Search

New types of bladeless wind turbines and electricity generation are currently one of the most interesting topics in engineering. Electricity generation using structural vibration due to the resonance phenomenon is the concept behind a vortex bladeless turbine. The present study numerically investigated the effects of the drag force on the body frequency of an oscillating bladeless wind turbine. A two-dimensional numerical simulation was performed for a cylinder with a semi-circular cross-flow cross-section in two different cases. This research was conducted for both uncontrolled and controlled oscillating cylinders. The controlling process was performed using a pair of ring magnets as springs with a variable coefficient. The flow field, vibration, vortex shedding, structural frequencies, and resonance phenomena were studied in this research. Finally, the controlled and uncontrolled frequencies of the cylinder are explored, and the power spectra for various velocities are analyzed in two different states, namely, with and without a tuning system. From the results, it can be concluded that the usage of the controlling system in these turbines can significantly regulate the oscillations and increase the frequency value by limiting the vibration amplitude. According to this principle, it can be inferred that increasing the frequency of fluctuations greatly increases the production capacity of these turbines.

Pumped storage units are the main parts in China’s power construction, as a hot issue concerned by the industry. The pump turbine involves the two-way flows and a multiple condition operation, and its operation flow pattern is very complex. The particle image velocimetry (PIV) is a very effective test technique to determine the internal flow field of pump turbines. This paper discusses the key problems of the pump turbine, based on the PIV experiments under typical conditions of the pump turbine, especially for problems such as the S-shape problem, the hump problem, the pressure fluctuation problem and the cavitation problem. In the internal flow fields under typical conditions are determined. The vortices induced and their development are observed in the PIV test. The flow phenomenon is shown at each operating point. The typical problems of the pump turbine are closely related to the vortex distribution in the internal flow field. From the PIV test results under several working conditions and from the comparisons between the optimal condition and the part load condition, it is seen that the vortex distributions are very different. Vortices at the vane-less area between the guide vane and the runner are closely related to the strong pressure pulsation, the first hump and the S-shape curve. From the PIV results of the cavitation working points, it is seen that the flow angle is changed in the vane-less region and the runner leading edge because of the cavitation bubbles and that the flow angle deviates from the optimal setting angle. From the computational fluid dynamics (CFD) result of the second hump working points, it is concluded that the vortex shedding on the runner leading edge is the main cause of the second hump.

Circular section tubular members with smaller wind load shape coefficient and higher stability are widely used in ultra-high-voltage (UHV) transmission towers. However, the tubular members, especially those with a large slenderness ratio, are prone to vortex-induced vibration (VIV) within a specific wind speed range. The sustained vibration of members can easily cause fatigue failure of joints and threaten the operational safety of transmission lines. Consequently, a novel countermeasure for the VIV of tubular towers using a new type of radial spoiler is proposed, whose mechanism is to change the vortex shedding frequency by destroying the large-scale vortexes into small-scale vortexes. Then, the parametric analysis of different variables is carried out based on the orthogonal experiment and numerical simulation, including the height H and length B of the spoiler and the distance S between adjacent spoilers. The results show that the above three parameters all have significant influences on vortex shedding frequency. Additionally, a practical design method of the new radial spoiler is proposed, and the recommended values of H, B, and S are 1D∼2D, 1.5H∼3H, and 5D∼12.5D, respectively, where D is the diameter of the tubular member. Finally, a numerical verification of the suppression effects is carried out, demonstrating that the proposed quick design method is simple and reliable, which can be widely used in the VIV design of tubular towers.

Turbulent phenomenon in braided rivers is much more complex compared to the straight and meandering rivers. The turbulent flow structure at locations upstream and downstream of the mid-channel are analysed experimentally and numerically by plotting the vertical profiles of velocity and turbulent parameters. Fluent Computational Fluid Dynamics (CFD) code is used to satisfactorily validating the experimental measurements. Reynolds stress model (RSM) is used in the numerical simulation as a turbulence model. The observation of the vertical profile of turbulent parameters found the submergence ratio greatly influences the flow structure in the vicinity of bar. The effect of submergence ratio (ratio of bar height to flow depth) on the flow structure is analysed by using the depth-wise profile of velocity and turbulent parameters. The studies concluded that: (1) For region upstream of bar, the low value of longitudinal velocity and negative value of vertical velocity leads to the region susceptible to scouring; (2) The bar height has a significant effect on the flow turbulence in its vicinity. The value of total turbulent intensity at points downstream of mid-channel bar is much more as compared to upstream points. This is mainly due to the vortex shedding in the downstream region of bar; (3) RSM CFD code is satisfactorily validating the depth-wise profile of turbulent and velocity distributions yielded from experimental results. 4) The sweep event is dominant in the near bed region (z/h < 0.1) for sections upstream of bar. For region near (z/h = 0.1), there is canceling effect (sweep vs ejection) which leads to the value of Su and Sw tends to zero in that region; 5) For points downstream of bar at region (z/h < 0.1), the positive value of Su and Sw indicate the dominance of outward interaction event.

Vortex shedding phenomenon behind bluff bodies and its destructive unsteady wake can be controlled by employing active and passive flow control methods. In this quest, researchers employed experimental fluid dynamics (EFD), computational fluid dynamics (CFD) and an analytical approach to investigate such phenomena to reach a desired outcome. This study reviews the available literature on the flow control of vortex shedding behind bluff bodies and its destructive wake through the modification of the geometry of the bluff body. Various modifications on the bluff body geometries namely perforated bluff bodies, permeable and porous mesh, corner modification and wavy cylinder have been reviewed. The effectiveness of these methods has been discussed in terms of drag variation, wake structure modifications and Strouhal number alteration.

