Numerical Prediction of Vortex-Induced Vibrations on Top Tensioned Riser in Consideration of Internal Flow

In consideration of the effect of the internal flowing fluid and the external marine environmental condition on the vortex-induced vibration （VIV） of top tensioned riser （Till）, the differential equation is derived based on work-energy principles and the riser near wake dynamics is modeled by Facchinetti＇ s wake oscillator model. Then Galerkin＇ s finite element approximation is implemented to derive the nonlinear matrix equation of the coupled equations and file corresponding numerical programs are compiled which solve the coupled equations directly in the time domain. The comparison of the predicted results with the recent experimental results and the prediction of SHEAR7 is performed. The results show the validity of the proposed method on the prediction of VIV of deep water risers. The effect of internal flow on the dynamic characteristics and dynmnic response of the riser is analyzed and several valuable conelusions are drawn.

Effect of Internal Flow on Vortex-Induced Vibration of Submarine Free Spanning Pipelines

At present, most researches on the vortex-induced vibration of submarine free spanning pipelines ignore the effect of internal flowing fluid; furthermore, there are no research reports considering the coupling effect of internal and external fluid with the free span. In this paper, combining Iwan's wake oscillator model with the differential equation derived for the dynamic response of submarine free spanning pipelines with inclusion of internal flow, the pipe-fluid coupling equations are developed to investigate the effect of internal flow on the vortex-induced vibration of the free spans. The finite element approximation is implemented to derive the matrix equations of equilibrium. The Newmark method combined with simple iteration is used to solve the system of equations. The results indicate that the internal fluid flow may cause the shift of resonance band to the lower frequency and a slight decrease in the peak value; the effect will be more pronounced with the increase of the span length and can be weakened in the presence of the axial tension.

Effect of Upward Internal Flow on Dynamics of Riser Model Subject to Shear Current

Numerical study about vortex-induced vibration （V/V） related to a flexible riser model in consideration of internal flow progressing inside has been performed. The main objective of this work is to investigate the coupled fluid-structure interaction （FSI） taking place between tensioned riser model, external shear current and upward-progressing internal flow （from ocean bottom to surface）. A CAE technology behind the current research which combines structural software with the CFD technology has been proposed. According to the result from dynamic analysis, it has been found that the existence of upward-progressing internal flow does play an important role in determining the vibration mode （/dominant frequency）, vibration intensity and the magnitude of instantaneous vibration amplitude, when the velocity ratio of internal flow against external current is relatively high. As a rule, the larger the velocity of intemal flow is, the more it contributes to the dynamic vibration response of the flexible riser model. In addition, multi-modal vibration phenomenon has been widely observed, for asymmetric curvature along the riser span emerges in the case of external shear current being imposed.

Nonlinear Analysis of Flexible and Steel Catenary Risers with Internal Flow and Seabed Interaction Effects

Flexible risers and steel catenary risers often provide unique riser solutions for today’s deepwater field development. Accurate analysis of these slender structures, in which there are high-speed HP/HT internal flows, is critical to ensure personnel and asset safety. In this study, a special global coordinate-based FEM rod model was adopted to identify and quantify the effects of internal flow and hydrostatic pressure on both flexible and deepwater steel catenary risers, with emphasis on the latter. By incorporating internal flow induced forces into the model, it was found that the internal flow contributes a new term to the effective tension expression. For flexible risers in shallow water, internal flow and hydrostatic pressure made virtually no change to effective tension by merely altering the riser wall tension. In deep water the internal pressure wielded a dominant role in governing the riser effective tension and furthering the static configuration, while the effect of inflow velocity was negligible. With respect to the riser seabed interaction, both the seabed support and friction effect were considered, with the former modeled by a nonlinear quadratic spring, allowing for a consistent derivation of the tangent stiffness matrix. The presented application examples show that the nonlinear quadratic spring is, when using the catenary solution as an initial static profile, an efficient way to model the quasi-Winkler-type elastic seabed foundation in this finite element scheme.

