Advantages And Disadvantages Of Unstructured Grid

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• Extra care has to be taken so that highly skewed and negative volume elements are not generated
Unstructured Grid:
Unstructured grid can consist of any type of elements ranging from triangle, quadrilateral to tetrahedral, prism, pyramids and hexahedral.

Advantages:
• The grid density can be controlled in any region without worrying much about the density getting increased in another region
• Easy to use with complex geometries. Can be computed within an hour
• Don’t have to worry much about skewness or bad quality since the grid generation algorithm takes care of that most of the time
Disadvantages:
• Every cell can be indexed as a function of only 1 variable which causes that variable to be very large and thus increases computational load
• Causes more numerical diffusion than structured grid

Chapter 3
The scramjet model
3.1 Configurations of scramjet engine
Scramjet engines can be used as a propulsion system for cruise missiles, military aerospace planes and space launchers. Scramjet propelled aerospace plane can propel small payloads using hydro-carbon as fuel in horizontal take off SSTO (Single Stage to Orbit) and propel large payloads when using hydro carbons as fuel and when it is likely to succeed on the second stage of TSTO (Two Stage to Orbit). Different types of Scramjet engines were developed during the research on it to overcome the demerits or disadvantages of the others. The types are classified on the basis of the inlet shape, the combustor shape and the fuel injection system.
3.2 The new model
The scramjet engines model used to perform analysis has three parts as follows:
i) The inlet ramp ii) The combustion chamber iii) The outlet
3.2.1 The Isometric views of Engine model (CATIA V5)

The ...

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...terested in capturing the internal flow physics of the configuration we created structured mesh inside the engine and unstructured otherwise.

Fig. 5.2: Structured Mesh in the internal domain near the ramp wall
To capture the flow physics near the engine wall we need to increase the density of the cells near the wall. A grid was generated in the computational domain of the simulations using grid generator package ICEM cfd (Ansys 14.5). The first grid layer near the engine wall was nearly 20microns and hence the grid is finer at the walls of the engine and relatively coarser away from the walls of the engine.

Fig. 5.3: Grid finer near the wall and courser away from it (Combustion chamber)
Other than the internal section of the configuration the external domain was created with the unstructured grid. So overall the simulations were carried out using hybrid grid.

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