Synthetic Turbulence Research Articles

Enrichment methods for inflow turbulence generation in the atmospheric boundary layer

We investigate the feasibility of introducing synthetic turbulence into finite-domain large-eddy simulations (LES) of the wind plant operating environment. This effort is motivated by the need for a robust mesoscale-to-microscale coupling strategy in which a microscale (wind plant) simulation is driven by mesoscale data without any resolved microscale turbulence. A neutrally stratified atmospheric boundary layer was simulated in an LES with 10-m grid spacing. We show how such a fully developed turbulence field may be reproduced with spectral enrichment starting from an under-resolved coarse LES field (with 20-m and 40-m grid spacing). The velocity spectra of the under-resolved fields are enriched by superimposing a fluctuating velocity field calculated by two turbulence simulators: TurbSim and Gabor Kinematic Simulation. Both forms of enrichment accurately simulated the autospectra of all three velocity components at high wavenumbers, with agreement between the enriched fields and the full-resolution LES observed at 400 m from the inflow boundary. In contrast, the spectra of the unenriched fields reached the same fully developed state at four times the downstream distance.

Numerical simulation of aeroacoustical noise from a wing-flap configuration

A noise-free volumetric synthetic turbulence generator has been applied for Embedded LES of the flow over the wing-flap configuration experimentally studied by Lemonie et al. Good agreement of CFD results with experimental data provides some evidence for the correctness of the chosen computational strategy, which includes a noise-free volumetric synthetic turbulence generator to create turbulent content, “sponge layers” to avoid the reflection of pressure waves from the external boundaries and the Ffowcs Williams, Hawkings acoustic model for calculating far-field noise.

Unsteady Simulations of a Fan/Outlet-Guide-Vane System: Aerodynamics and Turbulence

The Paper presents the aerodynamic and turbulent aspects of a project devoted to creating and validating a high-fidelity computational-fluid-dynamics-based system for the prediction of the performance and noise associated with the fan/outlet-guide-vane configuration typical of modern turbofan aeroengines. Such a tool will be of significant value for evaluating low-cost analytical methods needed for tradeoff studies and fine tuning of low-order computational models for engine performance and noise prediction. For turbulence representation, the system offers two approaches: the unsteady Reynolds-averaged Navier–Stokes equations, and a zonal approach that treats the part of the flowfield upstream of the fan by unsteady Reynolds-averaged Navier–Stokes and the remaining part (the interstage, outlet guide vane, and exhaust regions) by improved delayed detached-eddy simulation combined with a volume synthetic turbulence generator for triggering the scale-resolving capability of the improved delayed detached-eddy simulation. Details of the proposed methodology are outlined, including problem statements for simplified (“duct-only”) and full (including the entire nacelle and external flow) configurations. The results are presented, demonstrating the capabilities of the methodology and its validation by means of comparison of the mean and unsteady flow characteristics with experimental data for the NASA source diagnostic test. The numerical database will be used for the investigation of noise-generation/propagation phenomena.

Optimizing turbulent inflow conditions for large-eddy simulations of the atmospheric boundary layer

Large-eddy simulations (LES) of the atmospheric boundary layer (ABL) require the specification of a turbulent inflow condition with appropriate turbulence intensities and length scales. When using a synthetic turbulence generator, the statistics obtained downstream of the inlet might deviate considerably from the intended values. In the present work we propose a fully automated approach to modify the input parameters for the turbulence generator such that the desired turbulence statistics are obtained at the downstream location of interest. The method employs a gradient-based optimization in combination with the divergence-free version of the digital filter method developed by Xie and Castro [1, 2]. A sensitivity analysis showed that the spanwise and vertical Reynolds stresses and length scales are the most influential input parameters. Hence, the optimization adjusts these parameters until the desired turbulence statistics are obtained downstream in the domain. The results demonstrate the promising capabilities of the method: the mean velocity profile is correctly maintained using an appropriate wall function, while the optimization results in Reynolds stresses, integral length-scales and turbulence spectra that compare well to ABL wind tunnel measurements.

