Surrounding Flow Field Research Articles

The effect of dynamic twisting on the flow field and the unsteady forces of a heaving flat plate

Many marine animals can dynamically twist their pectoral fins while swimming. The effects of such dynamic twisting on the unsteady forces on the fin and its surrounding flow field are yet to be understood in detail. In this paper, a flat plate executing a heaving maneuver is subjected to a similar dynamic twisting. In particular, the effects of the direction of twist, non-dimensional heaving amplitude, and reduced frequency are studied using a force sensor and particle image velocimetry (PIV) measurements. Two reduced frequencies, , and , and two twisting modes are investigated. In the first twisting mode, the plate is twisted in the direction of the heave (forward-twist), and in the second mode, the plate is twisted opposite to the direction of the heave (backward-twist). Force sensor measurements show that the forward-twist recovers some of the lift that is usually lost during the upstroke of flapping locomotion. Additionally, the forward-twist maintains a near-constant lift coefficient during the transition between downstroke and upstroke, suggesting a more stable form of locomotion. PIV results show that forward-twist limits circulation and leading-edge vortex growth during the downstroke, keeping at the cost of the reduced lift. By contrast, backward-twist increases the circulation during the downstroke, resulting in large increases in both lift and drag coefficients. Force sensor data also showed that this effect on the lift is reversed during the upstroke, where the backward-twist causes a negative lift. The effects of each twisting mode are mainly caused by the changes in the shear layer velocity that occur as a result of twisting about the spanwise axis along the mid-chord. The twisting performed by forward-twist reduces the effective angle of attack through the upstroke and downstroke, resulting in a reduced shear layer velocity and lower circulation. The twisting performed by backward-twist does the exact opposite, increasing the effective angle of attack through the upstroke and downstroke and consequently increasing the shear layer velocity and circulation.

Physical Mechanism of the Development and Extinction of the China Southwest Vortex

In this paper, a typical vortex system based on quasi-linear thermal-dynamic equations to reflect the development and extinction of the China Southwest Vortex is established using the vortex motion stability method combined with the outer environmental field and cumulus convective latent heat release. The development and extinction of the China Southwest Vortex in catastrophic weather systems are studied from the aspects of stability and development mechanisms for the primary-stage China Southwest Vortex, the transition mechanism from the primary-stage China Southwest Vortex to the mature vortex, and stability and development mechanisms of the mature China Southwest Vortex. The results show the following: (1) the convergence and divergence of the surrounding flow field is the main factor influencing the development and extinction of the primary-stage China Southwest Vortex, while gravity wave disturbance is the main driving force for the maintenance and development of the primary vortex. Based on the convergence of the external flow field, the gravity wave disturbance must exceed the critical frequency, or the vortex will tend to die out. (2) The convergence and divergence of the surrounding flow field is also the main factor for the transition from the primary vortex to the mature vortex. Based on the convergence of the surrounding flow field, the primary vortex transforms into a mature vortex only when the gravity wave disturbance strongly exceeds the critical frequency and causes the vertical disturbance to become unstable. (3) The convergence and divergence of the external flow field is also the main factor for the development and extinction of the mature China Southwest Vortex. In the early stage, the vortex can be maintained and developed as long as the surrounding flow field converges. In the case of the divergence of the external flow field, the vortex may be maintained for a short time, but eventually dissipates when the gravity wave disturbance exceeds the critical frequency. In the later stage, under the convergence of the surrounding flow field, the vortex can be maintained when the gravity wave disturbance exceeds the critical frequency. However, with the divergence of the surrounding flow field, the vortex may be maintained for a short time, but it will eventually dissipate when the gravity wave disturbance is extremely strong. In addition, the observations of the evolution of China Southwest Vortexes and gravity wave activities under the influence of southwest airflow and atmospheric disturbance in the Western Sichuan Plateau–Sichuan Basin are explained by the above physical mechanism. It is also pointed out that the heating effect can be an obstacle to the development of the China Southwest Vortex by increasing the critical frequency of gravity waves during unstable layer formation, and the divergent environment flow field under the condition of stable layer formation. Therefore, this paper deepens the understanding of the evolution process and anomalous mechanisms of the China Southwest Vortex.

