Unsteady RANS Model Research Articles

The influence of thermal radiation on the free convection inside enclosures

Abstract Natural convection is the dominant flow regime inside containments following the short-term blowdown phase. To simplify CFD computations in such flows, thermal radiation has traditionally been ignored due to the modest containments temperatures expected in typical severe accident scenarios. Recently, however, some reduced scale experiments have shown that radiation may have profound effects on the flow field even at relatively low temperature levels and differences. We will summarize two series of computations conducted with CFD tools. The first exercise consists of simulating turbulent air flow inside the DIANA cubical differentially heated cavity at PSI (Switzerland). A large eddy simulation (LES) is performed with and without wall-to-wall radiation modeling. Including radiation significantly improves the prediction of the flow field, correctly displaying a reduced level of thermal stratification and an enhanced level of turbulence. Secondly, the technical scale THAI (Becker Technologies, Germany) natural circulation test TH24 is analyzed. In this experiment, a stratified air-steam atmosphere is remobilized due to a natural circulation flow induced by wall heating in the lower part of the facility and steam condensation inside the stratified cloud. An unsteady RANS model is applied with and without consideration of the gas radiation heat transfer within the steam rich atmosphere. The comparative evaluation clearly highlights the effect of gas radiation on the overall energy balance as well as the natural circulation and mixing process. In comparison to the experimental data, a significantly improved consistency is obtained if gas radiation is considered.

CFD simulation of flow behind overflooded obstacle

Abstract This paper deals with studying of two topics – measuring of velocity profile deformation behind a over-flooded construction and modelling of this velocity profile deformation by computational fluid dynamics (CFD). Numerical simulations with an unsteady RANS models - Standard k-ε, Realizable k-ε, Standard k-ω and Reynolds stress models (ANSYS Fluent v.18) and experimental measurements in a laboratory flume (using ADV) were performed. Results of both approaches showed and affirmed presence of velocity profile deformation behind the obstacle, but some discrepancies between the measured and simulated values were also observed. With increasing distance from the obstacle, the differences between the simulation and the measured data increase and the results of the numerical models are no longer usable.

Assessment of different URANS models for the prediction of the unsteady thermal mixing in a T-junction

Turbulent thermal mixing of fluids at different temperature in T-junctions represents a major concern for the safety of nuclear reactors. This is due to the significant thermal fluctuations that can arise under such circumstances, consequently leading to cyclic thermal stress and thermal fatigue within the pipe wall. Computational Fluid Dynamics (CFD) can be employed in order to obtain useful insights on the characteristics of the transient behaviour of the turbulent heat transfer in T-junctions. Although LES has been found to be the most accurate approach to turbulence modelling for this type of flow, its application at high Reynolds numbers is limited by its considerable computational costs. In this respect, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach can be considered as a less computationally demanding option. In the present work, different URANS models are applied for the simulation of the thermal mixing in a T-junction at high Reynolds numbers. The numerical results are thoroughly assessed against the available experimental data. It is shown that, despite its limitations, a proper use of the URANS approach can give reasonable results for the considered flow configuration; in particular, a good prediction of the temperature fluctuations near the wall has been obtained, which is important for the evaluation of the cyclic thermal stress induced within the pipe wall. Therefore, it is concluded that URANS models can be regarded a pragmatic approach for the evaluation of temperature fluctuations in T-junction pipes. Finally, general guidelines for the application of the URANS approach for the simulation of such configurations are given.

