RANS Computations Research Articles

On modeling added mass and circulation terms with fast potential-flow methods for oscillating-foil propulsors

The design of cycloidal thrusters and other oscillating foil propulsors requires fast tools that provide accurate estimates of propulsor hydrodynamic performance and mechanical loads. Potential-flow theory coupled with empirical formulae that account for viscous effects is the best choice for fast multi-parameter optimization. Within this context, oscillating foils are usually approximated as thin wings with flow solutions derived from small-disturbance theory. Added mass (non-circulatory) effects may be relevant even under such assumptions. They can be easily included in the flow equations assuming that the foil is a flat plate heaving and pitching. However, for blade motions involving complex trajectories like those in Voith Schneider propellers, the foils describe cycloidal loops, and the assumption of small pitch angles is no longer valid. Therefore, a more accurate modeling of added mass and circulatory terms is required.This paper presents extensions to a previous model, which allows for a more precise representation of the added mass terms in the potential flow equations, combining low computational cost and accurate modeling. The results show clear improvement in the evolution foil forces in time when the new approach is used. The extensions are especially needed for the simulations of foils describing complex paths with large variations of pitch angle. Applications to transverse-axis cycloidal propulsors are presented as well as to unconventional counter-flapping propulsors that have recently appeared in the scientific literature. Validation of the method with RANS computations is provided.

Anomalous intensification of vortex heat transfer in the case of separated air flow over an inclined groove in a hot isothermal region of a flat plate

Anomalous heat transfer intensification in turbulent separated air flow over a long groove of moderate depth made in a plate inclined at an angle of 45°to the freestream is revealed both experimentally and numerically. The region under investigation includes a rectangle heated to 100°C by saturated water vapor. The Reynolds number varied from 103to 3×104. Using the gradient heatmetry the twofold increase, as compared with the case of a flat plate, of the heat transfer coefficient on the groove bottom is established at the Reynolds number Re = 3×104. The relative Nusselt number in different regions of the groove is determined both in the physical experiment and in the RANS calculations with the application of multiblock computational technologies and the SST model in the VP2/3 software package. The results are in good agreement in the turbulent flow regime at Re = (5, 10, and 30) ×103.

Reverse Thrust Aerodynamics of Variable Pitch Fans with Inlet Distortion

Abstract Variable pitch fans can improve the operability of low pressure ratio fan systems by re-pitching the rotor blades. If a variable pitch fan can generate sufficient reverse thrust on landing it also eliminates the need for heavy, cascade-type thrust reversers. However, any reverse thrust generation is impacted by high levels of inlet distortion generated as air is drawn into the exhaust nozzle. This paper uses RANS computations and low-speed rig experiments to explore how a representative inlet distortion from the engine installation affects the aerodynamics and performance of a variable pitch fan operating in reverse thrust mode. The simulations and the experiments show that the distortion from the engine installation leads to a highly three-dimensional flow field with a large recirculation region within the bypass duct. The distortion substantially redistributes the mass flow, thrust and power in the engine. Streamline tracking combined with a power balance analysis reveals highly radial flow within the fan rotor, with almost all the fan power used to drive the recirculating flow in the bypass duct, generating high loss and high total temperature. The recirculation reduces the net mass flow through the engine to around 5% of a uniform inflow case and greatly reduces the effective reverse thrust. With uniform inflow, the net reverse thrust was found to be 35% of the nominal takeoff thrust. With inlet distortion, this was reduced to 20%. This demonstrates the importance of designing and operating variable pitch fan systems to minimise reverse thrust inlet distortion.

Effect of the hole configurations on effusion cooling effectiveness under swirl impact in gas turbine combustor

Effusion cooling characteristics of the cylindrical and fan-shaped hole configurations are studied under realistic swirl flows at blowing ratios ranging from 1.2 to 6.0. RANS computations with the k-ω SST model are used to evaluate the interaction between swirl mainstream and cooling air. The results show that the cooling effectiveness distribution for the cylindrical and fan-shaped hole configurations are similarly controlled by swirl impact. Two high-temperature regions emerge near the impact location of the swirl main flow on the liner wall. The fan-shaped hole configuration has higher cooling effectiveness, and the difference is relative to location. Quantitatively analyzing, the fan-shaped holes are 19.7 %–53.2 % higher than the cylindrical holes in impact zones. In the corner recirculation zone, the difference ranges from 39.1 % to 84.2 %. The computations reflect the interaction between swirl flows and cooling jets is stronger for fan-shaped holes due to lower outlet velocity. Therefore the cooling air is easier to be suppressed by swirl impact under low BR, while the increasing blowing ratio can enhance the resistance of cooling air against swirl flows.

