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A novel modeling strategy is proposed which allows high-accuracy predictions of aerodynamic and aeroacoustic target values for a low-pressure axial fan, equipped with serrated leading edges. Inspired by machine learning processes, the sampling of the experimental space is realized by use of a Latin hypercube design plus a factorial design, providing highly diverse information on the analyzed system. The effects of four influencing parameters (IP) are tested, characterizing the inflow conditions as well as the serration geometry. A total of 65 target values in the time and frequency domains are defined and can be approximated with high accuracy by individual artificial neural networks. Furthermore, the validation of the model against fully independent test points within the experimental space yields a remarkable fit, even for the spectral distribution in 1/3-octave bands, proving the ability of the model to generalize. A metaheuristic multi-objective optimization approach provides two-dimensional Pareto optimal solutions for selected pairs of target values. This is particularly important for reconciling opposing trends, such as the noise reduction capability and aerodynamic performance. The chosen optimization strategy also allows for a customized design of serrated leading edges, tailored to the specific operating conditions of the axial fan.
Extensive research efforts in the aeroacoustics community have firmly established the benefits of porous trailing edges to achieve low-noise radiation. However, most studies of porous treatment are based on the use of very complex, open-cell structures to manipulate turbulent flow. Although this implementation has been shown to improve the aeroacoustics performance, the exact physical mechanisms that can be drawn from such a geometry are limited due to their complex topology. This study aims to draw from previous works and to develop an optimised experimental method that utilises a 3D-printed array of rectilinear, structured permeable trailing edges on a NACA-0012 aerofoil based on a Box-Behnken experimental design. The essence of the work is to isolate individual porous parameters, and investigate the interdependencies of these parameters on target values such as the overall sound power level, the Strouhal number of the maximum noise reduction and many other characteristics of the far field. Twenty-eight porous trailing edges were produced based on the initial experimental design. Each is unique with the combination of streamwise and spanwise separation distance between the pores, pore size and porous coverage. The experiment was conducted over various angles of attack and Reynolds numbers. The results show that many of these trailing edges can indeed achieve low-noise radiation, and acceptable prediction accuracies are obtained for all the response variables except the total sound power reduction, ΔOAPWL, and the lower Strouhal limit of the noise reduction. This paper will establish the findings, discuss the results and detail the next stage of the experiment for the improvement of the statistical model.
Potential and Evolution of Miniatures Compressed Air Energy Storage Plants Based on Impulse Turbine
(2022)
This paper describes the work carried out to develop an impulse turbine for miniatures compressed air system. This study hypothesizes the question; what is the effect of combining an impulse turbine loss model into a compressed air energy storage system analysis? The miniatures power system has lower mass flow rates which lead to a small turbine size. The miniature impulse turbine has relatively low efficiency and is highly sensitive to operating conditions at a low mass flow rate due to all losses in terms of passage, trailing edge, incidence, and clearance becoming higher amounts compared to the total losses of the percentage foundation.
The development of a novel impulse turbine configuration is presented based on one-dimensional design and three-dimensional simulations. The impulse turbine in single-stage configuration was designed and analyzed for a range of operating conditions in terms of pressures, temperatures, mass flow rate, and rotational speeds. The simulations results showed that the maximum efficiency and power were 65.93% and 4.019 kW respectively with a mass flow rate of 0.2 kg/s. The energy analysis revealed that the system efficiency was 10.3%. The miniature compressed air energy storage system driven by an impulse turbine can be used to generate electricity for small power applications.
Passive air-jet blowing is an effective yet simple technique to control flow-induced noise due to vortex shedding of bluff bodies. The current study investigates the effect of the specific slot angles of passive jets in connection with the suppression capabilities of vortex shedding and the byproduct of vortex-induced noise in the wake region of a circular cylinder. Aeroacoustic tests for a baseline case and 10 different slotted cases with slot angles of 80°≤𝜃≤125° are performed for Reynolds numbers 6.6×103≤R≤3.3×104. This is supplemented by numerical computational fluid dynamics (CFD) analyses to identify the underlying aerodynamic mechanisms. The results obtained reveal that using the current passive control method results in a significant reduction of the vortex shedding tonal noise for slot angles of 90°≤𝜃≤125° and high Reynolds numbers. The numerical results showed good agreement with a remarkably reduced kinetic energy for slot-end angles of 115°≤𝜃≤125°. At low Reynolds numbers, however, the identified aeroacoustic benefits tend to cease.
