Quantification of resolution

Mesh resolution

The mesh resolution is quantified by obtaining a relation between the mesh characteristic length ( ${\Delta }$ ) and the characteristics lengths, i.e., the Taylor microscale ( ${\eta _{T}}$ ) and Kolmogorov length scale ( ${\eta }$ ). The former relation is shown in Fig. 1 while the latter is reported in Fig. 2. As it can be seen, both relations indicate that the resolution achieved by the present grid is at DNS level. In particular, it is commonly accepted that DNS is achieved when ${\Delta /\eta _{T}\leq 5}$ , as shown in Fig. 2.

Figure 1: Stanford double diffuser, Alya DNS-250M DoF, relation between the mesh size and the Taylor microscale.

Figure 2: Stanford double diffuser, Alya DNS-250M DoF, relation between the mesh size and the Kolmogorov length scale.

Solution verification

This section aims to provide an insight on the dataset freely available in this wiki this wiki. Simulation results are compared with the DNS of Ohlsson et al. (2010) and the experimental data of Cherry et al. (2008). The grid resolution has been proven to be fine enough to be at DNS level. An animation of the fluctuations of this flow can be seen in this link.

Velocity profiles and RMS quantities

The 10000 Reynolds number diffuser has been simulated and compared with the results with previous DNS data from Ohlsson et al. (2010) and experimental data from Cherry et al. (2008).


Figure 2: Stanford double diffuser, Re=10000, preliminary validation. Average streamwise velocity, Ohlsson et al. (2010) vs preliminary data obtained with Alya.

Figure 3: Stanford double diffuser, Re=10000, preliminary validation. Average streamwise velocity fluctuations, Ohlsson et al. (2010) vs preliminary data obtained with Alya.

In Fig. 2 and Fig. 3 results are presented with reference data from Ohlsson et al. (2010). Fair agreement is observed between both calculations, proving that the approach to be used in the present proposal is an optimal strategy. Turbulent inlet was generated in a precursor domain using a long enough duct with a roughness element to trigger the laminar to turbulent transition.