Velocity Pulsations Research Articles

Experimental research of incipient turbulent flow frequency spectra in hydrodynamic unsteadiness

Hydraulic and heat transfer processes play a very important role in the design and prototyping of aerospace technology. Unsteady conditions are the peculiarity of mostly aerospace systems. Flow acceleration and deceleration may significantly affect the heat transfer and hydrodynamic process in channels of aerospace systems. For unsteady process modeling, a fundamental research of unsteady hydrodynamic turbulent flow structure., Moscow Aviation Institute National Research University (MAI) has been building unsteady turbulent flow structures since 1989. An experimental facility was designed to provide gas flow acceleration and deceleration. Experimental data of a turbulent gas flow structure during flow acceleration and flow deceleration are presented. The frequency spectra of axial and radial velocity pulsations are based on experimental data. The results of experimental turbulent flow research demonstrate the fundamental hydrodynamic unsteadiness influence on the flow structure. The main results of the flow acceleration and deceleration experimental research show that there are tangible differences from the steady flow structure. The analysis of unsteady conditions influence on the turbulent pulsations generation and development mechanisms is presented. The results show the unsteady conditions influence onto turbulent vortexes disintegration tempo. The present paper describes a method of experimental research, methodology of data processing and turbulent accelerated and decelerated flow spectra results.

Inverted conical methane/air flame shape transformation under acoustic excitation

An inverted conical plane-symmetrical premixed methane–air flame under acoustic excitation was investigated experimentally. The flame was a conical one stabilized by a thin transverse rod. The Reynolds numbers were varied within 400–3000, and the fuel equivalence ratios were 0.8–1.4. Such a flame can take the shape of two fundamentally different types: M—when the flame front attaches both to the nozzle edge and stabilization rod; V—only to the stabilization rod. The transition conditions from one front configuration to another (M–V and V–M) were studied under various excitation parameters. Acoustic disturbances with a frequency of up to 420 Hz and an amplitude of longitudinal velocity pulsations up to 10% of the mean flow velocity excited the flame. New experimental data were obtained on the conditions corresponding to the M–V and V–M transitions. Qualitative and quantitative parameters of the hysteresis effect at which M–V and V–M transitions are realized, similar to the case of swirling flames, were revealed. The frequencies at which the acoustics most strongly affects the M–V and V–M transitions conditions were found. The hysteresis deterioration for M–V and V–M transitions was shown for the first time. It was noted that the conditions for the hysteresis deterioration are close to the theoretical frequency of the descent of vortices from the nozzle edge. The local maxima of the characteristic Strouhal numbers corresponding to the features of the minimum divergence of the flow velocities at the M–V and V–M transitions were determined.

Effect of Sinusoidally Varying Flow of Yield Stress Fluid on Heat Transfer From a Cylinder

Abstract The effect of pulsating laminar flow of a Bingham plastic fluid on heat transfer from a constant temperature cylinder is studied numerically over wide ranges of conditions as Reynolds number (0.1 ≤ Re ≤ 40) and Bingham number (0.01 ≤ Bn ≤ 50) based on the mean velocity, Prandtl number (10 ≤ Pr ≤ 100), pulsation frequency (0 ≤ ω* ≤ π), and amplitude (0 ≤ A ≤ 0.8). Results are visualized in terms of instantaneous streamlines, isotherms, and apparent yield surfaces at different instants of time during a pulsation cycle. The overall behavior is discussed in terms of the instantaneous and time-averaged values of the drag coefficient and Nusselt number. The size of the yielded zone is nearly in phase with the pulsating velocity, whereas the phase shift has been observed in both drag coefficient and Nusselt number. The maximum augmentation (∼30%) in Nusselt number occurs at Bn = 1, Re = 40, Pr = 100, ω* = π, and A = 0.8 with respect to that for uniform flow. However, the increasing yield stress tends to suppress the potential for heat transfer enhancement. Conversely, this technique of process intensification is best suited for Newtonian fluids in the limit of Bn → 0. Finally, a simple expression consolidates the numerical values of the time-averaged Nusselt number as a function of the pertinent dimensionless parameters, which is consistent with the widely accepted scaling of the Nusselt number with ∼Pe1/3 under these conditions.

