AEROLAB

The calibration was completed in March of 2000 by Bryan Feldman, Artie Nathan, Frank Bräuchle and Dora Yen. The calibration results were used in various reports in the same year. Instead of realigning the balance itself, which can take months for a competent crew, an interaction matrix method was applied to verify the interactions of the balance and load cells and to correct for them during the data acquisition process. This process is a faster and more efficient way to do a balance calibration. A brief summary is presented here and the full report is also available.

Background

The UCD Wind Tunnel uses an external, six-component, pyramidal balance with six interacting load cells. Traditional calibration methods basically require an alignment of the balance and the load cells in such a way that every load cell reads only one component. Instead a method using an interaction matrix is applied to calibrate the wind tunnel balance. A 2-D calibration process is described. This leads to the simplification that only three components need to be considered in the calibration process:

• Side Force, S (corresponds to lift in 2-D test)
• Drag, D
• Yawing moment, n (corresponds to pitching moment in 2-D test)

The method can be easily extended to include the full 3-D calibration by including all 6 components and generating a 6×6 interaction matrix. The underlying principle of the interaction matrix is straightforward. For the pyramidal balance, when applying a "true" load in one direction, on one component, ideally zero readings are desired for the other components. In reality however, non-zero readings result. These readings are interaction readings due to misalignment of the tunnel balance. For example, part of the lift is read as drag.

Test Rig

A listing of equipment used in the calibration is provided in the following equipment list. All calibration equipment is available in the wind tunnel except for the height gauge and the set of calibration weights. The height gauge can be checked out from the Ag Shop and the calibrated weight sets are available from the Civil Engineering Concrete Lab (English and metric units).

• calibration rig - front side and back side markers for orientation
• pulleys - 4 pulleys
• center rod - 20" long aluminum rod with grooved sections
• center rod cap - 4 screws on top for locking down cable
• side stands - used in yawing moment testing
• cables and cable hoops - 3/64 inch diameter stainless steel wire, breaking strength 270 lbs
• weight hangers - 1 lb, 10" hangers from McMaster Carr
• calibrated weight set - borrow from concrete lab
• 18" height gauge - borrow from Ag Shop.

Results

A detailed force balance calibration has been completed for the UC Davis Wind Tunnel Facility. Ideally calibrations should be performed prior to each major test. Although this process can be tedious and time consuming, it must be done to ensure the validity of the gathered data. By using an interaction matrix method, the calibration process has been greatly simplified. Uncertainty analysis results for force and moment coefficients show that experimental data measured using the facility is within expected tolerances (within 0.1% for Cl, 0.005% for Cm and within 4% of a drag count for Cd at 95% confidence levels). Based on the analysis, it can also be concluded that variations in velocity due to line noise, temperature variations and other experimental factors must be minimized since this variable most significantly affects the overall results. The procedure followed for calibration, alignment and uncertainty analysis has been documented in this report and will be placed in the Wind Tunnel Facility for future reference.