NASA/TM-2001-209621

An Experimental Study of the Ground Transportation

System (GTS) Model in the NASA Ames 7- by 10-Ft Wind

Tunnel

Bruce L Storms, James C. Ross, James T. Heineck,

Stephen M. Walker, David M. Driver, Gregory G. Zilliac

February2001

https://ntrs.nasa.gov/search.jsp?R=20010038028 2018-05-15T06:53:53+00:00Z
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NASA/TM-2001-209621

An Experimental Study of the Ground Transportation

System (GTS) Model in the NASA Ames 7- by 10-Ft Wind

Tunnel

Bruce Storms

AerospaceComputing, Inc., Los Altos, California

James C. Ross, James T. Heineck, Stephen M. Walker,

David M. Driver, and Gregory G. Zilliac

Ames Research Center, Moffett Field, California

National Aeronautics and

Space Administration

Ames Research Center

Moffett Field, California 94035-1000

February 2001
Acknowledgments

This work was sponsored by the Department of Energy, Office of Heavy Vehicle Technology. The hot-film measurement system and sensors were provided by Tao of Systems Integration, Inc., under SBIR 95-1.02.06. This test

would not have been possible without the hard work of the resident mechanic, Joel Gunter, and instrumentation

technician, Dave Pfluger. Thanks also to Kambiz Salari from Sandia National Laboratories for lending a hand

during the early phase of testing.

Available from:

NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076-1320

(301) 621-0390

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

(703) 487-4650
An Experimental Study of the Ground Transportation System (GTS) Model in

the NASA Ames 7- by 10-Ft Wind Tunnel

BRUCE L. STORMS, JAMES C. Ross, JAMES T. HEINECK, STEPHEN M. WALKER,

DAVID M. DRIVER, GREGORY G. ZILLIAC

Abstract

The l/8-scale Ground Transportation System a

(GTS) model was studied experimentally in the c

NASA Ames 7- by 10-Ft Wind Tunnel. Designed for CD

validation of computational fluid dynamics (CFD),

the GTS model has a simplified geometry with a cab- Cf

over-engine design and no tractor-trailer gap. As a Cp

further simplification, all measurements of the GTS D

model were made without wheels. Aerodynamic h

boattail plates were also tested on the rear of the Lp

trailer to provide a simple geometry modification for M

computation. The experimental measurements include OD

body-axis drag, surface pressures, surface hot-film p

anemometry, oil-film interferometry, and 3-D particle q

image velocime_'y (PIV). The wind-averaged drag u

coefficients with and without boattail plates were Re

0.225 and 0.277, respectively. PIV measurements S

behind the model reveal a significant reduction in the U

wake size due to the flow turning provided by the V

boattail plates. Hot-film measurements on the side of W

the cab indicate laminar separation with turbulent

x,y,z

reattachment within 0.08 trailer width for zero and hu

±10 degrees yaw. Oil-film interferometry provided x

quantitative measurements of skin friction and v

qualitative oil flow images. The methodology for p

calculation of the force and pressure coefficients is

included to facilitate comparison between

computation and experiment. A complete set of the

experimental data and the surface definition of the

model are included on a CD-ROM for further analysis

and comparison.

Nomenclature

speed of sound = x/_p/p

wind-tunnel contraction ratio

body-axis drag coefficient = D / qS

skin friction coefficient = x / q

pressure coefficient = (p - pot) / q

body-axis drag

trailer height

pressure level

Mach number = U / a

outside diameter

static pressure

dynamic pressure = 1/2 pU 2

local velocity

Reynolds number = U*w / v

trailer cross-sectional area = w * h

free-stream velocity

hot-film voltage

trailer width

right-handed coordinate system

yaw angle (positive nose right)

skin friction

kinematic viscosity

air density

Subscripts

max

tins

r

s

t

W

to

maximum value

root mean square

settling chamber value

static

total

wall

free-stream value

* AerospaceComputing, Inc., Moffett Field, California.
Introduction

Because of the enormous size of the U. S.

national trucking fleet, a small reduction in fuel

consumption equates to a considerable cost savings as

well as a reduced environmental impact. For a typical

class-8 tractor-trailer, the horsepower required for

rolling friction and accessories is greater than that of

aerodynamic drag until a highway speed of

approximately 50 mph (80 km/hr) (Ref. 1). However,

because drag generally varies with the square of

velocity, the impact of aerodynamic drag is much

more significant at typical highway speeds. At 70

mph (113 km/hr), for example, the power required to

overcome aerodynamic drag is 65% of the total fuel

consumption. It is estimated that a 25% reduction in

aerodynamic drag would yield a 10-15% reduction in

fuel consumption (Ref. 2). This savings is

considerably greater than the estimated possible

savings of other technology improvements, such as

improved engine efficiency and weight reduction.

The trucking industry currently relies on windtunnel testing and field experiments for aerodynamic

design and analysis. However, this process is costly

due to the expenses for both detailed scale models and

wind-tunnel occupancy. The use of CFD analysis

could significantly reduce the cost of aerodynamic

design by reducing the number of models tested in the

near term and eventually replacing most wind-tunnel

testing in the future. This experimental study is part

of a multi-year program focused on the development

of a validated aerodynamic simulation capability for

tractor-trailer configurations. The specific goal of this

experiment is to provide high-quality aerodynamic

data for direct validation of computational tools being

developed by national labs, academia, and industry.

A previous experimental study of the GTS model

was conducted in the Texas A&M University

(TAMU) Low-Speed Wind Tunnel (Ref. 3). This

facility has a 7-ft x 10-ft (2.13-m x 3.05-m) test

section similar to that used in the current study except

that the corners have 1-ft (0.305-m) fillets that house

lamps for photographic lighting. The l/8-scale GTS

model was tested at a Reynolds number of 1.6

million based on trailer width of 12.75 in (32.38 cm).

The measurements included surface pressures, forces

and moments, oil flow, tufts, smoke, and wake

measurements with a 7-hole probe and a tufted grid.

For the GTS model configuration without wheels, the

body-axis drag coefficients at zero yaw and wind

averaged were 0.246 and 0.266, respectively. Data

were also obtained for five add-on drag reduction

devices consisting of two ogival boattails and three

slants similar to those used by Ahmed et al (Ref. 4).

For the configuration without wheels, the largest

overall drag reduction (wind averaged) of 10.1% was

obtained by the 8-ft ogive configuration.

