Unsteady Simulations Research Articles

Aerodynamic noise characteristics of a centrifugal fan in high-altitude environments.

In high-altitude areas, the air is thin and the atmospheric pressure is low, which can affect the performance of centrifugal fans and aerodynamic noise. In this paper, steady and unsteady simulations of a centrifugal fan flow field are performed at altitudes of 0, 1000, 2000, 3000, 4000, and 5000 m, and the Ffowcs Williams-Hawkings equation is used to predict the aerodynamic noise of the fan. The results indicate that the tonal and broadband noise generated by the fan decrease with increasing altitude, and the A-weighted sound pressure level of each frequency band of the fan decreases when the air volume is held fixed. The maximum sound power level Lwmax, sound pressure pulsation interval, and total noise sound pressure level Lp decrease linearly with increasing altitude. For every 1000 m increase in altitude, Lwmax and Lp decrease by 0.45 dB and 1.05 dB respectively. The fan noise characteristics, performance parameters, and human auditory perception are the main factors that affect the establishment of fan noise standards in high-altitude areas.

Extension and Validation of the Turbomachinery Capabilities of SU2 Open Source Computational Fluid Dynamic Code

Abstract Computational Fluid Dynamic (CFD) tools have revolutionized the way to design engineering systems, but most established codes are proprietary and closed-source, making it difficult, if not impossible, to modify, debug, or add new features to the code. To provide a freely available open-source CFD code for turbomachinery aerodynamics and aeroelasticity, this paper enhances the turbomachinery capabilities of the open-source SU2 code and demonstrates its capabilities of single-passage steady simulation, full-annulus unsteady simulation, and aeroelasticity analysis in two high-speed compressors, namely NASA Stage 35 and TUDa-GLR-OpenStage, and a linear cascade SC1. For the single-passage steady simulation of NASA Stage 35, the SU2 results are validated against the measured data and verified against the commercial solver ansys cfx, and the performance characteristics results are in reasonably good agreement with each other. For the single-passage steady simulation of TUDa-GLR-OpenStage, grid and turbulence model sensitivity studies are performed and results are validated against the measured data, and SU2 can predict both the performance characteristics and the radial profiles with sufficient accuracy. For the full-annulus unsteady simulation of NASA Stage 35, it is demonstrated that SU2 can predict the propagation of inlet distortion equally well as ansys cfx. For the linear cascade, SU2 can predict the unsteady pressure and aerodynamic damping coefficient accurately. The presented results demonstrate the turbomachinery aerodynamics and aeroelasticity capabilities of SU2. The major modifications of SU2 made in this work will be shared with the code maintainer and the community in the future.

Effects of Different Turbulence Models on Prediction of Oscillating Hydraulic Jump at a Drop Structure with a Trench

Open-channel flows over a drop structure with a trench, often seen in urban rivers, may cause an oscillating phenomenon of hydraulic jumps depending on the trench geometry and hydraulic conditions. The oscillating hydraulic jumps are accompanied by large-scale water-surface variations with wave breaking, and numerical predictions of such flows are challenging in hydraulics. Using unsteady Reynolds-averaged Navier-Stokes simulations of air-water two-phase flows, we attempted to predict an oscillating hydraulic jump at a drop structure with a trench observed in our laboratory experiment. Several high-Reynolds-number turbulence models were applied, and the effects of turbulence models and grid resolutions on predicting the oscillating hydraulic jump were investigated. Turbulence models other than the k–ω SST model resulted in a steady-state flow with no oscillations regardless of the grid resolution. The k–ω SST model succeeded in predicting the oscillating hydraulic jump using a fine grid while the flow became a quasi-steady state using a coarse grid. Moreover, the period of the oscillation and the flow fields are in good agreement with the experimental results. The k–ω SST model has the ability to predict oscillating hydraulic jumps, including wave breaking, because this model accurately simulates flows with separations or stagnations under adverse pressure gradients.

