Large Eddy Simulation Approach Research Articles

On the spatio-temporal dynamics of cavitating turbulent shear flow over a microscale backward-facing step: A numerical study

The influence of cavitation on the mean characteristics and unsteady behavior of turbulent separated flows was comprehensively investigated in this study over a microscale backward-facing configuration at the Reynolds number (ReD) of 7440. The computational approach took both compressibility and finite mass transfer (Thermodynamic non-equilibrium) into account, to accurately capture the effects of shock waves, as well as to capture baroclinic phenomena on vortex dynamics within the turbulent separated flow. The compressibility effects were handled by using appropriate equation of states for each phase and for the mixture. Phase-change was considered through a transport equation for the vapor volume fraction, allowing for finite mass transfer contributions. Additionally, a wall adaptive large eddy simulation (LES) approach was utilized for simulating turbulent structures and their effects. The findings reveal that vapor development diminishes the mean growth rate of the shear layer and delays its reattachment to a longer distance from the step. Moreover, analysis of Reynolds normal and shear stresses, as well as the root mean square (RMS) of pressure fluctuations, demonstrates that the formation and collapse of vapor packets significantly influence turbulence decay and production in the second half of the shear layer and reattachment. It was also observed that both mean pressure and pressure fluctuations increased in vicinity of the reattachment region when cavitation was present, which was attributed to the condensation and collapse events. Spectral analysis further indicates the emergence of two dominant low frequency modes, linked to the displacement of the reattachment point. In the presence of cavitation, the frequencies associated with dominant Power Spectral Densities (PSDs) were smaller than those in the absence of cavitation. Additionally, each of these low frequencies corresponded to a specific vapor transport mechanism within the Turbulent Separation Bubble (TSB). Furthermore, it is shown that cavitation leads to a significantly higher spectral energy of high frequency fluctuations within the reattachment region in comparison to the condition where cavitation is absent. This can be attributed to the frequent collapse of bubbles in this region. At the end, we employed Spectral Proper Orthogonal Decomposition (SPOD) for modal analysis. This method offers valuable insights into the coherent structures and associated frequencies that arise in both the presence and absence of cavitation, which provides a deeper understanding of the effect of cavitation on the coherent structures and their dynamics.

A detached-eddy simulation study on assessing the impact of extreme wind conditions on load and wake characteristics of a horizontal-axis wind turbine

This study utilizes improved delayed detached-eddy simulation (IDDES) to investigate the impact of extreme wind conditions on a horizontal-axis wind turbine, as specified by the IEC-61400 standard. The employed IDDES has a hybrid formulation, where SST K-ω is used to model the turbulence in the boundary layers and large-eddy simulation (LES) is used to resolve turbulence in separated flow regions and the wake area. It also allows for switching to a wall-modeled LES approach, depending on the incoming turbulence content. The research focuses on extreme operating gust (EOG), extreme horizontal shear (EHS), and extreme vertical shear (EVS) conditions at two Tip Speed Ratios (TSR). The loadings and wake characteristics are compared to uniform inlet velocity results to highlight the effects of extreme winds. The novelty of this research lies in its comprehensive investigation of wake behavior under extreme wind conditions, providing valuable insights for improving wind farm layout design and reliability in challenging operational environments. The analysis shows that the impact of the TSR on wind turbine loadings during EOG, EVS, and EHS events is significant. During EOG events for a turbine operating at low TSR, the thrust force and bending moment reach 2.38 and 2.57 times their respective values in uniform inflow conditions, while at the optimal TSR, the thrust force and bending moment increase by only 25%. In all extreme wind cases, there is an escalation in turbulence within the wake, and wake meandering is observed in the EHS and EVS cases.

