Fluid Dynamic Phenomena Research Articles

Nonlinear dynamic behaviors of the generalized (3+1)‐dimensional KP equation

AbstractUnder investigation in this article is a generalized (3+1)‐dimensional Kadomtsev–Petviashvili (KP) equation, which is usually used to describe nonlinear phenomena in fluid dynamics. First, multiple waves solutions of this considered equation by means of the linear superposition principle are constructed. Second, the parameters of this considered model control multiple waves solutions are being investigated. We find that the parameters mainly affect the wave shapes, amplitude, and bright/dark nature. Third, we apply a polynomial function to construct rational solutions and rogue wave solutions when the parameters contain in the polynomial function are equal to zero and nonzero, respectively. Finally, the obtained results are demonstrated graphically to illustrate the nonlinear dynamics behaviors of multiple, rational, and rogue wave solutions in fluid mechanics.

A DMST-based tool to establish the best aspect ratio, solidity and rotational speed for tidal turbines in real sea conditions

A Double Multiple Stream Tube (DMST) routine to predict the performance of cross-flow hydrokinetic turbines in real environments is presented, along with a site assessment application to identify the most efficient turbine aspect ratio, solidity and configuration (single, or paired) for a selected area of the Northern Adriatic Sea. The peculiarity of this DMST tool is its 3D character, since it allows to reproduce the vertical distribution of the torque generated by the turbine. To this end, correlations for fluid dynamic phenomena, based on high-fidelity fully CFD simulations, were implemented. The marine circulation code SHYFEM is adopted to obtain velocity profiles for a half lunar cycle period. The sites with the highest mean kinetic power were identified. The DMST routine is equipped with an iterative process able to establish which rotational speed maximizes the power output. Indeed, a spatially non-uniform velocity profile requires to determine the flow velocity more suitable to obtain the rotational speed via Tip Speed Ratio (TSR) definition. To this end, the section of the blades working at optimal TSR varies from top to bottom, until the maximum power is reached. It works as a virtual Maximum Power Point Tracking system able to adapt the turbine operating conditions for the different turbine geometries, and for changes in flow conditions. The results show that for the case study, the performance curve shape influences the optimal TSR blade section: the latter is often located in the upper part of the turbine for the low solidity, whereas, for high solidity turbines, in the bottom half part.

Analysis of condensation and ventilation phenomena for double skin façade units

This paper presents a study of the thermo-hygrometric behaviour of a Double Skin Façade (DSF) unit. The study aims (i) at comparing currently used calculation procedures according to European and American standards (UNI EN ISO 10077, UNI EN ISO 12631:2018, ISO 15099:2003, ANSI/NFRC 100 for the thermal performance and ISO 13788:2012 (2012) for the condensation risk), and (ii) at assessing the 2D hygrothermal performance of a double skin module through a Finite Element Method (FEM)-based model. According to the current standards, a detailed characterization of thermal and fluid dynamic phenomena in closed and ventilated cavities is neglected and a simplified approach is proposed, which tends to overestimate the overall U-value of the curtain wall (UCW) due to an incremental thermal resistance that depends on the thickness of the air gap layer and the level of ventilation. The potential risk of this simplification is that the DSF estimated design performance, whilst complying with regulatory requirements, present inconsistencies respect to the real behaviour, impacting energy, comfort, material degradation, etc. Accurate assessments could be done already during design through detailed FEM multi-physic analyses. Nevertheless, those require a specific knowledge, are cost and time-consuming. As a first step, this study focuses on comparing the normed calculation approach for the design, against a detailed FEM-based multi-physics methodology. Specifically, this couples CFD, hygrothermal and Ray Tracing physics in a tool for the calculation of thermal transmittance, g-value and relative humidity of a DSF with a customizable geometry. As a second step, given a real DSF unit that showed unforeseen phenomena of surface condensation inside the cavity during several hours in spring and autumn, the multi-physic tool has been used to evaluate the condensation risk with the current and modified DSF design, under static and time-dependent boundary conditions.

