Flow Velocity Fluctuations Research Articles

Experimental Investigation Of Shock Induced Panel Flutter With Simultaneous Use Of DIC And PIV

In this experimental study, both classical and shock induced supersonic panel flutter are investigated at a freestream Mach number of 2, using a combination of planar particle image velocimetry (PIV) and stereographic digital image correlation (DIC) to obtain simultaneous full-field structural displacement and flow velocity measurements. Highspeed cameras are employed to obtain a time-resolved description of the panel motion and of the flow dynamics. In order to prevent interference between the PIV and DIC systems, an optical isolation is implemented using fluorescent paint, dedicated light sources, and camera lens filters. For the shock induced case, the effect of the panel motion on the SWBLI behavior is assessed, by comparing it with the SWBLI on a rigid wall. The results show that panel oscillations occur with an amplitude of ten times the panel thickness. The dominant frequencies observed in the panel oscillation (424 Hz and 1354 Hz) match the main spectral content of the reflected shockwave position. In absence of shock impingement, the oscillation amplitude is on the order of the panel thickness, while the dominant frequency contribution is at 730 Hz. The fluid-structure coupling is studied by identifying the flow regions of maximum correlation between the panel displacement and the flow velocity fluctuations. In presence of shock impingement the results obtained proved that the inviscid flow region upstream of the SWBLI is perfectly in phase with the panel oscillation, while the downstream region has a delay of one quarter of the flutter cycle. Instead, no delay is observed for the configuration in absence of shockwave impingement.

Numerical Simulation of Molten Pool Dynamics in Laser Deep Penetration Welding of Aluminum Alloys

In this paper, the numerical simulation of molten pool dynamics in laser deep penetration welding of aluminum alloys was established based on the FLUENT 19.0 software. The three-dimensional transient behavior of the keyhole and the flow field of molten pool at different welding speeds were analyzed, and the influence of the welding speed on the molten pool of aluminum alloys in laser welding was obtained. The results indicated that the generation of welding spatters was directly related to the fluctuation of the diameter size in the middle of the keyhole. When the diameter in the middle of the keyhole increased by a certain extent, welding spatters occurred. When welding spatters occurred, the diameter in the middle of the keyhole became smaller. In addition, the size of the spatters at the welding speed of 9 m/min was larger than that of the spatters at the welding speeds of 3 m/min and 6 m/min. The welding spatter formed in laser deep penetration welding included: spatter created by an inclined liquid column behind the keyhole; splash created by a vertical liquid column behind the keyhole; small particles splashed in front of the keyhole. With the increase of the welding speed, the tendency of the welding spatter to form in front of the keyhole and to form a vertical liquid column behind the keyhole became weaker. When the welding speed was 9 min, only an obliquely upward liquid column appeared on the molten pool surface behind the keyhole. Compared with the welding speeds of 6 m/min and 9 m/min, the maximum flow velocity fluctuation of the molten pool at the welding speed of 3 m/min was obviously higher.

Flow over a confined mounted fence at low and moderate Reynolds number: A numerical study

Direct numerical simulations of a flow over a wall mounted fence confined by two parallel walls were conducted at low and transitional Reynolds number (50⩽ReD⩽2000) for blockage ratios ranging from 0.25 (low blockage) to 0.9 (high blockage). A primary recirculation bubble forms downstream of the fence and the length of this recirculation region increased with Reynolds numbers. A secondary recirculation bubble appeared on the opposite wall to the mounted fence for the laminar cases with higher blockage ratios at sufficiently large Reynolds number. The separation and reattachment locations were documented and compared with numerical and experimental data. The length of the primary recirculation bubble in current simulations are consistent with the literature while the comparison of the secondary recirculation bubble with experimental data had limited agreement. This secondary recirculation bubble disappeared for the cases at transitional Reynolds number. Instead, the strong primary recirculation bubble induced a corner eddy at the rear of the fence. ‘Packets’ of cross stream velocity fluctuations were observed as the initially 2D, steady, laminar shear layer breakdowns to a 3D turbulent flow. The space–time diagram also showed ripple-like fluctuations of the shear layer further downstream of the fence with a single distinct peak observed in the streamwise and cross flow velocity fluctuations spectra. This peak in the spectra can be predicted by linear stability analysis of a mixing layer (Yasuo et al., 1986). In the near wake, scale-like features were observed while patches of fluctuations of a longer timescale were noted in the recirculation region.

