Search

Non-blade-order flow disturbances, also referred to as pre-stall disturbances or tip flow unsteadiness, are closely related with compressor instabilities. The present work provides a comprehensive investigation on multi-scale nature of non-blade-order disturbances and the underlying flow physics in axial compressors. By applying full-annulus Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, along with space-time correlation and spatial Fourier decomposition, to the disturbed pressure, the propagating feature of the non-blade-order disturbances is obtained. Further, a bridge between non-blade-order disturbances and the evolution of unsteady vortex has been set up. The results show that non-blade-order disturbances, featured as short-length-scale (35 modes across annulus), first appear as the occurrence of tip leakage vortex fluctuation, while the compressor still operates far from stall. Leading-edge radial vortex appears at near stall condition, and its movement induces a circumferential propagating disturbance overlaying on the one induced by oscillating tip leakage vortex. The interaction of the short-scale disturbances with a low-amplitude long-scale (of circumference) disturbance is observed, which results in disturbances with multiple scales of consecutive spatial modes, along with multiple frequency peaks in spectra. The compressor falls into stall as the circumferential nonuniform scattering of the leading-edge vortexes occurs. The densely- and sparsely-scattered leading-edge radial vortexes induce a high-amplitude long-scale (of circumference) disturbance, i.e. stall disturbance.

Quasi-static and dynamic stall control experiments, using dielectric barrier discharge plasma actuators, were performed on a square-tipped aspect-ratio-one wing, with a NACA 0015 profile, under harmonic pitching, at a Reynolds number of 3×105. Relatively low 풪(1) and relatively high 풪(10) pulse-modulated reduced excitation frequencies were introduced at the leading edge; the former produced the largest poststall lift improvements, whereas the latter produced the highest maximum lift. A reduced frequency, based on laminar separation bubble shedding, was enlisted to explain the differences between the two results. Under conditions of dynamic pitching, evidence of a dynamic stall vortex was absent, due to its interaction with the strong tip vortices, consistent with prior experimental and numerical investigations. As a general rule, performance benefits produced by excitation under quasi-static conditions were reproduced under harmonic pitching. Low-order dynamic stall and dynamic stall control models, based on a modified version of the Goman–Khrabrov delta-wing concept, were evaluated. Model constants were obtained from quasi-static data sets, and a single baseline dynamic case was employed to determine the time constants. The model produced excellent results for both baseline and controlled cases, which was attributed to similar lift and stall mechanisms exhibited by delta and rectangular low-aspect-ratio planforms.

Natural ice accretion on the lifting surface of an aircraft is detrimental to its aerodynamic performance, as it changes the effective streamlined body. The main focus of this work considers the optimization design of airfoils under atmospheric icing conditions for the Unmanned Aerial Vehicle (UAV). The ice formation process is simulated by the Eulerian approach and the three-dimensional Myers model. A three-equation turbulence model is implemented to accurately predict the stall performance of the iced airfoil. In recognition of the real atmospheric variability in the icing parameters, the medium volume diameter of supercooled water droplets is treated as an uncertainty with an assumed probability density function. A technique of polynomial chaos expansion is used to propagate the input uncertainty through the deterministic system. The numerical results show that the multipoint/multiobjective optimization strategy can efficiently improve both the ice tolerance and the cruise performance of an airfoil. The reason for the focus on robust optimization is that the ice angle of the optimized airfoil becomes less critical to the incoming flow. The optimized airfoils are applied to a UAV platform, in which the performance improvement and the relevant key flow feature are both preserved.

It is confirmed that tandem-blade configurations have potential to enlarge the flow turning in two-dimension (2D) studies. However, the potential of tandem blades to enlarge the design space for highly loaded axial compressors was rarely investigated in open literatures. The present work aims to show the capability of tandem blades to break the loading limit of conventional blades for highly loaded compressors. The 2D models of the maximum static pressure rise derived in previous work were validated by a large amount experimental data, which showed a good agreement. An E parameter was defined to evaluate the stall margin of compressor based on the theoretical models, which indicated that the tandem blade was able to increase the loading limit of axial compressors. A single-blade stage with a loading coefficient of 0.46 (based on the blade tip rotating speed) was designed as the baseline case under the guidance of the E parameter. A tandem-blade stage was then designed by ensuring that the velocity triangles were similar to the single-blade stage. The performances of both stages were investigated experimentally. The results showed that the maximum efficiency of the tandem-blade stage was 92.8%, 1% higher than the single; the stall margin increased from 16.9% to 22.3%. Besides, the maximum pressure rise of tandem rotors was beyond the loading limit of 2D single-blade cascades, which confirmed the potential of tandem blades to break the loading limit of axial compressors.

This paper presents the flow fields and aerodynamic loading of a two bladed H‐type vertical axis wind turbine with active variable pitch for load and circulation control. Particle Image Velocimetry is used to capture flow fields at six azimuthal positions of the blades during operation, three upwind and three downwind. Flow phenomena such as dynamic stall and tower shadow are captured in the flow fields. The phase‐averaged velocity fields and their time and spatial derivatives are used to calculate the normal and tangential loading at each position for each pitching configuration using the Noca formulation of the flux equations. The results show the effect of load shifting from the upwind to downwind region of the actuator using pitch and the effects of dynamic stall on the blades. The results also provide an unique database for model validation.

Hump characteristic is one of unique hydraulic instabilities of pump-turbines, which restricts the stable and safe operating range of the units. High-amplitude pressure fluctuations could be observed in the hump region, leading to hydraulic vibration of pumped storage power plants. To reduce the high-amplitude pressure fluctuation in the hump region, two optimization strategies for high-pressure edge shape of runner blades have been proposed. One is to increase the outlet angle near the shroud and the other increases the radius near the shroud. A large eddy simulation, which has been validated using performance and pressure fluctuation experiments, was performed to study the optimization effect. The valley point (0.65QBEP) in the hump region was selected for conducting time and frequency analyses under different optimization strategies. Analyses show that the pressure fluctuations were primarily caused by the rotation of three low-pressure regions at the circumference of the guide vanes. Both optimization strategies reduced the stall vortices formed in the three low-pressure regions and mitigated the rotating stall phenomenon. The influence of the runner-outlet geometries on pressure fluctuations is primarily reflected in the amplitude reduction of the dominant low frequency 0.2fn (where fn denotes the rotational speed) in the vaneless region and guide/stay vanes. Compared with strategies involving larger outlet angles near the shroud, this strategy of increasing the radius can more effectively reduce the amplitude of the dominant low frequency, and it makes the circumferential distribution of pressure fluctuations more uniform. These findings can help guide efforts in pump-turbine design optimization, which have been applied in a 700 m head pump-turbine design.

