Flow Angle Research Articles

A zero-net-mass-flux wake stabilization method for blunt bodies via global linear instability

A rectangular cylinder, with an aspect ratio of 5, is a widely used bluff body in engineering practice. It undergoes intricate dynamical behavior in response to minute alterations in the flow angle of attack (α). These modifications invariably precipitate the failure of wake control for classical flow control methods with various α values. In this study, global linear instability, adjoint method, and sensitivity analysis are employed to identify the optimal position for flow control. It is found that the sensitive region gradually transitions from the leeward side to the downwind side of the model as α and Reynolds number (Re) increase. So, we set up airflow orifices for flow control in both positions. Jet flow control on the leeward side effectively inhibits vortex shedding (α ≤ 2°). High-order dynamic mode decomposition is employed to reveal the inherent mechanism of control. Suction control on the downside effectively mitigates the shear layer separation phenomenon induced by the altered spatial structure associated with higher α. A novel zero-net-mass-flux wake control, bionics-based breathe-valve control (BVC), is proposed to optimize the control effect. BVC is applicable for various α and Re, with optimal effectiveness achievable through jet velocity adjustments. The prediction-control approach in this investigation provides a targeted method to mitigate flow-induced vibration.

Development of the Universal Air Collector Design for Measuring the Flow Rate of Swirling Air Flow Using an Integral Thermoanemometer

Measurement of the swirling air flow rate and average speed of air at air distribution units of ventilation systems is a rather difficult task due to the fact that the air flow enters the measuring device at different angles. As a result, significant measurement errors can occur when using point thermoanemometers or other flow meters. The use of integral anemometers facilitates the measurement process. However, it is necessary to eliminate the errors associated with changes in the angle of the air flow to the sensing element of the measuring device. To do this, it is necessary to ensure a rectilinear structure of the air flow using a transitional air collector and rectifier. The aim of this paper is to develop a design of an air collector device that will allow to measure the flow rate of a swirling air flow. The objectives of the paper are to optimize the geometrical parameters of the air collector to ensure a rectilinear flow, minimize its dimensions and aerodynamic drag. The correctness of the air collector design was evaluated by matching the calibration characteristic of the probe of an integral thermoanemometer in the presence of a vortex diffuser in front of the air collector and in its absence. The proposed device has a rectangular shape and consists of a receiver, a rectifying grid, a chamber, an accelerating and stabilizing section, at the outlet of which a thermoanemometer probe is installed. The receiver and the accelerating section are tapered along their length, while the chamber and the stabilizing section have a constant cross-section. The rectifying grid is installed inside the chamber and has a square-shaped honeycomb structure. Several options of the air collector design with different geometrical parameters were studied using computer modelling. The dependence of aerodynamic drag on air flow was plotted. The optimal design of the air collector was chosen, for which a life-size physical model was created. The calibration characteristics of the measuring probe with an air collector were experimentally obtained when a swirling air flow was applied. The developed universal air collector allows the air flow rate measurement at the outlets of almost any air distribution devices of ventilation systems of buildings.

Effect of overheat and direct flow loading on the MEMS bistable flow sensor

We present the findings from an experimental study of a MEMS flow sensor in which an initially curved, double-clamped bistable microbeam is the primary sensing element. Our research explores how the overheat ratio, direct flow loading, and turbulence-induced vibration affect the sequential snap-through (ST) buckling and snap-back (SB) release of an electrostatically actuated beam heated by an electric current. The sensor is fabricated from highly doped single-crystal silicon using a silicon-on-insulator (SOI) wafer. Positioned at the chip’s edge, the microbeam is exposed to airflow, enabling concurrent dynamic response measurements with a laser Doppler vibrometer and a video camera.Our research demonstrates that the overheat ratio can be significantly lower for this sensing principle than conventional thermal sensing elements, pointing to the potential for substantial energy savings. We also emphasize the significant impact of flow angles and vibrations on the critical ST and SB voltages, which are vital for the flow sensor’s output. Additionally, we introduce the first direct experimental observation of the beam profile’s time history during the snap-through/snap-back transition.The potential impact of this research lies in developing more robust MEMS flow sensors with enhanced sensitivity and a better understanding of their response to environmental factors, which could have broader applications in fields such as aerospace, environmental monitoring, and industrial process control.

