Stagnation Point Research Articles

Heat transfer characteristics of methane-air premixed jet flames with flat/hemispherical walls

Abstract The in-depth study of the mutual coupling between the flame and the wall can significantly enhance the efficiency of actual combustion devices. A two-dimensional numerical model of the heat transfer characteristics of methane-air premixed jet flames with flat/hemispherical walls has been developed. An examination of the effects of wall shape on the heat transfer characteristics of methane/air flames was conducted as a function of the equivalence ratio (ϕ=0.9-1.5), the mixture Reynolds number (Re=300-800), and the burner-to-plate distance (H/d=1-6). As the equivalence ratio and Reynolds number increase, the flame temperature increases on the surface near the wall, and the temperature near the flame centerline is higher under the influence of a hemispherical wall than it is under the influence of a plate. In addition, the wall's heat flux increases as both the equivalence ratio and the Reynolds number increase. It is observed that the heat flux of the hemispherical wall is greater than that of the flat plate near the stagnation point, whereas it is smaller at a distance from the stagnation point. Due to the burner-to-plate distance, thermal efficiency is maximized when the flame premixed cone contacts the impact surface, which is the desired condition for optimal performance. Due to different operating conditions, the efficiency of heat transfer is always higher under the action of a flat plate than under the action of a hemispherical wall.

Transverse vortex-induced vibration of a sphere with the application of Lorentz force at low Reynolds number

In this study, the transverse vortex-induced vibration (VIV) of an elastically mounted sphere with the application of a streamwise Lorentz force is investigated through direct numerical simulation. The research parameter range is 300 ≤ Re ≤ 1100 and −0.8 ≤ N ≤ 1, where Re is the Reynolds number and N is the interaction parameter of the Lorentz force. The dependence of sphere responses, forces, and wake structures on Re and N is illustrated in detail. Within this range, two oscillation patterns are identified: VIV and desynchronization. Three wake patterns are identified: two-sided hairpin vortex emerges in the VIV region, while one-sided hairpin vortex and double-threaded wake structures are observed in the desynchronization region. The evolution of these wake patterns is related to the motion of the rear stagnation point (RSP) and separation line (SL) on the sphere surface. A large positive or negative Lorentz force suppresses the motion of RSP and SL, leading to the one-sided hairpin vortex or double-threaded wake structures replacing the two-sided hairpin vortex. Finally, the oscillation patterns are summarized on a map of amplitude response contours in the Re-N space.

Investigation of stagnation point and microstructure evolution in forming Al-Mg-Si alloy sheets with synergistic strength-ductility

Plastic flow machining (PFM), an efficient process of severe plastic deformation, aims at preparing gradient-structured sheets in one step without cumbersome equipment. Nevertheless, material stagnation points and microstructure evolution in sheet forming process are often neglected in previous studies. Based on simulation and experiments, this paper reported in detail the forming mechanism, stagnation point position, and mechanical properties of Al-Mg-Si alloy sheets under different thicknesses prepared by PFM. Furthermore, the EBSD, TEM, and XRD methods were used to deeply analyze the microstructure evolution. The results indicated that there existed a material stagnation zone (MSZ) during the PFM process, and the stagnation point position was closely related to sheet thickness. Additionally, a prediction expression for the stagnation point position was proposed, which generally agreed with the actual results. As sheet thickness increased, the MSZ area enlarged and the stagnation point moved left, leading to more material extruding into the forming channel. Compared to initial sheets, the grains, effective strain and hardness of the prepared sheets exhibited a distinct gradient along the thickness direction, improving the strength without compromising much ductility. Accordingly, the Al-Mg-Si alloy sheets fabricated by PFM exhibited excellent strength-ductility synergy, providing insights into the clean processing of lightweight materials with gradient structure.

