Circular Free Jet Research Articles

Investigation into fresh air delivery performance with vortex ring

An air supply mode based on vortex ring structures was proposed to deliver fresh air for good indoor air quality. This study focused on the influence of air supply parameters, including formation time (T*), supply air velocity (U0), and supply air temperature difference (ΔT), on the fresh air delivery performance of an air vortex ring using numerical simulations. The cross-sectional average mass fraction of fresh air (Ca) was proposed to evaluate the fresh air delivery performance of the air vortex ring supply. The results showed that the convective entrainment of the vortex ring resulted from the combined effect of the induced velocity generated by the rotational motion of the vortex core and the negative pressure zone. When the formation time T* < 3.5, Ca increases with an increase in the formation time but decreases with an increase in the formation time for T* > 3.5. The variation in Ca is insignificant for U0 < 3 m/s or U0 > 3 m/s but decreases with an increase in the supply air temperature difference (ΔT). Thus, the optimal air supply parameters for air vortex ring supply are identified as T* = 3.5, U0 = 3 m/s, ΔT = 0 °C. Compared with the traditional circular and square free air jets, within 25Ds (Ds: the diameter of air supply inlet) downstream from the air inlet, the fresh air delivery performance of the air vortex ring supply is increased by a maximum of nearly 200 %, with an average improvement of approximately 120 %.

Scaling law for shock-cell length and its correlation with shock-associated noise of circular and elliptic supersonic free jets

A supersonic core length Lc and an average shock-cell length Ls¯ are experimentally investigated for the acoustic properties of supersonic jets. Following Kumar and Rathakrishnan [“Scaling law for supersonic core length in circular and elliptic free jets,” Phys. Fluids 33(5), 051707 (2021)], a proper scale for the average shock-cell length Ls¯ of circular and elliptic supersonic free jets is found as Ls*¯AR0.2e−0.15γMj2, where Ls*¯ is the average shock-cell length normalized by nozzle throat diameter Dth, AR is the nozzle aspect ratio, γ is the specific heat ratio, and Mj is the ideally expanded jet Mach number. The scaling law developed for Ls*¯ exhibits a close similarity with the scaling law for supersonic core length Lc*AR0.5e−0.11γMj2, where Lc* is the nozzle throat diameter normalized supersonic core length. The scaling laws Ls*¯AR0.2e−0.15γMj2 and Lc*AR0.5e−0.11γMj2 are valid across a wide range of parameters, such as γ from 1.28 to 1.6, the nozzle design Mach number Md from 1 to 2, AR from 1 to 6, and Mj corresponds to the absence of Mach reflection. Discussion conducted on the scaling laws provides important insight into the choice of AR. Furthermore, the relationship between Lc* and Ls*¯ with the intensity of shock-associated noise is also discussed; hence, new correlation functions are developed for Lc* and Ls*¯ that provide an analogous measure of the shock-associated noise.

Reynolds Number Effect on the Flow Demeanorin a Vertical Circular Free Turbulent Jet with Cross Flow

This paper deals with studying numerically two circular turbulent jets impinging on a flat surface with a low velocity cross flow by using ANSYS CFX 16.2, with the aim of proving the effect ofReynolds number on the flow demeanor in a vertical circular free turbulent jet with cross flow. Five turbulence models of the RANS (Reynolds Averaged Navier–Stokes) approach were tested and the k -ω SST model was chosen to validate CFD results with the experimental data. Average velocity profiles, velocity and turbulent kinetic energy contours and streamlines are presented for four case configurations. In the first three cases, the following parameters have been varied: Reynolds number at the level of the two jets ( ), wind velocity at the level of the cross-flow ( ), and the distance between the two jets (S = 45mm, 90mm and 135mm). In the last case, a new configuration of the phenomenon not yet studied so far was treated, where horizontal cross-flows were introduced from both sides in order to simulate gusts of wind disrupting a VSTOL aircraft which tries to operate close to the ground. This case was carried out for Reynolds number based on the crossflow of 4 104, 10 104 and 20 104 .The numerical results obtained show that the deflection of the jets is minimal when the Reynolds number at the level of the jets is greater than that of the cross-flow. The increase of Reynolds number at the level of the cross-flow reveals a significant deviation of the two jets with an intensity which always remains less for the second jet. As for the space parameter between the two jets, it turns out that the fact of further spacing the two jets makes the first jet even more vulnerable and leads to a greater deflection. Finally, the simulation of the wind gusts from the front and the back caused a zone of turbulence which resulted from a form of "interlacing" of the two jets under the effect of the transverse current imposed by the two sides.