Exploration of digital-filter and forward-stepwise synthetic turbulence generators and an improvement for their skewness-kurtosis

The performance of four synthetic turbulence generators that represent the majority of capabilities of i. digital-filter-based (DFM) and ii. forward-stepwise-based (FSM) generator categories is evaluated prior to transferring generator outputs into computational fluid dynamics simulations. In addition, a cheap-to-run and easy-to-code piecewise closed-form function that transforms one-spatial-point skewness-kurtosis of a synthetic time-series to a target value is derived and presented. The two main purposes of the study are to support model users in their decision process for choosing the most convenient type and their understanding of the models through a systematic exploration of model variables and modeling stages, and to extend the Gaussian nature of these models at a spatial point into non-Gaussianity for the first time. The evaluation test-bed contains three benchmarks, each of which focuses on an isolated aspect of turbulent flows: i. decaying homogeneous isotropic turbulence, ii. homogeneous shear turbulence and iii. plane channel flow with smooth walls. Results obtained reveal that: (i) the original DFM provides the highest level of reconstruction for input one-spatial-point second-order correlation tensors and two-spatial/temporal-point correlation functions; (ii) FSM yields the best trade-off between the computational cost and the level of reconstruction; (iii) the use of exponential-form correlation functions as a model approximation is more advisable than that of Gaussian-form, as the former removes the premature, sharp, flow-type-independent drop in power spectra observed for the latter; (iv) the proposed non-Gaussian functionality reconstructs the target one-spatial-point skewness-kurtosis pairs of the test-bed flows virtually without altering their already-embedded statistics; (v) the Lund transformation changes existing statistics only in statistically inhomogeneous lateral directions of a flow when anisotropic Reynolds stresses are present; and (vi) a spatial variation of correlation functions on turbulence generation plane improves the overall reconstruction fidelity in terms of correlation functions and power spectra.

An analytically-based method for predicting the noise generated by the interaction between turbulence and a serrated leading edge

This paper considers the interaction of turbulence with a serrated leading edge. We investigate the noise produced by an aerofoil moving through a turbulent perturbation to uniform flow by considering the scattered pressure from the leading edge. We model the aerofoil as an infinite half plane with a leading edge serration, and develop an analytical model using a Green's function based upon the work of Howe. This allows us to consider both deterministic eddies and synthetic turbulence interacting with the leading edge. We show that it is possible to reduce the noise by using a serrated leading edge compared with a straight edge, but the optimal noise-reducing choice of serration is hard to predict due to the complex interaction. We also consider the effect of angle of attack, and find that in general the serrations are less effective at higher angles of attack.

A Band-Width Filtered Forcing Based Generation of Turbulent Inflow Data for Direct Numerical or Large Eddy Simulations and its Application to Primary Breakup of Liquid Jets

Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) of spatially inhomogeneous flows strongly depend on turbulent inflow boundary conditions. Realistic coherent structures need to be prescribed to avoid the immediate damping of random velocity fluctuations. A new turbulent inflow data generation method based on an auxiliary simulation of forced turbulence in a box is presented. The new methodology combines the flexibility of the synthetic turbulence generation with the accuracy of precursor simulation methods. In contrast to most auxiliary simulations, the new approach provides full control over the turbulence properties and computational costs remain reasonable. The lack of physical information and artificiality attested with pseudo-turbulence methods is overcome since the inflow data stems from a solution of the Navier-Stokes equations. The generated velocity fluctuations are by construction divergence-free and exhibit the non-Gaussian characteristics of turbulence. The generated inflow data is applied to the simulation of multiphase primary breakup.

