Reynolds Averaged Navier-Stokes Numerical Simulations Research Articles

Numerical modelling of ship-generated solitary waves

A ship sailing at the wave speed in shallow water produces a complex wave pattern including a downstream disturbance in addition to a periodically generated upstream disturbance. The upstream component consists of solitary waves which are generated at the ship bow and emitted forward as soon as the local water depth is sufficiently modified enabling them to bypass the shallow water wave speed limit. Reynolds Averaged Navier-Stokes numerical simulations are carried out to explore this phenomenon in a fully non-linear and viscous virtual towing tank in cases where a ship sails at the waterway centreline as well as off-centreline conditions. Results indicate that friction attains no more than approximately 25 % of the total resistance coefficient, justifying the use of inviscid methods by previous studies in the literature. Water depth may have a significant impact on the frequency and amplitude of ship-generated solitary waves, but the manner in which this occurs is highly sensitive to the width of the waterway.

Assessment of URANS and LES methods in predicting wake shed behind a vertical axis wind turbine

In order to shed light on the Vertical-Axis Wind Turbines (VAWT) wake characteristics, in this paper we present high-fidelity CFD simulations of the flow around an exemplary H-shaped VAWT turbine, and we propose to apply Proper Orthogonal Decomposition (POD) to the computed flow field in the near wake of the rotor. The turbine under consideration was widely studied in previous experimental and computational investigations. In the first part of the study, multiple Reynolds-Averaged Navier–Stokes (RANS) simulations were performed at the Tip Speed Ratio (TSR) of peak power coefficient, to select the most accurate turbulence model with respect to available data. In the following step, further RANS numerical simulations were performed at different TSRs to compare the power coefficient against experimental data. Then, Large Eddy Simulation (LES) was applied for multiple TSR conditions. The spatial and temporal POD modes along with modal energy for the RANS and LES results were extracted, and the performance of the turbulence models was assessed. Also, an interpretation of the POD modes with respect to the flow structures was given to highlight the most significant time and length scales of the predictions considering the different dynamical levels of approximations of the computational models.

Approach and Landing Aircraft Wake Encounter Risk Based on Reynolds-Averaged Navier-Stokes Numerical Simulation

In order to shorten the wake safe separation, numerical simulation technology with aerodynamic response models and strip models have been combined to calculate wake hazard zone. As a realistic case, a medium aircraft ARJ21 following a heavy aircraft A330-200 is considered, and the Reynolds-averaged Navier-Stokes (RANS) method is used to explore the wake vortex evolution process of the leading aircraft at the decision height. A strip algorithm is proposed to calculate the rolling moment coefficient and overload increment of the ARJ21 after encountering the wake of the front aircraft in the three-dimensional space. The proposed algorithm identifies the area where the wake of the front aircraft can cause risks to the following aircraft and analyzes the evolution process of the hazard zone of the section where the decision height is located. The minimum safe separation of the ARJ21 following the A330-200 is 1.32 nmile, which is 26.4% of the ICAO separation standard of 5 nmile. When the average runway occupancy time (ROT) is reduced to match the separation of 1.32 nmile, the capacity of runway 02R/20L in Tianfu International Airport could theoretically reach 102.37 sorties/h under this aircraft pair combination. Compared to original 27 sorties/h, the runway capacity can be improved up to 279.14%, which will increase the airport operation efficiency.

URANS numerical simulations of pulsating flows considering streamwise pressure gradient on asymmetric diffuser

The paper focuses on implementing the wall model developed by Manhart, in Reynolds Averaged Navier - Stokes (RANS) turbulence models used in the field of Computational Fluid Dynamics (CFD). This wall model considers the influence of the streamwise pressure gradient in addition to the existing wall models used in the usual CFD codes. In the present work, two RANS numerical simulations are carried out using the k-ω Shear Stress Transport (SST) turbulence model on an asymmetric diffuser geometry. One numerical simulation is carried out using the implementation of the Manhart wall model in the k-ω SST turbulence model, and the other numerical simulation is performed using the standard formulation of the k-ω SST turbulence model. The numerical simulations carried out using the Manhart wall model and the standard formulation of the k-ω SST are compared with experimental measurements made on the asymmetric diffuser experimental installation. The numerical simulations are carried out using a free, open-source CFD tool, Code_Saturne. The comparisons between numerical simulations and the experimental data are in good agreement in the boundary layer of the flow inside the diffuser. The Manhart wall model had a faster convergence resulting in a shorter simulation time.

