Two-dimensional Wall Jet Research Articles

Performance analysis of various wall treatments of RANS models for prediction of near-wall turbulence transport characteristics of plane wall jet

PurposeThe present study scrutinizes the relative performance of various near-wall treatments coupled with two-equation RANS models to explore the turbulence transport mechanism in terms of the kinetic energy budget in a plane wall jet and the significance of the near-wall molecular and turbulent shear, to select the best combination among the models which reveals wall jet characteristics most efficiently.Design/methodology/approachA two-dimensional steady incompressible plane wall jet in a quiescent surrounding is simulated using ANSYS-Fluent solver. Three near-wall treatments, namely the Standard Wall Function (SWF), Enhanced Wall Treatment (EWT) and Menter-Lechner (ML) treatment coupled with Realisable, RNG and Standard k-e models and also the Standard and Shear-Stress Transport (SST) k-ω models are employed for this investigation.FindingsThe ML treatment slightly overestimated the budget components on an outer scale, whereas the k-ω models strikingly underestimated them. In the buffer layer at the inner scale, the SWF highly over-predicts turbulent production and dissipation and k-ω models over-predict dissipation. Appreciably accurate inner and outer scale k-budgets are observed with the EWT schemes. With a sufficiently resolved near-wall mesh, the Realisable model with EWT exhibits the mean flow, turbulence characteristics and turbulence energy transport even better than the SST k-ω model.Originality/valueThree distinct near-wall strategies are chosen for comparative performance analysis, focusing not only on the mean flow and turbulence characteristics but the turbulence energy budget as well, for finding the best combination, having potential as a viable and low-cost alternative to LES and DNS for wall jet simulation in industrial application.

Effect of initial velocity distribution and Reynolds number on two-dimensional wall jet of transition region impinging

Effect of initial velocity distribution and Reynolds number on two-dimensional wall jet of transition region impinging

Scaling and similarity laws in three-dimensional wall jets

Wall jets appear in many situations of technological and scientific interest. In gas turbines, flows produced by the film as well as impingement cooling devices are three-dimensional wall jets. High-lift devices produce flows that can easily be represented by two-dimensional wall jets. It has been known for a long time that wall jets in both stagnant and moving environments display a layered structure and only partially obey similarity laws. In this paper, we derive scaling laws and obtain self-similar velocity defect and Reynolds stress profiles for the outer part of three-dimensional wall jets in the high-Reynolds-number limit. The scaling laws are derived from prime principles under realistic assumptions about the behavior of the flow. We show that the leading term in an expansion of the turbulent kinetic energy (TKE) as a series of powers of the distance from the source must scale like the transversal velocity causing the jet to spread laterally. Only the second term in the TKE expansion is shown to scale like the square of the velocity defect. The scaling laws are tested on numerical and experimental data representing two commonly used film cooling devices.

Scaling laws for two- and three-dimensional wall jet scour based on the phenomenological theory of turbulence

The scour induced by wall jets may cause serious bed erosion and thus damage to hydraulic structures. Previous studies are largely empirical, providing only correlations of experimental data. At present, it is not clear what difference exists in the physical mechanisms of two-dimensional (2D) and three-dimensional (3D) wall jet scour. This study first summarizes previous efforts for predicting wall jet scour depths. Then, a scaling analysis is presented for investigating scour depths by applying the phenomenological theory of turbulence. It is shown that for wall jet cases, the dimensionless equilibrium scour depth can generally be expressed as a power function of the densimetric Froude number and relative roughness height, with the power index depending on jet configurations. The predictions of scour depth using the resulting formulas agree well with published data. The present analysis provides new insights into the understanding of the underlying physical mechanisms of wall jet scour as well as the difference between 2D and 3D configurations.

Analysis of Jet Wall Flow and Heat Transfer Conveying ZnO-SAE50 Nano Lubricants Saturated in Darcy-Brinkman Porous Medium

The problem of 2D (two-dimensional) wall jet flow, along with heat transfer incorporated by nanofluid in a Darcy-Brinkman medium, while recognizing the requirement for efficient heating and cooling systems. Following the use of similarity variables, the resultant system of ODEs (ordinary differential equations) is solved using the well-known and efficient bvp4c (boundary-value problem of the 4th order) technique. The significance of physical quantities for the under-consideration parameters is illustrated and explained. The findings show that the nanoparticle volume fraction and porosity parameters decrease the velocity, but increase the temperature. In addition, the temperature uplifts in the presence of radiation effect. The suction parameter initially decreases and then increases the velocity near the surface, while the temperature declines.

