Fundamentals of Fluid Dynamics: Elementary Viscous Flow Introductory Course on Multiphysics Modelling

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow

TOMASZ G.ZIELINSKI´

bluebox.ippt.pan.pl/˜tzielins/

Institute of Fundamental Technological Research of the Polish Academy of Sciences Warsaw • Poland 2 Navier–Stokes equations Continuity equation Cauchy’s equation of motion Navier–Stokes equations of motion Boundary conditions (for incompressible flow) Compressible Navier–Stokes equations of motion Small-compressibility Navier–Stokes equations Complete Navier–Stokes equations Boundary conditions for compressible flow 3 Reynolds number 4 Features of viscous flow Viscous diffusion of vorticity Convection and diffusion of vorticity Boundary layers

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Outline

1 Newtonian fluids Newtonian fluids and viscosity Constitutive relation for Newtonian fluids Constitutive relation for compressible viscous flow 3 Reynolds number 4 Features of viscous flow Viscous diffusion of vorticity Convection and diffusion of vorticity Boundary layers

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Outline

1 Newtonian fluids Newtonian fluids and viscosity Constitutive relation for Newtonian fluids Constitutive relation for compressible viscous flow 2 Navier–Stokes equations Continuity equation Cauchy’s equation of motion Navier–Stokes equations of motion Boundary conditions (for incompressible flow) Compressible Navier–Stokes equations of motion Small-compressibility Navier–Stokes equations Complete Navier–Stokes equations Boundary conditions for compressible flow 4 Features of viscous flow Viscous diffusion of vorticity Convection and diffusion of vorticity Boundary layers

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Outline

1 Newtonian fluids Newtonian fluids and viscosity Constitutive relation for Newtonian fluids Constitutive relation for compressible viscous flow 2 Navier–Stokes equations Continuity equation Cauchy’s equation of motion Navier–Stokes equations of motion Boundary conditions (for incompressible flow) Compressible Navier–Stokes equations of motion Small-compressibility Navier–Stokes equations Complete Navier–Stokes equations Boundary conditions for compressible flow 3 Reynolds number Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Outline

1 Newtonian fluids Newtonian fluids and viscosity Constitutive relation for Newtonian fluids Constitutive relation for compressible viscous flow 2 Navier–Stokes equations Continuity equation Cauchy’s equation of motion Navier–Stokes equations of motion Boundary conditions (for incompressible flow) Compressible Navier–Stokes equations of motion Small-compressibility Navier–Stokes equations Complete Navier–Stokes equations Boundary conditions for compressible flow 3 Reynolds number 4 Features of viscous flow Viscous diffusion of vorticity Convection and diffusion of vorticity Boundary layers Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Outline

1 Newtonian fluids Newtonian fluids and viscosity Constitutive relation for Newtonian fluids Constitutive relation for compressible viscous flow 2 Navier–Stokes equations Continuity equation Cauchy’s equation of motion Navier–Stokes equations of motion Boundary conditions (for incompressible flow) Compressible Navier–Stokes equations of motion Small-compressibility Navier–Stokes equations Complete Navier–Stokes equations Boundary conditions for compressible flow 3 Reynolds number 4 Features of viscous flow Viscous diffusion of vorticity Convection and diffusion of vorticity Boundary layers Non-Newtonian fluids For a non-Newtonian fluid the viscosity changes with the applied strain rate (velocity gradient). As a result, non-Newtonian fluids may not have a well-defined viscosity.

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Newtonian fluids and viscosity

Definition (Newtonian fluid) A Newtonian fluid is a viscous fluid for which the shear stress is proportional to the velocity gradient (i.e., to the rate of strain): du τ = µ . dy

Here: τ [Pa] is the shear stress (“drag”) exerted by the ﬂuid, µ [Pa · s] is the (dynamic or absolute) viscosity, du h 1 i dy s is the velocity gradient perpendicular to the direction of shear.

u,τ u,τ u0 moving surface u0 no slip no slip u0 u0 u(y) = y h h y 2i u(y) = u 1 − µ u pressure 0 h τ(y) = 0 h y y −h 0 h 0 fluid h 2µ u fluid τ(y) = − 0 y h2 Non-Newtonian fluids For a non-Newtonian fluid the viscosity changes with the applied strain rate (velocity gradient). As a result, non-Newtonian fluids may not have a well-defined viscosity.

