Bluff Body Shapes Research Articles

Harvesting wind energy with a bi-stable configuration integrating vortex-induced vibration and galloping

This paper proposed a wind energy harvester that integrates the merits of vortex-induced vibration (VIV), galloping and multi-stability. Two different shapes of bluff bodies are introduced to induce VIV and galloping for incoming flow, respectively. Three magnets are placed on the beam and fixture so as to generate bi-stability by magnetic attraction. As the wind blows through, the induced aero-dynamical instability leads the piezoelectric beam to oscillate and execute snap-through motion, which could generate a large electric output. The validation experiment was conducted. The experimental results show that as the flow speed increases from v= 1.0 m s−1 to 7.0 m s−1, the harvester could begin snap-through motion from a low speed v = 1.5 m s−1, and maintain the coherence resonance motion within a broad range of wind speed. Thus, it could maintain the large output in the environment of variable-speed wind.

External acoustic control of the laminar vortex shedding past a bluff body

This paper deals with the active control of the compressible flow past a bluff-body by means of external acoustic sources. We successfully suppress the transition leading to the von-Karman vortex street. The derived adjoint-based framework allows the determination of the corresponding acoustic sources for generic bluff-body shapes. In particular, we determine the optimal spatial position for the control, i.e. where the acoustic source is most efficient, and the associated temporal signal. For the proposed example, a two-dimensional cylindrical body, the resulting acoustic actuation is anti-symmetric and harmonic. We found that the limits of our control approach are related to the limits of linear acoustics. The methodology and the results proposed in this paper can be used to design a control strategy coupled with a continuation procedure able to obtain the unstable solutions beyond the critical threshold.

Experimental investigation of flow past a confined bluff body: Effects of body shape, blockage ratio and Reynolds number

This paper presents a Particle Image Velocimetry (PIV) study of the two-dimensional flow past a bluff body confined in a channel. Three typical shapes of bluff body with the same height, namely, circular cylinder (CC), flat plate (FP), and square cylinder (SC), but with varying blockage ratio (β = 0.25, 0.3, 0.4 and 0.5) and Reynolds number (Re = 3200, 4600 and 5400) are investigated. It is shown that for each body shape, vortex shedding patterns are relatively insensitive to Reynolds number (most notably for the case of FP), but strongly depend on the blockage ratio. As β increases, the mean recirculation length of the wake is initially constant and then displays a sharp increase at about β ≥ 0.4–0.5, which corresponds to the flow transition from periodic vortex shedding state to steady state. The strength of vortex shedding generally decreases with the increase in β except for CC at β = 0.25–0.3. On the other hand, the vortex shedding frequency expressed in terms of dimensionless Strouhal number (St) increases monotonically with blockage ratio until vortex shedding is suppressed. The critical blockage ratio for the cessation of vortex shedding, βCri, follows the order of βCri_FP<βCri_SC<βCri_CC.

Reynolds number effects on the wind pressure distribution on spherical storage tanks

Spherical storage tanks are important industrial equipment with typical bluff body shape, the aerodynamic loads of which are significantly affected by the Reynolds number (Re). In the present research, wind tunnel tests are carried out on spherical storage tanks with 4–20 supporting columns and a range of surface roughness (ks/D ​= ​0, 0.002 and 0.004) under uniform and atmospheric boundary layer (ABL) flows with Re ranging from 105 to 106. The Reynolds number effects on the aerodynamic drag and uplift coefficients, mean and RMS wind pressure distributions are analyzed. The separation angle on the upper hemi-sphere shifts from 135° (super-critical) to 120° (trans-critical) at Re ​≈ ​6.5 ​× ​105 for a smooth tank in the uniform flow. The trans-critical self-modeling Re range criteria are determined by analyzing the similarity of mean and RMS wind pressure coefficient distribution patterns. It is indicated that in the ABL flow, when ks/D ​≥ ​0.002, the mean and RMS wind pressure distribution of a spherical tank in the trans-critical self-modeling Re range can be simulated with merely Re ​> ​2.0 ​× ​105, thus providing a guidance for wind tunnel simulation of sphere-like bodies. Moreover, a piecewise mean wind pressure coefficient model in the trans-critical self-modeling Re range for spherical storage tanks is established for engineering reference.

