The Measurement of Wake and Gap Flows of the
Generic Conventional Truck Model (GCM) using
Three-Component PIV
James T. Heineck, Stephen M. Walker, and Dale Satran
Experimental Physics Group, Aeronautical Projects and Programs Office,
NASA Ames Research Center, Moffett Field, CA, 94035
Abstract
Particle Image Velocimetry (PIV) measurements were acquired in the wake of the
trailer and in the gap between the tractor and trailer of the Generic Conventional
Model (GCM) truck for the US Department of Energy. The data will be used both
for validation of computational fluid dynamics (CFD) codes and for understanding
the flow physics. The GCM is a 1/8th-scale, moderate-fidelity model of a full-scale
truck. The test was performed in the Army/NASA 7x10 wind tunnel at NASA
Ames Research Center. Surface pressure and force measurements were made prior
to the PIV measurements. PIV measurements were made at two yaw angles and at
three horizontal planes for three model configurations, each at a free-stream velocity of 52 m/s (Mach 0.15), which corresponds to a Reynolds number of 1 x 106,
based on the width of the tractor. This paper discusses the PIV system, samples of
flow data and some of the observed features that may have contributed to the measured drag.
Introduction
A multi-agency effort led by the US Department of Energy (DOE) is underway to
help truck and trailer manufacturers increase fuel efficiency of heavy vehicles. An
important part of the program is to reduce aerodynamic drag. This part of the effort
includes the development of computational fluid dynamics codes and the performance of experiments in both wind tunnels and full-scale road tests. The GCM is a
moderate fidelity 1/8th-scale model of a modern engine-forward tractor and trailer.
The level of fidelity is high enough to represent the flow of the modern full-scale
truck, though not so detailed as to make CFD gridding prohibitive. Figure 1 is a
photograph of the model installed in the test section.
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Fig.1. Photograph of basic configuration of the Generic Conventional Model.
The PIV measurements were made on the basic model configuration (Fig. 1),
then the basic model tractor with side-extensions that spanned 50% of the gap
width (Fig. 2A), and then basic model with trailer boat-tail plates (Fig. 2B). For the
basic configuration, the gap was 5 inches, The trailer back face was flat and no
skirts covered the wheels along the trailer. The balance measurements from the
aerodynamics testing phase of these experiments showed these devices reduced
drag.
A
B
Fig.2. Photographs of the glass side extenders (A) and the trailer boat-tail (B).
The PIV test matrix included combinations of the parameters. There were two
flow areas of interest; the wake and the gap. Two model configurations were tested
for each area; the basic and the basic with drag reduction treatment. Then two yaw
angles for each location; 0 degree and 10 degrees. The data set for each flow area
consisted of three vertical locations of the horizontal data planes: 1,4, 1/2 and 3/4 of
the trailer height of each area of interest was measured with and without the drag
reduction devices at two yaw angles. Extra data sets were acquired in response to
preliminary analysis of the drag data. A feature in the balance data showed that
drag suddenly decreases when the model was at approximately 11 degrees. Also, a
The Measurement of Wake and Gap Flows of the Generic Conventional Truck Mode
l75
weak vortex generated off the top of the trailer was observed when the model was
yawed at angles greater than 5 deg. An extra data set was acquired with the plane
oriented in a vertical cross-stream manner, 9 inches aft of the trailer. All conditions
were for M=0.15 (52 m/s). The Reynolds number for this condition was 1 x 106.
The PIV System
The three-component PIV technique requires the use of two digital cameras, each
viewing a laser light sheet from an oblique angle and imaging the same area of interest simultaneously (Arroyo and Greated, 1991, Willert, 1997). The cameras record the motion of particles as they move within the laser light sheet by making
separate exposures at Time 1 and Time 2. The two time-separated images from
each camera are cross-correlated to determine the displacement of the particle images. These vectors are calculated using these displacements. By viewing the same
area of interest from two viewpoints, the perspective difference seen by each camera permits a mathematical derivation of the cross-plane component of the velocity
vector. Figure 3 is an illustration of the stereoscopic three-component PIV concept.
Fig.3. Illustration of the camera and laser plane relationship for three-component PIV.
In this illustration, three flow conditions are represented as particle pair examples. Pair 1 indicates a positive flow direction with a significant in-plane displacement. The two camera views render that motion quite differently, which demonstrates the perspective difference that is exploited to calculate the third component.
Pair two represents cross-plane flow. Pair 3 represents reverse (relative to Pair 1)
flow.
