MOOC@TU9

Models in Civil Engineering: From Cardiology to

Fishery

Univ.- Prof. Dr.-Ing. Gerhard Müller

Dr.-Ing. Martin Buchschmid

Lehrstuhl für Baumechanik, Technische Universität München

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Motivation

Can we “hear” the thickness of ice?

We need information about the

following components:

- load

- ice layer

- sound field

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Motivation

Ice layer under jump, plate under impulse loading

acoustic fluid

plate in bending

support

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Can we hear the thickness of ice?

Content

1. Vibrations of beams and plates in bending

2. Wave propagation in a beam

a) Wavelength, wavenumber

b) Dispersion

3. Wave propagation in an acoustic fluid

4. Sound radiation

a) Far field

b) Evanescent field

5. Fourier analysis and spectral content of dynamic forces

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Vibration of beams and plate in bending

Plate in bending action (neglecting the impedance of the underlying water)

𝐵: bending stiffness

Differential equation of a plate (considering the Kirchhoff theory):

PDE of 4th order

Approach for solution• Harmonically oscillating in space (x-,y- coordinate)• Harmonically oscillating in time (t-coordinate)

harmonic in time

sin, cos

Excursus: Describing trigonometric functions with complex numbers (Euler‘s formula)

two conjugate complex solutions

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Vibration of beams and plate in bending

Plate in bending action

Approach for the computation of one part of the conjugate complex solution(the complete solution is deduced easily)

with wavenumbers

angular frequency

taken from wikipedia.org

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Vibration of beams and plate in bending

Homogeneous solution

Therefore the wave propagation speed in dependence of the frequency can be calculated:

with

with

Plate in bending action

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Visualisation: Dependency of the wave propagation speed and wavelength compared to the frequency

Vibration patterns of a finite plate under single load with different frequencies of excitation

Vibration of beams and plate in bending

𝑓

𝑐𝐵 𝜆 𝐵

𝑓

dispersive propagation

depending on frequency

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Vibration of beams and plate in bending

Computation results: Eigenmodes of an elastically supported plate

Plate in bending action

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Vibration of beams and plate in bending

Plate in bending action

Ω1 = 4.43 𝐻𝑧 Ω2 = 6.39 𝐻𝑧

Computation results: Response to a harmonic single load in dependency of the frequency

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Motivation

Ice layer under jump, plate under impulse loading

acoustic fluid

plate in bending

support

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Acoustic fluid

Differential equation (wave equation)

PDE of 2nd order

Approach:

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Acoustic fluid

Differential equation (wave equation)

PDE of 2nd order

Approach:

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Acoustic fluid

Case 1:

propagating wave

(far field)

Case 2:

evanescent field

Discussion of the component of the sound-field, which is orthogonal to the

x-y plane (z-coordinate)

x

z

x

z

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Coupling of plate and acoustic fluid

x

zvelocity of the acoustic fluid

velocity of the vibrating plate

𝜆𝑝𝑙𝑎𝑡𝑒 > 𝜆𝑎𝑖𝑟 in case of sound

radiation (far field)

𝜆𝑝𝑙𝑎𝑡𝑒 < 𝜆𝑎𝑖𝑟 in case of

evanescent field (near field)

limit case: 𝜆𝑝𝑙𝑎𝑡𝑒 = 𝜆𝑎𝑖𝑟𝑐𝑝𝑙𝑎𝑡𝑒 = 𝑐𝑎𝑖𝑟

⇒ 𝑓𝑐𝑟𝑖𝑡

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Coupling of plate and acoustic fluid

Radiation of sound is (in case of infinite layers) just possible for frequencies of excitation with 𝑓 > 𝑓𝑐𝑟𝑖𝑡

Investigation of the frequency content of the load necessary

Velocity and wave length in dependency of frequency

𝑐

𝑓 𝑓

𝜆

𝑓𝑐𝑟𝑖𝑡 𝑓𝑐𝑟𝑖𝑡

radiation of sound

plate

air

plate ~1

𝑓

air ~1

𝑓

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Coupling of plate and acoustic fluid

