Numerical Simulation Of Unsteady Flow Research Articles

Influence of cylinder rotation in close to a free surface on flow characteristics

This study investigates the numerical simulation of unsteady flow over a rotating circular cylinder near a free surface using the Volume of Fluid (VOF) method. The hydrodynamic characteristics and flow pattern are analyzed for various Froude numbers (0.3, 0.55, and 1), submergence depths (0.4, 0.6, and 1), and rotation rates (±0.5, ±1, and ±2). The results indicate that both the rotation direction and rate significantly influence flow instability. Counterclockwise rotation reduces free surface instability and drag coefficient. At a rotation rate of 2, the average drag coefficient decreases by 93% compared to a stationary cylinder at Froude number 0.3. Clockwise rotation induces free surface instability and increases the lift and drag coefficients. Higher Froude numbers result in more significant free surface variations, and increased submergence depth leads to stretched vortices.

Spatiotemporal Evolution and Fluctuation Characteristics of a Centrifugal Compressor under Near-Stall Conditions and High Mass-Flow Rate

Spatiotemporal evolution and fluctuation characteristics of a centrifugal compressor are investigated by numerical simulation under near-stall conditions and with a high mass-flow rate. The large-eddy simulation (LES) for unsteady computations is implemented in the numerical simulation of unsteady flow. The internal flow physical mechanism of the centrifugal compressor is presented at a high mass-flow rate (1.1 Qn) and low mass-flow rate (0.8 Qn, near-stall). The spatiotemporal evolution of the velocity and streamline for the internal flow of the centrifugal compressor demonstrates that a lot of large-scale eddies near the tongue are transformed into small-scale ones at high mass-flow rates. High mass-flow rate resulted in excessive fluid velocity in the impeller. A large amount of impact loss massive backflow appears near the tip clearance, and boundary layer separation of the suction surface emerges firstly and at a low mass-flow rate. Considerable flow loss occurs in the centrifugal compressor at the two non-designed operating flow rates. Several pressure and velocity fluctuations in the key position of the compressor are presented by the two deviations from design conditions. The analysis of the fast Fourier transform (FFT) and amplitude spectrum show that the starting point of flow instability in the impeller is different for the two deviations from design-condition flow rates. Understanding the spatiotemporal evolution and spatiotemporal characteristics of pressure and velocity fluctuations can provide insight into the unsteady internal flow of centrifugal compressors at high mass-flow rates (1.1 Qn) and near-stall conditions (0.8 Qn).

Чисельне моделювання нестаціонарної течії в гідравлічній системі за каві-туючою шайбою

This paper addresses the construction of an efficient mathematical model to be used in the numerical simulation of unsteady liquid flows in hydraulic systems with cavitating restrictors. Existing approaches to cavitation simulation are based either on accounting for a two-phase flow structure or on representing a cavitating flow as a homogeneous medium of variable density. In the latter case, the pressure and the density are related via the barotropic equation of liquid–vapor mixture state. The goal of this work is to verify the applicability of a cavitation model based on the barotropic equation of liquid–vapor mixture state to the numerical simulation of an unsteady flow in a hydraulic system with a cavitating ring plate. The method employed is a numerical flow simulation in the axisymmetric approximation using the complete averaged Navier–Stokes equations. It is shown that the use of the barotropic equation of liquid–vapor mixture state provides a satisfactory agreement between the computed results and the experimental data available in the literature. In agreement are the peak-to-valley values of the oscillating pressure on the pipe wall immediately downstream of the cavitating ring plate and the presence of a pronounced periodic component in the pressure vs. time relationship. It is shown that the parameters of the unsteady flow downstream of the cavitating ring plate vary when going from the ring plate to the cavity collapse location: the peak-to-valley value of the oscillating pressure on the pipe wall increases and so does the contribution of high-frequency periodic components to the pressure vs. time relationship. It seems desirable that the turbulence model employed be refined further to correctly simulate cavitation oscillations generated by periodically detached cavitation in Venturi tubes, which are used in various cavitation pulse plants.