Control of the von Kármán vortex street with focusing and vectoring of jet using synthetic jet array

In the present study, a novel flow control technique based on jet focusing and vectoring from a synthetic jet array (SJA) for controlling the wake of a bluff body is proposed and demonstrated. A numerical investigation into the flow past a square cylinder modified by the SJA has been carried out at a free stream Reynolds Number of 100. The SJA consists of four independently controlled synthetic jet actuators operating at a peak velocity of eight times the free stream and fifteen times the natural vortex shedding frequency of the square cylinder. The SJA is operated in two different regimes; a focusing regime involving phase delay (Δφ) with non-linear variation between the actuators and a vectoring regime with a linear phase delay without changing the geometric or operating parameters of the SJA. It has been found that jet focusing is able to reduce the coefficient of drag by as much as 43% for Δφ=90°. Focusing is also observed to reduce the fluctuations in the wake velocity with the maximum reduction in fluctuations also corresponding to Δφ=90°. Jet vectoring is able to deflect the von Kármán vortex street in a singular direction along with shifting of the front stagnation point with maximum deflection for Δφ=60°. Furthermore, vectoring leads to an asymmetry in the wake velocity field with the shifting of the velocity deficit region in the direction of the vectoring along with an asymmetry in the wake velocity fluctuations. This novel approach toward synthetic jet induced active flow control allows for greater manipulation of the flow field characteristics of bluff bodies than present methods with applications in areas of underwater and micro air vehicle maneuvering, automobile, and building aerodynamics among others.

Actuation manifold from snapshot data

We propose a data-driven methodology to learn a low-dimensional manifold of controlled flows. The starting point is resolving snapshot flow data for a representative ensemble of actuations. Key enablers for the actuation manifold are isometric mapping as encoder, and a combination of a neural network and a $k$ -nearest-neighbour interpolation as decoder. This methodology is tested for the fluidic pinball, a cluster of three parallel cylinders perpendicular to the oncoming uniform flow. The centres of these cylinders are the vertices of an equilateral triangle pointing upstream. The flow is manipulated by constant rotation of the cylinders, i.e. described by three actuation parameters. The Reynolds number based on a cylinder diameter is chosen to be $30$ . The unforced flow yields statistically symmetric periodic shedding represented by a one-dimensional limit cycle. The proposed methodology yields a five-dimensional manifold describing a wide range of dynamics with small representation error. Interestingly, the manifold coordinates automatically unveil physically meaningful parameters. Two of them describe the downstream periodic vortex shedding. The other three describe the near-field actuation, i.e. the strength of boat-tailing, the Magnus effect and forward stagnation point. The manifold is shown to be a key enabler for control-oriented flow estimation.

Magnetohydrodynamic Stagnation Point Flow and Heat Transfer of Casson Fluids Over a Stretching Sheet

This research investigates the effects of heat transfer on the stagnation-point flow of a non-Newtonian Casson fluid in a two-dimensional magnetohydrodynamic (MHD) boundary layer over a stretched sheet, considering thermal radiation impacts. By employing similarity transformations, the governing partial differential equations are transformed into nonlinear ordinary differential equations. The obtained self-similar equations are numerically solved using the Optimal Homotopy Analysis Method (OHAM). The numerical results are graphically represented, showcasing the influence of various parameters on fluid flow and heat transfer characteristics. The study uncovers important dynamics in transport phenomena. Examining and illustrating the effects of dimensionless parameters on velocity, temperature, and concentration profiles reveal significant insights. Moreover, skin friction and Nusselt number results for Casson fluids are analyzed and presented. The findings indicate that the Casson parameter and Hartman number act in opposition to fluid momentum, while the thermal conductivity parameter enhances fluid temperature. Thus, this research provides valuable insights into MHD boundary layer flows of non-Newtonian Casson fluids with thermal radiation effects, and the OHAM solution method proves effective in predicting flow transport properties.

Layer-by-Layer Deposition of Kraft Lignin and PEDOT:PSS.