Study on the influence of flow on parametric acoustic array modal response in surface fluctuation waveguides

This paper analyzes the parametric sound field in a waveguide and studies the influences of the flow velocity and surface fluctuation on the order of parametric difference-frequency sound selective excitation. The Green’s function solution for the difference-frequency wave describing the propagation of difference-frequency sound in a low Mach number and surface fluctuation waveguides is given. The difference-frequency sound field in the waveguide by discussing the effect of flow on the virtual source of the parametric array in the pump wave’s difference grazing angle. It is shown that the waveguide surface wave will affect the horizontal wavenumber of the difference-frequency sound field of the parametric array, causing the difference-frequency sound field’s amplitude and phase to fluctuate. Simultaneously, the flow velocity in the waveguide will affect the relationship between the order of modal selective excitation and the pump wave’s grazing angle when there is a flow in the waveguide, resulting in the change of the modal order and the intensity of the modal excitation of the difference-frequency sound field. It has an excellent guide on the long-distance propagation of the parametric difference-frequency sound field in the waveguide.

Experimental investigation of expansion effect on shock wave boundary layer interaction near a compression ramp

The experiment is conducted to investigate the effect of expansion on the shock wave boundary layer interaction near a compression ramp. The small-angle expansion with an angle degree of 5° occurs at different positions in front of the compression ramp. The particle image velocimetry and flow visualization technology show the flow structures, velocity field, and velocity fluctuation near the compression ramp. The mean pressure distribution, pressure fluctuation, and power spectral density are measured by high-frequency response pressure transducers. The experimental results indicate that the expansion before the compression ramp position affects the shock wave boundary layer interaction to induce a large-scale separation. But the velocity fluctuation and pressure fluctuation are attenuated near the large-scale flow separation region. When the expansion occurs closer to the compression ramp, the expansion has a more significant impact on the flow. The fluctuation of velocity and pressure is significantly attenuated, and the wall pressure rise of the separation point is reduced obviously. And the characteristic low-frequency spectrum signal related to the unsteadiness of the shock wave boundary layer interaction is significantly suppressed. In addition, variation of the separation region scale at different compression angle degrees is distinctive with the effect of expansion.

Independent component analysis of streamwise velocity fluctuations in turbulent channel flows

Independent component analysis (ICA) is used to study the multiscale localised modes of streamwise velocity fluctuations in turbulent channel flows. ICA aims to decompose signals into independent modes, which may induce spatially localised objects. The height and size are defined to quantify the spatial position and extension of these ICA modes, respectively. In contrast to spatially extended proper orthogonal decomposition (POD) modes, ICA modes are typically localised in space, and the energy of some modes is distributed across the near-wall region. The sizes of ICA modes are multiscale and are approximately proportional to their heights. ICA modes can also help to reconstruct the statistics of turbulence, particularly the third-order moment of velocity fluctuations, which is related to the strongest Reynolds shear-stress-producing events. The results reported in this paper indicate that the ICA method may connect statistical descriptions and structural descriptions of turbulence.

Cartesian grid method with a consistent direct discretization approach for numerical simulation of non-Newtonian fluid flow

This paper proposes an extended version of the discretization scheme for a consistent direct discretization approach in a Cartesian grid method for flow simulations of incompressible non-Newtonian fluids. Based on the concept of the original method, which was developed for Newtonian fluid flow, the Navier–Stokes and the pressure Poisson equations are discretized directly in the vicinity of a solid boundary while maintaining the consistency between the velocity and pressure fields. The main difficulty in the discretization arises from the evaluation of the velocity gradients on the fluid–solid interface, which is necessary for handling the viscous term for spatially distributed shear-dependent viscosity. In the extended method, a new procedure for the calculation of the velocity gradient using the relation that holds on a rigidly moving solid surface is proposed. The validity of the method is ascertained in fundamental test problems of non-Newtonian fluids: laminar Poiseuille/Couette flows in a concentric annulus and flows around a circular cylinder. It is found that a second-order spatial accuracy with the desired numerical stability is achieved in both the shear-thinning and shear-thickening viscous properties, and the numerical error is considerably decreased compared with some conventional methods. As a realistic application of the extended method, a polymer mixing simulation inside a co-rotating twin-screw extruder is demonstrated by applying the method to the moving boundary problem. In a screw arrangement with a sequential combination of a kneading disk (KD), a cylindrical ring (CR), and a backward mixing screw (BMS), large- and small-scale structures of bifurcating and converging flows are respectively formed in the KD and BMS zones, whereas elongational flow structures and velocity fluctuations are still maintained in the CR zone due to flow interaction with the upstream and downstream screw elements, which presumably contributes to moderate mixing at low shear stress. • A Cartesian grid method for non-Newtonian fluid flow is developed. • The present method extends a consistent direct discretization approach. • A procedure for evaluating velocity gradients on an immersed boundary is proposed. • Second order spatial accuracies for velocity and its derivatives are ascertained. • A polymer mixing simulation in a co-rotating twin-screw extruder is demonstrated.

