Hybrid Reynolds-averaged Navier–Stokes Large Eddy Simulation Research Articles

Downstream wake features of a Rood wing predicted by different turbulence models

AbstractComputational fluid dynamics is used to analyze the influence of the horseshoe vortex on the wake features of a simplified geometry representing an underwater vehicle sail (i.e. Rood wing). The sail wake features are of interest as they influence the performance of the downstream components of an underwater vehicle such as the aft appendages and propeller. This paper uses the Rood wing, a generic wing body, mounted on a flat plate as its low aspect ratio is comparable to the underwater vehicle sail and there are substantial published experimental data for validation. Two main simulation schemes were adopted in this paper, i.e. the Reynolds-averaged Navier–Stokes (RANS) and hybrid RANS–large Eddy simulation (LES) incorporating several turbulence models. Both schemes were also examined in their ability to predict the downstream wake features as the literature available to date have primarily focused only on the near-field flow features around the wing root. Three main parameters were investigated including the pressure distribution along the wing’s body, the mean streamwise velocity, and its root mean square fluctuation at three different downstream planes, two in the near field and one in the far field. Results show that the RANS and the hybrid RANS–LES models are capable of predicting the wing-body pressure distribution and the paths of the horseshoe vortex (HSV) as it moves downstream with acceptable numerical dissipation. It was found that different models provided higher accuracy when compared to the experiment depending on the downstream location of the plane. The re-normalization group k-epsilon model with enhanced wall treatment (RNG KE-EN) model captured the wake properties with the highest accuracy within the near field, while further downstream (in the far field), the scale adaptive simulation (SAS) model predicted the flow field with the highest accuracy followed by the RNGKE-EN model.

It’s About Time: Jet Interactions in an Asymmetrical Plenum

Hybrid Reynolds-Averaged Navier-Stokes–Large Eddy Simulation was used to reveal detailed flow information and timescales in an isothermal reactor cavity cooling system plenum four-jet configuration. Plenum asymmetry and nonuniformity work together to cause premature jet merging. Bulk stirring in the plenum causes lateral jet vortex shedding, strong jet-jet interactions, swirl, and premature confluence. Two dominant transient modes exist: a jet flow timescale and then a plenum circulation timescale that is nearly three orders of magnitude larger. A primary consequence is that frequencies far less than the presumed 10 Hz threshold for thermal striping are pervasive. A second result is that scale-resolved computational fluid dynamics (CFD) models (as well as experimental rigs) need hundreds of seconds of statistically stationary flow time (tens of thousands of jet timescales) to produce stationary time averages. Fluid typically arrives at positions on the laser sheet in less time than it spends at those positions fluctuating in the streamwise and lateral directions. Also, a previously undocumented, but experimentally confirmed, vortex trap was identified via CFD. Finally, two-point velocity correlation analyses demonstrated a few dozen strong correlations across positions on the laser sheet. Expected close-proximity correlations emerged, but others across larger spaces also were connected. Most of these correlated at timescales close to that of the jet.

Enhancing respiratory comfort with fan respirators: Computational analysis of carbon dioxide reduction, temperature regulation, and humidity control

Respirators provide protection from inhalation exposure to dangerous substances, such as chemicals and infectious particles, including SARS-COVID-laden droplets and aerosols. However, they are prone to exposure to stale air as masks create a microclimate influenced by the exhaled air. As a result, exhaled air from lungs accumulating in the mask produces a warm and humid environment that has a high concentration of carbon dioxide (CO2), unsuitable for re-inhalation. Fans are a favorable option for respirators to ventilate the mask and remove the stale air. This study utilized computational fluid dynamics simulation consisting of a hybrid Reynolds-averaged Navier–Stokes-large eddy simulation turbulence method to compare the inhalation flow properties for different fan locations (bottom, top, and side) with regular respirator breathing. Three mask positions, top, side, and bottom, were evaluated under two breathing cycles (approximately 9.65 s of breathing time). The results demonstrated that adding a fan respirator significantly decreased internal mask temperature, humidity, and CO2 concentration. The average CO2 concentration decreased by 87%, 67%, and 73% for locations bottom, top, and side, respectively. While the top and side fan locations enhanced the removal of the exhaled gas mixture, the bottom-fan respirator was more efficient in removing the nostril jet gas mixture and therefore provided the least barrier to respiratory function. The results provide valuable insight into the benefits of fan respirators for long-term use for reducing CO2 concentration, mask temperature, and humidity, improving wearer safety and comfort in hazardous environments, especially during the COVID-19 pandemic.

