Velocity In Turbulent Flow Research Articles

An urban dispersion inspired scenario for CFD model validation

Momentum, advection, diffusion, and turbulence are component physics relating to fire simulation tools like computational fluid dynamics (CFD). Magnetic Resonance Velocimetry and Magnetic Resonance Concentration MRV/MRC techniques can produce heretofore unrivaled detailed measurements of three-component velocity and concentration fields in turbulent flows. This study exhibits 3D flow comparisons between velocity and concentration fields obtained using MRC/MRV and SIERRA/Fuego for an urban geometry based on a section of downtown Oklahoma City. A 1:2500 scale water flow scenario provides 0.8 mm resolution data. Various techniques are employed to quantify the accuracy of the simulation results. The techniques all generally suggest a good comparison between the model and experiments throughout the compared volume. The selected metrics provide benchmark accuracy measures that can be used to indicate quantitative accuracy of the simulations, as well as for targets for future simulation improvements.

Three-dimensional thermal calculations of the radiation chamber of a cylindrical heating tube furnace

The paper briefly describes the method of thermal calculations of radiation chambers of tube furnaces on the basis of modelling interconnected processes taking place in the furnace by three-dimensional differential energy equations, the transfer of momentum components, mass concentrations of methane and air, continuity, a two-parameter model of turbulence and the equation of energy transfer by radiation. A software package using this method is applied to carry out numerical analysis of the spatial temperature distribution, the averaged values of velocities of the combustion products turbulent flow in the combustion chamber of a cylindrical heating tube furnace of the oil-processing industry. A furnace diagram with diffusion burners on the radiation chamber floor and differential equations describing physical processes taking place in the furnace are given. Methods for solution of the initial equations are described. The obtained distributions of the surface densities of heat fluxes to the coils along the chamber perimeter at different heights of the furnace are demonstrated. Graphs of the temperature change of the combustion products and the tubular reactor walls for different values of the angular coordinate are given.

An explicit expression for velocity profile in presence of secondary current and sediment in an open channel turbulent flow

The present study revisits the determination of vertical distribution of streamwise velocity in an open channel turbulent flow considering the effect of secondary current in the presence of sediment together with a concentration dependent settling velocity and von Karman constant κs. The work mainly modifies a previous study that introduced a lot of assumptions to obtain an analytical solution of the velocity distribution. The present study overcomes those assumptions in the model and though not fully analytical, attempts to present a semi-analytical solution that is explicit and in the form of a convergent series. Homotopy analysis method is used for this purpose and it is validated with numerical solution as well as with available laboratory data from the literature. How the secondary current and concentration dependent κsinfluence the velocity profile, is also discussed.

Analytical expression for predicting the reduced settling velocity of small particles in turbulence

The influence of turbulence on the settling velocity of small particles remains an inconclusive research subject. Both enhanced and retarded particle settling compared to quiescent settling have been reported in previous literature, and several theories have been proposed. Among the mechanisms that account for reduced settling velocities in turbulence, the loitering effect is an important one that generally exists in various conditions. This study focuses on an analytical prediction of reduced particle settling velocities due to the loitering effect. By considering the velocity autocorrelation function as a step function analogous to the free path theory, and using a modified integral time scale to capture the essence of the loitering effect, an analytical expression is derived for predicting the reduction in particle settling velocities in turbulent flows. Calculation results of the expression are then examined by comparing with the results of a random walk model, direct numerical simulations that conditionally captured the reduction of particle settling velocities due to the loitering effect, and several representative experiments. Major possible influencing factors on the prediction of the analytical expression and applicable conditions of the expression are then further discussed. The proposed analytical expression is shown to be suitable for predicting the reduced settling velocities of small particles with relatively weak inertia in turbulent flows and could provide a reasonable explanation for reported cases in which small to moderate reductions in particle settling velocities were observed.

