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NoiseCon 2014 September 8-10, 2014

Advancing the Technology and Practice of Noise Control Engineering

Atmospheric Absorption Effects on the Propagation of Aircraft Noise

Victor W. Sparrow and Rachel A. Romond

Graduate Program in Acoustics Penn State

Opinions, findings, conclusions and recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of PARTNER and ASCENT sponsor organizations.

Paper NC14_110

2

Outline

•! Motivations •! Physical basis for atmospheric absorption •! Effect of medium inhomogeneity

o! Pure tone o! Spectrum

•! Experience with a recent benchmarking exercise •! Preliminary observations and recommendations

Acknowledgments

•! Work supported by FAA’s Office of Environment and Energy •! Special thanks to Lou Sutherland

3

Motivation for Noise Source Emission and Propagation Studies

•! Want to do best possible job when making aircraft sound level predictions

•! Critical issues for noise source emission and propagation –! Need the radiation pattern (level & directionality) of the source –! Fast AND accurate noise prediction –! Integration with dispersion & other environmental models –! Real-world atmospheric models (match model & experiment)

4

One Key Piece: Atmospheric Absorption

•! Important / fundamental since must include absorption for correct propagation modeling

•! For long distance propagation, a small change in the absorption coefficient can have a substantial cumulative effect on the received sound level

•! En-route noise o! Aircraft at 30,000 feet [10 km] altitude or higher o! Long distance propagation compared to airport-

vicinity distances •! Needed to look at absorption as a part of modeling

longer distance noise propagation through more sophisticated weather/atmospheric conditions

5

Dominant Absorption Mechanisms

•! Classical absorption and rotational relaxation

(From PHYSICS)"

f 2

6

Dominant Absorption Mechanisms

•! Classical absorption and rotational relaxation

•! Oxygen vibration relaxation

(From PHYSICS)"

f 2

O2

7

Dominant Absorption Mechanisms

•! Classical absorption and rotational relaxation

•! Oxygen vibration relaxation

•! Nitrogen vibration relaxation

(From PHYSICS)"

f 2

O2

N2

8

!

Dominant Absorption Mechanisms

•! Classical absorption and rotational relaxation

•! Oxygen vibration relaxation

•! Nitrogen vibration relaxation

•! Absorption from other trace component gases

o! Sutherland and Bass (2004)

Key reference: Bass, Sutherland, Piercy, Evans (1984)"

(From PHYSICS)"Pierce book Fig. 10-13:"

9

Available Standards

•! General outdoor sound propagation o! ANSI S1.26-1995 (Reaffirmed 2009) o! ISO 9613-1:1994 (Confirmed 2010)

•! Specific for aircraft noise o! SAE ARP 866 (1964) – used in INM 7 o! SAE ARP 866A (1975) and ARP866B (2012) o! SAE ARP 5534 (2013) – will be added in AEDT

-! Best we have at this time -! Includes all ANSI S1.26-1995 absorption effects -! Includes path length attenuations greater than 50 dB -! Includes Volpe Method/SAE Method for faster calculation of levels

in third-octave bands -! Does not include trace component gas effects since not in ANSI

or ISO standards

(By CONSENUS)"

10

Atmospheric Inhomogeneity

•! Some noise propagation models assume the atmosphere is homogeneous, but it is not

•! One must be CAREFUL in using the absorption standards

for aircraft noise since o! absorption coefficient is a strong function of temperature and

humidity o! temperature and humidity are strong functions of altitude

•! The standards will let you vary the atmosphere along the ray path, but there is little guidance on how to do so o! Exception: ANNEX C of ANSI S1.26-1995 gives one example of

an inhomogeneous atmosphere

11

Inhomogeneity and Pure Tones

12

Inhomogeneity and Pure Tones

Octave Band Frequencies [Hz]

125 250 500 1000 2000

!cum [dB]

