Particle dynamics Particle overview Particle system Forces - PowerPoint PPT Presentation

Particle dynamics

• Particle overview � • Particle system � • Forces � • Constraints � • Second order motion analysis

Particle system • Particles are objects that have mass, position, and velocity, but without spatial extent � • Particles are the easiest objects to simulate but they can be made to exhibit a wide range of objects

A Newtonian particle • First order motion is sufficient, if � • a particle state only contains position � • no inertia � • particles are extremely light � • Most likely particles have inertia and are affected by gravity and other forces � • This puts us in the realm of second order motion

Second-order ODE What is the differential equation that describes the behavior of a mass point? f = m a What does f depend on? x ( t ) = f ( x ( t ) , ˙ x ( t )) ¨ m

Second-order ODE x ( t ) = f ( x ( t ) , ˙ x ( t )) ¨ = f ( x , ˙ x ) m This is not a first oder ODE because it has second derivatives Add a new variable, v ( t ), to get a pair of coupled first order equations { ˙ x = v v = f /m ˙

Phase space ⎡ ⎤ x 1 x 2 Concatenate position and ⎢ ⎥ ⎢ ⎥ � � x x 3 ⎢ ⎥ velocity to form a 6-vector: = ⎢ ⎥ v v 1 ⎢ ⎥ position in phase space ⎢ ⎥ v 2 ⎣ ⎦ v 3 � ˙ � v � � First order differential equation: x = f ˙ velocity in the phase space v m

Quiz • A mass point attached to a spring obeys Hooke’s Law: f = -k (x - x0). What is the ODE that describes this motion?

• Particle overview � • Particle system � • Forces � • Constraints � • Second order motion analysis

Particle structure Particle x position a point in the phase space v velocity f force accumulator mass m

Solver interface solver � system interface solver 6 GetDim particle x x v Get/Set State v f v m Deriv Eval f m

Particle system structure system x 1 x 2 x n v n v 1 v 2 particles ... f n f 1 f 2 m n m 1 m 2 n time

Particle system structure solver � system solver interface 6n GetDim particles x 1 x 2 x n n time Get/Set State . . . v 1 v 2 v n v 1 v 2 v n Deriv Eval f 1 f 2 f n . . . m 1 m 2 m n

Deriv Eval Clear forces: loop over particles, zero force accumulator Calculate forces: sum all forces into accumulator Gather: loop over particles, copy v and f /m into destination array

• Particle overview � • Particle system � • Forces � • Constraints � • Second order motion analysis

Forces • Constant � • gravity � • Position dependent � • force fields, springs � • Velocity dependent � • drag

Particle systems with forces system x 1 x 2 x n v n v 1 v 2 particles ... f n f 1 f 2 m n m 1 m 2 � n time ... forces F 1 F 2 F m

Force structure • Unlike particles, forces are heterogeneous (type-dependent) � • Each force object “knows” � • which particles it influences � • how much contribution it adds to the force accumulator

Particle systems with forces system x 1 x 2 x n v n v 1 v 2 particles ... f n f 1 f 2 m n m 1 m 2 n time ... forces F 1 F 2 F m

Gravity x 1 x 2 x n Unary force: f = m G v 1 v 2 v n f 1 f 2 f n m 1 m 2 m n F Exerting a constant p force on each particle . . . sys apply_fun particle system G p->f += p->m*F->G

Viscous drag At very low speeds for small x 1 x 2 x n v 1 v 2 v n particles, air resistance is f 1 f 2 f n m 1 m 2 m n approximately: F p f drag = − k drag v . . . sys apply_fun particle system k p->f += p->v*F->k

Attraction Act on any or all pairs of particles, depending on their positions l f p = − k m p m q l x p | l | 2 | l | x q f q = − f p l = x p − x q

Attraction x p x q v p v q l f p = − k m p m q f p f q m p m q | l | 2 | l | F p sys apply_fun particle system k

Damped spring x p � � ˙ l · l l f p = − k s ( | l | − r ) + k d | l | | l | r f q = − f p | l | l = x p − x q x q

Damped spring x p x q v p v q � � ˙ l · l l f p f q f p = − k s ( | l | − r ) + k d | l | | l | m p m q F r p k d sys apply_fun particle system k s

Quiz For an ideal spring, what is the force it applies to two particles, p and q, attached to it. Write down the pseudo code for its “apply_fun”. x p x q v p v q f p f q m p m q F r p sys apply_fun particle system k s

