Transition SST Research Articles

Polydispersed droplet spectrum and exergy analysis in wet steam flows using method of moments

In steam turbine flow, the complex droplet spectrum caused by nonequilibrium condensation is necessary to be modeled accurately to predict the droplet behavior and estimate the exergy destruction and erosion rate. This study built and validated a polydispersed model with Quadrature method of moments (QMOM), consisting of transition SST model, the moments and entropy generation. A spline-based algorithm was used to reconstruct the shape of the probability density function (PDF) of radius. It’s proved that the polydispersed model has a better prediction result for Sauter radius compare with monodispersed model. Then, the distributions of moments and droplet spectra in the nozzle with effects of asymmetric lambda shock and evaporation were investigated. The shape of droplet spectrum is closer to gamma distribution in nucleation zone and log-normal distribution in growth zone when outflow is supersonic. In the turbine, because the oblique shock induces complex evaporation and secondary condensation, the reconstructed shape is closer to gamma distribution. Finally, the obtained maximum exergy destruction is 25.293 kJ/kg. The rate of exergy destruction increases from 1.04% to 4.45%. The range of Baumann factor is 0.574–1.312. Besides, the erosion rate in polydispersed model is only 58.4–64.3% of monodispersed model. The polydispersed model used in this study can predict the droplet spectrum and energy loss of the turbine systems more accurately.

A novel CFD analysis to minimize the spread of COVID-19 virus in hospital isolation room.

The COVID-19 is a severe respiratory disease caused by a devastating coronavirus family (2019-nCoV) has become a pandemic across the globe. It is an infectious virus and transmits by inhalation or contact with droplet nuclei produced during sneezing, coughing, and speaking by infected people. Airborne transmission of COVID-19 is also possible in a confined place in the immediate environment of the infected person. Present study investigates the effectiveness of conditioned air released from air-conditioning machines to mix with aerosol sanitizer to reach every point of the space of the isolation room so as to kill the COVID-19 virus which will help to protect the lives of doctors, nurses and health care workers. In order to numerically model the laminar-transitional flows, transition SST k-ε model, which involves four transport equations are employed in the current study. It is found from the analysis that high turbulent fields generated inside the isolation room may be an effective way of distributing sanitizer in entire volume of isolation room to kill the COVID-19 virus.

Investigation of airflow at different activity conditions in a realistic model of human upper respiratory tract

In the present study, the turbulent flows inside a realistic model of the upper respiratory tract were investigated numerically and experimentally. The airway model included the geometrical details of the oral cavity to the end of the trachea that was based on a series of CT-scan images. The topological data of the respiratory tract were used for generating the computational model as well as the 3D-printed model that was used in the experimental pressure drop measurement. Different airflow rates of 30, 45, and 60 L/min, which correspond to the light, semi-light, and heavy activity breathing conditions, were investigated numerically using turbulence and transition models, as well as experimentally. Simulation results for airflow properties, including velocity vectors, pressure drops, streamlines, eddy viscosity, and turbulent kinetic energy contours in the oral-trachea airway model, were presented. The simulated pressure drop was compared with the experimental data, and reasonable agreement was found. The obtained results showed that the maximum pressure drop occurs in the narrowest part of the larynx region. A comparison between the numerical results and experimental data showed that the transition () SST model predicts higher pressure losses, especially at higher breathing rates. Formations of the secondary flows in the oropharynx and trachea regions were also observed. In addition, the simulation results showed that in the trachea region, the secondary flow structures dissipated faster for the flow rate of 60 L/min compared to the lower breathing rates of 30 and 45 L/min.

