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# Designing a particle engine

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Not sure this is the right forum, but here goes . Im currently in process of designing a general purpose particle engine. Im trying to determine what control parametres will give the user the best balance between control and ease of use. At this point im leaning towards modelling the engine dynamics by a range of simplified laws of physics. Below is an example list of parametres i imagine could be of use. Surroundings/medium: Viscosity Gravity wind/current (velocity) Particles: Initial position Initial velocity Weight(t) Resistance(Edit: friction) factor (sorry, i dont know the english word) Noise effect (eg Perlin). Lifespan Any thoughts on this approach ? Will it grant the right level of control to the user ? Whats the "normal" way ? Thanks in advance Edited by - newdeal on February 6, 2002 11:38:45 AM

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I don''t know much about physics, or particle engines, but just off the top of my head: If you have gravity, and Resistance factor (is "air friction" something like what you meant?), I don''t see why you need Weight. In a vacume two objects fall at the same rate regardless of weight. (if I remember what little physics I know correctly)

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true but wind affects heavier objects less than light ones and if the particle effect was to be used for bubbles it would be in water and heavy objects sink while light ones dont.

-ando15

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Also i think you should perhaps add elasticity if you want the particle to be able to bounce (ie a spark or something) so you can calculate how much the particle should bounce. Besides that i think the variables you mentioned would give the user enough control over the particle.

-ando15

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What AP said.

And yeah, "friction" is the word i was looking for (disappeared from my vocabulary somehow)

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Elasticity... I like that

Thanks

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quote:
Original post by Anonymous Poster
true but wind affects heavier objects less than light ones and if the particle effect was to be used for bubbles it would be in water and heavy objects sink while light ones dont.

-ando15

Actually, objects that float or sink is an issue of buoyancy rather than air resistance which are two completely different physical processes, even though they look similar (a pressure gradient on the surface of an object).

Timkin

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quote:
Original post by Anonymous Poster
true but wind affects heavier objects less than light ones and if the particle effect was to be used for bubbles it would be in water and heavy objects sink while light ones dont.

In a way you're correct to say that wind affects heavier objects less than light ones. That is to say that the same force due to wind on a heavier object causes less acceleration than it causes for a light object. But mass and weight have nothing to do with the actual forces applied to the object by the wind. Only shape, surface properties (roughness), and stiffness/rigidness of the shape (is it deformable) affect the wind forces. (Plus oddities such as temperature and permeability). Oh, and yeah, the properties of the air and wind itself.

Floating objects are basically not moving relative to the fluid that surrounds them (water and air), and so there is no wind! Or at least the wind is very small. Even if the air does blow around a bit, its effect is usually negligible in the direction of gravity.

quote:
Original post by Timkin
Actually, objects that float or sink is an issue of buoyancy rather than air resistance which are two completely different physical processes, even though they look similar (a pressure gradient on the surface of an object).

Timkin's right, although we could get into a big philosophical discussion of what is wind, etc. And actually the physical processes are exactly the same, in that they are both governed using the same full Navier-Stokes equations. Buoyancy forces are often ignored for objects moving in air that are heavier than air. When objects are floating, you have two fluids and a fluid/fluid interface, but the governing physics are the same, just with different boundary conditions.

Buoyancy forces are really a difference in pressure from one side of an object to the other. Water pressure (and air pressure) is greater at lower altitudes than at higher altitudes. Thus, there is more water or air pressure on top of an object than on the bottom. There is a net buoyancy force upwards, and if that buoyancy force is equal to the weight of the object, the object will stay afloat (even in air, as in hot air balloons). Even a small difference in height, say a few inches representing the diameter of a beach ball, can create enough buoyancy force for the object to float in water.

But buoyancy forces and air resistance are not without their common aspects. Air resistance has a few different components. There is surface or skin friction, which is fluid resistance created tangential to the surface (and normally what people think of when they think drag or air resistance). And then there is pressure drag, fluid resistance caused by a separation of smooth flow off of the surface creating a wake region of lower pressure turbulence at the rear of the object.

The pressure drag is similar to buoyancy. Both are due to a balance or imbalance in pressure forces, which act normal to the surface of the object. It just happens that buoyancy forces exist even when objects are not moving, while pressure drag is only present when the object is moving through the fluid (e.g., when there is a wind!). When an object is moving, you actually have both buoyancy and pressure drag forces. And when the object velocity has a nonzero component in the direction of buoyancy forces (really, in the direction of gravity, + or -), its difficult to distinguish between the two. They sort of become one collective pressure effect.

I won't even begin to talk about wave drag, .

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

Edited by - grhodes_at_work on February 7, 2002 11:16:27 AM

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