RANS Simulations Research Articles

Computational Fluid Dynamics Investigation of the Flow of Trailing Edge Cooling Slots

Slot film cooling is a popular choice for trailing edge (TE) cooling in high pressure (HP) turbine blades because it can provide more uniform film coverage compared to discrete film cooling holes. The slot geometry consists of a cutback in the blade pressure side connected through rectangular openings to the internal coolant feed passage. The numerical simulation of this kind of film cooling flows is challenging due to the presence of flow interactions such as step flow separation, coolant-mainstream mixing, and heat transfer. The geometry under consideration is a cutback surface at the trailing edge of a constant cross-section aerofoil. The cutback surface is divided into three sections separated by narrow lands. The experiments are conducted in a high-speed cascade in Oxford Osney Thermo-Fluids Laboratory at Reynolds and Mach number distributions representative of engine conditions. The capability of computational fluid dynamics (CFD) methods to capture these flow phenomena is investigated in this paper. The isentropic Mach number and film effectiveness are compared between CFD and pressure sensitive paint (PSP) data. When compared with the steady k − ω shear stress transport (SST) method, scale adaptive simulation (SAS) can agree better with the measurement. Furthermore, the profiles of kinetic energy, production, and shear stress obtained by the steady and SAS methods are compared to identify the main source of inaccuracy in RANS simulations. The SAS method is better to capture the unsteady coolant–hot gas mixing and vortex shedding at the slot lip. The cross flow is found to affect the film significantly as it triggers flow separation near the lands and reduces the effectiveness. The film is nonsymmetric with respect to the half-span plane, and different flow features are present in each slot. The effect of mass flow ratio (MFR) on flow pattern and coolant distribution is also studied. The profiles of velocity, kinetic energy, and production of turbulent energy are compared among the slots in detail. The MFR not only affects the magnitude but also changes the sign of production.

Computational investigation on secondary flows in a linear turbine cascade with tapered dual fence

The focus of the present work is to minimize the secondary flow losses inside a linear turbine cascade by means of a novel design of streamwise dual fence. The leading edge and trailing edge of the fences have been modified so as to reduce the total pressure loss coefficient in the passage. The study has been carried out computationally based on RANS simulations with SST turbulence model. Numerous simulations have been undertaken with single fence and dual fence models and compared with the base case model. The dual fence model with tapered trailing edge exhibits significant loss reduction compared to the base case. A suitable fence height ratio (FHR) has been identified for the dual fence model. The FHR = 2 configuration reduces the secondary flow kinetic energy by 78 % within the blade passage and it reduces the exit angle deviation significantly throughout the span. Detailed flow field analysis has been carried out to understand the physical mechanism behind the loss reduction with dual fence models. It is observed that fence-1 breaks the pressure side leg of the horse shoe vortex at the beginning of their formation itself. The radial penetration of the suction side leg of the horse shoe vortex is restrained by fence-2. These combined effects prevent the formation and mixing of two prominent loss core regions thereby avoiding the accumulation of low energy fluid near the suction side of blade.

Development of a model for unsteady conjugate heat transfer simulations

Recent developments in CFD simulations go towards higher accuracy by using time-resolved simulations instead of steady RANS simulations and adding solid structures to the simulation instead of assuming constant wall temperatures or heat fluxes at wall boundaries. This means a very large number of time steps are necessary to both accurately resolve the turbulent fluctuations in the fluid parts and have a converged temperature field in the solid parts of the computational domain. Another problem in near-wall heat transfer is the often used assumption of a constant turbulent Prandtl number, which is not correct for many flow configurations. The present work therefore focuses on the development of a methodology for accelerated unsteady conjugated heat transfer simulations. Furthermore, the implementation of a near-wall heat transfer model that does not rely on a constant turbulent Prandtl number was evaluated. The models are implemented in the CFD solver ANSYS CFX. The validation of the turbulent heat transfer model shows improved agreement with the experimental results compared to a constant turbulent Prandtl number approach. The improved unsteady conjugate heat transfer methodology gives the same accuracy of the results as the implicit unsteady conjugate heat transfer model but with considerable reduction of the computational time.

