Hazard Assessment via CFD
1. PurposeCHAM's Consultancy Team is frequently called upon to
give advice on the possible consequences, and their relative probability,
of hazards such as oil spills, air pollution, fires in car-parks, and the
The purpose of the present document is to set out, and to illustrate, the
principles to be observed.
Those principles include:
- Since hazard assessments always involve uncertainties about the what,
where and how-much of the initiating event, simulation of a large
number of scenarios is essential, in order that a worst-case
scenario can be given special attention.
- Because CFD simulations of turbulent flows are never more than plausible
approximations, and the presence of turbulence adds further
uncertainty, quantitative predictions which are made must always be
presented as being reliable only between estimated +/- % limits.
- When smoke-production and consequent visibility-impairment are in
question, the error-band width must be especially wide.
- Although the numerical accuracy of CFD simulations can in principle be
improved by the use of very fine grids, it is counter-productive
their associated expense only to achieve gains which are negligible compared
with the uncertainties just enumerated.
- To summarize, the best analysis is a well-balanced one in
which the amount of attention paid to an individual factor is proportional
to its probable influence on the outcomes which are of main interest.
Two dangers to beware of
- CFD is sometimes said, by its critics, to be an acronym for
Colourful Fluid Dynamics, the allusion being to its
employment of realistic-looking images and videos to display its
results; and to their power to induce far
too much faith in their realism. Such displays should be sparingly
used and carefully presented.
- The PHOENICS VR front end is good at setting up single-instance
runs; but it has no facility for launching parameter-varying multi-runs
of the kind which are essential to hazard assessment. Until the
deficiency is corrected (as it soon will be by introduction of the
PQ1Maker module), hand-editing of the Q1 file is needed; and run-control
batch files must be used. The danger is that, the necessary skills
being rare, only
single-instance runs are made; and far too few of them.
2. Establishing the customer's needs
Properly establishing the customer's needs is not an easy task; for he may
have little understanding of the limitations of CFD, or indeed of the
independent-of-CFD principle (a) above.
Sometimes the customer may be seeking to provide evidence that his
equipment or process satisfies some authority-set quantitative
criterion. Error bounds are clearly relevant in such cases.
Alternatively the target may itself be subject to uncertainty, as
when the inflammability limits of mixtures of some gas released into
air are uncertain. And in such cases it behoves CHAM to make clear that
the magnitude of the fluctuations of fuel-air ratio should be
calculated; for a spark may ignite an briefly-existing pocket of
mixture which differs in composition from the time-average.
In variably the customer has a limited budget to spend; and he will expect
to be provided by CHAM with the most informative CFD simulations which
can be created within that budget.
The customer must inform CHAM about:
Then CHAM should consider, and advise the customer about:
- The size, location and contents (buildings, partitions, cars, people, fans,
etc) of the space in which happenings are to be investigated.
- What predictions are required (temperature, concentration of
pollutant, force exerted by wind, etc.) and for what locations in
space and time, and within what limits of accuracy.
Thereafter an informed discussion can take place, leading to agreement
as to what will give the customer best value for money.
- the ranges of
conditions (e.g. inlet-wind speed, angle, temperature, turbulence
level; times of day and night) for which simulations should be performed;
- the ranges of grid and time-step fineness which can be employed
- the various ways in which results can be displayed;
- the assoociated costs to the customer.
It is important that input-range, grid-fineness and reporting options
should be costed separately, so that both sides can see that the best
value is usually provided by many runs, modest fineness,
and minimal reporting.
Examination of recent hazard-assessment projects performed by CHAM reveals
that decisions have been arrived at which entailed few runs,
excessive fineness, and extravagant reporting.
Evidently, the above described principles have not been observed;
and 'best value for money' has not been provided.
3. How the investigation should proceed
As has been mentioned above (item d.) the VR Editor does not lend itself
to the making of the connected multi-runs which hazard assessments
require. Therefore, although it may be convenient to employ VRE
(perhaps with SPPNAM=FLAIR) to create the very first Q1, this file must
be parameterised before the hazard-assessment runs can begin.
PQ1Maker will automate at least the simpler stages of this process;
but, until it is released for general use, the VRE-written Q1 must be
edited by hand. This is not difficult, requiring knowledge of only the
rudiments of PIL; and these are easily learned by copying fragments
of existing PQ1s.
Working with PQ1s entails using VRE thereafter for only the visual
display of the scenarios beng studied. and decidedly not for
entering and saving new data. The reason is that VRE does not
understand the advanced PIL which parameterisation employs; and indeed
is liable to over-write irrecoverably what the human editor has just
The computations of which results will be presented below have all been
conducted by way of a Q1 file originally created by VRE-FLAIR. The runs
have taken no more than a few minutes in an HP net-book.
This is how hazard-assessment studies should always be carried out:
with many short runs, and much reality-based thinking. Reporting should
also be similarly minimal, with just enough visual to support the
conclusions which are drawn.
4. Some facts about turbulence
It is well-known to all experienced CFD practitioners that turbulent flows are
represented only approximately by its so-called 'turbulence models. These
compute time-average values and cannot do justice to their true nature,
by the video hyperlinked here.
turbulent air near buildings waver throughout building-sized volumes.
This entails that coarse-grid prediction may be
more realistic than fine-grid one, rather than less so.
