Turbulence In Incompressible Fluid Research Articles

Employment of roughened absorber palate and jet nozzles with different hole shapes for performance boost of solar-air-heaters

In this study, roughened absorber plate and jet nozzle are simultaneously employed to enhance the efficiency of a solar based air heater. The impact of jet nozzle hole shape is comprehensively investigated for the mentioned system which have not been investigated before. Three-dimensional steady-state simulations of incompressible turbulent fluid flow and heat transfer are performed using the Reynolds-Averaged Navier-Stokes (RANS) equations to model the jet solar air heater (JSAH). Five different jet hole shapes (circular, triangular, square, hexagonal, and rectangular) are examined and results are compared with conventional structure. The thermal and hydraulic performance of the JSAH is assessed using a variety of geometric and operating parameters, including jet diameter ratio (dj/Dh), jet hole streamwise pitch (Lj/Dh), and Re number (Re). The results show that the ratio of (Nu/Nus) initially increases and then decreases as the hydraulic diameter of the jet holes increases. Circular jet holes consistently outperform other shapes, while rectangular and triangular jet holes perform poorly. The pressure penalty (f/fs) decreases with increasing (dj/Dh), with circular jet holes exhibiting the lowest value. Increasing the Reynolds number enhances heat transfer and pumping power but leads to a decrease in (Nu/Nus) and negatively affects the THPP of the JSAH compared to a conventional smooth solar air heater. The study also reveals that the THPP initially increases with streamwise pitch (Lj/Dh) but then declines, following the trend of (Nu/Nus). The results demonstrate up to 4.5 times higher Nusselt numbers with a jet solar air heater versus a smooth solar heater. A maximum thermal-hydraulic performance parameter of 1.64 indicates the proposed design's efficacy. This research furnishes critical insights into optimizing solar air heater performance by examining jet nozzle hole shape, streamwise pitch, and Reynolds number. It enables enhancing thermal efficiency and thermo-hydraulic performance.

Simulation of macrosegregation in continuous casting of steel based on large-eddy simulation turbulence model and meshless solution procedure

The formation of macrosegregation pattern in steel is explored for the continuous casting process. The related solidification problem is described by incompressible turbulent fluid flow, governed by mass, momentum, energy, and species conservation equations. The solid-liquid system is designed in a mixture continuum model formulation, and the mushy region is approximated with the Darcy model. The lever rule describes the formation of microsegregation of carbon. Turbulence is modelled with a large-eddy simulation approach implemented with the Smagorinsky model and van Driest correction. The synthetic isotropic fluctuations of the inlet velocity are generated by the von Karman Pao spectrum. The solution of partial differential equations is implemented locally by using collocation with radial basis functions for spatial discretization and the explicit Euler method for time discretization. The two-dimensional large eddy simulation model solutions are compared to the previously calculated examples obtained with the low-Re k − ε turbulence model.

Development of Three-Dimensional LES Based Meshless Model of Continuous Casting of Steel

A large-eddy simulation (LES) based meshless model is developed for the three-dimensional (3D) problem of continuous casting (CC) of steel billet. The local collocation meshless method based on radial basis functions (RBF) is applied in 3D. The method applies scaled multiquadric (MQ) RBF with a shape parameter on seven nodded local sub-domains. The incompressible turbulent fluid flow is described using mass, energy, and momentum conservation equations and the LES turbulence model. The solidification system is solved with the mixture continuum model. The Boussinesq approximation for buoyancy and the Darcy approximation for porous media are used. Chorin’s fractional step method is used to couple velocity and pressure. The microscopic model is closed with the lever rule model. The LES model is compared to the two-equation Low Re k−ε turbulence Reynolds Averaged Navier–Stokes (RANS) model in terms of temperature, velocity and computational times. The LES model resolves transient character of vortices which RANS-type turbulence models are unable to tackle. The computational cost of LES models is considerably higher than in RANS. On the other hand, it results in a much lower computational cost than the direct numerical simulation (DNS). The paper demonstrates the ability of the method to solve realistic industrial 3D examples. Trivial adjustment of nodal densities, high accuracy, and low numerical diffusivity are the main advantages of this meshless method.

Swift generator for three-dimensional magnetohydrodynamic turbulence.

