Aircraft Wake Detection Using Bistatic Radar: Analysis of ...

R. J. IANNUZZELLI ET AL.
(a)
Height (m)
(b)
Height (m)
120
100
80
60
40
20
0
–60 –40 –20 0
Crosstrack (m)
20 40
120
100
80
60
40
20
0
–60 –40 –20 0
Crosstrack (m)
20 40
Figure 14. Contours of turbulence kinetic energy for run 9 at
times late of (a) 1.4 s, and (b) 31.0 s.
the larger are these temporary local variations in the
refractive index.
The physical variable most identified with radar
signatures is the structure parameter 2
C discussed pre-
n
viously. To effectively use the model predictions to
interpret the radar observations, a means must be
found to relate 2
C to the mean-flow and turbulence
n
variables predicted by AWEC.
An algebraic expression for 2
C in terms of selected
n
AWEC model variables has been derived for a special
case of zero moisture content in the atmosphere, 11
2 2
2 a ( 77. 6R)
Cn= − ′ v′
w ,
1/3 4
10
y z
∂ ⎛ ⎞
⎜ − ′
′∂ ⎟
⎝ ∂ ∂ ⎠
60
60
(17)
Figure 15. Vortex trajectories for (a) run 9 and (b) run 15. The
position of the vortex cores (solid curve) is plotted as a function of
time late. The heavy dashed line indicates where the vortex
positions would be if the wake descended at a constant velocity
given by the induced velocity computed from Eq. 16. The actual
vortex descent begins near this speed but then slows. The fixed
upper and lower boundaries of the common volume are indicated
by a pair of solid horizontal lines. The outlying lighter dotted lines
indicate the first contour level above ambient of the turbulence
kinetic energy distribution.
where is the turbulence dissipation rate, ′ v ′ and
′ w ′ are turbulence density-velocity correlations, and
∂/ ∂y
and ∂/ ∂z
are gradients of the mean potential
density14 in the horizontal (cross-track or y) and vertical
directions, respectively. Constants appearing in
Eq. 17 are the universal gas constant R and a2 , which
ranges between 0.4 and 4. A value of a2 = 1.0 was
2
chosen for estimating Cn in this study.
Figure 16a shows that the wake boundary as determined
by the structure parameter roughly coincides
with the outer contour of turbulence kinetic energy for
run 15, which is shown for comparison in Fig. 16b.
Unlike the turbulence kinetic energy, however, the
2
plot of Cn shows evidence of structure within the
vortex wake. Values of 2
C range from 30 to 70 dB
n
over the extent of the recirculation cell, with the
312 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 3 (1998)
Height (m)
(a)
(b)
Height (m)
120
100
80
60
40
20
0
0
120
100
80
60
40
20
0
0
20
20
Time (s)
Time (s)
40
40
60
60