IMSIA, ENSTA ParisTech828, boulevard des Marechaux

91120 Palaiseau

Internship offer : Large Eddy Simulations of the effect ofinflow turbulence on a laminar separation bubble for airfoil

at low Reynolds number

Study context

Low Reynolds number (i.e. Re ≤ 5 × 105 [1]) aerodynamic applications such as small windturbines or drones appear to be more and more present for civil or military use. The flow on bladescan be subject to complex phenomena such as laminar boundary layer separation, transition to-wards the turbulence, boundary layer reattachment or even stall [1]. Boundary layer separation andreattachment form the well-known Laminar Separation Bubble (LSB) which can lead to the wingstall when the LSB bursts at high angles of attack. In this situation, the underlying mechanismsstill demand to be understood [2, 3].

The inflow turbulence strongly impacts the aerodynamics. Indeed, it is experimentally sugges-ted that the transition towards the turbulence of the LSB on an airfoil, its length and its burst arehighly affected by inflow perturbations [2, 3]. The effect of inflow turbulence on the LSB separation,transition and reattachment points has also been numerically studied using different methods, tur-bulent intensities or integral length scales [4, 5]. The numerical approach has the advantage to givea lot of insights for flow understanding. However, only a few numerical studies used experimentaldata for comparison and no simulation has been done at very low Reynolds number (Re ≤ 2×104),when the LSB is the most dependent on the inflow perturbations.

The LSB mechanisms will be studied using Large Eddy Simulations carried out with the EDFR&D solver Code Saturne. The Navier-Stokes incompressible equations are solved in an airfoilcase, using a synthetic turbulence generator at the inlet to produce realistic inflow perturbations.The numerical features such as mesh quality, mesh resolution, airfoil span or computational time,have to be carefully chosen because of the complex flow structure. Some preliminary computationshave been carried out at IMSIA [6] and some results are illustrated on Fig .1.

Internship objectives

The objective of the internship is to get a better understanding of the inflow turbulence effecton a LSB at low Reynolds numbers using airfoil numerical simulations. The applicant will see thecomplete chain of numerical simulation, from the geometry to the results analysis. More precisely,the applicant will do the following tasks :

• make a bibliography study on LSB mechanisms, experiments and simulations, in particularwith inflow turbulence.

• generate high quality airfoil grids• prepare and carry out Large Eddy Simulations using Code Saturne for different angles of

attack, Reynolds number (Re ≤ 2 × 104), turbulent intensities and grid parameters, inaccordance with experimental configurations

• extend the post-processing tools to relevant quantities for LSB understanding, based on thelitterature and analyse the simulation results

Parameters Expe. Simu. (Last Iteration)

α = 12 , It = 0%

α = 12 , It = 6%

α = 8 , It = 6%

Figure 1 – Comparison of (a) experimental LIF visualisations from Wang et al. [3] and (b)simulation streamlines at the last iteration for different parameters. The NACA0012 airfoil isimmersed in a water flow with a Reynolds number Re = 5.3 × 103, for different angles of attack αand inflow turbulent intensities It.

In particular, the first simulations will give the opportunity to precisely study the effect of themesh grid resolution and quality on the LSB results. Thus, the applicant will begin by computingthe flow for a NACA0012 airfoil at Re = 2 × 104 for α = 12 and It = 2.6% with different wallmesh sizes and boundary layer mesh size progressions. To know the spanwise correlation length,different spans will also be considered.

Practical details

• Duration : 4 to 6 months• Desired profile : Master 2 or final year engineering school student. Strong knowledge required

in fluid mechanics/aerodynamics and associated numerical methods. The applicant must beproactive and must have good scientific analysis skills.

• Supervisors : Benjamin Cotte (Assistant professor) and Tommy Rigall (PhD candidate)• To submit an application : send a resume, a cover letter and a transcript of grades to

benjamin.cotte@ensta.fr and tommy.rigall@ensta.fr

References

[1] T. J. Mueller and J. D. DeLaurier. Aerodynamics of small vehicles. Annual Review of FluidMechanics, 35 :89–111, 2003.

[2] M. M. Alam, Y. Zhou, H.X. Yang, H. Guo, and J. Mi. The ultra-low Reynolds number airfoilwake. Experiments in Fluids, 48 :81–103, 2010.

[3] S. Wang, Z. Zhou, Md. M. Alam, and H. Yang. Turbulent intensity and Reynolds numbereffects on an airfoil at low Reynolds numbers. Physics of Fluids, 26, 2014.

[4] S. Hosseinverdi, W. Balzer, and H. F. Fasel. Direct numerical simulations of the effect offree-stream turbulence on ”long” laminar separation bubbles. In 42nd AIAA Fluid DynamicsConference and Exhibit, New Orleans, Louisiana, June 2012.

[5] S. Schmidt and M. Breuer. Source term based synthetic turbulence inflow generator for eddy-resolving predictions of an airfoil flow including a laminar separation bubble. Computers andFluids, 147 :1–22, 2017.

[6] T. Rigall, B. Cotte, and P. Lafon. Airfoil Noise Numerical Simulations with Direct NoiseComputation and Hybrid Methods Using Inflow Synthetic Turbulence. In 25th AIAA/CEASAeroacoustics Conference, Delft, Netherlands, May 2019.