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Newtonian fluids and viscosity

Definition (Newtonian fluid) A Newtonian fluid is a viscous fluid for which the shear stress is proportional to the velocity gradient (i.e., to the rate of strain): du τ = µ . dy

Definition (Kinematic viscosity) The kinematic viscosity of a fluid is defined as the quotient of its absolute viscosity µ and density %:

µ h 2 i ν = m . % s Non-Newtonian fluids For a non-Newtonian fluid the viscosity changes with the applied strain rate (velocity gradient). As a result, non-Newtonian fluids may not have a well-defined viscosity.

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Newtonian fluids and viscosity

Definition (Newtonian fluid) A Newtonian fluid is a viscous fluid for which the shear stress is proportional to the velocity gradient (i.e., to the rate of strain): du τ = µ . dy

Deﬁnition (Kinematic viscosity)

µ h 2 i ν = m % s

fluid µ 10−5Pa · s ν 10−5m2/s air (at 20◦C) 1.82 1.51 water (at 20◦C) 100.2 0.1004 Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Newtonian fluids and viscosity

Definition (Newtonian fluid) A Newtonian fluid is a viscous fluid for which the shear stress is proportional to the velocity gradient (i.e., to the rate of strain): du τ = µ . dy

Non-Newtonian fluids For a non-Newtonian fluid the viscosity changes with the applied strain rate (velocity gradient). As a result, non-Newtonian fluids may not have a well-defined viscosity. 1 τ should be linear function of the velocity gradient; 2 this relationship should be isotropic, as the physical properties of the fluid are assumed to show no preferred direction; 3 τ should vanish if the flow involves no deformation of fluid elements. Constitutive relation for Newtonian fluids

T σ = µ ∇u + (∇u) −p I or σij = µ ui|j + uj|i − p δij . | {z } τ for incompressible This is a relation for incompressible ﬂuid (i.e., when ∇ · u = 0).

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for Newtonian fluids

T σ = µ ∇u + (∇u) −p I or σij = µ ui|j + uj|i − p δij . | {z } τ for incompressible This is a relation for incompressible ﬂuid (i.e., when ∇ · u = 0).

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for Newtonian fluids

The stress tensor can be decomposed into spherical and deviatoric parts: 1 1 σ = τ − p I or σ = τ − p δ , where p = − tr σ = − σ ij ij ij 3 3 ii is the (mechanical) pressure and τ is the the stress deviator (shear stress tensor). Using this decomposition Stokes (1845) deduced his constitutive relation for Newtonian fluids from three elementary hypotheses: 1 τ should be linear function of the velocity gradient; 2 this relationship should be isotropic, as the physical properties of the fluid are assumed to show no preferred direction; 3 τ should vanish if the flow involves no deformation of fluid elements. Moreover, the principle of conservation of moment of momentum T implies the symmetry of stress tensor: σ = σ , i.e., σij = σji. Therefore, the stress deviator τ should also be symmetric: τ = τ T, i.e., τij = τji (since the spherical part is always symmetric). Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for Newtonian fluids

1 1 σ = τ − p I or σ = τ − p δ , where p = − tr σ = − σ ij ij ij 3 3 ii

1 τ should be linear function of the velocity gradient; 2 this relationship should be isotropic, as the physical properties of the fluid are assumed to show no preferred direction; 3 τ should vanish if the flow involves no deformation of fluid elements;

T 4 τ is symmetric, i.e., τ = τ or τij = τji.