Reconstruction of Nusselt number in RANS computations with wall function approach

Two novel methods for reconstruction of Nusselt number (Nu) are proposed when the wall function approach is used in the RANS framework for defining wall boundary conditions for high Reynolds number flows. The work is motivated by the observation that the velocity and temperature fields are often well predicted when the wall function formulations are used, while at the same time Nu is not. The reason for discrepancy in predictions of flow fields and temperature, and prediction of Nu, is because Nu estimation is solely dependent on the wall function, while the variables obtained from the transport equations are determined also by the convection and pressure force in the momentum transport. The method based on the correction factor Ψ∗ originates from the simplified analytical wall function approach of Popovac and Hanjalić (2007) and modifies the formulation of so-called wall viscosity. The method based on a wall shape function of exponential type approximates temperature variation in the near-wall region. Once the shape function’s coefficients are calculated using values of temperature in three near-wall consecutive cells, the wall values of temperature or temperature gradient, needed for calculation of Nu, can be determined. The methods are tested for four standard benchmark cases, backward-facing step, impinging jet, differentially heated square cavity, and Rayleigh-Bénard convection. The method based on the correction factor significantly improved Nu predictions for the cases where the velocity field is strongly impacted by pressure force. The method based on the wall shape function improves predictions of Nu in all tested cases, with somewhat less accurate predictions of the maximum value of Nu compared to the correction factor method.

Impeller Design and Performance Analysis of Aviation Fuel Pump Based on the Inverse Method

Centrifugal pumps have a wide range of applications in the aviation field. The present work focuses on the optimal design of aviation fuel pump impellers by means of an inverse method. The fuel pump impeller is designed here by solving an inverse problem, in which the impeller geometry is found by imposing a target blade loading. As the inverse procedure is inviscid, an iterative process based on RANS is then applied to finally converge to a fully viscous solution. Three representative loading distributions have been investigated, and the final performances are evaluated by RANS computations. Since flow variables, rather than the blade geometry, are imposed on the target flow field, it is found that the impellers designed by way of the inverse method have high efficiency under the conditions without cavitation; among them, the pump impeller with a higher loading at the hub maintains a high efficiency for a wide range of flow conditions and also has better anti-cavitation performances under low inlet pressure conditions. Moreover, cavitation resistance can be improved by adjusting the loading distribution near the blade leading edge using the inverse design tool.

Experimental Validation of a Multi-Purpose Exhaust System Designed for a Naval Propulsion Group

This paper describes the effort of designing an unconventional exhaust manifold for a marine gas turbine engine, with an integrated passive ventilation port for cooling the engine housing. The study is part of a larger program to substitute the propulsion gas turbines for the T22R defense frigate and make the proper aerodynamic adaptations. The system in question is unique, in the sense that it uses the exhaust gas momentum to entrain outside air and ventilate the engine enclosure. In achieving this, RANS computation was used to test various concepts and dimensions for the ventilation system. Based on these analyses, the design that provided adequate air circulation with minimum pressure losses was chosen and the parts were integrated in the overall assembly. The experimental campaign performed on the entire aero-package showed good synergies of the ventilation system with the other adaptations and the engine itself. Performance was evaluated with pressure and temperature probes distributed around the aero-package and were found to be within 3.5% of the data predicted by CFD. This brings further studies closer to a technology readiness level vital for insitu testing on board the ship itself.

A Study on the Scale Effect According to the Reynolds Number in Propeller Flow Analysis and a Model Experiment

The demand for new propeller designs has increased alongside the development of new technology, such as urban aircraft and large unmanned aerial vehicles. In order to experimentally identify the performance of a propeller, a wind tunnel that provides the operating flow is essential. However, in the case of a meter class or larger propeller, a large wind tunnel is required and the related equipment becomes heavy; therefore, it is difficult to implement in reality. For this reason, propeller studies have been conducted via reduced models. In this case, it is necessary to investigate the different performance outputs between the full- and model-scale propellers due to the size difference. In the current study, a method is proposed to investigate the difference in the aerodynamic performance caused by the difference in propeller scale using VLM and RANS calculations, and the differences are analyzed. The wind tunnel test also verified the propeller performance prediction method. The boundary of aerodynamic performance independent of the Reynolds number could be predicted through the VLM based on the ideal fluid assumption. From the RANS calculations, it was possible to present the difference in the aerodynamic performance when propellers of the same geometry with different ratios were operated using different Reynolds numbers. It was confirmed that each numerical method matched well with the wind tunnel test results in the range of the advance ratio that produced the maximum efficiency, and from the results, it was possible to observe the change in aerodynamic performance that differed according to the scale change.

