Browsing by Author "Poll, D. I. A."

Now showing 1 - 8 of 8
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    The aerodynamics of slender aircraft forebodies at high angle of attack.
    (2001-08) Petterson, Kristian; Poll, D. I. A.; Sillén, M.; Arlinger, B.
    High angle-of-attack aerodynamics has been the focus of much attention as a result of the drive to increase combat aircraft manoeuverability and thereby improve survivability. A key factor in this regime is the behaviour of the complex vortical wake generated by the forebody. Even at zero yaw, the shear layers and vortex pairs which are symmetric at moderate angles of attack (alpha) can become highly asymmetric as alpha is increased. This can lead to large sideforces and yawing moments which may exceed available control power. Computationally simulating high-alpha forebody flowfields is known to be a challeng­ing problem. This thesis details the evaluation and enhancement of the CFD code NSMB with the objective of improving qualitative and quantitive predictions of the flowfield around fighter-type forebodies throughout the angle-of-attack range. Results on a tangent-ogive body confirmed that computing asymmetric flow required the introduction of a space- and time-fixed surface excrescence or the use of a non-symmetric solution algorithm to simulate flow instabilities via transient numerical error. Although solution stability problems were encountered, results with the non-symmetric algorithm showed promise. Suspected turbulence modelling issues were addressed by implementing the k-w family. Having established a methodology, solutions were obtained for the forebody of a cur­rent fighter aircraft, the Saab JAS-39 Gripen. The computed data shows excellent exper­imental agreement for 0° ≤ a ≤50° over the clean geometry but the inclusion of a nose pitot probe was seen to destabilise the calculation and prevent convergence. Finally, a single vertical nose strake or ‘rhino horn’ was added. When undeflected, this stabilised the flowfield, reduced solution oscillation and negated sideforce. Deflecting the hom pro­duced a stable flowfield with non-zero sideforce. Similar devices may be used, together with engine thrust-vectoring, in the next generation of combat aircraft and may also be added to existing airframes as a MLU.
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    Effect of surface suction on crossflow instability and transition occuring near a swept attachment-line
    (1998) Smith, A.; Poll, D. I. A.
    The work contained in this report is concerned with the use of transpiration at the leading edge of swept wings to modify and control transition due to cross-flow instability. Transition conditions are identified on both porous and non-porous surfaces, with and without suction, and the effect of two-dimensional excrescences is considered. Throughout, the application of the results to vehicle design is stressed and, where possible, simple design criteria are presented.
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    New models for the prediction of attachment-lines behaviour at hypersonic speeds
    (Cranfield University, 2002-01-24) Bellone, Franck; Poll, D. I. A.
    This investigation concentrates on the attachment-line flow behaviour under wind tun- nel and flight conditions. A turbulence model for incompressible, equilibrium flow'was developed for low Reynolds number down to and including relaminarisation. This was then extended to compressible flow using CFD to analyse experiments in which the heat transfer was measured. The extension is based upon the introduction of two Reynolds numbers to correlate turbulence in the inner and outer regions of the viscous layer. This requires the addition of just one constant, which is determined from the experimental heat transfer data. The turbulence model was then used to calibrate an approximate heating prediction method based upon the reference temperature concept. This method was then used to determine the minimum conditions for sustainable turbulent attachment-line flow in presence of large sources of disturbance, such as trip wires. It has been shown that existing criteria for the minimum conditions provide excessively optimistic predictions. Therefore new, more accurate criteria were proposed.
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    On the conditions for absolute minimum fuel burn for turbofan powered, civil transport aircraft and a simple model for wave drag
    (Cambridge University Press (CUP), 2024-06-01) Poll, D. I. A.; Schumann, Ulrich
    In a recent series of papers, Poll and Schumann have been developing a simple model for estimating fuel burn for turbofan powered, civil transport aircraft for a given mass, Mach number and flight level and in a specified ambient temperature profile for all phases of flight. This paper focuses upon the combination of Mach number and flight level at which an aircraft cruises with the absolute minimum fuel burn. For a given aircraft type, the information necessary to determine these conditions must be specified and this poses a challenge. An initial attempt to obtain these data has been described previously by the first author. In this paper, the optimum conditions are found using a completely different approach. Starting from first principles and using established theory, the equations governing the situation where engine overall efficiency and airframe lift-to-drag ratio both have local maxima at the same flight condition are developed. This special case is termed the “design optimum” condition and, for a specified aircraft mass and a specified atmospheric temperature versus pressure profile, it gives the lowest possible fuel burn for any aircraft and engine combination. The design optimum occurs at a particular Mach number and Reynolds number, and it is a fixed characteristic of the aircraft. The analysis reveals the significance of Reynolds number variations, wave drag, including its derivatives with respect to both lift coefficient and Mach number, and the atmospheric properties. Whilst wave drag is notoriously difficult to determine accurately, it is found that solutions to the equations are not particularly sensitive to the accuracy of this quantity. Consequently, a simple, physically realistic model can give good results. An appropriate model is developed and a complete, approximate solution is obtained. Taking the International Standard Atmosphere as the design atmosphere, results are presented for the 53 aircraft types previously considered by Poll and Schumann. Relative to the design optimum conditions, when Reynolds number is constant and wave drag is zero, compressibility alone reduces L/D by about 5%, reduces lift coefficient by about 1.5% and increases drag coefficient by about 3.5%. Reynolds number variation has little effect upon L/D, but it reduces lift coefficient and drag coefficient by a further 7% and 8% respectively. The reduction in lift coefficient has a significant impact on the optimum cruise flight level. In general, an aircraft’s operating optimum will not coincide with its design optimum, but deviations are expected to be small. Therefore, using the design optimum solution as a reference point, an improved version of the operating optimum estimation method described by Poll and Schumann in previous work is developed. This allows the estimation of the conditions for absolute minimum fuel burn for an aircraft of given mass flying thorough any atmosphere. Updated coefficients for the 53 aircraft types are given.
