[rajibroy]

Hi,
Could anyone help with mesh generation and simulation experience of an airfoil with surface tripped?

I am working on hybrid LES-RANS simulation of Naca4412 airfoil at an angle of attack of 12 deg which produces maximum lift (Re= 1.2E6) (according the experiment of Wadcock, 1987). This configuration produces a trailing edge separation (separated turbulent flow phenomenon).

In the experimental investigation, the airfoil is tripped at leading edge on both pressure and suction side.

My attempt to simulate without a trip predicts less boundary layer growth and later* separation. May be a simulation with a trip could provide better outcome.

I am using pointwise for mesh generation (open to structured or unstructured mesh) and OpenFoam for CFD simulation. Any suggestion or comments is highly appreciated.

Best,
Rajib

[cnsidero]

Are you wanting to physically create a protrusion/protuberance that trips the BL? You'll have to better describe the mechanism.

A couple of thoughts. Since this is LES-RANS (ie. DES?), the simulation will/should be 3D. To keep the cell count down AND to get the most reliable results, I would suggest using a structured mesh. Which, fortunately for airfoils, isn't terribly difficult. Although that may be depend on the tripping mechanism (above).

Therefore if you want to trip the BL with a physical mechanism (protrusion etc) and not have this affect the solution, i.e. it's small enough to trip the BL but in of itself not effect drag/lift, you might require a fairly large number of cells in the resultant grid. Let me qualify that last statement by saying I have never attempted this before - I'm just thinking aloud - so it's just my uneducated opinion.

If you want more specific guidance, please fill in some of the details.

-Chris

[rajibroy]

Hello Chris,
Appreciate your kind comments and input.
Please have a look at attached plots from 2D RANS simulation of the Naca4412 airfoil. Here RSMeb is the Reynolds Stress Model with Elliptic Blending near the wall. The hybrid LES-RANS (doesn't generate any encouging result either compared to experiment).

Ideally, trip is expected to work in boundary layer to force the laminer to turbulent transition. However, I'm not sure, if a trip is larger than the BL thickness, how it will affect the flow. The "Wadcock'87 experiment report" doesn't include the trip height.

A tripped airfoil is expected to have a higher turbulent BL and separate later in adverse pressure gradient. From the experimental velocity profiles, the boundary layers are much thicker than that predicted from the model.However, it seperates earlier than model predition !!! (i.e. lower lift and higher pressure drag).

My thought is:
1. the RSM or RANS-LES models aren't able to capture the physics and comparing model perfornmance with a trip will be interesting.

2. the Trip employed in the experiment was high enough to interrupt the overall flow. Similation could be also directed in comparing effect of various level of trip height.

Simulating a trip physically is challengeing. I don't have much knowledge about how the RANS or LES model impose laminar-to- turbulence transition, and does placing a trip matter in simulation.

I am thinking about a "rope" trip (semi circle over the surface) and make an strctured mesh.

Please, let me know your thought in this regard.

My Best,
Rajib

https://copy.com/QfaHCxejwwaO3xzn