Computational study of turbulent flow interaction between twin rectangular jets

Turbulent jets are commonly used in engineering applications. Systems of multiple parallel jets have an important flow structure that could accomplish rapid and efficient mixing. The mixing feature of parallel jets has several engineering applications, such as its application in Generation IV very-high-temperature nuclear reactors, where the coolants merge in the upper or lower plenum after passing through the reactor core. Computational fluid dynamics (CFD) simulations are extensively employed in the study of the mixing phenomenon of parallel jets. Therefore, the validation of various turbulence models is of great importance to the process of ensuring that the numerical results are trustworthy and that they serve as a guide for future designs.In this study, an open source CFD library, namely OpenFOAM, was utilized to conduct the numerical simulation of the twinjets. This work consists of two parts; one part focuses on steady-state simulations, and the other on transient simulations. In the first part, the Reynolds-averaged Navier–Stokes (RANS) models, such as the realizable k–ε and the shear stress transport (SST) k–ω, were used for the steady-state validation study. Steady-state simulations showed that with proper boundary conditions at the inlets, the mean velocity data agreed with the experimental data well within an engineering accuracy (14%). In the second part, the partially averaged Navier–Stokes (PANS) models were implemented in the code, and were utilized to conduct transient simulations. Experimental fluctuating inlet boundary conditions were employed. The results obtained from the PANS and the unsteady Reynolds-averaged Navier–Stokes (URANS) models were compared with the experimental data. The PANS model presented good agreement in terms of the merging point (4.3%). In addition, the k–ε PANS model was compared with the k–ε URANS model. Power spectrum density (PSD) analysis was performed based on the velocity at four sample locations to compare resolved frequencies between the PANS and the URANS models. It was observed that PANS model presented better capabilities in resolving higher turbulence flow frequencies compared with the URANS, based on the PSD analysis.

Numerical investigation on the buoyancy-driven infiltration airflow through the opening of the cold store

With the rapid growth of the total gross of the cold store, the energy consumption problem of the cold store is attracting increasing attentions. Infiltration through the door, which accounts for a very large part of the total heat load, has been highlighted in the previous simulation studies. However, the transient simulation of the infiltration in the cold store is not sufficient and the complex condition such as infiltration coupled with the cooling fans on is not covered. In this paper, a transient infiltration simulation model is established based on the unsteady RANS model. The model is validated by the experimental data of a cold store under conditions with different temperature differences, opening sizes and operation mode of the cooling fans. The results show that the predicted value and change trend of the infiltration air volume, local wind speed and temperature by the established model agree well with the experimental data. By using this model, the characteristics of the infiltration is analyzed. The results show that the position of the velocity boundary around the doorway plane and the tendency of the velocity distribution are almost invariable and are not affected by the door opening sizes and the temperature differences of the cold store.

Inverse modeling of indoor instantaneous airborne contaminant source location with adjoint probability-based method under dynamic airflow field

Accurate and prompt identification of airborne contaminant source location in indoor environment is of vital importance for building operation safety. Successful inverse tracking algorithm to identify airborne contaminant location usually requires limited sensor readings and indoor airflow information as input. Such mathematical algorithm under steady-state indoor airflow scenario has been intensively investigated. However, in many built environment scenarios, air velocity direction and magnitude keeps changing with time, making the transportation process of airborne contaminant significantly different from it under steady-state airflow. This paper mainly focuses on the airborne contaminant source location identification under dynamic airflow, by employing an adjoint probability-based inverse tracking method. The mathematical model and process of indoor instantaneous contaminants source location identification under dynamic air flow field is investigated and presented. Case studies using Computational Fluid Dynamics (CFD) tool by unsteady RANS simulation are conducted on a subway station model as well as an aircraft cabin, in which case experimental validation is also conducted. The capability of the new method is verified for air contaminant source location identification under dynamic indoor airflow.Practical implications: Successful identification of airborne contaminant source location under dynamic airflow field implies the capability of the method to locate an airborne contaminant source by limited sensor information and time-dependent airflow field in real-time. Practically the time-dependent airflow field data can be obtained through CFD simulation with unsteady RANS model with monitored time-dependent boundary condition.

