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Article
Publication date: 11 February 2019

Van Luc Nguyen, Tomohiro Degawa and Tomomi Uchiyama

This study aims to provide discussions of the numerical method and the bubbly flow characteristics of an annular bubble plume.

Abstract

Purpose

This study aims to provide discussions of the numerical method and the bubbly flow characteristics of an annular bubble plume.

Design/methodology/approach

The bubbles, released from the annulus located at the bottom of the domain, rise owing to buoyant force. These released bubbles have diameters of 0.15–0.25 mm and satisfy the bubble flow rate of 4.1 mm3/s. The evolution of the three-dimensional annular bubble plume is numerically simulated using the semi-Lagrangian–Lagrangian (semi-LL) approach. The approach is composed of a vortex-in-cell method for the liquid phase and a Lagrangian description of the gas phase.

Findings

First, a new phenomenon of fluid dynamics was discovered. The bubbly flow enters a transition state with the meandering motion of the bubble plume after the early stable stage. A vortex structure in the form of vortex rings is formed because of the inhomogeneous bubble distribution and the fluid-surface effects. The vortex structure of the flow deforms as three-dimensionality appears in the flow before the flow fully develops. Second, the superior abilities of the semi-LL approach to analyze the vortex structure of the flow and supply physical details of bubble dynamics were demonstrated in this investigation.

Originality/value

The semi-LL approach is applied to the simulation of the gas–liquid two-phase flows.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 29 no. 3
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 9 August 2021

Zongyao Yang, Yong Shan and Jingzhou Zhang

This study aims to investigate the effects of exhaust direction on exhaust plume and helicopter infrared radiation in hover and cruise status.

Abstract

Purpose

This study aims to investigate the effects of exhaust direction on exhaust plume and helicopter infrared radiation in hover and cruise status.

Design/methodology/approach

Four exhaust modes are concerned, and the external flow field and fuselage temperature field are calculated by numerical simulation. The infrared radiation intensity distributions of the four models in hovering and cruising states are computed by the ray-tracing method.

Findings

Under the hover status, the exhaust plume is deflected to flow downward after it exhausts from the nozzle exit, upon the impact of the main-rotor downwash. Besides, the exhaust plume shows a “swirling” movement following the main-rotor rotational direction. The forward-flight flow helps prevent the hot exhaust plume from a collision with the helicopter fuselage generally for the cruise status. In general, the oblique-upward exhaust mode provides moderate infrared radiation intensities in all of the viewing directions, either under the hover or the cruise status. Compared with the hover status, the infrared radiation intensity distribution alters somewhat in cruise.

Originality/value

Illustrating the influences of exhaust direction on plume flow and helicopter infrared radiation and the differences of helicopter infrared radiation under hover and cruise statuses are identified. Finally, an appropriate exhaust mode is proposed to provide a better IR signature distribution.

Details

Aircraft Engineering and Aerospace Technology, vol. 93 no. 10
Type: Research Article
ISSN: 1748-8842

Keywords

Article
Publication date: 11 July 2019

Van Luc Nguyen, Tomohiro Degawa and Tomomi Uchiyama

This paper aims to provide discussions of a numerical method for bubbly flows and the interaction between a vortex ring and a bubble plume.

Abstract

Purpose

This paper aims to provide discussions of a numerical method for bubbly flows and the interaction between a vortex ring and a bubble plume.

Design/methodology/approach

Small bubbles are released into quiescent water from a cylinder tip. They rise under the buoyant force, forming a plume. A vortex ring is launched vertically upward into the bubble plume. The interactions between the vortex ring and the bubble plume are numerically simulated using a semi-Lagrangian–Lagrangian approach composed of a vortex-in-cell method for the fluid phase and a Lagrangian description of the gas phase.

Findings

A vortex ring can transport the bubbles surrounding it over a distance significantly depending on the correlative initial position between the bubbles and the core center. The motion of some bubbles is nearly periodic and gradually extinguishes with time. These bubble trajectories are similar to two-dimensional-helix shapes. The vortex is fragmented into multiple regions with high values of Q, the second invariant of velocity gradient tensor, settling at these regional centers. The entrained bubbles excite a growth rate of the vortex ring's azimuthal instability with a formation of the second- and third-harmonic oscillations of modes of 16 and 24, respectively.