A rapid and low noise switch from RANS to WMLES on curvilinear grids with compressible flow solvers

A turbulent inflow for a rapid and low noise switch from RANS to Wall-Modelled LES on curvilinear grids with compressible flow solvers is presented. It can be embedded within the computational domain in practical applications with WMLES grids around three-dimensional geometries in a flexible zonal hybrid RANS/LES modelling context. It relies on a physics-motivated combination of Zonal Detached Eddy Simulation (ZDES) as the WMLES technique together with a Dynamic Forcing method processing the fluctuations caused by a Zonal Immersed Boundary Condition describing roughness elements. The performance in generating a physically-sound turbulent flow field with the proper mean skin friction and turbulent profiles after a short relaxation length is equivalent to more common inflow methods thanks to the generation of large-scale streamwise vorticity by the roughness elements. Comparisons in a low Mach-number zero-pressure-gradient flat-plate turbulent boundary layer up to Reθ=6100 reveal that the pressure field is dominated by the spurious noise caused by the synthetic turbulence methods (Synthetic Eddy Method and White Noise injection), contrary to the new low-noise approach which may be used to obtain the low-frequency component of wall pressure and reproduce its intermittent nature. The robustness of the method is tested in the flow around a three-element airfoil with WMLES in the upper boundary layer near the trailing edge of the main element. In spite of the very short relaxation distance allowed, self-sustainable resolved turbulence is generated in the outer layer with significantly less spurious noise than with the approach involving White Noise. The ZDES grid count for this latter test case is more than two orders of magnitude lower than the Wall-Resolved LES requirement and a unique mesh is involved, which is much simpler than some multiple-mesh strategies devised for WMLES or turbulent inflow.

Effect of Inflow Turbulence on an Airfoil Flow with Laminar Separation Bubble: An LES Study

The present paper is concerned with numerical investigations on the effect of inflow turbulence on the flow around a SD7003 airfoil. At a Reynolds number Rec = 60,000, an angle of attack α = 4∘ and a low or zero turbulence intensity of the oncoming flow, the flow past the airfoil is known to be dominated by early separation, subsequent transition and reattachment leading to a laminar separation bubble with a distinctive pressure plateau. The objective of the study is to investigate the effect of inflow turbulence on the flow behavior. For this purpose, a numerical methodology relying on a wall-resolved large-eddy simulation, a synthetic turbulence inflow generator and a specific source term concept for introducing the turbulence fluctuations within the computational domain is used. The numerical technique applied allows the variation of the free-stream turbulence intensity (TI) in a wide range. In order to analyze the influence of TI on the arising instantaneous and time-averaged flow field past the airfoil, the present study evaluates the range 0% ≤ TI ≤ 11.2%, which covers typical values found in atmospheric boundary layers. In accordance with experimental studies it is shown that the laminar separation bubble first shrinks and finally completely vanishes for increasing inflow turbulence. Consequently, the aerodynamic performance in terms of the lift-to-drag ratio increases. Furthermore, the effect of the time and length scales of the isotropic inflow turbulence on the development of the flow field around the airfoil is analyzed and a perceptible influence is found. Within the range of inflow scales studied decreasing scales augment the receptivity of the boundary layer promoting an earlier transition.

Analysis on the effects of turbulent inflow conditions on spray primary atomization in the near-field by direct numerical simulation

It is widely acknowledged that the development of sprays in the near-field is of primary importance for the spray formation downstream, as it affects both the spray angle, as well as the intact core length. In this frame, the present work aims to study the effects of turbulence inlet boundary condition on the spray formation by means of Direct Numerical Simulations on a real condition at low Reynolds number. To this extent, the code Paris-Simulator has been used, while a digital filter-based algorithm was used in order to generate synthetic turbulence at the inlet boundary condition. The influence of turbulence intensity and lengthscale on the atomization process has been studied and analyzed through 3 simulation for which these parameters have been varied. The results clearly highlight how the atomization is heavily affected by the inlet turbulence configuration. An analysis of the different atomizing conditions has been conducted, aiming to understand how the variation introduced by the inlet boundary condition on the velocity field is affecting the local atomization dynamics.