Numerical Investigation of Prediction on the NASA High-Lift Common Research Model

An assessment of a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation (HRLES) approach for prediction is presented for the NASA High-Lift Common Research Model (CRM-HL). Both free-air and wind tunnel configurations of the CRM-HL are investigated, and the results are compared to the QinetiQ wind tunnel experiments and two other numerical approaches: RANS and wall-modeled LES (WMLES). For the free-air configuration, HRLES is shown to address some of the known shortcomings with RANS and prevent inboard and outboard flow separation particularly in the region of and poststall. To achieve these improvements over RANS, LES-appropriate grids and numerical discretizations are required. It was also found that, when applying HRLES to a RANS best practice grid and numerics, the HRLES method significantly underperformed RANS. For the in-tunnel configuration, HRLES showed good agreement with the loads, surface pressure, and oil-flow photographs obtained in the experiment. HRLES was able to improve upon the RANS simulations, which showed a sharp loss of lift at the two highest angles of attack due to large-scale inboard and outboard separation on the wing, by correctly predicting the corner flow separation and showing remarkably close agreement in the flow topologies with the experiment.

Hybrid RANS–LES Simulation of Turbulent Heat Transfer in a Channel Flow With Imposed Streamwise or Spanwise Mean Temperature Gradient

Abstract Computational fluid dynamics (CFD) results are presented for turbulent flow and heat transfer in a plane channel. This study investigates an idealized fully developed planar channel flow case for which the mean velocity gradient is nonzero only in the wall-normal direction, and the mean temperature gradient is imposed to be uniform and nonzero in the streamwise or spanwise direction. The objective is to evaluate the accuracy of turbulent heat flux predictions using hybrid Reynolds-averaged Navier–Stokes (RANS)–large eddy simulation (LES) models in wall-bounded flows. Results are obtained at Prandtl number of 0.71 and Reynolds number of 180 based on wall friction velocity and channel half-height and are compared to available direct numerical simulation (DNS) data and to a well-validated RANS model (k–ω shear-stress transport (SST)). The specific hybrid RANS–LES (HRL) models investigated include delayed detached eddy simulation (DDES), improved delayed detached eddy simulation (IDDES), and dynamic hybrid RANS–LES (DHRL). The DHRL model includes both the standard formulation that has been previously documented in the literature as well as a modified version specifically developed to improve predictive capability for flows in which the mean velocity and mean temperature gradients are not closely aligned. The modification consists of using separate RANS-to-LES blending parameters in the momentum and energy equations. Results are interrogated to evaluate the performance of the three different model types and specifically to evaluate the performance of the new modified DHRL variant compared with the baseline version.

Numerical Simulation of a Three-Dimensional Axisymmetric Hill: Performance Evaluation of Hybrid RANS–LES Models

Abstract Computational fluid dynamics (CFD) simulation of flow over a three-dimensional (3D) axisymmetric hill presents a unique set of challenges for turbulence modeling. The flow past the crest of the hill is characterized by boundary layer separation, complex vortical structures, and unsteady wake flow. As a result, traditional eddy-viscosity Reynolds-averaged Navier–Stokes (RANS) models have been found to perform poorly for this benchmark test case. Recent studies have focused on the use of large eddy simulation (LES) and hybrid RANS–LES (HRL) methods to improve accuracy. In this study, several different HRL models are investigated, and results from the different models are evaluated relative to each other, to an eddy-viscosity RANS model, and to previously documented high-fidelity LESs and experimental data. Results obtained from the simulations in terms of mean flow statistics, surface pressure distribution, and turbulence characteristics are presented and discussed in detail. Results indicate that eddy-viscosity based HRL models can improve predictions over comparable eddy-viscosity based RANS models such as the shear stress transport (SST) k–ω model used in this study, but only when the development of turbulent velocity fluctuations in the separated shear layer and recirculation region are well resolved.

