Bluff Body Structures Research Articles

Tangential Mode Combustion Oscillation of a Bluff-Body-Stabilized Diffusion Flame: Experiments

Addressing the issue of tangential mode combustion oscillation prevalent in aviation engine afterburners, this study targets bluff body diffusion flames. Innovatively, acoustic cavities were installed on either side of the bluff body flame, forming a tangential mode oscillation. The study employed experimental methods to study the flame pulsation characteristics under tangential oscillation, identifying the phase distribution of the flame front as a distinct feature differentiating it from conventional longitudinal mode oscillation. Furthermore, it was noted that the multiperiodic pulsation of the flame surface significantly contributes to the complex variability of the flame transfer function/flame description function. The research then established a phase relationship among velocity pulsation, pressure pulsation, and heat release rate pulsation, uncovering the phase coupling mechanism inherent in oscillatory combustion. It also delved into the impact of equivalence ratio and bluff body structure on tangential mode oscillation. Modifications to the bluff body structure led to a significant suppression of oscillation, reducing the amplitude by up to 81.3%. Thus, the study concluded that the flame dynamic response characteristics play a crucial role in determining the thermoacoustic coupling intensity, especially when the combustion system’s acoustic properties are stable. This finding lays a theoretical foundation for future endeavors to mitigate tangential mode oscillatory combustion in afterburners.

A Numerical-Experimental Framework to Separate the Effects of Different Turbulence Components on the Buffeting Forces of Bluff-body Structures

In conventional buffeting analyses of flexible civil engineering structures, the differences between the effects of longitudinal (u-) and vertical (w-) turbulence components were usually ignored. This assumption could cause unpredictable errors and needs to be refined. To improve the accuracy of buffeting analyses, this paper proposes a framework combining Computational Fluid Dynamics simulation and wind tunnel test to analyze the buffeting forces considering the differences between turbulence components. First, the Aerodynamic Admittance Functions (AAFs) with respect to u- and w- turbulence are numerically evaluated. Next, the total buffeting forces are experimentally measured. Finally, the numerical and experimental results are combined following a theoretical scheme to separate the effects of u- and w-turbulence. Results show that u- and w- turbulence have different contributions to the buffeting forces, directly indicating the inaccuracy of the conventional assumption. In the turbulence field investigated, the buffeting lift force at zero angle of attack (AoA) are all contributed from the w-turbulence, while the contribution from u-turbulence increase as AoA increases. This numerical-experimental framework overcomes the limitations of the conventional method by quantitatively describing the different contributions of u- and w- turbulence.

Flow characteristics of a trapped-vortex pilot cavity under the influence of radial structure

In this study, using a laboratory-scale combustor model, we examined the flow modes in the trapped-vortex cavity under the influence of a radial bluff-body structure by particle image velocimetry (PIV) technology. The tested area included both inside the cavity and the mainstream zone. It is found that, under different inlet parameters, structure width, and circumferential positions, the flow characteristics change significantly for this structure. The number of vortexes, the distribution of vorticity, and the Q-value differ at different circumferential positions; while the increase in radial structure width, incoming flow velocity, and total pressure will all lead to stronger vortex flow. The flow pattern inside the cavity and behind the radial structure are coupled with each other, forming six typical flow modes. Further theoretical analysis summarized the causes of the six flow modes according to two aspects: how the mainstream influences the cavity flow, and the influence degree of the mainstream on the cavity flow. Considering the effect of the radial structure, the mainstream at different positions will have different effects on the cavity, which can be represented by the ratio of the radial position to the width of the structure. The influence degree was related to the inlet parameters and the width of the radial structure, which can be represented by the dynamic pressure of cavity inlet jets.

