Fundamental Flow Research Articles

MHD Slip Flow of CNT-Ethylene Glycol Nanofluid due to a Stretchable Rotating Disk with Cattaneo–Christov Heat Flux Model

This article deals with carbon nanoliquid flow due to stretchable rotating disk with the effect of Cattaneo–Christov heat flux model. Both SWCNTs and MWCNTs are considered with ethylene glycol as the base fluid. The effects of nanoparticle volume friction, normally applied magnetic field, stretching factor, velocity, and thermal slip factors are examined. The fundamental flow governing equations are transformed into dimensionless system of coupled nonlinear ordinary differential equations, and they are solved numerically using spectral quasi-linearization method (SQLM). Employing graphs and tables, the results of velocity and temperature fields as well as skin friction coefficient and local heat transfer rate are analyzed and presented via embedded parameters. The results reveal that higher velocity fields and lower temperature fields are noticed in the MWCNT nanofluids than SWCNT nanofluids. The higher incidence of magnetic field improves the thermal boundary layer thickness. A growth in velocity slip factor reduces the momentum boundary layer thickness of the nanoliquid flow. Generally, radial stretching of the disk is helpful in improving the cooling process of the rotating disk in practical applications.

Radial flow velocity profiles of a yield stress fluid between smooth parallel disks

In rock grouting, idealized 2D-radial laminar flow of yield stress fluids (YSF) is a fundamental flow configuration that is used for cement grout spread estimation. A limited amount of works have presented analytical and numerical solutions on the radial velocity profiles between parallel disks. However, to the best of our knowledge, there has been no experimental work that has presented measured velocity profiles for this geometry. In this paper, we present velocity profiles of Carbopol (a simple YSF), measured by pulsed ultrasound velocimetry within a radial flow model. We describe the design of the physical model and then present the measured velocity profiles while highlighting the plug-flow region and slip effects observed for three different apertures and volumetric flow rates. Although the measured velocity profiles exhibited wall slip, there was a reasonably good agreement with the analytical solution. We then discuss the major implications of our work on radial flow.

Modeling of Oil/Water Interfacial Dynamics in Three-Dimensional Bistable Electrowetting Display Pixels.

Electrowetting has drawn significant interest because of the potential applications of displays, lab-on-a-chip microfluidic devices, electro-optical switches, and so forth. However, electrowetting display (EWD) is monostable, which needs extra continuous voltage supply to keep contracting the oil. This paper is concerned with the simulation of two-phase liquid flow in three-dimensional EWD pixels with two electrodes (E1 and E2) demonstrating bistability, where power is only needed to move the oil droplet between two stable states. The effects of E1 geometry, E2 geometry, and E2 pulse characteristics on the dynamics of the oil droplet motion have been analyzed. Also, predictions of the transient states in four stages of the reversible bistable operation process have been carried out by employing the finite element method, in qualitative agreement with our experimental results of the monostable EWD and the existing literature. We seek to shed more light on the fundamental two-phase liquid flow in three-dimensional pixels exhibiting bistability for low power EWD and guide optimizing the electrodes to the perfect patterns with the aid of rigorous modeling.

Stochastic turbulence modeling in RANS simulations via multilevel Monte Carlo

A multilevel Monte Carlo (MLMC) method for quantifying model-form uncertainties associated with the Reynolds-Averaged Navier-Stokes (RANS) simulations is presented. Two, high-dimensional, stochastic extensions of the RANS equations are considered to demonstrate the applicability of the MLMC method. The first approach is based on global perturbation of the baseline eddy viscosity field using a lognormal random field. A more general second extension is considered based on the work of [Xiao et al. (2017)], where the entire Reynolds Stress Tensor (RST) is perturbed while maintaining realizability. For two fundamental flows, we show that the MLMC method based on a hierarchy of meshes is asymptotically faster than plain Monte Carlo. Additionally, we demonstrate that for some flows an optimal multilevel estimator can be obtained for which the cost scales with the same order as a single CFD solve on the finest grid level.