From Single Measurement 2.1 2.1 2.6 4.6 12.8

From Table C1 T, P, h Profiles 4.5 10.1 22.8 53.9 132.1

Table C1 ! Values 4.5 10.1 22.8 53.9 131.7

Example measurement versus “average value” from ANSI:"o! Cumulative absorption over 10 km vertical path"o! Measurement from late night, winter conditions"

Note: huge differences in absorption between measurement""and “average value”"

13

Inhomogeneity and Spectrum

Poulain, et al. (2010) calculations:"o! Cumulative absorption over 10 km vertical path"o! Summer versus winter profile"o! Hypothetical turbofan jet aircraft spectrum"

" Octave Band Frequencies [Hz] LA

[dBA] 63 125 250 500 1000 2000 4000

LW [dB]

source at at z = 10 km

125.5 135.5 143.5 148.6 149.6 147.8 136.6 154.1

Lp [dB]

winter humidity profile 32.3 39.1 41.4 40.3 -14.8 -114.7 -339.7 45.4

summer humidity profile 33.7 42.2 46.5 44.7 41.5 -52.3 -245.9 50.3

difference 1.4 3.1 5.1 4.4 56.3 62.4 93.8 4.9

Note: don’t get large differences in overall SPL even though""have large differences in levels at higher frequencies"

15

Preliminary Observations and Recommendations (Part I)

•! Trace gas components should be considered for inclusion in updates to ANSI S1.26 and ISO 9613:1 o! CO2 is most significant atmospheric component not yet included o! Merits further investigation

•! The new SAE ARP 5534 includes modern absorption algorithms o! Many people worked hard to develop this new standard.

•! There is still more work to do o! The new SAE ARP 5534 does not include trace gas absorption

mechanisms since they are not in ANSI S1.26-1995 or ISO 9613:1

(On absorption standards)"

16

Preliminary Observations and Recommendations (Part II)

•! Humidity has a huge impact on atmospheric absorption

•! Need to develop “standard” humidity and temperature profile models to include or be used alongside the absorption standards

•! Shall we begin an international dialog to develop standardized models?

•! Clearly more research and communication is needed

(On humidity)"

The effective sound speed approximation and its implications for en-route propagation

Victor W. Sparrow, Kieran Poulain, Rachel A. Romond

Graduate Program in Acoustics, Penn State

Opinions, findings, conclusions and recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of PARTNER sponsor organizations.

Paper 4pPA3 ASA Providence

May 8, 2014

2

Outline

  Background on aircraft en-route noise   Comparison of modeling approaches to en-route noise   Limitations of the Effective Sound Speed (ESS)

Approximation   Example results with and without ESS   Conclusions

Acknowledgments

  Work supported by FAA’s Office of Environment and Energy   Special thanks to FAA’s Bill He (project manager), Volpe’s

Eric Boeker, and Purdue’s Kai-Ming Li

3

Motivation for en-route noise

U.S. National Parks and other Quiet Areas:

Figure C-8. All Flights over GCNP

Daytime (1085 flights)

  Famous Grand Canyon National Park figure from 2005 FICAN report, Fleming, et al. “Assessment of tools for modeling aircraft noise in national parks”

�

4

Available approaches

  Finite difference time domain / linearized Euler equations

  Ray tracing

  Fast field program (FFP, wavenumber integration)

  Parabolic equation (PE)

  Semi-analytical (Example: NORD2000)

5

Options for modeling the wind

  Include the vector wind field exactly -  Difficult to get this information -  Vertical components are small compared to horizontal

components

  Assume the wind is horizontally stratified -  Agrees with our experience

  Assume the wind can be included in an effective sound speed profile

ceff(z)! c(T (z))+w(z)

6

Effective sound speed (ESS) approximation

  It’s an approximation

  Very convenient, if it does what you want it to do -  Must be careful with this, and it has been known for many years -  Reiterated by V. Osteshev in 1997.