The geometry of the model dictated the placement of the cameras. The gap flow
could only be viewed from above and to the side to permit off-normal imaging of
the laser plane. The plane for the wake was oriented horizontally in order to capture
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the wake convergence. Thus, the PIV system design parameters followed these
constraints.
The PIV system consisted of two Kodak ES 4.0 cameras (2048x2048 pixels),
two New Wave Gemini PIV laser systems, IDT Provision software for calibration,
acquisition and data reduction, hardware for synchronization signal timing and remote focusing, sheet forming optics and a micro-particle generator. Two lasers
were used so that both cameras received forward-scattered illumination of the seed
particles. The forward scatter position receives more than 20 times the light than
the back scatter position. The cameras were unable to render the particles in backscatter position; therefore the second laser was necessary. The light sheet from each
laser was matched for output and formed with identical optics.
Figure 4 is an illustration of the system position relative to the model. This figure shows how the two laser sheets were aligned to each other: the sheets were
made level, then the support frames were moved vertically to place the planes at the
proper height.
Cameras
Laser plane
Lasers and projection optics
Fig. 4. Illustration of PIV system design looking upstream.
Spatial resolution of the region of interest is determined by the magnification of
the region and the pixel resolution of the cameras. This value, given in mm/pixel,
indicates the minimum particle pattern displacement to render a flow feature. The
image area for the gap was approximately 14 in (0.36m) and was limited to the
width of the gap, which was 5 in. (0.13m) in the streamwise direction. Thus, the
spatial resolution was 0.15mm/pixel. In the wake, the image area was considerably
larger: 1100 mm (43 in.) in the span-wise direction x 600 mm (24 in.) in the
stream-wise direction. This makes the spatial resolution for the wake 0.5mm/pixel.
The larger image area permitted us to image the convergence of the wake for both
the 0 degree case and the 10 degree case without having to move the cameras. Figure 5 is an illustration of the laser planes and regions of interest.
The Measurement of Wake and Gap Flows of the Generic Conventional Truck Model
177
Fig. 5. Laser planes (green) and regions of interest (white boxes).
Referring to Figure 3 once again, the laser sheet thickness is shown as 3 mm.
The highest probability for image-to-image correlation occurs when the the highest
number of particle pairs from image to image remain within the interrogation area.
The particles moving through the laser plane must remain in the laser plane for this
to occur. The optimal cross-plane displacement should be 1/4 - 1/3 of the sheet
thickness (Willert, 1997). Therefore, the maximum cross-plane displacement of the
particle motion was limited to 1mm. The actual thickness of the sheet projected for
this experiment was 3mm as well. The magnitude of the measurable in-plane particle displacements is limited by the cross-plane component. When examining the
gap data, even the worst case permits sufficient particle displacement to allow for
an accurate measurement. In that example, a 1 mm cross plane displacement is recorded as a displacement of 5 pixels. The same case for the wake measurement
would get recorded by slightly less than 2 pixels, due to the lower magnification;
therefore the local measurement uncertainty increases. Purely cross-plane flow occurred in a small region in the wake where the wake converged. The rest of the
flow was primarily in the in-plane direction. The measurement uncertainty was approximately 2% for the in-plane components and 4% for the cross-plane component
for any instantaneous measurement.
Results and Discussion
The following plots are averages of 50 instances. Each instance has more than
10,000 individual vectors calculated. Only the 1/2 height planes are presented here
to conserve space, with the exception of Fig.12, which is from the 1/4 height data
set. The plots are oriented in model coordinates whose axes are depicted in Figure
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J.T. Heineck, S.M. Walker, and D. Satran
6. In each plot, the cross-plane component is in contour, which in model coordinates is V. The flow is from bottom to top in each of the plots. The in-plane velocities are not apparent in the streamlines, but the streamlines clearly indicate the
change in the flow structure.
Fig.6. Axes of plots.
Wake Flow Data
Comparisons of the flow-field data that result from the basic and boat-tail configurations quickly demonstrate the efficacy of this device. In Figures 7 and 8, the view
of the wake is from above, with the back of the trailer located at x=2460 mm and
centered z=0. The width of the trailer is 325 mm. The wake is shown to have reduced cross-flow velocities (V) in both the 0 and 10 degree cases when the boat-tail
is attached. The length of the wake in the 0 degree case is reduced by approximately 540 mm in the basic case to 440 mm in the boat-tail case, which is a 15%
reduction.
