Counter-rotating Vortex Pair Research Articles

Influences of microjet pressure and number of microjets on the control of shock wave/boundary layer interaction

Boundary layer separation is a common phenomenon in supersonic/hypersonic vehicles, and can adversely affect the lift coefficient. Currently, active flow control based on microjets is used to inhibit the separation of the turbulent boundary layer. The obtained results predicted by the three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations and two-equation shear stress transport (SST) k-ω both with Realizable k-ε turbulence models show that the control mechanism of the microjets method is the counter-rotating vortex pair (CVP) generated by the microjet mixes the low-energy flow within the boundary layer with the high-energy flow near the boundary layer. The size of the vortex core in this CVP is the key to controlling SWBLI. The vortex nucleus that is larger and closer to the wall enables better SWBLI control. A lower or higher ratio of microjet pressure to inflow static pressure increases the total volume of the separation bubble and produces a negative gain. When the ratio of the triangular microjet hole bottom edge length to the spacing between the holes is too small, leading to a deterioration in the control effect. The addition of microjets reduces the total pressure recovery coefficient.

Experimental investigations on fuel spatial distribution characteristics of aeroengine afterburner with all typical components

The fuel space distribution is a key factor in the combustion performance of afterburner. We have investigated the fuel space distribution and flow field in afterburner with all typical components. In this study, the complete afterburner test section of the afterburner is composed of a typical mixing diffuser, fuel supply system, flame stabilizer and heat shield. At the same time, we built an experimental system that enables a wide operating range of the afterburner. A temperature measuring rake, a pressure measuring rake, a Particle Image Velocimetry (PIV) system and a laser particle sizer were used to measure the flow field and fuel distribution in the combustor. It was found that the fuel particle size in the combustor space along the flow direction decreases, then increases and then gradually decreases. This is because the fuel particle size is smallest at the vortex nucleus in the recirculation zone after the stabilizer. The increase of the incoming Mach number will lead to an increase in the range of the recirculation zone behind the stabilizer, thus making the fuel distribution in the entire combustor more uniform. When the diffusion ratio is increased from 1.0 to 1.4, a significant asymmetric counter-rotating vortex pair can be formed after the stabilizer. Compared with the blockage ratio of 0.2 and 0.4, when the blockage ratio is 0.3, the fuel particle size will be reduced by 24%–32%. The spatial distribution of the fuel is not sensitive to pressure changes, and under the experimental conditions, the fuel particle size will increase by 9–25 µm with the decrease of pressure. In summary, when the diffusion ratio is 1.4 and the blockage ratio is 0.3, the afterburner has the most reasonable fuel distribution and flow field, which is conducive to the stable and efficient operation of the afterburner..

Experimental investigation on the flow patterns in a T-junction under steady and rolling motion conditions using PLIF

Flow mixing experiments are performed on the Ocean Conditions Simulation Platform (OCSP) test facility to investigate the flow pattern evolution mechanisms in a T-junction with branch jet under steady and rolling motion conditions. Plane Laser-Induced Fluorescence (PLIF) technology is used to display and test the instantaneous flow pattern in the T-junction. The effects of the main fluid Reynolds number, inflow rate ratio and rolling Reynolds number on the flow pattern characteristics are discussed at a range of 2110–15850, 1–20 and 0–3520, respectively. Based on the experimental results, new classification criteria of flow pattern in the T-junction under steady condition are proposed from the perspective of the evolution of vortex structure. The flow patterns are classified into the single shear vortex, co-rotating vortex pair, transition region and counter-rotating vortex pair according to the fluid Reynolds number and inflow rate ratio. Under rolling motion condition, the branch jet periodically sweeps up and down in the T-junction due to the effect of additional inertial force. The sweeping phenomenon of the branch jet caused by rolling motion enhances the flow instability in the T-junction that can be inhibited by increasing the fluid Reynolds number or decreasing the inflow rate ratio and rolling Reynolds number. In addition, the influence mechanisms of fluid Reynolds number, inflow rate ratio and rolling Reynolds number on the flow pattern in the T-junction are investigated. These results can inform the analysis, design, safe operation and performance improvement of equipment in various industrial applications involving flow mixing in a T-junction.

