Unstable Flow Field Research Articles

Rapid identification and early warning of axial compressor stall based on multiscale CNN-SVM-FC model

Early prewarning of compressor stall and surge is crucial to avoid aircraft engine instability, yet it is challenging due to the complex and unstable flow field characterized by multiple modes and multiscale features. To enhance the multi-scale feature representation capability of Convolutional Neural Network-Support Vector Machine (CNN-SVM) algorithm, a novel classifier modelling method combined multiscale windows with CNN-SVM is introduced for stall prewarning in this paper, named Multiscale CNN-SVM-FC. Multiscale detection windows are utilized to adaptively identify various pressure features during the stall process. Additionally, to reduce the false alarm rate, a fuzzy control algorithm is integrated with the temporal accumulation of prediction results from the multi-branch network for joint analysis. A series of test data from a five-stage axial compressor at different operating speeds is used to verify this method. The results indicate that the proposed Multiscale CNN-SVM-FC method enhances the accuracy of classification and reduces the false alarm rate compared to the standard CNN-SVM model, achieving over 99% accuracy in identifying unstable states under various speeds. Compared to three traditional stall prewarning methods, the Multiscale CNN-SVM-FC model provides an average warning signal 164 milliseconds ahead of stall, and reduces the uncertainty associated with threshold selection, which typically relies on engineering experience.

Study on Wind Farm Flow Field Characteristics Based on Boundary Condition Optimization of Complex Mountain Numerical Simulation

The accurate prediction of the flow field characteristics of complex mountains is of great practical significance for the development and construction of wind farms, but it is not yet fully understood. The main purpose of this study is to propose a method for the study of flow field characteristics under complex mountain conditions, which can optimize the boundary conditions required for numerical simulation through the wind acceleration ratio and, at the same time, couple the numerical simulation and wind measurement data to reflect the real mountain flow field distribution. The results show that the proposed method has good applicability in complex mountain wind farms, can reproduce the real flow field distribution, and has a certain practical value. Wind speed distribution and turbulence intensity are greatly affected by boundary conditions such as wind speed and wind direction and are also affected by the shielding effect brought by terrain changes. The contrast between 120° and 150° wind direction is more obvious. When the incoming wind moves to the top of a mountain or the ridgeline, it will form a low-speed wake area behind it, resulting in reduced wind speed, increased turbulence intensity, and an unstable flow field.

Comparison of Fluid Flow and Tracer Dispersion in Four-Strand Tundish under Fewer Strand Casting and Sudden Blockage of Strand Conditions

The study focuses on the four-strand tundish as the research object, aiming at the phenomenon of fewer strand casting (stable blockage) and sudden blockage of the tundish in industrial production. Numerical simulation methods are employed to compare the velocity vectors, flow fields, residence time distribution (RTD) curves, and outflow percentage curves under stable blockage and sudden blockage of the tundishes with a double-weir structure, U-shaped weir structure, and U-shaped weir structure with holes in the front. The results indicate that, after sudden blockage of the tundish strands, the flow field transitions from an unstable four-strand flow field to a stable three-strand flow field. Both the double-weir tundish and the U-shaped weir tundish reach a stable state after 200 s, while the U-shaped weir tundish with holes in the front reaches stability after 150 s. Additionally, compared to other structures, the tundish strands of the U-shaped weir with holes in the front are less affected by blockage, showing better consistency among strands and better adaptability under non-standard casting conditions.

Experimental and computational investigation on flow characteristics of rotating cavities with high inlet pre-swirl axial throughflow