The measurement of flow in fluidic channels is an essential task required for the proper operation of devices in many interesting applications. In this work, an optical fiber sensor based on a single mode - multimode – single mode (SMS) fiber structure is used to measure the flow in a millimeter-sized Polydimethylsiloxane (PDMS) fluidic channel. The measurement is based on the phenomena of vortex shedding, where the immersed fiber section acts as an obstructing strut. The sensing principle assumes slight fiber bending with vortex shedding, resulting in intensity variations of the light transmitted through the fiber sensor. To the best of our knowledge, this is the first time modal interference in an SMS fiber structure is explored for flow detection in a microfluidic device. The flow-related shedding frequency was detected through intensity variations of the transmitted light at a suitable laser wavelength. The linearity of the detected frequency with flow rate is confirmed over the investigated flow range with high R2 values up to 99%. The frequencies detected are compared to the theoretically expected frequencies based on the theory of vortex shedding past a rigid strut. Nevertheless, their relation to the flow velocity is found to deviate in slope from that of the theoretical relation, which could be due to the interaction of the flow with the flexible optical fiber shedding strut. The good linearity of this sensor makes it a promising component for macro and microfluidic flow measurement in many applications.

An overview of previous researches related to the problem of flow around a bluff-body, using experimental and numerical methods, is presented in the paper. Experimental investigation was performed by a laser doppler anemometer, measuring velocity components of the water flow around a smooth sphere, and a sphere with dimples in square channels. Measurement results in subcritical velocity flow field, velocity fluctuation components, lift, drag and pressure coefficients, and 2-D Reynolds stress at quasi-stationary flow are conducted using 1-D laser doppler anemometer probe. The obtained experimental results are compared with numerical simulations, which are performed using the ANSYS-CFX software. For the numerical simulations of quasi-steady-state flow, k-? turbulent model was used, while for numerical simulation of unsteady fluid-flow and for the comparison of results related to the eddy structures, vortex shedding and Reynolds stresses, detached eddy simulation were used. Since the obtained results of experimental and numerical investigation of flow around smooth sphere and sphere with dimples showed good agreement, the considered flow problem was expanded by introducing the influence of a transverse magnetic field with a slight modification of the electrical conductivity of the working fluid. The other physical properties of the fluid remained the same, which also corresponds to realistically possible physical conditions. Numerical simulations were performed for three different values of Hartmann number and very small values of Reynolds magnetic number (inductionless approximation). Comparisons and analyzes of the results were made for the cases containing a magnetic field and those with an absence of a magnetic field.

In depth studies on the vortex formation process as well as the periodic behavior of the reattachment profile have been mainly sparked by broad discussions on the flow topology. While the topology methodology is quite well established, the majority of airfoil types have still not been covered. In this study, six low Reynolds number airfoils were considered to investigate the separation bubble structure, vortex shedding and reattachment point. This was done by means of computational fluid dynamics simulations involving the airflow passing several Eppler and Selig models. The method was validated against an established mathematical relationship to indicate that both simulations and analysis technique were reliable. It was observed that the phenomena under investigation directly affect the airfoils aerodynamic performance. In particular, vortex shedding and big reattachment length contribute to the minimum lift. Moreover, high number of vortices in the separation bubble as well as secondary separation bubble result in maximum drag. The results which correspond to low Reynolds number airfoils are applicable in micro aerial vehicles field.

Dynamic stall often occurs under conditions of simultaneous unsteady pitching and surging (e.g., rotorcraft and wind turbines), but many models employ a dimensionless time base that implicitly assu...

In this study, the aerodynamic characteristics of tall buildings with corner modifications (e.g., local wind force coefficients, mean pressure distributions, normalized power spectrum density, and extreme local pressure) were examined. Wind tunnel experiments were conducted to measure the wind pressures on building models with different heights and recessed corners of different ratios. At a wind direction of a = 0° (i.e., wind blowing on the front of a building), corner modifications effectively reduced wind forces in all cases. Specifically, small corner modification ratios reduced wind forces more effectively than their larger counterparts. However, corner modifications resulted in extreme local pressure on building surfaces. In addition, for small corner modification ratios, the probability of extreme local pressure occurring at a = 0° was high. This probability was also high for large corner modification ratios at a = 15° (i.e., wind blowing slightly obliquely on the front of a building) because wind blowing obliquely creates substantial vortex shedding on one side surface and extreme negative pressure over one building side surface. Results of computational fluid dynamic modeling were adopted to determine details of the aerodynamic characteristics of tall buildings with corner modifications.

De-icing fluid flow-off from a flat plate subjected to an accelerating airflow was studied both experimentally and using computational fluid dynamics (CFD) simulations. The experimental tests were ...

Aeroacoustic pressure oscillation is one of the important challenges in segmented solid rocket motors with high slenderness ratio. The reason for these oscillations can be searched in vortex shedding due to grain burning surfaces, holes and slots. In this paper, a novel sub-scaled motor was used for evaluation of aeroacoustic pressure oscillations. First, the related parameters to scale down using Buckingham’s Pi-theorem were determined and then the sub-scaled motor was designed and manufactured. After this, Strouhal number in various grain forms and vortex shedding prediction criteria have been discussed. Then, one-dimensional linear and non-linear solution approaches have been presented. To understand the motor internal flow and vortex shedding formation, steady state computational fluid dynamic performed for seven regression steps and finally, two static tests have been performed. Results show that various definitions for Strouhal number are useful only for primarily glance on vortex shedding and pressure oscillations and so CFD solution and the test program are inevitable for a correct understanding of the ballistic operational condition of the motor. Experimental results demonstrated the existence of such oscillations with frequencies nearly equal to axial modes. It seems that non-linear parameters have small effects on aeroacoustic pressure oscillation and therefore the linear solution is acceptable to obtain approximate data. Of course, it should be emphasized that linear solution represents oscillations on overall motor action time, whereas the output of non-linear solution depends on thermochemistry properties of solid propellant and combustion products. Therefore, with a non-linear solution, oscillations maybe occur in some intervals of action time. FFT (Fast Fourier Transformation) results demonstrated that although both first and second acoustic modes have been excited, the position of longitudinal oscillation has an important role on which one is dominant. Keywords: Aeroacoustic, Pressure oscillation, Solid rocket motor, Strouhal number, Sub-scale, Vortex shedding