Leading edge serrations are well known for their ability to reduce turbulence-induced noise of single aerofoils while also providing aerodynamic advantages under certain operating conditions. Continuatively, applying leading edge serrations to rotating machinery such as axial fans proved the validity to generally transfer the obtained aeroacoustic benefits of single aerofoils. However, for the rotating applications the noise reduction potential highly depends on the point of operation. The current work aims at assessing the aeroacoustic effects of serrated leading edges under the increased geometrical complexity of the fan blades through blade skew. Therefore, the question is whether combining two potentially effective noise-reducing treatments through blade skew and leading edge serrations results in leveraging or obstructing effects. By varying the skew angle from 0 deg to 38 deg, four different prototypes of the fan impeller are tested experimentally in a test rig according to ISO 5136 and ISO 5801. All configurations are tested with original blades of straight leading edges plus five sets of serrations each, parameterised by the serration amplitude and the serrations wavelength. The intensity of the incoming turbulence ranges from 2.6% to 12.1%. The results obtained show the skewed blades to improve both the aerodynamic performance and the noise radiation after exceeding an initial skew angle, complemented by a significant onset of stall. Moreover, no contraindication between blade skew and serrated leading edges is encountered, showing the potential to further extend the noise reduction capabilities by combining effects of blade skew and leading edge treatment.
Low-Noise Design of Axial Fans Through Optimized Spanwise Application of Leading Edge Serrations
(2023)
Recent studies have shown that leading edge serrations can effectively reduce broadband noise in axial fans and expand their operational range. However, additional aerodynamic losses have to be considered alongside the benefits of improved acoustic performance. The majority of studies focused on applying leading edge serrations along the entire span of the rotor blades regardless of local differences in radial inflow angles, deflection effects, and blade loading. In this regard, local and spanwise varying applications of leading edge serrations are considered crucial to reconciling maximum aeroacoustic and aerodynamic performance. In making these applications, a fan blade span is divided into five sub-segments. Each segment can then be equipped with either an acoustic or aerodynamic optimum as well as a baseline straight leading edge. Through testing at three different inflow turbulence levels along the full characteristic curve, an extensive statistical experimental design is executed. Ultimately, a simplified statistical model is derived. The model provides information on the optimum spanwise shape of a serration design. The results indicate significant effects of the local spanwise selection of varying serration designs. Discrepancies between aerodynamic and aeroacoustic optimum solutions can be satisfactorily resolved where, primarily, a significant extension of the working area is observed. Moreover, an extended potential in the delay of stall entry and the associated improvement of total pressure as well as a significant reduction of aeroacoustic signature is found. The underlying mechanisms are attributed to the compartmentalization effects of the serrations which restrict separation phenomena to local cells of finite spanwise extension. Furthermore, the spanwise geometrical parametrization of the serration segments correlates well with radial blade loading as well as respective flow conditions. By taking the radial distribution of aerodynamic quantities into account, an individual design of locally applied leading edge serrations can be integrated into the fan design process, enabling improved balancing between aeroacoustic benefits while maximizing aerodynamic performance.
Controlling flow around a circular cylinder is essential for reducing drag, alleviating lift fluctuations, and enhancing aerodynamic efficiency in various engineering applications. In the present work, the effect of localized passive air-jet injection at different azimuthal angles on the near-wake region of circular cylinder was numerically investigated at subcritical Reynolds numbers Re = 3900 and 20,000 using ANSYS-FLUENT commercial software. The slot angle was varied from 70° to 130° with an increment of 10°. To insure the reliability of the current numerical model, the simulation results were validated with available experimental and numerical literature. The results revealed that the slot can significantly affect the flow structure in the near-wake region and pressure distribution around the cylinder at slot angles 100° ≤ θs ≤ 130°. It was found that the optimum slot angle was 130° at which a good pressure recovery on the rear surface of the cylinder was achieved. Moreover, turbulent kinetic energy in the very near-wake region showed a substantial decrease of 40 % compared to the BL case at lower tested Reynolds number, as well as an 11 % reduction in the extent of the recirculation region. The drag analysis showed that the drag can be reduced by 26 % when the slot inclined at 130°. It can be concluded that the position of the slot induced jet plays a vital role on the flow structure, pressure distribution, and drag coefficient. Additional experimental studies are required to further explore the impact of the slot on the wake region. Specifically, investigating the combined effect of 70° and 130° slot angles could provide deeper insights, potentially leading to a more significant influence on the wake region