Contents
Why are simulations of turbulent flows so challenging?
Why turbulent flows are challenging? Unsteady aperiodic motion Fluid properties exhibit random spatial variations (3D) Strong dependence from initial conditions Contain a wide range of scales (eddies) The implication is that the turbulent simulation MUST be always three-dimensional, time accurate with extremely fine grids ME469B/3/GI 6
What is the objective of a numerical simulation?
Direct Numerical Simulation The objective is to solve the time-dependent NS equations resolving ALL the scale (eddies) for a sufficient time interval so that the fluid properties reach a statistical equilibrium Grid requirement: N ~ (Re τ )9/4~ 1x107for Re
Why is it important to keep water velocity low?
Bigger pipe is more expensive, but keeping the water velocity low is important to limit pressure losses due to friction, water hammer, and pipe movement due to water momentum changes inside the pipe.
What is the velocity profile of a turbulent flow?
Turbulent Velocity Profile: The Logarithmic Velocity Profile: The shape of the velocity profile within a turbulent boundary layer is well-established by theory and experiment. The profile has specific characteristics very close to the bed where viscosity controls the vertical transport of momentum, and different characteristics farther
Which is the transport equation for turbulent kinetic energy?
• All models use the transport equation for the turbulent kinetic energy k • Several transport variables are used ε: turbulence dissipation rate L: turbulent length scale ω: inverse of turbulent time scale ω2 g ψ τ Example: ME469B/3/GI 17
Why are wall quantities important in turbulent flows?
In engineering applications: Wall quantities (velocity gradients, pressure, etc.) are very important in several applications Flow separation and reattachment are strongly dependent on a correct prediction of the development of turbulence near walls ME469B/3/GI 22 Damping Functions Approach (Low-Re k-ε)
What is the structure of the turbulent boundary layer?
Structure of the Turbulent Boundary Layer Universal Law (velocity profile) At High Reynolds number the viscous dominated layer is so thin that it is very difficult to resolve it ME469B/3/GI 20 Wall Function Approach (High-Re k-ε)
Which is the gateway to turbulent flow simulation in MATLAB?
(i) The very compact Matlab implementation of present Navier–Stokes solvers provides a gateway to efficient learning of both, physics of turbulent flows, and simulation of turbulence. (ii) Only relatively minor prerequisites on fluid dynamics and numerical methods are required for using DNSLab.
What kind of simulation is used in CFD?
Computational fluid dynamics (CFD) research is increasingly much focused towards computationally intensive, eddy resolving simulation techniques of turbulent flows such as large-eddy simulation (LES) and direct numerical simulation (DNS).