Simulation of the flow field and scour evolution by turbulent wall jets under a sluice gate

This numerical study of scour process tested the skills of computational fluid dynamics in modeling the unsteady flow field during the scour development stage by two-dimensional turbulent wall jets under a sluice gate. The modeling was found to well describe the experimentally observed flow patterns, that is, the main jet diverged to a returning jet and a tail jet. The model also correctly predicts the evolution of the scour depth and length. We examined the self-similarity of the profiles of scour bed and overlying velocities throughout the entire scour development and equilibrium stages. We found self-preserved profiles of velocities and scour beds using local jet parameters. Four growth curves were compared in describing the temporal evolution of scour depth. Finally, non-dimensional scaling of the equilibrium maximal scour depth was investigated. We used the theory of wall jet, and suggested that a modified jet Froude number can be used to predict the equilibrium scour depth, which accounts for the attenuation of the jet velocities along the apron.

Effect of thermal radiation on heat transfer in plane wall jet flow of Casson nanofluid with suction subject to a slip boundary condition

A Glauert type laminar wall jet issuing into a stationary liquid medium lying above a wall has technical uses in wall cooling and flow control. It plays a vital role in industrial applications like cooling/heating by impingement of jet, turbine blades, film cooling, mass and heat transfer phenomena. In this regard, a steady incompressible two-dimensional laminar Glauert kind wall jet is scrutinized in this study by considering nanoparticles suspension in the base liquid sodium alginate with suction and wall slip boundary conditions. Further, a comparative study is done by considering aluminum alloy and single-walled carbon nanotube as nanoparticles. The reduced ordinary differential equations (ODEs) are numerically solved by applying Runge–Kutta–Fehlberg fourth fifth-order (RKF-45) technique along with the shooting method. Results reveal that Casson nanofluid shows enhanced heat transfer than Casson nanoliquid for increased values of radiation parameter. The rising values of the Casson parameter deteriorate the heat transfer rate of both nanoliquids but an inverse trend is seen for improved values of radiation parameter.

Experimental Study of the Diffusion of a Confined Wall Jet through a Perforated Plate: Influence of the Porosity and the Geometry

This paper investigated lateral diffusion of a confined two-dimensional wall jet (air inlet height: 5 cm) through a perforated plate. We considered two plates with porosities of and . The plates were positioned at distances of 10 cm and 20 cm below the jet inlet. The experiments were realized using 2D Laser Doppler Anemometer (LDA). Different profiles of mean and fluctuating velocities are presented. The presence of a perforated plate strongly modified the airflow pattern compared to an empty enclosure. The velocities above and below the plate depend on several parameters, including the porosity and the plate’s position relative to the inlet slot and the longitudinal position. The difference between the flow velocity above and below the plates could not be related using a universal formula that depends on these parameters. We also investigated the influence of a porous media of a height of 20 cm (a stack of spheres having a diameter of 3.75 cm) located below the perforated plate. The results highlight that the porous medium strengthens the effects of the perforated plate on the flow.

Impact of thermophoretic particle deposition on Glauert wall jet slip flow of nanofluid

The heat transfer technology is growing significantly for desired solutions as the need for efficient heating and cooling systems in the aerospace, automotive, and chemical sectors. Bearing this in mind the need for effective cooling/heating systems, the steady, two-dimensional Glauert wall jet flow with thermophoretic particle deposition is investigated in the presence of Al2O3 nanoliquid. The Glauert transformations are used to transform the system of partial differential equations (PDEs) that reflect conservation, continuity, temperature, and concentration, as well as boundary conditions. The fourth fifth order Runge-Kutta-Fehlberg (RKF-45) method along with the shooting approach is used to solve the reduced system of ordinary differential equations (ODEs). The role of physical quantities for the parameters in concern are visually depicted and described in depth. Results reveal that, rise in values of velocity slip parameter upsurges the velocity profile away from the wall and declines near the surface. The increase in radiation parameter inclines the heat transfer. The growing values of both radiation parameter and volume fraction advances the heat transfer rate. The growing values of thermophoretic parameter improves the mass transfer rate.

