Search Results (24)

We proposed a bilateral parallel offset jet model that enables jet vectoring control without the need for an active high-pressure secondary flow. Flow characteristics, including deflection force, wall pressure distribution, and flow structures, were investigated. The evolutions of key flow structures during jet deflection were investigated, including the passive secondary flow, the shear layer, the boundary layer, and the separation bubble. By analyzing the formation, dissipation, and interactions of the key flow structures, as well as their relationship with pressure characteristics, the mechanism of the jet deflection control was further deduced. The fundamental driving force of the jet deflection stems from the unbalanced pressure difference on either side of the jet, and the valve can control the flow rate of passive secondary flow, thereby altering the near-wall pressure on its side and further generating a pressure that propels the jet to deflect. For walls of different lengths, at a moderate wall length, where L* = 1.5, with the valve controlling the passive secondary flow, a maximum jet vectoring angle of 6.4° can be continuously achieved at a low Reynolds number. Within the range where 20% < δ_v < 100%, the nonlinear error of jet vectoring control is 5.7%. At a short wall length, where L* = 0.5, the driving force generated by the valve to deflect the jet is insufficient, and the maximum vector angle is 0.3°. For longer walls, the impact of the jet against the trailing edge of the wall obstructs jet deflection; therefore, extending the wall is not conducive to jet vectoring control. Featuring a non-expanding wall structure, the bilateral parallel offset jet model provides a new thrust vectoring control scheme characterized by a compact afterbody, no need for a high-pressure secondary air source, and a simple structure. Full article

A numerical study was conducted to quantify the entropy generation in a fluidic oscillator operating at Reynolds numbers of 30,000, 40,000, and 50,000. Both the local entropy production rate and total entropy were calculated under these operating conditions. Transient computational fluid dynamics (CFD) simulations were carried out using the $k-\omega$ shear stress transport (SST) turbulence model. The total entropy was compared with the pressure and driving-force coefficients to establish its relationship with force dynamics. The total entropy showed a periodic evolution synchronized with the jet switching process, while its amplitude increased with Reynolds number and showed a slight phase delay. The pressure and driving-force coefficients exhibited weak fluctuations at the end and beginning of each oscillation period, matching the secondary peaks in total entropy and indicating that these variations arise from residual dissipative effects linked to the jet reattachment stages. The local entropy production rate was concentrated near the feedback channels, Coanda surfaces, and the interaction zone where the jet from the inlet nozzle met the returning flow from the feedback channels. Regions of elevated entropy were detected at the outlet corners due to expansion and pressure drop. The high-velocity jet core exhibited minimal entropy, which increased toward the flanks as the flow decelerated. The results show that entropy generation follows the jet switching motion, reflecting the variations in viscous dissipation and flow dynamics inside the oscillator. Full article

Fluidic thrust vectoring control (FTVC) enables highly agile flight without the mechanical complexity of traditional vectoring nozzles. However, a robust onboard identification of the jet deflection state remains challenging when only limited measurements are available. This study proposes a sparse reconstruction of the pressure field method for a wedge passive FTVC nozzle and validates the approach experimentally on a low-speed jet platform. By combining the proper orthogonal decomposition (POD) algorithm with an ${\mathcal{l}}_1$-regularized compressed sensing method, a full Coanda wall pressure distribution is reconstructed from the sparse measurements. A genetic algorithm is then employed to optimize the wall pressure tap locations, identifying an optimal layout. With only four pressure taps, the local pressure coefficient errors were maintained within $\left|\Delta Cp\right|$ < 0.02. In contrast, conventional Kriging interpolation requires increasing the sensor count to 13 to approach the reconstruction level of the proposed POD–compressed sensing method using 4 sensors, yet still exhibits a reduced fidelity in capturing key flow structure characteristics. Overall, the proposed approach provides an efficient and physically interpretable strategy for pressure field estimation, supporting lightweight, low-maintenance, and precise fluidic thrust vectoring control. Full article