Numerical investigation on combustion flow characteristics of a micro gas turbine swirl combustor with different protruded bluff body structures

A micro gas turbine swirl combustor with protruded bluff body is proposed. Compared with no bluff body, the protruded bluff body can significantly improve the combustion performance. When the bluff body height is 40 mm and the length is 30 mm, the average outlet NO emission is reduced by 14.43%, the pressure loss is decreased by 5.96%, and the outlet temperature distribution factor (OTDF) is dropped by 31.93%. The influence of different bluff body lengths (15–30 mm) and heights (10–50 mm) on the combustion flow are numerically analyzed further. The results show that the change of bluff body structure will affect the performance of combustor. In the range of 15–25 mm, with the increase of the bluff body length, the scope of central recirculation zone becomes narrower, velocity near the inlet and pressure loss decrease, the OTDF and field synergy angle β both become larger. When the bluff body length grows from 15 mm to 30 mm, the average outlet NO emission increases by 36.74%. The range of the central recirculation zone is gradually widen, and the average reaction rate grows up with the increase of bluff body height. The synergistic effect becomes better, and the heat transfer capacity is enhanced. When the bluff body height grows from 30 mm to 50 mm, the average outlet NO emission reduces from 35.84 ppm to 25.65 ppm.

Research on the performance of a valveless piezoelectric pump with a herringbone bluffbody

Aiming to improve the output performance of a valveless piezoelectric pump, this article presents a valveless piezoelectric pump with a herringbone bluffbody. The bluffbody is herringbone shaped and distributed in a tapered chamber. The tapered chamber and the bluffbody create a large reverse resistance in the chamber, thus effectively mitigating the backflow problem of the valveless pump. The theoretical analysis determined the relationship between the flow rate and the flow resistance coefficient as well as the variation of the pump chamber volume. It was also concluded that the piezoelectric pump has the best output flow at intrinsic frequencies. Through simulation calculations, the effectiveness of the bluffbody structure in mitigating backflow in piezoelectric pumps is analyzed to provide a reference for experimental prototype design parameters. Finally, a range of prototypes is produced for experimentation. The experimental results show that the designed bluffbody shape can increase the return energy loss to effectively mitigate the return flow issues of the valveless piezoelectric pump, thus improving the output performance. The optimum output flow rate is 158.5 ml/min at 200 V and 52.5 Hz and the tapered chamber angle of 6°, and the bluffbody height, angle, and quantities are 2 mm, 40°, and 2, respectively. The construction of the valveless piezoelectric pump proposed in this research can be used as a reference for subsequent improvements in the performance of valveless piezoelectric pumps, and due to the high output performance, experimental studies can be carried out in applications such as dispensing and heat dissipation in electronic products.

Numerical Simulation of the Combustion Characteristics in a Flue Gas Internal Recirculation Burner

The combustion characteristics and NOx emissions of a newly designed flue gas internal recirculation low-NOx burner (FIR) were studied. In the study, experimental and numerical simulations of the FIR low-NOx burner were conducted under natural inlet air conditions at three different powers. Results show that the fuel inlet strongly influences the jet effect, thus influencing the flue gas recirculation rate, flame stability, and NOx emissions. With a medium power of 20 kW, the NOx emission of a FIR low-NOx burner is lower than 30 mg/N m3. Higher or lower power will increase the NOx emissions or induce combustion instability. The swirling flow and bluff body structure can effectively improve the combustion stability. Three dominant frequencies of 54, 264, and 448 Hz can be observed from the power spectral densities of axial velocity, corresponding to the shedding vortex in the shear layer, the swirl frequency of processing vortex core (PVC), and the vortex induced by the PVC structure, respectively. The influences of vortex shedding and PVC structure are weak and inadequate to affect the overall flame stability.

Bluff body vortex-induced vibration control of floating wind turbines based on a novel intelligent robust control algorithm

Given the great potential of the offshore wind power generation in renewable energy sources, it will bring unprecedented significant development opportunities. Meanwhile, the installed capacity of floating wind turbines (FWTs) is huge. However, as one of the important parts of that, FWTs are always subjected to complex environmental loads during operation, which will critically affect the stability of wind power generation. Hence, it is urgent to analyze and control its stability for the safe operation of wind turbines. It is accepted that vortex-induced vibration (VIV) of a bluff body structure is the leading cause of structural damage to FWTs. For this reason, a radial basis function neural network sliding mode control (RBFNNSMC) is proposed to improve the modeling accuracy of bluff body VIV control. Then, the joint numerical analysis system was designed to achieve the completely coupled fluid structure vibration control of bluff body. The numerical results indicate that RBFNNSMC can better control the forward/cross-flow vibration of bluff body. In addition, the controller is not responsive to changes in system parameters and has strong robustness.