Low Speed Aerodynamic Analysis of the N2A Hybrid Wing–Body

Reduction of atmospheric emissions is currently a mandatory requirement for aircraft manufacturers. Several studies performed on Blended Wing–Body configurations showed a promising capability of reducing fuel consumption by increasing, at the same time, passengers’ transport capabilities. Although several aerodynamic studies are available at transonic speeds, low-speed evaluations of aerodynamic performances of Blended Wing Body aircrafts are less investigated. In this framework, the present paper deals with the aerodynamic performance of the N2A aircraft prototype at low-Mach number conditions. Aircraft longitudinal aerodynamics is addressed at M∞=0.2 with steady state three-dimensional RANS simulations carried out at two Reynolds numbers equal to 6.60×106 and 1.27×108, respectively. The former refers to an experimental test campaign performed at NASA Langley 14-by-22 foot subsonic tunnel, while the latter is related to free-flight conditions close to an approach and landing phase. Flowfield simulations are performed using the Computational Fluid Dynamic code FLUENT and the SU2 open-source code, currently adopted for research applications. Numerical solutions are validated by using available experimental data with reference to lift, drag, pitching moment and drag polar estimations. Pre-stall and post-stall aerodynamic behaviour through mean flow-field visualization along with the comparison of pressure distributions at several AoAs is addressed. Furthermore, the effect of convective discretization on a numerical solution for SU2 is discussed. Results indicate a good agreement with available experimental predictions. The present study aims to bridge existing computations at a Eulerian low-Mach number, with RANS computations and constitutes a further test-case for SU2 code with respect to a full aircraft configuration.

Numerical simulation of NACA4412 airfoil in pre-stall conditions

PurposeThe purpose of this paper is to study turbulent flow separation at the airfoil trailing edge. This work aims to improve the knowledge of stall phenomenon by creating a QDNS database for the NACA412 airfoil.Design/methodology/approachQuasi-DNS simulations of the NACA 4412 airfoil in pre-stall conditions have been completed. The Reynolds number based on airfoil chord and freestream velocity is equal to 0.35 million, and the freestream Mach number to 0.117. Transition is triggered on both surfaces for avoiding the occurrence of laminar separation bubbles and to ensure turbulent mixing in the wake. Four incidences have been considered, 5, 8 10 and 11 degrees.FindingsThe results obtained show a reasonably good correlation of the present simulations with classical MSES airfoil simulations and with RANS computations, both in terms of pressure and skin-friction distribution, with an earlier and more extended flow separation in the QDNS. The database thus generated will be deeply analysed and enriched for larger incidences in the future.Originality/valueNo experimental or HPC numerical database at reasonable Reynolds number exists in the literature. The current work is the first step in that direction.

VPP Coupling High-Fidelity Analyses and Analytical Formulations for Multihulls Sails and Appendages Optimization

A procedure for the optimization of a catamaran’s sail plan able to provide a preliminary optimal appendages configuration is described. The method integrates a sail parametric CAD model, an automatic computational domain generator and a Velocity Prediction Program (VPP) based on a combination of sail RANS computations and analytical models. The sailing speed and course angle are obtained, with an iterative process, solving the forces and moment equilibrium system of equations. Analytical formulations for the hull forces were developed and tuned against a matrix of CFD solutions. The appendages aerodynamic polars are estimated by applying preliminary design criteria from aerospace literature. The procedure permits us to find the combination of appendages configuration, rudders setting, sail planform, shape and trim that maximise the VMG (Velocity Made Good). Two versions of the sail analysis module were implemented: one adopting commercial software and one based on the use of only Open-Source codes. The solutions of the two modules were compared to evaluate advantages and limitations of the two approaches.

