Shear Stress Transport K-omega Research Articles

Numerical study on the effect of smoke emitted from the vents on the roof of a diesel train on the intake of downstream air-conditioning units

Constrained by economic development and geographical features, numerous railway lines remain unelectrified, underscoring the expansive potential of diesel trains. Diesel engine emissions discharged from the roof of trains pose a challenge as some of the smoke infiltrates the cabin through the intake of roof-mounted air-conditioning units (ACUs). This intrusion diminishes the indoor air quality, posing health risks to passengers and potentially jeopardizing their safety. This study employs the shear stress transport k-omega turbulence model to formulate a multiphase flow model for simulating smoke diffusion in diesel trains. Additionally, we conducted an optimization design to minimize smoke entry into the ACUs. This study defined six cases based on variations in the shape and height of the cover and the spacing of the smoke vents. The results show that the effect of the diffusion characteristics decreased with the cover height. With the progression of airflow diffusion, the effect of the smoke vent structure on the concentration diminished farther from the vents. The minimum smoke mass flow rate into the intake occurred with the vent spacing of 2.14 m and without a cover, resulting in a 57.0% decrease compared with the maximum. Thus, a smoke vent spacing of 2.14 m without a cover was deemed to be the optimal configuration. The research results provide certain engineering guidance significance for the design and operation of train-smoke vent structures.

Optimization of Flanged Diffuser for Small-Scale Wind Power Applications

The development of renewable and clean energy has become more crucial to societies due to the increasing energy demand and fast depletion of fossil fuels. A state-of-the-art design for an augmented wind turbine has been introduced in the past years to increase the efficiency of compact horizontal axis wind turbines, exceeding the ideal Betz’s limit of the maximum energy captured from the wind. The optimization of the flanged diffuser - so-called diffuser augmented wind turbine DAWT - is investigated numerically using the multi-objective genetic algorithm “MOGA”. A 2D computational model is developed using ICEM CFD and solved by ANSYS Fluent. The Turbulence model selected is shear stress transport K-omega, with a pressure-based solver and a coupled algorithm scheme. The optimization objectives are to maximize the velocity ratio at the shroud throat and minimize shroud form dimensions. 517 design points were solved, and the design dimensions were categorized into four types: compact, small, medium, and large design. The results showed that the diffuser dimensions are the main parameters to increase velocity inside the shroud throat, where a long diffuser with a low converging angle drags more air inside the shroud, reaching in some cases more than double the upwind velocity. While the nozzle and flange are also effective in the different design types. It was found that a super long diffuser with a length ratio of 2.9 LD to throat diameter D is optimal with a diverging angle of 7.6˚, accompanied by a nozzle of ratio 1.2 LN/D and 12.6˚ converging angle and a flange length ratio of 0.6 LF/D. This optimal design increased the velocity ratio by almost 2.5 times.

Transient aerodynamic behavior of a high-speed Maglev train in plate braking under crosswind

The test speed of high-speed maglev trains (HSMT) exceeds 600 km/h, requiring higher braking performance and technology. Plate braking technology, which is a suitable choice, has been applied for engineering the high-speed test vehicles. However, the unsteady aerodynamic response during the opening process of HSMT under crosswind needs to be studied. This study explores the unsteady aerodynamic characteristics of a HSMT with a train speed of 600 km/h during plate braking at different crosswind speeds. The plate motion is achieved based on the dynamic grid technology, and the unsteady flow field around the train is simulated using the unsteady Reynolds time averaged equation and the shear stress transport k-omega (SST k–ω) turbulence model. This calculation method was verified using wind tunnel test data. The peak aerodynamic drag (AD) of the braking plates overshot during opening. Under a crosswind of 20 m/s, the AD peak of the first braking plate was 11% larger than that without crosswind. The middle braking plates were significantly affected by upstream vortex shedding, and the AD fluctuation was the most severe. The AD of the head and tail coaches is significantly affected by crosswind. With an increase in the crosswind speed, the AD of the head and tail coaches decreased and increased, respectively. Compared with no crosswind, under a crosswind of 20 m/s, the AD of the head coach decreased by 43%, and the AD of the tail coach increased by a factor of approximately 1.1 times. Furthermore, the AD fluctuation of the tail coach was the most severe.