Combining active and passive wind tunnel tests to determine the aerodynamic admittances of a bridge girder

In conventional buffeting analyses for long-span bridges, the aerodynamic admittance functions (AAF) with respect to the longitudinal (u-) and vertical (w-) turbulence components are usually assumed to be equivalent. This assumption could cause unpredictable errors due to complicated turbulence-structure interactions. This study proposes an experimental method to determine the u- and w- AAFs in the framework of three-dimensional buffeting theory. First, passive wind tunnel test is conducted to measure necessary aerodynamic parameters. Next, the turbulence characterized only by u-velocity fluctuations is generated in a multi-fan active-controlled wind tunnel to reproduce the u-velocity component in the passive wind tunnel test. The aim is to identify the u-AAF separately. Finally, the w-AAF is evaluated by combining the parameters measured in the active and passive wind tunnel tests. For a streamlined bridge girder, results show significant discrepancies between the u-AAF and w-AAF. The derivation degrees depend on the frequency and the angle of attack (AoA). This indicates the inaccuracy of the conventionally invoked assumption. In detail, the w-AAF is higher than the u-AAF at low frequencies, whereas the relationship reverses at higher frequencies. The proposed method is also applicable for other bluff-body structures, it is useful for improving the accuracy of buffeting analyses.

A 3D numerical and experimental parametric study of wave-induced scour around large bluff body structures

This is the first study to undertake a comprehensive parametric study of wave-induced scour around a large bluff body structure, and also presents experimental data to validate a CFD model for wave-induced scour around the structure. A parametric study is undertaken using the REEF3D CFD model for varying incident wave condition (depth, wave amplitude, frequency, and direction), structure geometry (width, length, and corner curvature), and sediment size. The numerical model is validated against experimental datasets, demonstrating that the model can accurately replicate scour around large bluff body structures.Two new equations are proposed for prediction of non-dimensional equilibrium scour depths at the front and back corners of a large bluff body structure using the Keulegan-Carpenter (KC) number based on 18 and 24 datapoints respectively. The equations are valid within the following limits: 0.37 ≤ KC ≤ 1.75, 0.10 ≤ B/λp and 0.14 ≤ L/λp, where B and L are the structure width and streamwise length, and λp is the peak wave length. It is further demonstrated that the non-dimensional scour depth does not vary significantly with the sediment size (for 0.075 ≤ θ ≤ 0.117 where θ is the Shields parameter), or the structure width/length for the limits stated above.

Experimental measurements of wave-induced scour around a scaled gravity-based Oscillating Water Column Wave Energy Converter

Experimental measurements of wave scour are reported for a 1:20 scaled model of an operating 200 kW Oscillating Water Column (OWC) Wave Energy Converter (WEC) in King Island, Tasmania, Australia. Test cases include different wave conditions (Keulegan-Carpenter (KC) number range of 0.45-1.02) and multiple geometric changes to the device, which are compared to scour measurements taken in the field around the King Island OWC WEC device. The measurements therefore provide the first ever comparison of field data with a scaled laboratory model of scour around an OWC WEC.Results highlight that scour forms at the back corners of the device before undermining the structure with depths up to 64 mm (scour depth to structure width ratio, S/B, of 0.094), although the magnitude of this scour is shown to be reduced by 23% by curving the back corners. Scour at the front of the device is highly sensitive to the configuration of the front chamber door, with very different scour scenarios depending on whether the chamber door is open, closed, or removed entirely. Comparison with field data shows that undermining scour at the back of the device is reproduced in both settings, and this leads to settlement of the structure in both the laboratory and field. This suggests that the physical modelling forms an accurate representation of a real-world site, and therefore demonstrates scour processes with applicability to OWC WECs and other gravity-based bluff body structures such as caissons or gravity-based foundations.