Large eddy simulation of flow past a circular cylinder of low aspect ratio ( $AR=1$ and $3$ ), spanning subcritical, critical and supercritical regimes, is carried out for $2\times 10^3 \le Re \le 4\times 10^5$ . The end walls restrict three-dimensionality of the flow. The critical $Re$ for the onset of the critical regime is significantly lower for small aspect ratio cylinders. The evolution of secondary vortex (SV), laminar separation bubble (LSB) and the related transition of boundary layer with $Re$ is investigated. The plateau in the surface pressure due to LSB is modified by the presence of SV. Proper orthogonal decomposition of surface pressure reveals that although the vortex shedding mode is most dominant throughout the $Re$ regime studied, significant energy of the flow lies in a symmetric mode that corresponds to expansion–contraction of the vortex formation region and is responsible for bursts of weak vortex shedding. A triple decomposition of the time signals comprising of contributions from shear layer vortices, von Kármán vortex shedding and low frequency modulation due to the symmetric mode of flow is proposed. A moving average, with appropriate size of window, is utilized to estimate the component due to vortex shedding. It is used to assess the variation, with $Re$ , of strength of vortex shedding as well as its coherence along the span. Weakening of vortex shedding in the high subcritical and critical regime is followed by its rejuvenation in the supercritical regime. Its spanwise correlation is high in the subcritical regime, decreases in the critical regime and improves again in the supercritical regime.

This paper provides a mathematical model for the bladeless wind turbines working on vortex shedding principle. Unlike conventional wind turbines that work with rotary inputs, bladeless turbines require to generate power from vortex induced vibrations. Hence, it is required to have a mathematical model and equivalent circuit representation to help analyze the bladeless wind turbines and estimate different performance metrics. This may also be used in developing efficient control methodologies. A generalized mathematical model with three different equivalent circuit representations for the electro-mechanical system of a bladeless wind turbine is developed in this paper.

Despite their simplicity, photovoltaic (PV) modules are often arranged in structures that can be affected by severe and complex wind loads: in this context, the wind flow and the dynamic excitation induced by vortex shedding can introduce unexpected aeroelastic responses. This work introduces a novel wind tunnel application of experimental techniques to address this issue by the use of flow visualisation and video postprocessing, through the optical flow algorithm. Numerical simulations based on unsteady Reynolds-averaged Navier–Stokes (RANS) models are performed and compared against the experimental wind tunnel tests on a PV panel that was also instrumented with pressure taps. A setup with a 65∘ tilt angle was examined because, based on preliminary analyses, it was considered interesting for the free flow–wake transition associated with the dynamic response of the PV panel. The comparison of the experimental and numerical average wind fields supported that the proposed optical flow method was appropriate for characterising the wake of the panel, because there was enough seeding to perform the video postprocessing. Experiments and numerical predictions were compared as regards the average pressure distribution on the panel surfaces, and the average percentage was in the error of 7%, this supports that the URANS method was capable of reproducing the average behaviour of the panel, as well as for the selected configuration, which is particularly challenging. Furthermore, the simulated and measured power spectral densities of the wind speed were compared, and this resulted in the numerical model quite faithfully reproducing the frequency of the peak at 5 m/s, while the error was in the order of 20% for the 10 m/s case, this supports that, despite the URANS approach being affected by well-known critical points regarding the simulation of instantaneous quantities, it can be employed to elaborate information that can be particularly useful for the structural design of the panel. This kind of result can be considered as a first step, obtained with simplified and affordable methods, towards a characterisation of the dynamic behaviour of a PV panel in a real-world setup.

The influence of turbulent inflow, as it occurs in complex terrain, on the unsteady surface pressure distributions on a wind turbine is investigated numerically. A method is presented that enables an accurate reproduction of the inflow to the turbine in the complex terrain in Perdigao, Portugal. For this purpose, a precursor simulation with the steady-state atmospheric computational fluid dynamics (CFD) code E-Wind and a high-resolution Delayed Detached Eddy Simulation (DDES) with FLOWer is performed. The conservation of the flow field is validated by a comparison with measurements from the 2017 field campaign in Perdigao. Then, the resolved fluid-structure coupled generic wind turbine I82 is included in the FLOWer simulation to investigate the impact of the complex terrain inflow on the surface pressure fluctuations on tower and blades. A comparison with simulations of the same turbine in flat terrain with simpler inflows shows that the turbine in complex terrain has a significantly different vortex shedding at the tower, which dominates the periodic pressure fluctuations at the tower sides and back. However, the dominant source of periodic pressure fluctuations on the upper part of the tower, the blade-tower interaction, is hardly altered by the terrain flow. The pressure fluctuations on the blade have a rather broadband characteristic, caused by the interaction of the leading edge with the inflow turbulence. In general, it is shown that a sophisticated DDES of the complex terrain plays a decisive role in the unsteady aerodynamics of the turbine, due to its specific flow characteristic.

Wind field (e.g., wind speed and wind direction) has the characteristics of randomness, nonlinearity, and uncertainty, which can be critical and even destructive on a long-span bridge’s hangers, such as vortex shedding, galloping, and flutter. Nowadays, the finite element method is widely used for model calculation, such as in long-span bridges and high-rise buildings. In this study, the investigated bridge hanger model was established by COMSOL Multiphysics software, which can calculate fluid dynamics (CFD), solid mechanics, and fluid–solid coupling. Regarding the wind field of bridge hangers, the influence of CFD models, wind speed, and wind direction are investigated. Specifically, the bridge hanger structure has symmetrical characteristics, which can greatly reduce the calculation efficiency. Furthermore, the von Mises stress of bridge hangers is calculated based on fluid–solid coupling.

Vortex shedding in the wake flow generated by moving bodies exerts considerable influence on pollutant dispersion. This study investigated the effects of different body shapes using scaled models 1/5th of realistic size, including thin and wide shapes, standing and walking poses. The airflow from moving bodies was simulated using computational fluid dynamics (CFD) with dynamic meshing to account for the manikin movement. Experimental data from a smoke visualisation technique provided validation data for computational simulations which included flow separation angle over the head computed through image processing. Vortex structures were visualised using an Omega vortex identification method and compared with experimental visualisations. The main objective of this study is to verify the CFD simulations with smoke visualisation in terms of predicting motion-induced vortex structures, thus helping identify contaminant transport around different shaped bodies during walking and when coming to a stop. The results showed matching locations and patterns of vortex structures between the smoke visualisation and CFD simulations. After the manikin came to a stop, the flow induced by the larger body was characterised by a longer residence time for airborne contaminants in the breathing region while a reduced flow residence time for the thinner bodied manikin.

