Unsteady Flow Calculations Research Articles

Exploring cavitation dynamics and noise reduction in hydraulic machinery with bionic leading edge impeller

Cavitation and induced noise are common issues in the operation of mixed-flow pumps, affecting their stability. Studying these phenomena is of both academic and engineering interest. Previous research has mainly focused on modifying the impeller blade profile or optimizing the pump structure to reduce cavitation and noise. In this paper, we apply a bionic design to the mixed-flow pump impeller by mimicking the natural shape of a humpback whale's pectoral fin on the leading edge of the blade. We compare the cavitation suppression and noise reduction effects of this design with those of a conventional mixed-flow pump. Unsteady flow calculations for the original pump at different flow rates revealed that at its rated speed, increasing flow rates led to lower pressure in the low-pressure region on the suction surface of the impeller, expanding the cavitation-prone area. By applying the bionic design, we studied the unsteady cavitation flow and its induced noise. Results showed that the impeller and guide vane experienced larger pressure pulsations at the rotation frequency and its multiples. The bionic modification increased the cavitation initiation speed and reduced the cavity volume within the runner. With an NPSH (net positive suction head) of 6.76, the void volume in the bionic pump was only 79% of that in the original pump, and the head increased by 16%. Additionally, the bionic design reduced the maximum far-field sound pressure level by 12.5%, achieving significant noise reduction. This study demonstrates that the bionic design effectively controls dynamic flow separation, reduces flow-induced noise, and enhances the mixed-flow pump's performance. These findings have important theoretical and practical implications for optimizing mixed-flow pump design, improving energy efficiency, and reducing noise.

Study on the integrated self-priming process of a vehicle-mounted self-priming pump

In order to explore the self-priming characteristics of the self-priming pump at the mobile pump truck, this paper established a complete three-dimensional circulatory piping system including the self-priming pump, tank, valves, inlet pipe and outlet pipe. The UDF(User Defined Functions) was used to realize the acceleration-constant speed operation process of the impeller, thus reflecting the actual changing state of the rotational speed. Based on the VOF(Volume Of Fluid) multiphase flow model and the Realizable k-ε turbulence model, a coupled numerical calculation of unsteady incompressible viscous flow was conducted for its self-priming process. The results show that the self-priming process of the pump can be roughly divided into four stages: the rapid suction stage, the shock exhaust stage, the rapid exhaust period and the pump residual gas discharge stage. The proportion of each stage in the total self-priming time showed an increasing trend. During the rapid suction stage, the water level in the vertical section of the inlet pipe showed a slow and then fast-rising pattern. During the shock exhaust stage, the average gas-phase volume fraction in the volute is lower than that of the impeller, and the gas content at the volute outlet is lower than that of the impeller inlet. The region at the inlet and outer edge of the impeller consistently experience significant energy losses.

Adaptive conservative time integration for unsteady compressible flow

Unsteady flow computations typically rely on time integration schemes which employ the same step size everywhere. However, in cases with strong variations of wave speed and/or spatial resolution, local stability criteria and truncation errors would allow for much larger time steps in parts of the domain. To exploit this potential of improving the computational efficiency, adaptive time-stepping methods have been developed. Recently, an adaptive conservative time integration (ACTI) scheme for explicit finite volume methods was devised. Opposed to previous methods, ACTI guarantees exact conservation and periodic synchronization. Both are achieved by splitting a global time step into 2L (L≥0 is a cell dependent level) sub-steps, whereas the order of cell updates is critical. It has been demonstrated for flows in fractured porous media and for the Euler equations that ACTI can reduce the computational cost dramatically while preserving second order accuracy in space and time. In this paper, the ACTI scheme is extended to the compressible Navier-Stokes-Fourier system, and special attention is required for the spatio-temporal discretization of the convective and viscous fluxes. Numerical studies of unsteady flows involving shock boundary layer interaction demonstrate that the same accuracy is achieved with the new compressible ACTI flow solver as with classical time integration, but at much lower cost. Moreover, since the method is explicit, it has the potential for efficient computations on multiple CPUs and GPUs.

