Variation Of Mean Velocity Research Articles

Double-reflection symmetry of thermal convection for Rayleigh number up to 1010

The Rayleigh–Bénard convection system exhibits certain known symmetries at low Rayleigh numbers that are broken as the Rayleigh number increases. In this study, we investigate the statistical symmetry of Rayleigh–Bénard convection at moderately high Rayleigh numbers through direct numerical simulations. The simulations are conducted for a fluid confined within two-dimensional walls, with an aspect ratio of unity and a fixed Prandtl number. Although elliptical large-scale circulations break both left-right and top-down reflection symmetries, we observe the emergence of a restored double-reflection symmetry. This symmetry is evident in the velocity and temperature fields, as well as in the variations of mean velocity and temperature profiles along the streamwise direction and the characteristics of the kinetic and thermal boundary layers. For Rayleigh numbers ranging between 107 and 1010, our results demonstrate a remarkable data collapse under this double-reflection transformation.

Mean value and velocity variation of ions in different magnetic field at constant obliqueness


 
 
 
 The variation of mean values of different components of velocities concerning the magnetic field has been numerically investigated using a kinetic trajectory simulation model at a constant angle and electric field. At an angle of 30◦, mean values of different components of velocities at the different magnetic fields have been studied. As the magnetic field changes, the mean values of the three velocity components also change, but as the magnetic field increases from 2.5 mT to 5.5 mT mean value is almost constant.
 
 
 

3D CFD simulation and analysis of transient flow in a water pipeline

Abstract A three-dimensional (3D) computational fluid dynamics (CFD) approach is developed to elaborate the water-hammer pipe flow and 3D detailed dynamic characteristics of a closing ball valve. The proposed CFD approach considers the water compressibility and the viscous sublayer, which are sometimes neglected in previous studies. Comparisons of the CFD results, the measured pressures and the one-dimensional results, demonstrate that the current 3D CFD approach better reproduces the experimental pressure oscillations while helping to visualize the associated physical processes and to further explore the 3D transient characteristics. The mean velocity distributions in the radial direction significantly change as the pipe transient progresses, which is closely associated with transient shear stress. Mean velocity variations at the valve during the closing process undergo three distinct stages: slight change, then drastic reduction, and finally slowing down. Head loss coefficient and discharge coefficient of the valve change as the valve closing time shortens.

Comparative Analysis of Flow Rate Pattern, Velocity of Seminal Plasma, and Spermatozoa in Premature Ejaculation Category for Identification of Infertile Conditions Using MATLAB Software

The aim of this study is to identify the variation in flow rate pattern, maximum and mean velocity of sperm movement between premature ejaculation category, and normospermia conditions using MATLAB software. Materials and methods: Semen samples were collected for normospermia (N=28), premature ejaculation (N=28), and semen analysis report has been done by standard world health organization protocol. Flow rate pattern and velocity was evaluated using MATLAB software for both the groups using standard values and assumed values. Results: Independent sample test was done which reveals that the flow rate (P= 0.193), max velocity (P= 0.374), and mean velocity (P= 0.067) was found to be statistically insignificant for normospermia and premature ejaculation category. Conclusion: Flow rate in normospermia condition (0.09817 m³/ sec), mean velocity (is 0.03125m2/sec), and max velocity (0.0625m3/sec) found to be better in case of normospermia than premature ejaculation category in mathematical modeling. This indicates the flow rate and velocity plays a major role in premature conditions and in the case of erectile dysfunction.

Gravitational circulation in straits and estuaries

A coupled set of partial differential equations and associated boundary conditions is written to describe circulation and salt-flux processes for estuaries in which turbulent mixing results primarily from tidal currents. Similarity solutions, motivated by characteristic salinity distributions observed in estuaries, are obtained for this set of equations and are compared with observational data. The circulation is separated into modes analogous to the barotropic, baroclinic, and Ekman modes of oceanic circulation. The salinity distribution, although coupled to the velocity distribution, is found to vary independently of it as well. The theoretical results are discussed in regard to: (i) correlation between the vertical variations of mean velocity and salinity, (ii) the role of this correlation in maintaining the steady-state salinity distribution in estuaries, and (iii) some implications for computations of flushing and dispersion of contaminants.

