Complicated Flow Field Research Articles

Deformable microswimmer in a swirl: capturing and scattering dynamics.

Inspired by the classical Kepler and Rutherford problem, we investigate an analogous setup in the context of active microswimmers: the behavior of a deformable microswimmer in a swirl flow. First, we identify new steady bound states in the swirl flow and analyze their stability. Second, we study the dynamics of a self-propelled swimmer heading towards the vortex center, and we observe the subsequent capturing and scattering dynamics. We distinguish between two major types of swimmers, those that tend to elongate perpendicularly to the propulsion direction and those that pursue a parallel elongation. While the first ones can get caught by the swirl, the second ones were always observed to be scattered, which proposes a promising escape strategy. This offers a route to design artificial microswimmers that show the desired behavior in complicated flow fields. It should be straightforward to verify our results in a corresponding quasi-two-dimensional experiment using self-propelled droplets on water surfaces.

Numerical and performance analysis of one row transonic rotor with sweep and lean angle

In this study, aerodynamic behaviors of swept and leaned blades were investigated. Axial and tangential blade curvatures impacts on compressor’s operating parameters were analyzed separately. A commercial CFD program which solves the Reynolds-averaged Navier-Stokes equations was used to find out the mentioned impact and the complicated flow field of transonic compressor-rotors. The CFD method that was used for solving flow field’s equation was validated by experimental data of NASA Rotor 67. The results showed that the compressor with curved rotors had higher efficiency, rotor pressure ratio and stable operating range compared to the compressor with un-curved rotors. Using curved rotors mostly had higher impact on the overall stable operating range compared to the other operating parameters. Operating range involves choking point and stall point that were changed separately by using of bended blade. For finding the detailed impact of sweep and lean angle on transonic blades, various forms of lean and sweep angles were exerted to basic rotor. It was found that sweep angles increased overall operating range up to 30%, efficiency up to 2% and pressure ratio up to 1%. Leaning the blades increased the safe operating range, the pressure ratio and efficiency by 14%, 4% and 2% respectively.

Assessment of unsteady-RANS approach against steady-RANS approach for predicting twin impinging jets in a cross-flow

AbstractA complex flow field is created when a vertical/short take-off and landing aircraft is operating near ground. One major concern for this kind of aircraft in ground effect is the possibility of ingestion of hot gases from the jet engine exhausts back into the engine, known as hot gas ingestion, which can increase the intake air temperature and also reduce the oxygen content in the intake air, potentially leading to compressor stall, low combustion efficiency and causing a dramatic loss of lift. This flow field can be represented by the configuration of twin impinging jets in a cross-flow. Accurate prediction of this complicated flow field under the Reynolds averaged Navier–Stokes (RANS) approach (current practise in industry) is a great challenge as previous studies suggest that some important flow features cannot be captured by the Steady-RANS (SRANS) approach even with a second-order Reynolds stress model (RSM). This paper presents a numerical study of this flow using the Unsteady-RANS (URANS) ap...

Conjugate heat transfer study on a ventilated disc of high-speed trains during braking

The heat dissipation of a ventilated disc on high-speed trains during emergency braking is studied to improve heat dissipation performance. The conjugate heat transfer method is employed to understand the distribution and variation of convective heat transfer coefficients on the disc surfaces during braking. Finite element models of the ventilated disc and the complicated air flow field under the train are built. Boundary conditions are derived based on real working conditions. Heat transfer simulation is carried out using the FLUENT computer code. Simulation results, including temperature rise of the disc, convective heat transfer coefficient distribution, and heat transfer rate, are presented and analyzed. Using materials with high thermal conductivity coefficients and reducing the heat transfer wall thickness of the disc are proposed to improve the heat dissipation performance of the disc based on the simulation results. Both methods are effective in improving the heat transfer rate of the disc with a 10% improvement in the improved thermal conductivity case and a 30% improvement in the reduced wall thickness case.