Effect of fuel temperature on mixing characteristics of a kerosene jet injected into a cavity-based supersonic combustor

To explain the phenomenon observed in previous experiments of kerosene-ignition failure in scramjet combustors as the kerosene temperature increases, we numerically investigate the mixing characteristics of a kerosene jet injected into a cavity-based supersonic combustor at different injection temperatures by using a compressible two-phase flow large-eddy simulation based on the Eulerian–Lagrangian approach. The results indicate that, upon injecting kerosene at high temperatures, the flow field preceding the leading edge of the cavity is similar to a typical gas jet in supersonic crossflow. The wall counter-rotating vortex pair (CVP) develops more fully and eventually becomes the main vortex pair. This evolution of the wall CVP modifies the cavity shear layer and alters the local flow-field characteristics near the cavity. Upon injecting kerosene at high temperatures, its evaporation rate increases sharply and the cavity recirculation zone enlarges, which causes more kerosene vapor to be entrained into the cavity. Because the kerosene-vapor temperature is lower than that of the low-speed fluid in the cavity, a significant amount of kerosene vapor entering the cavity not only makes the mass fraction of kerosene in the cavity exceed the fuel stoichiometric mass fraction but also reduces the temperature in the cavity, which negatively impacts the ignition process. The ignition delay time is much longer when the injection temperature is high, which is consistent with the inability of the initial flame kernel to form in the experiment.

Effect of inclination angle on the film cooling in a serpentine nozzle with strong adverse pressure gradient

The serpentine nozzle is widely used in military unmanned aerial vehicles to improve their viability. The film cooling technology should be used in the serpentine nozzle to cope with excessive thermal load. Strong adverse pressure gradient (APG) in the serpentine nozzle induces the recirculation zone near the upper wall downstream of the film hole, such that the effect of inclination angle on the film cooling characteristics is quite different from that in other studies. A numerical investigation was conducted to study the influence of the film hole inclination angle (α = 20°, 30°, 45°, 60°) on the film cooling characteristics in a serpentine nozzle with strong APG. Results show that the inclination angle changes the region of recirculation zone, then affects the development of counter rotating vortex pair and anti-counter rotating vortex pair (ACRVP), and finally results in different distributions of the film cooling effectiveness (FCE). For blowing ratio, M, = 0.5, 1, and 1.5, with the increase in the inclination angle, the recirculation zone expands, and the vorticity of the ACRVP increases. For M = 2, with the increase in the inclination angle, the recirculation zone first shrinks and then expands, and the vorticity of the ACRVP first decreases and then increases. Taking all the four blowing ratios into account, we find that the 30° inclination angle gives the best film cooling effect, and the area-average FCE under conditions α = 20°, α = 45°, and α = 60° is 7.5%, 5.6%, and 5.0%, respectively, lower than that under condition α = 30°. Therefore, the 30° inclination angle is the optimal choice for the film holes near the APG region of the serpentine nozzle.

INVESTIGATION OF THE PROPERTIES OF MICROPARTICLES IN THE GLOW DISCHARGE STRATUM IN A CROSSED ELECTRIC AND MAGNETIC FIELD

In this work, the behavior of charged micron-sized particles in the DC glow discharge stratum at low pressure in a crossed magnetic and electric field was experimentally studied. The experiment was conducted in a vertically oriented gas-discharge glass tube. A homogeneous magnetic field was created using a two-section Helmholtz coil. The results showed that the micron-sized dust particles move in the opposite direction to the ExB drift as the magnetic field induction increases. Once the induction reaches a specific threshold (B>10 mT), the dust particles start rotating and forming counter-rotating vortex pairs on the horizontal plane. Moreover, it was observed that the shape of the dust structures changes from a disk to an ellipsoid. The PIV (particle image velocimetry) method was employed to analyze the dust vortices' dynamic behavior, and the generation of the co-vortex rotation was explained through the dust particles' charge gradient, which was orthogonal to the ion drag force.

