Slow Attractive Canonical Invariant Manifolds for Hydrogen-Air Kinetics J. M. Powers and S. Paolucci Department of Aerospace and Mechanical Engineering University of Notre Dame, USA Fifth International Workshop on Model Reduction in Reacting Flows Berlin, Germany 29 June 2015 partial support from C-SWARM/NNSA 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 1 / 27
Taxonomy We seek Slow Attractive Canonical Invariant Manifolds (SACIMs) for realistic spatially homogeneous gas phase kinetic systems. Invariant Manifolds (IMs) are sets of points which are invariant under the action of an underlying dynamic system. Any trajectory of a dynamic system is an IM. IMs may be locally or globally fast or slow, attracting or repelling. Slow or fast does not imply attracting or repelling and vice versa . We will evaluate the fast/slow and attracting/repelling nature of Canonical Invariant Manifolds (CIMs) constructed by connecting equilibria to determine heteroclinic orbits (Davis-Skodje, J. Chem. Phys. , 1999). 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 2 / 27
Taxonomy, cont. It is relatively easy to construct CIMs by numerical integration. Many CIMs exist, but we are only interested in those that connect to physical equilibrium. It is desirable to identify those CIMs to which dynamics are restricted to those which are slow , and neighboring trajectories are rapidly attracted . We call such CIMs Slow Attracting Canonical Invariant Manifolds (SACIMs). A global SACIM may represent the optimal reduction potentially enabling dramatic accuracy and efficiency in multiscale problems. Manifolds identified by Davis-Skodje construction are guaranteed to be CIMs; they are not guaranteed to be SACIMs, even locally! See Powers, Paolucci, Mengers, Al-Khateeb, J. Math. Chem. , 2015 for a full exposition of our results here. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 3 / 27
Theoretical framework for spatially homogeneous combustion within a closed volume d z z , z o , f ∈ R N . dt = f ( z ) , z (0) = z o , z represents a set of N specific mole numbers, assuming all linear constraints have been removed. f ( z ) embodies the law of mass action and other thermochemistry. f ( z ) = 0 defines multiple equilibria within R N . f ( z ) is such that a unique stable equilibrium exists for physically realizable values of z ; the eigenvalues of the Jacobian J = ∂ f ∂ z , are guaranteed real and negative at such an equilibrium (Powers & Paolucci, Am. J. Phys. , 2008). 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 4 / 27
SACIM construction strategy: heteroclinic orbit connection Davis and Skodje suggested a CIM construction strategy. Sink It employs numerical M I C A S integration from a saddle to the sink. This guarantees a CIM. Saddle It may be a SACIM. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 5 / 27
Failure of SACIM construction strategy It may not be a SACIM. The CIM will be attracting in Sink the neighborhood of each CIM equilibrium. The CIM need not be attractive away from either Saddle equilibrium. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 6 / 27
Sketch of a volume locally traversing a nearby CIM Sink CIM Saddle The local differential volume 1) translates, 2) stretches, and 3) rotates. Its magnitude can decrease as it travels, but elements can still be repelled from the CIM. All trajectories are ultimately attracted to the sink. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 7 / 27
Local decomposition of motion d z = f ( z ) , z (0) = z o , z o ∈ CIM , dt d dt ( z − z o ) = f ( z o ) + J s | z o · ( z − z o ) + J a | z o · ( z − z o ) + . . . . � �� � � �� � � �� � translation stretch rotation Here, we have J = ∂ f ∂ z = J s + J a , J s = J + J T J a = J − J T , . 2 2 The symmetry of J s allows definition of a real orthonormal basis. In 3d, the rotation vector ω of the anti-symmetric J a defines the axis of rotation; can be extended for higher dimensions. Geometric interpreta- tion for 4d or greater is unclear ; we confine to 3d or lesser. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 8 / 27
Stretching rates See Adrover, et al. J. Comp. Phys. , 2007. The local relative volumetric stretching rate is 1 dv ˙ dt ≡ ln v = tr J = tr J s . v The stretching rate σ associated with any unit vector α is σ = α T · J · α = α T · J s · α . The above result is general; α need not be an eigenvector of J or J s , and σ need not be an eigenvalue of J or J s . If they were eigenvalue/eigenvector pairs of J s , they would represent the principal axes of stretch and the associated principal values. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 9 / 27