Numerical Simulation of Weakly Ionized Dynamic Plasma for Reentry Vehicles

Past studies of plasma sheaths enveloping vehicles have shown that the distribution of plasma is time-varying and results in communication and navigation problems. This paper aims to investigate the dynamic characteristics of the plasma sheath through the simulations of the Radio Attenuation Measurements-C II vehicle and the Apollo command module flowfields. The numerical methodology is based on the unsteady Reynolds-averaged Navier–Stokes model and the detached-eddy simulation model, and the thermochemical nonequilibrium effect is considered. The stable flows of both vehicles with a steady plasma sheath widely distributed in the shock-wave layer are first obtained. For the Radio Attenuation Measurements-C II vehicle, the dynamic plasma arising from the vehicle pitching oscillation has the same frequency as the vehicle oscillation, and the magnitude of the disturbance to plasma is related to the oscillation velocity. For the Apollo command module, the oscillation period of the electron density is the same as the evolution period of the vortex structures in the leeward regions. The time-varying surface electron density and pressure reveal a similar variation trend with the phase difference. In addition, the periodicity of the dynamic plasma disappears as the flow instability intensifies in the Apollo command module flowfields, whereas periodic plasma exists for a long time in the Radio Attenuation Measurements-C II vehicle oscillation flowfields.

Scale-resolving simulations in engine combustion process design based on a systematic approach for model development

The increasing availability of high-performance computing resources will allow scale-resolving simulations such as large eddy simulations to be used instead of unsteady Reynolds-averaged Navier–Stokes approaches, not only in academic research but also for engine combustion process development. The scope of this work is to highlight and discuss this transition to scale-resolving simulations and to propose a systematic approach for model development and application. The current and future scope of industrial and academic research is discussed especially with respect to cycle-to-cycle variations, which cannot be identified with unsteady Reynolds-averaged Navier–Stokes models. The individual processes along the cause-and-effect chain leading to cyclic variations of the combustion process are identified, and the current state of scale-resolving simulations and the required models with respect to these processes are discussed.

The Vertical Axis Wind Turbine Efficiency Evaluation by Using the CFD Methods

In this paper, numerical studies have been conducted in order to evaluate the efficiency of a vertical axis wind turbine by using CFD methods. For these studies, it has been used SIMPLE type pressure-based coupled solver. The complexity and the unsteadiness character of the flow in such a machine, requires the use of unsteady RANS models. To determine the accuracy of the solution obtained by URANS models, this paper makes a comparative analysis with the same case and LES modeling. Analyses were performed with a 2D grid consisting of two regions, a rotor and a stator. The airfoil of the VAWT is a classic NACA0018. Results indicate that the poor accuracy of the URANS CFD method is mainly due to its limitation in the vortex modeling. In general the LES modeling have a better agreement with the experimental results from the wind tunnel.

3-D hybrid LES-RANS model for simulation of open-channel T-diversion flows

The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage. To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows, a hybrid LES-RANS model, which combines the large eddy simulation (LES) model with the Reynolds-averaged Navier-Stokes (RANS) model, is proposed in the present study. The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel, as well as the downstream region of a branch channel. The LES model was used to simulate the channel diversion region, where turbulent flow characteristics are complicated. Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence. A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations. This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions. Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T -diversion flows.

Unsteady Predictions of Mixing Enhancement with Steady and Pulsed Control Jets

Unsteady Reynolds-averaged Navier–Stokes predictions are reported for a single round jet at high Reynolds number and high subsonic Mach number (, ) excited by steady and pulsed control jets. Comparison has been made with experimental validation data to assess the ability of -based unsteady Reynolds-averaged Navier–Stokes modeling for predicting control-jet-driven flow control of near-field jet mixing and potential core-length reduction. The well-known overprediction of the clean (unexcited) core length with this level of turbulence closure remains, but taking this into account, the relative effect of control jets on core-length reduction was predicted remarkably well. For example, steady control jets and pulsed control jets in symmetric/antisymmetric modes indicated a core-length reduction (relative to the unexcited case) of in predictions, compared to in measurements. Comparison of radial profiles showed that unsteady Reynolds-averaged Navier–Stokes modeling was also able to predict the three-dimensional near-field behavior induced by control jets throughout the jet cross section. The vortex structures produced in the pulsed-control-jet predictions were compared. Symmetric and antisymmetric modes produced different vortex structures; these caused different levels of enhanced mixing in the two azimuthal modes and explained the better performance of antisymmetric pulsing.