ГИДРОДИНАМИЧЕСКИЕ ОСОБЕННОСТИ ТЕЧЕНИЯ СУСПЕНЗИЙ И ЭМУЛЬСИЙ

This scientific article is devoted to the problems associated with the flow of suspensions and emulsions and some simplifications of the real picture of the flow of a polydisperse medium are made. It is also stipulated that differential equations characterizing the motion of suspensions and emulsions should take into account the fundamental discontinuity of the medium and the physicochemical processes of heat and mass transfer occurring in it. Taking into account all these factors, a general equation for multiphase systems is proposed with certain simplifications that do not change. The behavior of particles in two-phase systems, their concentration, collision and coagulation are considered. As a result, it was concluded that there is a multifactorial interaction and mutual influence of both phases in a dispersed flow. A differential equation of motion of a single i-th spherical particle in suspension was proposed, and an equation describing the drag force of a solid spherical particles. Equations of conservation of mass and momentum are presented for one-dimensional laminar motion of two incompressible phases in a gravity field with the same pressure in the phases. Having studied the parameters of the flow of fine particles in a turbulent gas flow, some assumptions were made. It was found that the pulsating motion of particles, performed by them during one period of gas pulsations, can be represented as a change in the pulsating gas velocity in time. The parameter of entrainment of particles by a pulsating medium is an important characteristic in determining the transport coefficients in a turbulent flow. It is concluded that the presence of various kinds of particles in the liquid complicates the problem of solving hydromechanical problems in turbulent and laminar flow, and the assumptions given in the work facilitate the study of this problem.

Numerical Study on the Unsteady Film Cooling Characteristics of Turbine

In view of the main flow pulsation phenomenon and the new pulsating film cooling mode in the aeroturbine engine, the numerical calculation method is used to study the film cooling characteristics under the condition of the main flow velocity pulsation and cooling gas velocity pulsation with the classical circular film hole as the research object. In order to eliminate the influence of other factors on the film cooling characteristics of turbine blades as much as possible, a circular film hole of flat plate model similar to the small curvature section of turbine blades was established, and the influence of different cooling air pulse frequency, blowing ratio and mainstream pulse frequency on the film cooling characteristics was analyzed in depth. The results show that: with the increase of cooling air pulse frequency, the film cooling efficiency of cooling gas pulsation scheme can be achieved approximately the steadystate condition with blowing ratio of 1.0 while only half of the cooling air flow is consumed; at high blowing ratio, the cooling effect will be better when using the cooling air pulsating film cooling scheme compared with the steady-state scheme; in the main pulsating frequency range of the study, the frequency of main flow pulsation has little effect on film cooling efficiency.

Aeroacoustic Sound Source Characterization of the Human Voice Production-Perturbed Convective Wave Equation

The flow-induced sound sources of human voice production are investigated based on a validated voice model. This analysis is performed using a hybrid aeroacoustic workflow based on the perturbed convective wave equation. In the first step, the validated 3D incompressible turbulent flow simulation is computed by the finite volume method using STARCCM+. In a second step, the aeroacoustic sources are evaluated and studied in detail. The formulation of the sound sources is compared to the simplification (neglecting the convective sources) systematically using time-domain and Fourier-space analysis. Additionally, the wave equation is solved with the finite element solver openCFS to obtain the 3D sound field in the acoustic far-field. During the detailed effect analysis, the far-field sound spectra are compared quantitatively, and the flow-induced sound sources are visualized within the larynx. In this contribution, it is shown that the convective part of the sources dominates locally near the vocal folds (VFs) while the local time derivative of the incompressible pressure is distributed in the whole supra-glottal area. Although the maximum amplitude of the time derivative is lower, the integral contribution dominates the sound spectrum. As a by-product of the detailed perturbed convective wave equation source study, we show that the convective source term can be neglected since it only reduces the validation error by 0.6%. Neglecting the convective part reduces the algorithmic complexity of the aeroacoustic source computation of the perturbed convective wave equation and the stored flow data. From the source visualization, we learned how the VF motion transforms into specific characteristics of the aeroacoustic sources. We found that if the VFs are fully closing, the aeroacoustic source terms yield the highest dynamical range. If the VFs are not fully closing, VFs motion does not provide as much source energy to the flow-induced sound sources as in the case of a healthy voice. As a consequence of not fully closing VFs, the cyclic pulsating velocity jet is not cut off entirely and therefore turbulent structures are permanently present inside the supraglottal region. These turbulent structures increase the broadband component of the voice signal, which supports research results of previous studies regarding glottis closure and insufficient voice production.

