High-resolution flow field reconstruction is prevalently recognized as a difficult task in the field of experimental fluid mechanics, since the measured data are usually sparse and incomplete in time ...and space. Specifically, due to the limitations of experimental equipment or measurement techniques, the expected data cannot be measured in some key areas. In this paper, a practical approach is proposed to reconstruct flow field with imperfect data based on the physics informed neural network (PINN), which integrates those known data with the physical principles. The wake flow past a circular cylinder is taken as the test case. Two kinds of the training set are investigated, one is the velocity data with different sparsity, and the other is the velocity data missing in different regions. To accelerate training convergence, the learning rate schedule is discussed, and the cosine annealing algorithm shows excellent performance. Results reveal that the proposed approach not only can reconstruct the true velocity field with high accuracy, but also can predict the pressure field precisely, even when the data sparsity reaches 1% or the core flow area data are truncated away. This study provides encouraging insights that the PINN can serve as a promising data assimilation method for experimental fluid mechanics.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In this study, a centrifugal pump with a specific speed of 39.12 m×min
-1
×m
3
s
-1
is treated to analyze the flow instability under part-load conditions by numerical simulation and experimental ...test. For calculations, the RANS method, coupled with the
k-ω
SST turbulence model, is adopted. Numerical results at different operation points are compared with available experimental data, such as hydraulic performance and flow field information by particle image velocimetry. The numerical and experiment results agree well. The flow simulation indicates a strong reverse flow at the passage upstream impeller inlet, and the energy loss in the impeller is the largest under part-load conditions among all flow components in the pump. In one impeller revolution, one blade-to-blade flow passage is always nearly blocked off by the rotating stall occurring at the impeller inlet for each instant, and the blockage induces a jet flow with large velocity at the next blade-to-blade flow passage along the rotational direction of the impeller. The blockage and the jet flow in the blade-to-blade flow passages will make the flow unstable inside the impeller and cause performance breakdown and pressure vibration under part-load conditions for the pump.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The waterjet propulsion is widely applied in the marine vessels over 30 knots, and the intake duct is considered as an essential component that strongly relates to the propulsion performance. This ...paper sheds light on the flow features inside an intake duct under mooring conditions by using the particle image velocimetry (PIV) technique with three-dimensional (3D) numerical simulations. The hydraulic loss gradually increase as the flow-rate increases. According to analyses via the Bernoulli equation, the hydraulic loss is composed of the frictional head loss (
h
f
~
V
1.75
) and the local head loss (
h
j
~
V
2.0
). A recirculation region is observed near the duct lower wall with a high-velocity flow near the upper wall, and subsequently a shear flow presents in the horizontal straight pipe with an obvious velocity gradient. Three-dimensional simulations demonstrate that the vortex pair is very strong in the recirculation region and then it gradually decreases as the fluid flows downstream. With the flow-rate increasing, the non-uniformity at the duct outlet firstly increases to a peak and then slightly decreases, while the perpendicularity at the duct outlet dramatically decreases to a minimum and then increases. This work not only reveals some physics of the waterjet propulsion under mooring conditions, but also promotes its efficient operation.
Graphic abstract
The hydraulic loss gradually increase as the flow-rate increases. The hydraulic loss is composed of the frictional head loss (
h
f
~
V
1.75
) and the local head loss (
h
j
~
V
2.0
). A recirculation region is observed near the duct lower wall with a high-velocity flow near the upper wall, and a shear flow presents in the horizontal straight pipe with an obvious velocity gradient. Numerical simulations demonstrate that the vortex pair is very strong in the recirculation region and then it gradually decreases as the fluid flows downstream.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In order to maintain a uniform distribution of pareto-front solutions, a modified NSGA-II algorithm coupled with a dynamic crowding distance（DCD） method is proposed for the multi-objective ...optimization of a mixed-flow pump impeller. With the pump meridional section fixed, ten variables along the shroud and hub are selected to control the blade load by using a three-dimensional inverse design method. Hydraulic efficiency, along with impeller head, is applied as an optimization objective; and a radial basis neural network（RBNN） is adopted to approximate the objective function with 82 training samples. Local sensitivity analysis shows that decision variables have different impacts on the optimization objectives. Instead of randomly selecting one solution to implement, a technique for ordering preferences by similarity to ideal solution（TOPSIS） is introduced to select the best compromise solution（BCS） from pareto-front sets. The proposed method is applied to optimize the baseline model, i.e. a mixed- flow waterjet pump whose specific speed is 508 min^-1·m^-3s^-1·m. The performance of the waterjet pump was experimentally tested. Compared with the baseline model, the optimized impeller has a better hydraulic efficiency of 92% as well as a higher impeller head at the design operation point. Furthermore, the off-design performance is improved with a wider highefficiency operation range. After optimization, velocity gradients on the suction surface are smoother and flow separations are eliminated at the blade inlet part. Thus, the authors believe the proposed method is helpful for optimizing the mixed-flow pumps.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Turbulent cavitating flows in a mixed-flow waterjet pump were numerically investigated using the
