CONVECTION IN EXTERNAL TURBULENT FLOW

Fundamental character of forced convection heat transfer to the endwall surface of a gas turbine passage can be simulated by using a 90° turning duct. Elevated operating temperatures in gas turbines require a thorough understanding of the turbulent thermal transport process in the three-dimensional end-wall boundary layers. The current study uses an in-house developed three-dimensional viscous flow solver to computationally investigate the heat transfer character near the endwall surfaces. Extensive heat transfer experiments also illuminate the local heat transfer features near the endwall surface and form a baseline data set to evaluate the computational method used. Present experimental effort at Re=342,190 employs a prescribed heat flux method to measure convective heat transfer coefficients on the end-wall surface. Local wall temperatures are measured with liquid crystal thermography. Local convective heat flux is determined by solving the electric potential equation with constant uniform potential that is applied along the inlet and exit boundaries of the endwall. The viscous flow and heat transfer computation algorithms are based on the same methods presented in Part I. The influence of the two primary counter-rotating vortices developing as a result of the balance of inertia and pressure forces on the measured and computed endwall heat transfer coefficients is demonstrated. It is shown that the local heat transfer on the endwall surface is closely related to the structure of the three-dimensional mean flow and the associated turbulent flow field. A comparison of the molecular/ turbulent heat diffusion, wall shear stress, visualization of local turbulent kinetic energy are a few of the tasks that can be performed using a numerical approach in a relatively time efficient manner. The current numerical approach is capable of visualizing the near wall features that can not be easily measured in the flow field near the end-wall surfaces.

Analysis of turbulent flow and thermal structures in low-Prandtl number buoyant flows using direct numerical simulations

Experimental study and numerical analysis of heat transfer enhancement and turbulent flow over shallowly dimpled channel surfaces

Hydraulic and heat transfer processes play a very important role in the design and prototyping of aerospace technology. In most cases this technique works under non-isothermal conditions. Non-isothermal conditions may significantly affect heat transfer and hydrodynamic process. Fundamental research of Non-isothermal turbulent flow is required for further engineering modeling. Models for unsteady processes calculation must be based on fundamental turbulent structure research. Moscow Aviation Institute National Research University (MAI) has been building non-isothermal turbulent flow structures since 1989. An experimental facility was designed to provide gas flow heating. Experimental data of a turbulent gas flow structure in isothermal and non-isothermal conditions are presented. The frequency spectra of axial and radial velocity pulsations are based on experimental data received. The results of experimental turbulent flow research demonstrate fundamental non-isothermal processes influence on the flow structure. The main results of non-isothermal experimental research show that there are three specific zones in turbulent flow structure: wall area, maximal turbulent structure transformation and flow core. The analysis of non-isothermal conditions influence on turbulent pulsations generation and development mechanisms is presented. The results show significant distinction in turbulent flow spectra between isothermal and non-isothermal conditions. The present paper describes a method of experimental research, methodology of data processing and non-isothermal turbulent flow spectra results.

Nowadays, the global increase of energy demand and the necessity to satisfy high safety standards have led engineers and scientists to focus their efforts in order to understand and describe fundamental phenomena that are crucial for a correct design of the new generation nuclear power plants. In this framework, the present thesis aims at providing a first insight of the mechanisms of deposition of aerosol particles inside a closed geometry where relatively strong currents are present due to turbulent natural convective flows. Direct Numerical Simulations were conducted coupling high-order pseudo-spectral code with a Lagrangian particle tracker. Laminar flows were computed in two and three dimensions in order to benchmark the code with published reference data. A parametric study was performed for three different aerosol micro-size particle diameters and two super-critical Rayleigh numbers in a square cavity. An extended analysis of the turbulent flows is provided in terms of first and second order statistics, time-averaged momentum and energy budgets, and moreover, important terms appearing in the transport equations of turbulent kinetic energy and temperature variance are also briefly discussed. Furthermore, the evolution in time of particle concentration for the three different diameters is considered. The text provides information about the deposition velocity, the deposition patterns on the cavity surfaces, the influence of lift and thermophoretic forces and the fractal dimension. The same size dependent parametric study for the three different sets of micro-size particles was carried out in a fully three-dimensional closed cubic cavity for one super-critical Rayleigh number. A detailed investigation of the turbulence was performed by means of statistical quantities, signal processing and conditional averaging, in order to get a general view of the complexity of the flow and its characteristics. Further on, the sedimentation process is studied in the same way as for the two dimensional case. Finally a simple theoretical deposition model is provided in order to interpret the numerical results for the aerosol phase.