PHYSICAL AND MATHEMATICAL MODELING OF UNSTEADY COUPLED HEAT AND MASS TRANSFER IN CVD-DIAMOND WINDOWS OF THE SIBERIAN CIRCULAR PHOTON SOURCE

The results of physical and mathematical modelling in the ANSYS software package of three-dimensional unsteady coupled heat and mass transfer in the previously developed diamond vacuum window (DVW) of the workstation of the Siberian Circular Photon Source under construction are presented. Previous simulations have convincingly shown: the possibility of using a mini-channel cooling system to implement reliable and proper temperature management of this optical device, the advantage of a mini-channel cooling system over a one channel system in terms of fulfilling the optical requirements for DVW and its design. Here are detailed calculations of unsteady three-dimensional flow in the mini-channels of the cooling system, which will prevent possible dramatic destruction of the workstation due to local overheating of the CVD-diamond foil and its recrystallization.

Numerical calculation of unsteady MHD nanofluid flow across two fluctuating discs with chemical reaction and zero mass flux

AbstractThe fluid flow across spinning discs has several applications in different fields of engineering such as flywheels, gas turbine engines, brakes, and gears. The significance of heat source, Hall current, and magnetic field on the nanofluid flow between two spinning disks is studied in the present analysis. The upper disk spins as well as rotates, which generates the 3D flow. The evaluation and modeling of the mass density, flow motion, and energy transfer are performed using a system of partial differential equations (PDEs). The computational technique parametric continuation method (PCM) is employed to solve the modeled equations. The outcomes are relatively compared to the published literature for validity purposes. It has been noticed that during the downward motion of the upper disc, the effect of the magnetic field declines, while the influence of the hall current improves the velocity field. The temperature field rises with the ascending motion of the upper disk, whereas it shrinks with the downhill oscillation. Moreover, the mass diffusion ratio develops with the impact of hall current, while lessens with the influence of chemical reaction.

A Computationally Efficient Method for the Thermo-Structural Analysis of a Turbocharger Turbine Wheel Under Engine Transient Operation

Abstract The accurate evaluation of thermally-induced stresses is of particular importance for life-cycle analysis of modern turbochargers. Conjugated heat transfer (CHT), steady-state calculations represent the state-of-the-art, whose temperature distribution is usually imported into a structural solver as a thermal load for carrying out thermo-structural analyses. In the case of transient analyses, however, issues arise. Indeed, due to the different timescales of convective and conductive heat-transfer mechanisms, unsteady CHT simulations are burdened by high computational costs. Moreover, in the specific case of turbocharged engines, the timescales of engine load transients and turbocharger response may differ by a factor of up to 105. The present study proposes an approach for carrying out transient thermo-structural analyses of the solid domain only, characterized by a physical time in the order of 101 s, without the need for unsteady fluid flow calculations, which would imply the use of a solver time-step in the order of 10−5 s. In the proposed methodology, the effects of the fluid flow in the heat-transfer process are modeled by imposing adequate boundary conditions to wet surfaces of the solid domain with minimum computational effort. Indeed, by performing two steady-state CHT calculations and one steady-state computational fluid dynamics (CFD) calculation, time-dependent boundary conditions can be derived in terms of convective heat transfer coefficient and near-wall fluid temperature. The case study used for model development is a high-performance turbocharger turbine for automotive applications, responding to engine load transients of tenths of seconds. Initially, steady-state CFD and CHT models of the turbine stage are validated against experimental data. Then, transient analyses on the turbine wheel are carried out, and their results are imported as thermal loads into an unsteady structural solver, allowing for accurate transient thermo-structural analyses at sustainable computational cost.

Efficient numerical prediction of blade forced response under inlet distortion

The compressor blade in an aircraft gas turbine engine may suffer from forced vibration problems under the unsteady aerodynamic force due to inlet distortion. This paper develops an efficient numerical method combination to predict the forced response. The computational fluid dynamics (CFD) method is combined with the Fourier transform method to simulate the unsteady flow field under inlet distortion. Then, the forced response of the blade is obtained by the frequency-domain method based on the modal force. The results show that the developed method combination improves the unsteady flow calculation efficiency by 20 times and the response calculation efficiency by 48 times at the cost of less than 5% accuracy compared with the traditional method combination, including the unsteady flow field calculation method based on the three-row full-annulus model and the transient analysis method for calculating the forced response. Finally, the developed method combination is used to analyze the forced response of the blade under different inlet total pressure distributions. The results indicate that the forced response can be effectively decreased by decreasing the total pressure difference and increasing the sector angle difference between the high-pressure and low-pressure areas.