Rankine-vortex model based assessment of CFD methods for simulating the effect of gas entrainment observed in the hot-pool of sodium coold fast breeder reactors

The flow behavior in the hot pool of a compact Sodium Cooled Fast Breeder Reactor (SFR) is investigated in the MICAS facility of the CEA. Computational Fluid Dynamics (CFD) is used to assist the interpretation of the experiments by multi-scale calculations. Special attention is given to the risk of gas entrainment into the reactor core due to the formation of drain-type vortexes at the intermediate heat exchanger intakes. After theoretical considerations based on the Rankine-vortex model, the single effect gas entrainment experiment of Monji is analyzed by different CFD methods. The well-known incapacity of linear RANS models to simulate swirling flow is emphasized and limitations regarding the representation of gas cores by single-phase models are discussed. Then, the flow in the MICAS facility is analyzed by CFD using RANS and LES. The quantitative comparison between experiment and calculation shows that RANS can simulate correctly the global flow patterns in the hot pool. Small-scale drain-type vortexes have not been observed in the RANS calculations. LES can predict the onset of the formation of drain-type vortexes. However, single-phase models cannot simulate stable drain-vortexes.

Numerical analysis on the conceptual design of an air-breathing engine inlet working in mach number 3∼5.5

Based on the equal-intensity shock theory, this article designed a supersonic inlet working in Mach number 3.0∼5.5 with the background of an air-breathing engine. The inlet applied the four-shock train mixed compression configuration and inserted a sidewall compression at the beginning of the isolator. Through developing effective 3D RANS computations validated by current experiments, the performance of the designed inlet was identified. The designed inlet self-starts at freestream Mach number Ma∞ = 3.0 under which the total pressure recovery coefficient has dramatic increment, and the aerodynamic choking at the inlet throat no longer presents; the inlet keeps working at all studied flight states with zero angle of attack (AoA) and achieves shock-on-lip at the design point Ma∞ = 5.0. Both positive and negative AoAs can disrupt the equal-intensity shock allocations, which degrade the inlet performance. The inlet obtains maximum total pressure recovery coefficient at zero AoA. The maximum back pressure at Ma∞ = 3.0∼5.5 obtained by the inlet surpasses the requirements and keeps a certain margin. The inlet performance basically meets all the goals proposed by the engine.

Simulation of flow over high-lifted turbine cascade at low Reynolds numbers

This paper is dedicated to the simulation of flow in the T106C high loaded turbine cascade. A presence of a laminar-turbulent transition at the suction side of the blade caused by the separation of the boundary layer leads to a significant increase in kinetic energy losses. Predictions using both RANS and scale-resolving methods are carried out. The RANS computations are performed by the SST and BSL-EARSM models with Abu-Ghannam and Shaw criterion of laminar-turbulent transition or the model with different closing correlation dependencies. The hybrid RANS-LES IDDES method is used for scale-resolving simulations. The validation is done by comparing the numerical results with the experimental data on the isentropic Mach number along the blade surface and the relative total pressure losses at different Reynolds numbers. The RANS and IDDES results are comparatively analyzed. A strong dependence of all the numerical results on the incoming turbulent flow conditions is observed.

An evaluation of wall functions for RANS computation of turbulent flows

An evaluation of wall functions for RANS computation of turbulent flows

Insight into a fluid-to-fluid similarity theory for heat transfer at supercritical pressure: Results and perspectives

Recent papers by the Authors proposed a successful rationale to establish a similarity theory for heat transfer at supercritical pressures, making use of original definitions that seemingly solved the problem of finding a closely similar phenomenological behaviour with very different fluids. As promised in the conclusions of the latest publication, this paper reports on the result of the work performed by further exploring the features of the theory, confirming its choices in front of additional evidence.In particular, several further RANS calculations, made by the same improved turbulence model adopted in recent works, have been performed, assessing various additional features of the theory and finding excellent confirmations of its capabilities. While waiting for experimental confirmations, RANS analyses are here used as a suitable workbench to compare data obtained by four different fluids (water, carbon dioxide, ammonia and R23) for “similar” conditions. The predicted behaviour, in addition to be conditioned to the limited capabilities of the adopted turbulence model, can be attributed to the different thermodynamic and thermo-physical properties, i.e., to intrinsic features of supercritical fluids. The obtained results further clarify relevant issues, as the most appropriate selection of supercritical pressure for each fluid in front of a given reference condition, together with the level of accuracy achieved in establishing similarity and the role of the most important dimensionless numbers.It is shown that the reported results depict a very clear picture of the role played by the different boundary conditions in determining the relevant heat transfer regimes, with specific reference to deterioration and recovery, opening the possibility for a thorough and physically based revision of the only partially successful engineering correlations for heat transfer proposed up to now.