Magnetohydrodynamic turbulence is central to laboratory and astrophysical plasmas, and is invoked for interpreting many observed scalings. Verifying predicted scaling law behavior requires extreme-resolution direct numerical simulations (DNS), with needed computing resources excluding systematic parameter surveys. We here present an analytic generator of realistically looking turbulent magnetic fields, that computes three-dimensional (3D) O(1000^{3}) solenoidal vector fields in minutes to hours on desktop computers. Our model is inspired by recent developments in 3D incompressible fluid turbulence theory, where a Gaussian white noise vector subjected to a nonlinear transformation results in an intermittent, multifractal random field. Our B×C model has only few parameters that have clear geometric interpretations. We directly compare a (costly) DNS with a swiftly B×C-generated realization, in terms of its (1) characteristic sheetlike structures of current density, (2) volume-filling aspects across current intensity, (3) power-spectral behavior, (4) probability distribution functions of increments for magnetic field and current density, structure functions, and spectra of exponents, and (5) partial variance of increments. The model even allows to mimic time-evolving magnetic and current density distributions and can be used for synthetic observations on 3D turbulent data cubes.

Dissipation-range fluid turbulence and thermal noise.

We revisit the issue of whether thermal fluctuations are relevant for incompressible fluid turbulence and estimate the scale at which they become important. As anticipated by Betchov in a prescient series of works more than six decades ago, this scale is about equal to the Kolmogorov length, even though that is several orders of magnitude above the mean free path. This result implies that the deterministic version of the incompressible Navier-Stokes equationis inadequate to describe the dissipation range of turbulence in molecular fluids. Within this range, the fluctuating hydrodynamics equationof Landau and Lifschitz is more appropriate. In particular, our analysis implies that both the exponentially decaying energy spectrum and the far-dissipation-range intermittency predicted by Kraichnan for deterministic Navier-Stokes will be generally replaced by Gaussian thermal equipartition at scales just below the Kolmogorov length. Stochastic shell model simulations at high Reynolds numbers verify our theoretical predictions and reveal furthermore that inertial-range intermittency can propagate deep into the dissipation range, leading to large fluctuations in the equipartition length scale. We explain the failure of previous scaling arguments for the validity of deterministic Navier-Stokes equationsat any Reynolds number and we provide a mathematical interpretation and physical justification of the fluctuating Navier-Stokes equation as an "effective field theory" valid below some high-wave-number cutoff Λ, rather than as a continuum stochastic partial differential equation. At Reynolds number around a million, comparable to that in Earth's atmospheric boundary layer, the strongest turbulent excitations observed in our simulation penetrate down to a length scale of about eight microns, still two orders of magnitude greater than the mean free path of air. However, for longer observation times or for higher Reynolds numbers, more extreme turbulent events could lead to a local breakdown of fluctuating hydrodynamics.

Numerical and convergence analysis of the stochastic Lagrangian averaged Navier–Stokes equations

The primary emphasis of this work is the development of a finite element based space–time discretization for solving the stochastic Lagrangian averaged Navier–Stokes (LANS-α) equations of incompressible fluid turbulence with multiplicative random forcing, under nonperiodic boundary conditions within a bounded polygonal (or polyhedral) domain of Rd, d∈{2,3}. The convergence analysis of a fully discretized numerical scheme is investigated and split into two cases according to the spacial scale α, namely we first assume α to be controlled by the step size of the space discretization so that it vanishes when passing to the limit, then we provide an alternative study when α is fixed. A preparatory analysis of uniform estimates in both α and discretization parameters is carried out. Starting out from the stochastic LANS-α model, we achieve convergence toward the continuous strong solutions of the stochastic Navier–Stokes equations in 2D when α vanishes at the limit. Additionally, convergence toward the continuous strong solutions of the stochastic LANS-α model is accomplished if α is fixed. Neither of the mentioned convergences involves the Skorokhod theorem.

