EN221 Summary

1 Tensor Stuﬀ

• Divergence : T ∇ · u = ∂iui ∇ · u = u nda, ZR Z∂R T ∇ · T = ∂iTije j ∇ · T = T nda. ZR Z∂R ∇ ⊗ u = Jacobian ∇ ⊗ u = u ⊗ nda, ZR Z∂R (matrix divergence: columns stay separate) • Box product : [ a , b, c] = a · ( b ∧ c) • Levi-Civita tensor:

δi, 1 δj, 1 δk, 1 1 ( ijk) an even permut. of ( 123) , εijk = det δi, 2 δj, 2 δk, 2  = [ ei , e j , ek ] =  − 1 ( ijk) an odd permut. of ( 123) , δ δ δ  0 if not.  i, 3 j, 3 k, 3  e j ∧ ek = εijkei.  det( abc) = εijkaibjck

εijkεilm = δjl δkm − δjmδkl ,

εijkεijl = 2δkl ,

εijkεijk = 6 • Principal Invariants:

IA = λ 1 + λ2 + λ 3 = ([ Aa, b, c] + [ a , Ab, c] + [ a , b, Ac])/[ a , b, c] = tr A, 1 II = λ λ + λ λ + λ λ = ([ Aa , Ab, c] + [ Aa , b, Ac] + [ a , Ab, Ac])/[ a , b, c] = [ tr2 A − tr A2 ] , A 1 2 2 3 1 3 2 IIIA = λ 1 λ 2 λ 3 = [ Aa , Ab, Ac]/[ a , b, c] = det A.

T • Adjugate/Cofactor of a Tensor: A∗ ( a ∧ b) = ( Aa) ∧ ( Ab) ⇒ A∗ = det A( A− ) . 1 ∂t det A( t) = det A tr(( ∂tA) A− ) • Tensor Product : TO ⊗ FROM

T ei ⊗ e j = eie j ( u ⊗ v) a = u( v · a) ( u ⊗ v)( w ⊗ x) = v · w( u ⊗ x) ( u ⊗ v) A = u ⊗ ( ATv) • Skewsymmetric matrices: Rotation around axis Ω given by orthogonal matrix Q( t) . T x˙ = Q˙ x ⇒ ∂t( Q Q) = 0, W = Q˙ Q T, W = − WT. Wx = Ω ∧ x.

2 Kinematics

2.1 Static • Reference and deformed conﬁgurations.

1 2 Section 2

• Deformation gradient : assumed regular. J = det F 0. x( X) = X + u( X) , F = ∇X ⊗ x( X) , 1 F− = ∂x jXβEβ ⊗ ei • Isochoric : J = 1 . • Polar decomposition : ◦ F = RU, T 2 1 F F = U , R = FU − . ◦ F = V R. Features: ◦ Is unique. ◦ R is rotation of principal axes. ◦ R average of all rotations.

◦ Principal axes of V are Rui. ◦ σ( V) = σ( U) .

◦ R = vk ⊗ uk .

◦ F = λ kvk ⊗ uk . • Left/Right Cauchy-Green Deformation Tensor: FFT/FTF SPD. • Strain : 1 E = ( FTF − Id) ( Lagrangean: | dx | 2 − | dX | 2 = 2dX · EdX) , 2 1 E = ( Id − F TF 1 ) ( Eulerian: | dx | 2 − | dX | 2 = 2dx · E dx) . ′ 2 − − ′ • Stretch: λ( M) = ( M · FTFM) 1/2 = | UM | . Has local maxima and minima when M is an eigenvector of U. • Transformation of area elements:

nda = F∗ NdA • Deformation gradient in cylindrical coordinates: Given r  θ  = f( R, Θ,Z) , z we have   1 F = ∂ x ⊗ E + ∂ x ⊗ E + ∂ x ⊗ E . R R R Θ Θ Z Z

Also expressible as mixed tensor from E( R, Θ ,Z ) to E( r,θ,z) :