The paper presents the results of a controlled experimental study to investigate the effect of mutual interference of the bridge piers placed in a staggered arrangement on local scour depth. Experiments were conducted in a standard recirculating tilting bed water flume 12 m long, 0.60 m wide, and 0.70 m deep. The size of the pier models used was 62 mm and 42 mm. For the mobile bed experiments, the uniform-sized river sand used had an average mean size, d50 = 0.23 mm and 0.30 mm. Estimation of the scour depth of the staggered arrangement of the group of piers with a constant angle of attack of 45° but varying radial pier spacing had been achieved using 54 laboratory datasets. The results of the experimental studies were compared with artificial neural network functions and random forest model analysis. From experimental results, it is found that the downstream piers were placed at a radial spacing greater than five times the diameter of the bridge pier; the vortex shedding effect produced by the front pier on the rear piers is negligible.

A general numerical framework to estimate the stress range versus number of cycles curves experienced by a cable due to wind-induced vibrations is presented in this paper. The algorithm associated to the proposed numerical framework relies on a probabilistic approach to account for the uncertainties in the cable properties, initial conditions, damping, and wind excitation which are the variables that govern the wind-induced vibration phenomena in cables. Monte Carlo simulations and the concept of importance sampling techniques are used to propagate aforementioned uncertainties in which the latter is used to significantly reduce the computational costs when new scenarios with different probabilistic models for the uncertainties are evaluated. The proposed numerical framework also utilizes a high fidelity cable model that accounts for the helical geometry of cable components and is capable of capturing changes in cable curvature due to transverse vibrations. A 15 mm diameter high-strength steel strand under vortex shedding excitation mechanism is considered as an illustrative example to show the robustness and computational efficiency of the proposed numerical framework.

Unsteady aerodynamic forces on large, slender aerospace structures are established in wind-tunnel tests that employ a limited number of dynamic pressure sensors and use a correlation-based approach...

The flow-induced motion (FIM) of platforms is a vital phenomenon issue to be investigated to help the designers and to decrease the costs related to the mooring and/or riser systems. On this matter, when talking about multi-column configurations, captive tests (also called fixed or stationary tests) can be a methodology worth of examining the vortex detachments, the source for the oscillatory lift, the drag force, and the interference of the wake of the upstream column on the downstream ones. Computational fluid dynamics (CFD) evaluations on the flow around a fixed array of one, three, and four columns with low aspect ratio, $${\text{AR}} = H/L = 1.5$$ (the ratio between the column draft, $$H$$, and the column face dimension, $$L$$) were conducted. Three different shapes of the column sections were tested, namely, circular, square, and diamond. For the multi-column configurations, the spacing ratio tested was $$S/L$$ (where $$S$$ is the distance from center to center of each column). The Reynolds number tested was 100,000. Lift and drag forces were monitored in each column aiming at the integral quantities of the hydrodynamic to compare with the experimental results. Time averages of velocities and vorticity fields were also observed. The calculations were performed with the flow solver StarCCM+; the code solves the multi-phase incompressible Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations. Simulations were carried out with the Improved Delayed Detached Eddy Simulation (IDDES) method. The IDDES estimates the small turbulence scales near the body boundary with the Reynolds-Averaged Navier–Stokes (RANS) model and the large scales with the Large Eddy Simulation (LES) model. For the volume of fluid (VoF), an interface-capturing approach was used. The multi-column arrays presented no regular vortex shedding pattern when compared with the single-column case. The vortex shedding behavior was mainly determined by the column section. For the multi-column configurations, the number of columns, 3 or 4, was not the main issue. CFD calculations compared with the experiments showed good agreement for the circular and square cases and a significant difference for the diamond cases, which need to be further investigated.

The Mars Helicopter is part of the NASA Mars 2020 rover mission scheduled to launch in July of 2020. Its goal is to demonstrate the viability and potential of heavier-than-air vehicles in the Marti...

The simulation of fluid flow around moving rigid bodies has proven to be a difficult task for traditional computational fluid dynamics solvers. Decomposing the global domain into overlapping subdomains and allowing each subdomain to move independently, permits solutions to many flow problems with complex moving geometries to be determined in a straightforward manner. The present development of the moving overlapping grid method is built within a spectral element method incompressible flow solver, and uses an Arbitrary Lagrangian-Eulerian formulation of the governing equations to prescribe subdomain motions. The method maintains global spectral spatial accuracy across the subdomains with the polynomial refinement. The global high-order temporal accuracy of the method is also maintained through subdomain coupling enforced by an explicit interface temporal extrapolation scheme. The method produces aerodynamic forces and vortex shedding around two- and three-dimensional moving rigid bodies that is in line with published experimental and computational data. Additionally, the method achieves near linear computational scaling to thousands of cores.

The appurtenances effects of high-rise building on wind loads are investigated through a series of wind tunnel experiments with 21 facade appurtenances simulated by different configurations of horizontal splitter plates. “Zigzag variations” of layer forces can be found in both along-wind and cross-wind when the vertical separation distance ratio of continuous plates is larger than 4%. The appurtenances can significantly reduce fluctuating pressures on the side facade but with different effect on the fluctuating cross-wind layer force. The reason of this will be discussed in this study. The power spectra of response of the building model indicate that the horizontal facade appurtenances have negligible effects on the period of vortex shedding. But the peaks in the power spectra become slightly higher than those from a smooth surface model, and the bandwidths are also narrower. Such results indicate that the horizontal plates may result in better organization of the vortex shedding by restricting the flow in vertical direction and reduce the intensity of vortex. Further check on the correlation coefficients between pressures at the same level reviews that the correlation of response from different positions of the building models with appurtenances is higher than that for building model with smooth surface. It is also noted that discontinuous appurtenances can reduce the cross-wind base moment by approximately 5%.

