Investigation Of Unsteady Flow Research Articles

Numerical investigation of unsteady flow and heat transfer of a free convective second-grade fluid passing through exponentially accelerated vertical porous plate

Numerical investigation of unsteady flow and heat transfer of a free convective second-grade fluid passing through exponentially accelerated vertical porous plate

Effect of the number of blades on diffuser unsteady loss of centrifugal pump

Abstract In recent years, an unshrouded impeller is being developed for rocket turbopumps to reduce production costs and disk friction losses. However, the internal flow structure in a diffuser influenced by an impeller has not been clarified yet. In this study, we focused on the investigation of unsteady flow in turbopumps with unshrouded impellers and vaned diffusers by experiments and CFD. Furthermore, we investigated the effect of changing the number of blades of the impeller and diffuser on the unsteady losses in the diffuser. First, we measured the static pressure at the shroud side in the impeller and the velocity at the diffuser inlet and outlet. Second, we confirmed the accuracy of the CFD by comparing CFD results with experimental results. Third, we investigated unsteady losses in diffusers by CFD. We could confirm increased entropy at the suction surface and shroud side because of the tip leakage and the vortex. Finally, we changed the number of impeller blades or diffuser blades. In conclusion, the loss region at the suction surface and the high-pressure region at pressure surface in the unshrouded impeller were mixed and caused unsteady losses through the diffuser. And diffuser efficiency decreased because channel width in diffuser became narrower by the number of diffuser blades increasing.

Dynamic Instability Analysis of a Double-Blade Centrifugal Pump

The flow instability of a double-blade centrifugal pump is more serious due to its special design feature with two blades and large flow passages. The dynamic instabilities and pressure pulsations can affect the pump performance and operating lifetime. In the present study, a numerical investigation of unsteady flow and time variation of pressure within a complete double-blade centrifugal pump was carried out. The time domain and frequency domain of pressure pulsations were extracted at 16 monitoring locations covering the important regions to analyze the internal flow instabilities of the pump model. The frequency spectra of pressure pulsations were decomposed into Strouhal number dependent functions. This led to the conclusion that the blade passing frequency (BPF) related vibrations are exclusively flow-induced. Large vortices were observed in the flow passages of the pump at low flow rate. It is noted that high vorticity magnitude occurred in the vicinities of the blade trailing edge and tongue of the volute, due to the rotor-stator interaction between impeller and volute.

Numerical investigation of unsteady cavitating turbulent flows around a three-dimensional hydrofoil using stress-blended eddy simulation

The mechanism of flow instability, which involves complex gas–liquid interactions and multiscale vortical structures, is one of the hot research areas in cavitating flow. The role of turbulence modeling is crucial in the numerical investigation of unsteady flow characteristics. Although large-eddy simulation (LES) has been used as a reliable numerical method, it is computationally costly. In this work, we used a hybrid Reynolds-averaged Navier–Stokes (RANS) and LES model, that is, stress-blended eddy simulation (SBES), to improve the prediction capability for the cloud cavitating flow. Our hybrid approach introduces a shielding function to integrate the RANS model with the LES applied only regionally, such as to large-scale separated flow regions. The results showed that the periodic shedding of cavity growth, break off, and collapse around a three-dimensional Clark-Y hydrofoil was reproduced in accordance with experimental observations. The lift/drag coefficients, streamwise velocity profiles, and cavity patterns obtained by the SBES model were in better agreement with the experimental data than those obtained by the modified RANS model. The re-entrant jet dynamics responsible for the break off of the attached cavity were discussed. Further analysis of vorticity transportation indicated that the stretching and dilatation terms dominated the development of vorticity around the hydrofoil. In conclusion, the SBES model can be used to predict cavitating turbulent flows in practical engineering applications.

Numerical investigation of unsteady flow across tandem square cylinders near a moving wall at Re = 100

Flow across tandem square cylinders placed close to a plane moving wall, has been studied extensively for various cylinder inter-spacing ratio (0.5≤S/D≤8) and cylinder-to-wall gap ratio values (0.1≤G/D≤4) at a fixed value of Reynolds number, Re = 100. Numerical experiments are performed using the computational package ANSYS FLUENT®. Results indicate that unsteady flow past tandem cylinders relies primarily on the presence of a moving wall. Exhaustive details on the effects of varying S/D and G/D values on the onset and suppression of vortex shedding behind the tandem cylinders are given. A detailed description of the underlying flow physics is provided through instantaneous vorticity and streamline contours. Forces such as the lift and drag acting on the cylinders are reasoned qualitatively through time-averaged pressure, lift, and drag coefficient plots. A remark on existence and suppression of flow unsteadiness is given through Strouhal number variation. A rigorous comparison of the present flow field with flow past an isolated square cylinder, square cylinder near a moving wall, and unbounded tandem cylinders is given. Overall, unlike the lift, drag coefficient values happen to be highest for single cylinder case, followed by the upstream cylinder, and they are least for the downstream cylinder.