Kraft lignin (KL) is a sustainable carbon-based substance with a potential use in photovoltaic materials. However, its conductivity is low, but it can be improved via incorporation with a conductive polymer, such as poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS). This study examines the factors affecting the interaction of KL and PEDOT:PSS (PS) in a solution state using a quartz crystal microbalance with dissipation (QCM-D) and a stagnation point refractometer (SPAR). The results confirmed that aqueous environments, e.g., pH and ionic strengths, considerably affected particle size and zeta potential of KL and PS due to protonation, deprotonation, particle aggregation, and charge screening. The polymers exhibited the largest adsorbed mass and thickness at pH 6 and 10 mM NaCl on a solid surface, which was attributed to the relatively linear structure of PEDOT chains, exposing more adsorptive sites for interaction with KL. A 10 mM NaCl concentration facilitated the screening of charges on PS and KL surfaces, diminishing repulsive forces and enabling hydrophobic and cationic-π interaction, which led to increased adsorption. Contact angle and SEM investigations of the adsorbed layer revealed the water contact angle increasing and the morphology changing from a smoother layer to a porous surface, providing further evidence of adsorption. Furthermore, the conductivity was improved by the introduction of a PS adlayer on ITO glass when it was sandwiched between KL adsorbed layers. These findings provide insight into KL and PS interaction and suggest that KL can be used with PS for conductive materials, such as photovoltaics, imparting the waterproofness of the films.

Numerical simulation of two‐phase flow towards a stagnation point in the Jeffrey nanofluid across a porous deformable disc with zero mass flux condition and Lorentz forces

Numerical simulation of two‐phase flow towards a stagnation point in the Jeffrey nanofluid across a porous deformable disc with zero mass flux condition and Lorentz forces

Mixed convection flow and heat-mass transfer of cross liquid near a stagnation region with Dufour, Soret effects and chemical reaction

Heat-mass transfer in a liquid flow over a stretching surface at a stagnation regime is significant in optimal manufacturing and quality assurance depending on the rheological behavior of viscoelastic fluids in process machinery. Stretching sheet flow at a stagnation point is a standard procedure used in the metallurgical industries for manufacturing of different equipment and cooling systems. Due to its various applications in industrial operations, the current study aims to explore the heat-mass transfer of buoyancy-driven flow of cross liquid over a stretching sheet at a stagnation point. The heat and mass transfer under the framework of Dufour and Soret effects, radiant heat, and chemical reactions are assessed. Furthermore, the analysis also takes into account the linear and nonlinear convection-diffusion phenomena. The model findings are obtained by merging the Bvp4c in MATLAB using a local similarity technique up to the fourth truncation threshold. The findings illustrate that the action of magnetic field can be used to control the flow across the sheet. Temperature increases with thermal radiation, Biot number and Dufour number. The action of buoyancy ratio parameter result in improvement in fluid flow. With increasing trend of magnetic and Weissenberg number, the flow field and skin friction coefficient drops.

Influence of solar thermal radiations and convective boundary on Al2O3/H2O transient model efficiency

Riga plate is a new, sophisticated magnetic field device that may be created by adjusting a group of permanent magnets and a different electrode over a plane surface. The heat transport and fluid movement in such physical setup are of paramount interest and have numerous engineering and industrial application particularly in submarines technologies. Due to the fixed magnetics the field produced which imperatively affect the model dynamics. Keeping in mind the influential applications of such physical setup, the purpose of this study is to introduce a new model based scrutinization of heat transport of stagnation point flow of Al2O3/water along vertically oriented convectively heated Riga surface. The conventional stagnation point flow model extended for nanofluid via radiative heat flux, dissipation effects and the first order thermal slip. The values of thermal conductivity values estimated via Corcione model and achieved the final nanoliquid model. For the results interpretation, the numerical scheme used and portrayed the results using different parametric ranges. The fluid motion is observed very slow due to stronger mixed convection and higher concentration of Al2O3 nanoparticles. The temperature is determined very high against the stronger convection effects and radiation number. However, the fluid layers in the vicinity of Riga surface have high heat transmission ability. Further, thermal slip and viscous dissipation effects also boost the temperature of Al2O3/water. Further, buoyancy number δ resists the fluid movement and thermal boundary region reduces in the presence of buoyancy factor.

