Detailed Flow Field Research Articles

An advanced aero-thermodynamic study of a heart-shaped dimpled pipe

The detailed flow field and local heat transfer enhancement in a pipe with heart-shaped dimples on the internal surface were experimentally studied. The flow structures were investigated at a flow regime of Re=20k in five different measurement planes around a dimple in the streamwise and spanwise directions using both planar PIV and stereo PIV. A ray tracing-based image correction method was utilized to overcome the important image distortions caused by the optically complex geometry of the dimple for the planar PIV measurements. Furthermore, the convective heat transfer coefficient on the wall of the dimple was investigated utilizing steady-state liquid crystal thermography (LCT) and infrared thermography within the flow regime range of Re=20k-60k. A comprehensive uncertainty analysis was conducted to account for the multiple measured quantities and corresponding error sources on such a complex geometry. The resulting heat transfer and flow fields revealed that maximum heat transfer enhancement is concentrated along the reattachment line downstream of the dimple where higher kinetic energy levels are observed. The enhancement factor, which is a measure of the heat transfer improvement relative to a smooth pipe, was evaluated on average at 1.7 across the dimpled surface, with local maxima reaching values up to 2.8. The dimpled pipe under examination had a relative skin friction coefficient of approximately 1.8 compared to a smooth pipe. This outperforms some of the roughness elements, such as ribs and spherical dimples, previously studied on similar circular channels in literature.

Numerical analysis of H-Darrieus vertical axis wind turbines with varying aspect ratios for exhaust energy extractions

In this paper, a three-dimensional numerical study has been conducted by solving the unsteady Reynolds Averaged Navier Stokes equations with shear stress transport (SST) k-omega turbulence model to investigate the power production efficiency of a three-bladed H-Darrieus vertical axis wind turbines (HDWT) using S-1046 airfoil under accelerated wind conditions of the cooling tower exhaust system. Aiming to extract the waste energy from the exhaust air, four HDWTs having different aspect ratios were tested by varying the turbine diameter; and the moment and power coefficient values were calculated at tip speed ratios (TSR) in the range of 1.5–3.5. The results showed that the HDWT with a smaller aspect ratio (AR) of 0.44 gives a better power coefficient of 0.294 at TSR 2.5 as compared to other testing conditions. Further, a detailed flow field analysis around the HDWT for different ARs and TSRs has been provided in this paper to discuss the turbine's performance. Finally, a comparative study with the free-stream flow condition showed that HDWT provides 58.5% lower performance than the flow in the exhaust system at lower AR, indicating a positive guided flow effect on the turbine performance under exhaust flow systems.

Super-resolution reconstruction of turbulent flows with a transformer-based deep learning framework

Details of flow field are highly relevant to understand the mechanism of turbulence, but obtaining high-resolution turbulence often requires enormous computing resources. Although the super-resolution reconstruction of turbulent flow fields is an efficient way to obtain the details, the traditional interpolation methods are difficult to reconstruct small-scale structures, and the results are too smooth. In this paper, based on the transformer backbone architecture, we present a super-resolution transformer for turbulence to reconstruct turbulent flow fields with high quality. It is supervised and has a broader perceptual field for better extraction of deep-level features. The model is applied to forced isotropic turbulence and turbulent channel flow dataset, and the reconstructed instantaneous flow fields are comprehensively compared and analyzed. The results show that SRTT can recover the turbulent flow fields with high spatial resolution and capture small-scale details. It can obtain either the isotropic or the anisotropic turbulent properties even in complex flow configurations.

Steady-state error estimation and calibration of a shielded total temperature probe for turbomachinery

