The Nature of Engineering Knowledge

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The Nature of Engineering Knowledge
September 15, 2008
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Laws of Engineering

• Big machines are more efficient than
• small machines

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How efficient is the Sun?
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Efficiency (tons of steel out) /(tons of coal in)
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Major source of inefficiency is
Bremsstrahlung X-rays emitted when
high-velocity electrons strike the
walls. Bremsstrahlung losses go as surface
area, energy generation goes as Tokamak volume.
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Laws of Engineering

• Big machines are more efficient than
• small machines

2. Its easier to do the same thing a thousand
times than to do a thousand different things.
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When threads were hand-cut, each nut fitted a
single bolt, and vice-versa
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Whitney set up a demonstration for
President Adams and Vice-President Jefferson. He
had piles of musket parts on a table. There were
10 similar parts in each pile He went from pile
to pile picking up a part at random, Using these
completely random parts, he quickly put together
a working musket.
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The same idea underlies container shipping,
object-oriented programming, and nanotechnology
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Laws of Engineering

• Big machines are more efficient than
• small machines
• 2. Its easier to do the same thing a thousand
  times
• than to do a thousand different things

3. In mass production, unit cost depends only
on unit mass and material.
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• Cost per unit Fixed costs/N unit costs
• Fixed costs research, design, tooling
• Unit costs materials, labour

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Corollary to third law
If youre making a billion of them, its worth
spending five million dollars to cut unit costs
by a cent.
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Laws of Engineering

• Big machines are more efficient than
• small machines
• 2. Its easier to do the same thing a thousand
  times
• than to do a thousand different things

3. In mass production, unit cost depends only
on unit mass and material.
4. No measurement has more than 6 significant
figures, and the sixth is very expensive.

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Schrodingers Equation
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Engineering Modelling
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Scale Model of Boat Hull at Towing Tank (2005)
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Navier-Stokes Equations
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The Froude Number

• Fr V2/Lg

If the Froude number is the same for a ship and
its model, both will behave the same way (e.g.,
capsize and sink)
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Catamaran model in a towing tank
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Reynolds Number
At a critical value of Reynolds number, flow
changes from laminar to turbulent
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Reynoldss Experiment
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Karman Vortex Street

• Characteristic of turbulent flow

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The Mach Number

• M v/c

If the Mach number is the same for an aeroplane
and its model, both will be in the same sonic
regime (e.g., both supersonic)
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NASAs Supersonic wind tunnel at Glenn Research
Center
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Weber Number
Indicates the ratio between inertial forces and
surface-tension forces (this is why you cant
design bugs with a towing tank)
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Water strider on a pond
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• Detailed attention to non-dimensional
• numbers made pre-CGI monster movies more
  realistic

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Usefulness of the Non-Dimensional Numbers
Fluid friction in a pipe is affected by its
diameter, and by the fluids speed and
viscosity. Using Reynolds number, we
can investigate all these in one series of
experiments.
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Why arent there anynon-dimensional numbersin
electrical engineering?
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Laplaces Equation

• applies to heat conduction and electrostatics.
• So an electrostatic problem can model a thermal
  problem.

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Teledeltos paper
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Finite-Element Analysis
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Fuzzy Control

• Fuzzy logic employs models of systems that
• are deliberately imprecise for example, a
• car may be modelled as having three possible
  speeds, too slow, OK, too fast.
• This can yield simple, robust control
• Algorithms.

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Qualitative Physics

• In making predictions about the world, we
• employ mental models. These are neither
• exact nor numerical, but they work.
• Qualitative physics attempts to get computers
• to do the same thing.

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Conclusions

• Engineering has a range of strategies, not
  limited to the application of scientific
  knowledge
• New non-scientific strategies are continuing to
  be developed, and may be used in preference to
  older, more scientific methods.

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Conclusion
Engineers know stuff that scientists dont This
knowledge may be specific to one culture and one
period in history Approximate rather than
exact Useful rather than true