Originally Posted by Yota Toy
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.,
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
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.
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