Aerodynamic and Acoustic Interaction Effects of Adjacent Propellers in Forward Flight

Distributed electric propulsion systems are an emerging technology with the potential of revolutionizing the design and performance of aircraft. When propellers are located in close proximity, they can be subjected to aerodynamic interactions, which affect the far-field noise. In this paper, we study an array of three co-rotating and adjacent propellers to describe both the aerodynamic and acoustic installation effects. A scale-resolving CFD simulation based on the Lattice-Boltzmann/Very-Large-Eddy-Simulation method is used to solve the flow field around the propellers. An acoustic analogy integral approach calculates the far-field noise. Findings show that the helical vortical structures, generated at the tip of each blade undergo a flow deformation at the location of interaction. This causes the loading of each blade to vary during the rotation. Consequently, the unsteady loading noise becomes a dominant noise generation mechanism, driving the noise levels and directivity. It is also shown that introducing a non-zero relative phase angle between the propellers results in a reduction of the unsteady thrust, leading to a mitigation of the unsteady-loading tonal components along the rotation axis. Additionally, the relative phase angle causes constructive/destructive acoustic interference, as demonstrated by analyzing the noise emitted simultaneously by the three propellers

Can “publishing game” pressures affect the research topic choice? A survey of European accounting researchers

This study explores the uninvestigated area of research agenda setting, which has considerable influence on the societal impact of accounting academia, which the paying-off mentality stemming from a "publish or perish" culture risks jeopardizing. More specifically, it investigates the research topic choice of accounting researchers to ascertain whether and how the "publishing game" pressures induced by the governance principles of new public management influence this crucial decision. Survey evidence shows that European accounting researchers choose their research topics by considering (i) explicit research requests, (ii) short-term publishing opportunities, (iii) practical and educational needs, and (iv) the intellectual needs of the academic community. In this respect, researchers seem to form a heterogeneous community that places varying importance on these factors, suggesting different effects of "publishing game" pressures. The three clusters aim at societal impact through diverse avenues, while the probability of rapid publishing seems to be the primary driver of another cluster, thus revealing a substantial risk of goal displacement. This study contributes to the debate on publishing pressures in accounting academia by complementing the contextualized reflections of previous literature with evidence documenting their effects on what (in addition to how) accounting researchers study. These findings have policy and practical implications that can help policymakers, university managers, gatekeepers of the publishing process, and our entire academic community

Business Model in Accounting: An Overview

Despite the interest in business model (BM), the academic debate in the accounting field seems still in the early stage. We investigate the use of BM in accounting through a literature review and discuss the findings considering management and banking research. Specific streams and areas of improvement are identified

Turbulence-distortion analysis for leading-edge noise-prediction enhancement

The analytical model for leading-edge noise prediction formulated by Amiet, developed for a flat plate, relates the far-field acoustic pressure to the upstream inflow conditions, modeled by canonical turbulence spectra. The inaccurate results provided by this low-fidelity method when applied to thick airfoils has been attributed to the distortion experienced by turbulent structures when approaching the airfoil, not modeled in the original formulation of Amiet. The first attempts to account for the effects of this physical mechanism consisted of modifying the term representing the incoming turbulence by means of the analytical results of the rapid distortion theory, obtaining a promising improvement of the noise-prediction accuracy. This paper aims to set up the physical framework to investigate the relation between turbulence distortion and noise-generation mechanisms with the purpose of enhancing inflowturbulence noise modeling. A numerical database obtained for a rod-airfoil configuration has been chosen to allow the analysis of the vortex dynamics when interacting with a body. The analysis of the velocity field near the leading edge has highlighted that the extension of the region where turbulence distortion occurs depends on the size of the incoming turbulence structures. Furthermore, surface pressure fluctuations have been observed to peak at the same position along the airfoil where the pressure gradient in the streamwise direction is maximum. A novel approach has been proposed to account for turbulence distortion in Amiet’s model by using as input the turbulence spectrum directly sampled in this position. A satisfactory agreement with the prediction provided by the solid formulation of the Ffowcs-Williams and Hawkings analogy has been obtained