Constitutive relation for Newtonian ﬂuids

T σ = µ ∇u + (∇u) −p I or σij = µ ui|j + uj|i − p δij . | {z } τ for incompressible This is a relation for incompressible ﬂuid (i.e., when ∇ · u = 0). Newtonian ﬂuids are characterized by a linear, isotropic relation between stresses and strain rates. That requires two constants: the viscosity µ – to relate the deviatoric (shear) stresses to the deviatoric (shear) strain rates:

1 τ = 2µ ε˙ − 3 (tr ε˙) I ,

the so-called volumetric viscosity κ – to relate the mechanical pressure (the mean stress) to the volumetric strain rate: 1 p ≡ − 3 tr σ = −κ tr ε˙ + p0 .

Here, p0 is the initial hydrostatic pressure independent of the strain rate. Constitutive relation for compressible Newtonian ﬂuids

1 2 σ = 2µ ε˙ − 3 (tr ε˙) I − p I = 2µ ε˙ − p + 3 µ tr ε˙ I ,

and after using the deﬁnition for strain rate:

T 2 2 σ = µ ∇u+(∇u) − p+ 3 µ ∇ · u I or σij = µ ui|j+uj|i − p+ 3 µ uk|k δij .

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for compressible viscous flow

Deﬁnition (Rate of strain)

1 T ε˙ = 2 ∇u + (∇u)

1 The deviatoric (shear) and volumetric strain rates are given as ε˙ − 3 (tr ε˙) I and tr ε˙ = ε˙ · I = ∇ · u, respectively. Constitutive relation for compressible Newtonian ﬂuids

1 2 σ = 2µ ε˙ − 3 (tr ε˙) I − p I = 2µ ε˙ − p + 3 µ tr ε˙ I ,

and after using the deﬁnition for strain rate:

T 2 2 σ = µ ∇u+(∇u) − p+ 3 µ ∇ · u I or σij = µ ui|j+uj|i − p+ 3 µ uk|k δij .

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for compressible viscous flow

Deﬁnition (Rate of strain)

1 T ε˙ = 2 ∇u + (∇u)

1 The deviatoric (shear) and volumetric strain rates are given as ε˙ − 3 (tr ε˙) I and tr ε˙ = ε˙ · I = ∇ · u, respectively. Newtonian ﬂuids are characterized by a linear, isotropic relation between stresses and strain rates. That requires two constants: the viscosity µ – to relate the deviatoric (shear) stresses to the deviatoric (shear) strain rates:

1 τ = 2µ ε˙ − 3 (tr ε˙) I ,

the so-called volumetric viscosity κ – to relate the mechanical pressure (the mean stress) to the volumetric strain rate: 1 p ≡ − 3 tr σ = −κ tr ε˙ + p0 .

Here, p0 is the initial hydrostatic pressure independent of the strain rate. Constitutive relation for compressible Newtonian ﬂuids

1 2 σ = 2µ ε˙ − 3 (tr ε˙) I − p I = 2µ ε˙ − p + 3 µ tr ε˙ I ,

and after using the deﬁnition for strain rate:

T 2 2 σ = µ ∇u+(∇u) − p+ 3 µ ∇ · u I or σij = µ ui|j+uj|i − p+ 3 µ uk|k δij .

Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for compressible viscous flow

Newtonian ﬂuids are characterized by a linear, isotropic relation between stresses and strain rates. That requires two constants: the viscosity µ – to relate the deviatoric (shear) stresses to the deviatoric (shear) strain rates:

1 τ = 2µ ε˙ − 3 (tr ε˙) I ,

the so-called volumetric viscosity κ – to relate the mechanical pressure (the mean stress) to the volumetric strain rate: 1 p ≡ − 3 tr σ = −κ tr ε˙ + p0 .