Combustion Performance of Methane/Air in a Micro Combustor Embedded Hollow Hemispherical Slotted Bluff Body

With the rapid development of micro-energy power systems, the performance of micro-combustors as key components is in urgent need of further improvement. Aimed at enhancing combustion performance, a hollow hemispherical bluff body was used to analyze the methane combustion process. In this paper, we exploited the detailed reaction mechanism of methane/air with a laminar finite-rate model; the numerical analysis of methane combustion in the micro-combustor was carried out by Ansys Fluent software. The combustion, flow and thermal characteristics of the micro-combustor embedded with a hollow hemisphere bluff body (MCEHB) and the micro combustor embedded with a slotted hollow hemisphere bluff body (MCESHB) are compared, and the effect of slot width ratio on the combustion characteristics and thermal performance is discussed in detail. The results showed that the bluff body slotting treatment is not only beneficial to improving the velocity and temperature distribution behind the bluff body but also can improve the conversion rate of methane, especially at high inlet velocities. However, the conversion rate of methane is also affected by the slot width. When the slot width ratio below 0.5, the slot width corresponding to the peak methane conversion increased with the inlet velocity. Moreover, the bluff body slotting treatment can improve the wall temperature distribution, meanwhile expanding the high temperature area at the inner wall, thereby reducing the wall temperature fluctuation in the rear part of the micro-combustor. In addition, the optimal slot width ratio B increases with the inlet velocity. Since the inlet velocity is lower than 0.5 m/s, the optimal slot width ratio B is in the range of 0.3–0.375. However, as the inlet velocity exceeds 0.5 m/s, the optimal slot width ratio B moves to the range of 0.375–0.553. Furthermore, both large and small slot widths bring obvious temperature fluctuations to the micro combustor; the uneven wall temperature distribution phenomenon is detrimental to working performance. Therefore, the slot width ratio B of 0.375 only brings slight temperature fluctuations, indicating this is an optimal slot width ratio that should be chosen. This work has reference value for optimizing the design of the bluff body structure and improving the combustion performance of methane in the micro combustor.

Numerical comparison of H2/air catalytic combustion characteristic of micro–combustors with a conventional, slotted or controllable slotted bluff body

The flame instability and low combustion efficiency are still two vital problems to be solved in micro–combustion power field. In this work, the detailed chemical reaction mechanism of H2/O2 is used to numerical study the combustion characteristics of micro–combustors under three kinds of bluff body structures. The results show that a recirculation zone is formed in the conventional bluff body (CBB) micro–combustor, while two symmetric recirculation zones are formed in the slotted bluff body (SBB) micro–combustor and controllable slotted bluff body (CSBB) micro–combustor. Notably, the recirculation zones in the CSBB micro–combustor is significantly prolonged with the increase of controllable flow ratios. Meanwhile, the corresponding equivalence ratio reaching a maximum combustion efficiency is 0.8 in the CSBB micro–combustor, while the CBB and SBB micro–combustors reach the highest combustion efficiency at φ = 1.0. This means that the CSBB micro–combustor exhibits superior combustion performance in the fuel lean condition (φ = 0.8). Additionally, the adjustment of the controllable flow ratios can significantly affect the combustion efficiency of the CSBB micro–combustor. Moreover, the blow–off limits of CBB, SBB and CSBB micro–combustors achieve the maximum at φ = 1.0, which are about 540, 456 and 600 cm3/s, respectively. Thus, the proposed CSBB micro–combustor is of notable advantages compared with the other two micro–combustors in terms of combustion efficiency, temperature distribution, blow–off limit, and so on. Another interesting finding is that the CSBB micro–combustor with a gap width of 0.6 mm has better combustion performance. In conclusion, the controllable slotted bluff body is a good way to stabilize the flame in micro–combustors. This design offers us another way to design such kind of micro–combustors.