On the Continuous Adjoint of Prominent Explicit Local Eddy Viscosity-based Large Eddy Simulation Approaches for Incompressible Flows

On the Continuous Adjoint of Prominent Explicit Local Eddy Viscosity-based Large Eddy Simulation Approaches for Incompressible Flows

Modelling Turbulent Flows near Coastal Structures Using Various Turbulence Models

Introduction. The reduction in beach width due to erosion is a significant issue that can either be mitigated or exacerbated by coastal protection structures. Modelling breaking waves near the coast and around coastal structures can be used to determine their impact on the dynamics of the coastal zone. The objective of this study is to model and analyze the dynamics of turbulent structures around a single breakwater, obtained using two turbulence modelling schemes: RANS and LES.Materials and Methods. Turbulence induced by breaking waves was investigated. The modelling was based on bathymetric measurements conducted along the Azov Sea coast and a three-dimensional wave hydrodynamics model supplemented with various turbulence calculation configurations.Results. Modelling results of wave processes generating turbulent flows in the presence of coastal protection structures using different turbulence models were obtained. Results obtained based on Reynolds-averaged Navier-Stokes (RANS) equations are compared with the results of Large Eddy Simulation (LES) approach with Smagorinsky dynamic subgrid-scale model (DSM).Discussion and Conclusions. The results showed that wave heights simulated by LES were higher than those simulated by RANS in the front and leeward regions of the coastal protection structure and were lower in its upper part. Thus, according to LES, a greater amount of wave energy was preserved after passing over the breakwater. Velocity vectors of the water medium showed the formation of a vortex when LES was used, whereas no evidence of such turbulent vortices was detected in the case of RANS, confirming the better performance of LES for turbulence modelling in the coastal zone. According to the presented results, LES is the preferred tool for generating turbulence under incoming wave conditions in engineering practices.

Computational study of thermal and fluid mixing behaviour of a dual jet flow using LES approach

The primary goal of any thermal system which demands efficient cooling is to improve heat transmission. For a turbulent jet, this condition becomes far more significant. The present work has employed large-eddy simulation (LES) for an offset ratio of 2.0 to investigate the transfer in heat and flow characteristics of a dual jet. An isothermal boundary wall condition is considered for a Reynolds number of 7500. The present findings are evaluated by comparing with the published experimental and numerical studies on wall jets. A comparative analysis here exhibits the profiles of Reynolds normal stresses, Reynolds shear stresses, turbulence kinetic energy, Nusselt number and turbulent heat fluxes. A comprehensive investigation of the mixing and flow properties of the dual jet is presented by using the Q-criterion of the iso-surface for mean velocity, pressure and vorticity. The present outcomes show that an efficient thermal transmission from the wall occurs in the recirculation region. The magnitudes of the profiles of Reynolds normal stresses, Reynolds shear stresses, r.m.s. temperature and turbulent heat fluxes are observed to be low in the combined region. Roller-like structure formation and near-wall eddies are noticed which spread in the combined region indicating large heat loss due to the turbulent flow.

Large eddy simulations of a turbulent flow with hybrid nanofluid subjected to symmetric and asymmetric heating

The present study includes a comprehensive numerical analysis of the usage of nanofluids inside a thermal receiver to assess the possibility of enhancing the efficiency of this device. All simulations are performed on a small portion of a thermal receiver, similar to a bi-periodic channel subjected to symmetric and asymmetric heating. To generate conventional and hybrid nanofluids, nanoparticles of Al2O3 and Al2O3-Cu are dispersed in the base fluid H2O. The values of nanoparticles' diameter is 20nm and volume fraction of the nanoparticles is 2%. The Reynolds numbers used in this study are 12 000 and 18 000, which respectively correspond to mean friction Reynolds numbers of 140 and 260. The thermal Large Eddy Simulation (T-LES) approach is used to simulate the fluid flow and heat transfer inside the computational domain. The results are compared with those of prior numerical and experimental studies to validate the method and the numerical setup. A high level of agreement is achieved. In the case of symmetric and asymmetric heating, the dimensionless findings reveal that adding nanoparticles to the base fluid has a negligible impact on the velocity fluctuations. Nanoparticles make the base fluid denser and more viscous but also more conductive, allowing it to minimize kinetic energy while boosting heat transfer rate. Results also show that using hybrid nanofluids contributes better to improving the heat transfer rate compared to the base fluid and the conventional nanofluid. Introducing nanoparticles into the base fluid decreases its temperatures slightly, especially when Al2O3-Cu nanoparticles are used instead of Al2O3 nanoparticles. In the asymmetric case, water's turbulent heat flux values are greater than nanofluids and hybrid nanofluid, implying that more conductive and viscous liquids have the weakest turbulent heat flux. In the circumstances of symmetric and asymmetric heating, the variation of the velocity fluctuations is almost the same, but the behavior of the temperature–velocity correlation is entirely different. Finally, results show that symmetric heating leads to a higher heat transfer rate than asymmetric heating. The suggested research's findings may be utilized to promote the usage of nanofluids in Concentrated Solar Power (CSP) systems.

