helicopter exhaust flow

8/11/2019 Helicopter Exhaust Flow

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*Corresponding author. Tel.:#1-970-221-3371; fax:#1-970-221-3124.

E-mail addresses: [email protected] (L. Cochran), [email protected] (J. Peterka), rpeter-

[email protected] (R. Petersen)

Journal of Wind Engineering

and Industrial Aerodynamics 83 (1999) 347}360

Physical modelling of roof-top helicopter exhaust

#ow dispersion

Leighton Cochran*, Jon Peterka, Ron Petersen

Cermak Peterka Petersen Inc., 1415 Blue Spruce Drive, Fort Collins, CO 50524, USA

Abstract

A technique for physically modelling the interaction of helicopter rotor #ows with the natural

wind in a boundary-layer wind tunnel is described. This technique is used to measure the

dilution of helicopter exhaust fumes at the roof-level air intakes near the heliports on top of the

new American Stores Company Headquarters building in Salt Lake City, Utah. The dilution

values may then be compared to odor and health threshold limits in the literature. Physicalmodelling of the interaction of the helicopter rotor downwash and the ambient wind around

a complex architectural roof shape was used to determine the suitability of the locations chosen

for the building air intakes. This approach is appropriate where any heliport operation interacts

with the HVAC system of an o$ce building or hospital. 1999 Elsevier Science Ltd. All

rights reserved.

Keywords: Helicopter exhaust; Physcial modelling; Flow modelling; Architectural aerodynamics; Pollutant

dispersion; American Stores; Wind engineering

1. Introduction

Roof-top heliport locations are becoming more popular on hospitals and corporate

headquarter buildings (see Fig. 1). There is the potential for exhaust fumes during

take-o!, landing and idling to be ingested into the fresh-air intakes of the building

air-conditioning system [1,2], with the resulting indoor pollutant concentrations

being above odor thresholds. The architect and mechanical engineer need to establishwhere to place the air intakes in order to minimize exhaust intrusion into the building.

0167-6105/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 6 7 - 6 1 0 5 ( 9 9 ) 0 0 0 8 4 - 7


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Fig. 1. Typical roof-top heliport near air-conditioner intakes. This may cause indoor odors from engine

exhaust.

The ine$cient combustion of hydrocarbons during engine idling means that this is

actually the worst scenario for odors, even though the fuel consumption, and sub-

sequent #ow rate, is lower than at take-o![3]. In fact, the data collected by Lozano etal. [4] lead them to write that the required `. . . odor dilution threshold is the greatest

for the fan-jet engine at the `idlea power settinga.

2. Physical modelling

Techniques have been developed which permit boundary-layer wind-tunnel model-

ling of dispersion around buildings that include the interaction of an operating

helicopter. The physical modelling parameters for#ow caused by the natural wind are

well established [5}7,16]. These criteria are satis"ed by constructing a scale model of

the structure and its surroundings, and performing the tests in a wind tunnel speci"-

cally designed to model atmospheric boundary-layer #ows. Reynolds number sim-

ilarity requires that the quantity ;/ be similar for model and prototype. The

quantities ; and are the representative velocity and typical dimension in the model

and full-scale condition. Since , the kinematic viscosity of air, is identical for both

model and full-scale, Reynolds numbers cannot be made equal with reasonable wind

velocity, for such a velocity would introduce unacceptable compressibility e!ects.However, for su$ciently high Reynolds numbers '1.110 [8], the #ow at any

location around a sharp-edged structure will be essentially unchanged for a large

range of Reynolds numbers. Typical values encountered are 10}10for the prototype

and 10}10for the model. In this range acceptable #ow similarity is achieved without

Reynolds number equality. All model tests reported herein were performed at a su$-

ciently high velocity to maintain Reynolds number independence.

348 L. Cochran et al./J. Wind Eng. Ind. Aerodyn. 83 (1999) 347}360


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Given that the atmospheric #ows can be physically modelled adequately, it is the

added complication of the rotor #ow that is the focus of this discussion. To model

helicopter exhaust dispersion in the wind tunnel, the out#ow pro"le from a computer

cooling fan (see Fig. 3) was matched to the analytical helicopter pro"les for the Bell

412 helicopter. The comparison of the mean and peak #ow speeds at one of the fourradial distances is made in Fig. 5. These data were collected with a hot-"lm anemom-

eter calibrated to measure the mean speed and turbulent properties of the out#ow.

