Background Introduction
This animation simulates the dynamic response of a wing under high-speed impact of an external object using LS-DYNA in ANSYS Workbench to determine the structural damage of the airfoil.
Many students who are new to explicit dynamics will encounter such confusions:
- Why does the simulation suddenly crash halfway through?
- Why do the meshes inexplicably distort and fold?
- How should the various energy curves in the results be interpreted to be reliable?
In fact, behind these problems lie the most core pitfalls in the simulation process: geometry simplification, material parameters, hourglass control, time step, energy conservation, and other issues. Today, we will use this complete case of a wing being struck by a bird to break it down step by step: from model, material constitutive, calculation settings, to result post-processing, how to turn an impact problem that is prone to failure into one that is as credible, usable, and understandable as possible.
Case Operation Process
Use the LS-DYNA module in ANSYS Workbench.
In the geometric model, the wing is simplified using a shell, and the bird is simplified using an ellipsoid solid model.
In the material settings, an Equation of State (EOS), Shock EOS Linear, is added to the bird model material.
It is a state equation model used in ANSYS to describe the material under
high pressure, high strain rate
(such as impact, explosion, high-speed collision) conditions
volumetric deformation and pressure relationship
. Tensile pressure failure (0 Pa), fails under tension.
The wing uses aluminum material with the Johnson Cook strength model, defining initial yield stress, strain hardening modulus, strain hardening index, strain rate sensitivity coefficient, thermal softening index, melting point, and reference strain rate.
Enter Model for model setup. For this bird model, its Reference Frame is set to Particle, based on the meshless particle method SPH, where each particle acts as an independent material point, with position and state updated as it moves, without relying on a fixed mesh.
The default inter-body interaction (frictionless, all objects) can meet the needs of this simulation.
Apply mesh control to the bird model using the Particle control method.
When selecting the
Particlemethod, ANSYS does not generate traditional finite element meshes (such as tetrahedra, hexahedra), but discretizes the entire component into a series of independent
particles (Particles). These particles act as material points, moving and deforming with the object, completely avoiding the mesh distortion problem under large deformation. Therefore, it is particularly suitable for extreme working conditions such as high-speed impact, fragmentation, explosion, and fluid-structure interaction.
Check the maximum and minimum
CFL Time Step in the model.
Set an initial velocity for the bird model, 250000 mm.
The wing model is meshed using shell elements, and hourglass control is required here. The hourglass mode is a non-physical zero-energy deformation mode, common in first-order reduced integration elements (such as shell elements, solid elements). When element nodes undergo shear or bending deformation, the strain energy is zero, the element stiffness disappears, manifesting as distorted, folded, hourglass-shaped abnormal mesh deformations, leading to inaccurate stress and strain calculations, and even simulation divergence. For thin-walled structures like wings discretized with shell elements, under impact conditions, the bending and shear deformation of shell elements can easily excite hourglass modes.
Here, Flanagan-Belytschko Stiffness Form is used for control. This is one of the most commonly used hourglass control types in LS-DYNA, particularly suitable for shell elements. It constrains zero-energy modes by introducing artificial stiffness, and is more suitable for large deformation and high-speed impact scenarios than viscous-type control. It can effectively suppress bending and shear hourglass while maintaining the physical response of the element, avoiding over-hardening. It is specifically optimized for the in-plane shear and bending degrees of freedom of shell elements, making it more precise than general-purpose control.
The total calculation time is 0.005s, without enabling mass scaling, and the time step is entirely determined by the minimum element CFL. In the SPH control, the approximation theory is program-controlled.
After the calculation is completed, you can view the output information in Solution Information.
Through the energy information, judge the rationality and accuracy of the calculation.
The kinetic energy gradually decays from the initial high value, which is consistent with the physical process of kinetic energy converting into internal energy when the structure is subjected to impact/load. The internal energy continues to rise, representing the elastoplastic deformation and strain energy accumulation of the structure, which is the main destination of kinetic energy conversion. In the absence of external energy input, if the sum of kinetic energy and internal energy fluctuates dramatically, it indicates that the simulation has numerical problems (such as hourglass, contact instability).
The hourglass energy should be much smaller than
the total energy (typically required to be < 5%), and the curve should remain at a very low level with no significant growth. From the figure, the hourglass energy is almost zero, indicating that the previously set hourglass control is very effective.
The contact energy represents the friction, slip, and collision dissipation between contact surfaces. Under normal conditions, the contact energy should remain at a relatively low level, and the curve should be smooth.
The hourglass energy should typically be controlled within 5%–10% of the Internal Energy.
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