Cavitation Passive Control on Immersed Bodies Javadi, Khodayar; Dorostkar, Mohammad Mortezazadeh; Katal, Ali
Journal of marine science and application,
03/2017, Volume:
16, Issue:
1
Journal Article
Peer reviewed
This paper introduces a new idea of controlling cavitation around a hydrofoil through a passive cavitation controller called artificial cavitation bubble generator （ACG）. Cyclic processes, namely, ...growth and implosion of bubbles around an immersed body, are the main reasons for the destruction and erosion of the said body. This paper aims to create a condition in which the cavitation bubbles reach a steady-state situation and prevent the occurrence of the cyclic processes. For this purpose, the ACG is placed on the surface of an immersed body, in particular, the suction surface of a 2D hydrofoil. A simulation was performed with an implicit finite volume scheme based on a SIMPLE algorithm associated with the multiphase and cavitation model. The modified k-ε RNG turbulence model equipped with a modification of the turbulent viscosity was applied to overcome the turbulence closure problem. Numerical simulation of water flow over the hydrofoil equipped with the ACG shows that a low-pressure recirculation area is produced behind the ACG and artificially generates stationary cavitation bubbles. The location, shape, and size of this ACG are the crucial parameters in creating a proper control. Results show that the cavitation bubble is controlled well with a well-designed ACG.
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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
An optimal performance film cooling implies a high cooling effectiveness along with a wide surface cooling coverage. During the past decades, film cooling effectiveness has been well defined with a ...specific formula to evaluate the quality of a cooling system. However, despite numerous research on film cooling, there is still no explicit parameter to quantify how well a coolant film is spread over a hot surface. This work introduces a new coefficient namely cooling uniformity coefficient (CUC) to evaluate how well a coolant film is spread over a surface that is being cooled. Four different cases at two blowing ratios of 0.5 and 1.5 are studied. Three cases are ordinary single jets with different cross sections whereas the fourth one is a novel combined-triple-jet, sometimes called anti-vortex-holes, introduced in a previous work. The cross sections of all four cases were kept the same to have equal coolant mass flow rate. This article suggests that CUC is a necessary parameter in order to have a well performance film cooling system, in addition to the film cooling effectiveness. Indeed, CUC evaluates how well coolant films are spread over hot surfaces which is very important from thermal loads and thermal stress points of view.
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BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The rupturing process of a membrane, located between two fluids at the center of a three-dimensional channel, is numerically investigated. The smoothed particle hydrodynamics (SPH) and the finite ...element method (FEM) are used, respectively, for modeling the fluid and solid phases. A range of pressure differences and membrane thicknesses are studied and two different rupturing processes are identified. These processes differ in the time scale of the rupture, the location of the rupture initiation, the level of destruction and the driving mechanism.
•In contrast to freely rising bubbles, attached bubbles experiences different acceleration/deceleration fields.•wall hydrophobicity strongly affects the bubble profile.•Depending on the wall ...wettability and Bond numbers, the wall-bounded bubbles are influenced by different forces.•Different Bubble profile formation results in rise up with different velocities.•Bubbles with contact angles less than 90° move with a speed even higher than freely rising bubbles.
This paper deals with the dynamics of rising bubbles attached to a vertical wall under different wettability conditions. Even though, bubbles rising freely in a liquid have extensively been studied, bubbles attached to a wall have not been fully understood. Therefore, in this work, rising bubbles attached to a vertical wall were numerically investigated by applying the ALE method along with adaptive mesh refinement schemes to properly resolve the bubble interface and its deformation. To consider wall wettability effects, different contact angles of 45°, 90°, and 105° were considered along with the case of freely rising bubbles. The problem was carried out at different Bond numbers of 0.27, 0.4, and 0.635 for different Reynolds numbers 250, 1000, and 2000. It was shown that in contrast to freely rising bubbles, which have a smooth monotone deformation and motion behavior during the rising, attached bubbles experiences different acceleration/deceleration fields with complicated oscillations and deformations. The results show that wall hydrophobicity strongly affects the bubble profile and consequently the dynamics of bubble rising. In addition, depending on the wall wettability and Bond numbers, the wall-bounded bubbles are influenced by different forces leading to different profiles and hence rise up with different velocities along the wall. The results illustrate that bubbles with contact angles less than 90° move with a speed even higher than freely rising bubbles.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•We present a method to control various destructive effects of cloud cavitation such as vibration and high-pressure peaks.•Unsteady cloud cavitating flow around a hydrofoil was predicted using ...Partially-averaged Navier–Stokes (PANS) method.•Addition of the PANS Model in the OpenFOAM package.•Combination of the PANS method with Schnerr–Sauer cavitation model for obtaining the most accurate results.•Using a proper cavitation control a significant reduction in high wall-pressure peaks and vibration has been achieved.