Accurate measurement of the total gas temperature plays a vital role in the performance research of turbomachinery. The total temperature thermocouple (TC) used in the measurement often needs to be specially designed and calibrated. In the past, the calibration was usually completed by a complex and highly costly experiment. The present work developed a new shielded total temperature probe (NTTP) for relatively low-temperature gas measurement. Meantime, this new design's characteristics and different error sources are analyzed based on the Conjugate Heat Transfer (CHT) simulation method. The CHT results show that the thermal conduction error is kept at a small value in the range of Mach number from 0.2 to 0.6; The velocity error contributes more than 90% of the total error. Four standard temperature probes are selected for the comparison. The maximum measurement error is 1.13 K, which is 331.62%, 119.4%, 61.6% and 59.5% lower than the standard type-A, B, C and D, respectively. Moreover, the detailed flow field and the influence of TC junction position on measurement accuracy are discussed. With the increases of the junction away from the supporting, the total measurement error of the NTTP increases first and then decreases. To vindicate the applicability of the proposed design, the measurement error of the NTTP is verified by a new experimental method with the relatively static fluid as the gas flow source. The testing results show that when the inflow's Mach number is 0.52, the measurement error of the NTTP is about 40% lower than that of the type-C total temperature probe.

Numerical investigation of the evolution of aerodynamic behaviour when a high-speed train accelerates under crosswind conditions

Crosswind degrades the aerodynamic behaviour of trains, especially during train acceleration, and an understanding of the flow field and train safety details requires more research attention. In the present study, the improved delayed detached eddy simulation (IDDES) method is applied to investigate the aerodynamic behaviour when a high-speed train accelerates under crosswind conditions. The spatial–temporal evolution of eddies and pressure distributions on the train surface based on eight discrete moments are explored. Moreover, the effects of three acceleration values on the aerodynamic force/moment coefficients and safety indicators are evaluated. The results show that the distance between the eddy shedding from the head car and the train body becomes smaller and the negative pressure of the eddy core increases in the process of acceleration. The pressure on the windward side of the train is stable, while the pressure on the leeward side shows an increasing trend with increasing train speed. An increase in the acceleration value increases the maximum side force and rolling moment coefficients, most prominently for the head car. In addition, a larger acceleration value may cause stronger pulsation of the train. Although the acceleration values do not increase the maximum values of the safety indicators, they have a slight effect on the local maximum values.

Toward the Utilization of 3D Blading in the Cantilevered Stator from Highly Loaded Compressors

Three-dimensional blading is an efficient technique in compressor aerodynamic design, and its function mechanism in the cantilevered stator needs to be addressed. This paper focuses on the sweep and dihedral in the cantilevered stator and seeks to expose their effects through detailed flow field analysis. Results show that the forward sweep could alleviate the corner flow separation by preventing the accumulation of the secondary flow toward the corner region, resulting in stronger flow separation at the blade trailing edge; in summary, forward sweep with appropriate parameters could increase static pressure rise by 14.3%. The positive dihedral will carry the endwall flow to the upper-span sections, thereby reducing blade corner separation; hence, as much as 23.5% improvement in static pressure rise could be obtained with the appropriate dihedral. Moreover, the combination of a relatively large sweep height and a moderate sweep angle with a low dihedral height and a moderate sweep angle provides optimum aerodynamic performance; the static pressure rise coefficient sees an increment of 25.5% at the near stall point. An experiment is then performed to further validate the theory, which shows a 2% improvement in efficiency of 3D blading at small mass flow rates. However, the secondary leakage should be given attention at high mass flow coefficients, while the corner separation needs further elimination at small mass flow rates.

Influences of geometric parameters on serpentine nozzles for turbofan

Serpentine nozzles possess excellent advantages in improving the stealth ability of aero-engine exhaust. For a real turbofan, the flow characteristics of the serpentine nozzle are evidently affected by the geometric parameters and the complex exhaust mixer. This is an issue that needs to be seriously considered. The aim of this paper is to investigate the influences of the critical geometric parameters on the serpentine nozzles for turbofan. The detailed flow field of the serpentine nozzle for a small-scale model is precisely measured. The influence of the ratio of axial length to inlet diameter on the flow field and on the aerodynamic performance of the serpentine nozzle is numerically explored. Results show that the pressure on the upper wall first drops rapidly and then ascends with the local minimum occurring at the first inflection. The relative error between the numerical prediction and the experimental data is less than 2%. The calculated distributions of the expansion-shock waves and the jet shear layers are highly consistent with the schlieren flow visualization data. Thus, the reliability of the numerical method is effectively confirmed. The internal flow and the external jet features of the serpentine nozzle are extremely non-uniform. Such poor uniformity is reflected in the phenomena including the existence of the local high-shear-stress regions and the high-velocity regions, the multiple bending of the limiting streamlines, as well as the velocity fluctuation of plume core. The aerodynamic performance of the serpentine nozzle first enhances greatly and then stays unchanged as the value of L/D rises. The vortex loss and the shock loss both reduce evidently owing to the flow separation and the disappearance of the shock wave during the rise of L/D from 2.2 to 2.4. The value of the discharge coefficient and the thrust coefficient increases by 1.5% and 2.1%, respectively. The length of the plume core is effectively shortened due to the disappearance of the shock wave inside the nozzle.

