Circular Wall Jet Research Articles

Effects of Secondary Current on the Local Erosion Induced by Circular Turbulent Wall Jets in Cross Flow

This paper presents the results of a series of laboratory experiments, by placing a turbulent wall jet in the upstream and downstream of a 90° channel bend, to investigate the effects of secondary flow on formation of local sediment erosion and deposition. The effects of jet and current flow intensities on the erosion pattern were characterized by the jet’s Reynolds number, Re, channel Froude number, Fr, and the ratio of water depth to nozzle diameter, h/do. The geometrical characteristics of local erosion such as jet trajectory, maximum scour depth, ds, and maximum mound height, hm, were measured. The area and volume of scours and mounds were calculated from measurements, and the results were correlated with the controlling parameters. Experimental observations indicated the existence of distinct scour patterns due to variations of channel Froude number. Two additional scour holes were identified due to the effect of stream interactions and formation of vortex regions immediately downstream of sediment mound. It was found that both channel and jet flows contributed to trajectory of the jet and the footprint of erosion. A threshold Froude number of Fr=0.091 was identified at which the current and jet flow were in equilibrium. The results showed that for a constant value of Fr, all scour parameters increased with increasing Reynolds number. However, Re did not have any effects on the jet’s deflection. Comparing the erosion profiles in the upstream and downstream bend indicated that the secondary current and turbulent redistribution altered the erosion and mound formation in the downstream of the bend in relatively high Reynolds (i.e., Re≥23,000), high channel Froude numbers (i.e., Fr>0.091) and high jet submergence (i.e., h/do>10.53).

Effect of armor layer on the local scour formation induced by a deeply submerged circular wall jet in confined channels

A series of laboratory experiments was conducted in a confined channel to study the effect of armor layer on the scour formation induced by a deeply submerged circular turbulent wall jet. A wide range of particle sizes was utilized to form an armor layer and the local erosion downstream of a circular wall jet was measured. The time evolution of scour dimensions was recorded and the scour profiles of the mixed bed were compared with the scour profiles of a relatively uniform sand bed with the same average particle size. Three regimes were identified as the initial stage (Regime I), the quasi-steady state (Regime II), and the steady state (Regime III). The characteristic lengths of the scour showed rapid growth at the early stage of scour development and the growth rate decreased until the scour reached the steady-state condition. The effect of armor layer on scour dimensions was pronounced, which significantly reduced the maximum scour depth due to formation of an armor layer. The characteristic parameters affecting the maximum scour depth and width were identified and empirical correlations were introduced for the prediction of scour dimensions. The scour profiles in the mixed bed showed two consecutive scour depths. It was found that the area occupied by the armor layer increased by increasing the Froude number of the jet.

Circular turbulent wall jets in quiescent and coflowing surroundings

The dynamics and mixing of circular turbulent wall jets released into both a quiescent background and coflowing stream have been investigated experimentally. The statistics of the velocity field (measured by way of acoustic Doppler velocimetry) for the wall jets emitted into a quiescent background agree well with those of the other studies. The experiments involving coflowing wall jets were undertaken at three different jet-to-coflow velocity ratios. The coflowing wall jets were found to decay and spread at slower rates and have lower mean lateral velocities compared to wall jets in quiescent surroundings. Moreover, the decay and spreading rates of the coflowing wall jets increased with increasing jet-to-coflow velocity ratios. The wall jets issued into a coflow also developed more slowly and reached self-similarity at farther downstream distances relative to those emitted into a quiescent background. Given the decreased decay rate, spreading rate, and mean lateral velocities of wall jets in the presence of a coflow, it was inferred that the entrainment into, and mixing of, the wall jets was reduced, presumably due to the suppresion by the coflow of the vortical structures that characterize wall jets in quiescent backgrounds. Finally, the root-mean-square velocities of the wall jets increased when a coflow was present, and were found to be nearly self-similar in the range of measurements studied herein, in contrast with coflowing jets (that are not released in the vicinity of a wall).