Influence of asymmetric inflow on the transient pressure fluctuation characteristics of a vertical mixed-flow pump

In order to investigate the influence of asymmetric inflow on the internal flow fields of a vertical mixed-flow pump, numerical simulation of unsteady flow was carried out under four inflow angles ( θ = 0°, 10°, 20°, and 30°). The reliability of the calculation results was validated through an external characteristic experiment. The fluctuation value of the pressure was defined to obtain the transient pressure fluctuation intensity distribution. Three monitoring points were arranged to compare pressure fluctuation characteristics under different inflow angles. The results suggested that the increase in the inflow angle caused the design operation points to deviate to the over-load flow rate. When the inflow angle increased from 0° to 30°, the axial velocity distribution coefficient and weighted average angle at the inflow runner outlet both decreased. The overall transient pressure fluctuation intensity inside the impeller increased, especially on the pressure side of the blade. Additionally, the transient pressure fluctuation intensity on the pressure side of the guide vane also increased significantly. As a result, the amplitude of pressure fluctuation at the dominant frequency of each monitoring point increased. Relatively speaking, the negative influence of asymmetric inflow on the operational stability of the pumping station was further extended under the part-load flow rate.

Numerical Investigation of Unsteady Flow Dynamics in a Packed Bed

Numerical simulation of unsteady flow of a compressible fluid in a fixed bed filled with porous elements has been performed. The research was carried out via ANSYS Fluent software. The scientific substantiation and verification of the physical and mathematical approaches incorporated in ANSYS Fluent for the problem of unsteady flow in a fixed bed has been carried out. For the computational domain, the interfaces of the flow area and the surface of porous particles are coupled by combining the contacts into a component part. The numerical results were verified using experimental data. The study was carried out in the range of velocity from 0.25 to 3.25 m/s. An expression is proposed for determining the pressure drop in a fixed bed, in which the pressure drop depends on the velocity, flow properties and the linear coefficient of local resistance. The values of the linear coefficient of local resistance are determined for the most common nozzle shapes in the industry: cylinder, Raschig ring, convex cylinder with 7 holes, sphere with 7 holes. It was found that with an increase in velocity, the value of the linear coefficient of local resistance decreases.

A Nonintrusive Parametrized Reduced-Order Model for Periodic Flows Based on Extended Proper Orthogonal Decomposition

The periodic flows, such as vortex shedding and rotating flow in turbomachinery, are very common in both scientific and engineering fields. However, high-fidelity numerical simulations of unsteady flows are usually time-consuming, particularly when varying flow parameters need to be considered. In this paper, a novel nonintrusive parametrized reduced order model (PROM) approach for prediction of periodic flows is presented. The establishment of this ROM is based on two techniques, proper orthogonal decomposition (POD) and discrete Fourier transform (DFT), where the first one can extract the spatial features and the second has the ability to quantify the temporal effects of parameters. A prediction model based on artificial neural networks (ANNs) is used to map the flow parameters with DFT coefficients. Flows past a cylinder and two dimensions turbine flows are used to demonstrate the effectiveness of the proposed PROM. It is shown that the proposed POD-DFT-ANN (PDA) ROM are both efficient and accurate for the predictions of periodic flows with varying flow parameters.

Numerical Analysis of Unsteady Laminar Flow past a Pentagonal Obstacle

The current article dispenses the numerical investigation of a two dimensional unsteady laminar flow of incompressible fluid passing a regular pentagonal obstacle in an open rectangular channel. The centre of attention of this work is the comparison of drag coefficients estimated for two distinct cases based on the orientation of face and corner of an obstacle against the flow direction. The numerical results shows that the corner – oriented obstacle bring about 42% larger value of drag coefficient at Re = 500 than face – oriented obstacle. The substantial growth in the expanse of vortex behind obstacle (presented as a function of fluid inertia 25 < Re < 500) is analyzed through the contours and streamline patterns of velocity field. The two eddies in the downstream become entirely unsymmetrical at Re = 500 for both the cases, whereas; the flow separation phenomena occurs a bit earlier in the face – oriented case at Re = 250. Two dimensional Pressure – Based – Segregated solver is employed to model the governing equations written in velocity and pressure fields. The numerical simulations of unsteady flow are presented for 50 seconds time frame with time step 0.01 by using one of the best available commercial based Computational Fluid Dynamics (CFD) software, ANSYS 15.0.

