Anisotropic Turbulence Research Articles

Wall-modelled large-eddy simulation of three-dimensional turbulent boundary layer in a bent square duct

We conduct wall-modelled large-eddy simulation (WMLES) of a pressure-driven three-dimensional turbulent boundary layer developing on the floor of a bent square duct to investigate the predictive capability of three widely used wall models, namely, a simple equilibrium stress model, an integral non-equilibrium model, and a partial differential equation (PDE) non-equilibrium model. The numerical results are compared with the experiment of Schwarz & Bradshaw (J. Fluid Mech., vol. 272, 1994, pp. 183–210). While the wall-stress magnitudes predicted by the three wall models are comparable, the PDE non-equilibrium wall model produces a substantially more accurate prediction of the wall-stress direction, followed by the integral non-equilibrium wall model. The wall-stress direction from the wall models is shown to have separable contributions from the equilibrium stress part and the integrated non-equilibrium effects, where how the latter is modelled differs among the wall models. The triangular plot of the wall-model solution reveals different capabilities of the wall models in representing variation of flow direction along the wall-normal direction. In contrast, the outer LES solution is unaffected by the type of wall model used, resulting in nearly identical predictions of the mean and turbulent statistics in the outer region for all the wall models. This is explained by the vorticity dynamics and the inviscid skewing mechanism of generating the mean three-dimensionality. Finally, the LES solution in the outer layer is used to study the anisotropy of turbulence. In contrast to the canonical two-dimensional wall turbulence, the Reynolds stress anisotropy exhibits strong non-monotonic behaviour with increasing wall distance.

Large eddy simulations of fully developed turbulent flows over additively manufactured rough surfaces

In the last decade, with the growing demand for efficient and more sustainable products that reduce our CO2 footprint, progresses in Additive Manufacturing (AM) have paved the way for optimized heat exchangers, whose disruptive design will heavily depend on predictive numerical simulations. Typical AM rough surfaces show limited resemblance to the artificially constructed rough surfaces that have been the basis of most prior fundamental research on turbulent flow over rough walls. Hence, current wall models used in steady and unsteady three-dimensional (3D) Navier–Stokes simulations do not consider such characteristics. Therefore, a high-fidelity Large Eddy Simulation (LES) database is built to develop and assess novel wall models for AM. This article investigates the flow in rough pipes built from the surfaces created using AM techniques at Siemens based on Nickel Alloy IN939 material. We developed a code to generate the desired rough pipes from scanned planar surfaces. We performed high-fidelity LES of turbulent rough pipe flows at Reynolds number, Re = 11 700, to reveal the influence of roughness parameters on turbulence, mainly the average roughness height and the effective slope. The equivalent sand-grain roughnesses, ks, of the present AM rough surfaces are predicted using the Colebrook correlation. The main contributors to the skin friction coefficient are found to be turbulence and drag forces. In the present study, the existence of a logarithmic layer is marked even for high values of ks. The mean flow, the velocity fluctuations, and the Reynolds shear stresses show turbulence's strong dependence on the roughness topography. Profiles of turbulence statistics are compared by introducing an effective wall-normal distance defined as zero-plane displacement. The effective distance collapses the shear stresses and the velocity fluctuations outside the roughness sublayer; thus, Townsend's similarity of the streamwise mean velocity is marked for the present roughnesses. Furthermore, a mixed scaling is introduced to improve the collapse of turbulence statistics in the roughness sublayer. In addition, an attempt to investigate the impact of surface roughness on flow physics using the acquired LES results based on quadrant analysis of the Reynolds shear stresses and anisotropy of turbulence is made.

