# 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.

• Mixing.

• A Reynolds Number between 5x105 and 1.2x106.

**Application Uncertainties**

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.

**Physical Modelling**

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.

**References**

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

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