Sensitivity of aerodynamic forces in laminar and turbulent flow past a square cylinder

We use adjoint-based gradients to analyze the sensitivity of the drag force on a square cylinder. At Re = 40, the flow settles down to a steady state. The quantity of interest in the adjoint formulation is the steady asymptotic value of drag reached after the initial transient, whose sensitivity is computed solving a steady adjoint problem from knowledge of the stable base solution. At Re = 100, the flow develops to the time-periodic, vortex-shedding state. The quantity of interest is rather the time-averaged mean drag, whose sensitivity is computed integrating backwards in time an unsteady adjoint problem from knowledge of the entire history of the vortex-shedding solution. Such theoretical frameworks allow us to identify the sensitive regions without computing the actually controlled states, and provide a relevant and systematic guideline on where in the flow to insert a secondary control cylinder in the attempt to reduce drag, as established from comparisons with dedicated numerical simulations of the two-cylinder system. For the unsteady case at Re = 100, we also compute an approximation to the mean drag sensitivity solving a steady adjoint problem from knowledge of only the mean flow solution, and show the approach to carry valuable information in view of guiding relevant control strategy, besides reducing tremendously the related numerical effort. An extension of this simplified framework to turbulent flow regime is examined revisiting the widely benchmarked flow at Reynolds number Re = 22 000, the theoretical predictions obtained in the frame of unsteady Reynolds-averaged Navier–Stokes modeling being consistent with experimental data from the literature. Application of the various sensitivity frameworks to alternative control objectives such as increasing the lift and reducing the fluctuating drag and lift is also discussed and illustrated with a few selected examples.

Numerical simulation of large dunes in meandering streams and rivers with in-stream rock structures

The evolution and migration of large dunes in a realistic intermediate-size experimental stream, the Saint Anthony Falls Laboratory (SAFL) Outdoor StreamLab (OSL), and two large-scale meandering rivers with in–stream rock structures are studied numerically using the SAFL Virtual StreamLab hydro-morphodynamic (VSL3D) model. Due to the challenges arising from mesh quality and large disparity in time-scales, coupled morpho- and hydro-dynamics simulations of bed forms has, for the most part, been restricted to sand wave amplitudes of few centimeters. In this work, we overcome such difficulties by employing the immersed boundary approach and a dual time–stepping technique of the VSL3D model [63]. The VSL3D employs the curvilinear immersed boundary (CURVIB) method along with a suspended sediment load module and is capable of simulating turbulent stratified flows coupled with bed morphodynamic evolution in realistic riverine environments with arbitrarily complex hydraulic structures. Turbulence is handled either via large-eddy simulation (LES) with the dynamic Smagorinski subgrid scale model or unsteady Reynolds-averaged Navier Stokes (URANS) equations closed with the k-ω turbulence model. Simulations in the intermediate-scale OSL channel, in which we also collected experimental morphodynamic data, show that LES can capture the evolution and migration of bed forms with characteristics that are in good agreement with experimental measurements. The URANS model, however, fails to excite the bed instability in the OSL channel but captures realistic dune evolution in the two large-scale meandering rivers. This finding is especially important as it demonstrates the potential of the VSL3D model as a powerful tool for simulating morphodynamic evolution under prototype conditions. To our knowledge, our work is the first attempt to simulate large-scale bed forms in waterways with an order of magnitude disparity in spatial scales, from the ∼2.7m wide OSL channel to the 27m wide rivers. Accordingly, the height of the simulated dunes ranges from ∼0.2m to 2.0m and the wavelength ranges from ∼0.1m to 50m for the OSL and large-scale rivers, respectively. For all cases the statistical properties of the simulated bed forms are shown to agree well with those of bed forms observed in nature.

