Unsteady Analysis Research Articles

Numerical analysis of flow instability with vortex cavitation in venturi tube

The swirling flow with cavitation can cause pressure pulsations in a draft tube of a hydro turbine at off-design operating conditions. Especially at the full load condition, the cavitation volume may fluctuate greatly, causing pressure pulsations with flow rate fluctuations known as cavitation surge throughout the hydraulic system. The cases that predicted the phenomenon qualitatively using the CFD analysis have been reported previously, whereas the methods to predict the cavitation volume and the fluctuation frequency in the cavitation surge condition quantitatively have not yet been established. Therefore, in this study, unsteady CFD analysis was conducted for the experimental apparatus that simulates the flow in the draft tube downstream of a runner using the stationary guide vane and the venturi tube. In CFD analysis, the influence of the turbulence models by using LES and SST and the influence of the number of meshes in LES simulation were investigated. It was founded that LES simulation captured the vortex generation finely and improved the prediction accuracy of the cavitation volume than SST simulation. On the other hand, it was founded that too fine mesh has a small effect on the prediction accuracy of the cavitation volume, although the resolution of the vortex is improved.

Unsteady interaction and dynamic stability analysis of a two-stage-to-orbit vehicle during transverse stage separation

Hypersonic stage separation is a significant process for the future two-stage-to-orbit (TSTO) vehicle. Strong and complex interstage aerodynamic interference may result in drastic aerodynamic forces and moments, potentially conflicting with the safe separation. Therefore, the dynamic stability of the vehicle during separation is critical to aerospace safety. In this study, numerical analysis of a hypersonic flow with Ma = 6.7 past a parallel-staged TSTO vehicle during stage separation is performed by laminar flow simulations. The TSTO vehicle consists of a wave-rider and a spaceplane as booster and orbiter, respectively. Considering the different centers of gravity for the orbiter during separation, the longitudinal dynamic stability of the orbiter is analyzed based on the dynamic characteristic of the center of pressure (CoP) in different cases. The dynamic separation behavior, aerodynamic characteristics, and typical flowfield patterns are clarified. Moreover, the derivative of CoP to time is proposed and analyzed in detail, which serves as an indicator to determine the dynamic stability and the safe stage separation. A safety separation judgment criterion is also proposed based on the CoP dynamic characteristics for the TSTO vehicle, and the mechanism of CoP variations associated with the unsteady aerodynamic interference during stage separation is revealed.

Performance verification of pipe‐embedded wall system in school building and proposal of its optimal control strategy

AbstractIn this study, a thermally activated building system (TABS) installed on the exterior wall as the pipe‐embedded wall (PEW) system in a school building was investigated. The basic performance of the PEW system utilizing well water cascade is verified by unsteady CFD analysis and field measurements. Moreover, in order to improve the utilization efficiency of the thermal potential from well water, the MPC‐based optimal control method with the PEW system to maximize the heat extraction is proposed and verified by unsteady CFD analysis. The analysis results showed that: (1) The PEW system can reduce the peak load from the exterior wall facing different directions at different times. In addition, the PEW system has the effect of stabilizing the indoor thermal environment. (2) The proposed optimal operation strategy improves the energy efficiency, extracting up to 30% more heat and reducing the total heat transmission, which significantly develops the operation efficiency of the PEW system.

Low-frequency large-scale unsteadiness analysis of lateral jet interactions on a slender cylinder in hypersonic flows using fast-responding pressure-sensitive paint

