Volumetric velocimetry for fluid flows

In recent years, several techniques have been introduced that are capable of extracting 3D three-component velocity fields in fluid flows. Fast-paced developments in both hardware and processing algorithms have generated a diverse set of methods, with a growing range of applications in flow diagnostics. This has been further enriched by the increasingly marked trend of hybridization, in which the differences between techniques are fading. In this review, we carry out a survey of the prominent methods, including optical techniques and approaches based on medical imaging. An overview of each is given with an example of an application from the literature, while focusing on their respective strengths and challenges. A framework for the evaluation of velocimetry performance in terms of dynamic spatial range is discussed, along with technological trends and emerging strategies to exploit 3D data. While critical challenges still exist, these observations highlight how volumetric techniques are transforming experimental fluid mechanics, and that the possibilities they offer have just begun to be explored.SD was partially supported under Grant No. DPI2016-79401-R funded by the Spanish State Research Agency (SRA) and the European Regional Development Fund (ERDF). FC was partially supported by the U.S. National Science Foundation (Chemical, Bioengineering, Environmental, and Transport Systems, Grant No. 1453538)

Characterisation of turbulence in an open channel flow and in a fountain with tomographic PIV.

This work aims to improve the understanding of the fundamental characteristics of environmental flows by interpreting the turbulence in a 3D measurement domain. This thesis primarily describes the Tomographic PIV technique and the results of three experimental investigations of environmental flows. Two experiments were conducted in an open channel flow, divided into four sequential, identical pools, by a combination of regular grids. The first set of TPIV measurements were in the water column, while the second set of measurements were made along the channel bottom. The instantaneous structures in the flow were visualised and the turbulent kinetic energy k, energy dissipation ε and vorticity ω were analysed; their decay along the streamwise direction was revealed. Ejections (Q2) and sweeps (Q4) were identified along the channel bottom. A major contribution that resulted from the investigation pertains to the vibration correction of the cameras. TPIV measurements were taken of a regime of turbulent, forced fountain flows. The fountains were created by injecting a salt-water solution through a circular opening into the bottom of a reservoir of a water-ethanol solution, with their refractive indices carefully matched. The evolution of the fountain in its initial stages was captured and described in a series of chronological measurement volumes. Measurements of the fully developed fountains captured the large scale structures and their characteristics were analysed by considering the topology of the invariants of the velocity gradient tensor. The TPIV system was designed and built in-house at the University of Sydney. The experimental investigations described in this work revealed some interesting features of the environmental flows. The applicability and versatility of TPIV for these flows were demonstrated. The measurements allowed for the quantification and visualisation of the turbulence in the flows and hence shed light on the physics behind them

Characterisation of turbulence in an open channel flow and in a fountain with tomographic PIV.

This work aims to improve the understanding of the fundamental characteristics of environmental flows by interpreting the turbulence in a 3D measurement domain. This thesis primarily describes the Tomographic PIV technique and the results of three experimental investigations of environmental flows. Two experiments were conducted in an open channel flow, divided into four sequential, identical pools, by a combination of regular grids. The first set of TPIV measurements were in the water column, while the second set of measurements were made along the channel bottom. The instantaneous structures in the flow were visualised and the turbulent kinetic energy k, energy dissipation ε and vorticity ω were analysed; their decay along the streamwise direction was revealed. Ejections (Q2) and sweeps (Q4) were identified along the channel bottom. A major contribution that resulted from the investigation pertains to the vibration correction of the cameras. TPIV measurements were taken of a regime of turbulent, forced fountain flows. The fountains were created by injecting a salt-water solution through a circular opening into the bottom of a reservoir of a water-ethanol solution, with their refractive indices carefully matched. The evolution of the fountain in its initial stages was captured and described in a series of chronological measurement volumes. Measurements of the fully developed fountains captured the large scale structures and their characteristics were analysed by considering the topology of the invariants of the velocity gradient tensor. The TPIV system was designed and built in-house at the University of Sydney. The experimental investigations described in this work revealed some interesting features of the environmental flows. The applicability and versatility of TPIV for these flows were demonstrated. The measurements allowed for the quantification and visualisation of the turbulence in the flows and hence shed light on the physics behind them

Experimental investigation of inertial particle transport in a turbulent boundary layer

