthogonal decomposition of the hydrodynamics in a stirred tank

The present study attempts to characterize the time-dependent three-dimensional hydrodynamics in a stirred tank reactor using computational fluid dynamics (CFD). The rotating propellers produce a complex, unsteady, three-dimensional turbulent flow field. This results in efficient mixing and is therefore widely used in various process engineering applications. The time-dependent turbulent single-phase flow is computed using large eddy simulation, relying on the sliding mesh approach. The unresolved subgrid scales are treated using the Smagorinsky-Lilly model. The dominant coherent flow structures are characterized in the entire three-dimensional computational domain using the 3D proper orthogonal decomposition (POD) technique. The design of the propeller is evaluated in a separate POD analysis in a rotating frame which encloses the propeller. The most energetic POD modes characterize the organized large scale structures, the so-called coherent flow structures, while the higher modes correspond to the small-scale disorganized turbulence. It was found that the dynamics of the main flow structures can be reconstructed using only 3 modes corresponding to 98% of the overall energy in the entire 3D inner rotating domain and 21 modes are necessary for the same amount of energy in the outer stationary region.Furthermore, the macro instability (MI) was characterized by monitoring the velocity in more than 1 million computational cells, as well as using FFT analyses of the three-dimensional POD temporal coefficients. In the outer stationary domain, both approaches showed characteristic frequencies around one-eighth and one-fifth of the blade passage frequency.

Experimental investigation of cross flow mixing in a randomly packed bed and streamwise vortex characteristics using particle image velocimetry and proper orthogonal decomposition analysis

The study of flow and heat transfer through porous media or randomly packed beds is important as these configurations are widely used in many engineering applications, for example, heat energy storage, chemical catalytic reactors, and nuclear reactors. The flow mixing characteristics in a cross-flow plane of a facility with randomly packed spheres at an aspect ratio of 6.3 were experimentally investigated. The velocity fields at several regions of the cross-flow plane located in the vicinity of the wall and in the pores between spheres were obtained by applying the matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers ranging from 700 to 1700. The TR-PIV results revealed various flow patterns in the transverse plane of the packed spheres, including swirling flow structures aligned with the axial flow direction, a strong bypass flow near the enclosure wall, and a circulation region created when the bypass flow ejected into a large spatial gap. When the Reynolds number was increased, the peaks of root-mean-square fluctuating velocities, urms′ and vrms′, were found to increase approximately at the same ratio as the increase in Reynolds number, and the magnitude of the Reynolds stress increased considerably. In addition, the characteristics of flow mixing in different flow regions were investigated via the two-point cross-correlation of fluctuating velocities. Using Taylor’s hypothesis, the vorticity iso-surfaces were constructed. Thus, constructed iso-surfaces showed that shear layers generated from the bypass flow gaps were stretched, broken into smaller flow structures, and then evolved as vortex pairs when entering the neighboring gaps. The results obtained by applying proper orthogonal decomposition (POD) analysis to the velocity fields showed that the statistically dominant flow structures had approximately the same size and shape as those depicted by Taylor’s hypothesis. Vortex characteristics, such as populations, spatial distributions, and strengths, for various spatial regions and Reynolds numbers were obtained by a combination of POD analysis and vortex identification.

Phase-locking particle image velocimetry measurement of unsteady flow behaviors: Online dynamic mode decomposition using field-programmable gate array

A novel online dynamic mode decomposition (DMD) approach using a field-programmable gate array (FPGA), which takes full advantage of the DMD to extract multiple unsteady events and the FPGA system for signal sampling and fast computation, was developed for phase-locking particle image velocimetry (PIV) measurements of unsteady flow behaviors. The turbulent separated and reattaching flow around a finite blunt plate with a length-to-height-ratio L/D = 6.0 was examined to demonstrate this novel approach. The wall-pressure field and the velocity field were measured using arrayed microphones and the conventional planar PIV setup, respectively. Offline DMD analysis of the wall-pressure fluctuation field was first used to identify the dominant modes corresponding to the energetically unsteady events. For each mode, the eigenmode and its mode coefficient reflected the spatial footprint pattern and temporal strength of the unsteady event, respectively. Next, trained machine learning of the mode coefficient was used to establish a phase prediction strategy. Finally, in the online analysis, the relevant eigenmode was cast into the FPGA device to serve as the reference mode for reconstruction with the sampled wall-pressure data, determining the phase signal to fire the PIV setup. High-resolution spatiotemporal evolutions of the dominant flow structures (i.e., the flapping separation bubble, the impinging leading-edge vortex, and the trailing-edge vortex street) were separately assembled. Further measurements demonstrated a clear panoramic view of the synchronous behavior of the enlarging separation bubble and the impinging leading-edge vortex. The proposed online FPGA-DMD approach can serve as a sophisticated strategy for phase-locking PIV measurements of unsteady flow behaviors.