The influence of the adverse pressure gradient on the flow characteristics of a serpentine nozzle with film cooling

With the increase of the turbine inlet temperature, the thermal load of serpentine nozzles increases greatly, leading to the stiffness degradation and structure deformation. Therefore, efficient cooling technologies are required. There are adverse pressure gradients (APGs) and strong swirl characteristics in the serpentine nozzle. Therefore, the film cooling design of serpentine nozzles faces greater difficulty than that of other exhaust systems. This paper aims to obtain the flow characteristics of serpentine nozzles with film cooling and to provide theoretical support for the film cooling design of serpentine nozzles. We obtained the influence of the APG and the favorable pressure gradient (FPG) on the flow characteristics of a serpentine nozzle with film cooling via the employment of numerical methods. The results show that the APG obstructs the flow of the cooling airflow, and causes a part of the cooling airflow to decelerate, to stagnate, and eventually to flow in the reverse direction, thus forming a recirculation zone. The counter rotating vortex pair (CRVP) induces the anti-counter rotating vortex pair (ACRVP) near the boundary layer of the wall, and the ACRVP develops rapidly in the recirculation zone. With the development of the ACRVP, the CRVP leaves the wall and gradually dissipates. As the blowing ratio increases, the recirculation zone moves downstream, and the recirculation zone first expands and then shrinks, such that the location where the ACRVP develops also moves downstream. There is no recirculation zone under the influence of the FPG, and the CRVP occupies the core of the cooling airflow along the flow direction.

Identification of turbulent structures of inclined negatively buoyant jets with bed effects

Present study aims at exploring turbulent structures of inclined negatively buoyant jets (INBJs) using large eddy simulation (LES). Four types of turbulent structures are observed using the Q criterion and vorticity; namely, shear layer vortices, buoyancy-induced vortices, counter-rotating vortex pair (CRVP), and gravity current structures. The shear layer vortices emerge and develop in the momentum-driven regime of INBJ, where the jet core concentration reduces by nearly 20% due to the small amount of entrainment. Further downstream, where the negative buoyancy dominates the INBJ, the buoyancy-induced vortices form, which are larger and weaker than the momentum-driven structures. The velocity magnitude of the entrained flow also increases by a factor of 4, and the relative concentration decay rises to 70% in the buoyancy-driven region. A CRVP is also detected in the jet cross-sections. The vortex pair may be induced by a secondary flow inside the jet sections due to the jet deflection by gravity. The size of CRVP constantly grows from upstream to downstream, while the vorticity magnitude of it has an increasing-decreasing trend due of the changes in the jet trajectory curvature. A density current forms with big rollers on its edges when the plume-like flow impacts the bottom. Similar to previous studies of density currents, the streaky structure is also observed on the bed. Furthermore, it is found that increasing the bed slope results in bigger and more dispersed turbulent structures, especially in the plume-like regime after the maximum height point.

A combined cooling structure consisting of serrated trench and V-shaped surface for film holes: Flow structure and effectiveness improvement

Film cooling is a commonly used thermal protection technology for turbine airfoils. However, due to the counter-rotating vortex pair (CVP) generated by the interaction between the cooling jet and the mainstream, the coolant gradually breaks away from the wall, which is particularly evident at high blowing ratios. For the sake of improving the film cooling effectiveness (FCE) under high inlet temperature conditions, a novel film cooling structure (SerrTrench-VSS) that combines serrated trench film holes with shark skin-inspired V-shaped surface (VSS) is proposed. The RANS method and realizable k-ε model are used for the simulations. The flow characteristics, adiabatic FCE, and resistance loss are analyzed in detail when the blowing ratio (BR) is 0.5–1.5 and then compared with the traditional cylindrical film hole and transverse trench film hole. In addition, the impact of VSS height on the cooling performance is investigated and the best height of the structure is determined. The results show that the SerrTrench-VSS has better FCE. The serrated trench can destroy the CVP and improve the spanwise spreading ability of the coolant. Also, the VSS can converge the cooling jet downstream of the film hole toward the centerline, thereby improving its extension ability along the flow direction. In the studied range of blowing ratio, the SerrTrench-VSS has the best FCE, and the spanwise-averaged FCE increases by up to 10.50% when BR = 0.5. The empirical correlations between the globally-averaged FCE and the height ratio within the range of the present study are also given. For the serrated trench, the cooling jet has the best spanwise spreading and flow extension ability when the VSS height is 0.5D. The proposed novel film cooling structure (SerrTrench-VSS) can effectively improve the FCE and provide feasible means for the efficient cooling of a turbine blade under practical conditions.

