Staff publications (SoE)

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Browsing Staff publications (SoE) by Publisher "Cambridge University Press"

Now showing 1 - 4 of 4
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    Are There Good Air Traffic Management Safety Indicators For Very Safe Systems?
    (Cambridge University Press, 2007) Brooker, Peter
    European Air Traffic Management is extremely safe. The drawback to this safety record is that it is very difficult to estimate what the ‘underlying’ accident rate for mid-air collisions is now, or to detect any changes over time. The aim is to see if it possible to construct simple ATM safety indicators that correlate with this underlying accident rate. A perfect indicator would be simple to comprehend and capable of being calculated by a checklist process. This problem has been examined by a combination of analogies with simple ‘defensive’ systems with Markov process properties. An important concept is that of ‘system control’: the ability to determine the outcome against reasonably foreseen changes and variations of system parameters The statistical distribution of future incidents has been analysed by focusing on an index – the CPI – of separation at the Closest Point of Approach. A promising indicator is ‘Incident Not Resolved by ATC’, INRA, incidents in which the ground ATC defences have been ‘used up’. ATM Incidents can also be categorised in other ways: two examples are reviewed: the risk-bearing category for Airproxes and ‘risk of collision/severity’ scores. The second is more promising conceptually, but the existing scoring system has not been demonstrated to have the properties necessary to derive risk estimates. The key question is: if someone says they know how to make a good estimate of the underlying accident rate, then how could this claim be tested? If it correlates very well with INRA, then what would be the argument for saying that it is a better indicator?
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    The influence of initial conditions on turbulent mixing due to Richtmyer-Meshkov instability
    (Cambridge University Press, 2010-07-10T00:00:00Z) Thornber, Ben; Drikakis, Dimitris; Youngs, D. L.; Williams, R. J. R.
    This paper investigates the influence of different three-dimensional multi-mode initial conditions on the rate of growth of a mixing layer initiated via a Richtmyer-Meshkov instability through a series of well-controlled numerical experiments. Results are presented for large-eddy simulation of narrowband and broadband perturbations at grid resolutions up to 3 x 10(9) points using two completely different numerical methods, and comparisons are made with theory and experiment. It is shown that the mixing-layer growth is strongly dependent on initial conditions, the narrowband case giving, a power-law exponent theta approximate to 0.26 at low Atwood and theta approximate to 0.3 at high Atwood numbers. The broadband case uses a perturbation power spectrum of the form P(k) proportional to k(-2) with a proposed theoretical growth rate of theta = 2/3. The numerical results confirm this; however, they highlight the necessity of a very fine grid to capture an appropriately broad range of initial scales. In addition, an analysis of the kinetic energy decay rates, fluctuating kinetic energy spectra, plane-averaged volume fraction profiles and mixing parameters is presented for each case.
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    Longitudinal collision risk for ATC track systems: a hazardous event model
    (Cambridge University Press, 2006) Brooker, Peter
    This paper presents a collision risk model and operational concepts for longitudinal separated aircraft in the North Atlantic Region air traffic control track system, and indicates how it might be used to reduce separation minima safely, and hence deliver cost savings. It is an event-based model: it is easy to see what is being assumed, to understand the role of the main parameters, and to incorporate collision detection and hazard analysis. A potential future operation, with a 7-minute separation and a strategic lateral offset system, is sketched using the model.
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    On ice-induced instability in free-surface flows.
    (Cambridge University Press, 2007-04) Shapiro, Evgeniy; Timoshin, Sergei
    The problem of stability of a water-coated ice layer is investigated for a free-surface flow of a thin water film down an inclined plane. An asymptotic (double-deck) theory is developed for a flow with large Reynolds and Froude numbers which is then used to investigate linear two-dimensional, three-dimensional and nonlinear two-dimensional stability characteristics. A new mode of upstream-propagating instability arising from the interaction of the ice surface with the flow is discovered and its properties are investigated. In the linear limit, closed-form expressions for the dispersion relation and neutral curves are obtained for the case of Pr = 1. For the general case, the linear stability problem is solved numerically and the applicability of the solution with Pr = 1 is analysed. Nonlinear double-deck equations are solved with a novel global-marching-type scheme and the effects of nonlinearity are investigated. An explanation of the physical mechanism leading to the upstream propagation of instability waves is provided.