Effect of cavitation on vortex-induced vibration of hydrofoil

Vortex induced vibration (VIV) is frequently observed in marine structures in offshore platforms, ships and other structures. Under specific circumstances, the vortex shedding frequency is locked into the natural frequency of the structure, resulting in significant and destructive vibration. Moreover, the cavitation in the wake vortex also cause strong trailing edge vibration. In this study, the interaction between VIV and cavitation of two-dimensional hydrofoil was adopted the Shear Stress Transport K-omega model and the Schnerr–Sauer cavitation model. The research findings indicate that the frequency of the wake vortex increases with the decrease of the cavitation number, but the vibration amplitude decreases.

Study on transient flow behaviors in a three-stage helico-axial multiphase pump under slug flow condition from wellhead

Oilfield recovery often has extreme flow patterns of slug flow, and its gas volume fraction (GVF) still fluctuates strongly after being buffered by a homogenizer. After entering the multiphase pump, it causes transient fluctuations in pump performance, which challenges the stable operation of the multiphase pump. This paper simulates the flow field in a three-stage multiphase pump using the Euler–Euler two-fluid model and the shear stress transport k-omega (SST k−ω) turbulence model with the buffered slug flow as the boundary conditions. The results show that the differential pressure and torque of each impeller vary with the fluctuation of the inlet gas volume fraction (IGVF) and the maximum fluctuation rate reaches 36.48% and 32.97%, respectively; the maximum fluctuation rate of axial force and radial force is 1.02% and 51.18%, respectively, and the buffered slug flow has limited effect on the axial force. The main factor of pressure pulsation is the fluctuation of GVF flow and the interaction of static and dynamic. The main frequency in the impeller is the fluctuation frequency of the GVF or the diffuser blade frequency. The main frequency in the diffuser is the impeller blade frequency and its double frequency. The fluctuation frequency of the GVF is the secondary frequency, and the amplitude is relatively stable.

Applied Machine Learning to Estimate Length of Separation and Reattachment Flows as Parameter Active Flow Control in Backward Facing Step

Recently, large amounts of data from experimental measurements and simulations with high fidelity have extensively accelerated fluid mechanics advancement. Machine learning (ML) offers a wealth of techniques to extract data that can be translated into knowledge about the underlying fluid mechanics. Backward-Facing Step (BFS) is well-known for its application to fluid mechanics, particularly flow turbulence. Typically, a numerical approach can be used to understand the flow phenomena on BFS. In some instances, numerical investigations have a computational time limitation. This paper examines the application of ML to predict reattachment length on BFS flow. The procedure begins with a simulated meshing sensitivity of 1.27 cm in step height. This numerical analysis was conducted in the turbulent zone with a Reynolds number between 35587 and 40422. OpenFOAM® was used to perform numerical simulations using the turbulence model of k-omega shear stress transport. ML employed information in the form of Velocity and Pressure at every node to represent the type of turbulence. Using Recurrent Neural Networks (RNNs) as the most effective model to predict reattachment length values, the reattachment length was predicted with a Root Mean Square Error of 0.013.

The optimization of biomimetic sharkskin riblet for the adaptation of drag reduction

The biomimetic sharkskin riblet has attracted more attention in engineering and research owing to the economic benefit of drag-reducing and antifouling properties. Although several works have optimized the parameters for a number of riblet designs, there is not yet a study dedicated to optimizing for rectangular cuboidal riblet parameters. Hence, this work proposes a new numerical optimization of riblet orientations and arrangements of a three-dimensional rectangular cuboidal riblet. The shear-stress transport k-omega was adopted as an appropriate turbulence model for the simulation. The results showed that the riblets efficiently improved drag reduction regardless of the flow state. Riblets oriented perpendicularly to the flow direction showed the greatest performance of 11.3% and 6% in the laminar and turbulent flow, respectively. Moreover, the analysis of the flow field characteristics near the wall revealed a significant improvement in terms of reduction in near-wall velocity and shear stress. This work offers new perspectives on the role of the intricate multifunctional architecture of shark-scale structures in improving swimming speed and opening new doors for marine and underwater applications.