Intercontinental predictions of river hydraulic geometry from catchment physical characteristics

Hydraulic geometry is used to describe spatio-temporal variations in wetted width, mean depth and mean velocity with discharge in stream reaches. Models describing downstream (spatial variations) and at-a-station (temporal variations) hydraulic geometry are required for efficient and effective management of flow regimes, sediment and nutrient transport, and physical habitats at catchment to global scales. Data describing observed hydraulic conditions at 1327 stream reaches in France and New Zealand were compiled alongside reach descriptors and upstream catchments extracted from available GIS databases with nationwide coverage. Three contrasting empirical methods were developed to predict hydraulic geometry at unvisited sites. The first method applied basic models in which downstream hydraulic geometry parameters depend on discharge only. The second method applied separate country-specific models whose predictors were selected from the available nationwide GIS data from country. The third method applied an intercontinental model in which all observed hydraulic data were lumped together and predictors were selected from those available across both countries. Flexible regression models that account for interactions were used to parameterize second and third methods. Cross-validation tests showed that downstream parameters were well predicted for independent sites regardless of country. Model performance was higher for width than for depth. At-a-station parameters were less well predicted than downstream parameters, but the models provided satisfactory predictions of observed reach hydraulics. Importantly, the lumped intercontinental model performance was comparable to that of the country-specific models. This suggests that our approach can be applied in other regions where similar explanatory environmental variables are available.

Determination of the factors of variation of mean velocity in natural channels at steady state.

From the Chezy-Manning equation, considered valid for the "steady state" of a natural channel, as an extension of the condition of "uniform flow," it is interesting to determine the degree of variation of the average flow velocity, depending on the variations of the factors involved. That is why this article presents a theoretical analysis which yields a first approximation of the sensitivity of the variation of the magnitude of the average speed to changes in the geometry or geomorphology of a section of the channel. It is found that the concept of "stable state" for the thermodynamic state of a natural channel can be assimilated to that of "dynamic equilibrium," in which the values of the state parameters vary slowly, in a "regional" sense.

Laboratory study of the effect of vertically varying vegetation density on waves, currents and wave-current interactions

Under the influence of vegetation, waves, currents, turbulence as well as sediment processes are all changed, which significantly affect aquatic environment. Vegetation appears spatially heterogeneous in terms of vegetation density in most natural wetlands. However, how vertical heterogeneity of vegetation canopy affects turbulence under waves and currents is still not well understood. In this paper, the effects of different vegetation densities (i.e. sparse, dense and vertically-varying density) on vegetation-induced wave attenuation, mean flow adjustment as well as turbulence are studied through laboratory experiments. For regular waves, higher vegetation density results in smaller wave transmission coefficients and local wave heights. The difference between the effects of vertically uniform and varying vegetation density on wave height decay is significant for deep water waves and insignificant for intermediate water waves. The vertically varying vegetation density slightly changes the wave-averaged velocity distribution, producing reverse flows within the canopy. Turbulence intensity decreases more significantly with increasing water depth in the case with vertically varying vegetation density. For unidirectional flow, the vertically varying vegetation density increases the vertical gradient of mean streamwise velocity compared with the emergent uniform vegetation. The time series of u oscillates relatively stronger near bottom, and causes higher TKE, which is different with that in oscillatory flow. Compared with the submerged uniform vegetation, the vertical variation of mean velocity affected by vegetation with vertically varying density is smaller in both pure current and wave-current flow. Affected by dense submerged vegetation, the mean velocity of combined wave-current flow is significantly lower than that of pure current, while it is reduced near the bottom and increased in the upper layer comparing with that of pure current under the influence of sparse submerged vegetation. TKE values in wave-current flow is promoted in the upper layer with coexisting waves in the case of submerged uniform vegetation, while it is barely affected near the bed. In the case with vertically varying vegetation density, TKE values in the wave-current flow are slightly decreased than that in the pure current condition, especially near the bottom.