Discussion of “Discharge Coefficients for Baffle-Sluice Gates” by P. K. Mishra, Wernher Brevis, and Cornelia Lang

Discussion of “Discharge Coefficients for Baffle-Sluice Gates” by P. K. Mishra, Wernher Brevis, and Cornelia Lang

Conjugate Heat Transfer of an Internally Air-Cooled Nozzle Guide Vane and Shrouds

In order to assess the life of gas turbine critical components, it is essential to adequately specify their aerothermodynamic working environments. Steady-state analyses of the flow field and conjugate heat transfer of an internally air-cooled nozzle guide vane (NGV) and shrouds of a gas turbine engine at baseline operating conditions are numerically investigated. A high-fidelity CFD model is generated and the simulations are carried out with properly defined boundary conditions. The features of the complicated flow and temperature fields are revealed. In general, the Mach number is lower and the temperature is higher on the NGV pressure side than those on the suction side. There are two high temperature regions on the pressure side, and the temperature across the middle section is relatively low. These findings are closely related to the locations of the holes and outlets of the cooling flow passage, and consistent with the field observations of damaged NGVs. As a technology demonstration, the results provide required information for the life analysis of the NGV/shrouds assembly and improvement of the cooling flow arrangement.

Simulation and Understanding of the Aerodynamic Characteristics of a Badminton Shuttle

This paper presents a study in the simulation and understanding of the aerodynamic characteristics of a badminton shuttle, using Computational Fluid Dynamics. Modeled shuttle geometry was based upon a high quality synthetic shuttle, a Yonex Mavis 370. The study investigated the use of Reynolds Averaged Navier Stokes (RANS) simulation in comparison to Scale Resolving Simulation (SRS), for the prediction of the complicated flow fields that are associated with the bluff body aerodynamics of badminton shuttles. RANS are known to struggle with predicting large bluff body separations, in comparison to SRS modeling. However SRS modeling can require significant computational resource, and the models require careful application with consideration made to spatial and temporal resolution. It was found that unsteady RANS was capable of predicting comparable time averaged flow field data to the SRS models. Both approaches produced feasible drag coefficient values for the shuttle. However RANS was incapable of predicting the complicated turbulent vortex structures within the shuttle wake, which were captured by SRS.

Experimental Investigation of Impinging Shock – Cavity Interactions with Upstream Transverse Jet Injection

Mixing between the injected fuel and high speed free stream air is challenging at supersonic speeds. Placing cavities downstream of injection holes or slots addresses the problem of flame holding and stabilisation, however there are still open questions related to mixing enhancement, uniformity and efficiency. The present study examines experimentally the flow field interactions due to a transverse jet - cavity combination with shock impingement at supersonic speeds using PIV, Schlieren photography, and oil flow surface visualisation. The oblique shock lifts the shear layer over the cavity and combined with the instabilities generated by the transverse jet injection creates a highly complicated flowfield with numerous vortical structures. The interaction between the oblique shock and the jet leads to a relatively uniform velocity distribution within the cavity. The lifting of the shear layer is also believed to reduce the drag created by the cavity.

Aerodynamic characteristics of human movement behaviours in full-scale environment: Comparison of limbs pendulum and body motion

The aerodynamic effects of human movement can significantly influence the airflow motion and contaminants transmission in enclosed environments such as in aircraft cabins, cinema and conference room, and so it is necessary and important to study the characteristics of these aerodynamic effects. This work focuses on the aerodynamic characteristics of human movement behaviours including limbs pendulum and body motion. A thermal manikin is used in the corresponding environments to simulate these behaviours. The step frequencies of 20, 30, 40, 50 and 60 double steps per minute for limbs pendulum and the moving speeds of 0.5, 0.75, 1.0, 1.25 and 1.5 m/s for body motion are investigated in the experiments. In each case, the velocity distribution around the human body is measured by using hot-wire anemometers. The experimental results show that the characteristics of the velocity distribution induced by limbs pendulum depend on pendulum frequency and spatial location, and for body motion, it depends on moving speed, moving distance and spatial location. The analysis results show that body motion has a significant influence on the flow field, and its aerodynamic effects are greater than those due to limbs pendulum. When a human moves, more detailed profile of the human body leads to more complicated flow field in the nearby area.

