Uniform Cross Stream Research Articles

A laboratory investigation into the influence of a rigid vegetation on the evolution of a round turbulent jet discharged within a cross flow

The study of buoyant jets, those between pure jets and plumes, has been carried out with ever greater frequency over recent years due to its application in different practical engineering fields, i.e. appropriate design of outfalls for the disposal of municipal and industrial waste waters. The dispersion of waste and the related dilution of pollutants are governed by the mean-flow and turbulence characteristics of the resulting jets, which themselves depend on environmental conditions.The present study deals with how a uniform cross-stream with a channel bed surface covered by rigid emergent stems affects the behaviour of a circular turbulent buoyant jet. The time-averaged temperature and velocity fields are investigated in order to understand jet diffusion and penetration within the ambient fluid.The examination and comparison of the measured scalar and vector quantities show that the presence of emergent vegetation in the receiving environment affects both the average flow field and the jet structure, reducing the mean channel velocity, with a notable increase in jet penetration height and dilution compared to the test case without vegetation. This result is confirmed by the several vertical profiles of the mean scalar concentration and the normalized vertical velocity component along the channel centre plane. Moreover, the rigid emergent vegetation and its driven instabilities promote a distortion of the mean concentration and normalized axial velocity component profiles in the trajectory-based coordinate system.

Effect of cylinder rotation during mixed convective flow of nanofluids past a circular cylinder

The mixed convective flow and heat transfer of nanofluids past a rotating cylinder placed in a uniform cross stream is investigated via SUPG (streamline upwind Petrov–Galerkin) based finite element method. Nano sized copper (Cu) particles suspended in water are used with Prandtl number (Pr) = 6.9. The computations are carried out at a representative Reynolds number (Re) of 100. The dimensionless cylinder rotation rate (α) is varied between 0 and 2. The range of nanoparticle volume fractions (ϕ) considered is 0 ≤ ϕ ≤ 5%. Effect of aiding buoyancy is brought about by considering two fixed values of the Richardson number (Ri) as 0.5 and 1.0. The thermophysical properties of nanofluid is predicted using a model which considers the fundamentals of interparticle interaction kinetics according to the details of agglomeration–deagglomeration of nanoparticles, variations in the effective particulate dimensions and agglomerate size (n), as well as the volume fractions of the particulate system. Increase in cylinder rotation and nanoparticle agglomeration shows suppression of vortex shedding. Completely steady solution is obtained at α=2. At Ri = 0.5, ϕ=5%, and n = 5, vortex shedding disappears and the flow achieves a steady state. A von Karman street is seen at α=1.5, for n > 1 and ϕ ≤ 3%. For ϕ=4% and 5%, vortex shedding ceases at n = 5. Characterisation of vortex shedding by POD (proper orthogonal decomposition) shows that increase in the values of n institutes coherence in the wake vortical structures. The average Nusselt number decreases with increasing α and increases with increasing n. At n = 5, a significant enhancement in heat transfer is seen.

Analysis of Entropy Generation During Mixed Convective Heat Transfer of Nanofluids Past a Rotating Circular Cylinder

The entropy generation due to mixed convective heat transfer of nanofluids past a rotating circular cylinder placed in a uniform cross stream is investigated via streamline upwind Petrov–Galerkin based finite element method. Nanosized copper (Cu) particles suspended in water are used with Prandtl number (Pr) = 6.9. The computations are carried out at a representative Reynolds number (Re) of 100. The dimensionless cylinder rotation rate, α, is varied between 0 and 2. The range of nanoparticle volume fractions (ϕ) considered is 0 ≤ ϕ ≤ 5%. Effect of aiding buoyancy is brought about by considering two fixed values of the Richardson number (Ri) as 0.5 and 1.0. A new model for predicting the effective viscosity and thermal conductivity of dilute suspensions of nanoscale colloidal particles is presented. The model addresses the details of the agglomeration–deagglomeration in tune with the pertinent variations in the effective particulate dimensions, volume fractions, as well as the aggregate structure of the particulate system. The total entropy generation is found to decrease sharply with cylinder rotation rates and nanoparticle volume fractions. Increase in nanoparticle agglomeration shows decrease in heat transfer irreversibility. The Bejan number falls sharply with increase in α and ϕ.