Analysis of turbulence and blocking effects on loading capacity for elementary texture cells of infinite width under water lubrication

PurposeThe loading mechanism of textures considering turbulence has not been fully covered. This paper aims to investigate the effect of turbulence on the textured loading capacity under water lubrication and to analyze the causes of the turbulence effect.Design/methodology/approachComputational fluid dynamic models with different textured shapes are established after validation. The transition shear stress transport (SST) model, which is suitable for predicting the transition process of fluid from laminar state to turbulent state, is adopted in the present study. To illustrate the effect of turbulence, the loading capacity of textures predicted by transition SST model and laminar model is compared.FindingsThe loading capacity is higher after considering turbulence because more lubricant enters into textures and the flow rate of lubricant to textured outlet increases. There exists an optimal textured depth ratio and density for loading capacity and the change of flow state would not affect the optimal values. The degree of fluid blockage at textured outlet has a dominant influence on loading capacity. As the textured shape changes to triangle or ellipse from rectangle, the vortices at the textured bottom move forward and the blockage at a textured outlet is enhanced, which makes loading capacity improved under the action of blocking effect.Originality/valueThe enhancement of the blocking effect is found to be crucial to the improvement of textured loading capacity after considering turbulence. Present research provides references to understand the loading mechanism of textures under turbulent conditions.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2020-0149/

Analysis of heat transfer around bluff bodies with variable inlet turbulent intensity: A numerical simulation

In the present study, a numerical simulation is carried out to analyze the effect of turbulent intensity on the flow behavior of flow past two dimensional bluff bodies. Triangular prism, diamond and trapezoidal shaped bodies with the same hydraulic diameter D, a dimensionless length scale are taken into consideration as bluff bodies. The objective of the numerical analysis is to cover cross flow at both turbulent and laminar regime with varying Reynolds number upto 200,000 and inlet intensities ranging between 5% and 40%. The flow medium used is air at a constant Prandtl number. The energy, momentum and continuity equations are dealt with transition SST Model for closure of turbulence. The results obtained through numerical simulation are validated with other published results by researchers and show good agreements. This present study reveals that transition SST Model can be efficiently used to cover both laminar and turbulent flow regimes to estimate the heat transfer. The impact of inlet turbulent intensity on augmentation of heat transfer using bluff bodies has been evaluated. It is observed from the study that the turbulent intensity significantly affects drag coefficient.

Heat transfer simulation of annular elliptical fin-and-tube heat exchanger by transition SST model

In this study, thermo-fluid characteristics of elliptical annular finned tube heat exchanger were numerically studied in detail. Transition SST model was utilized to simulate turbulent flow. Effects of air velocities, horizontal to vertical fin diameter ratios, and fin densities were examined in detail. The simulations indicate superior performance of elliptical fin layout. It was shown that pressure drop of annular elliptical fin can be only one half of that of a circular annular fin while containing comparable heat transfer performance. The vertical elliptical annular fin may even contain a higher heat transfer performance over circular fin. Correlations are proposed to estimate the Nu number and pressure drop based on the annular circular fin. The maximum deviations between the proposed correlations and simulations regarding pressure drop and heat transfer coefficient are 5.6% and 3.2%, respectively. For further elaboration of the superiority of the elliptical layout from the second law perspective, normalized entropy generation was also studied. In all cases, the entropy generation rate in circular fin was higher than that of an elliptical fin.

Numerical analysis of unstable turbulent flows in a centrifugal pump impeller considering the curvature and rotation effect

In this study, a modified partially averaged Navier-Stokes model (MSST PANS) is proposed by treating a modified shear stress transport (SST) k-ω model as the parent turbulence model. The unstable turbulent flow in a centrifugal pump that considers the curvature and rotation effect is investigated as the test case to evaluate the performance of the MSST PANS model and analyze the flow instability in a centrifugal pump. The SST k-ω and the standard k-e PANS models are also evaluated for comparison. Results show that the MSST PANS model exhibits excellent performance and delivers the most satisfactory prediction results of the positive slope of the characteristic curve, time-averaged internal flows, and velocity profiles. The energy loss based on the energy balance equations is adopted to provide an explanation of the internal flow evolutions in pumps. The findings also indicate energy loss distribution is associated with the positive slope phenomenon. The high-velocity gradient flows at the entrance of the blade-to-blade passage and the reverse flows at the impeller exit are the main reasons for the high turbulent kinetic energy in the impeller. The MSST PANS model demonstrates promising applications in the field of hydraulic machinery, where unstable turbulent flows are prevalent.

