Prediction of Elastic-Plastic Behaviour of Structures at Notches

ABSTRAC

Prediction of Elastic-Plastic Behaviour of Structures at Notches

Under the condition of elastic-plastic deformation, aero engine casings experience local stress and strain concentrations along with associated variations in load paths and stiffness. The accurate prediction of such behaviour is clearly necessary for design optimisation, potentially leading to beneficial weight savings. The present research seeks to tackle the objective of accurate characterisation of elasticplastic casing behaviour. The objective is to develop approximate techniques for predicting the elasticplastic behaviour, for both generalised load-displacement responses (i.e. for global response) and notch stress-strain responses. Accurate prediction of the stress-strain distribution at a notch is difficult and existing notch prediction techniques can only be used for small strains. This paper seeks to develop novel techniques for predicting large elastic-plastic notch strain and associated stresses, with application to aero engine casing notches. The repeated local joints at the spoke-shell casing are of particular interest as they are the most likely sites for plastic deformation and possibly crack initiation. These local joints incorporate realistic notch-type features and the load cases cover a range of loading combinations, to develop insight and understanding of the elastic-plastic behaviour. This work analyse a double edgenotched flat bar with semicircular notches and a representative case of actual aero engine casing-type structures in a more simplified form. The investigation was carried out for structures for which stress and total strain are related by a power law. The equivalent stress at a notch can be estimated, given the value of n, by a linear interpolation between the stresses for a cases n=1 and n=0. The application of the notch stress-strain prediction method is illustrated through use of examples of notch components. The predictions are compared with results obtained using finite element analyses and approximate methods proposed by Nueber and Glinka

Minimizing Assembly Errors by Selecting Optimum Assembly Sequence in the Assembly of a Rigid Circular Structure

In order to increase the performance and to produce environment friendly aero engine, it is necessary to reduce the weight of the engine. A significant proportion of an engine\'s weight is due to the large castings used to form the main non-rotating structures. Replacement of these large castings with circular structures fabricated by assembling uniformly segmented circular components could provide a significant weight reduction. Due to various advantages of fabricated rigid structure of an aero engine casing, this article proposes that components should be assembled while considering optimum mating sequence to minimize assembly error and to reduce assembly time and cost. The algorithm based on connective assembly model is proposed to predict stage-by-stage assembly variations. A computer program is also developed to perform automated tolerance analysis and to find optimum mating sequence of component. The assembly variations are minimized based on three criteria of selection of assembly mating sequence. The results have shown that the three criteria may be used every time for tolerance analysis and to minimize variation propagations in the assembly of rigid circular structures

Axisymmetric Predictions of Fluid Flow inside a Rotating Cavity System

Accurate prediction of fluid flow in the rotating cavity system is of practical interest as it is most commonly used in the gas turbine engines and compressors. This paper presents the numerical predictions of a rotating cavity flow system for Reynolds numbers of the range 1x105 < Re? < 4x105 and two different mass flow rates Cw=1092 and 2184. A finite-difference technique is employed for a Steady-state solution in the axisymmetric cylindrical polar coordinate frame of reference. The two low Reynolds number turbulence models, the low Reynolds number k-? model and the low Reynolds number second moment closure have been used to compute the basic characteristics of the flow inside the rotating cavity flow system. Different flow regions have been identified by computing flow structures and dimensions of those regions have also been studied under different flow rates. A comparison of the computed variation of moment coefficient of both the turbulence models are presented for the above mentioned parameters and the parametric effects on the moment coefficients have been discusse

Modelling and Predictions of Isothermal Flow Inside the Closed Rotor-Stator System

This paper describes the numerical predictions of isothermal closed rotor-stator flows. Steady-state finite-difference solutions are sought for two gap ratios and two rotational Reynolds number in the axisymmetric cylindrical polar coordinate frame of reference. Low Reynolds number models, low Reynolds number k-? and second moment closure models have been used to compute the necessary description of the flow inside the rotor-stator system without superpose flow. The most important dissimilarities among the computational calculations of both the turbulence models obtain at the lower radial locations, where k-? model predicted the premature transitional predictions from laminar to turbulent flow. The major feature of this computational work is the emergence of four regions of the flow i.e. source, sink and two boundary layers. Computed velocity components of both models are compared against the experimental measurements. Low Reynolds number second moment closure shows the improved level of matching with data, particularly on apex of the boundary layers and recirculating core in the middle of the rotor-stator cavity

Numerical Prediction of Closed Contra-Rotating Disc Flows

This paper provides the finite-difference solutions for closed contra-rotating discs flows at different disc speed ratios for a fixed value of rotational Reynolds number of order five. The flow structure reveals that when two discs rotate in opposite directions, the fluid mass outside the boundary layers is divided between two regions, which give rise to the formation of two-cell flow structure. In order to assess the different level of closures, two turbulence models low Reynolds number k-? model and low Reynolds number second moment closure have been employed to predict the essential features of the closed contra-rotating disc system. The most significant differences between the predictions of the two turbulence models occur at the peak of slower disc boundary layer, because flow is more complex and turbulent on this side. The comparison of predicted velocity profiles of two turbulence models show that the low Reynolds number second moment closure produce a better agreement with measurements, particularly along the boundary layers and in the central core region

Potential contribution of ethanol fuel to the transport sector of Pakistan

Pakistan is an energy-deficient country. The indigenous reserves of oil and gas are limited and the country is heavily dependent on the import of oil. The oil import bill is a serious strain on the country's economy and has been deteriorating the balance of payments situation. The country has become increasingly more dependent on fossil fuels and its energy security hangs on the fragile supply of imported oil that is subject to disruptions and price volatility. The transport sector has a 28% share in the total commercial energy consumption in Pakistan. About 1.15 million tonnes of gasoline was consumed by this sector during 2005-2006. The gasoline consumption in the transport sector is also a major source of environmental degradation especially in urban areas. Consequently, Pakistan needs to develop indigenous, environment-friendly energy resources, such as ethanol, to meet its transport sector's energy needs. Pakistan produces about 54 million tonnes of sugarcane every year. The estimated production potential of ethanol from molasses is about 500 million liters per annum. Ethanol can be used in the transport sector after blending with gasoline, in order to minimize the gasoline consumption and associated economical and environmental impacts. This paper presents the assessment of the potential contribution of ethanol in the transport sector of Pakistan. It is concluded that 5-10% of the annual gasoline consumption in transport sector could be met from ethanol by the year 2030 under different scenarios. About US$200-400 million per annum could be saved along with other environmental and health benefits by using gasol in the transport sector.Ethanol Gasol Transport sector Renewable energy Pakistan