Re-entrant Flow Research Articles

Investigation of cavitation with air injection using Eulerian-Lagrangian method

Abstract Air injection can be used to reduce the cavitation erosion in fluid machinery. However the inverse result that the injected air indeed increases the size of cavitation may be obtained. To find out the interaction between air injection and cavitation, the present work proposes a Eulerian-Lagrangian method to investigate the effect of air injection on the characteristics of cavitating flow. The Eulerian method is used to describe the liquid and vapor distribution, and the morphology of large cavities are tracked by the Volume of Fluid (VOF) method in the Eulerian frame. On the other side, the Lagrangian method is used to track both air and vapor bubbles in small scale. With the Eulerian-Lagrangian coupling, discrete micro bubbles in sub-grid scale as well as the large cavities can be well captured and simulated. This work takes two cavitation situations in different cavitation numbers of σ = 0.81 and 0.70 into account. The cavitation features in σ = 0.81 are mainly characterized by the intermittent shedding of small-scale cavities under re-entrant flow mechanism. For σ =0.70, the injected air flow plays an important part which enhances the scale of cavity. For both cases, simulated results of the cavity shapes and distribution of discrete bubbles anastomoses well with experimental results, which verifies the accuracy of the algorithm. The interaction between injected air and vapor is also analysed.

Automated generation of digital models for manufacturing systems: The event-centric process mining approach

Digital and simulation models support the design and management of complex systems. However, system modelling is a time-demanding and knowledge-intensive activity. Moreover, modern manufacturing systems are subjected to frequent changes in production plans and subsequent reconfigurations. Therefore, the quick regeneration of the digital models is necessary to align digital twins and cyber-physical systems. This paper proposes a novel event-centric process mining paradigm, a process discovery algorithm, and a set of Key Performance Indicators for the fast and automated generation of digital models and their benchmarking. The discovery algorithm is based on the Event Relationship Graph of the conceptual model of the physical line. The algorithm is tested in four realistic systems of increasing complexity to verify the accuracy in modelling multi-product systems with re-entrant flows and random reworks in the presence of the assembly, disassembly, and split processes beyond the processing operations, and multi-operation workstations. The Event Relationship Graphs of the four systems are presented through the equivalent Petri nets models. The proposed approach is suitable for systems where the sensor positions are known and meaningful, like manufacturing systems, and it is effective for the quick automated generation of digital models for the activities of production planning and control as it requires a few seconds of computation time and a few hours of system observation.

Investigation of Workload Control Methods for Shops with Re-Entrant Flows

Abstract The re-entrant flow with an unpredictable nature of arrival would apparently harm production plans and schedules in flow type of shops. The re-entrant flow with varied arrival frequencies in rotor blade manufacturing is quite complicated and results in disproportionate workloads. Hence, an attempt has been made to study the significant influence of disproportionate workloads and research on an innovative order release method to enhance performance. The manufacturing process was observed thoroughly to incorporate the uncertain events that cause disturbance in the production. A simulation model was developed on a discrete event simulation platform by analysing problem phenomena right from the conceptualization phase. The model has been verified and validated to ensure the accuracy. The model was subjected to 288 experiments representing different scenarios that a flow shop undergoes in reality. The factors considered in the experimentation were re-entrant frequency, re-entrant proportions, order release methods and priority dispatching rules. A refined load release policy for disproportionate loads has been proposed to judge its effectiveness in terms of profit computation by comparing it with other relevant policies. Results of the experiment revealed that the order release methods contribute 95.93% to throughput performance, in addition, the use of the new re-entrant method policy in the above scenario was productive in improving the overall shop performance.