Effect of URANS and Hybrid RANS-Large Eddy Simulation Turbulence Models on Unsteady Turbulent Flows Inside a Side Channel Pump

Abstract In this work, the unsteady Reynolds-averaged Navier–Stokes (URANS) and three hybrid Reynolds-averaged Navier–Stokes-large eddy simulation (RANS-LES) models are employed to resolve the vortical flows in a typical single-stage side channel pump, to evaluate the suitability of these advanced turbulence models in predicting the pump hydraulic performance and unstable swirling flows. By the comparison of the overall performance, it can be observed that the results obtained by scale-adapted simulation (SAS) are closer to test data than shear stress transport (SST), detached eddy simulation (DES) and filter-based model (FBM). Simultaneously, the distribution of axial velocity on the plane near the interface is used to describe the position and intensity of internal fluid exchange between impeller and side channel. It is obvious that the intensity of mass flow exchange is strong near the inner and outer edges. Then, the vortex core region illustrates that the vortex is easily produced near the interface due to internal fluid exchange. Finally, the evolutions of circumferential in-plane vortical structures are presented to further account for the process of fluid exchange and the main vortex flows. It reveals that the recirculation flow presents a strong instability during 6–7 blade pitches as the fluid enters into the impeller and the flow is stable in downstream 7–8 blade pitches. Besides, the flow turns to be unsteady near outlet affected by the sudden change of fluid direction. This work could provide some suggestions for the choice of appropriate turbulence model in simulating strong swirling flows.

Blending of Algebraic Transition Model and Subgrid Model for Separated Transitional Flows

Accurate and efficient prediction of separated transitional flows is always a big challenge for computational fluid dynamics (CFD), especially in engineering applications. The recently proposed multiple hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation (HRLES) versions (such as DDES, SAS, and LDKM) of the transition model have shown good performance for such flows. But in terms of computational costs, they are still not saving enough. This is mainly because the above closures are driven by nine partial differential equations (PDEs) for a three-dimensional flow. Along with this, a novel hybrid closure is proposed that combines the SA model with BC algebraic transition function and the WALE subgrid model. It only consists of six PDEs and requires less than 30% CPU time per iteration as compared with the aforementioned counterparts. After calibration and basic validation for widely used zero pressure gradient flat plate, decaying isotropic turbulence test cases, and semi-infinite S809 wing, the new hybrid closure is applied to the flow over a circular cylinder from to . The mean drag results are well in line with the HRLES versions of model, but with less computational costs. More detailed simulations are also implemented for the circular cylinder at and rod-airfoil configuration at . The results further indicate that the new closure can capture the physics associated with the mean and fluctuating velocity of shedding vortex, frequency, and power spectral of vortex-induced noise.

Numerical Simulations for the Wake Prediction of a Marine Propeller in Straight-Ahead Flow and Oblique Flow

This paper presents the capability of a numerical code, isis-cfd, based on the solution of the Navier–Stokes equations, for the investigation on the hydrodynamic characteristics of a marine propeller in open water. Two propellers are investigated: the Istituto Nazionale per Studi ed Esperienze di Architectura Navale (INSEAN) E779A model in straight-ahead flow and the Potsdam Propeller Test Case (PPTC) model in oblique flow. The objectives of this study are to establish capabilities of various turbulent closures to predict the wake propeller and to predict the instability processes in the wake if it exists. Two Reynolds-averaged Navier–Stokes (RANS) models are used: the k–ω shear stress transport (SST) of Menter and an anisotropic two-equation explicit algebraic Reynolds stress model (EARSM). A hybrid RANS–large eddy simulation (LES) model is also used. Computational results for global flow quantities are discussed and compared with experimental data. These quantities are in good agreement with the measured data. The hybrid RANS–LES model allows to capture the evolution of the tip vortices. For the INSEAN E779A model, the instability of the wake is only predicted with a hybrid RANS–LES model, and the position of these instabilities is in good agreement with the experimental visualizations.

