Dynamic Overset Grid Research Articles

Multi-Physics Coupled FEM Method to Simulate the Formation of Crater-Like Taylor Cone in Electrospinning of Nanofibers

Crater-like Taylor cone electrospinning is a novel, simple, and powerful approach to mass produce nanofibers. The Taylor cone, crater-like liquid bump on the free liquid surface, in this electrospinning process plays a key role to produce multiple fluid jets which finally solidifies nanofibers. A multi-physics coupled FEM method was employed to simulate the dynamic formation process of crater-like Taylor Cone in crater-like electrospinning. A blended k−ω /k−ε model for turbulence and dynamic overset grids to resolve large amplitude motions were used to simulate two-dimensional uncompressed flow, which was described in axisymmetrical coordinates. The numerical calculation results were obtained by a computational fluid dynamics (CFD) method. The effect of gas flow on the formation of crater-like Taylor cone and the production of nanofibers were also discussed. The experiments were carried out to validate the numerical results. The Polyvinyl Alcohol (PVA)/ distilled water solution with 18wt% and the air pressures ranged varied from 4 to 50kPa were used in our experiments. The results showed that the numerical results were in good agreement with the experimental results. This work provides a deep understanding of the mechanisms of micro fluid jets production in electrospinning processes and two-phase flow in specific type of industrial equipment.

Turn and zigzag maneuvers of a surface combatant using a URANS approach with dynamic overset grids

Unsteady Reynolds averaged Navier–Stokes (URANS) computations of standard maneuvers are performed for a surface combatant at model and full scale. The computations are performed using CFDShip-Iowa v4, a free surface solver designed for 6DOF motions in free and semi-captive problems. Overset grids and a hierarchy of bodies allow the deflection of the rudders while the ship undergoes 6DOF motions. Two types of maneuvers are simulated: steady turn and zigzag. Simulations of steady turn at 35° rudder deflection and zigzag 20/20 maneuvers for Fr = 0.25 and 0.41 using constant RPM propulsion are benchmarked against experimental time histories of yaw, yaw rate and roll, and trajectories, and also compared against available integral variables. Differences between CFD and experiments are mostly within 10 % for both maneuvers, highly satisfactory given the degree of complexity of these computations. Simulations are performed also with waves, and with propulsion at either constant RPM or torque. 20/20 zigzag maneuvers are simulated at model and full scale for Fr = 0.41. The full scale case produces a thinner boundary layer profile compared to the model scale with different reaction times and handling needed for maneuvering. Results indicate that URANS computations of maneuvers are feasible, though issues regarding adequate modeling of propellers remain to be solved.

Numerical simulation of rotor-airframe aerodynamic interaction based on unstructured dynamic overset grids

A method of unstructured dynamic overset grids is developed for the numerical simulation of helicopter unsteady rotorairframe aerodynamic interaction. For the effective treatment of the relative motion between the rotor and the airframe, the domain of flowfield is divided into two overset subzones, namely, a rotational subzone containing the blades and a stationary subzone containing the airframe. The overset part of two subzones is used to convect the flow variables of the two zones. The Taylor series expansion is used to obtain a second-order spatial accuracy, and dual-time stepping is adopted to improve the solution accuracy. Mesh deformation from the blade motion in forward flight is treated by using a spring analogy. Validation is made by numerically simulating the flows around a wind tunnel configuration and comparing the predicted time-averaged and instantaneous inflow and airframe surface pressure distributions with the experimental data. It shows that the present method is efficient and robust for the prediction of complicated unsteady rotor-airframe aerodynamic interaction phenomena. unstructured grid, overset grids, rotor-airframe interaction, spring analogy, relative motion

Numerical Study of Propeller Slipstream Based on Unstructured Dynamic Overset Grids