Parametric Stability Analysis of Marine Risers with Multiphase Internal Flows Considering Hydrate Phase Transitions

This study investigates the effects of multiphase internal flows that consider hydrate phase transitions on the parametric stability of marine risers.A numerical model of the multiphase internal flow that considers a hydrate phase transition is established.The model first solves the flow parameters and subsequently obtains the natural frequencies of risers with different gas intake ratios.The stability charts of marine risers with different gas intake ratios are plotted by applying Floquet theory,and the effects of the gas intake ratio on the instability and vibration response of the risers are identified.The natural frequency increases with an increase in the gas intake ratio;thus,instability zones move to higher frequency ranges in the stability charts.As the increasing gas intake ratio reduces the damping effect of the Coriolis force,the critical amplitude of the heave in the unstable region decreases,especially when hydrodynamic damping is not considered.As a result,higher-order unstable regions are excited.When in an unstable region,the vibration response curve of a riser with a high gas intake ratio excited by parametric resonance diverges quickly due to parametric resonance.

APPROXIMATE NONREFLECTING INFLOW-OUTFLOW BOUNDARY CONDITIONS FOR THE CALCULATION OF NAVIER-STOKES EQUATIONS IN INTERNAL FLOWS

In this paper, the nonreflecting boundary conditions based upon fundamental ideas of the linear analysis are developed for gas dynamic equations, and the modified boundary conditions for Navier-Stokes equations are proposed as a substitute of the nonreflecting boundary conditions inside boundary layers near rigid walls. These derived boundary conditions are then applied to calculations both for the Euler equations and the Navier-Stokes equations to determine if they can produce acceptable results for the subsonic flows in channels. The numerical results obtained by an implicit second-order upwind difference scheme show the effective- ness and generality of the boundary conditions. Furthermore, the formulae and the analysis performed here may be extended to three dimensional problems.

CFD Simulation of Reactor Internal Flow in the Scaled APR＋

A series of 1/5 scale reactor flow model tests have been conducted in order to determine the hydraulic characteristics of the APR＋ （advanced power reactor plus）. The objective of test was to determine the core inlet flow field of the model reactor in order to provide input information required by the open core thermal margin analysis code such as TORC. In this study, in order to examine the validity of the results of reactor flow model tests and the applicability of CFD （computational fluid dynamics） in the simulation of reactor internal flow, CFD simulation was conducted with the commercial multi-purpose CFD software, ANSYSCFX V. 14. It was found that the velocity field in the downcomer had the inhomogeneous feature. Relative high velocity region was located in the core region. This result was different from measurement and this difference may result from the fact that some internal structures were not modeled with the real geometry but treated as the porous domain.

Analysis and Calibration of Internal Flow Force of EjectorPowered Engine Simulator System in Wind Tunnels

The ejector-powered engine simulator(EPES)system is an important piece of equipment in conducting an influence test of the intake and jet flow in low-speed wind tunnels.In this work,through the analysis of the structure and principle of EPES,three parts of the internal flow force were obtained,namely,the additional resistance before the inlet,the internal flow force in the inlet and the thrust produced by the ejector.On the assumption of one-dimensional isentropic adiabatic flow,the theoretical formulae for calculating the forces were derived according to the measured total pressure,static pressure and total temperature of the internal flow section.Subsequently,a calibration tank was used to calibrate the EPES system.On the basis of the characteristics of the EPES system,the process and method of its calibration were designed in detail,and the model installation interface of the calibration tank was reformed.By applying this method,the repeatability accuracy of the inlet flow rate calibration coefficient was less than0.05%,whereas that of the exhaust flow rate and velocity was less than 0.1%.Upon the application of the calibration coefficients to the correction of the wind tunnel experiment data,the results showed good agreement with the numerical simulation results in terms of regularity and magnitude before stall,which validates the reasonableness and feasibility of the calibration method.Analysis of the calibration data also demonstrated the consistency in the variation law and trend between the theoretical calculation and actual measurement of internal flow force,further reflecting the rationality and feasibility of the theoretical calculation.Nevertheless,the numerical difference was large and further widened with a higher ejection flow rate mainly because of the accuracy of flow measurement and the inhomogeneity of internal flow.The thrust deflection angle of EPES is an important factor in correcting this issue.In particular,the thrust deflection angle becomes larger with small ejection flow and becomes smaller with an increase in flow rate,essentially exhibiting a general change of less than 10°.