A hardware accelerated unstructured overset method to simulate turbulent fluid flow

Hardware acceleration consists of offloading computational work to devices such as graphics processing units (GPUs) to produce overall speed-up. Algorithms and numerical methods must be constructed to suit the available hardware in order to effectively produce speed-up. In this work a numerical method is presented which can effectively use hardware acceleration to simulate incompressible turbulent fluid flow. The method is an unstructured overset method where unstructured meshes are attached to individual bodies and connected throughout the flow domain to produce a single domain solution through an overset assembly process. The unstructured overset method shown in Horne and Mahesh [1] and Horne and Mahesh [2] was found capable of scaling to O(105) computational cores for O(105) moving bodies in turbulent flow fields while producing accurate flow results. This highly scalable method is modified and extended to effectively utilize on-node hardware acceleration. Overset assembly algorithms which use hardware acceleration are presented based on successful accelerated algorithms in real-time ray tracing and computational geometry. Timing results for core overset assembly operations are presented showing a maximum O(100x) speedup when using hardware acceleration. A novel method for turbulent fluid flow is presented which utilizes over-decomposition of the flow domain to produce task-parallelism allowing asynchronous calculation of the different steps of the method while also providing overlap between data transfer and computation. A mixed precision solver is utilized which provides a balance between optimal performance and numerical accuracy. A cost effective and accurate artificial compressibility pressure regularization is used which has minimal memory complexity and minimizes computational cost while maintaining accuracy. A primal-dual Laplacian operator is introduced which produces accurate results on skewed meshes. Results for canonical flow cases with overset meshes are shown illustrating the method's accuracy and numerical properties. Substantial speed-up is demonstrated for the numerical method reaching upwards of 50 times as fast as the non-accelerated method for high cell loadings.

The Mathematical Description of Homogeneous Turbulence for Incompressible Fluids

The present article establishes the mathematical description of turbulence for incompressible fluids as a response to an important question: “Is a complete mathematical description of turbulence possible?” in ResearchGate.net website discussion started August 26, 2018. Since inhomogeneous turbulence is too complex, this article will limit the discussion to homogeneous turbulence. This article provides the necessary propositions for a mathematical description of homogeneous turbulence for incompressible fluids which is achievable via a probabilistic formulation, deterministic chaos, and the Kolmogorov’s -5/3 law.

Erratum in “On the Holographic Principle Description for the Viscous Incompressible Fluids” and “The Mathematical Description of Homogeneous Turbulence for Incompressible Fluids

This note is to acknowledge errors made in previous article: “On the Holographic Principle Description for the Viscous Incompressible Fluids” due to the Laplacian of the Lagrangian fails to be zero as expected when a using G.I. Taylor's 2-D solution of the Navier-Stokes momentum equations. Unfortunately, the article “The Mathematical Description of Homogeneous Turbulence for Incompressible Fluids” was also affected due to the use of the Laplacian of the Lagrangian in order to obtain the power spectrum relationship of the pressure which obviously now is incorrect.

Clebsch confinement and instantons in turbulence

The turbulence in incompressible fluid is represented as a field theory in 3 dimensions. There is no time involved, so this is intended to describe stationary limit of the Hopf functional. The basic fields are Clebsch variables defined modulo gauge transformations (symplectomorphisms). Explicit formulas for gauge invariant Clebsch measure in space of generalized Beltrami flow compatible with steady energy flow are presented. We introduce a concept of Clebsch confinement related to unbroken gauge invariance and study Clebsch instantons: singular vorticity sheets with nontrivial helicity. This is realization of the “instantons and intermittency” program we started back in the 1990s.1 These singular solutions are involved in enhancing infinitesimal random forces at remote boundary leading to critical phenomena. In the Euler equation vorticity is concentrated along the random self-avoiding surface, with tangent components proportional to the delta function of normal distance. Viscosity in Navier–Stokes equation smears this delta function to the Gaussian with width [Formula: see text] at [Formula: see text] with fixed energy flow. These instantons dominate the enstrophy in dissipation as well as the PDF for velocity circulation [Formula: see text] around fixed loop [Formula: see text] in space. At large loops, the resulting symmetric exponential distribution perfectly fits the numerical simulations2 including pre-exponential factor [Formula: see text]. At small loops, we advocate relation of resulting random self-avoiding surface theory with multi-fractal scaling laws observed in numerical simulations. These laws are explained as a result of fluctuating internal metric (Liouville field). The curve of anomalous dimensions [Formula: see text] can be fitted at small [Formula: see text] to the parabola, coming from the Liouville theory with two parameters [Formula: see text], [Formula: see text]. At large [Formula: see text] the ratios of the subsequent moments in our theory grow linearly with the size of the loop, which corresponds to finite value of [Formula: see text] in agreement with DNS.