Thermohydraulic performance of thermal system integrated with twisted turbulator inserts using ternary hybrid nanofluids

Abstract Mono, hybrid, and ternary nanofluids were tested inside the plain and twisted-tape pipes using k-omega shear stress transport turbulence models. The Reynolds number was 5,000 ≤ Re ≤ 15,000, and thermophysical properties were calculated under the condition of 303 K. Single nanofluids (Al2O3/distilled water [DW], SiO2/DW, and ZnO/DW), hybrid nanofluids (SiO2 + Al2O3/DW, SiO2 + ZnO/DW, and ZnO + Al2O3/DW) in the mixture ratio of 80:20, and ternary nanofluids (SiO2 + Al2O3 + ZnO/DW) in the mixture ratio of 60:20:20 were estimated in different volumetric concentrations (1, 2, 3, and 4%). The twisted pipe had a higher outlet temperature than the plain pipe, while SiO2/DW had a lower T out value with 310.933 K (plain pipe) and 313.842 K (twisted pipe) at Re = 9,000. The thermal system gained better energy using ZnO/DW with 6178.060 W (plain pipe) and 8426.474 W (twisted pipe). Furthermore, using SiO2/DW at Re = 9,000, heat transfer improved by 18.017% (plain pipe) and 21.007% (twisted pipe). At Re = 900, the pressure in plain and twisted pipes employing SiO2/DW reduced by 167.114 and 166.994%, respectively. In general, the thermohydraulic performance of DW and nanofluids was superior to one. Meanwhile, with Re = 15,000, DW had a higher value of η Thermohydraulic = 1.678.

Numerical and Experimental Analysis of Shell and Tube Heat Exchanger with Round and Hexagonal Tubes

Shell and tube heat exchangers are used to transfer thermal energy from one medium to another for regulating fluid temperatures in the processing and pasteurizing industries. Enhancement of a heat transfer rate is desired to maximize the energy efficiency of the shell and tube heat exchangers. In this research work, we performed computational fluid dynamics (CFD) simulations and experimental analysis on the shell and tube heat exchangers using round and hexagonal tubes for a range of flow velocities using both parallel flow and counter flow arrangements. In the present work, the rate of heat transfer, temperature drop, and heat transfer coefficient are computed using three turbulence models: the Spalart–Allmaras, the k-epsilon (RNG), and the k-omega shear stress transport (SST). We further utilized the logarithmic mean temperature difference (LMTD) method to compute the heat transfer and mass flow rates for both parallel and counter flow arrangements. Our results show that the rate of heat transfer is increased by introducing the hexagonal structure tubes, since it has better flow disruption as compared to the round tubes. We further validated our simulation results with experiments. For more accurate results, CFD is performed in counter and parallel flow and it is deduced that the rate of heat transfer directly depends upon the velocity of fluids and the number of turns of the tube.

Quantitative evaluation on the suitable flow region inside the shielding device of submersible aquaculture facility

The primary purpose of the aquaculture facility is to provide a comfortable living environment and achieve welfare aquaculture for cultured fish in the open sea. Nevertheless, it has been a challenge to accurately assess the suitable flow region of the cultured fish inside the aquaculture facility for the severe ocean environment, which is a pressing concern for aquaculture farmers. First, the maximum domed swimming speed (MDSS) is used as an indicator to assess the swimming ability of Sciaenops ocellatus in this study, which can be measured through a vertical circulation water channel. Subsequently, the porous media theory is adopted and applied to predict the flow field inside the shielding device of the submersible aquaculture facility based on the shear stress transport k-omega turbulence model using the computational fluid dynamics (CFD) approach. In addition, combining the swimming ability of Sciaenops ocellatus and current speed distribution inside the shielding device of a submersible aquaculture facility, the suitable flow region is quantitatively assessed. The results indicate that when middle height, the inclined angle and net solidity are 0.4 m, 3π/12 and 0.65, respectively, the current speed inside the shielding device attenuates below the maximum domed swimming speed of Sciaenops ocellatus - 0.76 m/s and meets the survival of Sciaenops ocellatus for the actual sea condition with an incident current speed of 1.5 m/s. In addition, a suitable flow region for Sciaenops ocellatus, which is 204.512 m3 and accounts for 17.76% of the total aquaculture volume, is formed and located inside the shielding device, close to the downstream sides, and gradually extends towards the middle of the shielding device. Based on the swimming ability of Sciaenops ocellatus, using numerical means to describe the suitable flow region in detail is to provide clear and macroscopic guidance for aquaculture farmers.

