✈️Aerodynamics Unit 7 – Wind Tunnel Testing: Experimental Methods

Wind Tunnel Basics

• Wind tunnels generate controlled airflow to study aerodynamic effects on objects
• Consist of a contraction section, test section, diffuser, and drive system (fan or compressed air)
• Test section houses the model and allows for uniform, low-turbulence flow
  • Typically has a constant cross-sectional area to maintain flow velocity
  • Equipped with windows or transparent walls for observation and measurements
• Contraction section accelerates and smooths the airflow before entering the test section
• Diffuser gradually expands the flow area to decelerate airflow and recover pressure
• Drive system generates the airflow, either by a fan or compressed air
• Flow conditioning elements (honeycomb, screens) improve flow quality and reduce turbulence
• Closed-circuit wind tunnels recirculate air, while open-circuit tunnels draw from and exhaust to the atmosphere

Types of Wind Tunnels

• Subsonic wind tunnels operate at Mach numbers below 0.8
  • Used for studying low-speed aerodynamics (aircraft takeoff and landing, wind turbines)
• Transonic wind tunnels cover Mach numbers from 0.8 to 1.2
  • Investigate flow phenomena near the speed of sound (transonic drag rise, shock waves)
• Supersonic wind tunnels achieve Mach numbers between 1.2 and 5
  • Study high-speed aerodynamics (supersonic aircraft, missiles)
• Hypersonic wind tunnels operate at Mach numbers above 5
  • Used for researching hypersonic flight (spacecraft reentry, scramjets)
• Atmospheric boundary layer (ABL) wind tunnels simulate wind flow near the Earth's surface
  • Study wind effects on buildings, bridges, and other structures
• Climatic wind tunnels control temperature, humidity, and precipitation
  • Investigate the impact of weather conditions on vehicles and structures
• Automotive wind tunnels are designed for testing ground vehicles (cars, trucks, motorcycles)

Test Models and Scaling

• Test models are scaled-down representations of the full-size object
• Scaling ensures dynamic similarity between the model and the actual object
  • Maintains geometric, kinematic, and dynamic similarity
• Reynolds number (Re) is a key parameter for scaling
  • Ratio of inertial forces to viscous forces: $Re = \frac{\rho VL}{\mu}$
  • Matching Re ensures similar flow patterns and force coefficients
• Mach number (M) is crucial for compressible flow scaling
  • Ratio of flow velocity to the speed of sound: $M = \frac{V}{a}$
• Blockage ratio is the ratio of model frontal area to test section cross-sectional area
  • Blockage effects can influence the flow field and must be minimized or corrected
• Model materials should be rigid, dimensionally stable, and have a smooth surface finish
• Mounting systems (stings, struts) support the model while minimizing interference
• Instrumentation (pressure taps, force balances) is integrated into the model for measurements

Measurement Techniques

• Force and moment measurements using strain gauge balances
  • Internal balances mounted inside the model
  • External balances connected to the model support system
• Pressure measurements using pressure taps and transducers
  • Surface pressure distributions provide insights into flow characteristics and load distribution
• Velocity measurements using pitot tubes, hot-wire anemometers, and laser Doppler velocimetry (LDV)
  • Pitot tubes measure local flow velocity based on the difference between total and static pressure
  • Hot-wire anemometers measure velocity by sensing changes in wire resistance due to cooling by the airflow
  • LDV uses laser beams to measure velocity components based on the Doppler shift of scattered light
• Temperature measurements using thermocouples and resistance temperature detectors (RTDs)
• Accelerometers and gyroscopes for measuring model motion and vibrations
• Particle image velocimetry (PIV) for whole-field velocity measurements
  • Tracks the motion of seeded particles in the flow using laser sheets and high-speed cameras
• Pressure-sensitive paint (PSP) for surface pressure mapping
  • Paint luminescence varies with local air pressure, allowing for high-resolution pressure measurements