An intelligent control method based on artificial neural network for numerical flight simulation of the basic finner projectile with pitching maneuver

In this paper, an intelligent control method applying on numerical virtual flight is proposed. The proposed algorithm is verified and evaluated by combining with the case of the basic finner projectile model and shows a good application prospect. Firstly, a numerical virtual flight simulation model based on overlapping dynamic mesh technology is constructed. In order to verify the accuracy of the dynamic grid technology and the calculation of unsteady flow, a numerical simulation of the basic finner projectile without control is carried out. The simulation results are in good agreement with the experiment data which shows that the algorithm used in this paper can also be used in the design and evaluation of the intelligent controller in the numerical virtual flight simulation. Secondly, combined with the real-time control requirements of aerodynamic, attitude and displacement parameters of the projectile during the flight process, the numerical simulations of the basic finner projectile’s pitch channel are carried out under the traditional PID(Proportional-Integral-Derivative) control strategy and the intelligent PID control strategy respectively. The intelligent PID controller based on BP(Back Propagation) neural network can realize online learning and self-optimization of control parameters according to the acquired real-time flight parameters. Compared with the traditional PID controller, the concerned control variable overshoot, rise time, transition time and steady state error and other performance indicators have been greatly improved, and the higher the learning efficiency or the inertia coefficient, the faster the system, the larger the overshoot, and the smaller the stability error. The intelligent control method applying on numerical virtual flight is capable of solving the complicated unsteady motion and flow with the intelligent PID control strategy and has a strong promotion to engineering application.

Numerical Modeling and Geometry Enhancement of a Reactive Silencer

Internal combustion engines and blowers frequently utilize silencers to reduce exhaust noise. In the current paper, the transmission loss of reactive silencers is predicted using the plane wave decomposition method and a three-dimensional (3-D) time-domain computational fluid dynamics (CFD) approach. A mass-flow-inlet boundary condition is first used to perform a steady flow computation, which serves as an initial condition for the two subsequent unsteady flow computations. At the model's inlet, an impulse (acoustic excitation) is placed over the constant mass flow to perform the first unstable flow computation. Once the impulse has fully propagated into the silencer, the non-reflecting boundary condition (NRBC) is then added. For the scenario without acoustic excitation at the inlet, a second unsteady flow computation is performed. During the two transient computations, the time histories of the pressure and velocity at the upstream measuring points as well as the history of the pressures at the downstream measuring point are recorded. The related acoustic quantities show variations between the two unsteady flow computational findings. As a result, the transmitted sound pressure signal is just the sound pressure downstream, while the incident sound pressure signal is obtained by utilizing plane wave decomposition upstream. The transmission loss (TL) of the silencer is then calculated after the Fast Fourier Transform (FFT) converts the two sound pressure signals from the time domain to the frequency domain. The numerical calculations and the reported data are in good agreement for the published results, in addition to geometry enhancement by increasing number of holes in the cross section for muffler.

The acceleration effect of pump as turbine system during starting period

In order to reveal the influence of starting acceleration on starting process of a pump as turbine system, this paper carries out a numerical calculation of the three-dimensional viscous unsteady flow of pump as turbine circulating piping system under three starting acceleration conditions, and obtains the external and internal flow characteristics of each overflow component during the starting process, and also analyzes the energy loss of each component in the piping system in depth with the help of entropy production method and Q criterion method. The results show that during system start-up, the flow rate and outlet static pressure curves of the pump as turbine are hysteresis relative to the rotational speed, the head curve is similar to a linear rise during slow and medium speed start-up, while it shows a parabolic rise during rapid start-up, the entropy production and vorticity in the impeller domain of the pump as turbine are mainly distributed between the blades, and the distribution decreases during start-up. In addition, the pump similarity law does not apply to the performance prediction during the transient start of the pump as turbine.