Modernizatsiya izmeritel'nogo kompleksa «Sigma-1» dlya issledovaniya tsirkulyatsiy Lengmyura i obrusheniy poverkhnostnykh voln v Chernom more

The purpose of this article is to describe modernization of SIGMA-1 measuring complex. The modernization was performed to improve the quality of research of the wave breaking mechanisms and Langmuir circulations at the stationary oceanographic platform MHI. The article deals with the effects of natural oscillations of the device and changes in the housing position on the measured components of the flow velocity vector. The device natural oscillations are recorded by a position monitoring system consisting of a set of sensors: accelerometers and a magnetometer. A model study of permissible rotations and maximum tilt angles showed that in the absence of an angular velocity sensor, the uncorrected measured components of the flow velocity vector can differ significantly from the actual ones. To solve this problem, MPU-9250 module with an accelerometer, gyroscope and magnetometer was chosen; Arduino Nano was used as a microcontroller system. Based on the theory of calculating spatial angles, a program for correcting the measured components of velocity pulsations for the new module was developed in Arduino IDE programming environment. The optimal program (Magneto) for calibrating the magnetometer was selected as the most accurate and satisfying the conditions of use at the oceanographic platform. The main stages of sensor calibration are described. The flow velocity components measured in the device coordinate system are adjusted for the values of tilt angles, angular and linear velocities, the calculation is performed using the eddy correlation method. The developed system has been tested on specialized equipment in laboratory setting and has the following characteristics: within the range –30…+30° the maximum pitch error is 0.31°; the maximum roll error is 0.42°; the maximum magnetometer error is 2.09°. The achieved characteristics allow significant increase in the accuracy of measuring the velocity vector and reliability estimate of influence of various physical processes on the vertical exchange.

Measurement Method for Fluid Pressure Fluctuation in Fluid-Conveying Pipe Using PVDF Piezoelectric Film Sensor

As a representative fluid-transporting system, fluid-conveying pipes play an essential role in many fields. For a fluid-conveying pipe system in operation, fluid pulsation in pipes contains much information about fluid flow parameters (flow velocity, fluid pressure, etc.). Therefore, the measurement of fluid pulsation is important for understanding the internal fluid flow. To use polyvinylidene fluoride (PVDF) piezoelectric film sensors to indirectly measure the pressure fluctuation of the internal fluid, we simulated a fluid-conveying pipe with PVDF piezoelectric film sensors attached to the outer pipe wall. The simulation results showed that the variation of voltage signal of PVDF, circumferential stress and strain of the pipe wall, and the pressure fluctuation of internal fluid were highly positively correlated, which proved that the PVDF piezoelectric film sensor can be applied to indirectly measure the pressure fluctuation of internal fluid. We also studied the influences of flow velocity pulsation and mechanical vibration caused by the pipeline pump during operation. It is found that the flow velocity pulsation had little influence on the measurements of the variation of circumferential stress and strain of the pipe wall and the internal fluid pressure fluctuation. When both ends of the pipe were fixed by hoops, mechanical vibration had little influence on the measurement of the variation of circumferential stress and strain of the pipe wall as well as the fluid pressure fluctuation. Finally, simulation results were verified by experiments.

MHD grid-turbulence in a closed decaying flow of liquid sodium

Grid-induced turbulent flows of liquid sodium in a toroidal channel at high Reynolds numbers and moderate magnetic Reynolds numbers are studied experimentally. Grids with a pitch of 15 mm and 31 mm were used, and one or two grids were installed in the channel. It is shown that the dynamics of the pulsed turbulent flow in the channel with grids differs significantly from that observed in a free channel, where the turbulence develops on the scale of the channel cross-section. Measurements of the characteristics of velocity and magnetic field pulsations have shown that induction effects in the unsteady pulse flow have time to manifest themselves in full. The spectra of velocity and magnetic field pulsations are obtained.