∂r 1 ∂r ∂r ∂R R ∂Θ ∂z  r ∂θ r ∂θ r ∂θ  F = . 1 ∂R R ∂Θ 1 ∂z  ∂z 1 ∂z ∂z     ∂R R ∂Θ ∂Z 

Caveat for mixed tensors: tr( F) Fii. However det, V, U as usual. Also works for spherical basis, but more complicated. Kinematics 3

2.1.1 Static Examples 1 • Pure shear: F = λe1 ⊗ e1 + λ − e2 ⊗ e2 .

• Simple shear: F = Id + λe1 ⊗ e 2 . • Pure bending: x ( R − Y) sin α( x)  y  =  R − ( R − Y) cos α( x)  ,J = ( R − Y) α ′. z Z     • Tension and torsion : X cos α λZ − Y sin α λZ x  √λ l √λ l  X α Y α  y  = sin λZ + cos λZ .  √λ l √λ l  z      λZ    • Turning a cylinder inside out .

2.2 Dynamic • Steady motion : ∂/∂t v( x , t) = 0. • Material/Lagrangean POV : focus on particle, expressions in terms of X and t → Solids. • Spatial/Eulerian POV : focus on point in space, expressions in terms of x and t → Fluids. • Lines: ◦ Path line : Curve traced by a ﬁxed particle. ◦ Streamlines: Field lines of velocity in Eulerian POV. Both coincide under steady motion. • Material derivative : ∂ϕ ϕ˙ = + ∇xϕ · v , ∂t ∂w w˙ = + ( ∇x ⊗ w) v , ∂t ∂T T˙ = + ( ∇x ⊗ T) v. ∂T • Acceleration : a = v˙ .

• Velocity gradient : L = ∇x ⊗ v ⇒ F˙ = LF (chain rule). F requires a “reference state”, L does not. • dx˙ = F˙ dX = LFdX = Ldx. Assume dx = m| dx | . | dx | Strain rate : • = m · Lm = m · Dm | dx | m˙ = Lm − m( m · Lm)

• Stretch and Spin : L = D + W, D = DT, W = − WT. D11 : stretching rate of a line element along the 1-direction D12: (roughly) change in angle between the 1- and 2-direction. Principal axes pi of D are rigidly rotating about 1 ω = curl v 2 with Wpi = ω × pi. • Vorticity : curl v = 2 · angular velocity. (Letter here is also ω.) 4 Section 3

• J˙ = J tr L = J div v. • Integrals over moving contours:

v · dx = v( x , t) · FdX ICt ICR d v · dx = ( v˙ ( x , t) · F + v( x , t) · LF) dX dt ICt ICR = v˙ ( x , t) + LTv( x , t)dx ICt T • Integrals over moving surfaces: Similar, taking into account that F∗ = JF− . d u · nds = ( u˙ + u tr( L) − Lu) · nds. dt ZSt ZSt

• Integrals over moving volumes/Reynolds’ Transport Theorem :

d ϕ( x)dv = ϕ˙ + ϕ tr( L)dv. dt ZR t ZRt Observe that tr( L) = div v, which is zero in the incompressible case. • Circulation :

v · dx = curl v · ds = ω · ds ICt ZSt ZSt 1 LTv = ∇v2 2 if circulation-preserving d 0 = v · dx = v˙ ( x , t) + LTv( x , t)dx dt ICt ICt 1 = v˙ ( x , t)dx + ∇v2 dx ICt ICt 2 = a( x , t)dx ICt = curl a · ds ZSt If a = ∇ψ, then the motion is circulation-preserving. If circulation-preserving, then curl a = ω˙ + ωtr( L) − Lω = 0. Then consider the product rule on d 1 ( JF ω) = = 0 dt − to ﬁnd Cauchy’s vorticity formula : 1 ω = Fω . J ref Field lines of vorticity are vortex lines. If the motion is circulation-preserving, these are material curves.