Generation of divergence free synthetic inflow turbulence with arbitrary anisotropy

The paper presents an improvement to the Turbulent Spot method for generation of synthetic turbulence. Earlier versions of the Turbulent Spot method and the Synthetic Eddy method were only able to produce turbulence which does not obey the continuity constraint in general. This was only possible for special cases (isotropic or near-isotropic turbulence).The proposed extension enables the generation of arbitrary anisotropic turbulence and fulfillment of the continuity constraint at the same time. This is made possible by the introduction of a new type of turbulent structure. The derivation of the internal velocity field is given. Also, expressions for the statistical properties of the new structure are derived.The performance of the proposed extension is illustrated on generic test cases.

Synthetic generation of equilibrium boundary layer turbulence from modeled statistics

An improved synthetic digital filtering method (SDFM) for the generation of equilibrium turbulence is described, with emphasis on the ease of application to complex configurations. This is achieved by facilitating the use of modeled (RANS) statistics for the mean flow and Reynolds stress profiles, instead of simulated (DNS or LES) statistics that are not readily available for three-dimensional flows. The effects on boundary layer relaxation with modeled statistics are first quantified for a well-known canonical situation, specifically, Mach 2 equilibrium turbulent boundary layers over 250 < Reθ < 2220. Various convergence criteria are assessed in terms of the streamwise coordinate (x) normalized by the incoming boundary layer thickness (δ0). These include skin-friction, which requires x/δ0 ≈ 12 for equilibrium, inner scale mean velocity (x/δ0 ≈ 16), inner scale Reynolds stresses (x/δ0 ≈ 18), as well as outer scale mean velocity, Reynolds stresses, and spatio-temporal correlations (x/δ0 ≈ 40–50). Development lengths are shown to be inversely related to Reynolds number. In terms of boundary layer relaxation length, initializing the method with RANS (versus simulated) statistics incurs a negligible penalty in inner coordinates and a mild penalty (x/δ0 ≈ 5–10) in outer coordinates. The effectiveness of this approach is then demonstrated by application to significantly more complex three-dimensional flows, including internal single and multi-stream flows, for which synthetic turbulence generation techniques are necessary, but precursor high-fidelity simulations are not available. Thus, the RANS-initialized SDFM provides a rapidly adaptable method for the synthetic generation of wall-bounded turbulence.

A stochastic vortex structure method for interacting particles in turbulent shear flows

In a recent study, we have proposed a new synthetic turbulence method based on stochastic vortex structures (SVSs), and we have demonstrated that this method can accurately predict particle transport, collision, and agglomeration in homogeneous, isotropic turbulence in comparison to direct numerical simulation results. The current paper extends the SVS method to non-homogeneous, anisotropic turbulence. The key element of this extension is a new inversion procedure, by which the vortex initial orientation can be set so as to generate a prescribed Reynolds stress field. After validating this inversion procedure for simple problems, we apply the SVS method to the problem of interacting particle transport by a turbulent planar jet. Measures of the turbulent flow and of particle dispersion, clustering, and collision obtained by the new SVS simulations are shown to compare well with direct numerical simulation results. The influence of different numerical parameters, such as number of vortices and vortex lifetime, on the accuracy of the SVS predictions is also examined.

Synthetic turbulence modeling for evaluation of ultrasonic cross-correlation flow measurement

Performance of an ultrasonic cross-correlation flow measurement instrument may be significantly affected by turbulence at the location of the ultrasonic sensors. In this paper, a new method of generating Synthetic Turbulence is presented, to provide an effective tool for creating a variety of turbulent fields, which can be used to model and analyze instrument performance under different flow conditions. In the proposed method, a turbulent field is presented as a Fourier time-series in each point in space. Turbulence structures are defined by a spatial distribution of phase functions for each harmonic. Principles of designing a phase function to achieve the desirable distribution of turbulence scales, and two-point correlations, are outlined by considering the example of Uniform Isotropic Turbulence. One application of this method, presented in this work, is the mathematical modeling of ultrasonic cross-correlation flow measurement. Results predicted by the proposed mathematical model show good agreement with experimental data.