Computational Fluid Dynamics and Experimental Analysis of a Wind Turbine Blade’s Frontal Section with and without Arrays of Dimpled Structures

Horizontal axis wind turbines are used for energy generation at domestic as well as industrial levels. In the wind turbines, a reduction in drag force and an increase in lift force are desired to increase the energy efficiency. In this research work, computational fluid dynamics (CFD) analysis has been performed on a turbine blade’s frontal section with an NACA S814 profile. The drag force has been reduced by introducing an array of dimpled structures at the blade surface. The dimpled structures generate a turbulent boundary layer flow on its surface that reduces the drag force and modifies the lift force because it has greater momentum than the laminar flow. The simulation results are verified by the experimental results performed in a wind tunnel and are in close harmony with the simulated results. For accurate results, CFD is performed on the blade’s frontal section at the angle of attack (AOA) with a domain of 0° to 80° and at multiple Reynolds numbers. The local attributes, lift force, drag force and pressure coefficient are numerically computed by using the three models on Ansys fluent: the Spalart-Allmaras, the k-epsilon (RNG) and the k-omega shear stress transport (SST).

Simulations of thermal phase changes and bacterial inactivation in a superheated steam dishwasher

The use of superheated steam in dishwashers as a means of reducing water consumption and cleaning time without the use of chemical cleaning agents has great future potential for the restaurants, hotels, and hospitals. In these sectors in particular, hygienic safety is an important concern in addition to the removal of food residues. To evaluate this potential application, the heat transfer and phase change characteristics of superheated steam associated with bacterial inactivation are investigated in an idealized three-dimensional dishwasher with a nozzle and a plate at a temperature of 180 °C and a pressure of 10 bar. Transient OpenFOAM simulations were performed using the interThermalPhaseChangeFoam solver. The k-omega shear stress transport turbulence model was used to capture the turbulent flow conditions. Bacteria inactivation was described using first-order Arrhenius kinetics. The flow pattern of the steam jet, the shape of the steam plume, the steam condensate, and the separation of the boundary layer are affected by the structure of the shock interaction, and vortices occur near the nozzle exit, around the perimeter of the plate, and on the side walls of the dishwasher. Strong steam shocks result in a temperature increase, higher steam condensation rate, and lower bacterial concentration on the plate surface. The bacteria on the plate surface are killed within a short time of 25 s, proving the effectiveness of superheated steam in dishwasher cleaning. This study provides a basis for future development and optimization of next-generation superheated steam dishwashers.

Numerical Study of Thermal-Hydraulic Performance Improvement of Enhanced Surface Tubes

A computational fluid dynamics study was carried out to compare the thermal-hydraulic performance of two enhanced heat transfer tubes (Vipertex models 1EHT-1 and 1EHT-2) to the performance of a smooth surface tube while using R410A as the working fluid. All tubes had an equivalent internal diameter of 8.32 mm and were made of stainless steel. Analysis was performed using an SST (shear stress transport), k-omega numerical model, with the Reynolds number ranging from 15500 to 32500. There was an increase in the heat transfer coefficient and frictional pressure drop for increasing Reynolds number. Heat transfer coefficient relative error (RE) percentage for all tubes evaluated was below 10%, while for the pressure drop the RE was below 16%. Additionally, an analysis of the boundary layer in the case of the enhanced surface tube shows a fluid flow disruption and it reveals increased fluid mixing with enhanced heat transfer. Additionally, it was discovered that dimple height has more of an effect on the thermal hydraulic performance than dimple diameter. It is possible to conclude that the SST k-omega model can be used to simulate and predict heat transfer within an acceptable error tolerance, allowing the optimization of operational parameters.