Transient Characteristics of Fluidic Pintle Nozzle in a Solid Rocket Motor

The fluidic pintle nozzle, a new method to control the thrust of a solid rocket motor, has been proposed in recent years by combining the pintle with the aerodynamic throat (fluidic throat). The study of static characteristics has proved that it has a remarkable effect on thrust control. To study the transient characteristics of the fluidic pintle nozzle, 2D transient simulations of a fluidic pintle nozzle propulsion system were conducted, employing dynamic meshing techniques. The Reynolds-averaged Navier–Stokes equations were meticulously solved, implementing a k–ω SST turbulence model. The thrust control principle of the fluid pintle nozzle was studied, and the wave structure was summarized. The transient characteristics of the secondary flow opening, secondary flow closing, pintle moving forward (pressure rise), and pintle moving backward (pressure drop) were obtained, and the effects of the injection angle and injection port position were studied. The response process after injection can be roughly divided into three stages: pressure propagation, pressure oscillation, and equilibrium stability, with time distributions of 0.4%, 5.39%, and 94.21%, respectively. In the process of the pintle moving forward, the rate of combustion chamber pressure increases and thrust decreases gradually because of the arc wall of the nozzle throat upstream, and the process of throats moving backward is just the opposite. Compared with the condition with a maximum throat opening and no secondary flow, the thrust of the condition with a minimum throat opening and a 0.3-flow-ratio secondary flow is increased by 80.95%. Under conditions of constrained flow ratio, the injection angle of the secondary flow ostensibly exerts negligible influence on the dynamic modulation of thrust. Nevertheless, it remains evident that a reduction in throat opening accentuates the impact of reverse injection. Furthermore, the proximity of the injection port to the head of the pintle is directly proportional to the efficacy of thrust control.

Heat Transfer and of Flow Structure of a Turbine Blade’s Air-cooled Leading Edge Considering Different Hole Shapes and Additional Flow Angles

Heat Transfer and of Flow Structure of a Turbine Blade’s Air-cooled Leading Edge Considering Different Hole Shapes and Additional Flow Angles

Application of cohesive soil flow limit and slope critical flow angle

To investigate the relationship between moisture content in cohesive soils during spontaneous liquefaction and their corresponding critical slope failure angles. This study modified the liquid limit test apparatus originally developed by A. Atterberg in 1911, adapting it into a liquid-flow limit test apparatus. On-site soils from three actual cases of slope failures in Xiaolin Village, Jiasian District, Kaohsiung City, as well as soils from slope failures along the banks of Meilong River and Zengwun River were used for experiments involving different soil moisture content and slope failure angles. Based on the experimental findings, the moisture content which cohesive soils undergo spontaneous liquefaction was designated as the Flow Limit (FL), corresponding to the downslope angle at which circular failure occurs. This value represents the critical sliding angle at which spontaneous liquefaction occurs as a results of soil saturation. It also represents the maximum slope angle for exposed mudrock or clay slopes, and establishes the critical slope angle for vegetated slopes. Beyond this critical FL angle, all vegetation is susceptible to loss due to soil liquefaction. Furthermore, this study conducted FL tests on soils from thirteen tree holes in Kaohsiung City, two mountain slopes in Fengshan District, and the silty clay responsible for property damage and injuries during a landslide in Yanchao District on September 29, 2016, as a means of validation. The consistent test results observed reaffirm that different soils exhibit similar spontaneous liquefaction angles and flow limit behavior. These FL test results demonstrate a consistent pattern, reinforcing the notion of distinct curve characteristics that can be replicated across diverse soil compositions.

