Fluid Dynamic Point Of View Research Articles

The Development and Optimization of a New Wind Tunnel Design for Odour Sampling

The characterization of passive area sources, emitting odours due to wind-driven convection, poses significant challenges. The present experimental study aims to evaluate the performance, in terms of fluid dynamics and mass transfer, of a recently developed wind tunnel, with a more compact design and reduced weight, compared to the one proposed by the Italian regulations. The results show that the new design outperforms the Italian standard in several aspects. From a fluid dynamic point of view, the new wind tunnel exhibits a slightly more homogenous and uniform velocity distribution, and it does not reveal airflow preferential channels inside the central body. The pressure tests highlight that the presence of fillers in the new wind tunnel does not significantly alter the pressure inside the hood and therefore the gas–liquid equilibrium conditions; actually, the slight overpressure may help to prevent the infiltration of external air. Finally, mass transfer tests on the standard device show a vertical concentration gradient along the outlet duct, highlighting concentration values that differ up to a factor of two depending on the measurement point. The new design has almost completely solved this issue, thanks to the use of fillers that promote mixing of the outlet flow.

Design And Optimization Of Aeronautical Components And Digital Twins Development

In this work, the feasibility of using reduced models (ROMs) for optimizing a scoop air intake for aeronautical applications was evaluated. Since the air intake is exposed to aerodynamic loads, a two-way fluid-structure interaction workflow was used to characterize the component. The goal is to create an optimization dashboard that allows both scalar quantities (the parameters that are intended to be monitored during the design and optimization of the air intake) and field quantities to be evaluated in real time. In this way, the designer can have a full understanding of the physics of the problem and make more informed design choices. In addition, in this way it is possible to visualize results from different physics in a single dashboard, linking different components, interacting with models in real time. A mesh morphing technique based on Radial Basis Functions (RBFs) was used. The result was very interesting both from a structural point of view (mass reduction over 90% and maximum strain reduction of 36%) and from a fluid dynamic point of view (outlet pressure 86% higher and drag 32% lower) and the generated ROMs proved to be a very accurate (ROM relative error in the order of 7%) and flexible tool.

Mechanical and Computational Fluid Dynamic Models for Magnesium-Based Implants.

Today, mechanical properties and fluid flow dynamic analysis are considered to be two of the most important steps in implant design for bone tissue engineering. The mechanical behavior is characterized by Young's modulus, which must have a value close to that of the human bone, while from the fluid dynamics point of view, the implant permeability and wall shear stress are two parameters directly linked to cell growth, adhesion, and proliferation. In this study, we proposed two simple geometries with a three-dimensional pore network dedicated to a manufacturing route based on a titanium wire waving procedure used as an intermediary step for Mg-based implant fabrication. Implant deformation under different static loads, von Mises stresses, and safety factors were investigated using finite element analysis. The implant permeability was computed based on Darcy's law following computational fluid dynamic simulations and, based on the pressure drop, was numerically estimated. It was concluded that both models exhibited a permeability close to the human trabecular bone and reduced wall shear stresses within the biological range. As a general finding, the proposed geometries could be useful in orthopedics for bone defect treatment based on numerical analyses because they mimic the trabecular bone properties.

Fluid dynamic parameters of naturally derived hydroxyapatite scaffolds for in vitro studies of bone cells

Hydroxyapatite scaffolds obtained from the biomorphic transformation of wood are characterized from a fluid dynamic point of view. Such material of recent introduction offers great advantages for the in vitro study of bone cells, mostly in virtue of its peculiar porous structure. Determining the flow resistance and morphological parameters of these scaffolds is an essential step towards their practical use in bioreactors and microfluidic devices. To this aim, a series of tests involving a draining fluid are performed on a set of disc-shaped scaffolds, followed by the microscopy analysis of the pores visible on the sample faces. Contrarily to what expected, a temperature dependence is observed for the flow resistance, even after normalizing it by the fluid properties. The interpretation of the experimental results is assisted by numerical outcomes from Computational Fluid Dynamics modelling, which underline some limitations in the application of classical laws to the present problem. While the complex and variable internal structure of the scaffolds prevents the systematic use of simplified formulae, a correlation is found between the flow resistance and the pore geometry, which can facilitate the characterization of further samples.