EFFECT OF SUMP GEOMETRY AND SUBMERGENCE DEPTH ON FREE SURFACE VORTEX FORMATION OF PUMP INFLOW

This paper examines and discusses pump sump design as it affects unsteady pump inflow. It is shown that an improvement of the unsteady inflow characteristics can be accomplished by introducing some modifications to the pump sump design. The paper introduces a 3D numerical computation of unsteady flow through a water sump into the intake of a large vertical pump. The numerical simulation is performed using ANSYS CFX. Numerical simulations performed use the shear stress transport model to account for the effect of turbulence. The results obtained were verified by comparison with published experimental data. The effect of the sump geometry and water level on the internal flow in pump sump is presented. Four sump geometries were studied: a rectangular sump, a sump with inclined edge at 30°, a sump with inclined edge at 45°, and a half-circle-ended sump. Also, three submergence levels were studied: S/D = 1.015, 0.771, and 0.54. It is concluded that sumps of rectangular geometry are the best to avoid surface and floor vortex formation.

LiDAR DTM Application for Flood Inundation Modeling and Visualization (Case Study: Batu City, East Java)

LiDAR DTM has been widely used for simulating flood inundation from river overflows. This flood inundation simulation needs to be done as an alternative step in the flood control system and flood disaster mitigation to determine the estimated area affected by the flood. Flood simulation is a form of unsteady flow that is simulated with the HEC-RAS program that allows the user to perform one-dimensional steady flow, one and two-dimensional unsteady flow calculations, sediment transport/mobile bed computations, and water temperature/water quality modeling, and visualizes flooded areas and flood-affected areas using a geographic information system approach. Hydrological data is obtained from daily rainfall processing at the time of station rain. Hydrograph analysis was performed using the SCS CN model on the HEC-HMS software to estimate the flow rate and peak discharge. Watershed area, curve number, Impervious value, Initial ion, and time lag are parameters that affect hydrograph analysis. From the results of the hydrograph analysis, the peak discharge of the model is 119.992 m2/s. Meanwhile, hydraulic analysis using HEC-RAS resulted in an inundation of 364 m2 which inundated three villages, namely Bulukerto, Bumiaji and Sidomulyo villages. The depth of the inundated area ranged from 0-15 m with an average depth of 4.534 m. The Sidomulyo area affected by the flood inundation scenario is 4,924 meters with an area of 0,02 ha. Bumiaji village was flooded with an area of 0,007 ha with a scenario of inundation depth of 4,049 m. Bulukerto village which was affected by the flood has an area of 20,972,832 m2 with a scenario of inundation depth reaching 4,343 m. Then some land cover such as residential land that is inundated by this flood simulation is 0,002 ha. Buildings from residential areas that were flooded reached 209 buildings. Then the area of agricultural land affected is 0,02 ha. In addition, there are also green open spaces affected by this simulation with an inundation area of 0,003 ha.

Mathematical Justification for Optimizing Operating Conditions of Gas and Gas Condensate Producing Wells

The paper investigated the problem of selecting/finding the optimal process conditions for gas condensate wells. The well process conditions imply a set of parameters that characterize its operation. The optimization of process conditions provides for the efficient operation of an oil and gas field while meeting the defined boundary and initial conditions, and allows for the process/production goal to be achieved. This paper proposed using the tree-structured Parzen estimator (TPE), which allows for the results from previous iterations to be considered, in order to identify the most promising region of conditions, thereby increasing the optimization efficiency. The movement of multiphase fluid inside the pipeline system (also in the borehole) must be calculated to solve the process optimization problem. The optimization module was integrated into the hydraulic and unsteady state multiphase flow calculations inside the well and the pipeline. The platform created allows for the process conditions at gas condensate fields to be identified via the use of numerical methods. The proposed optimization algorithm was tested in delivering the task of optimizing the process conditions in 13 producing wells in a part of a real gas condensate field in Western Siberia. The engineering problem of optimizing the production of gas and the gas condensate was solved as a consequence of the calculations performed.

