Bob’s Research Corner: Grid Def – More Bending

For the last several months, I’ve been working on an algorithm to increase the amount of bending my grid deformer can handle before the grid becomes invalid. I’m finally to the point where I believe I have a reliable algorithm that unfolds folded faces resulting from grid deformation. For this article, I will focus on bending only. I will show the increased bending limit for both the Onera M6 wing and the F-18C vertical tail shown previously in “Bob’s Research Corner: Grid Def – Bending”.Continue reading

Bob’s Research Corner: Grid Def – Twist

In this post, I’ll present the twisting results of my grid deformation code.  I’ve taken the same grids (Onera M6 wing and the F-18C vertical tail) and applied a twist along the mid-chord.  For information about the grids and the bending results, you can read “Bob’s Research Corner: Grid Def – Bending”.  Twisting along the mid-chord is based on the square of the span location times a maximum twist angle.

On the Onera M6 wing, the maximum twist angle achieved before encountering folded faces was 10.5 degrees. The maximum twist occurs at the wing tip.  This is an ok twist amount just like the bending result.  The results are shown below, first with a video of the wing twisting over 25 time-steps followed by still images comparing the original versus deformed grid.  The original grid is shown in red while the deformed grid is shown in blue.Continue reading

Bob’s Research Corner: Grid Def – Bending

For the last month or so, I’ve been focusing on 3-D grids with deforming boundaries.  I split this into two main categories: bending and twisting.  This article will focus on bending only.  I will make a post about twisting in the near future.

The goal of this project is to develop a grid deformation algorithm capable of handling larger than real world deformations.  And by handling, I mean producing valid Cobalt grids.  If I’m successful, then our customers can simulate real world cases such as aeroelastic deformations.

The first test case I’m going to show is an Onera M6 wing grid.  The grid contains 2,276,261 cells (1,832,756 tets, 17,642 pyramids and 425,863 prisms) and is valid for Navier-Stokes simulations.  The bending function used is a quadratic displacement based on an amplitude multiplying the square of the span location.  The maximum deflection of the tip I could get was 9.25% of the span; anything larger resulted in the dreaded folded faces.  For those not familiar with a folded face, first, define a face as the interface between two cells.  When flow leaves one cell across a face, it enters the other cell.  When a face becomes folded, then the centroids of both cells lie on the same side of the face.  This means flow leaving one cell through the face, also leaves the other cells.  This is VERY bad as conservation cannot be conserved!  That being said, a 9.25% of the span deflection is not bad.  This might be good enough for real world cases.  Of course, I’d like to do better.  The video below shows the bending of the wing compared to the original.  And the images show the original grid versus the deformed grid.Continue reading

Evaluation of Dynamic Pressure-Sensitive Paint for Improved Analysis of Cavity Flows and CFD Validation

Cavity flows have been the subject of aerodynamic research since the early days of high-speed aircraft with internal store carriage in the 1950s. Low observability design requirements for current combat aircraft has renewed the interest in this field. The presence of an open cavity, such as a weapon or undercarriage bay, exposed to a high-speed grazing flow can result in severe pressure fluctuations. These pressure fluctuations can reach intensities of up to 165dB SPL and may cause damage to both the aircraft and to any sensitive components within the bay. Results for this study were generated using an existing generic rectangular planform cavity wind tunnel model, designated as M219/1. This small scale model was the forerunner for the more widely known larger scale M219 generic cavity model.

Numerical simulations were performed for two geometries, the 38mm and 80mm deep cavities, at a freestream Mach number of 0.80. In order to capture the unsteady flow behaviour in the cavity, a time accurate detached eddy simulation (DES) model was run which uses Reynolds averaged Navier-Stokes (RANS) near to the wall to reduce computational cost and a large eddy simulation (LES) model in the separated flow region far from the walls.Continue reading

Bob’s Research Corner: Grid Deformation – Part I

Welcome to Bob’s Research Corner!  In the past, we did not have a convenient way to provide research news to you – unless you considered spamming your email occasionally a good way :).  When I redesigned the website, I made it very easy to post quick news updates.  You can thank Bill for the idea (or maybe blame him — your choice!)  I plan to provide updates on my research once or twice a quarter.  It’s up to you if you want to read my ramblings…..

With the release of v7.0, I’ve been able to shift my focus from Overset to a new project.  I had several ideas/interests and decided to look into Grid Deformation.  Cobalt has been able to calculate solutions on deforming grids since the release of v6.0 (February 2013).  Our plan was always to interface with a 3rd party grid deformer.  Unfortunately, that has not happened yet.  So I decided it was time to see if I could develop our own grid deformation tool…

I’ve spent the last month looking at grid deformation methods, writing little research codes to test out ideas, etc.  I’m finally ready to provide a little preview.  It’s only 2-D with rigid boundaries — but the interior of the grid must deform. Every step has a valid Cobalt grid…I’m ok with +/-90 degree rotation! If you have any questions, leave a comment or send me an email. Check back in a few weeks when I hope to have more to show you.

Cobalt v7.0 Released

Oscillating Aileron Geometry

Isosurface of Vorticity

Cobalt Solutions, LLC announces the release of Cobalt v7.0, the latest version of their unstructured flow solver.  The changes include a new Overset module along with speed-up in post-processing output and the addition of a few turbulence models.

Cobalt v7.0 introduces a new Overset module which provides improved hole-cuts and improved efficiency. The hole-cut interface is calculated to lie midway between non-cuttable boundaries of overset grids.  The result is a very smooth hole-cut - whether it is a full scale aircraft with tiny gaps in control surfaces or an aircraft with multiple weapons cluster in a weapons release simulation.  The implementation of the module follows the same guidelines as found in the flow solver Cobalt – robust and user-friendly.  The new Overset module is discussed here along with some examples: F-18 JDAM Weapon Separation, Argus Missile Paddle Deployment, Wing/Aileron/Flap.

The output of flow visualization data has been sped-up.  Depending on how many flow variables requested, a speed-up of 2x has been seen in average cases. 

Three turbulence models have been added: Hellsten’s EARSM (Explicit Algebraic Reynolds Stress Model) and EARSM-CC  (CC = curvature correction), and Menter’s SST-SAS model (SAS = Scale Adaptive Simulation).  This brings the total number of turbulence models in Cobalt to sixteen.   The complete list and references can be found in the User's Manual.