Research on the effects of complex terrain on the hydrodynamic performance of a deep-sea fishlike exploring and sampling robot moving near the sea bottom

Deep-sea exploring and sampling technologies have become frontier topics. Generally, the movable exploring mode near the seabed with low disturbance is an important way to improve the measurement accuracy and expand the measurement range. Inspired by fish, the fishlike propulsion method has the characteristics of low disturbance and high flexibility, which is very suitable for near-seabed detection under complex terrain conditions. However, the swimming mechanism and surrounding flow field evolution law of the robotic fish under the constraints of complex terrain are still unclear. In this paper, the confined terrain space is constructed with an undulating seabed and a narrow channel, and the hydrodynamic changing law and flow field evolution law of the autonomous swimming process of the fishlike swimmer in the confined space are analyzed. Moreover, the influence mechanism of the terrain on the motion performance of the robotic fish is revealed, and the optimal motion mode of the robotic fish under a complex terrain constraint is discussed. The results show that the propulsion force, Froude efficiency, and swimming stability of the robotic fish vary with the distance from the bottom under the undulating seabed condition lightly. When the distance from the bottom exceeds a certain value, it can be considered that the undulating seabed no longer affects the swimmer. Furthermore, when the robotic fish swims through a narrow channel with certain width, the swimming performance obviously varies with the distance from the boundary surface. During swimming in the confined terrain space, the propulsion force and swimming stability of robotic fish will decrease. In order to maintain the forward speed, the robotic fish should improve the tail-beat frequency in real time. However, considering the swimming stability, the tail-beat frequency is not the larger the better. The relevant conclusions of this paper could provide theoretical support for the development of low-disturbance bionic exploring and sampling platforms for deep-sea resources and environments.

Simulated flow field environment of the giant mud crab <i>Scylla serrata</i> based on a behavioral model and computational fluid dynamics

Simulated flow field environment of the giant mud crab <i>Scylla serrata</i> based on a behavioral model and computational fluid dynamics

3D ray tracing solver for communication blackout analysis in atmospheric entry missions

During the atmospheric entry phase at hypersonic speed, the radio communication from/to a space vehicle can be disrupted due to the formation of a plasma sheath within the surrounding flow field. In order to characterize such communication blackout phases, this work presents a numerical methodology combining Computational Fluid Dynamic (CFD) simulations of ionized chemically reacting entry flows by means of Computational Object-Oriented Libraries for Fluid Dynamics (COOLFluiD) and a ray tracing analysis by means of the newly developed BlackOut RAy Tracer (BORAT). The latter is based on the numerical solution of the 3D Eikonal system of equations, offering a fast, efficient and accurate method to analyse the interaction between electromagnetic signals and weakly ionised plasmas. The proposed methodology, and BORAT in particular, is first verified on popular benchmark cases and then used to analyse the European Space Agency (ESA) 2016 ExoMars Schiaparelli entry flight into Martian environment. The corresponding results demonstrate the validity of the proposed ray tracing approach for predicting communication blackout, where signals emitted from the on-board antenna undergo reflection and refraction from the plasma surrounding the entry vehicle, and the advantage of a 3D approach for analysing real flight configuration.

How fluid-mechanical erosion creates anisotropic porous media

Using a Cauchy integral formulation of the boundary integral equations, we simulate the erosion of a porous medium comprised of up to 100 solid bodies embedded in a two-dimensional Stokes flow. The grains of the medium are resolved individually and erode under the action of surface shear stress. Through nonlinear feedback with the surrounding flow fields, microscopic changes in grain morphology give way to larger-scale features in the medium such as channelization. The Cauchy-integral formulation and associated quadrature formulas enable us to resolve dense configurations of nearly contacting bodies. We observe substantial anisotropy to develop over the course of erosion; that is, the configurations that result from erosion generally permit flow in the longitudinal direction more easily than in the transverse direction by up to a factor of six. These results suggest that the erosion of solid material from groundwater flows may contribute to previously observed anisotropy of natural porous media.