Here, p0 is the initial hydrostatic pressure independent of the strain rate. Volumetric viscosity There is little evidence about the existence of volumetric viscosity and Stokes made the hypothesis that κ = 0 . This is frequently used though it has not been definitely confirmed.£ ¢ ¡ Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Constitutive relation for compressible viscous flow

1 τ = 2µ ε˙ − 3 (tr ε˙) I ,

the so-called volumetric viscosity κ – to relate the mechanical pressure (the mean stress) to the volumetric strain rate: 1 p ≡ − 3 tr σ = −κ tr ε˙ + p0 .

Here, p0 is the initial hydrostatic pressure.

Constitutive relation for compressible Newtonian ﬂuids

1 2 σ = 2µ ε˙ − 3 (tr ε˙) I − p I = 2µ ε˙ − p + 3 µ tr ε˙ I ,

and after using the deﬁnition for strain rate:

T 2 2 σ = µ ∇u+(∇u) − p+ 3 µ ∇ · u I or σij = µ ui|j+uj|i − p+ 3 µ uk|k δij . Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Outline

1 Newtonian fluids Newtonian fluids and viscosity Constitutive relation for Newtonian fluids Constitutive relation for compressible viscous flow 2 Navier–Stokes equations Continuity equation Cauchy’s equation of motion Navier–Stokes equations of motion Boundary conditions (for incompressible flow) Compressible Navier–Stokes equations of motion Small-compressibility Navier–Stokes equations Complete Navier–Stokes equations Boundary conditions for compressible flow 3 Reynolds number 4 Features of viscous flow Viscous diffusion of vorticity Convection and diffusion of vorticity Boundary layers For incompressible flows the density does not change (% = %0 where %0 is the constant initial density) so D% = 0 → ∇ · u = 0 . Dt This last kinematic constraint for the velocity field is called the incompressibility condition.

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Continuity equation

Continuity (or mass conservation) equation The balance of mass ﬂow entering and leaving an inﬁnitesimal control volume is equal to the rate of change in density:

D% ∂% + % ∇ · u = + ∇ · (% u) = 0 . Dt ∂t Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Continuity equation

Continuity (or mass conservation) equation The balance of mass ﬂow entering and leaving an inﬁnitesimal control volume is equal to the rate of change in density:

D% ∂% + % ∇ · u = + ∇ · (% u) = 0 . Dt ∂t

For incompressible ﬂows the density does not change (% = %0 where %0 is the constant initial density) so D% = 0 → ∇ · u = 0 . Dt This last kinematic constraint for the velocity ﬁeld is called the incompressibility condition. Apply the Cauchy’s formula: t = σ · n, and the divergence theorem for the surface traction term: Z Z Z t dS = σ · n dS = ∇ · σ dV .

S S V Now, the global (integral) form of equation of motion is obtained which, being true for arbitrary V and provided that the integrand is continuous, yields the local (differential) form.

Use the Reynolds’ transport theorem and the continuity equation for the inertial term: D Z Z D(% u) Z Du % u dV = + % u ∇ · u dV = % dV . Dt Dt Dt V V V

Cauchy’s equation of motion

Du Du % = ∇ · σ + b or % i = σ + b Dt Dt ij|j i

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Cauchy’s equation of motion

The general equation of motion valid for any continuous medium is obtained from the principle of conservation of linear momentum: D Z Z Z % u dV = b dV + t dS Dt V V S where b is the body (or volume) force, and t is the surface traction. Apply the Cauchy’s formula: t = σ · n, and the divergence theorem for the surface traction term: Z Z Z t dS = σ · n dS = ∇ · σ dV .