As an integral part of the internal air system of aero-engines, the axial throughflow of the cooling air can interact with the cavity flow between the rotating compressor disks, forming a three-dimensional, unsteady, and unstable flow field. The flow characteristics in an engine-like rotating multi-stage cavity with throughflow were investigated using particle image velocimetry, flow visualization technology and three-dimensional unsteady Reynolds-Averaged Navier-Stokes (RANS) simulations. The focus of current research was to understand the distribution of the mean swirl ratio and its variation with a wide range of non-dimensional parameters in the co-rotating cavity with high inlet pre-swirl axial throughflow. The maximum axial Reynolds number and rotational Reynolds numbers could reach 4.41 × 104 and 1.24 × 106, respectively. The velocity measurement results indicate that the mean swirl ratio is greater than 1 and decreases with an increase in the radial position. The flow structure is dominated by the Rossby number, and two different flow patterns (flow penetration and flow stratification) are identified and confirmed by flowvisualizationimages. In the absence of buoyancy, the flow penetration caused by the precession of the throughflow makes it easier for the throughflow to reach a high radius region. Satisfactory consistency of results between measurements and numerical calculations is obtained. This study provides a theoretical basis and data support for toroidal vortex breakdown, which is of practical significance for the design of high-pressure compressor cavities.

Influence of nose angle on performance of dense medium cyclones with volute inlet

Dense medium cyclones (DMCs) are widely used in coal mine sorting processes to improve the quality of coal products and reduce pollution. The conventional linear inlet DMCs has the problems of unstable flow field, high turbulence density and low separation efficiency. Based on these concerns, the volute inlet DMCs was proposed, however the nose angle as one of the most critical parameters has not yet been investigated in depth. In this paper, a DMC with volute inlet is investigated, and the effect of nose angle is studied by a computational fluid dynamics model in terms of flow field, particle motion and the forces acting on the particles. The performance of the DMC is evaluated with respect to the inlet nose angle. As the nose angle increases from 0 to 180 deg., the separation efficiency has a tendency to increase and then decrease with the inflection point founding when nose angle is 180 deg. Then the flow field, particle distribution and particle force are analyzed respectively. The results show that the symmetry and stability of the flow field can be significantly improved as nose angle increases from 0 to 180 deg., but they remain almost constant when nose angle is 270°. In addition, a quantification method of the short-circuiting flow is established and it has been found that increasing the nose angle can notably reduce the short-circuiting flow. In terms of particle phase, increasing the nose angle causes heavier particle distribution tends to concentrate. During this period, the variation pattern of forces on particles with different nose angles is also analyzed. The DMC with a nose angle of 180 deg. has a longer acceleration zone at the entrance height and an increased radial force on the particles, which results in better performance. This study provides a theoretical basis and reference for the structure optimization of dense medium cyclones.

Airborne Pollutant Removal Effectiveness and Hidden Pollutant Source Identification of Bionic Ventilation Systems: Direct and Inverse CFD Demonstrations

A healthy and efficient ventilation system is essential for establishing a comfortable indoor environment and significantly reducing a building’s energy demand simultaneously. This paper proposed a novel ventilation system and applied it to the IEA Annex 20 mixing ventilation enclosure to verify its feasibility through mathematical modeling and CFD simulations. First, two bionic ventilation systems, single-side and dual-side ventilations, were compared to a conventional constant-volume supply system using CFD simulations, with the results demonstrating that the bionic ventilation system could provide higher ventilation efficiency and more effective pollutant removal from stagnant regions. Furthermore, the present work exercised these two bionic ventilation systems with different temporal periods of sine and rectangular wave functions, identifying a turning point at a period of 0.06 τ n , which could contribute to further enhancement of these bionic ventilation systems. Finally, a methodology depending on the Bayesian inference algorithm was developed for identifying pollution sources in the bionic ventilation system with unstable flow fields, and factors influencing source identification accuracy were discussed. The results show that the peaks of the KDE distributions and the sampling average values of both the source location and intensity are all consistent with the actual source parameters. The potential of the proposed bionic ventilation systems has been well demonstrated by direct and inverse CFD models, paving the way for further engineering applications.

Comparison of dimensionality reduction techniques for multi-variable spatiotemporal flow fields

In the field of fluid mechanics, it is a potential consensus that nonlinear dimensionality reduction (DR) techniques outperform linear methods. However, this conclusion has been obtained based on simple fluid phenomena and an incomplete evaluation system of dimensionality reduction algorithms. In this study, we use an improved evaluation system of DR methods to compare and evaluate the performance of four DR methods, including two linear techniques: Principal Component Analysis (PCA) and Independent Component Analysis (ICA), and two non-linear techniques: Isometric Mapping (ISOMAP) and Locally Linear Embedding (LLE). The four methods are applied to analyze a complex hydrodynamic flow field with cavitation by considering the joint features of multiple variables. Results show that LLE can capture redundant features that do not contribute to understanding the characteristics of flow fields, while ISOMAP is more suitable for handling datasets with multiple scales than LLE. PCA and ISOMAP can successfully capture the characteristics and evolution of flow fields. In addition, 3D supplementary information can assist ICA in improving the problem of identifying unstable flow field states.

