Three-dimensional Wall Jet Research Articles

Bed roughness effects on characteristics of turbulent confined wall jets

In this paper, effects of bed roughness on the flow pattern and turbulence characteristics, such as velocity contours, shear stresses, Reynolds stresses and turbulence intensities on a three-dimensional wall jet were investigated experimentally. The experiments have been conducted in a channel which is made of metal floor and glass walls, width 0.57 m, height 1 m and length 10 m. Jet was settled using a pipe with internal diameter of 20 mm over a horizontal bed. A velocimeter was used to measure the flow velocity at three directions. The time-averaged values of velocities were used to calculate the turbulence characteristics. The results showed that with increasing surface roughness, the rate of growth length of longitudinal scale decreases, while decay rate and jet velocity increases. Moreover, the maximum value of turbulence intensity is augmented with increasing the surface roughness. When surface roughness increases, the rate of length scale decreases and decay of jet velocity enhances. Whereas u-component of turbulence intensity increases by surface roughness increment. Also, when distance increases from jet exit, bed shear stress decreases and when surface roughness increases, surface shear stress enhances.The vertical distribution of ŵ for various x̂ showed that the vertical direction of velocity has changed from descending to ascending in x/d = 45 to x/d = 55 due to the existence of backflow and consequently the creation of a vortex. Furthermore, with an increase in surface roughness, the decay of W component of velocity has increased.

Quasi-radial wall jets as a new concept in boundary layer flow control

ABSTRACTThis work aims to introduce a novel concept of wall jets wherein the flow is radially injected into a medium through a sector of a cylinder, called quasi-radial (QR) wall jets. The results revealed that fluid dynamics of the QR wall jet flow differs from that of conventional wall jets. Indeed, lateral and normal propagations of a conventional three-dimensional wall jet are via shear stresses. While, lateral propagation of a QR wall jet is due to mean lateral component of the velocity field. Moreover, discharged Arrays of conventional three-dimensional wall jets in quiescent air lead to formation of a combined wall jet at large distant from the nozzles, while QR wall jet immediately spread in lateral direction, meet each other and merge together very quickly in a short distance downstream of the jet nozzles. Furthermore, in discharging the conventional jets into an external flow, there is no strong interaction between them as they are moving parallel. While, in QR wall jets the lateral components of the velocity field strongly interact with boundary layer of the external flow and create strong helical vortices acting as vortex generators.

Plane boundary effects on characteristics of propeller jets

The flow properties of a propeller jet in the presence of a plane bed boundary were investigated using the particle image velocimetry technique. Three clearance heights, Z b = 2D p, D p, and 0.5D p, where D p = propeller diameter, were used to examine boundary effects on the development of the jet. In each case, the mean flow properties and turbulence characteristics were measured in a larger field of view than those used in past studies. Both the streamwise and transverse flow fields were measured to obtain the three-dimensional characteristics of the propeller jet. Similar to a confined offset jet, the propeller jet also exhibits a wall attachment behavior when it is placed near a plane boundary. As a result, in contrast to its unconfined counterpart, the confined propeller jet features three regions, namely the free jet, impingement and wall jet regions. The study shows that the extent of each region varies under different clearance heights. The development of the mean flow and turbulence characteristics associated with varying clearance heights are compared to illustrate boundary effects in these regions. In the impingement region, the measured transverse flow fields provide new insights on the lateral motions induced by the impingement of the swirling jet. In the wall jet region, observations reveal that the jet behaves like a typical three-dimensional wall jet and its axial velocity profiles show good agreement with the classical wall jet similarity function.