The effectiveness of aerodynamic boattail plates

was investigated on a full-scale tractor-trailer

configuration in the 80- by 120-Foot Wind Tunnel at

NASA Ames Research Center in 1988 (Ref. 5). The

boattail plates consist of four panels that are mounted

perpendicular to the aft-facing trailer door. Typically,

the plates are inset from the edges of the trailer on

three sides and mounted flush with the trailer edge on

the bottom. This passive device traps recirculating

flow between the plates and the rear corners of the

trailer. As a result, the flow separating from the rear

of the trailer is turned inward slightly. This yields an

increase in the base pressure and a reduction in the

overall drag. The optimum boattail plate

configuration reduced the overall drag by about 10%

at zero yaw angle. Relative to the baseline

configuration (without plates), measurements

indicated that the base pressure between the boattail

plates was significantly increased whereas the pressure

between the plates and the edge of the trailer was

reduced. Unsteady pressure measurements were also

made for pressure taps at the center of the trailer rear

panel. A consistent peak was observed between 1.8

and 2.0 Hz both with and without boattail plates.

This frequency corresponds to a Strouhal number of

0.12 based on trailer width. The only significant and

repeatable effect of the boattail plates was an increase

in the high-frequency content of the fluctuating

pressure.

The goal of the present study was to provide

high-quality experimental data on a simplified tractortrailer geometry for CFD validation. In addition to the

force, moment, and pressure measurements made in

the TAMU test, additional on-body and off-body

details were provided by hot-film anemometry, oilfilm interferometry, and particle image velocimetry.

Measurements were made at various yaw angles to

study the influence of crosswind and at various wind

speeds to determine the effect of Reynolds number.

An additional configuration was tested with

aerodynamic boattail plates to provide a simple

geometry modification for CFD simulation. The

entire data set has been archived on CD-ROM for easy

reference.
Experimental Setup

The experiment was conducted in the 7- by 10-

Foot Wind Tunnel #1 operated by the Army in

cooperation with the NASA Ames Research Center.

This atmospheric facility is a closed-circuit wind

tunnel incorporating a 15-ft (4.57-m) long test

section with a constant height of 7 feet (2.13 m) and

a nominal width of 10 feet (3.05 m) with a 1% wall

divergence. The boundary layer thickness at the test

section entrance is 2.1 in (5.3 cm), which corresponds

to a displacement thickness of 0.6 in (1.5 cm). The

boundary layer profile resembles that of a turbulent

flat plate described by the l/7%power law (Ref. 6) and

is included on the CD-ROM. The multiple

turbulence-reducing screens in the circuit yield empty

test-section turbulence intensities in the longitudinal,

lateral, and vertical directions of 0.1%, 0.3%, and

0.3%, respectively, for a test condition of M = 0.22.

These turbulence levels correspond to an RMS

turbulence intensity of 0.25% and a turbulence factor

of 1.2.

The baseline geometry for this investigation is

the GTS model without wheels, which is

representative of a generic l/8-scale class-8 tractortrailer geometry with a cab-over-engine design.

Designed for CFD validation, this geometry includes

a simple tractor design and no tractor-trailer gap,

which facilitates grid generation and avoids the

associated flow complexities. The model was

fabricated and tested at Texas A&M University and

subsequently loaned to NASA Ames for further study.

All flat model surfaces are aluminum sheet metal, and

all curved surfaces were machined from Ren Shape

450. Figure 1 shows the GTS model installed in the

NASA Ames 7- by 10-Ft Wind Tunnel. Figure 2

shows the GTS model dimensions nondimensionalized by the trailer width of 12.75 in

(32.38 cm). Note that all data presented in the current

study were obtained without wheels. The model was

instrumented with 79 pressure taps, as shown in

Figure 2. A tabular list of the tap locations is

included in Ref. 3 and on the CD-ROM.

Because of the model length and the interest in

wake details, the GTS model was located at a position

5.25 in (13.33 cm) downstream of the beginning of

the test secdon, as shown in Figures 3-4, Mounted

level in the test section, the bottom of the model was

located 3.0 in (7.6 cm) above the wind-tunnel floor.

Four cylindrical struts connected the model to the

scale system, and 1.5-in (3.8-cm) diameter cylindrical

fairings (non-metric) extended from the floor to

within 0.25 in (0.63 cm) of the bottom of the model.

As a simple geometry modification for

computations, aerodynamic boattail plates were

provided by Continuum Dynamics, Inc. A

photograph of the boattall plates installed on the rear

of the trailer is shown in Figure 5. The plates

extended 3.75 in (9.52 cm) from the end of the trailer

and were inset from the sides and top of the trailer by

0.625 in (1.59 cm), The bottom plate was mounted

flush with the bottom of the trailer.

The installation side view (Figure 4) also shows

the location of the three rows of pressure taps

mounted flush with the right test-section wall

(looking upstream). For ease of comparison with

CFD, the test-section static pressure was obtained

from a single wall-pressure tap located at x/w = 4.5,

y/w = 2.6, and z/w = -4.7, as shown in Figure 4.

This wall-pressure, Psw, was used to calculate pressure

coefficients as follows:

P-- PSW

q_

where the tunnel dynamic pressure, q_, was calculated

as the difference between the settling-chamber static

pressure (Pr) and the reference wall tap static pressure

(Psw)- A correction for the difference between the

settling-chamber total and static pressure was included

as follows:

C 2

q-- = P, -- P_ = c---T'___I (Pr -- P_)"

The contraction ratio, c, is the ratio of the crosssectional area of the settling chamber (upstream of the

contraction) to that of the test section (c = 14.14 for

the NASA Ames 7- by 10-Ft Wind Tunnel). The

surface pressures were measured with an electronically

scanned pressure system, and time averaging provided

an uncertainty in the calculated pressure coefficients

of +0.002.

The model was mounted on the facility scale

system, which includes a turntable for remote model

positioning. Because the facility scales measure windaxis forces (parallel and perpendicular to the axis of

the wind tunnel), a coordinate transformation was

employed to determine the body-axis drag (the force

along the longitudinal axis of the model). The drag
measurements areaccurateowithin+1 lb (4.45 N).

The repeatability of the drag coefficient measurements

was :t-0.001 for q' > -5 degrees and _-0.01 for W <

-5 degrees. These error bands include both

measurement resolution and point-to-point

repeatability. All drag measurements are reported in

the body-axis coordinate system. Due to erroneous

readings in the lift measurements, no lift, pitching

moment, or rolling moment data are presented. In

addition, no sideforce or yawing-moment data are

presented, but these data are available on the CDROM for further analysis.