k
-ω SST turbulence model and the mass transfer cavitation model based on the Rayleigh-Plesset ...equation to provide a comprehensive understanding of the cavitation-vortex interaction mechanism. The predicted hydraulic performance, as well as the cavitation performance, exhibits a reasonable agreement with the experimental results. The vorticity distributions under three operation conditions were illustrated together. Based on the illustration, cavitation development enhances vorticity production and flow unsteadiness in a mixed-flow waterjet pump. Vortices are basically located at the cavity interface, particularly at the downstream interface, during cavitation. Further analyses using the relative vorticity transport equation in cavitating turbulent flows indicate that vortex dilation and baroclinic torque exhibit a steep jump as cavitation occurs. In addition, vortex stretching contributes mainly to large-scale vortex generation.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This paper presents the implementation and assessment of a modified Partially averaged Navier-Stokes (PANS) turbulence model which can successfully predict the transient cavitating turbulent flows. ...The proposed model treats the standard
k-ε
model as the parent model, and its main distinctive features are to (1) formulate the unresolved-to-total kinetic energy ratio (
f
k
) based on the local grid size as well as turbulence length scale, and (2) vary the
f
k
-field both in space and time. Numerical simulation used the modified PANS model for the sheet/cloud cavitating flows around a three-dimensional Clark-Y hydrofoil. The available experimental data and calculations of the standard
k-ε
model, the
f
k
= 0.8 PANS model, the
f
k
= 0.5 PANS model are also provided for comparisons. The results show that the modified PANS model accurately captures the transient cavitation features as observed in experiments, namely, the attached sheet cavity grows in the flow direction until to a maximum length and then it breaks into a highly turbulent cloud cavity with three-dimensional structures in nature. Time-averaged drag/lift coefficients together with the streamwise velocity profiles predicted by the proposed model are in good agreement with the experimental data, and improvements are shown when compared with results of the standard
k-ε
model, the
f
k
= 0.8 PANS model and the
f
k
= 0.5 PANS model. Overall, the modified PANS model shows its encouraging capability of predicting the transient cavitating turbulent flows.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
•Dynamic mode decomposition (DMD) is applied in unsteady cavitating flow.•The frequencies of cavitating flow are extracted to analyze the characteristic of cavitating flow.•The velocity field of ...cavitating flow is reconstructed with high accuracy using less information.
The transient cavitating flow around the Clark-Y hydrofoil is numerically investigated by the dynamic mode decomposition with criterion. Based on the ranking dominant modes, frequencies of the first four modes are in good accordance with those obtained by fast Fourier transform. Furthermore, the cavitating flow field is reconstructed by the first four modes, and the dominant flow features are well captured with the reconstructed error below 12% when compared to the simulated flow field. This paper offers a reference for observing and reconstructing the flow fields, and gives a novel insight into the transient cavitating flow features.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Reducing vessel resistance by using ventilated cavities has been a highly researched topic in the marine industry. There is limited literature on ventilated supercavities near the free surface, which ...indicates that their dynamic behavior is more complex than conventional ventilated cavities due to the effect of the free surface. This paper employs numerical simulations to study the dynamic behavior of the ventilated supercavity, taking into account the effect of the free surface. Numerical simulations can predict gas leakage behaviors, cavity geometry, and internal flow structures. The influence of the free surface shortens the length of the ventilated cavity and increases the diameter. The presence of the free surface mainly changes the vertical velocity distribution between the free surface and the cavity. The results show that there are two typical gas leakage mechanisms under different immersion depths: twin-vortex tube leakage mode and re-entrant jet leakage mode. The internal flow field of ventilated supercavity is classified into three regions: the internal boundary layer, the ventilation influence region, and the reverse flow region. As the distance between the free surface and the ventilated supercavity decreases, the ventilated supercavity is affected by both the free surface effect and the gravity effect.
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