Experimental study on the slip velocity of turbulent flow over and within porous media

The phenomenon of coupled flow between free flow and porous media is characteristic of fluid flowing across porous media, but the slip characteristics at the coupling interface need to be further studied. The purpose of this work is to investigate the velocity distribution of turbulence with a high Reynolds number above and in porous media. In this paper, a visual flume test bench is built to simulate porous media as an accumulation of spherical glass beads with a diameter of 10 mm. The free flow velocity of fluid crossing the porous media and the velocity inside the porous media are measured by ultrasonic Doppler velocimetry. The effects of Reynolds number, relative water depth, and porosity parameters on the slip velocity and momentum transfer near the interface are studied. The results show that the slip coefficient of a porous-media bed with 0.331 porosity ranges from 0.000 082 to 0.000 594, while that of a porous-media bed with 0.476 porosity ranges from 0.000 034 to 0.001 068. The slip velocity increases with the increase in Reynolds number but decreases with the increase in relative water depth and porosity. The thickness of the transition layer in the porous-media region is insensitive to the Reynolds number and relative depth, but sensitive to porosity and increases with the increase in porosity. In this study, the influence of effective parameters on turbulent velocity is studied by experiments, which provides an important reference value for the development of a theoretical model in turbulent flow.

The concept of turbulent «vortex backfill» - models and methods. Power engineering: research, equipment, technology

Models and methods for studying turbulence based on the concept of turbulent "vortex backfill" are presented. The essence of this concept is that the turbulent flow is considered as laminar, flowing through a "vortex backfill ", which creates internal resistance. This resistance can be considered either as distributed, or as locally concentrated. Based on the first representation, a modified Navier-Stokes equation, its approximate analytical and numerical solutions are obtained. Based on the second concept and the local fluctuation method developed for these purposes, a computer model of the turbulent flow in the pipes is obtained. Using simulation, it is shown that, when a certain system of local viscosity fluctuations is specified, the calculated flow profile corresponds to the profile of the turbulent flow velocity. The magnitude and profile of the turbulent viscosity of the flow are completely determined by the structure and properties of the "vortex backfill ". The results of the work confirm the possibility and efficiency of considering turbulence based on this concept.

Assessment of models for velocity distribution in turbulent smooth-wall open channel flows

ABSTRACT This study attempts to provide a more precise and comprehensive model for stream-wise velocities in turbulent smooth-wall open channel flows. Velocity profiles are obtained from the resolution of two ordinary differential equations. The first is derived from the Reynolds-averaged Navier-Stokes (RANS) equation while the second is from the equilibrium between turbulent kinetic energy (TKE) production and dissipation. We assessed different analytical models for eddy viscosity, TKE and mixing length. Computation results for velocity profiles were assessed and compared to experimental data for different flow conditions and the well-known linear, log and log-wake laws. Our results show that the model based on the RANS equation provides more accurate velocity profiles. In the viscous sublayer and buffer layer, the method based on Prandtl’s eddy viscosity model and Van Driest mixing length gives more precise results. For the log layer and outer region, a mixing length equation derived from Von Karman’s similarity hypothesis, provides the best agreement with measured data except near the free surface where we used an additional correction based on a damping function for eddy viscosity. This method allows more accurate velocity profiles with the same value of the damping coefficient that is valid under different flow conditions.

Calculation of probability density function of particle velocity in turbulent flow

The random motion of inertial particles in uniform isotropic turbulence is considered. Fluctuations of the gas velocity at the particle trajectory are modelled as a random Gauss process with a finite decay time of the autocorrelation function. A closed equation for the probability density function (PDF) of the random particle velocity is derived. An analytical solution of the equation for the PDF is found. The equation for the PDF is solved by two numerical methods. The first method of solution is based on the finite-difference approximation of the equation for the PDF. The second method is based on the calculation of empirical PDF, which is obtained by averaging over an ensemble of random trajectories of particles. The results of the comparison of analytical numerical solutions are presented.

Saddle-node bifurcations of traveling waves in the asymptotic suction boundary layer flow

Finite-amplitude traveling wave solutions in the asymptotic suction boundary layer are computed for different wall permeabilities. The permeability has negligible effect on the Reynolds number at which they emerge, but it affects their shape and amplitude and the turbulent flow velocity profile.