Bio‐mediated soil improvement: The way forward

AbstractBio‐mediated soil improvement involves the usage of microbes to improve soil engineering performance through a series of bio‐geochemical processes. In particular, Microbially Induced Calcite Precipitation (MICP), a ubiquitous bio‐geochemical process that occurs in soil and results in permanent inorganic cementation between soil grains, has received the greatest research focus. While substantial progress has been made to develop MICP as a mainstream soil improvement technique, we still need to: (a) improve our understanding of the fundamental microbial, chemical and flow processes involved; (b) achieve multi‐functionality by coupling engineering performance enhancement with ecological, environmental and carbon footprint benefits; and (c) maintain ecological balance and environmental friendliness, avoid long‐term deterioration and lower the energy demand.

Fundamental flow problems considering non-Newtonian hyperbolic tangent fluid with Navier slip: Homotopy analysis method

The presented research article deals with the classical fundamental flows (Poiseuille and Couette) of an incompressible hyperbolic tangent fluid while considering the Navier slip at the walls. The governing equations are solved using the homotopy analysis method. The velocity expressions are obtained for each problem and the effect of the flow parameters are discussed while being supplemented by graphical displays. Increasing the slip parameter reduces the fluid velocity for both problems, respectively. An increase in the Weissenberg number shows that skin friction at the lower wall reduces. This shows that a dominant elastic force is crucial in reducing the skin friction.

Total Factor Productivity and Idiosyncratic Volatility Trends

Firms’ idiosyncratic stock return volatility has become more volatile in the US since the 1960s. This paper investigates why individual stocks became more volatile over the 1964–2013 period using firm-level total factor productivity (TFP). On average, the volatility of idiosyncratic TFP growth rate has increased, being associated with higher idiosyncratic return volatility. The connection between TFP growth and economic profits provides an explanation for the increase in the idiosyncratic volatility of fundamental cash flows. The results are robust when using time-series and panel regressions and controlling for cash flow and earnings variability, size, book-to-market, leverage, profitability, age, dividend yield, and stock illiquidity.

Experimental study on two-dimensional cross-flow impeller with a minimal splitter plate instead of casings

In the present study, the authors investigate the simplest model of cross-flow impellers, in order to provide the fundamental flow features of cross-flow impellers. The model is a two-dimensional cross-flow impeller with a very-high aspect ratio, and rotates in stationary fluid. More strictly, we occasionally place a splitter plate with minimal influence instead of casings for more precise observations and measurements. Instantaneous measurements of flow-velocity vectors on the mid-span plane are conducted using a CCD camera and a double-pulse YAG laser with a particle-image-velocimetry technique, in addition to conventional flow visualisations and hot-wire-velocimetry measurements. As a result, the authors show the revolutional motion of such an eccentric vortex as a large-scale re-circulating flow structure inside the cross-flow impeller, and reveal the detail of flow features at Reynolds number Re = 300-2500. Furthermore, the authors show the relations between Re and out-flow rate from the impeller.

A NURBS-based finite element formulation for incompressible fluid dynamics and fluid-structure interaction with rigid bodies

Abstract A numerical investigation is performed here using a NURBS-based finite element formulation applied to classical Computational Fluid Dynamics (CFD) and Fluid-Structure Interaction (FSI) problems. Model capabilities related to refinement techniques are analyzed using a finite element formulation with NURBS (non uniform rational B-splines) basis functions, where B-splines and low-order Lagrangian elements can be considered as particular cases. An explicit two-step Taylor-Galerkin model is utilized for discretization of the fundamental flow equations and turbulence is considered using Large Eddy Simulation (LES) and the Smagorinsky’s sub-grid scale model. FSI is considered using an ALE kinematic formulation and a conservative partitioned coupling scheme with rigid body approach for large rotations is adopted. CFD and FSI applications are analyzed to evaluate the accuracy associated with the different refinement procedures utilized. Results show that high order basis functions with appropriate refinement and non-uniform parameterization lead to better predictions, compared with low-order Lagrangian models.

Remote sensing-based river discharge estimation for a small river flowing over the high mountain regions of the Tibetan Plateau

