Physics question...propulsion in a swimming pool

It's probably a mistake for me to come back to this thread. But let's focus on what we agree on.

Rocket engines work in a vacuum. Because they eject particles having both mass and velocity.

Propellers do NOT work in a vacuum. Because they operate off of Bernoulli's principle of pressure differential related to flow of a liquid or gas over paths of differing length.

We agree on these things.

Now - you guys are drilling in on the finer point of "does a rocket operate more (or less) efficiently in a vacuum vs in a medium. And if so, what physical principle makes it so?

At this point - I punt. I took basic newtonian physics in college. I never took any classes in hydrodynamics and fluid dynamics. I will say this - when studying the finer points of these principles, don't count on your intuition to guide you as that will usually lead to an incorrect conclusion. (e.g. the "push against" fallacy.)

 

wrong

wrong principal, we are discussing FLUID dynamics, not shotguns which use expanding gas.

We never said that there would be any more or less energy. Our discussion is how fluid dynamics work when in or above the waterline.

Never a mistake to come have fun! lol. Again though, we are not (at least I am not) trying to say anything about how a rocket moves. I am specifically referring to fluid dynamics.

YotaToy, I suggest you grab a physics book. I deal with fluid dynamics on a regular basis in my job, and I can assure you that your information is incorrect.

 

Did you just claim that the laws for expanding gases and fluid dynamics are somehow different? They are not.

You were discussing whether or not the raft would move, how is that not a talk about more or less energy and how it affects the raft?

I have enough physics to know that this entire argument is actually the "push off of" fallacy.

 

A: Uh, yes, they most certainly are different. Or at least different in their reaction rate and percentages. Again, recheck your information.

B: We are discussing if the raft would move differently in the fluid exhaust were below or above the waterline. It has nothing to do with more or less energy, but how that energy is transferred. Anyone with rudimentary science knowledge knows that energy is neither created nor destroyed, but merely changes state, or direction.

C: MY arguement is about fluid dynamics. nothing more. While the excerpt below appears to be airflow, it is actually an explanation of fluid dynamics, and how it differs between compressible(in air discharge) versus incompressible(under water discharge) and fully backs up exactly what I have been saying.



Compressible vs incompressible flow

All fluids are compressible to some extent, that is, changes in pressure or temperature will result in changes in density. However, in many situations the changes in pressure and temperature are sufficiently small that the changes in density are negligible. In this case the flow can be modeled as an incompressible flow. Otherwise the more general compressible flow equations must be used.
Mathematically, incompressibility is expressed by saying that the density ρ of a fluid parcel does not change as it moves in the flow field, i.e.,
where D/Dt is the substantial derivative, which is the sum of local and convective derivatives. This additional constraint simplifies the governing equations, especially in the case when the fluid has a uniform density.
For flow of gases, to determine whether to use compressible or incompressible fluid dynamics, the Mach number of the flow is to be evaluated. As a rough guide, compressible effects can be ignored at Mach numbers below approximately 0.3. For liquids, whether the incompressible assumption is valid depends on the fluid properties (specifically the critical pressure and temperature of the fluid) and the flow conditions (how close to the critical pressure the actual flow pressure becomes). Acoustic problems always require allowing compressibility, since sound waves are compression waves involving changes in pressure and density of the medium through which they propagate.
Viscous vs inviscid flow


Potential flow around a wing


Viscous problems are those in which fluid friction has significant effects on the fluid motion.
The Reynolds number, which is a ratio between inertial and viscous forces, can be used to evaluate whether viscous or inviscid equations are appropriate to the problem.
Stokes flow is flow at very low Reynolds numbers, Re<<1, such that inertial forces can be neglected compared to viscous forces.
On the contrary, high Reynolds numbers indicate that the inertial forces are more significant than the viscous (friction) forces. Therefore, we may assume the flow to be an inviscid flow, an approximation in which we neglect viscosity completely, compared to inertial terms.
This idea can work fairly well when the Reynolds number is high. However, certain problems such as those involving solid boundaries, may require that the viscosity be included. Viscosity often cannot be neglected near solid boundaries because the no-slip condition can generate a thin region of large strain rate (known as Boundary layer) which enhances the effect of even a small amount of viscosity, and thus generating vorticity. Therefore, to calculate net forces on bodies (such as wings) we should use viscous flow equations. As illustrated by d'Alembert's paradox, a body in an inviscid fluid will experience no drag force. The standard equations of inviscid flow are the Euler equations. Another often used model, especially in computational fluid dynamics, is to use the Euler equations away from the body and the boundary layer equations, which incorporates viscosity, in a region close to the body.
The Euler equations can be integrated along a streamline to get Bernoulli's equation. When the flow is everywhere irrotational and inviscid, Bernoulli's equation can be used throughout the flow field. Such flows are called potential flows.

