The cantilevered strain gauge sting was constructed at ATLSS Instrumentation by Edward Tomlinson. All details of the strain gauge design are documented in Strain Gage Stings (Tomlinson, 1996). The sting is constructed of a 3.175 mm brass stock. A strain gauge (Measurements Group, Inc.; EA-13-125BT-120) is mounted on each face of the stock in order to measure the lift and drag components of the fluid loading. The strain gauge sting has tree layers of waterproof and protective coating, which enables operation of the sting under the water.
The same ALT program that was used for control of stepper motors was used for force data acquisition. The lift and drag signals were amplified by Clip AE 301 S6 measurement amplifiers and then passed through analog filters (Krohn-Hite 3750) which operated in the low-pass mode with a cut-off frequency of 20 Hz. The filter gain was set at 20 db and the slope was 24 db per octave. The ALT software acquired the force data through a Data Translation DT-2801 series A/D converter. For each of the force signals (lift and drag), a total of 4096 data points were acquired at a Nyquist frequency of 454.5 Hz. The gain of the A/D converter was set at 8.
Several tests were developed to ensure proper operation of the strain gauge sting during the experiment. The zero level of the output voltage was carefully adjusted and monitored before and immediately after each experiment. In addition, known in-line and transverse forces are required for suitable calibration of the strain gauge sting. The cylinder was oscillated at the Keulegan-Carpenter numbers of 1 and 10, and the rms of the force coefficient CFrms was measured for both in-line and transverse motions of the cylinder, then compared with the results of Bearman et al. (1985). Herein, instantaneous force coefficients Cx and Cy are employed. They are defined as Cx = Fx/[½rU2Dl] and Cy = Fy/[½rU2Dl], where Fx and Fy are the horizontal and vertical dimensional forces, U is the maximum velocity of the orbital motion of the wave, and D and l are the cylinder diameter and length respectively.
To obtain the expression for the force coefficient Cf of the form where E is the voltage output of the strain gauge, r is the density of the water, U is the maximum velocity of the flow, and k is the calibration coefficient, a static calibration was performed for both vertical and horizontal directions by hanging weights from the end of the cylinder. The calibration coefficient k was found to be equal 0.813 for the vertical direction and 0.794 for the horizontal direction.
For the strain gauge configuration in the present apparatus, the polarity of the lift and drag forces are positive meaning that an upward force results in a positive voltage, and a drag force in the downstream direction results in the positive voltage.