Using the variation of drag with yaw angle,

wind-averaged drag coefficients were computed using

the SAE Recommended Practice of Ref. 7. This

practice assumes that the mean wind speed in the

United States of 7 mph (11.2 km/hr) has an equal

probability of approaching the vehicle from any

direction. This mean wind speed and the vehicle

velocity were used to calculate a weighted average of

the drag coefficient at various yaw angles. The windaveraged drag coefficients reported in this paper were

computed for a typical highway speed of 55 mph (88

km/hr).

Force and pressure measurements were made at

yaw angles ranging from -14 to 14 degrees in 2.5-

degree increments between +12.5 degrees. A majority

of the results that follow focus on yaw angles of 0,

10, and -10 degrees. Detailed data sets were obtained

at Reynolds numbers of 2 million and 740,000,

which correspond to wind-tunnel velocities of 205

mph and 75 mph, respectively (full-scale Reynolds

numbers vary from 4 to 6 million). The high-velocity

condition was chosen to maximize Reynolds number

while avoiding compressibility effects. The lowvelocity condition was chosen to minimize

compressibility effects for incompressible

computational algorithms. In addition, a Reynoldsnumber variation study was conducted at zero yaw for

Re = 300,000 to 2 million. Hot-film anemometry,

oil-film interferometry, and particle image

velocimetry were conducted at limited yaw angles and

tunnel speeds.

Results and Discussion

The results that follow do not include any

corrections for wind-tunnel wall effects. Therefore,

only computational grids that include the tunnel walls

will yield comparable results. Also, the computations

must use the same static pressure reference (detailed

above) for comparison with the experimental force

and pressure measurements.

I. Drag Measurements

The variation of the body-axis drag coefficient

with yaw angle for the model with and without

boattail plates is presented in Figure 6. Because of the

poor repeatability for yaw angles less than -5 degrees,

only data for positive yaw angles are shown. The

poor repeatability at negative yaw angles is due to

facility scale issues and is not an indication of flow

irregularities. The zero-yaw value of drag agrees well

with the results from Ref. 3, but the increase in drag

with yaw angle is larger than that of the previous

measurements. The differences between the current

results and those presented in Ref. 3 can be attributed

to the location of the static pressure reference used to

calculate tunnel dynamic pressure. These differences

are also evident in the static pressure measurements

given in the next section.

Relative to the GTS baseline configuration, the

addition of boattail plates yields an average of 20%

reduction in drag for yaw angles from zero to 14

degrees. The wind-averaged drag coefficients were

calculated using the data at non-negative yaw angles

and assuming symmetry of the drag curves about zero

degrees. The resulting wind-averaged coefficients with

and without boattail plates are 0.225 and 0.277,

respectively, which correspond to a 19% reduction in

drag. This reduction is almost twice that measured in

the full-scale test (Ref. 5) because of simplified

geometry of the GTS model (i.e., no wheels, no gap,

no minors, cab-over design, etc.). The drag reduction

from the boattail plates is also almost double that

measured with the best add-on device of Ref. 3 (8-fi

ogival boattail). This may be due to the effects of the

stagnation point where the flow reattaches to the

boattail plates. The local stagnation point on the

boattail plates provides more flow turning than is

possible from an aerodynamically contoured

afterbody.

The variation of the drag coefficient with

Reynolds number is presented in Figure 7 for a yaw

angle of zero degrees. The Reynolds number was first

varied from 300,000 to 2 million with increments of

100,000 and then reduced to 300,000 in larger

increments to check data repeatability. The differences

in the two curves at the lower Reynolds numbers

represent the uncertainty in the data due to the facility

scale system resolution as well as possible

mechanical and/or aerodynamic hysteresis. Note that
thereisasignificant variationinthedragcoefficient

forReynoldsnumbersbelow1million.Abovethis

value,theReynolds-number effectisminimal.The

variationoftheaveragebasepressurecoefficient on

thetrailerdoorislessthan0.005forthesame

Reynolds-number range.Thissuggeststhatthe

significant increaseindragcoefficient atReynolds

numbersbelow1millionmustbeduetoflow

variationsaroundthefrontofthemodel.Becauseflow

separationfromtheaftofabluffbodyisrelatively

insensitivetoReynoldsnumber,theresultsobtained

forReynoldsnumbersabove1millionareexpectedto

berepresentative ofthefull-scaleflowfield(Re=4-6

million).

II. Surface Pressures

Seventy-nine surface pressures were located on

the model, and 43 additional surface pressures were

located on the right wall of the test section (looking

upstream). In addition, one unsteady pressure

transducer was located on the rear of the trailer.

Pressure coefficients were calculated using a selected

wall pressure as the test-section static pressure (as

detailed above). By using the same static pressure

reference in the CFD simulations, a direct comparison

can be made between experiment and computation.

However, it should be noted that the selected wall

pressure varies with yaw angle, as shown in Figure 8,

due to the position of the model relative to the

reference pressure tap. Because of this variation, the

surface pressures on the model symmetry plane do not

agree at corresponding positive and negative yaw

angles.

Horizontal-Plane Surface Pressures: A

longitudinal row of pressure taps was located at half

trailer height (y/w = 0.696) on the left side of the

model. The variation of the horizontal plane pressures

with yaw angle is presented in Figure 9 for the

baseline configuration (no boattail plates). The tap at

x/w = 0 is on the front of the tractor and indicates a

pressure near stagnation, as expected. There is a

suction peak as the flow negotiates the comer, and

then the pressure is relatively constant until near the

end of the trailer. At this point, the flow again

accelerates due to the converging streamlines after

flow separation from the aft of the trailer. The effect

of boattail plates at zero yaw (Figure 10) is to reduce

the pressures near the end of the trailer because of the

increased flow curvature in the wake.

Top Surface Pressures: Another

longitudinal row of pressure taps was located along

the model centerline (z/w = 0) on the top of the model

(y/w = 1.392). The effect of the model surface

curvature above the cab is seen as a suction peak of

about Cp = -0.4 in Figure 11. Further aft, the pressure

is relatively constant until near the end of the trailer,

where the flow is accelerated due to the same effect

noted for the horizontal surface pressures. The reduced

pressures for the non-zero yaw angles correspond to

higher-velocity flow over the top of the truck. This

velocity increase is due to the increase in projected

area of the truck with increasing yaw angle. The

asymmetry between the +10- and -10-degree yaw

angles is due to the variation of the reference wall

pressure, as detailed above. Again, the effect of the

boattail plates at zero yaw (Figure 12) is to reduce the

pressure near the end of the trailer.

