Small Vortices Research Articles

Influence of phase difference and amplitude ratio on Kelvin–Helmholtz instability with dual-mode interface perturbations

A two-component discrete Boltzmann method (DBM) is employed to study the compressible Kelvin–Helmholtz (KH) instability with dual-mode interface perturbations, consisting of a fundamental wave and a second harmonic. The phase difference is analyzed in two distinct ranges, and the amplitude ratio is studied by varying the amplitude of either the first or second harmonic. The global average density gradient and the global mixing degree are analyzed from a hydrodynamic non-equilibrium perspective. The thermodynamic non-equilibrium (TNE) intensity is probed as a thermodynamic non-equilibrium variable. The system is also explored from a geometric perspective, with a focus on the rotation of two vortices, the mixing layer width, and the non-equilibrium area. Physically, under the influence of shear velocity, the fluid interface becomes distorted and progressively elongated, resulting in the formation of two small vortex structures and an enhancement of the physical gradient. The two vortices then begin to interact and merge into a single large vortex with complex fluid structures. Consequently, the physical gradient decreases, and the local TNE intensity weakens. Subsequently, the material interface elongates further, increasing the non-equilibrium region and enhancing the local TNE intensity. Finally, the physical gradient decreases due to dissipation and/or diffusion, weakening the local TNE intensity.

Evolution of vortices at the merging of an ethanol droplet with water in an intrusive mode

The evolution of vortices formed when a freely falling drop of a 95% aqueous solution of ethanol, tinted with brilliant green, merges with water in the intrusive mode has been traced by method of high-speed video recording. The drop smoothly flows into the liquid and forms a subducting lenticular intrusion, in which a weakly expressed ring vortex is formed if the potential surface energy is greater than or of the same order as its kinetic energy. Gradually, the intrusion of lighter liquid begins to float up and contracts around the cavern, which takes on a conical shape. From the center of the pointed bottom of the cavity, which has reached its maximum depth, a compact volume containing a light liquid of droplet is pushed into the thickness of the liquid. After the cavern collapses, the primary intrusion spreads along the free surface of the target fluid. In this case, the submerging volume is transformed into a small spherical vortex, which reaches its maximum depth, and then stops and forms a compact secondary intrusion elongated vertically. Next, the central part of the secondary intrusion begins to flow up and gradually transforms into a new ring vortex. As it approaches the free surface, the diameter of the vortex increases. The slowly rising shell of the intrusion forms the bottle-shaped base of the cylindrical trace of the ring vortex, colored with droplet pigment. Changes in the sizes of the main structural components during the evolution of the flow pattern were traced.

Scale-Up of Continuous Metallaphotoredox Catalyzed C–O Coupling to a 10 kg-Scale Using Small Footprint Photochemical Taylor Vortex Flow Reactors

Scale-Up of Continuous Metallaphotoredox Catalyzed C–O Coupling to a 10 kg-Scale Using Small Footprint Photochemical Taylor Vortex Flow Reactors

Stabilization of electroconvective vortices by membrane corrugation: Thresholds, transitions, and impacts on ion transport

In this work, the impact of geometrical membrane corrugations on ion transport and concentration polarization-induced electroconvection was investigated. A high-precision micro-particle-tracking velocimetry setup enables to visualize the temporal evolution of electroconvective vortices on different membrane surface geometries while simultaneously evaluating ion transport through the monitoring of current–voltage curves. A significant change in the electro-hydrodynamic response of a laboratory-scale electrodialysis cell was observed at overlimiting current densities for different corrugation sizes on the membrane. Experiments reveal that there is a relationship between the size of the corrugation features, the size of electroconvective vortices, and their effect on ion transport. We identified for the first time the coexistence of small and large electroconvective vortices, as foreseen by recent simulations, each moving at unique velocities. Additionally, it was found that the usually chaotic electroconvective vortices are stabilized by the electroosmotic forces induced by the membrane corrugation. More extensive membrane features led to more stable vortices, aligning their counter-rotation with the membrane’s geometry. A threshold for stable and unstable vortices was determined, and a characterization of transitioning to a chaotic and destabilized state was derived. Finally, the contribution of stable vortices to ion transport was evaluated, finding that stable, larger vortices lead to enhanced transport. These findings contribute to the proposal of optimizing membrane designs for electrodialysis application as a possible solution to overcome ion transport limitations at overlimiting current densities.

