Unsteady Numerical Analysis Research Articles

Investigating the unsteady flow mechanism at the lip of a ducted fan in crosswind

The unsteady numerical analysis method was utilized to investigate the unsteady flow structure of a contra-rotating ducted fan under crosswind conditions. The development and evolution mechanism of the lip vortex under the combined action of crosswind and rotor were studied. Two perspectives, namely the time-averaged flow field and unsteady time-frequency analysis, were employed for the examination. The results indicate that the unsteady aerodynamic forces exerted on the lip of the ducted fan are primarily influenced by four sets of frequencies, ranked in descending order of magnitude: 1 BPF (Blade Passing Frequency) of the upstream rotor, 1 BPF of the downstream rotor, the combined effect of the 1 BPF of the upstream rotor and 1 BPF of the downstream rotor, and 2 BPF of the upstream rotor. The maximum velocity occurs at the position where the inner surface of the windward side lip is inclined 40° from the freestream velocity direction, and a stable separation vortex is formed below this region. The lip separation vortex triggers the generation of blade suction side separation vortex in the upstream rotor, and its periodic formation, growth, shedding, and dissipation are the primary factors contributing to the unsteady flow. The research findings lay the groundwork for further advancements in active and passive flow control technologies under crosswind conditions.

Evolution of unsteady secondary flow structures near the onset of stall in a tip-critical transonic axial flow compressor stage

The aerodynamic stability of an axial compressor stage depends on the rotor and stator. However, due to the specific design requirements, the evolving flow field and the resulting secondary flow structures are different in both components. This study investigates the evolution of dominant secondary flow structures occurring in the rotor and stator of a tip-critical transonic compressor stage at the near-stall condition using unsteady numerical analysis and validates performance characteristics with the experimental data. The investigation reveals that the presence of rotor tip shock creates a large difference in the pressure gradient across the pressure surface and the suction surface, intensifying tip leakage flow and shock-induced boundary layer separation. The higher incidence angle near the hub leading edge creates a local separation and reattachment zone. The radial pressure gradient causes the low momentum flow from this local separation zone to migrate radially upward. This migrated flow interacts with the tip leakage flow and separated blade boundary layer, eventually creating a colossal recirculation zone and subsequent rotor blockage of around 46%. The increasing streamwise adverse pressure gradient pushes the tip shock upstream, and at the onset of the stall, the flow directly separates from the rotor leading edge avoiding the shock formation. The stator flow field is dominated by the asymmetric hub corner separation induced by the streamwise adverse pressure gradient and the tip corner separation caused by the vortex structures convected downstream from the rotor tip region leading to stator blockage of around 48%. Along with the blade passing frequency, four other dominating frequencies (0.06×BPF, 0.12×BPF, 0.44×BPF, and 0.84×BPF) related to the aerodynamic instabilities are observed at the inception of stall.

Unsteady Numerical Analysis of a Dual-Mode Scramjet Combustor with a Cavity

Unsteady Numerical Analysis of a Dual-Mode Scramjet Combustor with a Cavity

Assessment of unsteady cavitating flow around modified NACA4412 hydrofoil

Cavitation is a complex two-phase flow phenomenon that includes unsteady characteristics. This work performs an unsteady 2D numerical analysis of cavitating flow on (i) cambered NACA4412 hydrofoil and (ii) the same hydrofoil geometry modified with dimple. The analysis is done with the objective of estimating the effect of the dimple on the hydrodynamic performance of the cavitating hydrofoil. The numerical simulations are performed using a flow solver by employing Realizable κ-ε turbulence model and Zwart-Gerber-Belamri (ZGB) cavitation model. The hydrodynamic performance is quantified in terms of coefficient of lift, coefficient of drag, lift to drag ratio, static pressure distribution, and cavity length at various cavitation numbers and angles of attack. From the assessment, it is observed that for 8⁰ angle of attack, the coefficient of drag on NACA4412 hydrofoil with the dimple is less as compared to NACA4412 without dimple. For higher angles of attack (α = 12⁰&16⁰) NACA4412 hydrofoil with the dimple has higher coefficient of lift compared to NACA4412 without dimple.

