The master´s thesis deals with the flow induced by rotation of cavitating fluid in converging-diverging nozzle, which simulates the vortex rope in impeller of water turbines. Measurement is performed on an experimental circuit in laboratory. Results from experimental measurements are compared with CFD simulation of single and two-phase flow. The main focus is to compare the difference of hydraulic losses and shapes of cavitating structures identified in the experiment and in the simulation.

Numerical modeling of cavitation using computational software is a highly pursued topic of research due to its impact in different industrial sectors. While some industrial applications such as wastewater treatment and mineral processing are known to advantageously use this phenomena, it remains an unwanted process in others where it is known to induce vibration, reduce performance and cause structural damage. The main objective of the current research study is to investigate the accuracy to which cavitating flow behavior inside oil-hydraulic systems can be computationally modeled, what limitations exist and how to improve numerical predictions. An experimental test-rig has been built in the preceding years with plexiglass tube to observed the vapor formations during cavitation and the pressure readings at three points have been recorded. The current study uses a computational model with the same geometry as the experimental test-rig, and uses the experimentally recorded pressure values for validating numerical results. Two main software are used to setup the simulation framework. The first is Hopsan, one open source simulation software for hydraulic systems developed by Link\"oping University and the second is ANSYS Fluent, a commercial software for modeling complex fluid flow applications. Four different orifices are used to create different outlet pressures. For orifices of diameter 2 mm, 3 mm, and 5 mm, good correlation between numerical and experimental results were observed. Further investigations into complex cavitating flow behavior of repetitive water hammers were also carried out. Different valve profile movements were used to investigate what the impact of having and not having vapor bubbles in the plexiglass tube would have on the pressure distribution when oil starts to re-circulate in the system. Furthermore, repetitive water hammer flow behavior for oscillations of 2, 3, and 4 water hammers were investigated. This investigation revealed several important findings.  The first is that if valve opens to the point that the flow starts to re-circulate in the system while vapor bubbles already exist in the plexiglass tube, massive pressure peaks, as high as 350 bar, will be created in the plexiglass pipe. The strength of this pressure surge will be dependent on the amount of vapor in the pipe when flow is re-introduced. The second is that if the valve starts to re-open (move backwards) while no vapor exists in the plexiglass tube, this movement will result in the formation of vapor. However, this vapor only lasts for a small duration and disappears before the valve reaches a point that allows flow move again. The third and final finding for repetitive water hammers was that the strength of the pressure surges will reduce with each sequential water hammer.

In this thesis computational models for cavitating flows around orifice plates has been studied and compared. The goal was to fit a model with experimental data and this was done with some success, although problems with numerical stability, long calculation times and geometry overfitting remain. Cavitation is a complex fluid phenomenon that can occur in pressurized liquid flows. It starts when the liquid pressure is lowered below the boiling pressure and water that undergoes cavitation forms vapor which later implodes violently. This process can cause problems such as noise, vibrations and corrosion in piping systems. Loud noise is a nuisance, however powerful vibrations and corrosion can have serious consequences for the structural integrity of pipes. The for example lessened performance, leakages or even failure. Therefore the minimization of cavitation is often a goal in orifice and piping design. Vattenfall AB, together with Forsmark and Ringhals nuclear plants have studied cavitating flows around orifice plates used for flow limitation. A set of data from laboratory tests made by Vattenfall was used as the basis of analysis. Existing computational models in OpenFOAM were tested and evaluated based on their ability to model the experimental data accurately, as well as their computational performance and stability. The cavitation phenomenon was difficult to simulate using established methods so a new method was created and verified. It is based on the Kunz cavitation model together with Large Eddy Simulations, but with turbulence as a predictor of cavitation. The new computational model will serve as a tool for knowing how to design orifices in the future, so that laboratory experiments will not have to be conducted for each new piping design.

Many existing dams in the United States were built without hydroelectric generating accessories and are now being considered for hydroelectric installations. A bifurcation is regularly used as the method for diverting the water to the new generators. With a bifurcation installed as part of the new piping system, cavitation could become a problem. Although widely used, there are no published data on cavitation characteristics or head loss coefficients for these bifurcations. Dimensional analysis has not been adequate for experimentally quantifying the cavitation potential and full scale testing is prohibitive for many large geometries. Therefore this study utilized Computational Fluid Dynamics (CFD) in conjunction with a physical model to predict conditions that would cause the onset of cavitation. Head loss coefficients were also calculated from the CFD simulations and physical model. Based on these results, the authors have produced recommended operating conditions that will allow bifurcations to operate within safe limits of cavitation. This study was not exhaustive but presents data that has previously been unavailable and will assist designers and operators to better understand the performance of such bifurcations.

