Distortion Of Flow Field Research Articles

Binocular Scheimpflug light-field PIV

This work presents theoretical and experimental studies on a binocular Scheimpflug light-field based PIV. To investigate the effects of key parameters such as magnification, camera separation angle and tilt angle, a simulation study and quantitative comparison between conventional and binocular Scheimpflug light-field PIV have been performed. To validate the performance of binocular Scheimpflug light-field PIV, a series of vortex ring experiments have been carried out with different parameters. A comparison of the reconstructed volume and quality of the resolved vortex ring structures shows the advantages of binocular Scheimpflug light-field PIV. With Scheimpflug imaging, the focal plane of two light-field cameras can be fully overlapped which results in an extended reconstructed particle volume and less distortion in the resolved flow field than conventional binocular light-field PIV.

Study on the Influence of Volute Throat Jet on Excitation Vibration Alleviation of Radial Turbine Blade

Abstract Turbine blade fracture, resulting from high-cycle fatigue (HCF), is the most commonly observed failure mode of turbochargers. In a vaneless turbine, the radial turbine blade is susceptible to HCF under excessive aerodynamic excitation due to the flow field distortion caused by the volute tongue. The present study explores a technique to mitigate blade excitation by reducing pressure disturbance in the volute tongue area. The gas with higher pressure in the inlet section of the volute is transferred to the low-static pressure area upstream of the volute tongue through a throat jet hole. The usage of high-pressure gas improved the homogeneity of the flow field near the volute tongue. The study investigates the mitigation of high-pressure gas jet on turbine blade excitation, the impact of jet hole geometry on the suppression effect of the blade vibration, and the effect of high-pressure jet on turbine performance using numerical simulation of unsteady flow fields. The results indicate that the jet flow has a substantial impact on dampening the blade vibrations. The harmonic pressure amplitude at the monitor point located within the high blade vibration amplitude region is reduced by up to 56%. Generalized pressure analysis reveals that the aerodynamic excitation of the turbine blades is significantly reduced. Experimental evidence verifies the effectiveness of the jet hole scheme in reducing blade vibration. Moreover, the jet hole has a negligible effect on turbine efficiency. However, there is a degree of increased stress in the volute tongue with a jet hole which needs to be evaluated in engineering applications.

Analysis of aerodynamic and stealth characteristics of aircraft under the action of microwave plasma

This paper investigates the impact of plasma on the aerodynamic and stealth characteristics of hypersonic aircraft. Utilizing the finite volume method, the governing equations of the fluid mechanics were discretized, and the seven-element air chemical reaction model was employed for computations. A technique involving the alteration of the chemical reaction source term rate was employed to replicate the actual plasma concentration distortion phenomena induced by microwaves around hypersonic vehicles, enabling an analysis of the influence of plasma density distortion on the aerodynamic characteristics of these vehicles. In addition, the method of Piecewise Linear JE Recursive Convolution Finite-difference Time-domain (PLJERC-FDTD) is used to calculate the radar scattering cross-section of the target within both the initial and distorted flow fields. This allowed an examination of the variation in the aircraft’s stealth characteristics when exposed to distortion in an ionized environment.

Proper Orthogonal Decomposition Based Response Analysis of Inlet Distortion on a Waterjet Pump

This study addresses the challenge of performance degradation in waterjet pumps due to non-uniform suction flow. Utilizing the Proper Orthogonal Decomposition (POD) method, it decomposes and reconstructs the flow features within a waterjet pump under non-uniform inflow into a series of modes ranked in descending order of energy. By analyzing the modes with dominant energy, which contain complex information about the flow field, it is revealed that modes 1 and 2 predominantly represent the formation of a concentrated vortex, whereas modes 3 and 4 illustrate its spatial offset. Notably, in the hub section, mode 3 exhibits a delayed flow separation caused by the reduction of circumferential vortex (CV), with a consequent lift in blade loading at the leading edge and a higher head compared to mode 1. In the shroud section, the delayed flow separation in mode 3 suppressed reverse flow and the concentrated separation vortex (CSV) and then increased the blade loading, ultimately enhancing the pump head. The findings provide significant insights into optimizing waterjet pump performance by detailing the interactions between various flow structures and pump components, effectively filling a knowledge gap in applying dimensionality reduction techniques within the distorted flow fields of water jet pumps.

