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Combustion Instability in Liquid Rocket Engin...
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Work on liquid rocket engine combustion instabilities began in the early 1940s (Culick and Yang 1995). One of the most critical concepts in liquid rocket combustion instability, that of time lag (as a coordinating factor in influencing organized oscillations in liquid rocket combustion chambers) originated around this time in von Kármán‟s group at the Jet Propulsion Laboratory around 1941 (cf. Summerfield 1951) shortly after oscillations were observed in early tests in liquid rocket engines in the United States. The essential idea was that there existed a finite time delay when an element of propellant entered the combustor and when heat was released from it. This delay controlled the phasing between heat release and pressure oscillations thereby making the system stable or unstable as per Rayleigh‟s criteria. In the years that followed, this model was applied to various studies involving combustion instability in liquid rocket engines. Gunder and Friant (1950), Yachter (1951) and Summerfield (1951) analyzed low frequency chugging instability arising from the interaction between feed system and combustion process using a constant time lag model. Crocco (1951;1952) introduced the time varying combustion time lag and use... |
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Adaptive Equalization Techniques using Recurs...
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In this project, we extend the use of methods of least squares to find a recursive algorithm solution of adaptive transversal filter. Given the LS solution at any time instant n-1, we find the solution at time n recursively using past solution and newly arrived data. This algorithm is known as Recursive Least Squares (RLS) algorithm. We show the convergence rate of RLS algorithm is faster than LMS algorithm by comparing the learning curves of two algorithms for specified channel response. |
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Supersonic Nozzle design by Method of Charact...
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Using the 2D Method of Characteristics (MOC), the contour of a two dimensional minimum-length nozzle for the expansion of air to a design exit Mach number of 2 is computed and graphed. The problem is extended to include a nozzle with a finite expansion section which is a circular arc with a diameter equal to three throat heights. C++ source codes for the 2 nozzles are included. By inputting the values of gas constant, test section mach number, throat height and level of precision one can design nozzles for different working fluids at different test section mach numbers. The report was generated as a part of a graduate course on compressible aerodynamics taught that semester by Dr John D Anderson. His book entitled 'Modern Compressible Flow - with Historical perspectives' contain a good chapter on the use of the MOC in designing supersonic nozzle contours (Chapter 11 of the third edition). |
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Rayleigh criteria and Combustion Instability
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A short literature review on Rayleigh's Criteria is provided. This criteria is widely used in mechanical and aerospace engineering areas to understand the phenomena of combustion instability. In continuously running combustion systems where combustion occurs inside a volume of relatively low losses, small amplitude pressure disturbance and small amplitude heat release fluctuations can couple with each other in a positive feedback loop leading to very large amplitude pressure oscillations often ending up in limit cycle oscillations. Such oscillations are detrimental to the combustion system (power or propulsion) for it exposes the system to large mechanical fatigue, thermal loads and can lead to catastrophic failures. |
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Global Continuity in Stokes’ Flow
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Creeping flow (flow with Re << 1) past a sphere is described by Stokes’* celebrated solution (1851). The solution is formulated with the help of the streamfunction which ensures mass conservation on a differential level. However, if the streamlines and velocity contours for this flow are plotted, the image suggests that global continuity is violated. This can often lead to some confusion and doubt over the validity of the solution, so a simple demonstration follows to show that Stokes’ solution also satisfies global mass conservation, despite the illusion presented by the streamlines and velocity contours. |
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JPEG Progressive mode vs Sequential mode
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JPEG standard generally use two type of scan to transmit the image using entropy coding. One is known as progressive mode, while other is known as sequential mode. In sequential mode, every block of image is encoded in a zigzag fashion from low frequencies to high frequencies and transmitted. So, at the receive, we get information in a sequential manner from left to right blocks and top most block to bottom most. While in progressive mode, the encoding for all blocks are done at the same time for same frequency components. So, the DC component of each block is encoded together, followed by first frequency components of all the blocks and so on. Since, progressive mode uses the information from all blocks at the same time, it is useful to implement JPEG using progressive mode for large image sizes. As in that case, information about DC and low frequency components can give average information about image. User can then download the complete image, if he feels the need to store that image. Progressive scan is also efficient in coding because different Huffman table can be used to encode different frequency components. Hence, performance of progressive scan is expected to be better than sequential mode. |
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Laminar Separation Bubble
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This essay focuses on the phenomenon of Laminar Separation Bubble observed at Low Reynolds Numbers. The effect of this bubble on the airfoil characteristics is discussed. Airfoils used at low Reynolds numbers have to be designed taking into consideration the finite space and time occupied by the bubble. Active as well as passive mechanisms to control/delay the formation of the bubble are also mentioned.