Experimental Measurement of the Velocity Field of Round Wall Jet in Counterflow

Experimental investigations showing the mean and fluctuating velocity fields of a three-dimensional round wall jet in counterflow are presented. The ratios of jet to counterflow velocity ranged from 2.5 to 25 and the jet Reynolds numbers were from 1,000 to 10,000. The jet penetration length, which is the axial distance that the wall jet penetrates into the counterflow before stopping, was measured and scaled with a power-law relation. The decay of the jet velocity during its penetration into the oncoming flow was analyzed with proper scaling parameters. The results show that up to 70% of the wall jet penetration length, the velocity decay, and the spreading rate are effectively identical to those of a wall jet in quiescent ambient flow. Above this distance, the velocity decay is linear and the jet spreading rate increases sharply. The mean axial velocity profiles show self-similarity between 25 and 80% of the penetration distance and are similar to the case with no counterflow. The vortical structures from the jet and the opposing turbulent boundary layer interact in the stagnation area to provide a large recirculation region with stochastic oscillations that enhance the mixing of the wall jet and counterflow. These interactions result in a region of high turbulence intensity near the upstream extents of the wall jet. The profiles of Reynolds stresses are presented and discussed. It was observed that the normalized velocity fluctuation profiles show self-similarity for the region within 30 and 70% of the wall jet penetration length, which is shorter than the region for which the axial velocity profiles are self-similar.

Reynolds stress distribution and turbulence generated secondary flow in the turbulent three-dimensional wall jet

The lateral half-width of the turbulent three-dimensional wall jet is typically five to eight times larger than the vertical half-width normal to the wall. Although the reason for this behaviour is not fully understood, it is caused by mean secondary flows that develop in the jet due to the presence of the wall. The origin of the secondary flow has been associated previously with both vorticity reorientation and also gradients in the Reynolds stresses, although this has not been directly quantified as yet. The present investigation focuses on a wall jet formed using a circular contoured nozzle with exit Reynolds number of 250 000. Stereoscopic particle image velocimetry measurements are used herein to measure the three-component velocity, thereby allowing access to the full Reynolds stress tensor that contributes to the secondary flow in a turbulent three-dimensional wall jet. Throughout the jet, the Reynolds normal stress ($\overline{u^{2}}$) makes the largest contribution to the Reynolds stress field whereas Reynolds shear stress ($\overline{vw}$) is found to be negligible when compared with other stresses. In particular, the differences in the Reynolds normal stresses ($\overline{v^{2}}-\overline{w^{2}}$) are found to be significantly larger than $\overline{vw}$; these terms are important for the generation of turbulence secondary flow in the wall jet. Above all, the differences in the Reynolds normal stresses are oriented to reinforce the near-wall streamwise vorticity, and thus contribute to the large lateral growth of this flow. The contours of the turbulent kinetic budget indicate that the turbulent energy budget obtained on the jet centreline is different from that obtained off of the jet centreline.

Turbulence Measurements in a Corner Wall Jet1

The corner wall jet is similar to the standard three-dimensional wall jet with the exception that one-half of the surface has been rotated counterclockwise by 90 deg. The corner wall jet is selected for study as the geometry occurs in practical applications and is an ideal benchmark case for numerical code validation. The corner wall jet investigated here is formed using a long round pipe with a Reynolds number of 159,000. Contours of the mean and turbulent flow field were measured using hot-wire anemometry from x/D = 0 to 40. The results indicate that the ratio of lateral-to-vertical growth in the corner wall jet is approximately half that in a standard turbulent three-dimensional wall jet. The results indicate that this behavior is not simply tied to a slower development of the corner wall jet.

Prediction of Scour Profile Caused by Submerged Three-Dimensional Wall Jets

Prediction of Scour Profile Caused by Submerged Three-Dimensional Wall Jets

One-Equation Extended Nonlinear Turbulence Modeling in Predicting Three-Dimensional Wall Jets

To predict the flowfield of three-dimensional wall jets, an extended nonlinear constitutive relation is incorporated into the Spalart–Allmaras turbulence model. A modification of the model is made to improve the predictability of free jets. The FaST aerodynamic routines code, developed at the Japan Aerospace Exploration Agency, is used as a compressible-flow solver. First, a turbulent boundary-layer flow along a flat plate is computed to confirm the validity of the extended nonlinear constitutive relation. The obtained logarithmic velocity profile agrees well with the analytical solution. As a result, the modification is confirmed to have no adverse effects on the prediction of the typical wall-bounded flow. Second, computations of three-dimensional free and wall jets are performed, the results of which are compared with observations of the velocity field up to the far region from the jet nozzle exit, which are made by the experimental facility of Nagoya University. In free jets, reasonable computed results are obtained with respect to the streamwise decay and the half-width of the velocity, confirming the applicability of the modification to free jets. In wall jets, the modified model properly reproduces the secondary flow observed in the experiment, and its qualitative success is shown by the conservation of momentum.

