Reynolds-averaged Navier-Stokes Computations Research Articles

Computational simulation of turbulent flows around a marine propeller by solving the partially averaged Navier–Stokes equation

This study concerns the characteristics of the partially averaged Navier–Stokes method for local flow analysis around a rotating propeller. Partially averaged Navier–Stokes, resolving crucial large-scale structures of turbulent flow at a given computational grid resolution, is a bridging turbulence closure model between the Reynolds-averaged Navier–Stokes equation and the direct numerical simulation. A detailed comparison between partially averaged Navier–Stokes and Reynolds-averaged Navier–Stokes models is made to achieve a better understanding of partially averaged Navier–Stokes characteristics for predicting the coherent structures in turbulent flow. The two-equation k-ω shear stress transport model and the seven-equation Reynolds stress model are selected for Reynolds-averaged Navier–Stokes computations. The problem of interest is the flow around a rotating KP505 propeller in open water conditions at an advance ratio of 0.7. Near the leading edge, the partially averaged Navier–Stokes results are similar to those of Reynolds stress model in terms of the vortical structures. Vorticity predicted by different turbulence models, however, shows significant differences. For a more detailed analysis, the velocity gradient constituting the vorticity is identified at the leading edge. It is proven that partially averaged Navier–Stokes is able to capture the anisotropic characteristics of the flow at the leading edge, where both the geometric and flow characteristics change abruptly.

프란시스 수차 모델의 블레이드 내 와류 특성에 대한 수치해석적 분석

In this study, a numerical analysis of the inter-blade vortex characteristics of a Francis hydro turbine model was performed with various flow rate conditions in the operating range. The model turbine demonstrated notably different internal flow characteristics for different operating conditions. Particularly, an inter-blade vortex between the runner blade passages was observed at lower flow rate conditions. An inter-blade vortex can cause reduction in performance, and vibration and instability in the operation of a turbine system. Therefore, understanding inter-blade vortex characteristics is important for the safe and stable operation of turbines. To investigate internal flow and unsteady pressure characteristics, three-dimensional steady and unsteady Reynolds-averaged Navier-Stokes calculations using a shear stress transport turbulence model were performed with two-phase flow analysis. Inter-blade vortices were captured at the leading and trailing edges of the near runner hub between blade passages. This vortex region exhibited flow separation and lower velocity and contributed to decreased hydraulic performance and higher unsteady pressure characteristics.

The effects of pressure treatment on the flamelet modeling of supersonic combustion

The flamelet method has been used extensively as an affordable turbulence-combustion interaction model that obviates the need to solve the evolution equations for the species mass fractions during a large-eddy or Reynolds-averaged Navier–Stokes calculation of a reactive flow field, leading to substantial savings in the simulation time and enabling modeling with relatively complex kinetic mechanisms. The canonical problem analyzed and stored in a look-up array in the flamelet procedure usually assumes some baseline fields; in particular, the pressure is often specified at a fixed value that is characteristic of the examined configuration. However, pressure in supersonic combustion has significant dynamical roles, unlike in low-Mach number or incompressible flows, and a constant pressure field will not be adequate for the former. To remedy this problem, reaction rate in the combustor is often assumed to scale squarely with pressure. This approach, which is probably acceptable for low-speed, high pressure combustors, is not suitable for dealing with the variable pressure conditions in supersonic combustion. This paper focuses on the assessment of the aforementioned scaling, in absolute sense, and also relative to an approach where pressure is added as a control parameter in the flamelet library. To achieve this, three classes of reactive systems with different levels of modeling complexities are investigated to show that representative chemical variables do not scale squarely with pressure. For the case of supersonic combustion, the scaling treatment in general leads to over-prediction of pressure and combustion and also tends to stabilize the flame. To the knowledge of the authors, no previous studies have reported on the issues addressed in the present paper.

