NASA Technical Reports Server (NTRS) 19750009309: An investigation of rooftop STOL port aerodynamics

NASA CR-1 32570

(N ASA-CR- 1 32570) AN INVESTIGATION OF N75-17

ROOFTOP SIOLPOR 1 AEaODYN AfllC J (Virginia
Jniv. ) 32 p HC 44.75 C3CL C 12

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03/v5 12191

AN >N tSTIGATION OF ROOFTOP STOLPORT AERODYNAMICS

By Jeffrey N. Blanton and Hermon M. Parker

Distribution of this report is provided in the interest of
information exchange. Responsibility for the contents
resides in the author or organization that prepared it.

Prepared under Grant No. NGR-47-005-146 by
THE UNIVERSITY OF VIRGINIA
Charlottesville, Virginia

for

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

ABSTRACT

An Investigation into aerodynamic problems associated with large building
rooftop STOLports has been performed. Initially, a qualitative flow visualiza-
tion study indicated two essential problems: (l) the establishment of smooth,

steady, attached flow over the rooftop, and (2) the generation of acceptable
crosswind profile once (I) has been achieved. This study indicated that (1)
could be achieved by attaching circular-arc rounded edge extensions to the
upper edges of the building and that crosswind profiles could be modified by
the addition of porous vertical fences to the lateral edges of the rooftop.
Important fence parameters associated with crosswind alteration were found to
be solidity, fence element number and spacing. Large scale building induced
velocity fluctuations were discovered for most configurations tested and a
possible explanation for their occurrence was postulated. Finally, a simple
equation relating fence solidity to the resulting velocity profile was developed
and tested for non-u.iiform single element fences with 30 percent maximum solidity.

AN INVESTIGATION OF ROOFTOP STOLPORT AERODYNAMICS

Jeffery N. Blanton and Hermon M. Parker
University of Virginia

SUMMARY

An investigation into aerodynamic problems associated with large building
rooftop STOLports has been performed. A qualitative flow visualization program
carried out at the Langley Research Center indicated two essential problems:

(1) the establishment of smooth, steady, attache flow over the rooftop; and,

(2) the generation of an acceptable crosswind profile once (I) has been
achieved. It was found that the configuration of the building itself determines
the gross nature of the flow over the rooftop and that relatively smooth
attached flow could be achieved by the addition of circular-arc rounded edge
extensions to the upper edges of the building. It was further determined that
crosswind profiles could be altered by the addition of vertical fences along
the lateral edges of the building rooftop.

A more quantitative program was undertaken at the University of Virginia.
Effects of various fence parameters such as screen wire and mesh size, number
of elements, etc., were investigated. Also a brief study of building induced
turbulence was carried out and a qualitative description of the generation of
the turbulence was hypothesized.

In an effort to isolate fence effects from building effects, a study of
simple one element fences in the absence of a building model showed that
downstream flow velocity was reduced over a height equal to that of the
fence. In contrast to this, a fence installed on a building model affects
the downstream flow up to approximately two fence heights. Finally, a simple
equation relating the velocity at any height behind a one element fence to
the fence solidity (ratio of projected wire area to projected total area)
was derived and validated for low solidity fences (up to 0.30 area blockage).

INTRODUCTION

During recent years, the rapid increase in the number of air passenger
miles traveled has imposed considerable hardships on contemporary air transpor-
tation systems. Both airports and airspace are congested to the extent that
some immediate relief is needed. This is particularly true of short-haul
operations where times required for ground transportation to and from the
terminal areas often compare in magnitude to flight time. With the development
of STOL aircraft has come the possibility of supplying some needed relief.

Many variations of possible STOL transportation systems have been proposed, and
many points of view have been expressed concerning the advantages and disadvan-
tages of various components of these systems. One of the few issues upon which
there is agreement is that the STOL passenger would definitely be subjected to
conditions quite different from those to which he has been accustomed in
traveling between major cities. The uncertainty concerning the acceptability

of these factors to the traveling public has undoubtedly been an important
factor contributing to the hesitation which has been evident in making a
commitment to implement a STOL system.

In order to help alleviate some of these uncertainties, a program was
initiated at the University of Virginia, through its Center for the Application
of Science and Engineering to Public Affairs, to develop modeling techniques to
predict human acceptance of proposed STOL systems and to develop acceptability
criteria for the many variables and their trade-off possibilities. This
requires a thorough understanding of the nature of the variables and a quanti-
tative formulation of response to them.

The work reported herein represents one facet of this overall program
which arose *rom the following considerations. STOLcraft can be accommodated
in relatively small terminal areas; it is, therefore, possible that STOLports
can be placed nearer the population centers of cities. Such an arrangement
would have two major advantages. First, it would improve short-haul air trans-
portation directly by decreasing ground travel time and consequent total trip
time. Second, it would aid long-naul air transportation by relieving congestion
at conventional airports currently caused by short-haul operations.

One means of placing STOLports near city centers is to utilize the roofs
of very large downtown buildings as landing and takeoff areas. This would
minimize disruption of normal downtown economic activities since the building
itself could be used for varied revenue producing activities, as well as for
the STOL terminal. An artist’s conception of such a STOLport is shown in
figure 1. There are, however, many types of problems associated with the
development of such an unconventional airport, and the present work is directed
toward those related to the quality of the airflow over such a structure. There
are two important issues involved. One is the nature of the local flow near the
landing surface induced by the design of the structure. This will have a direct
influence on the motion experienced by the aircraft (and its passengers) in the
landing and takeoff maneuvers - as well as in the safety of these activities.