A High-resolution Simulation of Protoplanetary Disk Turbulence Driven by the Vertical Shear Instability

A high-resolution fourth-order Padé scheme is used to simulate locally isothermal 3D disk turbulence driven by the vertical shear instability (VSI) using 268.4 M points. In the early nonlinear period of axisymmetric VSI, angular momentum transport by vertical jets creates correlated N-shaped radial profiles of perturbation vertical and azimuthal velocity. This implies dominance of positive perturbation vertical vorticity layers and a recently discovered angular momentum staircase with respect to radius (r). These features are present in 3D in a weaker form. The 3D flow consists of vertically and azimuthally coherent turbulent shear layers containing small vortices with all three vorticity components active. Previously observed large persistent vortices in the interior of the domain driven by the Rossby wave instability are absent. We speculate that this is due to a weaker angular momentum staircase in 3D in the present simulations compared to a previous simulation. The turbulent viscosity parameter α(r) increases linearly with r. At intermediate resolution, the value of α(r) at midradius is close to that of a previous simulation. The specific kinetic energy spectrum with respect to radial wavenumber has a power-law region with exponent −1.84, close to the value −2 expected for shear layers. The spectrum with respect to azimuthal wavenumber has a −5/3 region and lacks a −5 region reported in an earlier study. Finally, it is found that axisymmetric VSI has artifacts at late times, including a very strong angular momentum staircase, which in 3D is present weakly in the disk’s upper layers.

Numerical simulation on metallurgical transport phenomena during DC casting of large-size Mg-Gd-Y alloy billet under various magnetic fields

The manufacture of large-size rare-earth Magnesium alloy billets by direct-chill (DC) casting process is now facing great technical challenges, mainly due to the high cost of casting experiments, and such casting defects like severe cracking, macrosegregation, and non-homogenized solidified structure. Based on the low-cost and high-efficiency numerical simulation method, the present study systematically explores the effects of single pulsed and alternating magnetic fields (PMF and AMF), differential-phase pulsed and alternating magnetic fields (DPMF and DAMF) on DC casting metallurgical transport phenomena of Φ750mm large-size Mg-Gd-Y magnesium alloy billet. The electromagnetic characteristics, flow behavior, and temperature distribution during casting process are clearly visualized. The results demonstrate that traditional DC casting process is optimized by applying magnetic fields. For one thing, pulsed magnetic fields (PMF and DPMF) generate larger and faster changing instantaneous Lorentz forces, which induce huge transient impact and oscillation effects on the melt. For another, alternating magnetic fields (AMF and DAMF) have greater time-average Lorentz forces, which can force the melt to flow continuously and steadily. Additionally, differential-phase magnetic fields (DPMF and DAMF) enhance the melt’s axial flow strength while reducing the small vortex generated at the edges, resulting in improved heat transfer efficiency and uniform temperature field. Among those magnetic fields, DPMF shows dual advantages of transient oscillation and homogenized temperature distribution. Based on the present simulated research, the Φ750mm large-sized rare earth Mg-Gd-Y alloy billet with high-quality and non-cracking has been manufactured during DC casting process under DPMF.