Investigation of unsteady flow in the unscalloped radial turbine cavity

This paper presents an unsteady numerical study of the unsteady flow in the unscalloped radial turbine cavity considering the effect of computational sector size and sealing flow rate. A simplified U-shaped cavity with vanes and blades was simulated and the sealing efficiency was obtained using additional variables method. Unsteady low-pressure structures exist under a certain sealing flow rate, which is similar with axial turbine cavity, while the impact area is much larger. A comparison of computational domains sizes for 60° and 360° shows that whether the sector model can accurately predict the flow field depends highly on the number of low-pressure structures. As the sealing flow rate increases, the number and the speed of low-pressure structures at high radius decrease, and the corresponding low-frequency amplitude increases initially but decreases subsequently. The speed of low-pressure structure is positively correlated with its number, which seems contrary to the phenomenon observed in the axial turbine cavity.

A Coupled Artificial Compressibility Method for Free Surface Flows

Modeling free surface flows in a CFD context typically requires an incompressible approach along with a formulation to account for the air–water interface. Commonly, pressure-correction algorithms combined with the Volume of Fluid (VOF) method are used to describe these kinds of flows. Pressure-correction algorithms are segregated solvers, which means equations are solved in sequence until convergence is accomplished. On the contrary, the artificial compressibility (AC) method solves a single coupled system of equations. Solving at each timestep a single system of equations obviates the need for segregated algorithms, since all equations converge simultaneously. The goal of the present work is to combine the AC method with VOF formulation and prove its ability to account for unsteady flows of immiscible fluids. The presented system of equations has a hyperbolic nature in pseudo-time, thus the arsenal of the hyperbolic discretization process can be exploited. To this end, a thorough investigation of unsteady flows is presented to demonstrate the ability of the method to accurately describe unsteady flows. Problems of wave propagation on constant and variable bathymetry are considered, as well as a fluid structure interaction problem, where viscous effects have a significant impact on the motion of the structure. In all cases the results obtained are compared with theoretical or experimental data. The straightforward implementation of the method, as well as its accurate predictions, shows that AC method can be regarded as a suitable choice to account for free surface flows.

Numerical Investigation of Unsteady Flow and Aerodynamic Noise Characteristics of an Automotive Axial Cooling Fan

Low-speed axial cooling fans are frequently used to manage engine temperature by ensuring that adequate quantities of air pass through heat exchangers, even at low vehicle speeds or in the idle condition. This study aims to provide a better understanding of the unsteady flow behavior around an automotive axial cooling fan with seven blades and its impact on the aerodynamic noise generation. Large Eddy Simulation (LES) near the near-field region and the Ffowcs-Williams and Hawkinbygs (FW-H) method were performed to analyze the flow characteristics around the fan and predict the aerodynamic noise emitted from the fan under a constant rotational speed of 2100 rpm. The simulation results for the velocity distributions and aerodynamic noise were compared with the experimental data measured by single hot-wire probe and in a dead-sound room. The results showed a comparatively good agreement upstream and downstream from the fan and at two different receivers of 0.5 m and 1.0 m. When the fan was rotating, a strong tonal noise numerically existed near the leading edge of the blades at the tip and amounted to 110 dB sound pressure level (SPL) caused by the increasing angles of attack with the increasing radial velocity near the ring, which caused the entire air foil to emit a low-frequency noise. Furthermore, the different SPL decay characteristics of approximately 5 dB in the near-field region and 6 dB in the far-field region were observed each time the distance from the fan doubles. The findings of this research can provide important insights into the design of axial fans with low noise and high performance.

Experiments on flow and heat transfer characteristics of a rectangular channel with a built-in adiabatic square cylinder

Experimental investigation of unsteady flow past a square cylinder confined in a channel and the associated heat transfer phenomena in laminar flow regime is carried out using purely non-intrusive diagnostic technique. A Mach-Zehnder interferometer has been employed to make the path-averaged measurements of the quantities of interest. Five different Reynolds numbers are considered and the heat transfer rates in the channel with and without the presence of square cylinder have been compared in the wake region of the built-in cylinder. Vortex shedding frequencies are determined using schlieren technique. Both the temperature field and the vortex shedding frequencies have been completely determined using non-intrusive techniques. The whole field experimental results depict significant augmentation in the wall heat transfer rates in the presence of a cylinder when compared with its counterpart (channel without cylinder). Flow instabilities induced by the square cylinder alter the thermal boundary layer profile and hence temperature gradients near the channel walls become steeper and consequently the wall heat transfer is significantly increased.

