RANS Results Research Articles

Fluidic sealing effect on flow structure in labyrinth shroud with slotted fins

The paper analyses the flow structure of the recently introduced fluidic sealing technique that can limit the leakage flow through the labyrinth seal. The main idea behind the concept is to introduce a high-velocity jet into the gap. The jet is introduced through the slot in the fin of the seal, connecting the upstream surface of the fin with the gap. The jet is driven by the naturally occurring pressure difference, thus the solution is passive. The concept was verified using test section measurements and numerical simulations, proving effectiveness. However, the physical phenomenon standing behind the effectiveness was not studied thoroughly. A deep analysis of the flow structure in the labyrinth seal with fluidic sealing was performed based on RANS results. In addition, LES simulations were performed to allow for even more insight into the flow. The analysis revealed that the fluidic sealing introduces much non-uniformity into the flow, in comparison to the reference configuration, causing intensive mixing. The energy spectrum reveals that increased mixing raises the levels of high-frequency kinetic energy oscillations, which additionally promotes dissipation.

Aerodynamic design optimization: Challenges and perspectives

Antony Jameson pioneered CFD-based aerodynamic design optimization in the late 1980s. In addition to developing the fundamental theory, Jameson implemented that theory in codes that were practical enough to be used in industry. As a result of Jameson’s seminal efforts, a research community has been established in aerodynamic design optimization. This research area has experienced sustained improvements in CFD solvers, mesh deformation, sensitivity computation, and optimization tools. We review recent developments for each of these components and present open-source tools available for aerodynamic shape optimization. A variety of applications is presented, including the optimization of a supercritical airfoil starting from a circle, a web application that optimizes airfoils within a few seconds, aircraft aerodynamic and aerostructural optimization, and aeropropulsive optimization. We also review the Aerodynamic Design Optimization Discussion Group (ADODG) benchmarks and other aerodynamic shape optimization problems. Among the ADODG benchmarks, we focus on the RANS-based problems and discuss some of the issues encountered, including comparing Euler and RANS results and design-space multimodality. The availability of these benchmarks and the open-source tools is expected to enable further studies and benchmarks in CFD-based aerodynamic design optimization and MDO.

Numerical analysis of escort tug manoeuvrability characteristics

The adoption of RANS techniques is widely spread in terms of resistance and self-propulsion prognosis for merchant and military vessels; however, on the other hand, the captive evaluation of hydrodynamic manoeuvring forces across numerical procedures still concerns great computational effort and challenge. When it comes to deal with Escort or tractor tugs, characterised by rather full hull forms, typical of harbour tugs, with the addition of extensive foiled shaped fore-skegs solutions, the ability of predicting manoeuvring and handling of such vessels becomes a great added value at an early design stage, as an alternative to physical model scale testing, since very scarce data are present in open literature. Escort vessels in particular, both operate at low and high speed, in addition to the typical harbour tasks entailed, making the manoeuvring modelling even more complicated. In present work, a low-speed and high-speed numerical methodology based on the OpenFOAM finite volume method on polyhedral unstructured mesh is investigated and validated against purposely designed model tests carried out in towing tank: Pure yaw, pure drift, drift and yaw, and drift and heel simulation results are compared with experimental data in terms of forces first, and impact on manoeuvring lastly, showing satisfactory agreement with experiments. Finally, Escort capability tests run at tank are compared with a purposely calibrated manoeuvring model on the RANS results.

Very Large Eddy Simulation of turbulent flow and heat transfer for single cylinder and cylindrical pin matrix

The study of turbulent flow and heat transfer around a cylinder and cylindrical pin matrix is important for turbine blade cooling. A newly developed VLES method is applied to investigate the unsteady flow and heat transfer mechanisms. DDES and unsteady RANS methods are also used in the simulations for comparisons. Two typical cases are selected, i.e. the flow around a single cylinder, and cylindrical pin matrix of wall-bounded 8 × 8 round pins, with Re = 10000. The results show that the VLES predictions agree well with the experiments for both the flow velocities and heat transfer Nusselt numbers for the two cases, which are even better than the DDES and RANS results. The VLES method gives satisfactory predictions of mean endwall Nusselt number of the complex pin matrix, which is within 15% difference from the experiments. The unsteady three-dimensional vortex characteristics play an important role for the heat transfer, especially in the pin matrix due to complex pin disturbance and wall confinement. Accurate representation of the vortex structures is the key for good heat transfer prediction. The results demonstrate the reliability and accuracy of the present VLES method for complex turbulent convection heat transfer of industrial relevance.

