Reducing the vibratory stress due to the forced response excitation of turbomachinery blades is an important engineering challenge facing designers. Detailed knowledge of the unsteady forces, the damping within the system, and the structural stiffness is required to predict the vibrational response and hence the high cycle fatigue life of a component. This study is focused on understanding the physical parameters influencing the unsteady forces causing the blade excitation in a transonic vaneless counter-rotating turbine, consisting of a vane row, a High Pressure (HP) spool, and a Low Pressure (LP) spool. Time averaged and time resolved measurements of the unsteady surface pressures on the HP and LP rotor blades are presented for a full scale rotating rig, using the actual engine components. Measurements were made and analyses performed at three different engine corrected aerodynamic conditions and with reduced frequencies (based on half blade chord) of approximately 10 for the unsteady aerodynamics. By varying the high-pressure rotor exit Mach number (1.44, 1.20, 1.05), the effects of varying the shock excitation to the LP blade row was studied. Extensive comparisons with CFD codes were obtained to determine flow-modeling requirements for the flow regimes studied. Comparison shows that for steady loading on the LP blade, 2D, single blade row Euler solvers are sufficient to achieve engineering accuracies. For the 1st harmonic unsteady loading, this level of modeling is adequate in the mid and lower half of the blade, but in the outer diameter region, three-dimensional effects require 3D modeling. The inclusion of nonlinear/viscous modeling shows moderately improved predictions.Copyright © 2000 by ASME