flow acoustic boundary element solver and the noise ...
Transcript of flow acoustic boundary element solver and the noise ...
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Accelerated flow‐acoustic boundary element solver and the noise generation of fish
JUST IN W. JAWORSK I , NATHAN WAGENHOFFER , KE ITH W. MOORED
LEH IGH UNIVERS ITY, BETHLEHEM, USA
FL INOV IA – PENN STATE
27 APR I L 2017
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MotivationMotivation
Design tool for high‐performance, silent devices
Goal
Estimate the acoustic signatures due to vortex‐body interactions of an individual or collective
Approach
Develop and validate a time‐domain, 2D acoustic boundary element solver Essential step towards integration with fluid BEM solver
Present advancement
First integration of a time‐domain, double‐layer acoustic BEM formulation accelerated by the fast multipole method
School of Fish
Autonomous Drones
Silent Owl Flight
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Formulation of the external 2D acoustics problem The acoustic field is governed by the wave equation in 2D space:
V is the exterior domainSb is the boundary of a solid body T is the total time periodgN is a prescribed Neumann BC
Transform to a constant strength dipole BEM formulation
Double‐layer (dipole) enables direct application of Neumann BC Can combine single‐/double‐layer potentials in present framework Time domain problem recast as convolution of many frequency‐domain problems
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Fast multipole method Accelerates the boundary element solution and evaluation
of the acoustic field
The method rapidly sums potentials of the form
by breaking them into near‐field and far‐field sums:
Fast multipole method (FMM) employs a tree structure to reduce number of operations for N boundary elements from O(N2) to O(N log N)
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Direct BEM Direct BEM builds full matrices to determine the influence of discrete points on each other
O(N) operations * N elements = O(N2) operations
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Direct BEM Direct BEM builds full matrices to determine the influence of discrete points on each other
O(N) operations * N elements = O(N2) operations
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Fast multipole method: far fieldProcedure
Build a quadtree structure
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Fast multipole method: far fieldProcedure
Build a quadtree structure
Sum the potential of leaves into a multipole
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Fast multipole method: far fieldProcedure
Build a quadtree structure
Sum the potential of leaves into a multipole
Find the influences of multipoles on one another
Repeat up/down the tree structure
One bottom/top/bottom pass per time step
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Fast multipole method: far fieldProcedure
Build a quadtree structure
Sum the potential of leaves into a multipole
Find the influences of multipoles on one another
Repeat up/down the tree structure
One bottom/top/bottom pass per time step
The multipole structure represents the far‐field influence
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Fast multipole method: near field Near‐field calculations require the direct method to account for local influences
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Acoustic BEM validation: rigid scatterer
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Scatter of harmonic field by rigid cylinder
13
Plane wave scattering by a cylinder, exp , cos
The scattered wave is
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Numerical convergence
1024 elements512 time steps/period
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Time savings of FMM
1 to 3 orders of magnitude speed up
Interpolated speeds Compare direct and FMM methods for single scatterer
256 time steps per period
Computation remains on order of seconds using FMM approach
Advantage for high‐frequency problems requiring many elements
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Scatter of soliton by multiple bodies
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Time‐dependent scatter of single wave
Current method manages multiple closed bodies in arbitrary configurations
Primary and secondary scattering events handled by BEM solver
Scatter of soliton by multiple bodies
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Scatter of soliton by multiple bodies
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Scattered Pressure
Γ 4π
0.40.50.2NACA 0012 airfoil
/
0
Vortex‐airfoil noise generation
Difference between BEM and exact solutions attributed to airfoil thickness
Doublet BEM enables investigation of arbitrarily thin airfoils
Neumann condition difficult with single‐layer potential in this geometric limit
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Find vortex street parameters:
0.3
a = Γ∗ 2 tan
Noise generation in swimmer’s wake
Vorticity contours
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Scattered Field – 2S
Vorticity
Scattered acoustic field
Directivity
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Scattered Field – 2P
Vorticity
Scattered acoustic field
Directivity
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Scattered Field – 2S
Vorticity
Scattered acoustic field
Directivity
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Scattered FieldContinuous 2S street
Vorticity
Scattered acoustic field
Directivity
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SummaryAchievements and Conclusions
A double‐layer, time‐dependent, 2D acoustic boundary element solver is developed and accelerated using the fast multipole method Needed to rapidly predict and evaluate the noise generation from high‐performance swimmers and fliers
Validation for radiators and scatterers, involving single‐ and multiple‐body arrangements
Preliminary comparison of 2S/2P wake structures suggest similar noise level and dipole directivity Collections of bodies in a wake modify the far‐field directivity in a non‐trivial way
Future work
Incorporate fluid BEM solver to feed vorticity field into acoustic BEM AIAA Aviation (Wagenhoffer, Moored, Jaworski): Integral Methods AA26, CAA V (Tuesday, 2:30pm)
Extension of methods to handle 3D acoustics and fluid dynamics of animal and robotic collectives
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Additional Slides
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BEM Flow Solver – Vortex Particle WakeTransform Laplace’s Eqn into a Boundary Integral Equation
Enforce Kutta Condition at the trailing edge (finite velocity)
Wake terms are Gaussian vortex blobs
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Fencing scheme
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• Can add vorticity to any flow
• Fence placed around bodies to prevent penetration
• Exaggerated fence here
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Wake evolution
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Pure pitching foil
0.25rad
2 5
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Noise due to pure pitching
30
PSPL(dB) above ambient