A Reduced Order Model based on Large Eddy Simulation of Turbulent Combustion in the Hybrid Rocket Engine

A combined method of large eddy simulations for non-premixed combustion in a turbulent boundary layer coupled with proper orthogonal decomposition of instantaneous velocity, pressure and temperature fields is developed in order to obtain a reduced order model. First, we investigate a channel turbulent reacting flow using Large Eddy Simulations (LES) technique. Polypropylene/O2 has been considered as fuel/oxidant pair. The turbulence-combustion interaction is based on a combination of finite rate/eddy dissipation model applied to a reduced chemical mechanism with four reactions. The LES numerical results are analyzed with respect to RANS simulations and with other reference data. The second part of the paper refers to the derivation of a Reduced Order Model (ROM) based on proper orthogonal decomposition (POD) technique for the unsteady flow field. In order to achieve that, the eigenmodes of the flow are computed from several snapshots of the instantaneous fields uniformly spaced and the most energetic ones are used to set up the Reduced Order Model. Constant regression rate of the fuel grain is considered. The flow and thermal fields obtained with ROMs are compared with the ones obtained from the full simulation and an analysis on the number of modes required to achieve the desired accuracy is presented.

Study on 2D RANS simulations of underexpanded sonic jets

Study on 2D RANS simulations of underexpanded sonic jets

Unsteady RANS Simulation of Air Distribution in a Ventilated Classroom with Numerous Jets

The study is devoted to the Unsteady Reynolds-Averaged Navies-Stokes (URANS) simulation of ventilation in an isothermal room with numerous jets supplied from ceiling diffusers. The computations of the airflow under the test conditions considered were carried out in the classroom of the Technical University of Sofia with no occupants. The room floor has a simple rectangular form, but several columns, beams, window sills, and four radiators are located inside the room that makes the geometry more complex. Air is supplied to the room through four ceiling fan coils, the Reynolds number is 2×104. Calculations were carried out using the ANSYS Fluent 18.2 software with the standard k-ε turbulence model chosen. Computational meshes of up to 33 million hexahedral cells clustered to the inlet and outlet sections were used. The main aim of the study presented is to analyze and discuss the complicated 3D flow structure in the room and to give foundation for future measurements of air velocity field in the room.

A wind tunnel and numerical study on the surface friction distribution on a flat roof with solar panels

This work aims to contribute for a better understanding of how solar panels, installed on a flat roof, affect the potential of aeolian erosion of loose particles deposited on it, influencing processes such as snow drifting. Two methodologies are employed, namely wind tunnel experiments and RANS simulation. Several configurations are studied, varying the distance between successive panels placed in tandem, and gap between the panels and the roof, for two wind orientations. The accuracy of the simulations is examined by comparing predicted results against wind tunnel experiments. It is confirmed that computational simulations, by means of k–ω SST turbulence model, yield the tendency of the static pressure, and also of wall shear stress, distributions along the roof provided that mesh satisfies y+ < 5. It is found that wall friction is considerably affected by the presence of panels and, consequently, the wind erosion process and, therefore, snow drifting on a roof. The computational wall shear stress results are confronted against experimental data, and, taking into account the uncertainty associated to the probes employed, it is found that, despite of high flow complexity, numerical simulations provide reliable information for this research field.

Numerical study of uneven wall-heating effect for a one side rib-roughened cooling channel subject to rotation

ABSTRACTThis paper investigates the impact of the wall-heating conditions on the heat transfer performance of a rotating channel with one side smooth and one side roughened by 45° inclined ribs. Previous experimental and numerical studies for single-ribbed wall-heated channels showed that rotation has a significant negative impact on heat transfer performance. In order to investigate this uncommon behaviour, RANS simulations were conducted under three different wall-heating conditions in the present study: ribbed wall heated, all walls heated and adiabatic conditions. Numerical results show that the presence of uneven wall-heating conditions has a negligible impact on the stationary case, but it has a large influence on rotational cases, in both, the heat transfer and the flow field. The underlying reason is that in rotating cases, uneven heating results in different buoyancy effects on the trailing and leading walls of the channel that alter the main flow velocity profile. As a consequence, also secondary flows and heat transfer performance are affected.