Analysis of Flow Field Distortion in Ship Inlet System and Its Effect on Compressor Performance

Analysis of Flow Field Distortion in Ship Inlet System and Its Effect on Compressor Performance

Flow characteristics of ejector nozzles with different auxiliary intake valves

The ejector nozzle requires a third auxiliary airflow to improve its thrust performance at transonic speed. However, different auxiliary air intake structures will affect the third auxiliary flow, which in turn may change its thrust performance and internal flow field characteristics. Herein, we simulated four nozzles with different intake valve structures (sector angles of the open-close interval structure were 30°, 22.5°, 18°, and 15°, respectively) at transonic speed (Ma = 1.2). The results show that there is significant lateral flow inside the nozzle, which induces a multi-pair vortex. In addition, the nozzle with an open-close sector angle of 30° has the largest flow rate of 9.352 kg/s, which produces the largest thrust and the slowest dissipating flow vortex, but the distortion of the flow field in nozzles is the most obvious. However, with the reduction of the sector angle of the open-close intake valves (i.e., 22.5°, 18°, and 15°), the flow rate and the thrust of the nozzle decrease. Meanwhile, the dissipation of the flow vortex is accelerated, resulting in a weakening of the flow field distortion.

Relative size of the effective regurgitant orifice to right ventricular cavity contributes significantly to the underestimation of 2D PISA method in tricuspid regurgitation quantitation

Abstract Background 2D proximal isovelocity surface area (PISA) method underestimates tricuspid regurgitation (TR) severity largely due to the distortion of proximal flow geometry under low jet velocity. Previous studies demonstrated that the confining wall geometry also strained the streamline distribution in the proximal flow field. This could add to the variability when applying 2D PISA in TR as TR presents with wider range of effective regurgitant orifice area (EROA). Purpose We aim to examine (1) the impact of ventricular wall constraint to proximal flow region on TR quantitation and (2) its relative contribution to the underestimation of 2D PISA compared with the distortion of proximal flow field under low jet velocity. Methods 150 pts with more than mild TR underwent comprehensive echocardiographic session. The standard 2D PISA EROA (EROASD)was derived as per guideline with angle correction applied for tethering leaflets. The underetimation caused by low jet velocity was corrected by multiplying the standard EROA (EROASD) with VOrifice/(VOrifice-VNyquist), where the VOrifice stands for the peak regurgitant velocity and the VNyquist represents the color Doppler aliasing velocity (EROAVo-VN). The volumetric EROA and vena contracta area serves as two independent reference methods. The ventricular wall constraint was quantified as the ratio of reference EROA to right ventricular end-diastolic volume (RVEDV). Results A consistent significant underestimation by standard 2D PISA methods, either corrected for low jet velocity or not, was observed as a function of the EROA (Figure 1). In Bland-Altman analysis, correction for low jet velocity lessened the systematic underestimation but still leaves minor while could-be-important error (VCA, bias = 19.6 and 15.1 mm2; volumetric EROA, bias = 10.1 and 5.6 mm2 for EROASD and EROAVo-VN respectively). The accuracy of both methods to differentiate severe TR was similar but the cut-off values were lower compared with guideline recommendation (AUC = 0.905 and 0.903, cut-off = 0.26 and 0.29 cm2 for EROASD and EROAVo-VN respectively). In multi-linear regression model (Table 1), the difference between standard 2D PISA EROA and reference EROA (mean of volumetric EROA and vena contracta area) was significantly related to etiology, the jet velocity-related correction coefficient, and the reference EROA/RVEDV ratio. The ventricular wall constraint contributes to the underestimation of 2D PISA to a greater extent than the low jet velocity (beta = -0.269 and 0.592 for VOrifice/(VOrifice-VNyquist) and reference EROA/RVEDV respectively). Conclusion Correction for low jet velocity lessens the extent of underestimation of TR severity by 2D PISA method but still leaves important measurement error. The confining wall geometry to proximal flow region contributes significantly to the understimation but is not accounted for in current recommendation. This could be especially important in massive and torrential TR.