S S V Now, the global (integral) form of equation of motion is obtained which, being true for arbitrary V and provided that the integrand is continuous, yields the local (differential) form. Cauchy’s equation of motion

Du Du % = ∇ · σ + b or % i = σ + b Dt Dt ij|j i

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Cauchy’s equation of motion D Z Z Z % u dV = b dV + t dS Dt V V S Use the Reynolds’ transport theorem D Z Z Df f dV = + f ∇ · u dV , Dt Dt V V and the continuity equation D% + % ∇ · u = 0 , Dt for the inertial term: D Z Z D(% u) % u dV = + % u ∇ · u dV Dt Dt V V Z Du D% Z Du = % + u + % ∇ · u dV = % dV . Dt Dt Dt V | {z } V 0 Now, the global (integral) form of equation of motion is obtained which, being true for arbitrary V and provided that the integrand is continuous, yields the local (differential) form. Cauchy’s equation of motion

Du Du % = ∇ · σ + b or % i = σ + b Dt Dt ij|j i

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Cauchy’s equation of motion D Z Z Z % u dV = b dV + t dS Dt V V S Use the Reynolds’ transport theorem and the continuity equation for the inertial term: D Z Z D(% u) Z Du % u dV = + % u ∇ · u dV = % dV . Dt Dt Dt V V V Apply the Cauchy’s formula: t = σ · n, and the divergence theorem for the surface traction term: Z Z Z t dS = σ · n dS = ∇ · σ dV .

S S V Cauchy’s equation of motion

Du Du % = ∇ · σ + b or % i = σ + b Dt Dt ij|j i

S S V Now, the global (integral) form of equation of motion is obtained: Z Du % − ∇ · σ − b dV = 0 , Dt V which, being true for arbitrary V and provided that the integrand is continuous, yields the local (differential) form. Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Cauchy’s equation of motion D Z Z Z % u dV = b dV + t dS Dt V V S Use the Reynolds’ transport theorem and the continuity equation for the inertial term: D Z Z D(% u) Z Du % u dV = + % u ∇ · u dV = % dV . Dt Dt Dt V V V Apply the Cauchy’s formula: t = σ · n, and the divergence theorem for the surface traction term: Z Z Z t dS = σ · n dS = ∇ · σ dV .

Du Du % = ∇ · σ + b or % i = σ + b Dt Dt ij|j i Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Navier–Stokes equations of motion

On applying the constitutive relations of Newtonian incompressible ﬂuids to the Cauchy’s equation of motion of continuous media, the so-called incompressible Navier–Stokes equations are obtained.

Incompressible Navier–Stokes equations

Du Du % = µ 4u − ∇p + % g or % i = µ u − p + % g , 0 Dt 0 0 Dt i|jj |i 0 i (+ the incompressibility constraint:) ∇ · u = 0 or ui|i = 0 .

Here, the density is constant % = %0, and the body force b has been substituted by the the gravitational force %0 g, where g is the gravitaty µ acceleration. Now, on dividing by %0, using ν = , and expanding the %0 total-time derivative the main relations can be written as

∂u 1 ∂ui 1 + u·∇ u = ν 4u− ∇p+g or +uj ui|j = ν ui|jj − p|i +gi . ∂t %0 ∂t %0 They differ from the Euler equations by virtue of the viscous term. Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Boundary conditions (for incompressible flow)

Let n be the unit normal vector to the boundary, and m(1), m(2) be two (non-parallel) unit tangential vectors.

Let uˆ, ˆun, ˆp be values prescribed on the boundary, namely, the prescribed velocity vector, normal velocity, and pressure, respectively.

Inﬂow/Outﬂow velocity or No-slip condition: u = uˆ (uˆ = 0 for the no-slip condition) .

Slip or Symmetry condition:  u · n = ˆun (ˆun = 0 for the symmetry condition) ,  (σ n) · m(1) = 0 , (σ n) · m(2) = 0  (1) (2)  (or: (τ n) · m = 0 , (τ n) · m = 0) . Pressure condition: σ n = −ˆp n (or: p = ˆp , τ n = 0) .

Normal flow: ( (1) (2) u · m = 0 , u · m = 0 , (σ n) · n = −ˆp (or: p = ˆp , (τ n) · n = 0) . These equations are incomplete – there are only 4 relations for 5 unknown fields: %, u, p. They can be completed by a state relationship between % and p. However, this would normally introduce also another state variable: the temperature T, and that would involve the requirement for energy balance (yet another equations). Such approach is governed by the complete Navier–Stokes equations for compressible flow. More simplified yet complete set of equations can be used to describe an isothermal flow with small compressibility.