Numerical investigation for effects of actuator parameters and excitation frequencies on synthetic jet fluidic characteristics

Numerical investigation is performed to further address the effects of geometric parameters and excitation frequency of the actuator on the synthetic jet fluidic characteristics by utilizing a two-dimensional unsteady Reynolds-averaged Navier–Stokes model. The vibrating diaphragm is modeled as a movable wall varying in sinusoidal mode. Computations are carried out by using FLUENT software with the coupled user definition function (UDF) describing the diaphragm movement. The results show that the geometric parameters of the actuator, such as the cavity depth and diameter, as well as orifice thickness and diameter, have important influences on the synthetic jet fluidic characteristics. The velocity output could be maximized if the geometric parameters of the synthetic jet actuator are designed to ensure that the cavity acoustic Helmholtz resonance frequency is coincided with the diaphragm excitation frequency. For a fixed actuator cavity, when the diaphragm excitation frequency is consistent with the Helmholtz resonance frequency of actuator cavity, the relative pressure insides the cavity is obviously great during the ejection stroke while low during the suction stroke. In the presented actuator parameters, the synthetic jet is enhanced as the decrease of cavity depth for the fixed orifice. The change of cavity diameter in the vicinity of corresponding cavity acoustic resonance diameter has relatively weaker influence on the synthetic jet. When the diaphragm is excited at high frequency, small orifice diameter will restrict the ejection and suction capacity of the synthetic jet actuator.

Influence of human movement on the transport of airborne infectious particles in hospital

The impacts of human movement on the distribution of airborne infectious particles in hospital environment are investigated numerically. In the case of airborne infection isolation room, the influence of different walking speeds on the distribution of respiratory droplets is investigated by adopting the Lagrangian method for tracing the motion of droplets, the dynamic mesh model for describing human walking and the Eulerian unsteady Reynolds-averaged Navier–Stokes model for solving the airflow. In the case of operating theatre, the impact of surgeon bending movement on the distribution of bacteria-carrying particles (BCPs) is investigated by using a similar approach, except that the drift-flux model is used for modelling BCPs distribution. The adopted models are successfully validated against reported experimental data. The results show that both walking speed and bending posture change considerably the suspended droplets concentration in a room. The key factors regarding the simulation techniques are discussed.

Internal flow numerical simulation of double-suction centrifugal pump using DES model

It is a challenging task for the flow simulation for a double-suction centrifugal pump, because the wall effects are strong in this type of pumps. Detached-eddy simulation (DES), referred as a hybrid RANS-LES approach, has emerged recently as a potential compromise between RANS based turbulence models and Large Eddy Simulation. In this approach, the unsteady RANS model is employed in the boundary layer, while the LES treatment is applied to the separated region. In this paper, S-A DES method and SST k−ω DES method are applied to the numerical simulation for the 3D flow in whole passage of a double-suction centrifugal pump. The unsteady flow field including velocity and pressure distributions is obtained. The head and efficiency of the pump are predicted and compared with experimental results. According to the calculated results, S-A DES model is easy to control the partition of the simulation when using near wall grid with 30 < y+<300 control approach. It also has better performance on efficiency and accuracy than SST k − ω DES method. S-A DES method is more suitable for solving the unsteady flow in double-suction centrifugal pump. S-A DES method can capture more flow phenomenon than SST k − ω DES method. In addition, it can accurately predict the power performance under different flow conditions, and can reflect pressure fluctuation characteristics.

Film-Cooled Trailing Edge Measurements: 3D Velocity and Scalar Field

Aircraft turbine blade trailing edges commonly are cooled by blowing air through pressure-side cutback slots. The surface effectiveness is governed by the rate of mixing of the coolant with the mainstream, which is typically much faster than predicted by CFD models. Three-dimensional velocity and coolant concentration fields were measured in and around a cutback slot using a simple uncambered airfoil with a realistic trailing edge cooling geometry at a Reynolds number of 110,000 based on airfoil chord length, which is lower than practical engines but still in the turbulent regime. The results were obtained using magnetic resonance imaging (MRI) techniques in a water flow apparatus. Magnetic resonance concentration (MRC) scans measured the concentration distribution with a spatial resolution of 0.5 mm3 (compared to a slot height of 5 mm) and an uncertainty near 5%. Magnetic resonance velocimetry (MRV) was used to acquire 3D, three-component mean velocity measurements with a resolution of 1.0 mm3. Coupled concentration and velocity measurements were used to identify flow structures contributing to the rapid mixing, including longitudinal vortices and separation bubbles. Velocity measurements at several locations were compared with an unsteady RANS model. Concentration measurements extrapolated to the surface provided film cooling effectiveness and showed that the longitudinal vortices decreased effectiveness near the lands and reduced the average film cooling effectiveness.

