Typical Turbulent Flow Research Articles

Study on the influence of typical turbulent flow field on the metering performance of cross four channel ultrasonic flowmeter

Abstract In this paper, influence of typical turbulent flow field on a multi-channel ultrasonic flowmeter was studied. Experimental sample of the cross four-channel ultrasonic flowmeter were designed and processed, and CFD simulation models of flowmeter and right-handed spoiler, gate valve and velocity profile spoiler were established. Through simulation, the turbulent flow field behind typical spoilers such as right-handed spoiler, gate valve and velocity profile spoiler, and the influence of turbulent flow field on the metering performance of the cross four-channel ultrasonic flowmeter were studied. On this basis, the CFD simulation results and the real flow experiment results were compared and analyzed, and the reliability of the CFD simulation analysis results was verified, which provides the research basis for improving flowmeter performance.

Large-eddy simulation of separated turbulent flows over a three-dimensional hill using WRF and OpenFOAM

It is not clearly known about the limitations of the large-eddy simulation (LES) mode of an atmospheric model (WRF) in predicting the microscale flows for engineering purpose. This study chooses a typical separated turbulent flow past a three-dimensional axisymmetric hill and investigates the performance of WRF-LES in comparison with the popular CFD solver (OpenFOAM). The numerical models and conditions are set similarly between the two codes. The instantaneous visualization shows that both WRF-LES and OpenFOAM-LES can produce the primary flow features, including hairpin vortices, horseshoes, and surface-shear-induced vortices at different scales, with high similarity. Nevertheless, the turbulent kinetic energy in the near wake produced by WRF-LES is underestimated, in comparison with WRF-LES. The energy spectra suggest that WRF-LES using the high-order advection schemes has a stronger capacity of generating and maintaining small-scale turbulent motions than OpenFOAM-LES. Furthermore, the deviation of numerical dissipation behavior is examined between the two solvers.

Numerical simulations of air-assisted primary atomization at different air-to-liquid injection angles

Numerical simulations have been performed for a coaxial, twin-fluid nozzle to study the influence of the angle α between the central liquid jet and the annular airflow on the primary atomization process. A glycerol/water mixture with a high dynamic viscosity of 200 mPas is used and the gas-to-liquid ratio is 0.6. The simulations show good agreement with experiments for the breakup morphology. The liquid jet breaks up quickly and its core length LC decreases with α from 0° to 30°, which is attributable to a reinforced aerodynamic interaction. The flow velocity of the gas phase close to the liquid jet increases with α in this case, which is confirmed by corresponding PIV measurements. This is due to the formation of a high pressure zone at the base of the liquid jet, which results in a favorable pressure gradient in the bulk flow direction. However, further increase of α from 30° to 60° leads to a decreased gas flow velocity along the liquid jet and an increase of LC. The same behavior has been found for the integral specific kinetic energy kL in the liquid phase, which represents a measure for the momentum transfer between the gas and liquid phases. kL increases from α=0∘ to 30° and it decreases again with higher α. Moreover, kL yields a similar distribution compared with the turbulent kinetic energy (TKE) of a typical turbulent flow in the spectral domain. This is attributed to the local concentration of TKE of the liquid phase in a small region around the tip of the liquid jet. The results reveal that, in addition to the common dimensionless operating parameters, the flow direction has an essential impact on the atomization process. According to the current work, the best atomization performance is achieved at an angle of α=30∘. The spectral correlation of kL with TKE of the gas flow may be used to assess the dynamics of the liquid phase.

