Time-Step Analysis of Sliding Mesh Rotation Modeling

Effect of time-step size on flow around generic car models

Numerical computations using the Unsteady Reynolds Averaged Navier-Stokes (URANS) and Delayed Detached-Eddy Simulations (DDES) approaches are carried out to investigate the complex three-dimensional flow in the root region of a generic 10 MW wind turbine rotor. Preliminary studies regarding the time step size and the number of rotor revolution required for the time averaging procedure are conducted. In the blade outer region, URANS is sufficient to predict the general flow characteristics, but small discrepancies are observed in the blade root area where the flow is massively separated.

The trailing edge of high pressure gas turbine blade in aeroengine is usually designed as thin as possible to achieve higher aerodynamic efficiency. However, as the inlet temperature of modern gas turbine is continuously increasing, thermal stress in a thin trailing edge will become much significant, resulting in possibilities of erosion and creep problems. To find a balance between these two conflicting goals, one method is the use of pressure-side cutback, which extends into the coolant slot to get film cooling and also achieves a thin trailing edge. Due to the interactions between mainstream and coolant flow, film cooling effect on trailing edge cutback is significantly affected by the vortex shedding downstream the cooling slot. To resolve the coherent flow structures and understand their role on film cooling effect on trailing-edge cutback, this paper implemented a Very Large Eddy Simulation (VLES) model into the solver ANSYS Fluent with user defined functions. By introducing a resolution control factor, the turbulence viscosity predicted by transient SST k-ω model was corrected and the VLES computations were realized in the whole computational region. With the VLES method, film cooling effectiveness distributions on trailing-edge cutback at three blowing ratios were computed and compared against the experimental data. The coherent unsteadiness in cutback region was visualized to reveal the mixing process between mainstream and coolant flow. The numerical accuracies between different unsteady prediction methods, i.e. URANS (Unsteady Reynolds Averaged Navier-Stokes), SAS (Scale-Adaptive Simulation), DES (Detached Eddy Simulation), DDES (Delayed-Detached Eddy Simulation), SBES (Stress-Blended Eddy Simulation), and VLES were compared with respect to the resolutions of cooling effect and vortex shedding. The results show that the periodic vortex shedding induced by the interactions between mainstream and coolant is the main factor that affecting the cooling performance on cutback. VLES method has a comparable accuracy in predicting the film cooling effect on trailing edge cutback with DDES and SBES approaching. In the detached shear layer, VLES method exhibits a good ability to resolve coherent unsteadiness caused by vortex shedding. Compared with URANS and SAS methods, the VLES method has a higher accuracy in resolving the periodic vortex shedding and film cooling effectiveness distributions, especially in low blowing ratio cases.

Accurate prediction of high Reynolds number flow fields around complex geometries requires turbulence models that have favorable characteristics with regard to numerical stability, computational cost, mesh sensitivity, and accuracy. In order to investigate the feasibility of a new Hybrid RANS/LES (HRL) model with respect to these characteristics, numerical simulations were performed over a rearward facing step at ReH = 37,000 and around a GLC-305 airfoil configured with a 22.5-minute glaze ice accretion at Re = 3.5x10 6 , M = 0.12, and α = 6°. In the case of rearward facing step flow, comparisons were made between simulation results that employed a RANS model (Shear Stress Transport (SST)), the Delayed Detached Eddy Simulation (DDES) model, and the new HRL model. Mean velocity profiles of the new HRL model predictions showed better agreement with experimental data, in comparison with RANS and DDES model predictions. Both RANS and the new HRL model predictions of the turbulent kinetic energy profiles exhibited qualitatively good agreement with the experimental measurements, while the DDES model results agreed less well. Simulations using the Detached Eddy Simulation (DES) model, DDES model, and the new HRL model were performed for the flow field analysis of the airfoil with ice shape. Computations of DES, DDES, and new HRL model are still not rigorously comparable to the experiments as these computations have not yet attained statistically stationary flow fields. However, the qualitative assessment of the computations shows that the new HRL model enhances shear layer breakup and rollup in the separated flow region more so than do either DES or DDES.