Aerodynamic and aeroacoustic investigation of vertical axis wind turbines with different number of blades using mid-fidelity and high-fidelity methods

The aerodynamics and aeroacoustics of small-scale Darrieus vertical axis wind turbine (VAWT) are investigated at chord-based Reynolds number below 4e5. A statistical temporal and grid convergence study is conducted to analyse the thrust and torque coefficients. Four different VAWTs with different numbers of blades (1, 2, 3, and 4) are investigated using both the high-fidelity Lattice Boltzmann Method (LBM) and mid-fidelity Lifting Line Free Vortex Wake (LLFVW) method. The statistical temporal convergence is achieved much earlier for the 1-bladed rotor than for the 3-bladed rotor, using both methods. Power performance analysis reveals that VAWTs with more blades generate more power at lower Tip Speed Ratios (TSRs), while VAWTs with fewer blades generate more power at higher TSRs. The aerodynamic efficiency of each blade decreases as the number of blades increases, leading to a decreased amplitude of rotor loading variation in a single rotation. Both the mid-fidelity LLFVW and high-fidelity LBM capture these physical trends well. However, LLFVW predicts lower peak thrust and torque values in a single rotation and higher streamwise velocities in the wake, as compared to LBM. Moreover, the former predicts higher average power output than the latter, and the discrepancy increases as the number of blades increases. In terms of noise, at constant TSR, low-frequency BPF noise is found to be higher in VAWTs with fewer blades, while high-frequency noise is found to be higher in VAWTs with more blades. Overall Sound Pressure Level values revealed that overall noise increased with an increase in the number of blades except for the 4-bladed VAWT for which the noise decreased

Measuring the Effectiveness of National Enforcers in the IFRS Context: A Proactive Approach

It is widely acknowledged that an effective enforcement system represents a crucial element to achieve significant improvements in financial reporting through the adoption of high-quality accounting standards. Indeed, the quality of financial reporting is considerably influenced not only by the standards to be adopted but also by their actual implementation, and consequently by enforcement mechanisms.The topic has generated considerable interest among scholars, who devoted their attention to developing different measures of the quality of the enforcement system. Building upon this literature, this paper aims at exploring the accounting enforcement system and focuses on controls over financial reporting considering two levels, namely the auditing activity and the controls performed by national enforcers.This paper extends the prior literature by proposing a dynamic measure of the accounting enforcement system capturing controls at those two levels. More specifically, the index here proposed focuses on the quality of the accounting enforcement operated by national enforcers in terms of proactivity, intended as the national enforcers’ capability to detect problems not highlighted in the auditors’ opinions, thus shifting the focus from an input to an output perspective. Indeed, the activities of auditors and national enforcers are strictly connected, given that the auditors’ opinion is the first public output of accounting controls and that is normally one of the bases for further investigation by national enforcers. An illustrative empirical analysis is carried out on the German and the Italian contexts to show the potential of the index for enforcement studies.</jats:p

A prospective evaluation on external jugular vein cut-down approach for TIVAD implantation

BACKGROUND: Totally implantable venous access devices can be implanted both by percutaneous approaches and by surgical approaches with cephalic vein or external jugular vein cut-down techniques that are related to low intraoperative complication rates. The authors report a prospective evaluation of 83 consecutive external jugular vein cut-down approaches for totally implantable venous access devices implantation. METHODS: Eighty three consecutive patients (28 M, 55 F, mean age 54.2) suffering from solid tumors (58) or hematologic diseases (25) were consecutively submitted to totally implantable venous access devices insertion through external jugular vein cut-down approach (75 on right side, 8 on left side). RESULTS: All devices were surgically implanted; no instances of intraoperative complications were detected. After a minimum follow-up of 150 days, only one case of wound hematoma and one case of device malfunction due to incorrect catheter angulation were noted. Postoperative patient satisfaction was evaluated by the use of specific questionnaire that demonstrated a good satisfaction and compliance (92.8 %) of patients with implanted devices. CONCLUSIONS: Despite the lack of controlled studies comparing external jugular vein cut-down approach vs other approaches, this approach should be considered as a tool for long-term central vein catheters positioning, both as an alternative and for primary approach

