CH2: 1-D Gas Dynamics Flashcards
True or False
For one-dimensional flow, if there are sections over which the flow conditions are not uniform, it is still possible to apply the one-dimensional gas dynamics equations between sections where they are uniform and one-dimensional.
True.
Elements of Gas Dynamics, Roshko, Pg. 39
If the flow is steady, the mass of the fluid which passes a given section must eventually pass all the other sections farther downstream. What is this a statement of?
The law of conservation of mass.
Elements of Gas Dynamics, Roshko, Pg. 40
True or False
The first law of thermodynamics is used to relate the equilibrium conditions in a throttling process.
True
Elements of Gas Dynamics, Roshko, Pg. 41
For a flowing fluid, the basic thermodynamic quantity is the _______.
Enthalpy.
Elements of Gas Dynamics, Roshko, Pg. 41
How is enthalpy defined in gas dynamics (reference frame)?
It is defined as the enthalpy that would be measured by an observer moving with the fluid.
Elements of Gas Dynamics, Roshko, Pg. 41
If a gas is thermally perfect, the enthalpy (h) depends only on _____.
The absolute Temperature.
Elements of Gas Dynamics, Roshko, Pg. 43
If a gas is calorically perfect, cp is a _______.
Constant.
Elements of Gas Dynamics, Roshko, Pg. 43
For a callorically perfect gas, h = ______.
A constant value that equals cp*T
Where:
>cp = Specific heat at a constant pressure
>T = Absolute Temperature
Elements of Gas Dynamics, Roshko, Pg. 44
True or False
For adiabatic flow (q=0), the total enthalpy is constant.
True.
Elements of Gas Dynamics, Roshko, Pg. 43
(See Eq. (2.13) in section 2.4)
For a perfect gas, the total temperature between states 1 and 2 is always ______.
Equal.
T_01 = T_02
Elements of Gas Dynamics, Roshko, Pg. 44
An increase in ______ at constant stagnation enthapy must result in a _______ of stagnation pressure.
entropy
decrease
Elements of Gas Dynamics, Roshko, Pg. 44
(bottom of the page)
The irreversible increase of entropy and the corresponding decrease of stagnation pressure from point 1 to 2 in a flow is due to _________.
The production of entropy in the flow between points or reservoirs due to currents.
Note: Only if there are no dissipative processes, that is, only if the flow is in equilibrium throughout, will there be no production of entropy.
Elements of Gas Dynamics, Roshko, Pg. 45
A condition in which the entropy and total pressure do not vary between two points in a flow is referred to as ________.
Isentropic.
Elements of Gas Dynamics, Roshko, Pg. 45
Describe (physically) how total conditions are defined.
The local total conditions at any point in the flow are the conditions that would be attained if the flow were brought to rest isentropically and the flow is in equilibirum.
Elements of Gas Dynamics, Roshko, Pg. 45
True or False
For stagnation conditions to exist, it is not enough that the velocity is zero; it is also necessary that the equilibrium conditions exist (e.g., there are no currents).
True
Elements of Gas Dynamics, Roshko, Pg. 45
What is an isentropic flow?
A flow that is both in equilibrium (reversible and has no currents including friction) and adiabatic.
Elements of Gas Dynamics, Roshko, Pg. 49
True or False
Entropy per unit mass (s) varies for adiabatic flow.
False. Entropy is a constant for adiabatic flow.
Elements of Gas Dynamics, Roshko, Pg. 49
True or False
An adiabatic, non-viscous, non-conducting flow, that is in chemical equilibrium can be considered isentropic.
True.
Elements of Gas Dynamics, Roshko, Pg. 50
True or False
In adiabatic flow, an increase in velocity (u), will result in an increase in Mach number (M).
True.
Elements of Gas Dynamics, Roshko, Pg. 51
A flow in which the speed of sound is infinite, and the Mach number is zero is also called an ________.
Incompressible flow.
Note: a^2 := (dp/drho)_s = const.
Elements of Gas Dynamics, Roshko, Pg. 52
For a convergent/divergent nozzle, M=1 can only be obtained in the _______.
Throat.
Elements of Gas Dynamics, Roshko, Pg. 53
For compressible flow, the dynamic pressure depends not only on the difference between stagnation and static pressure but also on _______.
The Mach number.
Elements of Gas Dynamics, Roshko, Pg. 55
It is generally believed that bulk viscosity is negligible except with the structure of __________.
Shockwaves.
At a given pressure, as the temperature increases to around 800 K and beyond, the ________ perfect gas assumption is no longer valid.
Calorically
Lecture 2 PP, Slide 13
When the calorically perfect gas assumption begins to break down due to a temperature increase, what mode is first excited? Specific heats become a function of what TD state variable?
The Vibrational mode
Temperature
Lecture 2 PP, Slide 13
Departure from the perfect gas assumption for air begins around what temperature?
800 K
Lecture 2 PP, Slide 13
What are the two requirements for stagnation conditions to be valid?
Hint: How must the flow be brought to rest, and what condition must the entire flow meet?
- The flow must be brought to rest isentropicly.
- Equilibrium conditions exist (no currents)
Lecture 2 PP, Slide 19
For an inviscid fluid, what force provides the acceleration to a fluid element (assuming body forces are neglected)?
The pressure force.
Lecture 2 PP, Slide 20
What is a “homentropic” flow field?
In an isentropic flow in which the total pressure and density are also constant throughout the entire flow.
Lecture 2 PP, Slide 32
How is the Bernoulli equation derived? Is it an energy equation?
A momentum balance along a streamline. No
Lecture 2 PP, Slide 38
What six conditions must be present for the Bernoulli equation to be applied?
- Steady flow
- Along a streamline
- Incompressible
- Irrotational
- Inviscid
6 No heat or work
Lecture 2 PP, Slide 51
From what equation is the Bernoulli equation derived assuming steady flow?
Eulers Equation
Elements of Gas Dynamics, Roshko, Pg. 45-47