Numerical study of vortex–bubble interaction mechanism in bubble breaker using the volume of fluid and large eddy simulation method

Effectively removing or reducing the number of bubbles doped in a material is of great significance to stabilize product characteristics and improve production safety. In this paper, the volume of fluid method and large eddy simulation method are used to numerically simulate the bubble dynamics in unsteady turbulence near a four-blade propeller in a bubble breaker at different speeds. The results show that the final rupture position of the bubble occurs in the main vortex region. The interaction mechanism between the vortex and the bubble involves not only the impact of the small vortex on the bubble but also the influence of the tail vortex pair, which exerts two strong torsional stresses in opposing directions. This interaction ultimately results in the breakup of the bubble. In addition, three different fracture modes were observed: Ring Broken Mode (RBM), Dented pull out Broken Mode (DBM), and Tear Broken Mode (TBM). The bubble breaking process is quantitatively described in terms of vorticity change rate and critical Weber number (We). It is found that DBM does not occur at high rotational speed, and TBM occurs faster and more violently with increase in vorticity change rate. After multiple cycles, the vorticity change in RBM is similar to that at medium speed, and when the change rate reaches its maximum, the size of the fragmentation bubble becomes smaller.

Unsteady flow analysis in a pump as turbine impeller based on proper orthogonal decomposition and dynamic mode decomposition methods

Pump as turbine (PAT) is an efficient, simple, and cost-effective equipment combining pump and turbine and is one of the excellent energy recovery devices. It is helpful to master the flow characteristics of the key component impeller for the further optimization and design of the PAT. To analyze the unsteady flow features in the impeller of a double-suction pump operating as a turbine, numerical simulations were conducted using the shear stress transport (SST) k-ω turbulence model at the designed operating conditions. By utilizing proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods on the unsteady velocity field of a single cycle, the dominant modes up to the fourth order, along with their respective space–time information, can be extracted. The velocity field and vorticity field analysis were performed on the first four modes extracted using two different methods. Additionally, the vortex structures were extracted using the Ω method. The analysis demonstrates that the POD and DMD methods effectively decompose the intricate flow characteristics within the impeller into dynamic–static interference modes, fundamental modes, and dissipative modes. The dynamic–static interference mode is dominant, reflecting the flow characteristics influenced by the stationary components within the impeller. The vortex structure is mainly small tubular vortex and point vortex. The fundamental mode captures the steady flow field characteristics caused by the blade channel geometry. The vortex structure is mainly continuous tubular vortex and the diameter becomes larger. The dissipative mode reflects the flow separation generated on the blades by disturbances from the stationary components. The vortex structure is dominated by point vortex and discontinuous tubular vortex. Comparing the outcomes of the two modal analysis methods shows that the POD method has a distinct advantage in showcasing key changing nodes. In contrast, the DMD method is superior in isolating modes with a single frequency and in determining their stability.

Spatiotemporal structure of flow field by a dielectric-barrier-discharge plasma actuator using phase-resolved particle image velocimetry

A typical dielectric-barrier-discharge plasma actuator (DBD-PA) operating in continuous mode generates a self-similar starting vortex followed by a quasi-steady wall jet. In the burst mode, it generates periodic vortices, leading to a spatially wider mean flow field. The characteristics of periodic vortices can be controlled by adjusting the duty cycle α and burst frequency fb of the actuation signal. In the present study, the flow field generated by the actuator in burst mode is studied for 50%≥α≤90%, and 10 Hz ≥fb≤90 Hz. The Reynolds number of mean flow based on the maximum induced velocity and jet half-width ranges from 171 to 524. High-speed laser-sheet visualization and time-resolved particle image velocimetry are used for flow field measurements. The flow field generated during burst mode operation shows the strong influence of the burst frequency. Periodic vortices with comparable size and convection speed of starting vortex are generated at low fb. Dipoles and small-scale vortices are formed at moderate fb similar to the behavior of a transitional wall jet. Kelvin–Helmholtz instability is observed at higher burst frequency. An ornamental vortex consisting of several small periodic vortices is formed at high values of fb during the starting period. Based on the temporal evolution of momentum, the flow field generated by the DBD-PA can be classified into (i) starting flow regime, (ii) transitional flow regime, and (iii) periodic flow regime. The self-similarity of the mean flow field generated by the DBD-PA is verified by comparing the mean velocity profile at multiple downstream locations with the self-similar solutions of laminar and turbulent wall jets. The entrainment coefficient of the mean flow field is similar to that of an axisymmetric turbulent wall jet. The bi-orthogonal analysis shows that both coherent mode energy content and the entropy are minimum (about 5% only) for the continuous mode actuation. For the burst mode actuation, both the coherent mode energy content and the entropy are higher in magnitude compared to that of continuous mode and decrease with increasing burst frequency. The vortices in the periodic flow regime are locked in with fb. The distance between subsequent vortices, λx and flow frequency, f follow a power-law relationship, λx=cf−n, where c is a constant and n can be approximated as 2/3.