Lateral jet interactions, commonly encountered in high-speed vehicles, are typical complex three-dimensional (3D) shock wave/boundary layer interactions. The spatiotemporal features of surface pressure are crucial for understanding such complex interactions. In this study, fast-responding pressure-sensitive paint measurements are implemented at a sampling rate of 3000 Hz and a spatial resolution of 0.35 mm/pixel to investigate global pressure fields in lateral jet interactions on a slender cylinder under hypersonic flow conditions. Tests are conducted at three angles of attack (0°, 5°, and 10°) and two sideslip angles (0° and 2.5°) at Mach 6 and a Reynolds number of 1.35 × 107/m. The mean pressure results indicate the presence of a 3D high-pressure region beneath the bow shock feet, the area and pressure level of which depend on the cylinder attitude. Moreover, significant low-frequency broadband unsteadiness associated with the large-scale motion of bow shock is found in the high-pressure region. Statistical, coherence, and modal analyses are performed to clarify the characteristics of the large-scale unsteadiness. The results indicate that the dominant pressure mode is induced by streamwise oscillation of the bow shock, which exhibits two subregions with opposite pressure-fluctuation variations. Furthermore, the effects of the cylinder attitude on the unsteadiness extent and intensity are discussed.

Numerical Modelling of the 3D Unsteady Flow of an Inlet Particle Separator for Turboshaft Engines

Helicopter and turboprop engines are susceptible to the ingestion of debris and other foreign objects, especially during take-off, landing, and hover. To avoid deleterious effects, filters such as Inlet Particle Separators (IPS) can be installed. However, the performance and limitations of these systems have to be investigated before the actual equipment can be installed in the aircraft powerplant. In this paper, we propose different numerical methods with increasing resolution in order to provide an aerodynamic characterization of the IPS, i.e., from a simple semi-empirical model to 3D large eddy simulation. We validate these numerical tools that could aid IPS design using experimental data in terms of global parameters such as separation efficiency and pressure losses. For each of those tools, we underline weaknesses and potential benefits in industry practices. Unsteady flow analysis reveals that detached eddy simulation is the trade-off choice that allows designers to most effectively plan experimental campaigns and mitigate risks.

Unsteady aeroelastic performance analysis for large-scale megawatt wind turbines based on a novel aeroelastic coupling model

Unsteady aeroelastic performance analysis for large-scale megawatt wind turbines based on a novel aeroelastic coupling model

Uncertainty analysis on flood routing of embankment dam breach due to overtopping failure

The breach risk of reservoir dam exists objectively while exerting benefits. There is great uncertainty in the overtopping breach process of embankment dam and the dam break flood routing which affects the accuracy of dam risk analysis and may cause unnecessary waste in emergency scheduling. However, the uncertainty analysis of the breach process and its consequence is currently inadequate. Therefore, a stratified sampling Monte Carlo method is proposed to simulate uncertain overtopping breach flood of embankment dams. The main sources of uncertainty are analyzed and determined as uncertain dam breach and flood routing processes. The uncertain breach process of dam is studied by presenting a sensitive study between 3 mechanism breach models and 5 parametric breach models. The random dam breach process is restored using HEC-RAS semi-empirical breach model by estimating breach characteristics through multiple common breach models. The random flood routing is carried out through 1D–2D coupled unsteady flow analysis in which the random breach process is adopted as an upper inflow boundary condition. According to the case study results, though parameters have been controlled in a limited range, the flood routing results in the early stage of dam overtopping failure present greater uncertainty. As the flood progresses further downstream, the uncertainty will gradually decrease. This study could serve as a reference for dam breach risk map making.

Influence of Door Gap on Aeroacoustics and Structural Response of a Cavity

Numerical aeroacoustic analysis using the Shear Stress Transport–Scale Adaptive Simulation turbulence model was conducted on a weapon bay model based on the M219 geometry with doors incorporating radar cross-section reduction features. The effect of the introduction of a gap between the doors and the cavity edge on the aeroacoustic and structural response of the cavity was analyzed at Mach 0.85. The effect of introducing 3 deg of sideslip was also investigated. Both mean and unsteady flow analyses were conducted. The results showed a strong palliative effect of the door gap with and without sideslips. The overall analysis of the spectral signature on the forces and moments acting on the doors indicated the possibility of fluid–acoustic coupling, as all acoustic spectra showed a predominant tone located at the same frequency of the first structural mode.