Research on Unsteady Characteristics of Pump Turbine in Pumping Mode

A study on the unsteady characteristics of pump turbine in pumping mode was made. Basing on the existing research, this paper is aimed at providing valuable reference for hydraulic design, structure optimization, fault diagnosis of pump turbine. CFX simulation verified by experiment was made to analyze the distribution law and propagation characteristic of pressure and its pulsation in the whole fluid field. Research shows that the pressure pulsation owns different propagation characteristic from the flow velocity pulsation. The pressure pulsation component with guide blade frequency has very special properties in stationary flow passages, while the pressure pulsation component with runner blade passing frequency is absolutely different. Blade constraints have significant effect on the unsteady properties of pressure and flow velocity. Besides, some other analysis was talked about, such as the external characteristics, pressure pulsation near the volute tongue, and so on.

VARIOUS APPROXIMATIONS OF VERTICAL TURBULENT EXCHANGE PARAMETERIZATION FOR THE ANALYSIS OF THE WAVES HYDRODYNAMIC IMPACT ON THE BOTTOM OF THE RESERVOIR

The article discusses the possibilities of using various types of approximations for parametrization of vertical turbulent exchange for calculating and evaluating the hydrophysical characteristics of the wave regime in the accumulative coastal zone of the southwestern corner of the Tsimlyansk reservoir. It is impossible to carry out these studies without using various types and classes of approximations for parametrization of vertical turbulent mixing. Algebraic models for calculating the coefficient of vertical turbulent exchange and semi-empirical turbulence models are compared. Using ADCP data on velocity pulsations for several stations to measure hydrological characteristics, the results of parameterization of the vertical turbulent exchange coefficient were analyzed. The developed numerical algorithms and the software package implementing them are used to study the pressure field, the velocity vector field of the aquatic environment and the prediction of the baric field for this section of the reservoir water area.

The influence of the structure of laminar flows on the characteristics of equipment

The new method of visual diagnostics of liquid motion processes in physical models showed a high degree of the flow structure organization. Visual pictures made it possible to develop a hydraulic experiment to reveal the dimensions of the transverse structure in the form of layers and zones of flow separation from the channel walls. Visual diagnostics is the basis for comprehensive equipment design. Visual studies of the flow structure provide information for improving equipment by changing the geometry of the flow paths. Hydraulic studies show the change in the resistance of the equipment channels. Based on the results of visual and hydraulic studies, the wave character of the distribution of the pulsation velocity components was revealed. The regularities of the velocity distribution allow predicting the minimum or maximum values of the resistances of the flow paths of the equipment.

Study on the Influence of Temperature on the Temporal and Spatial Distribution Characteristics of Natural Cavitating Flow around a Vehicle

Cavitation involves complex multiphase turbulence and has important research significance. In this study, the Schnerr–Sauer cavitation model was used to model cavitation, and the detached-eddy simulation (DES) method was used to calculate the unsteady natural cavitating flow. The predicted results are in good agreement with experimentally measured cavity evolution and pressure values, demonstrating the effectiveness of this numerical method. Low temperature causes changes in the properties of water. The density of water at 0° is 999.84 kg/m3 and the density of water at 25° is 997.04. Cavitation evolution and shedding are analyzed at temperatures of 0 °C and 25 °C. The results showed that lower temperature increased the frequency of cavitation and enhanced pressure pulsation. At the same time, low temperature also increases the frequency of cavity shedding and shortens the cycle. In addition, based on the Ω method, the difference between vortex dynamics at various temperatures was studied, and it was found that different cavity stages showed different vortex structure characteristics, and lower temperature would aggravate the change of wake vortex structure. At the same time, the analysis of the turbulence characteristics in the downstream of the cavity shows that the lower temperature reduces the velocity pulsation and reduces the turbulence integral scale. At the end of the model, large-scale pulsations are transformed into small-scale pulsations.