For conventional aircraft wings, whose Reynolds number exceeds a million, the flow is typically turbulent with the boundary layer able to strengthen itself by ‘mixing’. Consequently flow doesn’t separate until high angles of attack are encountered. For lower Reynolds numbers, the flow is initially laminar and is prone to separate even under mild adverse pressure gradient. Under certain flow conditions, the separated flow reattaches and forms a Laminar Separation Bubble (Fig. 1) while transitioning from laminar to turbulent state. Laminar separation bubble could modify the effective shape of the airfoil and consequently influence the aerodynamic performance, generally in a negative manner.
The need to understand low Reynolds number (104 to 106) aerodynamics is driven by variety of applicat... |
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The Behaviour of Air (Nitrogen) inside Carbon...
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This paper shows how Air (Nitrogen) is not a gas inside a Carbon nanotube, as inferred from the results of a Monte Carlo simulation of Nitrogen molecules in the presence of 2 surfaces. The surface under study was composed of Carbon, and the length scale was a few dozen nanometers, which made the problem representative of air inside a carbon nanotube. The surfaces have been shown to increase the compressibility of the gas, but the gas almost completely adsorbs onto the surface at room temperature due to the strong surface interaction potential. Therefore, it can also be deduced that Nitrogen is not quite a gas inside a Carbon nanotube. |
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A Technical Essay on the Gyroplane
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A study of “Gyroplane” and its historical evolution, general characteristics, flight characteristics, various designs, potential applications and aerodynamics explaining its flight is attempted. “Gyroplane” is an official term designated by the Federal Aviation Administration (FAA) describing an aircraft that gets lift from a freely turning rotary wing, or rotor blades, and which derives its thrust from an engine-driven propeller. The focus is on highlighting the differences between a Gyroplane and a conventional helicopter, relative merits and demerits and to trace the development to helicopters from autogiros. What lies ahead in the future for gyroplanes is also discussed.
Cierva thought of designing a flying machine that remains stable, safe and controllable irrespective of its forward speed. He segregated the function of lift and forward propulsion, where the former was done by a freely rotating rotor (and not wings) and latter by a conventional pusher or tractor engine.The rotor of an Autogiro (term coined and patented by Cierva) always works in a state of autorotation and a small upward flow is sufficient to rotate the disk. Thus, as long as the machine has forward motion the rotor would produce sufficient lif... |
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Physics of flow about lifting bodies
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Physics of lifting bodies is studied in some depth using the NASA Foilsim software freely available on website.With this software we can investigate how an aircraft wing produces lift by changing the values of different factors that affect lift.There are several different versions of FoilSim II which require different levels of experience with the package, knowledge of aerodynamics, and computer technology. An offline version can be downloaded and being interactive a variety of problems can be analyzed using it. In the present work a detailed analysis of ‘Physics of flow about lifting bodies' is studied. FoilSim is a computer simulator program that calculates the total "lift" of a specified wing. By using this program, the user can prove that the lifting capacity of the wing can remain the same at different airspeeds-as long as the angle of attack is changed to compensate. If the airspeed remains unchanged, an increase in the angle of attack will initially increase the lifting capacity of the wing. |
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