Originality/value

A semi-Lagrangian–Lagrangian approach is applied to simulate the interactions between a vortex ring and a bubble plume. The simulations provide the detail features of the interactions.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 29 no. 9
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 7 August 2020

Yogesh Jaluria

This paper aims to discuss inverse problems that arise in a variety of practical thermal processes and systems. It presents some of the approaches that may be used to obtain…

141

Abstract

Purpose

This paper aims to discuss inverse problems that arise in a variety of practical thermal processes and systems. It presents some of the approaches that may be used to obtain results that lie within a small region of uncertainty. Therefore, the non-uniqueness of the solution is reduced so that the final design and boundary conditions may be determined. Optimization methods that may be used to reduce the uncertainty and to select locations for experimental data and for minimizing the error are presented. A few examples of thermal systems are given to illustrate the applicability of these methods and the challenges that must be addressed in solving inverse problems.

Design/methodology/approach

In most analytical and numerical solutions, the basic equations that describe the process, as well as the relevant and appropriate boundary conditions, are known. The interest lies in obtaining a unique solution that satisfies the equations and boundary conditions. This may be termed as a direct or forward solution. However, there are many problems, particularly in practical systems, where the desired result is known but the conditions needed for achieving it are not known. These are generally known as inverse problems. In manufacturing, for instance, the temperature variation to which a component must be subjected to obtain desired characteristics is prescribed, but the means to achieve this variation are not known. An example of this circumstance is the annealing, tempering or hardening of steel. In such cases, the boundary and initial conditions are not known and must be determined by solving the inverse problem to obtain the desired temperature variation in the component. The solutions, thus, obtained are generally not unique. This is a review paper, which discusses inverse problems that arise in a variety of practical thermal processes and systems. It presents some of the approaches or strategies that may be used to obtain results that lie within a small region of uncertainty. It is important to realize that the solution is not unique, and this non-uniqueness must be reduced so that the final design and boundary conditions may be determined with acceptable accuracy and repeatability. Optimization techniques are often used for minimizing the error. This review presents several methods that may be applied to reduce the uncertainty and to select locations for experimental data for the best results. A few examples of thermal systems are given to illustrate the applicability of these methods and the challenges that must be addressed in solving inverse problems. By considering a variety of systems, the paper also shows the importance of solving inverse problems to obtain results that may be used to model and design thermal processes and systems.

Findings

The solution of inverse problems, which frequently arise in thermal processes, is discussed. Different strategies to obtain the conditions that lead to the desired result are given. The goal of these approaches is to reduce uncertainty and obtain essentially unique solutions for different circumstances. The error of the method can be checked against known conditions to see if it is acceptable for the given problem. Several examples are given to illustrate the use of these methods.

Originality/value

The basic strategies presented here for solving inverse problems that arise in thermal processes and systems, as well as the optimization techniques used to reduce the domain of uncertainty, are fairly original. They are used for certain challenging problems that have not been considered in detail earlier. Several methods are outlined for considering different types of problems.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 31 no. 10
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 25 August 2021

Lorris Charrier, Mathieu Jubera, Grégoire Pont, Simon Marié, Pierre Brenner and Francesco Grasso

The design of a space launcher requires some considerations about the unsteady loads and heat transfer occurring at the base of the structure. In particular, these phenomena are…

Abstract

Purpose

The design of a space launcher requires some considerations about the unsteady loads and heat transfer occurring at the base of the structure. In particular, these phenomena are predominant during the early stage of the flight. This paper aims to evaluate the ability of the unstructured, high order finite-volume CFD solver FLUSEPA, developed by Airbus Safran Launchers, to accurately describe these phenomena.

Design/methodology/approach

This paper first performs a steady simulation on a base flow around a four-clustered rocket configuration. Results are compared with NASA experiments and Loci-CHEM simulations. Then, unsteady simulations of supersonic H2/air reacting mixing layer based on the experiment of Miller, Bowman and Mungal are performed. Three meshes with different cells number are used to study the impact of spatial resolution. Instantaneous and time-averaged concentrations are compared with the combined OH/acetone planar laser-induced fluorescence imaging from the experiment.

Findings

FLUSEPA satisfactorily predicts the base heat flux at the base of a four-clustered rocket configuration. NASA Loci-CHEM reactive simulations indicate that afterburning plays an important role and should not be neglected. The unsteady reactive computation of a supersonic mixing layer shows that FLUSEPA is also able to accurately predict flow structures and interactions. However, the complexity of the experiment and the lack of details concerning the facility prevents from obtaining satisfactory converged results.

