Materi 6 Topik Aliran Kompresibel

Summary

This video delves into the concepts of shock waves and expansion waves within the context of compressible flow. It explains the formation of shock waves, especially normal shock waves in supersonic flow, and their effects on fluid properties. The script covers the relationship between pre- and post-shock properties, using conservation laws of mass, momentum, and energy. Additionally, it introduces key concepts like stagnation enthalpy, entropy changes, and the Vanno and Rilake lines on an enthalpy-entropy diagram. The video also explores practical examples and how shock waves are relevant to various systems, including nozzles and aircraft engines.

Takeaways

• 😀 Sound waves are created by small pressure disturbances and propagate at the speed of sound.
• 😀 Shock waves in compressible flow are caused by sudden changes in fluid properties, particularly in supersonic flow conditions.
• 😀 Normal shock waves occur in a plane perpendicular to the flow direction, and the flow through them is irreversible.
• 😀 The conservation laws of mass, momentum, and energy are used to analyze the flow before and after a shock wave.
• 😀 The stagnation enthalpy remains constant across a shock wave, but stagnation pressure decreases due to irreversibility.
• 😀 The static temperature increases drastically across a shock wave as kinetic energy is converted into enthalpy.
• 😀 The flow before a shock wave is supersonic, and it becomes subsonic after passing through the shock wave.
• 😀 Shock waves can occur in supersonic nozzles, aircraft engine intakes, or diffusers, and can also result from explosions.
• 😀 The Vanno line and the Rilake line on an enthalpy-entropy diagram represent conditions where stagnation enthalpy and mass flux are constant.
• 😀 The shock wave phenomenon becomes more intense as the Mach number increases, and when the Mach number is equal to 1, the shock wave turns into a sound wave.

Q & A

What are shock waves in compressible flow?

Shock waves are sudden changes in fluid properties that occur in compressible flow, particularly under supersonic conditions. These waves form when there is a rapid transition in pressure and other fluid properties, typically in a convergent-divergent nozzle.

What is the difference between normal shock waves and expansion waves?

Normal shock waves occur in a plane perpendicular to the flow direction, and they result in a drastic change in flow properties such as pressure, temperature, and velocity. Expansion waves, on the other hand, involve a gradual change in these properties and occur when the flow expands, typically in a divergent nozzle.

What happens to the flow properties as a fluid passes through a normal shock wave?

When a fluid passes through a normal shock wave, its velocity decreases significantly, leading to an increase in static temperature and pressure. The stagnation pressure also decreases, but the stagnation enthalpy remains constant across the shock.

Why does the stagnation pressure decrease across a shock wave?

The stagnation pressure decreases because the shock wave is an irreversible process. This irreversibility leads to energy dissipation, which causes a drop in pressure, despite the stagnation enthalpy remaining constant.

What does the conservation of energy in shock waves imply for stagnation enthalpy and temperature?

The conservation of energy in shock waves implies that stagnation enthalpy remains constant across the shock, while the static temperature increases significantly. This increase occurs because the kinetic energy of the fluid is converted into thermal energy (enthalpy).

How does the Mach number change across a shock wave?

The Mach number decreases as the flow transitions from supersonic to subsonic after passing through the shock wave. The greater the Mach number before the shock, the stronger the shock wave formed.

What is the significance of the Vanno line and Rilake line in thermodynamics?

The Vanno line and Rilake line are curves plotted on an enthalpy-entropy diagram. The Vanno line represents states with the same stagnation enthalpy and mass flux, while the Rilake line represents states with constant stagnation enthalpy and momentum. These lines intersect at points representing conditions before and after a shock wave.

What happens when the Mach number equals 1 in the context of shock waves?

When the Mach number equals 1, the shock wave transforms into an ordinary sound wave. This marks the transition from supersonic to subsonic flow without the characteristic sharp changes in fluid properties seen in normal shock waves.

How does entropy change across a shock wave?

The entropy increases across a shock wave because the flow through the shock is adiabatic but irreversible. This increase in entropy is a key characteristic of shock wave behavior.

In which other contexts, besides supersonic nozzles, can shock waves occur?

Shock waves can also occur in supersonic aircraft engine intakes or diffusers, where they help slow the airflow before it enters the compressor. Additionally, explosions can create powerful spherical shock waves, which can be highly destructive.