Evolution mechanism of wind turbine wake structure in yawed condition by actuator line method and theoretical analysis

Through deflecting the wind turbine wake and reducing the influence of wake effect, wind farm cooperative control can enhance the total power generation of wind farm. In addition to wake deflection, the wake of yawed wind turbine shows complex distortion characteristics, which cannot be explained sufficiently by traditional momentum conservation. In this paper, numerical simulation using Actuator Line Method and theoretical analysis were adopted to investigate the wake characteristics of NREL 5 MW wind turbine in yaw. The wake of yawed wind turbine shows a velocity deficit distribution with kidney-shape. Due to the periodic change of tip vortex shedding velocity, the vorticity carried by tip vortex changes periodically, leading to the formation of an asymmetric counter-rotating vortex pair (CVP), which induced the complex wake structure of yawed wind turbine. On this basis, a new theoretical model for the wake structure of yawed wind turbine is proposed, which can accurately predict the main characteristics of the CVP. This paper reveals the wake evolution mechanism of yawed wind turbine and establishes the corresponding theoretical model, which is conducive to the development of high-precision engineering wake model and the optimization of wind farm cooperative control.

Three-dimensional modeling of dilute sediment concentration around the synthetic sponge inspired by the tubular sponge mechanism

Marine tubular sponges use horizontal suction to receive the nutrients from the perforated body and pump the undigested materials from the top mouth. The idea of a synthetic sponge was inspired by the tubular one. It was introduced to manage the sediment-flow hydrodynamics for enhancing sediment transport in coral reefs and access channels. In the current study, synthetic sponge effects on the dilute sediment concentration were investigated numerically. The Mixture method based on Three-Fluids Modeling (TFM) was selected. The effects of pore area distributions (0–0.42) and jet flow discharges (0–7500 L/h) on the sediment concentration were evaluated. The surface Line Integral Convolution (surfaceLIC) was used as an image processing technique to assess the fluid hydrodynamics. The results indicate that the effects of perforation area distribution are more significant than suction/pumping discharges on the sediment concentration (94.87% vs. 5.13% contribution). So increasing the distribution of perforation areas can effectively reduce water turbidity (between 20% and 50%). The combination of the wide re-circulation zone, Counter Rotating Vortex Pair (CRVP), spiral flow, and dipole formation are the reasons for this. Therefore, the combination of vortices facilitates sediment transport due to the unique shape and mechanism of synthetic sponge.

Large-eddy simulation of gas-particle two-phase jet into a supersonic crossflow

Large eddy simulation is used to study the gas-particle two-phase transverse jet in supersonic flow. In this paper, the Eulerian method is used to simulate the fluid, and the Lagrangian method is used to simulate the particles. The numerical study is carried out for different particle mass fractions and Stokes numbers with the jet-crossflow momentum flux ratio and the Mach number of crossflow being kept constant. It is found that the larger particles (St = 5) are mainly located above the counter-rotating vortex pair, and the smaller particles (St = 0.9) are mainly located in the counter-rotating vortex pair. Due to the Kelvin–Helmholtz instability, several characteristic phenomena have been observed, including particle trailing and the wave structure constructed of the particle cloud, and the particles also make the Mach disk lower.

Learning time-averaged turbulent flow field of jet in crossflow from limited observations using physics-informed neural networks

Physics-informed neural networks (PINNs) are becoming popular in solving fluid mechanics problems forwardly and inversely. However, under limited observations, the application of PINNs was found to be difficult in solving the inverse problems of three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations. In this study, the classical turbulent case of jet in crossflow was representatively adopted into the investigation. The dataset was obtained from a high-fidelity large-eddy simulation. The tensor-basis eddy viscosity (t-EV) model was imported first into the structure of PINNs as prior knowledge. Observations of five measured planes were preliminarily used to reconstruct the time-averaged turbulent flow field. After embedding the t-EV model, the highest absolute error and the relative L2 error of streamwise velocity were reduced by 11.1% and 31.4%, respectively. To cut down the volume of limited observations, a more effective training dataset containing only two planes and two pairs of lines was determined based on the flow characteristics (e.g., shear layer and counter-rotating vortex pair). Compared with those of five planes, the highest absolute error and the relative L2 error of streamwise velocity were further reduced by 30.0% and 6.4%, respectively. The investigation in this study provided an alternative to resolve the inverse problems of three-dimensional RANS equations with limited observations, which extended the deep learning application in fluid mechanics.