Particles dispersion and deposition in inhomogeneous turbulent flows using continuous random walk models

The suitability of the normalized Langevin stochastic equation with appropriate drift correction for simulation of instantaneous fluctuation velocities in inhomogeneous turbulent flows was studied. The Reynolds Stress Transport turbulence model of the ANSYS-Fluent code was used to evaluate the inhomogeneous turbulent flow properties in a two-dimensional duct flow. The simulation results were then used in an in-house Matlab particle tracking code and the trajectories of about 2 × 105 randomly distributed particles in the channel were evaluated by solving the particle equation of motion including the drag and Brownian forces under the one-way coupling assumption. The performance of the Continuous Random Walk (CRW) stochastic model using the conventional nonnormalized, as well as the normalized Langevin equations without and with the drift term for predicting a uniform distribution for the fluid-tracer particles in an inhomogeneous turbulent flow was examined. The accuracy of these models in predicting the deposition velocities and distribution of solid particles with diameters ranging from 10 nm to 30 μm was also carefully examined. In addition, the effects of including the finite-inertia coefficient in the drift term and using the corrected root mean square normal velocity in the near-wall region on the accuracy of the results were emphasized. By exploring the concentration profiles and deposition velocities of particles resulting from different CRW models, it was concluded that the Normalized-CRW model including the appropriate drift term leads to the most accurate results.

Refinement of the logarithmic law of the wall

A refinement of the logarithmic law for the mean velocity in canonical wall-bounded turbulent flows is presented. The relevant length scale in the overlap region is found to be based on the weighted mean velocity gradient instead of the classical length scale based on the wall friction velocity.

Prediction of mean turbulent flow velocity in a permeable-walled pipe

Determining the characteristics of pipe flow and the effect of friction resistance is relevant to a variety of applications in hydraulic and hydrological engineering such as flood control, river restoration and ground runoff prediction. In the Nikuradse’s experiments, the relative roughness and the bulk Reynolds number are used to scale the Darcy friction factor thereby determining the mean velocity. Recent observations using permeable-walled pipes however indicate that the friction factor can significantly deviate from the results obtained in Nikuradse’s approach. Here, we propose a concise model to estimate the mean velocity of the pipe flow over permeable walls. The model is formulated using a data-driven approach based on the analytical solution of the mean velocity and the similarity analysis (dimension conservation) of the two extreme cases: impermeable and porous pipe flow. We show that the proposed approach can derive a physically meaningful formula for the mean velocity. Furthermore, we propose a simple momentum transfer model which confirms the findings from the data-driven approach. The momentum transfer model indicates that the friction factor can be scaled with two different relative hydraulics radius scales, which are intrinsically determined by two different eddy sizes. Lastly, the present approach is examined by comparing it with another approximate formula and the limited experimental data in the literature.

Highly sensitive 3C-SiC on glass based thermal flow sensor realized using MEMS technology

This paper presents a silicon carbide (SiC) based thermal flow sensor on a transparent and electrically insulating glass substrate via anodic bonding process. The paper elaborates on the fabrication steps of the thermal flow sensor. Three resistive heater size configurations of dimensions 100 μm × 100 μm, 300 μm × 300 μm, and 1000 μm × 1000 μm were fabricated. The thermoresistive properties of 3C-SiC on glass were investigated from ambient temperature to 443 K. The characterization of the SiC heater and temperature sensors revealed a high thermoresistive effect with a temperature coefficient of resistance (TCR) of approximately −20,716 ppm/K at ambient temperature(298 K) and −9367 ppm/K at 443 K respectively. The performance of the sensors was evaluated based on the sensitivity of the flow sensor. For a turbulent flow velocity of 7.4 m/s, the sensitivity of the sensor operating in the constant -voltage mode is 0.091 s/m with a power consumption of 133.50 mW for the 1000 μm × 1000 μm heater. Finally, a study on the flow direction was conducted to confirm the operation of 2-D direction independent hot-film flow sensor. Results indicated that the performance of the sensor remained the same when the flow direction was perpendicular to SiC heater and sensor respectively. However, the best sensitivity was achieved by passing air flow perpendicular to the sensing elements. The high TCR of the single crystalline 3C-SiC material, the relatively low power consumption on the order of milliwatts and the high sensitivity of our sensor demonstrates its potential use for high temperature flow sensing applications.