Чисельне моделювання нестаціонарних потоків холодної плазми при роботі плазмового актуатора

The numerical simulation of unsteady flows of cold plasma is considered in this article. A low-temperature non-equilibrium ideal plasma is formed when the plasma actuator interacts with the air. The mathematical model has been developed to describe the behavior of low-temperature plasma. It is based on non-stationary equations describing the dynamics of charged particles and plasma electrodynamics equations. The 14 types of particles: metastable and excited nitrogen and oxygen atoms, positive and negative ions, electrons and atomic oxygen are considered. Volumetric and surface chemical reactions describing processes in a barrier discharge that occur above the dielectric surface are considered. For non-stationary equations of plasma dynamics, an implicit numerical algorithm with pseudo-time iteration has been developed, which is based on a finite-volume approach. The equation for the electrostatic potential with sources was solved using the generalized minimal residual method with incomplete LU preconditioning. In non-stationary equations for the density of plasma particles, the drift derivatives were approximated using the TVD scheme with the MinMod limiter function. The derivatives in the equation for the electric potential were calculated using finite-volume relations taking into account the upwind approximation of the concentration of charged plasma particles. The numerical results of the generation, propagation and destruction of a streamer during a dielectric barrier discharge are obtained. The unsteady plasma characteristics in the region above the dielectric surface are analyzed, including the distribution of the particles density, electric potential and the Lorentz force components. The results of numerical simulation of unsteady flows of low-temperature plasma are in good agreement with the available experimental data.

Numerical simulation of unsteady flow of a viscous incompressible liquid in flat channels of arbitrary shape of heat exchangers

Reasonable development and creation of any device in which there is an interaction between the fluid flow and the elements of the flow parts (for example, heat exchangers, transport and power machines, main pipelines), is impossible without detailed information about the characteristics of the flow, about the forces on the surfaces that are around, about vibroacoustic phenomena, etc. Among the various methods of obtaining information about the characteristics of the flow, about the forces on surfaces that are flown around, about vibroacoustic phenomena, an important role is played by theoretical methods that rely on the equation of hydrodynamics and numerous ways to solve them. In this case, the main efforts are aimed at solving the system of Navier-Stokes equations. In this paper, a general method is described for the numerical solution of the problem of unsteady flow of a viscous incompressible fluid in flat channels of an arbitrary shape of heat exchangers. An effective solution to the problem is achieved by using adaptive networks. The mathematical model of the flow is based on the two-dimensional Navier-Stokes equations in the variables "flow function - vortex" and the Poissonequation for pressure, which are solved on the basis of the finite-difference method. A numerical simulation of the fluid flow in a flat curvilinear elbow is carried out at the Reynolds number Re = 1000. This form reflects the most characteristic features of the flow paths of various hydraulic machines, heat exchangers, hydraulic and pipeline systems. The presentation of the numerical results was carried out on the basis of the VISSIM graphic processing package. One of the main problems (difficulties) in the numerical solution of problems of mathematical physics is the representation of boundary conditions for regions of arbitrary shape. The implementation of various artificial methods that are now used in the approximation of both the curvilinear boundaries themselves and the boundary conditions on them can lead to significant losses in the accuracy of the solution. This is especially evident in problems in which solutions in the boundary region have maximum gradients. An effective method for solving this problem is the use of adapted grids for the computational domain. The essence of this method lies in the fact that such a coordinate system, not necessarily orthogonal, is found in which the boundary lines (surfaces) of the region coincide with the coordinate lines (surfaces). In the flat case, the computational domain is transformed into a rectangular one, and the limit curve is displayed on the sides of the rectangle. In practice, the problem of constructing an adapted mesh is reduced to finding functions that describe the mappings of the canonical (rectangular) region onto the region for which the problem was originally formulated, that is, for the two-dimensional case, the functions x (ξ, η), y (ξ, η) are determined.

Numerical simulations of unsteady flows in a rotating channel using a novel eigenfunction expansion method

Starting flows of a viscous incompressible fluid, modeled by the time-fractional derivatives, within a rotating channel due to an impulsive pressure gradient are studied. Using the eigenfunction expansion, the analytic solutions in series form are obtained. The flow of the ordinary fluid is studied as a special case of the time-fractional problem. The convergence of series solutions is proved. In addition, using the classical analytical method, coupled with the Laplace transform and Stehfest’s algorithm, an approximate solution is found. The flow rates in x- and y-directions are determined. In the case of the ordinary fluid, the steady-state and transient components of velocities are obtained. The numerical calculations are carried out by using the Mathcad software. It is found that, for fractional fluids, the reversal flow is much attenuated if the values of the fractional parameter are less than 1.