Multi-objective aerodynamic optimization of an axisymmetric variable-geometry inlet with a Mach 5 design point

The inlet is a critical component of the air-breathing system of an aircraft engine and an appropriate inlet design can address the issue of aerodynamic contradiction between low and high Mach numbers, restricting aircraft flight over a wide speed range. Therefore, in this study, an axisymmetric variable-geometry inlet with a Mach 5 design point was designed. A multi-point multi-objective optimization design, based on the Kriging surrogate model and non-dominated sorting genetic algorithm-II algorithm, was performed. Wind tunnel tests and numerical simulations were conducted to investigate the inlet performance and flow field details. The steady-state pressure distribution was recorded, and shadowgraph images were obtained simultaneously to comprehensively evaluate the inlet performance. The experimental and numerical results indicate that the design and optimization methods of the inlet are effective. The inlet operated appropriately and stably over a wide range of speeds, and the total pressure recovery (TPR) coefficient reaches 0.421 at the design point. The optimization results show that a 21.36% gain in the TPR coefficient at Ma = 3.0 was achieved at the cost of a 2.44% loss of total pressure recovery at Ma = 5.0. It is also beneficial in terms of the total pressure distortion at Ma = 5.0, which is reduced by 6.54%. This indicates that the inlet design and optimization framework developed in this study is promising for wide use in practical engineering applications.

Physics-informed deep learning cascade loss model

The design procedure of aero-engine compressor requires empirical models such as cascade loss model and deviation model. With accurate predictions of basic cascade performance by empirical model, better designs would be obtained and the cost would be reduced. A number of empirical models have been developed and most of them build mathematical equations based on experimental data. It is difficult to modify and broaden the scope of application for empirical models because of its inadequate ability to fit strong nonlinear function relationship and lack of data. Neural networks, which are widely used nowadays, are applied to solve this problem but a simple end-to-end neural network only slightly improves the model performance due to the neglect of flow field details and the similarity of different samples. A promising way to solve the problems is to embed the physics mechanism or flow field details into neural networks as intermediate variables to guide the learning of models. A physics-informed deep learning cascade loss model is proposed. By adjusting the architecture of neural network and the data structure, model can build the relationship of input and output in a more reasonable way while providing a prediction of flow fields. This paper verified the superiority of deep learning model, which has no physics informed, with a 22.3% error reduction when compared to empirical models. A physics-informed model is built and trained with the velocity distribution in boundary layer is chosen as intermediate variable and the influencing factors of physics-informed models are explored. The error of physics-informed model decreases 57.4% overall, and over 60% at high Mach numbers and high incidences compared to empirical model. The physics informed deep learning model is promising to replace the empirical models in compressor design since it shows better performance and gives a local description of the cascade flow field, which can be utilized in subsequent cascade design.

Numerical simulation on the shock wave focusing detonation process in the semicircle reflector

Shock wave focusing is considered a new type of initiation method that does not require external ignition devices. For this method, understanding the shock wave/flame interaction accurately is one of the critical issues for revealing the ignition and triggering mechanisms during shock wave focusing processes. In this study, numerical simulations were carried out to investigate the detailed flow field evolution of the focusing process with kerosene as fuel. Two detonation initiation modes were found during the shock wave focusing processes, which were the direct initiation mode and the reflected shock wave collision initiation mode, respectively. At the detonation wave propagation stage, the primary detonation wave decouples and fails to propagate out of the cavity. The multiple initiation phenomenon occurs in the cavity, and it dominated detonation waves to propagate outside of the cavity to accomplish one thermal cycle. There is no positive correlation between jet temperature and detonation wave propagation velocity. When the jet temperature is low, the detonation wave velocity dominated by the multiple initiation mode is the fastest. The analysis of the shock wave focusing process under different jet velocities showed that when the jet velocity was lower than 2 Ma, the decoupling of the primary detonation wave failed to induce multiple detonation waves. Driven by the vortex in the semicircular cavity, the flame is almost stationary, which means the failure of the detonation wave propagation process.