The speci"c additional modelling requirement to obtain the correct dilution ratios (d )

between model and full-scale helicopter exhaust is guided by continuity consider-

ations at the engine exhaust and away from the helicopter at a receptor site of interest.

Speci"cally, the mass #ow rate (m) of pollutant out of a control volume, encompassing

the source, and of side length () is given by:

(m)"CM;M, (1)

whereCMis the mean ambient concentration [kg/m] and ;Mis the mean velocity [m/s]

across one face of the control volume of area [m]. The exhaust pollutant mass

#ow rate at the engine may also be described locally as:

(m)"QC

(2)

whereQis the engine volume #ow rate [m/s]. Eqs. (1) and (2) may be equated by the

conservation of mass. In addition, the dilution ratio (d) may be de"ned as the ratio of

the exhaust concentration (C) and the mean downwind concentration (CM) at thereceptor site of interest.

Speci"cally,

d"CCM"

;M

Q . (3)

By requiring that the model (m) and full-scale (f) mass#owrate ratios be equal we get

the relationship:

d"

;M;M

QQ

d

, (4)

where ;M may be de"ned as the mean roof-top wind speed, is a typical length scale

and Q is the #ow rate of the helicopter's jet engines.

3. Wind-tunnel characteristics

The wind-tunnel test was performed in the boundary-layer wind tunnel shown inFig. 2. This wind tunnel has a 23-m long test section covered with roughness elements

to reproduce, at model scale, the atmospheric wind characteristics required for the

model test. The wind tunnel has a #exible roof, adjustable in height, to maintain a zero

pressure gradient along the test section and to minimize blockage e!ects.

A model of the building under study was constructed of architectural styrofoam at

a scale of 1 : 125 which was consistent with the modelled helicopter #ow. Other nearby

L. Cochran et al./J. Wind Eng. Ind. Aerodyn. 83 (1999) 347}360 349


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Fig. 2. The open-circuit dispersion boundary-layer wind tunnel at CPP Incorporated.

Fig. 3. View of the 1 : 125 model and cooling fan `helicoptera in the open-circuit boundary-layer wind

tunnel at CPP Inc.

buildings, protruding above a 61 m pseudo-ground plane, were also modelled (Fig. 3).

In this way the blockage of a full-height 1 : 125 model could be avoided. Fig. 8 shows

the original 1 : 400 turntable used for an earlier pressure study, and also illustrates the

expanded portion used for the current 1 : 125 dispersion study. The model was

mounted on the turntable located near the downstream end of the wind-tunnel test

section. The turntable permitted rotation of the modelled area for examination of

#ows from any approach wind direction.

The wind-tunnel #oor upstream from the modelled area was covered with rough-ness elements constructed from cubes (see Fig. 3 and Fig. 8). A two-dimensional trip

and roughness elements were designed to model the lower portion of the atmospheric

boundary layer. Velocity pro"le measurements were taken to verify that appropriate

boundary-layer #ow approaching the site was established (Fig. 4). Wind pro"le

measurements were made using a single hot-"lm anemometer mounted on a com-

puter-controlled vertical traverse. The hot"lm was oriented horizontally transverse to



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Fig. 4. Flow properties for the truncated 1 : 125 model: (a) vertical pro"le of the mean wind speed, and (b)

vertical pro"le of the longitudinal turbulence intensity.

the #ow. The instrument used was a TSI, Inc., constant-temperature anemometer

(Model 1053b) with 50 m diameter platinum-"lm sensing element. The calibration of

the hot-"lm anemometer was done using a pitot tube and the heat transfer properties

described by King [9]. A mean velocity power-law exponent (n"0.23 in Fig. 4)

similar to that expected to occur in the region approaching the model was developed.

A turbulence structure in the modelled atmospheric boundary layer similar to that

expected in the full-scale wind was also produced in the wind tunnel. In particular, the

turbulence intensity at the roof of the 1 : 125 model was about 12%. This compared

well with that used in the 1 : 400 pressure study and what was expected at this

elevation in the prototype #ow.

4. Out6ow pro5les for the Bell 412 helicopter

Velocity measurements were also made vertically across the fan out#ow, at various

radii, to determine the appropriate modelling for the full-scale Bell 412 helicopter.



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Fig. 5. Model and analytical #ow pro"les for the Bell 412 helicopter at 30 m radius. Model #ow matched at

the peak maximum at 450 mm above the ground.

