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Swirling flow at vertical shaft spillways results in flow fluctuations, reduces hydraulic system efficiency, and causes structural damage. Optimizing the shape of the inlet is a practical means to decrease the swirling flow strength. In the present study a comprehensive experimental investigation was performed to examine the effects of a new type of inlet over vertical shaft spillways, namely the circular piano-key (CPK) inlet, for free, orifice and full-pipe or black water flow regimes. Head-discharge relationships, threshold and critical submergence depths and the effects of different geometrical parameters of the CPK inlets on the spillway flow discharge coefficient were assessed. Results reveal that increases in the height of the inlet and outlet keys increase the released capacity of the shaft spillway. Given its increased total crest length and the effects of the radially placed walls of a CPK against the swirling flow, angles of the inlet and outlet keys of the CPK are found to significantly affect the spillway flow discharge. The effect of inlet and outlet overhangs on the efficiency of CPK inlets is not noteworthy, being negligible. Considering the destructive effects of swirling flow, the CPK inlets significantly decrease the range of submergence for an unfavourable swirling flow regime condition, increasing the threshold submergence depth and decreasing the critical submergence depth. The flow discharge through the CPK inlets is higher than those of the other circular-type spillways. The present modified CPK inlets more effectively divert water, serving as an economical alternative to the conventional circular spillways. [ABSTRACT FROM AUTHOR]

In hydraulic turbines, either Francis or axial turbines, a high swirling flow is generated at PL conditions at the inlet of the draft tube resulting in the formation of a rotating vortex rope (RVR). This leads to pressure fluctuations which limit the operating range of single regulated turbines. In the present study, an axial hydraulic turbine has been numerically simulated by the k − ϵ and SST-based Scale-Adaptive Simulation model (SST SAS) turbulent models to capture helical RVR. Considering the formation of the RVR as the result of a global instability, linear global stability analysis of the time-averaged turbulent flow field has been conducted. For the first time in axial hydraulic turbines, how boundary conditions affect the unstable mode and which frequency, plunging or rotating, is related to the vortex rope instability have been studied. It is found that the flow inside the draft tube is sensitive to the asymmetrical disturbances with a frequency close to the rotational component. [ABSTRACT FROM AUTHOR]

Pre-swirl in the approach flow towards a pump leads to a decreased efficiency or even to malfunctioning of a pump. Especially for pumping stations with large rotodynamic pumps, the risk of malfunctioning should be minimized. For this reason, hydraulic investigations are performed prior to the construction of pumping stations to assess the amount of pre-swirl in the approach flow and if needed, to design local measures to decrease it. This article presents a test set-up of a simplified pump sump in which the pump performance characteristics are measured at a real pump for a wide range of predefined corotating and counterrotating swirl angles. The swirl angles are measured in a transparent model pump at scale 1:1 with a swirl meter at four bell diameters above the bell-mouth. The findings discussed in this paper provide insight in the relation between pre-swirl in the approach flow and pump performance. [ABSTRACT FROM AUTHOR]

Scroll vortex intakes are commonly used in water supply, drainage and sewerage systems. Water flows into the intake via an eccentric approach channel, forming a swirling vortex flow with a stable air core along the dropshaft. Previous design theories have largely been based on unjustified assumptions on the velocity and pressure distributions of the vortex flow at the intake bellmouth. To clarify these assumptions, a validated three-dimensional computational fluid dynamics model has been developed. Model predictions compare well with independent measurements of flow profile, velocity and air core size. While the vortex flow in the scroll chamber can be described by a free vortex with constant circulation equal to that at the inlet, the tangential velocity and the pressure distribution at the bellmouth outlet have to be approximated by a forced vortex. A revised theory is proposed, providing significant improvement to the prediction of air core sizes and head–discharge relationship. [ABSTRACT FROM AUTHOR]

Hydropower is a key energy conversion technology for stabilizing electrical power. When running at full load, a 3-D cavitation vortex rope develops in Francis turbines that acts as an internal source of energy and instability. 1-D models allow this phenomenon to be predicted and the range of safe operating points to be defined. These models involve three parameters: the mass flow gain factor, the wave speed and the second viscosity that must be calibrated. For the first time, the present work aims at identifying these parameters using 3-D URANS cavitating simulations. Two cavitation test cases are considered: a 2-D Venturi and a reduced scale model of a Francis turbine at full load operating conditions. RANS simulations allow the mass flow gain factor to be determined, whereas URANS simulations coupled with an optimization process allow the determination of the wave speed and the second viscosity. The new methodology shows its ability to identify the three parameters. [ABSTRACT FROM AUTHOR]

Hydraulic characteristics of swirling flow at shaft spillways with an innovative inlet, namely the marguerite-shaped inlet, is investigated based on model experimentation. Detailed measurements of head–discharge relationship, flow regimes, water free-surface profiles, flow circulation, discharge coefficient and threshold and critical submergence depths were performed for shaft spillways with marguerite-shaped inlets. Results indicate that the strength of swirling flow through a marguerite-shaped inlet is significantly lower than that of a simple shaft and a morning glory spillway. Marguerite-shaped inlets increase the developed crest length by at least 6 times the global size of the original shaft spillway. Therefore, they produce exceptional hydraulic performances in increasing the weir overflow length without changing the global size of the shaft spillway, thereby increasing the flow discharge up to 6, 4 and 1.5 times the simple shaft, morning glory and circular piano-key spillways, respectively. Finally, based on the detailed experimental data, different empirical correlations are derived for estimating the threshold and critical submergence depths and discharge coefficient of the marguerite-shaped inlets for different flow regimes using regression analysis. [ABSTRACT FROM AUTHOR]