ABSTRACT River discharge, as one of the most essential climate variables, plays a vital role in the water cycle. Small-scale headwater catchments including high-mountain regions of Tibetan Plateau (TP) Rivers are mostly ungauged. Satellite technology shows its potential to fill this gap with a high correlation of satellite-derived effective river width and corresponding in-situ gauged discharge. This study is innovative in estimating daily river discharge using modified Manning equation (Model 1), Bjerklie equation (Model 2), and Rating curve approach (Model 3) by combining river surface hydraulic variables directly derived from remote-sensing datasets with other variables indirectly derived from empirical equations, which greatly contributes to the improvement of river flow measurement information especially over small rivers of TP. We extracted the effective width from Landsat image and flow depth via hydraulic geometry approach. All the input parameters directly or indirectly derived from remote sensing were combined and substituted into the fundamental flow equations/models to estimate discharges of Lhasa River. The validation of all three models’ results against the in-situ discharge measurements shows a strong correlation (the Nash–Sutcliffe efficiency coefficient (NSE) and the coefficient of determination (R 2) values ≥0.993), indicating the potentiality of the models in accurately estimating daily river discharges. Trends of an overestimation of discharge by Model 1 and underestimation by Model 2 are observed. The discharge estimation by using Model 3 outperforms Model 1 and Model 2 due to the uncertainties associated with the estimation of input parameters in the other two models. Generally, our discharge estimation methodology performs well and shows a superior result as compared with previously developed multivariate empirical equations and its application for other places globally can be the focus of upcoming studies.

Modernization of a Photochemical Reaction for the Undergraduate Laboratory: Continuous Flow Photopinacol Coupling

A continuous flow photopinacol coupling of benzophenone has been developed and implemented in the undergraduate laboratory. This photoreduction was accomplished using 2-propanol as an H atom donor and cosolvent in the reaction. The simple and inexpensive UV light source and flow reactor design (less than $350/photochemical flow system) provide an operationally accessible experiment. Students successfully compared the efficiencies (percentage conversion) of both flow and batch methods and explored fundamental continuous flow concepts to explain how and why continuous flow can improve the efficiency of a photochemical reaction. The percentage conversions for the flow reactions (average 82%) were more than 8 times higher than the batch reactions, based on irradiation of equal reaction volumes for 10 min.

Analysis of a civil aircraft wing transonic shock buffet experiment

The physical mechanism governing the onset of transonic shock buffet on swept wings remains elusive, with no unequivocal description forthcoming despite over half a century of research. This paper elucidates the fundamental flow physics on a civil aircraft wing using an extensive experimental database from a transonic wind tunnel facility. The analysis covers a wide range of flow conditions at a Reynolds number of around . Data at pre-buffet conditions and beyond onset are assessed for Mach numbers between 0.70 and 0.84. Critically, unsteady surface pressure data of high spatial and temporal resolution acquired by dynamic pressure-sensitive paint is analysed, in addition to conventional data from pressure transducers and a root strain gauge. We identify two distinct phenomena in shock buffet conditions. First, we highlight a low-frequency shock unsteadiness for Strouhal numbers between 0.05 and 0.15, based on mean aerodynamic chord and reference free stream velocity. This has a characteristic wavelength of approximately 0.8 semi-span lengths (equivalent to three mean aerodynamic chords). Such shock unsteadiness is already observed at low-incidence conditions, below the buffet onset defined by traditional indicators. This has the effect of propagating disturbances predominantly in the inboard direction, depending on localised separation, with a dimensionless convection speed of approximately 0.26 for a Strouhal number of 0.09. Second, we describe a broadband higher-frequency behaviour for Strouhal numbers between 0.2 and 0.5 with a wavelength of 0.2 to 0.3 semi-span lengths (0.6 to 1.2 mean aerodynamic chords). This outboard propagation is confined to the tip region, similar to previously reported buffet cells believed to constitute the shock buffet instability on conventional swept wings. Interestingly, a dimensionless outboard convection speed of approximately 0.26, coinciding with the low-frequency shock unsteadiness, is found to be nearly independent of frequency. We characterise these coexisting phenomena by use of signal processing tools and modal analysis of the dynamic pressure-sensitive paint data, specifically proper orthogonal and dynamic mode decomposition. The results are scrutinised within the context of a broader research effort, including numerical simulation, and viewed alongside other experiments. We anticipate our findings will help to clarify experimental and numerical observations in edge-of-the-envelope conditions and to ultimately inform buffet-control strategies.