Development of a novel vibro-wind galloping energy harvester with high power density incorporated with a nested bluff-body structure

A two degrees of freedom (DOF) vibro-wind galloping energy harvester with a nested structure is proposed to improve wind energy harvesting efficiency. Unlike traditional wind energy harvesters configured in a single bluff body-beam structure, the proposed system comprises outer and inner square cross-sectional bluff bodies in tandem, connected with elastic cantilevered beams to allow separate oscillations. The generated power performance of a conventional single-DOF wind galloping energy harvester and the proposed system is compared through wind tunnel tests. Aerodynamic interactions between the two oscillating bluff bodies are analyzed by varying the gap distances and the coupling mechanism is determined by analyzing their power spectral density (PSD). Experimental results show that the outer bluff body has a dominant influence on the inner bluff body at low wind speed, which is a secondary cause of inner body vibration after the plain galloping directly induced by the incoming wind. The proposed system exhibits excellent performance with a significant power density increase of 27.8%. Moreover, the interference effects between the two bluff bodies can be reinforced by reducing their gap distance. We conclude that the inner bluff body plays a key role in enhancing output power, specifically at relatively high wind speed.

FlowSculpt: software for efficient design of inertial flow sculpting devices.

Flow sculpting is a powerful method for passive flow control that uses a sequence of bluff-body structures to engineer the structure of inertially flowing microfluidic streams. A variety of cross-sectional flow shapes can be created through this method, offering a new platform for flow manipulation or material fabrication useful in bioengineering, manufacturing, and chemistry applications. However, the inverse problem in flow sculpting - designing a device that produces a target fluid flow shape - remains challenging due to the complex, diverse, and enormous design space. Solutions to the inverse problem have been constrained to single-material fluid streams that are shaped into top-bottom symmetric shapes due to the bluff-body structures available in current libraries (pillars) that span the height of the channel. In this work, we introduce multi-material design and symmetry-breaking flow deformations enabled by half-height pillars, presented within an extremely fast simulation method for flow sculpting yielding a 34-fold reduction in runtime. The framework is deployed freely as a cross-platform application called "FlowSculpt". We detail its implementation and usage, and discuss the addition of enhanced search operations, which enable users to more easily design flow shapes that replicate their input drawings. With FlowSculpt, the microfluidics community can now quickly design flow shaping microfluidic devices on modest hardware, and easily integrate these complex physics into their research toolkit.

Numerical investigation of wake induced vibrations of cylinders in tandem arrangement at subcritical Reynolds numbers

Wake-induced vibrations are considered distinctively different from well-known vortex induced vibrations where bluff body structures vibrate under vortex shedding in the wake of flows passing through them. Understanding characteristics of wake-induced vibrations plays an important role in design of marine-offshore structures such as platform columns, risers operating at high Reynolds number currents. This work presents an extensive study of wake-induced vibrations using a computational fluid dynamics approach. The numerical model is based on an incompressible Navier-Stokes flow solver with a hybrid detached eddy simulation (DES) approach for turbulence modelling of high Reynold number flows. The hybrid DES approach is first validated for wake-induced vibrations at subcritical Reynolds number ranging from 103−105. Comparison between numerical results and experiment shows very good agreement in prediction of downstream cylinder responses including amplitudes, frequency and phase angle. Data from numerical simulations is then used to characterize and study mechanisms of wake-induced vibrations by looking at detailed force components and their frequencies in relation to cylinder responses. The effect of different Reynolds number flow conditions in tandem cylinder responses are also analysed in the present work. It is found that Reynolds number has strong influence in changing response amplitude and frequency of cylinders in wake-induced vibrations.