Coherent Streamwise Vortex Structures in the Near-Field of the Three-Dimensional Wall Jet

A turbulent three-dimensional wall jet with an exit Reynolds number of 250,000 was investigated using stereoscopic particle image velocimetry (PIV) in the near-field region (x/D = 5). The proper orthogonal decomposition (POD) was applied to all three components of the velocity field to investigate the underlying coherent structures in the flow. A low-dimensional reconstruction of the turbulent velocity field using the first five POD modes showed the presence of coherent streamwise vortex structures formed in the outer shear-layers of the wall jet, not unlike those found in the near-field of free jets. The instantaneous streamwise vorticity reconstructed from the low-dimensional reconstructed velocity field indicates the presence of a persistent vortex pair close to the wall and on either side of the jet centerline that appear similar to the mean streamwise vorticity. These regions do not appear to be directly related to the positioning of the streamwise vortex structures in the outer shear-layer.

0140 三次元壁面噴流の数値解析に向けた非等方乱流モデルの構築(OS1 噴流,後流および剥離流れ現象の探求と技術革新,オーガナイズドセッション)

For Shear Stress Transport (SST) turbulence model, an anisotropic algebraic constitutive relations for the Reynolds stress tensor are integrated in CFD solver developed in JAXA called FaST Aerodynamic Routines (FaSTAR) that is three-dimensional compressible Reynolds averaged Navier-Stokes code. Using these relations, we calculate computations of three-dimensional wall-jet. As compared with experimental data, it is found that a wide spreading in spanwise direction in cross section of jet axis can be predicted and the obtained ratio of velocity half-width is improved by taking into account the anisotropy. The obtained results of SST turbulence model with algebraic constitutive relations agree quite well with those obtained in experiment.

Shock associated noise generation in curved turbulent Coanda wall jets

Curved three-dimensional turbulent Coanda wall jets are present in a multitude of natural and engineering applications. The mechanism by which they form a shock-cell structure is poorly understood, as is the accompanying shock-associated noise (SAN) generation. This paper will discuss these phenomena from both a modeling and experimental perspective. The Method of Characteristics will be used to rewrite the governing hyperbolic partial differential equations as ordinary differential equations, which will then be solved numerically using the Euler predictor-corrector method. The effect of complicating factors such as radial expansion and streamline curvature on the shock-cell location predictions will be discussed. Comparison of the theoretical calculations of the shock wave structure with associated schlieren flow visualization results will be presented. Related acoustical measurements will also be communicated. In this way, critical flow characteristics for shock-cell formation will be identified, and their influence on SAN discussed.

Characteristics of Turbulent Three-Dimensional Wall Jets

Three-dimensional turbulent wall jet was investigated using a particle image velocimetry technique. Three Reynolds numbers based on the jet exit velocity and diameter of 5000, 10,000, and 20,000 were studied. Profiles of the mean velocities, turbulence intensities, and Reynolds shear stresses as well as two-point velocity correlations and proper orthogonal decomposition analyses were used to document the salient features of the wall jets. The decay and spread rates are independent of Reynolds numbers in the self-similar region. The estimated values of 1.15, 0.054, and 0.255 for the decay rate, wall-normal spread rate, and lateral spread rate, respectively, are within the range of values reported in the literature. The two-point correlation analysis showed that the inclination of the streamwise velocity correlation contours in the inner layer is 11±3 deg in the wall region, which is similar to those of canonical turbulent boundary layers. The results from the proper orthogonal decomposition indicate that low-order modes contribute more to the turbulence statistics in the self-similar region than in the developing region. The Reynolds shear stresses are the biggest benefactors of the low-order mode contribution while the wall-normal turbulence intensities are the least.

