Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
MAE 3303 Exam 1
Compressible Flow
| Question | Answer |
|---|---|
| What is the definition of a compressible flow? Are all liquids incompressible? Are all gas flows are compressible? | - Significant density change of 5% relative density change or more. - All liquid flows are considered incompressible. - Not all gas flows are considered compressible. |
| Does M --> ∞ imply V --> ∞? | No bc M = V/a |
| Above what Mach number will changes in air density exceed 5%? | M > 0.3 |
| What are the most important and distinctive effects of compressibility on flow? | 1. Choking: Wherein the duct flow rate is shapely limited by the sonic condition 2. Shock Waves: Which are nearly discontinuous property changes in a supersonic flow. |
| How is kinetic energy distinguished from internal energy? | Kinetic Energy: Bulk Motion of the fluid Internal Energy: Random motion of molecules of a system. |
| Why does thermodynamics play an important role in the study of compressible flows? | The motion of a compressible fluid is directly affected by its thermodynamic state. |
| What is the principle of state? | For a simple compressible system, composed of a single chemical species of gas, the thermodynamic state is completely defined by two independent state variables. |
| The thermodynamic state of a unit mass pure substance is determined by how many state variables? | cp and cv |
| State the thermal equation of state. | p = ρRT |
| What is a calorically perfect gas? State the caloric equations of state. When does this assumption fail? | - Calorically Perfect Gas: Constant specific heats (cp and cv) cp = R/ (γ-1) and cv = γR/ (γ-1) - Assumption fails if there's a very large temperature changes in the flow or if the gas temperature is very high. |
| What is an isothermal process? What is an adiabatic process? | Isothermal Process: Constant temperature. Adiabatic Process: No heat transfer between system and surroundings. |
| What is a reversible process? What common agent is required for irreversibility? | Reversible Process: No dissipative phenomena (friction, heat transfer, mass diffusion, shock waves) The common agent required for irreversibility is a significant spatial gradient of the field. |
| An isentropic process is always | Adiabatic and Reversible |
| State the 1st law of thermodynamics applied to a system. | For a closed system: Q + W = Δ E |
| In a real process the entropy change can be divided into what two categories? | ds = dse + dsi dse = entropy change caused by the actual heat transfer between the + system and its surroundings. dsi = entropy change portion caused by irreversibilities. |
| Can the entropy of a system ever decrease? If so, give an example. Can the entropy of an adiabatic system ever decrease? If so, give an example | The entropy of a system can decrease if dse is negative in coffee the heat leaves the system to surroundings. - The entropy of an adiabatic system will always increase plus of its surroundings or stay the same. shaft work |
| Name the three governing equations of fluid dynamics. | 1. Mass Conservation 2. Momentum Conservation 3. Energy Conservation |
| State the 2nd law of thermodynamics applied to a system. | ds = δqrev / T = dse + dsi |
| What is the shaft work? | Mechanical energy an irreversibility providing work done to a system ( + to it). |
| What is the physical interpretation of the time rate of change of a fluid property contained in a fixed volume in the integral form of governing equations? | Accumulation/ decumulation of a flow quantity in the CV due to flow unsteadiness. |
| What is the physical interpretation of the surface integral in the integral form of governing equations? | Net efflux of the property across the control surface. |
| What is the advantage of the control volume approach over the differential equation approach for flow analysis? | It is not necessary to know the details of the flow inside the control volume. (like to compute useful information like thrust and efficiency of a jet engine.) |
| What is the limitation of the control volume approach? | Doesn't predict the details of the flow within the control volume. |
| What is the one-dimensional flow assumption? | At any given section all fluid properties are constant/uniform across the cross section (can still change from section to section). |
| State the energy equation for steady one-dimensional single stream flow. | h2 - h1 + ((V2)^2 - (V1)^2)/2 = q + ws |
| Why does the familiar Bernoulli Equation not hold in a compressible flow? | The internal energy component is not compatible with compressible flow. |
| How are stagnation properties defined? How are they determined? | Stagnation Properties: Thermodynamic properties of a fluid in stagnation state where the process must be isentropic without shaft power. ---> Determined by a reference state if the fluid were decelerated to zero speed relative to an observer. |
| Is there any difference between stagnation property and total property for gas flow? | No same they are |
| What is the difference between static and stagnation properties of a moving fluid? | Static Properties: Fluid motion is independent of any observers. Stagnation Properties: Fluid motion is dependent to the observer (for 0 speed). |
