✈️Aerodynamics Unit 10 – Unsteady and transient aerodynamics

Key Concepts and Definitions

• Unsteady aerodynamics involves time-dependent flow fields where properties vary with time
• Transient aerodynamics focuses on the transition between different flow states or conditions
• Characteristic time scales determine the relative importance of unsteady effects compared to steady-state behavior
• Reduced frequency ($k = \frac{\omega c}{2U_{\infty}}$) is a dimensionless parameter that quantifies the degree of unsteadiness
  • $\omega$ represents the angular frequency of the unsteady motion
  • $c$ is a characteristic length scale (e.g., airfoil chord)
  • $U_{\infty}$ is the freestream velocity
• Quasi-steady approximation assumes the flow instantly adapts to changing conditions, neglecting time-dependent effects
• Unsteady flow phenomena include dynamic stall, flutter, buffeting, and vortex shedding
• Unsteady aerodynamic forces and moments are influenced by the history of the flow field and the motion of the body

Fundamental Equations

• Unsteady flow is governed by the time-dependent Navier-Stokes equations
  • Conservation of mass: $\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \vec{u}) = 0$
  • Conservation of momentum: $\rho \frac{D\vec{u}}{Dt} = -\nabla p + \nabla \cdot \tau + \rho \vec{f}$
  • Conservation of energy: $\rho \frac{De}{Dt} = -p\nabla \cdot \vec{u} + \nabla \cdot (k\nabla T) + \Phi$
• Unsteady potential flow theory simplifies the equations by assuming inviscid, irrotational flow
  • Unsteady Bernoulli equation: $\frac{\partial \phi}{\partial t} + \frac{1}{2}|\nabla \phi|^2 + \frac{p}{\rho} + gz = C(t)$
  • Unsteady boundary conditions account for the motion of the body and the wake
• Unsteady thin airfoil theory provides analytical solutions for small-amplitude oscillations and gusts
  • Theodorsen's function ($C(k)$) relates the unsteady lift to the quasi-steady lift and the wake-induced downwash
• Unsteady panel methods discretize the surface into panels and solve for the time-dependent singularity strengths

Types of Unsteady Flows

• Periodic unsteady flows exhibit a regular, repeating pattern over time (e.g., oscillating airfoils, rotors)
• Aperiodic unsteady flows have irregular or non-repeating variations in flow properties (e.g., gusts, turbulence)
• Transient flows involve a sudden change in flow conditions or geometry (e.g., gust encounters, rapid maneuvers)
• Separated flows occur when the boundary layer detaches from the surface, leading to unsteady vortex shedding
  • Dynamic stall is a complex unsteady phenomenon involving flow separation, vortex formation, and reattachment
• Unsteady wakes are characterized by time-dependent vorticity distributions and induced velocities
• Unsteady shock waves can form and move in transonic and supersonic flows, affecting the aerodynamic loads

Time-Dependent Effects

• Added mass effect accounts for the additional force required to accelerate the fluid surrounding a moving body
• Unsteady boundary layers experience time-dependent growth, separation, and reattachment
  • Unsteady separation can lead to dynamic stall and increased drag
• Unsteady wake effects influence the pressure distribution and the overall aerodynamic forces
  • Wake vorticity induces a time-dependent downwash on the lifting surface
• Unsteady compressibility effects become significant at high reduced frequencies and transonic/supersonic speeds
  • Unsteady shock motion and shock-boundary layer interaction can cause flow instabilities and buffeting
• Unsteady flow-structure interaction couples the aerodynamic forces with the structural dynamics
  • Flutter is a self-excited oscillation that can lead to structural failure if not properly controlled

Analytical Methods

• Unsteady thin airfoil theory provides closed-form solutions for small-amplitude oscillations and gusts
  • Theodorsen's function relates the unsteady lift to the quasi-steady lift and the wake-induced downwash
  • Wagner's function describes the indicial response of an airfoil to a step change in angle of attack
• Unsteady lifting-line theory extends the steady-state lifting-line concept to account for time-dependent effects
  • Unsteady vortex lattice methods discretize the lifting surface into vortex elements and solve for the time-dependent circulation
• Unsteady panel methods represent the surface as a collection of panels with time-varying singularity strengths
  • Boundary element methods (BEM) solve the unsteady potential flow equations using Green's theorem
• Reduced-order models (ROMs) simplify the governing equations by projecting them onto a lower-dimensional subspace
  • Proper orthogonal decomposition (POD) identifies the most energetic modes of the unsteady flow field

Numerical Simulation Techniques

• Unsteady Reynolds-averaged Navier-Stokes (URANS) simulations solve the time-dependent RANS equations
  • Turbulence models (e.g., k-ε, k-ω) are used to close the system of equations and account for the effects of turbulence
• Large eddy simulations (LES) directly resolve the large-scale unsteady motions while modeling the smaller scales
  • Subgrid-scale (SGS) models represent the effects of the unresolved scales on the resolved scales
• Detached eddy simulations (DES) combine URANS in attached regions with LES in separated regions
  • Hybrid RANS-LES approaches aim to balance computational cost and accuracy for unsteady flows
• High-order methods (e.g., spectral elements, discontinuous Galerkin) provide increased accuracy for unsteady simulations
  • Adaptive mesh refinement (AMR) dynamically adjusts the grid resolution based on the local flow features

Experimental Approaches

• Unsteady wind tunnel testing measures the time-dependent aerodynamic forces and moments
  • Dynamic testing rigs (e.g., oscillating models, gust generators) simulate unsteady flow conditions
  • Pressure-sensitive paint (PSP) and temperature-sensitive paint (TSP) provide surface pressure and temperature distributions
• Particle image velocimetry (PIV) measures the instantaneous velocity field in a plane using tracer particles and laser sheets
  • Stereoscopic PIV (SPIV) captures all three velocity components in a plane
  • Tomographic PIV (Tomo-PIV) reconstructs the 3D velocity field from multiple camera views
• Hot-wire anemometry measures high-frequency velocity fluctuations using thin, electrically heated wires
  • Constant temperature anemometry (CTA) maintains a constant wire temperature and relates the voltage to the velocity
• Unsteady pressure measurements using high-frequency pressure transducers capture the time-dependent surface pressures
  • Kulite and PCB sensors are commonly used for unsteady pressure measurements

Real-World Applications

• Helicopter rotors experience unsteady aerodynamics due to the cyclic pitch changes and the interaction with the fuselage wake
  • Unsteady rotor aerodynamics affects the performance, vibration, and noise characteristics of helicopters
• Wind turbines operate in unsteady atmospheric conditions, including wind shear, turbulence, and gusts
  • Unsteady blade aerodynamics influences the power output, loads, and fatigue life of wind turbines
• Flapping-wing micro air vehicles (MAVs) rely on unsteady aerodynamic mechanisms for lift generation
  • Leading-edge vortices (LEVs) and clap-and-fling are examples of unsteady lift enhancement techniques used by MAVs
• Turbomachinery blades experience unsteady flow due to the relative motion between the rotor and stator rows
  • Unsteady blade-row interactions affect the efficiency, noise, and structural integrity of turbomachines
• High-performance aircraft maneuvers involve rapid changes in angle of attack and sideslip
  • Unsteady aerodynamics during maneuvers can lead to dynamic stall, wing rock, and tail buffeting
• Aeroelastic phenomena, such as flutter and buffeting, result from the interaction between unsteady aerodynamics and structural dynamics
  • Unsteady aeroelastic analysis is crucial for ensuring the safety and performance of aircraft and other structures