Scaling law for supersonic core length in circular and elliptic free jets

We present here the experiments demonstrating the scaling law for supersonic core length in circular and elliptic free jets. A proper scale for the supersonic core length of free jets is found as Lc*AR e−0.11γMj2, where Lc* is the core length normalized by nozzle throat diameter Dth, AR is the aspect ratio of the elliptic nozzle, γ is the specific heats ratio, and Mj is the ideally expanded jet Mach number. The scaling law developed for supersonic core length shows a close dependence with nozzle expansion ratio pe/pa and agrees well with the results reported in open literature, including the data reported for γ = 1.28–1.67. In spite of their uniqueness, the present results refer to circular and elliptic jets only. Thus, other similar studies on rectangular and square jets for a wide range of parameters are needed for a full comprehension of the universality of the scaling law for non-circular jets.

Simulation of a free circular jet using the vortex particle intensified LES (VπLES)

This paper presents validation and verification study of a novel VπLES method which is based on the decomposition of the flow structures in large scale ones, resolved on the grid, and small scale ones, represented by vortex particles for the turbulent circular free jet at Re=104. Comparison with experiments for mean velocities and fluctuations shows that the VπLES simulation with 1.5 · 105 cells attains the similar accuracy as the LES with the Dynamic Smagorinsky model on the Grid with 6.04 · 106 cells. The model is capable of reproducing the backward energy transfer, anisotropy of fine scales velocities and is automatically switched off when the flow is laminar. Numerical scheme parameters and the model reduction are studied and discussed.

Effect of sand particles on flow structure of free jet from a nozzle

The Characteristics of single and two- phase flow from a circular turbulent free jet from a nozzle of 10 mm diameter were investigated experimentally and numerically. The measurements were conducted for ReJ = 10007 - 31561. The velocity was measured at location from the nozzle y/D (0-8) in axial and radial directions. The two phase measurement were done by using natural construction sand as a solid phase of sizes (220,350,550) µm and loading ratios (mass flow ratio of sand to mass flow rate of air) in the range (0.18-1.38). Two phase air velocity of jet showed that the introducing of natural sand particles gives lower jet velocity attributed to momentum transfer to particles. The smaller particle size leads to lower values of velocity. The velocity found to be decreased with loading ratio increase. The numerical simulation was performed for single and two phase jet flow. RNG K-ε turbulence model was used to simulate the flow of fluid and the discrete phase model to simulate the particles flow. The results form numerical simulation showed a good agreement with experimental results.&#x0D;

Circular Free Jets: CFD Simulations with Various Turbulence Models and Their Comparison with Theoretical Solutions

The theory of turbulent free jet is fundamental for the design of comfort ventilation, as free jets frequently occur in mixing and personalized ventilation systems and their characteristics strongly influence the air quality in the breathing zone of an occupant. The aim of this research is to provide recommendations that help researchers and practitioners improve the accuracy and reliability of their computational models of ventilation systems involving circular free jets. To accomplish this, a review of existing theoretical calculation models is performed, and these models are subsequently investigated by computational fluid dynamics. The theoretical solutions of free jets are compared with CFD simulations using various turbulent models such as the standard k-epsilon model, the k-epsilon realizable model, the standard k-omega model, the shear stress transport (SST) k-omega model, and the Reynolds stress model (RSM). The simulated models are represented by profiles of the centreline velocity for a free jet emanating from a round nozzle, because such presentation of the data proved to be particularly helpful for the comparison of the turbulence models. The k-omega SST turbulence closure scheme with standard coefficients produced results of the centreline velocity closest to the average of theoretical solutions investigated, whereas the discrepancy between the simulations and the theoretical models was about 60 % with the k-epsilon standard turbulence model.

