Numerical Simulation Of Turbulence Research Articles

High-fidelity simulations of boundary layer transition on an underwater axisymmetric body with stability theory

During the cruise status of an underwater high-speed vehicle, the flow around the vehicle's head typically transitions from a laminar to a turbulent state, triggering flow noise that can interfere with the normal operation of sonar. In order to accurately investigate this flow noise through numerical simulation, a high-fidelity turbulent flow field solution is essential. Common traditional turbulence numerical simulation methods, such as unsteady Reynolds-averaged simulation and large eddy simulation (LES), struggle to capture high-frequency turbulent fluctuations accurately due to their inability to directly resolve small-scale eddy structures, which results in compromising the simulation accuracy of high-frequency flow noise. To address this issue, this paper employs direct numerical simulation (DNS) to achieve high-fidelity resolution of the turbulent flow field, thereby enabling a more accurate assessment of flow noise distribution on the vehicle's surface. Meanwhile, considering significant computational resources required to solve the entire flow field in an underwater high Reynolds number environment, this study also incorporates the fixed transition modeling method and stability theory to confine the DNS computational domain to the vicinity of the transitional zone to improve simulation efficiency. Comparative analysis of flow noise monitoring results in the laminar, transitional, and turbulent zones revealed that the flow noise source in the laminar zone exhibits the lowest amplitude across all frequencies, while the flow noise source in the transitional zone features the highest amplitude, approximately 10 dB higher than that in the turbulent zone. Moreover, significant amplitudes in high-frequency components (above 30 kHz) are detected in both the transition and turbulent zones. Additionally, this study employs LES with the Smagorinsky model to simulate the flow field within the same computational domain as DNS, demonstrating the limitations of the Smagorinsky model-based LES in capturing high-frequency flow noise.

High-fidelity simulations of boundary layer transition on an underwater axisymmetric body with stability theory

During the cruise status of an underwater high-speed vehicle, the flow around the vehicle's head typically transitions from a laminar to a turbulent state, triggering flow noise that can interfere with the normal operation of sonar. In order to accurately investigate this flow noise through numerical simulation, a high-fidelity turbulent flow field solution is essential. Common traditional turbulence numerical simulation methods, such as unsteady Reynolds-averaged simulation and large eddy simulation (LES), struggle to capture high-frequency turbulent fluctuations accurately due to their inability to directly resolve small-scale eddy structures, which results in compromising the simulation accuracy of high-frequency flow noise. To address this issue, this paper employs direct numerical simulation (DNS) to achieve high-fidelity resolution of the turbulent flow field, thereby enabling a more accurate assessment of flow noise distribution on the vehicle's surface. Meanwhile, considering significant computational resources required to solve the entire flow field in an underwater high Reynolds number environment, this study also incorporates the fixed transition modeling method and stability theory to confine the DNS computational domain to the vicinity of the transitional zone to improve simulation efficiency. Comparative analysis of flow noise monitoring results in the laminar, transitional, and turbulent zones revealed that the flow noise source in the laminar zone exhibits the lowest amplitude across all frequencies, while the flow noise source in the transitional zone features the highest amplitude, approximately 10 dB higher than that in the turbulent zone. Moreover, significant amplitudes in high-frequency components (above 30 kHz) are detected in both the transition and turbulent zones. Additionally, this study employs LES with the Smagorinsky model to simulate the flow field within the same computational domain as DNS, demonstrating the limitations of the Smagorinsky model-based LES in capturing high-frequency flow noise.