 

More information supporting exactly what I have been saying....

The Navier–Stokes equations (named after Claude-Louis Navier and George Gabriel Stokes) are the set of equations that describe the motion of fluid substances such as liquids and gases. These equations state that changes in momentum (force) of fluid particles depend only on the external pressure and internal viscous forces (similar to friction) acting on the fluid. Thus, the Navier–Stokes equations describe the balance of forces acting at any given region of the fluid.
The Navier–Stokes equations are differential equations which describe the motion of a fluid. Such equations establish relations among the rates of change of the variables of interest. For example, the Navier–Stokes equations for an ideal fluid with zero viscosity states that acceleration (the rate of change of velocity) is proportional to the derivative of internal pressure.
This means that solutions of the Navier–Stokes equations for a given physical problem must be sought with the help of calculus. In practical terms only the simplest cases can be solved exactly in this way. These cases generally involve non-turbulent, steady flow (flow does not change with time) in which the Reynolds number is small.
For more complex situations, such as global weather systems like El Niño or lift in a wing, solutions of the Navier–Stokes equations can currently only be found with the help of computers. This is a field of sciences by its own called computational fluid dynamics.

 

lol. I WISH she was my gf, but no such luck. Looks a lot like Tara Reid, which I one of the many reasons I have kept this sig for so long!

 

Females do not FART--YEA RIGHT
Looked like one to be proud of.
Who did that--whats that smell?

 

Wow... you're hopeless. Stay in school.

 

too many posts to read through since my last vist ... but, in skimming, it seems some folks might be letting some false intuition get into the mix... the genius of many of the fundamental theorems/laws such as newtons laws, benoulli's law, the navier stokes equation, the laws of thermodynamics, ...etc... is they hash the problem in different ways, but, they basically all support them same end result... IMO, the best expression for describing a fixed system, called "control volume" is the 1st law of thermodynamics, in it's expanded form since it enumerates the various types of energy transfer and mass flow/transfer.. ...so, i'd implore some to do some more research, ...look at these laws/theorems and pay attention to bernoulli's law, ...basically for a given exit flow, assuming the stream isn't being constricted or in some other way affected by external factors - the "pushing off of other particles", or "pushing off a solid wall" thing just doesn't substantiate a physical relevance, ...the mass/velocity of the flow exiting the control volume wholly encompasses the dynamics... the rocket analogy made by another poster is a prime example... same principle that allows satellites to make adjustments to their orbit...

...now is the flow affected by exhausting into water vs. air, ...probably - did hear mention of boundary layer flow, reynolds #, etc... while that is getting more into the weeds, i will contend that this would affect the flow, assuming a fixed "forcing" from the system, ...but is probably pretty negligible in this situation since we are dealing w/ a situation <<<< than supersonic speeds...

 

Every action has an equal and opposite reaction.
You will have to suck and blow in opposite directions to move.

May be a slight difference in speed.

No difference unless the tip of the blaster is in direct contact with the side (which increases pressure). As soon as you begin to move, that advantage is lost.


Now, you mentioned jet propulsion, so I imagine the argument went further than the above and some "examples" were given such as the Harrier where the air is drawn in at the front and exhausted downward, and even in a way that allows the plane to move backwards.

The engine is adding fuel and heat to the incoming air. The pressure of the exhaust is significantly greater than the suction of the intake. An open propeller with a simple 180-degree duct placed behind it would not produce motion as the sucking and blowing would cancel.

Increasing the size of the intake, and reducing the size of the exhaust, will increase the pressure at the exhaust relative to the intake and then allow for rearward movement... Models of the Harrier have been successfully flown using ducted fans.


So to follow that example, your blaster COULD suck and blow in the same direction and provide forward movement if a nozzle were installed during the "blow" stroke.

 

correct. or you just blow harder than you suck.

 

Yes and no....

With the same nozzle size, the volume drawn and expelled is the same.
The difference in pressure from blowing harder is offset by the shorter time.

You actually need to increase the pressure while not reducing the time of the burst.

 

this is what I was thinking about.

http://video.search.yahoo.com/video...sigr=12c5m5p6a&newfp=1&tit=Octopus+Swimming+2