Multiple-arc cylinder under flow: Vortex-induced vibration and energy harvesting

The shape of a cylindrical cross-section affects the vibrational performance. The vortex-induced vibration (VIV) phenomena of multiple-arc cylinders were numerically investigated to assess their impact on hydrodynamic energy harvesting and potential vibration suppression across a flow velocity range of 0.2 m/s to 1.4 m/s (1.767 × 104<Re < 1.237 × 105). The study involves five types of multiple-arc cylinders: 4-arc, 8-arc, 16-arc, 24-arc, and circular cylinders. The accuracy of the numerical method was validated through comparison with experimental data. Specifically, increasing the number of arcs generally enhances overall energy conversion efficiency. Then, the VIV response and energy conversion results of the 24-arc cylinder are similar to those of the circular cylinder with maximum efficiency. Notably, the 4-arc cylinder achieves a global maximum amplitude of 0.074 m (A∗ = 0.83) and a power output of 4.4 W with the new P + T mode, making it the most effective configuration for flow velocities between 0.7 and 0.9 m/s. For vibration suppression of multiple-arc cylinders, the appropriate arc structure effectively reduces amplitudes. The small vortices generated by the arc structures disrupt the separation of normal vortices from the boundary layer, leading to approximately a 50 % reduction in amplitude responses for 8-arc and 16-arc cylinders.

Numerical analysis of performance and internal flow characteristics of a gas-liquid multiphase rotodynamic pump

Abstract Inlet Gas Void Fraction (IGVF) and rotating speed are key parameters influencing the flow characteristics of gas-liquid multiphase pumps. To explore the influence of these two factors on the flow characteristics of gas-liquid multiphase rotodynamic pumps, CFD simulations were carried out based on the Euler two-fluid model to analyze the flow characteristics in an axial-flow pump at three different rotating speeds (1450 r/min, 2200 r/min, 2950 r/min) with different IGVFs. The numerical results indicate that the pump efficiency and head are first increased and then decreased with the increase of IGVF. When the IGVF is 5%, the head and efficiency reach their maximum values. This is attributed to the minimum vorticity and turbulence kinetic energy in the impeller passage at IGVF= 5%, and the small vortex range at the hub of the guide vane, thus resulting in the stable two-phase flow state in the impeller and guide vane. At the same IGVF, with the increase of the rotating speed, the head and efficiency of the multiphase pump increase accordingly. This is closely related to the smallest vortex range formed by gas-liquid flow separation at n=2950 r/min, indicating a better flow state.

Growth characteristics of the mean shear layer in pipe bends with and without a guide vane