Investigation of flow and heat transfer around internal channels of an air ventilation vest

The effects of a fabric’s ventilation and cooling are important to human comfort in the design of clothes. Persons in a high-temperature situation can experience imbalance of thermoregulation, which in turn results in the symptoms of heat exhaustion and heat stroke. The purpose of this research was to achieve cooling enhancement by designing air ventilation vests as an effective measure to control human thermoregulation in hot environs. In this study, the Taiwan Textile Research Institute devised a new air ventilation vest characterized by seven internal flow channels. The computational fluid dynamics software ANSYS/Fluent® was applied using the three-dimensional conservation equations of mass, momentum and energy to simulate the airflow and heat transfer phenomena within the internal channels. The incompressible turbulent flow was also treated with a standard k-ɛ two-equation model for turbulence closure. We compared the predicted steady axial velocities at the centers of channel exits with the measured data for software validation. The simulated results were employed to investigate the complicated flowfield and heat transfer phenomena around internal channels of the air ventilation vest. In practice, the design with the outlet arranged along the airflow direction produces a higher flow rate due to a relatively lower flow resistance, and thereby achieves better air-cooling outcomes. To enhance the cooling performance, simulations were also conducted to examine the influences of channel outlet design, inlet temperature and velocity on the convective heat transfer coefficient distributions over the inner channel surface of the vest.

Integrated assessment for route selection of river-crossing pipeline using structural and hydraulic approach

An optimised route design procedure for a river-crossing pipeline that minimises disturbance to the structure and stream environment during its lifetime is presented in this article. The work covers both the assessment of hydrological conditions and mechanical performance of the pipeline and its protective structure. The assessment techniques for crossings involve a numerical and analytical method for simple integrity estimation. The procedure provides a feedback link to select the optimised crossing route and inform the structural repair design step. As a case study, the proposed route selecting technique is applied to a river with an exposed section in Gyeonggi Province, Korea. Complicated hydraulic flow fields are apparent in this area due to the narrow shortcut directly upstream of the pipeline. In the scour state analysis, an installed encasement, and not an alternative route, exhibits sufficient structural robustness under flood conditions.

A-priori study of subgrid-scale models for the flow field in the rotor exit region of a centrifugal turbomachine

The present article deals with a-priori evaluation of two popular subgrid-scale (SGS) models, i.e. Smagorinsky and similarity, for the complicated flow field of a centrifugal turbomachine. Comparison of Smagorinsky and similarity models for various turbulent flows shows that accuracy of these models is inferior for complicated turbomachinery flow. The estimated SGS model coefficients, correlation coefficient among exact and modeled SGS quantities and the functionality between SGS stress/dissipation and resolved flow parameters are different features of SGS models which are examined for Smagorinsky and similarity models. The calculated model coefficients for the rotor exit flow are significantly smaller than their classical values to avoid over-estimation of SGS dissipation. Back-scattering of turbulent energy and spectral shortcut mechanisms are two possible reasons for this reduction of models coefficients. Investigation of instantaneous SGS dissipation shows that about 40% of flow samples in the rotor exit region have back-scattering of energy. This large density of back-scattering significantly reduces the performance of fully-dissipative models such as the Smagorinsky model. Joint probability density functions of exact vs. modeled SGS stress/dissipation show that the similarity model is capable of back-scattering prediction and has a considerably larger correlation coefficient than that of the Smagorinsky model. The present article shows that the Smagorinsky model performance improves in the presence of straining (in the jet–wake interacting regions) while the minimum correlation coefficient occurs in the core region of jets and wakes with smallest straining. Weak functionalities between Smagorinsky-modeled SGS stress/dissipation with the resolved strain rate tensor, particularly in the case of cross components, show the necessity for modifying Smagorinsky model in such a complicated flow field to allow for spectral shortcut and energy back-scattering mechanisms.