Numerical Assessment of the Hydrodynamic Behavior of a Volute Centrifugal Pump Handling Emulsion.

Although emulsion pumping is a subject of growing interest, a detailed analysis of the fluid dynamic phenomena occurring inside these machines is still lacking. Several computational investigations have been conducted to study centrifugal pumps carrying emulsion by analyzing their overall performance, but no studies involved the rheological behavior of such fluids. The purpose of this study is to perform a computational analysis of the performance and flow characteristics of a centrifugal pump with volute handling emulsions and oil–water mixtures at different water cuts modeled as a shear-thinning non-Newtonian fluid. The studied pump consists of a five-bladed backward curved impeller and a volute and has a specific speed of 32 (metric units). The rheological properties of the mixtures studied were measured experimentally under a shear rate ranging from 1 s−1 to 3000 s−1 and were fitted to conventional Cross and Carreau effective viscosity models. Numerical results showed the flow topology in the pump is directly related to the viscosity plateau of the pseudoplastic behavior of emulsions. The viscosity plateau governs pump performance by influencing the loss mechanisms that occur within the pump. The larger the ν∞, the less recirculation loss the fluid experiences, and conversely, the smaller the value of ν0, the less friction loss the fluid experiences.

Inquest of material dissipation from cavitation bubble in laser-irradiated solid–liquid interface

The role of fluid dynamical phenomena – shockwaves and cavitation bubble for laser irradiation in liquid/solid–liquid interface – is thoroughly investigated in the literature considering idealized fluid dynamical conditions. The dynamics outside the cavitation bubbles are mostly ignored in laser-irradiated pure liquids. But this proves pivotal at the solid–liquid interface when entities from ablated targets are dispersed in liquids from an early breakdown via an explosive boiling, diffusion-assisted mass transfer through cavitation bubble wall or via releasing of trapped nanomaterials from a cavitation bubble collapse. The localized conditions (pressure and temperature) of these ablated species found outside the cavitation bubble favors chemical interactions for underwater-pulsed laser-induced material processing. To gain an insight into the materials dispelling from the cavitation bubble, a simplified model is employed to extract the localized conditions (pressure and temperature) in liquid, diffusion coefficient and diffusion-assisted distance traversed by the laser-ablated agglomerates.

Tracer gas techniques for airflow characterization in double skin facades

Monitoring airflow rates and fluid dynamics phenomena in the ventilated cavity is a challenging aspect of the experimental assessment of the performance of double-skin facades (DSF). There are various methods to characterize the fluid-dynamics behavior of DSF, but each of these has its advantages and drawbacks. This paper presents the airflow characterization in the cavity of a double-skin façade installed in a full-scale outdoor facility through various methods, and, more specifically, it compares two tracer gas methods with the velocity traverse method. In the paper, we highlight how different characterization results can be explained by considering the features of each method, and how these differences are linked to velocity ranges and airflows in the cavity. By discussing (i) the challenges of these methods and their applicability, (ii) the requirements in terms of experimental set-up and (iii) the limitations linked to instrumentation, we aim to enhance the discussion on experimental methods for advanced building envelope characterization and contribute to a more grounded understanding of the suitability of tracer gas methods for in-field characterization of airflows in facades.

A comparison of plunging- and pitching-induced deep dynamic stall on an SD7003 airfoil using URANS and LES simulations