Computational Fluid Dynamics Analysis of 3D Wind Flow around Rhizophora Mangrove Tree and Drag Force Acting on the Wall of the Tree

Many natural disasters, such as cyclones, typhoons, mudslides, and tsunamis, are currently plaguing the world. These tragedies, which have destroyed the coastal settlements, claim a great number of lives each year. The characteristics and structure of the Mangrove tree have the significant impact on protecting the coast from heavy wind, floods, mudslides, and other kinds of natural disasters. This research work investigates how Mangroves can protect the area near the coast from a fluid dynamics point of view. Complex real world problems require intelligent systems that combine knowledge and techniques. Hence Computational Fluid Dynamics (CFD) technique namely the finite volume method is employed as a tool in this study. In this research, a model Rhizophora Mangrove tree from Pichavaram Mangrove forest, Tamilnadu, India is generated using Ansys Workbench design modeler. A computational domain is created around this tree to reduce the boundary effect. Grid-independent research was carried out using various mesh sizes to determine the best grid resolution for the analysis. An unstructured triangular mesh was generated for the simulation. The focus of this study is to determine the 3D flow around the tree in order to determine the wind velocity, as well as the coefficient of drag and drag force on the wall tree. This research helps to give a clear understanding of wind flow around the 3D Rhizophora Mangrove tree. The velocity profile, pressure distribution, drag force, and drag coefficient around the tree are captured, analysed, and presented in detail. The results show that the Rhizophora mangrove trees can significantly reduce the flow velocity of the wind and will be able to safeguard the coast and communities nearby from natural disasters.

Influence of obstruction’s unilateral length on flow and heat transfer performance of micro-channel heat sinks with flow obstructions

As the density and integration of electronic equipment continue to increase, it becomes necessary to build micro-channel heat sinks with more efficient cooling effects to solve the heat removal issue. This paper proposes a series of obstructions established on a smooth water-cooled micro-channel and analyzes the heat transfer and flow performance from the viewpoint of fluid dynamics. By adjusting the unilateral side length of the obstructions, the simulation conducted a comparative analysis between the seven different obstruction shapes and smooth micro-channel heat sinks, focusing on Nusselt number, thermal resistance, and pressure loss across a range of Reynolds numbers. The findings demonstrate that the heat dissipation capacity of micro-channel heat sinks with obstructions is consistently superior to that of the smooth heat sink, and the performance of these heat sinks is affected by the length of unilateral side of the obstructions. With no consideration of pumping power, the paper argues that the heat sink with obstructions, specifically those with a unilateral side length of 0.06 mm, has the best heat dissipation capacity at Reynolds numbers below 460. At this Reynolds number, the Nusselt number increases by 99% and the thermal resistance decreases by 33% compared to a smooth micro-channel heat sink. However, when the pumping power is considered, the heat sink with obstructions whose unilateral sides are the shortest has the best performance due to its minimal pressure loss. While meeting the thermal resistance requirement of 0.033, it requires only one-fourth of the pumping power needed by a heat sink with obstructions whose unilateral sides are longest.

Analysis of airborne sputum droplets flow dynamic behaviors under different ambient conditions and aerosol size effects

The coronavirus (COVID-19) is becoming more threatening with the emergence of new mutations. New virus transmission and infection processes remain challenging and re-examinations of proper protection methods are urgently needed. From fluid dynamic viewpoint, the transmission of virus-carrying droplets and aerosols is one key to understanding the virus-transmission mechanisms. This study shows virus transmission by incorporating flow-evaporation model into the Navier-Stokes equation to describe the group of airborne sputum droplets exhaled under Rosin-Rammler distribution. Solid components and humidity field evolution are incorporated in describing droplet and ambient conditions. The numerical model is solved by an inhouse code using advection-diffusion equation for the temperature field and the humidity field, discretized by applying the total-variation diminishing Runge-Kutta method. The results of this study are presented in detail to show the different trends under various ambient conditions and to reveal the major viral-transmission routes as a function of droplet size.

Diffusion of false Information on Fluid Mechanics

Elementary school textbooks say that in a curved river, the flow is slow on the inside and fast on the outside. On the other hand, from a fluid-dynamic point of view, the inside is fast and the outside is slow. We have investigated why such a contradiction occurs. Previous measurements in downstream rivers have confirmed that sedimentation occurs inside bends in plain rivers, and many rivers slow inside. In this report, measurements were taken in the upstream river. As a result, we observed several cases in which the inner part was faster and the outer part was slower in the upper reaches. Further experiments are needed to confirm this result. In addition, since most people usually see rivers in the lower reaches, it is speculated that elementary school textbooks have described that the inside of the river is slow.