Experimental Study on Vortex-Induced Vibration of Marine Riser Model with Coupling Interference Effect Under Combined Internal and External Flow

A partition model of interference efficiency was constructed to study the coupling interference effect under combined internal and external flow.The concept of“internal flow efficiency”,“velocity ratio”and“interference efficiency”were introduced to quantify the effect of internal flow and interference,and reveal the coupling mechanism among internal flow,external flow and interference effect.The results showed that the dynamic response of risers under variable angles was significantly different after considering the effect of internal flow.When the external flow velocity was smaller than 0.25 m/s,the vibration of risers was promoted by the internal flow.With the increase of external flow velocity,the effect of internal flow was weakened and the dynamic response of riser mainly depended on the external flow and interference effect.Under the effect of different internal flow,the interference efficiency had similar change trend.The interference effect amplified the complex secondary flow effect inside the riser,making the dynamic response of riser complex and random.In this paper,the overlap area and subdivision criterion of interference effect were constructed within the range of experimental velocity ratio,and the change curve of interference efficiency was obtained with an average meaning,which may have important practical meaning.

Control of Nozzle Flow Using Rectangular Ribs at Sonic and Supersonic Mach Numbers

This study deals with base pressure management in a duct for various values of the Mach number(M),namely,Mach number corresponding to sonic and four supersonic conditions.In addition to the Mach number,the nozzle pressure ratio(NPR),the area ratio,the rib dimension,and the duct length are influential parameters.The following specific values are examined at M=1,1.36,1.64,and 2,and NPRs between 1.5 and 10.The base pressure is determined by positioning ribs of varying heights at predetermined intervals throughout the length of the square duct.When the level of expansion is varied,it is seen that the base pressure initially drops for overexpanded flows and increases for under-expanded flows.When ribs are present,the flow field in the duct and pressure inside the duct fluctuate as the base pressure rises.Under-expanded flows can achieve a base pressure value that is suitably high without experiencing excessive changes in the duct flow in terms of static pressure if a rib height around 10%of the duct height close to the nozzle exit is considered.Rectangular rib passive control does not negatively affect the duct’s flow field.

The Effect of Blade Outlet Angle on Performance and Internal Flow Condition of Mini Turbo-Pump

Mini turbo-pumps having a diameter smaller than 100mm are employed in many fields;automobile radiator pump,ventricular assist pump,cooling pump for electric devices,washing machine pump and so on.Further,the needs for mini turbo-pumps would become larger with the increase of the application of it for electrical machines.It is desirable that the mini turbo-pump design is as simple as possible due to restriction to make precise manufactures.But the design method for the mini turbo-pump is not established because the internal flow condition for these small-sized fluid machineries is not clarified and conventional theory is not conductive for small-sized pumps.Three types of rotors with different outlet angles are prepared for an experiment and a numerical analysis.The performance tests are conducted with these rotors in order to investigate the effect of the blade outlet angle on performance and internal flow condition of mini turbo-pumps.It is clarified from the experimental results that head of the mini turbo-pump increases and maximum efficiency flow rate shifts to larger flow rate according to the increase of the blade outlet angle,however the maximum efficiency decreases with the increase of it.In the present paper,the performance of the mini turbo-pump is shown and the internal flow conditions are clarified with the results of the experiment and the numerical flow analysis.Furthermore,the effects of the blade outlet angle on the performance are investigated and high performance design with simple structure for the mini turbo-pump would be considered.