Aerodynamic analysis of a step adjustment method for blade pitch of a VAWT

Vertical-axis wind turbines (VAWTs), which are considered an important wind energy conversion device, have recently received renewed attention. Researchers are investigating the pitch angle control method to improve the wind energy utilization efficiency of VAWTs. In this study, ANSYS Fluent is used to investigate the aerodynamic performance of two-dimensional VAWTs. Different pitch angles are investigated using unsteady Reynolds-averaged Navier–Stokes calculations, and turbulence is modeled with the shear stress transport k–ω model. Four quartiles are defined for the azimuthal angle of a rotation period: upwind (45° <θ < 135°), leeward (135°<θ< 225°), downwind (225°<θ < 315°), and windward (315° <θ < 45°). Furthermore, the sliding-mesh technique and a user-defined function are applied to adjust the pitch angle of the wind turbine blades in different azimuthal zones. This study proposes a pitch angle control method that effectively improves the efficiency of VAWTs by comprehensively analyzing the aerodynamic performance of the wind turbine and power coefficient (Cp) variation in each zone. The results indicate that an appropriate pitch angle control solution can significantly improve the efficiency of the wind turbine. Compared to a fixed pitch angle of 0°, an approximately 17.0% increase in Cp can be achieved and, compared to the optimal fixed pitch angle −1°, an approximately 12.0% increase in Cp can be achieved.

Characterization and Impact of Secondary Flows in a Discrete Passage Centrifugal Compressor Diffuser

Truncating the exit of a discrete passage centrifugal compressor diffuser is observed to enhance a research compressor's stall line. By interrogating the experimental data along with a set of well-designed Reynolds-Averaged Navier–Stokes computations, this improvement is traced to the reduced impact of secondary flows on the truncated diffuser's boundary layer growth. The secondary flow system is characterized by counter-rotating streamwise vortex pairs that persist throughout the diffuser passage. The vortices originate from two sources: flow nonuniformity at the impeller exit and separation off the leading edge cusps unique to a discrete passage diffuser. The latter detrimentally impacts the diffuser pressure rise capability by accumulating high loss flow along the diffuser wall near the plane of symmetry between the vortices. This contributes to a large passage separation in the baseline diffuser. Using reduced-order modeling, the impact of the vortices on the boundary layer growth is shown to scale inversely with the diffuser aspect ratio, and thus, the separation extent is reduced for the truncated diffuser. Because the diffuser incidence angle influences the strength and location of the vortices, this mechanism can affect the slope of the compressor's pressure rise characteristic and impact its stall line. Stall onset for the baseline diffuser configuration is initiated when the vortex location and the corresponding passage separation transition from pressure to suction side with increased cusp incidence. Conversely, because the extent of the passage separation in the truncated diffuser is diminished, the switch in separation side does not immediately initiate instability.

Numerical Investigations of S-Shaped Air Inlet for Embedded Engines

In this paper, compressible Reynolds-averaged Navier–Stokes computations are conducted for an S-shaped duct, in a clean configuration and also with a single row of vortex generators. The numerical simulations are performed using a hybrid unstructured grid. The influence of including the geometric representation of the probe instrumentation is investigated by considering the rake fairing geometry and also the geometry of the complete rake with the probes. Some aspects of the probe modeling are also discussed. A direct comparison of the numerical simulation results with the experimental values is performed for the wall static pressure along the duct, total pressure recovery, and distortion indices at the aerodynamic interface plane. Three different eddy-viscosity turbulence models are evaluated: the one-equation Spalart–Allmaras, and the two-equation realizable and shear stress transport. A virtual 200-probe rake is also proposed, beyond the standard 40-probe rake, to enrich the total pressure and distortion indices comparisons. An overall good agreement is observed among experimental results and the numerical wall static pressure and the total pressure topology at the aerodynamic interface plane. Some discrepancies for the distortion indices are verified and discussed in detail.