The characteristics of square twin surface jets at various offset heights from the free surface were studied using a particle image velocimetry technique. The offset heights were varied from 1 to 4 nozzle widths at a fixed Reynolds number of 3890. The entrainment and mixing characteristics were examined using the potential core length, merging point, combined point, maximum velocity decay, half-velocity width and were found be nearly independent of offset height in near field. The jet–surface interaction was examined by surface velocity, vorticity thickness and surface turbulence intensities. The growth rate of the vorticity thickness reduced in the interaction region; and was more severe for shorter offset height. Turbulent structures were examined using weighted joint probability density function and two-point cross-correlations between swirling strength and velocity fluctuations. Weaker turbulent events were observed for deeper jet close to a free surface at the streamwise location beyond the attachment point. [ABSTRACT FROM AUTHOR]

The present study investigates a transient condition, i.e. load variation from the best efficiency point to the part load, of a Francis turbine. A complete turbine is considered for the study. The dynamic mesh approach is implemented to close the guide vanes and to perform load variation numerically. The primary goal is to investigate the inception, propagation and interaction of vortex rings as load on the turbine changes. The secondary goal is to determine characteristic frequencies. The continuous change in flow angle, due to the closing of the guide vanes, induces strong boundary layer separation in the runner. Flow decelerates along the blade and establishes an adverse pressure gradient due to steep curvature of the blades. Separated flow from the boundary layer creates two swirling regions attached to the blade suction side. Fully developed vortex rings exiting from the adjacent blade channels interact and induce high-amplitude stochastic fluctuations at the runner outlet. [ABSTRACT FROM AUTHOR]

Well-resolved detached eddy simulation (DES) and unsteady Reynold-averaged Navier–Stokes (URANS) simulations are conducted for a water pump-intake bay. Three cases are analysed: a high-submergence, low discharge case (Case 1), a low-submergence, low-discharge case (Case 2) and a low-submergence, high-discharge case (Case 3). The numerical model uses the volume of fluid (VOF) to account for the free surface deformations. In Case 1, for which the free surface deformations in the experiment were small, DES predictions are in better agreement with previous experimental measurements, compared to URANS predictions of the mean flow position, circulation, mean azimuthal velocity and vorticity distribution inside the cores of the main free-surface and submerged vortices forming inside the pump-intake bay. Decreasing the submergence depth in Case 2 results in an intensification of the coherence and unsteadiness of the main free surface vortex and in notable deformations of the free surface compared to Case 1. In Case 3, where the discharge is five times larger compared to Case 2, DES predicts the formation of an even stronger free-surface vortex whose core, at times, penetrates inside the pump column and entrains air. The effects of decreasing submergence and increasing discharge on the mean position, strength and general structure of the main submerged vortices are discussed. DES results for Case 3 are also used to describe the dynamics of the main pump-bay vortices. Results also show that the differences between the DES and URANS predictions of the mean-flow vortical structure increase dramatically for Case 3. Despite large deformations of the free surface, no well-defined free-surface vortex is present in the URANS. [ABSTRACT FROM AUTHOR]

Swirling flow at vertical shaft spillways with an innovative geometry, circular piano-key (CPK) inlet, is investigated based on model experimentation. Detailed measurements of head–discharge relationships, threshold submergence, and flow hydraulic characteristics were performed for CPK spillways. Conditions at the beginning of the submergence are analysed, because this situation is related to a condition in which flow instabilities appear on the spillway crest. The results show that the swirling flow strength for flow through the CPK spillway is several times lower than that for simple shaft spillways. This structure yields excellent hydraulic performance, e.g. increasing the weir overflow length by at least 200% without changing the global size of the shaft spillway, thereby increasing the flow discharge up to six times compared with a simple shaft spillway. Finally, two correlations are derived for determining the threshold submergence depth and discharge coefficient of CPK spillways. [ABSTRACT FROM PUBLISHER]

Operating hydraulic turbines over an extended range of discharge values requires an optimized runner that minimizes the weighted-average draft tube losses. It is shown that the draft tube losses can be minimized before actually designing the (new or refurbished) runner blades by optimizing the runner outlet flow. For this, a new runner surrogate model is introduced, where the relative flow angle is expressed via the so-called swirl-free velocity profile (represented with two parameters). Given a swirl-free velocity profile, as well as a turbine discharge value, the model provides swirling flow profiles consistent with the hydraulic turbine operation. With this swirling flow as inlet condition, the draft tube flow computation provides the relationship between the swirl-free velocity parameters and the hydraulic loss. It is shown that this approach can be used for optimizing the runner for a range of turbine operating regimes. [ABSTRACT FROM PUBLISHER]

Pre-swirl occurring in pump intake basins influences pump efficiency and lifetime. The exact effect on a pump depends on the pump design. In order to optimize the approach flow towards the pump, physical scale modelling is often applied following the guidelines formulated in pump intake design standard ANSI/HI 9.8-2012. This standard provides guidelines on measurement equipment for, and acceptance criteria of, swirl. As this standard does not elaborate on the measurement technique, an approach is proposed through which a swirl can be quantified with a known accuracy in a sump model test. The proposed method is demonstrated using two scale models where swirl was measured for 6 hours. Furthermore, conditions where the acceptance criteria of ANSI/HI 9.8-2012 are not strictly defined are outlined and quantifiable acceptance criteria for future sump model tests proposed. [ABSTRACT FROM PUBLISHER]