A computational study of the effects of vocal fold stiffness parameters on voice production

A three-dimensional flow-structure interaction model of voice production is used to investigate the effect of the stiffness parameters of vocal fold layers on voice production. The vocal fold is modeled as a three-layer structure consisting of the cover, ligament, and body layers. All the three layers are modeled as transversely isotropic materials for which the stiffness parameters include the transverse elastic modulus and longitudinal elastic modulus. The results show that, in addition to the obvious monotonic effects on the fundamental frequency, flow rate and glottis opening, the stiffness parameters also have significant and nonmonotonic effects on the divergent angle, open quotient, and closing velocity. It is further found that the longitudinal stiffness parameters generally have more significant impacts on glottal flows and vocal fold vibrations than the transverse stiffness parameters. The sensitivity analysis shows that, among all the stiffness parameters, the transverse and longitudinal stiffness of the ligament layer have the most dominant effect on most output measures.

Reliability Modeling of AC Filter in UHVDC Link Based on Reactive Power Compensation and Harmonic Performance Evaluation

Considering reactive power compensation and harmonic performance evaluation, the reliability model of the AC filter (ACF) in ultra high-voltage direct-current link is newly proposed. The state-space models of the filters are combined by matrix description to obtain transfer rates among the states that represent the configuration schemes, i.e., the type and number of the available filters. The sensitivity model of the combined ACF subsystem is proposed to find out the weakest filter. Due to failure of the filters, both lack of reactive power compensation and harmonic current, which exceed the limit, cause loss of the transmission capacity. The fundamental and harmonic power flow models are proposed to quantify the compensated reactive power and evaluate the harmonic indices, by which the configuration schemes are grouped with the capacity levels (CLs) of the transmitted power and regrouped to satisfy the harmonic standards. The numerical results demonstrate that the transitions among 100% CL, 75% CL, and 50% CL to 0% CL appear, due to which, the configuration schemes, which are grouped by the reactive power compensation but do not satisfy the harmonic standards, are removed to the group of 0% CL.

Generalized Hypersonic Equivalence Principle

The concept of the generalized hypersonic equivalence principle for viscous flows is proposed in this paper. The traditional hypersonic equivalence principle applies only to the inviscid flows and pressure prediction. The present generalized principle indicates that the skin friction and the heat flux resulting from a steady viscous flow over a slender body could also be approximately analogous to the counterparts in an equivalent unsteady flow in one less space dimension. The key point is that the hypersonic velocity component perpendicular to the reduced unsteady flow plane should be retained and its evolution with time be properly treated to produce strong shear and dissipation effects. The generalized principle is specifically implemented by using the direct simulation Monte Carlo method to study the fundamental boundary-layer flow models and a practical complex viscous-interaction flow over a typical vehicle model. The results of the pressure, skin friction, and heat flux from the steady flows and the corresponding reduced unsteady flows are compared and analyzed. This study points out an inherent similarity between the hypersonic steady flow and the dimension-reduced unsteady flow, and also shows a potential application value in the engineering practice.

Relation between oil-water interfacial flow structure and their separation in the oil-water mixture flow in a curved channel: An experimental study

We investigate the oil-water separation in a curved channel under a range of inflow conditions, focusing on the fundamental multiphase flow physics occurring at the oil-water interface. With a silicone oil (viscosity: 10 mm2/s, density: 935 kg/m3), we perform a series of water-tunnel experiments to measure the evolution of oil-water mixture flow inside a curved channel, devised for realizing a continuous and effective oil recovery (separation). The oil-water mixture velocity at the channel inlet is 1.35–2.35 m/s (Reynolds and Froude numbers based on the inlet height and length of the channel are 0.6–2.0 × 104 and 0.14, respectively), and we vary the inlet oil volume fraction as 0.49–0.85. We find that the efficiency of oil separation is strongly affected by the oil-water interfacial flow structures and identify two typical flow patterns: wavy oil-water interface and dispersed-oil flow. When the inlet oil fraction is high ( > 0.74), the instability occurring at the wavy oil-water interface plays a dominant role in determining the oil recovery rate (which is as high as 80% in general). As the inlet oil fraction becomes smaller ( < 0.7), on the other hand, the oil dispersion starts to appear vigorously, which interferes with the oil separation process and the recovery rate drops below 60% at the oil fraction smaller than 0.5. Through the quantitative analysis of the optically measured oil-water interface phenomena, we suggest theoretical models to predict the oil recovery rate as a function of the interfacial fluctuation of wavy oil-water interface and the fraction of dispersed oil phase, depending on the inlet flow conditions. We also propose a strategy to maximize the oil recovery rate of large-scale oil-water separation device, which is expected to be beneficial in many applications.