Local Scour on Noncohesive Beds by a Submerged Horizontal Circular Wall Jet

Local scour on noncohesive beds caused by horizontal circular wall jets in submerged environments is investigated in this study. The effects of the ratios of sediment size-to-hydraulic radius, Ks=d50/Rh, on the jet scour are considered. A total of 420 datasets of previous observations are compiled and 12 laboratory experiments in the Ks range of 0.44–2.44 are conducted to investigate the effect of Ks on scour dimensions. A dimensional analysis shows that both the particle gravity and bed resistance to the flow play important roles in the scour. Analysis on the data shows that the maximum scour dimensions vary linearly with the densimetric Froude number F84 and nonmonotonically with a manner of a Gaussian function of Ks, respectively. Finally, the formulas to predict the maximum scour dimensions of a submerged horizontal circular wall jet are proposed, which are fairly well in line with existing laboratory and field data.

Large eddy simulations of turbulent circular wall jets

Abstract Turbulent circular wall jets have been extensively investigated both experimentally and numerically in the past decades. Most of the previous studies focus on the kinematic characteristics of the jet flows, while the mixing characteristics including the scalar transport have not been well examined. The current study performs a comprehensive investigation of the turbulent circular wall jet including both the flow and mixing characteristics using the large eddy simulations (LES) approach with proper near-wall modelling. The LES results are compared to the existing experimental measurements, as well as numerical results from two other Reynolds-averaged Navier–Stokes (RANS) models: the standard k -e and standard k – ω models, with enhanced wall functions. The comparison focuses on the velocity and scalar distributions, rates of velocity and scalar decay, variations of characteristic length scales, and turbulence intensities in different directions. Overall, the study shows that LES coupling with proper near-wall modelling can simulate both the kinematic and mixing characteristics of the turbulent wall jet in a satisfactory manner, and the accuracy is superior to the RANS models with enhanced wall functions for this three-dimensional wall-bounded shear flow. The advantages can be attributed directly to the better simulations of the anisotropic jet spreading near the wall.

OPEN TYPE QUAY STRUCTURES UNDER PROPELLER JETS

In recent years, dramatically increases in ship dimensions and installed engine power, introduction of new type of special purpose ships and use of roll-on/roll-of, ferries, container ships can cause damage which in many cases threatens to undermine berth structures. Vessel jets of these types of ships can change flow area and cause erosion and scour around foundation of berth structures. Due to the damages in berth structures maintenance and repair cost may increase and also cause management losses. For this reason vessel jet induced the flow area around the berth structures during ships berthing and un-berthing operations are extremely important factor for the port structure design. This study is related with investigation of the flow characteristics at the sea bed around the pile, experimentally. Vessel jets were simulated both as circular wall jet and also propeller jet. The objective of this study is to determine the sea bed shear stress and velocity profiles along the jet axis for open type wharf structures (around a cylindrical piles and also on the slopes). Hot film anemometers were used to measure the magnitude of the bed shear stresses. The results from propeller jet experiments explained the erosion over the slopes. Bed shear and velocity profile measurements were carried out on the rigid bed conditions.

Scour formation due to simultaneous circular impinging jet and wall jet

Experiments involving both a circular wall jet and an impinging jet were conducted, as occurring at irrigation control structures or at dams with combined crest and low-level spillways. The effects of the non-dimensional parameters on the scour hole dimensions have been investigated. It was found that the effect of the expansion ratio of the channel width to the jet diameter is negligible for values larger than 22.5 and 35, depending on the experimental conditions. The scour depth decreases with increasing tailwater depth and the horizontal distance between the impinging jet and the origin of the wall jet. The scour length and the distance between the end of the ridge and the wall jet origin increase with decreasing densimetric Froude number of the impinging jet. The opposite trend occurs for decreasing densimetric Froude number of the wall jet. An empirical relationship for estimating the scour depth due to the simultaneous jet action is given.