Solution of the multi-criteria problem of optimization of a gear pump by the method of research of the space of parameters

Introduction. Gear pumps are one of the main sources of hydraulic energy for hydraulic fluid power of a number of machines for national and special applications. Improving the characteristics of the gear pump of the external gearing is based on a search for the gearing parameters of the gear pump in terms of reducing the overall size while maintaining the value of the working volume set by the customer, which is the subject of the formulation and solution of multicriteria optimization problemsProblem. Improving the characteristics of the gear pump of the external gearing is based on the search for the parameters of the gearing of the gear pump in terms of reducing the overall size while maintaining the value of the working volume set by the customer. In the studies of a number of authors, the problems of reducing velocity and pressure pulsations, optimizing gearing parameters, and determining the geometry of gear pump cavities are separately solved. However, a multicriteria approach to gear pump is absent, which narrows the range of positive results obtained by studying the quality of the developed design.Method. A method for studying the parameter space for a gear pump is proposed. This method makes it possible to work with models of various nature due to the absence of any restrictions imposed on functions, including quality functions (smoothness, continuity), the designer is not required to rank criteria, assign weighting factors, limits and restrictions, it is possible obtaining information about the behavior of the criteria and restrictions in the parameter space under study with the minimum calculation time due to the simplicity of data analysis.The practical results. The mathematical model of the gear pump is improved by introducing the equations of the radial and normal lateral gaps, which increased its adequacy to the full-scale sample. As a result of the multicriteria optimization, the overall dimensions of the pump were reduced by 7% while ensuring a given value of the working volume.

Dynamic processes modeling in a peristaltic concrete pump with a hydraulic drive

Problem. Cast concrete occupies a leading place in modern construction. Peristaltic concrete pumps are good proved in this area. Developing of devices for performance of such activities is an actual task. One of contemporary efficient directions in its solving is an adequate modeling of dynamic processes in mentioned devices. Currently efforts of researches are focused on resolving of separate tasks connected with developing of new models of peristaltic pumps.Goal. The creation of a universal mathematical model for dynamic processes in a peristaltic concrete pump with hydraulic drive to determine the technological device qualities and the choice of its rational parameters. The model should contain the basic geometric, mass characteristics of the rotor, the dynamic characteristic of a hydraulic motor, the parameters of the hose and building mixture. The following tasks are solved: development of the method for generating a moment of resistance on a pump rotor from rollers, that deforms a hose; creation of a model for friction forces, which prevent the movement of the building mixture; obtaining an expression for resistance forces, which exist due to the gravity forces of the mixture particles in the deferent part of the hose; building of the model of the torque for the hydraulic motor as a function of the angular velocity of its rotor using catalog data. Methodology. For the research of rotation dynamic processes it is proposed the usage of differential equation of motion written in the Lagrange form. The angle of rotation of the pump rotor is taken as a generalized coordinate. Results. The mathematical model of dynamic processes in a peristaltic concrete pump with hydraulic motor are created in the form of a differential equation for the angle of rotation of the rotor. The method for generating of the resistance moment to the rotation of the pump rotor from rollers, that deform the hose, is developed. The model for resistance forces to the movement of a building mixture is proposed. The expression for resistance forces, which exists due to the gravity forces of the mixture particles in the deferent part of the hose, is obtained. The approach for usage of catalog data to represent the torque of a hydraulic motor as a function of the angular velocity of its rotor is proposed. Originality. Important technological regularities of the device functioning were established: the speed of the mixture motion can have a significant variable component; the speed of the mixture motion and pump’s performance increase with a decrease in the length of the discharge hose and a decrease in its height, a decrease in the rolling friction coefficients for the rollers and a rejection of side rollers. Practical value. The conducted research show that an increase of an altitude leads to a decrease in the average speed of the mixture and the frequency of pulsations. In this case the amplitude of velocity pulsations increases. With an increase in the length of a deferent hose, the amplitude of velocity pulsations changes insignificantly. The rejection of side rollers leads to an increase in the average speed of the mixture motion with minor pulsations. The walls of the hose, however, in this case experience more intense bending stresses, which reduce the product life of the hose.

Investigation of transfer processes in swirling flows in application to vortex furnaces for coal fuel