  Used in the following programs/formulations: -  “Generic” fast field program (FFP) -  “Generic” parabolic equation (PE) -  Semi-analytical (NORD2000)

Preliminary comparison of Poulain� - Stationary point source (zs=10 km)� - Realistic averaged weather data� - Same θlaunch�

7

  Use ray tracing program with correct wind effects   M.S. thesis work of K. Poulain, Penn State   Program AERNOM   Based on moving media ray equations

  Pierce formulation for rays with wind   Lamancusa and Daroux method for spreading losses in a refractive

atmosphere

  Compare ray tracing   No ESS: with full horizontal wind implementation   With ESS: with wind set to zero and wind included in ceff

Test of ESS approximation

ceff(z)! c(T (z))+w(z)

8

Benchmarking Penn State’s Code

NASA AERNOM

+ T lapse

Benchmark setting

o  Point source at 1,500 m�o  Linear T and Vspeed�o Vdir in +X direction�o  Geometrical + refraction� losses only� �

- T lapse

Stephanie L. Heath and Gerry L. McAninch, “Propagation effects of wind and temperature on acoustic ground contour levels”, Proc. 44th AIAA Aerospace Sciences Meeting and Exhibit, 9-14 January 2006, Reno, Nevada.�

9

Set Up for Test Case

  �  Zs=10 km)�  Flat and even terrain (grassland)�  SEL ground contours (dBA):��

40 km

150 km

10 km

SEL = Leq +10 ! log10(" )

10

Profiles for Test Case

  �  z in km�

v(z) = 5 ! z

11

Footprint: no ESS approx.

  SEL contours [dBA] using AERNOM�  track at zs=10 km�

!w

!w

dBA

12

Footprint: with ESS approx.

Looks almost exactly the same.��Wonʼt show it.�

13

Subtraction: no ESS – with ESS

10 km altitude�

!w

!w

ESS too low�

ESS too high�

14

Slice: no ESS – with ESS

10 km altitude: crosswind case�

ESS too low�ESS too high�

They agree undertrack�

15

Slice: no ESS – with ESS

But now at 5 km altitude: crosswind case�

ESS too low�

They agree undertrack�

16

Conclusions

  These are preliminary results.   The investigation team was surprised

–  The expectation was that the en-route predictions with ESS would be substantially different than without ESS.

–  But the undertrack with and without ESS results match fairly closely –  Team observed substantial differences near the carpet edges, but

this is when the en-route SELs are 20 dB (or more) below the peak values

  Clearly more research is needed –  Need to try other wind and temperature profiles –  It may be that en-route carpets are not so sensitive to ESS since

the carpets may be dominated by energy traveling nearly straight downward from the aircraft to the ground

18

1.  K. Poulain, “Numerical propagation of aircraft en-route noise,” M.S. Thesis (Penn State, Graduate Program in Acoustics, 2011). Thesis available at https://etda.libraries.psu.edu/paper/12491/ .�

2.  G. Fleming, K. Plotkin, C. Roof, B. Ikelheimer Senzig, “Assessment of tools for modeling aircraft noise in the national parks,” Federal Interagency Committee o Noise (FICAN), March 2005. Available on .�

3.  “Nord2000. Comprehensive outdoor sound propagation model., Part 2: Horsholm,

www.delta.dk .�4.  V. Ostashev, Acoustics in Moving Inhomogeneous Media (E & FN Spon,

�5.  A. Pierce, Acoustics: An introduction to its physical principles and

applications (McGraw Hill, 1981), Chap. 8�6.  H. He and E. Boeker, “Overview of aircraft en-route noise prediction using

an integrated model,” Proc. NoiseCon �  S. Heath and G. McAninch, “Propagation effects of wind and temperature

on acoustic ground contour levels”, Proc. 44th AIAA Aerospace Sciences Meeting, 9-14 January 2006, Reno, Nevada.�

8.  J. Lamancusa and P. , JASA 93 �

References

Thanks!!Contact Information:!–"Vic Sparrow!–"vws1@psu.edu!–"+1 (814) 865-6364!