LES of the Sandia flame series D-F using the Eulerian stochastic field method coupled with tabulated chemistry

In this paper, the Eulerian Stochastic Field (ESF) model in the Transported Probability Density Function (TPDF) class model is combined with the Flamelet Generated Manifolds (FGM) model. This method solves the joint probability density function transport equation by ESF method that considers the interaction mechanism between flame and turbulence with high precision. At the same time, by making use of the advantage of the FGM model, this model is able to incorporate the detailed chemical reaction mechanism (GRI 3.0) with acceptable computational cost. The new model has been implemented in the open source CFD suite-OpenFOAM. Validation of the model has been carried out by simulating the Sandia flame series (three turbulent piloted methane jet flames) issued by the National Laboratory of the United States. The accuracy and advancement of the ESF/FGM turbulent combustion model are verified by comparing the LES results of the new model with the rich experimental data as well as the RANS results. The results demonstrate that the model has a strong ability in capturing combustion phenomena such as extinction and re-ignition in turbulent flame, which is essential in the accurate prediction of the combustion process in real combustion devices, for example, aircraft engines.

CFD simulation of radially stirred baffled and unbaffled tanks

Stirred tanks typically employed in process industries are provided with baffles. Although the presence of baffles is known to guarantee good mixing rates, unbaffled vessels may be compulsory in some applications as crystallization, bioremediation, biotechnology and ore industry. A better understanding of unbaffled stirred vessels flow dynamics may allow (i) a proper design to be performed and (i) conditions/processes where baffle presence can be avoided to be recognized.In the present study, the k-? SST was used to simulate an unbaffled tank from early to fully turbulent regime (Re(600-33,000). The unbaffled tank simulated has a diameter T=0.19m and is stirred by a standard six-bladed Rushton turbine with diameter D=T/2 and clearance C=T/3. A corresponding baffled tank was also simulated in order to compare the the two systems. A time dependent Sliding Grid approach was employed for the baffled tank to account for the impeller-to-baffle relative rotation. Conversely, for the case of the unbaffled vessel, a reference frame rotating with the impeller was adopted. Experimental literature data concerning the power and pumping numbers were employed for the simulation validation. RANS results were in good agreement with the experimental data for the baffled case at the largest Re, whereas predictions for the unbaffled vessel exhibited a less satisfactory agreement with experimental data. The latter finding may be due to the poor capability of the two-equations model to manage the anisotropic turbulence typical of high swirling flows.

Bi-fidelity UQ with combination of co-Kriging and arbitrary polynomial chaos: Film cooling with back facing step using RANS and DES

Uncertainty quantification analysis of a film cooled flat plate is performed by Sparse Approximation of Moment Based Arbitrary polynomial chaos (SAMBA). To combine simulations of different fidelities, such as DES and RANS, a co-Kriging formulation is used and combined with the Non-Intrusive Polynomial Chaos (NIPC) framework. The co-Kriging allowed to have the same accuracy in the prediction as all the collocation points were obtained from DES simulation, leveraging low fidelity RANS cases.As test case, a film cooled flat plate is used. Such configuration is representative of coolant systems of gas turbines. The configuration used present a back facing step in front of the coolant hole. This design has been recently suggested to improve the cooling performance but is strongly affected by the geometrical variations of the step. This work uses a UQ formulation to quantify these effects.The co-Kriging formulation successfully estimates the DES results from the RANS results, and the probability density function of film cooling effectiveness obtained with mixed RANS/DES cases adequately matches with that obtained DES only simulations. This work shows the advantage of combining the SAMBA and co-Kriging methods to perform higher order UQ analysis with less computational cost. Such approach is particularly useful in industrial applications where a limited number of high fidelity simulations is available.

Comparison of RANS and LES in the Prediction of Airflow Field over Steep Complex Terrain

The present study compared the prediction accuracy of the three CFD software packages for simulating airflow around a three-dimensional, isolated hill with a steep slope: 1) WindSim (turbulence model: RNG k-e RANS), 2) Meteodyn WT (turbulence model: k-L RANS), which are the leading commercially available CFD software packages in the wind power industry and 3) RIAM-COMPACT (turbulence model: standard Smagorinsky LES), which has been developed by the lead author of the present paper. Distinct differences in the airflow patterns were identified in the vicinity of the isolated hill (especially downstream of the hill) between the RANS results and the LES results. No reverse flow region (vortex region) characterized by negative wind velocities was identified downstream of the isolated hill in the result from the simulation with WindSim (RNG k-e RANS) and Meteodyn WT (k-L RANS). In the case of the simulation with RIAM-COMPACT natural terrain version (standard Smagorinsky LES), a reverse flow region (vortex region) characterized by negative wind velocities clearly forms. Next, an example of wind risk (terrain-induced turbulence) diagnostics was presented for a large-scale wind farm in China. The vertical profiles of the streamwise (x) wind velocity do not follow the so-called power law wind profile; a large velocity deficit can be seen between the hub center and the lower end of the swept area in the case of the LES calculation (RIAM-COMPACT).