Evaluation of mean velocity and mean speed for test ventilated room from RANS and LES CFD modeling

The paper presents and discusses data for the ventilation airflow in an isothermal room corresponding to the Nielsen et al. (1978) test computed with Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) approaches. As LES computations provide directly both the speed and velocity components data, the difference between the mean speed and mean velocity values is computed and discussed. For the RANS computations that give the mean velocity data only, application of the velocity-to-speed conversion procedure based on the turbulence kinetic energy field provided by a turbulence model resulted in accurate mean speed evaluation.

Effect of blade thickness on the hydraulic performance of a Francis hydro turbine model

Francis turbines are the most commonly used turbines for hydroelectric power generation. Preliminary studies to verify turbine designs are often performed with small-scale models; however, when the runner blade of a full-size turbine is geometrically scaled down to prepare a model for evaluating the design variables and performance characteristics, the blades become very thin and difficult to manufacture. Hence, the blockage effect of the runner blade should be considered to find a suitable blade thickness that satisfies the required hydraulic performance. Furthermore, a clear understanding of the blockage ratio at the highest efficiency point and off-design condition is required to investigate different blade thicknesses and performance characteristics. Here, the blockage effect of the runner blade on the hydraulic performance and internal flow characteristics of a 300-class Francis hydro turbine was investigated. Three-dimensional Reynolds-averaged Navier–Stokes calculations were performed with a shear stress transport turbulence model to analyze the internal flow characteristics near the runner blade and compare the blockage effect with various blade thicknesses on major performance parameters such as the hydraulic efficiency. Flow analyses for the off-design conditions were also performed with various blade thicknesses. The obtained results indicated that the power and efficiency gradually decreased with increasing blockage ratio. The runner head loss increased due to the mismatches between the flow angle and blade angle with changing the inlet velocity triangle components according to blockage ratio. Especially the efficiency of approximate 3.4% decreased as the blockage ratio increased with 12.5%, compared to the reference model. It was verified that the blockage effect significantly affects the design of Francis turbine models.

프란시스 수차 축소 모델의 수력학적 성능 특성에 대한 형상 매개변수 연구

This paper presents a geometrical parametric study on the hydraulic performance characteristics of a small-scale Francis hydro turbine model. The upper and lower clearances on the variable guide vane and the surface roughness on each component of the turbine model were considered as geometric parameters. Three-dimensional steady-state Reynolds-averaged Navier-Stokes (RANS) calculations with a shear stress transport (SST) turbulence model were performed to evaluate the hydraulic performance characteristics by varying these geometric parameters. Variable guide vane clearances and surface roughness were applied, respectively and simultaneously, to conduct flow analysis of the small-scale Francis hydro turbine model. Additionally, the loss distribution for each component of the turbine model was analyzed to obtain a better understanding of the effects of hydraulic performance on surface roughness. The results indicated it could be applied to the design and manufacture of a small-scale Francis hydro turbine model as the major geometric parameters.

Nacelle Strake Design for Short Takeoff and Landing Configuration with Turboprop Engines

The scope of this Paper is to investigate the integration effects of turboprop engines on a high-lift wing with an internally blown plain flap system with Reynolds-averaged Navier–Stokes computations. It is shown that the resulting nacelle vortices can significantly reduce the high-lift performance at zero thrust conditions. To limit the negative impact, an inboard nacelle strake was designed with the aim of maximizing the lift coefficient of the landing configuration at zero thrust. To reach this goal, a sensitivity study on basic geometric strake parameters was carried out. The best nacelle strake is able to improve the maximum lift coefficient by and the maximum angle of attack by 3 deg. The study also reveals the further potential of improvement due to an additional outboard strake. First simulations with outboard strake lead to a further improvement of and 4 deg in the maximum angle of attack. It is also shown that the installation of an inboard strake does not negatively impact the high-lift performance at a moderate thrust level.