Multi-scale analysis of respiratory droplets transport within the breathing cloud

Infectious diseases are transmitted primarily through pathogen laden droplets commonly exhaled during respiratory events such as breathing, coughing, or sneezing. The transport and evaporation of droplets are governed by the fundamental laws of fluid mechanics and convection-diffusion. From these, analytical models can be created helping in better understanding pathogens transmission from a mechanical perspective. The droplet transport within the humid air breath cloud and the local ventilation are crucial for accurately predicting the droplet fate. Different levels of complexity are possible for modelling the breath cloud, from simple 1D models to complex unsteady 3D simulations including discrete phase models for the droplets simulation. The former are too simple to capture the fluid dynamics of intermittent jets in detail, while the latter are too computationally expensive. The current work presents a novel multi-scale approach where an analytical model of the droplet transport and evaporation is coupled to unsteady CFD simulations of warm humid puffs of exhaled air. The proposed model has the advantage of the accuracy of an analytical model and the computational cost of a relatively standard unsteady CFD simulation, and can be used to predict the trajectory of the exhaled droplets for a variety of respiratory events.

Design optimization of a hybrid casing treatment based on axial momentum budget analysis in the tip flow region

In this paper, a novel slot–groove hybrid casing treatment is designed and optimized to improve the stall margin of a low-speed axial compressor. A combination of the axial slot and circumferential groove casing treatments is utilized to increase the stall margin without incurring efficiency loss. The slot meridional profile is described with 2 B-spline curves. Circumferential grooves are parametrically described with groove height and width. An in-house optimization design platform is constructed based on the nondominated sorting genetic algorithm II and Kriging surrogate model. The optimization objectives are the stall margin and the peak efficiency of the compressor at the design rotating speed. To avoid the large number of unsteady simulations that are required to predict the stall margin, a stall margin improvement indicator is proposed based on the axial momentum budget analysis at the rotor tip region. The performance of the optimal slot–groove hybrid casing treatment design is tested and simulated. The experimental data show that the optimal slot–groove hybrid casing treatment improves the stall margin by 8.42% without generating efficiency loss. The flow details are captured by unsteady simulations and analyzed in depth. The application of the optimal casing treatment enhances the blade tip axial momentum and the interface between tip leakage flow and incoming main flow is pushed downstream. Consequently, the stability of the compressor is improved.

Generalized prediction for self-starting performance of two-dimensional hypersonic inlets

The acceleration self-starting performance of a hypersonic inlet is pivotal for ensuring the stable operation of a scramjet. While the geometric configuration of the internal contraction section (ICS) considerably influences the inlet's self-starting performance, the current prediction model solely considers the internal contraction ratio. To encompass the entire geometric configuration's influence on ICS, a generalized prediction model for the acceleration self-starting Mach number of critical hard unstart two-dimensional hypersonic inlets is proposed. This model calculates the theoretical reattachment pressure rise of the main separation bubble within the actual unstarted flow structure of the hypersonic inlet. Additionally, it computes the theoretical pressure rise assuming the main separation bubble is in a critical state. By comparing these pressures, the model evaluates whether the main separation bubble can be sustained under given incoming flow conditions, predicting the self-starting Mach number. This modeling approach offers broad adaptability to various ICS configurations and incoming flow Reynolds numbers. Each step of the prediction model and the final computational results underwent rigorous evaluation through unsteady numerical simulations. Remarkably, the prediction results demonstrated exceptional alignment with simulation outcomes, surpassing the accuracy of previous prediction methods.

On the double-sided shock diffractions in quiescent and supersonic crossflows

Shock diffraction is a widespread phenomenon in aerospace applications, such as shock tunnel nozzle and jet exciter exit, impacting their performance significantly. This paper focuses on the transient evolution of double-sided shock diffraction in both quiescent and supersonic crossflows by unsteady numerical simulations. The characteristics of the shock wave and the vortex are revealed. In the quiescent flow, the double-sided shock diffraction exhibits remarkable symmetry. The diffracted shock retains a self-similar nature, but its intensity distribution displays non-uniform characteristics, which gradually weakens from the center to both sides. The vortices on both sides also exhibit symmetrical behavior, with their trajectory behaving in linear tendency. When the supersonic crossflow interacts with the diffracted shock, an upward-moving separation shock and an asymmetric diffracted shock are generated. The vortices remain confined beneath the boundary layer and exhibit different shapes. Moreover, due to the rapid motion of the separation shock, the relative Mach number is introduced into the free-interaction theory (FIT) to predict the shock angle of the separation shock. The F(x¯) values corresponding to the separation point and pressure plateau are determined to be 3.04 and 4.68, respectively. The results evaluated by modified FIT show a good agreement with the values of simulation and experiment.