Streamwise self-similarity and log scaling in turbulent boundary layers

High Reynolds number is thought to be a fundamental condition essential for the occurrence of log scaling in turbulent boundary layers. However, while log variation of mean velocity is seen to occur at moderate Reynolds numbers in the traditional boundary layer literature, log variations of higher-order moments are evident only at much higher Reynolds numbers, as reported in recent experiments. This observation suggests that, underlying the occurrence of log scaling in turbulent boundary layers, there exists a more fundamental condition (apart from the largeness of Reynolds number) – the requirement of self-similar evolution of a mean-flow quantity of interest along a mean-flow streamline, i.e. the mean advection of the scaled mean quantity of interest is required to be zero. Experimental data from the literature provide strong support for this proposal.

An improved particle tracking velocimetry (PTV) technique to evaluate the velocity field of saltating particles

Velocity is a key parameter characterizing the movement of saltating particles. High-speed photography is an efficient method to record the velocity. But, manually determining the relevant information from these photographs is quite laborious. However, particle tracking velocimetry (PTV) can be used to measure the instantaneous velocity in fluids using tracer particles. The tracer particles have three basic features in fluids: similar movement patterns within a small region, a uniform particle distribution, and high particle density. Unfortunately, the saltation of sand particles in air is a stochastic process, and PTV has not yet been able to accurately determine the velocity field in a cloud of blowing sand. The aim of the present study was to develop an improved PTV technique to measure the downwind (horizontal) and vertical velocities of saltating sand. To demonstrate the feasibility of this new technique, we used it to investigate two-dimensional saltation of particles above a loose sand surface in a wind tunnel. We analyzed the properties of the saltating particles, including the probability distribution of particle velocity, variations in the mean velocity as a function of height, and particle turbulence. By automating much of the analysis, the improved PTV method can satisfy the requirement for a large sample size and can measure the velocity field of blowing sand more accurately than previously-used techniques. The results shed new light on the complicated mechanisms involved in sand saltation.

Effect of Kinematic Operating Conditions on Ridge Deformation—Numerical Study With Experimental Comparison

The behavior of roughness features under rolling–sliding inside highly loaded elastohydrodynamically lubricated (EHL) contacts is studied in detail for many years now. In particular, the roughness deformation was subject to different theoretical analyses as well as experiments. A recent experimental work developed by Šperka et al. (2016, “Experimental Study of Roughness Effects in a Rolling–Sliding EHL Contact—Part I: Roughness Deformation,” Tribol. Trans., 59(2), pp. 267–276) studied the effect of kinematic operating conditions (mean velocity and slide to roll ratio) on the deformed profile of a ridge. The current paper presents results of full numerical simulations and their direct comparison to experiments in order to study the dependency of roughness deformation on the operating conditions. The assumption of non-Newtonian lubricant behavior seems to have a significant influence on the results as well. Results indicate that, in agreement with experiments, the variation of mean velocity causes changes in the deformed profiles of roughness while, on the other hand, the magnitude of slide to roll ratio (for sliding larger than ±50%) does not have influence on the size of the deformation.

Mastering nonlinear flow dynamics for laminar flow control.

A laminar flow control technique based on spanwise mean velocity gradients (SVGs) has recently proven successful in delaying transition in boundary layers. Here we take advantage of a well-known nonlinear effect, namely, the interaction of two oblique waves at high amplitude, to produce spanwise mean velocity variations. Against common belief we are able to fully master the first stage of this nonlinear interaction to generate steady and stable streamwise streaks, which in turn trigger the SVG method. Our experimental results show that the region of laminar flow can be extended by up to 230%.

Analysis of pressure behavior in a temperature controlled molecular dynamic flow

Thermo-fluid properties are required for numerical modeling of nano/micro devices. These properties are mostly obtained from results of molecular dynamics (MD) simulations. Therefore, efforts have been put in developing methods for numerical evaluation of fluid properties, such as pressure. In this paper, the pressure behavior in a controllable nanochannel flow is investigated. The nanoflow field is created by imposing a gradient of a macroscopic property such as temperature. Details of the pressure calculation method in a molecular system and its sensitivity to the approximations made are described first. The effect of temperature rise in a uniform flow on the pressure field is studied next. Then, in the flow under a fixed mean velocity condition, the effect of temperature gradient as a controllable property on the pressure field of nanoflow is studied. Velocity, pressure and molecular density of nanoflows with various temperature gradients and different temperature levels are investigated as well. It has been found that the temperature level at which the temperature gradient is imposed, is important. A fixed temperature gradient will not always lead to the same pressure gradient at different temperature levels. Furthermore, quite interestingly, it is observed that at a fixed temperature gradient, with the variation of mean velocity the pressure field also varies.