Simulating Rayleigh-Taylor induced magnetohydrodynamic turbulence in prominences

Aims.Solar prominences are large-scale condensations suspended against gravity within the solar atmosphere. The Rayleigh-Taylor (RT) instability is proposed to be one of the fundamental processes that lead to the generation of dynamics at many spatial and temporal scales within these long-lived, cool, and dense structures, which are located in the solar corona. We aim to study such turbulent processes using high-resolution, direct numerical simulations of solar prominences.Methods.We ran 2.5D ideal magnetohydrodynamic (MHD) simulations with the open-sourceMPI-AMRVACcode far into the nonlinear evolution of an RT instability perturbed at the prominence-corona interface. Our simulation achieves a resolution down to ∼23 km on a 2D (x, y) domain of size 30 Mm × 30 Mm. We followed the instability transitioning from a multimode linear perturbation to its nonlinear, fully turbulent state. Over the succeeding ∼25 min period, we performed a statistical analysis of the prominence at a cadence of ∼0.858 s.Results.We find that the dominant guiding component,Bz, induces coherent structure formation predominantly in the vertical velocity component,Vy, consistent with observations, indicating an anisotropic turbulence state within our prominence. We find power-law scalings in the inertial range for the velocity, magnetic, and temperature fields. The presence of intermittency is evident from the probability density functions of the field fluctuations, which depart from Gaussianity as we consider smaller and smaller scales. In exact agreement, the higher-order structure functions quantify the multi-fractality, as do different scale characteristics and the behavior between the longitudinal and transverse directions. Thus, the statistics remain consistent with conclusions from previous observational studies, enabling us to directly relate the RT instability to the turbulent characteristics found within quiescent prominences.

Correction to: Anisotropic Turbulence in the Radiatively Driven Convective Layer in a Small Shallow Ice-Covered Lake: An Observational Study

Correction to: Anisotropic Turbulence in the Radiatively Driven Convective Layer in a Small Shallow Ice-Covered Lake: An Observational Study

Modulation transfer function variation through anisotropic turbulence in biological tissue

Analysis of the long-exposure modulation transfer function (MTF) is performed for optical imaging using plane and spherical waves through anisotropic turbulence in biological tissues. To obtain the MTF, the wave structure functions of plane and spherical waves are obtained in closed-form expressions. Results are presented depending on various parameters of the turbulent medium and optical beam. The positive effect of anisotropy on optical imaging is remarkable in turbulent biological tissues. Besides scattering and absorption, taking anisotropy into account as well as turbulence will lead to a more accurate description of the performance of the medical imaging systems that use optical spectrums in biological tissues.

Turbulent Anisotropy and Length Scale Variation Over Multiple Shaped Structure

AbstractThe turbulent flow characteristics over bed-mounted three different cubical shape bluff bodies are examined experimentally in the water channel facility. The steady and fluctuating flow fields are investigated to analyze the effect of corner radius and shapes of the bluff body on turbulent flow structure, particularly in the wake region. It is found that the sharp corner region significantly impacts the flow separation and alters the characteristics of the shear-layer flow. In particular, the relatively suitable change in geometry resulted in a remarkable variation of the mean flow in the wake is observed. The anisotropic nature of flow is analyzed using the turbulence triangle for the different cubical structures. The variation of the turbulent length scales is presented in the near- and far-wake regions of the submerged obstacles.

Turbulent flow mechanisms in meandering channels with sediment transport

Abstract This research aims to investigate the near-bed turbulent flow characteristics in a meandering channel with both mobile bed and immobile bed conditions. Experiments were performed in a prismatic rectangular meandering channel with a non-uniform sand bed of size d50 = 0.523mm. The three-dimensional instantaneous fluid velocity was collected using the Acoustic Doppler velocimeter which will provide important results related to the flow turbulence such as mean flow velocity, turbulence intensity, Reynolds shear stress, turbulent kinetic energy, skewness, kurtosis and turbulent anisotropy. The secondary current flow and the exchange of momentum in the form of turbulence kinetic energy, Reynolds shear stress and turbulent intensity at the inner layer of the flow are identified more in a mobile bed condition as compared to an immobile condition, which causes sediment transport. For the inner layer of the flow, turbulence intensity and turbulent kinetic energy are decreased in magnitude and gradually increase in the outer layer of flow for both the bed conditions. Higher turbulence anisotropy is noticed in the mobile bed condition than in the immobile bed condition, which shows more nonuniformities near the bed level for the mobile bed condition. This study may help in understanding the effect of sediment transport due to a turbulent flow structure in a sinuous alluvial channel.