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Compressible Navier–Stokes equations of motion

On applying the constitutive relations of Newtonian compressible flow to the Cauchy’s equation of motion, the compressible Navier–Stokes equations of motion are obtained. Compressible Navier–Stokes equations of motion Du µ Du µ % = µ 4u + ∇∇ · u − ∇p + % g or % i = µ u + u − p + % g Dt 3 Dt i|jj 3 j|ji |i i D% D% (+ the continuity equation:) + % ∇ · u = 0 or + % u = 0 . Dt Dt i|i Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Compressible Navier–Stokes equations of motion

These equations are incomplete – there are only 4 relations for 5 unknown fields: %, u, p. They can be completed by a state relationship between % and p. However, this would normally introduce also another state variable: the temperature T, and that would involve the requirement for energy balance (yet another equations). Such approach is governed by the complete Navier–Stokes equations for compressible flow. More simplified yet complete set of equations can be used to describe an isothermal flow with small compressibility. Small compressibility is allowed: density changes are, as a consequence of elastic deformability, related to pressure changes. Navier–Stokes equations for nearly incompressible flow

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Small-compressibility Navier–Stokes equations

Assumptions: 1 The problem is isothermal. 2 The variation of % with p is very small, such that in product terms of u and % the latter can be assumed constant: % = %0. Navier–Stokes equations for nearly incompressible ﬂow

Newtonian ﬂuids Navier–Stokes equations Reynolds number Features of viscous ﬂow Small-compressibility Navier–Stokes equations

Assumptions: 1 The problem is isothermal. 2 The variation of % with p is very small, such that in product terms of u and % the latter can be assumed constant: % = %0. Small compressibility is allowed: density changes are, as a consequence of elastic deformability, related to pressure changes: % ∂% 1 ∂p q d% = 0 dp → = where c = K K ∂t c2 ∂t %0 is the acoustic wave velocity, and K is the elastic bulk modulus. This relation can be used for the continuity equation yielding the following small-compressibility equation:

∂p 2 = − c %0 ∇ · u , ∂t |{z} K where the density term standing by u has been assumed constant: % = %0. This also applies now to the Navier-Stokes momentum equations of compressible flow (ν = µ ). %0 Newtonian fluids Navier–Stokes equations Reynolds number Features of viscous flow Small-compressibility Navier–Stokes equations

∂u ν 1 + u · ∇u = ν 4u + ∇∇ · u − ∇p + g ∂t 3 %0 ∂ui ν 1 or + uj ui|j = ν ui|jj + uj|ji − p|i + gi , ∂t 3 %0 ∂p ∂p (+ small-compressibility equation:) = −K ∇ · u or = −K u . ∂t ∂t i|i These are 4 equations for 4 unknown ﬁelds: u, p. p−p0 After solution the density can be computed as % = %0 1 + K . Thermal equation of state: % = %(p, T). Constitutive law for ﬂuid: σ = σ(u, p) = τ (u) − p I. Thermodynamic relation: e = e(p, T). Heat conduction law: q = q(u, T).

There are 5 conservation equations for 14 unknown ﬁelds: %, u, σ, e, q. The constitutive and state relations provide another 11 equations and introduce 2 additional state variables: p, T. That gives the total number of 16 equations for 16 unknown ﬁeld variables: %, u, σ (or τ ), e, q, p, T. Using the constitutive and state relations for the conservation equations leaves only 5 equations in 5 unknowns: % (or p), u, T.

Du T Momentum conservation: % Dt = ∇ · σ + % g , (here: σ = σ ). D 1 Energy conservation: Dt % e + 2 % u · u = −∇ · q + ∇ · (σ · u) + % g · u + h. Equations of state and constitutive relations:

Remarks: