Computational modeling of flow over an ogee spillway

This paper presents an investigation into the hydraulics of regular ogee-profile spillways. The free-surfaces of the fluid for several flow heads as measured in the hydraulics laboratory are used as benchmarks. The finite element computational fluid dynamics software, ADINA, was used to predict the free surface over an ogee spillway and thus model the flow field. Since the actual flow is turbulent the k-ε flow model was used. For the cases considered in this paper, ADINA predicted reasonable free surface results that are consistent with general flow characteristics over spillways. The results are also in close agreement with measured free-surface profiles over the entire length of the spillway.     

CFD assisted modeling for control system design: A case study

This paper presents a novel modeling approach of coupling transient computational fluid dynamics (CFD) simulation with system identification for control system involving fluid flow and heat transfer. In order to illuminate the feasibility of this method, a fluid flow and heat transfer related process, i.e. a three dimension (3-D) spatio-temporal air temperature distribution and input (inlet air temperature) dependent process in the desert climate chamber, is considered. The distributed parameter models of the chamber temperature are identified using transient CFD simulation results and are then validated against the results obtained from the CFD simulations with high RT² (more than 0.97) and negative Young’s information criterion (YIC, less than ?11.8). The PI controllers embedded in CFD simulation are then developed based on the models. The performance of the closed-loop systems is also evaluated within the full-scale CFD model. The results show that CFD-based system identification is feasible to model fluid flow and heat transfer related processes.     

Effectiveness charts for counter flow corrugated plate heat exchanger

Gasketed plate heat exchangers (PHEs) are used in refrigeration and heat pump plants and are extensively used in the processing of food and drinks, where the ease of plate cleaning and re-gasketting are important. Existing literatures report on the calculation procedure, performance analysis for different flow configurations, CFD modeling and simulation for different number of plates with various flow patterns as well as passes, analysis charts for flat PHE, etc. However, analysis charts prepared for corrugated PHE are not available in the open literature. In the present study, a 9-plate counter flow corrugated plate heat exchanger is modeled and simulated using computational fluid dynamics (CFD). The performance of the PHE is analyzed using the obtained simulated data in the form of charts such as effectiveness, ε versus number of transfer units, NTU at constant capacity ratio, R; Temperature effectiveness, P versus NTU at constant R and non-dimensional mean temperature difference, θ versus temperature effectiveness, P at constant NTU and R.     

CFD simulation of magnetorheological fluid journal bearings

Magnetorheological fluid journal bearing can be controlled by a steady magnetic field doing that very effective for attenuating and controlling the performance of the rotor bearing systems.An integrated simulation study, of a magnetorheological (MRF) fluid journal bearing, via computational fluid dynamics (CFD) and finite element method (FEM) is presented in this paper. The journal bearing characteristics such as, eccentricity, attitude angle, oil flow and friction coefficients are calculated and presented as functions of the magnetic field, and L/D bearing ratios.A specific procedure in order to simulate an MRF bearing operated in high eccentricity ratios is also presented and the meshing requirements are discussed.     

Towards simulating urban canyon circulations with a 2D lattice Boltzmann model

The Lattice Boltzmann (LB) method is a novel fluid modelling technique developed from cellular automata. Instead of numerically solving the continuum Navier–Stokes equations, it simulates the interactions of mesoscopic particle populations p_α using discrete speeds and positions to obtain the macroscopic velocity, density and temperature fields. Localised at neighbouring grid nodes, the method handles complex geometries and multiple fluids more easily than traditional continuum CFD methods.Rothman and Zaleski (Lattice-Gas Cellular Automata: Simple Models of Complex Hydrodynamics (1997) Cambridge University Press, Cambridge) discuss LB method theory and development in more detail.To demonstrate the power of the technique, a 2D LB model is first used to perform urban canyon configuration studies at Reynolds number Re=100 for Height to Width (H/W) ratios from 0.125 to 2. Then, thermal lid driven cavity simulations for Re=100 and Rayleigh number Ra=2000 are performed for different locations of a relatively hot wall. The simulated flow fields appear qualitatively consistent with physical flows observed in wind tunnel and field studies, and indicate that LB methods generate results comparable to traditional CFD methods for the selected flow situations.     

Single-pulse dynamics and flow rates of inertial micropumps

Bubble-driven inertial pumps are a novel method of moving liquids through microchannels. We combine high-speed imaging, computational fluid dynamics (CFD) simulations, and an effective one-dimensional model to study the fundamentals of inertial pumping. For the first time, single-pulse transient flow through U-shaped microchannels is imaged over the entire pump cycle with 4 \(\upmu\)s temporal resolution. Observations confirm the fundamental N-shaped flow profile predicted earlier by theory and simulations. Experimental flow rates are used to calibrate the CFD and one-dimensional models to extract an effective bubble strength. Then, the frequency dependence of inertial pumping is studied both experimentally and numerically. The pump efficiency is found to gradually decrease once the successive pulses start to overlap in time.     

Investigations on the flow behaviour in microfluidic device due to surface roughness: a computational fluid dynamics simulation

In the field of micro-fluidics device, as the cross section of micro-channel comes down to the scale of few tens of micro-meters, surface area to volume ratio increases significantly, and due to this, surface dependent phenomenon dominates during flow of the fluid. This surface dependent phenomenon is mainly governed by surface roughness as an important parameter which directly influences on flow and results in the loss of pressure head due to the building of localised pressure as well as eddy flow. To understand this mechanism, a computational fluid dynamics (CFD) simulation is carried out. In the present CFD simulation, fluid and solid interactions are modelled in two different types. The first is modelled as pure slip between them so that the effect of roughness can be investigated as a main source of friction factor. The second model consists the effect of the pure adhesion by maintain zero relative velocity on the surface of micro-channel. Behaviour of fluid flow and increase in pressure-drop are observed differently in the both types of model. It is observed that the rise in pressure-drop occurs exponentially as size of a channel reduces from 300 to 100 µm. This phenomenon reveals the science of the size effect on micro-channels. The surface roughness of micro-channel is simulated and it is also observed that the surface finish up to few tens of nanometers does not affect the fluid flow. However, the flow resistance increases as the surface roughness increases up to few hundreds of nanometers, and the pressure-drops along the channel length. In the present case, an elevated temperature of fluid mitigates the effect of surface roughness up to some extent for the efficient flow of fluid in a micro-fluidic device. Hence, micro-fluidic device with nano-finished micro-channel and elevated temperature of fluid is recommended for economic and efficient utilisation of the device.

     

Accelerated numerical simulations of a heaving floating body by coupling a motion solver with a two-phase fluid solver

This paper presents a study on the coupling between a fluid solver and a motion solver to perform fluid–structure interaction (FSI) simulations of floating bodies such as point absorber wave energy converters heaving under wave loading. The two-phase fluid solver with dynamic mesh handling, interDyMFoam, is a part of the Computational Fluid Dynamics (CFD) toolbox OpenFOAM. The incompressible Navier–Stokes (NS) equations are solved together with a conservation equation for the Volume of Fluid (VoF). The motion solver is computing the kinematic body motion induced by the fluid flow. A coupling algorithm is needed between the fluid solver and the motion solver to obtain a converged solution between the hydrodynamic flow field around and the kinematic motion of the body during each time step in the transient simulation. For body geometries with a significant added mass effect, simple coupling algorithms show slow convergence or even instabilities. In this paper, we identify the mechanism for the numerical instability and we derive an accelerated coupling algorithm (based on a Jacobian) to enhance the convergence speed between the fluid and motion solver. Secondly, we illustrate the coupling algorithm by presenting a free decay test of a heaving wave energy converter. Thirdly and most challenging, a water impact test of a free falling wedge with a significant added mass effect is successfully simulated. For both test cases, the numerical results obtained by using the accelerated coupling algorithm are in a very good agreement with the experimental measurements.     