Turbulence anisotropy analysis at the middle section of a highly loaded 3D linear turbine cascade using Large Eddy Simulation

This study analyzes the flow over a three-dimensional linear low-pressure turbine cascade blade using large eddy simulation at Re = 90,000. The computational model consists of one blade passage with periodic boundaries and synthetic turbulence is generated at the inlet of the domain. Various flow metrics, including isentropic Mach number distribution at mid-span and wake total pressure losses are compared with available experimental data and found to be in good agreement. A more detailed analysis of the turbulence with particular attention to the separation bubble region is subsequently presented. The analysis revealed that the turbulence is in a nearly two-component state very close to the wall region and gradually follows a certain anisotropy trajectory, as the distance from the wall increases. Even in the free-stream region no fully isotropic state is reached, due to large acceleration and flow turning. The results give a new insight into the state of turbulence within the separation region on the blade suction side and emphasize the deficiencies of the Reynolds-averaged Navier Stokes (RANS) turbulence models in reproducing the turbulence anisotropy. This insight is of relevance for the aerodynamic design of turbines, since large parts of the total pressure loss are generated in the separation region.

Partially coherent anomalous vortex beam in anisotropic turbulence

Propagation properties of a partially coherent anomalous vortex beam (PCAVB) through the anisotropic turbulent atmosphere are studied. Based on the generalized anisotropic von-Karman spectrum and Rytov approximation, the mathematical expressions of mode probability, energy content, and spiral spectrum of the PCAVB are obtained. The effects of anisotropic turbulence and source parameters on the PCAVB’s signal orbital angular momentum (OAM) modal crosstalk are investigated. According to numerical findings, the transmission performance of the PCAVB can be improved by increasing the anisotropy coefficient, the beam order, the partial coherence length of the beam source, or reducing the OAM mode. In some instances, the PCAVB also shows stronger resistance to turbulence than the partially coherent Laguerre-Gaussian beam (PCLGB). This research may be helpful for constructing optical vortex communication systems as well as for comprehending the optical transmission mechanism in turbulent fluids.

On the laminar–turbulent transition mechanism on megawatt wind turbine blades operating in atmospheric flow

Abstract. Among a few field experiments on wind turbines for analyzing laminar–turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic glove projects provide significant findings. The effect of inflow turbulence on boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD (computational fluid dynamics) simulations of increasing fidelity (Reynolds-averaged Navier–Stokes, RANS; unsteady Reynolds-averaged Navier–Stokes, URANS; and large-eddy simulations, LESs). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence, for example, such as that from wakes, while natural transition is observed in other instances under relatively low inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the eN method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is found in case of bypass transition. A condition to easily distinguish the region of transition to turbulence based on the Reynolds shear stress is also observed. Overall, useful insights into the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.

Anisotropic plasma sheath turbulence effects on the mode states of Whittaker–Gaussian beams

In this paper detection probability and crosstalk probability variance of the orbital angular momentum states of the diffraction-free Whittaker–Gaussian (WG) beams passing through an anisotropic plasma sheath turbulence (PST) are theoretically formulated. In this regard, using modified von Karman spectrum in the frame of Rytov theory the spiral spectrum of the WG beams is derived, and the role of the anisotropy parameters, refractive index fluctuation variance, initial topological charge, receiving aperture radius, the outer scale of turbulence, beam wavelength and waist in the mitigation of the turbulence-induced disturbance is numerically explored. The remarkable findings indicate that WG beams in anisotropic PST are less influenced than isotropic one, and increasing the anisotropy feature of plasma medium leads to alleviate modulated distortion. Furthermore, it is found out that the propagation performance of WG vortex beams with high-order initial topological charge is significantly better than those with lower order. We expect that our results will be helpful for designing optimal free-space communication systems.

X-Ray Polarimetry Reveals the Magnetic-field Topology on Sub-parsec Scales in Tycho’s Supernova Remnant

Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions regarding the physical conditions at shock fronts, such as the magnetic-field morphology close to the particle acceleration sites, remain open. Here we report the detection of a localized polarization signal from some synchrotron X-ray emitting regions of Tycho’s supernova remnant made by the Imaging X-ray Polarimetry Explorer. The derived degree of polarization of the X-ray synchrotron emission is 9% ± 2% averaged over the whole remnant, and 12% ± 2% at the rim, higher than the value of polarization of 7%–8% observed in the radio band. In the west region, the degree of polarization is 23% ± 4%. The degree of X-ray polarization in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic field or a larger maximum turbulence scale. The measured tangential direction of polarization corresponds to the radial magnetic field, and is consistent with that observed in the radio band. These results are compatible with the expectation of turbulence produced by an anisotropic cascade of a radial magnetic field near the shock, where we derive a magnetic-field amplification factor of 3.4 ± 0.3. The fact that this value is significantly smaller than those expected from acceleration models is indicative of highly anisotropic magnetic-field turbulence, or that the emitting electrons either favor regions of lower turbulence, or accumulate close to where the orientation of the magnetic field is preferentially radially oriented due to hydrodynamical instabilities.