Computational analysis of MHD hybrid nanofluid over an inclined cylinder: Variable thermal conductivity and viscosity with buoyancy and radiation effects

Due to their widespread use in engineering, hybrid nanofluids have been the primary focus of mathematical and physical research. Only the improvement of hybrid nanofluids’ variable heat conductivity and viscosity has been considered so far. Hybrid nanofluid flow across an inclined cylinder has many potential uses, including heat transfer and cooling in electrical devices, energy storage, refrigerants, and the automobile industry. Examining the effects of buoyant force, variable viscosity, variable thermal conductivity, mass suction, convective thermal conditions, and a magnetic field on the stagnation point flow of a Al2O3–Cu/H2O hybrid nanofluid in an inclined cylinder is our objective in this work. In order to find solutions to boundary-condition flow-describing partial differential equations, we turn them into ordinary differential equations using similarity transformations. We achieve this by employing a numerical strategy known as the fourth-order Runge–Kutta technique, which incorporates shooting techniques. A graphical representation of the findings emphasizes the influence of many physical parameters on flow dynamics. In addition, we address the influence of drag force and rate of heat transfer on various elements, such as the Biot parameter, magnetic variable, viscosity variable, and thermal conductivity variable. The mixed convection and magnetic parameters cause the velocity profile to rise while the temperature profile falls. The research’s results elucidate the cause behind the rise in thermal contour of hybrid nanofluids, which is seen when there is an increment in thermal conductivity, radiation parameter, and Biot number. The heat transfer rate exhibits a significant increase of 36.87% in the aiding flow scenario when a 2.0 mass suction is applied in conjunction with a 0.01 hybrid nanofluid, as compared to the conventional fluid. In the scenario of opposing flow, the heat transfer rate exhibits a significant increase of 36.96% when compared to that of ordinary fluid. Heat transfer increases 43.00% when Rd increases from 0.1 to 0.5 for both assisting and opposing flow.

Investigation of airflow thermal ice melting process under various flight conditions

The challenge of preventing and removing ice from exposed regions of aircraft is a significant engineering concern. Understanding the melting process of ice due to aerodynamic heating during flight is essential for developing UAV flight strategies in icy conditions. This paper analyzes the heat transfer mechanisms involved in airflow over ice and introduces a theoretical model to estimate the ice melting rate, using principles of turbulent heat transfer at the stagnation point. The study also discusses the impact of environmental conditions on ice melting rates. Findings indicate that the melting speed of the ice surface exhibits a negative linear correlation with the initial temperature of the ice, whereas it shows a nonlinear correlation with airflow velocity and total temperature of the incoming flow. Lower airflow velocity or total temperature of the incoming flow enhances the sensitivity of ice melting speed to changes. Additionally, lower ice density results in a higher melting speed, showing an exponential relationship with factors like average droplet diameter, airflow velocity, and airfoil leading edge diameter in the cloud field during icing. Experiments conducted using a small jet test bench and an icing wind tunnel confirmed the impact of varying conditions on ice melting rates. The deviation between experimental results and theoretical predictions was under 5 %. These conclusions offer valuable insights for flight safety planning of unprotected iced aircraft in challenging environments.

Perturbation amplification near the stagnation point of blunt bodies

Perturbation amplification near the stagnation point of blunt bodies

Intelligent computing applications to study the tri-hybrid nanofluid past over the stretched surface

In this study, a recurrent neural network with the Levenberg-Marquardt backpropagation algorithm is used to examine the effects of various parameters on cooling mechanisms involving tri-hybrid nanofluid consisting of Al2O3, Cu, TiO2 and magnetohydrodynamic stagnation point flow. This research focus on nanofluid in the existence of an asymmetrically stretched disc in water, which plays an imprortant role for understanding heat transfer in addition to the dynamics of momentum and thermal boundary surface. Transform the complex partial differential system through suitable similarity transformations into the ordinary differential equations for numerical computing. These equations have been solved numerically by using the Adams method in Mathematica to generate datasets for various parameters.The current study demonstrates that the velocity profile rises with higher magnetic, velocity slip, and mass suction parameters. The fluid thermal level tends to decrease with an increase in mass suction parameters, whereas an increase in the magnetic parameter and Eckert number raises the fluid’s thermal level. Furthermore, it is noticed that the flow resistance and internal friction of the fluid are proportional to the viscosity parameter. The proposed scheme has shown its effectiveness in a wide range of scenarios and cases. This was achieved by comparing the mean square error metric for the squeezing flow tri-hybrid nanofluid problem with the obtained results. In addition, various statistical measures like state transitions, fitness assessments, error histograms, and regression analyses, provide further evidence of the solver’s robustness and reliability. The excellent agreement between the reference datasets and refined numerical solutions through the scheme showcases the method’s effectiveness, achieving remarkable accuracy levels ranging from 10-6 to 10-12.