Several methods for generating rotor wash winds were tried. The objectives were to

retain as much of the physics of the problem as possible including the entrainment

of air into the top of the rotor system, a rotational blade out#ow generation,

and stability and repeatability of operation. Several model aircraft propellers were

tried and ultimately rejected on the basis of an inability to repeat conditions. A verystable power supply was found to be needed. The most satisfactory solution was to

use a computer cooling fan. At a scale of 1 : 125, a fan was found that would pro-

vide a reasonable simulation of the out#ow (i.e., matching that of the prototype).

The comparison of velocity pro"les observed in the physical model to those

predicted by the analytical #ow model [12] are shown in Fig. 5. The model pro"les of

Fig. 5 were scaled to match the full-scale data at about 450 mm above ground (typical

maximum velocity height in a helicopter pro"le) at the 30 m distance. Under this

scaling, the peak wind speeds match the analytical model reasonably well, but the

mean velocity is somewhat high at closer distances. These pro"les (and others at closer

distances, not shown) were used to establish the range of wind-tunnel speeds

(0.5}2.8 m/s) required to match the corresponding full-scale wind data. The compari-

son between physical and analytical model is about as good as the comparisons of

full-scale to analytical model given by Ferguson [10,11].

The Ferguson and Kocurek out#ow model [12] is based on a theoretical phenom-

enological model which uses inputs derived from tests on full-scale helicopters.

Variables in the Ferguson and Kocurek model required to obtain a pro"le include: the

helicopter gross weight, rotor diameter, helicopter model, height of the rotor abovethe ground, and the distance of the desired pro"le location from the helicopter.

A typical example of their full-scale data and derived analytical model results for



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Fig. 6. Typical single rotor out#ow comparison (at 12 m radius) between full-scale data and analytical

model from Ferguson and Kocurek [12].

a single rotor is presented for comparative purposes in Fig. 6. The Ferguson and

Kocurek model also includes the non-symmetric out#ow in the presence of wind for

a hovering aircraft. Since the main interest in this study was the idling helicopter on

the roof surface, the impact of the non-symmetrical output was not examined.

The validation of helicopter out#ow of (Fig. 5) was established on a #at ground

surface, with no wind, to match the Ferguson and Kocurek conditions. The helicopter

model was then installed on the building roof-top for testing in that con"guration. In

addition, the engine exhaust was assumed to be on the side of the helicopter facing the

receptor. For other orientations of the helicopter, measured odorous concentrations

were somewhat lower.

The data validation of the Ferguson and Kocurek model was limited and contained

only a few gust measurements of uncertain averaging time. In addition, the full-scale

data have considerable variation due to the e!ects of ambient wind, tail-rotor #ows,

hover height variations and other variables. The overall sense of the Ferguson and

Kocurek model is that it is a reasonable representation of helicopter out#ows for

horizontally rotating blades, and it is in this capacity that it was used to develop

a reasonable physical model of helicopter out#ow.

5. Data collection

By using a 100% pure ethane gas to simulate the exhaust under the rotor blades

and a Flame Ionization Detector (FID) to measure the ambient concentration at the



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Table1

Minimumdilutionratios,bywindazimuthandroof-topwindspeed,atthethree

intakesduetothesouthernh

elicoptersource(H2)

Meandilutionratiostobeappliedto

helicopterexhaust

Southheliport(H2)

Intake1

Intake2

Intake3

Tunnel

velocity

(m/s)

Full-scale

roof-to

p

velocity

(m/s)

Percent

velocity

exceeded

(%)

Worst

wind

dir.(deg)

Minimum

mean

dilution

(!

)

Worst

wind

dir.(deg)

Minimum

mean

dilution

(!

)

Worst

wind

dir.(deg)

Minimum

mean

dilution

(!

