Numerical study on the flow field inside and around a semi-submersible aquaculture platform

Second-law analysis (SLA) is an important concept in thermodynamics, which basically assesses energy by its value in terms of its convertibility from one form to another.[...]

Numerical simulation of the flow field inside and around gravity cages

As computational fluid dynamics (CFD) advances, entropy generation minimization based on CFD becomes attractive for optimizing complex heat-transfer systems. This optimization depends on the accuracy of CFD results, such that accurate turbulence models, such as elliptic relaxation or elliptic blending turbulence models, become important. The performance of a previously developed elliptic blending turbulence model (the model) to predict the rate of entropy generation in the fully developed turbulent circular tube flow with constant heat flux was studied to provide some guidelines for using this class of turbulence model to calculate entropy generation in complex systems. The flow and temperature fields were simulated by using a CFD package, and then the rate of entropy generation was calculated in post-processing. The analytical correlations and results of two popular turbulence models (the realizable k–ε and the shear stress transport (SST) k–ω models) were used as references to demonstrate the accuracy of the model. The findings indicate that the turbulent Prandtl number (Prt) influences the entropy generation rate due to heat-transfer irreversibility. Prt = 0.85 produces the best results for the model. For the realizable k–ε and SST k–ω models, Prt = 0.85 and Prt = 0.92 produce the best results, respectively. For the realizable k–ε and the SST k–ω models, the two methods used to predict the rate of entropy generation due to friction irreversibility produce the same results. However, for the model, the rates of entropy generation due to friction irreversibility predicted by the two methods are different. The difference at a Reynolds number of 100,000 is about 14%. The method that incorporates the effective turbulent viscosity should be used to predict the rate of entropy generation due to friction irreversibility for the model. Furthermore, when the temperature in the flow field changes dramatically, the temperature-dependent fluid properties must be considered.

To achieve efficient and sustainable marine aquaculture, STAR-CCM+ was used to simulate the internal and external field characteristics of a semi-submersible aquaculture platform based on a porous media model, focusing on the influence of incoming flow velocity and net solidity ratio. The results indicate that the flow field distribution around the platform exhibits no significant regularity and that low-velocity vortex regions are primarily concentrated near the pillars and nets. After velocity attenuation, the velocity reduction coefficients at the centers of the three cages are 90.26%, 63.65%, and 52.56%, respectively. Furthermore, the velocity attenuation inside the cages is minimally influenced by incoming flow velocity, with a maximum difference of 3.10%. In contrast, differences in net solidity ratio significantly affect velocity attenuation, particularly in downstream regions. The velocity reduction coefficient in the third cage varies by up to 43.25% depending on the net solidity ratio. These findings provide practical insights for the engineering design and application of aquaculture platforms.

Computational Study on the Influence of jet on Reduction of Drag Over Cone Flare Bodies in Hypersonic Turbulent Flow

In this paper, a boundary element method (BEM) is used to predict the unsteady performance of ducted propellers in open water and ship behind conditions. The model propeller adopted includes the non-axisymmetric duct appendages (e.g., gearbox, brackets, and vertical structure connected to the hub), which induce severe shedding vortices on the propeller plane. This study thus investigates the effects of separation from the duct appendages as well as the upstream hull on the unsteady ducted propeller performance under different loading conditions. To improve the accuracy of a potential flow solver for highly viscous problems with separated flow near a blunt body, the present method is coupled with a viscous Reynolds-Averaged Navier-Stokes (RANS) solver. The former solves the ducted propeller problem to produce the propeller-induced flow field and body forces, with which the latter solves the total flow field based on a finite volume method. This approach is implemented in an iterative manner until the predicted 3D effective wake on the propeller surface becomes fully converged. An automated interface is developed to facilitate this process. A complete analysis of the propeller performance (i.e., predicted effective wake, flow-field, unsteady forces, and circulations on the blade) is presented at various operating conditions to investigate how the flow field inside and outside the nozzle is influenced by the viscous interaction among the incoming flow, propeller, its appendages, and upstream hull. For the sake of validation, the predicted results are compared with experimental measurements and results from unsteady full-blown RANS simulations. The presented results show satisfactory agreement among the results from different approaches, which makes the BEM/RANS coupling scheme adequate and computationally efficient for practical applications.

