# Fluid dynamics (2019/2020)

Course code
4S00258
Name of lecturer
Simone Zuccher
Coordinator
Simone Zuccher
Number of ECTS credits allocated
6
FIS/07 - APPLIED PHYSICS
Language of instruction
Italian
Period
II semestre dal Mar 2, 2020 al Jun 12, 2020.

#### Learning outcomes

Derivation of the fluid-dynamic equations from conservation laws in Physics; discussion on the rheological structure of fluids and the model for Newtonian fluids; different flows and simplifications of the governing equations; Bernoulli theorem in all forms and for all cases; some exact solutions; vorticity dynamics; laminar boundary layer; stability and transition; turbulence; hyperbolic equations in fluid dynamics. Numerical resolution in Matlab / Octave of some typical problems of fluid dynamics.

#### Syllabus

1. Introduction to fluids: definitions, continuous hypothesis and properties of fluids; differences between fluid, flux, flow; some kinematics (stream-lines, trajectories, streak-lines), forces and stresses (Cauchy Theorem and symmetry of the stress tensor), the constitutive relation for Newtonian fluids (viscous stress tensor).

2. Governing equations: Eulerian vs Lagrangian approach; control volume and material volume, conservation of mass in a fixed volume, time derivative of the integral over a variable domain, Reynolds Theorem (scalar and vectorial forms), conservation of mass in a material volume, from conservation laws to the Navier-Stokes equations, the complete Navier-Stokes equations (in conservative, tensorial form), substantial derivative, conservative vs convective form of the equations, alternative forms of the energy equation, dimensionless equations, initial and boundary conditions.

3. Particular cases of the governing equations: time dependence, effect of viscosity, thermal conduction, entropy, compressibility, barotropic flows, incompressible flows, ideal flows, Euler equations irrotational flows, barotropic and non-viscous flows: Crocco's form, Bernoulli theorem in all cases and forms.

4. Some exact solutions: incompressible and parallel flows, infinite channel flow, Couette and Poiseuille flows, flow in a circular pipe, Hagen-Poiseuille solution.

5. Vorticity dynamics: preliminary definitions, vorticity equation in the general case, special cases (constant density, non-viscous flow with conservative external field), Kelvin's theorem, Helmholtz's theorems and their geometrical meaning.

6. Laminar boundary layer: Prandtl theory, boundary layer past a flat plate, derivation of Blasius' equation (similar solutions), boundary-layer thickness, drag due to skin-friction, characteristics of a boundary layer (displacement thickness, momentum thickness, shape factor), integral von Kàrmàn equation, numerical solution of the 2D steady equations for the boundary layer past a flat plate:
(a) parabolic PDE + BC (Prandtl's equations): marching in space
(b) ODE + BC (Blasius' equation): nonlinear boundary value problem
(c) comparison between the two methods.

7. Stability and transition: flow in a pipe - Reynolds' experiment, transition in a laminar boundary layer, linear stability for parallel flows (Orr-Sommerfeld equation),
Squire's theorem, non-viscous stability (Rayleigh's criteria), viscous stability, linear stability curves.

8. Turbulence: phenomenological characteristics, turbulent scales, energy cascade, Kolmogorov's theory, DNS (Direct numerical simulation), RANS (Reynolds-Averaged-Navier-Stokes equations), the problem of closure for the RANS, closure models, Boussinesq hypothesis for the tutbulent viscosity (models of order 0, 1 and 2), LES (Large Eddy Simulation).

9. Hyperbolic differential equations in fluid dynamics: main characteristics and comparison with parabolic and elliptic equations, conservation laws, transport equation, characteristic lines, Riemann problem, Burgers' equation, weak solutions, shock waves, rarefaction waves, comparison between conservative and non-conservative numerical methods, method of characteristics, usage of an applet for the visualization of shock and rarefaction waves, hyperbolic linear and non-linear systems, genuine nonlinearity, linear degeneration, contact discontinuity, solution of the Riemann for the Euler equations.

#### Assessment methods and criteria

The aim of the exam is to ensure that the student is able to produce and recognize rigorous demonstrations, mathematically formalize natural language problems and discuss mathematical models for fluid dynamics analyzing their limits and applicability. The exam consists of an oral interview on the course program and the discussion on the numerical exercises in Matlab/Octave assigned during the course. The discussion on the latter aims to ensure that the student is able to use computer tools, programming languages, and specific software.