Numerical investigation of bluff body for vortex induced vibration energy harvesting

In order to obtain a larger VIV response and harvest energy more efficiently in wider flow velocity ranges in long-distance transport pipelines, the effects of the elastic modulus of material, mass ratio and cross-sectional shapes (circular cylinder, squa-cir prism, cir-squa prism) of flow-around structure on the VIV performance are numerically investigated in this paper. URANS equations with k-ω SST turbulence model are employed to calculate the two-dimensional flow field with Reynolds number ranging from 2000 to 50000, and Newmark-β method is adopted to capture the one-degree-of-freedom motion of the system. The numerical results show that the elastic modulus of material and mass ratio are two important factors affecting the natural frequency of the system, which is closely related to the vibration response of system. The range of synchronization increases with an increase in elastic modulus, and the onset of synchronization is more gradual for systems with higher natural frequency. And the maximum amplitude ratio and the range of Reynolds number corresponding to the synchronization region all increase with the decrease of mass ratio. Moreover, the bluff body with squa-cir cross section shows highest amplitude response of 1.16D due to its forward vortex shedding position, indicating the direction for the design of the cross-sectional shapes of bluff body in the pipeline for VIV energy harvesting. Thereby, it can be concluded that the natural frequency of system and vortex shedding position caused by the cross-sectional shapes are important factors affecting the VIV response.

Influence of Shape and Piezoelectric-Patch Length on Energy Conversion of Bluff Body-Based Wind Energy Harvester

The technology of scavenging ambient energy to realize self-powered of wireless sensor has an important value in practice. In order to investigate the effects of piezoelectric-patch length and the shape of front bluff body on energy conversion of the wind energy harvester by flow-induced vibration, the characteristics of a piezoelectric wind energy harvester based on bluff body are experimentally studied in this work. Four different section shapes of the bluff body, including triangular cylinder, trapezoidal cylinder, reverse trapezoidal cylinder, and square cylinder, are tested. The piezoelectric patch is attached on the leeward side of the bluff body. The lengths of piezoelectric patch are considered as 1.0D–1.4D (D is the characteristic length of the bluff body). It is found that the length of the piezoelectric patch and the shape of the front bluff body play a vital role in improving the performance of wind energy harvester. For the reverse trapezoidal cylinder and square cylinder, the back-to-back vortex-induced vibration (VIV) and galloping phenomenon can be observed. In addition, the energy harvesting performance of the reverse trapezoidal cylinder piezoelectric harvester is the best. The maximum average peak voltage of 1.806 V and the output power of P=16.3 μW can be obtained when external resistance and the length of piezoelectric patch are 100 KΩ and 1.1D, respectively.

Enhancement of low-speed piezoelectric wind energy harvesting by bluff body shapes: Spindle-like and butterfly-like cross-sections

Fluid-structure interaction can be utilized to harvest the low-speed wind energy for sustaining the low-power sensors for structural health monitoring. To enhance the low-speed wind energy harvesting, this study proposes the novel spindle-like and butterfly-like bluff bodies by coupling both the vortex-induced vibration (VIV) and galloping phenomena. Comprehensive wind tunnel experiments are conducted to investigate the advantages of the two bluff bodies in terms of the bluff body cross-sections and installment directions. The experimental results demonstrate that for both the spindle-like and butterfly-like bluff bodies, the vertical installment and small width ratio are beneficial to the performance in a broad range of wind speeds. Compared to a conventional galloping-based energy harvester, owing to the coupling between the VIV and galloping, the vertical spindle-like bluff body with the smallest width ratio can reduce the threshold wind speed of activating the energy harvesting function by over 13%, and improve the maximum voltage output by over 160%. Finally, taking the spindle-like bluff body as an example, the computational fluid dynamics (CFD) studies are conducted by using XFlow software to interpret the physical insight of performance enhancement. The CFD results show that the vertical installment direction and a small width ratio play an important role. The two designs can lead to a stronger aerodynamic force due to the fast vortex shedding, which improves the energy conversion efficiency from the flow-induced vibrations.

