First steps in Aerodynamics

with CFX TASCflow


I. Study configuration

1. Physical description

The aim of the study is to make a simulation of transonic and supersonic flows about a NACA 0012 airfoil at zero angle of attack. It is carried out thanks to a CfxTascflow tutorial available with the software. Hence, the grid generation and some parameters have been fixed yet in order to simplify the first steps with CFX. The airfoil profile was modified slightly to make easier the grid generation. In addition, the grid is refined near the airfoil in order to model the large gradients in that region. Since the symmetry particularity of the airfoil and the boundary conditions (thanks to the zero angle of attack), the flow is simulated by modelling only half of the domain. Then, the grid used is represented on the presentation page (last screen page).

Furthermore, the problem parameters are :

Desired inlet Mach number : 0.8
Molecular viscosity : 1.983 x 10-5 kg.m-1.s-1
Thermal conductivity : 2.624 x 10-2 W.m-1.K-1
Specific heat at constant pressure : 1.0057 x 103
Specific heat at constant pressure : 7.1836 x 102
Airfoil chord length : 0.1 m
Domain length : 4.5 x airfoil length
Domain heigth : 2.5 x airfoil length
Domain depth : 0.1 x airfoil length

A low Mach number flow will be simulated first and the results used as the initial guess for successively higher Mach numbers flows until the desired flow is achievied. Thus, Mach 0.2 flow is modelled first, the resulting flow field is used as the initial guess for a Mach 0.5 flow. The same process is also used to get the Mach 0.8 flow results. It permits to make shorter the convergence time and to reduce errors.

2. Inflow boundary conditions

The inflow boundary is a Total Pressure Inlet. This boundary condition requires a specification for the total pressure and the total temperature. One can calculate these quantities as follows :

the isentropic coefficient and the sound velocity need to be set up :

where the statis temperature is equal to 300 K,

the total temperature is given by :

where the variable M represents the Mach number,

and the total presure is determined by :

The following table sums up all the numerical parameters necessary to perform each simulation with different Mach numbers :