지면효과 속에 놓인 이중로터의 공력 특성에 대한 수치적 연구

Multi-copters are used in various fields as unmanned aerial vehicles. Consequently, the stability of the operation conditions of the unmanned aerial vehicle and the improvement of aerodynamic characteristics are required. Therefore, this study is intended to examine the effect of the wake interaction between rotors in ground effect when a set of double rotors is positioned adjacent to the ground through an unsteady numerical analysis. For this reason, the rotor is positioned adjacent to the ground, and the thrust ratio is measured while changing the distance between the rotors which are rotating at a fixed rotational speed. With the ground height as z and the rotor radius as R, the smaller the height ratio(z/R), the higher the thrust. As the distance between the rotors became smaller, the collision and interaction between wing tip vortices of the rotor blades got stronger, and the flow was not properly propagated into the direction of the wake. It is shown that when the rotor gets closer to the ground, the flow could not be fed back in after flowing along the ground, so the thrust ratio due to the ground effect tends to decrease. With these results and analysis, the effect of wake interference between rotors on the ground effect could be confirmed.

Magnetorheological Fluid of High-Speed Unsteady Flow in a Narrow-Long Gap: An Unsteady Numerical Model and Analysis

To investigate the unsteady flow field generated by magnetorheological (MR) fluid of a high-speed unsteady laminar boundary layer flow in a narrow-long gap of the magnetorheological absorber (MRA), a new unsteady numerical model is proposed. The gap has magnetic-field-activated and inactivated regions, with MR fluid flowing as bi-viscous (non-Newtonian) and Newtonian fluid. The unsteady flow field is described by the unsteady incompressible governing partial differential equation (PDE) and initial-boundary conditions with the moving boundary. The space-time solution domain is discretized using the finite difference method, and the governing PDE is transformed into implicit partial difference equations. The volume flow rate function is constructed to solve numerical solutions of pressure gradient and fluid velocity based on mass conservation, the continuity equation, and the bisection method. The accuracy of unsteady numerical model is validated by the experiment data. The results show that the fluid acceleration profiles along the gap’s height are non-uniform distribution. Further, the volume flow rate and excitation current has a significant impact on the dynamic distribution of fluid velocity profiles, and the moving boundary makes the flow field asymmetric about the central plane. Furthermore, as the transition stress increases, the thickness of the pre-yield region in the activated region increases. There is also a transition flow phenomenon in the activated region as the volume flow rate increases. Finally, the unsteady numerical model has good stability and convergence.

자동차 비배기 미세먼지 집진기 개념설계 및 수치해석을 통한 집진기 위치 선정 연구

Brake and tire wear emissions from vehicles are generated by friction between tire-asphalt and brake pad-disk. Although the proportion of brake and tire wear emissions among the fine dust generated in the entire automobile emission is currently low, it is expected that most of the fine dust will eventually be caused by non-exhaust sources. Despite this expectation, methods of reducing brake and tire wear emissions have not been actively studied. In the present article, a fine dust collecting system for brake and tire wear emissions is developed and tested. Feasibility tests were conducted to evaluate the overall performance of fine dust collector. Using the developed fine dust collector, an average of 67.5% of fine dust can be collected. The location of installation was decided by investigating the complex flow around a rotating wheel using the unsteady numerical analysis technique. As a result of the analysis of the flow around the wheel, the velocity of the wheel rotating counterclockwise and the flow trying to rotate in the clockwise direction compete with each other, resulting in a stagnation point where the flow speed is slowed down at a certain portion. If an electric dust collector is installed near the stagnation point, it is expected that the collection efficiency of the dust collector becomes high.