Scaling mean velocity in two-dimensional turbulent wall jets

Studies in the literature on plane turbulent wall jets on flat surfaces, have invariably considered either the nozzle initial conditions or the asymptotic conditions far downstream, as scaling parameters for the streamwise variations of length and velocity scales. These choices, however, do not square with the notion of self similarity which is essentially a "local" concept. We first demonstrate that the streamwise variations of velocity and length scales in wall jets show remarkable scaling with local parameters i.e. there appear to be no imposed length and velocity scales. Next, it is shown that the mean velocity profile data suggest existence of two distinct layers - the wall (inner) layer and the full-free jet (outer) layer. Each of these layers scales on the appropriate length and velocity scales and this scaling is observed to be universal i.e. independent of the local friction Reynolds number. Analysis shows that the overlap of these universal scalings leads to a Reynolds-number-dependent power-law velocity variation in the overlap layer. It is observed that the mean-velocity overlap layer corresponds well to the momentum-balance mesolayer and there appears to be no evidence for an inertial overlap; only the meso-overlap is observed. Introduction of an intermediate variable absorbs the Reynolds-number dependence of the length scale in the overlap layer and this leads to a universal power-law overlap profile for mean velocity in terms of the intermediate variable.

Characteristics of two-dimensional counter-flowing wall jets

Wastewater plant effluents discharged into streams are expected to undergo dilution in a short reach. Jets provide rapid mixing and enhance dilution of the effluent discharged into the stream. The counter-flowing wall jet expansion angle, the geometry of the dividing streamline and the jet penetration length were found using the measured mean axial jet velocity data. These jet characteristics were expressed in terms of the jet velocity ratio (jet exit velocity against main stream velocity). A salt solution of known concentration was injected into the jet flow. The distribution of the salt concentration was determined at several cross-sections of the jet wake using a concentration meter. The standard deviation of the salt concentration provided a measure of jet mixing. When standard deviation reached zero at a cross-section, mixing was considered to be complete.

The Effect of the Nozzle Top Lip Thickness on a Two-Dimensional Wall Jet

The effect of the nozzle top lip thickness on a two-dimensional wall jet was examined experimentally in a wind tunnel using hot-wire anemometry. Lip thicknesses of 0.125b, 0.5b, 1b, and 2b, where b is the jet nozzle height, were considered at a Reynolds number of 30,700 based on the jet nozzle height and jet velocity. Noticeable differences in the flow profiles were observed at the jet outlet, but by 10b downstream these differences became insignificant. Different lip thicknesses resulted in different maximum velocity decay rates. The spread of the wall jet was found to be insensitive to the lip thickness.

Experimental study on the scour due to a water jet subjected to lateral confinement

The effect of lateral confinement due to very close side walls, on erosion caused by a water jet issuing from a rectangular sluice aperture, has been studied experimentally in a narrow laboratory flume. Velocimetry (PIV) in presence of a smooth, no erodible bottom, revealed self-similar velocity profiles with the structure of a wall jet, but upward-shifted with respect to the classical two-dimensional profile. The classical definition of the Shields number was modified, to include the influence of the relative roughness and the inlet aspect ratio. This last coefficient was also relevant in the definition of a suitable length scale to account for the effect of the lateral confinement. Scour patterns in the narrow flume have also shown differences with respect to erosion by two-dimensional wall jets: the downstream mound undergoes a morphological transition, from triangular to trapezoidal, at the early stage of the scour development, and grains move downhill driven by gravity inside the granular wedge, in a regime of steady recirculation (SR). Above this regime, a digging–refilling (DR) cycle, with periods of the order of seconds, triggers at a well-defined Shields number.

Evaluation of RANS and LES turbulence models for simulating a steady 2-D plane wall jet

PurposeThis paper examines various turbulence models for numerical simulation of a steady, two-dimensional (2-D) plane wall jet without co-flow using the commercial CFD software (ANSYS FLUENT 14.5). The purpose of this paper is to decide the most suitable and most economical method for steady, 2-D plane wall jet simulation.Design/methodology/approachSeven Reynolds-averaged Navier–Stokes (RANS) turbulence models were evaluated with respect to typical jet scaling parameters such as the jet half-height and the decay of maximum jet velocity, as well as coefficients from the law of the wall and for skin friction. Then, a plane wall jet generating from a rectangular slot of 1:6 aspect ratio located adjacent to the wall was investigated in a three-dimensional (3-D) model using large eddy simulation (LES) and the Stress-omega Reynolds stress model (SWRSM), with the results compared to experimental measurements.FindingsThe comparisons of these simulated flow characteristics indicated that the SWRSM was the best of the seven RANS models for simulating the turbulent wall jet. When scaled with outer variables, LES and SWRSM gave generally indistinguishable mean velocity profiles. However, SWRSM performed better for near-wall mean velocity profiles when scaled with inner variables. In general, the results show that LES performed reasonably well when predicting the Reynolds stresses.Originality/valueThe main contribution of this article is in determining the capabilities of different RANS turbulence closures and LES for the prediction of the 2-D steady wall jet flow to identify the best modelling approach.