The other arises from the fact that the need to restrict the STOLport to a single
runway will generally give rise to crosswind problems which could effect the
quality and safety of the landings and takeoffs and the reliability of the
system operation. Thus, the possibility of alleviating crosswind problems and
generally improving flow quality through the use of wind screens and proper
building design was deemed worthy of study.

2

SYMBOLS

arc length of rounded edge extensions, cm (In.)
building model width, cm (In.)

screen drag coefficient based on average Interstitial velocity and
blocked area (see eq. A-4)

building model height, cm (In.)

fence height, cm (in.)

height of measurement above model, cm (in.)

local drag coefficient of a variable solidity screen

total pressure drop across screen or total pressure drop from far
upstream to far downstream, N/m 2 (lb/ft 2 )

2 2

static pressure far upstream of a screen, N/m (lb/ft )

static pressure far downstream of a screen, N/m 2 (lb/ft 2 )

2 2

dynamic pressure, N/m (lb/ft )
pv cw b

Reynolds number, — ^ —

distance between upstream and downstream fences, cm (In.)
local velocity in flow field, m/sec (ft/sec)
free-stream velocity, m/sec (ft/sec)

aircraft air speed, kts

V cw crosswind velocity, m/sec/kts (ft/sec)

V local crosswind velocity* m/sec (ft/sec)

l

V ru maximum crosswind velocity, m/sec (ft/sec)

c max

Vg aircraft ground speed, kts

x horizontal distance measured from fence, cm (in.)

y vertical distance measured from bottom of screen, cm (in.)

*o

P

M

vertical distance measured on the screen, cm (in.)
density of air, gm/cm (slugs/ft )
viscosity of air. Nsec/m* (slug/ft-sec)
local turbulence.

rms-veloci ty fluctuation
mean velocity

£ local solidity of screen * projected blocked area/projected total area

QUALITATIVE INVESTIGATION

The initial study into the feasibility of utilizing large building roof
tops as elevated STOLports was begun in November 1969, in a joint effort by
the University of Virginia, Department of Aerospace Engineering and
Engineering Physics, and the Langley Research Center, Low Speed Vehicles
Branch. Initial studies were of a qualitative nature and were performed in
the 17-foot test section of the 300-mph 7-by 10-foot wind tunnel at the
Langley Research Center. The purpose of this program was the qualitative
observation of the overall flow field associated with a rooftop STOLport and
the influence of the building conf igurat ion changes on that flow.

3

Test Set Up

The primary experimental technique utilized in this initial investigation
was flow visualization achieved by the use of smoke and tufts. Figure 2 is a
photograph of one of the configurations tested, and figure 3 shows cross
sections of six of the configurations tested. The first configuration in
figure 3 is the solid rectangular parallelepiped building itself. Next is a
series of four configurations, each having an elevated rooftop (or raised deck).
The first in this series is the basic raised deck. The second has side ramps,
which were tried because they offered a method of lateral containment for land-
ing STOlcraft. A circular arc side overhang was also tried. (See third
configuration in the series.) Finally, porous side fences were added to the
raised deck with curved overhang in an attempt to modify the crosswind velocity
profiles; each side fence was constructed of three rather closely-spaced pieces
of stainless steel screening of different heights. Figure 2 is photograph
of this fourth elevated-rooftop configuration with its curved overhang and
porous side fences. The last configuration in figure 3 differs from the
preceding one in that the space under the raised deck is closed in order to
determine whether the raised deck is essential in achieving the desired flow
patterns.

The building models are 1.2 meters by Ij. meters (4 ft by 8 ft) by either
4) cm or SI cm (16 in or 20 in) high, depending on the configuration. If the
full-scale building is 1 52 meters (500 ft) wide, then the IS cm (6 in) height
of the model fences corresponds to an actual fence height of 19 meters (62. 5 ft)
and the full-scale building would be SI or 63 meters high (167 or 208 ft high).
The building model was mounted so that it could be placed at any yaw angle to
the tunnel flow; in figure 2, the tunnel flow is parallel to the model runway.

Qualitative Test Results

A motion-picture film supplement showing smoke flow over the various
building models has been prepared. * Six frames from this film are presented
as figures 4 through 9. Figures 4 through 7 show the flow over the four con-
figurations in the elevated-rooftop series. Figure 8 shows the flow over the
solid rectangular para) lelepiped building alone, and figure 9 is of the solid
building with rounded edges but not fences. In each case, the flow is from
left to right and the tunnel flow is at 90* to the runway (pure crosswind).

Figure 4 shows the flow over the model with the basic deck. (See the
first schematic in the elevated-rooftop series in figure 3.) The f’ow is not
good. It is separated at the leading edge, and reattachment does not occur.
Over the entire rooftop, the flow is highly turbulent and unsteady.

“This film supplement (16 mu, 23 min, color, narrated) may be obtained or
loan by requesting film serial number L-1114 entitled "Roof-Top STOL/Port
Flow Visualization" from NASA Langley Research Center, Attn: Photographic

Branch, Mail Stop 171, Hampton, Virginia 23665.

4

Flow over the second configuration in the elevated-rooftop series is
shown in figure 5. The addition of side ramps to the basic raised deck is
seen to be detrimental. The separation angle is larger. In the center region,
the flow is even more turbulent and unsteady than that over the basic rai-ed
deck. The unsteady flow region extends very high above the model. This con-
figuration has the worst flow of any of those tested.

Figure 6 shows the flow over the third configuration in the elevated-roof-
top series. The effect of adding rounded edges to the raised deck is dramatic.
Some turbulence can be seen, but the flow pattern Is considerably better than
for the first two elevated-rooftop configurations.

