Turbulent dispersed multiphase flows are common in many engineering and environmental applications. The stochastic nature of both the carrier-phase turbulence and the dispersed-phase distribution makes the problem of turbulent dispersed multiphase flow far more complex than its single-phase counterpart. In this article we first review the current state-of-the-art experimental and computational techniques for turbulent dispersed multiphase flows, their strengths and limitations, and opportunities for the future. The review then focuses on three important aspects of turbulent dispersed multiphase flows: the preferential concentration of particles, droplets, and bubbles; the effect of turbulence on the coupling between the dispersed and carrier phases; and modulation of carrier-phase turbulence due to the presence of particles and bubbles.

Turbulent Dispersed Multiphase Flow

The Lagrangian description of turbulence is characterized by a unique conceptual simplicity and by an immediate connection with the physics of dispersion and mixing. In this article, we report some motivations behind the Lagrangian description of turbulence and focus on the statistical properties of particles when advected by fully developed turbulent flows. By means of a detailed comparison between experimental and numerical results, we review the physics of particle acceleration, Lagrangian velocity structure functions, and pairs and shapes evolution. Recent results for nonideal particles are discussed, providing an outlook on future directions.

Lagrangian Properties of Particles in Turbulence

Digital holography is an emerging field of new paradigm in general imaging applications. We present a review of a subset of the research and development activities in digital holography, with emphasis on microscopy techniques and applications. First, the basic results from the general theory of holography, based on the scalar diffraction theory, are summarized, and a general description of the digital holographic microscopy process is given, including quantitative phase microscopy. Several numerical diffraction methods are described and compared, and a number of representative configurations used in digital holography are described, including off-axis Fresnel, Fourier, image plane, in-line, Gabor, and phase-shifting digital holographies. Then we survey numerical techniques that give rise to unique capabilities of digital holography, including suppression of dc and twin image terms, pixel resolution control, optical phase unwrapping, aberration compensation, and others. A survey is also given of representative application areas, including biomedical microscopy, particle field holography, micrometrology, and holographic tomography, as well as some of the special techniques, such as holography of total internal reflection, optical scanning holography, digital interference holography, and heterodyne holography. The review is intended for students and new researchers interested in developing new techniques and exploring new applications of digital holography.

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Principles and techniques of digital holographic microscopy

Background and purpose the purpose of this study was to identify significant morphological and hemodynamic parameters that discriminate intracranial aneurysm rupture status using 3-dimensional angiography and computational fluid dynamics. Methods one hundred nineteen intracranial aneurysms (38 ruptured, 81 unruptured) were analyzed from 3-dimensional angiographic images and computational fluid dynamics. Six morphological and 7 hemodynamic parameters were evaluated for significance with respect to rupture. Receiver operating characteristic analysis identified area under the curve (AUC) and optimal thresholds separating ruptured from unruptured aneurysms for each parameter. Significant parameters were examined by multivariate logistic regression analysis in 3 predictive models-morphology only, hemodynamics only, and combined-to identify independent discriminants, and the AUC receiver operating characteristic of the predicted probability of rupture status was compared among these models. Results morphological parameters (size ratio, undulation index, ellipticity index, and nonsphericity index) and hemodynamic parameters (average wall shear stress [WSS], maximum intra-aneurysmal WSS, low WSS area, average oscillatory shear index, number of vortices, and relative resident time) achieved statistical significance (P Conclusions all 3 models-morphological (based on size ratio), hemodynamic (based on WSS and oscillatory shear index), and combined-discriminate intracranial aneurysm rupture status with high AUC values. Hemodynamics is as important as morphology in discriminating aneurysm rupture status.

Hemodynamic-morphologic discriminants for intracranial aneurysm rupture.

Background and Purpose— Arterial bifurcation apices are common sites for cerebral aneurysms, raising the possibility that the unique hemodynamic conditions associated with flow dividers predispose the apical vessel wall to aneurysm formation. This study sought to identify the specific hemodynamic insults that lead to maladaptive vascular remodeling associated with aneurysm development and to identify early remodeling events at the tissue and cellular levels. Methods— We surgically created new branch points in the carotid vasculature of 6 female adult dogs. In vivo angiographic imaging and computational fluid dynamics simulations revealed the detailed hemodynamic microenvironment for each bifurcation, which were then spatially correlated with histologic features showing specific tissue responses. Results— We observed 2 distinct patterns of vessel wall remodeling: (1) hyperplasia that formed an intimal pad at the bifurcation apex and (2) destructive remodeling in the adjacent region of flow acceleration that resembled the initiation of an intracranial aneurysm, characterized by disruption of the internal elastic lamina, loss of medial smooth muscle cells, reduced proliferation of smooth muscle cells, and loss of fibronectin. Conclusions— Strong localization of aneurysm-type remodeling to the region of accelerating flow suggests that a combination of high wall shear stress and a high gradient in wall shear stress represents a “dangerous” hemodynamic condition that predisposes the apical vessel wall to aneurysm formation.