Turbulence Enhancement and Mixing Analysis for Multi-Inlet Vortex Photoreactor for CO2 Reduction

In this article, we describe a prototype photoreactor of which the geometrical configuration was obtained by Genetic Algorithms to maximize the residence time of the reactant gases. A gas reaction mixture of CO2:H2O (1:2 molar ratio) was studied from the fluid dynamic point of view. The two main features of this prototype reactor are the conical shape, which enhances the residence time as compared to a cylindrical shape reference reactor, and the inlet heights and position around the main chamber that enables turbulence and mass transfer control. Turbulence intensity, mixing capability, and residence time attributes for the optimized prototype reactor were calculated with Computational Fluid Dynamics (CFD) software and compared with those from a reference reactor. Turbulence intensity near the envisioned catalytic bed was one percentage point higher in the reference than in the optimized prototype reactor. Finally, the homogeneity of the mixture was guaranteed since both types of reactors had a turbulent regime, but for the prototype the CO2 mass fraction was found to be better distributed.

Forced convective heat tranfer in a metallic foam cylinder cooled by a slot jet flow and comparison with a smooth cylinder and a full flow

The aim of the present paper is to study experimentally the enhancement of heat transfer, due to the presence of a layer of aluminum foam covering a smooth circular cylinder. The measurements are carried on with the impingement of a rectangular jet of air, which has a transversal dimension equal or smaller than the cylinder diameter, or jet flow, and full flow (or uniform flow, which has a greater dimension than the cylinder diameter). Literature does not report any paper dealing with this multiple problem. Experimental heat transfer and fluid dynamics measurements are carried out at Reynolds number ranging from ReD=2200 to ReD=24,750. The layer of the aluminum foam, 8 mm thick, is glued onto the smooth cylinder, with an external diameter of D = 10 mm. The height of the slot jet is H = 10 mm, with a width of W = 135 mm. Jet flow visualizations are performed for the smooth and the foam-covered cylinder to explain the heat transfer results from the fluid dynamics point of view. The flow visualizations show that the jet flow always penetrates into the foam, wetting it completely, while the reattachment never occurs downstream. In the smooth cylinder, the jet completely envelops the cylinder, at greater Reynolds numbers and a distance of x/H = 7, with a small recirculation zone, showing that the rear surface also participates in the heat transfer. The heat transfer on the foam-covered cylinder increases, mainly, because of the large increase in the exchange surface, while it is somewhat penalized, at the higher Reynolds values and at the greater distance from the slot exit, because the rear surface is little involved in the heat transfer.

The Drake Passage opening from an experimental fluid dynamics point of view

Pronounced global cooling around the Eocene–Oligocene transition (EOT) was a pivotal event in Earth’s climate history, controversially associated with the opening of the Drake Passage. Using a physical laboratory model we revisit the fluid dynamics of this marked reorganization of ocean circulation. Here we show, seemingly contradicting paleoclimate records, that in our experiments opening the pathway yields higher values of mean water surface temperature than the “closed” configuration. This mismatch points to the importance of the role ice albedo feedback plays in the investigated EOT-like transition, a component that is not captured in the laboratory model. Our conclusion is supported by numerical simulations performed in a global climate model (GCM) of intermediate complexity, where both “closed” and “open” configurations were explored, with and without active sea ice dynamics. The GCM results indicate that sea surface temperatures would change in the opposite direction following an opening event in the two sea ice dynamics settings, and the results are therefore consistent both with the laboratory experiment (slight warming after opening) and the paleoclimatic data (pronounced cooling after opening). It follows that in the hypothetical case of an initially ice-free Antarctica the continent could have become even warmer after the opening, a scenario not indicated by paleotemperature reconstructions.

The fluid dynamics of collective vortex structures of plant-animal worms

Abstract

Modeling Fluidized Bed Combustion of Liquid Fuel.(Dept.M)