Investigation of Speed Matching Affecting Contrarotating Fan’s Performance Using Wireless Sensor Network including Big Data and Numerical Simulation

This paper describes the investigations performed to better understand two-stage rotor speed matching in a contrarotating fan. In addition, this study develops a comprehensive measuring and communication system for a contrarotating fan using ZigBee network. The investigation method is based on three-dimensional RANS simulations; the RANS equations are solved by the numerical method in conjunction with a SST turbulence model. A wireless measurement system using big data method is first designed, and then a comparison is done with experimental measurements to outline the capacity of the numerical method. The results show that when contrarotating fan worked under designed speed, performance of two-stages rotors could not be matched as the designed working condition was deviated. Rotor 1 had huge influences on flow rate characteristics of a contrarotating fan. Rotor 2 was influenced by flow rates significantly. Under large flow rate condition, the power capability of rotor 2 became very weak; under working small flow rate condition, overloading would take place to class II motor. In order to solve the performance mismatch between two stages of CRF under nondesigned working conditions, under small flow rate condition, the priority shall be given to increase of the speed of rotor 1, while the speed of rotor 2 shall be reduced appropriately; under large flow rate condition, the speed of rotor 1 shall be reduced and the speed of rotor 2 shall be increased at the same time.

Comment on “Large-eddy and unsteady RANS simulations of a shock-accelerated heavy gas cylinder” by B. E. Morgan, J. Greenough

An important model coefficient in the production term of turbulent kinetic energy equation was printed in error in the paper by B. E. Morgan and J. Greenough (Shock Waves 26:355–383, 2016). In this comment, the correct value is presented.

Response to “Comment on ‘Large-eddy simulation and unsteady RANS simulations of a shock-accelerated heavy gas cylinder’ by B. E. Morgan, J. Greenough”

In response to a comment by Y. Zhang, two previously unidentified typographical errors in the work by Morgan and Greenough (Shock Waves 26:355–383, 2016) are identified and corrected.

Wind Tunnel Support System Influence on NASA Common Research Model at Low-Speed Conditions

Computational studies were conducted in order to gain an understanding and to quantify the impact of the model support system and aeroelastic wing deformation on the Common Research Model at subsonic inflow conditions at . Comparisons between results obtained in the European Transonic Windtunnel and from simulations using TAU reveal the extent of the modeling error introduced by neglecting the support structure. RANS simulations with and without the support system show that it has a near-field effect on the model’s rear fuselage, significantly altering the pressure field and thereby impacting the force coefficients, most significantly drag and pitching moment. Aeroelastic wing deformation was also included in the investigation, showing non-negligible impact on the forces. The inclusion of the support system and wing deformation, as well as the use of advanced turbulence models, was shown to promise considerable error reduction potential in comparative studies between wind tunnel and simulation data.

Panel Method for Ducted Propellers with Sharp Trailing Edge Duct with Fully Aligned Wake on Blade and Duct

A low-order panel method is used to predict the performance of ducted propellers. A full wake alignment (FWA) scheme, originally developed to determine the location of the force-free trailing wake of open propellers, is improved and extended to determine the location of the force-free trailing wakes of both the propeller blades and the duct, including the interaction with each other. The present method is applied on a ducted propeller with sharp trailing edge duct, and the predicted results over a wide range of advance ratios, with or without full alignment of the duct wake, are compared with each other, as well as with results from RANS simulations and with measurements from an experiment.

3D CFD simulation of a CI engine converted to SI natural gas operation using the G-equation