One-dimensional mechanism of gaseous deflagration-to-detonation transition

A one-dimensional mechanism of deflagration to detonation transition is identified and investigated by an asymptotic analysis in the double limit of large activation energy and small Mach number of the laminar flame velocity. The unsteady analysis concerns the self-accelerating tip of an elongated flame in a smooth walled tube. The flame on the tip, considered as plane and orthogonal to the tube axis, is pushed from behind by the longitudinal flow resulting from the cumulative effect of the radial flows of burned gas issued from the lateral flame of the finger-like front (called backflow in the following). The analysis of the one-dimensional dynamics is performed by coupling the flame structure with the downstream-running compression waves propagating in the external flows. A critical elongation is identified from which the slightest increase in elongation leads to a pressure runaway producing the flame blow-off. The dynamics of the inner structure of the laminar flame on the tip which is accelerated by the self-induced backflow is characterized by a finite-time singularity of the reacting flow in the form of a dynamical saddle-node bifurcation.

Dynamic mode decomposition for the tip unsteady flow analysis in a counter-rotating axial compressor

Counter-rotating axial compressor (CRAC) is a promising potential technology to improve the thrust-to-weight ratio of aero-engines, but its special aerodynamic layout usually causes more pronounced flow unsteadiness. Understanding the unsteady flow features and mechanism in the CRAC contributes to the aerodynamic optimization design and flow control strategy organization. A data-driven dynamic mode decomposition method is introduced to investigate the tip flow unsteadiness in a CRAC, and the unsteady features of the tip flow at the design point (DP) and near-stall point (NSP) conditions are revealed. The results show that the 1.0 times blade passage frequency (BPF) and its multi-order harmonic frequency are the dominant frequencies for both rotors at the DP condition. At the NSP condition, the 1.0 BPF is no longer the dominant frequency causing the tip flow unsteadiness, and the low frequency fluctuation of the tip leakage flow becomes the dominant frequency to induce the flow unsteadiness. In the front rotor R1, the unsteady dominant frequency is 1.0 BPF, whereas in the rear rotor R2, the frequency (0.801 BPF and 0.803 BPF) of the tip leakage flow is the dominant frequency. By reconstructing the flow field under the NSP condition, the spatiotemporal evolution of the tip flow during the unsteady stable manifests that the interference effect between the rotors is an important source of the tip flow unsteadiness. The increase in flow unsteadiness leads to an increase in the reconstruction error, indicating that more modes are required to obtain a more accurate reconstruction flow field.

Unsteady analysis of a solar air heater with sensible heat storage using 2D finite difference approximation

Unsteady analysis of a solar air heater with sensible heat storage using 2D finite difference approximation

Irreversibility analysis and thermal radiative of Williamson (ZnO+MOS 2/C 3 H 8 O 2) hybrid nanofluid over a porous surface with a suction effect

The current approach considered the unsteady 2D laminar second law analysis on Williamson hybrid nanoliquid in a porous medium under the effect of the thermal radiative flow towards a convectively heated stretching sheet. Propylene glycol () is employed as the base liquid in this study, and the nanoparticles are Molybdenum disulfide and Zinc Oxide The purpose of the current study is to maximize the energy and mass transfer rate for industrial and engineering applications. The primary contribution of this study is to discuss heat transmission via thermal radiation, suction, and the porous parameter on the flow of a William hybrid nanoliquid. The constitutive Maxwell flow model, heat transport, and entropy generation can be reduced to ordinary differential equations by similarity transformations. The Bvp4c technique is used on these generating ODEs for the numerical technique in the methodology section. Fluid friction, heat transmission, and Joule heating are used to calculate the entropy generation number. Graphs and tables are used to investigate the effects of dimensionless parameters on flow variables and entropy generation. The primary results suggest that hybrid Maxwell nanofluid is a more efficient heat conductor than regular nanofluid. A significant drop in the velocity field is supported by the Williamson Unsteady and porosity factors The temperature profile is significantly raised by the Biot number thermal radiation and suction factor Moreover, it has been found that entropy increases with radiation but reduces in Brinkman number against the Bejan number profile. Our results for a few specific situations were compared with previously published data to establish the validity of the current study, and it was discovered that they were in strong agreement.