A mathematical model has been developed 10 predict the combustion of liquid fuel in a fluidized bed. From the fluid-dynamic point of view, the model is based on the modified two-phase theory of fluidization where the gas in the bed is assumed to have three phases. A jet-bubble phase which is fuel rich bubbles formed at fuel feeding jet, a distributor-bubble phase which is fuel free bubbles generated at distributor plate and an emulsion phase. Each phase has its composition and exchanges masses with the other two ones. The bubble is assumed to have a constant size throughout the bed. Gas mixing due to bubbles coalescence is taken into consideration based on an equivalent mass interchange coefficient between the two-bubble phases. The coefficient has been derived based on bubble size growth and probability of coalescence among jet and distributor bubbles. Two-step fuel reaction has been applied. The fuel firstly reacts to carbon monoxide and water vapor. In the second step, carbon monoxide reacts with oxygen to form carbon dioxide. Mass balance differential equations have been derived for various reacting species (CxHy, O2, CO, H2O and CO2) in the three phases. The heavy liquid fuel has been considered as a base case whereas gasoline and diesel fuel have been studied for comparison. The numerical solution of the model yields the axial concentration profiles of different gaseous species within the three phases. The in-bed fuel combustion has been estimated. The relative importance of gas mixing due to bubble coalescence has been assessed. The influences of different parameters on the combustion performance have been studied and discussed. Among the later parameters, jet velocity has a major effect on combustion performance. Experimental tests have been carried out to validate the model using a bubbling fluidized bed combustor of 300 mm inner diameter. The comparison between the model and experimental results shows a good agreement, especially, at higher jet velocity.

Stability of the radially symmetric stationary wave of the Burgers equation with multi‐dimensional initial perturbations in exterior domain

AbstractThe present paper is concerned with stability of the stationary solution of the Burgers equation in exterior domains in . In the previous papers [5, 6, 7] the asymptotic behavior of radially symmetric solutions for the multi‐dimensional Burgers equation in exterior domains in , has been considered. The results [5, 6, 7] are restricted to stability of radially solutions within the class of spherically one dimensional flow. However, from a viewpoint of fluid dynamics, it is the rare case that such a radially symmetric stationary wave remains to be a radial flow under the initial disturbance. Hence it seems to be natural to handle the non‐radially symmetric perturbed fluid motion even from the radially symmetric one. On the other hand, Kozono and Ogawa [8] showed the asymptotic stability of stationary solutions for the incompressible Navier–Stokes equation on multi‐dimensional spaces. In this paper we apply their method [8] to the multidimensional Burgers equation, and show the asymptotic stability for stationary wave on .

Intestinal flow after anastomotic operations in neonates

Stagnation of contents at the anastomotic site for intestinal flows after anastomotic operation is a critical issue in neonates. Although various anastomosis methods have been developed, in the clinical field, poor passage at the anastomotic site in cases of jejunal atresia is still observed. A CFD study was carried out to clarify the reasons for the stagnation and to find favorable anastomosis methods from a fluid dynamical point of view. Direct numerical simulations were performed using OpenFOAM. The boundaries of the computational domain were peristaltically moved to reproduce flow. The results reveal that the peristaltic motion on the distal side dominates the flow and that on the proximal side has a negligible influence. In particular, the contents do not pass the anastomotic site when the peristaltic motion on the distal side is not active. The flow rate as a measure of the driving force of the flow on the proximal side is large when the amplitude of the peristaltic motion is large and the diameter is small. It was also found that anastomosis methods do not affect flow resistance.

Case study: Evaluation of discrete frequency noise generated from object in wake of centrifugal impeller: Case study on tonal noise of a centrifugal fan Centrifugal Fan

In order to clarify the physical mechanism of unpleasant fan noise, the discrete frequency noise (DFN) generated from a centrifugal fan is evaluated quantitatively based on the wake characteristics. The volute tongue of the scroll casing is replaced by a cylinder as an object in order to simplify the mechanical model. Moreover, an analytical evaluation method for the DFN generated from the volute tongue of the scroll casing based on the wake characteristics of the centrifugal impeller is proposed. From the comparison on the prediction and measurement of the noise level, the influence of the clearance of the volute tongue on the DFN of a centrifugal fan was evaluated from the viewpoint of fluid dynamics. The aerodynamic noise source of the DFN was formed at the rear side of the impeller, which had large momentum in the wake. The momentum was rapidly lost by the diffusion in the near-wake field. Therefore, when the scroll casing of the fan is designed with suitable clearance for the volute tongue, the DFN will be decreased because of the diffusion of the wake. The evaluated noise indicated that the noise level of the DFN was lower in the far-wake field than the broadband noise level. These results imply that the sinusoidal pressure fluctuation caused by the impingement of the jetwake flow became weaker than the pressure fluctuation induced by the turbulence on the blade surface. © 2019 Institute of Noise Control Engineering.