Numerical Study on the Interaction between Ocean Current Power Generator and Unmanned Underwater Vehicle

The unmanned underwater vehicle (UUV) can effectively utilize marine renewable energy after equipping the UUV with an accompanying power generator that improve its long-range endurance. In order to study the interaction between the UUV and the ocean current power generator, a coupled hydrodynamic model of the counter-rotating type turbine and the UUV hull was established. Based on the RANS model and the sliding grid method, this paper analyzed the influence of the ocean current power generator on the straight-ahead resistance and hydrodynamic coefficient of the UUV in the non-working state, and calculated the UUV yaw moment and its surrounding flow field characteristics with the power generator in the working state. Then, the effect of the drift angle on the performance of the counter-rotating type turbine was explored. The results show that the straight-ahead resistance of the UUV increased slightly after equipping the power generator, but the increase was within 14%, and the characteristics of the surrounding flow field of the UUV did not changed greatly. The difference in the linear hydrodynamic coefficients before and after equipping the UUV with the power generator was within 7%. At most drift angles, when the generator was in the working state, the yaw moment of the hull was greater than the condition when the generator was not equipped. Nevertheless, the overall trend of the two sides was similar, so the rotation of the counter-rotating type turbine did not reduce the stability of the hull in the moored state. In addition, when the drift angle was greater than 50°, the UUV hull had a great impact on the performance of the counter-rotating type turbine; the power coefficient CP and thrust coefficient CT increased by about 35.7% and 17.8%, respectively. This paper provides a good reference value for the design of the power generation mode of the UUV equipped with a counter-rotating type turbine.

A BEM-Based Model of a Horizontal Axis Tidal Turbine in the 3D Shallow Water Code SHYFEM

We present a novel 3D implementation of a horizontal axis tidal turbine (HATT) in the shallow water hydrostatic code SHYFEM. The uniqueness of this work involves the blade element momentum (BEM) approach: the turbine is parameterized by applying momentum sink terms in the x and y momentum equations. In this way, the turbine performance is the result of both the flow conditions and the turbine’s geometric characteristics. For these reasons, the model is suitable for farm-layout studies, since it is able to predict the realistic behavior of every turbine in a farm, considering the surrounding flow field. Moreover, the use of a shallow water code, able to reproduce coastal morphology, bathymetry wind, and tide effects, allows for studying turbine farms in realistic environments.

Aerodynamic simulation and analysis of large ring-sail parachute

Parachutes have been closely linked to many space missions. Many vehicles launched into space have had parachutes on board for recovery or landing deceleration, and relative research mainly adopts computer simulation methods due to high test costs. In order to investigate the aerodynamic force and the surrounding flow field structure of the parachutes in the process of vehicle return, a physical model is established for the main parachute used in spacecraft return, using trimmed mesh for spatial division, and numerical simulations of the steady-state flow field of the large ring-sail parachute at different angles of attack are conducted. The results show that, the drag coefficient increases and then decreases, and the lift coefficient decreases and then increases with increasing angle of attack; the high-low-high-low-incoming flow velocity of the axial flow field generated by the ring-sail canopy is demonstrated and analyzed. The simulation results provide theoretical guidance for the structural design of the return parachute.

Numerical Simulation of the Maneuvering Motion Wake of an Underwater Vehicle in Stratified Fluid

When a vehicle moves underwater, disturbance is generated and a wake remains that destroys the original free surface and produces a new wake. In order to study the mechanism and characteristics of the wave-making wake generated by the maneuvering motion of an underwater vehicle in density-stratified fluid, the k-ε model and the VOF method that is based on the RANS equation were used in this paper to analyze the SUBOFF model in stratified fluid at different drift angles. Numerical simulation of the maneuvering motion was carried out under these angles, and the corresponding changes in flow field caused by this motion were analyzed. The results from the comparison and analysis of the surface wave wakes under different drift angles in stratified fluid show that with the increasing drift angle, the motion wake of the vehicle still exhibits obvious Kelvin wave system characteristics. However, there are significant changes in hydrodynamic performance. The asymmetry of the surrounding flow field will increase with the increase in the drift angle. The pressure of the underwater vehicle is inversely proportional to the velocity of the surrounding flow field, and the amplitude of the peak and trough of the free surface wave is linearly related to the change in the drift angle. The numerical simulation can serve as a reference for the non-acoustic detection of the motion heading of an underwater vehicle and the motion trajectory of anti-reconnaissance underwater vehicles under actual sea conditions.