Prediction of Flow Instabilities in an Atmospheric Low Swirl Burner Using URANS Models

Swirl-induced phenomena are used in gas turbine burners as a mechanism to stabilize the flame. The formation of coherent structures under turbulent swirling conditions plays a fundamental role in the stabilization and needs to be completely understood also in the absence of combustion. In this work, numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations for isothermal flow in an unconfined annular low swirl burner (50 kW) are reported. The standard k-ϵ and Reynolds stress models are used to run computational cases at a Reynolds number of 12,000 and two swirl numbers (S L = 0.57 and S H = 0.64). The numerical method is validated with the experiments reported by Legrand et al. [27]. Numerical results agree well with experiments for mean flow, temporal pressure measurements, and transient coherent structures. 2-D proper orthogonal decomposition (POD), 3-D iso-surfaces and advanced, vortex-related visualization methods are used to document the latter.

Detached-Eddy Simulation of Rotating Stall Inception for a Full-Annulus Transonic Rotor

Full-annulus detached-eddy simulation (DES) is conducted in this paper to investigate stall inception for the axial transonic rotor, NASA rotor 67. A low-diffusion E-CUSP scheme with a third-orderMUSCL reconstruction is used to discretize the inviscid fluxes, and second-order central differencing is used for the viscous terms. An implicit line Gauss–Seidel iteration with dual time-stepping method and second-order temporal accuracy is employed for time integration of the spatially filtered unsteady Navier–Stokes equations in a rotating frame. It is observed that the rotating stall is initiated by the local spike flow disturbance, which quickly induces the rotor to stall roughly over two rotor revolutions. The stall cell covering more than six blade tip passages propagates at 48% of rotor speed in the counter-rotor rotation direction. The process of rotating stall is captured by the full-annulus DES, which indicates that the blockage created by the low-energy vortical flow structure pushes the tip leakage flow to the adjacent blade behind the detached sonic boundary. The unsteady Reynolds-averaged Navier–Stokes (URANS) simulation is also performed for comparison. DES predicts the stall inception roughly one rotor revolution earlier than the URANS model. Overall, DES predicts the stall inception more realistically.

Experimental and computational investigation of local scour around bridge piers

Experiments and numerical simulations are carried out to study clear-water scour around three bridge piers with cylindrical, square, and diamond cross-sectional shape, respectively. To handle movable-bed channels with embedded hydraulic structures, the fluid–structure interaction curvilinear immersed boundary (FSI-CURVIB) method is employed. The hydrodynamic model solves the unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the k-ω turbulence model using a second-order accurate fractional step method. Bed erosion is simulated by solving the sediment continuity equation in the bed-load layer using a second-order accurate unstructured, finite-volume formulation with a sand-slide, bed-slope-limiting algorithm. Grid sensitivity studies are carried out to investigate the effect of grid resolution on the predictive capability of the model. Comparisons of the simulations with the experimental data show that for all three cases the agreement is reasonable. A major finding of this work, however, is that the predictive capability of the URANS morphodynamic model improves dramatically for the diamond shape pier for which sediment transport is driven primarily by the shear layers shed from the pier sharp edges. For piers with blunt leading edge, on the other hand, as the circular and square shapes, the URANS model cannot resolve the energetic horseshoe vortex system at the pier/bed junction and thus significantly underpredicts both the scour depth at the nose of the pier and the rate of scour growth. It is also shown that ad hoc empirical corrections that modify the calculated critical bed shear stress to enhance scour rate in the pier leading edge need to be applied with caution as their predictive capabilities are not universal but rather depend on the pier shape and the region of the flow.