Effects of a jet turbulator upon flame acceleration in a detonation tube

Allowing a fuel/air mixture to flow through bluff-body structures enhances the deflagration-to-detonation transition (DDT) process in pulse-detonation engines by increasing the turbulence intensity within the detonation chamber. However, these structures give rise to resistance and thrust loss. The use of jets as turbulators to reduce the obstacle-blockage ratio may reduce this loss. A jet turbulator composed of multiple evenly spaced jets on the tube wall was designed, and its effects upon flame acceleration were analysed. The results show that fuel/air-mixture jets combined with a low obstacle-blockage ratio can accelerate flame propagation whereas pure-air jets cannot. The jet turbulator offers an improvement over bluff-body structures in terms of acceleration of the DDT process.

Research on the Vortex Mass Flow Meter of Dual Bluff Body Based on Differential Pressure Principle

The lower limit of the existing vortex mass flow meter based on differential pressure is high. So the application of the existing vortex mass flow meter is limited in the measurement for low flow rate. This paper proposes a method using vortex mass flow meter of dual bluff body based on differential pressure principle. The differential pressure signal between the upstream and downstream can be amplified by the vortex overlap caused by the dual bluff body structure. The results of the simulation by Fluent show that this method can reduce the lower limit of measurement, and improve the measurement sensitivity effectively.

On the aerodynamic and aeroelastic response of a bridge tower

This work is concerned with the aerodynamic characterization of a cable-stayed bridge tower in free-standing configuration; experimental tests were performed at Politecnico di Milano Wind Tunnel under smooth and turbulent flow conditions. The aerodynamic behavior of the tower was investigated through static and dynamic tests on a 1:30 scale sectional model; the whole structure response has been studied using a 1:100 scale full aeroelastic model in stand-alone configuration. Aim of this paper is to compare wind tunnel test results obtained on the two models for different wind exposures, discussing the problems related to scale effects in scale model testing. Some inconsistencies found during the experimental campaign confirm that scale effects in wind tunnel testing represent a serious issue especially for bluff body structures whose vortex shedding response is strongly affected by Reynolds number.

DYNAMICS OF MIXING PROCESSES IN THE MICRO-MIXER WITH CURVED BLUFF-BODY STRUCTURE UNDER EXTERNAL EXCITATION

This paper investigates the effects of external excitation on the mixing performance of the micromixer with curved bluff-body structure. The micromixer was fabricated by the MEMS process of polydimethylsiloxane (PDMS). The mixing process and mixing efficiency were evaluated with a high speed camera. Results showed that the finger-spiked type flow patterns were generated in the mixing chamber under an excitation frequency of 5Hz. It turns out that the mixing efficiency as high as 85.6% is achieved at Re =0.25 with a single bluff-body structure. It demonstrates that the new design can be used to achieve complete mixing within ultra short length at mciroscale.

TIMPAN ‐ Technologies to IMProve Aiframe Noise

TIMPAN is a 3‐year Strategic Targeted Research Project of the 6th European Framework Programme, Priority "Aeronautics and Space", launched in 2006. It addresses the community noise reduction objective for commercial aircraft by focusing on main airframe noise sources: landing gear and high‐lift devices ‐ responsible, on recent aircraft, for about half of the total noise in approach situation. TIMPAN addresses both sources, within 3 main tasks: 1) Landing gear activity: investigation of both innovative low noise technologies on bluff body structures and the improvement of advanced low noise main gear design, as outcome from previous EC Technology Platform SILENCE(R). 2) High lift device activity: study of both innovative concepts based on flow control technologies and mid‐term noise reduction solutions as absorptive wing leading edge treatments and high‐lift settings optimization through computational aero‐acoustic methods. 3) Technology evaluation: aims to prepare the exploitation phase by evaluating the noise reduction technologies under consideration in terms of noise benefit, integration, cost and performance. TIMPAN brings together 14 actors from the European aeronautics industry including aircraft manufacturers (Airbus and Dassault Aviation), landing gear manufacturers (Messier‐Dowty and Messier Bugatti), key research institutes (DLR, ONERA, NLR, EADS‐IW), universities (University of Southampton, Technical University of Braunschweig) and SMEs (ATECA, Free Field Technologies).