FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

km/hr for a total of eight passes per vehicle. The time, accurate to within two seconds,
was recorded whenever the vehicle passed immediately in front of the sonic anemometer.
After all three vehicles completed this cycle, the sonic anemometer was moved to 2.4
meters and then to 5.5 meters AGL. The driving pattern (three vehicles, four speeds, two
passes at each speed) was repeated at each of those two heights.
Under stable atmospheric conditions, natural fluctuations in the vertical
component of velocity (W) are expected to be small. Thus, W was chosen to assess the
vertical extent of the turbulent wake generated by the passage of the vehicles. This was
done by making use of Reynold’s decomposition
W ( t)
= Wave + W '
Equation 5-2
where W(t) is the instantaneous velocity at time t, W ave is the average component of the
vertical velocity, and W’ is the fluctuating component of the velocity. Upon inspection of
the data, it became apparent that the effect of the passing vehicle could be seen from 7
seconds prior to the recorded vehicle pass time to 15 seconds after the recorded time.
This was not the case for every vehicle pass, but this “influence” interval (-7 to +15
seconds) was large enough to be inclusive of all vehicle passes. W ave in Equation 5-2 was
obtained by averaging the W-component of the velocity for 30 seconds prior to the
beginning of the interval influenced by the vehicle (i.e. –37 to –7 seconds) and for 30
seconds following the interval (+15 to +45 seconds). These two 30-second intervals were
considered “background” conditions. The background turbulence was quantified by
taking the average and standard deviation of J b over each 0.1 second measurement
obtained during the entire “background” period where
J
b
=
( W t)
−W
) 2
(
b ave
1
Equation 5-3
and the subscript b indicates that this operation is performed for the background intervals.
The turbulence generated by the passing of the vehicle is likewise quantified by obtaining
the average of J i
J
i
=
( W t)
−W
) 2
(
i ave
Equation 5-4
1 As an aside, note that J can be thought of as the vertical component of the turbulence
kinetic energy (KE) which is customarily defined as
2 2
KE = U ' + V ' +
W
2
'
where U and V represent the x- and y- component velocities.
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