Experimental and Numerical Study of Laminar Round Jet Flows Along a Wall

In the present study, both experimental and numerical techniques were employed to study three-dimensional laminar wall jet flows. The wall jet was created using a circular pipe of diameter 7×10−3 m and flows into an open water tank. The inlet Reynolds numbers based on the pipe diameter and jet exit velocity were 310 and 800. A particle image velocimetry (PIV) was used to conduct detailed measurements at various streamwise-transverse and streamwise-spanwise planes. The complete nonlinear incompressible Navier–Stokes equation was also solved using a collocated finite volume based in-house computational fluid dynamics (CFD) code. The computation was performed for three inlet Reynolds numbers, namely, 310, 420, and 800. From the PIV measurements and CFD results, velocity profiles and jet half-widths were extracted at selected downstream locations. It was observed that the numerical results are in reasonable agreement with the experimental data. The distributions of the velocities, jet spread rates, and vorticity were used to provide insight into the characteristics of three-dimensional laminar wall jet flows.

PIV Study of Laminar Wall Jets of Non-Newtonian Fluids

Three-dimensional laminar wall jet flows of shear-thinning non-Newtonian fluids have been studied using a particle image velocimetry technique. The non-Newtonian fluids were prepared from xanthan gum solutions of various concentrations. The velocity measurements were performed in various streamwise-transverse and streamwise-spanwise planes at various inlet Reynolds numbers. From these measurements, the maximum velocity decay, jet half-widths, and velocity profiles were obtained to study the effects of Reynolds number and fluid type on the characteristics of the wall jet flows. It was observed that the maximum velocity decay and jet half-widths depend on inlet Reynolds number and fluid but the similarity velocities profiles are independent of both Reynolds number and specific fluid type.

Spectral Linear Stochastic Estimation of the Turbulent Velocity in a Square Three-Dimensional Wall Jet

The instantaneous turbulent velocity field in a three-dimensional wall jet was estimated from the fluctuating wall pressure using a spectral linear stochastic estimation technique. The wall jet investigated issued from a long square channel with Reynolds number of 90,000. Two downstream positions in the intermediate field were examined, x/h=10 and x/h=20, owing to the rapid changes in wall jet development over this region. The results indicate that the passage of the large-scale structures cause large, coherent lateral sweeps of fluid across the entire span of the wall jet. These sweeps are caused by the passage of half horseshoe-like structures and appear to be responsible for the larger lateral development of this flow.

Numerical Investigation of Three-Dimensional Laminar Wall Jet of Newtonian and Non-Newtonian Fluids

Three-dimensional laminar wall jets of a Newtonian fluid and two shear-thinning non-Newtonian fluids were numerically investigated. The complete nonlinear incompressible Navier-Stokes equation was solved using a colocated finite volume based in-house computational fluid dynamics code. For each fluid, the computation was performed at three Reynolds numbers. The results showed that the streamwise velocity profiles for the Newtonian fluid became self-similar but the more shear-thinning fluid never achieved a self-similar condition. Significant differences were observed among the profiles for the various fluids in the inner region. Although the transverse and spanwise components of the velocity decreased substantially with increasing Reynolds number, the values for the non-Newtonian fluids were generally an order of magnitude larger than the corresponding values for the Newtonian fluid. Depending on the specific fluid and Reynolds number, the apparent viscosities were up to 4 orders of magnitude higher than the dynamic viscosity of water. Consequently, the spread of the jet in both the transverse and spanwise directions, decay of the maximum streamwise velocity, and the skin friction coefficient depend strongly on both Reynolds number and nature of the fluid. The results also show that the jet half-width in the transverse direction is significantly higher than in the spanwise direction.