Analysis of Cavitation-Induced Unsteady Flow Conditions in Francis Turbines under High-Load Conditions

Hydraulic vibrations in Francis turbines caused by cavitation profoundly impact the overall hydraulic performance and operational stability. Therefore, to investigate the influence of cavitation phenomena under high-load conditions, a three-dimensional unsteady numerical simulation is carried out for a Francis turbine with different head operating conditions, which is combined with the SST k-w turbulence model and two-phase flow cavitation model to capture the evolution of cavitation under high-load conditions. Additionally, utilizing entropy production theory, the hydraulic losses of the Francis turbine during cavitation development are assessed. Contrary to the pressure-drop method, the entropy production theory can quantitatively reflect the characteristics of the local hydraulic loss distribution, with a calculated error coefficient τ not exceeding 2%. The specific findings include: the primary sources of energy loss inside the turbine are the airfoil cavitation and cavitation vortex rope, constituting 26% and 71% of the total hydraulic losses, respectively. According to the comparison with model tests, the vapor volume fraction (VVF) inside the draft tube fluctuates periodically under high-load conditions, causing low-frequency pressure pulsation in the turbine’s power, flow rate, and other external characteristic parameters at 0.37 Hz, and the runner radial force fluctuates at a frequency of 1.85 Hz.

Numerical Analysis of Unsteady Internal Flow Characteristics in a Bidirectional Axial Flow Pump

A bidirectional axial flow pump that utilizes an S-shaped hydrofoil design exhibits low efficiency and hydraulic instability when operated in reverse. In order to understand the unsteady flow characteristics of this bidirectional axial pump under different operating conditions, the SST k–ω turbulence model was applied to carry out a three-dimensional unsteady numerical simulation of the full flow channel of the pump. The reliability of the numerical calculation model was verified by comparing it with the experimental head and efficiency. The pressure pulsation characteristics on the impeller surface and the pump device under different operating conditions and the transient forces on the impeller were analyzed. The results show that the head and efficiency in reverse operation were lower than in forward operation and the flow streamline of the impeller outlet area was more turbulent in the reverse operation condition. The monitoring points at the inlet and the top of the impeller surface showed the largest pressure pulsation amplitude. The radial and axial forces on the impeller in the reverse operation were greater than those in the forward operation. Under a reverse 1.0 Qdes condition, the average pressure pulsation amplitudes at the inlet of the impeller were 19.2 times and 5.7 times of that at the inlet of the guide vane and the outlet of the impeller, respectively. This study provides a reference for the hydraulic design and optimization of bidirectional axial flow pumps.

Study on the Unsteady Flow Characteristics of a Pump Turbine in Pump Mode

Extensive research has been conducted on the performance of pump turbines, particularly focused on understanding the generation mechanism of S-shaped characteristics. However, there has been a lack of research on unsteady flow characteristics in hump characteristics with small guide vane openings. This study focuses on the hump characteristics of a pump turbine in pump mode. The unsteady numerical simulation method is used along with experimental testing to examine the internal flow characteristics and induced pressure fluctuations under pump operating conditions. The results indicate that flow separation occurs in the impeller when the flow rate decreases to the valley operating condition, and recirculation flow occurs near the impeller inlet at the partial flow rate. Moreover, the unstable flow on the positive slope exhibits a low-frequency characteristic of 0.15fn. The pressure fluctuation from the hub to shroud areas of the guide vane region diminishes sequentially. Notably, distinct vortex structures emerge at the draft tube cone section under the valley operating condition. These structures extend toward the elbow section of the draft tube as the flow rate decreases. This phenomenon generates low-frequency pressure fluctuation originating from the primary frequency of the vortex and dean vortex on the surface, located at 0.4 D of the draft tube under conditions of low flow rate.