Investigation on the unsteady ventilation performance of oscillating jet air supply through both spontaneous vortex street effect and active modulation

Ventilation is one of the most vital measures to create a healthy and comfortable indoor environment, significantly influencing indoor air quality and building energy consumption. The conventional steady-state airflow field of a ventilated room, if not reasonably configured, is prone to cause air stagnation in certain regions, whereas the unsteady airflow improves the mixing effect, which favors the dispersion and purging of indoor air pollutants. This paper proposes a method to induce an unstable air flow field utilizing the spontaneous Karman vortex street effect on the air-supply jet and swinging modulation. The quasi-periodicity of the airflow field and its influence on the indoor pollutant purging effect were investigated. The effects of air velocity oscillation and pollutant concentration decay in a ventilated room were studied through experimental measurements and computational fluid dynamics analysis. The results show that the transverse oscillation jet has a dominant period of 1.6 s and the longitudinal oscillation jet has a dominant period of 2.987 s, and both jets have a width about twice that of the free jet. The airborne pollutant discharge efficiency of the oscillating jet was higher than that of the free jet at the occupancy level, and the longitudinal oscillating jet purged pollutants faster than the transverse oscillating jet. This study reveals that oscillating jets can provide an option to create periodic flow fields and improve the efficiency of ventilation.

Effect of chord length ratio on aerodynamic performance of two-element wing sail

This study aims to optimize the chord length ratio of a two-element wing sail in order to enhance the aerodynamic performance of a wind-powered unmanned sailboat. The viscous Navier-Stokes flow solver was used to solve for changes in the numerical aerodynamic performance with the chord length ratio and rear wing sail deflection angle of the two-element wing sail for different wind angles and angles of attack. The comparative results demonstrate that at a given angle of attack, increasing the chord length ratio between the front and rear sails leads to higher lift and thrust coefficients of the wing sail, thereby improving the sailboat's performance. Moreover, increasing the deflection angle of the rear sail at a given angle of attack results in an overall increase in the lift and thrust coefficients of the wing sail. It was also observed that a smaller chord length ratio and a larger deflection angle of the rear sail contribute to a more unstable flow field. Based on these findings, the optimum values of the chord length ratio, c1/c2, could range between 1.5 and 3. These findings have significant implications for optimizing the structures and control systems of wing sails.

An improved mode time coefficient for dynamic mode decomposition

Dynamic mode decomposition (DMD) is widely used for extracting dominant structures of unsteady flow fields. However, the traditional mode time coefficient of DMD is assumed to change exponentially over the time. Consequently, it cannot deal with the unstable flow fields whose modes present nonexponential evolution regularities. Also, the inaccurate mode time coefficient might cause an unreasonable rank of decomposed modes, leading to the dominant modes to be ignored. To overcome these shortcomings, an improved mode time coefficient based on the Moore–Penrose pseudoinverse is proposed for the DMD, and a new integrated parameter based on the improved mode time coefficient is defined to rank the decomposed modes. The DMD with the improved mode time coefficient (abbreviated as DMD-TC) is expected to accurately describe the temporal evolutions of modes in complex forms for unstable systems and results in a more reasonable rank for the modes. To validate the DMD-TC, two complex analytical functions (a continuous case and an intermittent case) and two typical unstable flows (the flow around a cylinder and the dynamic stall of a pitching airfoil) are investigated. The results indicate that the DMD-TC can accurately describe temporal evolutions of modes with complex nonlinear regularities, including exponential, logarithmic, linear, gradually intermittent, transiently intermittent, and other complex regularities. Also, due to the improved mode time coefficient, the DMD-TC can provide a more reasonable rank for unstable modes. These improvements help to identify instantaneous dominant dynamic modes (even with minor initial amplitudes) of real unstable flow fields and accurately describe their temporal evolutions.