Amplification and structure of streamwise-velocity fluctuations in compression-corner shock-wave/turbulent boundary-layer interactions

In this work, we study the effect of the compression-corner angle on the streamwise turbulent kinetic energy (sTKE) and structure in Mach 2.8 flow. Krypton tagging velocimetry (KTV) is used to investigate the incoming turbulent boundary layer and flow over$8^{\circ }$,$16^{\circ }$,$24^{\circ }$and$32^{\circ }$compression corners. The experiments were performed in a 99 %$\text{N}_{2}$and 1 % Kr gas mixture in the Arnold Engineering Development Complex (AEDC) Mach 3 Calibration Tunnel (M3CT) at$Re_{\unicode[STIX]{x1D6E9}}=1750$. A figure of merit is defined as the wall-normal integrated sTKE ($\overline{\text{sTKE}}$), which is designed to identify turbulence amplification by accounting for the root-mean-squared (r.m.s.) velocity fluctuations and shear-layer width for the different geometries. We observe that the$\overline{\text{sTKE}}$increases as an exponential with the compression-corner angle near the root when normalized by the boundary-layer value. Additionally, snapshot proper orthogonal decomposition (POD) is applied to the KTV results to investigate the structure of the flow. From the POD results, we extract the dominant flow structures and compare each case by presenting mean-velocity maps that correspond to the largest positive and negative POD mode coefficients. Finally, the POD spectrum reveals an inertial range common to the boundary-layer and each compression-corner flow that is present after the first${\approx}10$dominant POD modes.

The interaction between a spatially oscillating jet emitted by a fluidic oscillator and a cross-flow

This experimental study investigates the fundamental flow field of a spatially oscillating jet emitted by a fluidic oscillator into an attached cross-flow. Dominant flow structures, such as the jet trajectory and dynamics of streamwise vortices, are discussed in detail with the aim of understanding the interaction between the spatially oscillating jet and the cross-flow. The oscillating jet is ejected perpendicular to the cross-flow. A moveable stereoscopic particle image velocimetry (PIV) system is employed for the plane-by-plane acquisition of the flow field. The three-dimensional, time-resolved flow field is obtained by phase averaging the PIV results based on a pressure signal from inside the fluidic oscillator. The influence of velocity ratio and Strouhal number is assessed. Compared to a common steady wall-normal jet, the spatially oscillating jet penetrates to a lesser extent into the cross-flow’s wall-normal direction in favour of a considerable spanwise penetration. The flow field is dominated by streamwise-oriented vortices, which are convected downstream at the speed of the cross-flow. The vortex dynamics exhibits a strong dependence on the Strouhal number. For small Strouhal numbers, the spatially oscillating jet acts similar to a vortex-generating jet with a time-dependent deflection angle. Accordingly, it forms time-dependent streamwise vortices. For higher Strouhal numbers, the cross-flow is not able to follow the motion of the jet, which results in a quasi-steady wake that forms downstream of the jet. The results suggest that the flow field approaches a quasi-steady behaviour when further increasing the Strouhal number.

Effects of Nozzle Pressure Ratio and Nozzle-to-Plate Distance to Flowfield Characteristics of an Under-Expanded Jet Impinging on a Flat Surface

The current work experimentally investigates the flowfield characteristics of an under-expanded turbulent jet impinging on a solid surface for various nozzle-to-plate distances 2.46 D j , 1.64 D j , and 0.82 D j ( D j is the jet hydraulic diameter), and nozzle pressure ratios (NPRs) ranging from 2 to 2.77 . Planar particle image velocimetry (PIV) measurements were performed in the central plane of the test nozzle and near the impingement surface. From the obtained PIV velocity vector fields, flow characteristics of under-expanded impinging jets, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress profiles, were computed. Comparisons of statistical profiles obtained from PIV velocity measurements were performed to study the effects of the impingement surface, nozzle-to-plate distances, and NPRs to the flow patterns. Finally, proper orthogonal decomposition (POD) analysis was applied to the velocity snapshots to reveal the statistically dominant flow structures in the impinging jet regions.