Trajectory tracking control for winged eVTOL aircraft is complicated by the high-angle-of-attack aerodynamics experienced during navigational flight occurring immediately after takeoff and immediately before landing. The total energy use of the vehicle can be reduced and the control performance can be improved by appropriately considering the pitch angle of the vehicle in varying flight conditions. We present a review of high-angle-of-attack aerodynamic models as well as an algorithm for finding the optimal pitch and thrust of a winged eVTOL throughout its flight regime. We show simulation results demonstrating a 75% reduction in tracking error over our previous work while maintaining a similar average thrust and an 85% reduction in tracking error over using a multirotor-like controller. The results show that the method is robust to modeling error in the stall characteristics.

The reasons for the occurrence of an impact when closing a butt erfl y valve installed on pipelines that discharge sludge water from horizontal sedimentation tanks of treatment facilities are considered. The assumption about the possibility of water hammer was experimentally refuted. It is hypothesized that the cause of the impact is the disruption of the fl uid fl ow when fl owing around a fl at plate at critical angles of att ack. A numerical experiment was carried out, which consists in modeling the movement of a water fl ow in a completely fi lled, closed space of a pipe. As a result of the experiment, it was revealed that the fl ow stall was caused by the formation of zones of high and low pressure, respectively, before and after the valve. This provides additional energy to increase the closing torque, comparable to the force of a pneumatic actuator, and results in an impact.

The Beddoes-Leishman model (B-L model) was widely applied in the airfoil's dynamic loads prediction. But the need for numerous experimental parameters greatly limits its application range. In this study, numerical simulation was used to simulate the wind tunnel experiments of airfoils to obtain the required parameters. Take the S809 and NACA0012 airfoils as examples, Fluent software was used to calculate the static parameters, and dynamic mesh technology was used to simulate the sinusoidal oscillation wind tunnel experiments of airfoils to obtain the dynamic parameters. Use the modified B-L model with the obtained parameters to predict the dynamic stall characteristics of the airfoils. The predicted results are in good agreement with the experimental data of the wind tunnel, which means that it is promising to use numerical simulation to calculate the parameters required by the B-L model to predict the dynamic stall characteristics of an airfoil. It provides a simpler and more feasible method for predicting the dynamic loads of the blade of wind turbines and tidal current turbines.

Made available in DSpace on 2022-04-28T19:47:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2022-01-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) A nonlinear analysis is performed on a two-degree-of-freedom wing-based system. Structural nonlinearities are introduced in the pitch degree of freedom when considering two different forms of the aerodynamic loads. Comparisons between quasi-steady and unsteady aerodynamic representations are investigated and discussed including stall effects. The nonlinear aeroelastic response is carried out in the presence of quadratic and cubic nonlinearities in the pitch degree of freedom. The effects of the stall and unsteadiness of the flow on the type of Hopf bifurcation and the limit cycle oscillations of the system are investigated. A comparative study between the unsteady and quasi-steady representations from a nonlinear perspective is also carried out. Results from the linear analysis show that for this specific aeroelastic system, the effect of the noncirculatory terms on its linear characteristics is dependent on the aerodynamic representation and the system’s design. Additionally, the quasi-steady approximation of the aerodynamic loads results in an inaccurate prediction of the linear flutter speed. On the other hand, the nonlinear analysis shows the existence of an instability shift from the supercritical to the subcritical type, depending on the value of the stall coefficient, as well as that of the cubic and quadratic nonlinearities, more dominantly for the case when the quasi-steady representation is used to approximate the aerodynamic loads. The results show that, even for small, reduced frequencies and small angles of attack scenarios, the unsteady representation of aerodynamic loads is critical to correctly predict the flutter speed, instability type, and system’s dynamics. Department of Mechanical and Aerospace Engineering New Mexico State University Department of Aeronautical Engineering São Paulo State University (UNESP), SP Department of Aeronautical Engineering São Paulo State University (UNESP), SP

High-fidelity large-eddy simulations are performed to study the unsteady boundary layer sensitivity of a natural-laminar-flow airfoil (NLF-0414) undergoing dynamic stall in comparison to two tradit...

The majority of wind power is currently produced on high wind speed sites by large wind turbine, whereas small wind turbines often operate in light wind conditions. Small capacity wind turbines have not received the same engineering attention as their larger counterparts. This is partially due to a number of unique problems that small wind turbines experience. The most relevant are: low operating Reynolds number ( R e 500 , 000 ) , and poor performance at high angles of attack. Low and medium wind speed sites (Class II–IV) are more common than high wind speed sites, meaning there is a large source of energy not being taken advantage of. Several studies have suggested that flow control devices such as the spherical tubercle could be used to increase lift before stall and generate more power in such situations. The aim of this study is to determine the effect of tubercle amplitude on aerodynamic performance of an airfoil at low-Re numbers ( R e = 300 , 000 & R e = 400 , 000 ) . Three amplitudes were considered in this study: A 1 = 0 . 005 c , A 2 = 0 . 01 c , and A 3 = 0 . 03 c . A detailed 2D simulation study is carried out, using FLUENT (a commercial CFD software) and the T r a n s i t i o n S S T k − ω turbulence model, to obtain aerodynamic coefficients and flow characteristics. Results indicate that small tubercles perform better overall than larger tubercles. The airfoil with the smallest tubercle outperforms the unmodified airfoil at both studied Reynolds numbers at angles of attack 0° – 4 °. Moreover, the airfoil with the largest tubercle outperformed all of the airfoils at an angle of attack of 0° and R e = 300 , 000 . The analysis of the aerodynamic coefficients indicates that the improvement of the aerodynamic performance of airfoils with tubercles is due to the reduction of the drag coefficient. Pressure, intermittency and wall shear stress contours suggest that the overall drag reduction is achieved through the decrease of friction drag. The decrease in friction drag is attributed to the thickening of the laminar boundary layer, caused by a more favorable pressure distribution around the airfoils with the aerodynamic improvements. Moreover, the drastic deterioration in aerodynamic performance at higher angles of attack is attributed to the turbulence generated by the tubercles. This study suggests that spherical tubercles could have a potential application in small wind turbines.

