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Simulating turbulence behind a moving vehicle.

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Old   August 14, 2014, 01:57
Default Simulating turbulence behind a moving vehicle.
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Dear friends

Trying to capture the whirls and curls of streamlines behind a vehicle.

With current model air streamlines flow around the vehicle and away. But can not catch the curling streamlines caused by drag.

Any specific turbulence model settings, solver settings, or any tips at the post-processor, where I can capture whirls streamlines behind a vehicle.

Just for your information. The vehicle is actually stationary, air flows towards the front of the vehicle.

Thanks
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Old   August 14, 2014, 06:47
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I assume you want an image in post-processing of the recirculating flow behind the vehicle. The easiest way to do this is to define some seed points in the recirculating region (either as explicit points, or a line/surface/volume object and use that object as a seed point for a streamline object.

Or you can use the whole fluid domain as seed points for the streamline object. If you do enough of them you will get some seed points in the recirculation region.
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Old   August 14, 2014, 12:29
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Thanks Ghorrocks

Yes, in the postprocessor.

Regarding the first paragraph of your reply, how to I define local seed points for streamlines right behind vehicle?

Will try the second solution mentioned in your second paragraph.

Thanks.
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Old   August 14, 2014, 18:31
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Define point objects if you only have one or two - or a point cloud if you have lots. Or use a line object or volume object. These are all objects you can create in CFD-Post.
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Old   August 17, 2014, 06:28
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Managed to set up a volume;
Insert>Location>volume

created a volume of a radius around the object.

Streamlines just cross through.

Just FYI

Domain turbulence model SST, inlet turbulence intensity at 5%. Air ideal gas. I am not so familiar with CFX.

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Old   August 18, 2014, 04:22
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Apologies

It looks like I have wrong set up the volume, just looked at the software manual as well as your helpful explanation.

Now I have set up the volume behind the vehicle (space of interest), with a radius of volume-sphere sufficient to capture the area of interest behind the vehicle.

Turbulence air movement is showing now.

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Old   August 18, 2014, 06:31
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Good to hear.

But an important fluid mechanics point - be very careful with your definition of turbulence. The wake region behind the vehicle is not necessarily turbulent. You can get wakes in laminar flow, and even wakes in laminar flows which are transient. The thing which makes a flow turbulent is the flow possesses a turbulence spectrum from flow length scales down to the Kolmogorov length scale (and likewise for time scales). A laminar flow can have a wake, but it will not possess eddy length and time scales down to the Kolmogorov scales.
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