PhDThe first major part of the work was to commission and test the newly built 3 meter openchannel experimental rig. Various development stages have been carried to improve the design specifications to meet experimental requirements. The original 2 meter open-channel working section was replaced with a new 3 meter channel working section enabling measurements to be taken further downstream allowing the boundary layer to develop. The original bell mouth inlet was replaced with a hyperbolic tangent profile 3:1 contraction with a honeycomb, coarse and fine gauzes fitted upstream. 25% porous perforated plates were installed at the channel exit and also within the inlet plenum tank to reduce the turbulence level. LDV measurement in the freestream revealed that the turbulence level is below 1% and the boundary layer profile collapses well with DNS data of Schlatter (2010). A dip in the outer wake region of the velocity profile can be observed throughout the measurements and is attributed to the aspect ratio of the channel which is 1.7 at Fr = 0.33. Nevertheless, boundary layer profile and turbulence intensity profile collapse well with published DNS data. Good agreement was obtained between measurements carried out using the available LDV and time-resolved PIV systems. Time-resolved PIV measurements were performed in a dilute particle-laden flow, tracking nearly neutrally buoyant polymer microspheres within the measured velocity field of a near wall turbulent boundary layer. Data were taken 2100mm downstream of the inlet, in a vertical light-sheet aligned in the streamwise direction on the centerline of the horizontal, open-channel channel facility. High frame-rate measurements were taken to temporally and spatially track particle motion and instantaneous visualization clearly reveal a link between particle movement and near-wall coherent structures. Structures having 2D vorticity signatures of near-wall hairpin vortices and hairpin packets, directly affect particle motion. Statistical and instantaneous results agree well with published experimental and numerical work. Conditional statistics were investigated for the particles using the Quadrant method. Particles moving outwards from the channel floor are influenced by the Quadrant 2 ejection events and those that moves inwards towards the wall are influenced by the Quadrant 4 sweeps events. v Particle-fluid velocity correlations, rpf were calculated for each particle trajectory and averaged of all the particle-fluid velocity correlations, R, were determined for the whole dataset for Re=1000. This value is estimated to be 0.0261 and 0.000643 respectively for the particle-fluid streamwise and wall-normal velocity correlation

The Specialist Committee on Wake Fields Final Reports and Recommendations to the 25th ITTC

The recommended actions of 25th ITTC Specialist Committee on Wake-Fields, as stated above are focused on two main areas, the review of the numerical prediction and experimental measurement (methods) of wakefields and the review and development of ITTC procedures

A Review of Laboratory and Numerical Techniques to Simulate Turbulent Flows

Turbulence is still an unsolved issue with enormous implications in several fields, from the turbulent wakes on moving objects to the accumulation of heat in the built environment or the optimization of the performances of heat exchangers or mixers. This review deals with the techniques and trends in turbulent flow simulations, which can be achieved through both laboratory and numerical modeling. As a matter of fact, even if the term “experiment” is commonly employed for laboratory techniques and the term “simulation” for numerical techniques, both the laboratory and numerical techniques try to simulate the real-world turbulent flows performing experiments under controlled conditions. The main target of this paper is to provide an overview of laboratory and numerical techniques to investigate turbulent flows, useful for the research and technical community also involved in the energy field (often non-specialist of turbulent flow investigations), highlighting the advantages and disadvantages of the main techniques, as well as their main fields of application, and also to highlight the trends of the above mentioned methodologies via bibliometric analysis. In this way, the reader can select the proper technique for the specific case of interest and use the quoted bibliography as a more detailed guide. As a consequence of this target, a limitation of this review is that the deepening of the single techniques is not provided. Moreover, even though the experimental and numerical techniques presented in this review are virtually applicable to any type of turbulent flow, given their variety in the very broad field of energy research, the examples presented and discussed in this work will be limited to single-phase subsonic flows of Newtonian fluids. The main result from the bibliometric analysis shows that, as of 2021, a 3:1 ratio of numerical simulations over laboratory experiments emerges from the analysis, which clearly shows a projected dominant trend of the former technique in the field of turbulence. Nonetheless, the main result from the discussion of advantages and disadvantages of both the techniques confirms that each of them has peculiar strengths and weaknesses and that both approaches are still indispensable, with different but complementary purposes

A versatile scanning method for volumetric measurements of velocity and density fields