3 Balance Laws and Field Equations

• Conservation of Mass: Assumption:

Jρ = ρref ( referential) . Balance Laws and Field Equations 5

Therefore,

ρ˙ J + ρJ˙ = 0 ρ˙ J + ρJ div v = 0 ρ˙ + ρ div v = 0

∂tρ + div( ρv) = 0. • Transport Theorem with density : d ρΦdv = ρΦ˙ dv. dt ZRt ZRt • Linear Momentum : M = ρv

◦ Stress vector: t( n) is force/unit area. ◦ Balance law : d ρv = ρv˙ = ρbdv + t( n) da. dt ZRt ZRt ZR t Z∂Rt

T ◦ Stress tensor/Cauchy’s Theorem : σ n = t( n) . Derivation:

− t( n) = − t( n) by pillbox and balance law. − − Tetrahedron argument: n the general normal of the coordinate-system-boxed tetrahedron. Then other faces ai = ani, where a is the area of the complicated face. Volume h a/3. Apply 1/a · balance law, let h → 0. Assume continuity, derive linear depen- dence by assuming values are locally constant. ◦ Updated balance law: ρa = ρb + ∇ · σ ZR t ZR t ◦ Field equations:

ρrefx¨ = ∇X · s + ρrefb ( referential) ρa = ∇x · σ + ρb ( spatial)

1 T T • Nominal Stress/Conjugate stress: s = JF− σ. ( σ nda = s NdA can be directly verified.) Also called Piola-Kirchoff stress. s T is 2nd Piola-Kirchoff stress. • Angular Momentum : H = ρx ∧ v ◦ Non-polar material : no contact torques. ◦ Balance law : ( ) d ∗ ρx ∧ v = ρx ∧ v˙ = ρ( x ∧ b + c)dv + x ∧ t( n) da dt ZRt ZRt ZRt Z∂Rt c is body torque. Equality ( ∗ ) follows because x-derivatives vanish once ∧ v is applied. ◦ Subsituting Cauchy’s Theorem into the balance law gives

x ∧ ( ∇x · σ + ρb) = ρ( x ∧ b)dv + x ∧ σnda ZRt ZRt Z∂Rt

x ∧ ( ∇x · σ) x ∧ σnda ZR t Z∂R t T View in component form, apply Gauß, derive εijkσji = 0 ⇒ σ = σ . ◦ Field equations:

s TFT = F s ( referential) σT = σ ( spatial) 6 Section 3

• Vector identities: 1 ( A · ∇) A = ∇| A | 2 + ( ∇ × A) × A 2 ( A · ∇) A = ( ∇ ⊗ A) A. Use these identities to rewrite the v˙ as ∇( v 2 ) for irrotational flow. • Types of fluid flow : ◦ Inviscid : σ = − p Id ⇒ div σ = − ∇p. ◦ Incompressible : ρ˙ = 0 or div v = 0.

◦ Steady : ∂tv = 0. ρ˙ = v · ∇ ρ. ◦ Irrotational : ω = 0 or v = ∇ϕ. ◦ Elastic : p( ρ) ◦ Ideal=incompressible : div v = 0, J = 1 . • Rayleigh-Plesset equation : Begin with deformation of spherical shell (with extent!), assume J ≡ 1 . Derive ODE. • Conservative potentials: b = − ∇β ◦ Elastic or ideal ﬂow here is circulation preserving, i.e. a = − ∇something. − Have 1 a = − ∇p( ρ) + b ρ 1 = − p ( ρ) ∇ ρ − ∇ β. ρ ′ − Deﬁne ρ 1 ε( ρ) = p′( ρ′)dρ ′ Z0 ρ ′ ⇒ ∇ε( ρ) = ε ′( ρ) ∇ρ − Therefore a = − ∇( ε( ρ) + β) .