Simulation and Experiments on a Valveless Micropump With Fluidic Diodes Based on Topology Optimization

Micropumps are microelectromechanical system (MEMS) devices that pump small quantities of liquids. They are used in many applications such as electronics cooling and drug delivery. Valveless fluid diode micropumps with no moving parts have recently attracted great interest in the MEMS community. In this paper, topology optimization is used to design two-dimensional, fixed-geometry, fluidic diodes of high diodicity, which is the ratio of pressure drops of forward to reverse flows. One of the fluidic diodes, of the Tesla type, shows a diodicity of over five. Another is of the nozzle-diffuser type. Both are experimentally and computationally demonstrated herein. Then the numerical simulation was applied to simplify the structures, and the two-dimensional geometry was converted into three-dimensional model for micropumps. Three-dimensional and unsteady numerical analyses of micropump fluid flow with optimized diodes were conducted for pumps of each of the two diode designs. The micropump with the Tesla-type fluidic diode reached a measured flow rate of 34 ml/h, consistent with the computed results and 2.2 times that of the nozzle-diffuser type micropump. The performance results show a high dependence on internal channel geometry. The two types show highest flow rates with an internal channel thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200 ~\mu \text{m}$ </tex-math></inline-formula> . Demonstrated good repeatability and precise flow control show positive prospects for application. [2021-0144]

多目的最適化による遠心ポンプの低流量域における右上がり不安定特性の改善

Centrifugal pumps, which are widely used in industry for transporting liquids in plants and in water purification systems, and it is necessary to improve the instability characteristics at extremely low flow rate conditions. The negative slope of the characteristic curve is the evidence of instability of the flow, and it must be based on the local reverse flow zone and pre-whirl at impeller inlet. It is possible to predict the negative slope of the characteristics by unsteady numerical analysis at low flow rate, however, optimization design search requires many numerical analysis of individuals, it is not realistic solution to use the unsteady numerical analysis for the optimization.

５つの山頂部をもつ平板コルゲート翼の翼表面圧力特性

A two-dimensional unsteady numerical analysis of a corrugated airfoil was performed to clarify the variation of flow around the airfoil as a function of leading-edge angle. A corrugated airfoil with five crests was made to follow the outline of NACA4412, and three types of airfoils with leading edge angles of 25° and 35° were selected as the target airfoils. The Reynolds number was set to 1.0 × 104. Comparison of NACA4412 and the three types of corrugated blades showed that the lift coefficient was highest for NACA4412 at angles of attack of 4° or less and was significant for the corrugated blades at angles of attack higher than that. On the other hand, the drag coefficient was the lowest for NACA4412 at all angles of attack. Comparison of corrugated blades with different leading-edge angles revealed that at low angles of attack of 6° or less, the circulating flow in the first valley has a greater effect on aerodynamic characteristics than that in the second valley. On the other hand, at high angles of attack of 10° or more, flow cancellation was observed from the leading edge to the first crest of the upper surface of the wing, which is one of the reasons for the improved wing characteristics.

A numerical analysis to determine wake recovery distance for the longitudinal arrangement of hydrokinetic turbine in the channel system

ABSTRACT Hydrokinetic technology is developing rapidly from model to prototype testing. The principle of hydrokinetic technology involves the passage of flow through the hydrokinetic turbine, which reduces the velocity downstream of turbine due to the energy extraction and obstruction created by turbine operation. This reduced velocity regains its initial velocity after certain distance known as wake recovery distance due to mixing regime and available discharge. This study aims to analyze the wake recovery distance under different flow velocities considering various dimensions of turbine. In order to study the impact of turbine operation on the flow condition, a 3D unsteady and rotational numerical analysis has been carried out by incorporating free surface effect. It is a first prototype based study and the analysis has been carried out on actual dimensions of the turbine taking 0.5, 1.0, 1.5, and 2.0 m dia. operating at varying water depth (2, 3, 4, 5, 6, and 7 m). Impact of velocity, turbine diameter and water depth on wake recovery distance is analyzed in the study to define the distance between two units along the channel length under different operating conditions. The distance between two units should be the summation of upstream (back water effect) and downstream (wake recovery distance) disturbance created by the turbine operation. It is observed that more than 90% of initial flow velocity gets recovered at ‘25D’ downstream of rotor.