Machine-learning-based reduced-order modeling for unsteady flows around bluff bodies of various shapes

We propose a method to construct a reduced order model with machine learning for unsteady flows. The present machine-learned reduced order model (ML-ROM) is constructed by combining a convolutional neural network autoencoder (CNN-AE) and a long short-term memory (LSTM), which are trained in a sequential manner. First, the CNN-AE is trained using direct numerical simulation (DNS) data so as to map the high-dimensional flow data into low-dimensional latent space. Then, the LSTM is utilized to establish a temporal prediction system for the low-dimensionalized vectors obtained by CNN-AE. As a test case, we consider flows around a bluff body whose shape is defined using a combination of trigonometric functions with random amplitudes. The present ML-ROMs are trained on a set of 80 bluff body shapes and tested on a different set of 20 bluff body shapes not used for training, with both training and test shapes chosen from the same random distribution. The flow fields are confirmed to be well reproduced by the present ML-ROM in terms of various statistics. We also focus on the influence of two main parameters: (1) the latent vector size in the CNN-AE, and (2) the time step size between the mapped vectors used for the LSTM. The present results show that the ML-ROM works well even for unseen shapes of bluff bodies when these parameters are properly chosen, which implies great potential for the present type of ML-ROM to be applied to more complex flows

Prediction of Fluid Force Exerted on Bluff Body by Neural Network Method

With the development of artificial intelligence, artificial neural network (ANN) has been widely used in recent years. In this paper, the method is applied to the prediction of the fluid force exerted on the bluff body when flow passes around. Firstly, back propagation (BP) model and convolutional neural network (CNN) model are introduced; then the mapping relation between the shape of bluff body and the fluid force, which is calculated by computational fluid dynamics (CFD), is established by sample training. Finally, it is used to predict the fluid force of the new shape bluff body. By taking the CFD results as benchmark, CNN model is capable of predicting both the resistance and lift force, while BP model is incompetent to predict lift force. Furthermore, both CNN and BP models have a significant advantage in prediction efficiency, compared by CFD calculation method.

Investigation of combustion and emission performance of a micro combustor: Effects of bluff body insertion and oxygen enriched combustion conditions

Major challenges for micro combustors are high heat losses and inappropriate residence time. In this study, it was aimed to eliminate these challenges via placing bluff bodies into the combustion zone and combusting fuel with oxygen enriched air. To this end, micro combustor models with different geometries were constructed and in these models, premixed H2/air combustion was simulated by using ANSYS/Fluent CFD code to investigate effects of bluff body shape, location and thickness, and low level O2 enhancement on performance determining parameters such as rate of conversion of fuel to useable heat, temperature uniformity, pollutant emissions etc. To further analyze effects of micro combustor geometry, a perforated plate was also placed into the combustion zone. Thermal performance of the micro combustor with perforated plate insertion in O2 enriched conditions was found to be highest in terms of increased reaction kinetics and heat transfer characteristics. The trade-offs of respective design are increased NOx emissions and slightly decreased temperature uniformity.

Numerical Investigation of the Influence of Bluff Body’s Shape on Flow Characteristics in Vortex Flowmeters

The paper examines the influence of bluff body’s shape on vortex shedding stability in a liquid flow. It shows that the expansion of stable vortex existence domain enables the extension of velocity measurement range for vortex flowmeters. Numerical simulation of eddy flows generated by various bluff body shapes is undertaken with the appropriate comparative analysis of flow characteristics. A bluff body with the best flow parameters enabling the improvement of flowmeter’s measurement performance is selected.