Estimation of Hydrodynamic Forces on Cylinders Undergoing Flow-Induced Vibrations Based on Modal Analysis

Abstract The objective of the present study is estimating hydrodynamic forces acting on cylinders undergoing vortex-induced vibration (VIV) using dynamic mode decomposition (DMD). The cylinders are subjected to a uniform incoming flow at a laminar Reynolds number (Re = 250) and an upper transition Reynolds number (Re = 3.6 × 106) (Re = U∞D/ν defined based on the incoming flow U∞, the diameter of the cylinder D, and the viscosity of the fluid ν). Both a single cylinder and a configuration of piggyback cylinders are considered. Numerical simulations based on two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations combined with the k−ω SST turbulence model are carried out to obtain the snapshots of the surrounding flow fields for DMD analysis. The DMD method is a powerful tool to obtain the spatial–temporal evolution characteristics of the coherent structures in the wake flow behind the cylinders. In the present study, this modal decomposition method is combined with a moving reference frame around the cylinders. The dominant DMD modes with their corresponding frequencies of the wake flows are identified and are used to reconstruct the flow fields. The large-scale shedding vortices are captured by the dominant modes. The reconstructed wake flow behind the cylinders is used to estimate the drag and lift forces on the cylinders combined with a force partitioning analysis.

Prediction for the Influence of Guide Vane Opening on the Radial Clearance Sediment Erosion of Runner in a Francis Turbine

In this paper, the Eulerian–Lagrangian method and Tabakoff erosion model are used to study the solid–liquid two-phase flow in a Francis turbine. Through the analysis of the overall flow pattern, particle flow, particle concentration, and wear in the bladeless area of the unit under different guide vane openings, the influence of runner radial gap flow on the surrounding flow field characteristics and wear under different guide vane openings is revealed. The results show that the smaller the opening of the guide vane, the greater the influence on the vortices and flow pattern and the particle distribution in the runner. The overall wear in the hydraulic turbine unit with the optimal opening is the smallest. The long-term wear of the runner inlet and guide vane outlet will cause the loss of local structures, an increase in the radial clearance of the runner, an increase in the clearance leakage, an increase in the vibration of the unit, and a reduction in efficiency. The research results provide a basis for the structural and hydraulic optimization of the Francis turbine.

Effects of spalled particles thermal degradation on a hypersonic flow field environment

The presence of spalled particles might affect the flow field and the material, thereby influencing the aerodynamic heating rates of the thermal protection system. In order to study the impact of particles on the flowfield, a two-way coupling is performed between a Lagrangian particle trajectory model and a hypersonic aerothermodynamics flow solver. Time-accurate solutions are computed for argon and air flowfields. Single-particle and multiple-particle simulations are performed and results are studied. The studies in the argon environment indicated that the particles start sublimating soon after they cross the shock and keep sublimating during the remainder of their time in the flow. In the air environment, the particles start releasing carbon vapor soon after their ejection, and the magnitudes of different carbon products vary based on the particle’s location relative to the shock. Similar behavior is observed for multiple-particle simulations but with an increase in the amount of released carbon vapor that diffuses and convects over a larger area. A single-particle simulation is also run by adding additional gas phase reactions, and it is found that the production rate of certain carbon products (C1 and CN) increases. A parametric study is conducted based on parameters that affect the motion of the particles. The results of the comprehensive study show that the carbon vapor released by spalled particles tends to change the composition of the flow field, particularly the upstream region of the shock, which affects the heat flux incident on the test sample.

