FUNDAMENTAL EQUATIONS, CONCEPTS AND IMPLEMENTATION OF NUMERICAL SIMULATION IN FREE SURFACE FLOW
Governing Equations of Fluid Flow Navier-Stokes Equations • A system of 4 nonlinear PDE of mixed hyperbolic parabolic type describing the fluid hydrodynamics in 3D. • Three equations of conservation of momentum in cartesian coordinate system plus equation of continuity embodying the principal of conservation of mass. • Expression of F=ma for a fluid in a differential volume.
The acceleration vector contains local acceleration and covective terms • The force vector is broken into a surface force and a body force per unit volume. • The body force vector is due only to gravity while the pressure forces and the viscous shear stresses make up the surface forces.
The stresses are related to fluid element displacements by invoking the Stokes viscosity law for an incompressible fluid.
The three N-S momentum equations can be written in compact form as The equation of continuity for an incompressible fluid
Turbulence The free surface flows occurring in nature is almost always turbulent. Turbulence is characterized by random fluctuating motion of the fluid masses in three dimensions. A few characteristic of the turbulence are: 1. Irregularity Turbulent flow is irregular, random and chaotic. The flow consists of a spectrum of different scales (eddy sizes) where largest eddies are of the order of the flow geometry (i.e. flow depth, jet width, etc). At the
other end of the spectra we have the smallest eddies which are by viscous forces (stresses) dissipated into internal energy. 2. Diffusuvity The turbulence increases the exchange of momentum in flow thereby increasing the resistance (wall friction) in internal flows such as in channels and pipes. 3. Large Reynolds Number Turbulent flow occurs at high Reynolds number. For example, the transition to turbulent flow in pipes occurs at NR~2300 and in boundary layers at NR~100000
4.Three-dimensional Turbulent flow is always three-dimensional. However, when the equations are time averaged we can treat the flow as two-dimensional. 5. Dissipation Turbulent flow is dissipative, which means that kinetic energy in the small (dissipative) eddies are transformed into internal energy. The small eddies receive the kinetic energy from slightly larger eddies. The slightly larger eddies receive their energy from even larger eddies and so on. The largest eddies extract their energy from the mean flow. This process of transferred energy from the largest turbulent scales (eddies) to the smallest is called cascade process.
Turbulence . The random , chaotic nature of turbulence is treated by dividing the instantaneous values of velocity components and pressure into a mean value and a fluctuating value, i.e. Why decompose variables ? Firstly, we are usually interested in the mean values rather than the time histories. Secondly, when we want to solve the Navier-Stokes equation numerically it would require a very fine grid to resolve all turbulent scales and it would also require a fine resolution in time since turbulent flow is always unsteady.
Reynolds Time-averaged Navier-Stokes Equations These are obtained from the N-S equations and include the flow turbulence effect as well.
Reynold Stresses The continuity equation remains unchanged except that instantaneous velocity components are replaced by the time-averaged ones. The three momentum equations on the LHS are changed only to the extent that the inertial and convective acceleration terms are now expressed in terms of time averaged velocity components. The most significant change is that on the LHS we now have the Reynold stresses. These are time-averaged products of fluctuating velocity components and are responsible for considerable momentum exchange in turbulent flow.
Closure Problem • 3 velocity components, one pressure and 6 Reynold stress terms = 10 unknowns • No. of equations=4 • As No. of unknowns >No. of equations, the problem is indeterminate. One need to close the problem to obtain a solution. • The turbulence modeling tries to represent the Reynold stresses in terms of the time-averaged velocity components.
Turbulence Models Boussinesq Model An algebraic equation is used to compute a turbulent viscosity, often called eddy viscosity. The Reynolds stress tensor is expressed in terms of the time-averaged velocity gradients and the turbulent viscosity.
k-ε Turbulence Model Two transport equations are solved which describe the transport of the turbulent kinetic energy, k and its dissipation, ε. The eddy viscosity is calculated as the Reynold stress tensor is calculated via the Boussinesq approximation
RNS Equations and River Flow Simulation • RNS equations are seldom used for the river flow simulation. Reasons being • High Cost • Long Calculation time • Flow structure • Method of choice for flows in rivers, streams and overland flow is 2D and 1D Saint Venant equations or Shallow water equations
2D Saint Venant Equation • Obtained from RNS equations by depth-averaging. • Suitable for flow over a dyke, through the breach, over the floodplain. • Assumptions: hydrostatic pressure distribution, small channel slope,
1D Saint Venant Equation The friction slope Sf is usually obtained from a uniform flow formula such as Manning or chezy.
Relative Weight of Each Termin SV Equation Order of magnitude of each term In SV equation for a flood on river Rhone
Calculation Grid • Breaking up of the flow domain into small cells is central to CFD. Grid or mesh refers to the totality of such cells. • In Open channel flow simulation the vertices of a cell define a unique point (x,y,,z) * The governing equations are discretized into algebraic equations and solved over the volume of a cell.
Classification of Grids • Shape • Orthogonality • Structure • Blocks • Position of variables • Grid movements
Boundary Conditions • Inflow b. c If Fr<1, specify discharge or velocity. If Fr>1, specify discharge or velocity and depth • Outflow b.c Zero depth gradient or Newmann b.c Specify depth Specify Fr=1
Initial Condition • Values of flow depth, velocity, pressure etc must be assigned at the start of the calculation run. • Hot start
Wall Boundary Condition No slip condition require very fine meshing adjacent to the wall requiring lot of CPU time. Flow close to the wall is not resolved but wall laws derived from the universal velocity distribution are used.
Methods of Solution • Finite Difference Method • Finite element method • Finite volume method • Strategies • Implicit • Explicit • CFL condition