Numerical investigation of cavitation vortex dynamics in different cavitation patterns coupled implicit large eddy simulation and boundary data immersion method

The objective of this paper is to investigate the flow characteristics of different cavitation flow patterns around a NACA (National Advisory Committee for Aeronautics) 66 hydrofoil by applying the BDIM (boundary data immersion method) and ILES (implicit large eddy simulation) with an artificial code. Meanwhile, an artificial compressibility method is also employed to consider the effects of compressibility on cavitating flow. The results present that the numerical method can effectively capture different cavitation patterns, which agrees well with the previous experimental data. Subsequently, the detailed analysis of vortex structures and dynamics for the non-cavitation (σ = 3.0), sheet cavitation (σ = 2.0), and cloud cavitation (σ = 1.6) cases with the Liutex method and the vortex enstrophy transport equation have been investigated. When cavitation occurs, the degree of turbulence and the enstrophy in the flow field have been enhanced, due to the disturbance of the velocity field. For sheet cavitation, complex vortex structures appear in the attached cavity region with high-intensity enstrophy causing by the highly intense velocity and density gradient. As the cavitation pattern transits from the sheet cavitation to the cloud cavitation, more complex vortex structures can be observed in the cavitation region. Furthermore, the value and the fluctuation amplitude of enstrophy intensity increase significantly under the effect of reentrant jet. Analysis of the enstrophy transport equation indicates that the vortex stretching term and dilatation term for cloud cavitation increase relatively significantly with the movement of the reentrant flow and are highly dependent on the cavitation evolution. In addition, the region affected by the baroclinic torque also increases.

Stabilize cloud cavitation with an obstacle near hydrofoil's trailing edge and conduct local entropy production analysis

Cloud cavitation always causes severe damage to the efficiency and stability of the hydraulic machinery, resulting in extra energy losses in the system. We have observed an effective and simple way to prevent cloud cavitation formation by placing an obstacle near the hydrofoil's trailing edge. Cavitating flows around four different types of hydrofoils were simulated using the stress-blended eddy simulation turbulence model: the National Advisory Committee for Aeronautics (NACA) 66 hydrofoil and the NACA 66 hydrofoil with a 1 ×1 mm2 obstacle at 0.3c, 0.5c, or 0.7c. Sheet cavitation is the predominant mode of cavity flow when the obstruction is positioned at 0.7c. To find out why the cloud cavitation growth can be stopped when the obstruction is positioned at 0.7c, the velocity field, vorticity in the Z direction, and vortex structure of the Q-criterion were computed. To study the energy loss of the cavity flow and comprehend how obstacles affect it, the local entropy production rate was computed. It was discovered that the vorticity downstream of the obstacle, positioned at 0.7c, is restructured, which helps manage the flow separation upstream of the obstacle. Consequently, the hydrofoil's suction surface vorticity nearly rotates in the same direction as the obstacle at 0.7c, and the direction of Vx upstream of the obstacle is in the positive direction of the X axis, indicating that the reentrant flow has been controlled upstream of the 0.7c obstacle. Furthermore, cavitation shedding and the entropy production rate are strongly correlated, and regulating cloud cavitation growth is advantageous for energy conservation.

Makespan minimization in reentrant flow shop problem with identical jobs

Makespan minimization in reentrant flow shop problem with identical jobs

From shear bands to earthquakes in a model granular material with contact aging.

We perform molecular dynamics simulations of homogeneous athermal systems of poly-disperse soft discs under shear. For purely repulsive interactions between particles, and under a confining external pressure, a monotonous flow curve (strain rate vs. stress) starting at a critical yield stress is obtained, with deformation distributing uniformly in the system, on average. Then we add a short range attractive contribution to the interaction potential that increases its intensity as particles remain in contact for a progressively longer time, mimicking an aging effect in the system. In this case the flow curve acquires a reentrant behavior, namely, a region where shear stress decreases with increasing strain. Within this region the deformation is seen to localize in a shear band with a well defined width that decreases as the global strain rate does. At very low strain rates the shear band becomes very thin and deformation acquires a prominent stick-slip behavior. This regime can be described as the system possessing a fault in which deformation occurs with an earthquake-resembling phenomenology. In this way the system we are analyzing connects a regime of uniform deformation at large strain rates, a localized deformation regime in the form of shear bands at intermediate stain rates, and seismic phenomena at very low strain rate. The unifying ingredient of this phenomenology is the existence of a reentrant flow curve, originating in the aging mechanisms present in the model.