Improving adiabatic film-cooling effectiveness spanwise and lateral directions by combining BDSR and anti-vortex designs

In the present study, a numerical investigation was conducted to enhance the film cooling efficiency by using anti-vortex designs. Four configurations are considered in this paper, which are the configuration with the streamwise cylindrical injection, the case with an upstream Barchan dune shape ramp (BDSR), the case with sister holes and the configuration that combine the Barchan dune shape with sister holes. The effects of a blowing ration (M = 0.5, 0.85, 1.0 and 1.5) on the film cooling effectiveness are considered. The validation shows good agreement and almost all flow structures are well reproduced by the RANS computation. Results show that the Barchan dune shapes with sister holes have an influence on thermal and flow structures, this configuration substantially augments the film cooling efficiency.

Numerical investigation on the effect of shaft inclination angle on hydrodynamic characteristics of a surface-piercing propeller

The global demand for fast sea transportation has led to an ongoing development of high-speed vessels and surface-piercing propeller, as a high performance propulsor, has played an important role in this development. However, the complexity of the two-phase flow field around the propeller has made its numerical analysis enough challenging. The performance of surface-piercing propeller depends on propeller geometric parameters and flow conditions at propeller disk. Among these parameters, shaft inclination angle is a key parameter which significantly affects the flow conditions at propeller disk. In this paper, RANS computations were applied to investigate the unsteady flow around an optimized surface-pricing propeller in various shaft inclination angles. Likewise, the homogeneous Eulerian multiphase model was employed along with Volume of Fluid model to solve the two-phase flow field equations. Rotational motion of the propeller was simulated by CFX sliding mesh technique. The effect of shaft inclination angle on the hydrodynamic coefficients of the propeller and the behavior of the fluid flow around the propeller key blade are among the principal objectives clarified in this study. As the results indicated, the propeller thrust and torque coefficients were gone up with an increase in the shaft inclination angle. Moreover, as the shaft inclination angle increases, the maximum thrust and torque coefficients of the key blade take place at the lower rotation angles. Additionally, it was revealed that the effect of shaft inclination angle on the torque coefficient of the key blade depends on the angular position of the key blade. Furthermore, the flow patterns around the propeller were predicted in different shaft inclination angles. In order to verify the accuracy of the numerical method used in this paper, numerical simulations were run on SPP-841B propeller with available experimental data. The comparison between the simulated and measured SPP-841B open characteristics as well as the ventilation pattern of the key blade indicates a reasonable agreement with the experimental data.

Evaluation method of riblets effects and application on a missile surface

To overcome the difficulties of numerical simulation for riblets drag reduction effects on large-scale aircraft model, a riblet-equivalent boundary condition is introduced by modifying wall ω value, which makes the riblets effects can be calculated with a conventional smooth wall by RANS computation. Firstly, an empirical formula is deduced by analyzing various experiment results, and the function relationship between nondimensional geometry factor h+ and wall ω value is established. Secondly, the verifications for riblet-equivalent boundary condition are conducted on both zero and adverse pressure flow, by comparing the numerical simulation results with experimental data, which indicates a good reliability of this new method. Finally, an application of riblet-equivalent boundary condition is conducted on a typical configuration missile. Three riblets surface cases are introduced, with various riblets geometry factors. Flow parameters are analyzed for presenting the flow structure and performance of riblets surface. The positive conclusions are obtained that a 2.4% total drag reduction and a 4.6% skin friction reduction are gained.

On the heading stability of a ship in waves

ABSTRACTA procedure is presented for analysing the sway-yaw-heading stability of a ship in regular waves. A linear stability analysis of the motion equations of the ship, using a mathematical model for approximating the hydrodynamic forces acting on the ship including mean forces and moments due to waves, shows if the equilibrium state for a given wave, i.e. the corresponding combination of rudder and drift angle, represents a stable condition. RANS computations for the container ship DTC in calm water and in regular waves of diverse lengths coming from several directions have been used to determine all coefficients of the mathematical model in the showed sample application. Selected situations proven to be stable in theory have successfully been directly simulated afterwards with the RANS code, confirming the validity of the proposed procedure. Some preliminary experiments at the Hamburg Ship Model Basin HSVA support our approach.