Effects of combustion on the near-wall turbulence and performance for supersonic hydrogen film cooling using large eddy simulation

Supersonic film cooling using fuel on board is an effective way to simultaneously shield the huge heat and momentum flux transported from the mainstream to the wall in a scramjet engine. The self-ignition and combustion of the injected fuel film will significantly change the turbulent transport behavior in the boundary layer. To reveal the effects of the boundary layer combustion on the near-wall turbulence and wall fluxes, large eddy simulations (LES) of the Burrows–Kurkov supersonic combustion experiment using hydrogen as a film are performed based on the in-house solver scramjetFoam. The solver successfully captures the additional skin friction reduction phenomenon induced by the boundary layer combustion compared to other numerical works using LES in the public literature. The results reveal that further increased anisotropy of turbulence combined with the low-density region contributes to a remarkable suppression of turbulent transport processes in the wall-normal direction. The self-ignition point of the hydrogen film is found to oscillate back and forth in a span of 80 mm, which significantly enhances turbulence in the boundary layer. However, the increased turbulent fluctuating velocity is mainly concentrated in the streamwise direction, while the other two components are suppressed instead. The findings are also essential for improving engineering computations based on the Reynolds averaged simulation method.

Numerical investigations of vortex dynamics and loss generation in the corner separation region of a high subsonic compressor blade

Three-dimensional corner separation seriously deteriorates the aerodynamic performance of a compressor blade. In this study, the complicated vortex dynamics and loss mechanism in the corner region of a high subsonic compressor blade (the inlet Mach number is 0.67) are investigated using large eddy simulations (LESs) at a Reynolds number (Re) of 5.6 × 105. The results show that the predicted total pressure loss and outlet flow angle match well with the experimental data, indicating that the LES method can accurately predict the size and strength of corner separation in the compressor blade. With the passage vortex rolling up and further interacting with the low-momentum fluids originating from the end wall boundary layers and the local blade boundary layers, strong shear strain induces a large-scale concentrated shedding vortex (CSV) near the spanwise location of x/H = 0.3. The formation and rolling-up of the CSV not only cause the strongest flow blockage but also strengthen the turbulence anisotropy. As the CSV moves toward the trailing edge, the strong interaction with a pair of counter-rotating vortices shed in the wake region (wake shedding vortex) further accelerates the local generation of turbulent kinetic energy (TKE). Detailed TKE budget analysis shows that the streamwise Reynolds normal stress (w′w′¯) plays the most decisive role in the TKE production term. Furthermore, we demonstrate that the strength of turbulence anisotropy is positively correlated with the TKE budget. Therefore, turbulence anisotropy should be considered carefully in predicting the loss level in the corner region of a compressor blade.

Propagation of Lorentz-Gaussian elliptical multi-Gaussian correlated Schell-Model beam in anisotropic turbulence

The beam spreading, spectral degree of coherence and evolution behavior of the intensity profile of partially coherent Lorentz-Gaussian elliptical multi-Gaussian correlation Schell-Model beam propagating in anisotropic atmospheric turbulence are studied. The analytical expressions of cross-spectral density function, as well as root mean square (rms) beam width, are derived based on the extended Huygens-Fresnel principle and the relationship of Lorentz distribution and Hermitian Gaussian function. With the increases of propagation distance, the elliptical beam will first evolve into Gaussian beam and then change back to elliptical beam. In anisotropic atmospheric turbulence, the influence of the inner scale of turbulence on the spectral degree of coherence and rms beam width are obviously greater than that of the outer scale. For Lorentz-Gaussian elliptical multi-Gaussian correlated Schell-Model beams, better propagation performance was found in anisotropic atmospheric turbulence with larger anisotropic factor and smaller inner scale.