Guiding Actuator Designs for Active Flow Control of the Precessing Vortex Core by Adjoint Linear Stability Analysis

The fundamental impact of the precessing vortex core (PVC) as a dominant coherent flow structure in the flow field of swirl-stabilized gas turbine combustors has still not been investigated in depth. In order to do so, the PVC needs to be actively controlled to be able to set its parameters independently to any other of the combustion system. In this work, open-loop actuation is applied in the mixing section between the swirler and the generic combustion chamber of a nonreacting swirling jet setup to investigate the receptivity of the PVC with regard to its lock-in behavior at different streamwise positions. The mean flow in the mixing section as well as in the combustion chamber is measured by stereoscopic particle image velocimetry (SPIV), and the PVC is extracted from the snapshots using proper orthogonal decomposition (POD). The lock-in experiments reveal the axial position in the mixing section that is most suitable for actuation. Furthermore, a global linear stability analysis (LSA) is conducted to determine the adjoint mode of the PVC which reveals the regions of highest receptivity to periodic actuation based on mean flow input only. This theoretical receptivity model is compared with the experimentally obtained receptivity data, and the applicability of the adjoint-based model for the prediction of optimal actuator designs is discussed.

Flowfield measurements of reverse flow on a high advance ratio rotor

One challenge that plagues the high-speed operation of rotorcraft is the inherent increase in size of the reverse flow region on the retreating side of the rotor disk. The present work experimentally investigated a portion of the reverse flow region on a 1.7- m-diameter sub-scale slowed rotor at advance ratios up to $$\mu =0.9$$ and three shaft tilt angles $${\alpha _{\text {s}}=-\ 4^\circ ,\,0^\circ ,\,4^\circ }$$ . Two-component time-resolved particle image velocimetry was used to characterize the flow field around a blade element in the reverse flow region, nominally positioned at $$\psi =270^\circ $$ and $${y/R=0.4}$$ . Four dominant flow structures were observed: the reverse flow entrance vortex, the blunt trailing edge wake sheet, the reverse flow dynamic stall vortex, and the tip vortex. Analyses are focused on the reverse flow dynamic stall vortex due to its influence on unsteady airloads, and similarities to that of a leading edge vortex over a rotating or surging wing at high angle of attack. The number of semi-chords traveled by the blade element through the reverse flow region directly affects the strength, trajectory, and estimated vortex-induced pitching moment of the reverse flow dynamic stall vortex. Shaft tilt angle also has a strong effect on the evolution of the vortex, with forward shaft tilt resulting in dramatically increased strength and size. The results of this characterization and sensitivity study serve as a reference for evaluation of numerical simulations and aid in the development of low-order aerodynamic models of the reverse flow region and resulting force histories. Collectively, these tools will be used to better predict airloads, wake interactions, and vibrations of rotors operating at high advance ratios.

On the structure of the self-sustaining cycle in separating and reattaching flows

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

Unsteady Conjugate Heat Transfer Investigation of a Multistage Steam Turbine in Warm-Keeping Operation With Hot Air

In pursuit of flexibility improvements, General Electric has developed a product to warm-keep high/intermediate pressure steam turbines using hot air. In order to optimize the warm-keeping operation and to gain knowledge about the dominant heat transfer phenomena and flow structures, detailed numerical investigations are required. For the sake of the investigation of the warm-keeping process as found in the presented research, single and multistage numerical turbine models were developed. Furthermore, an innovative calculation approach called the equalized timescales method (ET) was applied for the modeling of unsteady conjugate heat transfer (CHT). In the course of the research, the setup of the ET approach has been additionally investigated. Using the ET method, the mass flow rate and the rotational speed were varied to generate a database of warm-keeping operating points. The main goal of this work is to provide a comprehensive knowledge of the flow field and heat transfer in a wide range of turbine warm-keeping operations and to characterize the flow patterns observed at these operating points. For varying values of flow coefficient and angle of incidence, the secondary flow phenomena change from well-known vortex systems occurring in design operation to effects typical for windage, like patterns of alternating vortices and strong backflows. Furthermore, the identified flow patterns have been compared to vortex systems described in cited literature and summarized in the so-called blade vortex diagram. The analysis of heat transfer in turbine warm-keeping operation is additionally provided.