Numerical study of unsteady boundary layer flow and heat transmission of a nano-fluid past a continuously stretching sheet immersed in porous media

PurposeThis paper aims to incorporate a numerical investigation of unsteady flow of nano-fluid near the boundary wall and heat transmission over a flat sheet in porous media under the assumption that sheet is continuously stretching. A comprehensive study is accomplished to probe the impacts of different physical parameters on heat transmission and fluid flow.Design/methodology/approachThe problem is designed mathematically by using Navier–Stokes equations and corresponding equations are transmuted into non-dimensional form, which are reduced to local non-similarity equations. Implicit finite difference scheme is used to attain a non-similar solution for values of similarity variable ranging in [0,1]FindingsThe reduced Nusselt number is a decreasing function of each dimensionless number, while the reduced Sherwood number is an increasing function of each considered value of parameters Pr, Sc, Nb and Nt.Originality valueMotivated by the industrial, technological and scientific importance, the objective of this study is to discuss the unsteady flow generated by linear stretching of the surface in Nano-fluid in a porous medium. However, the investigation of unsteady boundary layer flow and heat transfer of a Nano-fluid past a stretching surface immersed in a porous medium has not been discussed yet. Furthermore, finite difference scheme is used to solve the problem, and authenticate results are achieved which are in good agreement with published work.

Numerical Investigations of Unsteady Flows and Particle Behavior in a Cyclone Separator

Mechanism of particle separation in a cyclone separator is fully clarified by one-way coupled numerical simulations of large eddy simulation and particle tracking. The former resolves all the important vortical structures, while the latter inputs the computed flow fields and tracks trajectories of particles by considering Stokes drag as well as gravity. The computed axial and tangential velocities of the swirl flow in a cyclone compare well with the ones measured by particle image velocimetry (PIV). The precession frequency of the vortex rope computed for Stairmand cyclone also matches with the one measured by Darksen et al. The predicted collection efficiencies reasonably agree well with the measured equivalents for two cylindrical cyclones with different diameters and inflow conditions. Detailed investigations on the simulated vortical structures in the test cyclones and predicted trajectories of the particles have revealed that there are three major paths of trajectories for those particles that are not collected and exhausted from the cyclone. More than half of the exhausted particles are trapped by longitudinal vortices formed in the periphery of the vortex rope. Namely, the precession motion of the vortex rope generates a number of longitudinal vortices at its periphery, which trap particles and move them into the region of the upward swirl.

Investigation of unsteady flow in a centrifugal pump at low flow rate

Due to the characteristics of unsteady flow in the centrifugal pump at low flow rate is not revealed well, a simulation of the internal flow at different flow rates is carried out with renormalization group k–ε turbulence model and multiple reference frame. For analyzing the influence of flow rate, ratios of flow rate ( Q/ Qd) are set to 0.1, 0.3, 0.6, and 1.0 at this study. The hydraulic performance of the centrifugal pump obtained by numerical calculation has matched well with the corresponding experimental result. From the characteristics of the internal flow captured by the numerical simulation, it can be seen that backflow occurs in the inlet of impeller at low flow rate, which prevents fluid discharging into impeller passages and leads to vortical structures in suction region. With further decrease in flow rate, the strength of backflow has been intensified, and the number of vortex has significantly increased. A visualization experiment of the backflow evolution in suction pipe is carried out to validate the unsteady simulated results. Results show that the prerotation is an important factor for the deterioration of centrifugal pump performance.

Numerical investigation of MHD flow of blood and heat transfer in a stenosed arterial segment

A numerical investigation of unsteady flow of blood and heat transfer has been performed with an aim to provide better understanding of blood flow through arteries under stenotic condition. The blood is treated as Newtonian fluid and the arterial wall is considered to be rigid having deposition of plaque in its lumen. The heat transfer characteristic has been analyzed by taking into consideration of the dissipation of energy due to applied magnetic field and the viscosity of blood. The vorticity-stream function formulation has been adopted to solve the problem using implicit finite difference method by developing well known Peaceman–Rachford Alternating Direction Implicit (ADI) scheme. The quantitative profile analysis of velocity, temperature and wall shear stress as well as Nusselt number is carried out over the entire arterial segment. The streamline and temperature contours have been plotted to understand the flow pattern in the diseased artery, which alters significantly in the downstream of the stenosis in the presence of magnetic field. Both the wall shear stress and Nusselt number increases with increasing magnetic field strength. However, wall shear stress decreases and Nusselt number enhances with Reynolds number. The results show that with an increase in the magnetic field strength upto 8T, does not causes any damage to the arterial wall, but the study is significant for assessing temperature rise during hyperthermic treatment.

Analytical and numerical investigation of unsteady flow near a critical point

The problem of unsteady flow of a viscous incompressible fluid near a critical point on a plane boundary is investigated. A theorem on the existence and uniqueness of its solution in Hölder classes of functions on an arbitrary time interval with natural restrictions imposed on the initial function is proved. Qualitative properties of the solution are investigated. Results of a numerical analysis demonstrating the possibility of disappearance after a finite time of a counterflow zone existing at the initial time in the case of a negative pressure gradient at the rigid plane are presented. In the case when the pressure gradient is a periodic function, a periodic mode of motion as well as breakdown of the solution after a finite time is possible.