Symmetry-Plane Surface Pressures: All

the pressures along the symmetry plane (z/w = 0) are

presented in Figure 13 as a function of vertical

location. This format gives an indication of the drag,

which is related to the pressure difference between the

front and aft of the model. The pressure distribution at

10 degrees yaw reveals increased suction on the rear of

the trailer relative to the zero-degree case. This

increased suction corresponds to an increase in drag

for non-zero yaw angles. Figure 14 presents the effect

of boattail plates on the symmetry plane pressures at

zero yaw. The effect of the boattail plates is evident

only on the lower portion of the rear of the trailer

where the pressures are significantly increased. This

increased base pressure with boattail plates

corresponds to the reduction in drag noted above.

Base Surface Pressures: In addition to the

row of surface pressures along the symmetry plane,

two other vertical rows of surface pressures were

located on the back of the trailer at z/w = 0.22 and

0.44. The base pressure variation with lateral position

is presented in Figure 15 for yaw angles of zero and

+10 degrees. The zero-yaw measurements indicate

little variation in base pressure with lateral position.

A greater variation in the base pressures is observed at

a yaw angle of 10 degrees. With taps on the windward

side of the trailer, the base pressures are lower than at

zero yaw and the pressure increases with increasing

lateral distance from the symmetry plane. At -10

degrees yaw, the pressure taps are on the leeward side

of the trailer and the base pressures on the upper

portion of the trailer are less than those on the

windward side. The effect of boattail plates on the

base pressures at zero yaw is presented in Figure 16.

As noted above, the boattail plates significantly

increase the base pressure on the lower portion on the

5
rearofthetrailer.Similarto the baseline configuration at zero yaw, there is little variation in the

boattail-case base pressures with lateral position.

The effect of Reynolds-number variation on the

trailer base pressures is presented in Figures 17-18.

Although the variation of the average base pressure

coefficients (Figure 17) is small (less than 0.005 for

350,000 _<Re _<2 million), there is an obvious break

in the curve at 1 million. At Reynolds numbers

below l million, the reduced back pressure (increased

suction) on the trailer corresponds to the increased

drag measured for the same range (Figure 7).

However, the small change in base pressures (less

than 2%) is not sufficient to account for the 60%

difference in drag between Reynolds numbers of 1

million and 350,000. As mentioned previously, this

suggests that there must be significant Reynoldsnumber effects on the forebody flowfield. Figure 18

shows that the base pressure distribution is also

significantly affected by variations in Reynolds

number. In particular, the vertical pressure

distributions are more symmetrical at the lower

Reynolds number. This is likely due to the thicker

boundary layer at lower Reynolds numbers and its

effect on the flow underneath the truck. As with the

data for Re = 2 million (Figure 15), there is little

variation in the pressure distributions with lateral

position for Re = 740,000.

Test-Section Wall Pressures: Txm

horizontal rows and one vertical row of pressure taps

were installed in the north test-section wall (z/w =

-4.7), as illustrated in the installation drawing

(Figure 4). The pressure distributions measured at

yaw angles of 0, 10, and -10 degrees are presented for

each of these rows in Figures 19-21. As detailed

above, the wall tap at rdw = 4.5 and y/w = 2.6 was

chosen as the freestream static pressure for calculation

of pressure coefficients. This results in a pressure

coefficient of exactly zero for the chosen wall tap

which is included in Figure 19. Some of the scatter

observed in the wall pressures may be due to

imperfections in the pressure-tap installation or wallpanel construction.

Unsteady Base Pressure: A single unsteady

pressure transducer was installed flush with the aftfacing panel near the fight side at y/w = 0.63 and z/w

= -0.46. The signal was sampled at 1500 Hz for 210

seconds with anti-aliasing filters at 750 Hz. A sample

time series is presented in Figure 22, where some

signal periodicity is evident. Spectra for various

configurations are presented in Figures 23-26 for

frequencies below 150 Hz. The spectra at higher

frequencies are relatively featureless and therefore are

not presented. In general, the signal is broadband with

no obvious peaks from periodic shedding that might

be expected from a bluff body. Figure 23 shows that

the spectrum does not vary appreciably between zero

and l0 degrees yaw. At -10 degrees, however, the

spectrum of the sensor near the windward edge is

5-10-dB higher than at zero yaw (Figure 24). Figure

25 shows that the effect of boattail plates is to reduce

the level for frequencies below 90 Hz.

The effect of Reynolds number variation on the

fluctuating pressure spectra is presented in Figure 26.

Unlike the relatively broadband spectrum for Re -- 2

million, the spectrum for Re = 740,000 includes a

distinct peak at approximately 9 Hz. Using velocity

scaling, a corresponding local maximum at 24 Hz can

also be identified in the spectrum for Re = 2 million.

These peaks correspond to a Strouhal number of

0.085 based on trailer width. This Strouhal number is

of the same order as that measured at full scale (Ref.

5) and is indicative of relatively organized shedding.

IlI. Oil-Film Inteffemmetly (OFI)

The OFI technique is based on the principle that

oil on a surface, when subjected to shear, will thin at

a rate related to the magnitude of the shear. The

measurement of skin friction involves measuring the

oil-thickness distribution, recording a history of the

tunnel run conditions, and knowing the properties of

the oil. The oil film's thickness distribution is

determined from the interference patterns that can be

seen in the oil as a result of interference between

reflected light from the model surface and the reflected

light from the air-oil interface. The spacing of the

dark bands (or fringes) is a measure of the slope of the

oil front, and the fringes are contours of constant oil

thickness. The uncertainty of this technique is +5%;

more details can be found in Ref. 8.

OFI measurements were made on the top and

fight side of the GTS model without boattail plates.

An image of the fringe patterns observed on top of

the trailer is shown in Figure 27 for 10 degrees yaw.

In addition to the fringe patterns, the oil flow shows

the effect of the vortex that forms from the flow off

the windward side of the model. The vortex path is

also evident in the vector plot of skin friction

presented in Figure 28 for l0 degrees yaw. The

relatively short vectors in the first two rows indicate

laminar flow on the front of the model where the flow

accelerates along the curved surface. Transition to
turbulenceoccursnearx/w= 1.0,asindicatedbythe

higherskin-frictionvaluesinthatregion.Thevortex

pathextendsinboardfromtheleftsideofthemodel

aftofaboutrdw=2.0.