This paper reports based on a numerical study of multiphase fluid flow through a channel with a semi-submersible platform using computational fluid dynamics (CFD). The present study will be designing and analysing semisubmersible platform with pencil column and without pencil column. The computational fluid dynamics which solves simple differential equations and finite volume method (FVM) will be used. A turbulence model is considered i.e. large eddy simulation (LES). The semi-submersible model is designed as pontoons, columns, horizontal brace, pencil column and deck. The pontoons are horizontal placed stadium shaped structures which are submerged into the water. The columns are structures which connects the deck and pontoons. The horizontal braces are structures which connects the two or more columns which increases the rigidity of the columns. The pencil columns are columns with lesser diameter of about 0.25D which are placed between the columns. The deck are flat surfaces which provides workable area. This paper is a comparison of fluid flow by varying the model i.e. with and without pencil column in the semi-submersible platform. The velocity contours, pressure contours and streamline contours are plotted. The difference in pressure, velocity and streamline flow are tabulated and graphically represented. The percentage difference in pressure and velocity are calculated for structural design for various offshore platforms.

Shear stress transport (SST) k–ω model and detached eddy simulation (DES) have been widely applied in crosswind stability simulations for trains in the literature. In the previous research, the influence of the SST and DES approaches on the flow field around trains, which affects the surface pressure and consequently the aerodynamic forces of the train, was not properly investigated in terms of their influence flow field. The SST and improved delayed detached eddy simulation (IDDES) turbulence models have been tested in this study for their ability to predict the flow field around, surface pressure, and aerodynamic forces on a 1/25th scale Class 390 train subjected to crosswinds. Numerical simulation results were validated with experimental data. Results show that both SST and IDDES predict similar trends in the mean flow field around the train. However, there were some slight differences observed in the size of vortices, the position of separation points, and consequently, the separation and attachment lines. The SST results compared more closely to the experimental data than IDDES for pressure coefficient on the leeward surface and roof at certain loops. Slight differences were observed in force coefficients for SST and DES. The side force coefficients calculated using computational fluid dynamics (CFD) sit within the experimental uncertainty, whereas the lift force coefficients deviated greatly due to the omission of some underbody geometrical features. Both SST and IDDES approaches used the linear-upwind stabilized transport (LUST) scheme and were able to predict accurately the time-averaged surface pressure within the margin of the experimental uncertainty.