Characterization of a canonical helicopter hub wake

The current study investigates the long-age wake behind rotating helicopter hub models composed of geometrically simple, canonical bluff body shapes. The models consisted of a 4-arm rotor mounted on a shaft above a 2-arm (scissor) rotor with all the rotor arms having a rectangular cross section. The relative phase between the 2- and 4-arm rotors was either $0\deg$ (in-phase) or $45\deg$ (out-of-phase). The rotors were oriented at zero angle-of-attack and rotated at 30 Hz. Their wakes were measured with particle-image-velocimetry within a water tunnel at a hub diameter based Reynolds number of $820,000$ and an advance ratio of $0.2$. Mean profiles, fluctuating profiles and spectral analysis using time-series analysis as well as dynamic mode decomposition were used to characterize the wake and identify coherent structures associated with specific frequency content. The canonical geometry produced coherent structures that were consistent with previous results using more complex geometries. It was shown that the dominant structures (2 and 4 times per hub revolution) decay slowly and were not sensitive to the relative phase between the rotors. Conversely, the next strongest structure (6 times per hub revolution) was sensitive to the relative phase with almost no coherence observed for the in-phase model. This is strong evidence that the 6 per revolution content is a nonlinear interaction between the 2 and 4 revolution structures. This study demonstrates that the far wake region is dominated by the main rotor arms wake, the scissor rotor wake and interactions between these two features.

Energy harvesting from flow-induced vibration: a lumped parameter model

ABSTRACTThe traditional models for VIVPEH (piezoelectric energy harvesting from vortex-induced vibration) are focused on a circular cylinder. In the present paper, a VIVPEH model was built to solve an aero-electromechanical coupling problem during VIVPEH with different cross-sectional shapes. The model was verified by a wind tunnel test. One key benefit of the present model is that the region from the initial position to the threshold of the lock-in region along the voltage time history curve can be calculated quite accurately compared with traditional methods. The present method will significantly contribute to the design and optimization of the key factors of VIVPEH, e.g., the bluff body shape and the electrical circuit.

Research and design of underwater flow-induced vibration energy harvester based on Karman vortex street

With the increasing development of wireless sensor network (WSN), power supply for WSN nodes had attracted increasing attention, and the energy harvesting system based on Karman vortex street has been widely used in underwater WSN. But the research of the influences of affecting factors towards the energy harvesting system is yet to be completed. So, in this paper, an underwater flow-induced vibration energy harvesting system based on Karman vortex street was proposed and tested. The influence of bluff body geometry and flow velocity towards the performance of the energy harvesting has been researched. The results showed that the output voltage increased as the diameter of bluff body and the water velocity increase. The power generation efficiency was the best when the shape of bluff body was circular.

Use of bluff body for enhancing submicron particle agglomeration in plasma field

ABSTRACTA bluff body was installed to generate a vortex shedding in gas flow under nonthermal plasma field. Various shapes of bluff body were analyzed using computational fluid dynamics for their ability to enhance submicron particle agglomeration. The cylindrical bluff body produced the lowest pressure drop and the plate bluff body showed the widest amplitude of vortex shedding. In the experiment, exhaust gas with a velocity of 1–3 m/s were fed into the test section. The electrical pulse peak voltage was 35 kV, 10 kHz. The bluff body improved the reduction efficiency by 27% and 17% for flat plate and cylindrical bluff bodies, respectively, relative to no-bluff body.

Aerodynamic load maps of bluff bodies: Measurement and diagnostics

Bluff bodies may assume arbitrary attitudes in a flow, causing aerodynamic loads that are sensitive to attitude. The Continuous rotation technique obtains 6-component loads on bluff bodies with 1-degree azimuth resolution about selected axes at a rate of 1 revolution per minute. The load coefficient variation is Fourier transformed and the resulting complex series is truncated in order to obtain rapidly computable, analytical formulae. The method is applied to bluff body shapes including cylinders, a cuboid, a flat plate and a porous box. A cylinder whose length is equal to its diameter, is used to show that rate effects, hysteresis, vortex shedding and other unsteady aerodynamic phenomena are negligible below 10 revolutions per minute. Approaches to generalize the aerodynamic loads on yawed finite cylinders of various aspect ratio are studied. The reasons for differences in aerodynamic load behavior between 2 cylinder models, are analyzed. To complement experiments, the ROTCFD unsteady Navier Stokes code is used to perform diagnostic computations. Methods to generalize the predictions are explored. Maps of the leading coefficients of the Fourier series of each load component over the aspect ratios space, are interpolated. The interpolation varies sharply for aspect ratios between 0.5 to 1. The variation is more gradual beyond aspect ratio 2. By aspect ratio 4, a ‘high aspect ratio’ limit appears to be reached.

