# arkie87

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Rocket Scientist

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1. ## Viscous dissipation and isenthalpic expansion

This question pertains to "viscous dissipation" term in the energy equation in flows.I am a bit confused about viscous dissipation. It seems to me that viscous dissipation is essentially the same thing as pumping power, such that if I pump 1 m3/s of air at 1 kPa, it takes 1 kW. To get 1 kPa pressure drop, I need a lot of friction, and that friction will cause viscous dissipation, which will result in 1 kW of heating i.e. ~0.8 K temperature rise in the air. For instance, for the simple case of steady state, fully developed laminar flow between parallel plates, one case show that the viscous dissipation term exactly equals the pumping power. However, for an ideal gas, enthalpy is only a function of temperature. If I expand the air through a valve instead, thermodynamics tells me the temperature wont have changed if the process is isenthalpic.How do i reconcile these two? Why wont the air going through the valve heat up if it is dissipating the same amount of power?
2. ## Fan Thought Experiment

Ok, thanks! Sounds like something a Kerbal would do.

4. ## Best Place on Plane for Feeling the Least Turbulence?

My wife is not a happy flier, and particularly dislikes the feeling of turbulence. I've told her that she should probably book seats towards the center of the plane, since this should reduce the g-forces from turbulence. I was wondering: (a) Is this true, and if so, is the reduction significant? (b) Is the best place in the center, or the center of lift/drag? Is that likely to be in a specific location (center of wing? edge of wing? center of mass? etc... TIA
5. ## Maximum Acceleration Due to Buoyancy

Well, dont assume that shape. Assume a cylindrical shape... regardless, even for a cylinder water rushes in from directly below but also from the sides. But it is nearly impossible to know the relative balance of each without doing some sort of CFD. Agreed, that is my opinion.
6. ## Maximum Acceleration Due to Buoyancy

@Nightside Thanks again for your reply. Very often, just knowing what something is called provides a huge breakthrough in being able to learn about it i.e. knowing it is called "added mass" or "virtual mass" allowed me to find much more references out there. And interestingly, @Scotius, the navy has studied it way before ballistic missiles, to be able to predict acceleration of a submarine underwater, for instance (concept of added mass applies even without buoyancy). See these references below: https://core.ac.uk/download/pdf/36709212.pdf So, for a sphere, added mass is 1/2 of the mass of the displaced fluid, but for more streamlined bodies, added mass can approach 0, as shown here: https://www.researchgate.net/publication/233581794_Translational_Added_Mass_of_Axisymmetric_Underwater_Vehicles_with_Forward_Speed_Using_Computational_Fluid_Dynamics
7. ## Maximum Acceleration Due to Buoyancy

Not quite. The form drag force is zero (presumably) when the object isnt moving. I am asking about the acceleration obtained at the start. The quora answer @Nightside gave was pretty useful, and explains the answer to my question. TL;DR: there is a concept of "added/virtual mass" (it is basically the mass of fluid that gets dragged along with the rocket) that limits its acceleration at the start (when velocity and all forms of drag are zero). See @Nightside's answer, which directly conflicts with yours. maximum acceleration is 2g, even when velocity is zero (where form drag would be zero).
8. ## Maximum Acceleration Due to Buoyancy

There is form drag and viscous drag/skin friction. Viscous drag/skin friction can be zero (which is what i meant by zero drag), but form drag cannot i.e. the thought experiment would be ill-posed or self-contradictory. zero velocity... so skin friction is zero. yes, acceleration, and yes from standstill ( i never said not from a standstill)?
9. ## Maximum Acceleration Due to Buoyancy

lol, my bad in this instance. replies like that, i usually do ignore. its hard to ignore when they begin attacking your assumptions even though its a thought experiment...
10. ## Maximum Acceleration Due to Buoyancy

I agree, my question could have been more clear. But in no way did the original response actually respond to the approximately 2/3 of my question either... And while you might be upset at people who get annoyed at people who try to help them for free with their problems, it is also equally annoying to be on the other end of it, when you ask for help with a specific problem and someone answers a different question altogether without fully reading your question. Or worse-- they start a whole argument about one of your assumptions when its supposed to be a thought experiment... etc.... if i had a penny for every time that has happened on this forum (and others).... i'd have like 10 pennies (at least)
11. ## Maximum Acceleration Due to Buoyancy

I asked for maximum acceleration, which occurs the instant the object is let go, where drag is zero. Yeah, i didnt specifically state it, but i went ahead and discussed a lot of other items, which the responder didnt address. Just told me stuff i already knew. It can be very frustrating when you ask a specific and highly technical question, and someone responds with basic physics and doesnt address your question. @mikegarrison for instance, this response above... not sure what do with it...
12. ## Maximum Acceleration Due to Buoyancy

yeah, i've run CFD simulations and i have a phd in mechanical engineering in the fields of fluid flow & heat transfer, but because of the accelerating frame of reference, typical navier stokes equations are not applicable..
13. ## Maximum Acceleration Due to Buoyancy

arg... when the object is at rest, there is no drag. turbulence isnt an issue either for obvious reasons (Reynolds=0). sheesh. Yes, i didnt call it an air bubble since air bubbles can deform under drag force and also expand as the static pressure changes, as you noted. It's hard to tell a priori what minute details strangers on the internet will latch onto instead of answering the question at hand, so i opted for a volume of air with an infitinitely small, massless, rigid shell instead of a bubble. Yes, thank you for the quora link. Cant say i understand why the limit is 2g though. The answer also appears to indicate that shape would be a factor (since they solve potential flow around a sphere) i.e. 2g is the solution for a sphere, but what about a rocket-shaped object? And no, i'm not considering launching objects from the sea floor. just had a discussion where someone claimed "maximum acceleration due to buoyancy is 1g since 1g acceleration causes buoyancy," but that appears to be incorrect...
14. ## Maximum Acceleration Due to Buoyancy

Let's limit the fluid to water, let's limit the acceleration due to gravity to 1 g, and let's limit the object to being filled with air (and zero wall thickness). My main question is if the acceleration that you compute from a force balance i.e. (weight of displaced water - weight of object)/mass of object is physical, because the fluid underneath or on the sides of the object must accelerate to follow the object (or else you have cavitation), and this acceleration will "absorb" some of the available pressure drop or pressure gradient.
15. ## Maximum Acceleration Due to Buoyancy

My main question is *neglecting drag*, what is the maximum acceleration of the body. Theoretically, the limit should be the ratio of the densities of the fluid and the object times g. In reality, drag can be neglected as diameter --> infinity. Unless there is cavitation, there needs to be enough volume flow to fill the void left by the object. I dont know how you can say there isnt? I
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