A mean flow velocity estimation scheme based on flow field characteristics and distribution point optimization for carbon monitoring in coal-fired power plants: A numerical study

CO2 online monitoring provides a method to obtain carbon emissions by directly quantifying the flue gas flow rate and CO2 concentration. However, significant flow rate measurement errors arise due to the complexity of turbulence in large-diameter stacks. To address the issue, this study utilizes turbulence numerical simulation combined with mean velocity estimation error analysis to investigate the flow field of typical stacks in coal-fired power plants. The simulation results demonstrated that the evolution of turbulence exaggerates velocities. The flow field exhibits stable turbulence characteristics in a single-introduced stack, while in the double-introduced stack, the flow field dynamically evolves due to asymmetric collisions and abrupt expansions. Furthermore, the effects of load and temperature on the estimation error of flow velocity are established. The optimum monitoring height, the ideal chord angle and the stepped number of deployment points based on the log-linear method are determined. Within the single-introduced stack, a correction factor is introduced and a corresponding formula is established that links estimation errors with monitoring heights. This resulted in a dramatic reduction of the maximum estimation error from 14% to 0.3%. Finally, the systematic and simplified optimization scheme for estimating mean flow velocity is summarised, which provides a powerful reference for flow rate measurement for online CO2 monitoring in coal-fired power plants.

Numerical simulations of turbulence in prominence threads induced by torsional oscillations (Corrigendum)

Numerical simulations of turbulence in prominence threads induced by torsional oscillations (<i>Corrigendum</i>)

Numerical simulations of turbulence in prominence threads induced by torsional oscillations

Context. Threads are the main constituents of prominences. They are dynamic structures that display oscillations, usually interpreted as magnetohydrodynamic (MHD) waves. Moreover, instabilities such as the Kelvin–Helmholtz instability (KHI) have also been reported in prominences. Both waves and instabilities may affect the thermodynamic state of the threads. Aims. We investigate the triggering of turbulence in prominence threads caused by the nonlinear evolution of standing torsional Alfvén waves. We study the heating in the partially ionized prominence plasma as well as possible observational signatures of this dynamics. Methods. We modeled a prominence thread as a radially and longitudinally nonuniform cylindrical flux tube with a constant axial magnetic field embedded in a much lighter and hotter coronal environment. We perturbed the flux tube with the longitudinally fundamental mode of standing torsional Alfvén waves. We numerically solved the three-dimensional (3D) MHD equations to study the temporal evolution in both ideal and dissipative scenarios. In addition, we performed forward modeling to calculate the synthetic Hα imaging. Results. The standing torsional Alfvén waves undergo phase-mixing owing to the radially nonuniform density. The phase-mixing generates azimuthal shear flows, which eventually trigger the KHI and, subsequently, turbulence. When nonideal effects are included, the obtained plasma heating is very localized in an annulus region at the thread boundary and does not increase the temperature in the cool core. Instead, the average temperature in the thread decreases owing to the mixing of internal and external plasmas. In the synthetic observations, first we observe periodic pulsations in the Hα intensity caused by the integration of the phase-mixing flows along the line of sight. Later, fine strands that may be associated with the KHI vortices are seen in the synthetic Hα images. Conclusions. Turbulence can be generated by standing torsional Alfvén waves in prominence threads after the triggering of the KHI, although this mechanism is not enough to heat such structures. Both the phase-mixing stage and the turbulent stage of the simulated dynamics could be discernible in high-resolution Hα observations.

Exploring intermittency in numerical simulations of turbulence using single and multi-spacecraft analysis

The energy dissipation in collisionless plasmas as the solar wind is not yet fully understood. The intermittent nature of magnetic structures appears to be a fundamental part of the energy cascade. Understanding energy transfer and dissipation in the solar wind requires an accurate description of its intermittency. Upcoming multi-spacecraft missions will provide new insight on this matter. However, the use of multi-point data requires developing new data analysis techniques as well as cross-validating these techniques. In this study, we address the latter and explore the intermittency in a 3D simulation of anisotropic plasma turbulence using two approaches. We implement the standard single-spacecraft partial variance increments technique as well as a multi-point partial variance increments technique. We contrast these two techniques and explore their dependence on the angle between the spacecraft-configuration travel direction and the background magnetic field.