The growth characteristics of two identical pipe bends with and without a guide vane are investigated by means of large eddy simulation. The two pipe bends, with a radius of curvature slightly above the separation threshold, are subjected to two fully developed upstream flow conditions, with a corresponding Reynolds number of 11 700 and 24 000. A precursor computation method is employed to provide the fully developed turbulence inflow conditions for all cases. The growth of the mean shear layer in this work is characterized by the local momentum thickness, which measures the extent of momentum deficit confined under the mean shear layer. For both pipe bends, the initial growth of momentum thickness is observed in the first quarter of the bend. The onset location is almost independent of the Reynolds numbers. However, a clear Reynolds number dependence is observed in the onset magnitude, which strongly defines the growth rate thereafter. By examining the mean momentum balance in the bend section, the results show that rather than the adverse pressure gradient, the overall growth characteristics of the mean shear layer, which include the onset location and the growth rate, are better described by the balance between the centrifugal force and the radial pressure gradient. This balance manifests itself as a change in the swirling intensity of the secondary flow. The presence of guide vane significantly suppresses the swirling intensity in the bend section, leading to a noticeable reduction in the overall momentum thickness growth and the production of turbulence in the flow downstream of the bend. Further inspection also indicates that the initial mechanism leading to the suppression of separation at the inner bend is linked to the increasing dominance of the small vortices at the near-wall vicinity relative to the local adverse pressure gradient. Certain aspects pertaining to the turbulent statistics are also discussed.

Experimental study on aerodynamic performance of turbine blade squealer tip with different trailing edge designs under high-speed relative casing motion

Over-tip leakage (OTL) flow leads to great aerodynamic performance reduction in high-pressure turbine, reasonable blade tip design can effectively control the loss caused by OTL flow. The relative casing motion is one of the key boundary conditions that can significantly influence OTL flow. In this study, aerodynamical tests were conducted for a high-pressure squealer tip with different trailing edge designs of full cavity squealer tip, pressure-side cutback and suction-side cutback at both stationary and rotating conditions. Loss distribution and blade near tip loading of different trailing edge structures at high-speed rotating condition are firstly reported and evaluated. The result indicates that pressure-side cutback design significantly increases the aerodynamic loss compared with full cavity tip, while suction-side cutback design has close overall loss to full cavity tip. This conclusion was consistent with numerical simulation based on Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) analysis, which reveals that pressure-side cutback design causes the cavity vortex leaks earlier compared with full cavity tip, a small vortex formed at cut region forms and results in higher total pressure loss. The effect of relative motion between blade tip and shroud reinforces this trend. The total pressure loss difference can reach 42 % compared with 8 % in stationary condition, which indicates that relative casing motion needs to be take into consideration when ranking different tip sealing designs.

Numerical and experimental study on water-sediment flow in a lateral pumping station forebay

Multiple vortex flow patterns are commonly observed in lateral pumping station forebays, particularly in basins with high sediment concentrations. These patterns can lead to pump blockage, sediment deposition, and other issues that disrupt pump station operations. The water-sediment two-phase flow in lateral pumping station forebays is significantly influenced by the start-up combination, yet our understanding remains limited. To address this, the mixture multiphase flow theory is introduced to describe water-sediment dynamics, and the mathematical model is validated with experimental data. By analyzing the characteristics and formation mechanisms of large vortex and multiple small vortex regions in the original scheme, nine different start-up combination schemes were proposed. The research results indicate that, due to the narrow channel and slope effect in the lateral forebay, some of the gravitational potential energy of the water-sediment mixture is converted into kinetic energy upon entering the forebay, thereby increasing the velocity in the main flow area. Additionally, due to the friction and dissipation effects of the two sidewalls, a pressure difference is generated in the main flow area, resulting in the formation of multi-level vortices. Furthermore, the various types of proposed start-up combinations can optimize the flow patterns in the forebay to a certain extent. The preferred scheme improved the uniformity of flow velocity by 18.63% and increased the deviation angle by 17.62°, resulting in a 75.47% reduction in vortex area compared to the original scheme. These research results provide theoretical guidance for optimizing start-up combinations and reorganizing flow field structures to achieve hydrodynamic dredging effects.