Flow instabilities in circular Couette flow of wormlike micelle solutions with a reentrant flow curve

In this work, we numerically investigate flow instabilities of inertialess circular Couette flow of dilute wormlike micelle solutions. Using the reformulated reactive rod model (RRM-R) (Hommel and Graham, 2021), which treats micelles as rigid Brownian rods undergoing reversible scission and fusion in flow, we study the development and behavior of both vorticity banding and finger-like instabilities. In particular, we focus on solutions that exhibit reentrant constitutive curves, in which there exists some region where the shear stress, τ, has a multivalued relation to shear rate, γ̇. We find that the radial dependence of the shear stress in circular Couette flow allows for solutions in which parts of the domain lie in the region of the flow curve where ∂τ/∂γ̇>0, while others lie in the region where ∂τ/∂γ̇<0; this mixed behavior can lead to complex flow instabilities that manifest as finger-like structures of elongated and anisotropically-oriented micelles. In 3D simulations we find that the initial instability is 2D in origin, and 3D finger-like structures arise through the axial instability of 2D sheets. Finally, we show that the RRM-R can capture vorticity banding in narrow-gap circular Couette flow and that vorticity bands are linearly stable to perturbations.

Robust scheduling in a two-machine re-entrant flow shop to minimise the value-at-risk of the makespan: branch-and-bound and heuristic algorithms based on Markovian activity networks and phase-type distributions

Robust scheduling in a two-machine re-entrant flow shop to minimise the value-at-risk of the makespan: branch-and-bound and heuristic algorithms based on Markovian activity networks and phase-type distributions

Batch scheduling in a multi-purpose system with machine downtime and a multi-skilled workforce

The paper presents a discrete-time mixed-integer linear programming (MILP) model for a generalised flexible job-shop scheduling problem as represented by a state-task network. The problem is characterised by reentrant flow, sequence-dependent changeover time, machine downtime, and skilled labour requirements. Two preprocessing procedures are proposed to reduce the size of the MILP model, and represent a major contribution of the research. The procedures reduce the number of assignment variables by exploiting job precedence and workforce qualifications. Machine availability for each task is determined as a function of possible start and end times, given duration, and maintenance schedule. The overall objective is to maximise the number of scheduled tasks while minimising their total finish time. Computational experiments are conducted with real and randomly generated instances. The results show that optimal solutions can be obtained for medium-size problems within a reasonable amount of time, primarily due to the use of the preprocessing procedures.

Problems and Solution Methods of Machine Scheduling in Semiconductor Manufacturing Operations: A Survey

Machine scheduling problems associated with semiconductor manufacturing operations (SMOs) are one of the major research topics in the scheduling literature. Lots of papers have dealt with different variants of SMOs’ scheduling problems, which are generally difficult to tackle theoretically and computationally. In this paper, the single machine, parallel machines, flow shops, and job shops scheduling problems from SMOs have been reviewed, based on different processing constraints, e.g., batch processing, auxiliary resources, incompatible job families, and reentrant flow, etc., with the cycle time, flow time, and throughput-related performance measures. Given the vast and diverse nature of the current literature, it is urgently needed to make a systematic survey in order to identify the important research problems, research trends, and the progress of the related solution methods, as well as clarify future research perspectives. We hope the findings and observations could provide some insights to the researchers and practitioners in this domain.

Numerical study of the effects of ventilation rates on supercavity dynamics and acoustic characteristics

This paper presents a numerical simulation investigation of the hydrodynamic and acoustic characteristics of ventilated supercavities that operate at various ventilation rates. The large eddy simulation (LES) and Ffowcs Williams–Hawkings (FW-H) methods are used to solve the unsteady flow field and noise. The results show that the ventilation rate plays an important role in cavity closure forms and can improve hydrodynamic performance. There is a transition from re-entrant flow closure to a stable double-vortex tube closure form at the supercavity tail as the ventilation rate increases. Meanwhile, under the condition of a high ventilation rate, the re-entrant flow region in the cavity gradually shrinks and hairpin vortices appear, the body length of the cavity increases,and the unsteadiness at the supercavity tail is weakened. However, the turbulence intensity and integral scale both increase with the ventilation rate. Moreover, it is found that the noise is closely related to supercavity closure form. Ventilated supercavities with closure forms of double-vortex tubes have better noise performance. Interestingly, as the ventilation rate increases, the total sound pressure levels are improved. The wavenumber-frequency spectra of supercavity turbulent pulsating pressures under studied ventilation rates are similar, but there are some differences in some local areas.