Stretching rates, cont. Consider now the motion along a given CIM: The unit tangent vector, α t , need not be a principal axis of stretch. The tangential stretching rate, σ t = α T t · J s · α t , can be positive or negative. The normal stretching rates, σ n,i = α T n,i · J s · α n,i , can be positive or negative. The sum of stretching rates equals the relative volumetric stretching rate: ˙ ln v = tr J = tr J s = σ t + σ n, 1 + · · · + σ n,N − 1 . 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 10 / 27
Necessary conditions for a SACIM For a slow CIM, attraction to the CIM must be faster than motion on the CIM (a type of normal hyperbolicity ): κ ≡ min i | σ n,i | > 1 . | σ t | for an attractive CIM, either all normal stretching rates, σ n,i , must be negative, σ n,i < 0 , i = 1 , . . . , N − 1 , or, if some of the normal stretching rates are positive, then the relative volumetric stretching rate must be negative, ˙ ln v < 0 , and the component of the rotation velocity in the normal plane must be much greater than the largest normal stretching rate, max i σ n,i = || Q T n · J a · Q n || || J na || ω µ ≡ = max i σ n,i > 1 . max i σ n,i 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 11 / 27
Procedure for local SACIM identification Identify all equilibria f ( z ) = 0 . Determine the Jacobian, J = ∂ f /∂ z . Evaluate J near each equilibrium to determine its source, sink, saddle, etc. character. Numerically integrate from candidate saddles into the unique physical sink to determine a CIM, z CIM : a candidate SACIM. Numerically determine the unit tangent, α t , along the CIM: f ( z CIM ) α t = || f ( z CIM ) || . Determine the tangential stretching rate, σ t , via σ t = α T t · J s · α t = α T t · J · α t . 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 12 / 27
Procedure for local SACIM identifcation, cont. Use a Gram-Schmidt procedure to identify N − 1 unit normal vectors, thus forming the orthonormal basis { α t , α n, 1 , . . . , α n,N − 1 } . Note that α n,i are not eigen-directions of J , so the procedure works for non-normal systems, though questions remain for highly non-normal, near singular systems. Form the N × ( N − 1) orthogonal matrix Q n composed of the unit normal vectors . . . . . . . . . . . . . . Q n = . . α n, 1 α n, 2 α n,N − 1 . . . . . . . . . . . . 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 13 / 27
Procedure for local SACIM identification, conc. Form the reduced ( N − 1) × ( N − 1) Jacobian J ns for the motion in the hyperplane normal to the CIM: J ns = Q T n · J s · Q n . Find the eigenvalues and eigenvectors of J ns . The eigenvalues give the extreme values of normal stretching rates σ n,i , i = 1 , . . . , N − 1. The normalized eigenvectors of J ns give the directions associated with the extreme values of normal stretching, α n,i . We have thus σ n,i = α T n,i · J · α n,i = α T n,i · J s · α n,i , i = 1 , . . . , N − 1 . Identify J na and then ω . 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 14 / 27
Example: simplified H 2 -air kinetics From Ren, Pope, et al., J. Chem. Phys. , 2006, 6 reactions: O + H 2 H + OH , ⇌ H 2 + OH H 2 O + H , ⇌ O + H 2 O 2OH , ⇌ H 2 + M 2H + M , ⇌ O + H + M OH + M , ⇌ H + OH + M H 2 O + M . ⇌ 6 species: H 2 , O, H 2 O, H, OH, N 2 . 3 elements: H, O, N T = 3000 K, P = 1 atm. Constraints give reduction to 3 ODEs for z H 2 , z O , and z H 2 O . d z /dt = f ( z ). 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 15 / 27
Results for H 2 -air kinetics R 6 -3 ×10 R 7 4 z 3 (mol/g) -3 2 ×10 0 (mol/g) 0 CIM=SACIM? 4 R 1 6 z 4 2 2 0 -3 z 1 (mol/g) ×10 We identified a CIM for this system ( J. Chem. Phys. , 2009). Is this CIM a SACIM? 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 16 / 27
CIM identification: Time-evolution from two distinct saddles to the physical sink ×10 -3 ×10 -3 H 2 O 4 H 2 O 4 H 2 z i (mol/g) O z i (mol/g) 2 2 H×10 -2 0 O 0 H 2 ×10 -2 OH -2 OH H -2 0 2 4 6 8 10 0 1 2 3 a) ×10 -3 ×10 -3 b) t (s) t (s) Trajectories originating near both unphysical saddles approach the same unique physical equilibrium. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 17 / 27
Relative volumetric deformation rate along the CIMs ×10 7 -1 ×10 7 -1 ln v (1/s) ln v (1/s) -3 -2 -5 -3 0 2 4 6 8 10 0 1 2 3 a) ×10 -3 b) ×10 -3 t (s) t (s) Volumes are shrinking everywhere on the CIMs during the approach to physical equilibrium. 5th IWMRRF – Berlin, Germany SACIMs 19 June 2013 18 / 27
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