Practical examples for the flight data compatibility check

Accurate information about aircraft speed, altitude, and aerodynamic flow angles is essential for evaluating aircraft performance and handling qualities. These quantities are determined from air data measurements taken by sensors normally located near the aircraft cockpit. Since these sensors are affected by the distorted flow field around the fuselage, a correction must be applied. Before the first flight, a set of calibration parameters is usually determined from wind tunnel experiments or CFD calculations. However, the Data Compatibility Check (DCC) method allows a more accurate air data sensor calibration during the certification flight test. This method reconstructs air data quantities from inertial acceleration, angular rate measurements and the flight path. By comparing the reconstructed quantities with the measured ones, the structure and parameters of air data sensor models can be identified. In this paper, an introduction to the data compatibility check method and the setup used in a flight test for system identification is given. The DCC is applied on data gathered from a test campaign with the new DLR research aircraft Dassault Falcon 2000LX ISTAR. Use cases for the calibration of the nose boom airflow vanes and the correction of sensors during large sideslip maneuvers will be presented in this paper.

Assessment of Pseudoshock Models Against Experiment in a Low-Aspect-Ratio Isolator

A highly confined shock train is investigated in a direct-connect isolator facility with a Mach 2 inflow and a constant-area low-aspect-ratio rectangular test section. High-speed schlieren imaging, wall static pressure measurements, surface oil-flow visualization, and particle image velocimetry from this isolator are synthesized into a three-dimensional schematic of the shock train structure. Against this, the prevailing pseudoshock models in the literature are assessed to evaluate the validity of their underlying assumptions. None of the prevailing pseudoshock models are found to simultaneously model the pressure and Mach number profiles, indicating a gap in the model formation and underlying assumptions when applied to the experimental isolator of interest. The presence of distortion in the isolator flowfield, such as a wall-bounded vortex, is found to skew the structure of the shock train, altering the strength and distribution of the compressive pressure gradient. It is further observed that the separated flow morphology surrounding the shock train is not monolithic, as is typically assumed, adjusting the balance of compressive forces within the shock cells. These findings lead to the conclusion that existing flux-conserved modeling approaches require modification to be effective in distorted and highly confined cases, including closure models that capture the three-dimensional distorted structure of the approach flow and its evolution along the shock train.

Non-axisymmetric design and flow field analysis of boundary layer ingesting fans

The inlet distortion problem exists in a boundary layer ingesting propulsion system, and the distortion position is almost stable, which seriously affects the fan performance and stability margin (SM). To effectively improve the distortion resistance of the fan, three non-axisymmetric stator (NAS) schemes are designed according to the parameter distribution law of the distorted flow field in front of the stator under the design conditions in this study, which are based on the solidity matching method of the optimization of the stator inlet geometric angle, chord length, and pitch. The impact of the NAS on the fan performance and the distorted flow field parameters under distorted conditions are numerically studied. The results indicate that NAS can effectively enhance the SM of the fan. Among the three NAS designs, the third one demonstrates a remarkable increase of 74.91% in SM compared to the original fan. The NAS design can promote the fluid re-distribution in the distortion zone, make the aerodynamic parameters more uniform, and thus improve the flow state in the distortion zone. Among the three NAS schemes that are adopted in this research, the NAS design based on the pitch optimization method has the best effect. This design can effectively improve the stator flow field structure, thus significantly improving the SM and the aerodynamic performance of the fan under distortion conditions.