Adaptive thermal convective cloak via inverse design

Thermal metamaterials, known for their effective and directional heat control and management strategy, have garnered significant attention leading to the investigation of various functional thermal conductive metadevices. Nevertheless, conventional thermal convective metadevices resort to artificial structures in the flow field, which face the challenge of the coupling of heat and mass transfer and unstable flow field. Herein, we report an adaptive strategy for fabricating an active thermal convective cloak via tunable heat gain and loss. Adaptive body and surface heat sources, rather than graded structures inside functional regions, have been demonstrated to be feasible for heat convection. Accompanied by the active manipulation scheme, an inverse design process was constructed to calculate the heat source distributions based on the knowledge of the target temperature fields. The proof-of-concept heat source matrix, using well-laid out resistance heaters and thermoelectric components, experimentally validated the thermal cloaking pattern in the flow field. The active scheme overcomes the limitations of the complex engineered structures required for conventional thermal convective metadevices. Thus, in this study, we establish a new paradigm for thermal convective camouflage and pave the way for active thermal management in heat convection.

Surge margin monitoring of one turboshaft engine with inlet distortion

In order to explore the engine surge when inlet distortion index and engine surge margin, the relationship between the parameter characteristics of research engine surge, strengthen safety monitoring assessment engine, this paper designed a kind of a certain type of engine bench test surge monitoring system, and unstable flow field of a certain type of vortex axis engine working parameters for monitoring and analysis. In this paper, a plug-plate distortion generator is used to induce a surge engine, and the circumferential static pressure changes of the inlet and outlet of the engine, the axial compressor and the centrifugal compressor under different working conditions are obtained. The changes of circumferential unevenness and mean turbulence, the pressure ratio at working point and the converted air flow rate at working point are analyzed with the increase of the height of the plug-plate under different working conditions. The results show that the distortion index of the working point increases and the surge margin decreases with the increase of the height of the plug-plate at the same rotating speed. When the height of the plug-plate is similar, the higher the speed is, the greater the distortion index of the working point is, and the smaller the surge margin is.

Adaptive control mechanism of shock wave/boundary layer interaction induced by the secondary recirculation jet with outlet backpressure

The increased and unstable flow field backpressure will cause problems such as the non-starting of the inlet tract, and the widespread shock wave/boundary layer interaction (SWBLI) phenomena in the supersonic flow field exacerbates these problems. Hence, a powerful flow control system is required. In this paper, backpressure is introduced at the flow field outlet, and the effect of different backpressure ratios on the flow field is explored. An adaptive control scheme is also developed by using the optimized secondary flow recirculation configuration. The three-dimensional implicit Reynolds Averaged Navier–Stokes equations are utilized for numerical simulation of the flow field. The results show that the adaptive control of the secondary recirculation jet has a positive control effect on the SWBLI of the flow field when backpressure is applied. Moreover, the adaptive control mechanism under the backpressure condition is analyzed, which is applicable to different backpressure flow fields with Mach numbers between 2.5 and 3.5.

The Role of Hydrodynamic Instabilities on Near-Lean Blowout Flame Shapes in a Swirl-Stabilized Spray Combustor

Abstract This study investigates the role of hydrodynamic instabilities on near-lean blowout (LBO) flame shapes in a swirl-stabilized spray combustor. Hydrodynamic instabilities often manifest themselves in swirling flows as a helical vortex that winds around the vortex breakdown bubble. However, the heat released from combustion tends to suppress coherent vortex structures, which can limit the helical vortex to certain combustor geometries and operating conditions. Flame shape changes often accompany changes in hydrodynamic stability because they reposition the heat release and consequently modify the degree of coherent vortex suppression. In this study, laser diagnostics measurements were used to characterize the flow fields and spray patterns corresponding to different flame shapes that were observed in the Air Force Research Laboratory (AFRL) referee combustor. In particular, the flame fluctuated between its original shape, FS1, and a new flame shape, FS2, when the combustor operated on the threshold of LBO. Proper orthogonal decomposition (POD) was used to analyze the measurements. POD showed that the appearance of FS2 coincided with coherent vortex structures that resembled those in the hydrodynamically unstable nonreacting flow field. Furthermore, fuel Mie scattering measurements and phase-averages of the velocity field provided evidence that the FS2 spray was periodically disturbed by a helical vortex. Near the swirler exit, this helical vortex structure involved both outer and inner shear layer vortices that appeared to be synchronized with each other. However, the inner shear layer vortices decayed as the flow progressed downstream and only the outer shear layer vortices remained throughout the measurements' field of view. In contrast, there was no indication of a helical vortex structure in either the flow field or fuel spray measurements corresponding to FS1.

