vibration frequency. The fluid force on the cylinder is dominated by inertia contributions at the 

 cylinder frequency at low reduced velocities. Measurements by Bearman and Currie (14) of the phase 

 between the pressure at 90 degrees from the front stagnation point and the cylinder's displacement 

 confirm this large inertia effect at low V^. 



The drag or resistance force Q;, is plotted in Figs. 2.4 and 2.6 as a function of V^ at the same dis- 

 placements, F= 0.50 and 0.75. This component of the total fluid dynamic force is negative and 

 becomes dominant near V, = 5. As noted earlier, Qy, is the negative of the lift coefficient as it is usually 

 characterized, so that the negative values of Q/, near V,= S suggest a net transfer of energy to the 

 cylinder in that region. These forced-cylinder results are comparable to the vortex-excited forces which 

 act upon resonantly vibrating cylinders when the reduced damping is sufficiently small as in the left- 

 hand portion of Fig. 2.2. It should be noted that C„i, and Q/, are Fourier-averaged coefficients and 

 contain only the components of the total fluid force at the vibration frequency /. Considerable power is 

 contained in the fluid force spectrum at other frequencies (i.e. 2/, the Strouhal frequency /j) for 

 reduced velocities outside of the regime of locking-on between the vortex and vibration frequencies. 



The excitation component of the lift force is defined as 



CiE == Q sin (f) = -Cji, cos e^. (2.4) 



The measurements in Figs. 2.3 through 2.6 are now compared to previous measurements by other 



investigators. The minimum value of Q/, in Fig. 2.4 occurs at V^ = 5, and similar results were 



obtained at the displacements Y = 0.13, 0.25, and 0.75. The several values of Q/, and C^i, so obtained 



are listed in Table 2.3 together with the related values of the force components derived from them. 



For all of these cases of forced vibration the condition for self-excitation 



Cn,H- WC,, > 



is satisfied, thus assuring the possibility of the equivalent vortex-excited oscillation. The results for 

 Qf are plotted against the effective displacement in Fig. 2.7 together with a host of similar findings. 



The conditions under which the experiments were performed are described in Table 2.4. 



tSee Appendix E. 



11 