Numerical simulation of swirling flow effect on the first stage vane film cooling distribution

In advanced gas turbine technology, lean premixed combustion is an effective strategy to reduce peak temperature and thus, NO[Formula: see text] emissions. The swirler is adopted to establish recirculation flow zone, enhancing mixing and stabilizing the flame. Therefore, the swirling flow is dominant in the combustor flow field and has impact on the vane. This paper mainly investigates the swirling flow effect on the turbine first stage vane cooling system by conducting a group of numerical simulations. Firstly, the numerical methods of turbulence modeling using RANS and LES are compared. The computational model of one single swirl flow field is considered. Both the RANS and LES results give reasonable recirculation zone shape. When comparing the velocity distribution, the RANS results generally match the experimental data but fail to at some local area. The LES modeling gives better results and more detailed unsteady flow field. In the second step, the RANS modeling is incorporated to investigate the vane film cooling performance under the swirling inflow boundary condition. According to the numerical results, the leading edge film cooling is largely altered by the swirling flow, especially for the swirl core-leading edge aligned case. Compared to the pressure side, the suction side film cooling is more sensitive to the swirling flow. Locally, the film cooling jet is lifted and turned by the strong swirling flow.

Investigation of the Flow Structure in the Tundish with the Use of Rans and Les Methods

AbstractThe liquid steel flow structure in the tundish has a very substantial effect on the quality of the final product and on efficient casting conditions. Numerous model studies are being carried out to explain the effect of the tundish working conditions on casting processes.It is necessary to analyze the structure of liquid steel flow, which is strongly supported with numerical modeling. In numerical modeling, a choice of a proper turbulence model is crucial as it has a great impact on the flow structure of the fluid in the analyzed test facility. So far most numerical simulations has been done using RANS method (Reynolds-averaged Navier-Stokes equations) but in that case one get information about the averaged values of the turbulent flow. In presented study, numerical simulations using large eddy simulations (LES) method were used and compared to RANS results. In both cases, numerical simulations are carried out with the finite-volume commercial code AnsysFluent.

Entrained-flow gasification of coal under slagging conditions: Relevance of fuel–wall interaction and char segregation to the properties of solid wastes

Entrained-flow slagging gasifiers are characterized by operating conditions that promote ash migration/deposition onto the reactor walls, whence ash is drained as a molten phase. Experimental investigation on ashes generated by full-scale plants suggested that both char entrapment inside the melt and carbon-coverage of the slag can occur. Because of the wide range of spatial and temporal scales involved in these phenomena, numerical simulation of the fate of the flying fine char particles is a very difficult task. This work illustrates how different numerical modeling approaches can be jointly used to understand segregation patterns of char particles in full-scale entrained-flow coal gasifiers operated in the slagging regime. A multilevel approach has been developed for this purpose. RANS-based simulations of the full-scale geometry with coal particle injection and tracking aimed to obtain the general behavior of the flow field and particle trajectories. Simulations enabled to estimate the effect of swirl and tangential flow on the bulk-to-wall char particle deposition rate. Then, RANS results were adopted in a more detailed numerical model based on the solution of the filtered Navier–Stokes equations. In this last model, a turbulence LES approach for the Eulerian gas phase was applied. The equations of particles motion were solved via a Lagrangian particle tracking algorithm with the TrackToFace method. Simulations were performed involving a level of detail that allowed to obtain a clear picture of the multiphase flow behavior responsible for char deposition phenomena. This multilevel approach enabled the assessment of the char particle deposition rates and the nature of char–slag interaction (segregation/entrapment) that are likely to occur in full-scale slagging gasifiers. Results of numerical simulations have been critically discussed in the light of experimental findings. They represent a useful source of information for the implementation of constitutive equations and parameters in design-oriented reduced compartmental models.

Large eddy simulations of coal gasification in an entrained flow gasifier

In this paper, we investigate multi-phase reacting flow in a coal-fed entrained flow gasifier using large-eddy simulations and Reynolds-averaged Navier Stokes models, respectively. An axial-flow type lab-scale gasifier is investigated. The simulations are performed using a Lagrangian–Eulerian method in which the coal particles are modeled using a Lagrangian approach and the gas phase is solved using an Eulerian approach. We compare the performance of LES and RANS results. The coal particle models include devolatilization, char consumption that uses heterogeneous chemistry and two-way coupling of mass, momentum and energy with the surrounding gas phase. The gas phase combustion is modeled using homogenous chemistry and the effect of turbulence on combustion and gasification is modeled using a partially stirred reactor approximation. Results show that LES captures the unsteady flow structures inside the gasifier. We show that modeling the unsteady mixing is critical to the accurate predictions of the gas phase species and carbon conversion in these gasifiers.

