I am measuring terminal velocity of Composite seeds in a closed chamber (assumed to be steady air).
The falling seeds have some slight lateral movement, generating a slight velocity in the horizontal direction. However, of course, the vertical velocity is highly associated and very similar to the overall velocity. Therefore, when measuring the terminal velocity of a seed, should I use the overall velocity (combining both vertical and lateral movement) or just the velocity in the vertical direction?
I have not found any paper clearly mentions the direction of velocity or discusses this point. Any recommendation?
In my opinion, the lateral movement of falling seeds in a closed chamber is caused by seed spinning or air fluctuation in the chamber, or both. If the reason is the former one (seed spinning), I might think using overall velocity would be more reasonable since it describes the overall behaviours of a falling seed. However, there seems to be few ways to separate these two effects. Any comments on this unexpected lateral movement?
Many thanks!
Lateral movement of seed is usually caused by the presence of seed hairs or irregular geometry of seeds. It is impossible to avoid, except in a vacuum chamber. However, it doesn't affect the vertical velocity, so I would use overall velocity, which describes the natural situation :-)
I will also suggest the same as recommended by Mr. Pegman. Even I have also performed the same experiment in some other family and also by consulting the previous literature. The lateral movements is due to the pressure of accessory structures which is a natural phenomenon which facilitates their dispersal abilities. My suggestion is to place a white sheet of cloth on the ground surface and make quadrants (I,II,III & IV) on it with center of origin coinciding exact with the point of release. when the seed is dropped from certain height, it will landed somewhere on the sheet. Now, measure the distance from center of quadrant to the point where seed is actually landed with the help of a simple foot rule (scale). Thereby, you will be able to calculate even the lateral displacement of the fallen seeds.
Hi Sichong Chen!
I was wondering that measure the overall velocity is a better choice. This measure would capture the natural way the seeds fall to the ground, simulating what happens in nature, as the colleagues stated above . But the final decision will always rely on your question. All the best! Alexander
Dear
Sichong Chen
The goal is to pay attention in the concepts that involve physics. When the net force on the object is zero, the object ceases to accelerate and the result is that the velocity of the object remains constant. The drag or force of resistance will equal the gravitational pull on the object. Hereafter, we also need to understand the physic concept of vector. Thus, working on such subject requires some facilities available in physics laboratory. Congratulations, because it requires studies in physics, biology and statistics. Additional information for starting the better understanding of this subject, you can type terminal velocity, wikipedia. Next, the studies will guide you. Best regards. Walter.
The vertical component of velocity tells you how fast the seed reaches the ground (or how long it stays in the air, subject to lateral movement by the wind in nature). The horizontal component in still air tells you how far the seed could be dispersed under these conditions in nature, but most wind-dispersed species, including Asteraceae, don't release seeds into still air, so this is not ecologically very interesting.
Thank you all very much for the answers and comments.
We finally decided to use the terminal velocity in the vertical direction. Reasons below (for people may have similar questions later on):
We agree that the lateral movement is a part of the natural behaviours which may also be correlated with the dispersal of the seed. However, such observed lateral movement can be subjective to the release apparatus in the beginning part of seed dropping -- we are using different ways to release seeds (two rotating planes with different sizes, as well as direct release by tweezers; BTW, what kind of release apparatus you've ever used to minimize the induction of horizontal force?). In contrast, the terminal velocity in the vertical direction won't be changed by the force unrecognisably induced in the releasing. We think vertical terminal velocity (theoretically) stays consistent across droppings of the same seed, and be more comparable across seeds.
We also very agree with Richard Corlett that the horizontal component is less ecologically interesting in the context of seed dispersal ability across seeds of different origins.
Dear Sichong,
The lateral movement of the particle is due to the emergence of the Force of Magnus. Due to the deviation from the spherical form, the couple of forces gravity-pressure generates spin movement of the particle. And this generates a pressure gradient that behave as a side force, namely the force of Magnus. And due to the same deviation from the spherical form, the direction of this force is continuous oscillating with the frequency of the spin movement that is increasing and decreasing the floating speed of the particle. Therefore, a fixed floating speed for non-spherical particle can not be defined but only a domain. If you will use perfect spherical particles, the force of Magnus will not appear, but still appears by the collisions between the particle. However, for the perfect spherical particle, the force of Magnus, for the vertical testing pipe, will not affect the floating speed. I would recommend you to take a look at our published paper "Considerations over the floating speed of a particle in vacuum pneumatic conveying systems in flour milling". You can download it from here.
- Badges
- Science topic
- Similar topics
- Agriculture
- Horticulture
- Seeds