Detached Eddy Simulation Shear-stress Transport Research Articles

Detached eddy simulation of turbulent flow fields over steep hilly terrain

The verification and validation (V&V) of multi-fidelity turbulence models is crucial for the efficient and reliable industrial application of computational fluid dynamics (CFD). This study aims to evaluate the performance of detached eddy simulation (DES) modeling approaches embedded with various unsteady Reynolds Averaged Navier-Stokes (URANS) models in simulating the turbulent flow fields over a steep-sloped hill. And the high-fidelity numerical methodology based on the DES model was proposed and analyzed in the context of URANS modes, mesh resolution and sampling duration. In comparison with the experimental data, the shear stress transport (SST) k-ω based DDES (Delayed Detached eddy simulation) turbulence model (DDES SST k-ω) could better predict the profiles of mean velocity and turbulence fluctuation, while the Spalart-Allmaras (S-A) based DES and DDES turbulence models show its stronger capability to reproduce the spectral characteristics. Additionally, in contrast to DES S-A and DDES S-A, a larger time-averaged separation bubble was predicted by DES SST k-ω and DDES SST k-ω. Moreover, the instantaneous flow patterns such as the formation and evolution of turbulent eddies in the hill wake could be reasonably reproduced by DES S-A and DDES S-A. Furthermore, the numerical results obtained from DES models were sensitive to URANS modes and vertical mesh resolution but trivially affected by the increase in sampling time after reaching the statistical convergence.

Study on vortex characteristics and velocity distribution in small rotary engine

Abstract In this paper, vortex characteristics and velocity distribution in the chamber region were studied by numerical method. The three-dimensional moving grid and Improved Delayed Detached Eddy Simulation Shear-Stress Transport model which is a hybrid RANS/LES model were used to predict evolution processes of streamlines and velocity change in small rotary engine. The simulation results show that vortex has always existed in intake process and has gone through three stages, including generation, brakedown, regeneration. The generation is mainly due to intake charge. The brakedown and regeneration is because of the shear and friction of the rotor face. Vortex radius in the leading gradually increases and in the trailing decreases in intake stroke from 490 °C BTDC to 390 °C BTDC. And the position of vortex core in the leading has no obvious change and in the trailing moves towards the trailing apex along the rotor wall. In the intake process, the airflow velocity in the middle of chamber is much higher than the one at both sides, even up to hundreds of times. In the compression process, the large velocity gradient in the trailing and high variation frequency in the leading are due to the shear and friction of the rotor face.

A 2–D Mathematical Model of Vortex Induced Vibration Driven Bladeless Wind Turbine

Bladeless wind turbine (BWT) is a flexible cylindrical structure that extracts energy from wind by utilising vortex-induced vibration (VIV) - aerodynamic forces and the resulting structural vibration. This work focuses on taking possible advantage of the increase in lift forces in the similar fashion to birds flying in a V-formation. The purposes of the present study are 1) to study the flow pattern and characteristic around two BWTs which are cylindrical structures in the same flow field and 2) to study the extra lift force generation of the system. 2–D CFD models are used to simulate flow of stationary cylinder of BWTs at Re = 105. The two different turbulent models, Reynolds Averaged Navier-Stokes shear-stress transport k(RANS–SST k) and Detached Eddy Simulation shear-stress transport k(DES–SST k) are investigated. The results show that only DES–SST kgives converged results, therefore, DES–SST kis selected for the additional studies of two cylindrical structures. From 2–D CFD simulation, the one BWT in flow field case produces lift coefficient 0.851. In the case of two BTWs in same flow field (BWT no.2 is located at x = 3D, y = 0D, directly downstream of two cylinders), BTW no.1 and BWT no.2 show greater lift coefficients of 0.893 and 1.841, respectively. This result indicates that the kinetic energy generation of the downstream BWT in the two BWTs system in this study is greater than the baseline BWT with an increase of 116% of lift. Further work is needed to determine the optimum location of the behind wind turbine for greater lift and result to increasing of energy produce of the system.