Understanding turbulence in a stratified environment requires a detailed picture of both the velocity field and the density field. Experimentally, this represents a significant measurement challenge, especially when full three-dimensional data is needed to accurately characterise the turbulent fields. This paper presents a new approach to obtaining such data through well-resolved, near-instantaneous volume-spanning measurements. This is accomplished using a simultaneous PIV and PLIF scanning setup that is highly versatile while allowing precise overlapping light sheets in a volume. Moreover, the approach detailed here allows for complex scan sequences over varying volumes without the need for mechanical adjustment. To demonstrate the technique, preliminary results from the buoyancy-driven exchange flow through density fluids are shown.This work was supported by EPSRC Programme Grant EP/K034529/1

Application of Holographic Particle Image Velocimetry in Bubbly Flow

Measuring the velocity field of a bubbly flow is of major importance in many industrial applications like, bubble column reactors. Traditional PIV setup is limited to single plane measurements. The present study involved extension of the existing holographic PIV method to two-phase flow velocimetry. The objective of the present study was to conduct velocity measurements using in-line holographic microscopy arrangement to analyze a flow consisting of an air bubble rising through stagnant water in a rectangular column. Neutrally buoyant 8 μm sized hollow glass spheres are scattered uniformly in the continuous phase and are used as seed particles for image analysis. . Double-pulsed hologram is taken with a 2K by 2K pixels CCD sensor with 200μs in between the two frames, which allows only a small movement of both the particles (in the continuous phase) and that of the dispersed phase itself. Holograms are reconstructed at different distances to produce images at every millimeter of the measurement volume. An averaging technique is developed to process the images to reduce the noise in reconstructed holographic images and make it useful for cross-correlation analysis. The image pairs are cross correlated and velocity vectors are obtained for the continuous phase. The dispersed phase velocity is measured using the shift in the center of the bubble. After analyses of all the image pairs throughout the measurement volume, a two component three dimensional velocity field is produced for the two-phase flow.Mechanical Engineerin

Investigation of turbulence modulation in solid-liquid suspensions using FPIV and micromixing experiments

The focus of this thesis is the study of turbulent solid-liquid stirred suspensions, which are involved in many common unit operations in the chemical, pharmaceutical and food industries. The studies of two-phase flows present a big challenge to researchers due to the complexity of experiments; hence there is a lack of quantitative solid and liquid hydrodynamic measurements. Therefore, an investigation of turbulence modulation by dispersed particles on the surrounding fluid in stirred vessels has been carried out, via two-phase fluorescent Particle Image Velocimetry (FPIV) and micromixing experiments. The main property of interest has been the local dissipation rate, as well as root-mean-square (rms) velocities and turbulent kinetic energy (TKE) of the fluid. Initially a single-phase PIV study was conducted to investigate the flow field generated by a sawtooth (EkatoMizer) impeller. The purpose of this study was to gain insight into various PIV techniques before moving on to more complex two-phase flows. Subsequently stereo-, highspeed and angle-resolved measurements were obtained. The EkatoMizer formed a good case study as information regarding its hydrodynamics is not readily available in literature, hence knowledge has been extended in this area. An analysis of the mean flow field elucidated the general structure of fluid drawn into the impeller region axially and discharged radially; the latter characterised the impeller stream. The radial rms velocity was considered to represent best the system turbulence, even though the tangential rms velocity was greater close to the blade; however the radial component was more prevalent in the discharge stream. Due to differences in rms velocities, TKE estimates obtained from two and three velocity components deviated, being greater in the latter case. Integral (1-D and 2-D) length scales were overestimated by the quantity W / 2 in the impeller region. Ratios of longitudinal-to-lateral length scales also indicated flow anisotropy (as they deviated from 2:1). The anisotropy tensor showed that the flow was anisotropic close to the blade, and returned to isotropy further away from the impeller. Instantaneous vector plots revealed vortices in the discharge stream, but these were not associated with flow periodicity. Alternatively, the vortex structures were interpreted as low frequency phenomena between 0-200 Hz; macro-instabilities were found to have a high probability of occurrence in the discharge stream. Dissipation is the turbulent property of most interest as it directly influences micromixing processes, and its calculation is also the most difficult to achieve. Its direct determination from definition requires highly resolved data. Alternative methods have been proposed in the literature, namely dimensional analysis, large eddy simulation (LES) analogy and deduction from the TKE balance. All methods were employed using 2-D and 3-D approximations from stereo-PIV data. The LES analogy was deemed to provide the best estimate, since it accounts for three-dimensionality of the flow and models turbulence at the smallest scales using a subgrid scale model. (Continues...)