Study on Flow Characteristics of Francis Turbine Based on Large-Eddy Simulation

The research object was a Francis turbine, and the working conditions at 100%, 75%, 50%, 25%, and 1% opening were determined by the opening size of the guide vane. Large-Eddy Simulation (LES) was adopted as a turbulence model method to conduct three-dimensional unsteady turbulent numerical simulation of the entire flow channel of a Francis turbine, and the flow situation of various parts of the turbine under different working conditions was obtained. The flow characteristics of each component under different working conditions were analyzed, and the hydraulic performance of each part was evaluated. The factors that affected the stability of hydraulic turbines were identified, and their formation mechanisms and evolution laws were explored. The results show that the guide vane placement angle was reasonable in the guide vane area, and the hydraulic performance was fine. The impact on the stability of the hydraulic turbine was small. Further research showed that the hydraulic performance was poor in the runner area, and there were flow separation and detachment phenomena in the flow field. This created a channel vortex in the runner blade channel. The channel vortex promoted the lateral flow of water and had a significant impact on the stability of the hydraulic turbine. The diffusion section of the draft tube can dissipate most of the kinetic energy of the water flow in the draft tube area, and it had a good energy dissipation effect. However, the was a large pressure difference between the upper and lower regions of the diffusion section, and it generated a backflow phenomenon. It created vortex structures in the draft tube, and the stability of the hydraulic turbine was greatly affected.

Transient Characteristic Analysis of Variable Frequency Speed Regulation of Axial Flow Pump

In order to explore the influence of different rotational acceleration on the transient internal and external flow characteristics of the axial flow pump and improve the performance of the pump, numerical simulations and experiments were used to analyze the variable frequency speed regulation characteristics of the axial flow pump. Taking three-dimensional turbulent numerical simulation as the main research method and CFX as the calculation platform, three variable frequency speed regulation methods were used to conduct transient numerical simulation; keep the acceleration constant, increased and decreased; and obtain the real-time pump performance curve and pressure characteristic curve. Uniform acceleration and deceleration with constant acceleration maintained the stable change and good stability of head and shaft power, and the pressure change was the most stable in the process. The acceleration and deceleration with decreasing acceleration ran most smoothly at high speed, and the frequency conversion effect was the best. At the same time, the transition to steady-state operation was also the most stable. The research in this paper can provide reference for the stable operation of variable frequency speed regulation of the axial flow pump.

Effect of Parameters of Pool Geometry on Flow Structure in Nature-like Fishway.

Many uncertainties such as variable irregular structure and complex flow conditions bring difficulties to the design of a nature-like fishway. This study defines the main factors and parameters affecting flow conditions such as permeability ratio, offset ratio, bottom slope and pool length to simplify and generalize the irregular geometry of the nature-like fishway. According to the engineering requirements of the Mopiling nature-like fishway, the effect of the above parameters of pool geometry on the flow structure is investigated through a 3D turbulent numerical simulation, and the parameter thresholds are summarized according to the optimization of the flow conditions. The results show that under the same conditions, the maximum velocity of the control section increases with the increase in permeability ratio, bottom slope and pool length, and the offset ratio has limited effect on the maximum velocity of the control section. It is recommended that when the bottom slope is 1/250 and the pool length is 10 m, the permeability ratio should not be greater than 0.30 and the offset ratio should be located between 0.15 and 0.60. When the bottom slope is adjusted to 1/200, it is recommended to control the permeability ratio below 0.20, the offset ratio between 0.30 and 0.60, and the pool length can be adjusted to 8 m. Within the above threshold range of the design parameters, the maximum velocity in the fishway can be basically controlled at about 1.0 m/s. The mainstream in the pool is clear and the flow pattern is good, which can basically satisfy the requirements of fish passing. The relevant design parameters and optimization strategies can provide reference for similar projects.