Experimental and numerical research have been performed to investigate the Wavy Leading Edge (WLE) effect on the rectangular wing. The WLE is inspired by humpback whale flipper morphology which is blunt and rounded in certain form pattern. This flipper shape plays an important role for its behaviour specially capturing their prey. This advantage could be applied to other systems such as fin stabilizers or wind turbines. Steady cases in various aspect ratios were conducted to find out the optimum effect of WLE with baseline NACA 0018 profile at Reynolds number 1.4 x 105. The chord length of the wing (c) was 125 mm. The WLE shape defined as wavelength (W) 8% of c and amplitude (d) is 5% of c. The aspect ratio (AR) variations were 1.6; 3.9; 5.1; 7.9 and 9.6. A simple rectangular form of the wing was selected to analysis the WLE effect on the various ARs. The taper wing shape is applied to find out the WLE effect at the AR 7.9. three types of taper ratio (TR) are 0.1; 0.3 and 0.5. The results show that the WLE on the taper wing has better advantage to control the stall in steady case. Another impressive result was the WLE wing with AR 7.9 and TR 0.3 has the best lift coefficient and pressure distribution.Keywords: stall, wavy leading edge, steady case, rectangle wing, taper wing, aspect ratio.

A time-dependent numerical simulation is performed to examine the flow separation control with the action of a hybrid jet (the combination of synthetic and continuous jets) over a NACA23012 airfoil. The unsteady Reynolds-averaged Navier–Stokes (URANS) simulation is performed with Spalart-Allmaras (SA) turbulence model to simulate the flow field around the airfoil to analyse the effect of the hybrid jet. A combined jet is placed at the point of flow separation on the upper surface of the airfoil which is located at the 12% of the chord length from the leading edge of the airfoil for a given flow configuration. Flow simulations are performed at a chord-based Reynolds number of 2.19 × 106 for the hybrid jet oscillating frequency of 0.159 at a blowing ratio of 3.0. The contribution of the continuous jet in the hybrid jet is evident by the flow control. Variation in the continuous jet velocity is studied, which improved the aerodynamic characteristics of the airfoil. The maximum improvement in lift to drag ratio is observed from 11.19 to 22.14 at an angle of attack of 22 degree. The stall angle also shows an enhancement from 18 degree to 20 degree.

Inspired from the Humpback whale flipper, airfoils and wings incorporated with sinusoidal leading edge are much studied for their improved aerodynamic characteristics like delayed stall and higher lift in post-stall region. In this work, the performance of different swept wings equipped with leading edge tubercles is studied by comparing with their baseline models at a low Reynolds number of 100,000. Tubercles on swept wings can either be placed normal to leading edge or parallel to flow direction. First part of the study involves selecting the swept wing configuration which has superior aerodynamic performance from the two possible configurations based on the tubercle alignment on it. Wind tunnel experiment is conducted for the above-mentioned configuration of tubercles on different swept wings as well as unswept wing and compared to corresponding baseline wings. The orientation of tubercle relative to flow has a significance in aerodynamic performance. Aerodynamic efficiency is maximum when the tubercles are aligned with the flow direction. It is seen that tubercles on high swept wing are not as effective as it is on a unswept wing. On unswept and low swept, wing tubercles improve the stall characteristics by preventing abrupt stall and maintaining high lift in post-stall regime. The lift-to-drag ratio for these wings is improved when tubercles are introduced. However, for high swept, wing tubercles do not change the aerodynamic characteristics significantly.

In the effect of periodically varying wind speeds, a wind turbine in an external field environment often works under stall conditions. This paper explores the factors that average wind speed, wind speed amplitude, and oscillation frequency resulted in the changes in blade torque by dynamic stall. According to the blade surface flow field during the dynamic change of wind speed, the law of surface pressure change of the blade in the period of periodic stall is analyzed, and the power changes of the wind turbine blade in the stages of no stall, light stall, and deep stall are studied. As a result, as the oscillation frequency increases, the amount of torque change also increases, and the maximum torque also increases. When the average wind speed increases, the torque experienced by the blades decreases stepwise. As the wind speed amplitude increases, the area of the closed-loop curve where the torque varies with the wind speed continues to expand. Accurately understanding the impact of dynamic stalls and stall delays on wind turbines can further improve the efficiency of wind turbines.

In this paper, we review the concept of Lie brackets and how it can be exploited in generating motion in unactuated directions through nonlinear interactions between two or more control inputs. Applying this technique to the airplane flight dynamics near stall, a new rolling mechanism is discovered through nonlinear interactions between the elevator and the aileron control inputs. This mechanism, referred to as the Lie Bracket Roll Augmentation (LIBRA) mechanism, possesses a significantly higher roll control authority near stall compared to the conventional roll mechanism using ailerons only; it produces more than an order-of-magnitude stronger roll motion over the first second. The main contribution of this paper is to study the nonlinear flight physics that lead to this superior performance of the LIBRA mechanism. In fact, the LIBRA performance in free flight (six DOF) is double that in a confined environment of two-DOF roll-pitch dynamics. The natural feedback from the airplane motion (roll, yaw, and sideslip) into the LIBRA mechanism boosts its performance through interesting nonlinear interplay between roll and yaw, while exploiting some of the changes in the airplane characteristics near stall.

The use of Active Flow Control (AFC) technologies to modify the forces acting on streamlined bodies is one of the most active research fields in aerodynamics. For each particular application, finding the optimum set of AFC parameters which maximises lift, minimises drag or maximises lift-to-drag ratio (aerodynamic efficiency), has become a necessary design requirement. In the present paper, the AFC technology was applied to the Selig-Donovan 7003 (SD7003) airfoil at Reynolds number 6 × 104. Synthetic jets were employed to modify the lift and drag forces acting on the airfoil. Four angles of attack (AoA) of 4¿, 6¿, 8¿ and 14¿ were considered, alongside five AFC parameters: jet position, jet width, momentum coefficient, forcing frequency and jet inclination angle. A multi objective optimisation based on genetic algorithms (GA) was performed for each angle of attack to find the optimum combination of AFC parameters. Each GA generation was simulated using Computational Fluid Dynamics (CFD). A home-made GA package was linked with a mesh generator and the CFD solver, and the results were automatically fed back to the GA code. Over 2200 CFD simulations were performed in two dimensions, using the SpalartAllmaras turbulent model. The motivation behind the current study is to understand the dependence of the optimum set of AFC parameters on the AoA. Results show that, as AoA is increased, the potential benefits of AFC become more pronounced, which allows for considerable improvement in aerodynamic efficiency. The physics involved in the interaction between the main flow and synthetic jet are clearly presented and clarifies that the physical phenomenon to obtain maximum efficiency is completely different at pre-stall and post-stall AoA. In particular, the aerodynamic efficiency was increased by 251% from baseline (no actuation) by using a moderate/finite momentum coefficient at AoA=14¿, while a mere 39% increase was obtained at AoA=8¿. In addition, the interaction between the incoming flow and the synthetic jet pulsating flow at different injection angles has been thoroughly investigated This work was supported by the Spanish and Catalan Governments under grants FIS2016-77849-R and 2017-SGR-00785, respectively. Part of the computations were done in the Red Española de Supercomputación (RES), Spanish supercomputer network, under the grants IM-2019-3-0002 and IM-2020-1-0001. Dr. Jordi Pons-Prats acknowledges the support of the Serra Hunter programme by the Catalan Government