A variational flexible multibody formulation for partitioned fluid–structure interaction: Application to bat-inspired drones and unmanned air-vehicles

We present a three-dimensional (3D) partitioned aeroelastic formulation for a flexible multibody system interacting with incompressible turbulent fluid flow. While the incompressible Navier–Stokes system is discretized using a stabilized Petrov–Galerkin procedure, the multibody structural system consists of a generic interaction of multiple components such as rigid body, beams and flexible thin shells along with various types of joints and connections among them. A co-rotational framework is utilized for the category of small strain problems where the displacement of the body is decomposed into a rigid body rotation and a small strain component. This assumption simplifies the structural equations and allows for the incorporation of multiple bodies (rigid as well as flexible) in the system. The displacement and rotation constraints at the joints are imposed by a Lagrange multiplier method. The equilibrium conditions at the fluid–structure interface are satisfied by the transfer of tractions and structural displacements via the radial basis function approach, a scattered data interpolation technique, which is globally conservative. For the coupled stability in low structure-to-fluid mass ratio regimes, a nonlinear iterative force correction scheme is employed in the partitioned staggered predictor–corrector scheme. The convergence and generality of the radial basis function mapping are analyzed by carrying out systematic error analysis of the transfer of fluid traction across the non-matching fluid–structure interface where a third-order of convergence is observed. The proposed aeroelastic framework is then validated by considering a flow across a flexible pitching plate configuration with serration at the trailing edge. Finally, we demonstrate the flow across a flexible flapping wing of a bat modeling the bone fingers as beams and the flexible membrane as thin shells in the multibody system along with the joints.

Automatic generators of multi-patch B-spline meshes of blade cascades and their comparison

In this paper, we propose several approaches for automatic construction of planar multi-patch B-spline parameterizations around one or between two consecutive blade profiles in a blade cascade starting from given design parameters (blade cascade is obtained by unfolding a cylindrical cross-section of the runner), which have been implemented in C++ as fully automatic generators and are an integral part of our in-house automatic shape optimization tool for water turbines. These B-spline parameterizations have to be prepared for the definition of periodic boundary conditions on the corresponding parts of the boundary, which makes finding of analysis-suitable multi-patch B-spline parameterizations even more challenging. All approaches are compared with respect to the selected geometric quality measures and also with respect to the results from our in-house isogeometric incompressible turbulent fluid flow solver and we discuss pros and cons of these approaches.

On numerical simulation of flexibly supported airfoil in interaction with incompressible fluid flow using laminar–turbulence transition model

In this paper the problem of mutual interaction of the incompressible turbulent fluid flow with a flexibly supported airfoil is addressed. The problem is mathematically described with the aid of the Reynolds averaged Navier–Stokes equations. The turbulence is modeled with the aid of k-kL-ω transition model. In order to treat the motion of the domain the system of equations are written in the arbitrary Lagrangian–Eulerian. The structure motion is governed by a nonlinear system of ordinary differential equations, where two degrees of freedom for the rotation and the vertical translation are used. The problem is numerically discretized with the aid of the finite volume method using the linear reconstruction technique. The numerical results are shown.

Assessment and verification of mean effective diameter of internal channels fabricated by laser powder bed fusion

Channels with small diameters, used for example in conformal cooling, can nowadays be fabricated by laser powder bed fusion. Measurements of the mean effective diameter and surface topography are important to quantify the flow capabilities through the channel. In this paper, a new method using X-ray computed tomography is developed to obtain the mean effective diameter and mean surface topography height. The developed method is verified by determining the mean effective diameter using incompressible turbulent fluid flow simulations, whereupon the determined mean surface topography height is fed as an input to the simulation. The method is proved to offer a non-destructive and relatively fast approach to measure the mean effective diameter and mean surface topography height in circular channels.