Effects of hydrogen enrichment and turbulence intensity on the combustion mode in locally stratified dual-fuel mixtures of n-dodecane/methane

In dual-fuel (DF) engines, a high-reactivity fuel (HRF) ignites a low-reactivity fuel (LRF) premixed with air. Identification of the combustion mode after autoignition in DF engines with different fuel combinations and under various mixture stratification levels is of crucial importance since the combustion mode may affect the engine performance and emissions levels. However, to avoid expensive computational or experimental tests for different fuel combinations under various initial conditions, an analytical diagnostics tool can help identify the modes of combustion. Recently, we developed an a-priori tool (β-curve), which separates between the deflagration and spontaneous combustion modes in transient problems based on two parameters: HRF stratification amplitude and wavelength. However, the developed analysis was only evaluated for a single fuel combination and under 2D turbulent conditions. The present study extends our previous work by (1) evaluating the 1D β-curve analysis for more common fuel combinations including hydrogen, and (2) extending the combustion mode analysis to 3D turbulent combustion. First, using 1D numerical simulations, the combustion mode for a new DF combination of n-dodecane (HRF) and methane (LRF) with and without hydrogen enrichment in the LRF blend is investigated. Although hydrogen enrichment shortens the combustion duration, its role in changing the combustion mode remains subtle. Therefore, only n-dodecane/methane (no hydrogen enrichment) is considered in the 3D simulations to avoid highly expensive 3D numerical simulations with hydrogen enrichment. Second, using flame-resolved 3D numerical simulations, variations of the combustion mode for DF n-dodecane/methane mixtures at three different turbulence intensities are studied. It is observed that high turbulence levels can switch the combustion mode from deflagration to autoignition. For the first time, the diagnostic tool is evaluated compared to 3D turbulent numerical simulations and a correction factor is proposed in the β-curve equation.

Spectra of Magnetic Turbulence in a Relativistic Plasma

We present a phenomenological and numerical study of strong Alfvénic turbulence in a magnetically dominated collisionless relativistic plasma with a strong background magnetic field. In contrast with the nonrelativistic case, the energy in such turbulence is contained in magnetic and electric fluctuations. We argue that such turbulence is analogous to turbulence in a strongly magnetized nonrelativistic plasma in the regime of broken quasi-neutrality. Our 2D particle-in-cell numerical simulations of turbulence in a relativistic pair plasma find that the spectrum of the total energy has the scaling k −3/2, while the difference between the magnetic and electric energies, the so-called residual energy, has the scaling k −2.4. The electric and magnetic fluctuations at scale ℓ exhibit dynamic alignment with the alignment angle scaling close to . At scales smaller than the (relativistic) plasma inertial scale, the energy spectrum of relativistic inertial Alfvén turbulence steepens to k −3.5.

Numerical Study on Extent of Hazardous Area for Sonic Jet Release Using Equivalent Leak Diameter

In this study, three-dimensional K - E turbulence numerical simulations were conducted to analyze the extent of hazardous area for the sonic jet leakage of flammable gas. Incompressible fluid flows were simulated based on an inlet boundary condition estimated using the theory of “equivalent leak diameter” to prevent the direct simulation of sonic flows near the leakage hole. Numerical simulations of 20 methane leakage scenarios providing the lower explosive limit contour showed shapes of the hazardous area with a maximum height of approximately 12-14 times larger than the maximum width, owing to convection. The extents of hazardous area determined using computational fluid dynamics (CFD) were approximately 5%-10% lower than the results obtained with 1 m over based on IEC 60079-10-1. For scenarios in which quantitative data were not calculated using IEC 60079-10-1 due to low release rates, CFD provided quantitative data for the extent of hazardous area, showing nonlinear relationships with the pressure and diameter of leak holes.