— In the present study, numerical studies are carried out to investigate the heat transfer in rectangular duct roughened by square and trapezoidal shaped ribs on one wall using different fluids. The coolant fluids such as air, steam, air/mist and steam/mist were investigated. The computational results show that the shear stress transport (SST) turbulence model is selected by comparing the predictions of different turbulence models with experimental results. The heat transfer coefficients enhance in ribbed channel at injection small amount of mist. The heat transfer coefficients of air/mist, steam/mist increase by 14% and 104% than that of air, respectively in square shaped ribs. However the heat transfer coefficient of air, air/mist, steam and steam/mist increase by 9%, 16%, 68% and 118%, respectively for trapezoidal shaped ribs compared with air in square shaped ribs. Keywords - Heat transfer; Rib roughness; mist I. I NTRODUCTION Periodic ribs are frequently employed to enhance the heat transfer process in various cooling passages such as turbine blades, guide vanes and combustor walls. Heat transfer augmentation inside cooling channels is achieved by using repeated ribs as turbulence promoters. The periodic ribs break the laminar sub-layer and create local wall turbulence due to flow separation and reattachment between the ribs, greatly enhancing the heat transfer. Several researchers have studied the heat transfer characteristics in straight channels with various shaped ribs using air as coolant flow. Chandra et al. [1], Han et al. [2], Lanjewar et al. [3], Srinath et al. [4], Salameh and Sunden [5], Tanda [6] and Wang and Sunden [7] studied experimentally the effect of ribs configuration and angled ribs on heat transfer and friction. While by numerical predictions of the flow and heat transfer in rib-roughened passages have been conducted previously by several investigators: Kashmiri et al. [8] investigated the rib pitch effect on heat transfer. Taslim and Liu [9], and Haasenritter et al. [10] performed both numerical and experimental analyses on roughened square channel with sharp and round profile ribs using k- turbulence emodel. They found good agreement between modeling and experimental. Chaube et al. [11] obtained a good agreement of heat transfer predicted with experimental data for roughness plate using SST k-ω. Lu and Jiang [12] have performed both the numerical analysis and experimental study to investigate the heat transfer and fluid flow behavior in rectangular channel using SST k-ω and RNG k-e turbulence models. They have concluded that the SST k-ω turbulence model was more suitable for the convection heat transfer in such channels. Wang et al. [13] examined the capabilities of different turbulence models in predicting heat transfer and flow serpentine cooling channel. They compared performance of air and steam as coolant flow. They showed the better model was SSG turbulence model and the steam heat transfer efficiency is higher than that of air. Shui et al. [14] compared of k-e model, SST model and SSG model in prediction of heat and fluid flow of square ribbed channel. The simulation matched well with experimental results using SSG turbulence model and the steam is proper coolant than air. Moreover, many numerical studies have been published regarding comparison analysis of air and steam as a coolant such as Albeirutty et al. [15], Najjar et al. [16] and Sanjay et al. [17]. These studies showed that the closed loop steam cooling offers the highest plant efficiency. There have been experimental and numerical studies of tubes and flat plate cooling with air/mist including those of Sikalo et al. [18], Oisin et al. [19], Novak et al. [20], Kumari et al. [21], Shokouhmand and Ghaffari [22] and Pakhomov and Terekhov [23]. These studies concluded that the heat transfer coefficient can be increased with introduction of a fine water mist. However the experimental and numerical validation of heat transfer results of mist/steam cooling in heated horizontal tube introduced by Gou et al. [24] and Dhanasekaran and Wang

The stall margin and choke margin of centrifugal compressor could be increased by using Self-Adaptive Casing Treatment (SACT). The previous numerical research mainly focuses on making parametric optimization rather than the selection of turbulence model and flow field analysis of the compressor with SACT. In this work, the 3D steady state simulations were carried out to obtain the performance and flow field of the Krain impeller with and without SACT by ANSYS-CFX. Four turbulence models including k-Epsilon turbulence model, RNG k-Epsilon turbulence model, Shear Stress Transport (SST) turbulence model and BSL Reynolds Stress (BSL) turbulence model were used to simulate the Krain impeller with a vaneless constant area diffuser. The numerical data were validated by the experimental data in reference. The results of this study showed that different turbulence models led to differences in performance predictions and flow field characteristics, and the overall performance and flow field features could be predicted more accurately by using SST turbulence model. The bypass flow and reinjected flow were respectively observed in the hole when the Krain impeller with SACT worked at large and small mass flow rate conditions. And the stable working range of the Krain impeller was expanded by using SACT. In addition, the development of the low-velocity fluid at the blade tip region was restrained with the application of SACT.

The aim of this article is to understand numerically the flow and heat transfer characteristics under oscillating flow conditions for periodically corrugated wavy channel. For the same channel, under steady-state flow conditions, experimental and numerical studies were done under steady-state flow conditions by our two previous studies. Three turbulence models are used, namely the k–ω, the Shear Stress Transport (SST), and the transition SST. According to the previous study, the best agreement with experiments was obtained using the SST turbulence model. Therefore, the SST turbulence model is applied in this study on the oscillating flow. The finite volume method is used as the numerical method. Investigations are performed for air flowing through corrugated channel which has sharp wavy peaks with an inclination angle of 30° and 5 mm minimum channel height. Reynolds number is varied within the range 6294–7380, while keeping the Prandtl number constant at 0.70. Four different sinusoidal oscillating flow conditions are used. Variations of the Nusselt number, friction factor, and thermo-hydraulic performance factor with the Reynolds number are studied.