Investigation of cavitation shedding mechanisms including reentrant jet and shock wave by Eulerian–Lagrangian multiscale simulation

Sheet-to-cloud cavitation in a Venturi pipe is investigated experimentally and numerically in this work. The multiscale vapor structures are simulated by Eulerian–Lagrangian multiscale modeling. The volume of fluid method with adaptive mesh refinement is employed to capture the macroscale cavity features, utilizing the large eddy simulation approach. The results of mesh sensitivity study demonstrate that second-order refinement is capable of capturing phase details and cavitation shedding behaviors. A comparison with experimental results reveals the mechanisms of different types of cavitation shedding induced by reentrant jet and shock wave. Taking microscale bubbles into account using the Lagrangian approach and achieving Eulerian–Lagrangian coupling, the multiscale cavitation features, including the morphological evolution of large cavities and characteristics of microbubbles, are accurately reproduced. The evolution features of microbubbles during pressure wave or condensation shock propagation are well recognized by different changing rates. The results indicate that shedding mechanisms in Venturi pipe cavitation can be categorized into three types: reentrant jet, condensation shock, and pressure wave. In the three conditions investigated in this work, shedding cycles are found to be mainly induced by the reentrant jet (σ = 1.03), reentrant jet, condensation shock, and pressure wave (σ = 0.64), and condensation shock (σ = 0.39), respectively, with a decrease in the cavitation number.

A330-300 Wake Encounter by ARJ21 Aircraft

Today, aviation has grown significantly in importance. However, the challenge of flight delays has become increasingly severe due to the need for safe separation between aircraft to mitigate wake turbulence effects. The primary emphasis of this investigation resides in elucidating the evolutionary attributes of wake vortices in homogeneous isotropy turbulence. The large eddy simulation (LES) method is used to scrutinize the dynamic evolution of wake vortices engendered by an A333 aircraft in the atmospheric milieu and assess its ramifications on the ARJ21 aircraft. The research endeavor commences by formulating an LES methodology for the evolution of aircraft wake vortices, integrating adaptive grid technology to reduce the necessary grid volume significantly. This approach enables the implementation of axial and non-axial grid adaptive refinement, leading to more accurate simulations of both axial and non-axial vortices. Numerical simulations are conducted using the LES approach to scrutinize three distinct rates of turbulence dissipation amidst the ambient atmospheric turbulence, and the results are juxtaposed with Lidar measurements (Wind3D 6000 LiDAR) of wake vortices acquired at Chengdu Shuangliu International Airport (CTU). Subsequently, the rolling moment of the following aircraft is calculated, and three-dimensional hazard zones are determined for the A333. It is found that during the approach phase, the wake turbulence separation minima for an ARJ21 (CAT-F) following an A333 (CAT-B) is 3.35 NM, which represents a reduction of approximately 33% compared to ICAO RECAT (Wake Turbulence Re-categorization). The findings validate the dependability of the fine-grained mesh used in the vortex core region, engendered through the adaptive grid method, which proficiently captures the Crow instability and the interconnected phenomena of vortices in the numerical examination of aircraft wake. The safety of wake encounters primarily depends on the magnitude of environmental turbulence and the development of structural instability in wake vortices.