Large eddy simulation of flow structure in the near region of a circular wall jet

When there is a turbulent circular wall jet discharge from a nozzle into a stagnant ambient area with initial momentum, the velocity field changes greatly, especially near the wall or the interface between the jet and surrounding fluid. This accounts for the great velocity gradient produced in the entrainment between jets and any surrounding fluid and the appearance of large-scale coherent structure. This phenomenon is more distinct in the region near the jet exit. However, experimentation cannot capture this characteristic of the near region because of the restrictions inherent in experimental technology. Large eddy simulation (LES) provides the mean and turbulent velocity characteristics in the near region without disturbing the surrounding area and reproduces the coherent structure. Here, a LES study of the flow features in the near region of a circular wall jet based on the finite volume method with 8.42 million nodes is reported. The mean velocity characteristics in the near region and fully developed region simulated by LES including the decay of maximum velocity and the evolution of velocity half-width agreed well with the results of similar experiments. The mean and turbulent velocity distributions of the circular wall jet in the near region (x=0-20D) were also investigated. It was found that the distributions of normalized mean velocity in spanwise direction were similar to those in the far field (x≥20D), but not exhibited similarity in vertical direction. Moreover, the turbulent velocities in two directions were smaller than the ones in the far field. We also analyzed the probability density function (PDF) of the streamwise fluctuant velocity at several locations. Furthermore, we looked at turbulent Reynolds stresses profiles and coherent structure in the region of the jet exit vicinity and the near wall, which essentially prescribe hydrodynamic properties but are difficult to obtain from experimentation or other Reynolds average Novier-Stokes methods.

Local scour due to an upwards inclined circular wall jet

The results of experiments on the scour due to an upwards inclined circular wall jet are presented. Outflow water jets with a velocity of 0·5–1·67 m/s at different angles of 0–20° using two pipes of 1 and 2 cm inner diameter were considered. The tailwater depth varied from 3 to 22 cm. The scour process due to horizontal and inclined wall jets is presented, and the effect of wall jet angle, tailwater depth and densimetric Froude number on the scour hole parameters is investigated. By using available data from different sources and considering the jet angle, new equations for estimation of scour parameters are developed.

Scour by jets in cohesionless and cohesive soils

A technique is developed in this paper to unify the methods of analyzing scour by turbulent water jets in cohesionless and cohesive soils. Data from previous studies using circular turbulent impinging jets and circular turbulent wall jets are used to compare the scour in low void ratio cohesive soils to that in uniform sands and gravels. Scour by these jets is related to the dimensionless excess stress on the soil bed. It is seen that this parameter will likely work well for developing a method to predict scour for circular wall jets that is applicable to both materials. However, a circular impinging jet appears to vary appreciably in its interaction with the bed between the two types of soil, which makes developing a unified method to predict scour by impinging jets more difficult. Key words: erosion, scour, water jets, cohesionless sediments, cohesive sediments, fine-grained soils, coarse-grained soils.

Erosion of Finite Thickness Sediment Beds by Single and Multiple Circular Jets

Sediment management in reservoirs with the help of water jets has motivated this work. Erosion caused by single and multiple submerged circular turbulent wall jets on a noncohesive sediment bed of finite thickness lying on a fixed boundary was studied with the help of laboratory experiments. Different combinations of jet diameter, jet separation, and sediment thickness to jet diameter ratio were tested. Results show a relation between dimensionless parameters characterizing the steady state bed profile and the densimetric particle Froude number F0 given by the velocity at the nozzle and the effective diameter and submerged specific density of the sediment. Evolution of scour with time confirms previous studies where the erosion was found to initially grow with the logarithm of time up to a certain reference time t*. This time, made dimensionless with a time scale tc involving the volume of sediment scoured and the rate of erosion, was also related to the densimetric Froude number. A comparison with studies regarding erosion of a semiinfinite layer of sediment is also presented.