In Fig.8, the plot of the wake at 10 degrees demonstrates the reduction in the
magnitudes of the V component with the use of the boat-tail. A similar reduction in
the wake length is also realized. Another flow feature rendered in these plots is the
weak vortex formed on the top of the trailer. This vortex is oriented horizontally
and is propagated in the free stream. The data plane passes through the structure in
a horizontal cross-cut. The evidence of the vortex is the positive cross-plane flow
next to negative flow.
The Measurement of Wake and Gap Flows of the Generic Conventional Truck Model
179
Flow, U
Fig. 7. Streamline plots of the mean wake flows at 0 deg of the GCM without and with the
boattail. The color contour renders the vertical component, V.
Flow, U
Fig . 8. Streamline plots of the mean wake flows at 10 deg yaw without and with the
boattail plates. Color contour renders the vertical component, V.
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Gap Flow Data
The gap flow data show similar improvements. Fig.9 contrasts the flows without
and with side extenders for the 0 deg case. Fig.10 are plots of the 10 deg case. Note
in both cases, the vortex strength and the V component of the flow reduces dramatically with the use of side extenders.
Figure 11 is the plot of the drag vs. yaw angle from the aerodynamics data. PIV
data were collected at 11 degrees, where this sudden drop in drag occurs. Figure 9
shows the flow patterns of the high drag state at 10 degrees and the low drag state
at 11 deg.
Flow, U
Fig. 9, Streamline plots of 0 deg mean gap flows without and with side extenders. Color
contour plots the vertical component, V.
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181
Flow, U
Fig. 10, Streamline plots of 10 deg mean gap flows without and with side extenders.
Color contour shows vertical component, V in the same scale as Fig. 7.
0.8
0.75
- Yaw
0.7
0.65
+ Yaw
0.6
0.55
2.5" Side
Extenders
0.5
0.45
0.4
-16 -12
-8
-4
0
4
8
12
16
Beta
Fig.11, Drag polar for basic model and basic with 50% side extenders. (Satran, 2003)
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J.T. Heineck, S.M. Walker, and D. Satran
Flow, U
Fig. 12. Streamline plots of mean gap flow at 1/4 -height at 11 deg (low drag) and 10 deg
(high drag) with vertical component V in color contour.
In Fig.13, the two surface pressure plots from Satran, (2003) were manipulated
to offer a perspective view of the gap’s pressure distribution. The streamline plot
from Fig.8 of the gap flow of the basic tractor configuration was scaled and placed
in the appropriate position, at 1/2 height. This composite shows the viewer the relationship between the recirculation (or vortex) region to the low pressure region on
the back of the tractor.
This same style of composite is presented for the tractor side-extension case.
With both the velocity contours and pressure contours are plotted using the same
scale. It becomes clear that the reduced gap flow has reduced the pressure differential between the tractor back and the trailer front. The drag polar plot in Figure 11
demonstrates the difference in measured drag with and without extension. But the
pressure reduction and flow field change may be a result of some other flow
mechanism and not a cause of the drag hysteresis. Another mechanism may cause
the change in circulation, where the change in the flow through the gap is the result
of a change in the condition along the trailer that induces axial drag. These discussions are left to future articles.
The Measurement of Wake and Gap Flows of the Generic Conventional Truck Model
183
Tractor back face
(Cp in contour)
Flow field data, (V component in contour)
Trailer Front,
(Cp in contour for
both faces)
Fig. 13. Composite illustrations of the surface pressures in the gap (without and
with side extenders) and a flow field plot at 1/2 height. Surface pressure contours in
Cp and flow field contour of Vmean, in m/s.
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J.T. Heineck, S.M. Walker, and D. Satran
Conclusion
Flow measurements were made around a tractor-trailer model called the Generic
Conventional Model (GCM). Three-component PIV measurements of the wake and
gap flows clearly render the changes in flow patterns caused by aerodynamic drag
reduction devices. Furthermore, a vortex structure in the gap was shown to cause a
hysteresis in the drag polar measured by Satran in the first phase in these experiments. The goal of creating a database for the truck engineering and research community was advanced with the addition of these measurements.
References
Arroyo, M.P, Greated, C.A., “Stereoscopic particle image velocimetry”, 1991,
Measurement Science and Technology 2, pp. 1181-1186
Raffel, M, Willert, C.E., Kompenhans, J.; Particle Image Velocimetry, A Practical
Guide , 1998 Springer-Verlag
Willert, C., “Stereoscopic digital particle image velocimetry for application in wind
tunnel flows” 1997, Measurement Science and Technology 8, pp. 1465-1479