Large eddy simulation of cavitating flow around a pitching hydrofoil

This numerical investigation employs advanced computational fluid dynamics techniques to examine the intricate physics of cavitating flows around a pitching NACA 66 hydrofoil. Unsteady simulations are conducted using both large eddy simulation (LES) and K-Omega SST turbulence models. The pitching motions range from 0 to 15°, encompassing four cavitation numbers (3.25, 3.5, 3.75, 4) and a Reynolds number of 750,000. The study comprehensively analyzes various flow field characteristics, including pressure distribution, velocity, vorticity, shear stress, vapor volume fraction, and turbulence kinetic energy. The kinematics of pitching significantly influence hydrodynamic loads and the surrounding flow structures in cavitating conditions. Flow visualizations illustrate the evolution of sheet/cloud cavitation, reentrant jet, and vortex dynamics throughout a pitching cycle. Notable findings demonstrate the impact of cavitation number on cavity shape and the relationship between angle of attack, reentrant jet location, and vortex size. Additionally, negative vorticity dilatation is observed to be correlated with cavity size.

Low-Engine-Order Forced Response Analysis of a Turbine Stage with Damaged Stator Vane.

A damaged stator vane can disrupt the circumferential symmetry of the design flow for turbine assemblies, which can lead to a low-engine-order (LEO) forced response of rotor blades. To help engineers be able to better address sudden vane damage failures, this paper conducts a mechanism analysis of the LEO forced response of rotor blades induced by a single damaged vane using an in-house computational fluid dynamic code (Hybrid Grid Aeroelasticity Environment). Firstly, it is found that the damaged vane introduces a family of LEO aerodynamic excitations with high amplitudes by full-annulus unsteady aeroelastic simulations of a single-stage turbine. In particular, the LEO forced response of the rotor blades excited by 3EO is 2.01 times higher than the resonance response excited by vane passing frequency, and the LEO resonance risk of the rotor blades is greatly increased. Then, by analyzing the flow characteristics of the wake and potential field of the stator row with a damaged vane, the localized high transient pressure in the notch cavity and the radial redistribution of the secondary vortex at the stator exit are the main sources of the low-order harmonic components in the flow field. Importantly, the interaction mechanisms in two regions with high LEO excitation amplitude on the rotor blade surface are revealed separately. Finally, an evaluation and comparison of a single damaged vane in terms of aerodynamic performance and LEO forced response was carried out. The results of this paper provide a good theoretical basis for engineers to effectively control the resonance response of rotor blades caused by a damaged stator vane in turbine design.

Passive flow control at impeller radial bend for stall delay in centrifugal compressors with fishtail pipe diffusers

This paper proposes a new strategy for extending centrifugal compressor stall margin through shroud passive flow control placed at the impeller radial bend. Relative to the usual leading-edge location, the radial bend presents fewer geometrical constraints for flow control devices and allows them to have potentially more impact on low-momentum flow structures that lead to stall. Two centrifugal compressors with fishtail pipe diffusers are chosen for study: a high-speed transonic design exhibiting diffuser stall and a low-subsonic design with exducer stall. Four passive flow control techniques placed on the impeller shroud at the radial bend are evaluated for each compressor, in terms of stall margin improvement and efficiency penalty at design mass flow, using unsteady RANS CFD simulations. These are circumferential groove, slots, impeller recirculation pipe and diffuser-impeller recirculation pipe. The results indicate that passive flow control at the impeller shroud radial bend can be significantly more effective than at the impeller leading edge. For each compressor, the flow mechanisms associated with stall initiation and its delay by the most effective studied flow control technique as well as the associated efficiency penalty are elucidated. The findings indicate that techniques having flow injection with high spanwise penetration and low relative streamwise momentum are most effective for delaying diffuser stall, while the opposite applies to exducer stall. The design point efficiency penalty is due to mixing/shear losses from the jet-in-cross flow phenomenon in the impeller and shear losses from flow redistribution in the diffuser.