Transition Delay and Drag Reduction using Biomimetically Inspired Surface Waves

This paper explores the use of Two-Dimensional sinusoidal surface features to delay transition and/or reduce drag. The authors, in this paper demonstrated that the presence of low amplitude sinusoidal surface features might damp the disturbances in the laminar boundary layer, reduce wall shear stress and maintain laminar flow for longer than a conventional flat plate. The hypothesis of the paper is inspired by the simplification of the dermal denticle on the surface of the shark-skin. Simulations are carried out using the Transition SST model in FLUENT based on the evidences of the transition model being suitable for a wider variety of high curvature scenarios. The surface waves are simulated for different amplitudes and wavelengths and their impact on transition onset and drag reduction are quantified at different velocities. Results presented in this paper indicate that a transition delay of 10.8% and a drag reduction of 5.2% are achievable. Furthermore, this paper adds credence to the notion that biomimicry is a very promising avenue for future drag reducing methods.

Laminar-Turbulent Transition on a Cambered NACA 16-009 Airfoil at Low Speed

Infrared thermography, force measurements, and oil flow visualizations are used to investigate the flow patterns around a cambered NACA16-409 airfoil at low Mach number. This cambered profile is widely used for propellers despite the lack of knowledge concerning its flow characteristics. The post-processing of thermograms relies on the analysis of the surface temperature gradient and identification of inflexion points in the temperature distribution. The observations made on the thermograms, based on the distribution of the temperature and Stanton number, are substantiated by the oil flow visualizations. RANS simulations with a transitional SST k-ω & γ turbulence model corroborate the analysis and deliver detailed insight in the flow around the airfoil. Depending on the angle of attack, three distinct flow patterns have been identified: laminar flow with early separation, laminar separation bubble with trailing edge separation, and turbulent flow with trailing edge separation. The shift between the last two regimes occurs sharply. The prediction capability of the transitional RANS simulations and XFOIL in terms of separation as well as reattachment location are compared with the experimental results. The force-coefficients dependency on the angle-of-attack, obtained by experiments, XFOIL, and RANS simulations, bear the traces from these flow patterns.

On the influence of blade thickness-to-chord ratio on dynamic stall phenomenon in H-type Darrieus wind rotors

The present work examines the influence of blade thickness-to-chord ratio (t/c) on dynamic stall phenomenon in an H-type Darrieus wind rotor. A 2D incompressible numerical study is conducted on a single-bladed rotor at a tip speed ratio (TSR) of 2 with a motivation to understand the intricate flow physics around the blade by employing a transitional shear stress transport (TSST) model. Five different t/c ratios (9%, 12%, 15%, 18% and 21%) have been studied for symmetrical NACA airfoils. It is found that in case of thinner airfoils (t/c = 9% and 12%), the formation of the dynamic stall vortex (DSV) is preceded by the formation and bursting of leading edge (LE) laminar separation bubble (LSB) leading to a more abrupt LE type stall. For t/c = 15%, a mixed type stall is observed with LSBs distributed over the airfoil resulting in two coherent vortex structures that subsequently merge to form the DSV. A trailing edge (TE) type stall was observed in case of thicker airfoils (t/c = 18% and 21%). The largest peak in the lift and drag coefficients (Cl, Cd) values of 1.87 and 1.27, respectively are obtained with the thinnest airfoil. However, the largest peak in the moment coefficient (Cm) value is achieved with t/c = 12%. Further, due to the DSV separation at the lift stall point, a further increase of 6.3%, 7.1%, 1.5%, 8.94% and 11.8% in Cd values is observed for increasing order of t/c. Overall, the study sheds light on the capability of TSST model in capturing the dynamic stall phenomenon efficiently.