Two-dimensional hydrofoil hydrodynamic performance and waveform evolution studies in density stratification and homogeneous fluid

This paper presents a numerical simulation study of the hydrodynamic coefficients, surrounding flow and generated internal waves of the NACA0012 hydrofoil in two-dimensional unsteady incompressible viscous stratified and uniform flows using computational fluid dynamics (CFD) methods. The Reynolds-averaged Navier-Stokes equations are discretized using the finite volume method (FVM) and combined with the volume of fluid (VOF) method to capture the interface between different phases. The k-ω SST turbulence model is used to predict the turbulent flow around the hydrofoil under different conditions. In order to validate the numerical methods, the simulation values are compared with the experimental results in this paper. The hydrodynamic coefficients and generated flow fields of hydrofoils in a stratified flow environment, i.e. water velocities, angles of attack, freshwater layer thicknesses, density differences and submergence depths are investigated. The surrounding flow fields and hydrodynamic characteristics of the hydrofoil in stratified flow are revealed by comparing the force coefficients and flow fields in homogeneous flow environment. The results show that compared to uniform flow, the hydrofoil can have a maximum loss of lift coefficient of 10.29% and a maximum reduction in drag coefficient of 49.1% in a stratified flow environment. Choosing the right variable can significantly improve the lift-to-drag ratio.

Digital modeling and dynamic analysis of RAT systems

In this paper, the key factors that influence the dynamic characteristics of RAT systems are analyzed and identified, and the digital model of typical RAT systems is established by AMESim plat-form. On this basis, the dynamic characteristics of typical RAT systems are analyzed.by simulating the dynamic changes of pitch Angle and turbine speed of RAT system under different flow field environment, hydraulic dynamic load and electrical load, the adaptability of RAT system to working scene was analyzed, and the inherent characteristics of typical RAT system were identified. The results provided theoretical basis for RAT system design and flight profile planning.

Pulse picking of a fiber laser enables velocimetry of biomass-laden jets at low and ultra-high repetition rates

The introduction of fibrous aspherical biomasses for the substitution of coal as a sustainable solid fuel causes complex particle-turbulence interactions, which need to be captured using minimally invasive laser-based diagnostics. High-resolution two-phase particle image velocimetry (PIV) of high-velocity biomass-laden turbulent flows require laser inter-pulse times of only a few microseconds, which is usually achieved with separate laser systems for low-speed statistical and time-resolved measurements. In this work, we introduce a novel approach to achieve low-speed dual-pulse and ultra-high-speed time-resolved two-phase PIV using a single laser system. The flexible repetition rate laser output is realized by coupling a continuously pulsing fiber laser to an acousto-optic deflector (AOD) for pulse picking. The selective deflection of two adjacent laser pulses into the measurement volume is presented at a camera repetition rate of 25 Hz and laser repetition rate of 312.5 kHz resulting in an inter-pulse time of 3.2 µs. Temporal limitations of the AOD pulse picking system are evaluated beyond the access time limit for partially overlapping pulse pairs. To demonstrate the feasibility of the diagnostic system, low-speed measurements and additional ultra-high-speed measurements at 200 kHz are performed at high spatial resolutions to resolve individual biomass particles and their surrounding flow field in a non-reacting turbulent round jet. Simultaneous diffuse back-illumination measurements using a pulsed LED for particle size measurements of a high-aspect-ratio biomass (miscanthus) are conducted. The diagnostic system offers the capability of determining the slip velocity field and projected particle size, enabling a direct measurement of the local particle Reynolds number under high-velocity turbulent conditions at low and ultra-high repetition rates.

Estimating wind velocity and direction using sparse sensors on a cylinder

Using finite pressure measurements on a cylinder, we are able to estimate both the oncoming wind speed and direction of uniform flow over a cylinder at Reynolds numbers 20 000<Re<120 000. While reduced-order methods, such as proper orthogonal decomposition with QR factorization, require at least nine sensors to estimate the oncoming wind speed and direction with <10% error, other methods, such as probabilistic approaches or curve-fitting, can achieve similar results with as few as five sensors. A utility function, based on the Kullback–Leibler divergence, is used to determine the locally optimal location of the sensors to accurately estimate inlet conditions. It was found that sensor arrangement also plays a significant role, with unevenly distributed sensors being preferable than evenly distributed sensors. These techniques, when paired with existing flow field estimation approaches, allow the user to predict the surrounding flow field from any oncoming direction.