Numerical modelling and simulation of a rotating, tapered, and twisted three-dimensional blade with turbulent inflow conditions and separating flows is a challenging case in Computational Fluid Dynamics (CFD). The numerical simulation of the fluid flow behaviour over a wind turbine blade is important for the design of efficient machines. This paper presents a numerical validation study using the experimental data collected by the National Renewable Energy Laboratory (NREL). All the simulations are performed on the sequence S of the extensive experimental sequences conducted at the NASA/Ames wind tunnel with constant RPM and variable wind speeds. The results show close agreement with the NREL UAE experimental data. The CFD model captures closely the totality of the defining quantities. The shaft torque is well-predicted pre-stall but under-predicted in the stall region. The three-dimensional flow and stall are well captured and demonstrated in this paper. Results show attached flow in the pre-stall region. The separation appears at a wind speed of 10 m/s near the blade root. For V>10m/s, the blade appears to experience a deep stall from root to tip.

In order to study the effect of different numbers of impeller blades on the performance of mixed-flow pump “saddle zone”, the external characteristic test and numerical simulation of mixed-flow pumps with three different impeller blade numbers were carried out. Based on high-precision numerical prediction, the internal flow field and tip leakage flow field of mixed flow pump under design conditions and stall conditions are investigated. By studying the vorticity transport in the stall flow field, the specific location of the high loss area inside the mixed flow pump impeller with different numbers of blades is located. The research results show that the increase in the number of impeller blades improve the pump head and efficiency under design conditions. Compared to the 4-blade impeller, the head and efficiency of the 5-blade impeller are increased by 5.4% and 21.9% respectively. However, the increase in the number of blades also leads to the widening of the “saddle area” of the mixed-flow pump, which leads to the early occurrence of stall and increases the instability of the mixed-flow pump. As the mixed-flow pump enters the stall condition, the inlet of the mixed-flow pump has a spiral swirl structure near the end wall for different blade numbers, but the depth and range of the swirling flow are different due to the change in the number of blades. At the same time, the change in the number of blades also makes the flow angle at 75% span change significantly, but the flow angle at 95% span is not much different because the tip leakage flow recirculates at the leading edge. Through the analysis of the vorticity transport results in the impeller with different numbers of blades, it is found that the reasons for the increase in the values of the vorticity transport in the stall condition are mainly impacted by the swirl flow at the impeller inlet, the tip leakage flow at the leading edge and the increased unsteady flow structures.

Ice accretion on a helicopter’s aerodynamic surfaces during flight can lead to incidents and fatal accidents. An iced helicopter blade results in a decrease in stall angle, an increase in required ...

The flow control effects on the S809 airfoil produced by an oscillating micro-cylinder placed upstream of the airfoil suction surface were investigated using numerical simulations at Reynold number of 1 × 10 6 and high angles of attack (α) range from 20° to 24°. The oscillating mode and initial position of the micro-cylinder are two main parameters considered in this paper. The numerical results suggest that at the start of a heavy stall angle of attack (α 22°), its aerodynamic performance could only be lightly improved or even deteriorated because of the strong and large adverse separation which could not be controlled effectively by the small oscillating cylinder. It was also found that a better control effect can be obtained when the initial position of the cylinder was set adjacent to the separation point compared with that placed near the leading edge at α = 21°. Furthermore, our results also showed that higher dimensionless oscillating amplitudes and a larger frequency can obtain a higher lift-to-drag ratio when the micro-cylinder was placed at optimal initial position.

Vaned diffuser inlet flow uniform is challenging when the impeller is throttled to stall. In this study, we extend the stable operating range of the compressor by improving the uniform flow of the diffuser inlet. First, a numerical investigation of a transonic centrifugal compressor with a vaned diffuser is presented and compared against test data. Then, a new diffuser parameterization method is pro- posed, and the throat feature of a pipe diffuser is successfully applied to parameterized vane diffusers. The influence of the throat length and divergence angle of the diffuser on the performance of the centrifugal compressor is studied via steady and non-linear harmonic simulations. Throat length delays the time of fluid pressurization and accommodates large flow instabilities from upstream—this widens the stall margin but increases mixing loss. Divergence angle affects compressor performance. Stage peak efficiency increases by about 0.58% as the divergence angle increases from 3.79° to 5.79° but drops to about 2.46% as the divergence angle further increases from 5.79° to 11.79°. This is because the boundary layers in the diffuser channel thicken with increasing divergence angle; additionally, the fluid near the hub-pressure side first becomes unstable, then flow separation occurs along the flow direction, which results in a large flow loss. Detailed performance maps of centrifugal compressors with different throat lengths and divergence angles are given to provide a reference for designing transonic centrifugal compressors.

The current design of vertical axis wind turbines (VAWTs) suffers from inevitable change in tip speed ratio, λ, in variant wind conditions due to fixed rotor speed. At relatively high wind speeds, which are promising due to high wind power potential, VAWTs operate at low λ with poor power coefficient. Morphing airfoils can be a potential solution by modifying the airfoil shape to optimal at each λ. The optimal airfoil shape for VAWTs at low λ, where dynamic stall is present, has not yet been studied in the literature, therefore, the present study addresses this gap by focusing on this regime to serve as a step towards designing morphing airfoils for VAWTs by identifying the optimal airfoil shape at low λ. The present study performs a combined analysis of three shape defining parameters, namely the airfoil maximum thickness and its position as well as the leading-edge radius, to reveal the overall design space. The analysis is based on 252 high-fidelity transient CFD simulations of 126 identical airfoil shapes. The simulations are verified and validated with three experiments. The results show that the three shape defining parameters have a fully coupled impact on the turbine power and thrust coefficients. When λ reduces from 3.0 to 2.5, the optimal airfoil changes from NACA0018–4.5/2.75 to NACA0024–4.5/3.5, that is increasing the maximum thickness from 18%c to 24%c and shifting its position from 27.5%c to 35%c, while the leading-edge radius index, I, remains 4.5. In general, reducing I from the default value of 6.0 to 4.5 is found to increase the turbine CP.