Low-order acoustic prediction tool for estimating noise emissions from distributed propeller configurations

A low-order numerical tool for estimating noise emissions from distributed propeller configurations is presented. The paper describes the tool’s computational framework, which uses Hanson’s near-field theory to calculate the thickness and loading noise components. The formulation assumes steady blade loading, but an unsteady case can be handled numerically by redefining the pressure distribution over the blade at each new time step. Two representative cases are analyzed to validate the tool: an isolated propeller operating in uniform flow and an array of three propellers in a side-by-side configuration under aerodynamic interference caused by adjacent propellers. The results obtained from the low-fidelity tool are compared to high-fidelity data to evaluate the accuracy and differences in predicting the noise of a distributed propeller system. The low-fidelity tool provides accurate results for both cases, with less than a 1.5 dB difference up to the fifth blade-passage frequency (BPF) when comparing tonal noise predictions at an observer located 10 diameters away and at the propeller plane. When analyzing the source directivity at the first BPF, there is a difference of approximately 0.5 dB at the propeller plane. However, this difference increases to 6 dB as the observer moves toward the inflow direction. This difference is due to the dominance of broadband noise near the propeller axis. The paper concludes with a noise analysis of the distributed propeller system, examining the relative importance of aerodynamic interference in the noise emitted by a propeller. In this case, the unsteady blade loading generated a tonal component of 40 dB at the first BPF in the propeller axis, while it had an insignificant effect at the rotor plan

Mitigation of Flow-Induced Cylinder Noise Through the Control of Sound Diffraction

The sound emitted by the flow past a circular cylinder can be described by a quadrupole placed at the outbreak location of the shedding instability and diffracted surface into the far field by the body with a dipolar directivity. This mechanism is greatly stabilized for a cylinder coated with a porous material, which features a substantial downstream shift of the onset location of the shed vortices that leads, in turn, to a reduction in the efficiency of the sound scattering and consequent noise mitigation. In this research, a novel design for a porous treatment of the cylinder based on the enhancement of this effect is proposed. Far-field acoustics tests were performed at the Delft University of Technology for Reynolds numbers based on the cylinder diameter ranging in the subcritical regime. The outcomes of the analysis demonstrate that, when the aft part of the flow-permeable coating is modified to make the internal flow more streamlined, an additional sound attenuation of up to 10 dB is achieved in comparison with a uniform porous cover. Moreover, a significant noise decrease of up to 10 dB and potential drag reduction are obtained if these components are connected to the bare cylinder without the use of a porous coating. This result can open up interesting opportunities to design disruptive and more optimized sound-mitigation solution

Performance and noise prediction of low-Reynolds number propellers using the Lattice-Boltzmann method

This paper proposes a CFD/CAA-based approach to predict the aerodynamic performances and tonal/broadband noise radiation of low-Reynolds number propellers at engineering level. Broadband self-noise prediction of low-Reynolds number propellers is particularly challenging, due to the requirement for the employed computational method to emulate the complexity of the laminar/turbulent boundary-layer behavior on the blade. In this study, the numerical flow solution is obtained by using the Lattice-Boltzmann/Very Large Eddy Simulation method, whereas far-field noise is computed through the Ffowcs-Williams & Hawkings' acoustic analogy applied on the propeller surface. A zig-zag transition trip on the propeller blades is used in the numerical setup to reproduce resolved turbulent pressure fluctuations in boundary-layer for broadband noise computation at a relatively low computational cost. The effect of using a transition trip to simulate low-Reynolds number propellers, as well as the impact of its chordwise position on the calculation of performances and radiated noise, is outlined. The trip position marginally affects the thrust and to a slightly larger extent the torque prediction. Tonal noise at the blade-passing frequencies does not show a relevant sensitivity to it, whereas broadband noise is found to be slightly more influenced by the chordwise position of the trip, especially at high advance ratios. The low sensitivity of the numerical results to the trip location, as well as their good agreement with loads and noise measurements carried out in the A-Tunnel of TU-Delft, demonstrates the robustness of the proposed approach for industrial applications