Influence of swirl number on jet noise reduction using flat vane swirlers

In this work, jet noise reduction using a swirling flow surrounding a circular free jet has been demonstrated. The passive control scheme induces swirl from six flat vanes fixed in an annular passage with vane angles from 0 to 50°, with the corresponding swirl numbers ranging from 0 to 0.91. Noise measurements in terms of overall sound pressure levels, directivity patterns, acoustic spectra; and flow measurements in terms of centerline pitot survey and flow visualization have been carried out to evaluate the efficacy of the passive control scheme. The co-axial swirl jets always reduce the low frequency noise, irrespective of the nozzle pressure ratio. The screech tone is entirely eliminated and broadband shock associated noise mitigated by the co-axial swirl jets. The results of highly under expanded supersonic cases, show that, the weak swirl generates higher noise and high swirl generates lower noise for the same nozzle pressure ratio. The centerline pitot pressure measurements reveal that the co-axial swirl jets decrease the core lengths and the number of shock cells compared to the free jet. The flow visualization study shows that Mach disks are generated at lower pressure ratios for the co-axial swirl jets compared to free jet. The present work proposes swirl as an excellent passive tool for jet noise suppression.

Self-similarity of heat transfer characteristics in laminar submerged and free-surface slot jet impingement

Laminar slot jet impingement of free-surface and submerged jets is investigated numerically regarding self-similar behavior in local heat transfer. Two different scaling behaviors, one for the stagnation region and one for the fully developed wall-jet region are shown, and the validity of the scaling is confirmed by fully resolved numerical simulations. Within the stagnation region, self-similarity is obtained if the relaxation process of the velocity profile is considered, which is described by the dimensionless nozzle-to-plate distance h/W·ReW as introduced in Rohlfs et al. (2014) for circular jets. Further, it is shown that the Prandtl number dependency can be well described by an analytical relationship obtained by solving the classical similarity solution of a viscous flow near stagnation point. For the wall-jet region, we first derive the scaling behavior based on the work of Watson (1964) for circular free surface jets. This analysis leads to an additional stretching of the spatial coordinate in the wall-tangential direction by the Reynolds number, e.g. x∗=x/(W·ReW).Finally, correlations for the stagnation region heat transfer are introduced for free-surface and submerged slot jets, dependent on the dimensionless nozzle-to-plate distance and constant heat flux/wall temperature, valid over a wide range of laminar flow conditions (i.e., 0.07⩽Pr≤1307,100⩽ReW⩽1000 and 4⩽h/W⩽20). Thereby, analytic Prandtl dependencies for stagnation point flows are employed and corresponding correlations for impinging slot jets are presented, valid in a very broad range (0.0005⩽Pr⩽4500).

On the wake-like vortical arrangement and behaviour associated with twin jets in close proximity