Comparison of single-stage and multi-stage drainage cannula flow characteristics during venoarterial extracorporeal membrane oxygenation

Venoarterial extracorporeal membrane oxygenation is a form of artificial heart–lung therapy able to support patients undergoing refractory cardio-respiratory failure. Drainage cannulae are responsible for extracting venous blood from the body via a negative pressure gradient induced by the pump downstream. However, the unique designs of single- and multi-stage cannulae, such as the presence of small inlets on the walls of the cannula (side holes), result in complex flow dynamics. This study evaluated flow features of both cannula designs using a stress blended eddy simulation turbulence model, within a patient-specific geometry of the venous system. The wall-adapted local eddy viscosity subgrid-scale model was used to resolve the large eddies directly in the free stream region, while small eddies were modeled using the k–ω shear stress transport model in the near-wall region. Flow within both cannulae was dominated by turbulent structures, such as counter-rotating vortex pairs, followed by a region of flow separation created by the entering jet. This phenomenon was synonymous with a jet in a crossflow, but involved multiple tandem and opposing jets in an internal tubular environment. The single-stage cannula drained 38% of the total flow via the most proximal holes compared to the multi-stage cannula (52.8%). The single-stage cannula allowed for larger tip velocities and was able to extract more flow from the upper body. Overall, this study demonstrated notable differences in blood flow dynamics between single- and multi-stage cannulae, which can be applied in clinical selection and cannula design.

Effect of multi-hole trench cooling on an adiabatic flat plate for gas turbine application

Abstract A 3D numerical analysis on an adiabatic flat plate for multi-hole trench cooling with forward, backward and mixed injection holes is performed in the current investigation. The numerical setup is validated before the performances of different cooling configurations are compared. The effect of three different multi-hole trench arrangements, square-diamond, long-diamond, and super-long-diamond with constant perforated percentage (3.27%), on film cooling performance is studied at blowing ratio 1.0. The row-to-row interaction between coolant jets and mainstream is analysed, and lateral film cooling effectiveness is calculated downstream. The dimensionless temperature contour overlaid with streamlines concluded that the SLD trench hole arrangement with forward injection forms a developed effusion layer due to counter-rotating vortex pairs, which helps in proper mixing of coolant jets into the mainstream and improves film cooling effectiveness in lateral as well as in longitudinal direction. It is observed that super-long-diamond arrangement with forward injection provides the highest film cooling effectiveness than square-diamond and long-diamond arrangements and favours early development of the coolant film layer.

Temperature uniformity characteristics of array jet impingement cooling with the maximum cross-flow scheme

Temperature uniformity of the impingement cooling is commonly needed in many fields in order to enhance performance, reliability, and to ensure material quality. In particular, operating life of the hot components in gas turbine depends heavily on how uniformly they are being cooled to mitigate the development of thermal stresses. However, there are rather limited literatures addressing these, and the cooling uniformities of jet impingement in extreme conditions, such as the cooling of turbine vane and combustor liner, are seldom discussed. This study focused on the temperature uniformity of array jet impingement cooling with the maximum crossflow scheme, where the detailed distribution of the nonuniformly cooled areas on the target wall, as well as their correlations with the impinging flow field, and the effects of typical geometrical and fluid parameters on the uniformity were investigated. Results indicated that the arising of the high temperature zones and inverse temperature gradient pairs around them, essentially induced by the entrainment vortex and counter rotating vortex pair in the streamwise and spanwise directions respectively, are the sources of temperature nonuniformities over the impingement target surface. Moreover, increasing the impingement Reynolds number or shortening the jet-to-target spacing for higher jet Reynolds numbers could improve the temperature uniformity.