In this work, the swirling flow structure was studied experimentally and numerically in a wide range of operating parameters at a laboratory model of the promising furnace of pulverized-coal boiler with a four-vortex combustion circuit. The basic aerodynamic features of construction, affecting the efficiency of fuel combustion, were investigated by the example of air under the atmospheric pressure (Re ~ 104) for the isothermal case. The data on distribution of averaged and pulsation velocity characteristics were experimentally obtained in various areas of the model using the non-contact PIV method (Particle Image Velocimetry). The results of measurements carried out in a wide range of initial velocities allow distinguishing three flow regimes (β is the ratio of flow velocities at the outlet of the side and frontal nozzles): the flow regime with regular structure (four symmetric vortices, β ≥ 1.8), the transitional regime (1.8>β > 0.9) and regimes with irregular vortex structure (β ≤ 0.9). It is shown that formation of irregular structures is associated with uneven organization of air supply to the model. In regimes with β < 1.8, the flow becomes sensitive to asymmetry in the input conditions. At the same time, at high β ≥ 1.8, a stable four-vortex flow structure is formed, even if the input conditions are asymmetric within 10% (by velocity). In the transitional regime, when β decreases from 1.8 to 0.9, the flow begins distorting, and this is presented by the flow asymmetry. With a further decrease in β ≤ 0.9, the development of asymmetry is accompanied by the breakdown of large vortices into smaller vortices. A stable four-vortex flow structure at β ≥ 1.8 is important for practical applications, since in this regime, effective “washing” of the heat-exchange surfaces of the boiler will be ensured, thus avoiding their slagging. The results of laboratory measurements obtained in the range of flow regimes, self-similar by the Reynolds number (104<Re < 106), allowed us to verify the mathematical model. Numerical calculations of aerodynamics were performed using the AnsysFluent CFD package. All considered turbulence models (URANS k-w SST, URANS RSM, DES) reproduce well the averaged flow structure in the combustion chamber. The results of numerical calculations are in good agreement with experimental data. The verified mathematical model and calculation method allow us a future transition to simulate the combustion of coal fuel, taking into account all the main processes of heat and mass transfer in the furnace.

Selectivity of plasma actuators in a boundary layer transition control applications

The paper describes the results of the study of broadband velocity pulsations, induced by dielectric barrier discharge plasma actuator in the 2D subsonic boundary layer, and their role in a boundary layer excitation. The origin of the velocity pulsations is the stochastic dynamics of microdischarges. The presented data include the parametric study of the disturbances power on the supply voltage characteristics and their structure. Also, the preliminary study of the role of the broadband disturbances in the Tollmien-Schlichting wave excitation was performed.

The effect of periodic velocity variation on turbulent characteristics of a channel flow behind a rib

The kinematic structure of the flow behind a rib is experimentally investigated for the cases of steady-state and pulsating regimes of air flow. The frequency varies in the range of 0 – 30 Hz, and the normalized amplitude of forced velocity pulsations is 0.5. The profiles of velocity and turbulent characteristics are demonstrated at characteristic coordinates of the separation region.

Unsteady flow fluctuations in a centrifugal pump measured by laser Doppler anemometry and pressure pulsation

Turbulent flow generated by the intense rotor–stator interaction is detrimental to the safe running of the centrifugal pump. In order to gain more insight into unsteady velocity pulsation characteristics, this research applies the laser Doppler anemometry technique to capture velocity signals at various flow rates. Besides, pressure transducers are arranged on the model pump to sample transient pressure pulsation signals. The study paid particular attention to pulsation signals in the diffusion section of the volute. Results show that at low flow rates, a prominent hump phenomenon generated within the pump head curve, which indicates the development of rotating stall in the model pump. As noted from the spectra, the discrete blade passing frequency and impeller rotating frequency dominate the velocity and pressure spectra. Root mean square values of velocity signals increase rapidly at off-design flow rates, especially within the rotating stall region, and a similar trend is observed for pressure amplitude at the blade passing frequency. From the measuring point (P3) at the inlet of the discharge channel upstream of the volute tongue to the point (P10) at the volute outlet, pressure amplitude rises significantly. Meanwhile, the minimum point of pressure amplitude vs flow rate occurs around 0.4ΦN, and again, the findings show differences for comparisons between measuring points in the core flow region of the volute. This resulting phenomenon can be attributed to flow patterns in the diffusion section.

Experimental study of a flow structure in coplanar channels

The results of an experimental three-dimensional flow structure study in flat coplanar channels with mutually intersecting ribs on opposite walls are presented. The measurements were carried out for different angle values between ribs (2β = 60°, 90°, 120°) and Reynolds number variations. In the experiments, a two-component laser Doppler speed meter was used, which makes it possible to measure both averaged and pulsating velocity components. It is shown that a flow structure in vortex cells between ribs varies greatly from a cell location and an angle between ribs. The magnitude of hydraulic losses in coplanar channels significantly exceeds losses in smooth channels, and they increase with increasing angle between ribs.