Flow structure beneath periodic waves with constant vorticity under strong horizontal electric fields

While several articles have been written on Electrohydrodynamics (EHD) flows or flows with constant vorticity separately, little is known about the extent to which the combined effects of EHD and constant vorticity affect the flow. This study aims to shed light on this topic by investigating the combined influence of a horizontal electric field and constant vorticity on the free surface and the emergence of stagnation points. Using the Euler equations framework, we employ conformal mapping and pseudo-spectral numerical methods. Our findings reveal that increasing the electric field intensity eliminates stagnation points and smoothen the wave profile. This implies that a horizontal electric field acts as a mechanism for the elimination of stagnation points within the fluid body. Besides, we have identified regimes where three stagnation points appear on the free surface — something that cannot occur in purely gravity rotational waves.

Pharmaceutical aerosol transport in airways: A combined machine learning (ML) and discrete element model (DEM) approach

The continuum and discrete phase interaction could significantly affect the inhaled particle's transport behaviour. The accurate analysis of continuum and discrete phase interaction needs computational fluid dynamics (CFD) and Discrete Element Method (DEM) simulation, which is computationally expensive. Therefore, this study aims to develop a novel machine learning (ML) prediction model from CFD-DEM data to predict pharmaceutical aerosol transport in airways accurately. This study uses the CFD model for the continuum and DEM for the discrete phases. A soft sphere approach was used to calculate the overlap of the colliding particles. Proper validation was performed to ensure the accuracy of the present model. The CFD-DEM model analysed the particle transport in an idealised and realistic airway model, and different methods were used to analyse the transport behaviour. As the flow rate increased from 15 to 60 lpm, the deposition efficiency (DE) significantly improved due to particle interaction, rising from 20 % to 42 % without interaction and from 50 % to a remarkable 76 % with interaction, demonstrating the critical role of particle interaction in enhancing DE across varying flow rates. During the particle-particle interaction, a stagnation point and a high-pressure zone were observed at the airway model's carinal angle. Finally, a ML prediction model is developed from CFD-DEM data, which accurately predicts the pharmaceutical aerosol deposition in airways. The ML prediction model predicts the deposition pattern for different flow rates and particle sizes without CFD-DEM simulations. The present findings and more case-specific investigation would advance the knowledge of aerosol transport in airways and benefit more efficient targeted drug delivery devices.

An experimental study of a quasi-impulsive backwards wave force associated with the secondary load cycle on a vertical cylinder

Steep wave breaking on a vertical cylinder (a typical foundation supporting offshore wind turbines) will induce slam loads. Many questions on the important violent wave loading and the associated secondary load cycle remain unanswered. We use laboratory experiments with unidirectional waves to investigate the fluid loading on vertical cylinders. We use a novel three-phase decomposition approach that allows us to separate different types of nonlinearity. Our findings reveal the existence of an additional quasi-impulsive loading component that is associated with the secondary load cycle and occurs in the backwards direction against that of the incoming waves. This quasi-impulsive force occurs at the end of the secondary load cycle and close to the passage of the downward zero-crossing point of the undisturbed wave. Wavelet analysis showed that the impulsive force exhibits superficially similar behaviour to a typical wave-slamming event but in the reverse direction. To monitor the scattered wave field and extract run-up on the cylinder, we installed a four-camera synchronised video system and found a strong temporal correlation between the arrival time of the Type-II scattered wave onto the cylinder and the occurrence of this quasi-impulsive force. The temporal characteristics of this quasi-impulsive force can be approximated by the Goda wave impact model, taking the collision of the Type-II scattered waves at the rear stagnation point as the impact source.