Aeroelastic assessment of a highly loaded high pressure compressor exposed to pressure gain combustion disturbances

A numerical aeroelastic assessment of a highly loaded high pressure compressor exposed to flow disturbances is presented in this paper. The disturbances originate from novel, inherently unsteady, pressure gain combustion processes, such as pulse detonation, shockless explosion, wave rotor or piston topping composite cycles. All these arrangements promise to reduce substantially the specific fuel consumption of present-day aeronautical engines and stationary gas turbines. However, their unsteady behavior must be further investigated to ensure the thermodynamic efficiency gain is not hindered by stage performance losses. Furthermore, blade excessive vibration (leading to high cycle fatigue) must be avoided, especially under the additional excitations frequencies from waves traveling upstream of the combustor. Two main numerical analyses are presented, contrasting undisturbed with disturbed operation of a typical industrial core compressor. The first part of the paper evaluates performance parameters for a representative blisk stage with high-accuracy 3D unsteady Reynolds-averaged Navier-Stokes computations. Isentropic efficiency as well as pressure and temperature unsteady damping are determined for a broad range of disturbances. The nonlinear harmonic balance method is used to determine the aerodynamic damping. The second part provides the aeroelastic harmonic forced response of the rotor blades, with aerodynamic damping and forcing obtained from the unsteady calculations in the first part. The influence of blade mode shapes, nodal diameters and forcing frequency matching is also examined.

CFD Investigations of Ship Maneuvering in Waves Using naoe-FOAM-SJTU Solver

Ship maneuvering in waves includes the performance of ship resistance, seakeeping, propulsion, and maneuverability. It is a complex hydrodynamic problem with the interaction of many factors. With the purpose of directly predicting the behavior of ship maneuvering in waves, a CFD solver named naoe-FOAM-SJTU is developed by the Computational Marine Hydrodynamics Lab (CMHL) in Shanghai Jiao Tong University. The solver is based on open source platform OpenFOAM and has introduced dynamic overset grid technology to handle complex ship hull-propeller-rudder motion system. Maneuvering control module based on feedback control mechanism is also developed to accurately simulate corresponding motion behavior of free running ship maneuver. Inlet boundary wavemaker and relaxation zone technique is used to generate desired waves. Based on the developed modules, unsteady Reynolds-averaged Navier-Stokes (RANS) computations are carried out for several validation cases of free running ship maneuver in waves including zigzag, turning circle, and course keeping maneuvers. The simulation results are compared with available benchmark data. Ship motions, trajectories, and other maneuvering parameters are consistent with available experimental data, which indicate that the present solver can be suitable and reliable in predicting the performance of ship maneuvering in waves. Flow visualizations, such as free surface elevation, wake flow, vortical structures, are presented to explain the hydrodynamic performance of ship maneuvering in waves. Large flow separation can be observed around propellers and rudders. It is concluded that RANS approach is not accurate enough for predicting ship maneuvering in waves with large flow separations and detached eddy simulation (DES) or large eddy simulation (LES) computations are required to improve the prediction accuracy.

Isolator Dynamics During Unstart of a Dual-Mode Scramjet

The phenomenology of unstart in a dual-mode ethylene-fueled scramjet is studied in the presence of inflow distortion using carefully calibrated unsteady Reynolds-averaged Navier–Stokes computations with an emphasis on understanding the large-scale transients over the length scale of the flowpath. The computational approach is validated with experimental data for two combustor operating states that bound the dual-mode and unstart conditions. Unstart is induced via an imposed fuel-staging transient that emulates the experiments. Inlet distortion introduces three-dimensional effects in the isolator, which primarily manifest in the nature of side wall separation and flow curvature; however, localized regions of heat release are shown to contribute to unstart-inducing side wall separation. A wall-pressure-based metric used to track shock-train motion in experiments is adapted to identify a period of constant shock-train speed and an incipient unstart condition. During the constant-velocity phase, separation on the upper wall modulates the supersonic core, resulting in a change from an oblique to a normal-like shock-train configuration. The incipient unstart state precedes the ejection of the shock train from the isolator, which is suggestive of a precursor to unstart. This metric highlights the importance of the side wall corner region during the unstart process and complements other studies of three-dimensional effects in similar isolator geometries.