Once success in maintaining attached flow had been achieved by the addition
of rounded edges, fences were added to determine whether the flow would remain
smooth and whether the crosswinJ profile above the deck would change. The flow
over the resulting configuration (fourth in the elevated-rooftop series in fig.
3) is shown in figure 7. One must not confuse the spreading oi the smoke due
to turbulence and unsteadiness with the spreading due to low velocities. The
lazy drifting action of the smoke in this figure indicates low velocities.

Figure 8 shows that the flow over the solid parallelepipeo building alone
is separated and turbulent. In fact, it seems to be slightly worse than the
flow over the basic-raised-deck configuration (fig. 4).

Again, the effect of adding rounded edges is dramatic. Figure 9 shows
that over the solid building with rounded edges the flow is attached and fairly
smooth and steady everywhere; it seems to be as smooth as the flow over the
raised-deck configuration with rounded edges (fig. 6). At this stage of the
study, it was concluded that the raised deck is not necessary to obtain good
attached flow but that the rounded edges are the important factor. However,
later investigations showed that the elevated deck may be desirable. This
will be discussed in a later section. Although not illustrated, it should be
noted that the addition of fences to the solid building with rounded edges
produced favorable results. The flow remained smooth and the crosswind veloci-
ties near the building rooftop were greatly reduced. Most of the subsequent
work has been without raised decks on the models.

This first qual : t3tive phase of the research effort gave encouragement to
the expectation that smooth, attached flow over an elevated STOLport could be
achieved and that the crosswind profile could be modified in some desirable
fashion.

QUANTITATIVE INVESTIGATION

The next phase of the study was a souk / hat more detailed experimental in-
vestigation carried out at the University of Virginia. Results concerning the
influence of fence structure and geometry on changes in crosswind velocity are
presented.

ORIGlHii; »AOSB
Of POOR QtJALfTfl

5

Figure 10 shows schematically the type of effect one expects fences on
either side of a STOLport runway to have on the crosswind velocity profile.

The left-hand sketch shows the tunnel flow with the tunnel boundary layer. In
the right-hand sketch, the dashed line indicates the profile on the STOLport
model at the centerline with no fences. With fences, the profile is modified
as shown by the solid curve. The curves in this figure are just Illustrative.

The effects of a crosswind velocity on an aircraft attitude and airspeed
can be seen in figure 11, which is a schematic diagram of a STOLcraft, designed
to land at an airspeed of 60 knots, landing at a STOLport between fences in a
30-knot crosswind. The right-hand side of the figure indicates the situation
initially in the full 30-knot crosswind. The airspeed is 67 knots; the ground
speed is 60 knots; and the crab angle is 27 . The left-hand side indicates the
situation at a pi ace where the fences have reduced the crosswind speed to IS
knots, with the assumption of perfect decrabbing and a constant ground speed of
60 knots. The airspeed is now 62 knots, and the crab angle is 14 . The effect
of crosswind angles of other than 90 could be examined in a similar fashion.
Some concern had been expressed about the potential loss of airspeed as the
crosswind component is removed. It can be seen from figure II that even when
the relatively large crosswind of 30 knots (50 percent of ground speed) is
decreased by 50 percent, the airspeed is only decreased by about 7 5 percent.
Consequently, it seems that loss of airspeed is probably not a sufficient reason
for not seeking to control crosswind profiles.

It seems obvious that if a pilot had to land such a STOLcraft an an
elevated STOLport in a crosswind, he would be concerned about such factors as
the following: Mt what height does the crosswind component begin to change?

How rapidly does it change? At what height above the runway does the crosswind
become negligible?

It should be noted that the results to be presented herein are not given
as a "solution" to the crosswind problem. With a given, unfenced STOLport
building and a given flow over it, there will be a certain crosswind velocity
profile provided by nature. If this profile is acceptable to all concerned,
there is no problem. If it is not acceptable, then some mechanism of making the
crosswind velocity profile acceptable becomes of interest. The question to
which the present work is addressed is simply this: What are the possibilities

of crosswind profile modification by putting fences on either side of the STOL-
port building?

Crosswind Investigation

Figure 12 shows the test setup for a building model that Is 76 cm (30 in)
wide and 15 cm (6 in) high. The rounded edges of the model are circular and of
7 cm (2.75 in). If the 76 -cm (30-in) model width corresponds to a 152-meter
(500-ft) full-scale building width, then the 15-cm (6-in) model height corres-
ponds to a 30.5-meter (100-ft) building height. Also, the 7-cm (2.75-*n) model
overhang radius corresponds to a 14-meter (46-ft) building overhang radius, and
the 7.6-cm (3-in) rr,odel fence height corresponds to a 15-meter (50-ft) full-
scale fence height. The fences were constructed of one, two or three wire
screens.

6

Figures 13 and 14 show the measured profiles at the model centerline for
different fence configurations. The dashed line indicates the top of the fences.
The model is 76 cm (30 in) wide, and all the fence configurations are 7.6 cm
(3 in) high. It is obvious that increasing the number of screens causes the
centerline velocities near the deck to decrease and causes the height to which
the fence effect extends to become larger. It is apparent that fence structure
has a large influence on centerline crosswind velocity profiles.

Figure 15 shows results of efforts to generate varied velocity profiles by
proper fence design. The straight line profile is the result of a separate
experiment conducted at Langley (ref. 1) where a pair of three screen fences
were modified in a trial and error manner until a linear profile was produced.

The other profile was generated at the University of Virginia in an effort to
show that unusual profiles could be generated. These results indicate that
centerline profiles are adjustable over a considerable range and suggest that
one might learn how to predict profiles for given fence structures and
geometries.