Investigation of the mixing layer of underexpanded supersonic jets by particle image velocimetry

The present experimental study focuses on some properties of the turbulence and the shock-cell structure in underexpanded supersonic jets, which are of practical relevance in air transport. Choked jets at fully expanded Mach numbers Mj=1.10, 1.15, 1.35 and 1.50 are investigated using particle image velocimetry. The strength of the shock-cell structure is studied from mean velocity profiles, both in the jet core and in the mixing layer. The general geometry of the latter and its location relatively to the mean shock-cell structure are established. Furthermore, detailed accounts of mixing layer thickness, turbulence levels, spatial correlations and intrinsic turbulence length scales are given. While the mean velocity variations related to the shock-cell structure extend up to the subsonic part of the studied jets, their mixing layer is found to be mostly located in the subsonic region. Some of the observed turbulence properties, like the mixing layer thickness and turbulence levels, are close to what is found for subsonic jets. The effect of the shock-cell structure on turbulence is however visible for Mj⩾1.35. The spatial correlations of turbulence are used to estimate intrinsic turbulence length scales and these are found to be of the order of the shock-cell length. These data are used to make some comments upon the generation mechanism of shock-associated noise, a noise component produced by imperfectly expanded supersonic jets.

Multiscale Deformation and Dynamic Recrystallization in Shock Deformed Aluminum Alloy

Two regimes, equilibrium and non-equilibrium interaction of shock wave and inner structure of solid are studied. The theoretical analysis of the regimes is carried out by using the concept of the meso-macro momentum exchange. As a test material for the experiments, D16 Al alloy is taken, firstly because of its initial heterogeneity in equilibrium regime of dynamic straining and, secondly, due to increasing heterogeneity in non-equilibrium regime. Shock tests of D16 Al alloy within impact velocity range of 85÷450 m/s evidence that maximum dynamic strength is realized under conditions: (i) equilibrium regime of meso-macro momentum exchange, (ii) velocity defect equals to mean velocity variation. In non-equilibrium regime, the shock-induced dynamic recrystallization occurs, which is investigated with the metallography and X-ray analysis.

Identification of the Cold Flow Perturbation Sources in a Dump Combustor With a Tapered Exit

This paper reports on the experimental investigation of the flow inside a low aspect ratio (length less than the reattachment length of separated flow) dump combustor with a tapered exit. The flow field in the combustor is evaluated using wall pressure and velocity measurements at varying flow Reynolds numbers. The mean velocity and turbulence variation closer to the wall of the combustor was found to be different from that at other radial locations due to the presence of recirculation and possible thickening of the shear layer caused by a decrease in the strength of the potential core. The power spectral study of the wall pressure and velocity fluctuations suggested the dominant presence of acoustic perturbations with amplitude modulation of such perturbations due to viscous dissipation in the shear layer.

Comparative Analysis of Velocity Decomposition Methods for Internal Combustion Engines

Different signal processing technique performances are compared to each other with regard to separating the mean and fluctuating velocity components of a simulated one-dimensional unsteady velocity signal comparable to signals observed in internal combustion engines. A simulation signal with known mean and fluctuating components was generated using experimental data and generic turbulence spectral information. The simulation signal was generated based on observations on the measured velocity data obtained using LDV in a motored Briggs-and-Stratton engine at about 600 RPM. Experimental data was used as a guide to shape the simulated signal mean velocity variation; fluctuating velocity variations with specified spectrum and standard deviation was used to mimic the turbulence. Cyclic variations were added both to the mean and the fluctuating velocity signals to simulate prescribed cyclic variations. The simulated signal was introduced as input to the following algorithms: ensemble averaging; high-pass filtering; Proper-Orthogonal Decomposition (POD); Wavelet Decomposition (WD) and Wavelet Decomposition/Principal Component Analysis (WD/PCA). The results were analyzed to determine the best method in correctly separating the mean and the fluctuating velocity information, indicating that the WD/PCA performs better compared to other techniques.