Analysis of the effect of unsteady reservoir flow and sand excavation on the channel conditions of the Yangtze River (Yibin-Chongqing)

This paper systematically describes the current research methods for analyzing unsteady flows in an open channel. In particular, it focuses on elaborating a discrete method to solve the one-dimensional Saint–Venant equations for unsteady flow in an open channel, compares the advantages and disadvantages of different discrete methods, and proposes a new implicit iterative mathematical model applicable to the calculation of the multiwavelength unsteady flow in the Yangtze River from Yibin to Chongqing. This paper also simulates three typical unsteady flow processes and analyzes the effect of sand excavation on the water level in the upper reaches of the Yangtze River from Yibin to Chongqing considering flood, normal-water, and low-water conditions. The spreading characteristics of the daily adjusted unsteady flow were obtained in the Shuifu-Chongqing river reach. Under the conditions of the daily minimum unsteady discharge flow rate, the designed water level of the reach from Yibin to Chongqing increases by 0.19 m compared to the natural water level. With the increase in concentrated sand excavation in the upper reaches of the Yangtze River, a large number of shingle beaches have disappeared, and the riverbed has been drastically lowered. Affected by the excessive and unordered sand excavation in the river channel, the water level of the upper reaches of the Yangtze River from Chongqing to Yibin has decreased by 0.5–2.0 m.

Effect of Incident Angle of Wear-Ring Clearance on Pressure Pulsation and Vibration Performance of Centrifugal Pump

A wear-ring is an important part of the centrifugal pump. The leakage flow in the wear-ring clearance and main flow at the impeller inlet form crossed mixed flows perpendicular to each other. Large eddies and shocks are produced at the intersection of the two flows due to great velocity difference and different directions, resulting in flow losses, unsteady flow, and even flow-induced vibration. Consequently, the pump performance, pressure pulsation and vibration, and other characteristics will be greatly affected. In this paper, 5 incident angles between the incident section of the wear-ring clearance and the circumferential direction of impeller inlet, i.e., the original angle (90°), 75°, 60°, 45°, and 30°, were formed with a low-specific-speed centrifugal pump as the study object. Unsteady flow calculation and fluid–structure interaction calculation were performed on centrifugal pumps with different wear-ring clearances; the effect of the incident angle of the wear-ring clearance on the distribution of pressure pulsation and vibration performance of centrifugal pump was analyzed. The results showed that the improved efflux angle of the wear-ring clearance could effectively weaken the impact disturbance of the leakage flow in the wear-ring clearance to the main flow at the inlet. Accordingly, the flow status at the inlet of the centrifugal pump was improved, flow losses were reduced, the efficiency of the centrifugal pump was improved, and the vibration amplitude and vibration energy of the pump were also reduced.

Nonlinear simulation of mucociliary clearance: A three- dimensional study

In this study three-dimensional (3D) numerical simulation of mucociliary clearance from the lung has been studied. The major thrust of this investigation is 3D simulation of mucociliary clearance accounting for non-linear viscoelastic characteristics of the mucus layer which is one of the most complex fluids in our body. Using a numerical code, three-dimensional unsteady flow computations have been carried out to study ciliary motion in a two-layer fluid model of the (bronchial) airway surface liquid (ASL) consisting of a Newtonian lower periciliary layer (PCL) and a nonlinear viscoelastic upper mucus layer. The projection finite difference method is used to solve the governing and constitutive equations on a staggered Eulerian grid. The immersed boundary method (IBM) is also employed to study the effect of cilia propulsive force on the fluid flow. The 5-mode nonlinear Giesekus model has been used as the constitutive equation of mucus which is characterized by nonlinear (non-Newtonian viscosity) properties. Numerical results have been devoted to study the effect of various parameters such as cilia beat frequency, cilia length as well as PCL and mucus depth on mucociliary clearance. The present 3D model shows very good agreement with previous experimental studies, while ca. 2.5-fold larger clearance rates were found for 2D models. 3D results show that cilia beating frequency and cilia length are the most critical factors affecting mucociliary clearance, while PCL depth and mucus depth have little and intermediate effect on mucus flow and hence clearance rate respectively.

Study on Dynamic Characteristics of Mars Entry Module in Transonic and Supersonic Speeds

The aerodynamic configuration of the Tianwen-1 Mars entry module that adopts a blunt-nosed and short body shape has obvious dynamic instability from transonic to supersonic speeds, which may bring risk to parachute deployment. The unsteady detached eddy of the entry module cannot be accurately simulated by the Reynolds-Averaged Navier-Stokes (RANS) model, while the computational cost for direct numerical simulation (DNS) and large eddy simulation (LES) is huge. It is difficult to implement these methods in the coupled engineering calculation of unsteady flow and motion. This paper proposes the integrated numerical simulation method of computational fluid dynamics and rigid body dynamics (CFD/RBD) based on detached eddy simulation (DES) and calculates and studies the dynamic characteristics of attitude oscillation of the Mars entry module in free flight from transonic to supersonic speeds with one degree of freedom (1-DOF) at small releasing angle of attack. In addition, the unstable range of Mach number and angle of attack are determined, and the effect of different afterbody shapes on dynamic stability is analyzed.