2D numerical simulation of unsteady flows for large scale floods prediction in real time

The challenge of finding a compromise between computational time and level of accuracy and robustness has traditionally expanded the use simplified models rather than full two-dimensional (2D) models for flood simulation. This work presents a GPU accelerated 2D shallow water model for the simulation of flood events in real time. In particular, an explicit first-order finite volume scheme is detailed to control the numerical instabilities that are likely to appear when used in complex topography. The model is first validated with the benchmark test case of the Toce River (Italy) and numerical fixes are demonstrated to be necessary. The model is next applied to reproduce real events in a reach of the Ebro River (Spain) in order to compare simulation results with field data. The second case deals with a large domain (744 km2) and long flood duration (up to 20 days) allowing an analysis of the performance and speed-up achieved by different GPU devices. The high values of fit between observed and simulated results as well as the computational times achieved are encouraging to propose the use of the model as forecasting system.

Numerical Simulation of Unsteady Flow toward a Stretching/Shrinking Sheet in Porous Medium Filled with a Hybrid Nanofluid

The purpose of this study is to present simulation and numerical solutions to the unsteady flow and heat transfer near stagnation point over a stretching/shrinking sheet in porous medium filled with a hybrid nanofluid. Water (base fluid), nanoparticles of titania and copper were considered as a hybrid nanofluid. It is worth mentioning that evaluating the heat transfer enhancement due to the use of hybrid nanofluids has recently become the center of interest for many researchers. The coupled non-linear boundary-layer equations governing the flow and heat transfer are derived and reduced to a set of coupled non-dimensional equations using the appropriate transformations and then solved numerically as a nonlinear boundary value problem by bvp4c scheme from MATLAB. To validate the modeling of hybrid nanofluid and also numerical procedure, the value of the skin friction and the heat transfer rate for the limited cases of pure water, titania/water and copper/water is obtained and compared with previously reported results that demonstrate an excellent agreement. In the present investigation, the thermal characteristics of hybrid nanofluid are found to be higher in comparison to the base fluid and fluid containing single nanoparticles, respectively. It can be concluded that both skin friction coefficient and local Nusselt number enhance almost linearly with increasing the copper nanoparticle volume fraction (as second nanoparticle). Besides, the porosity and the magnetic effect amplify heat transfer rate, while the unsteadiness parameter has a reducing effect on heat transfer rate in problem conditions.

Numerical analysis of advection-diffusion problems on 2D general-shaped domains by means of a RBF Collocation Meshless Method

A Collocation Meshless Method based on local Radial Basis Function (RBF) interpolation is employed to solve two-dimensional advection-diffusion problems with particular reference to the incompressible Navier-Stokes equations in their transient form, i.e., unsteady flows, using primitive variables (u, p). A projection scheme is employed to decouple the continuity and momentum equations; particular attention is given to the choice of the required solvers. This approach is applied to the simulation of unsteady flows for two typical test cases, i.e., the lid-driven cavity problem and the flow past a circular cylinder between parallel walls. Numerical results compare very favorably with literature ones, confirming that this approach can be effectively employed in the numerical simulation of unsteady flows on practical geometries where complex node distributions and large number of nodes are required.

A Numerical Analysis of Pressure Pulsation Characteristics Induced by Unsteady Blood Flow in a Bileaflet Mechanical Heart Valve

The leaflet vibration phenomenon in bileaflet mechanical heart valves (BMHVs) can cause complications such as hemolysis, leaflet damage, and valve fracture. One of the main reasons for leaflet vibration is the unsteady blood flow pressure pulsation induced by turbulent flow instabilities. In this study, we performed numerical simulations of unsteady flow through a BMHV and observed pressure pulsation characteristics under different flow rates and leaflet fully opening angle conditions. The pressure pulsation coefficient and the low-Reynolds k-ω model in CFD (Computational Fluid Dynamics) software were employed to solve these problems. Results showed that the level of pressure pulsation was highly influenced by velocity distribution, and that the higher coefficient of pressure pulsation was associated with the lower flow velocity along the main flow direction. The influence of pressure pulsation near the trailing edges was much larger than the data obtained near the leading edges of the leaflets. In addition, considering the level of pressure pulsation and the flow uniformity, the recommended setting of leaflet fully opening angle was about 80°.

