Basic Concepts in Viscous Flow

Microhydrodynamics The fluid dynamic equations Properties of Stokes flow Three Stokes-flow theorems References

Basic concepts in viscous ﬂow

Elisabeth´ Guazzelli and Jeﬀrey F. Morris with illustrations by Sylvie Pic

Adapted from Chapter 1 of A Physical Introduction to Suspension Dynamics Cambridge Texts in Applied Mathematics

Elisabeth´ Guazzelli and Jeffrey F. Morris with illustrations by Sylvie Pic A Physical Introduction to Suspension Dynamics Basic concepts in viscous flow Microhydrodynamics The fluid dynamic equations Properties of Stokes flow Three Stokes-flow theorems References

1 The fluid dynamic equations Navier-Stokes equations Dimensionless numbers Stokes equations Buoyancy and drag Boundary conditions 2 Properties of Stokes flow Linearity Reversibility Instantaneity 3 Three Stokes-flow theorems Minimum dissipation Uniqueness Reciprocity 4 References

Microhydrodynamics

Microhydrodynamics, a term coined by G. K. Batchelor in the 1970s, deals with processes occurring in fluid flow when the characteristic length of the flow field is of the order of one micron

Many particles in a ﬂow

Navier-Stokes equations Equations for an incompressible ﬂuid

Continuity equation for an incompressible ﬂuid

∇ · u = 0

Equation for conservation of momentum

∂u ρ[ + (u ·∇)u] = f + ∇ · σa ∂t = f −∇pa + µ∇2u

The superscript a indicates an absolute pressure and a corresponding absolute stress tensor. The term ‘absolute stress’ is used to indicate the actual stress (with the absolute pressure being the true pressure) rather than a modiﬁed stress to be deﬁned in slide 12 in which the hydrostatic pressure is removed.

Navier-Stokes equations Newtonian ﬂuid

Constitutive equation for a Newtonian ﬂuid

a a σij = −p δij + 2µeij

Symmetric stress tensor

a a σij = σji

Symmetric rate-of-strain tensor

1 ∂ui ∂uj eij = eji = + 2 ∂xj ∂xi

Navier-Stokes equations Navier-Stokes equations

Incompressibility

∇ · u = 0

Momentum equation ∂u ρ + ρ(u ·∇)u = f −∇pa + µ∇2u ∂t

Dimensionless numbers Reynolds number

|ρ(u ·∇)u| UL Re = = |µ∇2u| ν

For sedimenting (spherical) grain of sand of size L ∼ 1µm settling in water at velocity U ∼ 1µm/s, Re ∼ 10−6

Dimensionless numbers Stokes number

|ρ∂u/∂t| L2 St = = |µ∇2u| T ν

Sphere of radius a = 1µm sedimenting in water at stationary regime when time T ≫ a2/ν ∼ 10−6 s

Stokes equations Stokes equations

Stokes equations: Re ≪ 1 and St ≪ 1

∇ · u = 0 ∇ · σa = −∇pa + µ∇2u = f

Homogeneous Stokes equations: f = ρ g ⇒ p = pa − ρ g · x

∇ · u = 0 ∇ · σ = −∇p + µ∇2u = 0

Stokes equations Other expressions for Stokes equations

Stress tensor σ

σij = σji = −pδij + 2µeij

Rate-of-strain tensor e

1 ∂ui ∂uj eij = eji = ( + ) 2 ∂xj ∂xi

Stokes equations

∇ · u = 0 or eii = 0 a a ∂σij ∇ · σ = f or = fi ∂xj

Buoyancy and drag Buoyancy and drag

drag a 3 F = (σ +ρg ·xI)·n dS = σ ·n dS = −4πa (ρp −ρ)g/3 ZSp ZSp

Boundary conditions Boundary conditions

No-slip boundary condition on the particles:

p p u(x)= U + ω × (x − xp)

at the surface of a particle with center of mass at xp

+ Outer boundary condition on a containing vessel or at inﬁnity

Linearity Linearity

Linearity of the Stokes equations: no non-linear convective acceleration term (u ·∇)u Principle of superposition: by adding diﬀerent solutions of the Stokes equations one obtains also a solution of the Stokes equations Reversibility: the motion is reversible in the driving force

Linearity Streamlines in a porous medium A doubling of the driving pressure gradient yields a doubling of the ﬂow rate but no change to the streamlines

Linearity Principle of superposition Summation of translation and rotation

(ﬂuid motions)

Reversibility Description of G. I.Taylor ﬁlm on reversibility

Reversibility Reversibility argument for a sphere settling near a wall A spherical particle falling adjacent to a wall falls at constant distance, as shown by the reversibility principle depicted visually here

Reversibility Reversibility argument for a sphere in a Poiseuille flow A single neutrally-buoyant spherical particle in Poiseuille flow stays at a fixed distance from the wall, as shown by the reversibility principle depicted visually here

Instantaneity Instantaneity

No history dependence ∂u/∂t The ﬂow is determined by the conﬁguration given by the boundary conditions, coming both from the particle positions and outer boundaries

Information from boundary motion communicated to inﬁnity instantly Divergence of the homogeneous momentum equation ⇒∇2p = 0. Curl of the homogeneous momentum equation ⇒∇2ω˜ = 0 withω ˜ = ∇× u. Pressure p and vorticityω ˜ are harmonic

Minimum dissipation Kinetic energy balance

Kinetic energy:

u2 K = ρ dV ZV 2 where V is the ﬂuid volume

Calculation for unsteady Stokes case: ∂u ρ = f + ∇ · σ ∂t

K ∂ · ∂u dV f u dV u ∂σij dV t = ρ u t = i i + i x ∂ ZV ∂ ZV ZV ∂ j u f u dV U nout dS − ∂ i dV = i i + i σij j x σij ZV IS ZV ∂ j

Minimum dissipation Rate of energy dissipation

Rate of energy dissipation due to viscosity

∂ui Φ= σij dV = eij σij dV = 2µeij eij dV ≥ 0 ZV ∂xj ZV ZV

Steady Stokes ﬂow (∂K/∂t = 0)

Du out ρu · dV = fi ui dV + Ui σij nj dS − Φ = 0 ZV Dt ZV IS

Rate of energy dissipation = rate of working by external forces

Φ= f · u dV + U · (σ · nout )dS ZV IS

Minimum dissipation Minimum dissipation theorem

Consider u and uS two velocity ﬁelds in the volume V such as ∇ · u = ∇ · uS = 0 in the volume V u = uS = U on the surface S limiting V uS satisfying the homogeneous Stokes equations (no external force f) The minimum dissipation theorem states that the Stokes ﬂow corresponds to the least dissipation

S S 2µ eij eij dV ≤ 2µ eij eij dV ZV ZV

Minimum dissipation Proof of minimum dissipation theorem