Flow analysis through a randomly packed pebble-bed geometry using computational fluid dynamics

This study presents a comprehensive analysis of the flow behavior in packed pebble-bed reactors using computational fluid dynamics (CFD) simulations. The pebble-bed geometry corresponds to an experimental facility located at the Texas A&M Thermal-Hydraulics Research Laboratory. The unsteady Reynolds-averaged Navier–Stokes (URANS) k−ω shear stress transport (SST) and the large eddy simulation (LES) approaches were selected to model the turbulence at different Reynolds numbers. The numerical models were first validated by comparing the pressure drop results obtained from the simulations against established correlations, finding the simulation predictions in accurate agreement. Secondly, the velocity first-order statistics from the URANS k−ω SST and LES calculations were also contrasted with the available experimental particle image velocimetry data to validate the numerical models. Results were found in reasonable agreement as the mean absolute error achieved values smaller than 10% of the inlet velocity for most of the analyzed velocity profiles. A comprehensive turbulence characterization was performed, including second-order statistics, Reynolds stress anisotropy, and turbulent kinetic energy production. The proper orthogonal decomposition of the fluctuating velocity was examined in the current flow domain. The turbulence characterization revealed the complex nature of turbulence in packed pebble-bed geometries, which is further complicated by the presence of an enclosing wall. Overall, the findings of this study provide a solid foundation for the development of more accurate CFD-based methodologies for predicting the behavior of flow through packed pebble-bed reactors.

A grid-independent dispersion approach for large-eddy simulation of spray

This study proposes a subgrid-scale (SGS) dispersion approach for large-eddy simulation (LES) of spray. Rather than associating the turbulence correlation timescale used in the dispersion approach with the computational grid, we put forward an approximation of this timescale proportional to the Kolmogorov timescale. The Kolmogorov scale kinetic energy is added to the variance of the stochastic part of the SGS dispersion velocity to account for the effects of micro-scale flows on droplets. The proposed approach supports the notion of grid-independent LES and is illustrated for two benchmark Engine Combustion Network's Spray-A cases, including a vaporizing spray case and a non-vaporizing spray case. We investigate the performance of the approach in LESs of two cases by comparing it with other dispersion approaches. In particular, the investigation mainly covers spray structures, vapor and liquid penetrations, and mass fraction profiles. Results obtained using the proposed approach shows the statistics of the simulated spray cases that exhibit both good agreement with measurements and a noticeable improvement over simulations based on other tested approaches. The approach also yields more consistent results under different grid resolutions, which shows its ability to achieve LESs in non-uniform grids.

LES investigation of the wavy leading edge effect on cavitation noise

This paper investigates the noise reduction performance of biomimetic hydrofoils with wavy leading edge and the corresponding mechanisms. We employ Large Eddy Simulation (LES) approach and permeable Ffowcs Williams-Hawkings (PFW–H) method to predict cavitation noise around the baseline and biomimetic hydrofoils. The results show that the wavy leading edge can effectively reduce the high-frequency noise, but has little effect on the low-frequency noise. Further analyses and discussions deal with the noise reduction mechanisms. The main source for the low-frequency noise is the cavity volume acceleration, while the wavy leading edge has little effect on it. The high-frequency noise sources, related to the surface pressure fluctuations and the turbulence characteristics, are significantly suppressed by the wavy leading edge, thus decreasing the high-frequency noise intensity. Our investigation indicates that the wavy leading edge has great prospects for cavitation noise reduction technique.