− For ideal ﬂuid substitute p/ ρ0 for ε. ◦ Bernoulli’s Theorem : − Flow irrotational (i.e. v = − ∇ ϕ):

v2 ∇ ∂tϕ + + ε( ρ) + β = 0. 2

Proof: Just rewrite, obtaining v2 /2 from second term of material derivative. − Flow steady: v2 • + ε( ρ) + β = 0, 2 i.e. this quantity is constant along streamlines. Proof: Exploit v · v˙ = v · ∇( v2 ) − Flow both irrotational and steady: v2 ∇ + ε( ρ) + β = 0. 2 Balance Laws and Field Equations 7

• Acoustic wave equation :

◦ Assume ρ = ρ0 + δρ, | v | ≪ 1 , |∇v | ≪ 1 .

◦ Start with ∂t ( ∇ · v) , use cons. of. momentum without second order term, cons. of mass as ∂tρ + ρ0div( v) = 0. 2 2 ◦ δρtt = c ∇ δρ, with c = ∂ρp. p • Mach number: assume steadiness b = 0, use v · v˙ in terms of c2 .

v2 v · ( ρv) = v · v˙ ρ 1 −

•  c2 

 m 4  ◦ Supersonic nozzle m < 1 , m > 1 . • Conservation of Energy : ◦ Balance law : d K( R ) = − S( R ) + P( R ) dt t t t d 1 ρv · vdv = − tr( σD)dv + ρb · v + σn · vda

dt 2 Z ZR t ZRt Z∂Rt

Kinetic energy K( t) Stress power S( Rt ) Power supplied P ( Rt )

Proof: Multiply Equation of Motion by v, integrate by parts in the σ term. ◦ Field equation : 1 •

ρ v · v + tr( σD) = ∇x · ( σv) + ρb · v. 2

Stress Power Rate-of-working Kinetic Energy

◦ Key words for more global energy conservation: internal energy U( Rt) , heat supply per unit mass H( Rt) , heat ﬂux through material surface. d { K + U } = P( R ) + H( R ) . dt t t Now, because there is a stress power loss above, there needs to be a gain here: d U( R ) = S( R ) + H( R ) . dt t t t • Jump conditions: For the balance law d ρπ = ρs + f( n) , dt ZR t ZR t Z∂R t we get

[ ρVπ + f( n) ] = 0.

Vn interface speed, V = Vn − v · n. Mass Mom. A.Mom. Energy π v x v ε 1 v v quantity per unit mass 1 ∧ + 2 · s supply of π per unit mass 0 b x ∧ b b · v + r f( n) inﬂux of π per unit area 0 t( n) x ∧ t( n) t( n) · v + h( n) so that for example

[ ρV] = 0,

[ ρVv + t( n) ] = 0.

Or for material jumps: [ t( n) ] = 0. 8 Section 4

Derivation: ◦ Modiﬁcation for moving boundary is

− [ ρπ] Vn. Zjump surface

◦ Then use pillbox that ﬂattens around surface. Examples: ◦ Free boundary: pressure must be continuous, because otherwise there’s a ﬁnite force on something massless. • Stokes waves: ◦ Assume v = ∇ ϕ. ◦ Conservation of mass ∇2 ϕ = 0. ◦ Bernoulli’s equation v2 p ∂tϕ + + + β = const 2 ρ0 ◦ BCs: z depthward, z = η free surface

− ϕz = 0 at bottom d z η z η ∂ ϕ ∂ η z − dt ( − ) = 0 at = → t = t at = 0 (!). − pressure continuous at interface. Use Bernoulli’s equation to rewrite as condition

∂tϕ + gη = 0 at z = 0.

◦ Surface tension : p1 − p2 = − γ · curvature. ◦ Rayleigh-Taylor instability : Large density over small density. ◦ Kelvin-Helmholtz instability : Wave formation.