Contrast between steady and time-averaged unsteady combustion simulations

Time-averaged predictions from unsteady solutions of the two-dimensional Navier–Stokes equations are contrasted with Reynolds-averaged results for a reacting flow problem in a high pressure combustor. The goal is to determine whether the two-dimensional unsteady approximation can be useful as an engineering analysis in problems for which time-averaged quantities are of primary interest. The conditions are taken from an experiment in which non-premixed gaseous oxygen and hydrogen were injected into a combustion chamber through coaxial channels. The resulting flowfield is dominated by a large recirculation zone arising from the back-step created by the injector. The results of steady and time-averaged, unsteady solutions are strikingly different. The unsteady simulation produces strong unsteady structures whose time-averaged results lead to a much wider flame zone, a different recirculation zone structure, and a substantially different wall heat flux than those obtained with a steady RANS procedure. The time-averaged calculations yield the correct combustor chamber pressure and compare considerably more favorably with heat flux measurements than do the RANS results. The two-dimensional approximation, however, overstates the unsteady vortex roll up and precludes large scale mixing across the axis of symmetry, thereby giving deficient predictions near the centerline. Overall, the present results indicate that capturing large-scale unsteady characteristics can provide more accurate predictions of recirculation dominated reacting flows and suggest that two-dimensional, time-averaged solutions represent a potentially useful engineering tool for problems of this nature while also serving as a precursor for full three-dimensional simulations.

Large eddy simulation modelling of spray-induced turbulence effects

This study is focused on the development of spray models for large eddy simulations (LESs) of high injection pressure diesel sprays. Basic governing equations are numerically solved using the Lagrangian—Eulerian approach for liquid and gas phases respectively, in the KIVA-3V code. Non-reacting simulations of evaporating spray are performed using a dynamic structure LES model in which the subgrid stress tensor is modelled with a non-viscosity tensor coefficient. This is a one equation based model, in which an extra transport equation for the subgrid kinetic energy ( k) is solved. Subgrid scale energy exchange between droplets and the gas phase is identified as an important mechanism to capture the correct scaling of k, which eventually feeds back to both the spray droplets and the gas phase turbulent mixing. Hence, to account for spray-induced gas turbulence, a LES spray source model for k is developed. This model requires subfilter scale velocities, which are obtained by defiltering the filtered velocity field available from LES. Results are compared with corresponding RANS results and available experimental data for various physical spray variables such as spray penetration, gas phase penetration, droplet distribution, etc. The LES spray model was found to be important in predicting the correct liquid spray evolution and responds correctly to varying parameters such as nozzle size and ambient density.

Stochastic modelling of inertial particle dispersion by subgrid motion for LES of high Reynolds number pipe flow

Aiming at the better prediction of inertial particle dispersion in LES of turbulent shear flow, a stochastic diffusion process of the Langevin type is adopted to model the time increments of the fluid velocity seen by inertial particles. This modelling is particularly crucial for dispersion and deposition of inertial particles with small relaxation times compared to the smallest LES-resolved turbulence time scales. Simple closures for drift and diffusion terms are described. The effects of inertia and external forces on particle trajectories are modelled. To numerically validate the proposed model, LES calculations are performed to track solid particles (glass beads in air with different Stokes' numbers) in a high Reynolds number, equilibrium turbulent pipe flow (Re = 50 000 based on the maximum velocity and the pipe diameter). LES predictions are compared to RANS results and experimental observations. Simulation findings demonstrate the superiority of LES compared to RANS in predicting particle dispersion statistics. More importantly, the use of a stochastic approach to model the subgrid scale fluctuations has proven crucial for results concerning the small-Stokes-number particles. The influence of the model on particle deposition needs to be assessed and additional validation for non-equilibrium turbulent flows is required.

자유수면을 포함한 사각기둥의 횡동요 유체동역학 수치해석

Accurate prediction of ship dynamics, particularly roll motion, is very important in ship safety. In the past, empirical or vortex based methods were commonly used for the hydrodynamic roll damping predictions but they could not be applied to practical ship roll motion cause of limitations about geometries ad design conditions. Recently RANS-based techniques are developed for the practical ship motion analysis. In this study, RANS based roil analysis about a rectangular cylinder with WAVIS developed by MOERI/KORDI are performed and compared with the experimental data and other RANS results.