Turbulent bubbly flows have attracted a lot of attention because of their importance for many practical applications such as flows in chemical plants and nuclear power plants. Enhancement of heat transfer by bubble-induced turbulence also attracts a lot of attention from the view point of energy saving. Many studies have been conducted for the motion of bubbles and the characteristics of heat-transfer in turbulent bubbly flows. It is expected that the research on turbulent bubbly flows is accelerated by fully resolved simulations of bubble-turbulence interaction (Tryggvason et al., 2011). The characteristics of bubbly upflow strongly depend on the motions of bubbles and resulting void fraction distribution in the flow. Serizawa et al. (1975) found that the local void fraction is high near the walls and is lower in the core region of upflow in a pipe. Liu (1993) also found in the experiments of turbulent bubbly upflow in a vertical channel that the void fraction has peaks near the walls for the bubbles smaller than 5-6mm, while it has a peak in the core of the channel for the bubbles larger than 5-6mm. Lu & Tryggvason (2008) also showed in their direct numerical simulations of turbulent bubbly upflow in a vertical channel that nearly spherical bubbles tend to concentrate on the near-wall regions, while strongly deformable bubbles tend to be expelled from the near-wall regions. They also showed that the turbulence structures are changed by the motions of bubbles. The detailed mechanism of turbulence modulation due to the bubbles, however, has not been fully clarified yet. Some experimental studies have been conducted for heat-transfer enhancement by the injection of bubbles. Tamari & Nishikawa (1976) showed in their experiments of laminar natural convection heat transfer in water from a vertical plate that the heat transfer is enhanced by the injection of air bubbles. The enhancement of heat transfer by bubble injection was studied further in detail by Tokuhiro & Lykoudis (1994) and Kitagawa et al. (2008, 2010). However, the mechanism of the heat-transfer enhancement has not yet been fully clarified especially in turbulent flows. In the present study, direct numerical simulations have been conducted for turbulent bubbly upflow between two parallel heating walls in order to clarify its heat transfer characteristics. The mechanism of the heat-transfer enhancement is examined by performing simulations with different values of control parameters. The performance of the heat-transfer enhancement is discussed based on the numerical results.

Freestream turbulence in rivers is a key contributor to the flux of dissolved nutrients, carbon, and other ecologically important solutes into porewater. To advance understanding of turbulent hyporheic exchange and porewater transport, we investigate flow over and through a rough bed of spheres using large eddy simulation (LES). We apply double averaging (combined space and time averaging) to the LES results to determine the mean velocity distribution, momentum balance, and drag forces. Our simulations show large‐scale freestream structures interacting strongly with vortices generated at the surfaces of individual spheres to control turbulent momentum fluxes into the bed. The transition between turbulent flow and Darcy flow occurs over the first row of spheres, where turbulence decays rapidly and turbulent kinetic energy, Reynolds stress, and drag forces peak. Below this region, turbulence is only present in the high‐velocity flow in open pore throats. Experimental observations suggest that minimum mean porewater velocity occurs in the first open pore space below the transition region, but our results show that the minimum occurs between the first and second pore spaces. The simulation mean porewater velocities are approximately half those captured in measurements because the model resolves the entire flow continuum while measurements can access high‐velocity fluid in open pores. The high‐resolution dual time‐space averaging of the LES resolves both turbulent and mean flow features that are important to interfacial solute and particle fluxes, providing a means to include turbulent hyporheic exchange in upscaled river models, which has not been achieved to date.

Both experimental and numerical studies of a turbulent flow in a bifurcating channel are performed to characterize the dynamical behavior of the flow and its impact on the convective heat transfer on the sides of the branch. This configuration corresponds to the radial vents placed in the stator vertically to the rotor–stator air gap in the electrical machines. Indeed, our analysis focuses on the local convective heat transfer on the vents internal surface under a turbulent mass flow rate. The flow field measurements were carried out with two components particle image velocimetry (PIV) system, and the local heat transfer on the sides of the bifurcation branch was measured using an infrared thermography device. The convective heat transfer and the flow dynamics through the geometry are investigated numerically considering a three-dimensional (3D) flow. The closure system of the Navier–Stokes equations for steady and incompressible flow is based on the low-Reynolds numbers Reynolds stress model (RSM) (RSM-stress-ω). The comparison of the 3D computed results with the measurements in the xy symmetry plane is satisfactory in the vertical and horizontal channels. The numerical prediction of the secondary flow in the vertical branch was analyzed and complements the experimental results. It was particularly noticed that the accelerated flow observed at the right side of the branch's inlet allows more pronounced heat transfer comparatively to the left side. Beyond approximately 7 hydraulic diameters from the entrance of the branch, the Nusselt number curves on the two sides of the branch tend to be the same developed Nusselt number, Nud.