A highly loaded axial flow compressor often leads to significant flow separation, resulting in increased pressure loss and deterioration of the pressure increase ability. Improving flow separation within a compressor is crucial for enhancing aeroengine performance. This study proposes adding a fin structure to the jet cavity of the Coanda jet cascade to improve flow separation at the trailing edge and corner area. The fin structure is optimized using response surface technique and a multi-objective genetic algorithm based on numerical simulation, enabling more effective control of the simultaneous separation of the boundary corner and trailing edge of the layer. The response surface model developed in this study is accurately validated. The numerical results demonstrate a 2.13% reduction in the optimized blade total pressure loss coefficient and a 12.74% reduction in the endwall loss coefficient compared to those of the original unfinned construction under the same air injection conditions. The optimization procedure markedly improves flow separation in the compressor, leading to a considerable decrease in the volume of low-energy fluid on the blade’s suction surface, particularly in the corner area. The aerodynamic performance of the high-load cascade is enhanced. Full article

Double feedback fluidic oscillators, which create oscillating fluid jets, are commonly used in flow control and thermal applications. The geometry of the Coanda surface affects the oscillation frequency, jet deflection angle, and pressure drop in the mixing chamber. This study numerically investigates the impact of rib locations on the Coanda surface on jet characteristics. Air, with an inlet velocity of 55.8 m/s, is used as the working fluid. Three cases—full ribs, upper ribs, and lower ribs—are compared to a smooth Coanda surface. The full ribs case achieves an increased oscillation frequency of 820 Hz, compared to 355 Hz for the smooth case. However, the jet deflection angles decrease when ribs are present. The upper ribs case achieves a larger 41.5° deflection angle, while the full ribs case achieves a relatively lower 33.8° angle. Interestingly, adding ribs to the Coanda surface reduces the pressure drop in the oscillator. Oscillators with upper ribs achieve a 76.1% increase in FDPR compared to smooth cases, making them the best solution for enhancing the combined effect of jet oscillation frequency and deflection angle. Full article

Ore collection devices are important for the collection of deep-sea polymetallic nodules. Based on the CFD-DEM solid–liquid two-phase flow coupling calculation method, this paper simulated the rise and transport phases of polymetallic nodules using the Coanda effect ore collection device. The validity of the numerical simulation method was confirmed through experimental testing. On this basis, the effects of different working and structural parameters on the collection rate were studied. The results indicate that the flow rate of the collection jet and the bottom clearance were the primary factors affecting the collection rate of the polymetallic nodules. An increase in the collection jet flow rate leads to a substantial rise in the collection rate of polymetallic nodules. Conversely, an increase in bottom clearance results in a decrease in the collection rate. A collection rate exceeding 90% can be achieved in both scenarios: a 10 mm bottom clearance with an 8 m/s collection jet flow rate, and a 30 mm bottom clearance with a 10 m/s collection jet flow rate. The collection nozzle slant angle has no substantial impact on the collection rate, and the recommended collection nozzle slant angle is 35° to reduce energy loss. Full article

The Coanda effect is a fluid dynamics phenomenon in which a fluid jet adheres to a convex or flat surface. This effect occurs when a liquid or gas jet emerging from an orifice clings to an adjacent surface and entrains the surrounding fluid, creating a lower-pressure region along its path that maintains its attachment to the surface. The Coanda effect accounts for the behavior of blood flow in the fetal right atrium and the dispersion of eccentric mitral regurgitation jets along atrial walls. This series of interesting images depicting a large 4 × 3.75 cm saccular intracranial aneurysm suggests that the Coanda effect may play a role in the pathophysiology of intracranial aneurysms and could be an underlying factor in their formation, progression, or rupture. Full article

This paper proposes a hybrid propulsion-based small underwater robot for robust trajectory tracking control in a harsh and complex underwater environment. The robot is equipped with a Coanda-effect jet thruster and a pair of propeller-based reconfigurable magnetic-coupling thrusters, allowing it to traverse safely in confined or cluttered spaces as well as cruise efficiently in the open water. To investigate the robot dynamic modeling, we first formulated its simplified mathematical model and estimated the hydrodynamic coefficients by performing the planar motion mechanism using CFD (computational fluid dynamics) simulation. Then, a double-loop trajectory tracking control architecture was designed considering the model uncertainties and environmental disturbances. Based on Lyapunov theory, the outer-loop kinematic control produces the virtual velocity command, while the inner-loop dynamic control adopts the full-state feedback L1-adaptive control to match the command. The asymptotic convergence of the tracking errors and the stability of the whole closed-loop system are guaranteed. Finally, comparative simulations in the presence of unknown disturbances and the variation of model parameters were carried out to verify the robustness of our proposed trajectory tracking control, which is also suitable for the separated son robots. Full article