A Review of Direct Drive Proportional Electrohydraulic Spool Valves: Industrial State-of-the-Art and Research Advancements

This paper reviews the state of the art of directly driven proportional directional hydraulic spool valves, which are widely used hydraulic components in the industrial and transportation sectors. First, the construction and performance of commercially available units are discussed, together with simple models of the main characteristics. The review of published research focuses on two key areas: investigations that analyze and optimize valves from a fluid dynamic point of view, and then studies on spool position control systems. Mathematical modeling is a very active area of research, including computational fluid dynamics (CFD) for spool geometry optimization, and dynamic spool actuation and motion modeling to inform controller design. Drawbacks and advantages of new designs and concepts are described in the paper.

Comparative Computed Flow Dynamic Analysis of Different Optimization Techniques in Left Main Either Provisional or Culotte Stenting.

Provisional and culotte are the most commonly used techniques in left main (LM) stenting. The impact of different post-dilation techniques on fluid dynamic of LM bifurcation has not been yet investigated. The aim of this study is to evaluate, by means of computational fluid dynamic analysis (CFD), the impact of different post-dilation techniques including proximal optimization technique (POT), kissing balloon (KB), POT-Side-POT and POT-KB-POT, 2-steps Kissing (2SK) and Snuggle Kissing balloon (SKB) on flow dynamic profile after LM provisional or culotte stenting. We considered an LM-LCA-LCX bifurcation reconstructed after reviewing 100 consecutive patients (mean age 71.4 ± 9.3 years, 49 males) with LM distal disease. The diameters of LAD and LCX were modelled according to the Finnet's law as following: LM 4.5 mm, LAD 3.5 mm, LCX 2.75 mm, with bifurcation angle set up at 55°. Xience third-generation stent (Abbot Inc., USA) was reconstructed and virtually implanted in provisional/cross-over and culotte fashion. POT, KB, POT-side-POT, POT-KB-POT, 2SK and SKB were virtually applied and analyzed in terms of the wall shear stress (WSS). Analyzing the provisional stenting, the 2SK and KB techniques had a statistically significant lower impact on the WSS at the carina, while POT seemed to obtain a neutral effect. In the wall opposite to the carina, the more physiological profile has been obtained by KB and POT with higher WSS value and smaller surface area of the lower WSS. In culotte stenting, at the carina, POT-KB-POT and 2SK had a very physiological profile; while at the wall opposite to the carina, 2SK and POT-KB-POT decreased significantly the surface area of the lower WSS compared to the other techniques. From the fluid dynamic point of view in LM provisional stenting, POT, 2SK and KB showed a similar beneficial impact on the bifurcation rheology, while in LM culotte stenting, POT-KB-POT and 2SK performed slightly better than the other techniques, probably reflecting a better strut apposition.

Lactation in the Human Breast From a Fluid Dynamics Point of View.

This study is a collaborative effort among lactation specialists and fluid dynamic engineers. The paper presents clinical results for suckling pressure pattern in lactating human breast as well as a 3D computational fluid dynamics (CFD) modeling of milk flow using these clinical inputs. The investigation starts with a careful, statistically representative measurement of suckling vacuum pressure, milk flow rate, and milk intake in a group of infants. The results from clinical data show that suckling action does not occur with constant suckling rate but changes in a rhythmic manner for infants. These pressure profiles are then used as the boundary condition for the CFD study using commercial ansys fluent software. For the geometric model of the ductal system of the human breast, this work takes advantage of a recent advance in the development of a validated phantom that has been produced as a ground truth for the imaging applications for the breast. The geometric model is introduced into CFD simulations with the aforementioned boundary conditions. The results for milk intake from the CFD simulation and clinical data were compared and cross validated. Also, the variation of milk intake versus suckling pressure are presented and analyzed. Both the clinical and CFD simulation show that the maximum milk flow rate is not related to the largest vacuum pressure or longest feeding duration indicating other factors influence the milk intake by infants.

Modification of a Steam Valve Diffuser for Enhanced Full Load and Part Load Operation Using Numerical Methods

The flow in a steam turbine inlet valve is investigated and improved by numerical methods. From the fluid dynamic point of view two requirements exist: Low pressure losses are desired at the fully opened valve position and dynamic fluid forces acting on the valve plug should be minimized to reduce valve vibration. Usually these undesired dynamic fluid forces occur when the flow is throttled at part load. It is found that these fluid forces are generated by separated jets in the diffuser. The attachment and the separation of the jet are related to the Coanda effect. By understanding the flow physics a way is found to modify the diffuser design in such a way that the flow separations are reduced. Bell-shaped diffusers are able to reduce the flow losses at full load operation. A diffuser contour that fulfils both requirements is developed.