Investigation of Unsteady Flow in an Axial Compression Based on Double Times Dual Time Stepping

A numerical investigate of unsteady (rotor 37) based on dual time step is carried out .Results were used to identify the existence of an interface between the approach fluid and the tip-leakage flow. This paper discusses mechanism of spike stall.

0212 旋回噴流の渦の振れ回りによって発生する非定常流れの調査

0212 旋回噴流の渦の振れ回りによって発生する非定常流れの調査

Simulation-based investigation of unsteady flow in near-hub region of a Kaplan Turbine with experimental comparison

This paper presents a detailed comparison of steady and unsteady turbulent flow simulation results in the U9 Kaplan turbine draft tube with experimental velocity and pressure measurements. The computational flow domain includes the guide vanes, the runner and the draft tube. A number of turbulence models were studied, including the standard , RNG , SST and SST-SAS models. Prediction of the flow behavior in the conical section of the draft tube directly below the runner cone is very sensitive to the prediction of the separation point on the runner cone. The results demonstrate a significant increase in precision of the flow modeling in the runner cone region by using unsteady flow simulations compare to stage simulation. The prediction of the flow in the runner cone region, however, remains delicate, and no turbulence model could accurately predict the complex phenomena observed experimentally.

Lock-in spectroscopy employing a high-speed camera and a micro-scanner for volumetric investigations of unsteady flows.

Spectroscopic methods are established tools for nonintrusive measurements of flow velocity. However, those methods are either restricted by measuring pointwise or with low measurement rates of several hertz. To investigate fast unsteady phenomena, e.g., in sprays, volumetric (3D) measurement techniques with kHz rate are required. For this purpose, a spectroscopic technique is realized with a power amplified, frequency modulated laser and an Mfps high-speed camera. This allows fast continuous planar measurements of the velocity. Volumetric data is finally obtained by slewing the laser light sheet in depth with an oscillating microelectromechanical systems (MEMS) scanner. As a result, volumetric velocity measurements are obtained for 256×128×25 voxels over 14.4 mm×7.2 mm×6.5 mm with a repetition rate of 1 kHz, which allows the investigation of unsteady phenomena in sprays such as transients and local velocity oscillations. The respective measurement capabilities are demonstrated by experiments. Hence, a significant progress regarding the data rate was achieved in spectroscopy by using the Mfps high-speed camera, which enables new application fields such as the analysis of fast unsteady phenomena.

Investigations of unsteady flow in the draft tube of the pump- turbine model using laser Doppler anemometry

The measurements and video observation of unsteady flow in the draft tube cone of the pump-turbine model were conducted in the Laboratory of Water Turbines, property of OJSC "Power machines" - "LMZ". The prototype head was about 250 m. The experiments were performed for the turbine mode of operation.Measurements were taken for the unit speed value n11 corresponding to rated head in the generating mode of operation, for a wide range of guide vanes openings at loads ranging from partial to maximum value. The researches of the velocity field in function of the Thoma number were carried out in some operating conditions.The mean values and RMS deviations of the velocity components were the results of laser measurements. The curves of the intensity of the vortex versus the guide vane opening and the Thoma number were plotted. The energy velocity spectra were presented for the points at which the most pronounced frequency precession of the helical axial vortex was observed. Video recording and laser Doppler anemometry were made in the operating conditions of the developed cavitation. Based on the results of video observations and energy spectra obtained via LDA, vortex frequencies were determined i.e. the frequencies of the vortex precession under the runner in the draft tube cone.

Investigation of Unsteady Flows and Noise in Rotor-Stator Interaction with Adjustable Lean Vane

The paper is a study of the noise reduction effects of the stator lean angle. Firstly, a numerical study has been carried out to investigate the unsteady characteristics of the flow induced by the rotor-stator interaction. To obtain the rotor‟s wake, the distribution of the RMS pressure and the pressure disturbance coefficient, the compressor with an adjustable stator lean angle has been numerically studied. Then the compressor was experimentally tested at 11 different lean angles to obtain the free field noise spectra. Both the unsteady flow property and noise characteristics are discussed on the basis of the Computational Fluid Dynamics (CFD) results. The results indicate that (1) the pressure fluctuation of stator is the main source of the interaction noise; (2) the phase of rotor‟s wake can be tuned by the lean angle of the stator; (3) the distribution of the first harmonic of tone noise decides the distribution of total Sound Pressure level (SPL); and (4) stator with positive lean angle has better noise reduction than negative one and the lean angle should exceed 10° for better noise reduction.