Airflow Structure Research Articles

Aerodynamic effects of the gap spacing between adjacent vehicles on wind tunnel train models

A certain gap spacing between adjacent vehicles is usually inevitable in wind tunnel force tests of high-speed trains under no crosswind, which may affect the wind tunnel test results. Thus, to understand the influence of gap spacings on the train aerodynamics, the aerodynamic drag, pressure distributions and airflow structures of 1/8th-scale high-speed train models with gap spacings of 0, 5, 8, 10, 20, and 30 mm were studied using RANS based on SST k-ω turbulence model. The simulation method was verified by the wind tunnel experiment data. The results show that the gap spacing significantly affects the airflow structure around inter-car gap and aerodynamic resistances of train models. For the high-speed train model scaled at 1/8th at zero yaw, compared with gap spacing of 0 mm, the gap spacings lead to a significant reduction in the aerodynamic drag of the head car and an increase in that of the tail car, whereas which of the middle car is not significant. The maximum difference of the drag coefficient of the entire train model is smaller than 2.0%. When the gap spacing does not exceed 8 mm, the discrepancies of the drag coefficients of three cars are within 6.15%.

CFD analysis of the flow structure in a monkey upper airway validated by PIV experiments

Inhalation exposure to airborne contaminants has adverse effects on humans; however, related research is typically conducted using in vivo/in vitro tests on animals. Extrapolating the test results is complicated by anatomical and physiological differences between animals and humans and a lack of understanding of the transport mechanism inside their respective respiratory tracts. This study determined the detailed air-flow structure in the upper airway of a monkey. A steady computational fluid dynamics simulation, which was validated by previous particle image velocimetry measurements, was adopted for flow rates of 4 L/min and 10 L/min to analyze the flow structure from the nasal/oral cavities to the trachea region in a monkey airway model. The low Reynolds number type k–ε model provided a reasonably accurate prediction of the airflow in a monkey upper airway. Furthermore, it was confirmed that large velocity gradients were generated in the nasal vestibule and larynx regions, as well as increased turbulent air kinetic energy and wall sheer stress.

Pore-ridge nanostructures on the surface of trichoid sensilla of the male silkmoth Bombyx mori: Aerodynamic trapping and transporting of the pheromone molecules

This paper tries to reveal the mechanism of the high-efficient adsorption of the sex pheromone by the trichoid sensilla of the male silk moth Bombyx mori. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to acquire the topographies and nanostructures of the surfaces of the trichoid sensilla. SEM and AFM images present mostly regular pore-ridge nanostructures on the sensilla, and all the pores are located at or near the feet of the ridges. AFM phase-shift images demonstrate that the variation of phase-shift, which appears along the ridge cannot simply be attributed to heterogeneity in surface lipid properties, for the phase-shift was present in the same region with the sudden difference in height. Simulations of computational fluid dynamics were applied to investigate the effects on the airflow velocity field and streamlines by the pore-ridge nanostructures and the antenna vibration. Simulation results indicate that the airflow vortexes that form on the sensillum surface are generated by the combined effect of ambient airflow and pore-ridge structure as well as spontaneous vibration of the antenna. We suggest that the vortex intercepts and traps the pheromone molecules passing nearby, and transports them through its periodical movement to the pore. We speculate that the vortex is the aerodynamic factor benefitting the highly efficient adsorption of pheromone molecules.

Research on transient aerodynamic characteristics of windshield wipers of vehicles

Purpose The main purpose of this paper is to gain a better understanding of the transient aerodynamic characteristics of moving windshield wipers. In addition, this paper also strives to illustrate and clarify how the wiper motion impacts the airflow structure; the aerodynamic interaction of two wipers is also discussed. Design/methodology/approach A standard vehicle model proposed by the Motor Industry Research Association and a pair of simplified bone wipers are introduced, and a dynamic mesh technique and user-defined functions are used to achieve the wiper motion. Finite volume methods and large eddy simulation (LES) are used to simulate the transient airflow field. The simulation results are validated through the wind tunnel test. Findings The results obtained from the study are presented graphically, and pressure, velocity distributions, airflow structures, aerodynamic drag and lift force are shown. Significant influences of wiper motion on airflow structures are achieved. The maximum value of aerodynamic lift and drag force exists when wipers are rotating and there is a certain change rule. The aerodynamic lift and drag force when wipers are rotating downward is greater than when wipers are rotating upward, and the force when rotating upward is greater than that when steady. The aerodynamic lift and drag forces of the driver-side wiper is greater than those of the passenger-side wiper. Originality/value The LES method in combination with dynamic mesh technique to study the transient aerodynamic characteristics of windshield wipers is relatively new.