Experimental and numerical investigations on the unsteady flow and film cooling characteristics of the trailing edge cutback

The film cooling characteristics of the trailing edge cutback slot of a high pressure turbine were experimentally and numerically investigated. The unsteady flows have an important effect on the mixing process of the mainstream and coolant flow, resulting in deteriorated film performance. To illustrate the interaction between the unsteady flows and coolant injection, eight blowing ratios ranging from 0.3 to 1.2 were considered for the baseline cases. Pressure sensitive paint measurements were conducted to acquire the adiabatic film cooling effectiveness distribution. The counter-intuitive decrease of the film cooling effectiveness was confirmed and further revealed. Particle image velocimetry showed that the unsteady flow patterns downstream of the trailing edge slot varied with increasing blowing ratio. Furthermore, the high-fidelity RANS/LES hybrid method was employed to resolve the detailed flow fields. The numerical method was carefully validated, and it successfully predicted the unsteady flows and mixing process. The spanwise vorticity showed that the two branches of the shedding vortices alternatively become strong. At the blowing ratio of 0.75, the strength of the two branches was comparative, yielding the worst cooling performance. Additionally, the density ratio and lip thickness to slot height ratio were investigated. Thus, the density ratio affects the coolant velocity ratio, causing the forward movement of the inflection point of the film cooling effectiveness map. The decrease of the lip thickness to slot height ratio leads to the reduction of the strength of the shedding vortices and therefore increases the film cooling effectiveness.

Experimental investigation of the distorted flow field in a new type of direct-connect facility for scramjet studies

A supersonic distorted flow field generated by a new type of direct-connect facility at Mach 2.1 is investigated experimentally via particle image velocimetry (PIV). This facility is designed to accurately reproduce the real flow field in scramjet especially the flow distortion within the isolator, while avoiding the unstart problem in traditional direct-connect facility. The time-averaged velocity fields reveal the principle of flow distortion generated by this facility and show the detailed flow field structures. The mean results suggest that the choose of target parameters distribution will directly affect the performance of this facility. Then, the distorted flow fields in the isolator with a specific non-uniform incoming flow condition are investigated in detail, with the method of proper orthogonal decomposition (POD). The POD analysis of the fluctuating velocity fields finds that there are complex flow field structures within the isolator, including the large-scale vortex structure and the small-scale structure near the wall surface. And these structures in the eigenmodes are associated with the shock wave and the low-velocity region close to the wall surface. Meanwhile, the distorted main flow is also considered to contribute to the flow field instability. Finally, according to the analysis results, the practical guidance of this new type of direct-connect facility is proposed for the subsequent experimental research about scramjet.

Investigation on the flow around a submarine under the rudder deflection condition by using URANS and DDES methods

Considering the complex flow features of a submarine under the maneuvering condition and their close relationship with the maneuverability, a numerical study is performed for the flow around the submarine under the rudder deflection condition. Taking the submarine model DARPA SUBOFF as the study object, the rudder force test with a wide range of rudder angles is simulated based on the open source CFD platform OpenFOAM. Two numerical methods of Unsteady Reynolds-Averaged Navier-Stokes (URANS) method and Delayed Detached Eddy Simulation (DDES) method are employed. A systematic convergence study is performed to clarify the effect of grid spacing and time step on the numerical solution. The computed hydrodynamic forces are compared with the available experimental data, and the effectiveness of the adopted numerical methods is validated. Besides, the obtained rudder forces and rudder parameters are compared and discussed. Based on the obtained flow field details of vortex structures, velocity fields, streamline distribution, turbulence levels, and pressure distribution, the flow mechanisms of the deflected rudders are analyzed. Compared with URANS method, DDES method presents a better capability of capturing the flow field details around the submarine under the tight maneuvering condition, where remarkable flow separation is involved.