Analyzing the Anti-Noise Performance of NAIRT Used in Deflection Tomography with Noised Projections

A new iteration reconstruction technique is suggested, which is named nonlinear auto-adjusting iterative reconstruction technique (NAIRT). Its anti-noise performance used in deflection tomography was tested with its projections added noises. A complicated air flow field, called model, was simulated, and was projected according to deflection tomographic algorithm. Thereupon, the real projections were obtained. A Series of random noises at different strength level were produced using randG function. Then, the noises were added to the real projections linearly. So, a series of noised projections were acquired. According to deflection tomographic algorithm, the noised projections were inversely projected to reconstruct the model using NAIRT. The reconstructive effect at the end of each cycle iteration was recorded with mean square error (MSE) index. The iteration stopped after one hundred and three cycles. As the results: First, at the noise level of 60dB S/N, NAIRT could reconstruct the model by a decent accuracy. The MSE declined to 6.49×10-5 at the end of 103 iteration cycles. Second, at the noise level of 30dB S/N, NAIRT could yet reconstruct the model by the level of its profile. The MSE stabilized at 2.67×10-4 at the end of 103 cycles. Last, at the noise level of 100dB S/N, NAIRT could accurately reconstruct the model. The MSE declined to 3.49×10-5 at the end of 103 iteration cycles. Both the reconstructed images and MSE analyses demonstrated that NAIRT had wonderful anti-noise performance when it was used in deflection tomography.

Steel sections subject to a long-duration blast

This paper examines the dynamic pressure effects of a long-duration blast load on small-diameter circular and angular structural steel members for a Mach stem region where a shock wave travels at near the speed of sound. In complicated flow fields, where fully coupled analyses are not reasonably practicable, the correct application of translational drag loads is vital. The variability in drag coefficient for members of the same shape can differ greatly, owing to stagnation of the flow and the level of flow field obstruction. To investigate this effect, this paper details a simplified experimental trial conducted in the world's most powerful blast tunnel, the US large-blast thermal simulator at White Sands Missile Range, New Mexico. Four assemblies were examined based upon hollow steel sections mounted at different angles and positions. The experiment indicated a drop in drag coefficient for shielded circular members in close proximity of approximately 40%. The drag force on unshielded sections was lower in magnitude than initially expected; this was attributed to an absence of flow stagnation and reduced boundary layer effects. Comparative square and angled sections displayed approximately 50% higher values than circular members.

Drag Reduction Using Spiked-Aerodisk & Reattachment Ring for Hypersonic Hemispherical Bodies

Rockets and Shuttles flying at hypersonic speeds experience severe drag and aerodynamic heating. While blunting the fore-body helps distribute the heat load over a large area, this produces, however, substantial amount of drag. A number of studies were dedicated to understanding the mechanism of drag and heat mitigation with spikes and maximizing the performance. This study aims at alleviating the severe pressure and heating occurring at the reattachment point by the introduction of the Reattachment Ring; a ring mounted at the reattachment point. Using the commercial code FLUENT, to solve the complicated flow field, this study seeks to maximize the amount of the total drag and heat reduction using spike-diskspike assembly. The models are tested at Mach 6, 8 & 10 at altitude 25 km in standard atmosphere. The Reattachment Ring and the spike-disk-spike assembly add up synergistically to achieve a maximum drag reduction of 69.82%.

Effect of time step size and turbulence model on the open water hydrodynamic performance prediction of contra-rotating propellers

A growing interest has been devoted to the contra-rotating propellers (CRPs) due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibration. Compared with the single-screw system, it is more difficult for the open water performance prediction because forward and aft propellers interact with each other and generate a more complicated flow field around the CRPs system. The current work focuses on the open water performance prediction of contra-rotating propellers by RANS and sliding mesh method considering the effect of computational time step size and turbulence model. The validation study has been performed on two sets of contra-rotating propellers developed by David W Taylor Naval Ship R & D center. Compared with the experimental data, it shows that RANS with sliding mesh method and SST k-ω turbulence model has a good precision in the open water performance prediction of contra-rotating propellers, and small time step size can improve the level of accuracy for CRPs with the same blade number of forward and aft propellers, while a relatively large time step size is a better choice for CRPs with different blade numbers.