Nozzle Flow Influence on Forebody Aerodynamics

In this study, the nozzle flow influence on the forebody aerodynamics has been numerically investigated and validated with experiment. To understand the nozzle flow aerodynamics on the forebody, five different nozzle-exit profiles, which are often encountered in the experiments, are considered one at a time. One flow condition is considered at a nominal freestream Mach number of 4. The CFD simulations are performed using a two-dimensional axisymmetric four-equation transition shear stress transport (SST) turbulence model. The numerical results are compared with the Ludwieg tube Mach 4 experimental data and showed good agreement. The numerical results showed that despite the slight effect on the wall static pressure depending on the nozzle profiles considered, the physical flow phenomena such as shock shape are directly influenced. With the undeveloped nozzle flow profile case, however, significant pressure differences are observed. The distance between the model and the nozzle exit may significantly affect the forebody aerodynamic characteristics, and so, the consideration of the distance between the model and nozzle exit needs to be placed during the model setup to reduce the unwanted uncertainty in the measurement.

Optimization of a Small Wind Turbine for a Rural Area: A Case Study of Deniliquin, New South Wales, Australia

The performance of a wind turbine is affected by wind conditions and blade shape. This study aimed to optimize the performance of a 20 kW horizontal-axis wind turbine (HAWT) under local wind conditions at Deniliquin, New South Wales, Australia. Ansys Fluent (version 18.2, Canonsburg, PA, USA) was used to investigate the aerodynamic performance of the HAWT. The effects of four Reynolds-averaged Navier–Stokes turbulence models on predicting the flows under separation condition were examined. The transition SST model had the best agreement with the NREL CER data. Then, the aerodynamic shape of the rotor was optimized to maximize the annual energy production (AEP) in the Deniliquin region. Statistical wind analysis was applied to define the Weibull function and scale parameters which were 2.096 and 5.042 m/s, respectively. The HARP_Opt (National Renewable Energy Laboratory, Golden, CO, USA) was enhanced with design variables concerning the shape of the blade, rated rotational speed, and pitch angle. The pitch angle remained at 0° while the rising wind speed improved rotor speed to 148.4482 rpm at rated speed. This optimization improved the AEP rate by 9.068% when compared to the original NREL design.

Capturing the Dynamic Stall in H-Type Darrieus Wind Turbines Using Different URANS Turbulence Models

Abstract The occurrence of dynamic stall phenomenon in an H-type Darrieus wind turbine with low tip speed ratio (TSR) has been numerically investigated on a single-bladed rotor with NACA 0012 airfoil. The Reynolds number (Re) ∼105 at TSR = 2 implicates complex turbulence environment around the blades of the turbine modeling which still remains a challenging problem. Thus, with a motivation to find out a suitable turbulence model to capture the dynamic stall, a comparative study is carried out between three unsteady Reynolds-averaged Navier–Stokes (URANS) models: Spalart–Allmaras (S-A), shear stress transport (SST) k–ω, and transition SST (TSST). It was found that the TSST model predicted the dynamic stall phenomenon the earliest, whereas, the S-A model predicted it the latest. The transitional phenomenon like formation and bursting of the laminar separation bubble (LSB) was best predicted by the TSST model. However, the TSST overpredicts the turbulent boundary layer (BL) roll up from the trailing edge (TE) toward the leading edge (LE). The percentage difference in the power coefficient (Cp) values with respect to the TSST accounted to 16.67% and 60% higher for SST k–ω and S-A models, respectively. The S-A model delays the torque coefficient (Ct) peak prediction by 5 deg and 11 deg azimuthal angle compared with SST k–ω and TSST models, respectively. Overall, it was found that the transitional aspect in TSST model is important in predicting the light stall regime; however, in the deep stall regime SST k–ω model performed well too.

Математическое и физическое моделирование влияния пульсаций скорости сносящего потока воздуха на структуру пламени при диффузионном режиме горения метана

The paper presents the results of calculation and experimental studies of the diffusion combustion of methane in the air cross-flow. We developed a mathematical model for describing a diffusion air-methane flame, the model being based on solving a system of averaged Navier --- Stokes equations in an unsteady setting. To calculate the combustion processes, we used the flamelet models and eddy dissipation concept (EDC) model. The mathematical model was supplemented by a detailed kinetic mechanism consisting of 325 elementary reactions involving 53 substances. Furthermore, we carried out calculations and comparative analysis of the flame characteristics using various turbulence models: k − ε, k − ω SST and Transition SST. The study introduces a diagram of the experimental setup for physical modeling of methane combustion in the air cross-flow, and presents the results of the calculation and experimental study of the cross-flow velocity pulsation effect on the flame structure, as well as the efficiency of methane combustion in the diffusion mode. We obtained data on temperature and concentration fields at pulsation frequencies of 0--100 Hz. Findings of research show that for the case under consideration, stable combustion occurs at pulsation frequencies of 0--90 Hz. The maximum observed flame lift-off is 3.2 times the diameter of the burner nozzle