Errors Incurred in Local Convective Heat Transfer Coefficients Obtained through Transient One-Dimensional Semi-Infinite Conduction Modeling: A Computational Heat Transfer Study

In typical turbulent flow problems, detailed heat transfer coefficient (h) maps obtained through short-duration experiments are based on inverse heat transfer methods that take the wall temperatures measured via liquid crystals or infrared thermography as input, and an error minimization routine is adopted to determine the best value of h that satisfies the wall temperature temporal evolution under a certain change in fluid temperature. A common practice involves modeling the solid as a one-dimensional semi-infinite medium by selecting the solid material that has low thermal conductivity and low thermal diffusivity. However, in certain flow scenarios, the neglection of the lateral heat diffusion may lead to significant errors in the deduced h values. It is imperative to understand the reasons behind large errors that may be incurred by using the 1D heat conduction assumption in order to accurately determine high-resolution h maps for better heat exchanger designs in a wide range of thermal management applications. This paper presents a computational heat transfer study on different jet impingement scenarios to demonstrate the errors incurred in the determination of h when calculated under the assumption of one-dimensional (1-d) heat conduction into a solid. To this end, three different cases are studied: (a) single jet, (b) array jet (theoretical distribution), (c) array jet (experimental distribution), along with three different mainstream temperature evolution profiles representing step change, moderately fast transient and slow transient nature of flow driving the heat transfer in the solid. A known distribution of heat transfer coefficient (“true h”) for each of the three cases is considered, and three-dimensional transient heat diffusion equations were solved to populate temperatures of each node in the solid at every time step. It is found that stagnation zones’ h1d calculations were lower than the “true h” while the low heat transfer zones exhibited significantly higher h1d compared to the “true h”. For the array jet (experimental distribution) case, it was observed that errors can be as high as 10% in certain low heat transfer zones. Different data reduction procedures, configurations, and conditions explored in this study indicate that a suitable balance can be achieved if shorter time durations in transient experiments are used as a reference for tracking in h1d calculations to keep the deviations from the “true h” low.

A grid-adaptive simulation model for turbulent flow predictions

Hybrid Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) methods, abbreviated as hybrid RANS-LES, have been rapidly developed and increasingly used for predicting complex turbulent flows. In this study, a new high-fidelity hybrid RANS-LES strategy that modifies the turbulent viscosity equation using the ratio of grid length scale to turbulent integral length scale based on the Kolmogorov energy spectrum, termed the grid-adaptive simulation (GAS) model, is proposed to achieve high accuracy for turbulent flows using different grid resolutions. Using the shear-stress transport (SST) k–ω model as the baseline turbulence model, the GAS-SST model is validated by predicting three typical turbulent flows with coarse and fine meshes, including periodic hill flow, circular cylinder flow, and simplified tip leakage flow. As a reference, the scale-adaptive simulation (SAS) and delayed detached-eddy simulation (DDES) models are also employed to predict the above three turbulent flows. Solutions of GAS-SST, SAS-SST, and DDES-SST are compared against the high-fidelity data from the experiments or LES solutions. Detailed comparisons show that the GAS-SST model could achieve high accuracy with different grid resolutions for all three validation cases, which means that the GAS model has strong grid-adaptive ability. The results predicted by the GAS-SST model using coarse meshes are usually much more in agreement with the high-fidelity data than those predicted by SAS-SST and DDES-SST models. The GAS model demonstrates the potential to address the accuracy and computational efficiency requirements for predicting turbulent flows.

Fundamentals of turbulent flow spectrum imaging.

PurposeTo introduce a mathematical framework and in‐silico validation of turbulent flow spectrum imaging (TFSI) of stenotic flow using phase‐contrast MRI, evaluate systematic errors in quantitative turbulence parameter estimation, and propose a novel method for probing the Lagrangian velocity spectra of turbulent flows.Theory and MethodsThe spectral response of velocity‐encoding gradients is derived theoretically and linked to turbulence parameter estimation including the velocity autocorrelation function spectrum. Using a phase‐contrast MRI simulation framework, the encoding properties of bipolar gradient waveforms with identical first gradient moments but different duration are investigated on turbulent flow data of defined characteristics as derived from computational fluid dynamics. Based on theoretical insights, an approach using velocity‐compensated gradient waveforms is proposed to specifically probe desired ranges of the velocity autocorrelation function spectrum with increased accuracy.ResultsPractical velocity‐encoding gradients exhibit limited encoding power of typical turbulent flow spectra, resulting in up to 50% systematic underestimation of intravoxel SD values. Depending on the turbulence level in fluids, the error due to a single encoding gradient spectral response can vary by 20%. When using tailored velocity‐compensated gradients, improved quantification of the Lagrangian velocity spectrum on a voxel‐by‐voxel basis is achieved and used for quantitative correction of intravoxel SD values estimated with velocity‐encoding gradients.ConclusionTo address systematic underestimation of turbulence parameters using bipolar velocity‐encoding gradients in phase‐contrast MRI of stenotic flows with short correlation times, tailored velocity‐compensated gradients are proposed to improve quantitative mapping of turbulent blood flow characteristics.