Time-resolved PIV measurements in a low-aspect ratio facility of randomly packed spheres and flow analysis using modal decomposition

This work experimentally investigated the flow characteristics in a facility with randomly packed spheres at a low-aspect ratio of 4.4. Velocity fields in the near-wall region and in the pores between spheres were obtained by employing the matched-index-of-refraction (MIR) and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of 340, 520, and 720. From the obtained TR-PIV velocity vector fields, flow characteristics including first- and second-order statistics, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress profiles, were computed. The effects of the wall enclosure and Reynolds numbers on the flow patterns were investigated by comparing the computed flow statistics and evaluating two-point cross correlations of the velocities measured adjacent to and far from the wall. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress component showed an increase in flow mixing and turbulent intensity in the gaps between spheres in the packed bed. The re-circulation-region sizes, however, were found to be independent from an increase in Reynolds numbers. Finally, flow modal decompositions, such as the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were applied to the vorticity fields extracted from sub-regions located near and far from the wall to reveal the most dominant POD and DMD flow structures.

Investigations of Helicopter Wake Flow Including Rotor-Head Motion

The present paper emphasizes on the characteristics of the wake flowfield emanating from a helicopter fuselage and rotor head, including cyclic and collective pitch motion. Predicting the convection of the coherent vortical flow structures generated by the fuselage as well as the rotor head is a difficult goal by the application of numerical methods. Furthermore, the accurate determination of the wake flowfield requires the detailed knowledge of the origin of the associated coherent structures. Therefore, numerical and experimental investigations have been conducted on a twin-engine light utility helicopter. The wind-tunnel model includes a fuselage and a rotating five-bladed rotor head. To obtain the wake flowfield, stereoscopic particle image velocimetry is applied. The numerical investigations are performed with a commercial flow solver. The flow solver is based on the unsteady Reynolds-averaged Navier–Stokes equations. Turbulence modeling is performed with the scale-adaptive simulation model. The motion of the rotor head is covered by the moving and deforming mesh methods. The wake-flow velocity distribution obtained by the experimental data and the numerical results shows a good agreement and allows a detailed analysis of the dominant vortical flow structures propagating downstream.

An aeroacoustic study of a leading-edge slat: Beamforming and far field estimation using near field quantities

The leading edge slat of a high-lift system is a large contributor to the overall radiated acoustic field from an aircraft during the approach phase of the flight path. This is due to the unsteady flow field generated in the slat-cove and near the leading edge of the main element. In an effort to understand the noise-source mechanisms, a suite of experimental measurements has been performed on a two-dimensional multi-element MD-30P30N airfoil, including PIV, steady and unsteady surface pressure, and phased microphone array measurements. Acoustic trends are given with angle of attack and flow speed which corresponds to a stowed chord Reynolds number range of 1.2 × 106 ≤ Rec ≤ 1.71 × 106. Spatially integrated beamforming is used to isolate the slat noise, while Kevlar sidewalls are utilized to minimize environmental influence. In addition, temporally-resolved estimates of a low-dimensional representation of the velocity vector fields are obtained through the use of proper orthogonal decomposition and spectral linear stochastic estimation and show good agreement with independent phase-locked PIV measurements. A time-resolved estimate of the pressure field is then computed via solution of the pressure Poisson equation. From this, Curle's acoustic analogy scaled for the frequency-dependent spanwise coherence length projects the time-resolved pressure forces in the slat region to the acoustic far field for comparison to the array measurements. The favorable results establish the connection between the dominant unsteady flow structures most responsible for the radiated noise and provide insight for future noise reduction efforts.

Characteristics of bubble-induced liquid flows in a rectangular tank

Bubbly flows are frequently encountered in many industrial applications where multiphase contact is used to promote heat, mass and momentum transfer. These include applications where both chemical and physical processes occur, such as wastewater treatment and biological aeration systems. We investigated the behaviour of underwater-generated bubble swarms, which were produced at the bottom of a 1-m3 square tank from a 5-mm nozzle and allowed to rise by buoyancy in still water. Instantaneous velocity fields around the bubbles were obtained using Particle Image Velocimetry (PIV) seeded with 10–15-µm poly-dispersed fluorescent particles and gas flow rates ranging from 2 to 15 L/min (1.7–12.8 m/s). A continuous laser was used to obtain the time-resolved field, and a pulse laser was used to obtain the mean velocity fields. Images were captured at up to 2000 fps. After interrogation, a post-processing validation algorithm was employed to identify and remove vectors produced by bubbles and the interface, essentially producing vector fields of the liquid phase only. Proper orthogonal decomposition analysis was carried out on 1000 realisations of each gas flow case to identify dominant flow structures, and the flow was decomposed into its constituent spatial and temporal modes. We established that induced vortices in the liquid phase more clearly manifest at far streamwise locations shown by the spatial mode at lower gas flow rates and are clearer in the temporal mode at high gas flow rates. The mean streamwise and spanwise liquid velocities increased with the gas flow rate, and the streamwise bubble velocities can be well described by a top-hat profile curve. Finally, an analysis was done to estimate the bubble entrainment coefficient using the slip velocity and the gas buoyancy flux.

