In this work, the laminar-to-turbulent transition phenomenon around the two- and three-dimensional ellipsoid at different Reynolds numbers is numerically investigated. In the present paper, Reynolds ...Averaged Navier Stokes (RANS) equations with the Spalart-Allmaras, SST
k
−
ω
, and SST-Trans models are used for numerical simulations. The possibility of laminar-to-turbulent boundary layer transition is summarized in phase diagrams in terms of skin friction coefficient and Reynolds number. The numerical results show that SST-Trans method can detect different aspects of flow such as adverse pressure gradient and laminar-to-turbulent transition onset. Our numerical results indicate that the laminar-to-turbulent transition location on the 6:1 prolate spheroid is in a good agreement with the experimental data at high Reynolds numbers.
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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
Passive methods of flow and cavitation control appear to offer some of the best prospects in the field of hydraulic engineering and marine applications. In this article, we aimed at an experimental ...examination of the effect of wall roughness/wettability on the occurrence of cavitation and turbulence structure in the cross flow around and in the wake of a circular cylinder in two characteristic regimes. For this, we used three test bodies with different surface morphologies: smooth (reference), micro-scale irregularities (rough) and regular large-scale (of the order of a millimeter) texture (finned). Using high-speed imaging to observe vapor cavities, we revealed that cavitation is noticeably suppressed by both types of roughness. Applying the method of vapor phase detection (Pervunin et al., 2021), this finding was then quantitatively confirmed through an in-depth analysis of an ensemble of instantaneous velocity fields measured by PIV, indicating that modification of wall morphology is an effective method of cavitation control. The procedure of statistical vector filtration (Heinz et al., 2004) allowed us to remove outliers from the velocity fields and, thus, calculate various turbulence characteristics, including higher-order moments (i.e., the coefficients of skewness and excess). Wall irregularities were found to significantly affect the turbulence structure of the wake flow, but the higher-order moments downstream of the modified-surface cylinders turned out to be unexpectedly insensitive to a change in the flow regime, as opposed to the smooth one. Regardless of the type of surface morphology, the influence of roughness on the mechanism of formation of large-scale vortices and their characteristics was weakened. However, it caused overall disorganization of liquid motion in the cylinder wake, thus making local flow conditions highly unsteady. In addition, this process became more chaotic with an increase in the scale of irregularities.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this work, we performed molecular dynamics simulations to study the dynamics of a shock wave-induced single nanobubble collapsing near one flexible and two rigid boundaries. The flexible boundary ...consisted of polyethylene, and the rigid boundaries were made of aluminum and iron. The shock waves impinging on the nanobubble inside a molecular system were generated using a momentum mirror approach. For two relative wall distances, we studied the dynamics of the shock-induced single nanobubble and its collapse near the flexible and the rigid boundaries. The atomic velocity contours surrounding the single nanobubble and the collapse-induced damage on the boundaries were analyzed. We obtained this collapse-induced damage from ten collapsing nanobubbles. Results showed that the relative wall distance affected the single nanobubble’s collapse dynamics near the boundaries. A generated nanojet was directed on the surfaces during the collapse process. From the collapse-induced damage point of view, the depth damage of the polyethylene, iron, and aluminum boundaries for the relative wall distance of γ = 1.3 were 6.0, 0.47 and 0.63 nm, respectively. It was observed that the extensive collapse-induced damage occurred only on the polyethylene boundary.
In this work, we conducted a numerical study on the cavitation flow around a circular cylinder with Re=200 and σ=1, through the implementation of a porous coating. The primary objective addressed the ...effectiveness of utilizing a porous surface to control cavitation. We analyzed the cavitation dynamics around the cylinder and the hydrodynamic performance at different permeability levels of the porous surfaces (K=10−12−10−10). The flow was governed by the density-based homogeneous mixture model, and the volume penalization method was used to deal with the porous layer. A high-order compact numerical method was adopted for the simulation of the cavitating flow through solving the preconditioned multiphase equations. The hydrodynamic findings demonstrated that the fluctuations in the lift coefficient decreased when the porous layer was applied. However, it is not possible to precisely express an opinion about drag because the drag coefficient may vary, either increasing or decreasing, depending on the permeability within a constant thickness of the porous layer. The results revealed that the application of a porous layer led to the effective suppression of cavitation vortex shedding. In addition, a reduction of the shedding frequency was obtained, which was accompanied by thinner and elongated vortices in the wake region of the cylinder. With the proper porous layer, the inception of the cavitation on the cylinder was suppressed, and the amplitude of pressure pulsations due to the cavitation shedding mechanism was mitigated.
In this paper, we performed the numerical and experimental study of unsteady cavitation surge around a semi-circular leading-edge flat plate using a passive flow control method. We mounted a ...miniature spanwise wedge-type vortex generator on the suction side of the model close to its leading edge. To mitigate the destructive impact of this type of cavitation on the hydrofoil performance, we analyzed the effects of the passive control on the dynamics of cavitation surge. First, we investigated experimentally the unsteady cavitating flow around the semi-circular leading-edge flat plate without passive control using high-speed visualization, acoustic measurements and particle image velocimetry method. Next, we simulated numerically the dynamics of unsteady flow under the cavitation surge conditions with an open source code and validated the numerical results using the experimental data. We used a proper interaction between turbulence and cavitation model to capture a highly unsteady behavior of cavitation surge. Finally, we considered the effects of the passive control device on the mechanism of the cavitation surge instability. Our results revealed that using the passive control method, it is possible to stabilize the attached cavity on the suction side of the flat plate, to hinder the development of the spanwise instability of the attached cavity and to mitigate large-scale cavity structures. Furthermore, high-pressure pulsations in the wake region induced by unsteady cavitation surge were considerably reduced.