The KRISO container ship model is used for numerical simulations to investigate hydrodynamic performance under high speeds. Unsteady Reynolds-Averaged Navier-Stokes (URANS) and delayed detached eddy simulation (DDES) approaches are used to resolve the flow field around the ship model. High-resolution Volume of Fluid (VOF) technique in OpenFOAM is used to capture the free surface. The present work focuses on the wave-breaking phenomena of high-speed ships. To study the speed effects on the phenomenon of ship bow wave breaking, three different speeds, i.e., Fn = .26, .35, and .40, are investigated for a fixed ship model in calm water. Predicted resistance and wave patterns under Fn = .26 are validated with available experimental data, and a good agreement is achieved. The breaking wave phenomena can be observed from both URANS and DDES results for Froude numbers greater than .35. And the Fn = .40 case shows more violent breaking bow waves. The process of overturning and breaking of bow wave is more complex in the DDES results, and some small-scale free surface features are also captured. The predicted bow wave is compared with the experiment conducted at the China Ship Scientific Research Center. It shows that the DDES results are more accurate. Wave profiles and vorticity field at several cross sections are presented to illustrate the relationship between bow waves and vortices. It is found that the free surface vorticity dissipates quickly in the URANS simulation, which leads to the difference compared with the DDES results.

The CFD-code FLUENT, version 5.4, has been used for the flow analysis of a test pump of end-suction volute type. FLUENT provides three calculation methods for analysis of turbomachinery flows: the Multiple Reference Frame method (MRF), the Mixing Plane method (MP) and the Sliding Mesh method (SM). In all three methods, the flow in the rotor is calculated in a rotating reference frame, while the flow in the stator is calculated in an absolute reference frame. In the MRF and MP methods steady flow equations are solved, while in the SM method, unsteady flow equations are solved. The SM method does not introduce physical approximations. The steady methods approximate the unsteady interaction between rotor and stator. The cost of the unsteady method is however typically 30 to 50 times higher than the cost of the steady methods. It is found that the MRF and MP methods lead to completely erroneous flow field predictions for flows far away from the best efficiency point. This makes the steady methods useless for general performance prediction.

This work aims at studying the influence of adding splitter blades on the performance of a hydraulic centrifugal pump. The studied machine is an ENSIVAL-MORET MP 250.200.400 pump (diameter=408 mm, 5 blades, specific speed=32), whose impeller is designed with and without splitter blades. Velocity and pressure fields are computed using unsteady Reynolds-averaged Navier-Stokes (URANS) approach at different flow rates. The sliding mesh method is used to model the rotor zone motion in order to simulate the impeller-volute casing interaction. The flow morphology analysis shows that, when adding splitter blades to the impeller, the impeller periphery velocities and pressures become more homogeneous. An evaluation of the static pressure values all around the impeller is performed and their integration leads to the radial thrust. Global and local experimental validations are carried out at the rotating speed of 900 rpm, for both the original and the splitter blade impellers. The head is evaluated at various flow rates: 50%, 80%, 100%, and 120%of the flow rate at the best efficiency point (BEP). The pressure fluctuations are measured at four locations at the BEP using dynamic pressure sensors. The experimental results match the numerical predictions, so that the effect of adding splitter blades on the pump is acknowledged. Adding splitters has a positive effect on the pressure fluctuations which decrease at the canal duct.