Research on Resistance Characteristics and Detailed Flow Field of Eccentric Orifice Plate

Research on Resistance Characteristics and Detailed Flow Field of Eccentric Orifice Plate

Experimental investigation of vortex evolution around oscillating water column wave energy converter using particle image velocimetry

Although the oscillating water column (OWC) wave energy converter has been extensively studied in recent decades, the understanding of the flow field around the OWC converter is still in its infancy due to the complexity of the wave–structure interaction, especially with the vortex evolution. In this study, the characteristics of the flow field around the OWC converter were investigated by using the time-resolved particle image velocimetry technique to analyze the temporal development of the vortex in the vicinity of the OWC converter. The Q criterion was calculated to highlight the vortex intensity, and the entropy was used to analyze the energy dissipation. The results indicated a close relation between the energy dissipation and the formation of vortices. The temporal processes of vortex generation and development were elaborated in terms of phase-averaged results, including their variations with wave heights and wave periods. It was found that the vortex intensity and structure were clearly affected by the wave heights. As the wave period increased, the position of the seaward vortex shifted downward, while that of the leeward vortex shifted upward and deeper into the chamber. In addition, the detailed flow field results provided insight into the effect of vortex evolution on the energy extraction rate of OWC, showing that the leeward vortex led to a decrease in the energy extraction rate, while the seaward vortex led to more energy dissipation as the water leaving the OWC converter and had a relatively small effect on the energy extraction rate.

Flow mechanism and heat transfer characteristic of sweeping jet impinging on confined concave surfaces

The sweeping jet actuator (SJA), a type of fluid oscillator, has attracted considerable attention in the areas of enhanced heat transfer and flow control. In this study, an unsteady Reynolds-averaged Navier–Stokes numerical investigation (k–ω baseline) was conducted to evaluate the flow mechanisms and heat transfer characteristics of a sweeping jet (SJ) on a confined concave surface. A conventional oscillating jet with a unit aspect ratio was used to generate the SJ. The heat transfer at the target surface and the flow details inside the flow field are discussed in detail to analyze the mechanism of the effect of SJ on impinging cooling. First, an appropriate numerical model was selected based on a comparison with a previous experiment. Subsequently, the time-averaged and time-resolved flow field and heat transfer results were investigated at three target distances for three different Reynolds numbers. The unsteady time-averaged results show that the SJ exhibits better heat transfer performance at higher Reynolds numbers and narrower target spacings than circle jet, which improves by 13% at Re = 30 000, H/D = 1. Subsequently, the turbulent kinetic energy and velocity loss theories were combined to compare the time-averaged and time-resolved flow field details of the two jets. The heat transfer characteristics of the target surface and the flow details inside the flow field in time and space were effectively correlated. Finally, the topology and three-dimensional (3D) vortex structure inside the confined channel were remodeled to better understand the unsteady sweeping process and provide theoretical support for subsequent applications.

Analysis of the Flow Field at the Tip of an Axial Flow Compressor during Rotating Stall Process Based on the POD Method

Rotating stalls are one of the most dangerous phenomena to be avoided in the designing and operating of axial flow compressors. An understanding of the evolution of the flow characteristics around the rotor tip region is important to study the process of stall development. In this paper, some critical characteristics of the stall-related structures, which could not be observed by the traditional analysis methods, are understood by using the proper orthogonal decomposition (POD) method. The detailed unsteady flow fields in a typical transonic axial flow compressor during the stall process are obtained by the validated numerical simulation. Thereafter, the proper orthogonal decomposition method, as a method for model reduction and decomposition, is adopted to extract the flow characteristics from the numerical results of the stall process. It is found that the flow characteristics during the stall process can be well decomposed by the POD method. The pre-stall POD results show that the important flow features can be extracted and revealed in the low-order modes, which are not obvious in the original flow field. When the stall cells are formed and developed, the flow characteristics are gradually determined by the modes, which are related to the features of the stall cells. When the compressor is operated under stable stall conditions, the low-order POD modes are composed of a series of harmonic modes, which are sinusoid-like in space and time with the frequency of the stall cell rotation.