Dynamic stall is a fluid dynamics phenomenon experienced by airfoils undergoing a rapid variation in their operating conditions, such as in the presence of pitching or plunging motions, when the effective angle of attack exceeds the static stall angle. The flow field near the airfoil is characterized by the presence of a strong clockwise vortex originating near the leading edge, which is convected downstream along the suction side, generating a low-pressure region and a continuous rise in lift. A controlled form of dynamic stall is involved in the flight mechanism of some natural flapping-wing flyers, and this is currently a matter of interest for the design of biomimetic micro aerial vehicles. Dynamic stall is also responsible for limitations in helicopters' forward-flight velocity, it can lead to flutter in fixed-wing aircraft and vibration-induced damage in wind turbine blades. In this work, 2D URANS and LES numerical simulations have been carried out for incompressible dynamic stall flow cases over an SD7003 airfoil at a Reynolds number of 6⋅104 and at a reduced frequency of 0.25, with the aim of assessing the validity and the role of low fidelity simulations in dynamic stall prediction, at operating conditions relevant for micro air vehicles. Simulations have been conducted both in the case of periodic pitching and plunging, in order to investigate similarities and differences in the resulting flow fields and aerodynamic coefficients. The effect of the reduced frequency on the agreement between 2D URANS and LES has also been evaluated for an airfoil under periodic plunging.

Development and application of a modularized geometry optimizer for future supercritical CO2 turbomachinery optimization

During the operation of supercritical CO (sCO) turbomachineries, large pressure ratios, supersonic conditions and non-ideal gas fluid dynamic phenomenon may happen, which will decrease the whole cycle efficiencies. Hence non-standard designs for the turbomachineries geometry are needed. Successfully simulations in capturing non-ideal gas fluid dynamics and coupled with optimization algorithm are hard and currently not available in any commercial CFD software. Therefore, the problem of optimizing sCO turbomachinery is difficult to solve, which has become a major obstacle to obtain compact and high-efficiency sCO turbomachineries. In this study, a modularized geometry optimizer is developed to obtain the non-standard geometric designs for sCO turbomachineries. Multiple techniques are applied to this optimizer, include the Nelder–Mead algorithm, Mahalanobis distance and stochastic algorithm. The newly developed optimizer can successfully find the optimum satisfying the objective function under given weighting factors. The computational cost can be reduced through a stochastic algorithm. To validate the optimizer, a convergent–divergent nozzle for air with a target Mach number equal to 2.4 is optimized. Different starting points and combinations of weighting factors are used to create a Pareto front. Adjust the weighting factors for different terms of the objective function leading the optimizer to go to different directions in n-dimensional spaces. Three optimized cases, one is Mach number optimized, one is outlet flow uniformity optimized and the other is compromised case, are picked out and analyzed. The results show that the optimizer can successfully find optimized geometry than the reference case and potentially save computational cost. Due to the modularized characteristics, the components of this optimizer can be replaced with any available techniques, which mean the optimizer can be applied to solve different types of optimization problems.

Mixed soliton solutions for the (2+1)-dimensional generalized breaking soliton system via new analytical mathematical method

In this research, we study the dispersive phenomena for the (2+1)-dimensional generalized breaking soliton system (GBSS), which describe the interaction phenomena between Riemann wave and long wave via two space variable in nonlinear media. The constructed solitary wave results for the (2+1)-dimensional GBSS via extended modified rational expansion (EMRE) method in the form of rational, elliptic, and trigonometric functions including mixed solitons, single bright and dark solitions, combined bright and dark solitons, kink and anti-kink wave solitions, traveling waves, and periodic waves. The physical phenomena of demonstrated results represented by higher dimensional and contour plots graphically with the help of symbolic computation. The obtained results are very helpful in the study of interaction phenomena in fiber optics, mathematical physics, fluid dynamics, geophysics, engineering and many other various areas of scientific fields.

Da Vinci's observation of turbulence: A French-Italian study aiming at numerically reproducing the physics behind one of his drawings, 500 years later

This research was started in 2019 for the 500th anniversary of Leonardo da Vinci's death. Our Italian-French research group focused its attention on a famous drawing by da Vinci in which a water jet impacts on a pool (RCIN 912660 The Windsor Collection). This particular drawing has often been used by many fluid dynamicists as the first important document concerning turbulent flows. It is worth noting that the word “turbulence,” one of the most important phenomena in fluid dynamics, was used for the first time by da Vinci in the “Atlantic codex.” After a detailed study of different historical documents, we sought to reproduce the flow drawn in the sheet RCIN 912660 using the Smoothed Particle Hydrodynamics model in order to better analyze the different descriptions that Leonardo reported beside his drawing.