Reconstruction of Supersonic Flowfield Using Physical Neural Network Based on Channel Interaction

The evolution of shock waves in a combustor is a complex process comprising multiple time series, which results in a long computational cycle for unsteady numerical simulations. Current data-driven methods integrating physical information provide novel opportunities to rapidly and accurately predict the flowfield in the combustor. In this study, we construct a dataset under Mach numbers from 2 to 5 based on a self-designed scramjet combustor model and propose a physical convolutional neural network flowfield reconstruction method based on channel interaction (PICNNCI). This method embeds Euler equations that characterize the flow characteristics of inviscid fluids as physical constraints to solve the low-accuracy problem of traditional convolutional neural networks arising due to insufficient prior information. The model first performs dimensional transformations on temporal and spatial information input and converts a one-dimensional column vector into a two-dimensional vector to meet the convolution operation rules. Then, convolution is used for feature extraction to output the predicted flowfield, including velocity, pressure, and density fields. Finally, the predicted flowfield is optimized using the Euler equation and compared with the results obtained by numerical calculation and traditional neural network methods. The average peak signal-to-noise ratio of PICNNCI reaches 34.01 dB, and the linear correlation reaches 0.994.

Evolution features and optimization of interface shape during CdZnTe crystal growth using Vertical Gradient Freeze (VGF) technique

The growth of CdZnTe crystal suffers from poor yield due to the low thermal conductivity, and a convex growth interface is usually essential to grow large-volume CdZnTe single crystal because it can lessen the adverse interactions between the growing crystal and crucible wall. The growth of 90 mm diameter CdZnTe crystal by VGF technique has been investigated using experimental and simulation methods. The influences of the growth parameters (such as temperature gradient and growth rate) and the heat transfer configurations on the growth interface shape have been inspected respectively by the unsteady simulation. The simulation results confirm that the interface shape exhibits the tendency to be “convex → flat → concave” due to the gradually deteriorated heat transfer environment with time during growth. In addition, a modified configuration has been designed and used in experiments based on the simulation results, with the aim of strengthening the conduction heat transfer of the center region of the configuration geometry. The modified configuration consists of the splicing tapered-contact quartz&graphite support with a shorter conical region and seed region, as well as the SiC holder. Experimental results demonstrate that the modified configuration would make the growth interface maintain a convex shape for a longer duration. The CdZnTe crystal grown by using the modified configuration possesses fewer grains compared with that using the initial configuration.

Study of multiphase pump front homogenizer on the performance of fluctuating gas-liquid two-phase incoming flow reforming

As an important component of multiphase mixing technology in oilfield, the homogenizer plays the role of buffering the fluctuation of gas rate of incoming flow and gas-liquid homogenization, providing good inlet conditions for multiphase pumps. In this paper, the homogenizer designed and manufactured by ourselves is connected into the multiphase pump loop for experiment. The results show that the designed homogenizer can effectively improve the homogeneity of gas and liquid phases at the inlet of the multiphase pump, which in turn improves the pressurization capacity of the multiphase pump. The gas flow is buffered under slug flow condition, and the effective running time of multiphase pump is extended. The unsteady numerical simulation of the flow field in the homogenizer is carried out for three kinds of slug flow conditions. The results show that the homogenizer suppresses the fluctuation of gas content rate for the three blockage conditions. The gas content fluctuations of the outlet fluid were reduced by 71.3%, 64.7%, and 69.4%, respectively, compared with the incoming flow. In the homogenizer, the liquid phase entering the perforated pipe produced a significant jet phenomenon, which significantly enhanced the mixing with the gas phase. In the horizontal tube near the outlet, the gas phase floats due to gravity. The guide vane at the outlet effectively suppressed the gas-liquid stratification phenomenon that occurs here, which further improves the gas-liquid mixing at the outlet.

The importance of unsteady phenomena of ammonia/methane combustion in an experimental swirl burner: Comparison of steady-state and transient simulation results

Ammonia is a potential advanced green fuel. Combustion simulations play a crucial role in facilitating the usage of ammonia in existing combustion systems as a drop-in fuel. Choosing between the unsteady and steady simulation methods should be done carefully depending on the investigated phenomena. In this study, steady-state and transient Computational Fluid Dynamics simulations were performed using detailed chemistry of the Okafor mechanism for a wide range of ammonia/methane blend combustion under lean conditions. The results revealed that the steady-state simulations could provide close values to the unsteady mean results at the outlet in magnitude, while outlet temperature and intermediate species concentration slightly differ with ammonia concentration-dependent extents. The unsteady simulations provided valuable insights into the temporal evolution of the combustion and flow field characteristics, and the Root Mean Square and mean properties were evaluated comprehensively, indicating that the oscillations highly depend on the fuel composition. The model was validated using measurement data from the literature, including Particle Image Velocimetry, OH* chemiluminescence, and pollutant emission. The velocity and OH* distribution matched acceptably, while the CO, NO, and NO2 emissions were underpredicted, which implies necessary reaction model improvements.