An experimental study of the turbulent structure of two-dimensional wall jet

An experimental study of the turbulent structure of two-dimensional wall jet

Further solutions for laminar boundary layers with cross flows driven by boundary motion

Laminar boundary layers with cross flows driven by transverse plate motions are considered. In each case, the cross flows are generated either by uniform plate motion or by transverse uniform shear. The three streamwise laminar boundary layers studied are the two-dimensional wall jet, the Blasius boundary layer flow with streamwise uniform plate motion, and uniform shear flow over a plate with streamwise stretching plate motion. In each case, a coordinate decomposition reduces the governing equations to a primary ordinary differential equation for the streamwise flow and a secondary linear equation coupled to the primary solution describing the fully developed cross flow. The one-parameter family of Blasius and uniform shear flow solutions, dependent on the streamwise wall motion parameter \(\lambda \), have dual solutions, and previous studies have shown that the upper branches of these dual solutions are stable, while the lower branches are unstable. This limits the range of admissible solutions for the new cross flows studied here.

Scour due to turbulent wall jets downstream of low-/high-head hydraulic structures

AbstractTo overcome over-prediction of the scour depths by existing methods, a mathematical model is developed based on a work transfer theory. This model predicts the equilibrium scour hole’s depth and length due to two-dimensional turbulent wall jets downstream of low-/high-head hydraulic structures. The work transfer theory states that the work done by the attacking jet flow is transferred to work done to remove the volume of the scoured bed material out of the scour hole. This results in an analytical nonlinear equation for predicting the equilibrium scour depth. This unique feature of the nonlinearity of the developed equation shows the mutual dependence of the scour geometry and flow hydrodynamics on each other. Non-circulating wall jet flows and re-circulating jet flows within the scour hole are both considered. A separate equation is developed for predicting the length of the scour hole. Field data at the Nile Grand Barrages in Egypt and the Shimen Arch Dam in China are used to validate the develo...

Laminar Wall Jet Flow and Heat Transfer over a Shallow Cavity.

This paper presents the detailed simulation of two-dimensional incompressible laminar wall jet flow over a shallow cavity. The flow characteristics of wall jet with respect to aspect ratio (AR), step length (X u), and Reynolds number (Re) of the shallow cavity are expressed. For higher accuracy, third-order discretization is applied for momentum equation which is solved using QUICK scheme with SIMPLE algorithm for pressure-velocity coupling. Low Reynolds numbers 25, 50, 100, 200, 400, and 600 are assigned for simulation. Results are presented for streamline contour, velocity contour, and vorticity formation at wall and also velocity profiles are reported. The detailed study of vortex formation on shallow cavity region is presented for various AR, X u, and Re conditions which led to key findings as Re increases and vortex formation moves from leading edge to trailing edge of the wall. Distance between vortices increases when the step length (X u) increases. When Re increases, the maximum temperature contour distributions take place in shallow cavity region and highest convection heat transfer is obtained in heated walls. The finite volume code (FLUENT) is used for solving Navier-Stokes equations and GAMBIT for modeling and meshing.

An experimental study of mixed convection over various thermal activation lengths of vertical TABS

This paper presents experimental study of supported mixed convection of two-dimensional wall jet over various thermal activation lengths of vertical TABS. It was found out that supported mixed convection upward the jet over vertical TABS is not influenced by thermally inactivated vertical TABS downwards of the wall jet. Practical consequences of this founding were after that tested using TRNSYS simulation tool in energy simulation study for contemporary modern office room having vertical TABS. Results show that office room can be sufficiently cooled by mixed convection even for TABS designs where only upper half of room wall is thermally activated by environmental high temperature cooling sources.

Flow Environment and Organized Turbulence Structures Near a Plane Below a Rotor

Time-resolved particle image velocimetry measurements were made in the turbulent flow environment below a hovering rotor near a ground plane that was covered with mobile sediment particles. The results were compared to the near-wall flow produced by a nominally equivalent two-dimensional wall jet. The objective was to understand the fluid dynamic mechanisms of how the mean flow, stochastic turbulence, and concentrated vorticity produced by the rotor affected the mobilization and pickup of particles from the bed. Another objective was to better understand the assumptions that would be required for the development of models that are applicable to rotor-induced particle mobilization. The mean flow in the boundary layer at the ground below the rotor was found to be similar to a wall jet. However, the instantaneous flowfield and turbulence characteristics between the two flows were significantly different. Measurements of the heterogeneous dual-phase flow environment using particle image and particle tracking velocimetry exposed the complex interplay between the carrier and dispersed phases of the flow. The turbulence characteristics were clearly modified by the presence of particles in the near-wall region. The processes of particle mobilization and pickup from the bed were found to correlate with the Reynolds stresses and discrete turbulence events, respectively.