Interference effect on vortex-induced vibration in a parallel twin cable-stayed bridge

A vortex-induced vibration (VIV), amplified by an interference effect caused by two parallel decks, was observed in the upstream deck of a twin cable stayed-bridge. This represents the first case of such an observation in an actual long-span cable-supported bridge. The observed VIV was successfully reproduced in a wind tunnel. Both decks were equipped with vanes and the single deck alone showed an allowable performance, in terms of VIV, based on wind tunnel tests that were carried out. It therefore appears that the observed vibration was affected by the parallel arrangement of the decks. A particle image velocimetry technique was successfully applied to investigate the complicated flow field between the upstream and downstream decks. Alternating eddies were formed in phase with the upstream deck motion and were transmitted to the downstream deck introducing “Ω” and “℧” shaped flow fields as a result. Following the alternating eddies, upward and downward wind streams were in turn fed into the gap and these flows amplified the vibration in the upstream deck of the bridge. Several modifications of aerodynamic additives were not effective in reducing this VIV. However, an increase in structural damping effectively mitigated the vibration.

Numerical Simulation of Muzzle Flow Field of Gun Based on CFD

Gunpowder was released in an instant when the pill fly out of the shell during the firing, and then formed a complicated flow fields about the muzzle when the gas expanded sharply. Using the 2 d axisymmetric Navier-Stokes equation combined with single equation turbulent model to conduct the numerical simulation of the process of gunpowder gass evacuating out of the shell without muzzle regardless of the pill’s movement. The numerical simulation result was identical with the experimental. Then simulated the evacuating process of gunpowder gass of an artillery with muzzle brake. The result showed complicated wave structure of the flow fields with the muzzle brake and analysed the influence of muzzle brake to the gass flow field distribution.

Real 3-D flow fields reconstruction by interferometric volume computerized tomography (IVCT)

Optical computerized tomography (OCT) has been considered as a powerful diagnostic tool for complicated flow fields as it is nonintrusive and instantaneous. However, the traditional reconstruction methods are 2-D, which have to split the whole field into several parallel slices. In this paper, a real 3-D reconstruction via interferometric volume computerized tomography (IVCT) is presented and discussed, which could reconstruct the measured flow field as a whole. In order to verify this, the temperature distribution of an actual flow field based on 3-D FBP by employing the IVCT is reconstructed. Compared with the traditional 2-D reconstruction methods, 3-D IVCT could describe the object more truthfully. In a word, the involved results prove that 3-D IVCT is feasible and reasonable.

Numerical Simulation on Head-On Binary Collision of Gel Propellant Droplets

Binary collision of droplets is a fundamental form of droplet interaction in the spraying flow field. In order to reveal the central collision mechanism of two gel droplets with equal diameters, an axi-symmetric form of the Navier-Stokes equations are firstly solved and the method of VOF (volume of fluid) is utilized to track the evolution of the gas-liquid free interface. Then, the numerical computation model is validated with Qian’s experimental results on Newtonian liquids. Phenomena of rebound, coalescence and reflexive separation of droplets after collision are investigated, and structures of the complicated flow fields during the collision process are also analyzed in detail. Results show that the maximum shear rate will appear at the point where the flow is redirected and accelerated. Rebound of droplets is determined by the Weber number and viscosity of the fluid together. It can be concluded that the gel droplets are easier to rebound in comparison with the base fluid droplets. The results also show that the alternant appearance along with the deformation of droplets in the radial and axial direction is the main characteristic of the droplet coalescence process, and the deformation amplitude attenuates gradually. Moreover, the reflexive separation process of droplets can be divided into three distinctive stages including the radial expansion, the recovery of the spherical shape, and the axial extension and reflexive separation. The variation trend of the kinetic energy is opposite to that of the surface energy. The maximum deformation of droplets appears in the radial expansion stage; in the case of a low Weber number, the minimum central thickness of a droplet appears later than its maximum deformation, however, this result is on the contrary in the case of a high Weber number.