Best Practice Advice AC3-10
Combining/dividing flow in Y junction
Application Challenge 3-10 © copyright ERCOFTAC 2004
Best Practice Advice for the AC
Key Fluid Physics
The application challenge on which this best practice advice is based is associated with the flow of water in a Y-junction, Reference 1. The flow is characterised by the following elements:
• Steady, turbulent and incompressible flow.
• Flow separation.
• A Reynolds Number between 5x105 and 1.2x106.
The application uncertainties associated with the Application Challenge are as follows:
• Turbulence at the Y-junction inlets. It was assumed that the upstream pipework was sufficiently long for the inlet flow to the computational domain to be fully-developed.
• Use of wall functions. Flow separation can occur in the Y-junction. Within separated regions and flow recirculations the standard wall function approach is unreliable.
• Choice of turbulence model. The simple k-epsilon turbulence model performs less well than more sophisticated methods, e.g. Differential Stress.
The sensitivity of the DOAP to these application uncertainties is relatively small.
Computational Domain and Boundary Conditions
With respect to the computational domain and boundary conditions, the following best practice advice is appropriate:
• It is acceptable to model half of the Y-junction, on one side of the symmetry plane.
• Use mass-flow boundaries at the inlets and exits of the Y-junction, with fully-developed velocity profiles.
• For best accuracy, use the Differential Stress turbulence model, with standard wall functions.
Discretisation and Grid Resolution
With respect to discretisation and grid resolution, the following best practice advice is appropriate:
• Standard (hybrid) spatial discretisation schemes are adequate.
• The grid must be sufficiently fine to resolve the details of the flow. A constant near-wall cell size is recommended.
With respect to physical modelling the following BPA is appropriate:
• Assume that the flow is turbulent.
• Assume that the fluid is isothermal and incompressible.
Recommendations for Future Work
The following recommendations for future work are appropriate:
• Do further CFD calculations using finer grids.
• Perform further experiments to measure velocities and turbulence quantities within the Y-junction.
1. Combining/Dividing Flow in a Y-Junction
AC3-10 D30, July 2002
© copyright ERCOFTAC 2004
Contributors: Alan Stevens - Rolls-Royce Marine Power, Engineering & Technology Division