Unsteady Behaviors of Steam Flow in a Control Valve With T-Junction Discharge Under the Choked Condition: Detached Eddy Simulation and Proper Orthogonal Decomposition

Due to the practical space limitation, the control valve in industrial utilities is usually immediately followed by a short flow passage, which would introduce considerable complexity into highly unsteady flow behaviors, along with the flow noise and structure vibration. In the present study, the unsteady behaviors of the steam flow inside a control valve with a T-junction discharge, when the valve operates under the choked condition, are numerically simulated. Toward this end, the detached eddy simulation (DES) is used to capture the spatiotemporally varying flow field in the serpentine flow passage. The results show periodic fluctuations of the aerodynamic forces on the valve spindle and periodic fluctuations of the pressure and flow rate at the two discharge outlets. Subsequently, proper orthogonal decomposition (POD) analysis is conducted using the velocity field and pressure field, identifying, respectively, the dominant coherent structures and energetic pressure fluctuation modes. Finally, the extended-POD method is used to delineate the coupling between the pressure fluctuations with the dominant flow structures superimposed in the highly unsteady flow field. The fourth velocity mode at St = 0.1, which corresponds to the alternating oscillations of the annular wall-attached jet, is determined to cause the periodic flow imbalance at the two discharge outlets, whereas signatures of the first three modes are found to be dissipated in the spherical chamber. Such findings could serve as facts for vibration prediction and optimization design. Particularly, the POD and extended-POD techniques were demonstrated to be effective methodologies for analyzing the highly turbulent flows in engineering fluid mechanics.

Experimental investigation of torque hysteresis behaviour of Taylor–Couette Flow

This paper describes the hysteresis in the torque for Taylor–Couette flow in the turbulent flow regime for different shear Reynolds numbers, aspect ratios and boundary conditions. The hysteresis increases with decreasing shear Reynolds number and becomes more pronounced as the aspect ratio is increased from 22 to 88. Measurements conducted in two different Taylor–Couette set-ups depict the effect of the flow conditions at the ends of the cylinders on the flow hysteresis by showing reversed hysteresis behaviour. In addition, the flow structure in the different branches of the hysteresis loop was investigated by means of stereoscopic particle image velocimetry. The results show that the dominant flow structures differ in shape and magnitude depending on the branch of the hysteresis loop. Hence, it can be concluded that the geometry could have an effect on the hysteresis behaviour of turbulent Taylor–Couette flow, but its occurrence is related to a genuine change in the flow dynamics.

Görtler vortices in low-Reynolds-number flow over multi-element airfoil

The low-Reynolds-number flow over a multi-element airfoil (30P30N) is investigated with time-resolved particle image velocimetry (TR-PIV) and flow visualization (FV). Dominant flow structures over the main element of the multi-element airfoil are explored with the variation of angle of attack ($\unicode[STIX]{x1D6FC}$). It is of great importance that Görtler vortices are first observed with this configuration at $\unicode[STIX]{x1D6FC}=2^{\circ }{-}12^{\circ }$, which is quite different from the high-Reynolds-number cases. The characteristics of the Görtler vortices are explored to determine the origin of these unexpected flow structures. It is found that these Görtler vortices travel in the spanwise direction. Secondary counter-rotating vortices are induced beneath the main Görtler vortices. The travelling property of the Görtler vortices is utilized to determine the positions of the main Görtler vortices and the secondary counter-rotating vortices. It is observed that Görtler vortices reside above the separated shear layer originating from the leading-edge separation of the main element. The secondary counter-rotating vortices are located within the separated shear layer, as a result of the interaction between the Görtler vortices and the separated shear layer. The relative positions of the Görtler vortices, the secondary counter-rotating vortices and the separated shear layer result in a special transition scenario within the separated shear layer. The position of Görtler vortices combined with the Rayleigh discriminant indicates the mechanism that the Görtler vortices are generated by a virtual curved boundary. The travelling property of the Görtler vortices, which is different from the classical stationary Görtler vortices, can also be interpreted by this mechanism. Ultimately, modified criteria for generating Görtler vortices with a virtual curved boundary are proposed to provide references for the follow-up works.