Fluid–Structure Interaction Simulation of Vortex-Induced Vibration of a Flexible Hydrofoil

Fluid–structure interaction (FSI) is investigated in this study for vortex-induced vibration (VIV) of a flexible, backward skewed hydrofoil. An in-house finite element structural solver finite element analysis nonlinear (FEANL) is tightly coupled with the open-source computational fluid dynamics (CFD) library openfoam to simulate the interaction of a flexible hydrofoil with vortical flow structures shed from a large upstream rigid cylinder. To simulate the turbulent flow at a moderate computational cost, hybrid Reynolds-averaged Navier–Stokes–large eddy simulation (RANS–LES) is used. Simulations are first performed to investigate key modeling aspects that include the influence of CFD mesh resolution and topology (structured versus unstructured mesh), time-step size, and turbulence model (delayed-detached-eddy-simulation and k−ω shear stress transport-scale adaptive simulation). Final FSI simulations are then performed and compared against experimental data acquired from the Penn State-ARL 12 in water tunnel at two flow conditions, 2.5 m/s and 3.0 m/s, corresponding to Reynolds numbers of 153,000 and 184,000 (based on the cylinder diameter), respectively. Comparisons of the hydrofoil tip-deflections, reaction forces, and velocity fields (contours and profiles) show reasonable agreement between the tightly coupled FSI simulations and experiments. The primary motivation of this study is to assess the capability of a tightly coupled FSI approach to model such a problem and to provide modeling guidance for future FSI simulations of rotating propellers in crashback (reverse propeller operation).

Inflow Turbulence Generation Methods

Research activities on inflow turbulence generation methods have been vigorous over the past quarter century, accompanying advances in eddy-resolving computations of spatially developing turbulent flows with direct numerical simulation, large-eddy simulation (LES), and hybrid Reynolds-averaged Navier-Stokes–LES. The weak recycling method, rooted in scaling arguments on the canonical incompressible boundary layer, has been applied to supersonic boundary layer, rough surface boundary layer, and microscale urban canopy LES coupled with mesoscale numerical weather forecasting. Synthetic methods, originating from analytical approximation to homogeneous isotropic turbulence, have branched out into several robust methods, including the synthetic random Fourier method, synthetic digital filtering method, synthetic coherent eddy method, and synthetic volume forcing method. This article reviews major progress in inflow turbulence generation methods with an emphasis on fundamental ideas, key milestones, representative applications, and critical issues. Directions for future research in the field are also highlighted.

Unsteady Computational Fluid Dynamics Investigation of Effusion Cooling Process in a Lean Burn Aero-Engine Combustor

This work describes the main findings of a computational fluid dynamics (CFD) analysis intended to accurately investigate the flow field and wall heat transfer as a result of the mutual interaction between a swirling flow generated by a lean burn injection system and a slot–effusion liner cooling system. In order to overcome some limitations of Reynolds-averaged Navier–Stokes (RANS) approach, the simulations were performed with shear stress transport (SST)–scale-adaptive simulation (SAS), a hybrid RANS–large eddy simulation (LES) model. Moreover, the significant computational effort due to the presence of more than 600 effusion holes was limited exploiting two different modeling strategies: a homogeneous model based on the application of uniform boundary conditions on both aspiration and injection sides, and another solution that provides a coolant injection through point mass sources within a single cell. CFD findings were compared to experimental results coming from an investigation carried out on a three-sector linear rig. The comparison pointed out that advanced modeling strategies, i.e., based on discrete mass sources, are able to reproduce the effects of mainstream–coolant interactions on convective heat loads. By validating the approach through a benchmark against time-averaged quantities, the transient data acquired were examined in order to better understand the unsteady behavior of the thermal load through a statistical analysis, providing useful information with a design perspective.

Impact of Swirl Flow on Combustor Liner Heat Transfer and Cooling: A Numerical Investigation With Hybrid Reynolds-Averaged Navier–Stokes Large Eddy Simulation Models