Assessment of improved delayed detached eddy simulation in predicting unsteady flows and sound around a circular cylinder

Unsteady flows in the field of engineering are usually calculated by the Unsteady Reynolds-Averaged Navier–Stokes (URANS) owing to the low requirements for computational efforts. However, the numerical resolution of URANS, especially in predicting the unsteady wake flows and sound, is still questionable. In this work, unsteady flow and sound calculations of a circular cylinder are carried out using Improved Delayed Detached Eddy Simulation (IDDES) and the Ffowcs Williams–Hawkings (FW-H) analogy. The predicted results of this calculation are compared with those from the previous studies in the literature in terms of the mean and RMS of the velocity components as well as the sound pressure. The results show that IDDES retains much of the numerical accuracy of the Large Eddy Simulation (LES) approach in predicting unsteady flows and noise while requiring a reduced computational resources in comparison to LES. It is believed that the IDDES can be applied to calculate the complex unsteady flows and flow generated sound with reasonable accuracy in engineering field, which can be used as a promising method for scale-resolving simulations to avoid the expensive computational requirements of LES.

Simulating unsteady flows in a compressor using immersed boundary method with turbulent wall model

Computations of unsteady turbulent flows in turbomachine are still challenging due to the complexity of geometry and relative motion between rotor and stator. A potential solution is using the immersed boundary method (IBM) for its advantages in simulating moving boundary problems, but it loses efficiency with increasing Reynolds number. In the present work, to accurately simulate unsteady flows in a compressor, the well-established turbulent wall model is adopted with the hierarchical Cartesian grid in IBM to alleviate the requirements of grid spacing for turbulent boundary layer simulation. First, verification of the flow solver on two-dimensional (2D) stationary cases of the non-inclined/inclined flat plate and zero attack NACA 0012 were conducted. Furthermore, the Tamaki et al. wall model which had been only validated in stationary problems was first extended to solve the moving boundary problem, i.e. plunging airfoil. Finally, the new immersed boundary solver employed Tamaki et al. wall model was used to simulate the classic rotor/stator interaction. The unsteady normal forces on stator blade of a low-speed compressor were examined with two different sizes of axial gap, i.e. 10% chord and 30% chord. With the large gap, the predicted phase and magnitude of the unsteady aerodynamic force agree with the measurements well. With the more challenging small gap, although the predicted phase of the peak force is lagged slightly with the measurements, the magnitude of the peak force is captured. The agreements between numerical results and measurements indicate the ability of the developed IBM solver based on Tamaki et al. wall model and hierarchical Cartesian grid in dealing with simulations of unsteady flows in turbomachine.

Study of BPF pressure pulsations reduction in centrifugal bladed machines using splitters

Presence of intensive pressure pulsations and noise on blade passing frequencies is a characteristic feature of all types of centrifugal bladed machines including pumps. Under definite conditions pressure pulsations can reduce the pump reliability and life cycle. High level of tonal noise can be inappropriate due to ecological requirements. The paper is devoted to the study of application of splitters for reduction of pressure pulsations in centrifugal pumps. 2D and 3D numerical methods are used in the study. Initial computational tests were performed with a two-dimensional formulation for a centrifugal pump with a simple volute using 2D discreet vortex method for the impeller flow and 2D acoustic-vortex approach for pressure pulsations prediction. 3D unsteady flow computations of an air model of the centrifugal pump are undertaken with the impeller having splitters. Calculations of pressure pulsations show that the use of additional blades reduces the amplitude of the first harmonic of blade passing frequency by more than three times compared to the design without splitters. It correlates with flow parameters distribution along the impeller outlet diameter that has less nonuniformity and a more balanced vorticity profile. Application of splitters reduces the amplitude of pressure pulsations by a factor of 3.