Theoretical and experimental investigation on the stall inception of low-speed axial compressors with swept rotor blades

The effect of rotor blade sweep on the flow stability of low-speed axial compressors is investigated by means of theoretical model, numerical simulations and experimental measurements. Four compressors with forward or backward swept rotors are carefully designed by modifying the tip element location of the baseline rotor. Steady simulation results show that the designed aerodynamic sweep in this work hardly changes the pressure rise and efficiency characteristics of the compressor, which ensures the comparability between these compressors regarding flow stability. The developed stall inception prediction model based on global linear stability analysis and eigenvalue theory is employed to evaluate the stall boundary of compressors after blade sweep designs, and results indicate that the forward sweep slightly improve the flow stability, while the backward sweep has significant deterioration effect. What's more, the circumferential propagation frequency of the perturbation wave at stall onset changes little after the sweep design. The steady and dynamic measurements in experiments for different rotor blades verifies the theoretical conclusions and the prediction accuracy of the theoretical model. Through steady and unsteady flow field analysis, it is found that the forward sweep reduces the diffusion factor, elementary work and blade loading at the rotor tip region, while the backward sweep has the opposite effect and enhances the interaction between the tip leakage flow and the primary flow, which is deemed the contributors to their effect on flow stability.

Unsteady cavitation dynamics and pressure statistical analysis of a hydrofoil using the compressible cavitation model

A compressible cavitation model is developed in this paper, in which the bubble wall velocity is obtained by solving the compressible Rayleigh–Plesset (R–P) equation. Additionally, vapor compressibility is also included during evaporation/condensation to correct the phase change rate. The predicted results around a National Advisory Committee for Aeronautics (NACA) 66 (mod) hydrofoil are compared with the available experimental data, and a satisfied agreement is obtained. By (mod), we mean the NACA 66 hydrofoil modified by Brockett [“Minimum pressure envelopes for modified NACA-66 sections with NACA a = 0.8 camber and BuShips type I and type II sections,” Technical Report No. 1780 (David Taylor Model Basin Washington DC Hydromechanics Lab, 1966)] and Valentine [“The effect of nose radius on the cavitation-inception characteristics of two-dimensional hydrofoils,” Technical Report No. 3813 (Naval Ship Research and Development Center, 1974)]. Several crucial flow properties, e.g., fluid compressibility, cavitation evolution features, and pressure statistical characteristics, are studied in detail. The results suggest that the developed compressible cavitation model is better suited for predicting the collapse behavior of cavitation. Moreover, our work captures the liquid re-entrant jet and bubbly shock waves well and reveals that these two mechanisms jointly dominate the cavity shedding dynamics. Shock-induced pressure pulses play a more important role in flow features, with a maximum amplitude exceeding 200 kPa, significantly larger than the pressure pulse caused by liquid re-entrant jets. Finally, the statistical analysis indicates that the pulsating pressure presents non-Gaussian nature with positive skewness, and shock waves exhibit high-frequency and high-energy characteristics.