Design, Modeling, and Control of a Composite Tilt-Rotor Unmanned Aerial Vehicle

Tilt-rotor unmanned aerial vehicles combine the advantages of multirotor and fixed-wing aircraft, offering features like rapid takeoff and landing, extended endurance, and wide flight conditions. This article provides a summary of the design, modeling, and control of a composite tilt-rotor. During modeling process, aerodynamic modeling was performed on the tilting and non-tilting parts based on the subcomponent modeling method, and CFD simulation analysis was conducted on the entire unmanned aerial vehicle to obtain its accurate aerodynamic characteristics. In the process of modeling the motor propeller, the reduction of motor thrust and torque due to forward flow and tilt angle velocity is thoroughly examined, which is usually ignored in most tilt UAV propeller models. In the controller design, this paper proposes a fusion ADRC control strategy suitable for vertical takeoff and landing of this type of tiltrotor. The control system framework is built using Simulink, and the control algorithm’s efficiency has been verified through simulation testing. Through the proposed control scheme, it is possible for the composite tiltrotor unmanned aerial vehicle to smoothly transition between multirotor and fixed-wing flight modes.

Multi-objective design optimization of a transonic axial fan stage using sparse active subspaces

In this paper, a multi-objective optimization strategy for efficient design of turbomachinery blades using sparse active subspaces is implemented for a turbofan stage design. The proposed strategy utilized sparse polynomial chaos expansion on a limited dataset to generate a function from which the differential and the covariance matrix can be obtained. Active subspace was used to compute the active variables via singular value decomposition and a hybrid polynomial correlated function expansion was used to construct a surrogate model on the active subspace. Coupled with freeform method and multi-objective genetic algorithm, an automated optimization loop was run at a single operating condition. An improvement in stage efficiency and total pressure ratio of 2.97% and 1.15% was achieved for the optimum design compared with the baseline. Additionally, total pressure loss coefficient decreased by 5.88%, exit flow angle by 34.65% and shock strength by 5.32%. The coupled effect of change in stagger angle, forward sweep, forward lean, and chord length reduced the recirculation at the hub, and blockage at the shroud due the tip leakage flow by decreasing the blade loading. The threshold value hyperparameter was found to be the most influential and must be accurately determined.

Design and Experiment of Precision Seed Metering Device for Flow Adsorption of Quinoa Seeds

This paper proposes a solution to the problem of tight population in the filling area of traditional air suction seed metering devices during quinoa sowing, which leads to inaccurate adsorption. The proposed method disperses the population into a stable seed flow and absorbs the seeds in a flow posture. The flow adsorption precision seed metering device is designed and improved, and the key structure parameters are optimized based on the shape and size parameters of quinoa seeds. A four-factor and three-level response surface orthogonal test is conducted using the Box–Behnken experimental design. The number of seeds, flow angle, negative pressure at the suction hole, and advancing speed are taken as experimental factors, and the qualified index of grain number per hole, qualified index of hole distance, and coefficient of variation of hole distance are used as evaluation indices. The results are optimized using extreme value theory, and it is found that when the seed amount is 5.82 mm, the flow angle is 31.08°, the negative pressure of the air chamber is 1.7 kPa, and the advancing speed is 3.82 km·h−1, and the qualified index of hole number is 94.37%, the qualified index of hole distance is 95.39%, and the coefficient of variation of hole distance is 4.51%. The results are in general agreement with the prediction through the bench validation test, which meets the requirements of quinoa seed metering devices.

On the vortex identification within a linear blade cascade - an experimental research

This paper deals with the identification of the individual vortical structures within a linear blade cascade based on H criteria. The experimental data obtained from the pressure measurement at the cascade outlet were evaluated by a standard procedure to obtain the velocity field in one plane. Then, the vorticity in the flow field was evaluated based on Crocco’s theorem, and in the final step, the helicity density was calculated. The impact of the inlet flow angle on the velocity field as well as on the vortical structures was investigated. The effect of the value of the H threshold on the vortex identification is discussed in the last part of the paper.