Evaluation of a modified URANS prediction of unsteady cavitating flow around a hydrofoil by comparing with LES results and experimental results

The research makes the analysis of cavitating flow field around hydrofoil NACA0009 through both experiments and numerical simulations. We previously proposed an interface effect-based correction model (IE model) for turbulent viscosity correction of cavitation flow, which performed well for hydrofoil cavitation simulation at small angles of attack. This research makes a further evaluation of the IE model. Firstly, the cavitation experiments of the NACA0009 blunt trailing edge hydrofoil with a 10-degree angle of attack are carried out on a closed test platform. The images of quasi-periodic shedding of partial cavitation are obtained. Secondly, the standard k-ε turbulence model, the IE model, and the large eddy simulation (LES model) are used to reproduce the cavitation flow around the hydrofoil. The results show that the standard k-ε model over-predicts the turbulent viscosity of the flow field, resulting in the cavitation region stably attached to the leading edge of the hydrofoil, which can't simulate the unsteady characteristics of the cavity bubbles. The LES model successfully reproduces the process of cavitation structure quasi-periodic shedding off. The IE model modifies the turbulent viscosity in the high shear flow region so that the reentrance flow can reach the leading edge of the hydrofoil and cut the attached cavity off. IE model can properly simulate the macro features of the cavity and the shedding frequency. In each stage of cavitation periodic shedding, the results of the IE model and the LES model on the pressure distribution and velocity field are consistent well. In addition, the calculation time cost of the IE model is basically the same as that of the standard k-ε model, only 0.14 times the LES model costs. The IE model is more suitable for cavitation evaluation in engineering applications.

Numerical investigation on the formation of the re-entrant flow and the transformation of varying shedding modes in cloud cavitation

The objective of this paper is to investigate two types of cloud shedding processes around the NACA66 (mod) hydrofoil with emphasis on the formation of the re-entrant flows and the transformation of varying shedding modes. The LES method combines the Schnerr-Sauer cavitation model in OpenFOAM to capture the detailed flow structures. Combining the experimental observation, the cloud shedding process can be divided into main re-entrant flow mode, transition mode, and side re-entrant flow mode. The re-entrant flow is primed from the middle of the attached cavity closure for the main re-entrant flow pattern. The velocity is more stable as it develops in a fan shape towards the leading edge of the hydrofoil. For the side re-entrant flow mode, the rotation and local collapse of cloud cavity in the previous cycle resulted in the incipient of the side re-entrant flow. Compared with the main re-entrant flow, the velocity of the side re-entrant flow fluctuates more significantly. The transition between the side re-entrant flow mode and the main re-entrant flow mode can be characterized by the normalized cavity length (lmax/c) and the cavity shedding frequency (St number).

Change in Cavitation Regime on NACA0015 Hydrofoil by Heating the Hydrofoil Surface