Analysis of Flow Field Characteristics and Pressure Pulsation in Horizontal Axis Double-Runner Francis Turbine

Horizontal axis double-runner Francis turbines have great advantages in the development of small hydropower plants, but the arrangement of double runners aggravates the complexity of the water flow between runners, and the mutual influence of the two runners cannot be ignored. In order to explore the relationship between the performance and the internal flow field and investigate the pressure pulsation characteristics of the double-runner Francis turbine, the steady and unsteady numerical analysis of the full flow channel of a prototype turbine was carried out based on the Realizable k-epsilon model and the polyhedral mesh method. The results show that the relationship between the average efficiency of the two runners and the flow difference between the runners is negatively correlated. As the flow rate difference between the runners on both sides increases, the average efficiency of the runners decreases. The draft tube flow of a horizontal-axis turbine has a profound effect on the flow field characteristics in the runner. When the working conditions change, the turning and converging timing of the mainstream at the outlet of the two runners will change. The movement of the mainstream promotes the change in location of the dead water zone. The existence of the vortex zone makes the pressure distribution at the outlet of the runner uneven, which is an important reason for the asymmetry of the flow in the runner. The analysis of pressure pulsation and its frequency spectrum shows that when the working conditions change, the low-frequency, strong pressure pulsation area on the surface of the guide vane will regularly migrate between the two runners, while the high-frequency pressure pulsation that occurs in the bladeless zone will dissipate in the runner. The doubling of the blade frequency on the pressure surface and back surface of the blades gradually attenuates with the increase of frequency. The pressure pulsation attenuation on the surface of the high-position blade conforms to the linear law, and the attenuation of the pressure pulsation on the surface of the low-position blade conforms to the exponential law. The research in this paper provides a certain reference value for revealing the flow field mechanism and pressure pulsation characteristics of the double-runner Francis turbine.

Revisit on the natural convection from horizontal multi-channel rectangular-fin heat sinks

Unsteady numerical analysis is conducted for horizontal 16-channel rectangular-fin heat sinks with heat sink length of L = 127 mm. Beside normal heat sinks, those with guard-fin blocks at both sides as in the experiments of Harahap and McManus (1967) and Harahap and Setio (2001) are also studied to clarify the effects of guard-fin blocks on the flow characteristics and heat transfer performance as compared to normal heat sinks. The inner channels of a normal heat sink with low fins of H = 6.4 mm exhibit moderate sliding oscillatory motions in both directions along and normal to the channel, due to strong interaction between the end flow and the side flow. The outer channels are obviously suppressed by the side flow. The guard-fin blocks can relieve the side-flow suppression on the outer channels of the low-fin heat sink. The average heat transfer coefficients for the fin channels are higher than those of normal heat sinks by about 20%. For a high-fin heat sink with H = 25 mm, the plume velocity increases due to stronger buoyancy so that the end-flow is dominant, and the plume appears to be a weakly oscillating single-chimney flow. The guard-fin blocks are of little effect for high-fin heat sinks. The traditional explanation for lower h‾s of sliding-chimney flow in that the low-pressure middle recirculation region draws in warmer air from above is shown to be incorrect when the cool side flow is involved. The reason for low h‾s is mainly because the low-h recirculation region in the middle of the heat sink is broader for a sliding-chimney flow.

Assessment of the Influence of Boundary Layer Ingestion (BLI) on the Axial Fan

Boundary Layer Ingestion (BLI) is a technology which is effective to reduce fuel consumption of future commercial aircrafts. However, the inlet distortion caused by BLI has a negative on the engine performance. This paper investigates the effect of the inlet total pressure distortion on the aerodynamic performance of the fan rotor. Unsteady numerical analysis was conducted on JAXA TechClean Fan with circumferential distortion caused by BLI. As the result, under the high distorted condition, the peak efficiency of the rotor decreases by 0.622 points. It is found that the main factor of the efficiency reduction is tip-wall passage vortex.