The efficiency of high-level water collecting cooling tower with the installation of cross wall affect by the evolution of aerodynamic field

The aerodynamic field around high-level water collecting natural draft wet cooling tower (HNDWCT), especially that below its special high-level water collecting devices (HWCDs), strongly affects the tower efficiency. Therefore, the aerodynamic fields below HWCDs and its evolution characteristics have been studied, then the corresponding impact on the performance of HNDWCT has been clarified in detail. The results demonstrate that the incidence angle and speed of crosswind have great influence on the air flow state below HWCDs, thus the air distribution in heat and mass transfer zones and air mass flow rate through tower, resulting in the degradation of tower cooling performance. To mitigate the adverse effect of crosswind on tower cooling performance, cross wall has been investigated as a means to improve tower air flow structure. It has been found that the cross wall with appropriate porosity is beneficial to increase the tower efficiency. While, the optimum porosity depends on the crosswind speed and its incidence angle, which ranges from approximately 0.33–0.53. Furthermore, the structure of heat and mass transfer zones of HNDWCT, especially the rain zone, are different with that of usual natural draft wet cooling tower (UNDWCT), which causes the different effect of impermeable cross wall.

New Assessment Scales for Evaluating the Degree of Risk of Wind Turbine Blade Damage Caused by Terrain-Induced Turbulence

The present study scrutinized the impacts of terrain-induced turbulence on wind turbine blades, examining measurement data regarding wind conditions and the strains of wind turbine blades. Furthermore, we performed a high-resolution large-eddy simulation (LES) and identified the three-dimensional airflow structures of terrain-induced turbulence. Based on the LES results, we defined the Uchida-Kawashima Scale_1 (the U-K scale_1), which is a turbulence evaluation index, and clarified the existence of the terrain-induced turbulence quantitatively. The threshold value of the U-K scale_1 was determined as 0.2, and this index was confirmed to not be dependent on the inflow profile, the influence of the horizontal grid resolution, and the influence of the computed azimuth. In addition, we defined the Uchida-Kawashima Scale_2 (the U-K scale_2), which is a fatigue damage evaluation index based on the measurement data and the design value obtained by DNV GL’s Bladed. DNV GL (Det Norske Veritas Germanischer Lloyed) is a third party certification body in Norway, and Bladed has been the industry standard aero-elastic wind turbine modeling software. Using the U-K scale_2, the following results were revealed: the U-K scale_2 was 0.86 < 1.0 (within the designed value) in the case of northerly wind, and the U-K scale_2 was 1.60 > 1.0 (exceeding the designed value) in the case of easterly wind. As a result, it was revealed that the blades of the target wind turbine were directly and strongly affected by terrain-induced turbulence when easterly winds occurred.

Simulation of two types fuel cell stack models to investigate the relationship between air flow distribution and structure

For solid oxide fuel cell (SOFC) stacks, it is generally agreed that the distribution qualities of the fuel and air flows would determine the temperature and pressure distribution within the stack. The flow distribution within the stack is considered to depend largely on the geometry of the stack. According to the direction of inlet flow and outlet flow, the designs of the stack can be divided into U-type structure and Z-type structure, and stacks with two structural characteristics should have some similar flow characteristics. Researchers generally have different degrees of understanding of Z- or U-type characteristics, but there are few intuitive experiments and reports on these two types of structures at a larger stack scale. In this paper, two typical one-in-one output stacks with 30 layers in Z-type and U-type models are developed to investigate the general flow distribution.