Numerical simulation of unsteady flows through a radial turbine

The article deals with the numerical simulation of unsteady flows through the turbine part of the turbocharger. The main focus of the article is the extension of the in-house CFD finite volume solver for the case of unsteady flows in radial turbines and the coupling to an external zero-dimensional model of the inlet and outlet parts. In the second part, brief description of a simplified one-dimensional model of the turbine is given. The final part presents a comparison of the results of numerical simulations using both the 3D CFD method and the 1D simplified model with the experimental data. The comparison shows that the properly calibrated 1D model gives accurate predictions of mass flow rate and turbine performance at much less computational time than the full 3D CFD method. On the other hand, the more expensive 3D CFD method does not need any specific calibration and allows detailed inspections of the flow fields.

Numerical simulation of unsteady flows with forced periodical oscillation around hydrofoils using locally power-law preconditioning method

Abstract In the present study, a developed progressive preconditioning approach called locally power-law preconditioning method is implemented to analyze unsteady flows around hydrofoils at fixed and oscillating angles of attack. The preconditioning matrix is adopted locally from the velocity flow-field by a power-law relation. The unsteady preconditioned governing equations are solved by means of Jameson’s cell-centered finite volume numerical method and a dual-time implicit algorithm. Solution stability is obtained through second- and fourth-order artificial dissipation terms. Explicit four-stage Runge–Kutta time integration is applied to achieve the steady state condition in each pseudo time step. The results focus on velocity profiles, drag and lift coefficients and the impact of locally power-law preconditioner on the convergence rate. The results indicate that the locally power-law preconditioning method has an acceptable accuracy and improves the convergence rate to a large extent in unsteady regimes.

Numerical Simulation of Unsteady Flows Affected by Stator-Rotor Blade Secular Change in Middle Pressure First-Stage Steam Turbines

Numerical Simulation of Unsteady Flows Affected by Stator-Rotor Blade Secular Change in Middle Pressure First-Stage Steam Turbines

Numerical Study of Pressure Pulsation of Centrifugal Pumps with the Compressible Mode

The compressible effect of water is often neglected in the simulation of hydraulic machinery. However, based on experimental and numerical study, it is found that the compressibility of water could influence the magnitude of the pressure pulsation at some frequency in the pump. Therefore, in order to investigate the influence of water compressibility, compressible model is established by using Tait equation. The internal flow of centrifugal pump under different conditions is calculated by this model. The calculated results are compared with the incompressible results, and it is indicated that the compressibility of water has little effect on the performance parameters. But it affects the amplitude of pressure fluctuations at some discrete frequency, especially at the outlet of impeller and volute tongue where significant jet-wake and rotor/stator interaction appears respectively. Meanwhile, water compressibility makes greater influence on the flow pulsation under off-design condition. Therefore, it is necessary to consider the compressibility of working medium in the numerical simulation of unsteady flow in centrifugal pumps, especially in area with strong unsteady flow and at off-design condition.

Investigation of noise generation by turbulent jets on the basis of numerical simulation of unsteady flow in mixing layers

In the previous experimental studies it was concluded that the turbulent jet noise is produced by large-scale motions in the mixing layer induced by turbulence intermittence. The burden of this numerical simulation is the validation of these conclusions. As a result of numerical calculations, the “instantaneous” flow patterns and the parameter distributions in the initial regions of turbulent jets are obtained. On the basis of this information the flow dynamics are investigated. In the jet flow there are observable slowly transforming low static pressure regions and zones of elevated static pressure. These regions are displaced at the convection velocity. The inflow induced by the low pressure in the mixing layer has streamlines entering into the low pressure zones and flowing around the elevated pressure zones. The motion of the zones of the static pressure varying along the flow produces velocity disturbances in the induced external flow. The succession of the transformations of the intermittence-induced static pressure disturbances into sound waves is determined. This transformation occurs in the regions occupied by the ejected air.

Structure overset grid method and its applications to simulation of multi-body separation

This paper proposes an automatic structure overset grid method, which utilizes the hole-surface optimization with one-step searching, wall-surface grid oversetting, and dynamic overset grid approaches to achieve the high adaptability of overset grids for complex multi-body aircrafts. Specifically, based on the automatic structure overset grids, the method first solves the coupling of Navier-Stokes (N-S) unsteady flow equation and 6DOF motion equation, and establishes the multi-body collision model. Then, the numerical simulation of unsteady flow for complex aircrafts’ multi-body separation, the simulation of multi-body separating trajectory and the separation safety analysis are accomplished. Thus, the method can properly handle practical engineering problems including the wing/drop tank separation, aircraft/mount separation, and cluster bomb projection. Experiments show that our numerical results match well with experimental results, which demonstrates the effectiveness of our methods in solving the multi-body separation problem for aircrafts with complex shapes.