Aplotoftheskin-frictionmagnitudealongthe

centerlineontopof the model is presented in Figure

29 for zero yaw. Also included in the figure is the

theoretical curve for skin friction on a turbulent flat

plate. Because of the shape of the front of the GTS

model, the boundary layer is laminar until near the

end of the surface curvature. As a result, the skin

friction is initially significantly less than that of the

turbulent fiat plate. The skin friction peaks during

transition, after which it approximates the theoretical

curve for a turbulent boundary layer. Quantitative

results are also available for the skin friction on the

right side of the model at zero and 10 degrees yaw

(included on CD-ROM).

The OFI measurements were made only at a

Reynolds number of 2 million. At lower Reynolds

numbers, the skin-friction distribution on the top of

the model is expected to be very similar except for the

location of boundary-layer transition. As the

Reynolds number is reduced, laminar-to-turbulent

transition will be delayed and the peak in the skin

friction distribution (Figure 29) will move aft. At

Reynolds numbers below 1 million, it is expected

that OFI would show a significant separation region

on the side of the tractor that would account for the

corresponding increase in drag coefficient.

IV. Hot-Film Anemometry

Similar to hot-wire anemometry, the hot-film

technique is based on the principle that a change in

temperature affects the current flow or voltage drop

through a fine metallic connection. For hot-film

anemometry, a thin metallic substrate, or "film," is

mounted flush with the surface of interest and is

heated electrically by a circuit that maintains either

constant current or constant voltage drop. When air

flows over the hot film, the cooling, which is a

function of the skin friction, can be detected as

variations in voltage (for constant current) or current

(for constant voltage).

Hot-film measurements were made on the right

side of the cab to determine the existence and extent

of flow separation in the vicinity of the front comer

radius. Mylar sheets with an array of hot-film sensors

were mounted to the side of the cab as shown in

Figure 30. Sixteen sensors were measured

simultaneously at 1024 Hz for 2 seconds. Several

sensor configurations were studied, but the results

that follow are for streamwise rows of hot-film

sensors located at y/w = 0.35 and 0.64 with clustering

near the comer radius. The hot-film sheets and

measurement system were provided by Tao of

Systems Integration, Inc., with funding from the

NASA Small Business Innovative Research (SBIR)

program. Further details can be found in the SBIR

Phase II final report (Ref. 9).

Spectra of four hot-film sensors (#2, 3, 4, & 5)

on the comer radius at y/w = 0.35 are presented in

Figures 31-32. In general, the spectra are relatively

fiat and devoid of obvious peaks, which suggests that

there is no extensive separation region. If separation

were present, periodic flow oscillations would likely

result in significant peaks in the spectra of sensors

near the separation region. At zero yaw (Figure 31),

the spectra for the first three sensors are at the same

level whereas the fourth sensor is 20 dB higher. This

is indicative of laminar-to-turbulent transition of the

boundary layer. At 10 degrees (Figure 32), only the

first two sensors are at the lower (laminar) level, so

the boundary layer transitions earlier than at zero yaw.

Spectra of three hot-film sensors near the comer

radius at y/w = 0.64 are presented in Figures 33-34

for 10 degrees yaw. At this height on the model, only

the first sensor, at x/w = 0.06, indicates laminar flow

(Figure 33). All subsequent sensors in the streamwise

row are at a higher level, which implies that

boundary-layer transition occurs between the first and

second sensors. Figure 34 presents a comparison of

the spectra for Reynolds numbers of 2 million and

740,000. At the lower velocity, there is a significant

increase in the spectra below 100 Hz. This is

indicative of large-scale flow unsteadiness in a

separation region. The remaining sensors in the

streamwise row are all at a level similar to that of the

sensor at x/w = 0.09. This low-frequency peak is

evident for Reynolds numbers less than 1 million and

corresponds with the Reynolds-number effect observed

in the pressure and drag measurements.

The statistics of all 16 hot-film sensors at y/w =

0.35 are presented in Figure 35 for yaw angles of 0,

10, and -10 degrees. The top plot shows the mean

value of the hot-film signal, which is proportional to

the third power of surface shear stress (Ref. 9). A drop

in the mean signal level occurs at all bifurcation

points (i.e., flow separation, reattachment,

stagnation). The plots of RMS and intermittency give
anindicationofthestateoftheboundarylayer.The

lowerplotshowsthepositionsofthesensorselative

tothe1.l-in.(2.85-cm)comerradiusatthefrontof

thecab.Asindicatedinthefigure,thefirstsensor

waslocatedonthefrontofthecab,thenexthree

wereonthe comer radius, and the remaining sensors

were on the side of the truck.

At all yaw angles, the mean voltage level

initially increases up to sensor #3 as the flow

accelerates around the comer radius. The subsequent

drop in the mean voltage at sensor #4 indicates flow

separation. The RMS and intermittency levels show

that the boundary layer is laminar at sensor #3 for all

yaw angles. The increase in RMS and intermittency

at sensor #4 for the 10-degree case indicates transition

to turbulence in advance of the other yaw angles. This

is to be expected due to the adverse pressure gradient

on the leeward side of the model. The mean, RMS,

and intermittency levels peak at or before sensor #5,

indicating laminar-to-turbulent transition in the

separated shear layer. All levels subsequently drop at

sensor #6 after flow reattachment. In summary, a

relatively small (0.08w) laminar separation bubble is

found at all three yaw angles. The extent of this

separation region on the leeward side is no larger than

that of the windward side, contrary to what was

expected. Oil-film interferometry measurements on

the side of the model (not shown) corroborate these

results.

V. Particle Image Velocimetry (PIV)

PIV measures fluid velocities by time-sequential

imaging of the displacement of individual tracer

particles in the flow. A pulsed-laser light sheet

illuminates the tracer particles, or seed, and a

correlation of two successive images yields the

distance and direction that the particles move. For 2-D

velocity measurements, a single camera oriented

normal to the light sheet measures the motion of the

tracer particles in a plane. To obtain three components of velocity, two cameras view the identical

region of interest obliquely and simultaneously. By

viewing the laser light sheetobliquely, both cameras

can record the movement of the particles through the

light sheet. This method exploits the difference in

perspective between the two cameras and derives the

third component of velocity mathematically. More

details on this PIV method can be found in Ref. 10.

Three-component PIV was conducted for several

measurement plane orientations, as shown in Figure

36. For each orientation, the laser light sheet was

moved to several locations in the trailer wake.

