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Lighter-than-air (LTA) aerial vehicles such as airships and aerostats can be found in various strategic and commercial applications, primarily due to their capability to hover and stealth. The mathematical model of these vehicles helps in understanding their complex dynamics and designing and developing proper stabilisation systems for them. Stability derivatives have been used for developing mathematical models for heavier-than-air aerial vehicles since their introduction. This paper presents a methodology to develop a mathematical model of an aerostat based on stability derivatives. One of the major contributions of this study is the estimation of aerostat's added mass terms expressed as longitudinal stability derivatives due to acceleration of the longitudinal motion variables. A longitudinally decoupled linear mathematical model of a single-tethered aerostat using stability derivatives is investigated in this study. A computational fluid dynamics (CFD)-based analysis of the 3D model of the vehicle is used to obtain the stability derivatives. The methodology presented considers the aerostat and tether models separately before coupling them to create the full model. The stability derivative analysis is carried out using ANSYS Fluent, and the coupled tethered aerostat model is investigated using MATLAB 2020. The negative pitch angle of the aerostat is caused by the selection of the pitching centre as the aerostat centre of volume instead of the tether confluence point. The tension force on the tether, which is proportional to the wind velocity, and aerostat velocity components are found to be stabilised within 200–400 s. [ABSTRACT FROM AUTHOR]

Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k-e/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh=5, 7, and 9) and separation distance (H/Dh=3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance. [ABSTRACT FROM AUTHOR]

An analysis of the effect of wind gusts on the aerodynamic characteristics of an aerostat is presented in this study. A three dimensional computational fluid dynamics based flow analysis of an aerostat using the realizable k − ϵ model is presented. The performance of the aerostat is analyzed by varying the angle of attack, gust amplitude, gust duration, gust velocity profile and the aerostat shape. Actual wind data is used to formulate the gust parameters used for the study. It is found that the aerostat drag is unaffected by the angle of attack because of the streamlined structure. A trade-off between the drag and lift values suggests the use of a slight positive angle of attack for the stable operation of the aerostat. A gust with a short rising or falling time with small amplitude is severe than the symmetric gust with larger amplitude and a slower rising or falling profile. A scaling methodology for the aerodynamic characteristics of the aerostat for different angles of attack, gust amplitudes, gust duration and aerostat shapes is presented. The scaling obtained for different angles of attack and gust amplitudes is linear, whereas for different gust duration the scaling is not linear. [ABSTRACT FROM AUTHOR]

This paper presents a study on the effect of solidity and airfoil profile on the performance of Vertical Axis Wind Turbines (VAWTs). A 1.1 kW commercially viable Darrieus VAWT was studied using ANSYS Fluent. Four different airfoils – NACA 0012, NACA 0015, NACA 0030 and AIR 001 – were considered in the analysis. The tip speed ratios (λ) were varied from 1 to 2.5 with an incoming wind velocity of 10 m/s. It was observed that NACA 0030 performed better at lower values of λ due to long duration of attached flow; while NACA 0012 performed better at λ > 1.8 with a wider range of λ. The shed vortex dissipates much faster for thinner airfoils than for thicker airfoils at higher values of λ. Two bladed VAWTs generated more power than the three bladed turbines. This indicated that turbines with lower solidity perform better at high λ. [ABSTRACT FROM AUTHOR]