Numerical Study on Tandem-Rotor Autorotation in Forward Flight

This work presents a systematic approach to analyzing the aerodynamic characteristics of tandem rotor forward autorotation considering rotor-to-rotor interference. The single-rotor computational model trimmed from a generic helicopter flight dynamics analysis program was used as the baseline model. The effectiveness of the baseline model is demonstrated by a comparison with data from wind tunnel tests performed in this work. The rotor disk angle of attack and driven moment distribution obtained by the modified model indicate the fact that the rotor acceleration is primarily caused by the higher angle of attack region of the disk. This is of great significance in the rotor blade design, in terms of the drag-to-lift ratio characteristics of the airfoil under different angle-of-attack ranges. The influence of wind speed, rotor shaft angle, and collective pitch on the steady-state rotor speed was then studied. The results show a nonlinear nature of the variation of steady rotor speed with collective pitch, which can cause a thrust control reverse problem during flight operations. To reveal the flow field details of rotor-to-rotor interference, the flow field Navier–Stokes equations of tandem rotor autorotation were solved. Computational results of both rotors’ inflow velocities were considered when deriving the empirical model of interference. The refined interference model was compared to the wind tunnel test data of the tandem rotor autorotation and showed good performance. This synthetical methodology, which combines mechanism analysis with CFD-aided refinement and experiment verification, achieves a balance between computational costs and accuracy and thus can be readily applied to engineering practices.

Critical diffraction of irregular structure detonations and their predictability from experimentally obtained [formula omitted] data

The present work reports new experiments of detonation diffraction in a 2D channel configuration in stoichiometric mixtures of ethylene, ethane, and methane with oxygen as oxidizer. The flow field details are obtained using high-speed schlieren near the critical conditions of diffraction. The critical initial pressure for successful diffraction is reported for the ethylene, ethane and methane mixtures. The flow field details revealed that the lateral portion of the wave results in a zone of quenched ignition. The dynamics of the laterally diffracting shock front are found in good agreement with the recent model developed by Radulescu et al. (Physics of Fluids 2021). The model provides noticeable improvement over the local models using Whitham’s characteristic rule and Wescott, Bdzil and Stewart’s model for weakly curved reactive shocks. These models provide a link between the critical channel height and the critical wave curvature. The critical channel heights and global curvatures are found in very good agreement with the critical curvatures measured independently by Xiao and Radulescu (Combust. Flame 2020) in quasi-steady experiments in exponential horns for three mixtures tested. Furthermore, critical curvature data obtained by others in the literature was found to provide a good prediction of critical diffraction in 2D. These findings suggest that the critical diffraction of unstable detonations may be well predicted by a model based on the maximum curvature of the detonation front, where the latter is to be measured experimentally and account for the role of the cellular structure in the burning mechanism. This finding provides support to the view that models for unstable detonations at a meso-scale larger than the cell size, i.e., hydrodynamic average models, are meaningful.

A novel hybrid CFD-DEM method for high-fidelity multi-resolution modelling of cross-scale particulate flow

Cross-scale polydisperse particulate-fluid flows are widely practised yet their multi-resolution simulations are not achieved due to the lack of reliable numerical methods. In this work, a novel so-called hybrid CFD-DEM model is developed for the high-fidelity multi-resolution simulations of cross-scale polydisperse particulate-fluid flow systems, for the first time featuring no limit on the grid size to particle diameter ratio (lm/dp). The hybrid CFD-DEM method collaboratively considers the fluid-coarse particles interaction (lm/dp < 0.1), fluid-medium particles interaction (0.1 < lm/dp < 3), and fluid-fine particles interaction (lm/dp > 3) by means of the FD (fictitious domain)-based resolved method, semi-resolved method, and unresolved method, respectively. Three typical polydisperse particulate-fluid flow scenarios are simulated for model validation and effectiveness demonstration, i.e., sedimentation of single particle in liquid; drafting, kissing, and tumbling (DKT) process of two particles; and the migration of fine glass particles through the pores of the coarse particles. The results confirm the ability of the hybrid CFD-DEM model of capturing both the detailed flow fields of fluid and particles and particle–fluid interactions in the particulate-fluid flow with a cross-scale size distribution. Moreover, the hybrid CFD-DEM model shows excellent capability in mesh convergence, wide applicability, and high accuracy. It provides a promising tool for the modelling of many cross-scale particulate-fluid flow systems.