Experiments on twin circular free jets with separation-distances of 1.5, 2.0 and 3.0 jet diameters (D) have been conducted using time-resolved particle-image velocimetry technique. Mean velocity results show that the merging of inner shear layers of the twin jets occurs nearer to the jet exit surface as the separation-distance decreases. Along the mean centre location of the twin jets, turbulent kinetic energy at 3.0D separation-distance increases monotonically up to 8.0D downstream of the jet exits, whereas it decreases gradually from a maximum level for smaller separation-distances. Frequency and proper orthogonal decomposition (POD) analyses show that two distinct peak frequencies are found to correlate with vortex formations associated with the outer and inner shear layers, regardless of the separation-distances. Velocity fields from POD analysis reveal that the inner shear layers vortices from the twin jets merge in a staggered configuration at 1.5D and 2.0D separation-distances, reminiscent of the wake vortex shedding phenomenon behind bluff-bodies. On the other hand, they remain essentially independent at 3.0D separation-distance. The present study indicates that the inner shear layer interactions between the twin jet flows are sensitive towards their separation-distance and they are governed by considerably different mechanisms. Lastly, comparisons of the frequencies associated with the outer shear layer, centreline and wake-like region between the present twin jets and annular jets investigated previously demonstrate good agreements in the overall trends.

A note on the exact temperatures of free two-dimensional and circular jets

A two-dimensional laminar free jet and a circular axisymmetric free jet issuing into an otherwise quiescent fluid are revisited. Making use of the well-documented flow results in the literature, exact temperature profiles of both jets are shown to exist, some of which are presented for a variety of Prandtl numbers of interest. In particular, closed-form solutions are presented valid for the entire Prandtl numbers for the circular-free-jet problem. These can be easily made use of as test cases in practical applications. Circular free jet is shown to admit higher temperature values as compared with the two-dimensional free jet. It is also ensured that for these profiles a temperature integral condition due to the thermal flux is satisfied.

Reynolds number influence on statistical behaviors of turbulence in a circular free jet

The present paper examines the effect of Reynolds number on turbulence properties in the transition region of a circular jet issuing from a smoothly contracting nozzle. Hot-wire measurements were performed for this investigation through varying the jet-exit Reynolds number Red (≡ Ujd/ν, where Uj, d, and ν are the jet-exit mean velocity, nozzle diameter, and kinematic viscosity) approximately from Red ≈ 4 × 103 to Red ≈ 2 × 104. Results reveal that the rates of the mean flow decay and spread vary with Reynolds number for Red &amp;lt; 104 and tend to become Reynolds-number independent at Red ≥ 104. Even more importantly, the small-scale turbulence properties, e.g., the mean rate of dissipation of kinetic energy (ɛ), the Kolmogorov and Taylor microscales, are found to vary in different forms over the Red ranges of Red &amp;gt; 104 and Red &amp;lt; 104. Namely, the critical Reynolds number appears to occur at Red,cr ≈ 104 across which the jet turbulence behaves distinctly. Two turbulence regimes are therefore identified: (i) developing or partially developed turbulence at Red &amp;lt; Red,cr and (ii) fully developed turbulence at Red ≥ Red,cr. It is suggested that the energy dissipation rate (DR) can be expressed as $\varepsilon \sim \nu U_c^2 /R^2$ɛ∼νUc2/R2 in regime (i) and $\varepsilon \sim U_c^3 /R$ɛ∼Uc3/R in regime (ii), where Uc and R are the centerline (or maximum) mean velocity and half-radius at which the mean velocity is 0.5Uc. In addition, the critical Reynolds number appears to vary from flow to flow.

STAGNATION REGION HEAT TRANSFER DUE TO A TURBULENT CIRCULAR IMPINGING AIR JETS

The heat transfer characteristics in a stagnation region were investigated experimentally for a five circular free jets impinging into a heated flat plate. The local temperature distribution, on a thin flat plate was measured using thermal infrared camera. To get precise heat transfer data over the plate, fully developed straight pipe jets were used in this study. Mean jet Reynolds number varies from 1000 to 45000 and vertical distance jet-to-plate, H/D, from 2 to 6, while the spacing jet-to-jet is, S/D, varies from 2 to 8. A geometrical arrangement of one jet surrounded by four jets in -line array was tested.