Cavitation bubble dynamics in a funnel-shaped tube

Control of cavitation in confining geometries is vital to the safety of hydraulic conduits and structures. Here, we investigate cavitation bubble dynamics in one typical type of open-ended tube with varying cross sections, i.e., a funnel-shaped tube, defined as a funnel-shaped section connected to a cylindrical section. In our experiments, single cavitation bubbles are generated by spark along the symmetric axis of the funnel-shaped tube at different positions, including in the funnel-shaped section (regime 1), near the throat (regime 2), and in the cylindrical section (regime 3). With high-speed photography, we observe that at the end of collapse, bubbles produce weak jets in regime 1, jets in a shape of an inverted cone in regime 2, and jets in a cylindrical shape with a rounded end in regime 3. With the help of OpenFOAM simulation, we better understand the surrounding flow fields during the bubble evolution. By capturing the main features of the flow fields, we derive dynamics equations for bubbles in each regime. Scaling laws are proposed for the bubble jet velocity, which depends on the position of bubble generation, the inner diameter of the cylindrical section of the tube, and the maximum distance where the bubble's upper surface can reach before the collapse. Our findings may be inspiring for understanding cavitation bubble dynamics in tubular conduits with complex geometries, e.g., cavitation in pipelines and draft tubes in hydrodynamic systems, and ultrasonic diagnosis in blood vessels.

Treating NOx emission of hydrogen fueled combustion engines by NOx storage and reduction catalysts: A transient kinetic study including PLIF measurements

NOx storage and reduction (NSR) catalysts are a well-known and broadly used technology to reduce NOx emissions from combustion engines, which may also be applied for hydrogen fueled engines in the future. In this study, Pt- and Pd-based NSR-catalysts were investigated in the absence and presence of water to understand how NO oxidation as well as the storage and reduction phases are influenced by the gaseous environment with H2 as a reductant. A planar channel configuration was chosen for conducting planar laser-induced fluorescence experiments during the storage phase in addition to steady-state oxidation measurements and transient lean/rich cycles in a packed bed reactor. The presence of steam significantly decreases the NO oxidation activity of both noble metal catalysts. The Pt/BaO/Al2O3 catalyst is more active during transient lean/rich cycles, however, it suffers an activity loss during repeated cycles, whereas the activity of the Pd/BaO/Al2O3 sample is slightly more stable in the wet gas feed over time. All experiments showed a strong correlation between the NO2 formation over the catalyst and its storage capability. The influence of water in the exhaust gas on the NSR-catalysts shows a strong temperature dependency on storage and reduction of NO for both catalysts containing Pt and Pd. The storage behavior is also strongly influenced by both the experimental configurations chosen revealing the significance of the interaction of intrinsic catalytic kinetics and mass transfer in the surrounding flow field.

Experimental and numerical studies on the two “lock-in” regions characteristic of vertical vortex-induced vibration of Π-shaped composite bridge deck

The Π-shaped bridge deck may easily occur vertical vortex-induced vibration, due to the bluff body aerodynamic configuration, which may cause the uncomforting of vehicles and pedestrians on the bridges. The present study mainly focused on the two “lock-in” regions characteristics of vertical vortex-induced vibration (VIV) of Π-shaped bridge deck, which can reveal the aerodynamic mechanism of vibration mitigation countermeasures. The vertical VIV response of original Π-shaped deck and those with different countermeasures are experimentally investigated detailly. Computational fluid dynamics (CFD) method is employed to numerically simulate the surrounding flow field of original girder section and those with different aerodynamic countermeasures. The numerical method is firstly verified through the comparison with the results of corresponding experimental results obtained in wind tunnel test. Afterwards, the flow pattern and vortex structure around the girder section simulated by CFD are utilized to interpret the effects of the mitigation countermeasures. The numerical results show that: (a)The vibration mitigation effect in the first vortex “lock-in” region is not sensitive to the position of the aerodynamic countermeasures, while the vibration mitigation effect in the second “lock-in” region is more sensitive to the position of the aerodynamic countermeasures; (b) The phase of aerodynamic lift and displacement response is close to synchronization in the ascending section of the VIV amplitude, greater than 90° in the descending section, and close to 180° outside the vortex-induced resonance range. The first vertical VIV region is related to the “Karman vortex street” at the trailing edge of the section, while the second vertical VIV region is related to the vortex shedding at the lower edge of the front end of the section and the secondary vortices on the upper surface.