Regression model for predicting the speed of wind flows for energy needs based on fuzzy logic

Renewable energy integration becomes an important criterion for the sustainable generation of electrical power. The high introduction of wind power plants into the power system leads to some inconveniences in the power system operators’ work due to the unpredictable and variable nature of the wind speed and the power generated by wind farms. Even though the power generated at the wind power plants is not regulated by the system operator, the accurate predicting of the wind speed and the angle of its direction could solve such problems leading to improving the reliability of power supply systems.This paper considers the prediction of both the speed and direction of wind flows for the Far East coast. It is proposed to use autoregression based on the fuzzy systems concept. The goal of this study is to find a regression model that satisfies all observable fuzzy data within the specified optimality criterion. Thus, the regression coefficients are fuzzy numbers that can be expressed as interval numbers with membership values. As result, the electric power generated by wind power plants could provide additional energy accumulation function, if the wind flow velocity fluctuations will be considered.

Periventricular- intraventricular hemorrhage in the premature infant- A historical perspective

The objective of this chapter is to trace the evolution of intraventricular hemorrhage in the premature infant highlighting the importance of the germinal matrix, a critical role for cerebral blood flow changes in the genesis of hemorrhage, clinical factors that increase the bleeding risk, and potential preventative strategies. In 1976, neuropathological studies demonstrated capillary rupture within the germinal matrix as the precursor of hemorrhage. In 1980, introduction of cranial ultrasound facilitated diagnosis of intraventricular hemorrhage. In 1979, loss of cerebral autoregulation in sick newborn infants was demonstrated. In the 1980’s, studies demonstrated the importance of intravascular factors in provoking hemorrhage. In 1983, the association of cerebral blood flow velocity fluctuations and subsequent hemorrhage was demonstrated. In 1994, antenatal steroids use to accelerate lung development was recommended. This was associated with an unanticipated reduction in hemorrhage. In the mid 1990’s early indomethacin administration was associated with a reduction of severe hemorrhage.

Revisiting the Reynolds-averaged Navier–Stokes equations

Abstract This study revisits the Reynolds-averaged Navier–Stokes (RANS) equations and finds that the existing literature is erroneous regarding the primary unknowns and the number of independent unknowns in the RANS. The literature claims that the Reynolds stress tensor has six independent unknowns, but in fact the six unknowns can be reduced to three that are functions of the three velocity fluctuation components, because the Reynolds stress tensor is simply an integration of a second-order dyadic tensor of flow velocity fluctuations rather than a general symmetric tensor. This difficult situation is resolved by returning to the time of Reynolds in 1895 and revisiting Reynolds’ averaging formulation of turbulence. The study of turbulence modeling could focus on the velocity fluctuations instead of the Reynolds stress. An advantage of modeling the velocity fluctuations is, from both physical and experimental perspectives, that the velocity fluctuation components are observable whereas the Reynolds stress tensor is not.

Effects of Velocity Fluctuations of Upper Flow on Turbulent Structure of Atmospheric Boundary Layer

The effects of velocity fluctuations of upper flow on turbulent structure of atmospheric boundary layer were experimentally investigated. The atmospheric boundary layer was reproduced in a wind tunnel. In order to produce various velocity fluctuations in the upper flow, three kinds of turbulence grids were set to the entrance of the test section. The streamwise and spanwise velocities were simultaneously measured by using hot-wires anemometer. Turbulence statistics were estimated. The vertical profiles of Reynolds shear stress, Ruw, for M25 and M50 decrease as z/L increase, on the other hands, those for M75 increase. The ratio of the integral length scale, λ(GT)/λ(ABT), for M75 were higher than those for M25, M50 at 0.33≦z/L ≦0.67 in the upper region of the atmospheric boundary layer. The joint probability density functions of u and w for M75 show the strong negative correlation in the upper region of the atmospheric boundary layer. It is suggested that the effects on the turbulent structure of the atmospheric boundary layer depends on the scale of velocity fluctuations of the upper flow.