The conversion of heavy-duty CI engines to natural gas (NG) SI operation have the potential to increase the use of NG in the transportation sector in the United States. More, the increased turbulence of a bowl-in-piston combustion chamber can increase the flame speed under more efficient lean conditions. The main objective of this study was to investigate if a 3D G-equation-based RANS simulation (i.e., reasonable computational costs and running times) can predict the efficiency and emissions of such converted engine, for various NG compositions and operating conditions. The model was validated with experimental data from a single-cylinder CI research engine that replaced the fuel injector with a spark plug and fumigated NG inside the intake manifold using a low-pressure gas injector. Using a unique set of model tuning parameters, the model was able to qualitatively predict the effect of NG composition on engine performance and emissions over a range of operating conditions that changed spark timing, equivalence ratio, and engine speed. The model also captured the double-peak heat release rate seen at advanced spark timing in the experiments. The results showed that a lower methane number (MN) increased peak pressure and indicated mean effective pressure. Higher H/C ratio advanced combustion phasing. More, higher MN lowered nitrogen oxides but increased unburned hydrocarbons emissions. However, while a lower MN increased carbon monoxide (CO) production during the combustion process, there was no clear trend for engine-out CO emissions. Overall, the predicted gas composition effects on engine efficiency and emissions were relatively small, at least for the range of operating conditions investigated here. However, the results suggest that the 3D CFD model described here is suitable for combustion phenomena analysis such the flame behavior in a bowl-in-piston combustion chamber.

Quantification of the axial induction exerted by utility-scale wind turbines by coupling LiDAR measurements and RANS simulations

The axial induction exerted by utility-scale wind turbines for different operative and atmospheric conditions is estimated by coupling ground-based LiDAR measurements and RANS simulations. The LiDAR data are thoroughly post-processed in order to average the wake velocity fields by using as common reference frame their respective wake directions and the turbine hub location. The various LiDAR scans are clustered according to their incoming wind speed at hub height and atmospheric stability regime, namely Bulk Richardson number. Time-averaged velocity fields are then calculated as ensemble averages of the scans belonging to the same cluster. The LiDAR measurements are coupled with RANS simulations in order to estimate the rotor axial induction for each cluster of the LiDAR data. First, a control volume analysis of the streamwise momentum is applied to the time-averaged LiDAR velocity fields to obtain an initial estimate of the axial induction over the rotor disk. The calculated thrust force is imposed as forcing of an axisymmetric RANS simulation in order to estimate pressure, radial velocity and momentum fluxes. The latter are combined with the LiDAR streamwise velocity field in order to refine the estimate of the rotor axial induction through the control volume approach. This process is repeated iteratively until achieving convergence on the rotor axial induction while minimizing difference between LiDAR and RANS streamwise velocity fields. This procedure allows to single out the reduction in thrust load while the blade pitch angle is increased transitioning from region 2 to 3 of the power curve. Furthermore, an enhanced thrust force is observed for a fixed incoming wind speed and transitioning from stable to convective stability regimes. The presented technique is proposed as a data-driven alternative to the blade element momentum theory typically used with current actuator disk models in order to quantify rotor aerodynamic thrust for different operative and atmospheric conditions.

Comparison of unsteady aerodynamics on wind turbine blades using methods of ranging fidelity

In this paper the unsteady aerodynamic loads for aeroelastic bend-twist coupling predicted by methods of various fidelity are compared. These are: a fast and simplistic strip theory model based on the analytical theories by Greenberg and Loewy, an Euler and a RANS unsteady CFD simulation. The test object is the DTU 10MW reference wind turbine. The comparison uses transfer functions with bending deflections and blade torsion as an input, aerodynamic thrust and pitching moment as an output. A structural feedback is not considered. The RANS simulation has the highest fidelity and is taken as a baseline for all other methods. The temporal discretization and simulation time of the CFD have to be chosen carefully as they influence the transfer behavior. As long as the flow separation is small, the Euler simulation is very accurate. The frequency response by the strip theory has a good agreement over a broad range of flow conditions in attached flow when a distributed torsion or a flap-wise deflection is applied. Severe deviations are found for an edge-wise deformation and in-plane forces.

Influence of Reynolds Number on the Unsteady Aerodynamics of Integrated Aggressive Intermediate Turbine Duct