Analysis of a Late-Autumn Rainstorm in the Sichuan Basin on the Eastern Side of the Tibetan Plateau

An abnormal heavy rainfall that occurred on 27 October 2014 in the Sichuan Basin (SB), China, is analyzed. An inverted trough at 850 hPa evolved into a Southwest China Vortex (SWCV), and strong upward motion caused by interaction between the low-level jet (LLJ) at 850 hPa and the upper-level jet (ULJ) at 200 hPa triggered the rainstorm process. Under a large-scale circulation system featuring a westerly trough and subtropical high, there were two cloud bands over the northeast side and south side of the Tibetan Plateau, respectively. Influenced by the eastward-moving trough, the inverted trough, LLJ, and the SWCV, a Mesoscale Convective System (MCS) was generated near the intersection of the two cloud bands, and it was the direct rainstorm system. The MCS strengthened under the situation of the 850 hPa inverted trough, but weakened when the inverted trough evolved to into the SWCV. Eventually, it formed the phenomenon known as “existing vortex without cloud.” Through analysis of the possible reasons why precipitation strengthened (weakened) under the situation of the inverted trough (SWCV), it was found that the strengthening of precipitation was due to a strong tilting updraft in the area of the ULJ and LLJ intersection. On one hand, the upward motion was related to the vorticity advection variation with height and the low-level warm advection forcing; while on the other hand, the dew-point front near the LLJ also played a lifting role in the upward flow of the lower-layer vertical circulation. Meanwhile, the LLJ “head” was a high-value area of water vapor convergence, which provided sufficient water vapor for the rainstorm. During the SWCV, the weakening of precipitation was due to the SWCV weakening gradually; plus, the ULJ was interrupted over the SB, the upper airflow presented downdrafts, and its superposition with the ascending branch of low-level vertical circulation. This airflow structure inhibited the development of strong upward motion, whilst at the same time, the LLJ retreated toward the south and the dew-point front ultimately weakened and disappeared. Subsequently, water vapor convergence weakened and no longer supported the occurrence of heavy rainfall. Therefore, the strong upward motion caused by the ULJ-LLJ intersection and the lower-level dew-point front were the key reasons for the occurrence of this late-autumn rainstorm.

Studying airflow structures in periodic cylindrical hills of human tracheal cartilaginous rings

The objective of this study is to assess tracheobronchial flow features with the cartilaginous rings during a light exercising. Tracheobronchial is part of human’s body airway system that carries oxygen-rich air to human’s lungs as well as takes carbon dioxide out of the human’s lungs. Consequently, evaluation of the flow structures in tracheobronchial is important to support diagnosis of tracheal disorders. Computational Fluid Dynamics (CFD) allows evaluating effectiveness of tracheal cartilage rings in human body under different configurations. This study utilizes Large Eddy Simulation (LES) to model an anatomically-based human large conducting airway model with and without cartilaginous rings at the breathing conditions at Reynolds number of 5,176 in trachea region. It is observed that small recirculating areas shaped between rings cavities. While these recirculating areas are decaying, similar to periodic 2D-hills, the cartilaginous rings contribute to the construction of a vortical flow structure in the main flow. The separated vortically-shaped zone creates a wake in the flow and passes inside of the next ring cavity and disturb its boundary layer. At last, the small recirculation flow impinges onto tracheal wall. The outcome of this impinge flow is a latitudinal rotating flow perpendicular to the main flow in a cavity between the two cartilaginous rings crest which appear and disappear within a hundredth of a second. Kelvin-Helmholtz instability is observed in trachea caused by shear flow created behind of interaction between these flow structures near to tracheal wavy wall and main flow. A comparison of the results between a smooth wall model named simplified model and a rough wall model named modified model shows that these structures do not exist in simplified model, which is common in modeling tracheobronchial flow. This study proposes to consider macro surface roughness to account for the separating and rotating instantaneous flow structures. Finally, solving trachea airflow with its cartilages can become one of major issues in measuring the validity and capability of solving flow in developing types of sub-grid scale models as a turbulence studies benchmark.

