Paper VII — Engineering

30 QUESTIONS

Scope

Answer in the idiom of the college: provocations invite a disciplined essay; technical problems require correct, quantitative reasoning; interdisciplinary questions require two fields to be brought together. No moral, political, or value-philosophical content.

1.
Is engineering applied science, or a distinct way of knowing? Argue from cases in which the artefact preceded the theory.
2.
Why do bridges stand? Give an answer that a physicist would accept and an engineer would find sufficient.
3.
The original Tacoma Narrows Bridge is popularly said to have failed by resonance with a periodic wind. State the correct modern explanation, and say precisely why the resonance account is wrong.
4.
The de Havilland Comet fuselage failed in service after a number of pressurisation cycles far below the number implied by static strength. Reconstruct the reasoning error, and identify the roles of stress concentration and metal fatigue.
5.
Fracture mechanics gives the Paris law for fatigue-crack growth per cycle. State its regime of validity, and identify at least two regimes in which it fails to describe crack growth.
6.
Is a safety factor a quantified confession of ignorance? Distinguish the components of a safety factor that could in principle be eliminated by better knowledge from those that could not.
7.
Design a machine that reports its own remaining life. What must it measure, and what must it assume about its own failure modes?
8.
Is failure the engineer's primary datum? Contrast the epistemic value of a structure that failed with that of a structure that has merely not yet failed.
9.
Reynolds-averaged and large-eddy simulation persist despite growth in computing power. Explain why direct numerical simulation of turbulence at high Reynolds number remains infeasible, using an estimate of the number of grid points required.
10.
State the Betz limit for a wind turbine in an open flow, derive the value 16/27, and explain physically why extracting all the wind's kinetic energy is impossible.
11.
A real heat engine falls short of the Carnot bound. Separate the loss attributable to finite-time operation from that attributable to irreversibility internal to the working fluid.
12.
State the Lawson criterion (triple product) for a fusion reactor. Given that ignition has been achieved at the National Ignition Facility and long high-performance plasmas sustained in magnetic-confinement devices, identify the principal engineering obstacles that still stand between these results and net electrical power.
13.
The rocket equation makes staged chemical launch a tyranny of exponentials. Derive the equation, and quantify the penalty paid for a delta-v requirement twice the effective exhaust velocity.
14.
The square-cube law limits the scaling of structures and organisms. Give one structural and one thermal consequence, and explain why a scaled-up model of a working machine may fail where the original stood.
15.
Weibull statistics describe the strength of brittle materials. Explain why the strength of a ceramic component is a property of the population of flaws rather than of the material, and why larger specimens are weaker.
16.
Reliability is statistical. Explain why "this component will not fail" is not an admissible engineering claim, and reformulate it as a claim that can be tested.
17.
Can a system be proved safe, or only not yet unsafe? Relate your answer to the distinction between verification against a specification and the adequacy of the specification itself.
18.
Shannon's capacity theorem sets a bound on reliable communication over a noisy channel. Explain how low-density parity-check and turbo codes approach that bound in practice, and what "approach" costs in block length and decoding effort.
19.
Control an underactuated system: a system with fewer actuators than degrees of freedom. Explain, using an example, why underactuation makes some trajectories dynamically inaccessible.
20.
Chaotic systems can be controlled by small, timely perturbations. Explain the principle by which sensitivity to initial conditions becomes a control resource rather than an obstacle.
21.
Landauer's principle sets a thermodynamic floor on the cost of erasing information. State the bound, and explain why logically reversible computation escapes it while logically irreversible computation does not.
22.
Dennard scaling has ended. Explain what Dennard scaling asserted, why its end decoupled transistor count from usable performance, and what "dark silicon" names.
23.
A power grid loses synchronous rotating generation. Explain the role of mechanical inertia in frequency stability, and why grids dominated by inverter-based sources require synthetic inertia.
24.
Design a self-healing material. Specify what "healing" must mean physically, and identify the trade-off between healing capacity and load-bearing capacity.
25.
Biomechanics: bone remodels in response to load. Treating bone as a control system with a set-point, identify the sensed variable, the actuator, and the failure mode when the loop is opened by disuse.
26.
Finite-element modelling discretises a continuum. Identify two distinct ways in which a converged finite-element solution can nonetheless be wrong about the physical structure.
27.
A pressure vessel must be designed against both yielding and fracture. Explain why a leak-before-break criterion can be safer than a design that merely maximises burst pressure.
28.
Communications and control share the concept of feedback but use it differently. Distinguish the role of feedback in a phase-locked loop from its role in a feedback amplifier.
29.
Process engineering: a continuous chemical reactor must be stable at its operating point. Explain how an exothermic reaction can produce multiple steady states, and how runaway is prevented by design rather than by operation.
30.
Aerospace structures are certified partly by test and partly by analysis. Argue for the proposition that certification is an epistemic activity: what does a certified structure entitle its designer to believe, and on what grounds?

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