Experimental study on the direct contact condensation of gaseous oxygen jets in crossflow of liquid oxygen

To ensure the safe and stable operation of the oxidant supply system in liquid oxygen (LOX)/kerosene liquid rocket engines, cryogenic visualization experiments on the direct contact condensation of gaseous jets in liquid crossflow are conducted using oxygen as the working fluid. The mass flux of gaseous oxygen (GOX) is 200–800 kg/m2s, while the temperature of LOX is 101 K. Under these operating conditions, violent oscillations occur. To address this problem, this paper proposes an image processing program to facilitate the study of flow patterns, oscillation frequency, plume length, and heat transfer coefficients in unstable flow fields. Three flow patterns are identified, namely condensation oscillation, chugging, and crawling. The effect of the GOX mass flow rate on chugging is investigated. In addition, the flow pattern map with the coordinates of nondimensional GOX mass flux and condensation driving potential is developed. The oscillation frequency, ranging from 120 Hz to 150 Hz, increases monotonically with the GOX mass flow rate and LOX pressure in the test section. The plume shrinks with the increase of the GOX mass flow rate, which differs from the experimental results reported for room-temperature working fluids. The dimensionless plume length varies between 4 and 6 and is fitted by a correlation relationship with discrepancies of ±5%. The heat transfer coefficient ranges from 0.1 MW/m2K to 0.5 MW/m2K and is also fitted by a correlation relationship with discrepancies of ±15%.

A simple yet efficient approach for electrokinetic mixing of viscoelastic fluids in a straight microchannel

Many complex fluids such as emulsions, suspensions, biofluids, etc., are routinely encountered in many micro and nanoscale systems. These fluids exhibit non-Newtonian viscoelastic behaviour instead of showing simple Newtonian one. It is often needed to mix such viscoelastic fluids in small-scale micro-systems for further processing and analysis which is often achieved by the application of an external electric field and/or using the electroosmotic flow phenomena. This study proposes a very simple yet efficient strategy to mix such viscoelastic fluids based on extensive numerical simulations. Our proposed setup consists of a straight microchannel with small patches of constant wall zeta potential, which are present on both the top and bottom walls of the microchannel. This heterogeneous zeta potential on the microchannel wall generates local electro-elastic instability and electro-elastic turbulence once the Weissenberg number exceeds a critical value. These instabilities and turbulence, driven by the interaction between the elastic stresses and the streamline curvature present in the system, ultimately lead to a chaotic and unstable flow field, thereby facilitating the mixing of such viscoelastic fluids. In particular, based on our proposed approach, we show how one can use the rheological properties of fluids and associated fluid-mechanical phenomena for their efficient mixing even in a straight microchannel.

Peak Ratio Characteristic Value Sequence Based Signal Processing Method for Transit-Time Ultrasonic Gas Flowmeter

The transit-time ultrasonic gas flowmeter plays a vital part in the measurement field with its unique advantages. In recent years, it has developed into a research hotspot in the field of gas flow measurement. However, while the ultrasonic signal propagates in gas, the amplitude fluctuation of the ultrasonic signal is produced under the condition of energy attenuation and unstable flow field. This leads to inaccurate transit time of ultrasonic signal that causes flow calculation errors. Aiming at this problem, a signal processing method is proposed in this paper for the transit-time ultrasonic gas flowmeter based on the peak ratio characteristic value sequence (PRCVS). Through the research on the mathematical model of ultrasonic signal, the ratio of the amplitude of adjacent peaks is defined as the peak ratio characteristic value (PRCV) of the peak. According to the corresponding relationship between the PRCV and the peak serial number, a set of reference PRCVS is established. By matching the characteristic value of the ultrasonic signal with the reference characteristic value sequence, the peak serial number can be determined. In this research, the PRCVS-based signal processing method is applied to the gas flow measurement system based on time-to-digital converter (TDC) that has strict requirements on the peak serial number which can verify the validity of the method. The calibration experiment of basic measurement performance test and the unstable flow field experiment of the curved pipe were performed on the gas flow standard device, which verified the stability and validity of the method proposed in this paper.

