✈️Aerodynamics Unit 3 – Compressible flow

Fundamentals of Compressible Flow

• Compressible flow involves significant changes in fluid density due to high-speed flow or large pressure variations
• Occurs when the Mach number (ratio of flow velocity to the local speed of sound) approaches or exceeds unity
  • Subsonic flow: Mach number < 1
  • Transonic flow: Mach number ≈ 1
  • Supersonic flow: Mach number > 1
• Compressibility effects become important when the Mach number exceeds 0.3
• Compressible flow is governed by the conservation of mass, momentum, and energy equations
• Ideal gas law relates pressure, density, and temperature in compressible flow ($PV = nRT$)
• Compressible flow exhibits phenomena such as shock waves, expansion waves, and choked flow
• Applications include high-speed aircraft, rocket engines, and gas pipelines

Thermodynamics Review

• First law of thermodynamics states that energy is conserved in a closed system
• Second law of thermodynamics introduces the concept of entropy and irreversibility
• Ideal gas law assumes that gas particles have negligible volume and no intermolecular forces
• Specific heat capacity at constant pressure ($c_p$) and constant volume ($c_v$) are important properties in compressible flow
• Ratio of specific heats ($\gamma = c_p/c_v$) is a key parameter in compressible flow equations
  • For air at standard conditions, $\gamma \approx 1.4$
• Isentropic flow assumes no heat transfer and no irreversibilities (such as friction or shocks)
• Stagnation properties (temperature, pressure, and density) represent the conditions that would exist if the flow were brought to rest isentropically

Speed of Sound and Mach Number

• Speed of sound ($a$) is the speed at which small pressure disturbances propagate through a fluid
• In an ideal gas, the speed of sound is given by $a = \sqrt{\gamma RT}$, where $R$ is the specific gas constant and $T$ is the absolute temperature
• Mach number ($M$) is the ratio of the flow velocity ($V$) to the local speed of sound ($a$): $M = V/a$
  • Mach number is a crucial parameter in compressible flow analysis
• Mach number determines the flow regime: subsonic ($M < 1$), transonic ($M \approx 1$), supersonic ($M > 1$), or hypersonic ($M \gg 1$)
• Mach angle ($\mu$) is the angle between the Mach wave and the flow direction, given by $\sin \mu = 1/M$
• Mach cone is a conical region formed by Mach waves emanating from a supersonic object
• Critical Mach number is the freestream Mach number at which the maximum local Mach number on an airfoil or wing reaches unity

Shock Waves and Expansion Waves

• Shock waves are thin regions of rapid changes in fluid properties (pressure, density, and temperature) that occur when a supersonic flow encounters an obstacle or a sudden change in flow direction
  • Normal shock waves are perpendicular to the flow direction
  • Oblique shock waves form at an angle to the flow direction
• Shock waves are irreversible and always result in an increase in entropy
• Rankine-Hugoniot equations relate the fluid properties across a shock wave
• Expansion waves occur when a supersonic flow encounters a convex corner or a sudden expansion in flow area
  • Expansion waves are isentropic and result in a decrease in pressure, density, and temperature, and an increase in velocity
• Prandtl-Meyer function relates the flow properties across an expansion wave
• Shock-expansion theory is used to analyze the flow over supersonic airfoils and wings

Quasi-One-Dimensional Flow

• Quasi-one-dimensional flow assumes that the flow properties vary only in the flow direction, while allowing for changes in cross-sectional area
• Continuity equation for quasi-one-dimensional flow: $\rho AV = \text{constant}$, where $\rho$ is the density, $A$ is the cross-sectional area, and $V$ is the velocity
• Momentum equation for quasi-one-dimensional flow: $P + \rho V^2 = \text{constant}$, where $P$ is the pressure
• Energy equation for quasi-one-dimensional flow: $h + \frac{V^2}{2} = \text{constant}$, where $h$ is the specific enthalpy
• Area-Mach number relation links the Mach number to the cross-sectional area in isentropic flow
• Choking occurs when the flow reaches sonic conditions (Mach number = 1) at the minimum cross-sectional area (throat) of a nozzle or duct
  • For isentropic flow, the maximum mass flow rate through a nozzle occurs at the choked condition

Nozzle Flow Analysis

• Nozzles are used to accelerate or decelerate a compressible flow by varying the cross-sectional area
• Converging nozzles accelerate subsonic flow and decelerate supersonic flow
• Diverging nozzles accelerate supersonic flow and decelerate subsonic flow
• Converging-diverging (CD) nozzles can accelerate a flow from subsonic to supersonic conditions
  • CD nozzles are used in rocket engines and supersonic wind tunnels
• Nozzle flow can be characterized by the pressure ratio (ratio of back pressure to total pressure) and the area ratio (ratio of exit area to throat area)
• For a given pressure ratio, there exists a unique area ratio that results in isentropic flow throughout the nozzle
• Off-design conditions in nozzles can lead to shock waves, flow separation, and losses in performance

Compressible Flow in Aircraft Design

• Compressibility effects play a crucial role in the design of high-speed aircraft (transonic and supersonic)
• Swept wings are used to delay the onset of shock waves and reduce wave drag in transonic flight
• Area rule states that the cross-sectional area of an aircraft should vary smoothly along its length to minimize wave drag
• Supercritical airfoils are designed to minimize the strength of shock waves and improve transonic performance
• Supersonic inlets are used to decelerate the flow and increase the pressure before entering the engine
• Convergent-divergent nozzles are used in jet engines to accelerate the exhaust flow and generate thrust
• Sonic boom is a shock wave generated by an aircraft flying at supersonic speeds, which can cause disturbances on the ground

Advanced Topics and Applications

• Hypersonic flow occurs at very high Mach numbers (typically $M > 5$) and involves additional physical phenomena such as chemical reactions, ionization, and rarefaction effects
• Magneto-hydrodynamics (MHD) studies the interaction between electrically conducting fluids and magnetic fields, with applications in plasma physics and propulsion systems
• Compressible turbulent flows are characterized by the interaction between turbulence and shock waves, and are encountered in high-speed boundary layers and jet flows
• Computational Fluid Dynamics (CFD) is widely used to simulate and analyze compressible flows, using techniques such as finite volume, finite element, and spectral methods
• Experimental techniques for compressible flows include schlieren imaging, shadowgraphy, and pressure-sensitive paint (PSP) for flow visualization and measurement
• Compressible flow principles are applied in various fields, such as aerospace engineering, automotive engineering, and industrial processes involving high-speed flows or large pressure variations