It is also interesting to examine crosswind velocity profiles at positions
other than the centerline. The investigations at the University of Virginia
included experiments to determine the effects of measurement position. Figures 16
through 21 compare profiles taken 11.4 cm (4.5 in) behind the leading-edge fence
to profiles taken above the centerline. In general, there is a shift to the
left and upwards with increasing distance. This indicates a slowing and diffu-
sion of the flow with distance behind the fence. Figures 22 through 26 show
variation of velocity profile with distance for different ratios of S/h^. The

trends shown by these graphs are similar to those seen in figures 16 through
21. Figure 22 corresponds to the maximum S/h^ tested. It can be seen that

there is less difference between the 1/2 S and the 3/4 $ profiles than between
the 1/4 S and the 1/2 S profiles. This seems to indicate that the flow is
approaching an equilibrium condition.

In an effort to determine the effect of screen mesh and wire size, similar
fence configurations were constructed from two different mesh and wire size
screens. The solidity or ratio of projected »Jire area to projected total are?
was constant at about 0.30 for each of the screens. Results for similar fence
configurations are shown on figures 27 through 32, The data taken above the
centerline show negligible variation with type of screen for the one screen and
two screen profiles. However, tMt taken at 11.4 cm (4.5 In) behind the screen
shows appreciable differences. This is in agreement with other similar tests
performed at the University of Virginia (ref. 2) which indicated that the equi-
librium profile behind a screen depends only on the solidity of the screen, but
that the distance to equilibrium depends on the wire and mesh size. The graphs
also hint that equilibrium distance increases as the effective solidity of the
fence increases by the addition of more screens. In other words, comparison
of the three fence configurations tested at the centerline indicates small
but increasing differences in velocity profiles with increasing number of
screens.

7

An additional experiment showed that the centerline profile was essential-
ly the same with and without the downstream fence. Thus, one tentatively
concludes that the profiles in the runway area are fixed by the upstream fence
and are nearly independent of the downstream fence. Some additional evidence
on this point is provided in figure 33. A single two-screen fence was placed
approximately at the centerline position, and profiles were measured 5 cm (2 in)
upstream and 5 cm (2 in) downstream of the fence (that is, 0.07b upstream and
downstream of the fence). The other curve (through the square symbols) is the
no-fence profile at the fence position. The data clearly show that a fence
alters the downstream flow velocity very much more than it does the upstream
velocity. These results provide some insight into the mechanism by which
fences alter the adjacent upstream and downstream flow fields.

Velocity Fluctuation Investigation

Throughout the quantitative test program, Hrge-scale velocity or dynamic
pressure fluctuations were observed above >. te STOLport deck. These fluctua-
tions appeared and were similar for all configurations tested. They usually
died out on the order of 0.5 building heights above the deck. The predominate
frequency associated with them was approximately one hertz, but slower and
faster frequencies were superimposed on this. Unfortunately, no spectral
analysis was carried out due to the lack of proper instrumentation. For most
configurations, the gross appearance of the fluctuation was invariant over the
small Reynolds number range investigated (Re = 100,000 to 750,000). Figure 3^
shows a typical variation of the fluctuations with height above the building.
These measurements were obtained using a pitot-static tube placed at different
heights above the centerline of the model. Approximately ±50 percent fluctua-
tion in dynamic pressure (±25 percent velocity fluctuation) can be seen in the
extreme case near the deck. It is realized that because of their relatively
slow response times pitot-static tubes are not the ideal instruments to use in
measuring flow unsteadiness. In this application, however, the low frequency
oscillations are of primary interest since they would have the greatest effect
on landing aircraft. The response time of the pitot-static tube is sufficient-
ly fast to observe these low frequency oscillations.

In an attempt, to determine scale effects, a single series of tests were
performed on a larger model (one that had been used in the earlier flow visual-
ization tests) in the Langley full-scale tunnel. The same instrumentation that
had been used at the University of Virginia was used in this investigation. A

Reynolds number one full order of magnitude higher (5 x 10^) was achieved.

Mean velocity profiles and velocity fluctuations were similar to those previous-
ly obtained at Re - 5 x 10^. Extrapolation of these results to a full-scale

o

building with a Reynolds number of 5 x 10 is not possible. Hence the full-
scale existence of this phenomena has not been verified. However, It can not
be discounted on the basis of tests completed to date.

Qualitative description of velocity f l uctuat ion . -One possible explanation
of the flow fluctuations is the existence of an unstable vortex located in
front of the building model. This is a f 1 ov pattern similar to that shown In

8

figure 35. The flow field is divided into two regions separated by a stagnation
streamline which is attached to the leading-edge overhang. Adjacent to the deck
and elsewhere in the upper region the speed is high. The speed below the stag-
nation streamline, however, is low. Associated with this difference in speed
is a pressure gradient between regions 1 and 2 in figure 35. Under equilibrium
conditions, this gradient is just balanced by the curvature of the stagnation
streamline. If the pressure gradient is raised above this equilibrium con-
dition, the stagnation streamline would detach as shown in figure 36. This
would allow a "bubble" of turbulence to escape up over the rounded edge. When
the bubble is released, the excess pressure gradient is relieved and the stag-
nation streamline reattaches. This allows the pressure gradient to rise again
and the cycle is repeated. This heuristic explanation has been substantiated
by the use of flow visual izat ion. Smoke injected under the rounded edge in
front of the building escapes in distinct bubbles. It was further hypothesized
that if the above explanation is true, the velocity fluctuation could be
eliminated simply by continuously relieving the pressure gradient. With this
in mind, a building configuration such as that shown in figure 37 was installed
in the tunnel. It was found that if the second deck is raised high enough, and
if proper rounded edges are used, the velocity fluctuations can be virtually
eliminated. Another interesting means of alleviating the fluctuation (although
it may not be practical in actuality) is shown in figure 38, The wedge in
front of the building can be adjusted so that the stagnation streamline has a
stagnation point at A and another at S. With this situation, the stagnation
streamline is stabilized and the vo*tex is stable. Consequently, the flow
over the deck is smooth.