The propeller slipstream is numerically simulated by solving three-dimensional unsteady Euler equations. The method of unstructured overset grids is applied to the simulation of relative motion between the propeller and airframe. The rotationalmotion of the propeller is simulated by establishing a rotational subzone, and the airframe is contained in the stationary subzone. During the unsteady simulation with the rotational subzone rotating, there is always an overlapping area between the rotational and stationary subzones, and the overlapping area is used to convect the flow variables of the two subzones. The governing equations are solved by using the dual-time method and standard explicit four-stage Runge-Kutta method. The unsteady flowfields of different working conditions are calculated, and the influence of propeller slipstream is analyzed. Some calculating results are compared with experimental data, and they have a good agreement with each other. It is demonstrated that the present method is efficient and robust for the simulation of an unsteady propeller slipstream flow.

Dynamic overset CFD simulations of wind turbine aerodynamics

Simulations of the National Renewable Energy Laboratory (NREL) phase VI wind turbine using dynamic overset grid technology are presented. The simulations are performed in an inertial frame of reference with the rotor consisting of the blades and hub. The geometries of the tower and nacelle are approximate but included in the computation. Computations of the effect of wind speed (5, 10, 15 and 25 m/s) at a fixed blade pitch angle of 3° with constant rotational speed using unsteady Reynolds-Averaged Navier–Stokes (RANS) and Detached Eddy Simulation (DES) turbulence models, both showing little difference in the averaged forces and moments. However, significant improvements in the transient response are seen when using DES. The effect of angle of attack is evaluated by dynamically changing the pitch from −15° to 40° at constant wind speed of 15 m/s. Extensive comparison against experimental results, including total power and thrust, sectional performance of normal force coefficient and local pressure coefficient, shows consistently good predictions. The methodology shows a promise for more complex computations including active turbine control by varying the pitch angle and fluid-structure interaction.

Numerical Simulation of Unsteady Flow Around Forward Flight Helicopter with Coaxial Rotors

Three-dimensional unsteady Euler equations are numerically solved to simulate the unsteady flows around forward flight helicopter with coaxial rotors based on unstructured dynamic overset grids. The performances of the two coaxial rotors both become worse because of the aerodynamic interaction between them, and the influence of the top rotor on the bottom rotor is greater than that of the bottom rotor on the top rotor. The downwash velocity at the bottom rotor plane is much larger than that at the top rotor plane, and the downwash velocity at the top rotor plane is a little larger than that at an individual rotor plane. The downwash velocity and thrust coefficient both become larger when the collective angle of blades is added. When the spacing between the two coaxial rotors increases, the thrust coefficient of the top rotor increases, but the total thrust coefficient reduces a little, because the decrease of the bottom rotor thrust coefficient is larger than the increase of the top rotor thrust coefficient.

Coaxial Rotor Helicopter in Hover Based on Unstructured Dynamic Overset Grids

C OAXIAL rotor configuration is one of the technological solutions for increasing helicopter forward speed, maneuverability, and load-carrying ability. Since two rotors produce the net thrust instead of a single rotor in the conventional design, the diameter of the rotors can be reduced to carry the same amount of weight. Themost attractive feature of a coaxial design is the resulting compactness and safety of the vehicle. Second, the antitorsional moment generated by the two rotors would be canceled due to the opposite rotational directions, and the tail rotor and tail boom can be eliminated, resulting in a smaller and lighter vehicle. Compared to the individual-rotor helicopter, the aerodynamics and flow physics of coaxial rotor helicopter are relatively less studied and understood. The method of slipstream theory was first used to study the coaxial rotor, and then the predicted wake vortex model [1,2], free wake vortex model [3], and vorticity transport model [4,5] were applied to the coaxial rotor analysis. All the methods above carry out some approximations to the rotor, and the detailed flow feature near the blades and blade vortex in the wake cannot be simulated. Recently, the unsteady Navier–Stokes equations were used to simulate the coaxial rotor flows [6–8] to provide a detailed understanding of the flow physics of coaxial rotor. In the present Note, the three-dimensional unsteady Euler equations were solved to simulate the unsteady flows around coaxial rotor helicopter and individual-rotor helicopter in hover based on the unstructured dynamic overset grids. The aerodynamic interaction property of the coaxial rotor helicopter in hover and the effect of fuselage on the unsteady performance were studied.