A parametric study for a flettner rotor in standalone condition using CFD

In this study, the performance prediction and parametric study of a Flettner rotor in standalone condition was conducted based on a computational fluid dynamics (CFD) approach. A method for predicting the rotor performance based on steady RANS simulation was established, and the present method was validated with a reference rotor model in which wind tunnel test results were published. The CFD results depending on the turbulence models including k-omega shear stress transport (SST), k-epsilon, Spallart-Almaras and Reynolds stress model, and the non-dimensional wall distance (y+) as well as boundary conditions were compared with the experimental results. A model scale condition with a similar rotor diameter was considered to apply the CFD analysis method to the target rotor dimension. The CFD results based on the Reynolds number (Re) and y + values were compared in the model scale. In addition, a comparative analysis was performed on the model and prototype size under the same Re condition. Consequently, an appropriate condition for the CFD analysis was proposed and the lift, drag, torque and energy efficiency of the target rotor design were predicted. Based on this, parametric investigations for the rotor aspect ratios from 4 to 6 and the diameter ratio of the disk and rotor cylinder from 1.5 to 8 were conducted. The effect of aspect ratio was examined by varying the rotor diameter while maintaining the height. Furthermore, the torque components generated by the rotor cylinder and disk owing to the aerodynamic load were analyzed.

Study on the effect of dimple position on drag reduction of high-speed maglev train

Abstract Transient numerical simulations were carried out by placing dimples at the top, sides and bottoms of the tail car streamline area of a high-speed maglev train. The results of an improved delayed detached eddy simulation turbulence model using three-dimensional compressible Navier-Stokes and shear-stress transport K-Omega double equations were compared to the results of a wind tunnel test to verify the numerical simulation accuracy, within 5% of the ground truth, which is an acceptable precision range. The results show that dimples arranged on the streamline area atop the train tail car affected the locations at which the airflow at the top and bottom of the train met and weakened the strength of the wake. The aerodynamic drag and lift coefficient decreased by 3.40% and 4.27%, respectively. When the dimples were arranged on the streamline area at the sides or bottoms of the train tail car, they had little effect on the top of the tail car, so they did not destroy the balance of the airflow at the top and bottom. They also had little influence on the development of wake topology. Therefore, the aerodynamic drag and lift of the train changed little.

Inducing swirl flow inside the pipes of flat-plate solar collector by using multiple nozzles for enhancing thermal performance

In this numerical study, an attempt has been made to improve the thermal performance of the flat-plate solar collector (FPSC) by inducing the swirl flow inside the tube by the considered nozzles. To this end, the effect of the number of circumferential nozzles and their inclination angles was taken into the account. The considered number of nozzles was "single", ''dual'', ''triple'', and ''quad''. For each of the said cases, the inclination angle of nozzles was taken 30°, 45°, 60°, and 90° (A30, A45, A60, A90). Moreover, the mass flow rate of single-nozzle pipe was considered 0.2 kg/s, 1 kg/s, and 2 kg/s. To analyze all of the cases under identical conditions, the said mass flow rates were distributed equally among all of the nozzles (for ''dual'', ''triple'', and ''quad''). All of the characteristics were defined in a form of "A…-D…-N…-M…'' where ''A…'', "D…", "N…", and "M…" stand for angle of injection, diameter of pipe, nozzle cross-section edge, and mass flow rate, respectively. Numerical simulation (3-dimensional) of the system was performed by Finite Volume Method (FVM). The turbulence nature of flow was simulated by the k-omega SST (shear stress transport) turbulent model. Results showed that the "single-nozzle'' swirl generator had the highest thermal performance factor (TPF) so that for all cases its values were greater than unit. Mass flow rate growth increases Nu, heat extraction rate, and kinetic energy rate (KER) while drops friction factor and outlet temperature. Increment of injection angle increases outlet temperature and friction factor and reduces KER. The maximum and minimum values of TPF are 4.19 and 0.44 which belong to "single; A30-D50-N12.5-M0.2" and "quad; A90-D50-N12.5-M0.5", respectively.

