771 research outputs found
Investigation of propeller characteristics at low Reynolds number with an angle of attack: A computational aeroacoustic study
No full textAdvancements in technology have made commercial unmanned aerial vehicles reliable and readily available, leading to an exponential rise in their market demand over the past few years. COVID-19 has further accelerated this growth through an increase in demand for contact-less delivery and crowd monitoring systems. However, despite these favorable conditions, their limited range, perceived threat, and concerns about noise pollution in urban environments have prevented them from being widely accepted by society. A recent study by NASA found that people perceive UAV noise to be more annoying than cars, and trucks at a similar sound pressure level, which highlights the need to understand the acoustic characteristic of these aircraft. These UAVs are generally powered by electric motors, making their propellers the most dominant source of noise. In the past, researchers have conducted several studies to understand and characterize the noise produced by aircraft propellers. However, these studies were limited to high Reynolds (>1,000,000) and Mach number operations for large commercial aircraft, creating a significant gap in the understanding of the aerodynamic and acoustic characteristics of propellers operating at low Reynolds (<200,000) and Mach number. This thesis aims to address the research gap by performing a high-fidelity computational simulation using Dassault Systèmes PowerFLOW®. The tool uses a lattice Boltzmann very large eddy simulation (LBM-VLES) based approach to compute the aerodynamic results and the Ffowcs-Williams and Hawkings (FWH) aeroacoustic analogy to calculate far-field acoustic values. The main objective of the thesis is: “To characterize and quantify the effect of non-axisymmetric inflow conditions on the aerodynamic and acoustic properties of propellers operating at low Reynolds numbers.” To meet the objective, a computational setup consisting of a twin-bladed propeller with a radius of 15 cm is designed in PowerFLOW®. The propeller is analyzed at 0º and 15º AoA, operating at 6000 RPM with a free stream velocity of 12 m/sec and the results validated against experimental data. Aerodynamic measurements and flow analysis revealed that the change in angle of attack (AoA) resulted in a 3.87% increase in the net thrust, and 1.16% increase in the net torque value of the propeller. Operating at an AoA, the propeller blade experiences asymmetric loading around the propeller plane, the loads fluctuate by 35% between the points of maximum and minimum loading. Further analysis of the propeller flow field is carried out by averaging the velocity field and performing a phase-locked analysis to visualize the vortex field. The analysis helps in understanding the effect of AoA on propeller wake and quantifies its a symmetric nature. Far-field acoustic data is acquired by two circular microphone arrays, with a polar angle resolution of 10º. The arrays are placed around the propeller plane and along the axial axis of the propeller. The change in AoA results in a 3 dB higher noise at an azimuthal angle (Ψ) of 90º and reduces by an equal magnitude at Ψ = 270º. The shift is attributed to the change in propeller tip Mach number and local blade AoA as a function of its azimuthal location and propeller AoA. Further analysis of the sound power level (PWL) produced by the propeller is carried out, showing a 1.5 dB increase in the PWL produced by the propeller blade at 15º AoA than 0º.Aerospace Engineerin
Directive speech acts in the New Testament
No full textDanuta Pytel-Pandey, WrocławThis article discusses the directive speech acts in the New Testament. The
author presents examples of such acts from the New Testament Scriptures, and
then carries out their pragmalinguistic [email protected]
Similarities between 2D and 3D convection for large Prandtl number
No full textUsing direct numerical simulations of Rayleigh-B\'enard convection (RBC), we perform a comparative study of the spectra and fluxes of energy and entropy for large and infinite Prandtl numbers in two (2D) and three (3D) dimensions. We observe close similarities between the 2D and 3D RBC, in particular the kinetic energy spectrum , and the entropy spectrum exhibits a dual branch with a dominant spectrum. We showed that the dominant Fourier modes in the 2D and 3D flows are very close
Evaluation and analysis of impact of subsidies on small scale renewable energy technologies dissemination: a case study of Nepal
No full textThe report has analysed the status of rural electrification using renewable energy technologies (RETs) and the current subsidy delivery modality of RETs in Nepal. As the Government of Nepal is subsidizing small scale renewable energy projects, it is important to evaluate and analyse the programme to make sure that a programme is running in an effective and efficient way while reaching the target group.
During the course of the research, primary information collected from the field has been compiled using appropriate tools/software (such as MS Office) and analyzed. Different sets of questionnaires were prepared, targeting different personnel ranging from the users’ level to the government policy level. The field survey methods include field observations of RETs installations and operations along with interviews with selected users on a random sampling basis to identify barriers to subsidy delivery and possible measures to overcome these barriers; to determine user satisfaction level and time period for subsidy delivery mechanisms.
The report started with the country background information and the rationale behind the study. This is followed by the literature review and information on the current status of small scale RETs in the country. Next it deals with the organizational structure and current subsidy delivery modality of RETs in the country. The evaluation of the effectiveness of the subsidy with the current subsidy delivery modality has been analyzed based on six parameters; Targets and Achievements, Impact of RETs, Subsidy Delivery System Efficiency, Transparency, Sustainability and Effectiveness.
The research shows that the subsidized government program has been successful in achieving its goal to provide basic energy services in rural areas through RETs in terms of its quantitative target. Although the subsidized program is successful in electrifying the rural communities via RETs, the poorest of the poor families are still excluded from the subsidy. The program seems to benefit more the rich and upper middle class families rather than the poor families for which the subsidy was intended. The high capital cost and long subsidy delivery process are the main important reasons for this failure. The author has proposed a new modality which might overcome the time barriers and bureaucratic process in the subsidy delivery mechanism and will decrease the lead time. This will in turn help to reduce the operational cost of the private companies and will encourage more players to enter the market, increase competition and result in lower system costs.
After identifying gaps in the current subsidy delivery modality, the project report ends with the recommendation of a new modality improving the current modality and filling the gaps identified. As this study has some limitations, as described in chapter one, the scope of further works has been listed at the end
Performance analysis of a compact heat exchanger
No full textCompact heat exchangers are one of the most critical components of many cryogenic components; they are characterized by a high heat transfer surface area per unit volume of the exchanger. The heat exchangers having surface area density (β) greater than 700 m2/m3 in either one or more sides of two-stream or multi stream heat exchanger is called as a compact heat exchanger. Plate fin heat exchanger is a type of compact heat exchanger which is widely used in automobiles, cryogenics, space applications and chemical industries. The plate fin heat exchangers are mostly used for the nitrogen liquefiers, so they need to be highly efficient because no liquid nitrogen is produced, if the effectiveness of heat exchanger is less than 87%. So it becomes necessary to test the effectiveness of these heat exchangers before putting them in to operation
this paper is the real work of the author
No full textthe author is the renouned economist and have an experience of 25 years as a reader and as a guest professor .unemployemnt
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