The Effect of y+ on CFD Prediction of Foil Performance - CFD Assignment

Internal Code: TOA55

CFD Assignment

This assignment investigates the effects of y+ on the CFD predicted the performance of a foil traveling through water at Re = 1x106 In Fluid Dynamics, you learned about the physics of foils and the associated flow phenomena that define their performance. In particular, one of the critical flow phenomena that dictates foil performance is flow separation. This necessitates appropriate wall treatment models and the associated mesh requirements (i.e. y+ ) for flow separation to be accurately captured in CFD. In your technical report, you will need to complete the tasks below. Part A – a) Using Design Modeller and/or appropriate CAD packages develop a foil with the following specifications (note: see the notes given in Fluid Dynamics, especially the NASA web link):

• Symmetric foil - select a NACA foil that has validation data for the cross-section.
• Chord – 5 cm, span – 10 cm
• Identify the significant characteristics and dimensions of the foil.

b) Conduct a mesh convergence for the foil model with regards to the hydrodynamic characteristics of the foil, e.g. drag and lift and moment at the quarter-chord. c) Conduct simulation runs with different y+ values and sufficient angles to identify the effects of y+ on following the three conditions, i.e. zero angles, before stall and after stall. Assume turbulent flow. Discuss the results, using figures, screen dumps, and plots. Include forces (lift, drag, etc.), and moments about a quarter chord, and provide suitable flow visualisations with explanations. This discussion must include the relevant fluid/wing theory, CFD theory/approaches, and the assumptions/limitations. Additionally, discuss the general observed hydrodynamic performance of the foil at the three conditions and relate this to reality, e.g. the performance of a rudder or a sailing hydrofoil. d) Compare and discuss the CFD results of the foil performance against predictions based on lifting-line theory, available experimental data or other methods. Part B e) Explain the difference between the standard wall function and low Reynolds wall treatment in CFD simulations in relation to boundary layer theory. Use y+ to aid your explanation. f) Discuss the advantages and disadvantages of CFD over model test experiments and potential flow solutions. g) Outline the derivation of the Navier-Stokes equations.