Self-propulsion computations using a speed controller and a discretized propeller with dynamic overset grids

A method that can be used to perform self-propulsion computations of surface ships is presented. The propeller is gridded as an overset object with a rotational velocity that is imposed by a speed controller, which finds the self-propulsion point when the ship reaches the target Froude number in a single transient computation. Dynamic overset grids are used to allow different dynamic groups to move independently, including the hull and appendages, the propeller, and the background (where the far-field boundary conditions are imposed). Predicted integral quantities include propeller rotational speed, propeller forces, and ship’s attitude, along with the complete flow field. The fluid flow is solved by employing a single-phase level set approach to model the free surface, along with a blended k−ω/k−ɛ based DES model for turbulence. Three ship hulls are evaluated: the single-propeller KVLCC1 tanker appended with a rudder, the twin propeller fully appended surface combatant model DTMB 5613, and the KCS container ship without a rudder, and the results are compared with experimental data obtained at the model scale. In the case of KCS, a more complete comparison with propulsion data is performed. It is shown that direct computation of self-propelled ships is feasible, and though very resource intensive, it provides a tool for obtaining vast flow detail.

Investigation into the Aerodynamic Performance of the Tiltrotor Unmanned Aerial Vehicle Proprotor

T HE Smart UAV is a tilt-rotor type of convertiplane, which can take off and land like a helicopter and convert to a propellerdriven fixed-wing aircraft through a tilting procedure. Therefore, unlike the propeller of a conventional fixed-wing aircraft or the rotor of a helicopter, the proprotor of a tilt-rotor aircraft experiences significant changes in aerodynamic loading during its overall flight modes. The differences in aerodynamic performance due to tilt angle and forward speed cause severe changes in thrust andmoments. They may also induce significant vibration due to asymmetric flow on the rotor disk. To evaluate the aerodynamic performance of the rotor under such varying flight conditions, numerical simulations and experimental tests were conducted in this study. The geometric specifications and operating conditions of the proprotor of a full-scale SmartUAVare summarized inTables 1 and2, respectively [1], and the computational grid and sign convention of the proprotor are shown in Fig. 1. This paper includes three parts. Section II describes the unsteady numerical simulation of airflow around the proprotor using Navier– Stokes equations with a dynamic overset grid technique. Section III introduces the structure of the proprotor test rig and the results of the ground test. And in Sec. IV, the results of the wind-tunnel test, conducted in a low-speed wind tunnel at the Korea Aerospace Research Institute, are discussed using a comparison with the numerical data mentioned in Sec. II. For all numerical and experimental tests in this study, the hub of the proprotor was modeled and fabricated as a rigid hub. In the interest of simplicity and cost effectiveness, the hub has no cyclic pitch control in either test.

Large-scale DES computations of the forward speed diffraction and pitch and heave problems for a surface combatant

This paper aims at presenting the most resolved solutions to date for the ship forward speed diffraction and pitch and heave problems, and discuss the method that enables these computations. Large-scale DES computations (60–115 million grid points, 276–500 processors) of ship hydrodynamics problems are presented for the DTMB model 5512 surface combatant. The forward speed diffraction problem is studied at Fr=0.28 with waves of amplitude a=0.006 and wavelength λ=1.5, with the ship static allowing the overset assembly to be a pre-processing step. In the pitch and heave problem the ship faces head waves at Fr=0.41 with waves of amplitude a=0.006 and wavelength λ=1.5, with the ship is allowed to pitch and heave, thus requiring dynamic overset grid processing. The code CFDShip-Iowa version 4 and the overset assembly code Suggar were modified to carry out some large scale simulations of free surface ship hydrodynamics. These modifications were focused on reducing the memory requirement and optimizing the per-processor and parallel performance at the implementation and algorithmic levels, plus the addition of a lagged mode for the overset domain connectivity computation. The simulation results show very significant improvements in the local flow and free surface results, but minor in forces and moments when compared with previous URANS computations performed with grids with about three million points.