Building induced turbulence measurements . -Since the magnitude and frequency
of the velocity fluctuations are such that it would be potentially hazardous
(depending o.. scaling, of course), it was decided that the phenomena should be
more carefully investigated. Consequently, an experimental program was initia-
ted to determine important parameters associated with the velocity fluctuation.
The test set up was similar to that used earlier at the University of Virginia,
the only difference being the use of hot wire anemometry for velocity
measurements. Both mean velocity and RMS turbulence were measured with height
above the deck for various building configurations. This program was not as
complete as anticipated largely because of difficulties in using hot wires in
the University of Virginia low-speed tunnel. The primary difficulty being the
inability to adequately temperature compensate for the large scale temperature
variations which occurred in this closed tunnel. Turbulence measurements,
however, are insensitive to this problem, and consequently these data are
presented.

Figures 39 and 41 show the height versus turbulence variation at stations
above the deck for various rounded edges. A basic building configuration (no
second elevated deck) was used for these tests. Each of the figures shows
greater differences between the leading edge and centerline data than between
the centerline and trailing edge data. This seems to indicate that some equili-
brium is being approached. Figures 42 and 44 show differences due to
different rounded edges. With increasing distance from the leading ed^e, data
due to the 0.75b. Dia and 0.916h. Dia rounded edges become more similar.

D D

9

Next an elevated deck was installed above the basic building deck. The
spacing between the basic building and the elevated deck was 7.6 cm (3 in) or 1/2
building height. Tests were made with various rounded edge extensions on both
the basic building and the deck. Figures 45 through 50 show the variation of
turbulence with height for a given model configuration at different tunnel
speeds, in each case there is a general decrease in turbulence with increasing
wind velocity. The shape of the profile, however, seems to remain the same for
a given configuration. There is no indication that * constant turbulence pro-
file, independent of Reynolds number, is being appro^-hed. It should be
remembered , however, that the maximum tested Re is two orders of magnitude
smaller than that for an actual building so no real conclusions about scaling
can be drawn. Figures 51 through 53 show turbulence profiles obtained by plac-
ing different rounded edges on the building edge while keeping a given rounded
edge on the upper deck. The profiles on each graph are similar. This indicates
a relative insensitivity to building rounded edges for a given deck edge.

Figures 54 through 56 show results for different deck edges and a constant build-
ing edge. In each of these figures, the profiles fall into two groups. In each
case, the similar profiles have similar arc length to radius ratios (a/r).

These graphs indicate, therefore, that the rounded edge on the upper deck has a
greater effect on the turbulence than the rounded edge on the lower section of
the building. They also indicate the quantity a/r is important and should be
investigated. Unfortunately, this could not be done in this program. The
spacing of the elevated deck above the building is another important parameter
that was not, but should be, investigated. It would also be desirable to per-
form frequency and spectral -energy analyses on the turbulence above the deck.

This investigation has slvwn that the building configuration has a marked
effect on the turbulence above the building.

Basic Fer-v Research

During the eK ited deck turbulence program, it was noticed that mean
velocity profiles took different shapes than corresponding profiles obtained
with similar fence configurations on the basic building model. This indicated
that the building configuration itself, or possibly the building-fence inter-
action, has an effect on the velocity profile. In an attempt to isolate this
effect and gain knowledge into the basic mechanism by which fences alone affect
velocity profiles, a basic fence investigation was initiated.

Experimental program . -The experimental part of this investigation
utilized the test set-up shown in figure 57- In this arrangement, the fence
was placed on a ground board above the tunnel boundary layer. Reasonably
uniform flow was therefore incident on the fence as long as the fence was fairly
close to the leading edge of the ground board. Figure 58 shows velocity profiles
taken at various distances behind a simple one element fence. Near the screen
the velocity profile is very irregular. This is because effects of individual
wires are being seen. As the distance behind the fence increases, these
differences are smoothed and a uniform profile is established. Figure 59 shows
results for a more complicated two element fence configuration. Data were taken for
this configuration up to 91 cm (36 in) (12 fence heights) downstream. As was
the case with the simple one element fence, an equilibrium profile is established
downstream and maintained until the boundary layer growing from the leading edge

10

of the ground board becomes thick enough to affect the profile. At 91 cm
(36 In) downstream, a large boundary-layer effect can be seen in these data up
to about 1/2 fence height. Figure 60 shows velocity profiles in front of the
fence for this 2 element configuration. It can be seen that there is very
little upstream effects of the fence at 23 cm (9 in) (3 fence heights) in front
of the fence. This is similar to results obtained in front of the fence on
the building model.

The striking difference between these results with fences on a ground
board and those obtained with similar fences on building models is that the
height to which the fence affects the flow is much less in the absence of the
building. This could have important practical ramifications since it is
structurally and economically desirable to build fences as low as possible.
Corresponding profiles obtained with fences on the building model and on the
rasied ground board also vary somewhat in appearance because of the different
characteristics of the boundary layer associated with each case.