Horizontal-axis propeller hydrokinetic turbine optimization by using the response surface methodology: Performance effect of rake and skew angles

The design of a horizontal-axis propeller hydrokinetic turbine (HPHT) depends on several geometric parameters affecting its hydrodynamic efficiency which is measured through the power coefficient (CP). In this study, a 1 kW turbine with 1.6 m of rotor diameter (D) was used as the prototype to know the relationship between the CP and the turbine design parameters, such as the skew (ϕ) and rake (γ) angles. A full-factorial design of experiments, as a response surface methodology technique, and computational fluid dynamics simulation were used to determine the significance of the factors considered and their interaction in the maximization of the response variable (CP). A 3D computational domain in ANSYS Fluent software and the k-ω SST turbulence model were utilized, for the unsteady flow simulations. Under optimal design conditions, i.e., when ϕ and γ were equal to 13.30° and -18.06°, respectively, the highest CP was 0.4571. For these optimal values, a scaled model with 0.24 m of diameter was numerical and experimentally studied and the findings were compared. A good agreement was found between the numerical results regarding the lab-scale turbine and the experimental data for the CP values obtained as a function of the tip speed ratio.

Optimization of a centrifugal pump with high efficiency and low noise based on fast prediction method and vortex control

This study aims to develop a rapid optimization design method for the centrifugal pump with high efficiency and low noise. The improved delayed detached eddy simulation and Powell's vortex sound method were employed to calculate the flow and sound field. The prediction models of hydraulic loss and noise based on vortex characteristic quantities were built using linear regression (LR) and artificial neural network (ANN) methods respectively. The Kriging surrogate model and the NSGA-II genetic algorithm were utilized for minimizing the entropy production rate and average total sound pressure in the pump, where the objectives of the training dataset for Kriging model were calculated by prediction model (scheme I) and conventional unsteady numerical simulation (scheme II). Results show that the structure and performance of optimized pumps under both schemes are nearly identical, but the computational cost of scheme Iis much lower than that of scheme II. The shear of the optimized pump is significantly reduced and the rigid rotational strength is enhanced, leading to head and efficiency improved by 3.2 % and 3.7 % respectively, under the design flow condition. The average total sound pressure level is reduced by 1.07 % due to the fluctuation suppression of shear and rigid vorticity.

A method for an efficiency and weight-optimised preliminary design of a hydrogen-powered fuel cell-based hybrid-electric propulsion system for aviation purposes

There is a worldwide effort to advance the usage of zero-emission propulsion systems for aircraft. Due to their high thermodynamic efficiency and the fact that they produce no CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{CO}_{2}$$\\end{document} and NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{NO}_{x}$$\\end{document} emissions, hydrogen-powered fuel cells are becoming increasingly popular for aviation purposes. However, fuel cell systems suffer from lower power density and higher cooling requirements when compared to conventional propulsion systems. Harnessing the high potential requires an optimised design of the whole propulsion system and its heat management system. This paper aims to present a method for the preliminary design and dimensioning of a fuel cell-based hybrid-electric propulsion system, which respects the limits of the heat management system and is weight and efficiency optimised. Thermodynamic models of the whole propulsion system are a crucial element to enable further investigations. Such a model has been developed, which is suitable for unsteady simulations of the propulsion and the heat management system performance of a short-range four-seater aircraft. A parameter study of the design parameters has been performed to display their impact on the system mass, the overall efficiency and the total hydrogen consumption. These results enable the identification of an overall optimised configuration. The study indicates that fuel cell-only configurations with an oversized fuel cell stack are beneficial for the analysed aircraft and flight mission.