Estimating the Reattachment Length by Realizing a Comparison between URANS k-Omega SST and LES WALE Models on a Symmetric Geometry

In this study, a water reattachment length was calculated by adopting two different models. The first was based on Unsteady Reynolds-Averaged Navier–Stokes (URANS) k-omega with Shear Stress Transport (SST); the second was a Large Eddy Simulation (LES) with Wall-Adapting Local Eddy-Viscosity (WALE). Both models used the same mesh and were checked with Taylor length-scale analysis. After the analysis, the mesh had 11,040,000 hexahedral cells. The geometry was a symmetrical expansion–contraction tube with a 4.28 expansion ratio that created mechanical energy losses, which were taken into account. Moreover, the reattachment length was estimated by analyzing the speed values; the change of speed value from negative to positive was used as the criterion to recognize the reattachment point.

Assessment of turbulent blood flow and wall shear stress in aortic coarctation using image-based simulations

In this study, we analyzed turbulent flows through a phantom (a 180^{circ } bend with narrowing) at peak systole and a patient-specific coarctation of the aorta (CoA), with a pulsating flow, using magnetic resonance imaging (MRI) and computational fluid dynamics (CFD). For MRI, a 4D-flow MRI is performed using a 3T scanner. For CFD, the standard k-epsilon , shear stress transport k-omega , and Reynolds stress (RSM) models are applied. A good agreement between measured and simulated velocity is obtained for the phantom, especially for CFD with RSM. The wall shear stress (WSS) shows significant differences between CFD and MRI in absolute values, due to the limited near-wall resolution of MRI. However, normalized WSS shows qualitatively very similar distributions of the local values between MRI and CFD. Finally, a direct comparison between in vivo 4D-flow MRI and CFD with the RSM turbulence model is performed in the CoA. MRI can properly identify regions with locally elevated or suppressed WSS. If the exact values of the WSS are necessary, CFD is the preferred method. For future applications, we recommend the use of the combined MRI/CFD method for analysis and evaluation of the local flow patterns and WSS in the aorta.

Hydrodynamic assessment of a semi-submersible aquaculture platform in uniform fluid environment

Predicting the distribution of flow velocity inside a semi-submersible aquaculture platform and taking measures to optimize the flow field are critical to provide a dependable inhabit for farmed fish. This paper examines the hydrodynamics of a semi-submersible fish farm under current using the porous media method combined with a rigid wall based on the shear stress transport k-omega turbulence model. The results show that the presence of nets expands the low flow area and provides a better inhibit for farmed fish in the severe currents. The increase of net solidity enhances the damping effect on the flow, and it also simultaneously increases the current loads acting on the net. The attenuation caused by different net solidities produces a 66.10 % deficit of the flow velocity in maximum inside aquaculture platform. A maximum increase of 36.82 % for the drag force acting on the aquaculture platform is obtained. Furthermore, the distribution of drag force acting on the aquaculture platform in current is also assessed. Although the drag force of the frame-only is approximately twice compared to the net-only case, the improvement in the strength of the net structure under the action of flow should not be ignored.

Performance Evaluation of Vertical Axis Wind Turbine on Highways

The consumption of electricity at highways is increasing over time, consequently, more fossil fuel is burned to generate more electrical energy. However, this expensive cost of burning fossil fuels can be saved by replacing them with renewable energy sources such as wind energy. Wind energy can be generated by using a wind turbine. Therefore, this study aims to develop a vertical axis wind turbine (VAWT) that can work effectively in highway wind speed conditions while evaluating its performance in terms of torque and power by using a simulation approach. The simulation involves the use of K-omega shear-stress transport (SST) as the turbulence viscosity model. The VAWT undergoes two types of tests during the simulation process. In Test 1, the wind turbine is tested with a various number of blades at constant wind speed to find which model has the best performance in terms of torque and power output. Then, the selected model with the better output proceeds to Test 2 where it is tested with different sizes of blade radius. The results have shown that a Savonius wind turbine with five blades produces more torque and power output compared to the other rotors with two, three and four blades reaching the maximum at 140 RPM. In terms of blade radius, the model with a 400 mm radius has a better performance and capable of producing higher torque and power output than the other blade sizes.