Effects of sediment characteristics on the erosion of Pelton turbine

As Pelton turbines are increasingly used in high mountain regions because of their unique advantage of high-water head adaptability, the hydraulic characteristics and multi-phase flow behaviour in Pelton turbines have attracted the attention of many scholars. Most notably, the presence of solid particles such as sand exists in natural river systems, which end up in the turbines, causing erosion. This has become an international academic issue which has not been fully developed yet. In this study, three-phase simulations based on Eulerian–Lagrangian methodology are conducted to investigate the erosion phenomenon, especially in the bucket region. Sediment particle characteristics such as size and concentration in both the distribution system and buckets region are analysed. The innovation of this study is that, unlike other erosion investigations, it focuses on the impact angle of the jet flow as this is a unique characteristic of Pelton turbines. Furthermore, the sediment particle's flow characteristics at different bucket rotating angles and how they influence the erosion mechanism are also investigated. Results reveal that larger particles cause greater erosion on the bucket. In small particle and standard diameter conditions, the impact angle is mostly below 5 ∘ . Finally, this study presents the first successful attempt in using rotating angle to describe the erosion in a Pelton turbine. A corrected erosion model adapted to the Pelton turbine is given through regression and proves effective in predicting erosion.

Leakage vortices and energy characteristics of semi-open sewage pump with various blade tip alterations

As one of the most commonly used fluid machineries, sewage pumps play a critical role in various sectors, including urban construction, industry, and environmental protection. Considering the clog-resistant performance, the impeller usually uses a semi-open structure, giving rise to a tip structure that introduces leakage flow. This exacerbates the instability of flow within the pump, thereby impacting the overall performance and operational stability of sewage pumps. In this study, four distinct blade tip structure schemes are developed through a combination of numerical calculations and experimental validation. The Liutex–Omega vortex identification method is employed to scrutinize the initiation and development mechanisms of the leakage vortex system. A comprehensive analysis is conducted to explore the influence of the tip structure on both the energy characteristics and the internal flow dynamics of semi-open sewage pumps. It is found that the complex mixing effect of the leakage flow with the main flow is the primary source of energy loss within the semi-open impeller. Additionally, the characteristic angle of the leakage flow exhibits a high correlation with the energy dissipation characteristics of the impeller. Meanwhile, the impeller with a rounded tip exhibits the poorest performance, with the most substantial reductions in head and efficiency amounting to 44.17% and 36.11%, respectively, compared to the original impeller. Conversely, the impeller featuring a T-shaped tip demonstrates a maximum increase in the head of up to 7.25% under conditions of a large clearance size. This study not only contributes to the theoretical understanding of the unsteady flow within the semi-open impeller but also offers valuable insights for the optimized design of semi-open impeller sewage pumps, holding both theoretical and practical significance.

Flow dynamics of train under turbulent inflow at different crosswind yaw angles

The turbulence intensity and yaw angle of crosswinds exert a substantial impact on the aerodynamic characteristics of trains traveling in windy regions. It is urgent to study how the yaw angle and turbulence intensity of incoming flow jointly affect the aerodynamic characteristics of the train and the corresponding flow field under turbulent crosswind. A high-speed train scaled at a ratio of 1/8 of its actual size at different yaw angles was investigated. Three inflow conditions were adopted, including uniform inflow, Iu = 0.05 inflow, and Iu = 0.2 inflow (Iu is turbulence intensity). The turbulent inflow was generated by the synthetic eddy method. The instantaneous and time averaged characteristics of aerodynamic loads and pressure loads of the train were analyzed. The vortex structures, vorticity, swirling strength, mean velocity, reverse flow, and Reynolds stresses are analyzed to explore the flow pattern and flow evolution. The results found that the fluctuation of the aerodynamic loads, the average side fore, and the average rolling moment of the train are remarkably enhanced under turbulent inflow. These results stem from the alterations in the flow field around the train induced by turbulent inflow, consequently leading to variations in surface pressure on the train. As the turbulence intensity of the inflow increases, the stability of the vortex structures decreases, and the position of the large-scale vortex structure has been changed. Moreover, the yaw angle (β) exerts a more significant influence on the vortex structure's flow pattern on the leeward side compared to the inflow turbulence intensity.