Effects of coarse riblets on air flow structures over a slender delta wing using particle image velocimetry

This research investigates the aerodynamic performance and flow characteristics of a delta wing with 65° sweep angle and with coarse axial riblets, and then compares with that of a smooth-surface delta wing. Particle Image Velocimetry (PIV) were utilized to visualize the flow over the wing at 6 cross-sections upright to the wing surface and parallel to the wing span, as well as 3 longitudinal sections on the leading edge, symmetry plane, and a plane between them at Angles of Attack (AOA) = 20° and 30° and Re = 1.2 × 105, 2.4 × 105, and 3.6 × 105. The effects of the riblets were studied on the vortices diameter, vortex breakdown location, vortices distance from the wing surface, flow lines pattern nearby the wing, circulation distribution, and separation. The results show that the textured model has a positive effect on some of the parameters related to drag reduction and lift increase. The riblets increase the flow momentum near the wing’s upper surface except near the apex. They also increase the flow momentum behind the wing.

A Pedagogical Study of Aerodynamic Feedback Control by Dielectric Barrier Discharge Plasma

Active flow control by means of plasma actuators has potential advantages over conventional strategies, e.g., mechanical or hydraulic components may be replaced by lightweight, compact, fast response plasma actuators. In this paper, several designs of dielectric barrier discharge (DBD) plasma actuators are presented for aerospace applications and the focus is on the associated feedback control implementation. The interdisciplinary nature of aerodynamic feedback control with plasma, however, makes direct experimental demonstrating an outstanding challenge. Here, we propose a realistic experimental control implementation afforded by commercial off-the-shelf electric products and the major achievement is the detailed instruction (in both electricity and aerodynamics) and the successful demonstration of the closed-loop design in controlling the dominant modes from a cylinder flow setup. The essence of our approach is to drive DBD plasma actuations by a downstream sensor and excite aerodynamic velocity perturbations, which are further amplified on the shear flow from the cylinder, leading to airflow structures, such as vortex roll-up and randomization, which are measured by the downstream sensor to complete the whole loop. We benchmark our control approach by comparing to the predicted dominant frequencies of the controlled flow system, which can be achieved by the Barkhausen stability criterion after establishing the corresponding transfer function of the whole flow control system. Overall, this paper shall assist a host of new applications in aerospace applications in the near future.

A Study on In-Cylinder Flow of Small Engine Using Steady-State Flow Benches

In-cylinder air flow structures are known to strongly impact on the performance and combustion of internal combustion engines (ICE). Therefore, the aim of this paper is to experimentally study an IC engine in-cylinder flow under steady-state conditions. Steady-state flow bench methods can be used to characterize the in-cylinder flow engine. The experiments were carried out on the small cylinder head with pressure difference across the inlet valves of 100 to 300 mmH2O. The experimental results are presented in terms of the measured air flow rate, flow coefficient and discharge coefficient. Moreover, the CFD used to illustrate the evolution of tumble motion in-cylinder engine. The results show that air volume flow rate and flow coefficient increased as the valve lift increases but the coefficient of discharge decrease if valve lift increase. The higher the pressure drop the higher the mass flow rate value. However, the higher the pressure drop the lower the value of flow coefficient and discharge coefficient.

The air-flow structure and gas dispersion behavior in gas tunnel construction through bench cut method

The inability to accurately predict the in rush of gas increases the fear of field workers security and unnecessary investments in tunnel ventilation. In this study, a 3-D numerical model was established to investigate the air-flow structure and gas diffusion characteristics of a gas tunnel during construction. The result showed that the flow inside the bottom bench mainly came from the re-circulation zone of the upper bench. Mutation in the tunnel section due to bottom bench excavation would cause the formation of some air circulation in the bottom bench. The circulation air in the bottom bench came from the re-circulation zone of the upper bench, while the air-flow in the considered zone was a polluted air. Based on the obtained results, there was no accumulation of CH4 inside the bottom bench. Nevertheless, gas gushing will still appear during the construction of the bottom bench surface, which can possibly lead to gas disaster. This study aims to provide some guidance for tunnel engineers on safety monitoring systems.