A dynamic wall modeling approach for large eddy simulation of offshore wind farms in realistic oceanic conditions

Due to the multitude of scales present in realistic oceanic conditions, resolving the surface stress is computationally intensive, motivating modeling approaches. In this work, a dynamic wave drag model is developed for large eddy simulation (LES) to quantify the effects of multiscale dynamically rough surfaces on the atmospheric boundary layer. The waves are vertically unresolved, and the total drag due to the horizontally resolved portion of the wave spectrum is computed through a superposition of the force from each mode. As LES can only resolve the horizontal wind–wave interactions to the filter scale Δ, the effects of the horizontally unresolved, subfilter waves are modeled by specifying a roughness length scale characterizing the unresolved wave energy spectrum. This subfilter roughness is set proportional to the subfilter root mean square of the wave height distribution, and the constant of proportionality is evaluated dynamically during the simulation based on the assumption that the total drag force at the wave surface is independent of the filter scale. The dynamic approach is used to simulate the airflow over a spectrum of moving waves, and the results are validated against high-fidelity phase-resolved simulations. The dynamic approach combined with the wave spectrum drag model is then used to study flow through a fixed-bottom offshore wind farm array, equivalent to an infinite farm, with each turbine represented using an actuator disk model. The dynamic model accurately adapts to the changing velocity field and accurately predicts the mean velocity profiles and power produced from the offshore wind farm. Furthermore, the effect of the wind–wave interactions on the mean velocity profiles, power production, and kinetic energy budget is quantified.

On the effectiveness of Reynolds-averaged and subgrid scale models in predicting flows inside car cabins

The aim of the present study is to analyze the performances of unsteady Reynolds-averaged Navier–Stokes (URANS) and large eddy simulation (LES) approaches in predicting the airflow patterns inside car cabins and to give insight in the design of computational fluid dynamics simulations of a real car cabin. For this purpose, one eddy viscosity-based turbulence model (shear stress transport k–ω) and two subgrid scale models (wall-adapting local eddy-viscosity and dynamic kinetic energy) were tested, and numerical results were compared with particle image velocimetry measurements carried out on a commercial car. The URANS model exhibited great accuracy in predicting the mean flow behavior and was appreciably outperformed by the LES models only far from the inlet sections. For this reason, it was deemed suitable for conducting further analyses, aimed at characterizing the airflow patterns in winter and summer conditions and performing a thermal comfort analysis. The thermal regime was found to have a very little effect on the air flow patterns, once the quasi-steady state regime is achieved; in fact, both in winter and in summer, the temperature field is fairly uniform within the car cabin, making the contribution of buoyancy negligible and velocity fields to be very similar in the two seasons. Findings also reveal that thermal comfort sensation can be different for passengers sharing the same car but sitting on different seats; this aspect should be considered when designing and operating the ventilation system, since the minimum comfort requirements should be met for all the occupants.

Assessment of a Flow-dependent Subgrid Characteristic Length for Large-Eddy Simulation on Anisotropic Grids

This paper presents the latest results of a long track development activity in the context of low-dissipative finite volume method for compressible flows. Specifically, here we focus our attention on the Large-Eddy Simulation (LES) approach which can be considered a good candidate for turbulent flow simulations over the next decades. One of the key ingredients of LES models is the subgrid length scale which is typically evaluated based on the local mesh size. This standard approach suffers from loss of accuracy on anisotropic grids that are commonly employed to obtain sufficient wall-normal resolution, whilst keeping the total cell count to a minimum. In order to avoid this issue, we assess the effectiveness of a velocity-gradient-based length scale, referred to as least square length (LSQ) [1]. In this paper, we present for the first time results obtained with the LSQ length scale in the context of compressible LES. The superiority of the LSQ approach over the standard cubic-root length scale is demonstrated in terms of accuracy and overall time to solution.

Acceleration of complex high-performance computing ensemble simulations with super-resolution-based subfilter models

