Lab 01: Wind Tunnel Characterization

Lab Overview

• Course: AE 315 – Experimental Aerodynamics, Section 04 (Robert Bryan) | February 7, 2025
• Objective: Learn to operate the ERAU open-circuit wind tunnel, collect airspeed data across varying fan frequencies and probe heights, and propagate measurement uncertainties using MATLAB.
• Instrumentation: Traverse pitot-static probe, analog manometer, analog-to-digital converter (ADC), data acquisition computer
• Ambient conditions: T = 21.5°C (294.65 K), P = 30.3 inHg (102,607.6 Pa), ρ = 1.213 kg/m³

Procedure & Methodology

The lab was divided into two experiments. In Experiment A, the traverse pitot probe was fixed at a constant height while fan frequency was increased in 5 Hz increments from 0 to 25 Hz. The 0 Hz baseline reading was subtracted from each measurement to isolate differential pressure, and airspeed was computed using V = √(2q/ρ). Reynolds number per unit length was then calculated at each frequency using Re/l = ρv/μ, where dynamic viscosity μ was determined via Sutherland's formula.

In Experiment B, the fan was held at 15 Hz (≈10 m/s) while the probe height was swept from 1 to 40 inches across the test section height. Velocity and turbulence intensity (I = STD_v / V̄ × 100) were computed at each increment. All data was acquired, processed, and visualized in MATLAB, with uncertainty propagated using standard error of the mean.

Results & Analysis

• Airspeed increased linearly with fan frequency: 2.89 m/s at 5 Hz up to 18.2 m/s at 25 Hz, confirming a direct correlation.
• Reynolds number per unit length ranged from 192,508 m⁻¹ (5 Hz) to 1,212,334 m⁻¹ (25 Hz).
• Velocity measurement uncertainty decreased with speed: 0.42% at 2.89 m/s down to 0.056% at 18.2 m/s.
• Pressure uncertainty was consistent at ≈1.27% across all fan frequencies, indicating stable sensor performance.
• At constant 15 Hz, airspeed decreased slightly as probe height increased toward the tunnel ceiling, consistent with boundary layer effects and wall proximity.
• Turbulence intensity mirrored the height–speed relationship: intensity increased as the probe approached the ceiling and the flow became less uniform.

MATLAB Code

Pressure data was loaded from CSV, converted from inH₂O to Pa (×248.84), and used to compute velocity, Reynolds number, and turbulence intensity. Below is a representative excerpt; the full script is AE315_INT.m.

% Subtract baseline (0 Hz) and convert inH2O → Pa
Hz_5  = (data(:,2) - data(:,1)) * 248.84;
Hz_15 = (data(:,4) - data(:,1)) * 248.84;

rho = 1.213; % kg/m^3  (ideal gas: P = rhoRT)
V_5  = sqrt(2 * Hz_5  / rho);
V_15 = sqrt(2 * Hz_15 / rho);

% Reynolds number per unit length
mu   = 1.821e-5; % N·s/m²  (Sutherland's formula)
Re_l = (rho * mean(V_15)) / mu;

% Turbulence intensity at each probe height
v_2       = mean(v, 2);           % mean velocity per height row
intensity = (std(v_2) ./ v_2) * 100;

% Uncertainty (standard error of mean, as %)
UC_V_5 = (std(V_5) / sqrt(length(V_5))) * 100;