Based on the complex flow field of vertical takeoff and landing (VTOL) aircraft with distributed propulsion, the influence of the turbulence intensity and gradient of turbulence intensity on the aerodynamic characteristics of two-dimensional airfoil under low Reynolds number was studied by solving the unsteady Reynolds averaged Navier-Stokes (URANS) Equation based on the c-type structural mesh and γ-Reθt transition model. The aerodynamic characteristics of NACA0012 airfoil at different turbulence intensities and Reynolds numbers are simulated and compared with the experimental data, which verifies the reliability of the low Reynolds number calculation method. Meanwhile, the effects of the different low Reynolds number and gradient of turbulence intensity on the aero-dynamic characteristics of airfoil are studied, and the effect mechanism of the turbulence on the flow field around airfoil is analyzed. It shows that the flow characteristics of the airfoil with high turbulence or Reynolds number are more stable, the separation bubble size is smaller, the flow separation is delayed, and the stall angle of attack is larger, but the effect of the two mechanisms on the earlier transition is different. The influence of the turbulence gradient on the airfoil is limited by the Reynolds number, and the flow separation, transition and reattachment of the airfoil with high turbulence gradient are advance. The generation and evolution of the laminar separation bubble are closely related to the turbulence intensity and Reynolds number, and its scale and location also affect the aerodynamic characteristics of the airfoil.

The hydrodynamic performance of the blended-wing-body underwater glider can be improved by opening a hole on the surface and applying the steady suction active flow control. In order to explore the influence law and mechanism of the steady suction active flow control on the lift and drag performance of the hydrofoil, which is the profile of the blended-wing-body underwater glider, based on the computational fluid dynamics (CFD) method and SST k-ω turbulence model, the steady suction active flow control of hydrofoil under different conditions is studied, which include three suction factors: suction angle, suction position and suction ratio, as well as three different flow states: no stall, critical stall and over stall. Then the influence mechanism in over stall flow state is further analyzed. The results show that the flow separation state of NACA0015 hydrofoil can be effectively restrained and the flow field distribution around it can be improved by a reasonable steady suction, so as to the lift-drag performance of NACA0015 hydrofoil is improved. The effect of increasing lift and reducing drag of steady suction is best at 90° suction angle and symmetrical about 90° suction angle, and it is better when the steady suction position is closer to the leading edge of the hydrofoil. In addition, with the increase of the suction ratio, the influence of steady suction on the lift coefficient and drag coefficient of hydrofoil is greater.

Dynamic stall, which has a significant effect on the aerodynamic performances of dynamic airfoils, is closely related to the physics of the dynamic stall vortex (DSV). The physics of the DSV on the NACA 0012 airfoil was experimentally studied using unsteady pressure measurements with high time accuracy. The experimental Reynolds number was Re = 1.5 × 106, and the reduced frequency was k = 0.069. The propagation of the unsteady pressure field during the dynamic stall process was analyzed in detail. The motion characteristics of the DSV were examined, including its near-wall development characteristics and near-wall evolution velocity. Moreover, the frequency characteristics of the near-wall DSV were studied using wavelet analysis combined with proper orthogonal decomposition (POD) technology. In addition, the effects of the mean angle of attack (AoA) and the amplitude on the DSV motion and frequency characteristics were examined in detail. The effects of the mean AoA on the near-wall DSV strength and the propagation velocity were linear, while the effects of amplitude were nonlinear. The mean AoA and amplitude had a significant influence on the frequency of the leading-edge vortex (LEV) at the initial stage of the DSV development (x/c = 0.10–0.20). This work allows the DSV physics to be understood more deeply. Graphic abstract

The influence of laminar-to-turbulence transition of the laminar separation bubble (LSB) on the dynamic stall process of a pitching foil was investigated using transition SST k-ω turbulence model. As the LSB moves to the leading edge, the strong strain rate produces a pronounced laminar-to-turbulence transition, resulting in a turbulent mixing, faster turbulent reattachment, and shrinkage of LSB. This transition leads to the enhancement of the friction coefficient after the reattachment point and delays the suction collapse and evolution of secondary vortex. A baseline SST k-ω model was also applied. Without the transition mechanism, the suction collapse occurs too early, and the downstroke relaminarization is absent. The oscillation of the downstroke lift curve is severer using the SST model than that using the transition SST model. This study extends the detailed validations of transition model and helps improve the understanding of the influence of the transition on the dynamic stall process.

Technologies inspired by the functioning and behavior of biological beings are commonly developed for aircraft flight. Among the bio-inspired approaches that have grown in interest, particularly for unmanned aerial vehicle flight, is based on the behavior of bird’s cover feathers under higher angles of attack. The covert feathers, when activated by separated flows, promote lift increment that helps in certain maneuvers. This work investigates the benefit in the stall and post-stall performance of employing bio-inspired covert feathers devices attached to an airfoil’s upper surface. To fill the gaps in the recent technical literature, experimental analysis of an SD7003 airfoil was executed in a wind tunnel with the application of bio-inspired covert feathers of different shapes and tapes in three chordwise positions. The bio-inspired devices were conceived to resemble the feathers’ lightness and discrete-distribution along with the wing model. Experiments were carried out measuring the aerodynamic forces and moment at Reynolds number around 170,000 for static and dynamic ramp-up and hold pitching motion. It has been confirmed that the use of bio-inspired covert feathers brought benefits to the stall and post-stall behavior of the airfoil. The maximum lift has increased, and the transition from attached to stalled flow around the airfoil tends to be smoother when the devices were used. Four shapes for the bio-inspired devices and three positions in chordwise direction were considered. The best performance among the case was encountered for a jagged bio-inspired device taped at a quarter-chord position. Indeed, the most forward position for all the devices resulted in higher maximum lift and increment to the respective angle of attack. Ramp-up and hold wind tunnel tests also confirmed the best performance of jagged bio-inspired devices nearer the leading edge. The aerodynamic response to the pitching motion showed that the stall and post-stall regime occur much smoother, indicating that the approach presents good potential for dynamic stall or gust response passive control.