The ultra-high bypass ratio turbofan engine attracts more and more attention in modern commercial engine due to advantages of high efficiency and low Specific Fuel Consumption (SFC). One of the characteristics of ultra-high bypass ratio turbofan is the intermediate turbine duct which guides the flow leaving high pressure turbine (HPT) to low pressure turbine (LPT) at a larger diameter, and this kind of design will lead to aggressive intermediate turbine duct (AITD) design concept. Thus, it is important to design the AITD without any severe loss. From the unsteady flow’s point of view, in actual operating conditions, the incoming wake generated by HPT is unsteady which will take influence on boundary layer’s transition within the ITD and LPT. In this paper, the three-dimensional unsteady aerodynamics of an AITD taken from a real engine is studied. The results of fully unsteady three-dimensional numerical simulations, performed with ANSYS-CFX (RANS simulation with transitional model), are critically evaluated against experimental data. After validation of the numerical model, the physical mechanisms inside the flow channel are analyzed, with an aim to quantify the sensitivities of different Reynolds number effect on both the ITD and LPT nozzle. Some general physical mechanisms can be recognized in the unsteady environment. It is recognized that wake characteristics plays a crucial role on the loss within both the ITD and LPT nozzle section, determining both time-averaged and time-resolved characteristics of the flow field. Meanwhile, particular attention needs to be paid to the unsteady effect on the boundary layer of LPT nozzle’s suction side surface.

RANSE-based simulation and analysis of scale effects on open-water performance of the PPTC-II benchmark propeller

This paper presents our numerical study of the scale effects on a tip-rake propeller, the PPTC-II, based on the RANS simulations using software FLUENT 6.3. The low Re option in SST k–ω model is adopted at model scale, together with fine prism grids to resolve the viscous sub-layer. At full scale, standard wall function is adopted. The scale-effect corrections yielded by our RANS simulations are compared with those obtained from the ITTC method. To explain the CFD results, an analysis of sectional forces is performed. To investigate how the tip rake influences propeller scale effects, the geometry of PPTC-II is modified by removing the tip rake only, and the RANS-predicted scale effects for the modified propeller, PPTC-II-m, are compared with those for the PPTC-II. The study indicates that the scale effect on propeller thrust can be as important as that on the torque; somehow the RANS- and ITTC-based predictions for full-scale efficiency agree quite well; the tip-rake reduces tip loading and tip vortex strength, and brings about large differences in the scale effects as compared with the propeller without tip-rake.

Turbulence generation after a monolith in automotive catalytic converters

This work reports theoretical studies of flow behaviour in a monolith outlet zone for different Reynolds numbers covering laminar and transitional/turbulent flow regimes. Monolith type substrate is the core part of the automotive catalytic converter. Due to computational limitations, most numerical models of the converter represent the monolith as a continuum, averaging the effect of the solid and the open space on the flow. This strategy is useful to study the macro-structure of the flow, however, it does not capture the exact behaviour of an actual honeycomb type structure, especially at its entrance and exit. In this work, which is a continuation of the publication by Cornejo et al. (2018), a series of 3D LES and RANS simulations are performed using different discrete channel geometry to study and quantify the velocity fluctuations of flow leaving a monolith. The results show that above a certain Reynolds number the instability of the flow after the monolith is significant, leading to turbulence generation. The velocity fluctuations are mainly explained by the flow past the outlet of the monolith, and their magnitude is related to the Reynolds number based on the thickness of the walls between channels. An expression for this critical Reynolds number has been designed and verified against numerical simulations. Parametric studies are carried out to illustrate the influence of the Reynolds number on the appearance of flow fluctuations at the outlet zone of the monolith.

Numerical Investigation of Secondary Flow and Loss Development in a Low-Pressure Turbine Cascade with Divergent Endwalls

Secondary flow and loss development in the T106Div-EIZ low-pressure turbine cascade are investigated utilizing (U)RANS simulations in cases with and without periodically incoming wakes at M a 2 t h = 0 . 59 and R e 2 t h = 2 × 10 5 . The predictions are compared to experimental data presented by Kirik and Niehuis (2015). The axial mid-span and overall loss development in the T106Div-EIZ and the T106A-EIZ in the steady case are analyzed regarding the effects caused by the different loading distributions and by the divergent endwall geometry. Furthermore, the entropy generation is analyzed in the T106Div-EIZ with periodically incoming wakes in several axial positions of interest and compared to the undisturbed steady case. It is found that in the front-loaded T106Div-EIZ, the incoming wakes cause a premature endwall loss production in the front part of the passage, resulting in a lower intensity of the secondary flow downstream of the passage and a redistribution of the loss generation components.