CHAM personnel need to understand this themselves. Moreover, they must convey
this knowledge to their customers, and make sure that its practical implications
This is a professional duty, both to the customer and to CHAM. When it is not
performed, CHAM can be held legally responsible for damage resulting from the
customer's too-credulous reliance on CHAM's unqualified predictions.
It is also well-known that the quantitative accuracy, and so the practical
reliability, of CFD-based predictions depend greatly on the fineness of the
This too needs to be conveyed to customers, not least so as to make clear
sufficiently-fine-for-accuracy grids may be unaffordably expensive.
CHAM's proposal writers need to understand it, too; else CHAM finds itelf
committed to the making of runs which cost far more than the customer is being
asked to pay.
The message to be conveyed to customers is therefore that, although CFD is good
at predicting trends, it can rarely predict absolute values with better than
+/- 50% accuracy.
Grid coarsened runs are essential in consultancy
The just-alluded-to facts lead logically to this section heading. Thus:
It follows that means must be devised of coarsening the fine grids which are
being used to define a particular scenario without losing the essential features
of that scenario.
- many runs must be made;
- fine-grid runs are very expensive;
- therefore the 'many runs' must be coarse-grid runs
Such a means does exist; and it should be used in all future hazard-assessment
consultancies. While not yet available by way of any GUI, it can be
activated by simple hand-editing of the so-called OBJINF files printed
automatically by the Satellite. It allows the number of intervals in each grid
region to be diminished, while the number and position of the regions remains
the same. It has been used in the calculations of which results are presented
Among the purposes of the exploratory runs is to distinguish the more-important
of the scenario-defining attributes from the less-important ones; and one way of
doing so is to examine the influences of each attribute separately.
This will now be illustrated by considering a steady-state pollutant- injection
situation with no buildings present at all.
The influence of wind direction
The next two image show the predicted positions of the surface on which the mass
fraction of HCl is 1.e-5 with wind directions of 90 degrees on the left and 45
degrees on the right.
Comparison of the the images shows that CFD has indeed predicted the
correctly: the plume of pollutant has changed direction by 45 degrees.
However the maximum value has changed in the ratio 12/8.4; and
lengths and widths of the plumes are radically different. For this there is no
physical reason; but there is a numerical one. It is
called 'numerical diffusion'; and it has been known since the earliest
days of CFD [Gosman et al, 1969, p.132] that its magnitude is of the order
of the cell dimension times velocity times the sine of twice the angle
of inclnation of the velocity vector. Its value is often greatly in excess of
the magnitude of the effective diffusivity calculated by the turbulence model.
This conclusion can be tested by performing a run with no turbulence
model at all, i.e. for laminar flow. The following image shows
The plume shape is almost the same, and the maximum value has risen
only from 8.40 to 8.86 .
One wonders how many results, which CHAM
has confidently presented to its customers, have been similarly dominated by
Several further remarks are in order relevant to the purpose of the present
- The runs just reported took a minute or two on a netbook, so of
trivial cost. It would therefore not have been costly to investigate
many more wind angles.
- Had the grid been the finest contemplated for recent CHAM contracts
(costing more), the results would not have been qualitatively different.
- For this reason, CHAM's CityScape SimScene provides for grids to
be aligned automatically with the wind direction.
- Contemplation of the differences between the above three images
raises serious doubts about the reliability of any results which CHAM
may be about to report.
The influence of choice of turbulence model
That turbulence models disagree with each other is well-known. For
long-forgotten reasons the Kim-Chen variant of the k-epsilon model is
the default choice of PHOENICS-FLAIR. However, for flows along smooth surfaces
such as the ground in the present problem, the LVEL model is likely to
fit reality more closely, especially if the grid is coarse. It is
therefore interesting to ask how different would be the result if it had
been chosen; of course for the 90-degree wind direction so as to
minimise numerical diffusion.
The next images show the Kim-Chen and LVEL solutions side-by-side.
The difference in the maximum value is around 10%; but the difference
in lateral spread is much greater.
Probably, with a finer grid, the difference would be smaller; but it is
clear that the choice of model is likely always to be influential.
The influence of grid fineness
The abiliy to hand-edit the infob2 file allows local grid refinement,
which is economically feasible, instead of whole-domain refinement,
which is not. In the first (left) image below, the number of intervals
in the z-direction region nearest to the ground has been increased
five-fold. In the second (image) the same has been done in x- and
y-direction regions just downwind of the pollutant injection point.
The maximum pollutant concentration has increased nearly five-fold in
both images. This is understandable because the material is being
supplied at the ground surface into cells which now receive one fifth of
the previous amount of air.
The lesser lateral spread of plume in the right-hand image is a
consequence of the reduced numerical diffusion coefficient, this being
proportional, as stated above to the size of cell.
The influence of the wind speed
The postulated speedof the wind is of obvious importance in
atmospheric-pollution problems; for, the lower it is, the higher will be the
concentration of the pollutant. The following two images show the
change of shape of the 1.E-4 pollutant-concentration surface downstream
of the source, for two wind speeds, respectively 2.1 and 1.9 m/s.
Evidently, the 10% reduction in wind speed has led to a
rise of maximum concentration of about 10% and to a
corresponding enlargement of the volume of polluted air.
To be continued