Non‐Circular Hydraulic Jump on a Moving Surface due to an Impinging Circular Free Surface Jet of Water

AbstractLiquid jet impingement has many industrial cooling applications such as metal manufacturing and steel cooling on run‐out tables (ROT). The development of the wetting front around the impingement point of a jet is central in jet impingement cooling. In this paper, the effects of moving target surface and jet Reynolds number on wetted zone and on the formation and location of hydraulic jump (HJ) are explored through a series of industrial‐scale experiments of an impinging circular free surface long water jet with high Reynolds number of 11 000–50 000 and industrial jet parameters. The moving test surface impacts the radial evolution of circular wetted zone in all directions and alter the circular HJ at the wetting front into a non‐circular contour that depends on the jet Re number. The limited relations in the literature do not represent these measured shapes and do not appropriately predict radii of HJ in industrial scale. A new correlation for radius of non‐circular HJ has been derived in this study that compared more accurately to the experimental data. Numerical simulations of radial impingement flow on moving surface were performed using a variant of k–ε turbulent model and results are compared to the experimental data. The computational results for the wetting front were found to be close to the experimental data indicating the appropriate performance of the turbulent model.

1701 形状の異なるノズルから噴出させられた噴流の近傍構造に関する実験(1)(OS17-1 超音波を利用した流動場の計測,OS17 超音波を利用した流動場の計測,オーガナイズドセッション)

1701 形状の異なるノズルから噴出させられた噴流の近傍構造に関する実験(1)(OS17-1 超音波を利用した流動場の計測,OS17 超音波を利用した流動場の計測,オーガナイズドセッション)

1701 形状の異なるノズルから噴出させられた噴流の近傍構造に関する実験(2)(OS17-1 超音波を利用した流動場の計測,OS17 超音波を利用した流動場の計測,オーガナイズドセッション)

1701 形状の異なるノズルから噴出させられた噴流の近傍構造に関する実験(2)(OS17-1 超音波を利用した流動場の計測,OS17 超音波を利用した流動場の計測,オーガナイズドセッション)

202 噴流中心間距離が異なる 2 円形自由噴流の混合拡散過程

202 噴流中心間距離が異なる 2 円形自由噴流の混合拡散過程

504 環状吸込流を付加した円形自由噴流の流動特性に及ぼす吹出し管径の影響(流体工学 : 噴流)

504 環状吸込流を付加した円形自由噴流の流動特性に及ぼす吹出し管径の影響(流体工学 : 噴流)

Fluid mechanics of regurgitant jets and calculation of the effective regurgitant orifice in free or complex configurations

The velocity fields of turbulent jets can be described using a single formula which includes two empirical constants: k core determining the length of the central core and k turb the jet widening. Flow models simulating jet adhesion, confinement and noncircular orifices were studied using laser Doppler anemometer and the modifications of the constants were derived from series of velocity profiles. In circular free jets, k core was found equal to 4.1 with a variability of 1.4%. In complex configurations, its variability was equal to 15.2%. For k turb, the value for free circular jets was of 45.2 with a variability of 6.0% and this variability in complex configurations was significantly higher (30.1%, p=0.025). The correlation between the actual orifice size and the jet extension was poor ( r=0.52). However, the almost constant value of k core allowed to define a new algorithm calculating the regurgitant orifice diameter with the use of outlines of the jet image ( r=0.89). In conclusion, the fluid mechanics of regurgitant jets is modified in complex configurations but, due to the relative independency of the central core, velocity fields could be used to evaluate the dimensions of the effective regurgitant orifice.

362複合旋回噴流の可視化実験

A compound swirling jet which consists of a circular free jet and an annular swirling jet mounted on the same axis is proposed and is visualized by smoking method to clarify its flow characteristics. As a result, the flow characteristics of the compound swirling jet, for example, the jet radius, changes widely with the flow ratio of the free jet to the annular swirling jet. The compound swirling jet can be controlled its jet radius continuously by changing the flow ratio and there exists the flow ratio showing the smallest jet radius which gives the highest energy concentration and the entrainment of the flow. The compound swirling jet is expected to apply for various industrial purposes.