# Impinging jet

## Best Practice Advice for the UFR

Key physics

&#x0095;The heat transfer in the impingement region is mainly driven by the turbulent heat flux in the wall-normal direction (<v theta>).

&#x0095;Therefore, the prediction of wall-normal velocity fluctuations is of primary importance.

&#x0095;These fluctuations results from a competition between generation terms (convection from the jet and production by normal straining) and destruction terms (wall-blocking).

Numerical Issues

Grid and grid resolution

&#x0095;Use grid clustering at the heated wall: y+=1 is necessary with low-Reynolds number models (tolerance depends on the model).

&#x0095;Grids of the order 100x100 cells appear adequate but a grid sensitivity analysis is highly advised.

Discretisation method

Use a second order convection scheme (convection terms are crucial).

Computational Domain & Boundary conditions for UFR Impinging Jet

Boundary conditions

&#x0095;Inlet conditions: use a separate fully turbulent pipe flow computation

&#x0095;Wall: avoid wall functions

&#x0095;Right outlet: pressure or convective outlet boundary conditions

&#x0095;Upper boundary: use constant pressure. Symmetry boundary condition is possible if sufficiently far to avoid confinement (x/D>5)

&#x0095;Computational domain

-Inlet at least 1D before the pipe exit

-Right outlet: r/D>10

Physical modelling for UFR Impinging Jet

Turbulence modelling

- Best Practice Advice for overall quantities (Nusselt number distribution and mean velocity)

Don&#x0092;t use eddy-viscosity models which do not reproduce the correct damping effect of the wall and RSM without wall echo terms or with standard wall echo terms (Nusselt number overestimated by a factor of about 2.8).

Use eddy-viscosity models reproducing the damping effect of the wall (V2F, k-eps-fmu and SST models) or linear RSM with modified wall echo terms and Yap correction.

- Best Practice Advice for flow details (Reynolds stresses and turbulent heat fluxes)'

Use RSM integrable down to the wall and accounting for the wall blocking effect (nonlinear Two-Component Limit model and linear Elliptic Blending Model)

Turbulent heat fluxes modelling

- Best Practice Advice for overall quantities (Nusselt number distribution)

Use a simple eddy-diffusivity model with constant Prt number (0.9) or with Kays & Crawford correlation.

- Best Practice Advice for flow details (turbulent heat fluxes)

Use heat fluxes transport equations

Application uncertainties

In this application, all the conditions are very well defined (boundary conditions, Reynolds number, geometry, &#x0085;). The only uncertainties come from the measurements methods : the uncertainty analyses given by the authors lead to low values (typically less than 5%).