Artefact Indicator for Flow‐based Temporal Interpolation of Power Doppler Ultrasound

AbstractPower Doppler ultrasound is a sonography technique to detect and visualise blood circulation. Its acquired image sequences suffer from poor temporal resolution and artefacts caused by aliasing, noise, large displacements, shearings or out‐of‐plane movements. These pitfalls distort flow fields and have unforeseen consequences on flow‐based temporal interpolations. We propose a method to detect artefacts caused by such distorted flow fields and to improve the visual quality by replacing regions with detected artefacts by locally better results. This is demonstrated on real data.

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.

Electroosmotic modulated unsteady squeezing flow with temperature-dependent thermal conductivity, electric and magnetic field effects

Modern lubrication systems are increasingly deploying smart (functional) materials. These respond to various external stimuli including electrical and magnetic fields, acoustics, light etc. Motivated by such developments, in the present article unsteady electro-magnetohydrodynamics squeezing flow and heat transfer in a smart ionic viscous fluid intercalated between parallel plates with zeta potential effects is examined. The proposed mathematical model of problem is formulated as a system of partial differential equations (continuity, momenta and energy). Viscous dissipation and variable thermal conductivity effects are included. Axial electrical distribution is also addressed. The governing equations are converted into ordinary differential equations via similarity transformations and then solved numerically with MATLAB software. The transport phenomena are scrutinized for both when the plates move apart or when they approach each other. Also, the impact of different parameters such squeezing number, variable thermal conductivity parameter, Prandtl number, Hartmann number, Eckert number, zeta potential parameter, electric field parameter and electroosmosis parameter on the axial velocity and fluid temperature are analysed. For varied intensities of applied plate motion, the electro-viscous effects derived from electric double-capacity flow field distortions are thoroughly studied. It has been shown that the results from the current model differ significantly from those achieved by using a standard Poisson–Boltzmann equation model. Axial velocity acceleration is induced with negative squeeze number (plates approaching, S < 0) in comparison to that of positive squeeze number (plates separating, S > 0). Velocity enhances with increasing electroosmosis parameter and zeta potential parameter. With rising values of zeta potential and electroosmosis parameter, there is a decrease in temperatures for U e > 0 for both approaching i.e. squeezing plates (S < 0) and separating (S > 0) cases. The simulations provide novel insights into smart squeezing lubrication with thermal effects and also a solid benchmark for further computational fluid dynamics investigations.

Effects of Nonaxisymmetric Volute on Rotating Stall in the Vaneless Diffuser of Centrifugal Compressors

Abstract Rotating stall is an important unstable flow phenomenon that leads to performance degradation and limits the stability boundary in centrifugal compressors. The volute is one of the sources inducing nonaxisymmetric flows in centrifugal compressors, which has an important effect on compressors' aerodynamic performance. However, the influence of volute on rotating stall is not clear. Therefore, the effects of volute on rotating stall behavior have been explored in this paper by experiments and numerical simulations. The frequency of the rotating stall captured by the experiments is 43.9% of the impeller passing frequency (IPF), while it is 44.7% of IPF calculated from the numerical results, which proves the accuracy and capability of the numerical method in this work to study the rotating stall behavior. The flow fields from computational fluid dynamics (CFD) simulations further reveal that one stall cell initializing in a particular location deforms into several stall cells while rotating along the circumferential direction and becomes much smaller in a specific location during the evolution process, and finally, it is suppressed in another specific location as a result of the distorted flow field caused by the volute. Moreover, stall cells' behavior, including shrinking and enlarging along the spanwise direction during rotation, is closely related to the circumferentially nonaxisymmetric pressure distribution caused by the volute. The existence of volute makes rotating stall incept in the flow rate 32% larger than that of the centrifugal compressor without volute. By optimizing volute geometry to reduce the distortion of the flow field, it is expected that the rotating stall can be weakened or suppressed, which is helpful to extend the stable operating range of centrifugal compressors.