Radiation of sound is (in case of infinite layers) just possible for frequencies of excitation with 𝑓 > 𝑓𝑐𝑟𝑖𝑡

Investigation of the frequency content of the load necessary

Velocity and wave length in dependency of frequency

𝑐

𝑓 𝑓

𝜆

𝑓𝑐𝑟𝑖𝑡 𝑓𝑐𝑟𝑖𝑡

radiation of sound

plate

air

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Coupling of plate and acoustic fluid

Radiation of sound is (in case of infinite layers) just possible for frequencies of excitation with 𝑓 > 𝑓𝑐𝑟𝑖𝑡

Investigation of the frequency content of the load necessary

Velocity and wave length in dependency of frequency

𝑐

𝑓 𝑓

𝜆

𝑓𝑐𝑟𝑖𝑡 𝑓𝑐𝑟𝑖𝑡

radiation of sound

plate

air

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Load

=

Remember: Fourier Series

with:

For an impulse an infinite number of cosine-members is needed

𝑝 𝑡 =

𝑘=−∞

∞

𝑐𝑘 ∙ 𝑒𝑖𝑘𝜋𝑡𝑇 𝑐𝑘 =

1

2𝑇

−𝑇

𝑇

𝑝(𝑡) ∙ 𝑒−𝑖𝑘𝜋𝑡𝑇 𝑑𝑡

𝑝(𝑡)

𝑡

𝑝(𝑓)

𝑓

𝑝(𝑡)

𝑡

Fourier Transformation

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Conclusion

• All frequencies are part of the signal.

• Just 𝑓 > 𝑓𝑐𝑟𝑖𝑡 radiate sound (and can be heard).

• High frequencies propagate faster (bending waves are dispersive) and can be perceived earlier at

different locations.

• The lowest frequency to be heard is 𝑓𝑐𝑟𝑖𝑡.

• 𝑓𝑐𝑟𝑖𝑡 is depending on the thickness of the ice layer.

We can „hear the thickness of an ice layer“.

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Example

Velocity in dependency of frequency

velo

city in

m/s

frequency

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Task of the Week

Given: Results of acoustic measurements for two different ice layers

Layer 1

Layer 2

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Task of the Week

Given: Results of acoustic measurements for two different ice layers

Layer 1

Layer 2

Time Domain Frequency Domain [0Hz; 5kHz]

0 0.2 0.4

-0.2

-0.1

0

0.1

0.2

Time [s]

Sound P

ressure

p(t

)

0 1000 2000 3000 4000 50000

2

4

x 10-3

Frequency [Hz]

Sound P

ressure

p(f

)

0 0.2 0.4 0.6 0.8

-0.2

-0.1

0

0.1

0.2

Time [s]

Sound P

ressure

p(t

)

0 1000 2000 3000 4000 50000

1

2

3x 10

-3

Frequency [Hz]S

ound P

ressure

p(f

)

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Task of the Week

Compute the thickness of the ice layers (Would you jump on the ice?)

Use the following material parameters for ice

Young's Modulus 𝐸 = 9.1 ⋅ 109𝑁

𝑚2

Density 𝜌 = 916.7𝑘𝑔

𝑚3

Poisson’s Ratio 𝜈 = 0.33

Use the following constant (frequency independent) wave velocity for air

Wave velocity 𝑐𝑎𝑖𝑟 = 340𝑚

𝑠

Is the applied model conservative (on the save side)? Please compare your results with the results

published by Lundmark (2001) and comment on the differences. Under what circumstances would you

trust the results in order to jump on the ice?

Estimate the stiffness of the water using the model of an elastically supported plate. Comment on the

model of an elastic support of the plate.

Given: Results of acoustic measurements for two different ice layers

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery

Published Results

Lundmark, 2001

Fre

quency

in H

z

Thickness in mm

MOOC@TU9: Models in Civil Engineering: From Cardiology to Fishery