The Study of Turbulent Fluctuation Characteristics in a Small Rotary Engine with a Peripheral Port Based on the Improved Delayed Detached Eddy Simulation Shear-Stress Transport (IDDES-SST) Method

In this paper, an improved delayed detached eddy simulation method combined with shear-stress transport (SST) model was used to study the three-dimensional turbulent characteristics in a small rotary engine with a peripheral port. The turbulent characteristics including instantaneous velocity, turbulent fluctuation, coherent structure and velocity circulation were analysed based on a dynamic model of the small rotary engine. Three sets of conclusions on the basis of computational results were obtained. First, it was found that large-scale vortex structures with high intensity were distributed in the center of the chamber in the intake process and broke into lots of small vortex structures in the compression process. Second, flow stability in the X direction decreased from the leading to the trailing in the small rotary engine. The fluctuation velocity of the Y direction showed the paraboloid feature and its peak position moved from the mid-back to the middle of the chamber during the operation process. Third, during the intake process, two vortices occurred in the cross section parallel to the covers and were located at the leading and trailing of the cross section, respectively. Compared to the intake process, more vortices occur at cross sections which were far away from the central section during the compression process.

Numerical investigation of boundary layer flow and wall heat transfer in a gasoline direct-injection engine

Near-wall turbulent boundary-layer has a substantial impact on the wall heat transfer process in internal combustion engines, and ultimately affects the engine performance. However, the heat transfer processes are not well understood because of a lack of information on gas side velocity and temperature distribution. In the present study, the transient velocity and thermal boundary layers in a motored spark-ignition direct-injection engine are predicted using a Delay Detached Eddy Simulation Shear-stress Transport (DDES-SST) model. The near-wall ensemble-averaged and fluctuation velocity and temperature fields at the cylinder head are investigated throughout the compression stroke. The numerical results of the flow field in engine core region agree well with high-speed Two Dimensional Three Components-Particle Image Velocimetry (2D3C-PIV) experimental data. The simulated data show that boundary-layer separation occurs around the closing phase of intake valves and the turbulence in the near-wall region is anisotropic. The thickness of thermal boundary layer shows a non-monotonic variation with time. The dimensionless velocity profiles agree well with the law of the wall in the viscous sublayer but deviate from the log layer. The dimensionless temperature profiles tend to be uniform in the log-layer. The maximum local wall heat flux and heat transfer coefficient are 70kW/m2 and 200W/(m2K) at top dead center (TDC), respectively. The spatial distribution of turbulent heat flux shows a strong link between the velocity and thermal boundary layers, and the peaks in the buffer layer occupy about 70–85% of the local wall heat flux, indicating that the forced convection heat transfer is the main heat transfer mechanism in internal combustion engines.

Investigation of an IC Engine Intake Flow Based on Highly Resolved LES and PIV

To reduce emissions and fuel consumption, the current generation of gasoline engines uses technologies such as direct injection, downsizing and supercharging. All of them require a strong vortical in-cylinder charge motion, usually described as “tumble”, to improve fuel-air mixing and enhance flame propagation. The tumble development strongly depends on the flow field during the intake stroke. This flow field is dominated by the intake jet, which has to be captured well in the simulation. This work investigates the intake jet on a steady-state flow bench, especially in the vicinity of the intake valve. At first, the general flow dynamics of the intake jet for three different valve lifts and three different mass flows were investigated experimentally. For the smallest valve lift (3 mm), flow-field measurements using Particle Image Velocimetry (PIV) show that the orientation of the intake jet significantly depends on the air flow rate, attaching to the pent roof for low flow rates. This phenomenon is less pronounced for higher valve lifts. An intermediate valve lift and flow rate were chosen for further investigations by scale-resolving simulations. Three different meshes (coarse, medium and fine) and two turbulence models (Sigma and Detached Eddy Simulation-Shear Stress Transport (DES-SST)) are applied to consider their effect on the numerical results. An ad-hoc post-processing methodology based on the ensemble-averaged velocity field is presented capturing the jet centerline’s mean velocity and velocity fluctuations as well as its orientation, curvature and penetration depth. The simulation results are compared to each other as well as to measurements by PIV.

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.

