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A Technique for Delayed Mesh Relaxation in Multi-Material ALE Applications ASME-PVP Conference - July 25-29 2004 K. Mahmadi, N. Aquelet, M. Souli.

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Presentation on theme: "A Technique for Delayed Mesh Relaxation in Multi-Material ALE Applications ASME-PVP Conference - July 25-29 2004 K. Mahmadi, N. Aquelet, M. Souli."— Presentation transcript:

1 A Technique for Delayed Mesh Relaxation in Multi-Material ALE Applications ASME-PVP Conference - July K. Mahmadi, N. Aquelet, M. Souli

2 The Challenges To apply a delayed mesh relaxation method to arbitrary Lagrangian Eulerian multi-material formulation to treat fast problems involving overpressure propagation such as detonations. To define relaxation delay parameter for general applications of high pressures, because this parameter is a coefficient dependent.

3 The Process Introduction Eulerian and ALE multi-material methods Multi-material interface tracking –VOF method Delayed mesh relaxation technique –Lagrangian phase –Mesh relaxation phase Numerical applications –Three-dimensional C-4 high explosive air blast –Three-dimensional C-4 high explosive air blast with reflection Conclusions

4 Introduction A problem of blast propagation Lagrangian Formulation The computational domain follows the fluid particle motion, which greatly simplifies the governing equations. Advantages The material may undergo large deformations that lead to severe mesh distortions and thereby accuracy losses and a reduction of the critical time step. Drawbacks Lagrangian schemes have proven very accurate as long as the mesh remains regular.

5 Introduction Multi-Material Eulerian Formulation The mesh is fixed in space and the material passes through the element grid. The Eulerian formulation preserves the mesh regularity. Advantages The computational cost per cycle and the dissipation errors generated when treating the advective terms in the governing equations. Drawbacks

6 Introduction Arbitrary Lagrangian Eulerian (ALE) Formulation The principle of an ALE code is based on the independence of the finite element mesh movement with respect to the material motion. The freedom of moving the mesh offered by the ALE formulation enables a combination of advantages of Lagrangian and Eulerian methods. Advantages For transient problems involving high pressures, the ALE method will not allow to maintain a fine mesh in the vicinity of the shock wave for accurate solution. Drawbacks

7 Introduction The method aims at an as "Lagrange like" behavior as possible in the vicinity of shock fronts, while at the same time keeping the mesh distortions on an acceptable level. The relaxation delay parameter must be defined for general applications of high pressures. Delayed mesh Relaxation in ALE method The method does not require to solve the equation systems and it is well suited for explicit time integration schemes.

8 Introduction u = 0 Eulerian approach u = v Lagrangian approach ALE approach v: Fluid particle velocity, u: Mesh velocity Conservation of momentum Conservation of mass Conservation of energy Equilibrium equations

9 Eulerian and ALE Multi-Material Method Operator split 2 phases of calculations Transport equation Second step: Remap phase Step n+1 Eulerian ALE Step n First step: Lagrangian phase Lagrangian

10 Multi-Material interface tracking In the Young technique, Volume fractions of either material for the cell and its eight surrounding cells are used to determine the slope of the interface. VOF

11 Delayed mesh relaxation technique Lagrangian phase Acceleration Lagrangian node coordinate Material velocity where Mesh relaxation phase Reference system velocity Node coordinate after relaxation is a node coordinate provided by a mesh relaxation algorithm, operating on the Lagrangian configuration at t n+1. is a relaxation delay parameter.

12 Numerical applications Jones Wilkins Lee equation of state A (Mbar)B (Mbar)R1R1 R2R2 E 0 (Mbar) C-4 high explosive JWL parameters Three dimensional C-4 high explosive air blast

13 Numerical applications Three dimensional C-4 high explosive air blast zoom Modeling

14 Numerical applications Three dimensional C-4 high explosive air blast with reflection zoom Modeling

15 Numerical applications Three dimensional C-4 high explosive air blast Pressure propagation

16 Numerical applications Three dimensional C-4 high explosive air blast with reflection Pressure propagation

17 Numerical applications Three dimensional C-4 high explosive air blast Pressure plot at 5 feet

18 Numerical applications Three dimensional C-4 high explosive air blast with reflection Pressure plot at 5 feet

19 Numerical applications Three dimensional C-4 high explosive air blast Overpressure according to relaxation parameter With elements Experimental overpressure = 3.40 bar With elements t 0 =1, µs

20 Numerical applications Three dimensional C-4 high explosive air blast with reflection Overpressure according to relaxation parameter Experimental Overpresure=2.2 bar t 0 =2, µs

21 Conclusions Delaying the mesh relaxation makes the description of motion more "Lagrange like", contracting the mesh in the vicinity of the shock front. In this study, the definition of the relaxation delay parameter has improved for general applications of shock wave: 0.001µs µs -1. Comparing numerical results using delayed mesh relaxation in ALE method to Lagrangian, Eulerian and classical ALE methods shows that this method is the best for problems involving high pressures. This is beneficial for the numerical accuracy, in that dissipation and dispersion errors are reduced.


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