DDES Study of Confined and Unconfined NACA Wing Sections Using Spectral Elements

TL;DR

This study develops hybrid RANS-LES turbulence modeling strategies within the spectral element code Nek5000 to accurately simulate flow around NACA wing sections, demonstrating improved convergence and flow dynamics capture over traditional methods.

Contribution

It introduces a hybrid RANS-LES approach using spectral elements for wing flow simulation, with validation against experimental data and analysis of flow discrepancies at high angles.

Findings

DDES captures flow separation and attachment accurately.

High-order spectral elements converge faster and more accurately.

Discrepancies at high angles are linked to wall effects and Reynolds number.

Abstract

We develop hybrid RANS-LES strategies within the spectral element code Nek5000 based on the $k-\tau$ class of turbulence models. We chose airfoil sections at small flight configurations as our target problem to comprehensively test the solver accuracy and performance. We present verification and validation results of an unconfined NACA0012 wing section in a pure RANS and in a hybrid RANS-LES setup for an angle of attack ranging from 0 to 90 degrees. The RANS results shows good corroboration with existing experimental and numerical datasets for low incoming flow angles. A small discrepancy appears at higher angle in comparison with the experiments, which is in line with our expectations from a RANS formulation. On the other hand, DDES captures both the attached and separated flow dynamics well when compared with available numerical datasets. We demonstrate that for the hybrid turbulence modeling approach a high-order spectral element discretization converges faster (i.e., with less resolution) and captures the flow dynamics more accurately than representative low-order finite-volume and finite-difference approaches. We also revise some of the guidelines on sample size requirements for statistics convergence. Furthermore, we analyze some of the observed discrepancies of our unconfined DDES at higher angles with the experiments by evaluating the side wall "blocking" effect. We carry out additional simulations in a confined 'numerical wind tunnel' and assess the observed differences as a function of Reynolds number.