URANS analysis of a broaching event in irregular quartering seas

Ship motions in a high sea state can have adverse effects on controllability, cause loss of stability, and ultimately compromise the survivability of the ship. In a broaching event, the ship losses control, naturally turning broadside to the waves, causing a dangerous situation and possibly capsizing. Classical approaches to study broaching rely on costly experimental programs and/or time-domain potential or system-based simulation codes. In this paper the ability of Reynolds averaged Navier–Stokes (RANS) to simulate a broaching event in irregular waves is demonstrated, and the extensive information available is used to analyze the broaching process. The demonstration nature of this paper is stressed, as opposed to a validated study. Unsteady RANS (URANS) provides a model based on first principles to capture phenomena such as coupling between sway, yaw, and roll, roll damping, effects of complex waves on righting arm, rudders partially out of the water, etc. The computational fluid dynamics (CFD) method uses a single-phase level-set approach to model the free surface, and dynamic overset grids to resolve large-amplitude motions. Before evaluating irregular seas two regular wave cases are demonstrated, one causing broaching and one causing stable surf riding. A sea state 8 is imposed following an irregular Bretschneider spectrum, and an autopilot was implemented to control heading and speed with two different gains for the heading controller. It is concluded that the autopilot causes the ship to be in an adverse dynamic condition at the beginning of the broaching process, and thus is partially responsible for the occurrence of the broaching event.

비정상 RANS 법과 중첩격자계를 이용한 횡파중 선박운동 수치해석

The present paper presents the CFD result for a beam wave test case. An ONR tumblehome ship model with bilge keels is used. The beam wave test is for zero forward speed and roll and heave 2DOF with wave slope <TEX>$a_k=0.156$</TEX> and wavelength <TEX>${\lambda}=1.12L_{PP}$</TEX>, with <TEX>$L_{PP}$</TEX> the ship length. The problems is solved numerically with an unsteady Reynolds averaged Navier-Stokes approach. The free surface flow is computed using a single-phase level-set method and the motions in each time step are integrated using a predictor-corrector iteration approach which uses dynamic overset grids moving with relative ship motion. The predicted CFD results for motions and forces are compared with experimental data, showing a reasonable agreement.

Semi‐coupled air/water immersed boundary approach for curvilinear dynamic overset grids with application to ship hydrodynamics

AbstractFor many problems in ship hydrodynamics, the effects of air flow on the water flow are negligible (the frequently called free surface conditions), but the air flow around the ship is still of interest. A method is presented where the water flow is decoupled from the air solution, but the air flow uses the unsteady water flow as a boundary condition. The authors call this a semi‐coupled air/water flow approach. The method can be divided into two steps. At each time step the free surface water flow is computed first with a single‐phase method assuming constant pressure and zero stress on the interface. The second step is to compute the air flow assuming the free surface as a moving immersed boundary (IB). The IB method developed for Cartesian grids (Annu. Rev. Fluid Mech. 2005; 37:239–261) is extended to curvilinear grids, where no‐slip and continuity conditions are used to enforce velocity and pressure boundary conditions for the air flow. The forcing points close to the IB can be computed and corrected under a sharp interface condition, which makes the computation very stable. The overset implementation is similar to that of the single‐phase solver (Comput. Fluids 2007; 36:1415–1433), with the difference that points in water are set as IB points even if they are fringe points. Pressure–velocity coupling through pressure implicit with splitting of operators or projection methods is used for water computations, and a projection method is used for the air. The method on each fluid is a single‐phase method, thus avoiding ill‐conditioned numerical systems caused by large differences of fluid properties between air and water. The computation is only slightly slower than the single‐phase version, with complete absence of spurious velocity oscillations near the free surface, frequently present in fully coupled approaches. Validations are performed for laminar Couette flow over a wavy boundary by comparing with the analytical solution, and for the surface combatant model David Taylor Model Basin (DTMB) 5512 by comparing with Experimental Fluid Dynamics (EFD) and the results of two‐phase level set computations. Complex flow computations are demonstrated for the ONR Tumblehome DTMB 5613 with superstructure subject to waves and wind, including 6DOF motions and broaching in SS7 irregular waves and wind. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Ship motions using single-phase level set with dynamic overset grids

