[X3D-Public] [X3D] X3D Comment on WindPhysicsModel

Don Brutzman brutzman at nps.edu
Wed Dec 5 08:28:18 PST 2012

Marcus:  thanks for your original comment.  I hope that this response answers your question, the specification appears to be adequately defined.

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On 12/2/2012 10:43 PM, Don Brutzman wrote:
> On 11/30/2012 12:14 PM, Leonard Daly wrote:> Justin,
>> Dick and I were going through the comments for X3D V3.3. There is one comment that we assigned to you because we felt you were most likely to have the correct answer. It is #464 at http://www.web3d.org/membership/login/mantis/view.php?id=464.
>> The comment has to do with the equation for calculating pressure. In the equation there is a term that represents speed. The question is whether the speed is the relative speed between the particles and the wind or the absolute (in the local reference frame) speed of the particles.
> http://www.web3d.org/files/specifications/19775-1/V3.3/Part01/components/particle_systems.html#WindPhysicsModel
> also 40.3.1 X3DParticleEmitterNode, which says:
> "The surfaceArea field value represents an average frontal area that would be presented to the wind, assuming a spherical model for each particle (i.e., the surface area is the same regardless of direction)."
> Since there is only one set of values provided for the wind speed probability distribution function, it would appear that the wind speed is defined relative to local reference frame.  There is no assumption of wind spatial variability, so the pressure force would be consistent for all particles at a given moment in time.  An implementation would likely apply the current wind direction, speed, gustiness and turbulence during each time step using vector addition.  Whether that constant wind vector is applied to the frame of all particles, or on a particle-by-particle basis, would seem to be up to the implementation.  The result should be the same.
> all the best, Don

all the best, Don
Don Brutzman  Naval Postgraduate School, Code USW/Br       brutzman at nps.edu
Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA   +1.831.656.2149
X3D graphics, virtual worlds, navy robotics http://faculty.nps.edu/brutzman

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