Measurement Of Excitation Force Research Articles

Further Study of Quasiperiodic Vibration Excitation Forces in Rotated Triangular Tube Bundles Subjected to Two-Phase Cross Flow

Two-phase cross flow exists in many shell-and-tube heat exchangers. Flow-induced vibration excitation forces can cause tube motion that will result in long-term fretting-wear or fatigue. Detailed vibration excitation force measurements in tube bundles subjected to two-phase cross flow are required to understand the underlying vibration excitation mechanisms. Some of this work has already been done. Somewhat unexpected but significant quasiperiodic forces in both the drag and lift directions were measured. These forces are generally larger in the drag direction. However, the excitation force frequency is relatively low (i.e., 3–6 Hz) and not directly dependent on flow velocity in the drag direction. On the other hand, much higher frequencies (up to 16 Hz) were observed in the lift direction at the higher flow velocities. The frequency appears directly related to flow velocity in the lift direction. The present work aims at (1) providing further evidence of the quasiperiodic lift force mechanism, (2) determining the effect of cylinder position on such quasiperiodic drag and lift forces, and (3) verifying the existence of quasiperiodic drag and lift forces in a more realistic larger tube array. The program was carried out with two rotated triangular tube arrays of different width subjected to air/water flow to simulate two-phase mixtures from liquid to 95% void fraction. Both the dynamic lift and drag forces were measured with strain gauge instrumented cylinders.

Correlation Between Vibration Excitation Forces and the Dynamic Characteristics of Two-Phase Cross Flow in a Rotated-Triangular Tube Bundle

Two-phase cross flow exists in many shell-and-tube heat exchangers. Flow-induced vibration excitation forces can cause tube motion that will result in long-term fretting wear or fatigue. Detailed flow and vibration excitation force measurements in tube bundles subjected to two-phase cross flow are required to understand the underlying vibration excitation mechanisms. Some of this work has already been done. The distributions of both void fraction and bubble velocity in rotated-triangular tube bundles were obtained. Somewhat unexpected but significant quasiperiodic forces in both the drag and lift directions were measured. The present work aims at understanding the nature of such unexpected drag and lift quasiperiodic forces. An experimental program was undertaken with a rotated-triangular array of cylinders subjected to air/water flow to simulate two-phase mixtures. Fiber-optic probes were developed to measure local void fraction. Both the dynamic lift and drag forces were measured with a strain gage instrumented cylinder. The investigation showed that the quasiperiodic drag and lift forces are generated by different mechanisms that have not been previously observed. The quasiperiodic drag forces appear related to the momentum flux fluctuations in the main flow path between the cylinders. The quasiperiodic lift forces, on the other hand, are mostly correlated to oscillations in the wake of the cylinders. The quasiperiodic lift forces are related to local void fraction measurements in the unsteady wake area between upstream and downstream cylinders. The quasiperiodic drag forces correlate well with similar measurements in the main flow stream between cylinders.

Vibration Excitation Force Measurements in a Rotated Triangular Tube Bundle Subjected to Two-Phase Cross Flow

Two-phase cross flow exists in many shell-and-tube heat exchangers. Flow-induced vibration excitation forces can cause tube motion that will result in long-term fretting-wear or fatigue. Detailed vibration excitation force measurements in tube bundles subjected to two-phase cross flow are required to understand the underlying vibration excitation mechanisms. An experimental program was undertaken with a rotated-triangular array of cylinders subjected to air/water flow to simulate two-phase mixtures over a broad range of void fraction and mass fluxes. Both the dynamic lift and drag forces were measured with strain gage instrumented cylinders. The experiments revealed somewhat unexpected but significant quasi-periodic forces in both the drag and lift directions. The periodic forces appeared well correlated along the cylinder with the drag force somewhat better correlated than the lift forces. The periodic forces are also dependent on the position of the cylinder within the bundle.

Prediction of flow-induced excitation in a pipe conveying fluid

Experimentally it is evident that the nature of the flow-induced excitation in a pipe conveying fluid is a broadband frequency excitation. It is also observed that the amplitude of excitation decreases with increase in frequency. However, there is no method to estimate such forces. The measurement of excitation force all along the length of a pipe using the pressure transducers may be difficult or perhaps impossible. Another possibility is to measure the structural responses using vibration transducers all along the pipe length and then estimate the flow-induced excitation forces (both amplitude and phase) using a finite element (FE) model of the pipe. However, the measured degree of freedoms (dofs) are always much smaller than the dofs in FE model, hence a method has been developed that uses non-linear optimization method involving the limited measured responses together with FE model to predict the excitation forces (both amplitude and phase) acting all along the pipe conveying fluid. The predicted excitation forces can then be used to perform safety related study by assessing the pipe responses at any location whether accessible or not. The theory of the proposed method and its validation has been presented in the paper through a long straight pipe conveying fluid. Typical applications of the proposed method are also discussed.

Identification of Single-Degree-of-Freedom Nonlinear Vibrating System by Piecewise Linear Approximation.

A new method for parameter identification of single-degree-of-freedom nonlinear vibrating systems is proposed in the time domain. The nonlinearity is approximated by piecewise linear characteristics. The nonlinear characteristics of restoring force and damping can be obtained from the linear spring constants and linear viscous damping coefficients identified on each piecewise linear section. Two experimental procedures, that is, the impact excitation method and the additional mass method without excitation, are performed. The response and excitation data for the impact excitation are required in the impact excitation method and the free vibration responses of the system with and without additional mass are required in the additional mass method. System parameters of the vibrating systems with various nonlinear characteristics, Duffing-type system and clearance system, can be accurately identified by both the procedures. It is shown that the additional mass method is valid for torsional nonlinear systems on which the measurement of excitation force is difficult.