Analytical program . - Analytical investigations of incompressible flow
through wire gauze have been performed by Taylor and Batchelor in 1949 (ref. 3)
and by Owen and Zienkiewicz in 1957 (ref. 4). The Taylor-Batchelor work deals
with the effect of woven wire gauze on small disturbances to a uniform stream.
In this work, the drag coefficient of a uniform screen is defined by

k

O

p i -

l/2cU 1
00

( 1 )

Owen and 2ienkiewicz's analytical work on screens was performed as part of
an attempt to produce uniform shear flow in a wind tunnel. They were able to
predict the solidity distribution necessary to generate constant shear.

The objective of the present work is to predict the fence solidity distri-
bution necessary to generate a predetermined nonlinear velocity profile. In
addition to differing in profile shape, this work differs from Owen and
Zienkiewicz's in the boundary conditions on the fence. There is essentially
no upper boundary on the fence in the current work. Owen and Zienkiewicz's
grid spanned the entire test section of the wind tunnel.

Qualitatively, the effect of a fence on a flow field can be described as
follows. As a streamline passes through the fence, fluid momentum is lost.

This corresponds to a decrease in stagnation pressure at the fence. Conserva-
tion of mass requires that fluid velocity through the fence be continuous,
therefore, the change in stagnation pressure must appear as a static pressure
drop across the fence. Farther downstream an equilibrium condition is reached
where static pressure does not vary vertically. At the equilibrium condition
an exchange between static pressure and velocity has occurred so that the
momentum (or stagnation pressure) decrease appears as a decrease in velocity.

1 1

Bernoulli's equation written for a stream) Ine passing through a fence is

P- i + 1 °«_ 2 - p„ - i p° 2 (t) - Pj - p 2

( 2 )

where p and p are the static pressures far upstream and far downstream,
r r ®2

respectively, and u(y) is the local equilibrium velocity downstream. In the
absence of an upper boundary on the fence, p and D are equal. This allows

r ®2 r <©2

equation (2) to be rearranged as

p i : i

1/2PU.

2

7

(3)

In a manner analogous to equation (I), a local drag coefficient can be defined
by the left hand side of equation (3) so that

*<»„>

co

It has been demonstrated that for practical solidities, streamlines passing
through the fence are "essentially" straight so that y Q a y.

For any arbitrary, single-element screen or parallel-rod grid in an in-
compressible flow, it can be shown that the drag coefficient, K(y Q ) is related

to the solidity, by

«*„> C D

U - C(y 0 >) 2

(5)

where Cp is a drag coefficient based on the blocked area of the grid and the

average interstitial velocity (see Appendix I). Experimental results have
shown that in the absence of an upper boundary Cp * 0.5 for uniform grids of

30 percent and 15 percent solidity. When the solidity is increased to 60 per-
cent, however, the Cp decreases to approximately 0.14. This indicates that Cp

is approximately constant at 0.5 for solidities less than approximately 30 per-
cent. For higher solidities, however, it becomes a function of fence solidity.
The following results are limited to maximum solidities of 0.3. Using Cp • 0.5.

and combining equations (4) and (5) leads to

£(y_) „ 2

Ti"- »V') ] ; ’ 2 0 - (§-) 1 «>

O 00

12

Figure 61 shows a nonlinear velocity profile measured behind a parallel-
rod grid installed in the University of Virginia Low Turbulence Wind Tunnel.

The ratio of grid height to tunnel test section height was approximately 0.14.
The experimental values of u/U^ from figure 61 were used in equation
(7) to compute the solidity distribution. The computed solidity is compared
to the actual measured solidity in figure 62. Figures 63 and 64 show similar
results for another fence of approximately the same height, but constructed
from a variable solidity screen. The average error is approximately 5 percent
for the parallel-rod arrangement and approximately 15 percent for the screen.
Host of this error can be attributed to wind tunnel blockage effects that
prevent and from being exactly equal. Results are also sensitive to
1 2

accurate measurement of U . It should also be pointed out that these results

OO

cannot be extended to higher Reynolds number cases until Reynolds number effects
on Cp can be determined.

CONCLUSIONS

The problem of rooftop STOLport aerodynamics is a large and complicated
one. Consequently, the initial investigation has generated more questions than
it has answers. There are, however, some relatively firm conclusions which
may be stated as follows.

1) At the scale of these experiments, it Is possible to modify STOLport
building models so that in a steady (wind tunnel) crosswind there is
smooth attached flow over the models. This is achieved by attaching
rounded-edge extensions to the upper lateral edges of the model.

2) At the scale of these experiments, it Is possible to modify the
steady crosswind profile above the building model, by attaching
vertical porous fences to the upper edges of the model. By varying
the fence geometry, a wide range of profiles can be obtained.

3) By a not we 1 1 -understood mechanism, some model configurations produce
large levels of turbulence at low frequencies (I Hz at this scale)
above the STOLport deck.

4) A simple theoretical analysis of the effect of a single
element fence on a uniform velocity profile has been
validated for low solidity fences (up to 0.30 area
blockage).

5) There are several important questions that have been raised, but left
unanswered because of the limited scope of this research. The most
important of these pertain to both fences and buildings and are:

a) The determination of scale effects

13

b) The determination of effects derived from the nature)
unsteadiness in actual atmospheric flow.

c) The determination of effects derived from the unique
character of the earth's boundary layer.

The above questions can only be answered by large scale atmospheric
testing.

14

APPENDIX A

Relationship Between Solidity and Drag Coefficient

-r he solidity of a uniform screen or parallel-rod grid is defined as the
ratio of blocked area to total area. If a wire screen has grid dimensions of
i by h where t is the grid length and h is the grid height and the diameter of
the wire is d, the following can be written for each grid:

Blocked Area = hd + id - d*

Total Area = hi

2

Open Area = h£ - (hd + id - d )

Consequently, the solidity, £, is given by

, dCh + i - d)

* ht

or if the grid is square so that h -l

, . d(2t - d)

For the case of a parallel-rod grid, the solidity can be written as

* “ S A3

A1

A2

where d is the rod diameter and S is the spacing between successive rod
centerlines. These ^'nations are valid local solidity expressions for a
non-uniform screen (or parallel-rod grid with variable spacing) if h and £

(or S) are givf"’ neir local values.