Unsteady flow separation in rotationally augmented flow fields plays a significant role in the aerodynamic performance of industrial rotors, including wind turbines. Current computational models un...

The lasting high fuel cost has recently inspired the resurgence in drag reduction research for vehicles, which calls for a thorough understanding of the vehicle wake. The acknowledged MIRA fastback vehicle model is characterized by similar real vehicle geometry, thus it is especially suitable for the above purpose. In spite of a considerable number of previous investigations, our knowledge of flow around this model remains incomplete. This paper aims to visit turbulent flow structure behind this model. An investigation has been conducted to measure the near wake flow structure of the MIRA 1/8 scale mode, using both Particle Image Velocimetry (PIV) experimental method and Computational Fluid Dynamics (CFD) simulation method. In order to capture the flow structure accurately, PIV measurement was performed in different sections along three orthogonal directions, and the CFD method acquired additional simulation results to catch the better flow status. Through the maps of the time-averaged vorticity, the instantaneous vorticity and the handled velocity vector from the PIV and CFD methods, we found out the formation mechanism of the transient flow of fastback model and summed up the schematic of flow structure. This study not only analyzed the vortex shedding characteristics of turbulent near wake, but more importantly provided insight into the complex three-dimensional features of the flow structure in the wakes of MIRA fastback model.

Flexible structures are common in nature and biomimetic applications. A detailed understanding of their interactions with fluid is essential for the optimisation of structural design. The material stiffness has a strong influence on the motion and flows around the body. In this study, the effect of material stiffness on the motion and flow was experimentally investigated. Cylinders with various rotational stiffnesses ranging from 0.610 N·mm to 0.274 N·mm were produced by changing the degree of cross linking of polydimethylsiloxane. The motion and flow near the free end of the cylinders were examined using a high-speed imaging technique, time-resolved particle image velocimetry, and a proper orthogonal decomposition (POD) modal analysis. Consequently, the primary direction of the motion is altered from the inline to cross-flow direction as the stiffness decreases. The cylinder motion is observed to strongly affect the on flows near the free end, and the length and width near the wake are affected by the vibration magnitude of the cylinder along the inline and cross-flow directions. Vortex shedding occurs near the free end when the motion along the cross-flow direction is larger than 0.1D. These flow structures only propagate downstream behind the flexible cylinders having a relatively weak oscillating motion along the inline direction. The present results could be utilised as basic data in the design of bio-inspired cylinder-shaped structural applications, and relevant computational fluid dynamics (CFD) analyses.

Numerical simulations of wind circumfluence around Tensairity cylindrical beams and predictions of their response are presented in the paper. An appropriate computational fluid dynamics model (CFD) and a fluid-structure interaction (FSI) method are used to study circumfluent wind fields and wind-induced vibrations of the Tensairity cylindrical beams. Navier-Stokes equations for the incompressible fluid dynamic simulations and FSI analysis of the Tensairity cylindrical beam subjected to the wind flow are applied. Constant and simulated fluctuating wind fields were assigned to the investigated structure. A Spalart-Allmaras (SA) one-equation turbulence model that uses an eddy-viscosity variable with a nonlinear transport equation was applied in the CFD analysis. The finite element method is used to simulate the aeroelastic behavior of the Tensairity cylindrical beam subjected to constant and fluctuating wind effects. Aeroelastic response characterized by wind velocity fields and vortex-shedding phenomena around the cross-section of the inflatable structure are presented. Results obtained from the CFD and FSI analysis were compared. The vortex shedding effects may produce significant lateral (cross-wind) vibrations with nonlinear redistributions of forces and displacements as well as introduce additional deformations to the individual structural members of the Tensairity system. The wind-structure interaction analysis of the Tensairity cylindrical beam revealed, in particular, the sensitivity of the pneumatic structure to vortex induced vibrations with galloping effects.

The suspended monorail (SM) vehicle–bridge system has been considered a promising modern transit mode due to its clear advantages: low pollution, high safety, convenient construction, and low cost. The wind-induced response can significantly affect the running safety and comfort of this type of vehicle due to its special suspended position from a fixed track. This study is the first to systematically investigate its aerodynamic characteristics and interference effects under various spacing ratios using wind tunnel tests and numerical simulations. A high level of agreement between the wind tunnel test and CFD (computational fluid dynamics) results was obtained, and the aerodynamic interference mechanism can be well explained using the CFD technique from a flow field perspective. A wireless wind pressure acquisition system is proposed to achieve synchronization acquisition for multi wind pressure test taps. The paper confirms that (1) the proposed wireless wind pressure acquisition system performed well, (2) the aerodynamic coefficients of the upstream vehicle and bridge were nearly unchanged for vehicle–bridge combinations with varying spacing ratios, (3) the aerodynamic interference effects were amplified when two vehicles meet, but the effects decrease as the spacing ratio increases, (4) the aerodynamic force coefficients, mean, and root mean square (RMS) wind pressure coefficients for the downstream vehicle and bridge are readily affected by the upstream vehicle, (5) the vortex shedding frequencies of vehicles and bridges can be readily obtained from the lift force spectra, and they decrease as the spacing ratio increases, and (6) a spacing ratio of 3.5 is suggested in the field applications to ensure the running safety and stability of the SM vehicle–bridge system under exposure to crosswinds.