)

0.00

Calm

96.9

*

12700

*

7700

*

8700

0.57

3.7

68.7

*

*

150

5000

*

*

1.14

7.5

25.9

140

4400

130

5000

150

7600

1.72

11.3

5.9

140

3300

140

5200

150

5700

2.26

14.7

1.0

140

2600

140

4900

150

4300

2.86

18.7

0.1

140

2800

140

3500

160

4100



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Fig. 7. 1 : 125 turntable showing the fresh-air intakes just below roof level and the two helicopter sources

(H1 and H2).

three air intakes, the e!ective dilution caused by the wind and rotor #ow was

measured. To "nd the worst-case concentration (i.e., lowest dilution), a variety of wind

directions (103 increments) and wind speeds were investigated. The fraction of time

that a given wind speed can be expected to be exceeded in the full scale is given in

Table 1. The three intakes were tested with the helicopter and wind operating together

for a variety of wind speeds. The remaining test was for calm conditions. Flow

patterns identi"ed with smoke (titanium dioxide) showed that the helicopter exhaust

is not easily ingested into the air-conditioning system due to the intake location under

the cantilevered roof. The intake locations are shown in Fig. 7. The #ow features

responsible are discussed in the following sections.

6. Results

A search of wind speeds and wind directions in Table 1 yields the lowest mean

dilution to be expected for one source (south heliport, H2) impacting on the three

intake sites shown in Fig. 7. The United States National Research Council report for



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Fig. 8. A portion of the original turntable used for the 1 : 400 cladding study was expanded to provide the

1 : 125 dispersion study turntable. A pseudo-ground plane at 61 m was used to reduce blockage.

the EPA [13] claims that the highest amount of dilution is required for the low

throttle or idle setting and dilution values from 500 to 1000 are appropriate depending

on the engine type. At this southern heliport the worst wind-induced, full-scale

dilution ratio of 2600 is above the most odor sensitive EPA standard of 1000. In calm

conditions the dilution is even more e!ective ('7700).Table 2 shows the results of the search for the lowest dilution ratios for the north

helicopter (H1). None of the wind-induced dilution ratios fall below the more sensitive

1000 value. The worst wind-induced full-scale dilution ratio on the American Stores

Headquarters building is 1200. However, in calm conditions there is a worst-case

dilution ratio between the two EPA suggested odor thresholds of 500}1000. It is

worth noting that in the interim 18 years, the e$ciencies of jet engines have improved

greatly [14] and so the proportion of poorly combusted fuel has been reduced. In fact,

recent work by NASA [3] shows that the Thrust Speci"c Fuel Consumption (TSFC)

has improved by about 25% in the last 25 years. Speci"cally they show a drop from

24 mg/J to about 18 mg/J for a whole variety of manufactures 'engines.



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Table2

Minimumdilutionratios,bywindazimuthandroof-topwindspeed,atthethree

intakesduetothenorthernh

elicoptersource(H1)

Meandilutionratiostobe

appliedtohelicoptere

xhaust

Northheliport(H1)

Intake1

Intake2

Intake3

Tunnel

velocity

(m/s)

Fuul-scale

roof-to

p

velocity

(m/s)

Percent

velocity

exceeded

(%)

Worst

win

dir

(deg)

Minimum

mean

dilution

(!

)

Worst

win

dir

(deg)

Minimum

mean

dilution

(!

)

Worst

win

dir

(deg)

Minimum

mean

dilution

(!

)

0.00

Calm

96.9

*

2100

*

6800

*

800

0.57

3.7

68.7

*

*

*

*

*

*

1.14

7.5

25.9

0

3100

290

3100

120

1400

1.72

11.3

5.9

330

1200

340

1900

120

1200

2.26

14.7

1.0

330

1600

330

2800

120

1200

2.86

18.7

0.1

*

*

*

*

120

1200



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Fig. 9. Salt Lake City, Utah, wind rose (1964}1990) taken from the NCDC CD-ROM.

Thus in the calm wind cases for the northern heliport, the overall minimum

full-scale dilution ratio (800) is between the two highest standards of 500}1000.

However, given the relatively infrequent occurrence of calm conditions (about 3% of

the time in Fig. 9) and the engine improvements in recent years noted above, the odor

from helicopter exhausts should not be noticeable inside the o$ce building.

7. Conclusions

During take o!and landing, the Bell 412 helicopter emits quantities of exhaust, and

the rotor #ow pattern may be modelled by the cooling fan shown in Fig. 3 which



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locally dominates the #ow. When the aircraft is idling, the exhaust volume is greatly

reduced, but the source of odor is increased due to the incomplete hydrocarbon

combustion [3]. Thus, the rotor #ow was combined with the lower-throttle settings

that produce the highest odor potential. From observing the #ows using an o!-center

exhaust source (taken closer to the building edge and the air intakes) tagged withtitanium dioxide, the following features were noted:

1. When the roof-level wind is at a comparable speed to the rotor out#ow (this is

a relatively rare event occurring less than 1% of the time), the exhaust air may be

forced back under the roof deck to the cavity containing the air intakes.