Representation model of wind velocity fluctuations and saltation transport in aeolian sand flow

Since the stream-wise and vertical turbulent fluctuations have an important influence on the movement of sand and dust particles, the accurate quantitative estimation of natural wind fluctuations has an important role in improving the forecast level of wind erosion and dust storm. The high Reynolds number turbulence characteristics of aeolian sand flow and the complexity of sand-laden flows are the main difficulties affecting the accuracy of prediction. In this study, the characteristics of the stream-wise and vertical wind velocity fluctuations in sand-laden flows are studied by analyzing the real-time data of the wind velocity field in the atmospheric surface layer (ASL). Then, a simple empirical model representing the stream-wise and vertical wind velocity fluctuations is proposed. The model not only considers the effects of large-scale and small-scale fluctuations, but also has the advantage of predicting the stream-wise and vertical wind velocity fluctuations at any height. Verification shows the results predicted by the model are in good agreement with the observation results, and the predicted saltation sand flux is closer to the experimental results than that without considering the turbulence effect or the vertical wind velocity fluctuations. Moreover, it is a convenient model for application especially in predicting aeolian sand transport.

Fluid flow and colloid transport experiment in single-porosity sample; tracking of colloid transport behavior in a saturated micromodel

Release of colloids and their subsequent transport into the subsurface environments takes place during a wide range of applications such as industrial, energy storage, and agricultural activities. Therefore, processes contributing to transport, attachment, and re-mobilization of colloids in porous media are attracting attention. A fraction of the released colloids may cross the soil vadose zone to reach the saturated groundwater. In this study, we explored colloid transport in a micromodel with high repulsion energy barrier where colloid retention is assumed to be low. Three major shortcomings were improved: pore space domain size, imaging resolution, and speed of imaging. The flow path of 1357 colloids with a size of 4 µm were tracked and these enabled precise determination of individual colloid transport mechanism as well as the integrated behavior of the system.Our direct observations have shown that even under unfavorable attachment conditions (defined based on the DLVO theory) colloids deposition occurred which was mainly due to the local flow velocity fluctuations and grain surface heterogeneity. Using the information from collective trajectories, we have quantified the contribution of differently behaved colloids in the observed breakthrough curve which show an integrated, macroscopic, behavior of the system and is often the only available information when performing column or field scale experiments to explore colloid transport in porous media. Furthermore, we have shown that attachment and remobilization of colloids increased the dispersion coefficient, and consequently the dispersivity value of the media.

Analysis of local hydrodynamic characteristics of wading buildings under tidal action

The tidal bore is a kind of nonlinear strong discontinuous flow that has a great impact on the wading buildings. During the tidal wave propagation, the local water flow structure of the wading buildings changes significantly. In order to analyze the evolution characteristics of local hydrodynamics of wading buildings under tidal action, a small-scale mathematical model of tidal current is established, and the local hydrodynamic changes of different forms of spur dikes and piers are studied. The results show that the flow velocity fluctuations in front of the non-submerged shoal dam and the dam head are more fluctuating than the submerged spur dikes, and the influence of the tidal dam on the submerged shoal dam area is more severe; the square piers under the tidal wave The turbulent energy and backwater recirculation zones are larger than circular and streamlined piers.

An investigation of particles effects on wall-normal velocity fluctuations in sand-laden atmospheric surface layer flows

Based on the high-quality observational data in the Qingtu Lake Observation Array (QLOA), the difference in the energy distribution, the scale of the coherent structures, and the amplitude modulation effect of the wall-normal velocity fluctuations between particle-free and particle-laden flow in the atmospheric surface layer are analyzed. The results show that the presence of particles enhanced the wall-normal turbulence intensity, especially the increase at the top of the logarithmic region is more significant though the particle mass loading decreases with the wall-normal distance. A further insight indicates that the increase in the length scale of the wall-normal fluctuating velocity coherent structure by particles is more significant further from the wall, which is supported by the premultiplied energy spectra and the two-point correlation. This leads to a drastic increase in kinetic energy of the large-scale coherent structures by the particle away from the wall and thus results in increased amplitude modulation effects of large-scale wall-normal velocity fluctuations onto small-scales.