In 2011, John Dabiri proposed the use of counter-rotating vertical-axis wind turbines to achieve enhanced power output per unit area of a wind farm. Since then, various studies in the wind energy and marine hydrokinetic (MHK) literature have been dedicated to pairs of vertical axis turbines in both co-rotating and counter-rotating configurations, in terms of their power production, wake characterization, and optimal array design. Previous experimental works suggest an enhancement of up to 27.9% in the system power coefficient of pair configurations compared to a single turbine. Additionally, previous numerical studies have indicated that the increased power output is correlated with higher torque on the turbine blades which correspondingly produces a stronger leading edge vortex. This paper presents an extended investigation into a pair of laboratory scaled cross-flow hydrokinetic turbines in counter-rotating configurations. Experiments were conducted to observe, compare, and discuss the leading edge vortex shedding from the turbine blades during their positive torque phase. The turbines operated in a small water flume at the diameter-based Reynolds number of 22,000 with a 0.316 m/s freestream velocity and 4% turbulent intensity. Using a monoscopic particle image velocimetry setup, multiple realizations of the water flow around each blade at their positive torque phase were recorded and phase-averaged. Results show consistent leading vortex shedding at these turbine angles while a correlation between the turbine power performance and the vortex size and strength was observed.

The present work investigates the flow-acoustics coupling in a half-dump combustor emulating methane-air non-premixed combustion for varying Reynolds numbers (Re) and fuel flow rates. The study uses hybrid computational aero-acoustics (CAA) simulation methodology, which is based on Lighthill’s acoustic analogy, to compute the acoustic flow field. The underlying turbulent flow is modeled using a hybrid Reynolds Averaged Navier Stokes/Large Eddy Simulation (RANS/LES) approach, while the turbulence-chemistry interaction is handled using steady laminar flamelet model (SFLM). Chemical kinetics for methane-air combustion is represented using the GRI-3.0 mechanism. In simulations, time-resolved data is analyzed to understand the flow physics in greater detail, while the comparison is made with the measurements to benchmark the numerical code used herein. In particular, the information of non-dimensional numbers, i.e., Helmholtz and Strouhal numbers, allows to characterize the instability modes and distinguishes between hydrodynamic (vortex shedding modes) and acoustics instabilities (acoustic modes). The observation yields that unsteady pressure fluctuations either come from the natural acoustics mode of the channel or vortex shedding process and thereby exhibiting multiple frequencies in the domain for lower Re case, while a single dominant frequency for higher Re case. Moreover, the computed Rayleigh index using the unsteady heat release and pressure oscillations from reacting conditions confirms the mode of instability, whether it is driving (positive net value) or damping (negative net value), at different frequency levels.

The rotary energy recovery device (RERD) is widely equipped in desalination to reduce the system energy consumption. In this study, the fluid dynamics and mixing performance of a typical structure RERD and a visualization apparatus of a RERD (V-RERD), had been compared by simulation. The effects of rotating components on fluid dynamics and mixing had been researched. Simulation results indicated that a swirling flow can be observed from flow fields in the device duct. In the RERD case, the swirling flow changed its rotating direction in the center of the duct, while in the V-RERD case its rotating direction unchanged. Then, a swirling number Sn was applied to characterize the degree of swirl intensity, and its formation mechanism in RERD had been discussed. In addition, the Q criterion was adopted to visualize the three-dimensional flow structures and identify vortex structures in the duct. The evolution of vortices in the working process had been investigated. It was found that vortices significantly affected the mixing performance, and the detached vortex could lead to high turbulence and mixing in the duct. Suppressing the vortex shedding may reduce the flow turbulence and gain a lower volumetric mixing rate.

The occurrence of low-frequency instability (LFI) appears to be related to multiple interactions among many complex physical processes, such as vortex shedding, boundary-layer oscillation, and additional combustion in the post-combustion chamber. In this study, two combustion tests were conducted to suppress LFI and to examine which physical processes its occurrence was most sensitive. In the first test, two fuel inserts were used to modify the formation of a boundary layer, vortex shedding at the end of the fuel, and vortex impingement. In the second test, the fuel insert located at the front end was replaced with swirl injection. The first test was aimed at controlling and suppressing the initiation of LFI using fuel inserts, through which a small step appeared gradually due to differences in the regression rates of the two materials, i.e., polymethyl methacrylate and high-density polyethylene. The test results confirmed that (i) there are physical connections among several processes, such as the thermoacoustic coupling between p′and q′ and the oscillations of the upstream boundary flow, and (ii) LFI suppression is possible by disrupting or eliminating the connections among these physical processes. The second test was also aimed to control LFI while minimizing the deviation in combustion performance using proper swirl injection along with a fuel insert. Even when replaced by swirl injection, LFI suppression was still possible and showed reasonable combustion performance without causing too much deviation from the baseline in terms of the oxygen-to-fuel ratio and the fuel regression rate.

Wind turbines can be a replacement for coal as an energy source. However, the conventional wind turbines are expensive and are very complicated due to many mechanical components translating to high manufacturing and maintenance costs. This study aims to help improve the efficiency and design of bladeless wind power generators through generation of power using vortex induced vibration. A bladeless wind power generator was designed using Autodesk Fusion 360 wherein the prototype was modeled to be approximately 1.35 m x 0.5m x 0.5m (overall height, width, and length). A Nonlinear Static Simulation, using ANSYS®Academic Student MechanicalTM, was performed to determine the effect of two different wind velocities, 4.5 m/s and 6.5 m/s, acting on the mast. The design of the bladeless wind turbine was focused on simplifying its manufacturability by using a helical spring to connect the mast to the base while also attaining maximum vortex shedding at a low velocity. The researchers of this study used a static simulation to simplify the study. The predetermined wind velocities were converted into a pressure value, allowing the researchers to obtain the total deformation, directional deformation, and maximum principal and shear stresses (in the spring). It has been determined that the maximum deformation experienced by the bladeless wind power generator was 131.800 mm and 253.270 mm for wind velocities of 4.5 m/s and 6.5 m/s resulting to a theoretical power output of 9.765W and 29.428W, respectively.

Fishes are talented swimmers. Depending on the propulsion mechanisms many fishes can use flapping tails and/or fins to generate thrust, which seems to be connected to the formation of a reverse von Kármán wake. In the present work, the flow past a 2D flapping foil is simulated by solving the incompressible Navier-Stokes equations in the open-source OpenFOAM platform. A systematic study by varying the oscillating frequency, peak-to-peak amplitude and Reynolds number has been performed to analyze the transition of vorticity types in the wake as well as drag-thrust transition. The overset grid method is used herein to allow the pitching foil to move without restrictions. Spatial convergence tests have been carried out with respect to grid resolution and the size of overset mesh domain. Numerical results are compared with available experimental data and discussed. The results show that the adopted methodology can be well applied to simulate large amplitude motions of the flapping foil. The transitions in the types of wake are consistent with the benchmark experimental data, and the drag-thrust transition of the pitching foil does not coincide with von Kármán (vK)-reverse von Kármán (reverse-vK) wake transition and it is highly dependent on the Reynolds number.