Complex Hemodynamics at the Apex of an Arterial Bifurcation Induces Vascular Remodeling Resembling Cerebral Aneurysm Initiation

Object. Few researchers have quantified the role of arterial geometry in the pathogenesis of saccular cerebral aneurysms. The authors investigated the effects of parent artery geometry on aneurysm hemodynamics and assessed the implications relative to aneurysm growth and treatment effectiveness. Methods. The hemodynamics of three-dimensional saccular aneurysms arising from the lateral wall of arteries with varying arterial curves (starting with a straight vessel model) and neck sizes were studied using a computational fluid dynamics analysis. The effects of these geometric parameters on hemodynamic parameters, including flow velocity, aneurysm wall shear stress (WSS), and area of elevated WSS during the cardiac cycle (time-dependent impact zone), were quantified. Unlike simulations involving aneurysms located on straight arteries, blood flow inertia (centrifugal effects) rather than viscous diffusion was the predominant force driving blood into aneurysm sacs on curved arteries. As the degree of arterial c...

Effects of arterial geometry on aneurysm growth: Three-dimensional computational fluid dynamics study

Holographic particle image velocimetry (HPIV) offers potentially the best solution to volumetric measurements of the three-dimensional velocity fields in complex flows. However, the traditional film-based HPIV measurement is rather cumbersome, limiting its use to only a handful of groups worldwide. The newly emerged digital HPIV revolutionizes flow measurement science by providing a practical 3D velocimetry tool. It commands simple hardware that is similar to regular two-dimensional particle image velocimetry (PIV), yet it provides continuous (time-series) three-dimensional, three-component flow field data. Not only is the need for chemical processing eliminated, but also the cumbersome optical reconstruction is completely replaced by numerical reconstruction algorithms. Several breakthroughs have led to the development of the first practical and integrated digital HPIV systems. To explain the transition from film to digital recording, fundamental issues in HPIV are reviewed in this paper. Axial accuracy in HPIV measurement is ultimately limited by an inherent depth-of-focus problem, while information capacity is limited by inherent speckle noise. Information capacity is an important concept in HPIV, comprising the maximum acceptable seeding density multiplied by the sample volume depth along the optic axis. Both the axial accuracy and the information capacity are limited by the effective hologram aperture. The pursuit of a large hologram aperture in the past has resulted in further complexity in film-based HPIV systems. Digital HPIV, on the other hand, enjoys great simplicity of implementation and operation. A digital HPIV is also far more compact and rugged compared to existing film-based HPIV systems, making it suitable for duplication and commercialization. However, since digital sensors suffer from inferior pixel resolutions compared to films, the effective hologram aperture is much smaller in digital HPIV than that achievable in film-based HPIV. Alleviating this problem, digital HPIV also presents new possibilities in data processing such as the use of the complex amplitude of the reconstructed light wave to improve depth sensitivity and signal-to-noise ratio. Two examples of digital HPIV systems and measurement results are given. We believe digital HPIV can revitalize holographic particle imaging and bring it into the mainstream in much the same way that digital PIV brought PIV into widespread use a decade ago.

http://iopscience.iop.org/article/10.1088/0957-0233/15/4/009/pdf

Holographic particle image velocimetry: from film to digital recording

This paper presents the first detailed comparisons between experiments and direct numerical simulations (DNS) of inertial particle clustering in nearly isotropic 'box turbulence'. The experimental system consists of a box 38 cm in each dimension with fans in the eight corners that sustain nearly isotropic turbulence in the centre of the box. We inject hollow glass spheres with a mean diameter of 6 μm and measure the locations of several hundred particles in a 1 cm 3 volume in the centre of the box using three-dimensional digital holographic particle imaging. We observe particle concentration fluctuations that result from inertial clustering (sometimes called 'preferential concentration'). The radial distribution function (RDF), a statistical measure of clustering, has been calculated from the particle position field. We select this measure because of its relevance to the collision kernel for particles. DNS of the equivalent system, with nearly perfect parameter overlap, have also been performed. We observe good agreement between the RDF predictions of the DNS and the experimental observations, despite some challenges in the interpretation of the experiments. The results provide important guidance on ways to improve the measurement.

Experimental and numerical investigation of inertial particle clustering in isotropic turbulence

Flow dynamics is a key player in the initiation and progression of vascular diseases such as atherosclerosis and cerebral aneurysms [1–4]. Numerous hemodynamic parameters, such as wall shear stress (WSS), pressure, oscillatory shear index, wall shear stress gradient, impingement size on the arterial wall, and residence time of blood, have been postulated to indicate the tendency for initiation or progression of these vascular pathologies and to evaluate the effectiveness of medical devices in the treatments of such diseases [1–4]. However, in vivo measurements of these hemodynamic parameters for patients are difficult and often cost prohibitive. The combination of noninvasive diagnostic tools (e.g. MRI, CT, or ultrasound) and image-based computational fluid dynamics (CFD) techniques provides an alternative to in vivo measurements to estimate these patient-specific hemodynamic parameters [5–12]. Such image-based CFD analysis has the potential to provide key hemodynamic feedback for prospective studies of vascular diseases and to plan for individual therapeutic options [6,11,13].