A Study of the Vortex Filament Pool Left by a Super Typhoon

AbstractEnhancing typhoon forecasts hinges on a deeper understanding of the upper‐ocean’s response and feedback mechanisms to typhoons. Presently, our knowledge of typhoon‐ocean interactions is largely derived from low‐resolution numerical simulations (often &gt;10 km) and limited observations, which inadequately capture submesoscale processes (SPs) in the ocean. Connecting extreme typhoons to upper‐ocean SPs remains a challenge. This study reveals the formation of a distinctive vortex filament pool (VFP) in high‐resolution (∼1.2 km) numerical experiments. The experiments show that under specific conditions, typhoons can generate this visually striking phenomenon, displaying SP dynamics and kinematics typical of the upper ocean, with Rossby numbers and the nondimensional strain and divergence rates exceeding 2. The VFP formation is mainly driven by strain‐induced frontogenesis linked to the flow generated by Typhoon Nangka after a major turn. Initially, the pool consists of many near‐parallel filaments, but processes such as merging, stretching, and destabilization subsequently occur lead to numerous small vortices with a mean radius of ∼13 km. While the high‐resolution numerical experiments highlight phenomena requiring observational validation, they suggest the presence of natural processes previously undetectable with low‐resolution models and limited observations. This study underscores the need for enhanced observations and numerical models to better understand refined ocean dynamical processes.

Comparison of vortex formation in expanded and curved aortic sinuses: The effect of sinus curvature

Aortic valve disease is a significant health issue. Research on the effects of aortic sinus geometry and aortic valve motion during a heartbeat is imperative, as numerous valvular diseases are associated with blood flow near the heart valve. Changes in the internal space of the sinus and the formation of blood clots have been reported after the implantation of a transcatheter aortic valve, an artificial heart valve. Although the sinus shape and leaflet motion significantly affect the hemodynamic characteristics and platelet aggregation, the blood flow behaviors near the heart valve associated with clot formation have not been fully elucidated. This study conducted in vitro experiments to investigate the flow behavior near an aortic sinus model based on the sinus shape. Additionally, hemodynamic changes associated with variations in the aortic sinus geometry and leaflet length within an aortic sinus model under pulsatile-flow conditions were elucidated. In a curved sinus model, a large single vortex formed near the center of the sinus region. Meanwhile, the center of the vortex in the sinus region shifted during a cycle in an expanded sinus model. Furthermore, a small vortex remained where the leaflet was initiated, thus increasing the likelihood of thrombus formation. Thus, the curved sinus model is advantageous for preventing material accumulation by maintaining a large vortex structure. The experimental results confirmed that the aortic sinus shape and leaflet length affect the likelihood of thrombus formation inside the aortic sinus.

Laser powder bed fusion sintering mechanism of phenolic resin investigated by ReaxFF molecular dynamics simulations

ABSTRACT This study presents an innovative approach utilising molecular dynamics simulations to elucidate the polymerisation and neck formation mechanisms of phenolic resin during laser powder bed fusion (L-PBF). By developing system and dual-particle models and employing infrared thermal imaging, we established a heat source model for phenolic resin under varying parameters. Our molecular dynamics simulations, using the ReaxFF reactive force field, indicate that high-power lasers significantly enhance cross-linking, achieving a peak polymerisation degree of 78.9% at 30W and 35W power levels. Conversely, increased scanning speed slows product degradation, reducing the polymerisation degree. The interplay of laser power and scanning speed influences polymerisation, with total input energy and heating rate impacting polymerisation and molecular size. Additionally, we investigated neck formation, revealing that small vortices evolve into larger ones during sintering, promoting neck formation. Optical microscopy confirmed diverse neck shapes under different conditions. Quantitative data showed that at a scanning speed of 1000 mm/s, the polymerisation degree reached 93%, with only 2 molecules remaining post-simulation. At 30W power, the largest individual molecule observed was C849H703O117. This research offers crucial insights for selective laser sintering processes, emphasising the importance of high energy input and optimal heating rates for well-formed necks.