AbstractAn experimental study of cavitating flow on a heated NACA0015 hydrofoil was conducted in a cavitation tunnel to investigate the influence of the hydrofoil surface temperature on the cavitating flow. The cavitation behavior under different heating conditions was examined using high-speed video, and an image processing method was used to obtain the periodic characteristics of the cavitating flow. The results revealed that attached sheet cavitation and supercavitation occurred on both heated and unheated hydrofoils. However, sheet-cloud cavitation was observed only on the unheated hydrofoil, whereas transient cavitation was observed only on the heated hydrofoil. Transient cavitation also exhibited periodic growth/collapse behavior; however, there was no shedding of a large vapor cloud. Moreover, with a further increase in the hydrofoil surface temperature, transient cavitation turned into open-type cavitation. The cavitating flow exhibited a quasi-steady cavity length with an open cavity closure. It was considered that the surface temperature promoted vapor generation at the cavity leading edge, which enlarged the vapor-filled fore part of the sheet cavity. This enlarged sheet cavity prevented the reentrant flow from moving upstream and thus turned the cavity closure into an open type. Once the cavity closure turned into an open type, the local disturbance led to a smaller adverse pressure gradient, which was not sufficiently strong to create a reentrant flow. In this case, if the vapor generation at the cavity leading edge was sufficiently large to reach a balance with vapor condensation at the open cavity closure, the cavity would be steady.

재투입이 존재하는 유연흐름공정에서 주문의 총 흐름시간을 최소화하기 위한 일정계획 방법론

This study focuses on a scheduling problem in a flexible flow shop, where there are serial stages, each with identical parallel machines. We suggest heuristic algorithms for the problem to minimize the total flow time of a given set of orders. Each order is composed of multiple lots and is processed on any parallel machines at each stage. This shop has reentrant flows since products for specific orders should visit the processing stages twice. We suggest algorithms of two types. The algorithms of the first type are three-phase algorithms, and the second type's algorithm is one-phase. In the first phase of three-phase algorithms, an initial sequence is obtained using priority dispatching rules, and then a construction algorithm is used to obtain sequences for each stage in the second phase. Finally, in the third phase, lots that visit the stages for the second time are scheduled using priority dispatching rules. Computational experiments are performed on randomly generated test problems to evaluate the performance of the suggested algorithms. Results show that the order-based algorithms perform better than the lot-based algorithms and an algorithm used in practice.

Partial cavity shedding on a hydrofoil resulting from re-entrant flow and bubbly shock waves

Partial cavity flows forming on a NACA0015 hydrofoil are visualized using high-speed cinematography and time-resolved X-ray densitometry. These observations reveal the underlying flow features that lead to the cloud cavity shedding. Previous studies have reported that both near-surface liquid re-entrant flow and bubbly shock waves can serve as the mechanisms causing cavity pinch-off and cloud shedding. We identify both mechanisms in the current study. The cavity shedding frequency was also examined and related to the underlying flow dynamics. The probability of re-entrant flow or bubbly shock-induced shedding processes are quantified, and the likelihood of each mechanism is shown to be a function of both the cavitation number and the Mach number of the bubbly mixture within the separated region of the cavity. When the Mach number of the two-phase mixture in the cavity exceeds unity, shock waves become the dominant mechanism that lead to large-scale cavity shedding and cloud cavitation.

Scheduling approach for a wafer fab with re-entrant flows and photo dedication constraints

Scheduling approach for a wafer fab with re-entrant flows and photo dedication constraints

Sequence- and Time-Dependent Maintenance Scheduling in Twice Re-Entrant Flow Shops

Sequence- and Time-Dependent Maintenance Scheduling in Twice Re-Entrant Flow Shops

Numerical Analysis and Experimental Investigation of Cavitating Flows Considering Thermal and Compressibility Effects

This article deals with the numerical simulation of unsteady cavitating flow around hydrofoils, supported by experimental research realized in a cavitation tunnel situated in the Centre of Hydraulic Research. Two straight NACA hydrofoils (NACA0020 and NACA2412) were employed. The comprehensive unsteady CFD analysis was based on scale-resolving simulations (hereinafter SRS) with the aim of capturing correctly the interactions between the cavitation structures and re-entrant flow as well as the compressibility and thermal effects of cavitation. The static pressure fluctuations during the cavity oscillation cycles and the evaporation and condensation processes are discussed in detail. To predict correctly the high-pressure peaks during the bubble cloud collapses and the pressure pulse propagation speed, the real properties of water and the mixture total energy conservation equation were considered. In addition, the estimated content of undissolved air was taken into account. The numerical simulations were validated by means of already published experiments or compared with experiments conducted by the authors, and with good agreement.