Unsteady RANS simulation of the ship forward speed diffraction problem

The forward speed diffraction problem for a surface ship is analyzed numerically, using a RANS approach with a single-phase level set method to compute the free surface and a blended k-ε/k-ω model for the turbulent viscosity. Simulations were run for a DTMB 5512 model under head incident waves at two speeds and two wavelengths with the same wave amplitude (a = 0.006L, with L the ship length). The medium speed case (Fr = 0.28) with long wavelength incident waves (λ = 1.5L) behaves linearly and has been extensively compared against available experimental data for resistance and heave forces and pitching moment, unsteady free surface elevations, and unsteady velocity fields at the nominal wake plane (x/L = 0.935). Quantitative verification and validation was performed for this case by running three grids and three time steps with refinement ratio of and the flow field analyzed in detail. The behavior of the boundary layer is analyzed to explain the origin of large first harmonic amplitudes on the axial velocity observed both experimentally and numerically. The high speed case (Fr = 0.41) with short wavelength incident waves (λ = 0.5L) exhibits non-linear behavior on the forces and moment with a strong second harmonic component and an unsteady breaking bow wave. The second harmonic has been reproduced by the CFD computations and the breaking wave predicted. Analysis of the flow indicates that the breaking wave could be responsible for the non-linear behavior on the forces and moment.     

Fabrication of the cyclical fluid channel using the surface acoustic wave actuator and continuous fluid pumping in the cyclical fluid channel

A surface acoustic wave (SAW) device has been reported as a micro fluid device such as a pump of a water droplet so far (Renaudin et al. in μTAS, pp 599–601, 2004, 1:551–553, 2005; Sritharan et al. in Appl Phys Lett 88:054102, 2006; Wixforth in Anal Bioanal Chem 379:982–991, 2004; Yamamoto et al. in μTAS, pp 1072–1074, 2005). The SAW device is an interdigital transducer (IDT) fabricated on the piezoelectric substrate only. IDTs are advantageous in terms of integration, miniaturization, free position setting on the substrate and simple fabrication process because of their simple structure. Therefore, the SAW device is easy to apply to integrated chemical system such as lab-on-a-chip. The SAW drives the liquid homogenously by the transmission of surface vibrations of the substrate. Thus, both ends of the channel for pressure loading are not necessary to pump the liquid by using the SAW. The SAW can pump the liquid in both of a closed channel and an opened channel, although continuous flow pumping using an external pump is difficult for no loading pressure in the closed fluid channel. In this paper, we proposed and fabricated the micro fluid devices combined cyclical fluid channel and SAW actuator for liquid pumping. This device is fabricated on a piezoelectric substrate (LiNbO₃) with UV photolithography and wet etching. Structure material of cyclical fluid channel is epoxy photoresist SU-8 100. Then, it is demonstrated to continuous flow pumping and reciprocal flow pumping in the channel. As a result of optimization of a SAW pump’s structural parameter, 32.5, 71.3 and 108.0 mm/s are achieved in the 500, 1,000 and 2,000 μm channel width as a maximum flow velocity.     

Measurement-integrated simulation of wall pressure measurements using a turbulent model for analyzing oscillating orifice flow in a circular pipe

The development of a highly functionalized orifice flowmeter with high accuracy under realistic conditions is desired. This paper presents a method for analyzing oscillating air flow through an orifice in a circular pipe. A measurement-integrated (MI) simulation using a standard k–ε model was used to reduce the computation time. In a previous study, the feedback law of the MI simulation was determined by considering the effect of the computational fluid dynamics (CFD) grid on contracted flow. However, the previous method required the measurement of inlet flow rate, which is not feasible in many applications. Therefore, an MI simulation was proposed that only requires wall pressures, which are much simpler to measure than flow rate. In this MI simulation, the wall pressure downstream of an orifice was measured, and a new proportional–integral controller feedback algorithm was developed to control the inlet flow rate in the computed flow field. The proposed MI simulations were performed for steady and oscillatory flow rates up to 10 Hz. It was found that this MI simulation provides accurate solutions at a significantly shorter computation time than conventional CFD analysis.     

Performance assessment of OpenFOAM and FLOW-3D in the numerical modeling of a low Reynolds number hydraulic jump

A comparative performance analysis of the CFD platforms OpenFOAM and FLOW-3D is presented, focusing on a 3D swirling turbulent flow: a steady hydraulic jump at low Reynolds number. Turbulence is treated using RANS approach RNG k-ε. A Volume Of Fluid (VOF) method is used to track the air–water interface, consequently aeration is modeled using an Eulerian–Eulerian approach. Structured meshes of cubic elements are used to discretize the channel geometry. The numerical model accuracy is assessed comparing representative hydraulic jump variables (sequent depth ratio, roller length, mean velocity profiles, velocity decay or free surface profile) to experimental data. The model results are also compared to previous studies to broaden the result validation. Both codes reproduced the phenomenon under study concurring with experimental data, although special care must be taken when swirling flows occur. Both models can be used to reproduce the hydraulic performance of energy dissipation structures at low Reynolds numbers.     