Machine learning based Reynolds averaged simulation of backward-facing step flows at different Reynolds numbers

To address the closure problem of Reynolds-averaged Navier–Stokes in numerical simulations of turbulence, the method of solving Reynolds-averaged Navier–Stokes equations based on artificial neural network is introduced in this paper. We establish the nonlinear mapping relationship between the average flow field and the steady-state eddy viscosity field. The machine learning (ML) surrogate model for the shear stress transport turbulence model is constructed. The solution process of replacing the original turbulence model equations with the predicted field variables is realized by coupling the ML algorithm with the CFD solver. The classical backward facing step problem is selected in our study to verify the simulation accuracy of the surrogate model. The comparative analysis is carried out on the six backward facing step flows simulations at different Reynolds numbers. The results of simulations show that the testing flows with the Reynolds numbers closest training datasets Reynolds numbers can obtain the best simulation accuracy. Then for the Reynolds number that is lower than the training datasets, the simulation accuracy will decrease as the Reynolds number decreases. On the contrary, the simulation accuracy of the test flow will increase as the Reynolds number increases. These results indicate the feasibility of the ML surrogate model to simulate at higher Reynolds number. It shows the great potential of applying ML algorithms to Reynolds-averaged Navier–Stokes simulation (RANS) turbulence model and also provides a new idea for industrial simulations of turbulent flows.

A Novel Automatic Method for the Urban Canyon Parametrization Needed by Turbulence Numerical Simulations for Wind Energy Potential Assessment

The energy transition to more sustainable forms is currently ongoing worldwide, because of the environmental impacts produced by the non-renewable energy sources employed in the last decades. Among the main alternatives, wind plays a key role and, nowadays, innovative instruments, such as small-scale turbines allow for installation of wind turbines in urban areas. Their energy potential assessment requires high-accuracy simulations of the turbulent flows in the urban canopy layer, which, in turn, require detailed information about the geometrical properties of the basic element to classify urban surfaces, i.e., the urban canyon, often not available. In this work, we propose a novel automatic method, based on Voronoi graph, to univocally identify urban canyons and to extract their geometrical parameters from online available GIS (Geographic Information System) data, and test it on four European cities that differ in size, story and location. Results show the capability of the method to identify the single urban canyon and to properly extract its geometrical parameters, which tend to assume similar values for the largest cities. Moreover, we first attempt to propose and test some curves to generally describe the data probability distribution, which may be useful for turbulence simulations for urban wind energy assessment and planning. The best results are found for the canyon aspect ratio.

A Low-Communication-Overhead Parallel DNS Method for the 3D Incompressible Wall Turbulence

This paper presents a low-communication-overhead parallel method for direct numerical simulations of the 3D incompressible wall turbulence. A fully explicit projection method with second-order space-time accuracy is adopted. Combined with fast Fourier transforms, the parallel diagonal dominant (PDD) algorithm for the tridiagonal system is employed to solve the pressure Poisson equation. The number of all-to-all communications is decreased to only two, in a 2D pencil-like domain decomposition. The resulting MPI/OpenMP hybrid parallel code shows excellent strong scalability up to 104 cores and small wall-clock time per timestep. Numerical simulations of turbulent channel flow at different friction Reynolds numbers () have been conducted and the statistics are in good agreement with the reference data and the recent scaling theories [Chen, X. and K. R. Sreenivasan. 2021. “Reynolds Number Scaling of the Peak Turbulence Intensity in Wall Flows.” Journal of Fluid Mechanics 908: R3. doi:10.1017/jfm.2020.991].

Effects analysis on aerodynamic noise reduction of centrifugal compressor used for gasoline engine

Miniaturization of gasoline engines equipped with turbochargers is an effective way to alleviate energy shortages and reduce emissions, whereas its compressor noise has become an issue. In order to research the effects of particular blade tip clearance and blade number on noise reduction of high-speed (100,500 rpm) centrifugal compressor used for gasoline engine under actual working conditions, the aerodynamic noise of a turbocharger compressor mounted on a passenger car engine is analyzed by using turbulence numerical simulation, aero-acoustic simulation and acoustic boundary element method (BEM). On the basis of the transient large eddy simulation calculation of the compressor flow field, the aero-acoustic analogy noise model and the acoustic boundary element method are used to calculate the aerodynamic noise of the compressor. The frequency spectrum of compressor BPF noise is analyzed, and the influence of different blade tip clearances and blade numbers on the broadband noise and BPF noise of the compressor at high speed is studied. The research of compressor related structural parameters on aerodynamic noise plays an important role in reducing compressor noise and improving compressor acoustic quality.