Definition of mutual thermal influence of containers with spent nuclear fuel at the open storage site

The problem of spent nuclear fuel handling in Ukraine is a key issue. A half of spent nuclear fuel is currently stored in Ukraine at the open-site dry storage facility at Zaporizhzhya NPP. Thermal safety analysis should be performed as a part of the storage facility safety assessment. Thermal analysis of a container group is a poorly investigated area. As literature review shows, current results do not clearly identify mutual influence of the containers and influence of weather conditions on the thermal condition of stored spent nuclear fuel. This type of analysis could be performed using the multi-stage methodology proposed by the authors. Thus, mutual thermal influence of the containers and boundary conditions at the inlets to the ventilation duct of each storage container should be identified. Thermal processes in the container group where spent nuclear fuel is stored that are described in this paper are analyzed by solving the conjugate heat transfer problems. A row of containers under wind influence is simulated and the structure of ventilation airflow inside the containers is identified. The mutual thermal influence of the containers is absent under calm conditions, and heated air does not come from one container to another. Resulting from the simulation, boundary conditions at the inlet of the ventilation duct are specified and can be used in the iterative modelling methodology for spent fuel thermal condition. The dependence of the velocity of the inlet ventilation air in the ventilation duct of each container in the row was defined. The container placement methodology with the purpose of decreasing the wind influence on the thermal condition of spent fuel storage was proposed. Thermal studies are carried out for the containers and storage conditions of the dry spent nuclear storage facility at Zaporizhzhya NPP.

Thermal buoyancy driven flows inside the industrial buildings primarily ventilated by the mechanical fans: Local facilitation and infiltration

The turbine hall houses a number of components with intense heat release, resulting in a hot and humid air environment and making it very unpleasant for plant staffs to conduct maintenance work there, particularly at a hot outdoor environment. The traditional pure buoyancy driven natural ventilated could fulfil the overall heat exhausting requirement, however, creating hot environment at several positions due to the vortex airflow structure. As only part of the building need to be cooled, a localized cooling system with assisting mechanical fans is used. Buoyancy driven natural ventilated flows primarily occupying this turbine hall have been investigated by a computational fluid dynamics (CFD) model and an on-site measurement. The ventilation airflow is solved by 3D steady state Reynolds Averaged Navier-Stokes (RANS) equations in conjunction with the SST k-ω turbulence model. The numerical codes and procedures were validated by comparing with on-site measurements. The concept of mean age of air is used to evaluate the natural ventilated efficacy. Following that, the potential of using mechanical vents and slabs to cool equipment work area at this buoyancy driven natural ventilated hall is analysed. The simulation results show that the mechanical vents could decrease air temperature and increase air exchange efficiency at work area effectively on condition that the local airflow pattern is well organized, which is strongly related to the ejection direction and velocity. Present research could benefit future development of the ventilation design in the power plant buildings.

Analysis of Crank Angle-Resolved Vortex Characteristics Under High Swirl Condition in a Spark-Ignition Direct-Injection Engine

In-cylinder air flow structure makes significant impacts on fuel spray dispersion, fuel mixture formation, and flame propagation in spark ignition direct injection (SIDI) engines. While flow vortices can be observed during the early stage of intake stroke, it is very difficult to clearly identify their transient characteristics because these vortices are of multiple length scales with very different swirl motion strength. In this study, a high-speed time-resolved two-dimensional (2D) particle image velocimetry (PIV) is applied to record the flow structure of in-cylinder flow field along a swirl plane at 30 mm below the injector tip. First, a discretized method using flow field velocity vectors is presented to identify the location, strength, and rotating direction of vortices at different crank angles. The transients of vortex formation and dissipation processes are revealed by tracing the location and motion of the vortex center during the intake and compression strokes. In addition, an analysis method known as the wind-rose diagram, which is implemented for meteorological application, has been adopted to show the velocity direction distributions of 100 consecutive cycles. Results show that there exists more than one vortex center during early intake stroke and their fluctuations between each cycle can be clearly visualized. In summary, this approach provides an effective way to identify the vortex structure and to track the motion of vortex center for both large-scale and small-scale vortices.

Turbulent Airflow and Wave-Induced Stress Over the Ocean

We examine the structure of turbulent airflow over ocean waves. Based on an analysis of wind and wave observations derived from a moored and floating Air–Sea Interaction Spar buoy during the Shoaling Waves Experiment field campaign, we show that the cospectra of momentum flux for wind–sea conditions follow established universal scaling laws. Under swell-dominant conditions, the wave boundary layer is extended and the universal cospectral scaling breaks down, as demonstrated previously. On the other hand, the use of peak wave frequency to reproduce the universal cospectra successfully explains the structure of the turbulent flow field. We quantify the wave-coherent component of the airflow and this clarifies how ocean waves affect momentum transfer through the wave boundary layer. In fact, the estimated wave-induced stresses for swell-dominant conditions explain the anomalous cospectral shapes observed near the peak wave frequency.