Numerical study of heat transfer in laminar and turbulent pipe flow with finite-size spherical particles

Experimental and numerical study about local heat transfer in a microchannel with a pin fin

In the context of flow visualization, a triple decomposition of the velocity gradient into irrotational straining flow, shear flow, and rigid body rotational flow was proposed by Kolář in 2007 [V. Kolář, “Vortex identification: New requirements and limitations,” Int. J. Heat Fluid Flow, 28, 638–652 (2007)], which has recently received renewed interest. The triple decomposition opens for a refined energy stability analysis of the Navier–Stokes equations, with implications for the mathematical analysis of the structure, computability, and regularity of turbulent flow. We here perform an energy stability analysis of turbulent incompressible flow, which suggests a scenario where at macroscopic scales, any exponentially unstable irrotational straining flow structures rapidly evolve toward linearly unstable shear flow and stable rigid body rotational flow. This scenario does not rule out irrotational straining flow close to the Kolmogorov microscales, since there viscous dissipation stabilizes the unstable flow structures. In contrast to worst case energy stability estimates, this refined stability analysis reflects the existence of stable flow structures in turbulence over extended time.

This study presents a numerical investigation of turbulent crude oil flows in circular curved pipelines, with a focus on the deposition patterns of solid particles within the fluid. Simulations were conducted using the Reynolds–Averaged Navier–Stokes equations coupled with the k–ε turbulence model to examine the influence of structural characteristics, such as bend curvature angle and bend curvature radius, on flow dynamics and particle deposition. The results reveal that the complex flow patterns generated by these geometric features significantly affect pressure gradients and particle trajectories. Specifically, the study shows that turbulent flows within bends exhibit intricate behaviors, with deposition patterns being strongly influenced by the pipeline’s geometric parameters. Sand particles, commonly present in petroleum flows, are found to be more effectively transported in pipes with larger curvature angles, while they exhibit a higher propensity to settle at smaller curvature angles and larger bend curvature radii. Furthermore, the simulations indicate that the majority of deposited particles accumulate on the inner wall immediately downstream of the elbow entrance.

A dynamic sub-grid scale model for large eddy simulation of turbulent flows in a lid-driven cubical cavity

Computational Fluid Dynamics (CFD) uses numerical methods to solve problems involving fluid flows, e.g. in automotive engineering, energy, civil engineering, and aerospace. One of these methods is the lattice Boltzmann method (LBM). Beside giving accurate results for a wide range of complex flows, the LBM lends itself to efficient implementations on massively parallel systems such as the general-purpose computing on graphics processing units (GPGPU). This work considers the cumulant LBM, which overcomes some problems of classical LBMs, such as the violation of the Galilean invariance, the spurious coupling of the degrees of freedoms, and the hyper-viscosity. In order to enable the cumulant LBM for complex CFD engineering problems, this work focuses on the discretization of the computational domain and the analysis of turbulent flows in the near-wall region.
\nThe grid generation for LBM deals with several issues such as the second order boundary definition, the free shape grid refinement, and the level wise load balancing for parallel meshes. While discretizing complex geometries, these issues have to be handled simultaneously.
\nTurbulent flows close to walls are characterized by high velocity gradients in a thin region called the boundary layer. In order to resolve numerically the boundary layer, a grid with a high resolution is required resulting in a high computational cost for the simulation. Empirical functions can be used for modelling the boundary layer, allowing to estimate the quantities at the wall even with coarse grids, and thus increasing the efficiency of the numerical simulation.
\nIn this work, a new grid generator that addresses the LBM discretization issues is implemented. It creates free shape multi-level three-dimensional meshes with second order accurate boundary definition for very complex geometries. It can be applied for generating grids of complex bodies, such as cars, porous media, and urban areas.
\nRegarding the near-wall region treatment, a new wall function is introduced. It uses local information at the boundary nodes for recovering the quantities at the wall. For generating a proper turbulence for wall bounded flows, a new set of relaxation parameters is introduced, which eliminates the spurious dependence of the error on the bulk viscosity.
\nIn conclusion, this work addresses two important aspects for enabling the cumulant LBM for complex fluid flow problems: grid generation and boundary treatment for turbulent flows.

Turbulent flow of air with variable properties in a set of regular polygonal ducts and circular tube have been numerically simulated. All the ducts have the same hydraulic diameter as their characteristic length dimension in the Reynolds number. The flow is modeled as three-dimensional and fully elliptic by using the finite volume method and the standard k-ε turbulence model is adopted. The performances of the flow and heat transfer have been thoroughly investigated. The results showed that comparatively strong secondary flow can be observed with variable properties fluid. For the regular polygonal duct, the local heat transfer coefficient along circumferential direction is not uniform and there is an appreciable reduction in the corner region. The use of hydraulic diameter for regular polygonal ducts to determine turbulent flow heat transfer from circular tube correlations leads to unacceptably large errors. Based on the simulation results, a correction factor Cϕ is proposed to ameliorate the previous correlations, and the error in the prediction of turbulent heat transfer with the new correlation is within 6%.