Flow Control of Swept Shock-Wave/Boundary-Layer Interaction Using Plasma Actuators

The effect of plasma actuators on a swept shock-wave/boundary-layer interaction induced by a sharp fin was investigated by means of Reynolds-averaged Navier–Stokes calculations. The work focused on a sharp-fin configuration in a Mach 5 flow, at an angle of attack of 12 deg, and with a momentum thickness Reynolds number of about 5100. The mean wall pressure and heat transfer predicted for the baseline flow were compared to experimental data. The plasma actuator was modeled semi-empirically with a heat source and a magnetic body force imposed in the boundary layer upstream of the mean separation line. A horseshoe vortex appeared with the presence of the model actuator and decayed some distance downstream. The flow heating by the actuator acted as a virtual fillet, which slowed the compression and made separation more gradual. Decreased intensity of the separation shock wave, reduced skin friction coefficient, and increased separation bubble size were observed with the application of control.

Validation of a Numerical Model for Predicting Stalled Flows in a Low-Speed Fan—Part II: Unsteady Analysis

This paper investigates the flow near the stall boundary for a low-speed/low-pressure ratio fan. Three-dimensional, Reynolds-averaged Navier–Stokes computations are performed for a modern low speed fan rig for which extensive measured data are available. Simulations are conducted at 80% corrected speed, for which the measured constant speed characteristic contains a part with positive slope. It is shown in this paper that by using an unsteady whole assembly approach, it is possible to predict the flow for all the points on the measured constant speed characteristic (including those on the positive slope part), which is not achievable by using a single passage strategy as it would result in premature “numerical stall.” The results of the computations reveal that for the operating points on the positive slope part of the characteristic, the flow structure becomes asymmetric and hence requires a whole assembly numerical model. The type of asymmetry which appears at lower flow coefficients is similar to the multicell, part span rotating stall, which can occur on the front stages of core compressors at stable operating conditions. The numerical results showed a good correlation with the measured data in terms of stall characteristics.

Three-Dimensional Modeling Shock-Wave Interaction with a Fin at Mach 5

The three-dimensional single-fin configuration finds application in an intake geometry where the cowl-shock wave interacts with the side-wall boundary layer. Accurate numerical simulation of such three-dimensional shock/turbulent boundary-layer interaction flows, which are characterized by the appearance of strong crossflow separation, is a challenging task. Reynolds-averaged Navier–Stokes computations using the shock-unsteadiness modified Spalart–Allmaras model is carried out at Mach of 5 at large fin angle of $$23^{\circ }$$ . The computed results using the modified model are compared to the standard Spalart–Allmaras model and validated against the experimental data. The focus of the work is to implement the modified model and to study the flow physics in detail in the complex region of swept-shock-wave turbulent boundary-layer interaction in terms of the shock structure, expansion fan, shear layer and the surface streamlines. The flow structure is correlated with the wall pressure and skin friction in detail. It is observed that the standard model predicts an initial pressure location downstream of the experiments. The modified model reduces the eddy viscosity at the shock and predicts close to the experiments. Overall, the surface pressure using modified model has predicted accurately at all the locations. The skin friction is under-predicted by both the models in the reattachment region and is attributed to the poor performance of turbulence models due to flow laminarization.