Extinction of non-equidiffusive premixed flames with oscillating strain rates

Extinction of premixed flames under non-uniform, unsteady strain is a phenomenon commonly observed in turbulent combustors. To assess the role of inequity in thermal and mass diffusion, represented by a global Lewis number (Le) - defined as the ratio of the mixture’s thermal diffusivity to the mass diffusivity of the deficient species, on such extinctions, we present a study of counterflow twin-flames with various Le under oscillating strain rates. Experimental results confirm that for low mean strain rates, the amplitude of strain rate oscillation required for extinction is so large that the flow temporarily alters its direction, leading to distortion of the counterflow flow-field, destabilization, and eventual extinction of the flame irrespective of Lewis number. However, for relatively large mean strain rates, extinction results from flame-response to the peak instantaneous strain rate. For Le>1 mixtures, the maximum strain rate that the flames can sustain is greater than the steady state extinction values, while Le≤1 flames extinguish at a maximum strain rate, approximately equal to the steady state strain condition. This distinctively disparate behaviors of extinctions depending on the Lewis number are analyzed and delineated using numerical simulations of unsteady flames.

Experimental validation of a new type of direct-connect facility for scramjet studies

Combustion chamber design has been important to the development of scramjet engines for a long time. Conventional experimental facilities of scramjet could be divided into two types: free-jet and direct-connect facilities. Free-jet facility can provide real incoming conditions to the combustion chamber, especially the real shock wave train structures formed in inlet and isolator. Unfortunately, it is relatively expensive. While direct-connect facility has a distinct advantage in experimental cost and period, so it has been used widely in relevant researches. However, traditional direct-connect facility usually can only provide a uniform incoming flow, which is much different from the distorted flow field in real scramjet, and this is a non-negligible disadvantage to relevant design. An inverse design method based on method of characteristics (MOC) is presented to solve this problem. The facility designed by this new method can produce the pre-assigned distorted flow field without encountering unstart phenomenon. In this paper, experimental investigation of the new direct-connect facility, which is designed to reproduce a specific distorted flow field with Mach number more than 2, is further presented. Test data of particle image velocimetry (PIV), flow visualization experiments and corresponding numerical simulation results are used to verify the reliability and accuracy of this new facility. Qualitative flow visualization and quantitative measurement results further prove that this new type of direct-connect facility can generate the target distorted flow field as pre-defined.

Experimental simulation methodology and spatial transition of complex distortion fields in a S-shaped inlet

The S-shaped inlet of the modern military aircraft propulsion system suffers from severe inlet distortion during flight maneuvering. The complex distorted flow field is generally decomposed into total pressure and swirl distortion, both of which could substantially degrade the performance and stability of an aero-engine. In the ground test of fans and low-pressure compressors, how to simulate the complex inlet distortion occurring in flight is non-trivial. In this paper, a subsonic S-shaped inlet test rig was constructed, and eight configurations of curved-edge plate-type distortion generators were installed in the intake channel to create a complex distorted flow field downstream. Results show that the distorted flow field is dependent on the circumferential position of the blocking plate due to the duct curvature. Prominent vortex structures caused by the pressure gradient and the cross secondary flow exist between the main flow and separated flow. Conventional distortion indices DC, CDI/RDI, and the SAE AIR1419/SAE AIR 5686 standard system are implemented to represent the total pressure distortion and swirl distortion and the effects of the size and location of the curved-edge plate on distortion intensity and distribution are investigated. Additionally, this paper also studied the spatial variation of the total pressure and swirl distortion between the duct circular exit section and the annular section, termed as spatial transition characteristics. This spatial transition process can be used in the future to specify a real inlet flow condition for stability assessment of a fan or low-pressure compressor. According to the experimental results, the circumferential total pressure distortion index is approximately unaltered along the contraction duct, while the radial index reduces by 62% on average. The spatial transition of the swirl distortion differs for bilaterally symmetrical and asymmetric swirling fields. The overall swirl intensity index decays by 37%∼70% only for bilaterally symmetrical modes, while being the same for bilaterally asymmetric approximately. At last, the generation and evolution mechanism of the above complex distorted flow field is analyzed theoretically.