With the rapid improvement of computing power, unsteady Reynolds-Averaged Navier-Stokes (U-RANS) solvers are increasingly used to get a deeper comprehension of the rotor-stator unsteady flows inside turbomachinery. The dual-time stepping method is adopted in most U-RANS solvers and the determination of the optimal size of physical time step is a crucial procedure for rotor-stator U-RANS simulations, and it is the primary purpose of this paper. The criteria for determining the size of physical time step for rotor-stator interactions with and without filtering mixing plane were proposed through theoretical analysis. For conventional no-filtering U-RANS simulations, the number of physical time steps is determined by the number of circumferential grid points and the characteristic propagation velocity of flow. While for U-RANS simulations adopting the filtering mixing plane method, the number of retained harmonics through the filtering mixing plane plays an important role. Then, three numerical cases including subsonic rotor-stator interactions, IGV-rotor interactions and rotor-stator interactions coupled with inlet distortion, were carried out to verify the effectiveness of proposed criteria. The results showed that the requirement of time step independence can be satisfied when the number of physical time steps exceeds the number estimated by the criteria, thus the effectiveness of the proposed criteria has been verified.

Detached eddy simulation of flow around two wall-mounted cubes in tandem

Solutions of unsteady Reynolds-averaged Navier-Stokes (URANS) equations are obtained using Reynolds stresses which are directly computed from direct numerical simulation (DNS) solutions for the same flow configuration. By comparing the solution obtained using this method with the conventional URANS results and DNS results, it is able to identify the contribution of the time-step size and the turbulence model to the URANS solution. It is shown that, to accurately predict separation, the magnitude of Reynolds stresses predicted from URANS technique is more important than its temporal variation. It is also shown that the solution starts to deviate from the DNS result even with the time-step size much less than the period of the organized unsteadiness.

The flowfield around a 60% scale rotating Formula 1 tire in contact with the ground in a closed wind tunnel at a Reynolds number of 500,000 was examined computationally and experimentally. The goal of this study was to assess the accuracy of unsteady Reynolds-averaged Navier–Stokes (URANS) equations and confirm the existence of large scale vortical and flow recirculating features. A replica deformable F1 tire model that includes four tire treads and all brake components was used to determine the sensitivity of the wake to cross flow within the tire hub as well as the flow blockage caused by the brake assembly. Several turbulence closures were employed and the one that matched closest to the experimental PIV data was the Reynolds stress model. The variability between the six turbulence closures is shown by comparing velocity profiles, pressure distributions, and vortex eccentricity. The sensitivity of the wake to four different hub geometries, contact patch boundary conditions, multiple reference frame (MRF) rotor and spoke treatment, and time step size are also discussed.

In the paper based on the Computational Fluid Dynamics (CFD) method three-dimensional flow fields are studied using FLUENT software. Sliding mesh method, RNG к - e turbulent model and second-order upwind difference scheme are used to perform the numerical simulation. Firstly, the prediction capabilities of the turbulence models used in the simulation are assessed in detail. Secondly, numerical study is focused on the velocity field of an actual quadrate stirred tank with a dual 45 degrees pitched four-blade turbine in given conditions of design and operation parameters. Finally, the influences of design and operation parameters on mixing effects are analyzed by the simulation of the turbulence intensity. The study provides reference aids for optimization of design and operation parameters.

Numerical assessment of a horizontal axis marine current turbine performance

No-load speed is an important performance factor for the safe operation of hydropower systems. In turbine design, the manufacturers must conduct several model tests to calculate the accurate value of no-load speed for the complete range of operating conditions, which are expensive and time-consuming. The present study presents steady and unsteady methods for calculating no-load speed of a Francis turbine. The steady simulations are implemented using a commercial flow solver and an iterative algorithm that relies on a smooth relation between turbine torque and speed factor. The unsteady method uses unsteady RANS simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step and friction torque. The main goal of this research is to evaluate and compare the two methods by calculating turbine dynamic parameters for three test cases consisting of high and medium head Francis turbines. Overall, the numerical results agreed well with experimental data. The unsteady method provided more accurate results in the opening angle range from 20 to 26 degrees. Nevertheless, the steady results showed more consistency than unsteady results for the three different test cases at different operating conditions.

A numerical investigation on hydrodynamic characteristics of the circulating water channel