Chapter 8 Flashcards
(42 cards)
Why does temperature increase across a shockwave even when an adiabatic flow is being considered?
Because KE is converted to internal energy across the shockwave.
True or False
Flow-through a shockwave is adiabatic.
True
The physical mechanism of sound propagation in a gas is based on ___________.
molecular motion.
The speed of sound in a calorically perfect gas is a function of what property?
Temperature only.
True or False
The lower the compressibility, the higher the speed of sound.
True
In a scenario where the shockwave is stationary and flow moves through a stationary control volume containing the shockwave; is the flow steady?
Yes, the flow, in this case, can be considered steady.
See the submarine example in FM textbook chapter 5 for a good explanation.
True or False
Flow-through a stationary normal shock wave is isentropic.
True
Pg. 569
The speed of sound in a theoretically incompressible flow is _____.
Infinite
Pg. 571
The speed of sound in a calorically perfect gas is a function of ____ only.
Temperature
Pg. 571
The critical and total speed of sound is constant along a streamline so long as what conditions are met?
If the flow is both adiabatic and inviscid.
Ref: Pg. 578
What is the general rule of thumb as far as treating density as a variable in a flow field?
If the flow is less than Mach 0.3 the flow can be treated as incompressible. Higher than that, the flow should be treated as compressible and always above Mach 1.0.
Ref: Pg 584
What is a Mach wave?
An infinitely weak normal shock wave.
Ref: pg 589
Does the Mach number in front of a Mach wave differ from the Mach number behind it?
No, a Mach wave does not change the flow properties as they go across it.
If M1 = 1 then M2 = 1.
Ref: Pg 589
For a normal shockwave, if M1>1 then M2 is …?
Less than Mach 1.
Ref: Pg 589
For a higher upstream Mach number (M1) the wave will be weaker or stronger?
Stronger
Ref: Pg 589
If the upstream Mach number is equal to one, the entropy change across the wave will be ____.
Zero
s2-s1=0 –> s2=s1
Ref: Pg 590
Why does entropy increase across a shockwave?
Within the shock wave itself, large gradients in velocity and temperature occur. In other words, the mechanisms of friction and thermal conduction are very strong. These are both dissipative, irreversible mechanisms that always acts to increase entropy.
Ref: Pg 591
True or False
The total temperature varies across a stationary normal shockwave.
False, total temperature is constant across a stationary normal shock wave.
Ref: Pg. 592
How does total pressure change across a normal shockwave?
Total pressure decreases across the shock.
Ref: Pg 593
How does the ratio of total pressure across a shockwave change with increasing Mach number?
It decreases
Ref: Pg 593
How does the ratio of pressure across a shockwave change with increasing Mach number?
Increases exponentially
Ref: Pg 593
How does the ratio of temperature across a shockwave change with increasing Mach number?
Increases
Ref: Pg 593
How does the ratio of density across a shockwave change with increasing Mach number?
Increases logarithmically.
Ref: Pg 593
If a pitot tube is immersed in sub-sonic flow, why can isentropic conditions be safely assumed?
Because of the nature of how the pitot tube works. Flow is brought from a high velocity to zero velocity without a source of heating. Therefore the flow can be considered isentropic and the isentropic relations can be used.
Ref: Pg 603