Deriv Eval 1. Clear force accumulators x 1 x 2 x n v n v 1 v 2 2. Invoke apply_force ... f n f 1 f 2 functions m n m 1 m 2 ... F 1 F 2 F m 3. Return derivatives to solver � ˙ � v � � x = f ˙ v m

ODE solver Euler’s method: x ( t 0 + h ) = x ( t 0 ) + hf ( x , t ) x t +1 = x t + h ˙ x t v t +1 = v t + h ˙ v t

Euler step solver � system interface solver x t +1 = x t + h ˙ x t GetDim 3. particles v t +1 = v t + h ˙ v t 4. 2. x 1 x 2 x n 5. Advance time Get/Set State . . . v 1 v 2 v n time 1. v 1 v 2 v n Deriv Eval Deriv Eval f 1 f 2 f n . . . m 1 m 2 m n

• Particle overview � • Particle system � • Forces � • Constraints � • Second order motion analysis

Particle Interaction • We will revisit collision when we talk about rigid body simulation � • For now, just simple point-plane collisions

Collision detection Normal and tangential components x v T v N = ( N · v ) N N v N v T = v − v N v

Collision detection Particle is on the legal side if ( x − p ) · N ≥ 0 Particle is within of the wall if � x v T N ( x − p ) · N < � v N v Particle is heading in if p v · N < 0

Collision response Before After collision collision v ′ − k r v N v T v T v N v v ′ = v T − k r v N coefficient of restitution: 0 ≤ k r < 1

Contact Conditions for resting contact: 1. particle is on the collision surface 2. zero normal velocity If a particle is pushed into the contact plane a contact force f c is exerted to cancel the normal component of f N x v p f f N f T

• Particle overview � • Particle system � • Forces � • Constraints � • Second order motion analysis

Linear analysis • Linearly approximate acceleration �  �  �  � d � x x x = f ( , t ) = A + a 0 v v v dt � �  �  �  � d x 0 I x + a 0 = v − K − D v dt � • Split up analysis into different cases � • constant acceleration � • linear acceleration

Constant acceleration • Solution is � v ( t ) = v 0 + a 0 t � x ( t ) = x 0 + v 0 t + 1 2 a 0 t 2 � • v ( t ) only needs 1st order accuracy, but x ( t ) demands 2nd order accuracy

Linear acceleration • When K (or D) dominates ODE, what type of motion does it correspond to? � � � � � � � � � � d x 0 I x x = A = v − K − D v v dt � � • Need to compute the eigenvalues of A

Linear acceleration � � u 1 Assume is an eigenvalue of A , is the α u 2 corresponding eigenvector � � � � � � 0 I u 1 u 1 = α − K − D u 2 u 2 � � u The eigenvector of A has the form α u Assuming D is linear combination of K and I (Rayleigh damping) That means K and D have the same eigenvectors

Linear acceleration � � u For any u , if is an eigenvector of A, the following α u must be true � � � � � � 0 I u u = α − K − D α u α u Now assume u is an eigenvector for both K and D − λ k u − αλ d u = α 2 u � α = − 1 (1 2 λ d ) 2 − λ k 2 λ d ±

Eigenvalue approximation • If D dominates � α ≈ − λ d , 0 � • exponential decay � • If K dominates � √ � α ≈ ± λ k − 1 � • oscillation

Analysis • Constant acceleration (e.g. gravity) � • demands 2nd order accuracy for position � • Position dependence (e.g. spring force) � • demands stability, oscillatory motion � • looks at imaginary axis � • Velocity dependence (e.g. damping) � • demands stability, exponential decay � • looks at negative real axis

Explicit methods • First-order explicit Euler method � • constant acceleration: � bad (1st order) • position dependence: � very bad (unstable) • velocity dependence: � ok (conditionally stable) • RK3 and RK4 � • constant acceleration: � great (high order) • position dependence: � ok (conditionally stable) • velocity dependence: ok (conditionally stable)

Implicit methods • Implicit Euler method � • constant acceleration: � bad (1st order) • position dependence: � ok (stable but damped) • velocity dependence: � great (monotone) • Trapezoidal rule � • constant acceleration: � great (2nd order) • position dependence: � great (stable and no damp) • velocity dependence: good (stable, not monotone)