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MAE 3303 - Exam 2
Quizzes 9-14
| Question | Answer |
|---|---|
| What is Mach wave? What is the Mach angle? | A Mach wave is the weak disturbance envelop in a supersonic flow (an infinitely weak shock wave with normal component of Mach number equal to 1). The Mach angle is the angle between a Mach wave and the flow direction denoted by the symbol µ. |
| Is the flow across a Mach wave isentropic? | Yes, the flow across a Mach wave is isentropic. |
| When does an oblique shock wave occur in a supersonic flow? | An oblique shock wave occurs in a supersonic flow when the supersonic flow turns into itself. It forms when compression Mach waves coalesce and reinforce each other forming a strong compression wave. |
| How is the oblique shock wave angle defined? | The oblique shock wave angle (ß) is defined as the angle between the incoming flow direction and the shock wave. ß > µ |
| **How do the fluid properties change across an oblique shock wave? | Mach number and velocity decrease. Pressure, density, and temperature increase. |
| **Changes across a normal shock wave area a function of what? Changes across an oblique shock wave are a function of what? | Across a NSW, changes are a function of the upstream Mach number. Across an OSW, changes are a function of the upstream Mach number and shock wave angle (i.e. changes are a function of the normal component of the free-stream Mach number). |
| Is the flow across an oblique shock wave isentropic? | No, the flow is NOT isentropic. |
| What is the difference between a normal shock wave and an oblique shock wave? | A normal shock wave is perpendicular to the flow direction, while an oblique shock wave is not. |
| What happens to the normal and parallel velocity components to an oblique shock as a supersonic flow passes through the shock wave, i.e. goes up, down , or stays the same? What happens to the Mach number components? | Goes down: normal component of velocity, normal component of Mach number, parallel (tangential) component of Mach number Stays the same: parallel (tangential) component of velocity |
| For a given Mach number, what is the meaning of the maximum deflection angle, Θ_max? | It's the maximum turning angle for which an attached oblique shock is possible for the given Mach number. |
| **What will happen if we attempt to turn a supersonic flow through an angle greater than Θ_max for a given Mach number? | A detached, curved (bow) shock wave will form ahead of the body. |
| The Mach number downstream of a normal shock is always subsonic. Is the same true for an oblique shock wave? Explain your answer. | No, the Mach number downstream of an oblique shock wave is given by Θ-ß-M1 relation. For a strong shock solution, M2 is subsonic. For a weak shock solution, M2 is supersonic except for a small region near Θ_max. |
| Which solution is usually observed, the weak or strong oblique shock? | Weak oblique shock. |
| How will the change in entropy compare for a M=2 normal shock and a M=3 oblique shock with a shock wave angle of 30°? | The M=2 normal shock wave will have a bigger entropy change than the M=3 oblique shock wave. |
| **How will the change in entropy compare for a M=2 normal shock wave and a M=2 oblique shock with a shock wave angle of 45°? | The normal shock wave will have a bigger entropy change than the oblique shock wave. |
| For a fixed deflection angle Θ, as the free-stream Mach number decreases from the high to low supersonic values, the oblique shock angle increases or decreases? The shock becomes weaker or strong? | OBS angle (ß) increases, shock becomes weaker. |
| For a fixed free-stream Mach number, as the deflection angle decreases from high to low values, the oblique shock angle increase or decreases? The shock becomes weaker or stronger? | OBS angle (ß) decreases, shock becomes weaker. |
| **Under what conditions do the oblique shock relations reduce to the normal shock relations? | When wave angle ß = π/2. |
| **Under what conditions do the oblique shock relations reduce to the change in properties across a Mach wave? | When the wave angle equals the Mach angle (ß = µ). |
| Why is the oblique shock reflected when it impinges on a solid wall? | Because the wall is impenetrable and the flow immediately adjacent to the wall must be parallel to the wall, so the supersonic flow behind the incident shock wave is turned into itself creating the reflected shock wave. |
| What is the Mach reflection? | When the flow Mach number and deflection angle behind the incident shock are in such a condition that no oblique shock solution exists, a single NSW is formed on the wall. A curved reflected shock branches from the NSW and propagates downstream. |
| What is the slip line behind the intersection point of two shock waves? Across the slip line, which flow properties are continuous, which are not? | The slip line divides the two shock wave regions behind 2 refracted shock waves. All properties are different except for pressure and flow direction. |
| How will the shock wave strength (or the change in entropy or total pressure) compare for a wedge and a cone of the same deflection angle and Mach number? Why? | Because of the three-dimensional relieving effect, there's less obstruction which makes the shock wave strength of a cone weaker than the shock wave strength of a wedge. |
| A bow shock wave is formed ahead of a blunt body in a supersonic flow field. Is the flow behind the bow shock supersonic or subsonic? | It's a mixed region of both subsonic and supersonic flow. The dividing line between the subsonic and supersonic regions is called the sonic line. |
| The waves generated in a supersonic flow over a concave surface of a body are compression waves or expansion waves? | The waves generated in a supersonic flow over a concave surface of a body are compression waves. |
| When will expansion waves occur in supersonic flow? | When a supersonic flow is turned away from itself. |
| Is the flow across a shock wave isentropic? Is the flow across expansion waves isentropic? | Flow across a shock wave is NOT isentropic. Flow across expansion waves IS isentropic. |
| **Does the Mach number increase, decrease, or stay the same across expansion waves? | The Mach number increases across expansion waves. An expansion corner is a means to increase supersonic flow Mach number. |
| **What is an expansion fan? | A continuous expansion region that is composed of an infinite number of Mach waves. |
| Indicate whether each of the following quantities goes up, down ,or remains the same as the flow crosses expansion waves: Mach number, velocity, density, pressure, temperature, entropy, stagnation temperature, stagnation pressure | Increase: Mach number, velocity Decrease: pressure, density, temperature Remains the same: entropy, stagnation temperature, stagnation temperature |
| Why can we say that expansion waves are the antithesis of shock waves? | Because changes in flow properties across expansion waves and across shock waves are opposite. |
| What are the two necessary conditions for boundary layer separation? | Viscosity and large adverse pressure gradient |
| Why can a strong shock wave/boundary layer interaction cause boundary layer to separate locally from a surface? | Because the large pressure rise across the shock wave acts as a severe adverse gradient imposed on the boundary layer which causes the boundary layer to locally separate from the surface. |
| What is wave drag? | The drag due to shock waves. |
| Does the wave drag of a supersonic airfoil increase or decrease as the thickness increases? | Wave drag increases as thickness increases. |
| What are the dimensions of the coefficients of lift, drag, and pitching moments? | They're dimensionless. |
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