Most studies of vortex shedding from a circular cylinder in a gas flow have assumed the no-slip condition on the cylinder surface. To investigate the effect of slip, we simulate vortex shedding using molecular gas dynamics (direct simulation Monte Carlo method) and computational fluid dynamics (incompressible Navier-Stokes equations with a slip boundary condition) and find the results of both methods to be in reasonable agreement. The critical accommodation coefficient below which vortex shedding ceases is found for Reynolds numbers of 60-100 at a Mach number of 0.3. Conditions to observe the effect of slip on vortex shedding appear to be experimentally realizable, although challenging.

The StreamVane™ swirl distortion generator, developed by Virginia Tech, can efficiently reproduce the boundary layer of an airframe or duct found in boundary layer ingesting (BLI) aircraft. Due to manufacturing limitations, the vanes within StreamVanes induce unsteady, vortical wakes, commonly known as a von Karman vortex street. This paper investigates the use of a commercial URANS code and SST turbulence model to predict the vortex shedding frequency from the vanes. The objective was accomplished in two main tasks. First, the CFD methodology was validated by modeling the fluid dynamics of a linear cascade experiment done by the von Karman Institute. Second, the same methodology was applied to airfoils used in StreamVane design to calculate the shedding frequency as a function of turning angle and TE thickness. It was predicted that an increase in turning angle exponentially increased the shedding frequency while an increase in TE thickness exponentially decreased the shedding frequency. The results provided a correlation between the shedding frequency and airfoil characteristics in StreamVanes as well as various turbomachinery components.

The fluid–structure--sound interaction in reducing the flow-induced noise of a circular cylinder with a back flexible splitter plate is investigated experimentally. The rigid splitter plate (RSP) and the flexible splitter plate (FSP) are individually attached to the rear of the cylinder with a diameter D = 30 mm. Three parameters are considered: the flexibility of the plates, the length of the plates (L = 0.5 D ~ 6.0 D) and the Reynolds numbers (Re = 3.83×104 ~ 9.57 × 104). Both RSP and FSP achieve the optimal reduction in the Aeolian tone at L/D = 1.0 regardless of Re, where the RSP results in a reduction of 14 dB on average and the FSP a superior reduction of 18 dB. Subsequently, the velocity fields are measured with two-dimensional particle image velocimetry for L/D = 1.0. The dynamic deformation of the FSP is captured with a single-camera three-dimensional digital image correlation system. The results show that the splitter plate elongates the recirculation region length (Lr) significantly, and it is preferable to normalize the streamwise coordinate x with Lr instead of D in this investigation. It is determined that the Aeolian tone is mainly induced by the fluctuations in the near wake region of 1 < x/Lr < 3. The splitter plates effectively suppress the overall fluctuations originating from vortex shedding in the wake, thereby reducing the Aeolian tone successfully. The movement of the FSP, which is dominated by low-frequency vibrations with broadband and a relatively energetic periodic vibration, has important effects on the secondary and high-frequency noise.

In order to prevent a turbine to reach its runaway speed when load rejection occurs, an emergency closing system must be devised in case the regulation system fails. For Kaplan turbines, fixed wheel gates located in the turbine intake or in the draft tube outlet are usually employed. Gates of this type are move by gravity and the closing velocity is controlled by gantry cranes. The closing maneuver is complex due to the high flow rates inherent to runaway conditions and the rotational deceleration during the gate's closing time. Research on this topic is scarce and limited, and numerical studies are usually clouded by uncertainties concerning the setting of proper boundary conditions. In this work, the closing maneuver of the emergency fixed wheel gates at the intake of a Kaplan turbine was studied with CFD two-phase transient simulation. The software used was ANSYS CFX, that solves unsteady Navier-Stokes equations (URANS) by means of the finite volume method. The simulated domain includes a 2D case from one of the span of the semi-spiral casing and a 3D case of a complete span. Two types of simulation were considered, namely: quasi-steady state, where the position of the gate is fixed; and full transient state, where the gate movement was modelled by an immersed solid model. In search of the optimum model layout with its set of boundary conditions, numerical results were compared and validated against experiments performed on a physical scale model in accordance with IEC 60193 norms for several turbulence models. Results show that the pull-up force on the gate increases as it is being closed. Analysis of pressure fluctuation at different points of the gate suggest that the main frequency component is the vortex shedding of the gate lip., Facultad de Ingeniería

Comparison study of unsteady Reynolds-averaged Navier-Stokes (URANS) and hybrid RANS-LES models is carried out for predicting the performance of three-straight-bladed vertical axis wind turbine operating within tip speed ratios (TSRs) ranges of (1.44–3.3). The evaluation is focused on power coefficient, moment coefficient and vortex structure generation, growth and transportation predictions. It was found that URANS turbulence modelling is sufficient for averaged power coefficient prediction and specific range of TSR evaluation, and it also benefits from short simulation run time. To further evaluate flow field details and to understand the underlying flow physics such as dynamic stall behaviour, hybrid RANS-LES turbulence modelling is necessary. Comparing between hybrid models adopted, stress-blended eddy simulation based on transition shear-stress transport (TSST) model has achieved an overall better performance, such as a reduction of simulation discrepancy by 50% in low TSRs range compared to TSST turbulence model. In both medium and high TSRs ranges, the modelling discrepancies are less than 3% compared to TSST turbulence models at about 25% extra computational time. Furthermore, in high TSRs range, hybrid RANS-LES models are able to predict the appearance of vortex shedding at high azimuthal angles (θ ≥ 180°) for which URANS models failed to capture.