Effect of Prandtl number and free-stream orientation on global parameters for flow past a heated square cylinder

In this study, an in-depth examination of the aerodynamic parameters involving forced and mixed convection around a heated square cylinder is presented. The ranges of Prandtl number (Pr), Richardson number (Ri), and flow orientation (α) are kept as 0.71 ≤ Pr ≤ 1000, 0 ≤ Ri ≤ 1.6, 0° ≤ α ≤ 90°, while the Reynolds number (Re) and the cylinder orientation (ϕ) are kept fixed as Re = 100 and ϕ = 0°, respectively. The flow is considered as two-dimensional (2D), steady, laminar, incompressible, and viscous. The buoyancy effects are taken into account through the Boussinesq approximation. At lower Pr, the flow shifts from unsteady to steady with increasing Ri. This transition persists at higher Ri with increasing Prandtl values. The flow remains consistently unsteady at α = 90°. Isotherm crowding intensifies with higher Pr and/or Ri across all flow inclinations. Across the complete spectrum of flow angles, it is noted that the mean lift coefficient rises as the Richardson number increases. Additionally, the mean drag coefficient reaches its peak at Ri = 1.6 when Pr = 0.71. The findings reveal that the Strouhal number (St) rises as the Richardson number (Ri) increases, and it decreases as the Prandtl number (Pr) increases. The mean Nusselt number (Nu¯) demonstrates an upward trend as the Prandtl number increases, with Ri held constant. It is also observed that Nu¯ is more sensitive to the Prandtl number than the Richardson number and is maximum at Pr = 1000 for the selected range of flow orientations.

An improved theoretical model for the dynamics and stability of a vertical cantilevered pipe aspirating fluid

In this paper, a new theoretical model for the dynamics and stability of a vertical cantilevered aspirating pipe is developed based on the Newtonian method. The effects of the hydrodynamic forces at the free end on the dynamics and stability of the cantilevered pipe are examined in detail. A new comprehensive governing equation is discretized using the Galerkin method. Numerical results are presented to demonstrate the effects of the axial flow velocity ratio, flow angle ratio, and tangential velocity ratio on the eigenfrequency and critical velocity of the cantilevered pipe system. The theoretical model results are also found to be in good quantitative agreement with experimental results.

Validation of 2D flow MRI for helical and vortical flows

PurposeThe main objective of this study was to develop two-dimensional (2D) phase contrast (PC) methods to quantify the helicity and vorticity of blood flow in the aortic root.MethodsThis proof-of-concept study...

Computational Study on Compressor-Combustor Interactions for Improved Matching of a Micro Gas Turbine

ABSTRACT Understanding complex aerodynamics/combustion interactions across major components of aeroengines, including the compressor, combustor and turbine is demanded for the improved performance of advanced propulsion systems. This study employs high-fidelity CFD simulations to investigate the coupled compressor-combustor interactions for the improved performance of a micro gas turbine. In the prototype configuration, the deficient compressor/combustor matching causes insufficient air/fuel mixing and delayed combustion inside the combustor chamber. By reducing the flow area of the compressor and redistributing the air holes on the combustor chamber, a stable united recirculation structure is formed to promote air/fuel mixing and efficient combustion, achieving a over 50% decrease in OTDF and RTDF at the combustor exit and a total 20% decrease of pressure loss in the entire engine through-flow. Compared with the single-component combustor simulation, the coupled compressor-combustor results exhibit stronger turbulence mixing, increased temperature uniformity inside the combustor chamber. Compared with the single-component compressor simulation, the compressor in the multi-component simulation shows the reduced flow angle and Mach number at the compressor outlet, leading to increased stall critical angles and delayed flow separation at the compressor diffuser and vanes. This demonstrates that the added combustor endues the compressor with an expanded operating range and a lower mass-flow rate instability boundary. This work implies that 3D multi-component CFD simulation is necessary to understand the aerodynamic and combustion interactions and improve the multi-component matching in gas turbine engines.