4 Constitutive Laws

• Observer: A reference frame/coordinate system w.r.t. which vectors and tensors are seen.

x∗ = c( t) + Q( t) x

so, for example, F∗ = QF, J∗ = J, U∗ = U, R∗ = Q R. • Objective ﬁelds:

ϕ∗ ( x∗ ) = ϕ( x)

u∗ ( x∗ ) = Qu( x) T A( x∗ ) = Q A( x) Q Examples: D, regions, normals, σ Non-examples: v = c˙ + Q v, L = Q L Q T + Q˙ Q T, W. • Constraint stress: ◦ Must be workless, i.e. tr( N D) = 0

T ◦ Constraint given as λ( C) = 0 → λ˙ = tr( λ CC˙ ) = 0, where C = F F. T T ◦ C˙ = 2F DF ⇒ N = αFλCF . • Fluid : σ = g( L) . Constitutive Laws 9

Cannot support shear stress at equilibrium. If ideal , also cannot support shear stress when in motion.

◦ Objectivity : σ∗ = g( L∗ ) . − Choose Q = Id, Q˙ = − W to obtain that g( L) = g( D) . ◦ Most general such g: σ( D) = αI + βD + γD2 , with α, β, γ functions of the invariants of D. Proof: Cayley-Hamilton. ◦ Incompressible fluid : σ = − p Id. ◦ Ideal fluid : σ = − p( ρ) Id ◦ Newtonian fluid : σ = − p( ρ) Id + 2 µD ◦ Navier-Stokes equation : ρa = − ∇ p + µ∆v + ρb plus conservation of mass. 2 ◦ Rescaling x˜ = x/l, v˜ = v/v, p = p/( ρ0 v ) , t˜ = t/l. Then kinematic viscosity is ν = µ/ p. ◦ Reynolds number: Re = l v/ν. High: Dominated by inertial effects. Low: Dominated by viscous effects. ◦ No-slip BCs apply only for viscous fluids. ◦ Wiggling plate : Watch for emergence of a boundary layer. • Solid : σ = f( F) ◦ Material Symmetry : P ∈S, where S is the symmetry group of the material. σ = f( F) = f( FP)

Isotropic Material : S = SO(3) . Then choose P = RT ⇒ σ = f( F) = f( V) .

◦ Objectivity : σ∗ = f( V∗ ) . Most general expression to satisfy this: σ( V) = αId + βV + γV2 , with α, β, γ functions of the invariants. ◦ Lamé constant/Young’s Modulus: Linearization! F = Id + ∇u 1 1 E = FTF − Id ≈ ∇u − ( ∇u) T 2 2 V ≈ Id + E 1 R ≈ Id + ∇u − ( ∇u) T 2 Use these in 2 σ = c0tr V Id + c1 V + c3V ≈ λtr( E) Id + 2 µE, where λ, µ are the Lamé constants. 10 Section 4

◦ Strain energy per volume : W( F) ← the usual way to specify constitutive relations for solids Then 1 ∂W 1 ∂W σ = · FT = · V

J ∂F J ∂V ∂W R ∂V Invoke objectivity: W( F) = W( U) Invoke isotropy: W( F) = W( V) ⇒ W depends only on invariants of V. ⇒ W’s principal axes line up with those of V, i.e. principal stresses k principal stretches:

1 ∂W( λ1 , λ 2 , λ 3) σα = λ α . J ∂λ α Incompressible: 1 ∂W( λ 1 , λ 2 , λ 3) σα = λ α − p. J ∂λ α Specifying W in terms of B = FFT: 2 σ = III W Id + ( W + I W ) B − W B2 , J B IIIB IB B IIB IIB where subscripts by IB , IIB , IIIB mean partial derivatives. ◦ Neo-Hookean material : 1 1 W = µ λ 2 + λ2 + λ 2 − 3 − 2ln( J) + µ ( J − 1) 2 , 2 1 2 3 2 ′ 2 unconstrained: σi = µ( λ i − 1) + µ ′J ( J − 1) , 2 incompressible: σi = µλ i − p. ◦ Solving a solids problem: − Calculate F (Kinematics) − Calculate B = FFT − Calculate σ − Apply conservation of momentum in deformed conﬁguration. Solve for unknowns x, p, using BCs.