Parametric CFD study of micro-energy harvesting in a flow channel exploiting vortex shedding

Abstract Miniature energy harvesting devices are increasingly used in various fields. For example, Wireless Sensor Networks have recently made great progress in many applications. However, their main drawback, i.e. the limited duration of operation, poses the requirement for an effective way to recharge their batteries. In this context, the presentwork focuses on the study of micro-energy harvesting from flow by exploiting vortex shedding behind bluff bodies, in order to cause oscillations to a piezoelectric film and generate the required electrical power. To this end, a Computational Fluid Dynamics (CFD) tool is validated on a particular miniature device configuration proposed in the literature and implemented for the numerical simulations of flow around bluff micro-bodies in a very small channel. Aiming to enhance vortex shedding, parametric studies corresponding to different bluff body shapes and arrangements for a fixed Reynolds number are performed, the main parameters involved in the phenomenon are highlighted and the potential for vortex shedding exploitation is qualitatively assessed.

Flow Field Predictions of Bluff Body Introduced Micro Combustor

Combustion in micro scale is the area having wide scope in this era of miniaturization of energy efficient devices. Some important areas of application are in the field of propulsion as a micro thruster and as an apt replacement for conventional batteries by accounting the high energy density of micro combustors. Numerical simulation of combustion is a challenging process due to associated chemical reaction, unsteadiness and non equilibrium nature. In this work a numerical study has been carried out in a typical non premixed micro combustor. The steady simulations of micro combustor without and with bluff body are analyzed for different fuel inlet velocities. For studying the effect of location of bluff body, the micro combustor with wall positioned bluff body is separately analyzed by changing the position of bluff body along axial direction. The effect of different bluff body shapes and significance on its location in combustion process is analyzed. The results are presented in the form of contours of velocity, temperature and mass fractions.

Effects of bluff body shape on the flame stability in premixed micro-combustion of hydrogen–air mixture

Combustion characteristics and flame stability of lean premixed hydrogen–air mixture in a micro-combustor with different shapes of bluff body (circle, ellipse, diamond, semicircular, half ellipse, triangle, crescent, arrowhead and wall-blade) under various physical and chemical circumstances were investigated by solving two-dimensional governing equations. The blow-off limit of different bluff bodies, combustion efficiency, wall temperature and exhaust gas temperature of micro-combustor were examined. The results illustrate that in moderate equivalence ratio (∅ = 0.5) and low velocity (V = 10 m/s), the maximum flame temperature occurs when a wall-blade is applied as a bluff body. When the inlet mixture velocity increases from V1 = 10 m/s to V2 = 20 m/s, the flame temperature of the micro-combustion rises in all cases. Apart from the wall-blade, the maximum flame temperature was recorded at V2 = 20 m/s for all cases and when the inlet velocity of the mixture increased from 20 m/s to 30 m/s, the flame temperature reduced in all studied cases. Therefore, the flame of micro-combustor with wall-blade bluff body is more stable than other cases. Moreover, the mean wall temperature of the micro-combustor with wall-blade bluff body is highest. Emitter efficiency is very high in the micro-combustor with wall-blade bluff body in lower velocities.

Effect of bluff body shape on the blow-off limit of hydrogen/air flame in a planar micro-combustor

We recently developed a micro-combustor with a triangular bluff body, which has a demonstrated 5-time extension in the blow-off limit compared to straight channel. In the present work, the effect of bluff body shape on the blow-off limit was investigated with a detailed H2/air reaction mechanism. The results show that the blow-off limits for the triangular and semicircular bluff bodies are 36 and 43 m/s respectively at the same equivalence ratio of 0.5. Analyses reveal that flame blowout occurs due to the stretching effect in the shear layers for both the triangular and semicircular bluff bodies. Moreover, it is found that the triangular bluff body has a smaller blow-off limit because of the stronger flame stretching as compared with the semicircular case. Calculations indicate that the two cases have negligible differences in heat losses because the reaction zones and high temperature regions are located in the combustor centers. Therefore, the heat losses have a negligible effect on the difference in the blow-off limit of the two micro-combustors.