Spectral Method of Detection of Laser Doppler Velocimeter Signals in Turbulent Flows

A spectral method of detection of laser Doppler velocimeter signals in turbulent flows is proposed. The method is based on estimating the signal/noise ratio adapted to the width of the power spectral density of the signal and comparing its value with the recognition threshold. Numerical simulations show that the error of the signal/noise ratio estimates is 1% for different velocities of the turbulent flow. Physical experiments aimed at measuring turbulent aerodynamic processes show that the proposed method offers a possibility of eliminating the errors of calculating the mean value and the velocity deviation equal to 15 and 78% by means of eliminating the signals with low signal/noise ratios from the processing system. Application of the proposed method also ensures lower nonuniformity of the signal/noise ratio estimates than that ensured by the famous method developed by Tropea for a wide range of turbulent velocities.

Reinvestigation on mixing length in an open channel turbulent flow

The present study proposes a model on vertical distribution of streamwise velocity in an open channel turbulent flow through a newly proposed mixing length, which is derived for both clear water and sediment-laden turbulent flows. The analysis is based on a theoretical consideration which explores the effect of density stratification on the streamwise velocity profile. The derivation of mixing length makes use of the diffusion equation where both the sediment diffusivity and momentum diffusivity are taken as a function of height from the channel bed. The damping factor present in the mixing length of sediment-fluid mixture contains velocity and concentration gradients. This factor is capable of describing the dip-phenomenon of velocity distribution. From the existing experimental data of velocity, the mixing length data are calculated. The pattern shows that mixing length increases from bed to the dip-position, having a larger value at dip-position and then decreases up to the water surface with a zero value thereat. The present model agrees well with these data sets and this behavior cannot be described by any other existing model. Finally, the proposed mixing length model is applied to find the velocity distribution in wide and narrow open channels. The derived velocity distribution is compared with laboratory channel data of velocity, and the comparison shows good agreement.

Dynamics of phytoplankton blooms in turbulent vortex cells.

Turbulence and coherent circulation structures, such as submesoscale and mesoscale eddies, convective plumes and Langmuir cells, play a critical role in shaping phytoplankton spatial distribution and population dynamics. We use a framework of advection-reaction-diffusion equations to investigate the effects of turbulent transport on the phytoplankton population growth and its spatial structure in a vertical two-dimensional vortex flow field. In particular, we focus on how turbulent flow velocities and sinking influence phytoplankton growth and biomass aggregation. Our results indicate that conditions in mixing and growth of phytoplankton can drive different vertical spatial structures in the mixed layer, with the depth of the mixed layer being a critical factor to allow coexistence of populations with different sinking speed. With increasing mixed layer depth, positive growth for sinking phytoplankton can be maintained with increasing turbulent flow velocities, allowing the apparently counter-intuitive persistence of fast sinking phytoplankton populations in highly turbulent and deep mixed layers. These dynamics demonstrate the role of considering advective transport within a turbulent vortex and can help to explain observed phytoplankton biomass during winter in the North Atlantic, where the overturn of deep convection has been suggested to play a critical role in phytoplankton survival.

Performance evaluation of a hydrocyclone with a spiral rib for separation of particles

Several design modifications have been done to improve particle separation efficiency in a hydrocyclone. The effects of a rib which is introduced into the cylindrical part of the hydrocyclone are discussed here. CFD (Computational fluid dynamics) is a useful tool to study the velocity and pressure distribution of complex turbulent flow in a hydrocyclone. Flow simulations are carried out using a three-dimensional double precision, segregated, steady-state solver tool. Reynolds stress model is employed for turbulent model which is suitable for the anisotropic turbulent flow. A comparison study for pressure drop and flow velocity for the conventional and ribbed hydrocyclone have done. The obtained CFD simulated results in correlation with experimental data shows that the pressure drop reduces by 13.9% at a velocity of 2.5m/s by using rib. An experimental finding shows that the cut size particle diameter for conventional and ribbed hydrocyclone are 36µm and 28µm respectively at the velocity of 2.5m/s.