Evolution of unsteady secondary flow structures near the onset of stall in a tip-critical transonic axial flow compressor stage

The aerodynamic stability of an axial compressor stage depends on the rotor and stator. However, due to the specific design requirements, the evolving flow field and the resulting secondary flow structures are different in both components. This study investigates the evolution of dominant secondary flow structures occurring in the rotor and stator of a tip-critical transonic compressor stage at the near-stall condition using unsteady numerical analysis and validates performance characteristics with the experimental data. The investigation reveals that the presence of rotor tip shock creates a large difference in the pressure gradient across the pressure surface and the suction surface, intensifying tip leakage flow and shock-induced boundary layer separation. The higher incidence angle near the hub leading edge creates a local separation and reattachment zone. The radial pressure gradient causes the low momentum flow from this local separation zone to migrate radially upward. This migrated flow interacts with the tip leakage flow and separated blade boundary layer, eventually creating a colossal recirculation zone and subsequent rotor blockage of around 46%. The increasing streamwise adverse pressure gradient pushes the tip shock upstream, and at the onset of the stall, the flow directly separates from the rotor leading edge avoiding the shock formation. The stator flow field is dominated by the asymmetric hub corner separation induced by the streamwise adverse pressure gradient and the tip corner separation caused by the vortex structures convected downstream from the rotor tip region leading to stator blockage of around 48%. Along with the blade passing frequency, four other dominating frequencies (0.06×BPF, 0.12×BPF, 0.44×BPF, and 0.84×BPF) related to the aerodynamic instabilities are observed at the inception of stall.

Numerical investigation of the dynamic stall reduction on the UAS-S45 aerofoil using the optimised aerofoil method

AbstractThis paper investigates the effect of the optimised morphing leading edge (MLE) and the morphing trailing edge (MTE) on dynamic stall vortices (DSV) for a pitching aerofoil through numerical simulations. In the first stage of the methodology, the optimisation of the UAS-S45 aerofoil was performed using a morphing optimisation framework. The mathematical model used Bezier-Parsec parametrisation, and the particle swarm optimisation algorithm was coupled with a pattern search with the aim of designing an aerodynamically efficient UAS-45 aerofoil. The $\gamma - R{e_\theta }$ transition turbulence model was firstly applied to predict the laminar to turbulent flow transition. The morphing aerofoil increased the overall aerodynamic performances while delaying boundary layer separation. Secondly, the unsteady analysis of the UAS-S45 aerofoil and its morphing configurations was carried out and the unsteady flow field and aerodynamic forces were analysed at the Reynolds number of 2.4 × 106 and five different reduced frequencies of k = 0.05, 0.08, 1.2, 1.6 and 2.0. The lift ( ${C_L})$ , drag ( ${C_D})$ and moment ( ${C_M})\;$ coefficients variations with the angle-of-attack of the reference and morphing aerofoils were compared. It was found that a higher reduced frequencies of 1.2 to 2 stabilised the leading-edge vortex that provided its lift variation in the dynamic stall phase. The maximum lift $\left( {{C_{L,max}}} \right)$ and drag $\left( {{C_{D,max}}} \right)\;$ coefficients and the stall angles of attack are evaluated for all studied reduced frequencies. The numerical results have shown that the new radius of curvature of the MLE aerofoil can minimise the streamwise adverse pressure gradient and prevent significant flow separation and suppress the formation of the DSV. Furthermore, it was shown that the morphing aerofoil delayed the stall angle-of-attack by 14.26% with respect to the reference aerofoil, and that the ${C_{L,max}}\;$ of the aerofoil increased from 2.49 to 3.04. However, while the MTE aerofoil was found to increase the overall lift coefficient and the ${C_{L,max}}$ , it did not control the dynamic stall. Vorticity behaviour during DSV generation and detachment has shown that the MTE can change the vortices’ evolution and increase vorticity flux from the leading-edge shear layer, thus increasing DSV circulation. The conclusion that can be drawn from this study is that the fixed drooped morphing leading edge aerofoils have the potential to control the dynamic stall. These findings contribute to a better understanding of the flow analysis of morphing aerofoils in an unsteady flow.