Comparison of Far-Field Turbulent Structure of a Rectangular Surface Jet to Three-Dimensional Free and Wall Jets

The turbulence structure of a rectangular surface jet is compared to that of the three-dimensional free and wall jets. The surface jet turbulence quantities are mapped using laser Doppler velocimetry. In general, the turbulence structure of these three jets is found to be significantly different. For the surface jet, the free surface kinematic condition has a predominant effect on the whole structure, while for the wall jet, the influence of wall kinematic constraint is contained in the wall layer. A surface current with a higher lateral spreading rate than the submerged portion of the jet is developed, which does not exist for the wall jet because of the no-slip boundary condition. Unlike free jets, the submerged portion of the rectangular surface jet is characterized by two length scales. The Prandtl hypothesis with constant eddy viscosity provides a good estimate for the shear stresses in the lateral direction, but fails in the vertical direction, where the velocity profiles are much flatter, due to the free surface condition, than those for the free and wall jets.

Three-Dimensional Turbulent Wall Jets Issuing from Moderate-Aspect-Ratio Rectangular Channels

The development of three-dimensional turbulent wall jets emanating from long channels with outlet cross-sectional aspect ratios from 1 to 8 was investigated by measuring the mean and turbulent flowfields using hot-wire anemometry. The turbulent velocity profiles indicate that the core of the jet behaves like a two-dimensional wall jet before the interaction of the lateral shear layers. Contours of the full flowfield indicate that the turbulent mechanism that causes the lateral growth of the three-dimensional wall jets is located in the lateral shear layers near the wall. Increasing the outlet aspect ratio separates the lateral shear layers, causing a wider core region of two-dimensional wall-jet development that, in turn, delays the onset of far-field three-dimensional wall-jet development. The development of the different aspect-ratio wall jets collapsed onto a single curve when the streamwise coordinate was normalized by the square root of the channel cross-sectional area and the vertical and lateral jet half-widths were normalized by the height and width of the channel, respectively.

The Asymmetry of the Large-Scale Structures in Turbulent Three-Dimensional Wall Jets Exiting Long Rectangular Channels

The development of the large-scale structures in three-dimensional wall jets formed using long rectangular channels with aspect ratios of 1 and 4 was investigated using measurements of the fluctuating wall pressure and point measurements of the turbulent velocity throughout the near and intermediate field. The instantaneous pressure fluctuations in both jets were laterally asymmetric causing the fluctuating wall pressure to be poorly correlated across the jet centerline. A frequency-dependent proper orthogonal decomposition (POD) of the fluctuating pressure measurements indicated that the first two mode shapes were opposite and each mode made similar contributions to the mean square fluctuations at all frequencies in order to capture the instantaneous asymmetry of the pressure field. The mode shapes in the intermediate field of both jets were strongly frequency dependent, and a subsequent wavelet analysis indicated that there are both large-scale horseshoe structures that span one-half of the jet and separate, smaller, near-wall structures located near the jet centerline. The initial development of the large-scale structures in the two jets differed, with the most energetic fluctuations being more antisymmetric in the square jet.

Experimental Study on a Three-Dimensional Wall Jet (Flow Field in a Cross-Section of Developed Region)

This study is to clarify the flow structure of a three-dimensional wall jet discharged tangentially on a flat surface from a circular nozzle. Distributions of the mean velocities and fluctuating velocities in a cross section of a fully-developed flow region were measured by using a single hot-wire probe and cross-wire probes. It is confirmed in any position of the lateral direction that the distributions of the mean velocities U and W in the direction normal to the wall are made similar to each other when they are made dimensionless by the maximum local velocity while the jet height remains almost invariable in the spanwise direction. The direction of the mean flow vector in the inner layer is remained constant while the direction of the mean flow vector outside the half-value height is coincided with the direction of the jet axis. The distributions of the turbulent statistics in the direction normal to the wall are almost similar to each other outside the half-value height due to the scaling by the maximum local velocity, and on the other hand, the magnitude of the turbulent energy and Reynolds stress in the wall-side region becomes larger at the position farther from the jet axis.