Scalars, Vectors and Tensors

Transcription

1 Scalars, Vectors and Tensors A scalar is a physical quantity that it represented by a dimensional number at a particular point in space and time. Examples are hydrostatic pressure and temperature. A vector is a bookkeeping tool to keep track of two pieces of information (typically magnitude and direction) for a physical quantity. Examples are position, force and velocity. The vector has three components. velocity vector v = v 1 x 1 + v 2 x 2 + v 3 x 3 = i v 1 v i x i = v 2 v 3 Figure 1: The Velocity Vector The magnitude of the velocity is a scalar v v What happens when we need to keep track of three pieces of information for a given physical quantity? We need a tensor. Examples are stress and strain. The tensor has nine components. σ 11 σ 12 σ 13 stress tensor σ ij = σ 21 σ 22 σ 23 (1-25) σ 31 σ 32 σ 33 For stress, we keep track of a magnitude, direction and which plane the component acts on.

2 The Stress Tensor σ 11 σ 12 σ 13 stress tensor σ ij = σ 21 σ 22 σ 23 (1-25) σ 31 σ 32 σ 33 The first subscript keeps track of the plane the component acts on (described by its unit normal vector), while the second subscript keeps track of the direction. Each component represents a magnitude for that particular plane and direction. Figure 2: Four of the nine components of the stress tensor acting on a small cubic fluid element. The stress tensor is always symmetric σ ij = σ ji (1-26) Thus there are only six independent components of the stress tensor. Tensor calculus will not be required in this course.

3 The Stress Tensor EXAMPLE: SIMPLE SHEAR σ 12 = σ 21 σ = F/A (1-27) σ 13 = σ 31 = σ 32 = σ 32 = (1-28) σ 11 σ σ ij = σ σ 22 (1-29) σ 33 EXAMPLE: SIMPLE EXTENSION σ 11 σ ij = σ 22 (1-3) σ 33 EXAMPLE: HYDROSTATIC PRESSURE P σ ij = P (1-3) P The minus sign is because pressure compresses the fluid element. Polymers (and most liquids) are nearly incompressible. For an incompressible fluid, the hydrostatic pressure does not affect any properties. For this reason only differences in normal stresses are important. In simple shear, the first and second normal stress differences are: N 1 σ 11 σ 22 (1-32) The Extra Stress Tensor is defined as N 2 σ 22 σ 33 (1-33) { σij + P for i = j τ ij for i j σ ij (1-35)

4 Strain and Strain Rate Tensors Strain is a dimensionless measure of local deformation. Since we want to relate it to the stress tensor, we had best define the strain tensor to be symmetric. γ ij (t 1, t 2 ) u i(t 2 ) x j (t 1 ) + u j(t 2 ) x i (t 1 ) (1-37) u = u 1 x 1 + u 2 x 2 + u 3 x 3 is the displacement vector of a fluid element at time t 2 relative to its position at time t 1. Figure 3: Displacement Vectors for two Fluid Elements A and B. The strain rate tensor (or rate of deformation tensor) is the time derivative of the strain tensor. γ ij dγ ij / (1-38) The components of the local velocity vector are v i = du i / (1-39). Since the coordinates x i and time t are independent variables, we can switch the order of differentiations. γ ij v i x j + v j x i (1-4)

5 Strain and Strain Rate Tensors EXAMPLE: SIMPLE SHEAR γ γ ij = γ Where the scalar γ = u 1 / x 2 + u 2 / x 1 (1-41) γ γ ij = γ (1-51) Where the scalar γ dγ/. EXAMPLE: SIMPLE EXTENSION 2ε γ ij = ε (1-47) ε Where the scalar ε u 1 / x 1. 2 ε γ ij = ε (1-48) ε Where the scalar ε dε/.

6 The Newtonian Fluid Newton s Law is written in terms of the extra stress tensor τ ij = η γ ij (1-49) Equation (1-49) is valid for all components of the extra stress tensor in any flow of a Newtonian fluid. All low molar mass liquids are Newtonian (such as water, benzene, etc.) EXAMPLE: SIMPLE SHEAR The normal components are all zero γ γ ij = γ (1-48) η γ τ ij = η γ (1-48) σ 11 = σ 22 = σ 33 = (1-53) and the normal stress differences are thus both zero for the Newtonian fluid N 1 = N 2 = (1-54) EXAMPLE: SIMPLE EXTENSION 2 ε γ ij = ε (1-48) ε 2 ε τ ij = η ε (1-55) ε

7 Vector Calculus (p. 1) velocity v = v x i + v y j + v z k i, j, k are unit vectors in the x, y, z directions a = d v acceleration a = d v = a x i + a y j + a z k involves vectors so it represents three equations: a x = dvx a y = dvy a z = dvz a = dv x i + dv y j + dv z k The vector is simply notation (bookkeeping). Divergence n. v v x + vy x y + vz z If vx x vy vz > and > and > y z and v x > and v y > and v z > then you have an explosion! the name divergence.

8 Vector Calculus (p. 2) Gradient n. P P x i + P y j + P z k 3-D analog of a derivative Gradient operator makes a vector from the scalar P. (whereas divergence made a scalar from the vector v). Laplacian n. 2 v 2 v x x v y y v z z 2 2 v = ( 2 v x x ( 2 2 v y + x ( 2 2 v z + x v x y v y y v z y 2 ) + 2 v x i z v y z v z z 2 ) j ) k Laplacian operator makes a vector from a the vector v. Also written as v, divergence of the gradient of v.

9 Conservation of Mass in a Fluid (p. 2) THE EQUATION OF CONTINUITY Consider a Control Volume ρ = density (scalar) v = velocity vector n = unit normal vector on surface ρ( v n)da = net flux out of the control volume through small area da. S V d ρ( v n)da = net flow rate (mass per unit time) out of control volume. ρdv = total mass inside the control volume. V ρdv = rate of accumulation of mass inside the control volume. Mass Balance: d ρdv = ρ( v n)da V S

10 Conservation of Mass in a Fluid (p. 2) d ρdv = ρ( v n)da V S Integral mass balance can be written as a differential equation using the Divergence Theorem. Divergence Theorem: S ρ( v n)da = V V (ρ v)dv ( ) ρ t + (ρ v) dv = Control volume was chosen arbitrarily ρ t + (ρ v) = Derived for a control volume but applicable to any point in space. Cornerstone of continuum mechanics!!! Applies for both gases and liquids.

11 Conservation of Mass in a Liquid A liquid is incompressible Density ρ is always the same ρ t = ρ = constant Integral Mass Balance becomes d V ρdv = ρ( v n)da S S ( v n)da = Continuity Equation ρ t + (ρ v) = becomes v = for incompressible liquids In Cartesian coordinates: v = v x i + v y j + v z k v x x + v y y + v z z = (1-57)

12 Incompressible Continuity Equation for Liquids Cartesian Coordinates: x, y, z v x x + v y y + v z z = Cylindrical Coordinates: r, θ, z 1 r r (rv r) + 1 r v θ θ + v z z = Spherical Coordinates: r, θ, ϕ 1 r 2 r (r2 v r ) + 1 r sin θ θ (v θ sin θ) + 1 v ϕ r sin θ ϕ = All are simply v =