Dual channels of helicity cascade in turbulent flows

Helicity, as one of only two inviscid invariants in three-dimensional turbulence, plays an important role in the generation and evolution of turbulence. From the traditional viewpoint, there exists only one channel of helicity cascade similar to that of kinetic energy cascade. Through theoretical analysis, we find that there are two channels in helicity cascade process. The first channel mainly originates from vortex twisting process, and the second channel mainly originates from vortex stretching process. By analysing the data of direct numerical simulations of typical turbulent flows, we find that these two channels behave differently. The ensemble averages of helicity flux in different channels are equal in homogeneous and isotropic turbulence, while they are different in other type of turbulent flows. The second channel is more intermittent and acts more like a scalar, especially on small scales. Besides, we find a novel mechanism of hindered even inverse energy cascade, which could be attributed to the second-channel helicity flux with large amplitude.

Euler–Lagrange study of bubble drag reduction in turbulent channel flow and boundary layer flow

The excellent drag reduction effect of the bubble drag reduction technique has been proved through many experiments since it was proposed. In this paper, the authors investigate the bubble-turbulence interaction and the corresponding drag reduction effect with a two-way coupled Euler–Lagrange code. The liquid phase is simulated by using a large eddy simulation method with the immersed bubbles treated using a nonlinear collision model to accurately simulate the bubble–wall interaction. A Gaussian distributed method is adopted to obtain the void fraction and interphase forces in the two-way coupled algorithm. Two typical wall-bounded turbulent flow problems (turbulent channel flow and boundary layer flow) are simulated to validate the accuracy and stability in bubbly flows and investigate the drag reduction mechanism. First, the effect of bubbles on the turbulent flow is studied in the channel flow cases in which the bubbles are observed attaching to the upper plate and swaying in the spanwise direction. In this case, Reynolds stress near the wall is decreased, which contributes to the drag reduction. Moreover, drag reduction of a turbulent boundary layer flow with bubble injection is studied in which the drag reduction under different air flow rates is in good agreement with experimental results. The contribution of turbulence and different liquid forces to the migration of bubbles away from the wall is investigated. The bubble trajectory in the turbulent boundary layer is divided into three distinct stages and discussed in detail finally.

First-in-human Study of Biotube Vascular Grafts: A 2 Year Follow-up in A Bypass Model of Stenosed AV Shunt

Introduction - An arteriovenous (AV) fistula is the current gold standard for chronic hemodialysis access. However, a substantial number of shunt will fail because of stenosis or obstruction at anastomotic site or venous outflow. Tissue-engineered blood vessels have been proposed for dialysis access as an alternative to prosthetic grafts. We developed autologous collagenous tubular tissues Biotubes based on in-body tissue architecture (iBTA), which is a novel regenerative medicine technology by using a patient's body as a bioreactor. This report presents the results in the first-in-human study up to 2 year follow-up in the first two patients bypassed with an autologous Biotube. Methods - Two female patients had end-stage renal disease and had been receiving heamodialysis with a high probability of failure, because of repeatable stenosis about every 2 or 3 months at venous outflow regions over 1 year. Biotube with 5 or 6 mm in diameter and 7 cm in length (2 in photos) was prepared as autologous collagenous tubular tissues with wall thickness of ca. 1 mm by subcutaneous embedding of the molds (1 in photos), assembled with a silicone center rod and a stainless steel pipe, into patients abdominal pouches for 2 months. Results -The Biotubes after stored for 1 day in a 70% alcohol solution and washing with a saline solution before surgery were bypassed by end-to-side anastomoses over venous stenosis region of an AV shunt (3 in photos). Palpable thrill and typical turbulent flow pattern by pulsed-wave Doppler were observed. Monthly angiography showed little change in the implanted grafts with no signs of dilation or stenosis with time points up to 3 months (4 in photos). Although shortening of the Biotubes occurred, dialysis was possible without requiring balloon expansion for 2 years. Conclusion - This long-term follow up study successfully supported the concept of creating dialysis access by Biotube grown in patient’s subcutaneous pouches.

Initial 2-Year Results of First Human Use of an In-Body Tissue-Engineered Autologous 'Biotube' Vascular Graft in Internal Shunt Restoration for Haemodialysis

This study presents the initial 2-year results of the first in-human study of internal shunt restoration using completely autologous vascular grafts, Biotubes, based on in-body tissue architecture. Biotubes (diameter, 6 mm; length, 7 cm) were prepared as autologous collagenous tubular tissues with approximately 0.5-mm wall thickness by embedding moulds (two per patient), assembled with a silicone rod and a stainless steel pipe with many slits, into the patients' abdominal subcutaneous tissue for 2 months. Two female patients with end-stage renal disease were undergoing haemodialysis with a high probability of failure due to repeated stenosis every few months at the venous outflow regions over 1.5 years. Biotubes formed in both patients and were bypassed over the venous stenosis region of the arteriovenous shunt. After bypass with Biotubes without living cells, palpable thrill and typical turbulent flow pattern were observed by pulsed-wave Doppler. Follow-up angiography in showed no signs of dilation or stenosis after implantation, and puncture could be performed easily without graft damage. In both cases, stenosis of Biotubes occurred after 3-4 months. In the first case, percutaneous transluminal angioplasty (PTA) was not required for over 2 years after implantation even after the development of Biotube stenosis. In the second case, stenosis at the proximal anastomotic site of the Biotube became prominent, and PTA was needed 7 months after implantation and then repeated at up to 2 years. This first human study successfully supporting the concept of internal shunt restoration for haemodialysis using an autologous Biotube. Funding Statement: The authors state: none. Declaration of Interests: The authors state: none. Ethics Approval Statement: The clinical research process was approved by the ethics committee of the National Cerebral and Cardiovascular Center (No. M27-084) and of Tenri Hospital (No. 677).

A new inflow turbulence generator for large eddy simulation evaluation of wind effects on a standard high-rise building

The generation of inflow turbulence is among the key issues that affect the large eddy simulation (LES) of atmospheric boundary layer (ABL) flow. An improved inflow turbulence generator for LES, the narrowband synthesis random flow generator (NSRFG), is proposed in this study based on previous random flow generation (RFG) techniques. The novel NSRFG technique yields a more concise expression and overcomes some defects in the previous study of RFG techniques by strictly maintaining a divergence-free condition, coherency function, and turbulent spectra of the ABL flow theoretically. The new technique was validated by simulating a typical turbulent ABL flow, and comparisons between the NSRFG method and the consistent discretizing and synthesizing RFG method are presented. The new technique was then applied to the numerical simulation of the turbulent flow around the CAARC standard building model, which was tested in the boundary layer wind tunnel of the South China University of Technology. A detailed comparison of the wind effects obtained from numerical simulations and a wind tunnel test, including wind pressures, base moments, and wind-induced responses, was finally performed to demonstrate the feasibility and effectiveness of the new technique.

Gas kinetic scheme for turbulence simulation

The extended gas-kinetic scheme (GKS) for turbulence simulations is developed based on the generalized BGK equation with the effective relaxation time. This relaxation time can be computed from the turbulent viscosity, through which turbulence models can be directly combined. For engineering low-cost simulations of high Reynolds number flows, common-used RANS models are applied, while the LES and hybrid RANS/LES (DES and IDDES) models as well as the minimized dispersion and controllable dissipation (MDCD) reconstruction are adopted in high fidelity turbulence simulations. In addition, the turbulent transport equations are solved in a strongly coupled way by using GKS with scalar transport. The extended GKS is applied in typical turbulent flow predictions including the RANS simulation of hypersonic compression ramp flow and the detailed simulation of multiscale turbulent structures in low-speed cylinder flow with hybrid models. The predicted results agree well with existing experimental measurements and numerical studies, which shows the good accuracy, resolution and robustness of the extended GKS and reveals the wide prospects in turbulence simulations on different model scales, including the multiscale models such as hybrid RANS/LES methods. Furthermore, the multiscale turbulence simulation methods are worthy of further studies based on the generalized BGK model.

A GPU-accelerated solver for turbulent flow and scalar transport based on the Lattice Boltzmann method

In this article, we apply the Vreman model with a dynamic procedure for subgrid scale modeling of turbulent flow and scalar transport under the Lattice Boltzmann framework. Numerical results of two types of typical turbulent flow systems are presented and discussed, i.e., the channel Poiseuille flow with heat transfer, and the Rayleigh-Taylor convection. Due to their intrinsic differences, analyses of the two turbulent phenomena are carried out mainly according to the statistical averaging property and the temporal evolution process, respectively. The feasibility and accuracy of the present turbulent solver is validated via comparisons with direct numerical simulation results and theoretical predictions. In addition, the GPU parallel technique is applied and yields a very efficient solver for turbulent flow and scalar transport, and the algorithm is introduced in detail. Overall, our work offers a useful extension of the current Lattice Boltzmann method and a powerful tool for turbulent modeling.

A Spalart–allmaras Turbulence Model Implementation for High-order Discontinuous Galerkin Solution of the Reynolds-averaged Navier-stokes Equations

A novel and robust approach has been proposed for the high-order discontinuous Galerkin (DG) discretization of the Reynolds-averaged Navier-Stokes (RANS) equations with the turbulence model of Spalart-Allmaras (SA). The solution polynomials of the SA equation are reconstructed by the Hermite weighted essentially non-oscillatory (HWENO) scheme. Several practical techniques are suggested to simplify and extend a positivity-preserving limiter to further guarantee the positivity of SA working variable. The resulting positivity-preserving HWENO limiting method is compact and easy to implement on arbitrary meshes. Typical turbulent flows are conducted to assess the accuracy and robustness of the present method. Numerical experiments demonstrate that with the increasing grid or order resolution, the limited results of the working variable are getting closer to the unlimited ones. And the most obvious improvement with proposed method is on the computation of the working variable field in wake regions.

Fully consistent CFD methods for incompressible flow computations

Nowadays collocated grid based CFD methods are one of the most efficient tools for computations of the flows past wind turbines. To ensure the robustness of the methods they require special attention to the well-known problem of pressure-velocity coupling. Many commercial codes to ensure the pressure-velocity coupling on collocated grids use the so-called momentum interpolation method of Rhie and Chow [1]. As known, the method and some of its widely spread modifications result in solutions, which are dependent of time step at convergence. In this paper the magnitude of the dependence is shown to contribute about 0.5% into the total error in a typical turbulent flow computation. Nevertheless if coarse grids are used, the standard interpolation methods result in much higher non-consistent behavior. To overcome the problem, a recently developed interpolation method, which is independent of time step, is used. It is shown that in comparison to other time step independent method, the method may enhance the convergence rate of the SIMPLEC algorithm up to 25 %. The method is verified using turbulent flow computations around a NACA 64618 airfoil and the roll-up of a shear layer, which may appear in wind turbine wake.

Numerical simulation of grid-generated turbulent particulate flow by three-dimensional Reynolds stress

A three-dimensional (3D) Reynolds stress turbulence model based on 3D Reynolds-averaged Navier-Stokes equations has been elaborated for grid-generated turbulence in particulate downward flow arranged in the channel domain of the square cross section. The model presented considers both the enhancement and attenuation of turbulence by means of the additional terms of the transport equations of the normal Reynolds stress components. It allows us to carry out calculations covering the long distance of the channel length without using algebraic assumptions for various components of the Reynolds stress. The results obtained show the effects of particles and mesh size of the turbulence generating grids on turbulence modification. In particular, the presence of solid particles at the initial period of turbulence decay results in the pronounced enhancement of turbulence that diminishes appreciably downwards in the area of typical channel turbulent flow. As the results show, the character of modification of all three normal components of the Reynolds stress taking place at the initial period of turbulence decay are uniform almost all over the channel cross sections. The increase in the grid mesh size slows down the rate of the turbulence enhancement which is caused by particles.