The problem of surface ships free to pitch and heave in regular head waves is analyzed numerically with an unsteady Reynolds averaged Navier Stokes (URANS) approach. The unsteady single-phase level set method previously developed by the authors was extended to include six degrees of freedom (6DOF) motions. The method uses rigid overset grids that move with relative motion during the computation, and the interpolation coefficients between the grids are recomputed dynamically every time the grids move. The motions in each time step are integrated implicitly using a predictor–corrector approach. An earth-based reference system is used for the solution of the fluid flow, while a ship-based reference system is used to compute the rigid-body equations of motion. Predicted results for sinkage and trim and resistance at two Froude numbers (medium, Fr = 0.28 and large, Fr = 0.41) were compared against experimental data, showing good agreement. Pitch and heave motions were computed for near-resonant cases at Fr = 0.28 and 0.41, with regular linear head waves with slope ak = 0.025 and wavelength λ = 1.5 L, with L the ship length. The predicted motions compare favorably with existing experimental data. A solution for a large amplitude head wave case ( ak = 0.075) was also obtained, in which the transom wave breaks and extreme motions are observed. The medium Froude number case was subject to a verification and validation analysis. A problem with two ships pitching and heaving one behind the other is demonstrated.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan Encountering Highly Turbulent Inflow

A large eddy simulation (LES) was applied to predict the unsteady flow in a low-speed axial-flow fan assembly subjected to a highly “turbulent” inflow that is generated by a turbulence grid placed upstream of the impeller. The dynamic Smagorinsky model (DSM) was used as the subgrid scale (SGS) model. A streamwise-upwind finite element method (FEM) with second-order accuracy in both time and space was applied as the discretization method together with a multi-frame of reference dynamic overset grid in order to take into account the effects of the blade-wake interactions. Based on a simple algebraic acoustical model for axial flow fans, the radiated sound power was also predicted by using the computed fluctuations in the blade force. The predicted turbulence intensity and its length scale downstream of the turbulence grid quantitatively agree with the experimental data measured by a hot-wire anemometry. The response of the blade to the inflow turbulence is also well predicted by the present LES in terms of the surface pressure fluctuations near the leading edge of the blade and the resulting sound power level. However, as soon as the effects of the turbulent boundary layer on the blades become important, the prediction tends to become inaccurate.

Large Eddy Simulation of Unsteady Flow in a Mixed-Flow Pump. 1st Report. Numerical Method.

Large eddy simulation of the complete stage of a mixed-flow pump is presented. The computation takes full account of the interaction between the rotating impeller and the stationary casing by using a multi-frame-of-reference dynamic overset grid approach. A streamline-upwind finite element formulation with second-order accuracy both in time and space is used for discretizing the governing equations. It is implemented in parallel by a domain decomposition programming model, and therefore, it can be applied to a large scale computation on a distributed-memory parallel computer. Internal flow of a high-specific-speed mixed-flow pump stage (ωs=2.1) is simulated to test the validity of the proposed method under four operating conditions: the design point (Q/Qd=100%) and three off-design points (Q/Qd=60%, 80% and 120%). For all the cases computed herein, the predicted total pump head and the unsteady fluid forces on the impeller agreed well with the measured values.