Applying Ensemble Kalman Filter to Transonic Flows Through a Two-Dimensional Turbine Cascade

Abstract Turbulence is one of the most important phenomena in fluid dynamics. Large eddy simulation (LES) generally allows us to analyze smaller eddies than when using simulations based on unsteady Reynolds-averaged Navier–Stokes equations (URANS). In addition, the numerical solutions of LES show good agreements with experiments and numerical solutions based on direct numerical simulation. URANS simulations are, however, frequently used in academia and industry because LES computations are much more expensive compared with URANS simulations. In this investigation, an optimization of unsolved coefficients of the k–ω two equations model is performed on the transonic flow around T106A low-pressure turbine cascade to improve the accuracy of turbulence prediction with URANS. For the optimization approach, two-dimensional URANS is combined with ensemble Kalman filter which is one of the data assimilation techniques. In the assimilation process, a time- and spanwise-averaged LES result is used as pseudo-experimental data. Three-dimensional URANS simulations are performed for the evaluation of the optimization effect. URANS simulations are also applied to a different turbine cascade flow for the evaluation of the robustness of the optimized coefficients. These URANS results confirmed that the optimized coefficients improve the accuracy of turbulence prediction.

Fluids Demonstrations III: Viscous Flow in Modified Thin Enclosures, Centrifugal Effect, Vortical Flow, and Turbulence

Several new devices that demonstrate a variety of known phenomena in fluid dynamics are presented. These add on to a recent collection of demonstrations and follow the same design features as documented earlier (self-contained, easy to use, low cost). Descriptions are provided to enable replication by others, using readily available materials. The significance of each demonstration is outlined and background information on the relevant physics can be found in the cited references. The devices can be generally used in classroom lecture demonstrations, outreach activities, or other student-based projects. However, pedagogical details are left to readers’ discretion, depending on the scope of local interests and constraints. Video samples of demonstrations are available online.

Lump, its interaction phenomena and conservation laws to a nonlinear mathematical model

We solve the Ostrovsky equation in the absence of the rotation effect using the Hirota bilinear method and symbolic calculation. Some unique interaction phenomena have been obtained between lump solution, breather wave, periodic wave, kink soliton, and two-wave solutions. All the obtained solutions are validated by putting them into the original problem using the Wolfram Mathematica 12. The physical characteristics of the solutions have been visually represented to shed additional light on the acquired results. Furthermore, using the novel conservation theory, the conserved vectors of the governing equation have been generated. The discovered results are helpful in understanding particular physical phenomena in fluid dynamics as well as the dynamics of nonlinear higher dimensional wave fields in computational physics and ocean engineering and related disciplines.

A Computational Fluid Dynamic Study of Developed Parallel Stations for Primary Fans

A Computational fluid dynamic (CFD) model was developed considering three geometries for primary parallel fan stations that have already been developed, implemented, and are currently in operation within Chilean mines. To standardize the comparison, the same primary fan was used in all the simulations with a unique set of settings (speed, blade angle, and density). The CFD representation was used to determine the operating point per configuration and compare the performances in terms of airflow and pressure delivered. This approach allowed ranking primary fan station geometry based on resistance curve and energy consumption of the fan. This paper presents the results obtained through the CFD simulations and the corresponding primary fans operating points of each configuration: symmetrical branches (SB), overlap branches (OB), and run around (RA) bypass. The RA configuration was identified as the best-performing station geometry on the lowest frictional and shock pressure losses, highest airflow delivery, and lowest energy cost. The results are discussed, considering pressure, velocity, and vector contours to understand the fluid dynamics phenomena occurring inside the station. The capital cost involved in the development of each primary parallel station was considered in the analysis in addition to the energy cost to determine the economic configuration over time.