The stalk of Vorticella convallaria, a sessile microorganism, is a model for calcium-powered cellular contraction and biomimetic actuators because of its high contraction speed and unique energy source. Usually, the stalk coiling follows the contraction of the cell body with a time delay (latent period), and it propagates from the cell body side to the root side. The dynamics and energetics of contracting V. convallaria have been studied by applying a stall force to the cell and thus slowing down the stalk contraction. This study exerted an extrinsic stall force to V. convallaria by applying drag force to V. convallaria cells using a microchannel and found that the propagation speed of the stalk coiling was not affected by the stall force, which is similar to the stalk coiling propagation of V. convallaria contracting in a stagnant viscous liquid. In contrast, the minimum latent period increased with the stall force, which enabled estimation of the maximum rate of contraction force development of the V. convallaria stalk.

Hydrofoil is the core unit of energy conversion for axial-flow pumps and hydro-energy turbines. The dynamic characteristics of hydrofoil significantly affect the operation stability of pumps and turbines, especially under the critical stall condition. In this paper, the unsteady flow field of NACA66 hydrofoil under critical stall condition at Reynolds number 0.75 × 10E6 is investigated by IDDES method. The low-frequency oscillations (LFO) under critical stall condition characterized by two typical frequencies with Strouhal number (St) 0.031 and 0.065 are reported in pressure pulsation and lift coefficient respectively. It is found that the merging phenomenon of separation vortices are the main reason of the wave peak of LFO with St 0.031, while the phenomenon of generation and shedding of separation vortices exist at the moment corresponding to the wave trough. The reason of the LFO of lift coefficient with St 0.065 is found to be the long-short switching of streamwise coverage of the leading edge separation vortex. The change of streamwise coverage is corresponding to the movement of the reattachment point of the separation area. The revelation of LFOs flow mechanism in hydrofoil under critical stall can provide reference for its unsteady flow control and applications.

Purpose The main purpose of this paper is to investigate the effects of icing on unmanned aerial vehicles (UAVs) at low Reynolds numbers and to highlight the differences to icing on manned aircraft at high Reynolds numbers. This paper follows existing research on low Reynolds number effects on ice accretion. This study extends the focus to how variations of airspeed and chord length affect the ice accretions, and aerodynamic performance degradation is investigated. Design/methodology/approach A parametric study with independent variations of airspeed and chord lengths was conducted on a typical UAV airfoil (RG-15) using icing computational fluid dynamic methods. FENSAP-ICE was used to simulate ice shapes and aerodynamic performance penalties. Validation was performed with two experimental ice shapes obtained from a low-speed icing wind tunnel. Three meteorological conditions were chosen to represent the icing typologies of rime, glaze and mixed ice. A parameter study with different chord lengths and airspeeds was then conducted for rime, glaze and mixed icing conditions. Findings The simulation results showed that the effect of airspeed variation depended on the ice accretion regime. For rime, it led to a minor increase in ice accretion. For mixed and glaze, the impact on ice geometry and penalties was substantially larger. The variation of chord length had a substantial impact on relative ice thicknesses, ice area, ice limits and performance degradation, independent from the icing regime. Research limitations/implications The implications of this manuscript are relevant for highlighting the differences between icing on manned and unmanned aircraft. Unmanned aircraft are typically smaller and fly slower than manned aircraft. Although previous research has documented the influence of this on the ice accretions, this paper investigates the effect on aerodynamic performance degradation. The findings in this work show that UAVs are more sensitive to icing conditions compared to larger and faster manned aircraft. By consequence, icing conditions are more severe for UAVs. Practical implications Atmospheric in-flight icing is a severe risk for fixed-wing UAVs and significantly limits their operational envelope. As UAVs are typically smaller and operate at lower airspeeds compared to manned aircraft, it is important to understand how the differences in airspeed and size affect ice accretion and aerodynamic performance penalties. Originality/value Earlier work has described the effect of Reynolds number variations on the ice accretion characteristics for UAVs. This work is expanding on those findings by investigating the effect of airspeed and chord length on ice accretion shapes separately. In addition, this study also investigates how these parameters affect aerodynamic performance penalties (lift, drag and stall).

Present study experimentally investigates the effects of ground clearance and Reynolds number on aerodynamic coefficients of smooth and sinusoidal leading-edge wings. Wind tunnel tests are conducted over a wide range of angles of attack from zero to 36 degrees, low Reynolds numbers of 30,000, 45,000 and 60,000, and also ground clearances of 0.5, 1 and ∞. Results showed that reduction of ground clearance and increment of Reynolds number cause the lift coefficient and the lift to drag ratio of both wings to be enhanced. Furthermore, the effects of Reynolds number and ground clearance on the smooth leading-edge wing are more than the sinusoidal leading-edge one. In addition, the sinusoidal leading-edge wing shows an excellent performance in the poststall region due to producing a higher lift and also by delaying the stall angle compared to the smooth leading-edge wing.

Boundary layer suction is one of the effective methods for the prevention of flow separtion. In this paper, a method is introduced to integrate the momentum equation, in which the suction velocity is assumed constant, and the velocity gradient varies. This particular type of flow can be used to form an estimate of the quantities of air that must be sucked on an aerofoil, so that separation of flow does not take place. In this case, although the boundary layer is very thin before separation, the velocity gradient may be very large, and it is obvious that only small quantites of air withdrawn are sufficient to delay the stall.

Icing has now become an important factor endangering flight safety. This paper takes the icing data of the NACA 23012 airfoil as an example, establishes an icing influence model for real-time simulation based on icing time and aircraft angle of attack, and analyzes the influence of different icing geometry on aircraft characteristics. The two-dimensional interpolation method is used to improve the model of the aircraft’s stall area, which is mainly divided into the correction of the lift-drag coefficient linear area and the stall area and the correction of the aircraft stability derivative and the control derivative. The aerodynamic equation of the airplane after icing is established, and the modal analysis of the airplane under different icing conditions is completed through the linearization of the flight equation. The closed-loop simulation system of the altitude holding mode and roll attitude holding mode is used to calculate and analyze the flight quality changes of the aircraft after the wing surface is frozen. The analysis results show that, under icing conditions, in the range of small angles of attack, icing has no obvious influence on the aircraft mode. As the degree of icing increases, the throttle skewness and the negative deflection angle of the airplane’s level flight requirements continue to increase. The case of icing flight in altitude hold and roll hold modes shows that flying in the autopilot mode under severe icing conditions is very dangerous and is prone to cause the aircraft to stall.