Statistical Lyapunov Theory Based on Bifurcation Analysis of Energy Cascade in Isotropic Homogeneous Turbulence: A Physical–Mathematical Review

This work presents a review of previous articles dealing with an original turbulence theory proposed by the author and provides new theoretical insights into some related issues. The new theoretical procedures and methodological approaches confirm and corroborate the previous results. These articles study the regime of homogeneous isotropic turbulence for incompressible fluids and propose theoretical approaches based on a specific Lyapunov theory for determining the closures of the von Kármán–Howarth and Corrsin equations and the statistics of velocity and temperature difference. While numerous works are present in the literature which concern the closures of the autocorrelation equations in the Fourier domain (i.e., Lin equation closure), few articles deal with the closures of the autocorrelation equations in the physical space. These latter, being based on the eddy–viscosity concept, describe diffusive closure models. On the other hand, the proposed Lyapunov theory leads to nondiffusive closures based on the property that, in turbulence, contiguous fluid particles trajectories continuously diverge. Therefore, the main motivation of this review is to present a theoretical formulation which does not adopt the eddy–viscosity paradigm and summarizes the results of the previous works. Next, this analysis assumes that the current fluid placements, together with velocity and temperature fields, are fluid state variables. This leads to the closures of the autocorrelation equations and helps to interpret the mechanism of energy cascade as due to the continuous divergence of the contiguous trajectories. Furthermore, novel theoretical issues are here presented among which we can mention the following ones. The bifurcation rate of the velocity gradient, calculated along fluid particles trajectories, is shown to be much larger than the corresponding maximal Lyapunov exponent. On that basis, an interpretation of the energy cascade phenomenon is given and the statistics of finite time Lyapunov exponent of the velocity gradient is shown to be represented by normal distribution functions. Next, the self–similarity produced by the proposed closures is analyzed and a proper bifurcation analysis of the closed von Kármán–Howarth equation is performed. This latter investigates the route from developed turbulence toward the non–chaotic regimes, leading to an estimate of the critical Taylor scale Reynolds number. A proper statistical decomposition based on extended distribution functions and on the Navier–Stokes equations is presented, which leads to the statistics of velocity and temperature difference.

Extending the Meshless Local Petrov–Galerkin Method to Solve Stabilized Turbulent Fluid Flow Problems

The aim of this paper is to extend the meshless local Petrov–Galerkin method to solve stabilized turbulent fluid flow problems. For the unsteady incompressible turbulent fluid flow problems, the Spalart–Allmaras model is used to stabilize the governing equations, and the meshless local Petrov–Galerkin method is extended based on the vorticity-stream function to solve the turbulent flow problems. In this study, the moving least squares scheme interpolates the field variables. The proposed method solves three standard test cases of the turbulent flow over a flat plate, turbulent flow through a channel, and turbulent flow over a backward-facing step for evaluation of the method’s capability, accuracy, and validity purposes. Based on the comparison of the three test cases results with those of the experimental and conventional numerical works available in the literature, the proposed method shows to be accurate and quite implemental. The new extended method in this study together with the previously published works of the authors (on extending the meshless local Petrov–Galerkin method to solve laminar flow problems) now, for the first time, empower the meshless method to solve both laminar and turbulent flow problems.

Renormalization of viscosity in wavelet-based model of turbulence

Statistical theory of turbulence in viscid incompressible fluid, described by the Navier-Stokes equation driven by random force, is reformulated in terms of scale-dependent fields $\mathbf{u}_a(x)$, defined as wavelet-coefficients of the velocity field $\mathbf{u}$ taken at point $x$ with the resolution $a$. Applying quantum field theory approach of stochastic hydrodynamics to the generating functional of random fields $\mathbf{u}_a(x)$, we have shown the velocity field correlators $\langle \mathbf{u}_{a_1}(x_1)\ldots \mathbf{u}_{a_n}(x_n)\rangle$ to be finite by construction for the random stirring force acting at prescribed large scale $L$. The study is performed in $d=3$ dimension. Since there are no divergences, regularization is not required, and the renormalization group invariance becomes merely a symmetry that relates velocity fluctuations of different scales in terms of the Kolmogorov-Richardson picture of turbulence development. The integration over the scale arguments is performed from the external scale $L$ down to the observation scale $A$, which lies in Kolmogorov range $l \ll A \ll L$. Our oversimplified model is full dissipative: interaction between scales is provided only locally by the gradient vertex $(\mathbf{u}\nabla) \mathbf{u}$, neglecting any effects or parity violation that might be responsible for energy backscatter. The corrections to viscosity and the pair velocity correlator are calculated in one-loop approximation. This gives the dependence of turbulent viscosity on observation scale and describes the scale dependence of the velocity field correlations.