Temporal dynamics of the alignment of the turbulent stress and strain rate

Energy transfer between scales in the turbulent cascade requires geometric alignment between scale-dependent turbulent stresses and strain rates. However, each of these tensors evolves dynamically and differently, leading to nontrivial temporal behavior of this alignment. A numerical simulation of turbulence is used to study this process and, by formulating the energy flux in a fully Lagrangian way, the important role played by deformation in the energy cascade is highlighted.

A comparative analysis of multi-machine learning algorithms for data-driven RANS turbulence modelling

To improve the speed or accuracy of Reynolds-averaged Navier-Stokes in numerical simulations of turbulence, five machine learning algorithms including Random Forests, eXtreme Gradient Boosting, K-Nearest Neighbours, Support Vector Machine and Artificial Neural Network are introduced in this paper. We establish the nonlinear mapping relationship between the average flow field and the steady-state eddy viscosity. The machine learning surrogate models for the Spalart-Allmaras turbulence model are constructed and used to solve the backward facing step problem. It is demonstrated that the machine learning surrogate models constructed by the five machine learning algorithms can effectively express the complex mapping relationship between the average turbulent flow field and the steady-state eddy viscosity. They can not only obtain reasonable simulation accuracy but also significantly accelerate the solution time of the flow field. The algorithms of eXtreme Gradient Boosting, K-Nearest Neighbours and Artificial Neural Network have better performance when considering the calculation accuracy, time cost and memory cost. It shows the great potential of applying machine learning algorithms to RANS turbulence model and also provides a new idea for industrial simulations of turbulent flows.

Comparative Study on Energy Dissipation Numerical Simulation of Different Energy Dissipators in Wide and Narrow Alternated Channels

This paper proposed clustered energy dissipators and ribbed energy dissipators in view of the energy dissipation of wide and narrow alternated river channels in mountainous areas. According to the structure and layout of energy dissipation piers, clustered energy dissipators are divided into several layout types: 1# “2-1 alternating” dissipator, 2# “2-1 inclined alternating” dissipator, 3# “4-3 alternating” energy dissipator; ribbed energy dissipator is divided into: 4# “ribbed” dissipator, 5# “ribbed + step-wise” dissipator. In this paper, we used a standard k-ε model to carry out 3D turbulent numerical simulation of energy dissipation of different energy dissipators in wide and narrow river alternated channels in mountainous areas, and analyzed its energy dissipation effect, which can provide design guidance for remediation and river bank slope protection.

Influence of suction chamber profile on flow field of annular jet pump

Jet pump is a kind of fluid machinery, which uses jet shear and turbulent diffusion to transfer mass and energy. It is widely used in water project, civil engineering and other fields. In order to investigate the influence of suction chamber profile on flow field of annular jet pump, the conical suction chamber and the streamlined suction chamber were compared by applying turbulence numerical simulation method. The performance, pressure characteristics, velocity and turbulent energy distributions of annular jet pumps with two suction chambers are analysed. The calculation shows that compared with the traditional conical suction chamber, the efficiency of annular jet pump with streamlined suction chamber is improved at low flow ratio, and the change is not obvious at high flow ratio. In the same section, the cross sectional area of streamlined suction chamber is slightly larger than that of conical suction chamber, so the primary flow velocity near the wall is slightly lower, which can reduce the friction loss caused by high speed wall-attached flow. In addition, the smooth turning of the primary flow in streamlined suction chamber can weaken the extrusion on the entrained flow field. Different from the conical suction chamber, the streamlined suction chamber is smoothly connected with throat without sudden corner change, which can eliminate the low pressure near the inlet wall of throat, improve the wall pressure distribution and reduce the possible cavitation. The research results of suction chamber profile of annular jet pump can provide scientific basis for the popularization and application of jet pump.