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The physical origin of severe low-frequency pressure fluctuation frequently observed in Francis hydraulic turbines under off-design conditions, which greatly damages the structural stability of turbines and even power stations, is analyzed based on the hydrodynamic stability theory and our Reynolds-averaged Navier-Stokes equation simulation (RANS) of the flow in the entire passage of a Francis turbine. We find that spontaneous unsteady vortex ropes, which induce severe pressure fluctuations, are formed due to the absolute instability of the swirling flow at the conical inlet of the turbine's draft tube.

In this study, large eddy simulation (LES) coupled with the homogeneous cavitation model is used to simulate the turbulent cavitating flow in the venturi with special emphasis on LES errors and pressure fluctuation analysis. The numerical results accurately predict the quasi-periodic behavior and frequency characteristics of the cavitation by comparing them with the experimental observations. The modified one-dimensional model is utilized here to figure out the relationship between cavitation and pressure fluctuation. A good coincidence between the predicted and monitored pressure is obtained to validate the consideration of the geometric and flow factors in the modified model. Further analysis indicated that the cavity volume acceleration is the main source of cavitation excited pressure fluctuation. Moreover, LES Verification and Validation (V&V) are involved to quantify the errors and uncertainties of the numerical results. It is found that the large magnitude of the errors often emerges in the region where the re-entrant jet and shedding cavity occurs, which demonstrates the influence of cavitation on the simulation accuracy. The modeling error has a larger magnitude than the numerical error and both often show opposite signs. To better understand the influence of cavitation on LES V&V, the interaction between cavitation and vortex is also discussed further.

We investigated the effects of outdoor pressure fluctuations on natural ventilation in a room with two openings. One opening is exposed to an oscillating outdoor pressure and the other is exposed to a fixed neutral pressure. The ventilation airflow rate depends on the amplitude and period of the outdoor pressure fluctuations, the room volume, and the sizes of the openings. Dimensionless parameters are derived from the governing equations that determine indoor pressure responses due to outdoor pressure fluctuations. The pressure responses and the airflow rates through the openings are obtained using a fourth-order Runge–Kutta method. The flow regions are categorized into a synchronized region, an opening resistance region, and a transition region, depending on the dimensionless parameters. Applications are considered using an example building space to investigate the effective air change rates depending on the size of the openings and the period of wind pressure fluctuations.

In this research, two diffuser types of volute were designed, the volute with tangential diffuser and the volute with radial diffuser. Normally, different diffuser types of volute will affect the performance of pump, while in this paper, little effect can be found. Two diffuser types of volute were designed to study the effect on pressure fluctuation features of centrifugal pump under part-load condition, with the same volute design parameters and impeller parameters. The unsteady, three dimensional turbulent flow in the pump was simulated. It shows that the periodic features in pressure fluctuation near its tongue are the same for two diffuser types of volute, however in the diffuser of the volute, the values of the pressure fluctuation in radial diffuser is greater than that in tangential diffuser. The results can provide a useful reference for designing the diffuser of volute in centrifugal pump.

The paper presents improved MPS methods for the prediction of wave impact pressure on a coastal structure. By focusing on the momentum conservation properties of original MPS formulations, a new pressure gradient term is proposed in a momentum conservative form. As the first modification the original MPS formulation for pressure gradient is replaced by the new term which conserves both linear and angular momentum. Second modification is made by introducing a new source term for Poisson Pressure Equation (PPE). By revisiting the derivation of the PPE in MPS method, a higher-order source term is derived based on calculating the time differentiation of particle number density. Both first and second modifications are shown to significantly reduce the spurious fluctuations in particle number density (and thus pressure) field. The improved performance of the improved methods is demonstrated through the simulations of: a static fluid, a dam break with impact, a flip-through impact, and a slightly-breaking wave impact.