Data Acquisition and Analysis

• Data acquisition systems (DAQ) convert analog signals from sensors to digital data
  • Typically include signal conditioning, amplification, and analog-to-digital conversion
• Sampling rate and resolution should be sufficient to capture relevant flow phenomena
• Time-averaging techniques used for steady-state measurements
  • Reduces the impact of flow fluctuations and noise
• Phase-averaging employed for periodic or unsteady flows
  • Aligns and averages data based on a reference signal (e.g., model motion or flow pulsation)
• Frequency analysis using Fourier transforms to identify dominant frequencies and flow structures
• Post-processing software for data visualization, analysis, and reporting
  • Includes tools for data filtering, statistical analysis, and curve fitting
• Uncertainty analysis to quantify the accuracy and reliability of measurements
  • Considers sources of error such as sensor calibration, data acquisition, and flow conditions

Flow Visualization Methods

• Smoke or fog injection to visualize streamlines and flow patterns
  • Commonly used in low-speed wind tunnels
• Tufts attached to the model surface to indicate local flow direction and separation
• Oil flow visualization to reveal surface flow patterns and separation lines
  • Mixture of oil and pigment applied to the model surface
  • Shear stress distribution and flow direction become visible as the air flows over the surface
• Schlieren imaging to visualize density gradients in the flow
  • Based on the refraction of light due to density variations
  • Particularly useful for compressible flows and shock wave visualization
• Shadowgraph technique to observe flow features with strong density gradients
  • Projects shadows of density gradients onto a screen or camera
• Surface flow visualization using pressure-sensitive paint (PSP) or temperature-sensitive paint (TSP)
  • Reveals surface pressure or temperature distributions related to flow characteristics
• Particle image velocimetry (PIV) for quantitative flow field visualization
  • Provides instantaneous velocity vector fields in a plane or volume

Error Sources and Corrections

• Solid blockage effects due to the presence of the model in the test section
  • Causes an effective reduction in test section area and an increase in velocity
  • Corrected using blockage correction factors based on model size and shape
• Wake blockage effects arising from the model's wake
  • Increases the effective velocity and dynamic pressure experienced by the model
  • Wake blockage corrections account for the momentum deficit in the wake
• Streamline curvature effects due to the presence of tunnel walls
  • Alters the streamline curvature around the model compared to free-air conditions
  • Corrected using streamline curvature correction methods
• Support interference effects caused by the model mounting system
  • Struts, stings, or wires can affect the flow around the model
  • Minimized through careful design and placement of support systems
• Boundary layer effects on tunnel walls
  • The presence of a boundary layer on the test section walls can influence the flow around the model
  • Mitigated by using boundary layer control techniques (suction, blowing) or corrections
• Turbulence intensity and flow angularity effects
  • Non-uniform or turbulent inflow can affect the accuracy of measurements
  • Addressed through flow conditioning (honeycomb, screens) and calibration procedures
• Measurement uncertainties due to sensor accuracy, calibration, and data acquisition
  • Quantified through uncertainty analysis and minimized by using high-quality instrumentation and proper calibration techniques

Real-World Applications

• Aircraft design and optimization
  • Evaluating aerodynamic performance, stability, and control characteristics
  • Optimizing wing and fuselage shapes, high-lift devices, and control surfaces
• Automotive aerodynamics
  • Reducing drag and improving fuel efficiency of cars and trucks
  • Studying wind noise, vehicle stability, and cooling airflow
• Wind engineering for buildings and structures
  • Assessing wind loads, pedestrian comfort, and pollutant dispersion
  • Optimizing building shapes and layouts for improved wind performance
• Turbomachinery and propulsion systems
  • Investigating flow in turbines, compressors, and jet engines
  • Improving efficiency, reducing noise, and ensuring reliable operation
• Sports equipment and athlete performance
  • Analyzing the aerodynamics of balls, bicycles, and other sports equipment
  • Optimizing athlete posture and gear for enhanced performance
• Environmental and atmospheric studies
  • Modeling atmospheric boundary layer flows, wind erosion, and pollutant dispersion
  • Studying the effects of wind on crops, forests, and other ecosystems
• Renewable energy applications
  • Designing and testing wind turbines for optimal power generation
  • Investigating the performance of solar panels under various wind conditions