Experimental investigation of lab scale solar powered Electrodialysis system with corrugated membrane configuration

An experimental laboratory-scale setup of a solar-powered electrodialysis (ED) water desalination system is investigated in this study. The introduced ED system maintains a coupled configuration of corrugated membranes for the first time. The corrugated membrane configuration allows for increasing saline water's flow velocity fluctuations, especially near the membrane surfaces. As a result, the flow turbulence and mixing at the membrane surfaces are promoted, and therefore, higher rates of ion exchange can be attained. The acquired concentration of dilute water was measured at the steady-state operation over a diversity of system parameters: input voltage (4–12) volts, flow rates (5–22 mL/s), and feed concentrations (15–35 g/L). The optimal current efficiency (CE) value was obtained at 70% with a flow rate of around 15mL/s and feed concentration of around 30g/L. High CE percentage values were obtained (60–70%) within the ED system, which indicates that the process of ions transfer through the exchange membranes is effective even if higher feeding flow and concentrations are applied. Regarding the obtained salt removal (SR) percentage values, the present ED model showed a practical operation scenario where an optimal salt removal value of 35% was achieved within 15 minutes. The findings in this study concluded that the present ED system is superior in desalinating saline water when compared to other ED systems in the literature. The energy demands required to power the current ED system are fully supplied by a photovoltaic solar panel. A larger scale of the current ED setup could be useful in the regions that suffer from the lack of freshwater while potential access to renewable energy sources is available, especially in off-grid areas.

Formation mechanism of high-frequency combustion oscillations in a model rocket engine combustor

We study the formation mechanism of high-frequency combustion oscillations in a model rocket combustor from the viewpoints of symbolic dynamics and complex networks. The flow velocity fluctuations in the fuel injector generated by the pressure fluctuations in the combustor give rise to the periodic ignition of the unburnt fuel/oxidizer mixture, resulting in a significant change in the heat release rate fluctuations in the combustor. The heat release rate fluctuations drive the pressure fluctuations in the combustor before a transition state, while the pressure fluctuations in the combustor gradually begin to significantly affect the heat release rate fluctuations during the transition to combustion oscillations. The directional feedback process during the transition and subsequent combustion oscillations is identified by the directionality index of the symbolic transfer entropy. The thermoacoustic power network enables us to understand the physical mechanism behind the transition and subsequent combustion oscillations.

Laboratory realization of an asymptotic wall flow

Abstract

On the capability of the curvilinear immersed boundary method in predicting near-wall turbulence of turbulent channel flows

The immersed boundary method has been widely used for simulating flows over complex geometries. However, its accuracy in predicting the statistics of near-wall turbulence has not been fully tested. In this work, we evaluate the capability of the curvilinear immersed boundary (CURVIB) method in predicting near-wall velocity and pressure fluctuations in turbulent channel flows. Simulation results show that quantities including the time-averaged streamwise velocity, the rms (root-mean-square) of velocity fluctuations, the rms of vorticity fluctuations, the shear stresses, and the correlation coefficients of u′ and v′ computed from the CURVIB simulations are in good agreement with those from the body-fitted simulations. More importantly, it is found that the time-averaged pressure, the rms and wavenumber-frequency spectra of pressure fluctuations computed using the CURVIB method agree well with the body-fitted results.

The Effect of the Degree of Premixedness on Self-Excited Combustion Instability

Abstract The use of lean, premixed fuel and air mixtures is a common strategy to reduce NOx emissions in gas turbine combustors. However, this strategy causes an increased susceptibility to self-excited instability, which manifests as fluctuations in heat release rate, flow velocity, and combustor acoustics that oscillate in-phase in a feedback loop. This study considers the effect of the level of premixedness on the self-excited instability in a single-nozzle combustor. In this system, the fuel can be injected inside the nozzle to create a partially-premixed mixture or far upstream to create a fully-premixed mixture, varying the level of premixedness of the fuel and air entering the combustor. When global equivalence ratio is held constant, the cases with higher levels of premixing have higher instability amplitudes. High-speed CH* chemiluminescence imaging shows that the flame for these cases is more compact and the distribution of the heat release rate oscillations is more concentrated near the corner of the combustor in the outer recirculation zones. Rayleigh index images, which are a metric for the relative phase of pressure and heat release rate oscillations, suggest that vortex rollup in the corner region is primarily responsible for determining instability characteristics when premixedness is varied. This finding is further supported through analysis of local flame edge dynamics.