The relationship between the drag coefficient and solidity for a screen
can be developed by considering the aerodynamic force on one grid as follows:

Force » K(-j pU J) hi * C D (y pU aye 2 )(hd + td - d 2 ) A4

where K is the drag coefficient based on total grid area and the freestream
velocity, J , and C_ is the drag coefficient based on the blocked area of the

oo y

grid an d the average interstitial velocity, U aye - The average interstitial
velocity can be found in terms of by considering conservation of mass in a

15

rectangular (h x t) stream tube intersecting the screen at the grid where K
is to be determined. For incompressible flows, this gives

U hi ■ U [hi - (hd + Id - d 2 )] AS

« eve

which can be combined with equation A4 to give

C D [(hd + Id - d 2 /(hl)]

K “ [1 - (hd + Id - d*)/(hl)) 2 A6

Similarly, it can be shown for a paral let-rod grid that,

C D d/S

* “ [1 - d/S] 2

A7

When written in terms of solidity, either of these equations reduces to the
general result

s

“ a"To 2

As mentioned in the text, ■ 0.5 for solidities less then 30 percent (for the
Reynolds numbers tested). “Equation A8 can therefore be used to determine
the resistance, K, of a screen or parallel-rod grid as a function of solidity.

16

REFERENCES

1. Parker, Hermon M. ; Blanton, Jeffrey N. ; and Grunwald, Kalman, J.: Some

Aspects of the Aerodynamics of STOl-ports. Proceedings of a Conference
on NASA Aircraft Safety and Operating Problems, Vol. 1, NASA Sp-270,
1971, pp. 263-276.

2. Hwang, Men Shiuh: Experimental Investigation of Turbulent Shear Flow.

Ph.D. Dissertation, University of Virginia, 1971.

3. Taylor, G.I.; and Batchelor, G.K. : The Effect of Wire Gauze on Small

Disturbances in a Uniform Stream. Quart. Journ. Mech. and Applied
Hath., Vol. II, Pt. 1, I9*»9. pp. 1-29.

k. Owen, P.R.; and Zienkiewicz, H.K. : The Production of Uniform Shear Flow

in a Wind Tunnel. J. Fluid Hech., Vol. 2, 1957, PP- 521-531.

17

«M<m.kh PAGE JH
m W» QtUUTtf

Figure T

.DING MODE

Figure 2

m

1

V

KS)

CL

£

<:

P"

£T

1 >

LJ

o

<

5

LT) I

1

=>

tt ^

CL ~

£ W
oo
c

>1

a:

<

>

<:

DC

J

o o
2! —

O c uO

2 x

— ce ^

O u

H? > O u_»

3 ° a O

® ^ oz
o Q> ^

E > x

__j CC *—

° => >
i/-> o >

o O

> o
o 4/1
o a.

U-l

> X
a: ►—

o ^

1

o

r. — ,

o

<

X

QC

uo

> —
O O

£2

> -Q

cc Cg

X °

<-> o’

>-

<

or

r._

o

’o

o

cr

o

£

c

>

•KJONAL JAOUlfl

(if 1 f*oCR

Figure 3

%#

L flgr

ORIGINAL PAOS W
Of HWR UUALIT^

y

SMOKE FLOW
SOLID BUILDING

^we 8

ORIGINAL PAGE IS

Of ICO* QlL. Ura

CROSSWIND VELOCITY PROFILE
AS A RESULT OF FENCE INSTALLATION

Figure 10

CROSSWIND LANDING FACTORS
30-Knot CROSSWIND

V *60 Knots V ■ 60 knots

Figure 11

Vcw moi

Fig. 16- Effect of distance from upstream fence on cross wind profile
3-Element fence. Each element construe ted from
009 m wire , 0 4 96 * grid screen

Fig. 20^ Effect of distonce from upstreom fence on cross wind profile
2-Element fence ; constructed from

0 02" wire , 0 12 5 H grid screen

Fig. 22- E f fee t of distance from upstream fence on cross wind profile

(Distance between upstream and downstream fences ,S) /(max fence height ,hf) = 867

V CW max

.Fig. 27- Effect of screen geometry 3-efemenf fences

•!/n

Top of fence

* - w

; O 0.009" wire ,0 0496” gr id screen
TT $ 0 02” wire, 00125” grid screen

i .2 4 .6 .8 1.0

V C Wj

v cw m o*

Fig. 31 - Effect of screen geometry 2-element fences

EFFECT OF FENCE LOCATION

Figure 33

q,

o 5 10 15 20 25 30

Time (sec)

Fig. 34-Effect of height above building on turbulence

Region of low pressure

Fig. 36- Flow over building model when stagnation streamline has detached.
PRECEDING PAGE BUM NOT FQJ4HD

Oi l i l_ i r, i, r i iti.i.i.i .