Kinetic Equation Study on Momentum Conservation of Aerodynamics Incompressible Fluid of High-Altitude Aircraft

This paper introduces an integrated aerodynamic design method for aircraft based on numerical solution of hydrodynamics and electromagnetics equations. By using Pro/Engineer, ANSA, Fluent and MATLAB, the geometric shape, unstructured fluid mesh, hydrodynamic parameters and unpowered flight trajectory data of an aircraft were processed and calculated. In this paper, N-S equations are studied, and a test scheme for a hydrodynamic material aerodynamic model is established. Then, relevant parameters of the aircraft are tested and studied. The feasibility of this method for unsteady aerodynamics analysis of aircraft is verified.

Modeling unsteady and steady 1D hydrodynamics under different hydraulic conceptualizations: Model/Software development and case studies

The stage-discharge relationship is affected by hysteresis, especially in 1D river cross-sections with very mild slopes, and/or where backwater effects occur. To model these cases, hydraulic property (HP) estimations (e.g., hydraulic radius and conveyance) are usually required as closed-form functions of stage. However, these are typically unavailable for complex cross-sections with overbanks. For no sediment-laden flows, we created the HydroHP - 1D tool to solve the 1D Saint Venant Equations aided by the cross-section HP stored in tables. It also simulates irregular and composite hydraulic cross-sections, allows hydrodynamic simulation using the single-channel method (SCM), and proposes the divided channel method (DCM) for steady and unsteady flow analysis. A wide range of applications in rivers such as the Little Washita River and the Upper Negro River, and comparisons with NOAA normal depth solver, HEC-RAS, and with an analytical solution shows a promising scenario of applying HydroHP - 1D to estimate 1D steady and unsteady hydrodynamics.

Investigation of Vaned-Recessed Casing Treatment in a Low-Speed Axial Flow Compressor, Part I: Time-Averaged Results

This paper investigates the effects of two modifications to a vaned recessed casing treatment. First, the shape of a circular curve was used in the top of the treated casing. Second, a fully curved guide vane was also applied. The goals of the modifications are to enhance the flow recirculation as well as to relieve the low-speed flow, which is normally accumulated within the corners of the vaned recessed casing treatment. The solid casing in addition to the two vaned recessed configurations with 23.2% and 53.5% rotor blade tip axial chord exposure have been studied numerically. The results indicated that two mechanisms are involved in the stall margin enhancement. First, the circumferential pressure gradient is reduced for both configurations. The reduction in pressure gradient largely reduces the development of tip leakage vortex and, thus, the generation of low-speed fluid is diminished. Second, the main flow/tip leakage interface moves toward downstream and the movement of interface toward the leading edge is delayed. The second configuration with a greater rotor blade tip exposure enables extra flow recirculation due to decreasing surface area and, therefore, could be superior to the application of the first casing treatment configuration. The major streamlines within the casing treatment are also discussed. The time-averaged results are presented in this paper, while the unsteady results including instantaneous flow fields, origins of the unsteadiness and frequency analysis are discussed in part II.

Analysis of the hydrodynamic damping of a hydrofoil with leading edge cavitation using the modal work approach

The objective of this investigation is to evaluate the applicability and accuracy of the modal work approach in predicting the hydrodynamic damping of a hydrofoil with leading edge cavitation. Simulations are carried out at a flow velocity of 6.64 m/s, an attack angle of 10°, and cavitation numbers between 1.04 and 2.02. The grid scale, time step, and mode shape amplitude are carefully verified. Results for the cavitation shedding frequencies and hydrofoil natural frequencies show good agreement between experiment and simulation, with relative errors within 8.76% and 7.12%, respectively. The unsteady characteristics of leading edge cavitation have a significant influence on the modal parameters, such as the variation in natural frequency can reach 32.57% in a cavitation cycle and the corresponding mode shape is also changed. Therefore, a new strategy including unsteady mode analysis and energy dissipation signal filtering is proposed to simulate the hydrodynamic damping ratio with leading edge cavitation. Then, the time-averaged hydrodynamic damping ratio in a cavitation cycle is obtained, with relative errors between 15.11% and 35.57% by comparing them with the experimental results. This study realizes the application of the modal work approach in leading edge cavitation conditions.