I am little confused about the meaning of thermal plumes and convection currents. As thermal plumes is flow driven by buoyancy effects, while convection currents is also flow driven by the density difference. Both are same or some difference is there ?
Dear Dr. Rajesh Choudhary,
Please be patient with my lengthy and boring explanation....here goes....
Assume a rectangular cavity with length four times it's height. Heat the bottom....insulate the side walls....keep the top at a lower temperature compared to the bottom. Assuming air inside the cavity, it will so happen that the (bottom) air adjacent to the hot wall will become lighter due to thermal expansion and move up (flow becomes buoyancy driven). Once at the top, the air would cool and become heavier and fall downwards towards the hot wall only to be reheated again and sent back to the top.
The ensuing vortical/rotational/circuitous movement of air inside the cavity is what is called a convection current and that vortex itself is called a convection cell.If the cavity is square....through an extremely crude assumption one could expect that it would have one convection cell with size comparable to the entire domain (not true at all times). But here's the twist....
At the outset we assumed that the cavity is rectangular with length four times the height. In that case, there are multiple convection cells generated within the same cavity and the flow pattern is characterized by slender vortices (that occupy the entire height of the cavity) placed side by side. This is where the concept of thermal plumes comes up. Since the bottom is heated, hot air from the bottom has to rise up....however, with the convection cells active within the cavity, an interplay emerges between (atleast) two forces.....(a) the buoyancy effect that tries to push the hot air upwards and (b) the inertia effect induced by the cells that opposes upward motion. The hot air would only rise up at points where the buoyancy effect dominates inertia or (put more correctly) get favorably aided by inertia (momentum) of the convection cells to move up. Turns out that there do exist such points of escape....
It is common knowledge that a vortex cannot be "breached" by flows directed towards it's center....it's like a balloon....but unlike pressure (that's inside the balloon) the vortex exerts centrifugal force which "forbids" a direct flow of momentum towards it's center. In fact, the path of least resistance for any such flow is around the periphery of the vortex where the centrifugal forces (F=mv2/r) are weakest (I hope I am not wrong on this one).
For the hot air, this path of least resistance lies between two adjacent vortices where the flow is directed upwards (since adjacent convection cells are counter-rotational). Hence, a jet of hot air shoots upwards through the junction of two adjacent vortices and blooms at the top due to (combined) advection-diffusion into a two-lobed pattern....resembling mushroom clouds that form during atomic bomb explosions. Such bi-lobed structures of hot air that routinely shoot-up from the heated wall (bottom) between adjacent convection cells is what is called a thermal plume.
I am attaching a picture from one of my simulations that will showcase everything that I have explained above.
I hope this helps.
Best regards.
Shaswat
Thank You @Vitalii Pestsevyi for your answer. I am clear on that both are different phenomenons from the sentence "The more addition of nano-particles will cause the strong convection currents that are mainly experienced in the plume regions to diminish which will cause a disappearance of the plume region". But what plume region means and how convection currents diminish the plume regions?
Dear Dr. Rajesh Choudhary,
Please be patient with my lengthy and boring explanation....here goes....
Assume a rectangular cavity with length four times it's height. Heat the bottom....insulate the side walls....keep the top at a lower temperature compared to the bottom. Assuming air inside the cavity, it will so happen that the (bottom) air adjacent to the hot wall will become lighter due to thermal expansion and move up (flow becomes buoyancy driven). Once at the top, the air would cool and become heavier and fall downwards towards the hot wall only to be reheated again and sent back to the top.
The ensuing vortical/rotational/circuitous movement of air inside the cavity is what is called a convection current and that vortex itself is called a convection cell.If the cavity is square....through an extremely crude assumption one could expect that it would have one convection cell with size comparable to the entire domain (not true at all times). But here's the twist....
At the outset we assumed that the cavity is rectangular with length four times the height. In that case, there are multiple convection cells generated within the same cavity and the flow pattern is characterized by slender vortices (that occupy the entire height of the cavity) placed side by side. This is where the concept of thermal plumes comes up. Since the bottom is heated, hot air from the bottom has to rise up....however, with the convection cells active within the cavity, an interplay emerges between (atleast) two forces.....(a) the buoyancy effect that tries to push the hot air upwards and (b) the inertia effect induced by the cells that opposes upward motion. The hot air would only rise up at points where the buoyancy effect dominates inertia or (put more correctly) get favorably aided by inertia (momentum) of the convection cells to move up. Turns out that there do exist such points of escape....
It is common knowledge that a vortex cannot be "breached" by flows directed towards it's center....it's like a balloon....but unlike pressure (that's inside the balloon) the vortex exerts centrifugal force which "forbids" a direct flow of momentum towards it's center. In fact, the path of least resistance for any such flow is around the periphery of the vortex where the centrifugal forces (F=mv2/r) are weakest (I hope I am not wrong on this one).
For the hot air, this path of least resistance lies between two adjacent vortices where the flow is directed upwards (since adjacent convection cells are counter-rotational). Hence, a jet of hot air shoots upwards through the junction of two adjacent vortices and blooms at the top due to (combined) advection-diffusion into a two-lobed pattern....resembling mushroom clouds that form during atomic bomb explosions. Such bi-lobed structures of hot air that routinely shoot-up from the heated wall (bottom) between adjacent convection cells is what is called a thermal plume.
I am attaching a picture from one of my simulations that will showcase everything that I have explained above.
I hope this helps.
Best regards.
Shaswat
Dear Mr. Shaswat,
Yes, it helps a lot. I am very thankful for your answer with a great elaboration. Your answer also cleared my all doubts regarding the thermal plume and convection currents. Thank you very much for your support.
Thanking You
Rajesh
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