Effects of different CFD modeling approaches and simplification of shape on prediction of flow field around manikin

Providing a comfortable and pleasant indoor environment for the occupants is the primary objective of an efficient ventilation process. Therefore, it is important to gain a proper understanding between the interaction of airflow and the occupants which determines the ultimate ventilation strategy and as one of the final outcomes, the architecture layout itself. The CFD (Computational Fluid Dynamics) methods provide an accurate and robust tool to simulate the complex airflow structure in enclosed spaces. Among the CFD simulation approaches, the large eddy simulation (LES) and its hybrid variants such as detached eddy simulation (DES) provides the information on the flow behavior by modeling the small turbulent structures of the airflow encountered in an indoor environment. These methods are highly sensitive to the boundary conditions which are the pivotal factors in an indoor airflow modeling and its accuracy. The main objectives of this paper are: (a) to assess the performance of different LES and DES models in capturing velocity and thermal fields (b) to investigate the effect of manikin’s shape on the accuracy of predictions. Three LES SGS (sub-grid scale) and one DES models are applied in this study considering the cases with and without the radiation from the manikin. Moreover, the effect of simplifying the manikins’ geometry on velocity and thermal distribution is investigated. Results showed that a detailed manikin shape can provide more accurate predictions (4–10%) at some locations, especially those close to the manikin’s body. As for performance of the applied models, Comparisons showed that ZEM and WALE models provide a better predictions of velocity and temperature fields by 3–10% at different locations.

Numerical investigation of a swirl diffuser with a novel design using large eddy simulations

In the present study, a swirl diffuser with a novel design is proposed, in which three geometrical parameters, the angle of the guide vanes φ, the height of the convergent chamber h, and the radius of the diffuser outlet r, can be varied independently. Large eddy simulations were conducted to examine the airflow characteristics downstream of the swirl diffuser. Grid independence was obtained for the grid number 4000,000, and this result was validated by comparison with an experiment. The vorticities, the instantaneous and averaged velocities, and the evolution of the age of air were investigated. The turbulent kinetic energy budget and the momentum balance in the time-averaged Navier–Stokes equations were then studied in an attempt to clarify the mechanism of the swirl diffuser's high diffusing performance. With the increasing swirling strength, the pressure gradient force could not balance the centrifugal force in the radial direction. As a result, the vortex core expanded, generating turbulence. A non-dimensional swirl number parameter was suggested which can be used for determining the structure of the swirl diffuser airflow. To validate this non-dimensional parameter, different combinations of φ, h, and r were examined. It was found that only if the swirl number is the same, will the airflow structure be similar. The results of our study are likely to be useful for obtaining deeper insights about the swirl diffuser airflow, and for better understanding of the effect of the controlling parameter on the swirl diffuser's performance.

Large eddy simulation of the flow pattern in an idealized mouth-throat under unsteady inspiration flow conditions

An excellent understanding of the airflow properties is critical to improve the drug delivery efficiency via the extrathoracic airway. The present numerical study focuses on the investigation the characteristics of important airflow structures such as the secondary vortices, the impinging jet and the recirculation zone under unsteady inspiration flow conditions in a circular idealized mouth-throat model using large eddy simulation (LES). Five inhalation cycles are simulated, the last one of which is analyzed in detail at five different times. Two times are chosen during the accelerating branch, one at the peak and two within the decelerating inhalation wave. The flow exhibits an extinct process of the flow transiting from laminar to turbulent during the accelerating phase and transiting back from turbulent to laminar in the decelerating phase. It is found that the flow is much more turbulent during the decelerating phase compared to the accelerating phase of the inspiration wave, which is associated with more smaller secondary vortices, a shorter and more unsteady laryngeal jet, a smaller and more unsteady recirculation zone, as well as an enlarged mixing zone. These differences during the unsteady inspiration require more attention in particular if particle transport and deposition in the upper airway are to be investigated.