Numerical study on ignition and failure mechanisms of hydrogen explosion accident in Fukushima Daiichi Unit 1

This paper studied ignition and failure mechanisms of hydrogen explosion accident in the container building of Fukushima Daiichi Nuclear Power Plant Unit 1, which occurred a day after the Great East Japan Earthquake in 2011. To understand the mechanism of hydrogen explosion, an unsteady three-dimensional numerical analysis of the flow field in the container building was conducted in real time by assuming that the high-temperature hydrogen gas (500 °C) issued from the head flange of the reactor vessel. The numerical results show that the hydrogen gas spread not only on the 5th floor but also on the 4th floor of the container building covering the stairs. It was found that the hydrogen concentration on the 5th and 4th floors was highly uniform owing to the long real-time accumulation (15 h) of hydrogen, and it spread gradually from the 5th to the 4th floor through the stair. As a result, the hydrogen concentration reached the ignition level on the 4th floor (5%), which could result in the ignition of hydrogen by a tank heater on the 4th floor and the flame propagated to the 5th floor, where fast deflagration and detonation might occur because of the high concentration of hydrogen gas (19%). This scenario is consistent with the observations of failure mechanism after the explosion accident.

Numerical evolution of unsteady wake characteristics of H-type Darrieus Hydrokinetic Turbine for a hydro farm arrangement

In this study, the CFD method is carried out in the wake region of an H-type Darrieus hydrokinetic turbine. The hydrodynamic investigation was examined with symmetric and non-symmetric foil shape to maximize output power coefficient and wake recovery. The SST (Shear Stress Transport) k-ω turbulence model was used on 2-D and 3-D unsteady numerical analyses. The obtained power coefficients from 2-D and 3-D analyses at different Tip Speed Ratios (TSR) were validated by comparing with the experimental data from the literature. As the experimental study, the maximum power coefficient was obtained at TSR=1.05 for a 3-D analysis of the NACA0018 foil configuration of the hydrokinetic turbine. It is found that the results of 2-D and 3-D calculated by the SST k-ω are in good agreement with the experimental data in terms of power coefficients curve trend at investigated TSR range between 0.9–1.4. The difference between the power coefficient results of 3-D and experimental data varies between 1.8-11% ranges. The performance and flow characteristics of the wake region were also compared in detail for different foil by CFD. Therefore, the SST k-ω model can be preferred as an acceptable model to analyze the performance and flow characteristics of hydrokinetic turbines.

Influence of curtain plates on the aerodynamic performance of an elliptical bladed Savonius rotor (S-rotor)

The S-rotor is a drag driven wind turbine, having lesser performance coefficients. Its various geometric and aerodynamic parameters have been optimized to improve its performance. Several rotor profiles and augmentation methods have also been progressed to improve its performance further. The augmentation method reduced the negative drag generated in the S-rotor. Very recently, the elliptical bladed profile has shown higher performance than the other profiles. Hence, in this study, the elliptical bladed profile has been considered along with the curtain plates augmenter before the rotor blades. The 2D unsteady numerical analysis is conducted through the SST k–ω turbulence model. The simulations are also performed for the elliptical profile without the curtain plates for direct comparison. From the 2D unsteady results, it was found that the elliptical bladed profile with curtains plates has higher aerodynamic performance (CDavg = 2.60 and CLavg = 0.65) than the elliptical profile without curtains (CDavg = 1.43 and CLavg = 0.95). Hence, there is an improvement of CDavg by 81.81% in the elliptical profile with curtain plates than the profile without the curtain plates.