Nonplanar flow-induced vibrations of a cantilevered PIP structure system concurrently subjected to internal and cross flows

Pipe-in-pipe(PIP)structures are widely used in offshore oil and gas pipelines to settle thermal insulation issues.A PIP structure system usually consists of two concentric pipes and one softer layer for thermal insulation consideration.The total response of the system is related to the dynamics of both pipes and the interactions between these two concentric pipes.In the current work,a theoretical model for flow-induced vibrations of a PIP structure system is proposed and analyzed in the presence of an internal axial flow and an external cross flow.The interactions between the two pipes are modeled by a linear distributed damper,a linear distributed spring and a nonlinear distributed spring along the pipe length.The unsteady hydrodynamic forces due to cross flow are modeled by two distributed van der Pol wake oscillators.The nonlinear partial differential equations for the two pipes and the wake are further discretized by the aid of Galerkin’s technique,resulting in a set of ordinary differential equations.These ordinary differential equations are further numeri cally solved by using a fourth-order Runge-Kutta integration algorithm.Phase portraits,bifurcation diagrams,an Argand diagram and oscillation shape diagrams are plotted,showing the existence of a lock-in phenomenon and figure-of-eight trajectory.The PIP system subjected to cross flow displays some interesting dynamical behaviors different from that of a single-pipe structure.

Shock Train and Pseudo-shock Phenomena in Supersonic Internal Flows

When a normal shock wave interacts with a boundary layer along a wall surface in supersonic internal flows and the shock is strong enough to separate the boundary layer, the shock is bifurcated and a series of shocks called “shock train” is formed. The flow is decelerated from supersonic to subsonic through the whole interaction region that is referred to as “pseudo-shock”. In the present paper some characteristics of the shock train and pseudo-shock and some examples of the pseudo-shocks in some flow devices are described.

Suction Performance and Internal Flow of a 2-Bladed Helical Inducer with Inlet Asymmetric Plate

It has been found in our past studies that the installation of asymmetric plate at the inlet of inducer is effective for the suppression of cavitation surge phenomenon.In the present study,the suction performance of 2-bladed helical inducer with an inlet asymmetric plate is experimentally investigated.It is observed that the suction performance in large flow rate conditions is not significantly influenced by the asymmetric plate,whereas the head of inducer with the asymmetric plate increases just before the head breakdown in partial flow conditions.To understand the mechanism of this additional head,the flow measurements and the numerical simulations are carried out.It is found that the circumferential component of absolute velocity at the exit of inducer slightly increases with the development of cavitation in both cases with and without the inlet asymmetric plate,indicating the increase of the theoretical head.The theoretical head increase with the inlet asymmetric plate is also confirmed by the unsteady numerical simulations,suggesting that the additional head is achieved through the increase of the theoretical head with the change of the exiting flow from the inducer associated with some amount of cavitation.

Synergy Methodology for Internal Flow of Turbomachinery

The complex curvature of turbomachinery rotor blade channels combined with strong rotational effect and clearance leakage brings on intricate internal flow phenomenon.It is necessary to study the internal flow and energy loss mechanism to reveal the influence law of the key parameters and to achieve its optimal design.Considering features of flow and temperature fields in rotor passage,the concept of synergy analysis derived from equation of energy conservation was put forward.Typical NASA low-speed centrifugal compressor(LSCC)rotor was chosen for analysis using CFD.Numerical results showed remarkable agreement with experiment datum in both the tendency of the performance characteristics and quantitative pressure values.Under different flow rates and inlet total temperatures conditions,thermal-fluid interaction effect and losses were studied by synergy analysis.Results showed that peak synergy positive value zones located around blade leading edge,across the shroud wall and hub wall,and at the position where tip-leakage flow was mixing with the bulk flow and high entropy zones existed.Increasing flow rate from design condition,positive and negative synergy areas both changed tiny around leading edge and trailing edge.Reducing flow rate,positive synergy areas tended to increase and negative areas decreased at same positions.The relationship between flow separation,heat transfer and losses in turbomachinery rotor can be revealed based on synergy analyses.