A butterfly web girder is a box girder that has discretely distributed side-surface openings. Due to these openings, butterfly web girders show better aerodynamic performance than conventional box girders. This study aims to investigate the effects of side-surface openings on the aerodynamic instability of a box girder when it is placed in tandem with another girder. In addition, this study aims to investigate whether the aerodynamic instability of one girder is affected by the side-surface openings of an adjacent girder. A series of wind tunnel tests will be carried out on a butterfly web girder and conventional box girder, with another rigidly supported butterfly web girder or conventional box girder, respectively, at its upstream or downstream side. The fixed downstream girder increased the galloping instability of the upstream girder, and the fixed upstream girder amplified the vortex-induced vibration of the downstream girder. The side-surface openings of the upstream girder stabilized its galloping and vortex-induced vibration, irrespective of the type of girder downstream. The side-surface openings of the downstream girder stabilized its vortex-induced vibration independent of the type of girder upstream.

Background The hybrid comprehensive stage 2 (HCS2) procedure is a novel palliative operation applicable to a select subset of single ventricle patients with adequate native antegrade aortic flow to the upper body. Flow to the descending aorta, through the pulmonary outlet and ductal arch, is influenced by a stented intrapulmonary baffle connecting the branch pulmonary arteries. We used computational fluid dynamics (CFD) to elucidate the hemodynamic characteristics of this reconstruction. Methods We used multiscale CFD analysis of a synthetic, patient-derived HCS2 anatomic configuration with unsteady laminar flow conditions and a non-Newtonian blood model to quantify the resultant hemodynamics. The 3-dimensional CFD model was coupled to a 0-dimensional lumped parameter model of the peripheral circulation to determine the required boundary conditions. Results For the specific anatomy studied, the intrapulmonary baffle did not obstruct flow from the pulmonary trunk to ductal arch as long as the distance between the anterior pulmonary artery wall and baffle wall exceeded ∼7 mm. Vortex shedding off of the baffle wall did not develop, because of the short distance to the ductal arch. The stented baffle experienced significantly uneven “inward” loading from the systemic side. Pulmonary outlet flow separation distal to the baffle produced a low-speed recirculation region. Conclusions Hemodynamic patterns in this complex anatomy are generally favorable. Low flow recirculation could be mitigated by preoperative shape optimization. Calculated inward stresses on the pulmonary baffle can be used in the future to study baffle stent deformation, which is expected to be small.

International audience; Direct contact membrane distillation (DCMD) is a promising means of concentrating brines to their saturation limit. During that process, membrane spacers play a key role in temperature polarization, concentration polarization, and mineral scaling. These interactions are not well understood, because they are difficult to study experimentally and numerically, and the flow regimes are not fully charted. We consequently develop a tailored in-house CFD code that simulates unsteady two-dimensional heat and mass transport in plate-and-frame DCMD systems with cylindrical spacers. The code uses a combination of finite-volume methods in space, projection methods in time, and recent advances in immersed boundary methods for the spacer surfaces. Using the code, we explore how the transition to unsteady laminar vortex shedding affects polarization and permeate production of DCMD systems. We show that the impact of spacers can be explained by examining the various steady and unsteady vortical flow structures generated in the bulk and near the membranes. Overall, we show that though unsteady vortex structures tend to mix temperature polarization layers with the bulk, they are not similarly able to mix the concentration layers. Rather, vortical structures tend to create regions of preferential salt accumulation. In the vortex shedding regime, the net result is that spacers often increase vapor production at the expense of increasing the risk of mineral scaling.

Thermoacoustic system is one of the alternative technologies that provides green working principles but the lack of understanding of the complex fluid flow and energy transfer interactions within structures inside the system is leading to difficulty in accurate analysis related to the system. This study presents fluid dynamic investigation of vortex shedding and velocity profile of an oscillatory flow across a parallel-plate structure inside a standing-wave thermoacoustic system by using a two-dimensional ANSYS FLUENT CFD (computational fluid dynamics) of SST k-ω turbulence model. The model was validated using experimental data and theoretical solution. Two different operating frequencies of 14.2 Hz and 23.6 Hz were investigated with drive ratios (defined as maximum pressure amplitude to mean pressure) from 0.3% up to 3%. The results revealed that velocity profiles and boundary layers within the area of parallel-plate stack changes with time and the changes followed the cyclic travel of vortex across the structure. Two layers of vortex formed near the surface of the solid structure. These layers, known as the main and secondary vortex layers, change with the cyclic flow, and are affecting the shape of velocity profiles within the channel. The appearance of an ‘m’ shape, a ‘slug’ shape and a ‘parabolic’ shape velocity profiles are also depending on the flow amplitude (drive ratio) and flow frequency. The ‘parabolic’ velocity profile is only found for a certain moment of flow with thick boundary layer. The results indicated that fully developed flow may not be likely for cases presented in this paper. Hence care should be exercised in the use of equations during the analysis of thermoacoustic system. As the parallel-plate structure is usually an important structure of the system, a proper understanding of the dynamics of flow within this structure is crucial. These findings are expected to give better insight for future design of thermoacoustic system.

Low-pressure turbine (LPT) is the last stage in a gas turbine engine where energy remnants are extracted from the exhaust air. The blade profile of an LPT is designed with a high camber to generate maximum lift from the upstream flow at a low Reynold's number. Such highly cambered blades are called high lifting blades. Airflow over a highly cambered blade maximizes boundary layer separation on the suction side of the blade, thereby resulting in vortex shedding which subsequently generates blade vibrations. These vibrations can be detrimental to the blade's fatigue life due to resonance. So, selecting a strong material for the blade is important and this can be done through modal analysis. This paper aims to investigate the modal parameters (natural frequencies, amplitudes and mode shapes) of a high lifting T106 blade under Nickel-based superalloys U-500, IN-738 and Mar-M246 using Euler-Bernoulli beam theory and validation of results through computational modal analysis. For each turbine material, parameters such as blade mass, natural frequency and maximum amplitude will be obtained, validated and compared and the most suitable material for a high lifting T106 blade will be proposed based on these results.