Measurements were made with and without boattail

plates for most locations. A summary of the

locations, yaw angles, and tunnel conditions is

presented in the table.

A sample of the horizontal plane measurements

at half height is presented in Figure 37. This figure

includes three components of velocity in the trailer

wake with and without boattail plates. The upper

figures present contours of the streamwise velocity

component, which reveal that the size of the wake is

significantly reduced by the presence of the boattail

plates. At the streamwise location of x/w = 8.6, for

example, the a&ttion of the boattail plates reduced

the width of the wake by 16% (from 0.94w to

0.79w). The lower figures present in-plane velocity

vectors that are colored by the magnitude of the outof-plane velocity. As in the streamwise velocity

contours, these figures illustrate that the boattail

plates produce a narrowing of the wake, which leads

to increased base pressure and reduced drag.

Orientation

Horizontal

Location

y/w = 0.35, 0.70, 1.05

Yaw, deg.

0, 10

Cross-stream x/w = 8.0, 8.35, 8.78 0, 10 2.0, 0.74

Streamwise z/w = 0, 0.25** 0

* without boattail plates only

** measured at --4-deg. yaw

2.0*, 0.74*
Summary

Experimental measurements were obtained for

CFD validation of a simplified l/8-scale tractor-trailer

configuration. To reduce complexity of the geometry

and resulting flowfield, the model design included a

cab-over-engine design, no tractor-trailer gap, and no

wheels. Add-on boattail plates were also tested on the

rear of the trailer to provide a simple drag-reduction

device for computation. The boattail plates provided a

19% reduction in wind-averaged drag and reduced the

size of the wake by turning the flow downstream of

the trailer. Particle image velocimetry measurements

provide details of the off-body flowfield in the model

wake. Hot-film measurements on the side of the

model revealed a laminar separation bubble in the

vicinity of the comer radius. Oil-film interferometry

yielded quantitative measurements of skin friction and

qualitative surface flow visualization. Due to

differences in the static-pressure reference, there were

some discrepancies between the current results and

measurements from a previous test of the same

model. To facilitate comparison to computation, the

methodology is provided for calculation of the force

and pressure coefficients. If not included with the

hardcopy of this report, a CD-ROM data archive is

available for further study.

Future Work

A follow-on wind-tunnel experiment will include

a conventional cab-behind-engine tractor design and a

tractor-trailer gap. The gap flow and the effect of

tractor side extenders will be studied in detail in the

NASA Ames 7- by 10-Ft Wind Tunnel. Although

this report shows little variation of drag for Re > 1

million, the conventional tractor design and the

tractor-trailer gap are expected to produce results with

significant Reynolds-number sensitivity. To

document this sensitivity up to full-scale Reynolds

numbers (4-6 million), the same model will later be

tested in the Ames 12-Ft Pressure

References

1 McCallen R., Couch, R., Hsu, J., Leonard, A.,

Brady, M., Brownad, F., Hammache, M., Salari, K.,

Rutledge, W., Ross, J., Storms, B., Heineck, J.T.,

Driver, D., Bell, J., Zilliac, G., Progress in Reducing

Aerodynamic Drag for Higher Efficiency of Heavy

Duty Trucks (Class 7-8), 1999-01-2238,Gov/Industry

Meeting, Washington, D.C., April 26-28, 1999.

2 McCallen R., Flowers, D., Dunn, T., Owens,

J., Leonard, A., Brady, M., Brownad, F., Hammache,

M., Salari, K., Rutledge, W., Ross, J., Storms, B.,

Heineck, J.T., Driver, D., Bell, J., Zilliac, G.,

Walker, S., Aerodynamic Drag of Heavy Vehicles

(Class 7-8): Simulation and Benchmarking, 2000-01-

2209, SAE Gov/Industry Meeting, Washington,

D.C., June 19-21, 2000.

3 Croll, R. H., Gutierrez, W. T., Hassan, B.,

Suazo, J. E., and Riggins, A. J., "Experimental

Investigation of the Ground Transportation Systems

(GTS) Project for Heavy Vehicle Drag Reduction,"

SAE Paper 960907, February 1996.

4 Ahmed, S. R., Ramm, G., and Faltin, G.,

"Some Salient Features of the Time-Averaged Ground

Vehicle Wake," SAE Paper 840300, 1984.

5 Lanser, J. M., Ross, J. C., and Kaufman, A.

E., "Aerodynamic Performance of a Drag Reduction

Device on a Full-Scale Tractor/Trailer," SAE Paper

912125, September 1991.

6 Schlichting, H., Boundary-Layer Theory,

Seventh Edition, McGraw-Hill, New York, 1979, pp.

636-640.

7 "SAE Wind Tunnel Test Procedure for Trucks

and Buses," SAE J 1252 JUL81, SAE Recommended

Practice, July 1981.

8 Driver, D. M., "Application of Oil Film

Interferometry Skin-Friction to Large Wind Tunnels,"

AGARD CP-601, Paper no. 25, 1997.

9 Tat of Systems Integration, Inc., "Interactive

Flow Measurement (IFM) System for High-Reynolds

Number Wind-Tunnel Testing," NASA SBIR Phase

II Final Report, Contract Number NAS2-97007,

March 1999.

10 Heineck, J. T., and Walker, S. M., "3D PIV

in Wind Tunnel Applications: Measurements of a

Truck Wake," SAE Paper 199-01-5600, October

1999.

9
Figure 1: GTS model installed in the NASA Ames 7- by 10-Ft Wind Tunnel.

ELLIPSE- X

MAJOR DIA .... 353\

MINOR DIA. = 1.373

y/w

BLEND RADIUS TO SHARP "'_

CORNEIR AT END OF CAB

[BOTH SIDES)

R.II8 j

z/w-._----

1.000

/Static pressure tap

_Unsteady pressure sensor

2.000

7.647 -"i

5.647

00004

-_---'_ • • • • O00Oe

.696

ooool

t

1.392

I -------_ x/w

• ooo° t

Figure 2: GTS geometry and pressure tap locations. All measurements non-dimensionalized by trailer width,

w = 12.75 in (32.38 cm).

10
0.41 --_

Airflow _ Turntable

/

/ N

\

\

GTS model \

/ \

-0.63 / _ 3.49 _ \

9.41

1

Figure 3: GTS model installation, top view. All measurements non-dimensionalized by trailer width,

w = 12.75 in (32.38 cm).