Scale effect of gas injection into a supersonic crossflow

This study focuses on the scale effect of transverse gas fuel injected into a Ma=2.95 supersonic flow. NPLS (Nanoparticle-based Planar Laser Scattering) and oil flow techniques were employed to obtain flow structures and jet penetration, while the RANS (Reynolds-averaged Navier-Stokes) model was utilized to investigate fuel mixing characteristics. In order to get the scale effect law, the diameters of jet orifice were chosen as D=2 mm and D=4 mm, and the jet-to-crossflow dynamic pressure flux ratio (J) were set as 2.3, 5.5, and 7.7. The jet penetration boundary could be extracted from NPLS images. By analyzing plenty of NPLS images, a corrected fitting formula was given, which could reflect the scale effect law of jet penetration. The oil flow experiments compared the shapes and the size of the V-shape recirculation zone of different scales. Results showed that the size of the recirculation zone amplifies proportionally with the diameter of the orifice growth. The RANS numerical simulations were conducted to study the mixing characteristics, and the detailed fuel mass fraction concentration and mixing efficiency analysis were given. It is concluded that the turbulent boundary layer plays an important role in the scale effect of the jet penetration and mixing characteristics, especially for the low jet-to-crossflow dynamic pressure flux ratio (J).

Formation and suppression of the instability in radial vaneless diffusers

The change of width radius ratio may lead to different physical mechanisms for the instabilities in vaneless diffusers. Reynolds-averaged Navier–Stokes numerical simulations were performed and the results were used to present the mean flow. A quasi-laminar mixed approach based was applied on the vaneless diffusers with narrow and moderate width. The frequency of the perturbations in the vaneless diffuser flow with different axial widths was validated against the experimental data. Moreover, in order to reveal the wavemaker region, the structural sensitivity analysis of the vaneless diffuser flow was carried out. The sensitivity of the eigenvalue to the mean flow modifications were used to guide the flow control. The results indicate that the viscosity tends to stabilize the narrow diffusers but destabilize the moderate width diffusers. For narrow diffusers, the wavemaker regions lie at the centerline near the intersection region of boundary layers. However, the wavemaker regions lie beside the centerline for moderate width diffusers. The occurrence of instabilities may be related with the inflection point of the velocity distribution in axial direction. Given the distribution of the sensitivity to mean flow modifications, the flow control effect may be associated with the amplitude of viscosity stress at the local flow field.

Ethanol turbulent spray flame response to gas velocity modulation

A numerical investigation of the interaction between a spray flame and an acoustic forcing of the velocity field is presented in this paper. In combustion systems, a thermoacoustic instability is the result of a process of coupling between oscillations in heat released and acoustic waves. When liquid fuels are used, the atomisation and the evaporation process also undergo the effects of such instabilities, and the computational fluid dynamics of these complex phenomena becomes a challenging task. In this paper, an acoustic perturbation is applied to the mass flow of the gas phase at the inlet and its effect on the evaporating fuel spray and on the flame front is investigated with unsteady Reynolds averaged Navier-Stokes numerical simulations. Two flames are simulated: a partially premixed ethanol/air spray flame and a premixed pre-vaporised ethanol/air flame, with and without acoustic forcing. The frequencies used to perturb the flames are 200 and 2500 Hz, which are representative for two different regimes. Those regimes are classified based on the Strouhal number St = (D/U)ff: at 200 Hz, St = 0.07, and at 2500 Hz, St = 0.8. The exposure of the flame to a 200 Hz signal results in a stretching of the flame which causes gas field fluctuations, a delay of the evaporation and an increase of the reaction rate. The coupling between the flame and the flow excitation is such that the flame breaks up periodically. At 2500 Hz, the evaporation rate increases but the response of the gas field is weak and the flame is more stable. The presence of droplets does not play a crucial role at 2500 Hz, as shown by a comparison of the discrete flame function in the case of spray and pre-vaporised flame. At low Strouhal number, the forced response of the pre-vaporised flame is much higher compared to that of the spray flame.