Experimental Observations on Flow Characteristics around a Low-Aspect-Ratio Wall-Mounted Circular and Square Cylinder

The mean wake structures of a cube (square cylinder) and circular cylinder of height-to-width aspect ratio 1.0, at a Reynolds number of 1.78 × 104 based on the obstacle width, were investigated experimentally. The boundary-layer thickness was 0.14 of the obstacle height. The study was performed using thermal anemometry and two-dimensional digital particle image velocimetry (DPIV). Streamwise structures observed in the mean wake for both cylinders included well-known tip- and horseshoe (HS)-,vortex pairs as well as additional structures akin to the base vortices. In addition to tip-, base-, and HS-vortices, in the near wake of the cube, two more counter-rotating pairs of streamwise structures, including upper and inboard vortices, were observed. The existence of base vortices formed in the near wake for both obstacles is a unique observation and has not been previously reported for such low-aspect-ratio obstacles in thin boundary-layers. A model of arch-vortex evolution was proposed, in which arch structures were deformed by the external shear-flow to explain the observed base-vortices in the cylinder wake. A weak dominant-frequency of St = f0D/U∞ = 0.114 was observed across the height for the cube, while no discernible spectral peaks were apparent in the wake of the cylinder. Cross-spectral analysis revealed the shedding to be symmetric (in-phase) arch-type for the cylinder and predominantly anti-symmetric (out-of-phase) Karman-type for the cube. The study makes fundamental contributions to the understanding of the flow-field surrounding low-aspect-ratio cylinders.

Detailed numerical simulation of multi-scale interface-vortex interactions of liquid jet atomization in crossflow

In this paper, a detailed numerical simulation of liquid jet atomization in crossflow is performed based on the volume of fluid method coupled with the adaptive mesh refinement technique. The multi-scale phase interface evolution and the interface-vortex interaction are reported. It is found that the momentum flux ratio, Weber number, density ratio and viscosity ratio are key variables for the empirical correlation to accurately predict the atomization characteristics. Three breakup regimes, i.e., column bag breakup, surface breakup and ligament breakup, occur due to the great mass and momentum exchanges at the interface. Specially, the Kelvin-Helmholtz instability induces axial surface waves that eventually develop into column bag breakup while the Rayleigh-Taylor instability and surface thinning can induce surface breakup. Relatively, the column bag breakup generates larger liquid structures, presenting a bimodal feature. The produced ligaments either shrink to droplets or further breakup into droplets depending on their size and shape, leading to a log-normal distribution of droplet size. The interface-vortex interaction is distinctive compared to single-phase flows. A vortex core is observed to simultaneously form, grow and dissipate within each bag during the life circle of the bag, and three types of counter-rotating vortex pair exist around the column root, bag membrane and liquid droplets. Vortical structures tend to concentrate near the interface with large deformations, possessing a strong perpendicularity between the phase interface and the vortex. This work offers fundamental basis for better understanding and organization of atomization.

Existence of traveling asymmetric vortex pairs in an ideal fluid

In this paper we study the existence of counter-rotating vortex pairs with different scales and different distributions for the Euler equations. We construct a family of traveling asymmetric vortex pairs using the variational method. The quasi-geostrophic shallow water equations and the generalized surface quasi-geostrophic equations are also discussed in a similar framework.

Leading-edge tubercle modifications to the biomimetic wings

An experimental investigation was conducted to better understand the effects of the humpback whale flipper's tubercles on biomimetic models. Different configurations of tubercles were investigated for five biomimetic flipper models by performing force measurement experiments at the Reynolds number of 5.0 × 104, 8.0 × 104, and 1.2 × 105 and surface oil flow visualization at Re = 1.2 × 105. The experiments were carried out with five different test models: two baseline models, one having a smooth and one having a tubercle leading-edge (LE); two simply designed tubercle models with uniform distribution; and a proposed tubercle model having a more realistic approach. It is proposed to create a tubercle pattern of a flipper model by summing two wave functions. The results indicated that the models with LE tubercles improved lift, delayed stall angle, and reduced drag compared to the baseline model. Irrespective of the Reynolds number, the model C3, which was created with a more realistic approach, performs better compared to baseline and other tubercle models. It has been seen that the maximum improvement in lift coefficient is achieved by approximately 18% with the C3 model at Re = 5.0 × 104. According to flow visualization results, the laminar separation bubble formed in the smooth baseline model was replaced by a counter-rotating vortex pairs (CRVPs) in the tubercle models. The improvement of the aerodynamic characteristics is due to the CRVPs formed by the interaction of the LE tubercles with each other and wavelike trailing-edge flow separation pattern. One of the significant findings to emerge from this study is that a more realistic approach has the potential to obtain better performance than a model with a uniform distribution of tubercles.