Effect of free-stream fluctuations on laminar forced convection from a straight tube

The effect of free-stream fluctuations on forced convection heat transfer from a straight tube of circular cross-section is investigated. The tube is assumed to have an isothermal surface and is placed in an unsteady but uniform cross stream. The free-stream fluctuations are represented by periodic (sinusoidal) fluctuations superimposed on the average stream velocity. The resulting unsteady velocity and thermal fields are obtained by solving the conservation equations of mass, momentum and energy. The main parameters involved are Reynolds number, Strouhal number, Prandtl number and the relative amplitude of fluctuations. The paper focuses on the effects of the amplitude and frequency of free-stream fluctuations on the local and average Nusselt numbers while keeping Prandtl number unchanged ( Pr = 0.7). The average flow Reynolds number ranges from 50 to 500, the Strouhal number ranges from π 4 to π and the amplitude ranges from 20 to 50% of the free-stream average velocity. The study revealed that the effect of fluctuations on the time-average Nusselt number becomes more pronounced with increasing Reynolds number. It also revealed that the rate of heat transfer increases with the increase of the amplitude but decreases with the increase of frequency. The time-average Nusselt number is tabulated for all cases studied and the details of flow and thermal fields are presented in the form of local Nusselt number and surface vorticity distributions as well as streamline and temperature contours for some selected cases.

Numerical Simulation of Vertical Forced Plume in a Crossflow of Stably Stratified Fluid

In this study, an investigation using a three-dimensional numerical model, which treats conservation of mass, momentum, and salinity simultaneously, was carried out to study the character of a vertical forced plume in a uniform cross-stream of stably linear stratified environment. A k-ε turbulence model was used to simulate the turbulent phenomena and close the solving problem. The performance of the three-dimensional model is evaluated by comparison of the numerical results with some available experimental measurements. Results indicate that the numerical computation simulates satisfactorily the plume behavior in a stratified crossflow. The secondary vortex pairs in the cross section induced by the primary one change as the plume flows downstream. This denotes the transformation of entrainment mechanism in stratified crossflow.

Vibrations of a pair of elastically supported tall building models in a uniform stream

Experimental investigations were carried out to examine the flow features around a pair of elastically supported square cylinders and the associated flow-excited vibrations. Square cylinders of finite height were placed side by side in parallel to model adjacent tall buildings. The undisturbed uniform cross stream is along the side of the buildings. The experiment was carried vit in a recirculating water channel. The flow field was visualized using hydrogen bubble technique. Time sequential photography was used to quantify the development of the vortex pattern and the vibrational characteristics of the building models.

Vibrations of a pair of elastically supported tall building models in a uniform stream

Experimental investigations were carried out to examine the flow features around a pair of elastically supported square cylinders and the associated flow-excited vibrations. Square cylinders of finite height were placed side by side in parallel to model adjacent tall buildings. The undisturbed uniform cross stream is along the side of the buildings. The experiment was carried vit in a recirculating water channel. The flow field was visualized using hydrogen bubble technique. Time sequential photography was used to quantify the development of the vortex pattern and the vibrational characteristics of the building models.

False diffusion in three-dimensional flow calculations

This paper presents the derivation of the three principle components of the false or numerical diffusivities which result from the use of first-order upwind difference schemes in the calculation of steady three-dimensional flows. The effects of the false diffusion are illustrated by comparison of the first-order calculations with those with a higher-order scheme and experimental data, for the flow of a row of turbulent jets issuing at right angles to a nearly uniform cross-stream. The adverse effect of the false diffusivities in suppressing the physics of the flow is shown.

Unsteady Boundary-Layer Flow of Power Law Fluids

It has been shown here that use of a skewed coordinate system, aligning the coordinate axes with the jet and the stream velocity vectors, can greatly simplify the description of the trajectories of turbulent jets discharging at various angles of inclination into a uniform cross stream and having various ratios between the initial jet velocity and the stream velocity. By aid of this skewed coordinate system and on the basis of dimensional and similarity considerations, the centerline trajectories of such jets appear to be very well correlated by a simple power law. The centerline trajectory and induced pressure field of the deflected jets also can be determined by integrating numerically the momentum equation from the given initial condition at the jet exit and with the entrainment coefficient and the crossflow drag coefficient chosen from the test data. Such calculations certainly are important for determining the pressure field induced by the deflected jet, but the present formula should provide a convenient and sufficient estimation of the centerline trajectory of the deflected jets.