Enhanced droplet collision rates and impact velocities in turbulent flows: The effect of poly-dispersity and transient phases

We compare the collision rates and the typical collisional velocities amongst droplets of different sizes in a poly-disperse suspension advected by two- and three-dimensional turbulent flows. We show that the collision rate is enhanced in the transient phase for droplets for which the size-ratios between the colliding pairs is large as well as obtain precise theoretical estimates of the dependence of the impact velocity of particles-pairs on their relative sizes. These analytical results are validated against data from our direct numerical simulations. Our results suggest that an explanation of the rapid growth of droplets, e.g., in warm clouds, may well lie in the dynamics of particles in transient phases where increased collision rates between large and small particles could result in runaway process. Our results are also important to model coalescence or fragmentation (depending on the impact velocities) and will be crucial, for example, in obtaining precise coalescence kernels in such systems.

POD-Galerkin reduced-order model for viscoelastic turbulent channel flow

ABSTRACTIn this work, with elasticity governed by the Giesekus constitutive equation, a proper orthogonal decomposition (POD) reduced-order model of viscoelastic turbulent channel flow is established for the first time. The established reduced-order model is based on small sets of basis functions from the POD of the sampling data obtained by direct numerical simulation (DNS) for the studied flow. The POD reduced-order model is tested on cases with that are different from the samplings for viscoelastic turbulent channel flow. The results show that the errors for root-mean-square (rms) velocity fluctuations are significant at the top and bottom walls. It is found that each basis function plays an important role in describing the studied turbulence which makes it unmanageable to obtain accurate velocity field (including mean velocity and velocity fluctuations) through solving the reduced-order model. It is of necessity to take all the basis functions into consideration to depict the flows more accurately. However, the mean velocity obtained from the reduced-order model is of high precision, which states that the POD-based reduced-order model is a potential approach to obtain an accurate mean velocity field for viscoelastic turbulent flow, which has great significance in academic study as well as engineering. The calculation speed of the established reduced-order model is much faster than that of DNS, which indicates that the POD is a highly efficient way of obtaining the statistic characteristics, such as mean velocity in turbulent channel flow.

Flow thresholds for leaf retention in hydrodynamic wakes downstream of obstacles

AbstractLeaves are the major component of terrestrial litter input into aquatic systems. Leaves are distributed by the flow, accumulate in low flow areas, and form patches. In natural streams, stable leaf patches form around complex structures, such as large woody debris. Until now, little is known about flow conditions under which leaf patches persist. This study aims to quantify flow conditions for stable leaf patches and entrainment of leaf patches. We hypothesize that entraining flow processes, such as turbulence, Reynolds stress, or lift forcing (vertical flow velocity), best explain local leaf retention. This study was performed in an unscaled flume experiment, which conditions coincide with conditions found in low‐energetic lowland streams. We positioned a wooden obstacle perpendicular to the flow on the bed of the flume. A leaf patch was positioned downstream from the wooden obstacle. The experiment was performed under 5 flow conditions. We monitored leaf patch cover and near‐bed flow conditions in the area downstream of the wooden obstacle. We showed that near‐bed flow velocities explain leaf retention better than more complex flow velocity derivatives such as turbulence, Reynolds stress, and vertical flow velocity. The entrainment near‐bed flow velocity for leaves ranges from 0.037 to 0.050 m/s. Flow velocities frequently exceed those values, even in low‐energetic lowland streams. Therefore, complex structures, such as woody debris, create flow conditions to support stable leaf patches. Thus, adding instead of removing obstacles may be a key strategy in restoring biodiversity in deteriorated streams.