Performance and Internal Flow of a Counter-rotating Type Tidal Stream Turbine

In the past decade, the tidal energies have caused worldwide concern as it can provide regular and predictable re- newable energy resource for power generation. The majority of technologies for exploiting the tidal stream energy are based on the concept of the horizontal axis propellers, which can be derived from the design and operation of wind turbines. However, there are some peculiar features such as the propeller working in the seawater with free surface and the possible occurrence of cavitation as compared with wind turbines. Especially, for a coun- ter-rotating type tidal stream power turbine, it is difficult to accurately predict the interaction between the front and rear blades at the design stage by blade element momentum theory. As a result, CFD shows its advantage to predict the performance of counter-rotating type propellers of the tidal stream turbi^le. In order to improve the accuracy of CFD predictions, the predicted results must be verified with experimental values. In this paper, a CFD model using block-structured grid was set up and experimental test was performed in a water tunnel for a tidal stream turbine with counter-rotating type propellers. The comparison between CFD predictions and experimental data shows quite good agreement on the power coefficients, which provides an evidence of validation of the CFD model. Such results offer the necessary confidence in the accuracy of the set up CFD model for the coun- ter-rotating type tidal stream turbine.

Manipulating internal flow units toward favorable plasticity in Zr-based bulk-metallic glasses by hydrogenation

This work intends to manipulate the internal flow units in Zr_(55)Cu_(30)Ni_(5)Al_(10)bulk-metallic glasses(BMGs)through plasma-assisted hydrogenation to generate a positive microalloying effect on plasticity.Based on the cooperative shear model theory,serration-flow statistics during nanoindentation loading and creep tests during the holding stage were used to analyze the influence of hydrogen on the behavior of flow units in BMGs.Experimental observations showed that the hydrogen in the Zr_(55)Cu_(30)Ni_(5)Al_(10)BMGs caused mechanical softening,plasticity improvement,and structural relaxation.Analysis also showed that the average volume,size,and activation energy of internal flow units in the BMGs all increased as a result of the increase in the hydrogen content.The hydrogenation in the BMGs was found to lead to a prolifera-tion of shear bands,which promoted plasticity.The aggregation of these internal flow units reduced the stress required for plastic deformation through shear bands,ultimately causing softening and structural relaxation.

INTERNAL FLOW IN CROSS-FLOW FAN FOR AIR CONDITIONER

A numerical analytic process is suggested combining direct boundary element methodwith discrete vortex method and internal flow in cross-flow fan calculated. The process considersnot only function of impeller, but also effect of volute casing. Internal flow field of cross-flow fan ispractically measured, and results compared with that from calculation. It shows that the process isexpected to be used in predicting performances of fluid machinery.

Internal Water Flows and Particles Abstraction in Daphnia

The operational function of the trunk limbs (thoracic appendages), of Daphnia, P3 and P4, is a long-term disputed definition between “solid walls”, sieving filters. Sieving is unlikely process for routine particle collection, particle capture is not a simple mechanical process and not by sieving alone. Analysis promotion supported by direct observational examination of the in-vivo cinematographic slow-motion film and magnified solid photos of tethered Daphnia by high-speed camera (250 frames per second) resulted in a definite interpretation presented in this paper. The Daphnia’s feeding mechanism achieves particles abstraction not by sieving. The existence of two internal alternate water flow routs was indicated: Lateral and Median. These micro flow structures are suggested as vulnerability reduction.

Analysis of Profile and Unsteady Flow Performance of Variable Base Circle Radius Scroll Expander

To study the complex internal flow field variation and output characteristics of a variable base radius scroll expander,this paper uses dynamic mesh techniques and computational fluid dynamics(CFD)methods to perform transient numerical simulations of a variable base radius scroll expander.Analysis of the flow field in the working cavity of a variable base radius scroll expander at different spindle angles and the effect of different profiles,speeds and pressures on the output characteristics of the scroll expander.The results of the study show that due to the periodic blocking of the inlet by the orbiting scroll,the fluid hits the internal walls of the expander at different flow rates,with excessive mechanical losses,resulting in an uneven distribution of the internal flow field.At the same temperature,pressure and scroll plate diameter,the variable base radius scroll expander increases the output torque by 0.046 N·m,the output power by 9.634 W and the isentropic efficiency by 3.8%compared to the fixed base circle scroll expander.As the speed is inversely proportional to the output torque,the isentropic efficiency of the expander tends to increase and then decrease as the speed increases.The density of the fluid is directly proportional to the pressure over a range of pressures.As the pressure increases from 0.6 to 0.9 Mpa,the average mass flow rate and isentropic efficiency increase by 0.02357 kg/s,and 6.61%,respectively.