We present an efficient method to control the evolution of unsteady cloud cavitation around the CAV2003 benchmark hydrofoil using passive cavitation controllers so called cavitation-bubble generators (CGs). Cavitation control may be used in many engineering applications, particularly in the marine and turbo machinery field. We first simulated the unsteady cavitating flow around the hydrofoil without CGs using a Partially-averaged Navier–Stokes (PANS) method, and validated the acquired results against experimental data. We coupled the turbulence model with a mass transfer model and successfully implemented it in the open source toolbox OpenFOAM. Next, we studied the effect of different CGs on the qualitative parameters, such as the cavitation structure and the cavity shape. We varied size and location of the CGs to find the proper control of the cloud cavitation. We also analyzed in detail the effect of CGs on various destructive mechanisms of cavitation, such as highly unsteady cloud cavitation, turbulent velocity fluctuations, wall-pressure peaks, and degrading hydrodynamic performances. Our results revealed that CGs can substantially reduce instantaneous high-pressure pulsations on the hydrofoil surface. We observed that the cyclic behavior of unsteady cloud cavitation was suppressed, and the hydrodynamic efficiency of the hydrofoil was increased. The local boundary layer on the hydrofoil surface was altered, and the turbulent velocity fluctuation was reduced significantly, confirming that the vortex structures on the suction side and the wake region of the hydrofoil were changed remarkably.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Three-dimensional unsteady flow field around a finite circular cylinder standing in a flat-plate boundary layer is studied. For this purpose, two different numerical turbulence approaches as wall ...adapted local eddyviscosity LES (LES-WALE) and the zonal hybrid RANS-LES approach of Detached-Eddy Simulation (Zonal-DES) are used. Analysis is carried out for a finite circular cylinder with diameter of D = 3 mm and length-to-diameter ratio of L/D=6 which leads to the Reynolds number 2x10 super(4). Numerical simulation has been performed based on the LES-WALE and Zonal-DES turbulence models using coarse and fine grids. Ability and accuracy of two models in capturing the complex physics of present phenomenon are investigated by comparing their results with each other and validated experimental results. Also, effect of several important parameters such as time-averaged pressure coefficient, velocity, vortex shedding frequency and performance of the LES-WALE and Zonal-DES turbulence models are studied.
•We present an efficient passive control method to stabilize the cloud cavitation instabilities using Cylindrical Cavitating-bubble Generators (CCGs).•Addition of the PANS Model in the OpenFOAM ...package.•Combination of the PANS method with Schnerr-Sauer cavitation model for modeling the unsteady cloud cavitating flow.•Using our passive control method a notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed.
Unsteady cloud cavitation phenomenon is an important subject due to its undesirable effects in various applications such as ship propeller, rudder and hydraulic machinery systems. We present an efficient passive control method to control the cavitation instabilities which may be caused by the shedding of cavity structures in the vicinity of the solid surface of an immersible body. We proposed a passive control method so called Cylindrical Cavitating-bubble Generators (CCGs) on the surface of a benchmark hydrofoil and analyzed the effects of this passive controller on the dynamics of the unsteady cloud cavitation. First we modeled the unsteady cavitating flow around the hydrofoil without CCGs using a hybrid URANS model which was implemented in an open source code. Next, we studied the effect of CCGs on the mechanism of the unsteady cloud cavitation. The results show that using this method, the unsteady cavity structure was changed to a quasi-stable cavity structure compared with the cloud cavity shedding in the case without CCGs. We observed that the instability behavior of the unsteady cloud cavitation was mitigated and only small-scale cavity may be shedded from the hydrofoil in the free stream flow away from the hydrofoil surface. A notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed.
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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 paper, flow separation control over a NACA 4415 wing section using arrays of discrete quasi-radial wall jets is numerically investigated. This novel flow control method is studied under ...conditions which the wing section angle of attack is α=18 degrees and Reynolds number based on chord length is Rec=550000. Contours of mean streamwise velocity indicated that quasi-radial wall jets cover wing surface rapidly. Therefore, arranging them in a spanwise row can be an effective way in adding momentum to the near wall fluid particles. According to the results, if blowing parameters of the jets be chosen correctly, arrays of quasi-radial wall jets would lead to considerable decrease in wake region extent and would bring enhancement of wing aerodynamic performance (up to 46% increment of lift coefficient). Moreover, investigation of streamlines and velocity vectors reveals the presence of two helical vortices on both edges of quasi-radial wall jets. These vortices, which form due to interactions between external stream and spanwise velocity component of jets, generate mixing in the surrounding fluid. It could be said that besides momentum addition, quasi-radial wall jets in an external stream act like vortex generators.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The torque exerted on the walls of a micro cylindrical Couette gas flow is studied using Direct Simulation Monte Carlo (DSMC) method and analytical solutions. An analytical solution for temperature ...filed under specified wall heat flux condition is proposed using a previously introduced power-law (PL) wall-scaling approach. The results of cylindrical Couette flow under specified wall heat flux condition are compared with those of isothermal flow. The effects of rarefaction, compressibility, tangential momentum accommodation coefficient (TMAC) and gas-surface heat exchange on the torque values are investigated for slip to free molecular flow regimes. The results indicate that the torque coefficient has a maximum value at the transitional flow regime for TMAC values less than unity. Lower TMAC values and higher Knudsen numbers lead to reduction and deviation of torque magnitudes from those of continuum limit. Thermal simulations of cylindrical Couette flow show that heating and cooling have significant effects on the torque values. Velocity profiles and normalized torque values demonstrate mixed increasing/decreasing behavior under heating/cooling processes for rarefied gas flow.
•Analysis of two non-dimensional torque parameters at rarefied conditions.•Simulation of rarefied gas between two cylindrical walls under specified wall heat flux condition using DSMC method.•Proposing a new analytical solution for temperature field under thermal boundary conditions.•Detection of a mixed behavior of rarefied gas in response to heating/cooling conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In this study, natural super-cavitating flow around three different conical cavitators with wedge angles of 30°, 45° and 60° is investigated. We apply the k - ... turbulence model and the volume of ...fluid (VOF) technique to numerically study the three-dimensional cavitating flow around the cavitators. The turbulence approach is coupled with a mass transfer model which is implemented into the finite-volume package. Simulations are performed for different cavitation numbers. Finally, the effects of some important parameters such as the cavitation index, inlet velocity, Froude number and wedge angle of cavitators on the geometrical characteristics of the super-cavities are discussed. The authors' numerical results are in good agreement with the experimental data.(ProQuest: ... denotes formulae/symbols omitted.)
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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