Direct numerical simulation (DNS) of fluid flow problems has been one of the most important applications of high-performance computing (HPC) in the last decades. For example, turbulent flows require the simultaneous resolution of multiple spatial and temporal scales as all scales are coupled, resulting in very large simulations with enormous degrees of freedom. Another example is reactive flows, which typically result in a large system of coupled differential equations and multiple transport equations that must be solved simultaneously. In addition, many flows exhibit chaotic behavior, meaning that only statistical ensembles of results can be compared, further increasing the computational time. In this work, a combined HPC/deep learning (DL) workflow is presented that drastically reduces the overall computational time required while still providing acceptable accuracy.Traditionally, all the simulations required to compute ensemble statistics are performed using expensive DNS. The idea behind the combined HPC/DL workflow is to reduce the number of expensive DNSs by developing a DL-assisted large-eddy simulation (LES) approach that uses a sophisticated DL network, called PIESRGAN, as a subfilter model for all unclosed terms and is accurate enough to substitute DNSs. The remaining DNSs are thus used in two ways: first, as data contributing to the ensemble statistics, and second, as data used to train the DL network. It was found that in many cases two remaining DNSs are sufficient for training the LES approach. The cost of the DL-supported LES is usually more than one order of magnitude cheaper than the DNS, which drastically speeds up the workflow, even considering the overhead for training the DL network.

Investigations into the Approaches of Computational Fluid Dynamics for Flow-Excited Resonator Helmholtz Modeling within Verification on a Laboratory Benchmark

This paper presents the results of a study of self-sustained processes excited in a Helmholtz resonator after a flow over its orifice. A comparative analysis of various approaches to the numerical modeling of this problem was carried out, taking into account both the requirements for achieving the required accuracy and taking into account the resource greediness of calculations, the results of which were verified by comparison with data obtained during a special experiment. The configuration with a spherical resonator with a natural frequency of 260 Hz and an orifice diameter (about 5 cm) in an air flow with a speed of 6 to 14 m/s was considered. A comparison of the calculation results with data obtained in experiments carried out in the wind tunnel demonstrated that the accuracy of calculations of the characteristics of the self-sustained mode using the simplest URANS class model tends to the accuracy of calculations within the large eddy simulation approach formulated in the WMLES model. At the same time, when using WMLES, it is possible to better reproduce the background level of pulsations. From the point of view of resource greediness, expressed in the number of core hours spent obtaining a solution, both models of the turbulence turned out to be almost equivalent when using the same grid models.

Analysis of the local equivalence ratio under fuel-feeding excitations in a lean gas turbine model combustor: Large eddy simulation approach

The synchronization process between the fuel-feeding excitation (ff) and the local equivalence ratio (ϕlocal) is a key issue that has not been well studied numerically under lean conditions. Therefore, in this study, a large eddy simulation (LES) with a wall-adapting local eddy-viscosity (WALE) subgrid model was utilized to analyze the interaction between various fuel-feeding excitations and the local equivalence ratio in a lean partially premixed gas turbine model combustor. The model combustor was simulated using five values of fuel-feeding excitation: 0 Hz, 15 Hz, 50 Hz, 100 Hz and 200 Hz. The results demonstrated that the complex interaction between the swirling flow and the fuel-feeding excitation affected the flow structure. These changes enhanced the excitation of toroidal counter-rotating vortices in the region between the fuel jet exit and shear layers, which improved the homogeneity of ϕlocal inside the inner recirculation zone (IRZ). Furthermore, the fuel-feeding excitation modulated the shear layer thickness by increasing the rolling-up of the instantaneous counter-rotating vortices, leading to spatial regularity for ϕlocal inside the IRZ. The coupling process between the fuel-feeding excitation and the precessing vortex core (PVC) significantly increased the fluctuations of ϕlocal around the fuel jet exit and the shear layer region. An analysis of the probability distribution showed a nonlinear relationship between ff and ϕlocal, where ϕlocal was high when ff = 50 Hz, enhancing the uniformity of ϕlocal inside the IRZ. However, ϕlocal was more spread radially when ff = 0 Hz in the downstream direction. The merging of vortex shedding and the PVC increasingly excited the self-oscillation of ϕlocal, while the fuel-feeding excitation dominated the oscillation of the counter-rotating vortices by increasing the subharmonic vortices. Moreover, the fuel-feeding excitation controlled and organized the small vortical structures, leading to stabilization of ϕlocal when ff = 50 Hz, 100 Hz, and 200 Hz.