Numerical investigation on flow and heat transfer characteristics of impingement/swirl cooling structures in a turbine blade leading edge

A numerical investigation is carried out to study flow and heat transfer (HT) performance of impingement/swirl cooling structures in the turbine leading edge (LE) adopting shear stress transport (SST) k-ω turbulent model. The influence of Re, the ratio of impingement cooling (IC) hole offset distance to IC hole diameter (e/d) and coolant outflow structure are analyzed. Results show that for a model without film holes, the first IC jet forms a large vortex because of the limitation imposed by one side wall of the LE. The remaining four IC holes all form a small entrainment vortex near the impact target surface (TS) due to the outlet diversion effect. When Re = 20 000 and e/d = 0, the impingement jet is not uniformly diffused to the left and right sides but flows more along the inclined plane. Both the turbulent pulsation and turbulent kinetic energy (TKE) in the wall region impacted by the jet increase as Re increases, and the region generates small entrainment vortices. When Re = 20 000, regardless of e/d, the region with a high level of TKE includes a region of vortex generation, a region of vortex collision in opposite directions and the wall where the jets make impact.

The Effect of Vortex Generators on Spray Deposition and Drift from an Agricultural Aircraft

Vortex generators (VGs) attached to the leading edge of an agricultural aircraft are purported to control airflow over the upper surface of the wing by creating small vortices that delay boundary layer separation, thereby improving the performance of the aircraft. These devices are commercially available for use in the aviation industry, primarily to increase pilot control of the aircraft. The benefits attributed to VGs remain largely descriptive and anecdotal in nature without rigorous empirical assessment in the field. The intent of this study was to evaluate whether this aerodynamic device could improve deposition or reduce drift when mounted on an agricultural aircraft. Airborne drift and ground deposition were measured with monofilament lines and Mylar cards, respectively. Deposits were expressed as percent of fluorometric response using a spectrofluorophotometer. There were 46% fewer downwind drift deposits on monofilament lines when VGs were installed than when VGs were not installed. Whether or not VGs were installed on the aircraft was the predominant factor which influenced deposition on monofilament lines. Spray deposits on Mylar cards placed at ground level downwind of the applications at three different locations (5, 10, and 20 m) varied significantly (p &lt; 0.0001) between treatments, with corresponding 31, 54, and 61% reductions in downwind deposits when VGs were installed. While these findings overall are positive, this is the first known study of its type, and more research is warranted to better understand the role of vortex generators in the reduction in drift relative to aerially applied sprays.

Morphological effects of leading-edge sawtooth on the vortex evolution and acoustic characteristic of an ultra-thin centrifugal fan

Serrations on the owl wings' leading edge (LE) are considered one of the critical characteristics leading to their silent flight. Inspired by this, LE sawtooth was innovatively induced on ultra-thin centrifugal fan blades, and the morphological effects of these teeth on the vortex evolution and aeroacoustic characteristics of the fan were studied using large eddy simulation and the Ffowcs Williams–Hawkings analogy. A single-passage model was adopted to finely simulate the flow mechanism between blades with an acceptable scale. Five sawtooth schemes with relative tooth width λ/b from 7.96% to 29.84%, as well as the prototype, were calculated and analyzed. It is found that the optimal λ/b ranges from 8% to 17.05%, which reduces the overall sound pressure level (SPL) by over 1 dB without impacting the blade pressure and efficiency. These sawteeth inhibited the LE separation, shattered the leading-edge vortex (LEV) into small vortices, and consequently weakened the pressure fluctuations on the blades. However, more prominent teeth (λ/b &amp;gt; 23.8%) intensify the interactions between LEV and other passage vortices, changing the dominant pressure pulsations to high frequency, in turn raising the overall SPL. Too small sawteeth are challenging to process on such ultra-thin blades, so the largest sawtooth among the suggested range was considered the optimal scheme (λ/b = 17.05%) and was manufactured to measure. The results show that the SPL of the fan with LE sawtooth is 0.24–0.57 dB lower than that of the prototype under the same flow rates, even though its rotational speed is increased.