A mixed flow pump is regarded as an important power facility in the hydropower field for renewable energy. Pump impeller design shows a great attempt to alleviate the hump characteristic and suppress the severe pressure fluctuations as the mixed flow pumps operate under part-load conditions. In this respect, an advanced turbulence model, i.e., modified SST k-ω partially averaged Navier-Stokes (MSST PANS) model, was adopted to numerically investigate the effect of forward skew angle blade on the hump characteristic using different impeller blades. Both experimental and numerical results indicate that the pump with forward skew angle blade shifts the hump region to the deeper part-load region. In the pump impeller with the forward skew angle blades, a synchronous stall cell is observed near the blade leading edge and near the hub side, while typical rotating stall cell evolution is observed in the conventional pump impeller. Analysis on the blade loading also indicates that under the unstable condition, the forward skew angle blade could switch the mid-loaded distribution to the fore-loaded distribution. Finally, the low frequency pressure fluctuations induced by the rotating stall cell evolution could be also eliminated successfully as the forward skew angle impeller blades are adopted.

When the aircraft is flying at high angle of attack, the local separation of the air flow on the surface of the aircraft, the linear change of aerodynamic force is destroyed, and the maneuverability and stability are reduced to varying degrees, resulting in the aircraft stalling or entering the spin. The stall characteristics and spin characteristics directly affect the safety of the aircraft. In order to analyze the stall and spin characteristics, a full-scale 6-DOF nonlinear high angle of attack dynamic model is established for a two-seater electric high aspect ratio aircraft based on the wind tunnel test data. The stall characteristics of an electric aircraft at high angle of attack are obtained by numerical simulation and analysis. According to the stall characteristics of simulation calculation, flight test is carried out to verify the reliability of numerical simulation, which provides reference for the later analysis of stall/spin test of large aspect ratio series electric aircraft at nonlinear high angle of attack.

Unusual flow characteristics such as reverse flow, rotating stalls, flow recirculation, and stationary vortexes can induce high dynamic forces and torque variations on the entire system, creating a positive slope in the discharge-head curve. To avoid these problems, the present study investigates the hydrodynamic characteristics of a lab-scale model of a Francis type pump-turbine unit in the transition region. To verify the simulation, its results were compared with those of a laboratory-scale experiment performed over various operating ranges. The differences between the experimental and numerical speed, discharge, and torque factors were compared. The numerical analysis was well-matched the experimental tendencies in the overall operating and transition regions. The reliability of the simulations was within 4%. The unsteady RANS equations in the SAS–SST model were discretized for a detailed analysis of the pressure and internal flow characteristics. Under the runaway condition and low-discharge conditions, the frequency spectra of the pressure fluctuations were remarkable at low-frequency related to the rotating stall and blade passing frequency. These results represent a rotating stall with a frequency propagation of approximately 60% of the rotational speed of the runner. In case of the internal flow field, some blade loading distributions developed a positive shape while others developed a negative shape under the runaway condition. Although a rotating stall formed under the low-discharge condition, the form under this condition differed from that developed under runaway conditions, owing to backflow and the single stall cell.

Aiming at the dynamic stall phenomenon of the retreating side of the rotor in forward flight, the existing flow control method of dynamic leading edge droop was applied to the flow control of forward-flying rotor at three-dimensional scale. A numerical simulation method based on variable droop leading edge is established in this paper. The seesaw rotor is taken as the research object, the moving overset mesh method and RBF grid deformation technology are used, the integral form of Reynolds average N-S equation is the main control equation. The influence of the dynamic leading edge at r/R=0.75~1 on the aerodynamic characteristics of the rotor when the forward ratio is 0.3 is investigated. It is found that variable droop leading edge on the retreating side can effectively inhibit the generation and development of separation vortices near the trailing edge, and has a significant effect on lifting lift coefficient and section normal force coefficient, reducing torque coefficient, and thus improving the equivalent lift-drag ratio of the rotor. In a certain range, the control effect is better with the increase of the droop amplitude under the leading edge.

This paper presents the design of the LQR (Linear Quadratic Regulator) and SDRE (State-Dependent Riccati Equation) controllers for the flight control of the F-8 Crusader aircraft considering the nonlinear model of longitudinal movement of the aircraft. Numerical results and analysis demonstrate that the designed controllers can lead to significant improvements in the aircraft's performance, ensuring stability in a large range of attack angle situations. When applied in flight conditions with an angle of attack above the stall situation and influenced by the gust model, it was demonstrated that the LQR and SDRE controllers were able to smooth the flight response maintaining conditions in balance for an angle of attack up to 56% above stall angle. However, for even more difficult situations, with angles of attack up to 76% above the stall angle, only the SDRE controller proved to be efficient and reliable in recovering the aircraft to its stable flight configuration.

In the present work, the numerical investigation of a wind turbine airfoil with suction flow control is initially performed to find the relative improvement in its erodynamic efficiency. Consequently the study of turbine performance is accomplished using 2D simulation results as extending the flow control to wind turbine blades. The latter is incorporated via definition of new input data file in Q-blade code including complete set of section aerodynamic coefficients for both with and without suction control against variety of angles of attack and velocities. The benchmark turbine in this study is NREL 5 MW. It is shown that using suction in 1/3 of chord length would lead in better enhancement. Simulation of the airflow with and without suction control has been presented in three different locations in the chord-wise direction (26%C, 33.5%C and 50%C) and for several intensities or suction speed ratios (0.022, 0.045, 0.11, and 0.155). The turbulence model is transitional 4-equation SST model. The aerodynamic improvement in section is dominant and this method could tolerate stall behavior. Wind turbine power and thrust coefficients were analyzed via Q-blade versus tip speed ratio and flow control parameter. The range of tip speed ratio studied here is 3.5 to 15. The results depicted that applying suction in certain part of the blades could increase output power at low tip speed ratio. Applying suction at 33.5% of chord length improves the average power coefficient by 4.1%.

The stall margin of tip-critical axial compressors can be improved by using circumferential casing grooves. From previous studies, in the literature, the stall margin improvement due to the casing grooves can be attributed to the reduction of the near casing blockage. The pressure rise across the compressor as the compressor is throttled intensifies the tip leakage flow. This results in a stronger tip leakage vortex that is thought to be the main source of the blockage. In this paper, the near casing blockage due to the tip region aerodynamics in a low-speed axial compressor rotor is numerically studied and quantified using a mass flow-based blockage parameter. The peak blockage location at the last stable operating point for a rotor with smooth casing is found to be at about 10% of the tip chord aft of the tip leading edge. Based on this information, an optimised single casing groove design that minimises the peak blockage is found using a surrogate-based optimisation approach. The implementation of the optimised groove is shown to produce a stall margin improvement of about 5%.