COMPUTATIONAL AERODYNAMICS OF THE C-130 PREDICTING AIRFLOW EFFECT ON AIRDROP

This paper presents a delayed detached-eddy simulation-shear stress transport (DDES-SST) analysis of the airflow in the opened cargo hatch region of the C-130H. Numerical simulations are performed on a full-scaled axisymmetric aircraft model with the standard airdrop method settings at a 5° angle of attack. Results show a large vortex core forming at the mouth of the cargo bay and diminishing in strength and vorticity with distance from the cargo ramp. An approximate safe distance for deployment of extraction parachutes is also measured with the help of streamline plots.

Comparison of Turbulence Models in Near Wake of Transport Plane C-130H Fuselage

This paper presents a computational fluid dynamics analysis of the delayed detached eddy simulation–shear-stress transport on a 1:16 scale simplified C-130H fuselage model in flight conditions and with a closed cargo ramp to observe the turbulent-flow structures that evolve behind its empennage region. The results of which are also compared against already existing -vorticity plots of the delayed detached eddy simulation–Spalart–Allmaras with rotation and curvature correction. With vorticity magnitude of , the delayed detached eddy simulation–shear-stress transport is shown to display a lot more detail in terms of the formation and unsteadiness of the upsweep and detached vortices. The simulations are performed with the finite volume solver Cobalt with up to 64 processors on a high-performance computer.

The Influence of Inflow Condition on the Generation of Tumbling Flow Using Detached Eddy Simulation

One of the objectives of car manufacturers is to improve engine performance, reduce consumption reduce emissions. To achieve this objective, it is important to understand the phenomena involved in the combustion chambers of engines. These phenomena are numerous and complex in nature such: the aerodynamics, fuel-air mixture, turbulence, combustion and the cycle to cycle instabilities that cause more problems. One of the factors responsible for the phenomenon of cycle to cycle variations is the instability of the characteristics of the vortex flow Tumble. This instability may be due to changes in initial conditions. This study is achieved in order to contribute in a better understanding of engine flow by using a Detached Eddy Simulation Shear-Stress Transport (DES SST) model, which is a hybrid RANS/LES model. These simulations have been performed with the commercial CFD (computational fluid dynamic) code (FLUENT) coupled with our own development based on UDF facilities given by FLUENT. To explore the suitability of the 3D DES STT to simulate the internal flow, the calculation is performed for a model tumbling flow at constant volume. This flow has been measured in an experimental set up and measurements are used to initiate and to validate simulations. For this case study, we consider simplified engine geometry. To generate tumbling motion, we use non-reacting DES with a single cycle (SC) strategy. Also, with this strategy we study the effect of initial conditions on the instabilities that accompany a vortex type tumble.

Numerical investigation of cyclic variations in gasoline engines using a hybrid URANS/LES modeling approach

Cycle to cycle variations are an important aspect in the development and optimization process of internal combustion engines. In this study the feasibility of using a detached eddy simulation (DES) SST model, which is a hybrid URANS/LES model, to predict cycle to cycle variations is investigated. In the near wall region or in regions where the grid resolution is not sufficiently fine to resolve smaller structures, the two-equation RANS shear-stress transport (SST) model is used. In the other regions with higher grid resolution an LES model is applied. First, the numerical requirements associated with the hybrid URANS/LES and the employed solver are studied in detail. The numerical dissipation of the spatial scheme and the choice of the temporal scheme including the step size are evaluated. In addition, the accuracy of the solver for moving meshes, which are required for engine calculations, is assessed. The modeling constant linking the grid size to the DES filter length scale is determined by calculating a decaying homogeneous isotropic turbulence test case for different grid resolutions. The final applications of the model are two different engine cases with increasing complexity. The first case is the statistically stationary flow through an engine intake port. The time resolved flow structure predicted by the DES SST model is analyzed and the resulting time-averaged velocity fields are compared to experimental data at different locations. The second application is a motored multi-cycle simulation of a series production engine. The instantaneous flow development during the intake and compression stroke of one single cycle is studied and the ensemble-averaged and the instantaneous velocity fields as well as the resolved velocity fluctuations are compared to optical measurements. Special emphasis is placed on the cyclic differences of the velocity fluctuations at the time of ignition in the vicinity of the spark plug and the expected influence on the combustion process.