Optimization Study of Guide Vanes for the Intake Fan-Duct Connection Using CFD

The connection between an intake fan and a ventilation shaft must be designed in such a way that it minimizes the energy waste due to singularity losses. As a result, the questions of which radius of curvature to use and if guide vanes have to be included need to be answered. In that case, the variables such as the number, upstream and downstream penetration length, radius of curvature, and width of the vanes, need to be defined. Although this work is oriented to mine ventilation, these questions are usually valid in other engineering applications as well. The objective of this study is to define the previously mentioned variables to determine the optimal design combination for the radius/diameter relationship (r/D). Computational fluid dynamics was used to determine the shock loss factor of seven elbow curvature ratios for a 3 m diameter duct and fan, with and without guide vanes to estimate the best performing configuration and, therefore, to maximize the fan airflow volume. The methodology used consisted of initially developing models in 2D geometries, to optimize the meshing and the CPU use, and studying separately the number of vanes, upstream and downstream penetration, radius of curvature, and width of the vanes for each curvature ratio (r/D). Then, the best-performing variable combinations for each curvature ratio were selected to be simulated and studied with the 3D geometries. The application of the guide vane designs for three-dimensional simulated geometries is presented, first without and then with guide vanes, including the shock loss factors obtained. The methodology and obtained results allowed quantifying the energy savings and to reduce the CFD simulations steps required to optimize the design of the elbow and guide vanes. The results obtained cannot be used with elbows in exhaust fans, because fluid dynamics phenomena are different.

Analysis of the thermo-fluidic transport around counter-rotating tandem circular cylinders

The work physically relates to the influence of thermal buoyancy on the flow and heat transfer of an incompressible fluid around two counter-rotating circular cylinders arranged in tandem configuration within an unconfined domain. Two-dimensional numerical simulations are conducted using a finite volume based computational fluid dynamics tool to explore the problem. The Reynolds number is taken as 100 with Prandtl number 0.71, keeping the non-dimensional spacing between the cylinders fixed at 1.5. The cylinder rotations are considered in the range of a dimensionless speed of 0 to 5. The upstream cylinder is rotating in the clockwise sense, whereas, the downstream one in the counter-clockwise sense. The buoyancy effect is analyzed for the Richardson number range 0 to 1. The flow is unsteady periodic characterized by vortex shedding around the stationary cylinders at the chosen value of the Reynolds number. The flow shows unsteadiness with vortex shedding initially with increasing rotational speed; however, at a critical value of the rotation, the flow becomes stabilized with suppression of vortex shedding. On the contrary, the cross thermal buoyancy effect destabilizes the flow into an unsteady periodic pattern. This complex interplay among the free stream flow, cross buoyancy, and counter-rotation produces intriguing fluid dynamic and thermal phenomena. The critical rotational speeds for the range of Richardson numbers are obtained as [Formula: see text] respectively for Ri = 0, 0.25, 0.5 and 1. A corresponding regime diagram is also constructed to depict the unsteady and steady zones of operation.

Optimum power generation assessment in an H-Darrieus vertical axis wind turbine via Exergy Destruction Minimization

In recent years, Vertical Axis Wind Turbines have emerged as a viable complement to the ever-expanding worldwide wind power generation. Compared to conventional wind turbines, they are independent of wind direction, emit less noise, and require a smaller installation area, making them attractive for urban and remote implementations. In this work, a Second Law of Thermodynamics approach is utilized to deepen the understanding of the fluid dynamics phenomena that reduce the turbines' power generation. The turbulent unsteady fluid flow around the turbine is computed using a two-dimensional Finite Volume approach via the commercial software ANSYS Fluent, which is validated using experimental data available from the literature. The Exergy Destruction and the Reversible Work grow in direct proportion to the flow speed, although their growth rate depends on the turbine operating regime: attached flow or dynamic stall. The latter explains how the Second Law Efficiency maximizes at an intermediate Tip Speed Ratio, which coincides with a more efficient power generation.