Originality/value

This study is the first step on the development of a cost-effective method aiming at predicting unsteady loads and heat transfer on space launchers using an unsteady and reactive model for the CDF calculations. It uses original techniques such as conservative CHIMERA-like overset grids, local re-centering of fluxes and local adaptive time-stepping to reduce computational cost while being robust and accurate.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 31 no. 11
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 17 April 2023

Christopher Stutzman, Andrew Przyjemski and Abdalla R. Nassar

Powder bed fusion processes are common due to their ability to build complex components without the need for complex tooling. While additive manufacturing has gained increased…

Abstract

Purpose

Powder bed fusion processes are common due to their ability to build complex components without the need for complex tooling. While additive manufacturing has gained increased interest in industry, academia and government, flaws are often still generated during the deposition process. Many flaws can be avoided through careful processing parameter selections including laser power, hatch spacing, spot size and shielding gas flow rate. The purpose of this paper is to study the effect of shielding gas flow on vapor plume behavior and on final deposition quality. The goal is to understand more fully how each parameter affects the plume and deposition process.

Design/methodology/approach

A filtered-photodiode based sensor was mounted onto a commercial EOS M280 machine to observed plume emissions. Three sets of single tracks were printed, each with one of three gas flow rates (nominal, 75% nominal and 50% nominal). Each set contained single-track beads deposited atop printed pedestals to ensure a steady-state, representative build environment. Each track had a set power and speed combination which covered the typical range of processing parameters. After deposition, coupons were cross-sectioned and bead width and depth were measured. Finally, bead geometry was compared to optical emissions originating in the plume.

Findings

The results show that decreasing gas flow rate, increasing laser power or increasing scan speed led to increased optical emissions. Furthermore, decreasing the gas cross-flow speed led to wider and shallower melt pools.

Originality/value

To the best of the authors’ knowledge, this paper is among the first to present a relationship among laser parameters (laser power, scan speed), gas flow speed, plume emissions and bead geometry using high-speed in situ data in a commercial machine. This study proposes that scattering and attenuation from the plume are responsible for deviations in physical geometry.

Details

Rapid Prototyping Journal, vol. 29 no. 7
Type: Research Article
ISSN: 1355-2546

Keywords

Article
Publication date: 20 November 2009

Diana F. Spears, David R. Thayer and Dimitri V. Zarzhitsky

In light of the current international concerns with security and terrorism, interest is increasing on the topic of using robot swarms to locate the source of chemical hazards. The…

Abstract

Purpose

In light of the current international concerns with security and terrorism, interest is increasing on the topic of using robot swarms to locate the source of chemical hazards. The purpose of this paper is to place this task, called chemical plume tracing (CPT), in the context of fluid dynamics.

Design/methodology/approach

This paper provides a foundation for CPT based on the physics of fluid dynamics. The theoretical approach is founded upon source localization using the divergence theorem of vector calculus, and the fundamental underlying notion of the divergence of the chemical mass flux. A CPT algorithm called fluxotaxis is presented that follows the gradient of this mass flux to locate a chemical source emitter.

Findings

Theoretical results are presented confirming that fluxotaxis will guide a robot swarm toward chemical sources, and away from misleading chemical sinks. Complementary empirical results demonstrate that in simulation, a swarm of fluxotaxis‐guided mobile robots rapidly converges on a source emitter despite obstacles, realistic vehicle constraints, and flow regimes ranging from laminar to turbulent. Fluxotaxis outperforms the two leading competitors, and the theoretical results are confirmed experimentally. Furthermore, initial experiments on real robots show promise for CPT in relatively uncontrolled indoor environments.

Practical implications

A physics‐based approach is shown to be a viable alternative to existing mainly biomimetic approaches to CPT. It has the advantage of being analyzable using standard physics analysis methods.

Originality/value

The fluxotaxis algorithm for CPT is shown to be “correct” in the sense that it is guaranteed to point toward a true source emitter and not be fooled by fluid sinks. It is experimentally (in simulation), and in one case also theoretically, shown to be superior to its leading competitors at finding a source emitter in a wide variety of challenging realistic environments.

Details

International Journal of Intelligent Computing and Cybernetics, vol. 2 no. 4
Type: Research Article
ISSN: 1756-378X

Keywords

Article
Publication date: 30 May 2008

Q.W. Wang, D.J. Zhang, M. Zeng, M. Lin and L.H. Tang

The purpose of this paper is to investigate the overall flow and temperature field of the air in the whole power plant, especially around the air‐cooled heat exchanger (ACHE) to…

3545

Abstract

Purpose

The purpose of this paper is to investigate the overall flow and temperature field of the air in the whole power plant, especially around the air‐cooled heat exchanger (ACHE) to evaluate the feasibility of the thermal plant project.