Investigation of Sub-Grid Scale Turbulence-Radiation Interaction Effects on Turbulence Energy Transport and Varying Thermophysical Properties Using Large Eddy Simulation

Abstract The main objective of this article is to investigate sub-grid scale turbulence–radiation interaction (SGS TRI) effects on SGS turbulence kinetic energy (TKE) fluctuations and varying thermophysical properties in a partially premixed combustion system for a laboratory-piloted methane/air flame. The large eddy simulation approach is employed to simulate the turbulence of the compressible reactive flow. SGS quantities, including turbulent stress and fluxes of enthalpy and species in the sub-grid scale, are computed using the standard Smagorinsky–Lilly model. The radiative transfer equation is modeled by applying the spherical harmonic P1 approximation by considering the radiative heat source related to the SGS TRI contribution. Optically thin fluctuation approximation is utilized to simplify the radiative absorption term. A chemical reaction mechanism comprising 41 steps and 16 species is applied to model methane–air mixture combustion. Diffusion flamelet-generated manifolds are employed to govern the species transport equation. About 87% of TKE is resolved by applying the finest grid consisting of 1,822,580 cells. Impacts of SGS TRI on the spatially filtered density, eddy viscosity, SGS velocity and TKE, overall radiative emission, RMS temperature fluctuations, and nitrogen monoxide (NO) formation are studied. The results reveal that considering SGS TRI in the simulation leads to remarkable discrepancies, particularly in SGS velocity and TKE by 6.70% and 7.40%, respectively. Meanwhile, SGS density and eddy viscosity deviate negligibly in the presence of SGS TRI. Also, the filtered mass fraction of NO reduces up to 17.54% on average by considering TRI.

The effect of segmentation depth in a helical fin tube bundle on the anisotropic turbulence state

The large eddy simulation approach was used to perform a numerical simulation of a flow around a helical segmented-fin tube bundle. This work focuses on the study of turbulence and the effect of segment depth on it. The simulation was conducted in an area away from the boundaries of the tube bundle, where the flow is fully-developed, and the use of periodic boundary conditions is possible. The computer-aided design of the helical segmented-fin tube geometry is incorporated into the computational grid from the immersed boundary technique. The Reynolds stress tensor was used to analyze the anisotropic state of turbulence. This flow is characterized by an anisotropic state generated by a flow with a preferential direction. A transfer of momentum in the inter-segment space and contiguous zones was observed. This intersegment flow generates a less anisotropic state of turbulence. This study aimed to understand a possible transformation of heat transfer with marginal geometrical changes and pressure drop increases.

Numerical Simulation of Turbulent Structures Inside Internal Combustion Engines Using Large Eddy Simulation Method

<div>Using two subgrid-scale models of Smagorinsky and its dynamic version, large eddy simulation (LES) approach is applied to develop a 3D computer code simulating the in-cylinder flow during intake and compression strokes in an engine geometry consisting of a pancake-shaped piston with a fixed valve. The results are compared with corresponding experimental data and a standard K-Ɛ turbulence model. LES results generally show better agreement with available experimental data suggesting that LES with dynamic subgrid-scale model is more effective method for accurately predicting the in-cylinder flow field. Representative Fiat engine equipped with moving valve and piston bowl is analyzed as the second case to assess the capability of the method to handle complex geometries and impacts of geometrical parameters such as shape and position of piston bowl together with swirling intake flow pattern on both turbulent structure of in-cylinder flow and engine performance using dynamic version of LES approach over a curvilinear computational meshed geometry. Results indicate that presence of piston bowl leads to eye-catching increment in both turbulent kinematic energy and tumble ratio amounts at the end of compression stroke by around 29% and 33%, respectively. The optimum swirl ratio found to be 4, leading to 67.9% increment in pre-injection turbulent kinetic energy in comparison with non-swirl pattern, whereas 20% eccentricity of cylinder bowl just led to 2% improvement in the pre-injection turbulent kinetic energy, which is not recommended due to small impact compared to noticeable manufacturing expenditures.</div>