PROPER ORTHOGONAL DECOMPOSITION: A TOOL TO STUDY THE UNDERLYING PHYSICS OF TURBULENCE

Wall bounded turbulence have been investigated by many authors to understand the underlying physics, experimentally as well as numerically with different techniques and methods. Enormous studies are reported in the literature in the field of turbulence to get the insight of near wall structure in a broad spectrum of Reynolds number. To recognize the contribution of different turbulent scales in a flow a well-known technique, Proper Orthogonal decomposition (POD) is used. Dynamical behaviour of coherent structure in a turbulent channel flow submitted to high temperature gradient is investigated via proper orthogonal decomposition (POD). The turbulent data is generated using thermal large eddy simulation (TLES) of channel flow at Re = 180 for two values of temperature ratio between hot and cold wall. The POD technique is applied to fluctuating part of the velocity to study the temporal evolution of the most energetic modes. It is observed that dominant flow structures are elongated in streamwise direction which further distorted due to the interaction with bean shaped propagating modes. The plotted average energy E(t) as a function of non-dimensional time shows a constant level of fluctuating energy with one little peak at t+ = 1000, all other smaller spikes are showing constant fluctuations about mean line. This behavior at quantitatively describe the role of temperature stratification which stabilizes the roll mode energy and prevent them from breakup into smaller scales thus affects the interaction process during turbulent burst event resulting in a chugging or relaminarizing phenomenon. It is observed that thermal stratification decreases the bursting rate by slowing the breaking mechanism of streamwise elongated modes to larger number of bean shaped modes which results in relaminarization of the flow near hot wall.

Stereoscopic PIV measurements of near-wall flow in a tightly packed rod bundle with wire spacers

In this study, we experimentally investigated the flow field characteristics in the complex geometry of a tightly packed 61-rod bundle with wire spacers by employing matched-index-of-refraction (MIR) and stereoscopic particle image velocimetry (SPIV) techniques. SPIV measurements were performed on a cross-flow plane at Reynolds numbers of 500, 2500, and 6300, and the measurement area covered the interior and edge sub-channels that were located near the enclosure wall. The SPIV results showed that peaks of the velocity magnitude occurred in the edge sub-channel, indicating the wall effects on the flow symmetry in the cross-flow plane. Mean velocities at the central areas of the edge and interior sub-channels showed flat profiles for Re=500 and local peaks around 0.15<y/Drod<0.3, due to the wire’s presence and near-wall location of the edge sub-channel. The rms fluctuating profiles velocities showed that local maximal peaks linearly shifted from y/Drod=-0.44 to −0.36 and −0.28 as Re increased. Taylor’s hypothesis was applied to the SPIV velocity vector fields for reconstructing quasi-instantaneous three-dimensional (3D) vortical structures in the wire-wrapped rod bundle. The visualization of 3D vorticity iso-surfaces showed that vortical structures were generated from shear layers of neighboring rods and traveled in the streamwise direction. Finally, proper orthogonal decomposition (POD) analysis was applied to SPIV velocity snapshots taken at the edge and interior sub-channels to extract the most statistically dominant flow structures. The POD velocity decomposition revealed coherent structures with sizes and shapes comparable to instantaneous vortical structures identified by iso-surfaces. Energy fractions of the first POD modes were found to reduce from 46% and 31% to 26% and 18% when Reynolds numbers increased from Re=500 to 6300, respectively. On the other hand, kinetic energy levels of low-order POD modes (modes 2, 3, 4, etc.) increased when the Reynolds number increased.

LES‐VOF Simulation and POD Analysis of the Gas‐Jet Wiping Process in Continuous Galvanizing Lines

Gas‐Jet wiping is a widely applied technology in continuous galvanizing mills, which enables the adjustment of the specific coating mass by an impinging turbulent gas‐jet. The unsteady impingement conditions, however, are reported to cause surface non‐uniformities such as waves. In this study, proper orthogonal decomposition (POD) is used to analyze an industrial gas‐jet wiping process. POD allows to objectively extract the most dominant flow structures (modes) and their dynamics from the impinging jet. The acquisition of the necessary temporally and spatially highly resolved flow data is done by a LES‐VOF simulation model. The POD analysis shows that jet flapping and axial fluctuations of the jet core are the most dominant spatial modes in the studied case. The frequency range of the modes as well as the frequency range of the height fluctuations of the impinged film are compared in a spectral analysis. Additionally, it is shown that slight changes, for example, the absence of the thin liquid film on the impinged surface, alters the frequency range of the dominant POD modes, whereas the modes themselves remain mostly unchanged.