Research on the influence of highly loaded design method on the stability of helium compressor

The highly loaded design method of helium compressor can effectively resolve the problem of helium compression in high temperature gas cooled reactor (HTGR). However, the alteration in design method has led to the variation of its characteristics at the same speed. In order to resolve this issue, in this paper, a three-dimensional, multi-passage unsteady numerical analysis is conducted to develop the understanding of the performance of a 1.5 stages highly loaded helium compressor. At first, the influence of the highly loaded design method on the unsteady flow of helium compressor is analyzed at the design point. Then, the instability characteristics of highly loaded helium compressor are studied by analyzing the flow field near stall point. Finally, the main factors causing the instability of highly loaded helium compressor are discussed. The result shows that radial separation which is caused by the inverse pressure gradient at the stator root is the main factor causing instability in highly loaded helium compressor.

Thermodynamics Investigation and Spike-stall Identification Based on Energy Loss of Centrifugal Compressor

Although quantities of investigations concerning the stall characteristics and thermal-dynamics of centrifugal compressor were carried out, rare discussions, however, were employed on the relationship between energy loss and flow unsteadiness, especially the losses originating from spike-type stall in the impeller. In the present work, a combined unsteady numerical and experimental analysis of energy loss features under stall condition in the impeller of centrifugal compressor was presented. Firstly, an energy loss prediction model based on one-dimensional equations considering multiple components was proposed and validated against experimental data. The dominating loss mechanisms in the impeller were quantitatively evaluated and the corresponding effect of each loss on various operating conditions was measured by proposing a weight coefficient. In order to prove the possibility of the potential application of the present approach, the weight coefficient method was also employed in another centrifugal compressor and the results were well validated. Then the incidence loss, which has the leading three weight coefficients during the occurrence of stall, were investigated with the distribution of inflow angle and entropy near stall condition. Then, applying blade loading variations and vortex transport analysis allowed the relevance of blade loading loss and clearance loss to be taken into account. Finally, with the synergy angles introduced, the criteria of spike stall through losses identification and qualification were established. The results imply that friction loss, clearance loss and incidence loss account for the major part of the total losses with the weight coefficient of 48%, 29% and 19% near stall condition. However, the weight coefficient of incidence loss increases remarkably with those of other losses decreasing from design condition to stall condition, indicating the significant contribution of incidence loss to the occurrence of stall. Correspondingly, the dramatic increase of inflow angle at limited circumferential region can be treated as a general criterion for spike-stall. The influence of tip clearance leakage on the leading edge of splitter blade results in the decrease of blade loading at reduced mass flow. Based on the synergy angle distribution, mutation region of field synergy angle accompanied with the increase of flow resistance at the impeller inlet may lead to a more efficient determination of the spike-stall for compressor impeller.

Effect of Blade Outlet Angle on Acoustics of Marine Centrifugal Pump

To research the effect of blade outlet angle on acoustics of marine centrifugal pump, the unsteady numerical simulation analysis of the four models is carried out and the dipole source of pump shell is extracted. The constraint modal of pump, interior sound pressure level (SPL), exterior acoustic directivity and sound power level are obtained. The results show that five and six SPL peaks appear at BPF and blade frequency multiplier in the pump inlet and outlet, respectively, the SPL peak value is the largest at blade passing frequency (BPF), and the interior acoustics propagates to inlet and outlet directions, and to outlet more significantly. The exterior acoustic directivity in the four models is the same in all directions, and the blade outlet angle has little influence on the changes of exterior acoustic directivity. The SPL at BPF and 2BPF in Y = 0 plane is significantly smaller than that in X = 0 and Z = 0 planes, and the difference of maximum SPL at BPF is 7.2 dB and 8.0 dB and that at 2BPF is 7.6 dB and 6.4 dB, respectively. The sound power level is maximum at 2BPF, and that at the BPF and 2BPF increases first and then decreases with the blade outlet angle increases. When the blade outlet angle is 40o, the interior and exterior acoustic performances are better. The experimental value of the SPL in model 35 is basically consistent with the simulated value, which fully indicates that the numerical simulation analysis has high accuracy and provides support for the credibility of the acoustic simulation results.