Research on the hydrodynamic performance of propellers under oblique flow conditions

The phenomenon of oblique water inflow is widespread in the operation of thrusters, which will cause adverse effects on the hydrodynamic performance of thrusters. In order to investigate the hydrodynamic performance and stress variation of the propeller in oblique flow, based on computational fluid dynamics theory, this paper takes the DTMB4119 propeller as the research object and conducts numerical simulation research on the propeller in oblique flow by solving the RANS equation. By calculating the open water performance curve and surface pressure distribution of the propeller, the rationality of the numerical method and the meshing are verified. Considering the flow field information such as velocity, oblique flow angles, flow line distribution, and pressure distribution, the changes in hydrodynamic characteristics of the propeller are simulated and analyzed. The results show that the uneven distribution of pressure on the propeller blade surface increases with a decrease in the advance coefficient. The force pulsation amplitude of a single blade increases with an increase in oblique flow angles. With the increase in oblique flow angles, the thrust, torque, and efficiency of the propeller show different increasing trends. With the increase in propeller advance coefficient, propeller thrust and torque decrease gradually, and propeller efficiency increases gradually. By using mature propellers, this paper has more reference value for studying the flow field around the ship hull and the hydrodynamic performance of propellers in the process of ship maneuvering.

CFD modeling of baijiu yeast spray drying process and improved design of drying tower

High-quality baijiu yeast – modified Sporidiobolus johnsonii A (MSJA) suffers from the problem of weak vitality, thus failing to combine high efficiency and high survival rate drying. Therefore, in this study, we firstly designed a real-time sampling experiment of MSJA spray drying process, and based on this, we established a New CFD model (N-CFD model) that can accurately simulate the variation of the spray drying process of microorganisms such as MSJA. Then, a new spray drying air inlet mechanism of "high-temperature cyclone in the center L-type guide plate + medium-temperature laminar flow in the peripheral holes and slots" was invented. Finally, the parametric scanning simulation was carried out to optimize the flow angle θ of the L-type guide plate using the N-CFD model. The results show that, firstly, the N-CFD model can accurately simulate the temperature distribution (P1 = 0.05) and the motion of drying particles in the drying tower. Secondly, the optimized new air inlet mechanism with the L-type guide plate flow angle θ = 47° can even out the temperature and particle distribution in the drying tower, which meets the "two-stage theory of spray drying." Finally, under the premise of ensuring the drying efficiency, the survival rate of MSJA spray drying was increased from 42.6% to 82.24%, which achieved the objectives of low sticking wall, high efficiency and high survival rate of spray drying MSJA.

Coupled SPH-FEM analysis of piled power transmission tower system subjected to debris flow

Debris flow is a type of highly dangerous geological hazard, which can pose significant threat to the safety of various infrastructures including power transmission tower system. However, the performance of piled power transmission tower system against debris flow has yet to be well investigated. A validated coupled SPH-FEM method was employed to investigate the behavior of a typical piled power transmission tower system subjected to various debris flows, accounting for influencing factors of slope angle, sliding distance, and density and thickness of debris flow. The findings revealed that the incorporation of power transmission tower was important to accurately estimate the impact force and horizontal displacement of the pile and the frontal piles undertook more than 70% of the total impact force; among the factors considered, the slope angle and density were found to be more influential. After examining the performance of existing prediction methods, two semi-empirical equations were proposed for respectively estimating the maximum impact force and horizontal displacement of pile based on multivariate regression analysis. Furthermore, it was found that debris flows did not likely cause steel tower to directly experience structural damages; however, its pile foundation could suffer moderate to severe damages depending on the debris flow conditions.