2. In the calm-wind case, the northern helicopter #ow causes the dispersed exhaust to

accumulate in the building's notch which houses air intake number three.

3. Generally, the presence of wind keeps the exhaust gases away from the air intakes

by sweeping the #ow to the downwind side of the helipad and building. Thepollutant is generally well dispersed by the time it reaches the lee of the structure

which may house an air intake. This observation is borne out by the high dilution

ratios measured in these circumstances.

4. The #ow in still air shows the common trend of the exhaust being ejected beyond

the roof edge by the rotor #ow alone.

8. Suggested further reading

For general background, see McKiney [17].

Acknowledgements

The authors wish to thank the American Stores Company [15] in Salt Lake City

and HKS Inc. of Dallas for permission to publish the results of the study on their new

corporate headquarters. Discussions with the Bell Helicopter Corporation wereinvaluable in establishing the engine #ow rates to be expected for various load

conditions on the Bell 412 helicopter. Much of the "ne drafting was performed by Mr.

Kevin Ott and Mr. Brian Moon of CPP Inc.

References

[1] R.L. Petersen, M.A. Ratcli!, C. Wisner: Helicopter fume entrainment evaluation. Eighty-Third

Annual Meeting and Exhibition of the Air and Waste Management Association, Number 90-149.3,Pittsburgh, Pennsylvania, 1990.

[2] H. McKew, Special rooms and special applications, Engineered Systems 13 (3) (1996) 42}56.

[3] Nasa Langley Research Center, Jet Engine Emission Database, Contract Report Number 46123,

1994.

[4] E.R. Lozano, W.W. Melvin, S. Hochheiser: Air pollution emissions from jet engines. J. Air Pollut.

Control Assoc. 18 (6) (1968) 392}394.

[5] J.E. Cermak, Aerodynamics of buildings, Ann. Rev. Fluid Mech. 8 (1976) 75}106.



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[6] J.E. Cermak, Applications of#uid mechanics to wind engineering } A freeman scholar lecture. Amer.

Soc. Mech. Eng. J. Fluids Eng. 97 (1) 1975, 38 pp.

[7] J.E. Cermak, Laboratory simulation of the atmospheric boundary layer. AIAA J. 9 (1971) 1746}1754.

[8] W.H. Snyder, Guideline for #uid modeling of atmospheric di!usion, United States Environmental

Protection Agency Report 600/8-81-009, Research Triangle Park, North Carolina, 1981.

[9] C.V. King, On the convection of heat from small cylinders in a stream of#uid, Philos. Trans. Roy. Soc.

London A214 (1914) 373.

[10] S.W. Ferguson, Evaluation of the rotorwash characteristics for the Bell/Boeing V-22, Bell 214ST and

Sikorsky S-76B, EMA Incorporated Report 91-1-1 for JPJ Architects, 1991.

[11] S.W. Ferguson: Evaluation of the rotorwash characteristics for Tiltrotor and Tiltwing Aircraft in

Hovering Flight, Federal Aviation Authority Report DOT/FAA/RD-90/16, December 1990.

[12] S.W. Ferguson, J.D. Kocurek, Analysis and recommendation of separation requirements for rotocraft

operation at airports and heliports. Final Report by System Technology Incorporated for the

Transportation Systems Center, United States Department of Transportation, 1986.

[13] United States National Research Council: Odors from stationary and mobile sources, Report

Number PB83-159186 for the United States Environmental Protection Agency, Department ofCommerce, 1979.

[14] R.E. George, J.A. Verssen, R.L. Chass, Jet aircraft} a growing pollution source, J. Air Pollut. Control

Assoc. 19 (11) (1969) 847}855.

[15] L.S. Cochran, M.A. Ratcli!, J.A. Peterka, J.E. Cermak, Dispersion study for the American Stores

heliport, CPP Report number 95-1277 for HKS Incorporated, 1996, 38 pp.

[16] American Society of Civil Engineers, Wind Tunnel Model Studies of Buildings and Structures,

Manual of Practice Number 67, New York, 1999, 214 pp.

[17] J.B. McKiney, Evaluating wind #ow around buildings on heliport placement. Federal Aviation

Authority Report DOT/FAA/PM-84/25, 1990, 37 pp.


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