Design optimization of supersonic jet pumps using high fidelity flow analysis

Supersonic jet pumps are simple devices with no moving parts, where a high velocity (primary) flow is used to pump a second fluid. In this paper, Computational Fluid Dynamics (CFD) is combined with an optimization framework in order to develop a tool for the rapid generation of jet pump designs. A key feature of the problem formulation is the transformation of the jet pump design parameters in terms of geometric ratios. This approach dramatically reduces the number of unrealistic designs covered by the Design of Experiments. Optimal Latin Hypercubes for surrogate model building and model validation points are constructed using a permutation genetic algorithm and design points are evaluated using CFD. Surrogate models of primary and entrained flow rates are built using a Moving Least Squares approach. A series of optimizations for various pump sizes are performed using a genetic algorithm and Sequential Quadratic Programming, with responses calculated from the surrogates. This approach results in a set of optimized designs, from which pumps for a wide range of flow rates can be interpolated.     

A computational study of wall friction and turbulence dynamics in accelerating pipe flows

A CFD model of turbulent flow in a smooth pipe accelerating uniformly from steady state is used to study the influence of turbulence and inertia on wall shear stresses. A low-Reynolds-number k-ε turbulence model is used in conjunction with a finite volume/finite difference discretization scheme. It is shown that the wall shear stress initially overshoots the corresponding quasi-steady value and this is attributed to inertial causes. Thereafter, the wall shear stress is shown to undershoot the quasi-steady value because inertial effects are more than counterbalanced by the cumulative influence of delays in the response of turbulence to flow changes. The dependence of the flow behaviour on the geometry, the fluid properties, the Reynolds number and the acceleration is studied and is shown to correlate well with a non-dimensional parameter based on the turbulence production timescale. The durations of the initial overshoots and the amplitudes of the overshoots and undershoots are smaller at high Reynolds numbers than at low ones.     

Ω形双螺杆泵流场仿真与实验研究

研究螺杆转子转动过程中的流场变化有助于对双螺杆泵运行参数的优化。采用计算流体力学(CFD)方法,对双螺杆泵流场进行三维瞬态动网格仿真分析。针对Ω形双螺杆泵,建立内流场数值模型,通过仿真得到一个转动周期内泵内压力分布,同时研究了不同粘度下泵外特性的变化规律。仿真结果表明,泵内压力由吸入端到排出端逐级增大,与容积腔推挤增压规律相吻合;在吸入端容积腔截面和螺杆啮合缝隙内均存在负压,且由于泄露的原因,密封腔两端的压力分布并非完全一致,存在轻微差异;流量随着压差的增大而呈近似线性下降趋势,粘度越大,流量受压差的影响就越小;泵效随压差的变化曲线呈抛物线形,达到峰值后先逐渐减小,最后趋于平稳。实验结果与仿真一致,证明了仿真方法的有效性。     

A fast numerical method for flow analysis and blade design in centrifugal pump impellers

A numerical methodology is developed to simulate the turbulent flow in a 2-dimensional centrifugal pump impeller and to compute the characteristic performance curves of the entire pump. The flow domain is discretized with a polar, Cartesian mesh and the Reynolds-averaged Navier-Stokes (RANS) equations are solved with the control volume approach and the k-ε turbulence model. Advanced numerical techniques for adaptive grid refinement and for treatment of grid cells that do not fit the irregular boundaries are implemented in order to achieve a fully automated grid construction for any impeller design, as well as to produce results of adequate precision and accuracy. After estimating the additional hydraulic losses in the casing and the inlet and outlet sections of the pump, the performance of the pump can be predicted using the numerical results from the impeller section only. The regulation of various energy loss coefficients involved in the model is carried out for a commercial pump, for which there are available measurements. The predicted overall efficiency curve of the pump was found to agree very well with the corresponding experimental data. Finally, a numerical optimization algorithm based on the unconstrained gradient approach is developed and combined with the evaluation software in order to find the impeller geometry that maximizes the pump efficiency, using as free design variables the blade angles at the leading and the trailing edge. The results verified that the optimization process can converge very fast and to reasonable optimal values.     