MR phase‐contrast velocity mapping methods for measuring venous blood velocity in the deep veins of the calf

To evaluate the feasibility of using un-gated, real-time MRI for venous blood velocity mapping in the calf, comparing an interleaved spiral k-space sequence (ISP) against a standard segmented gradient echo sequence (GRE). A flow phantom with a variable flow-rate was scanned using both GRE and ISP sequences for an in vitro comparison. Seven subjects were scanned prone, performing metronome guided breathing, using the (externally triggered) segmented GRE and real-time ISP sequences. The segmented GRE acquisition duration was 2.5 mins (22 guided respiratory cycles) and the ISP sequence ran continuously for 35 s, 4 full guided respiratory cycles. Mean velocity from each of the deep veins was measured and peak mean velocity, peak flow rate and cumulative volume flow over a respiratory cycle compared between sequences. The two sequences compared well both in vitro and in vivo. The real-time ISP sequence showed short-term variations in mean velocity superimposed on the respiratory induced flow, which were averaged out using the segmented GRE sequence. Real-time ISP provides comparable time-averaged flow results to the standard sequence with additional information on real-time flow variations and so could be used for further investigation into venous blood flow in the lower leg.

Development and characterization of a variable turbulence generation system

Experimental turbulent combustion studies require systems that can simulate the turbulence intensities [u′/U0 ~ 20–30% (Koutmos and McGuirk in Exp Fluids 7(5):344–354, 1989)] and operating conditions of real systems. Furthermore, it is important to have systems where turbulence intensity can be varied independently of mean flow velocity, as quantities such as turbulent flame speed and turbulent flame brush thickness exhibit complex and not yet fully understood dependencies upon both U0 and u′. Finally, high pressure operation in a highly pre-heated environment requires systems that can be sealed, withstand high gas temperatures, and have remotely variable turbulence intensity that does not require system shut down and disassembly. This paper describes the development and characterization of a variable turbulence generation system for turbulent combustion studies. The system is capable of a wide range of turbulence intensities (10–30%) and turbulent Reynolds numbers (140–2,200) over a range of flow velocities. An important aspect of this system is the ability to vary the turbulence intensity remotely, without changing the mean flow velocity. This system is similar to the turbulence generators described by Videto and Santavicca (Combust Sci Technol 76(1):159–164, 1991) and Coppola and Gomez (Exp Therm Fluid Sci 33(7):1037–1048, 2009), where variable blockage ratio slots are located upstream of a contoured nozzle. Vortical structures from the slots impinge on the walls of the contoured nozzle to produce fine-scale turbulence. The flow field was characterized for two nozzle diameters using three-component Laser Doppler velocimetry (LDV) and hotwire anemometry for mean flow velocities from 4 to 50 m/s. This paper describes the key design features of the system, as well as the variation of mean and RMS velocity, integral length scales, and spectra with nozzle diameter, flow velocity, and turbulence generator blockage ratio.

Numerical study of the turbulent flow in strongly curved stationary and rotating U‐ducts

AbstractA large‐eddy simulation (LES) study is undertaken to explore the complex flow of developing turbulent flow through stationary and rotating U‐ducts with strong curvature. Three flow cases are investigated: stationary (non‐rotating), positive and negative rotational cases. Stationary and positive rotational cases are shown to have similar flow characteristics in terms of the mean velocity variations, although the predicted separation zone is nearly doubled in size for positive rotational case. Unlike the positive rotation for which the laminarization effects are observed, turbulence is significantly enhanced for the negative rotation mainly due to the existence of strong secondary flow. Turbulence is found to be highly anisotropic throughout the duct apart from the far downstream regions of the bend for the negative rotational case. The stress–strain relation seems to be completely invalid in the U‐duct apart from the bend region. Copyright © 2009 John Wiley & Sons, Ltd.