Direct numerical simulation of turbulent flow over backward-facing at high Reynolds numbers

Direct numerical simulation (DNS) was performed for the first time to study the flow over a backward-facing step at a high Reynolds number on a coarse grid. The flow over backward-facing step is the typical turbulent flow controlled by large eddy, in which the effect of small eddy could be negligible as an approximation. The grid dimension could easily satisfy the resolution requirement to describe the characteristics of a large eddy flow. Therefore, direct numerical simulation of N-S equations to obtain the turbulent flow field on the coarse grid could be realized. Numerical simulation of a two-dimensional flow over a backward-facing step at a Reynolds number Re=44000 was conducted using Euler-Lagrange finite element scheme based on the efficient operator-splitting method (OSFEM). The flow field was descretized by triangle meshes with 16669 nodes. The overall computational time only took 150 min on a PC. Both the characteristics of time-averaged and instantaneous turbulent flow were simultaneously obtained. The analysis showed that the calculated results were in good agreement with the test data. Hence, the DNS approach could become the reality to solve the complex turbulent flow with high Reynolds numbers in practical engineering.

Stochastic subgrid modelling for atmospheric large eddy simulations

Dynamical subgrid scale parameterizations of stochastic backscatter and eddy dissipation have been calculated for typical atmospheric turbulent flows on the sphere. A methodology based on a stochastic model representation of the subgrid scale eddies in direct numerical simulations, and with wide applicability to fluid flows, has been employed. Large eddy simulations incorporating these subgrid scale parameterizations are found to have energy spectra that compare closely with the results of higher resolution direct numerical simulations for both barotropic and baroclinic turbulent flows. References O'Kane, T.J. and J.S. Frederiksen, Statistical dynamical subgrid-scale parameterizations for geophysical flows, Physica Scripta , 78 , 2008, in press. Frederiksen, J.S., Subgrid-scale parameterizations of eddy-topographic force, eddy viscosity, and stochastic backscatter for flow over topography, J. Atmos. Sci. 56 , 1999, 1481--1494. 2.0.CO;2>doi:10.1175/1520-0469(1999)056 2.0.CO;2 Frederiksen, J.S., and A.G. Davies, Eddy viscosity and stochastic backscatter parameterizations on the sphere for atmospheric circulation models, J. Atmos. Sci. , 54 , 1997, 2475--2492. 2.0.CO;2>doi:10.1175/1520-0469(1997)054 2.0.CO;2 Frederiksen, J.S. and S.M. Kepert, Dynamical subgrid-scale parameterizations from direct numerical simulations, J. Atmos. Sci. , 63 , 2006, 3006--3019. doi:10.1175/JAS3795.1 Frederiksen, J.S. and M.R. Dix and A.G. Davies, The effects of closure-based eddy diffusion on the climate and spectra of a GCM, Tellus A , 55 , 2003, 31--44. doi:10.1034/j.1600-0870.2003.201329.x

Efficient Generation of Inflow Conditions for Large Eddy Simulation of Street-Scale Flows

Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor. The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities, the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations.

On steady state computation of turbulent flows using k–ε models approximated by the time splitting method

AbstractThe time splitting method is frequently used in numerical integration of flow equations with source terms since it allows almost independent programming for the source part. In this paper we will consider the question of convergence to steady state of the time splitting method applied to k–ε turbulence models. This analysis is derived from a properly defined scalar study and is carried out with success for the coupled k–ε equations. It is found that the time splitting method does not allow convergence to steady state for any choice of finite values of the time step. Numerical experiments for some typical turbulent compressible flow problems support the fact that the time splitting method is always nonconvergent, while its nonsplitting counterpart is convergent. Copyright © 2005 John Wiley & Sons, Ltd.