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October 24, 2014, 11:20 |
simpleFoam sailboat aerodynamic simulations
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New Member
Jim Conger
Join Date: Jan 2014
Location: California, USA
Posts: 21
Rep Power: 12 |
I have been using simpleFoam to simulate airflow over sailboat sails/hull for the last 18 months. In general, this works very well. Here are a few tips for others who might be interested in doing this.
1) You can get useful results with 2D models as long as the sail configuration is not fully stalled. After that, 3D models are required as the airflow is intrinsically 3D. One of the attached images shows an extreme example of this with airflow streamlines in a downwind sailing configuration. In general, I have found the 3D models to be much better behaved than the 2D models, independent of the grid resolution. 2) SnappyHexMesh is a wonderful tool for creating the mesh around your model, but it has its own likes and dislikes. It is important to create a starting mesh where the grid cells are nearly rectangular. I use blockMesh with a uniform grid expansion from the center point to the edges to create the starting grid with a fine mesh near the center (where the model lies), uniformly expanding to a course grid at the edges. 3) For boundary conditions I fix the velocity and pressure at two 'inlet' sides and use zeroGradient on the two 'outlet' sides of the mesh cube. It is convenient to parameterize the velocity vectors so that it is easy to change conditions between runs. You can embed variables in your velocity (U) file using data from an include file. Something like: #include "velocityValues" internalField uniform ($XvelMPS 0 $ZvelMPS); ... inlet1 { type fixedValue; value uniform ($XvelMPS 0 $ZvelMPS); } 4) Unlike most aircraft wings, sails are flexible. You are inevitably going to want to change the shape and position of the sails many times in fine increments. I found it was easier to create the STL model of the sails directly in Python code than to use a 3D modeling tool like Blender. I combine a fixed model of the hull/mast with a dynamically generated model of the sails and boom as separate inputs into snappyHexMesh. 5) Like all other CFD models, a firm connection to reality is a requirement to have a model that you can trust. In my case I tested the model against the actual performance of a sailboat, using the boat speed and boat heel as the basis for comparison. This quickly pointed out some (stupid) errors in the model which were easy to fix. Once calibrated, the CFD model appears to predict the performance of the boat very well under all wind conditions, although I am only simulating situations with 'flat' seas. You can see further details at this link: https://sites.google.com/site/sailcfd/ |
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