This paper reports the main findings of a numerical investigation aimed at characterizing the flow field and the wall heat transfer resulting from the interaction of a swirling flow provided by lean-burn injectors and a slot cooling system, which generates film cooling in the first part of the combustor liner. In order to overcome some well-known limitations of Reynolds-averaged Navier–Stokes (RANS) approach, e.g., the underestimation of mixing, the simulations were performed with hybrid RANS–large eddy simulation (LES) models, namely, scale-adaptive simulation (SAS)–shear stress transport (SST) and detached eddy simulation (DES)–SST, which are proving to be a viable approach to resolve the main structures of the flow field. The numerical results were compared to experimental data obtained on a nonreactive three-sector planar rig developed in the context of the EU project LEMCOTEC. The analysis of the flow field has highlighted a generally good agreement against particle image velocimetry (PIV) measurements, especially for the SAS–SST model, whereas DES–SST returns some discrepancies in the opening angle of the swirling flow, altering the location of the corner vortex. Also the assessment in terms of Nu/Nu0 distribution confirms the overall accuracy of SAS–SST, where a constant overprediction in the magnitude of the heat transfer is shown by DES–SST, even though potential improvements with mesh refinement are pointed out.

Large eddy simulation of turbine internal cooling ducts

Large-Eddy Simulation (LES) and hybrid Reynolds-averaged Navier–Stokes–LES (RANS–LES) methods are applied to a turbine blade ribbed internal duct with a 180° bend containing 24 pairs of ribs. Flow and heat transfer predictions are compared with experimental data and found to be in agreement. The choice of LES model is found to be of minor importance as the flow is dominated by large geometric scale structures. This is in contrast to several linear and nonlinear RANS models, which display turbulence model sensitivity. For LES, the influence of inlet turbulence is also tested and has a minor impact due to the strong turbulence generated by the ribs. Large scale turbulent motions destroy any classical boundary layer reducing near wall grid requirements. The wake-type flow structure makes this and similar flows nearly Reynolds number independent, allowing a range of flows to be studied at similar cost. Hence LES is a relatively cheap method for obtaining accurate heat transfer predictions in these types of flows.

LES of heat transfer in electronics

Numerical methods based on the Reynolds Averaged Navier–Stokes (RANS) and Large Eddy Simulation (LES) equations are applied to the thermal prediction of flows representative of those found in and around electronics systems and components. Low Reynolds number flows through a heated ribbed channel, around a heated cube and within a complex electronics system case are investigated using linear and nonlinear LES models, hybrid RANS–LES and RANS–Numerical-LES (RANS–NLES) methods. Flow and heat transfer predictions using these techniques are in good agreement with each other and experimental data for a range of grid resolutions. Using second order central differences, the RANS–NLES method performs well for all simulations.

Detached-Eddy Simulations of Vortex Breakdown over a 70-Degree Delta Wing

An understanding of the vortical structures that comprise the vortical flowfield around slender bodies is essential for the development of highly maneuverable and high angle-of-attack flight. This is primarily due to the physical limits this phenomenon imposes on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current computational fluid dynamics methods in the high Reynolds number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. One of the goals of this paper is to demonstrate the ability of detached-eddy simulation, a hybrid Reynolds-averaged Navier―Stokes large-eddy simulation method, to accurately predict the vortical flowfield over a slender delta wing at Reynolds numbers above 1 × 10 6 . Although detached-eddy simulation successfully predicted the location of the vortex breakdown phenomenon in previous work, the goal of the current effort is to further validate the method with additional experimental data from the Office National d'Etudes et Recherches Aerospatiales, such as surface pressures and turbulent kinetic energy in the vortex core. The effect of grid density and an adaptive mesh refinement technique is also assessed through comparisons with the experiment. Detailed wind-tunnel geometry, such as tunnel walls and the sting mount system, are simulated and found to make a measurable difference. Finally, modeling the laminar-to-turbulent transition is demonstrated to have a significant effect on the vortical flowfield.

Zonal-Detached-Eddy Simulation of the Flow Around a High-Lift Configuration

A zonal hybrid Reynolds-averaged Navier-Stokes large-eddy simulation (RANS/LES) approach, called zonal-DES, used to handle a two-dimensional high-lift configuration with deployed slat and flap is presented. This method allows to reduce significantly the cost of an accurate numerical prediction of the unsteady flow around wings compared to a complete LES. Some issues concerning grid generation as well as the use of zonal-detached-eddy simulation for a multi-element airfoil are discussed. The basic planar grid has 250,000 points and the finest spanwise grid has 31 points with Az/c = 0.002. The effort is geared toward detailed comparison of the numerical results with the Europiv2 experimental particle image velocimetry data including both mean and fluctuating properties of the velocity field (Arnott and al)