Comparative analysis of marine traffic flow in classical models

Abstract Traffic flow theory has been applied to the study of marine traffic flow and underpins the phenomena being studied. This paper is intended to demonstrate the uses of classical fundamental diagrams, which are developed previously on the basis of highway traffic, by conducting empirical tests on marine traffic data. Marine traffic data are extracted from the Automatic Identification System (AIS) from Shanghai port in 2010 and 2018. Interpolation methods are used to estimate the three primary variables of fundamental diagrams, namely, flow rate, traffic density, and local speed. The relationships among these three variables are discussed and marine traffic data are further compared and contrasted with classical traffic flow models. The importance of the finding is to create consistency, predictability and uniformity of uses of fundamental diagrams on marine traffic flow.

Identifying eigenmodes of averaged small-amplitude perturbations to turbulent channel flow

Eigenmodes of averaged small-amplitude perturbations to a turbulent channel flow – which is one of the most fundamental canonical flows – are identified for the first time via an extensive set of high-fidelity graphics processing unit accelerated direct numerical simulations. While the system governing averaged small-amplitude perturbations to turbulent channel flow remains unknown, the fact such eigenmodes can be identified constitutes direct evidence that it is linear. Moreover, while the eigenvalue associated with the slowest-decaying anti-symmetric eigenmode mode is found to be real, the eigenvalue associated with the slowest-decaying symmetric eigenmode mode is found to be complex. This indicates that the unknown linear system governing the evolution of averaged small-amplitude perturbations cannot be self-adjoint, even for the case of a uni-directional flow. In addition to elucidating aspects of the flow physics, the findings provide guidance for development of new unsteady Reynolds-averaged Navier–Stokes turbulence models, and constitute a new and accessible benchmark problem for assessing the performance of existing models, which are used widely throughout industry.

Hierarchical geometry modelling using the immersed boundary method

The Immersed Boundary Method has been used to simulate a range of fundamental flows and turbulence. Such studies have demonstrated the method’s promising applicability for engineering analysis. However, to the authors’ knowledge, flows in coupled components or in scenarios with coupled physics, such as rotor–stator interaction, fan–intake interaction, aeroelastics, aeroacoustics, etc., are still rarely investigated using high-fidelity methods. Due to its high computational costs, the complexity of geometry meshing process and the requirement for moving boundaries limit the investigation of flows in such environments. Previous research suggests that high-fidelity simulations with an acceptable geometry modelling strategy may tackle these issues and provide useful insights. There exists a hierarchy of geometry modelling methods which includes the conventional Directly Mesh Resolving (DMR) method, the Immersed Boundary Method (IBM), and the IBM with Smeared Geometry (IBMsg, or eIBMg). The present research proposes an alternative to these approaches in the form of the Euler IBM with local force (eIBMl) by imposing a distribution function generated from blade configuration. Compared to the eIBMg, this method can include more realistic flow physics within each blade passage without smearing its geometry. This method is applied to the study of fan–intake interaction focusing on the transport of inlet distortion through blade passages and pressure wave propagation.

Width-Based Cell Transmission Model for Heterogeneous and Undisciplined Traffic Streams

Traffic streams in many developing countries consist of various modes of transport, with high heterogeneity in driver behavior. Modeling these types of traffic streams, in which traffic rules (speed limit, lane discipline, etc.) are not strictly followed, is a complex task. A review of the existing literature shows that there is a lack of traffic flow models that model the behavior of heterogeneous and undisciplined traffic streams. Like other undisciplined traffic streams, there are no speed limits (hence no speed enforcement) on most of the roads in Karachi, Pakistan. Lane discipline is also not observed by drivers, which results in a varying number of traffic lanes on a road. Therefore, most of the existing traffic flow models/simulation packages developed for disciplined traffic streams cannot appropriately model traffic streams without lane discipline. This research proposes a width-based cell transmission model (WCTM) by developing a fundamental flow-density diagram whose parameters are a function of the road width. Extensive field data have been collected from a selected arterial in Karachi for development of the fundamental traffic flow diagram. The values of the computed parameters are significantly different than the values reported in the literature. The piecewise-linear flow-density relation is developed by optimally estimating the breakpoints. Results show that the quadrilateral and pentagonal-shaped fundamental diagrams fit better with the collected data in comparison with the triangular-shaped fundamental diagram. The proposed WCTM is applied to selected segments of an arterial and results show that the WCTM was able to accurately model different traffic conditions.