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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
In this work, we experimentally investigated the cavitation effects on the hydrodynamic behavior of a circular cylinder at different cavitating flows. We analyzed the cavitation dynamics behind the ...circular cylinder using a high-speed camera and also measured the associated hydrodynamic forces on the circular cylinder using a load cell. We studied the cavitation dynamics around the cylinder at various types of the cavitating regimes such as cloud cavitation, partial cavitation and cavitation inception. In addition, we analyzed the cavitation dynamics at three different Reynolds numbers: 1 × 105, 1.25 × 105 and 1.5 × 105. The results showed that the hydrodynamics force on the circular cylinder can be increased with the formation of the cavitation behind the cylinder compared with the cylinder at cavitation inception regime. The three-dimensional flow caused complex cavitation behavior behind the cylinder and a strong interaction between vortex structures and cavity shedding mechanism. In addition, the results revealed that the effects of the Reynolds number on the cavitation dynamics and amplitude of the shedding frequency is significant. However the effects of the cavitation number on the enhancement of the amplitude of the shedding frequency in the cavitating flow with a constant velocity is slightly higher than the effects of Reynolds number on the enhancement of the amplitude of the shedding frequency at a constant cavitation number.
Experimental investigations were conducted to uncover the impact of cavitation control—through the use of biomimetic riblets on cavitating flows around a circular cylinder. First, the dynamics of ...cavitation in the flow behind a finite cylinder (without riblets) was unveiled by visualizing the cavitation clouds and measuring the lift force fluctuations acting on the cylinder. Second, in a significant step forward, a comprehensive explanation was provided for the cavitation control methods using two bio-inspired riblet morphologies positioned in different orientations and locations on the cylinder. For the first time, the impacts of these tiny formations on the flow dynamics and the associated cavitation process were scrutinized. This showed that scalloped riblets, with their curved design, induced secondary vortices near their tips and distorted primary streamwise vortices, and that high velocity gradients near the jagged pattern peaks of sawtooth riblets delayed flow separation, which affected cavitation.
In this work, we simulated a two-phase cavitating flow around one and two circular cylinders with the density based homogeneous mixture model. We discretized the governing equations with employing a ...high-order compact finite-difference scheme incorporating filtering scheme to account for the numerical instabilities and physical discontinuities. We applied a shock capturing sensor for discontinuities detection and switching between the second-order and high-order filtering and computed the far-field acoustic by the Ffowcs Williams-Hawkings surface integral method. We studied the flow features in the wake of a circular cylinder and two cylinders for noncavitating regime and cavitating conditions. In addition, we analyzed the cavitating flow for various gaps of two side-by-side cylinders. Results revealed that the shock waves due to the collapse of the cavity moving downstream of the cylinders is the main mechanism for the generation of the acoustic impulses. The results also showed that the noise peak can occur at lower frequencies for the smaller gap between the two cylinders. Furthermore, the wakes behind the side-by-side cylinders were merged together and a single vortex street was generated by the reduction of the gap between two cylinders.
•We simulated cavitating flow around one and two circular cylinders using a high-order simulation.•We analyzed the cavitating flow for various gaps of two side-by-side cylinders.•A shock capturing sensor for discontinuities detection and switching between the second- and high-order filterings was used.•Shock waves due to the collapse of the cavity was the main mechanism for the generation of the acoustic impulses.•Wakes behind the side-by-side cylinders were merged together by the reduction of the gap between two cylinders.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this work, we experimentally investigated effects of cavitation control on cavitating flows around a circular cylinder using mesoscale surface structuring. We considered two kinds of scalloped ...riblets, machined on the surface of a cylinder, to control cavitation. First, we tested a smooth cylinder placed in a cavitation tunnel at various cavitation numbers and Reynolds numbers. Then, we tested a cylinder with horizontal riblets and a cylinder with vertical riblets under the same conditions. We analyzed the effects of these scalloped riblets on cavitation dynamics behind the cylinder and on the cavitation-induced vibration. A high-speed camera captured the cavity structures behind the cylinders, and a load cell measured the associated pulsating lift force. Results showed that the riblets reduced the amplitude of a cylinder’s cavitation-induced vibration significantly compared to the amplitude of a smooth cylinder. In addition, the riblets mitigated tip-vortex cavitation and cavity structures in most cavitating regimes. At Reynolds numbers 1.0×105 and 1.5×105 and cavitation number 1.0, the vibration was decreased up to 60% for the cylinder with horizontal riblets.
•We experimentally studied cavitating flow around circular cylinders at different cavitation numbers.•We analyzed cavitation dynamics and pulsations of lift force using a high-speed camera and a load cell.•We used horizontal and vertical scalloped riblets on the cylinder’s surface to control cavitation.•Amplitude of cavitation-induced vibration on the cylinder with the riblets was significantly reduced.•Amount of cavitation behind the cylinder and tip-vortex cavitation were mitigated in most cavitating regimes.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
