Jost Fischer

I currently research at University of Hamburg on complex interactions and mechanisms of sound generation and sound radiation of musical instruments, for example organ pipes and reed instruments.

I recently finalized my PhD at the Department for Physics and Astronomy at University of Potsdam in the group on Statistical Physics and Chaos Theory. The topic of my dissertation was about nonlinear coupling mechanisms in acoustic oscillators which lead to synchronization. 

I'm working on nonlinear phenomena like synchronization in aeroacoustics and fluiddynamics. My diploma thesis was "About Synchronization Phenomena of Nonlinear Acoustic Oscillators". My supervisor was Priv.-Doz. Dr. Markus Abel.

Furthermore I think about synchronization aspects in complex communication setups, e.g. interaction of musicians and dancers.

A second topic of my work are investigations on flows and transport phenomena in turbulent layers, e.g. in the ocean, in the atmospheric boundary layer or, at the moment,  in shear layers in musical instruments.  

I also researched the dynamics of thermo-induced convection processes in buildings to find optimal control strategies for natural ventilation.

 

Current work

A) Nonlinear coupling mechanisms in acoustic oscillator systems which can lead to synchronization

 

Mutual interaction of two organ pipes P1 (left) and P2 (right). The pipes interplay via the radiated spherical sound waves. Shown is the pressure p. Numerical simulation, done with OpenFOAM 2.2., is calculated fully compressible, using a LES-one-equation-model for the sub-grid scales. Reynolds number: Re~2000-10000. Mesh size: (L x W x H) 400mm x 10mm x 300mm. Grid points ~1.25e6. The mutual coupling leads to synchronization of the pipe-pipe system after t=20ms. 

 

Mutual interaction of the jets (oscillating air sheets) of two organ pipes P1 (left) and P2 (right). The jets act via the radiated spherical sound waves (external coupling) generated by the sound generation regimes jet-resonator inside the organ pipes (internal coupling). Shown is the turbulent kinetic energy k which is scaled logarithmically. Numerical simulation, done with OpenFOAM 2.2., is calculated fully compressible, using a LES-one-equation-model for the sub-grid scales. Reynolds number: Re~2000-10000. Mesh size: (L x W x H) 400mm x 10mm x 300mm. Grid points ~1.25e6. The mutual coupling leads to synchronization of the pipe-pipe system after t=20ms.

 

B) Sound generation and sound radiation in acoustical wave guides

Numerical simulation of sound generation in an organ pipe. Shown is the turbulent kinetic energy k. Realized with the free, open source CFD software package OpenFOAM 2.1

Numerical Simulation of sound generation in an organ pipe. Shown is the pressure p at the initial inflow process.

 

Velocity magnitude in the jet region:

Velocity component v_x through the cross section along the resonator.  

 

C) Synchronization of organ pipes - Measurements in the 2D-plane

Setup:

Two organ pipes P1, P2, exactly tuned. P2 moveable stepwise in the 2D-plane. Grid 80 x 80 measurement points. (MP's). 

C1) Evolution of fundamental frequency and the amplitude (sound pressue level, SPL) of the synchronized 2-pipe-system:

a) at the first horizontal measurement line MP[1:80;1]  

b) at the first vertical measurement line MP[1;1:80]

 

 

C2) Evolution of first harmonic frequency and the amplitude (sound pressue level, SPL) of the synchronized two-pipe-system:

 

c) at the first horizontal measurement line MP[1:80;1]

 

d) at the first vertical measurement line MP[1;1:80]

e) Mean SPL-spectrum (Pegelspektrum) of the full domain 80 x 80 measurent points (for the ) 

 

At pipes distances of 0.5 x wave length (of the fundamental) in the 2D-plane the synchronized 2-pipe-system flips from anti-phase state into in-phase state. At pipes distances of 1 x wave length in the 2D-plane the synchronized 2-pipe-system flips back to the anti-phase state. At pipes distances of 1.5 x wave length the synchronized 2-pipe-system flips again back to the in-phase state. During the flips, located at sharp circular regions around the pipe P1, the  system de-synchronizes to find the other stable state. The fundamental frequencies of both pipes jump and the amplitudes (SPL) change significantly. At the process of re-synchronization, the energy transfer from the fundamentals to the higher harmonics play an important role.   

 

D) (Auto)-Synchronization of an jet via external acoustic field

Interaction of an jet (oscillating air sheet) with its own time-delayed sound signal, reflected by a convex, acoustically hard wall in front of the pipe. Left: the pressure p, right the turbulent kinetic energy k. Simulation (2D), done with OpenFOAM 2.1. is calculated fully compressible, using a LES-one-equation-model for the sub-grid scales. Reynolds number: Re~2000-10000, mesh size: (w x h) 180mm x 200mm, grid points ~260 000. 

 

 

E) Numerical simulation of a free ventilated building in the atmospheric boundary layer

Building in a turbulent flow (Re: 3.8e5). The numerical simulation (2D) is done by OpenFOAM 2.1. The fully compressible Navier-Stokes equations were calculated, using a LES-one-equation-model. Shown is the turbulent kinetic energy k. The size of the mesh is 120m x 40m (the buildings height is 10m), grid points ca. 150000.

Sim I: The mean velocity of the wind in the atmospheric boundary layer (0-40m) is 2.8m/s. (Beaufort 2-3).

 

Sim II: The mean velocity of the wind in the atmospheric boundary layer (0-40m) is 10.0m/s. (Beaufort 5-6):

 

Sim III: Free ventilated building in a turbulent atmospheric boundary layer. (Re: 3.8e5).
Simulation is done by OpenFOAM-2.2.2 using an LES model. The mesh has ca. 570000 grid points. Shown is the pressure p. The work is part of a reseach project at ATB Potsdam-Bornim, Germany, 2014.

 

Sim IIIA: Free ventilated building in a turbulent atmospheric boundary layer. (Re: 3.8e5). 
Simulation is done by OpenFOAM-2.2.2 using an LES model. The mesh has ca. 570000 grid points. Shown is the turbulent kinetic energy k. The work is part of a reseach project at ATB Potsdam-Bornim, Germany, 2014.

 

F) Thermo-induced convection in agricultural buildings

The numerical simulation is done by OpenFOAM-2.2.2. The Mesh has ca. 550000 grid points. Rayleigh number Ra~1e12, External temperature T_out=274K, Bottom temperature T_bottom=309.6K. The building is closed. The walls have constant heat flux. Shown is the vorticity magnitude in the time intervall t=0s-1000s. The vorticity is color coded in log scale. The whole simulation has t=1000s. Solved are the incompressible Navier-Stokes Equations with a one-equation LES-model. The solver buoyantPimpleFoam was used. The start in slow motion is shown here: http://youtu.be/O8imosN3IXU

 

The numerical simulation is done by OpenFOAM-2.2.2. The Mesh has ca. 550000 grid points. Rayleigh number Ra~1e12, External temperature T_out=274K, Bottom temperature T_bottom=309.6K. The building is closed. The walls have constant heat flux. Shown is the time intervall t=0s-100s in slow motion. Solved are the incompressible Navier-Stokes Equations with a one-equation LES-model. The solver buoyantPimpleFoam was used. The whole simulation is shown here:  http://youtu.be/mgtnCUdEL1M