Simulation of tungsten impurity transport by DIVIMP under different divertor magnetic configurations on HL-3

Tungsten (W) accumulation in the core, depending on W generation and transport in the edge region, is a severe issue in fusion reactors. Compared to standard divertors (SDs), snowflake divertors (SFDs) can effectively suppress the heat flux, while the impact of magnetic configurations on W core accumulation remains unclear. In this study, the kinetic code DIVIMP combined with the SOLPS-ITER code is applied to investigate the effects of divertor magnetic configurations (SD versus SFD) on W accumulation during neon injection in HL-3. It is found that the W concentration in the core of the SFD is significantly higher than that of the SD with similar total W erosion flux. The reasons for this are: (1) W impurities in the core of the SFD mainly originate from the inner divertor, which has a short leg, and the source is close to the divertor entrance and upstream separatrix. Furthermore, the W ionization source (S W0) is much stronger, especially near the divertor entrance. (2) The region overlap of S W0 and pointing upstream promote W accumulation in the core. Moreover, the influence of W source locations at the inner target on W transport in the SFD is investigated. Tungsten impurity in the core is mainly contributed by target erosion in the common flux region (CFR) away from the strike point. This is attributed to the fact that the W source at this location enhances the ionization source above the W ion stagnation point, which sequentially increases W penetration. Therefore, the suppression of far SOL inner target erosion can effectively prevent W impurities from accumulating in the core.

Flow dynamics and heat transfer characteristics of air/water mist jet impingement using Eulerian-Eulerian approach

The present numerical study focuses on a two-phase Eulerian-Eulerian approach to model the flow structure and heat transfer characteristics of an air/water mist jet emanating from a coaxial jet nozzle and impinging on an isothermal heated flat plate. A detailed parametric numerical study has been performed to investigate the effect of various operating parameters like air Reynolds number (2400-9142), mist Reynolds number(1.87–8.68), mist loading fraction (0 % - 0.96 %), droplet diameter (1 μm – 60 μm), and non-dimensional nozzle to plate spacing (10–42) on the flow structure and heat transfer characteristics of air/water mist jet. The present numerical results are in agreement with the experimental results, within 16 % accuracy. The heat transfer increases with an increase in the air Reynolds number or mist loading fraction. The increase in air Reynolds number provides better heat transfer enhancement as high as 94.5 % compared to 27.6 % in the case of an increment in the mist loading fraction. Additionally, the effect of droplet diameter on the flow structure and heat transfer characteristics of air/water mist jet is analyzed at depth. The increase in nozzle-to-plate distance from 10 to 42 leads to a decrease in stagnation point Nusselt number of the order of 31 %. Further, correlations have been proposed to predict the Nusselt number at the stagnation point, stagnation region, and average Nusselt number for the target plate.

Entropy generated nonlinear mixed convective beyond constant characteristics nanomaterial wedge flow

Background and objectiveNanomaterial flows at present have received much attention of the researchers. Such motivation stems for their utilization in pharmaceutical, industrial, petroleum, manufacturing and engineering applications including regenerative medicine, treatment of cancer, electronic device cooling, solar collector, mineral oil, antimicrobial agents, thermal storage system, transformer cooling and X-imaging etc. Higher thermal energy requirement in several electrical and mechanical systems is desired in recent time due to high advancement of science and technology. For important application here the stagnation point flow due to wedge with nonlinear convection is explored. For porous media, the Darcy-Forchheimer relation is considered. Characteristics of nanofluid is added by utilizing Buongiorno's model. Variable fluid characteristics are under consideration. Applied magnetic field is accounted. Energy expression comprises thermal radiation, Brownian motion, Ohmic heating, thermophoresis and heat generation. First order reaction and entropy rate are considered. MethodologyNonlinear differential expressions are changed into dimensionless ordinary systems through adequate transformations. The resultant nonlinear ordinary systems are solved through optimal homotopy analysis technique (OHAM). ResultsOutcomes for influential variables about temperature, velocity, concentration and entropy rate have been arranged. Here results show that for magnetic field the liquid flow and entropy rate have opposite scenarios. Thermal distribution and concentration have reverse effects for random motion variable while similar effect holds for thermophoresis parameter. Magnetic field and diffusion parameters contribute to augment entropy generation. Higher values of temperature dependent electrical conductivity variable lead to increase entropy rate. Thermal distribution for radiation and Eckert number is similar.