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    On the use of the boundary layer integral equations for the prediction of skin friction and heat transfer
    (Cranfield University, 1986-10) Hellon, C. S.; Poll, D. I. A.
    The usefulness of the energy equation integrated over the thickness of the boundary layer, in predicing heat transfer rates to smooth body surfaces in investigated. It is found that on assuming very simple closure relations, similar to those often used with the momentum equation, highly accurate predictions are made. It is shown further that the usefulness of these predictions extend into areas where the momentum equation-skin friction predictions, which have proved so popular, break down such as regions of reverse flow and shock/boundary layer interactions. The technique is has been tested in laminar transitional and turbulent flows with both experiment and other more complex theories. The technique is extended to three-dimensional laminar flows with the inclusion of a crossflow model.
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    Some aspects of the flow near a swept attachment line with particular reference to boundary layer transition
    (1978-08-01T00:00:00Z) Poll, D. I. A.
    The general properties of the flow at infinite swept attachment line have been investigated theoretically and experimentally. By using the concepts of reference temperature and Reynolds analogy it has been possible to produce a simple, semi-empirical calculation technique for skin friction and heat transfer rate which is valid for free stream Mach numbers up to eight both with and without heat transfer to the body surface. A consideration of the predictions made by this method shows that an understanding of the transition phenomenon is particularly important and therefore the response of the incompressible attachment line boundary layer to two-dimensional trip wires and turbulent boundary layers generated on bounding end plates has been investigated experimentally. A sufficiently wide range of parameters has been covered to ensure that the results are typical of full scale flight situations and simple prediction criteria have been derived. By re-evaluating existing data from several different sources it has been possible to extend the validity of some of the principal results to include flight at hypersonic speeds. In addition to the mechanism of transition via cross instability has been studies experimentally for incompressible flow conditions and prediction criteria which are compatible with advanced boundary layer calculation methods, have been developed with the aid of liner stability theory and results obtained from the re-evaluation of several early experimental investigations. In all cases sample calculations and the practical implications of the various predictions have been discussed.
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    Some effects of transpiration on the boundary layer at the leading edge of a swept wing
    (2000-03) Smith, A.; Poll, D. I. A.
    The effect of transpiration on the boundary layer near the leading edge of a swept wing has been investigated using a large swept cylinder model with a laser-drilled titanium leading edge to model the leading edge of a swept wing. In the region near the leading edge, boundary layer transition due to crossflow instability has been examined. Natural transition on a porous surface was compared with that on a non-porous surface, and it was found that transition occurred at lower R ’s on the porous surface (ie there was a performance penalty due to the porous surface). The effect of suction on transition due to crossflow instability was then studied. It was found that only moderate amounts of suction were required to delay the onset of crossflow-induced transition and a simple algebraic model has been derived, in terms of R , Rex, and Cq, to describe transition on the porous surface with or without suction. It was also found that two-dimensional trip wires had a negligible effect on crossflow transition, except where they caused attachment-line contamination. On the attachment-line, several subjects were addressed. The effect of attachment-line blowing was considered, and good agreement was obtained with previous work. The effect of spanwise blowing length was also addressed, and a simple algebraic model was derived, in terms of R , s/rj, and Cq, to describe attachment-line transition due to blowing. A comparison has also been made with linear stability theory. The effect of suction at the wing-fiiselage junction was examined as an alternative to suction on the attachment-line. However, it was found that applying suction on the attachment-line when the boundary layer had attained infinite swept conditions was much more efficient than applying suction in the junction region. Suction was successfully used to relaminarise a turbulent attachment-line at R values between 600 and 950, the magnitude predicted for the next generation of large transport aircraft. During the experiments, no sign of critical oversuction was found. Finally, the behaviour of a relaminarised attachment-line flowing onto a non-porous surface was studied. The conditions for natural transition on the non-porous surface were measured, and it was found that they were the same as those predicted by previous work on an entirely non-porous attachment-line.
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    The theory and application of heated films for the measurement of skin friction
    (Cranfield University, 1985-09) Mathews, J.; Poll, D. I. A.
    The use of hot surface films for measuring skin friction is examined. It is shown that all existing theories, which neglect heat conduction within the substrate, are inadequate in predicting the variation of heat loss from the film with skin friction. A more physically realistic theory is presented which accounts for the conductive and convective heat transfer into the flow and also heat conduction within the substrate. This leads to a more general relationship between skin friction and heat loss from the film. Experiments conducted in flat plate laminar and turbulent boundary layer flow show that this relationship is more accurate than previous forms. The time and temperature dependence of the heat loss from the hot film are also explored theoretically and experi- mentally. The effect of surface misalignment of the film is shown to alter significantly the convective heat transfer. This effect is more pronounced in laminar flow than in turbulent flow. Using a glue-on hot film probe, calibrations relating the heat loss to skin friction were found to be different in laminar and turbulent flow. An experimental operating procedure for the use of hot surface films for measuring skin friction is proposed which significantly increases the accuracy of the technique.