Large eddy simulation of the influence of synthetic inlet turbulence on a practical aeroengine combustor with counter-rotating swirler

To study the influence of inlet turbulence on the prediction of flow structure in practical aeroengine combustor, large eddy simulation with dynamic Smagorinsky subgrid model is used to explore the complex unsteady flow field in a single burner of a typical aeroengine combustor with two-stage counter-rotating swirler. The complex geometric configuration including all film cooling holes is fully simulated without any conventional simplification in order to reduce the modeling errors. First, unsteady process that flow developing from static to statistically stationary state is fully simulated under laminar inlet condition to obtain a fundamental understanding of flow characteristics in the combustor. Afterwards, synthetic eddy method is utilized to generate a turbulent inlet condition so that a perturbation with about 5% turbulence intensity is superimposed to the inlet plane. Simulation result shows that for the laminar inflow case, flow separation occurs in the near-wall region of the diffusion section, inducing a boundary layer transition and consequently introducing turbulence with nonuniformity in space before the swirler. In contrast, synthesized inflow generated under turbulent inlet condition by synthetic eddy method is more spatially homogeneous. Time-averaged flow field inside the swirler cup reveals that turbulent inflow ultimately causes the swirling flow with higher rotating speed in central region and more uniform distribution along the circumferential direction. It also enhances the transverse jet flow from primary holes and reverse flow in the central recirculation zone, and makes streamlines corresponding to the recirculation vortices more symmetrical on central profile. Maximum recirculating velocity predicted in central recirculation zone is −27.65 m/s and −17.86 m/s in turbulent and laminar case respectively, and corresponding total pressure recovery coefficient is 96.03% and 96.81%.

Novel approaches to estimating the turbulent kinetic energy dissipation rate from low- and moderate-resolution velocity fluctuation time series

Abstract. In this paper we propose two approaches to estimating the turbulent kinetic energy (TKE) dissipation rate, based on the zero-crossing method by Sreenivasan et al. (1983). The original formulation requires a fine resolution of the measured signal, down to the smallest dissipative scales. However, due to finite sampling frequency, as well as measurement errors, velocity time series obtained from airborne experiments are characterized by the presence of effective spectral cutoffs. In contrast to the original formulation the new approaches are suitable for use with signals originating from airborne experiments. The suitability of the new approaches is tested using measurement data obtained during the Physics of Stratocumulus Top (POST) airborne research campaign as well as synthetic turbulence data. They appear useful and complementary to existing methods. We show the number-of-crossings-based approaches respond differently to errors due to finite sampling and finite averaging than the classical power spectral method. Hence, their application for the case of short signals and small sampling frequencies is particularly interesting, as it can increase the robustness of turbulent kinetic energy dissipation rate retrieval.

The three-dimensional structure of swirl-switching in bent pipe flow

Swirl-switching is a low-frequency oscillatory phenomenon which affects the Dean vortices in bent pipes and may cause fatigue in piping systems. Despite thirty years worth of research, the mechanism that causes these oscillations and the frequencies that characterise them remain unclear. Here we show that a three-dimensional wave-like structure is responsible for the low-frequency switching of the dominant Dean vortex. The present study, performed via direct numerical simulation, focuses on the turbulent flow through a $90^{\circ }$ pipe bend preceded and followed by straight pipe segments. A pipe with curvature 0.3 (defined as ratio between pipe radius and bend radius) is studied for a bulk Reynolds number $Re=11\,700$, corresponding to a friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}\approx 360$. Synthetic turbulence is generated at the inflow section and used instead of the classical recycling method in order to avoid the interference between recycling and swirl-switching frequencies. The flow field is analysed by three-dimensional proper orthogonal decomposition (POD) which for the first time allows the identification of the source of swirl-switching: a wave-like structure that originates in the pipe bend. Contrary to some previous studies, the flow in the upstream pipe does not show any direct influence on the swirl-switching modes. Our analysis further shows that a three-dimensional characterisation of the modes is crucial to understand the mechanism, and that reconstructions based on two-dimensional POD modes are incomplete.