The present paper numerically investigates the rotating stall mechanism in the NASA CC3 transonic centrifugal compressor with wedge-type vaned diffuser. The three-dimensional compressible, unsteady...

Two-dimensional flow of the pulsed co-flow jet (CFJ) and the influence of the pulsed parameters on the lift and power consumption are investigated numerically. Firstly, the jet channel of traditional CFJ airfoil is improved. The stall margin is increased by 3° compared with the corresponding traditional CFJ airfoil, and the lift is increased, while the drag is reduced significantly. Then, the influence of the pulsed parameters, such as the pulse waveform, including sinusoidal and rectangular waves, the duty cycle, and the pulse frequency on the lift and power consumption are presented and analyzed in detail. It is concluded that, compared with the steady CFJ, the pulsed CFJ possesses much better ability in suppressing separation and can improve the lift characteristics significantly with limited cost of power consumption. For example, at an angle of attack of 20° flow separation occurs severely, when the steady CFJ is adopted; however, the airflow becomes fully attached on the upper airfoil, when the rectangular pulsed CFJ is employed. As a result, the lift corresponding to the rectangular wave is higher than that of the sinusoidal wave. The results also indicate that the lower duty cycle and pulse frequency can lead to the higher lift but with more power consumption.

A method is proposed to improve the aerodynamic performance of small-scale aircraft systems. The method is based on fundamental gas-dynamic phenomena, such as local separation of the boundary layer accompanied by the formation of the so-called separation bubbles, separation of the turbulent boundary layer, and flow stall from the leading edge of the airfoil, which change the entire flow structure around the body. Publications where the relationship of these phenomena is established and those that describe a control method eliminating the adverse consequences of flow separation with the use of a wavy surface of the airfoil are reviewed. The method is simple in implementation and offers many prospects. The areas of its applicability are determined, and the criteria of optimizing the wavy surface of the airfoil for particular operation conditions are provided. Results of investigations are reported, which show that the use of a wavy surface of wings or blades of flying vehicles can improve their aerodynamic characteristics. Various structural elements of the boundary layer are noted, such as local separation bubbles or a global separation region, and the criteria of their emergence are given, which assist in verification of numerical experiments.

The aerodynamic forces and flowfield on a delta-wing Unmanned Aerial Vehicle (UAV) with specific airfoil are studied here. The leading-edge protuberances inspired from the flippers of the humpback whale is introduced in present work to watch its effect on stall delaying and drag reduction. Two installing factors, which are amplitude and wavelength of the protuberances, are considered to study the interaction of leading-edge variation and de-tached vortex. Three changes in both amplitude and wavelength are investigated herein. Generally, the modified models have larger stall angle of attack than that of the baseline model; the difference exists in the attainable lift coefficient and drag reduction.

A low-order method is presented for aerodynamic prediction of wings operating at near-stall and post-stall flight conditions. The method is intended for use in design, modeling, and simulation. In this method, the flow separation due to stall is modeled in a vortex-lattice framework as an effective reduction in the camber, or "decambering." For each section of the wing, a parabolic decambering flap, hinged at the separation location of the section, is calculated through iteration to ensure that the lift and moment coefficients of the section match with the values from the two-dimensional viscous input curves for the effective angle of attack of the section. As an improvement from earlier low-order methods, this method also predicts the separation pattern on the wing. Results from the method, presented for unswept wings having various airfoils, aspect ratios, taper ratios, and small, quasi-steady roll rates, are shown to agree well with experimental results in the literature, and computational solutions obtained as part of the current work., 29 pages, 28 figures. Submitted to Journal of Aircraft

The aerodynamic forces and vortex characteristics of an H-type Vertical Axis Wind Turbine (VAWT) become complicated because of dynamic stall, particularly in the three-dimensional impact on the blade spanwise direction. This study focused on the evaluation of the aerodynamic performance and vortex characteristics of an H-type VAWT in the spanwise direction by numerical simulations and wind tunnel experiments. Pressure acting on the blade surface was obtained from multiport pressure measurement devices by wind tunnel. Meanwhile, the vortex field around different blade sections was investigated through numerical simulations. The stall behavior was analysed by comparing the results of numerical simulations and experiments. As a result, the tangential force of single blade was mainly contributed at the chordwise position of x/c≤0.4c and the power of single blade was mainly contributed at the azimuthal angle range of 60°≤θ≤150° in the blade section position region of 0≤z/(H/2)≤0.7. At the section position of z/(H/2)=0.5, the initial flow separation was found at the suction side and progressed forward to the leading edge. With the increase of the tip speed ratios, the decreasing position of the averaged local power coefficient of each section was closer to the middle section of z/(H/2)=0, and the attenuation speed became faster. The power coefficient reductions at the blade section position of z/(H/2)=0.9 were 38.29%, 46.78% and 56.42% when the tip speed ratios were 1.38, 2.19 and 2.58, respectively. The results of this study provided a better understanding of the development of the performance characteristics and vortex characteristics of H-type VAWT.

In order to further enrich the theory of rotational stall, this paper explored the mechanism of internal energy loss in the mixed-flow pump under stall condition based on the shear stress transport (SST) k-ω turbulence model, identified the vortices in the impeller by the Q-criterion method, and characterized the turbulence intensity by the turbulent kinetic energy (TKE). The results show that the SST turbulence model can well predict the positive slope characteristics of the energy performance curve of a mixed-flow pump, and there are obvious characteristics of rotating stall in impeller. The numerical results of the energy performance curve and internal flow field are in good agreement with the experimental results. Under stall conditions, there exist swirls and vortices at impeller inlet, which makes the flow inception angle increase by 38°. Meanwhile, the tip leakage flow causes certain energy loss in the rim region, but the high loss area caused by the leakage vortex does not coincide with the core region of the leakage vortex. The interaction between stall vortex and the main flow in the flow passage results in the large-scale blockage area, which is the most fatal effect on the head drop of the mixed-flow pump. The backflow appears on the suction surface of the blade outlet, which has a great impact on the main flow and leads to a large hydraulic loss. The typical characteristics of critical stall and deep stall are found. Under the critical stall condition, part of the leakage flow overflows the leading edge of the lower blade and affects the flow of the next passage. While under deep stall condition, the streamlines of the leading edge of stall vortex reaches the inlet of the next stage blade and overflows, causing a lot of energy loss. And the interference effect between tip leakage flow and stall vortex is strengthened. In general, under the deep stall condition, the average turbulent kinetic energy value in the passages is the highest, and the uniformity of flow rate distribution in the four passages is the worst, resulting in the largest energy loss, which is at the lowest head in the saddle area.