Flow pattern around an appended tanker hull form in simple maneuvering conditions

A detailed computational study is presented of the flow pattern around the Esso Osaka with rudder in simple maneuvering conditions: “static rudder” and “pure drift”. The objectives are: (1) apply RANS for maneuvering simulation; (2) perform verification and validation on field quantities; (3) characterize flow pattern; and (4) correlate behavior of the integral quantities with the flow field. The general-purpose code CFDSHIP-IOWA is used. The free surface is neglected and the two-equation k-ω turbulence model is used. The levels of verification of the velocity components for the “straight-ahead”, “static rudder” and “pure drift” conditions show ranges from 5.5% to 28.3% of free stream, U₀, for the axial velocity U and 2.5-29.1%U₀ for the cross flow (V, W). Qualitative validation against limited experimental data shows encouraging results with respect to trends and levels. The flow pattern is characterized by fore and aft body bilge and side vortices, which are similar for “straight-ahead” and “static rudder” conditions, except in close vicinity of the rudder. The “pure drift” condition shows strong asymmetry on windward vs. leeward sides and a more complex vortex system with additional bilge vortices. Similarities and differences with data for other tanker, container, and surface combatant hulls and relation between flow pattern and forces and moments are discussed. Future work focuses on influence of propeller.     

基于DLM法与CFD法的飞机平尾气动力计算比较

为提高飞机平尾的结构计算精度,利用HyperMesh同时建立结构网格与流场网格,在翼面结构与流场中物面节点坐标位置完全相同的情况下,分别采用偶极子格网法(Double-Lattice Method,DLM)与计算流体力学(Computational Fluid Dynamics,CFD)法计算飞机平尾负攻角变化时的升力变化情况.在HyperMesh中建立平尾有限元模型和流体模型,用FLUENT计算气动升力,用MSC Nastran进行静力分析.计算结果与实验结果的对比表明DLM更快,CFD法更精确,所以可以在翼面设计初期用DLM计算翼面载荷,在翼面设计后期用CFD法计算翼面载荷.     

Numerical modeling of wave overtopping a levee during Hurricane Katrina

In this paper, a numerical wave model based on the incompressible Reynolds equations and k-ε equations has been applied to estimate the impact of overtopping on levee during storm surge. The free surface locations are represented by volume of fluid function (VOF). The model was satisfactorily tested against empirical equation of overflow discharge at a vertical seawall, and experimental data of overtopping discharge at a sloping seawall. The validated model was used to simulate wave overtopping of the levee system during storm surge of Hurricane Katrina. The time history of wave profiles and velocity magnitude field in the vicinity of the levee are demonstrated and analyzed. It is concluded that the failure of parts of the levee system was caused by erosion during wave overtopping.     

Pseudo bond graph model of coupled heat and mass transfers in a plastic tunnel greenhouse

In greenhouses, models built are classified into two kinds; heterogeneous approaches based on computational fluid dynamics (CFD) codes and homogeneous one based on heat and mass balance equations. Bond graph modelling was not seriously introduced in greenhouse modelling in spite of the concordance of the energetic based approach of bond graphs with the nature of the greenhouse plant.In this research work, a pseudo bond graph model of a greenhouse was elaborated to simulate temperature and relative humidity inside. The model proposed is an energetic lumped approach which describes coupled heat and mass transfers in a plastic tunnel greenhouse. This model includes convection, evaporation/condensation phenomena, air change flow and soil heat and mass transfer. Multiport (3 ports) bond graph elements are introduced to describe the state of the two elements (dry air and water vapor) fluid. New bond graph schemes are used to characterize the coupling effect between heat and mass transfer, and for modelling free evaporation/condensation mechanism. Big leaf assumption was used to model the canopy. New boundary layer elements are added in the model, these elements allow a separation of the different phenomena inside greenhouse and thus a simplification of the modelling task. The practical results obtained from an experimental tunnel greenhouse are used here as validation elements for the greenhouse bond graph model. A good correlation is observed between measured and predicted samples.