Design/methodology/approach

The commercial computational fluid dynamics code FLUENT with standard kε turbulent model was used. The buoyancy of the air was also considered.

Findings

It is concluded that plume recirculation occurs in each case due to the wind effect and the suction of the fan. Installing a side board below or above the fan platform (side board I or side board II) is an effective method of avoiding the plume recirculation and, the higher the board, the better the effect. When the height of the side board I H1≥10 m or the height of the side board II H2≥12 m, the temperature distributions of the fan platform will be sufficient to meet the requirement.

Research limitations/implications

A proper distance between the adjacent high buildings and the ACHE should be found with further investigation.

Practical implications

The paper presents a very useful numerical method for the prediction of the flow and temperature field around ACHE or in a large space.

Originality/value

The paper provides the numerical simulation of the flow and heat transfer inside the whole thermal power plant. Suggestions which can effectively avoid the unfavorable influence and ensure the whole system in safe conditions are offered. The study gives some useful information to the design of a thermal power plant with an ACHE system.

Details

Engineering Computations, vol. 25 no. 4
Type: Research Article
ISSN: 0264-4401

Keywords

Article
Publication date: 1 August 1996

ABDULKARIM H. ABIB and YOGESH JALURIA

A numerical study of a two‐dimensional turbulent flow in a partially open rectangular cavity such as a room is carried out. The turbulent flow is induced by the energy input due…

76

Abstract

A numerical study of a two‐dimensional turbulent flow in a partially open rectangular cavity such as a room is carried out. The turbulent flow is induced by the energy input due to a localized heat source positioned on the floor of the cavity. This flow is of interest in enclosure fires where the flow in the cavity interacts with the environment through the opening or vents. The focus is on the stable, thermal stratification that arises in the room and on the influence of the opening height. A finite‐difference method is employed for the solution of the problem, using a low Reynolds number k — ε turbulence model for the turbulent flow calculations. This model is particularly suitable for flows in which the possibility for relaminarization exists. It was found that, for high Grashof numbers and for relatively small opening heights, particularly for doorway openings, a strong stable thermal stratification is generated within the cavity, with a cooler, essentially uniform, layer underlying a warmer, linearly stratified, upper layer. As a consequence, turbulence is suppressed and the flow in the upper region of the cavity becomes laminar with turbulence confined to locations such as the fire plume above the source and the shear layer at the opening. The penetration distance and the height of the interface are both found to decrease with a reduction in the opening height. The Nusselt number for heat transfer from the source is seen to be affected to a small extent by the opening height. The basic trends are found to agree with those observed in typical compartment fires. Comparisons with results available in the literature on turbulent buoyancy‐driven enclosure flows indicate good agreement, lending support to this model and the numerical scheme.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 6 no. 8
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 25 November 2019

Nuno Serra and Viriato Semiao

This paper aims to clarify the necessity of taking into account the commonly neglected radiation in built environments. Ignoring radiation within acclimatized spaces with moist…

Abstract

Purpose

This paper aims to clarify the necessity of taking into account the commonly neglected radiation in built environments. Ignoring radiation within acclimatized spaces with moist air, which is a participating medium, can yield inaccurate values of the relevant variables, endangering the Heating, ventilation, and air conditioning design accuracy and leading to energy inefficiencies and discomfort.

Design/methodology/approach

The paper uses computational fluid dynamics to predict non-isothermal flows with radiation, for both mixing and displacement ventilation strategies. The tool is applied to a lab-scale model (scale 1:30), and the results are compared with experimental data and predictions without radiation. Furthermore, the radiation influence is also assessed at real-scale level, including a parametric study on the effect of the air relative humidity on radiation.

Findings

The paper demonstrates the unequivocal impact of radiation on the flows thermal-kinematics at real-scale: ignoring radiation yields average air temperature differences of 2ºC. This becomes more evident for larger air optical thicknesses (larger relative humidity): changing it from 20 per cent to 50 per cent and 70 per cent yields maximum relative differences of 100 per cent for the velocity components and 0.4ºC for the air temperature. Nevertheless, the results for the lab-scale case are not so conclusive about the effect of moist air radiation on the thermal flow characteristics, but they evidence its impact on the flow kinematics (maximum relative differences of velocity components of 35 per cent).

Originality/value

The paper fulfills an identified need to clarify the relevant effects of air moisture on radiation and on the flow turbulence and thermal-kinematic characteristics for forced convective flows inside built environments.

Details

Engineering Computations, vol. 37 no. 4
Type: Research Article
ISSN: 0264-4401

Keywords

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