.01 0.1 10

Fig. 39- E f feet o f distonce from upstream building edge on turbulence profile
building rounded edges 0.916 h^ diom. and 0.5ht,ore length R e =6.3xl0 5

Fig. 41- Effect of distance from uf
building rounded edges 05

■01 01 1.0 10

°vu

Fig. 43- Effect of building rounded edge on turbulence profile ot centerline
R e -6 3x I0 5

S 2323 SCI

Bttssssai

Measurements mode
of I.Ob

is98l

ESaSM!

leat—a wi

memaaM

IFWOUBHI

O 0-9l6h b diom ,05bb ore length

A 0.75h|j diom ,0.543hbare length
□ 0-5hh diom. ,0.292hb ere length

Fig. 44- Effect of building rounded edge on turbulence profileat troilmgedge
R e = 6 3 x I0 5

Ele voted deckt

imanum

Measurements made

— — Mil

B«aB8BB — »

&HGSSBHHHB!

Reynolds Number

-0 0.8x105 r --

' □ I.84xI0 5 “

"O 2.7x105 TJj
'A 4.0x105
- 0 5.2x 105 —

: o 6.3 x 105

■auxi
naani

Fig. 45- Effect of Reynolds number on turbulence P r ofiie above building
center Ime. £ievoted deck configuration. Building rounded edge ■,
0.916 bedlam and 0.5hbore length. Elevated deck rounded edge ,
0.75 hb diam ond 0.543 hjj are length

mm

snnses

laaci

MM

Measurements made
above centerlme

Elevated deck

mnaiBi

EBBSBI

I12SSS1

irfSis

kZt

1BSI

Reynolds Number

paniiani

svsiKiiiSi

Fig. 46- Effect of Reynolds number on turbulence profile obove building centerline
elevated deck configuration Building rounded edge diom and 0.543 h^

ore length. Elevated deck rounded edge,Q. 7 5h^ diom and 0 543h^ore length

I8BS83SSBXMI

Measurements made
above centerline

Elevated deck

iWHiiKiiiiBHNWkiHinHl

IBBtSS

I* — \

Reynolds Number

_o

08xl0 5

□

I.84xl0 5

]0

2.75 xIO 5-

~ A

4.0x105

1 0

52xl0 5

1 o

6.3xl0 5

Ill

Tin

Fig. 47- Effect of Reynolds number on turbulence profile obove building centerline
elevoted deck configuration Building rounded edge , 0.916 hb dtom ond
0.5 h^ are length Elevated deck rounded - t 0.583 h^diam ond 0 438hbore

8- Effect of Reynolds number on turbulence profile obove build ng centerline
e leva ted deck configuration. Build ing rounded edge; 0.583 h^d'orn and
0.438hbure length.elevated deck rounded edge ; 0 75 d mm and
0 543 hb ore I ength

Fig. 49- Effect of Reynolds number on turbulence profile obove building centerline
elevoted deck configurotion.Building rounded edge;0.5h b diam and
0 292 h b are length. Elevoted deck rounded edge; 0 916 h b diam
and 05h b are length

Meosurements made
above centerline

■■■■B MWWM iWW ijW WWiMiiiilil

iBBnsai

ini

III

SESauaESIsaSSi

Reynolds Number; j
O 0.8x105 pi

n 1 .84xl0 5 ft

!0 2.75 x I0 5

A 4.0 x10 s 14-

. 0 5.2 xio 5 Lj-j

C 6.3 x 1 0 5 : :

i/v

Fig. 50- Effect of Reynolds number onturbulence profile obove building centerline
elevated deck conf igura lion. Bui Iding rounded edge ; 0.583 h^ diom and
0.483 h^ ore length E le voted deck rounded edge; 0.916 h^ diom ond
0 5 hb are length

Fig. 51- Effect of geometry of building rounded edge on turbulence profile obove
the centerline Elevated deck rounded edge; 05bb diom ,0292 ore length

Re -'6 3xl0 5

IB8HI

1SSSSBHBI

Measurements at <£

Elevoted deck

II!

Ill

iHMHSi

....

I'SPSSR!

wsssai

61

Building rounded edge
(> 0 9l6htj diom ond 0.5htjore length

O 0.5 h^ diam ond 0 292 h^ ore length
□ 0583hb diom ond 0 483hb ore length
a 0.75h b diem and 0543hb ore length

I.TjnUHII

lllllBI

PH

■KBfill

Fig. 52- Effect of geometry of building rounded edge on turbulence profile above
the centerline Elevoted deck rounded edge ; 0.75 hfc diom ond 0 543 are length

Re = 6. 3x105

gsaaas:
IflBBMSI

Measurements at <£

Elevofed deck

M

iBsasi

mu

Mfl

IBBl

piSgwfliil

Mp

■I in

Building rounded edge

0 0.9l6hb diom. and 0.5 hbore length
O 0.5 hfcd iom. ond 0.292 are iength
□ 0-583 hfc diom. ond 0.483 ore length
A 0-75hx diam ond 0543 hbore lengi.

til

Fig. 53- Effect of geometry of building rounded edge on turbulence profile obove

the centerline. Ele vo ted deck rounded edge ; 0 91 6 hfc diom ond 0 5ht>ore length

Re = 6 3xl0 5

Fig. 54- Effect of geometry of elevoted deck rounded edge on turbulence prof ile obove
the centerline. Building rounded edge ;09l6hbdiom.and0.5hb are length

R e =63x I0 5

Fig. 55- Effect of geometry ofelevated deck rounded edge on turbulence profile above
the center I ine Bu i Iding rounded edge ; 0.583 h^d iom ond 0.483 h^ ore le ng th

R e =6.3xl0 5

Fig. 56- Ef fee t of geometry of ele voted deck rounded edge on turbu lence prof ne above
the center ne. Bui Id ing rounded edge,0.5hb diam and 0 292 h^ ore length

R e = 63xl05

Wind tunnel test section

Fence

Configuration

Fig. 57- Experimental test set-up

u®

f

f

Fig. 62- Computed solidity compared to actual solidity for parallel rod grid