Pitch control of an unmanned aerial vehicle (UAV) using fluidic thrust vectoring (FTV) is a relatively novel technique requiring no moving control surfaces, such as elevators. In this paper, the authors’ previous work on the characterization of a static co-flow FTV rig is further validated using the free to pitch dynamic test bench. The deflection of a primary jet after injection of a high-velocity secondary jet was captured using the tuft flow visualization technique, along with the experimental recording of subsequent normal force impinged on the Coanda surface resulting in the pitching moment. The effect of primary and secondary flow velocities on exhaust jet deflection, as well as on the pitch angle of the aircraft, is examined. Aerodynamic gain as well as the inertia of a delta wing UAV test bench are computed through experiments and fed into the equation of motion (e.o.m). The e.o.m developed aided in the design of a model-based PID controller for pitch motion control using the multi-parameter root locus technique. The root locus tuned controller serves as a benchmark controller for performance evaluation of the genetic algorithm (GA) and particle swarm optimization (PSO) tuned controllers. Furthermore, the frequency domain metric of gain and phase margins were also employed to reach a robust control design. Experiments conducted in a simulation environment reveal that PSO-PID results in a better response of the UAV in comparison to the baseline pitch controller. Full article

The enhancement in the jet pressure ratio and jet velocity contributes to expanding the control efficiency and control boundary of circulation control airfoil under high subsonic incoming flow. However, because of an excessive jet pressure ratio, the jet separates prematurely on the Coanda surface, resulting in control failure. In a bid to improve the adhesion capability of the jet under a high pressure ratio, a circulation control airfoil with a converging nozzle and back-facing step structure at the trailing edge was numerically simulated based on the Reynolds averaged Navier−Stokes equation (RANS), and a study was conducted on the complex flow structure of the under-expansion jet on the Coanda surface and the impact of design parameters such as jet pressure ratio, ellipticity, and nozzle height on the jet separation. The results show that the back-facing step provides an expansion space for the under-expansion jet and changes the shock-boundary layer interaction form. As the jet pressure ratio and nozzle height increase, the size of the shock cell increases, the strength of the intercepting shocks on both sides increases, and Mach reflection occurs, resulting in jet stratification and in a decline in the adhesion capability of the jet. The combination design of proper ellipticity and the back-facing step contributes to forming a closed low-pressure vortex area behind the step and promote jet attachment. Reducing the nozzle height can improve the adhesion capability of the jet under a high pressure ratio. Full article

Precise prediction of ventilation flow is essential to create a comfortable, economical, and healthy indoor environment. In the past three decades, the computational fluid dynamics (CFD) method has been used more often, and it is now one of the primary methods for studying building ventilation. The most common CFD method is RANS simulation because of the low cost of computational resources and good accuracy. This paper presents a numerical investigation of a transitional ventilation flow with the Coanda effect, which makes the initial free jet transform into a wall jet. Six low-Reynolds number k-ε models proposed by Abid (AB), Lam and Bremhorst (LB), Launder and Sharma (LS), Yang and Shih (YS), Abe Kondoh and Nagano (AKN), and Chang, Hsieh and Chen (CHC) are applied. The performance of the six models is evaluated by comparing the computational results with the PIV measurements of Van Hooff et al. The predictions revealed that the LS model has a good approximation of velocity profiles because of its two extra terms in k and ε equations, and the AB model is in good agreement with experimental results for predicting the Coanda effect. The LB model provides the worst agreement with experimental data on account of the wrong prediction of turbulent kinetic energy and dissipation rate. Full article

Thrust, propulsion, vector control of supersonic jets has been applied to jet and rocket engines, ejectors, and other many devices. In general, there are two approaches to this type of control, namely mechanical moving systems and fluidic thrust vector control systems without moving parts, with mechanical moving systems being the most common. However, generally speaking, these systems are very complicated, and more simple methods and devices are desired. In this study, an extremely simple method for the thrust vector control of a supersonic jet by a fluidic Coanda nozzle (FC-nozzle) using the entrainment of the surrounding fluid and Coanda effect is newly proposed. The FC-nozzle consists of a pipe nozzle (Pi-nozzle), spacer, and linearly expanded Coanda nozzle (Co-nozzle) with eight suction pipes (Su-pipes) installed to surround the jet from the Pi-nozzle. The jet from the Pi-nozzle flows straight with the entrainment flow of the surrounding fluid. When some Su-pipes are closed, the pressure between the jet and Co-nozzle wall decreases, and subsequently, the jet deflects to the closed side of the Su-pipe and reattaches to the wall by the Coanda effect. The flow characteristics and deflection characteristics of the supersonic jet from the FC-nozzle are examined by the visualized flow pattern using the Schlieren method and measurements of the velocity distribution. As a result, it is shown that by changing the number of Su-pipes and the locations at which they are closed, the deflection angle and circumferential position of the jet from the Pi-nozzle can be easily controlled. Full article