Improving Accuracy of Airfoil Trailing Edge Noise Models with Turbulent Flow Anisotropy for Cambered Airfoil

In turbomachinery applications such as wind turbines, and aircraft airframe, flow-induced noise and vibration are significant noise sources. There are five main types of airfoil self-noise, with trailing edge noise (TEN) being the most dominant high-frequency broadband noise. The Amiet TE noise model is a low-order analytical model that requires the surface pressure (SP) near the trailing edge for TEN modelling. The accuracy with which the SP spectrum is calculated determines the accuracy of the TEN modelling. The SP spectrum is calculated using the physics-based TNO-Blake (TB) Model and its recently produced improved version. Most studies have focused on using the TB model for symmetric airfoil, with little attention paid to modelling the TEN of cambered airfoil. We use TB model variants on the SP spectrum prediction in this study, which are enforced with the Amiet TEN model for far-field noise modelling. The SP models' source comes from a high-fidelity Large-Eddy-Simulation simulation (LES) based on CABARET method. The simulation is carried out using the NACA65410 airfoil with a chord length of 0.3m and an angle of attack of 0,2, 4, 6, and 8 degrees at a Reynolds number of 5.2e5. The SP spectrum and velocity spectrum used to improve the TEN model's accuracy for cambered airfoil.

On the application of statistical turbulence models to the simulation of airflow inside a car cabin

Computational fluid dynamics simulations of airflow inside a full-scale passenger car cabin are performed using the Reynolds averaged Navier–Stokes equations. The performance of a range of turbulence models is examined by reference to experimental results of the streamwise mean velocity and turbulence intensity profiles, obtained using the hot-wire anemometry technique at different locations inside the car cabin. The models include three linear eddy-viscosity-based variants, namely, the realizable k–ε, the renormalization group k–ε, and the shear-stress transport k–ω models. The baseline Reynolds stress model (BSL-RSM), a second-moment-closure variant, and an Explicit Algebraic Reynolds Stress Model (BSL-EARSM) are also investigated. Visualization of velocity vectors and streamlines in different longitudinal planes shows a similar airflow pattern. The flow topology is mainly characterized by jet flows developing from the dashboard air vents and extending to the back-seats compartment resulting in a large vortex structure. Additionally, a comparison between numerical and experimental results shows a relatively good agreement of the mean velocity profiles. However, all models exhibit some limitations in predicting the correct level of turbulence intensity. Moreover, the realizability of the modeled Reynolds stresses and the structure of turbulence are analyzed based on the anisotropy invariant mapping approach. All models reveal a few amounts of non-realizable solutions. The linear eddy-viscosity-based models return a prevailing isotropic turbulence state, while the BSL-RSM and the BSL-EARSM models display pronounced anisotropic turbulence states.

Effects of temperature gradient and beam path height from heat source on phase structure and mutual coherence functions of a light beam propagating through convective air turbulence

In this paper, we investigate the effects of different values of temperature gradient (TG) and the distance of a plane wave path from a flat heat source generating an anisotropic and non-Kolmogorov convective air turbulence medium on the phase structure function (PSF) and mutual coherence function (MCF) of the light beam propagating through the medium. The non-Kolmogorov and anisotropic convective air turbulence is created using an electrical heater with a surface size of 50 cm × 100 cm. The heater temperature is changeable from room temperature to 100 °C. The laser beam with a diameter of 20 cm passes through the turbulent medium at different distances from the heater surface. At the end of the path, the wavefront of the light beam is reconstructed using a two-channel moiré wavefront sensor. Then, the PSF and MCF are calculated, and the effects of different TGs and the different beam path heights from the heater are studied. Results show that with increasing the TG and decreasing the beam path distance from the heat source, the turbulence intensity and PSF increase, while the MCF decreases.

Anisotropic Turbulence in the Radiatively Driven Convective Layer in a Small Shallow Ice-Covered Lake: An Observational Study

Anisotropic Turbulence in the Radiatively Driven Convective Layer in a Small Shallow Ice-Covered Lake: An Observational Study

The stability of inviscid Beltrami flow between parallel free-slip impermeable boundaries

We present a family of exact inviscid three-dimensional Beltrami flows in a horizontally periodic domain lying between two parallel free-slip boundaries. Significantly, these flows are not stress free: the horizontal vorticity varies on each boundary. Using direct numerical simulations (employing horizontal hyperdiffusion only for numerical stability), we find that the largest-scale member of the family is unstable and breaks down into anisotropic turbulence, with relatively large horizontal vorticity at and near each boundary, and associated surface frontal features. We conjecture that all members of the family are similarly unstable. The free-slip boundaries play an important role in the late stages of the instability by constraining the deformation of vortex lines near the boundaries. This study appears to be the first to consider the role of boundary horizontal vorticity in an inviscid context.