Aerodynamic shape optimization of guided missile based on wind tunnel testing and computational fluid dynamics simulation

This paper presents modification of the existing guided missile which was done by replacing the existing front part with the new five, while the rear part of the missile with rocket motor and missile thrust vector control system remains the same. The shape of all improved front parts is completely different from the original one. Modification was performed based on required aerodynamic coefficients for the existing guided missile. The preliminary aerodynamic configurations of the improved missile front parts were designed based on theoretical and computational fluid dynamics simulations. All aerodynamic configurations were tested in the T-35 wind tunnel at the Military Technical Institute in order to determine the final geometry of the new front parts. The 3-D Reynolds averaged Navier-Stokes numerical simulations were carried out to predict the aerodynamic loads of the missile based on the finite volume method. Experimental results of the axial force, normal force, and pitching moment coefficients are presented. The computational results of the aerodynamic loads of a guided missile model are also given, and agreed well with.

Active Flow Control in a Radial Vaned Diffuser for Surge Margin Improvement: A Multislot Suction Strategy

This work is the final step of a research project that aims at evaluating the possibility of delaying the surge of a centrifugal compressor stage using a boundary-layer suction technique. It is based on Reynolds-Averaged Navier-Stokes numerical simulations. Boundary-layer suction is applied within the radial vaned diffuser. Previous work has shown the necessity to take into account the unsteady behavior of the flow when designing the active flow control technique. In this paper, a multislot strategy is designed according to the characteristics of the unsteady pressure field. Its implementation results in a significant increase of the stable operating range predicted by the unsteady RANS numerical model. A hub-corner separation still exists further downstream in the diffuser passage but does not compromise the stability of the compressor stage.

3D pressure imaging of an aircraft propeller blade-tip flow by phase-locked stereoscopic PIV

The flow field at the tip region of a scaled DHC Beaver aircraft propeller, running at transonic speed, has been investigated by means of a multi-plane stereoscopic particle image velocimetry setup. Velocity fields, phase-locked with the blade rotational motion, are acquired across several planes perpendicular to the blade axis and merged to form a 3D measurement volume. Transonic conditions have been reached at the tip region, with a revolution frequency of 19,800 rpm and a relative free-stream Mach number of 0.73 at the tip. The pressure field and the surface pressure distribution are inferred from the 3D velocity data through integration of the momentum Navier-Stokes equation in differential form, allowing for the simultaneous flow visualization and the aerodynamic loads computation, with respect to a reference frame moving with the blade. The momentum and pressure data are further integrated by means of a contour-approach to yield the aerodynamic sectional force components as well as the blade torsional moment. A steady Reynolds averaged Navier-Stokes numerical simulation of the entire propeller model has been used for comparison to the measurement data.

Numerical investigation of unsteady aerodynamic characteristics of a pitching missile

Three-dimensional Reynolds averaged Navier–Stokes numerical simulations were carried out to predict the aerodynamic loads of a pitching winged missile based on the finite volume method. The Baldwin–Lomax eddy viscosity model with the modifications suggested by Degani and Schiff was used here. The computational results of the aerodynamic loads of a slender revolution body are also given, and agreed well with the experiment data. The unsteady aerodynamics about the winged missile, which is oscillated in pitching with different frequencies, with different center of rotation and at different range of angle of attack is shown in the paper. It is found that from the computational results, the unsteady hysteresis of the lift, drag and pitch moment coefficients vary not only with the frequency, but also with the position about which the missile performs the oscillatory motions. Although the angle of attack range at which the missile oscillate in pitching are different, the derivative of lift coefficients with respect to the angle of attack is the same for the same phase of oscillating under the same reduced frequency condition.