This master thesis project was done in collaboration with Epiroc Group Ab. Epiroc supplies high-quality drills of various types that can be used both above and below ground. A major problem in their percussive rock drills is that that cavitation is formed. Cavitation is a phenomenon that occurs when a fluid is subject to a sudden pressure drop. This pressure drop causes the liquid to vaporize and create gas bubbles. These gas bubbles will cause erosion to the walls when imploded. These cavitation damages lead to drills breaking and parts having to be replaced preserved. An experimental rig was used to create cavitation. From the experimental rig, it was possible to measure the hydraulic transients that are created when the valve was closed. In this study, we examined whether one can visually see these damages occurring inside the pipe on valve parts that are subjected to these cavitation damages. CFD simulations were used to re-create the closing of the valve in the experimental rig. By exporting pressure data from the experiments one could compare the numerical result to the experimental data. It was also investigated if it is possible to see some connection between the gas formation and the damages seen visually from the experimental part. For the simulation the realizable k − ε methods were implemented with enhanced wall treatment. The mixture model was used since we have a multi-phase flow. Some visual damages were recognized during the experiments. However, no distinguished pattern or specific areas was established. From the simulations, it could be determined that they generated gas when the valve was closed. However, the pressure transients could not be replicated in the numerical result.

Gels and hydrogels have attracted a great attention for potential applications in tissue engineering, drug delivery, actuators, and soft robots. There has been a significant progress to engineer hydrogels from both synthetic and natural precursors to be as tough as a solid and as stretchable as a rubbery material while maintaining high water/solvent content. Despite considerable advances in rationally designing hydrogels, our understanding of their complex nonlinear mechanical deformation behavior is incomplete. This is partially due to the difficulty in conducting mechanical characterization on slippery, soft and swollen gels. Thus, it is required to develop new experimental techniques in order to better characterize them. Further, analyzing the experimental observations and link it with the molecular networks is an important factor. With this perspective, in this dissertation, nonlinear mechanical properties of different gel like materials have been investigated. We chose different gels with varied molecular structure, from molecular gel to self-assembled copolymer gels with flexible chains, to semiflexible polysaccharide based polymers. By developing suitable experimental protocols, strain-stiffening behavior of these materials, similar to that observed in biological materials, have been captured. Chain flexibility is a dominant factor in mechanical behavior of gels. For example, gels with flexible chains dilate orthogonal to an external shear load, whereas gels with semilexible chains contract similar to biological gel-like materials. In order to investigate the failure mechanism in our gels, cavitation rheology technique was also applied. We found that cavitation phenomenon in gels is related to the molecular architecture of the gels. The present work provides a better understanding of the deformation behavior of soft gels when subjected to a large load.

A physical model was built to study the properties of the feeding strike of the mantis shrimp that are responsible for drag reduction and cavitation control. The model had three goals: 1) The model was to be outfitted with a method to collect kinematic, force and cavitation data. 2) The velocity and acceleration profile of the model were to be predicted with a mathematical model of the mechanism. 3) The model was to match as many drag and cavitation sensitive properties of the mantis shrimp strike as feasible and have a means to control the rest. The first iteration of the model met the first goal but not the second or third. It matched the strike in maximum velocity, appendage size and shape and environmental temperature and salinity but did not control acceleration profile, water quality or pressure. Data collected with high-speed video of strikes of the model and Gonodactylus smithii showed the model to cavitate at speeds at which no cavitation was seen in animal strikes. The model was redesigned to be driven by the stored elastic energy in the deflection of a beam spring. The redesigned model reached the animals maximum accelerations but not velocities. Environmental variation was found to not substantially contribute to the variation in cavitation onset velocity between the model and animal experiments.

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Three-dimensional two-phase unsteady CFD simulations were run on three and four-blade inducers for the purpose of analyzing differences in cavitation stability at design and off-design flow rates. At design flow rates, there were very small differences between the breakdown curves for the three and four-bladed inducers. However, at lower cavitation numbers, the three-bladed inducer exhibited up to three times the rotor forces than the four-bladed inducer. When moving to off-design flow rates, both inducers experienced multiple modes of cavitation instabilities including rotating cavitation, alternate-blade cavitation, and cavitation surge. The four-bladed inducer began experiencing the formation of these modes of instability beginning at a cavitation number of $sigma = 0.047$ whereas the three-bladed inducer began at a cavitation number of $sigma = 0.091$. Additionally, the three-bladed inducer showed rotor forces up to ten times higher than the four-bladed inducer at similar cavitation numbers.Three-dimensional single-phase steady CFD simulations were run on four-blade inducer geometries with $7^{circ}$, $9^{circ}$, $11^{circ}$ and $14^{circ}$ inlet tip blade angles with a stability control device (SCD) installed. The simulations were ran at multiple flow coefficients. Results show interesting flow effects from the SCD. For example, at lower flow coefficients, the incidence angle actually decreases at greater than 70\% span. This is due to a region of accelerated axial flow coming from the recirculation of the SCD which occurs near the shroud. Results also show strong correlations between efficiency and head rise to the local mass flow gain experienced due to the recirculating flow through the SCD. A best fit curve was generated to predict mass flow gain based on the inducer's inlet tip blade angle and flow coefficient. Based on this research, the ability to predict mass flow gain and consequently efficiency and head rise for similarly designed inducers with varying inlet blade angles has been demonstrated.