The application of body scanning, numerical simulations and wind tunnel testing for the aerodynamic development of cyclists

The aerodynamic efficiency of an elite cyclist is often evaluated and optimised using either one or a combination of field testing, wind-tunnel testing and numerical simulation. This study focuses on the processes and limitations involved in using a body scan to produce an accurate geometry for input to numerical simulation, with validation through drag comparisons from wind-tunnel tests and vortical wake-flow features reported in previous experimental studies. Transitional Shear Stress Transport Reynolds-Averaged Navier-Stokes simulations based on the scanned geometry were undertaken for a 180 ° half crank cycle at 15 ° increments. The sectional drag force contributions of 23 body subparts are presented, documenting the contribution and variation of each body/cycle component over the cycle. These methods are evaluated and the limitations of the approaches are discussed. The results from the numerical simulation and the wind tunnel measured drag force were very similar, differing by approximately 1%–7% for various crank angles.

Numerical Simulation of the Flow around a Straight Blade Darrieus Water Turbine

In this study, three-dimensional transient numerical simulations of the flow around a cross flow water turbine of the type H-Darrieus are performed. The hydrodynamic characteristics and performance of the turbine are investigated by means of a time-accurate unsteady Reynolds-averaged Navier–Stokes (URANS) commercial solver (ANSYS-Fluent v. 19) where the time dependent rotor-stator interaction is described by the sliding mesh approach. The transition shear stress transport turbulence model has been employed to represent the turbulent dynamics of the underlying flow. Computations are validated versus previous experimental work in terms of the turbine efficiency curve showing good agreement between numerical and experimental values. The behavior of the power and force coefficients as a function of turbine angular speed is analyzed. Moreover, visualizations and analyses of the instantaneous vorticity iso-surfaces developing at different blade rotational velocities are presented including a few movies as additional material. Finally, the fluid variables fields are averaged along a turbine revolution and are compared with the steady predictions of simplified steady approaches based on the blade element momentum theory and the double multiple streamtube method (BEM-DMS).

CFD-based improvement of Savonius type hydrokinetic turbine using optimized barrier at the low-speed flows

A Savonius turbine is a vertical axis hydrokinetic turbine (VAHT) utilizes in low-speed channels and rivers. In comparison with the other hydrokinetic turbines, the Savonius turbine is simple in construction and installation, and involves less installation cost; however, these turbines have low torque and power coefficients in comparison with other hydrokinetic turbines. The idea of this paper is utilizing a simple barrier to deviate the fluid flow from the reversing bucket of the Savonius turbine to enhance its generated power. In order to investigate the most appropriate length of the barrier, numerical modeling has been performed by applying computational fluid dynamics (CFD). The continuity, Reynolds Averaged Navier-Stokes – RANS equations and the SST transition turbulence model are numerically solved. The validation of the numerical simulation is assessed based on the experimental data of Sandia laboratories, and the results indicated good agreement with experimental data. Thereupon, the model is utilized for optimizing the length of the barrier in various cases. The power coefficient of different cases is compared with the obstacle-less conventional Savonius turbine. The results of this analysis reveal that utilizing a barrier in its optimum length increases the maximum generated power by about 18 percent.