Thermo-structural analysis of a honeycomb-type volumetric absorber for concentrated solar power applications

PurposeVolumetric air receivers experience high thermal stress as a consequence of the intense radiation flux they are exposed to when used for heat and/or power generation. This study aims to propose a proper design that is required for the absorber and its holder to ensure efficient heat transfer between the fluid and solid phases and to avoid system failure due to thermal stress.Design/methodology/approachThe design and modeling processes are applied to both the absorber and its holder. A multi-channel explicit geometry design and a discrete model is applied to the absorber to investigate the conjugate heat transfer and thermo-mechanical stress levels present in the steady-state condition. The discrete model is used to calibrate the initial state of the continuum model that is then used to investigate the transient operating states representing cloud-passing events.FindingsThe steady-state results constitute promising findings for operating the system at the desired airflow temperature of 700°C. In addition, we identified regions with high temperatures and high-stress values. Furthermore, the transient state model is capable of capturing the heat transfer and fluid dynamics phenomena, allowing the boundaries to be checked under normal operating conditions.Originality/valueThermal stress analysis of the absorber and the steady/transient-state thermal analysis of the absorber/holder were conducted. Steady-state heat transfer in the explicit model was used to calibrate the initial steady-state of the continuum model.

Investigating the haemodynamics of myocardial bridging

Myocardial bridging is a congenital anomaly wherein a segment of a coronary artery passes under a ‘bridge’ of heart muscle rather than resting upon the heart’s surface. Although it is usually benign, myocardial bridging has been associated with adverse clinical events including ischaemia, arrhythmia and sudden death. Moreover, there is a tendency for atherosclerotic lesions to develop upstream of the bridge. These lesions may be the result of adverse fluid dynamic phenomena induced by the periodic compression of the artery by the overlying myocardial bridge. It is not possible to visualise these phenomena in vivo, and in this study we present an in vitro model capable of replicating the bridging conditions. This model is comprised of a pressure-measuring guide wire and catheter, a piston pump, a scaled artery model, and a ‘myocardial bridging mechanism’ which periodically compresses the artery model. A proportional-integral-derivative (PID) controller allowed the piston pump to recreate a patient-specific aortic pressure waveform upstream of the occluded artery model segment for each study. Stationary occlusions—achieved by placing 3D printed ‘stenosis inserts’ within the artery model—induced globally reduced pressures downstream of the stenosis when compared against the upstream pressure waveform. Conversely, the pressures downstream of the dynamic stenoses generated by the bridging mechanism closely matched the upstream pressures at all stages of the cardiac cycle except at the end of systole. This divergent pressure behaviour at the end of systole was similarly observed in vivo within a patient with a myocardial bridge. Flow visualisation using a laser sheet enabled dynamic flow structures to be observed, including recirculating flow regions, which may be precursors to arterial dysfunction.

Multi-scale performance evaluation of ejector refrigeration systems

Despite the many advantages, ejector refrigeration systems have not been able to penetrate the market because of two prevailing reasons: low coefficient of performance and relevant influence of ejector operation on the performance of the whole system. Indeed, the performance of ejector refrigeration systems depends on the local flow phenomena occurring within the ejector. Thus, improving the performance of ejector refrigeration systems relies on the understanding of the fluid dynamic phenomena at the “component-scale” and on integrating such information at the “system-scale”. This paper contributes to the present discussion regarding the multi-scale modeling of ejector-based systems by proposing an integrated Computational Fluid Dynamic (CFD) - Lumped Parameter Model (LPM) ejector refrigeration system. In particular, ejector performances have been obtained by a validated CFD approach, whereas a LPM approach has modeled the refrigeration cycle. The refrigeration system’s performances, for different boundary conditions, have been evaluated, and the effects of the “local-scale” on the “system-scale” have been commented.