Research on a transit-time liquid ultrasonic flowmeter under unstable flow fields

A transit-time ultrasonic flowmeter calculates a flow rate by measuring the transit time of the ultrasonic signal. In flow measurement, there are energy attenuation and noise interference, and sometimes the ultrasonic signal will fluctuate or even distort, which makes it impossible to find the reference position and obtain the correct transit time under unstable flow fields. Therefore, based on the mathematical model of the ultrasonic signal, the characteristics of the peak ratios of adjacent peaks in the ultrasonic signal are studied, and a peak ratio matching method-based signal processing method is proposed for ultrasonic flowmeters. Firstly, through the analysis of signal quality, a standard peak ratio array is set up, then the peak ratios are calculated to obtain a real-time peak ratio array in the flow measurement, and finally a matching algorithm for elements in the standard peak ratio array and elements in the real-time peak ratio array are used to find the reference position. The transit time of the ultrasonic signal can be obtained by the reference position and positive zero-crossing points. The signal processing method is implemented in real time in the system where a time-to-digital converter timing module is used to measure zero-crossing times, an analog-to-digital converter sampling module is used to sample the ultrasonic signal at the same time, and two STM32 (32-bit ARM Cortex-M Microcontroller) processors are used to analyze the signal quality and data processing. The effectiveness of this signal processing method is verified by a large number of experiments.

A new method of dynamic mesh used in continuous guide vane closure of a reversible pump-turbine in generating mode

In this paper, a new method of dynamic mesh based on two functional controls is used in the continuous guide vane closure, and the three-dimensional numerical simulation is carried out to investigate the transient flow characteristics for a Francis-type reversible pump-turbine in the turbine mode in the load regulation scenario, with the detached eddy simulation (DES) turbulent model. The transient flow characteristics during the closure of the guide vanes are illustrated by analyzing the signals of the mass flow, the torque and the pressure fluctuations in the frequency and time-frequency domains. It is shown by the simulated results that a continuous assessment of the transient flow characteristics during the guide vane closure may be made by using the new method of the dynamic mesh. Furthermore, the flow field analysis involves both the onset and the development of the unsteady phenomena progressively based on an organized guide vane closure law. The flow pattern in the return channel maintains a relatively stable flow field before the last stage of the closure, as compared with the unstable flow field in other domains. To identify the unit variation under the fluid-dynamical conditions, the influence of the three-dimensional unsteady flow structures in the passage is analyzed and its evolution during this transient process is characterized by the fluid-dynamics and the spectral analysis.

Dynamic hydraulic characteristics of a prototype ball valve during closing process analysed by 3D CFD method

During the runaway process of high-head turbine-generator units due to wicket gate failure, the ball valve should be closed promptly to prevent accidents, and the dynamic hydraulic characteristics during the closing process should be studied. In this paper, the flows through a prototype ball valve are simulated by the latest 3D CFD method. First, the steady flows in different openings are calculated, and we find that the headloss coefficients agree well with experimental results. Then, the water hammer led by linear closing of the ball valve is simulated by both 3D CFD and 1D Method of Characteristic (MOC), and the reasonable agreements further validate feasibility of the 3D CFD method. After that, we investigate the evolution laws of flow patterns, pressure and forces around the valve, and find that the pressure variations before, within and behind the valve differ greatly. Besides, the unstable flow fields within the valve are essential causes of pressure and force fluctuations, bringing potential danger to the safety of ball valve operation. This research provides basic information for studying ball valve vibration and its impact on the turbine and hydraulic system.