Numerical aero-thermal study of high-pressure turbine nozzle guide vane: Effects of inflow conditions

Accurate predictability of high-pressure turbine nozzle guide vane aero-thermal performance is highly desired in the development campaign due to the exposure of the component to a frequent and high heat load. In this paper, the representative vane profile in modern aero-engines is numerically studied. Aerodynamics and aero-thermal validations of the blade profile have been performed in comparison with the available experimental data. It has been shown that a satisfactory agreement could be achieved with the use of the transitional turbulence model shear stress transport γ–θ due to its superiority in capturing the laminar–turbulent transition. Sensitivity studies on the increase in the inlet turbulence intensity, inlet endwall boundary layer thickness, and inlet total temperature profile have been performed to understand the impact of inflow conditions’ uncertainty on the aero-thermal predictability. Increasing the inlet turbulence intensity increases the pressure surface heat transfer coefficient and induces an earlier transition onset on the suction surface. Due to the rapid decay of turbulence intensity in the numerical model, the use of an artificially high inlet turbulence intensity has been shown to be effective in the prediction improvement. On the other hand, the change in the inlet boundary layer thickness influences the formation and strength of the secondary flow, namely, horseshoe vortex and passage vortex. These secondary flow phenomena affect the local blade surface heat transfer coefficient in the near-endwall region although the most significant rise in heat transfer is found on the endwall. The temperature distortion amplitude of a hot streak and its relative clocking position with the vane significantly affect the heat flux distribution. In contrast, the heat transfer coefficient is less sensitive to the change in hot streak conditions. However, it has been shown that increasing the temperature distortion amplitude could induce a larger difference among different clocking configurations. In addition, decreasing the difference between the fluid and wall temperature would delay the transition onset and stabilize the boundary layer. Further analysis of the unsteady effects has been carried out by comparing the steady and time-averaged flow solutions. It has been observed that the discrepancy between these solutions is attributed to the flow field nonlinearity. Thus, a significant discrepancy can be found in the laminar–turbulent transition as well as in the trailing edge region. However, since the contribution of these regions on the total area-averaged heat transfer is small, their influence on the total vane heat transfer is limited.

Experimental and numerical investigation of tip leakage vortex cavitation in an axial flow pump under design and off-design conditions

The interaction of tip leakage flow and main flow in an axial flow pump can induce the tip leakage vortex, which causes the unstable flow and complex cavitation structures. In the present research, the modified shear stress transport k–ω turbulence model was utilized to predict the cavitating flow of the model pump under design and off-design conditions. Validations were carried out using high-speed photography techniques. Results show that the simulation results about cavitation performance based on the modified shear stress transport k–ω turbulence model agree well with the experimental results. Cavitation inception occurs more possible at part-load conditions, and with the increase of the flow rate, the starting point of tip leakage vortex gradually moves to the back edge of the blade chord. Both the sheet cavitation and the triangular cavitation cloud that formed in the blade tip grow in size and intensity gradually with the decrease of cavitation number. Distributions of the cavity area fraction and axial velocity show that the tip leakage vortex cavitation locates at the radial coefficient r* = 0.95–1.0 with peaks at about r* = 0.97, while the sheet cavitation is found at r* = 0.5–0.9 on the suction side (SS). The cavitation structures lead to a significant decrease in the axial velocity, especially in the tip region of the blade.

Flow and thermal analysis of hybrid mini-channel and slot jet array heat sink

In this paper, experimental tests and numerical studies were performed for better understanding the flow and heat transfer characteristics of the hybrid mini-channel and slot jet array heat sink. The average heat transfer coefficients were experimentally obtained and compared with independent mini-channel heat sink and slot jet array heat sink with a dimension of 10 mm × 10 mm at a flow rate from 0.1 L/min to 1.2 L/min, heat flux of 35.74 W/cm2 and 62.22 W/cm2. The results showed that the hybrid scheme has a better heat transfer performance than other two heat sinks at the same flow rate. Numerical simulations were conducted to further analyze thermal-hydraulic performance of the hybrid heat sink at an inlet Reynolds from 230 to 2760. Compared with k-ε models, numerical results with transition SST flow model match better with the experimental results. According to the numerical results, we found vortexes in the channel of the hybrid heat sink and these vortexes are beneficial to the heat transfer. The influence of geometries on the overall thermal resistance and total pressure drop were discussed using numerical simulations. The results demonstrated that the base thickness, channel height, side wall to channel width ratio and channel number of the hybrid heat sink all have significant effect on the cooling performance and thermal resistance and pressure drop can be reduced obviously via geometry optimization.