| According to the steady energy equation for a single stream adiabatic flow free from shaft power, what thermodynamic properties are constant along a streamline? | h0,2 = h0,1 and T0,2 = T0,1 |
| According to the steady energy equation for a single stream adiabatic and reversible flow free from shaft power, what thermodynamic properties are constant along a streamline? | 6. Isentropic Relations but replace numerator with stagnation property and denominator with static property |
| Irreversibilities cause entropy and stagnation pressure to increase or decrease? | Entropy increase and a stagnation pressure decrease. |
| If the flow through a gas turbine is steady adiabatic and reversible, will stagnation temperature and stagnation pressure change from the inlet to the exit of the machine? | Yes, the stagnation temperature and pressure will change due to the shaft power. |
| The total to static property ratio is a function of what dimensionless quantities? | M and γ |
| What quantities does a pitot-static tube measure? What additional information is needed to measure the speed of a compressible subsonic flow? | A pitot-static tube can measure stagnation pressure and static pressure. Speed of sound (T) is necessary to measure the speed of a compressible subsonic flow. |
| What is the sonic reference state? What type of process is assumed to hold in bringing the fluid to M=1? | Sonic Reference State: If the fluid is speed up or slowed down to M = 1. Adiabatic, reversible, without shaft power process. |
| If a single stream calorically perfect gas flow is steady, adiabatic, and free of shaft power, which of the following quantities do not change along the flow direction? | Total Enthalpy, Total Temperature, Critical Temperature, Total Speed of Sound, Characteristic Speed of Sound (a*) |
| If a single stream calorically perfect gas flow is steady, adiabatic, reversible and free of shaft power, which of the following quantities do not change along the flow direction? | All stagnation and sonic (critical) variables. |
| What is a wave? | A disturbance that propagates energy through a medium without net mass transport. |
| What are the factors that affect wave speed? | Medium type, thermodynamics state and wave strength. |
| How do the possible disturbance propagation directions differ in subsonic and supersonic flows? | Subsonic flow disturbances can propagate in all directions and forewarn the flow about the presence of the body while supersonic flow disturbances cannot propagate upstream against the flow. |
| How does the speed of a shock wave compare with the speed of a sound wave. | Shock Wave Speed: Supersonic Sound Wave Speed: Sonic |
| Why do shock waves sometimes for in supersonic flow? | Disturbances cannot propagate upstream against the flow. |
| How is a shock wave formed in a supersonic flow? | Compression waves coalesce and reinforce each other forming stronger compression wave. |
| What is the relationship between the compressibility and the speed of sound? | a = sqrt ( 1 / (tau*rho) ) The smaller the compressibility means the higher the speed of sound. |
| Is the flow upstream of a stationary normal shock wave always supersonic or subsonic? Is the flow downstream of a stationary normal shock always supersonic or subsonic? | Flow upstream of a stationary NSW is always supersonic. Flow downstream of a stationary NSW is always subsonic. |
| Does the Mach number after a stationary normal shock wave decrease or increase as the Mach number before the shock wave increases? For a stationary normal shock wave M1 -> ∞ , does M2 -> ∞ ? | - As M1 (before NSW) increases M2 (after NSW) decreases. - As M1 -> ∞ , M2 -> sqrt((gamma-1)/(2*gamma)) = 0.378 for air |
| What is the relation between density ratio and velocity ratio across a normal shock wave? | 9. Normal Shock Relations ρ2/ρ1 = u1/u2 |
| In a steady adiabatic flow, why a "expansion shock" (pressure, temperature, and density decrease across the shock) is impossible? | Against the 2nd Law of Thermodynamics since p, T, ρ, and Δs can't decrease across a SW. |
| The loss in stagnation pressure across a shock wave is directly related to what other important thermodynamic variable? | 5. Entropy Change for problems. |
| Shock wave is a dissipative phenomenon. Across a shock wave, where did the lost useful energy go? | Internal Energy, irreversible entropy increase, aerodynamic drag force for supersonic flow. |
| The static property ratios across a normal shock wave are one-to-one functions of the Mach number before or after the normal shock wave. True or False? | True from 9. Normal Shock Relations |
| Indicate whether each of the following quantities goes up, down, or remains the same across a stationary steady adiabatic shock wave: Mach number, velocity, density. pressure, temperature, stagnation temperature, stagnation pressure, entropy, and enthalpy | M decreases, V decreases, density increases, P increases, T increases, To remains the same, Po decreases, entropy increases, h remains the same. |
| What complicating factor must be taken into account in reading a pitot-static tube in supersonic flow? | Total Pressure (p0,2/p1) |
| Describe the basic mechanism for sound wave propagation. | Molecules near the sound source gain excess translational kinetic energy and transfer the energy surplus to neighboring molecules creating a traveling wave. |
Created by:
Frandarth13
Popular Engineering sets