0.65

Turntable

centerline

5.29

_-- 0.47

• ° • • ° ° ° • ° I

Reference static pressure

onPressure tunnel taps wall l

2.82

1.41

n

6.59

r

Figure 4: GTS model installation, side view. All measurements non-dimensionalized by trailer w_dth,

w = 12.75 in (32.38 cm).

11
Figure5:Photographofboattailplatesinstalledonrearof trailer.

0.30

Baseline__--_

0.28i _ "_

Boattail plates Run14

0.22-

0.20

0 4 8 12 16

Yawangle,deg

Figure6:Effectofboattaiiplatesondragcoefficient.

Re= 2million.

0.26

CD

0.24

0.45

0.40

0.35

C

D

0.30

0.25

0.20

_ Decreasing

Velocity

_ Increasing _ No Boattail Plates

_ Velocity

I I I I

0.5 1 1.5 2

Reynolds number, millions

Figure 7: Effect of Reynolds number on baseline drag

coefficient.

12
0.024:

?,

9.

i

L_

0.012

0.000

-0.012

I I I I I

-10 -5 0 5 10 15

Yaw angle, deg.

Figure 8: GTS Baseline, Reference wall pressure variation with yaw angle. Pressure coefficients calculated using

facility static-pressure ring as static-pressure reference. Re = 2 million.

1.0

End of trailer j i

I

0.5- /lOOyaw_WindwardSide !

0.0- _

-0.5 _- Yaw

°°- Leeward Side

l Re = 2 million

-1.0 _ I l t ,

0 2 4 6 8

x/w

Figure 9: GTS Baseline, Side static-pressure coefficients

at y/w = 0.696 (half-height).

C

P

0.0

-0.1 -

-0.2 -

-0.3 -

f Re = 2 million Endof trailer/"

-0.4 nil t I I ,

0 2 4 6

x/w

Figure 10: Effect of boattail plates on side staticpressure coefficients.

0.0

-0.1 _-

C

P

-0.2 -

-0.3 -

-0.4

0

-10 ° Yaw

End of trailer /

Re = 2 million

L I L . ,

2 4 6

x/w

Figure 11: GTS Baseline, Top static-pressure

coefficients at z/w = 0.0 (centerline).

0.0

-0.1

Cp

-0.2

-0.3

-0.4

0

/ End of trailer /

/

/ 0° Yaw 1

n[ Re = 2 million

t J I ,

2 4 6

x/w

Figure 12: Effect of boattail plates on top staticpressure coefficients.

13
1.5

.......... 4 _- %po?t_ai-ler

0° Yaw / |

,o _ l_ ,0OYaw

0.5 /j _,,_ Back of trailer

_rf

Front _million

0.0 _of tractor ,

1.5 1 0.5 0 -0.5 -1 -

C

P

Figure 13: GTS Baseline, Symmetry plane staticpressure coefficients.

1.5

1.0

y/w

0.5

0° Yaw ---. 10° , , ---7 -10 °

-+-00o I

+ 0.22 I '_'_ "x_',,,_.

// :?//

Re= 2 r_llion ,_d 0.0

I I

0 -0.1 -0.2 -0.3

C

P

Figure 15: GTS Baseline, Trailer base static-pressure

coefficients.

-0.140

-0.141

-0.142

C

P

-0.143

-0.144 -

-0.145

0

No Boattail Plates

I I 1 I

0.5 1 1.5 2

Reynolds number, millions

Figure 17: Effect of Reynolds number on the average

trailer base pressure at zero yaw.

1.5

Boattaii plates/qt/ [

1.0 //_ ! Baseline[

y,w r f

0.,- _ _/_a_ °f_ler/

/ _1 / Re = 2 million /

Front __,_ 0° Yaw [

0.0 _oftractor __ _ [

I I 1 I 1 /

5 1.5 1 0.5 0 -0.5 -1 -1.5

Cp

Figure 14: Effect of boattail plates on symmetry plane

static-pressure coefficients.

1.5

• z/w

.... • .... 0.00

+ 0.00

1.0 + 0.22

"-,, ----o--- 0.44

y/w "

0.5 , j Baseline

! plates

0.0 •, Re = 2 million

I 0° Yaw

1.0

y/w

0.5

0 -0.1 -0.2 -0.3

Cp

Figure 16: Effect of boattail plates on base staticpressure coefficients.

1.5

o+ _ 0.00 0.00

"+ _ 0.22

0.44

z/w

++74i i

0.0 - 0 ° Yaw

I I

0 -0.1 -0.2 -0.3

C

P

Figure 18: Effect of Reynolds number on the trailer

base pressure distribution at zero yaw.

14
i

0.04-

/ -10° Yaw //'_

- / _ Reference tap )n/j

-0.02 "_,,, \ 0Oyaw End of trailer / ,

\ ,

-0.04 - 10° Yaw Re = 2 million _,

} I I ,

0 2 4 6

x/w

Figure 19: GTS Baseline, Wall pressure coefficients at

y/w = 2.59.

0.02

Cp

0.00

5.5

4.5

3.5

y/w

2.5

1.5

10° Yaw _-10 ° Yaw

...... _ _ _'r_ p_o_f g le_ 

, ,

0.5

0.04 0.02 0 -0.02 -0.04

C e

Figure 21: GTS Baseline, Wall pressure coefficients at

x/w = 4.94.

135

._. 120

¢',,1

m 105

90

0o Yaw /

10° Yaw 1

Re = 2milfion

i I

50 100 150

Frequency, Hz

Figure 23: GTS Baseline, Spectra of unsteady pressure

sensor on rear of trailer at W = 10 °.

0.04

0.02

Cp

0.00

-0.02

-0.04

_ , -10 ° Yaw

- _\ 0° Yaw End of trailer / ',

\ 10°Yaw Re=2million i

• l I I ,

0 2 4 6 8

x/w

Figure 20: GTS Baseline, Wall pressure coefficients at

y/w = 1.18.

0.10

Re = 2 million

I I I 0° Yaw

0.25 0.5 0.75 1

Time, sec.

Figure 22: GTS Baseline, Unsteady pressure signal

from sensor on rear of trailer.

_ ) ,_,_,/ -10 ° Yaw

' % i; : _ _r

135

_. 120

105

0° Yaw

90- Re = 2 million I

I I

0 50 100 150

Frequency, Hz

Figure 24: GTS Baseline, Spectra of unsteady pressure

sensor on rear of trailer at _g = -10 °.

15
120 140

,_110

e_

0

¢q

100

90

80

=oPla,os

Boattail Plates

Re = 2 million

0° Yaw

I

0 50 100 150

Frequency, Hz

Figure 25: Effect of boattail plates on unsteady pressure

spectra.