The Investigation on the Flow Distortion Effect of Header to Guarantee the Measurement Accuracy of the Ultrasonic Gas Flowmeter

The quantification of the flow distortion effect on the measurement accuracy of the ultrasonic gas flowmeter downstream of the header is important but an area that has been of less concern in the research. By experiments and computational fluid dynamics (CFD), the influence of flow field distortion was studied. Experimental results under three different installation conditions showed that when there was flow field distortion downstream of the header, the measurement results of the gas ultrasonic flowmeter were 1% higher than those when there was no distortion, while a flow conditioner could effectively eliminate flow field distortion. Based on the experimental tests, the flow field distribution was analyzed with CFD, which showed that the flow field distortion effect generated by the header had a significant influence on the parameter of nonconforming Profile factor, while the parameters of Symmetry and Cross-flow could be obviously eliminated by the double-cross-section designing.

Experimental and numerical investigation on total pressure distortion generated by a rectangular movable flat baffle

A distortion generator equipped with a motor-activated movable flat baffle was installed just upstream of a rectangular plenum entrance to investigate the effects of inlet total pressure distortion on the stability and performance of an auxiliary power unit (APU). Experiments and numerical simulations on a direct connect scale inlet model of the APU were carried out to obtain a quantitative relationship between the insertion depth of the flat baffle in the flow stream and the total pressure distortion intensity and region. In the experiments, the blocking coefficient and total pressure distortion coefficient were controlled by adjusting the insertion depth of the flat baffle and the mass flow. In the simulations, detailed flow field was analyzed based on the detached-eddy simulation (DES) method. The results show that the pressure distribution of the distorted flow on the aerodynamic interface plane (AIP) can be divided into a high-pressure region, a transition region, and a low-pressure region. The area affected by the distorted flow was larger than the inserting area of the flat baffle. That area was more related to the relative blocking coefficient, and less affected by the mass flow. The total pressure distortion coefficient had a linear relationship with the mass flow rate and is positively correlated with the relative blocking coefficient. As the relative blocking coefficient increased to a certain value, an exponential growth in the total pressure distortion coefficient occurred, and consequently, the flow field distortion was intensified. In the flow field, a pair of corner vortices were formed at the corner between the flat baffle and the bottom wall of the inlet pipe, and a large separation zone was formed behind the flat baffle and exhibits certain unsteady characteristics.

Impact of prosthetic mitral valve orientation on the ventricular flow field: Comparison using patient-specific computational fluid dynamics

Significant mitral valve regurgitation creates progressive adverse remodeling of the left ventricle (LV). Replacement of the failing valve with a prosthesis generally improves patient outcomes but leaves the patient with non-physiological intracardiac flow patterns that might contribute to their future risk of thrombus formation and embolism.It has been suggested that the angular orientation of the implanted valve might modify the postoperative distortion of the intraventricular flow field. In this study, we investigated the effect of prosthetic valve orientation on LV flow patterns by using heart geometry from a patient with LV dysfunction and a competent native mitral valve to calculate intracardiac flow fields with computational fluid dynamics (CFD). Results were validated using in vivo 4D Flow MRI. The computed flow fields were compared to calculations following virtual implantation of a mechanical heart valve oriented in four different angles to assess the effect of leaflet position. Flow patterns were visualized in long- and short-axes and quantified with flow component analysis. In comparison to a native valve, valve implantation increased the proportion of the mitral inflow remaining in the basal region and further increased the residual volume in the apical area. Only slight changes due to valve orientation were observed.Using our numerical framework, we demonstrated quantitative changes in left ventricular blood flow due to prosthetic mitral replacement. This framework may be used to improve design of prosthetic heart valves and implantation procedures to minimize the potential for apical flow stasis, and potentially assist personalized treatment planning.