Numerical Investigation on Windage Losses of High-Speed Gears in Enclosed Configuration

Increasing efficiency of gearing systems is a key point in developing innovative aeroengines with low specific fuel consumption. The transmission system has a direct impact on the engine overall efficiency; thus, a performance optimization is an essential issue. In high-speed gearing systems, windage effects are extremely important and can dominate the other loss mechanisms. In this work, a comprehensive numerical study on windage losses experienced by a single spur gear rotating in an oil-free environment was carried out. Steady-state Reynolds-averaged Navier–Stokes calculations of the gear in free configuration were performed with conventional eddy viscosity models, comparing numerical predictions with experimental data from the open literature. The resulting modeling was exploited for simulating the same gear enclosed within cylindrical casings of various sizes, to assess the effect of the boundary walls on windage losses. The volume of fluid within the system was found to affect windage loss; therefore, results collected from the numerical analysis were exploited for developing a correlation aimed at predicting windage loss reduction with respect to the free configuration when the gear works within cylindrical casings. Finally, the predictive capability of formulation was assessed by exploiting experimental data available in the open literature, achieving a satisfying agreement.

Film cooling optimization on leading edge gas turbine blade using differential evolution

This article reports the optimization of film cooling on a leading edge of a gas turbine blade model, with showerhead configuration, it is based on five input parameters, which are hole diameter, hole pitch, column holes pitch, injection angle, and velocity at plenum inlet. This optimization increased the Area-Averaged Film Cooling Effectiveness [Formula: see text] and reduced the consumption of coolant flow. Differential Evolution assisted by artificial neural networks was used as optimization algorithm. Reynolds Averaged Navier–Stokes computations were carried out to getting the net database and to evaluate the optimized models predicted by artificial neural network. The results show an effective increment of [Formula: see text] by 36% and a mass flow reduction by 66%. These results were reached by means of a better distribution of cooling flow at blade surface as function of the input parameters. To assure the reliability of the numerical model, particle image velocimetry technique was used for its validation.

Trailing edge noise prediction based on wall pressure spectrum models for NACA0012 airfoil

This paper compares several approaches for the prediction of the noise emitted by a NACA0012 airfoil at 0∘ and 4∘ of angle of attack and a Reynolds number Re=1.5×106. Amiet's semi-analytical model for trailing-edge noise is combined with two-dimensional Reynolds-Averaged Navier-Stokes (RANS) computations. The wall-pressure spectrum, which constitutes the cornerstone of Amiet's model, is obtained by processing the boundary layer data extracted from the simulations. The specific contribution of the paper is a comparison of two families of prediction methods: semi-empirical wall-pressure models that are fitted to experimental databases, and statistical approaches based on the integration of the Poisson equation across the boundary layer profile. The semi-empirical models that were calibrated on airfoil databases provide better predictions with experiments, while the models based on flat-plate boundary layer data fail to reproduce the measured spectra. Considering the statistical approach, it was shown to predict the general spectral features, but with an overall under-prediction of about 3 dB. It can be concluded from this study that the statistical approach proves indeed more robust than semi-empirical models when the latter were not precisely calibrated for the flow under consideration. Further improvements of the statistical approach are suggested for future work.

Unsteady aerodynamics of a diamond wing configuration

The damping derivatives associated with the pitching, yawing, and rolling motion of the SAGITTA flying wing configuration at low Mach number conditions are presented. A tailless variant of the configuration and a variant with attached double vertical tail are investigated. The damping derivatives are determined by means of the aerodynamic response to forced harmonic oscillations. The required data for the determination of the damping derivatives are obtained from time-accurate Reynolds-averaged Navier–Stokes computations. The calculation methodology for the pitch-, yaw-, and roll-damping derivatives for arbitrary freestream conditions is described and a short evaluation of the approach is presented. Angle of attack and sideslip angle trends as well as the effect of the double vertical tail on the dynamic stability are investigated. The damping derivative of every considered type of motion exhibits significant non-linearities with respect to the freestream condition for angles of attack larger than $$8^\circ$$ . The pitch-damping and roll-damping derivative indicate a dynamically stable behavior at all considered freestream conditions for the configuration with and without vertical tail. The yaw-damping characteristic is more critical. Both configurations exhibit unstable behavior at several freestream conditions. The vertical tail, however, considerably improves the yaw-damping characteristic of the SAGITTA configuration.