Experimental and Numerical Investigation of Trailing Edge Film Cooling by Circular Coolant Wall Jets Ejected From a Slot With Internal Rib Arrays

The present study concentrates on the experimental and computational investigation of a cooled trailing edge in a modern turbine blade. The trailing edge features a pressure side cutback and a slot, stiffened by two rows of evenly spaced ribs in an inline configuration. Cooling air is ejected through the slot and forms a cooling film on the trailing edge cutback region. In the present configuration the lateral spacing of the ribs equals two times their width. The height of the ribs, i.e., the height of the slot equals their width. Since the ribs are provided with fillet radii of half the slot height in size, circular coolant jets are exiting the slot tangentially to the trailing edge cutback. The adiabatic wall temperature mappings on the trailing edge cutback indicate that strong three-dimensional flow interaction between the coolant jets and the hot main flow takes place in such a way that two or more coolant jets coalesce depending on the blowing ratio. Experimental and numerical data to be presented in the present study include adiabatic film cooling effectiveness on the trailing edge cutback, the pressure distribution along the internal ribbed passage as well as slot discharge coefficients for different blowing ratios ranging from M=0.35 to 1.1.

Closure to “An Experimental Study on Turbulent Circular Wall Jets” by Adrian Wing-Keung Law and Herlina

Closure to “An Experimental Study on Turbulent Circular Wall Jets” by Adrian Wing-Keung Law and Herlina

Measurements of turbulent mass transport of a circular wall jet

The results of a laboratory investigation on the turbulence characteristics of a circular three-dimensional turbulent wall jet are presented. Measurements were taken up to 50 nozzle diameters using combined particle image velocimetry and planar laser induced fluorescence. The results showed that the induced turbulence was still evolving in the present range and had not achieved similarity. While the turbulent intensity for both velocity and concentration increased downstream, the turbulent mass transport showed a decline over distance for both the streamwise and spanwise directions, implying weakening dispersion from the jet core.

円筒壁面上を軸方向に流出する円形噴流

円筒壁面上を軸方向に流出する円形噴流

505 円筒壁面上を軸方向に流出する円形噴流(流体工学 : 噴流)

505 円筒壁面上を軸方向に流出する円形噴流(流体工学 : 噴流)

A105円筒壁面上を軸方向に流出する円形噴流

In order to reveal the intermittent phenomenon of circular wall jet issuing along cylinder over a time and frequency region, the wavelet cross spectrum was used to analyze the experimental velocity signals between two different fluctuating velocity components. From the wavelet cross spectrum analysis, the instantaneous contribution to Reynolds stress may be easily analyzed. The flow structure of the circular wall jet has been clarified using the wavelet cross spectrum method, comparing with that of the plane wall jet.

A104円筒壁面上を軸方向に流出する円形噴流

A104円筒壁面上を軸方向に流出する円形噴流

202 円筒壁面上を軸方向に流出する円形噴流(流体工学III)

202 円筒壁面上を軸方向に流出する円形噴流(流体工学III)

Generalized study of erosion by circular horizontal turbulent jets

Experimental data on erosion by circular wall jets from thirteen sources including the present experimental study were compiled and analyzed. The present experimental study consisted of circular air jets on a bed of canola seeds and circular water jets on sand and gravel beds. The compiled data, consisting of over 350 experiments, cover wide ranges of flow submergence, jet sizes and velocities, bed material size, relative channel width and relative density difference. Similarity of the scour hole was examined both in the unsteady and asymptotic states by determining the ratios between the characteristic dimensions of the scour hole. The effect of jet submergence on the asymptotic characteristic dimensions of the scour hole was found to be pronounced when the densimetric particle Froude number F o is greater than 10. The effect of elevating the jet outlet on scour depth was also investigated. Equations relating the asymptotic characteristic dimensions of the eroded bed to F o have been proposed. The effects of the relative density difference on the characteristic dimensions of scour were addressed.