,_ 120

::1.

o

t'-,I

100

80

_ " ,

i

_ ,I =

'_'ii _'i _ _ it

_4

%

Re = 740,000

0° Yaw

6O I I

0 9 24 50 100 150

Frequency, Hz

Figure 26: GTS Baseline, Effect of Reynolds number

on unsteady pressure spectra.

5.0 5.5 6.0 6.5 x/w

Figure 27: Oil-film interferometry image showing vortex roll-up on top of truck at 10 deg. yaw. Skin friction is

proportional to spacing between interference bands. Flow from left to right.

Figure 28: Skin friction vectors on top of the truck from oil-film measurements at 10 deg. yaw. Re = 2 million.

16
6

4

2

0

/_ Turbulent flat pllte

Transition GTS model contour 1

End of trailer J

0 2 4 6 8

x/w

Figure 29: Skin friction coefficients on top of trailer from oil-film interferometry measurements at

zero yaw. Re = 2 million.

Figure 30: Hot-film sensor sheets mounted on the side of the GTS model.

17
-20

-40

ha-60

'x3

,.d

-80

-100

0

./ x/w = 0.17

1 _ x/w = 0.09 Re = 2 million

1 I I 0° Yaw

150 300 450

Frequency, Hz

Figure 31: GTS Baseline, Spectra of adjacent hot-film

sensors on the fight side of the model at

= 0°(y/w = 0.35).

-20

x/w = 0.13

-40

m-60

M

-80

Re = 2 million

-100 I I 10° Yaw

0 150 300 450

Frequency, Hz

Figure 32: GTS Baseline, Spectra of adjacent hot-film

sensors on the right side of the model at

_u = 10O(y/w = 0.35).

-20

-40

m -60

2"

-80

-100

- / x/w = 0.13

. .. ,",';_ . , "'"',_., , :,',;,. ', ..'...., ,,"','" ...,_ .',

0 150 300 450

Frequency, Hz

Figure 33: GTS Baseline, Spectra of adjacent hot-film

sensors on the fight side of the model at

W = 10°(y/w = 0.64).

-100

10° Yaw

I I

0 150 300 450

Frequency, Hz

Figure 34: GTS Baseline, Spectra of adjacent hot-film

sensors on the fight side of the model at

= 10°(y/w = 0.64)

18
2.40

1.80

>

_ 1.20

0.60

0.00

0.06

> 0.04

0.02

0.00

1.00

>_ 0.75

e,9

•_ 0.50

'_ 0.25

0.00

1.5

1.0

y/w

0.5

0.0

_ [ _ Separation _ 0Oyaw

:_ i _h'_nt- --B--10° yaw

.... o..... 10° yaw

I I I I

0 0.25 0.5 0.75 1

x/w

i?':

I I [ I

0.25 0.5 0.75 1

x/w

m

[Z]! - _ _

t .................. _ ......

" i

[ I

0.25 0.5 0.75 1

x/w

[ sensors

I

I

I I I I I

0.25 0.5 0.75 1

X]W

Figure 35: Hot-film measurements on the right side of the truck for yaw = 0, 10, & -10 deg. Flow direction is from left

to right. Re = 2 million.

19
Horizontal Plane

VerticalCross-streamPlane

W

Laser fight sheet

(35 cm x 50_8 era)

Vertical Streamwise Plane

Figure 36: Measurement plane orientations for particle image velocimetry in the trailer wake.

20
Streamwise

Boattail Plates Component, m/s

I 38

32

25

19

13

6

0

-6

Baseline

Flow direction

Boattail Plates

Out-of-plane

Component, m/s Baseline

I 6

5

4

3

2

l

0

-1

2

-3

-4

-5

-6

i •

,m

,r.--,lmlMw'-- - --

Figure 37: Particle image velocimetry measurements in the wake of the trailer with and without boattail plates.

Horizontal plane at half height (y/w = 0.696), time averaged. Re = 2 million.

21
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REPORT DOCUMENTION PAGE ouaNo,o7o4-o188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

February 2001 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

An Experimental Study of the Ground Transportation System (GTS)

Model in the NASA Ames 7- by 10-Ft Wind Tunnel

,6. AUTHOR(S)

Bruce L. Storms,* James C. Ross, James T. Heineck, Stephen M.

Walker, David M. Driver, Gregory G. Zilliac

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Ames Research Center, Moffett Field, CA 94035-1000 and

*AerospaceComputing, Inc., Los Altos, CA 94022

9. SPONSORING/MONITORING AGENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001 and

U.S. Department of Energy, Washington, DC 20585

RTOP 522-21-72-52-98

8. PERFORMING ORGANIZATION

REPORT NUMBER

A-00V0031

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM- 2001-209621

11. SUPPLEMENTARY NOTES

Point of Contact: James C. Ross, Ames Research Center, MS 260-1, Moffett Field, CA 94035-1000

(650) 604-6722

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified--Unlimited Distribution: Standard

Subject Category - 02

Availability: NASA CASI (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The l/8-scale Ground Transportation System (GTS) model was studied experimentally in the NASA

Ames 7- by 10-Ft Wind Tunnel. Designed for validation of computational fluid dynamics (CFD), the

GTS model has a simplified geometry with a cab-over-engine design and no tractor-trailer gap. As a

further simplification, all measurements of the GTS model were made without wheels. Aerodynamic

boattail plates were also tested on the rear of the trailer to provide a simple geometry modification for

computation. The experimental measurements include body-axis drag, surface pressures, surface hot-film

anemometry, oil-film interferometry, and 3-D particle image velocimetry (PIV). The wind-averaged drag

coefficient with and without boattail plates was 0.225 and 0.277, respectively. PIV measurements behind

the model reveal a significant reduction in the wake size due to the flow turning provided by the boattail

plates. Hot-film measurements on the side of the cab indicate laminar separation with turbulent

reattachment within 0.08 trailer width for zero and _ 10 degrees yaw. Oil film interferometry provided

quantitative measurements of skin friction and qualitative oil flow images. A complete set of the

experimental data and the surface definition of the model are included on a CD-ROM for further analysis

and comparison.

,14. SUBJECTTERMS

Bluff Body Aerodynamics, Truck Drag, GTS Model

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

18, SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

15. NUMBER OF PAGES

24

i 16. PRICE CODE

A03

19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF ABSTRACT

Standard Form 298 (Rev. 2-89)