Overview of Turbulent Inflow Boundary Conditions for Large-Eddy Simulations

Generating realistic turbulent inflow conditions for large-eddy simulations and other large-scale-resolving approaches is essential to fully exploit the ever-increasing capabilities of modern computers in solving fluid flow problems of engineering interest. This research area has received much attention during the last two decades and will continue to grow as large-eddy simulation is applied to an even wider range of flows. It is deceptively difficult to generate turbulent inflow fluctuations that are realistic for a specific problem, and various methods have been designed to improve this realism. A review of such turbulent inflow boundary conditions is presented, broadly classified into transition-inducing approaches, turbulence library-based methods, recycling–rescaling-based methods, and synthetic turbulence generators. The last category is given more attention to cover a variety of creative approaches that are found in the literature as well as due to its practicality for problems of industrial interest. Selection of a particular method for a particular application is a compromise between the required fidelity and the allowable complexity and computational expense. Lack of quantitative knowledge about turbulence to be fed into the simulation is recognized as a limiting factor in the effective use of such inflow generators for problems of practical interest.

Plasma-wall interactions in the presence of plasma fluctuations—interpretation of line emission from sputtered tungsten in PSI-2

The analysis in this work essentially addresses the general question to what extent the temporal average of a particular quantity which is a highly nonlinear function of fluctuating quantities can be approximated by using the averages of the fluctuating quantities for its evaluation. The concrete case considered is the line emission intensity from sputtered impurities being a function of fluctuating electron density and temperature in a plasma beam of the PSI-2 device. A three-dimensional fluid model is employed to study the impact of plasma fluctuations on the distribution of particles and line emission in PSI-2 discharges and its interpretation in long-term measurements. In the model presented the solution of a vorticity equation to obtain a self-consistent electric field is avoided and a synthetic turbulent velocity field is included instead. This approach is based on a Langevin model including advection and allows numerically efficient parameter scans by controlling amplitude, correlation length and correlation time of plasma fluctuations known from extended 3D simulations and/or experiment. The synthetic turbulence model considered is an extension of established stochastic models used for studies of passive scalar advection and therefore, it is described in detail in a general framework. Numerical examples of PSI-2 applications show that a double log-normal probability density function for the electrons and impurity ions is likely to occur and that this supports the conclusion that very high levels of intermittency are required to find a significant impact on the experimental evaluation method which is based on temporal averages only. Consequently, for typical PSI-2 experiments the method of evaluation based on averaged plasma parameters is justified.

Simulation of device-scale unsteady turbulent flow in the Fundy Tidal Region

This paper specifically focuses on site-specific CFD modelling in the context of tidal energy development, and presents results for two sites from the Fundy Tidal Region (FTR) situated in Eastern Canada. The FTR is one of the best and most energetic areas in the world for tidal energy development. To efficiently meet the computational demands of modelling unsteady three-dimensional turbulent flow in complex tidal zone terrains, highly parallel GPU hardware are utilized. Strategically deployed Acoustic Doppler Current Profiler (ADCP) measurements at two tidal turbine locations provide validation of the predictions under ebb and flood flows. The processing methodology of ADCP devices is incorporated into the CFD simulation to improve predicted turbulence levels for validation. The simulations use detached eddy simulation (DES) turbulence modelling with engineered inlet boundary conditions and realistic bathymetry. The inlet boundaries use mean horizontal velocity values interpolated from regional tidal simulations and are perturbed by synthetic turbulence to accelerate boundary layer development. Simulations cover tidal developments from ∼1.5 to 3 square kilometres with spatial and temporal resolutions of 1–2 m and 0.1 s, respectively.