A 3D common-refinement method for non-matching meshes in partitioned variational fluid–structure analysis

We present a three-dimensional (3D) common-refinement method for non-matching unstructured meshes between non-overlapping subdomains of incompressible turbulent fluid flow and nonlinear hyperelastic structure. The fluid flow is discretized using a stabilized Petrov–Galerkin method, and the large deformation structural formulation relies on a continuous Galerkin finite element method. An arbitrary Lagrangian–Eulerian formulation with a nonlinear iterative force correction (NIFC) coupling is achieved in a staggered partitioned manner by means of fully decoupled implicit procedures for the fluid and solid discretizations. To begin, we first investigate the accuracy of common-refinement method (CRM) to satisfy the traction equilibrium condition along the fluid-elastic interface with non-matching meshes. We systematically assess the accuracy of CRM against the matching grid solution by varying grid mismatch between the fluid and solid meshes over a tubular elastic body. We demonstrate the second-order accuracy of CRM through uniform refinements of fluid and solid meshes along the interface. We then extend the error analysis to transient data transfer across non-matching meshes between the fluid and solid solvers. We show that the common-refinement discretization across non-matching fluid–structure grids yields accurate transfer of the physical quantities across the fluid–solid interface. We next solve a 3D fluid–structure interaction (FSI) problem of a cantilevered hyperelastic plate behind a circular bluff body and verify the accuracy of coupled solutions with respect to the available solution in the literature. By varying the solid interface resolution, we generate various non-matching grid ratios and quantify the accuracy of CRM for the nonlinear structure interacting with a laminar flow. We illustrate that the CRM with the partitioned NIFC treatment is stable for low solid-to-fluid density ratio and non-matching meshes for the 3D FSI problem. Finally, we demonstrate the 3D parallel implementation of the common-refinement with the NIFC method for a realistic engineering problem of drilling riser undergoing complex vortex-induced vibration with strong added mass effects and turbulent wake flow.

Spontaneous symmetry breaking, conformal anomaly and incompressible fluid turbulence

We propose an effective conformal field theory (CFT) description of steady state incompressible fluid turbulence at the inertial range of scales in any number of spatial dimensions. We derive a KPZ-type equation for the anomalous scaling of the longitudinal velocity structure functions and relate the intermittency parameter to the boundary Euler (A-type) conformal anomaly coefficient. The proposed theory consists of a mean field CFT that exhibits Kolmogorov linear scaling (K41 theory) coupled to a dilaton. The dilaton is a Nambu-Goldstone gapless mode that arises from a spontaneous breaking due to the energy flux of the separate scale and time symmetries of the inviscid Navier-Stokes equations to a K41 scaling with a dynamical exponent z=frac{2}{3} . The dilaton acts as a random measure that dresses the K41 theory and introduces intermittency. We discuss the two, three and large number of space dimensions cases and how entanglement entropy can be used to characterize the intermittency strength.

Small scale turbulence and the finite Reynolds number effect

Failure to recognize the importance of the finite Reynolds number effect on small scale turbulence has, by and large, resulted in misguided assessments of the first two hypotheses of Kolmogorov [“Local structure of turbulence in an incompressible fluid for very large Reynolds numbers,” Dokl. Akad. Nauk SSSR 30, 299–303 (1941)] or K41 as well as his third hypothesis [A. N. Kolmogorov, “A refinement of previous hypotheses concerning the local structure of turbulence in a viscous incompressible fluid at high Reynolds number,” J. Fluid Mech. 13, 82–85 (1962)] or K62. As formulated by Kolmogorov, all three hypotheses require local isotropy to be valid and the Reynolds number to be very large. In the context of the first hypothesis, there is now strong evidence to suggest that this requirement can be significantly relaxed, at least for dissipative scales and relatively low order moments of the velocity structure function. As the scale increases, the effect of the large scale motion on these moments becomes more prominent and higher Reynolds numbers are needed before K41 and K62 can be tested unambiguously.