Although the application of image-based CFD is rapidly advancing, the ever-present uncertainties in CFD analysis need to be evaluated. In particular, CFD outputs such as WSS or WSS gradients are extremely sensitive to the acquired in vivo geometry. The accuracy and reproducibility of the image-based CFD geometries depend on the orientation of the subject, physiological condition of the subject, variation in vascular structure, contrast-media injection techniques, and operator-dependent decisions in converting the medical images to CFD models [14,15]. Such variations produce poor geometric renditions that can generate as much as 60% variation in hemodynamic parameters obtained using CFD, especially in complex flow regions such as bifurcation apexes, anastomoses, flow separation zones, and aneurysm inflow zones [8,15–17]. Smoothing can reduce poor geometric renditions [16], but the process is somewhat subjective, resulting in an unknown deviation from the true in vivo geometry and subsequent in vitro and CFD models. In addition, the validity of CFD results relies heavily on both temporal and spatial boundary conditions [17–19], which are not routinely available in clinical practice. Significant efforts have been made to improve image-based CFD techniques [8,14,16,20–22], but there have been insufficient efforts to validate the CFD results with experiments.

In vitro experimental studies have often been employed to investigate the hemodynamics of vascular diseases and mechanical treatment devices, as well as assisting the development of CFD techniques. Rhee et al. and Imbesi et al. explored the influence of aneurysm geometry and intervention through flow visualization techniques [23,24]. Particle image velocimetry (PIV) and particle tracking velocimetry have been utilized to study the influence of intervention on aneurysmal flow [25–29]. Direct in vivo measurements of blood flow through x-ray imaging, MRI, and ultrasound techniques have also been explored [10,30–33]. MRI and ultrasound allow noninvasive measurement of instantaneous velocities in the 3D domain. Based on the field data, velocity gradients and strain rates are obtained to determine shear stresses and shear stress gradients. However, to date, in vitro studies have been limited to 2D flow fields and scaled-up models that provide optical access. These studies experience significant difficulties in reproducing the in vivo geometries [23] and in vivo biological environments. Direct in vivo measurements pose technical shortcomings, such as inadequate resolutions to derive the flow velocity near the wall, uncertainties in wall positions and insufficient velocity vectors to estimate the derivatives in complex flow regions [10,11,30,31,34–36].

One of the clinical applications of CFD is cerebral vascular diseases such as cerebral aneurysms. Subarachnoid hemorrhage following cerebral aneurysm rupture is often catastrophic, resulting in 12.4% of patient death before patients receive medical attention [37]. Recent studies using aneurysm geometry to predict the risks of aneurysm rupture relied heavily on the imaged aneurysm geometry [38–41]. Prospective patient-specific aneurysm studies that combine medical imaging, CFD analysis, and knowledge of biological responses to hemodynamics forces can provide insight into the hemodynamics of cerebral aneurysms, scientific assessments on therapeutic planning and, ultimately, positive clinical outcomes. However, quantitative validation of image-based CFD analysis against experimental and in vivo data has, to date, not been fully addressed. Following a prior CFD study, which focused on the characteristics of the hemodynamics in an aneurysm located on a curved vessel [42], a PIV validation effort was undertaken in our lab, leading to the investigation of flow sensitivity to aneurysm geometric variations. The goals of this study were to validate our CFD results with PIV experiments using an experimental aneurysm phantom and to quantify the influence of geometric variations on the aneurysmal flow dynamics.

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Validation of CFD simulations of cerebral aneurysms with implication of geometric variations.

Measuring the turbulent kinetic energy dissipation rate in an enclosed turbulence chamber that produces zero-mean flow is an experimental challenge. Traditional single-point dissipation rate measurement techniques are not applicable to flows with zero-mean velocity. Particle image velocimetry (PIV) affords calculation of the spatial derivative as well as the use of multi-point statistics to determine the dissipation rate. However, there is no consensus in the literature as to the best method to obtain dissipation rates from PIV measurements in such flows. We apply PIV in an enclosed zero-mean turbulent flow chamber and investigate five methods for dissipation rate estimation. We examine the influence of the PIV interrogation cell size on the performance of different dissipation rate estimation methods and evaluate correction factors that account for errors related to measurement uncertainty, finite spatial resolution, and low Reynolds number effects. We find the Re
 λ
 corrected, second-order, longitudinal velocity structure function method to be the most robust method to estimate the dissipation rate in our zero-mean, gaseous flow system.

Scott H. Woodward

Papers

Effects of arterial geometry on aneurysm growth: Three-dimensional computational fluid dynamics study

Holographic particle image velocimetry: from film to digital recording

Experimental and numerical investigation of inertial particle clustering in isotropic turbulence

Validation of CFD simulations of cerebral aneurysms with implication of geometric variations.

Dissipation rate estimation from PIV in zero-mean isotropic turbulence