The Coanda effect nozzle is a fluid thrust vectoring technology that uses the Coanda effect to control jet vector deflection. The jumping phenomenon often occurs in the process of controlling jet vector deflection. This phenomenon leads to the nonlinearity of thrust vector control. It destroys the control performance of the aircraft and brings potential dangers to the safety of the aircraft. The jumping phenomenon occurs in an unsteady flow field different from the traditional flow phenomenon. The flow structure in an unsteady flow field changes with time, so it is not easy to control by the traditional active flow control method. This paper explains the reasons for the jumping phenomenon from two aspects: flow field stability and flow structure. Secondly, the unsteady flow field with the jumping phenomenon is studied and analyzed by a flow visualization experiment and dynamic force measurement. Furthermore, the dynamic modal decomposition (DMD) method is used to extract the characteristic frequencies of the critical vortices causing jets to jump in unsteady flow fields. Finally, a pulsed jet with the same characteristic frequency is used to control the varying vortices in the unsteady flow field. The experimental results show that the active flow control method, which extracts the characteristic frequency of the critical flow field structure by DMD, effectively suppresses the jumping phenomenon in the unsteady flow field. It also linearizes the process of jet nonlinear vector deflection. Full article

When pressurized with a fluid, the sweeping jet actuator (SWJA) emits a self-induced and self-sustained temporally continuous, but spatially oscillating bi-stable jet at the outlet. The SWJA adds up local momentum using the Coanda extension without any moving parts and, therefore, can be a promising tool for suppressing aerodynamic flow separation. However, the SWJA needs to be integrated into curved aerodynamic surfaces with an angle. The present study focuses on investigating the effects of various exit nozzle geometries on the flow field. The geometric parameters considered were the exit nozzle angle, diffuser arm length, and curvature. The working fluid was air, and the mass flow rate was 0.015 lb/s. A set of time-dependent flow fields was computed using a two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) simulation. The time history of pressure was recorded inside the upper and lower feedback channels. The jet oscillation frequency was obtained by employing the fast Fourier transform (FFT) for all datasets. The results were compared against the baseline case and data available in the literature. The results showed that external geometric variations at the nozzle exit had a negligible impact on the oscillation frequency. However, there were notable effects on the pressure and velocity distribution in the flow field, indicating that the actuator had sensitivity towards the geometric variation of the exit nozzle—the wider the exit nozzle, the lower the downstream velocity. Notably, we observed that the mean velocity at the exit nozzle downstream for the curvature case was 40.3% higher than the reference SWJA. Full article

The lift of an aircraft can be effectively enhanced by circulation control (CC) technology at subsonic speeds, but the efficiency at transonic speeds is greatly decreased. The underlying mechanism of this phenomenon is not fully understood. In this study, Reynolds averaged Navier—Stokes simulation with $k-\omega$ shear stress transport model was utilized to investigate the mechanism of lift enhancement by CC in transonic flow. For validation, the numerical CC results were compared with the NASA experimental data obtained for transonic CC airfoil. Thereafter, the RAE2822 airfoil was modified with a Coanda surface. The lift enhancement effects of CC via steady blowing with different momentum coefficients were tested at $Ma=0.3$ and 0.8 at $\alpha =3^{\circ }$, and various fluid mechanics phenomena were investigated. The results indicate that the flow structure of the CC jet is insensitive to the incoming flow conditions because of the similarity to the local static pressure field around the trailing edge of the airfoil. Owing to the appearance of shockwaves on the airfoil surface in the transonic regime, the performance of the CC jet is restricted to the trailing edge of the airfoil. Transonic CC achieved a slight improvement in aerodynamic performance owing to a favorable shift in the shockwave pattern and accelerated flow in the separation region on the airfoil surfaces. Revealing the mechanism of lift enhancement of CC in the transonic regime can facilitate the rational design of new fluidic actuators with high activity and expand the potential applications of CC technology. Full article