For pulsating flow, the behaviours of the convective heat transfer and friction factor for a periodic corrugated channel are investigated numerically. The finite volume method is used in the numerical study. Three different Reynolds Averaged Numerical Simulation based turbulence models, namely the k-ω model, the Shear Stress Transport (SST) model and the transition SST model are used and compared. The results are also compared with the previous experiments for non-pulsating flow. Analyses are conducted for air flow through a corrugated channel which has sharp corrugation peaks with an inclination angle of 30° and a 5mm minimum channel height. Reynolds number is changed in the range 6294 to 7380, while keeping the Prandtl number constant at 0.70. A sinusoidal pulsatile flow condition which is F=400 and uA*=0.5 is used. Variations of the Nusselt number and the friction factor with the Reynolds number are studied. Non-pulsating flow results and pulsating flow results are compared with each other.

Flow instability in a centrifugal fan was studied using energy gradient theory. Numerical simulation was performed for the threedimensional turbulent flow field in a centrifugal fan. The flow is governed by the three-dimensional incompressible Navier-Stokes equations coupled with the RNG k-e turbulent model. The finite volume method was used to discretize the governing equations and the Semi-implicit method for pressure linked equation (SIMPLE) algorithm is employed to iterate the system of the equations. The interior flow field in the centrifugal fan and the distribution of the energy gradient function K are obtained at different flow rates. According to the energy gradient method, the area with larger value of K is the place where the flow loses stability easier. The results show that instability is easier to generate in the regions of impeller outlet and volute tongue. The air flow near the hub is more stable than that near the shroud. That is due to the influences of variations of the velocity and the inlet angle along the axial direction. With the decrease of the flow rate, instability zone in a blade channel moves to the impeller inlet from the outlet and the unstable regions in different channels develop in opposite direction to the rotation of impeller.

This numerical study focuses on the three-dimensional vortical structure of a flow field inside a side-ported rotary engine. Since both two-dimensional numerical simulation and experimental visualization methods may not be applicable to side-ported rotary engine where the flow field inside is essentially three-dimensional, the authors have developed a three-dimensional computer simulation code based on the compressible Navier-Stokes equations. A Chakraverthy third-order TVD scheme devised in the code successfully calculates a flow field in a light load region at 3000 rpm, since the macroscopic stroke characteristics show appropriate behaviors. The velocity vector fields alone do not seem to provide fundamental images of the flow fields, thus characteristic vortex tubes are sought. Although it is not easy to extract vortex tubes closely surrounded by the moving boundary walls, they are capable of providing useful information which otherwise may not be given. The shapes and their change in vortex tubes, in fact, refine perspectives on the flow fields, and hence the effects of the intake flow and the rotor motion on the flow structure are well understood.

The article states that a ship in the open sea is in a complex oscillatory motion due to the aerodynamic force of water flow and hydrodynamic pressure. Among the lateral, keel, horizontal, and vertical vibrations, only the vertical vibration of the vessel primarily affects the technological process of drilling a well. The article determines the magnitude of the hydrodynamic pressure that arises in the wellbore from the oscillation of the semi-submersible drilling platform in the absence of well flushing and a telescopic string, a compensator for vertical movement. When formulating the problem accurately, one should take into account the presence of natural vibrations of both the drilling tool and the semi-submersible drilling platform from which the well is to be drilled. The semi-submersible drilling platform was awarded by Caspian Drilling Company to two subsidiaries of Keppel offshore & Marine Company with Caspian Rigbuilders and Caspian Shipyard Company. This platform, named after Heydar Aliyev, is the first and the only drilling platform in the world with a 1,400-atmosphere system technology. Its carrying capacity is 5,600 tons, and its displacement is 47,500 tons. Vertical vibrations of the semi-submersible platform, caused by hydrometeorological conditions, set in motion the drilling tool associated with the semi-submersible drilling platform, along with the flushing solution that fills the well during drilling without a riser. This leads to a change in the hydrodynamic pressure in the well. Therefore, the change in hydrodynamic pressure in the well must be checked using a special installation that simulates the vibrations of the vessel and drilling tool in a well filled with drilling fluid.