Offset Jet Research Articles

プラズマアクチュエータを用いた二次元オフセット噴流の流動と熱伝達

プラズマアクチュエータを用いた二次元オフセット噴流の流動と熱伝達

Numerical Simulation of an Offset Jet in Bounded Pool with Deflection Wall

Thek-εturbulent model and VOF methods were used to simulate the three-dimensional turbulence jet. Numerical simulations were carried out for three different kinds of jets in a bounded pool with the deflection wall with angles of 0°, 3°, 6°, and 9°. The numerical simulation agrees well with the experimental data. The studies show that the length of the potential core zone increases with the increase of the deflection angle. The velocity distribution is consistent with the Gaussian distribution and almost not affected by the deflection angle in potential core zone. The decay rates of flow velocity in the transition zone are 1.195, 1.281, 1.439, and 1.532 corresponding to the unilateral deflection angles, 0°, 3°, 6°, and 9°, respectively. The decay rates of velocity in the transition zone are 1.928 and 2.835 corresponding to the bilateral deflection angles 3° and 6°. It is also found that the spread of velocity is stronger in the vertical direction as the deflection angles become smaller. The spread rates of velocity with unilateral deflection wall are higher than those with bilateral deflection walls in the horizontal plane in the pool.

Acoustic Doppler velocimeter measurements of a submerged three-dimensional offset jet flow over rough surfaces

ABSTRACTThe flow characteristics of a submerged three-dimensional offset jet over transverse square ribs and gravel are experimentally investigated using an acoustic Doppler velocimeter. The pitch-to-height ratio of the ribs was varied to achieve d-type, intermediate, and k-type roughness. The Reynolds number based on the nozzle height and jet exit bulk velocity was 53,000. The mean flow properties and turbulent statistics are compared to those obtained over a smooth surface. Both decay and wall-normal spread rates were independent of surface roughness with estimated values of 0.66 ± 0.03 and 0.138 ± 0.01, respectively, with these values influenced by the presence of lateral confinement of the flow. The distribution of the mean flow, within the inner shear layer, revealed a dependence on surface roughness after reattachment. A representation of the Reynolds normal stresses within the symmetry and lateral planes revealed the highly anisotropic nature of the turbulence field of offset jets.

Flow characteristics of an offset jet over a surface mounted square rib

ABSTRACTA three-dimensional offset jet flow over a surface mounted square rib was investigated using particle image velocimetry at Reynolds number of 7600. The square rib was mounted at three different locations downstream of the nozzle exit. Contour and profile plots in the symmetry plane were used to investigate the effect of the rib on the mean flow distribution and turbulence statistics. Changing the rib location enhanced the mixing and entrainment characteristics of the jet thereby leading to larger wall-normal spread rates. The quadrant analysis revealed a significant contribution of all events towards the Reynolds shear stress. The joint probability density function analysis showed that the structures within the outer shear layer of the jet differed from those of boundary layers. Results from the two-point correlation analysis indicated that the introduction of a surface mounted rib modified the flow structures within the developing region. However, there was no significant structural difference between the flow with mounted rib and the no rib case within the self-similar region. The proper orthogonal decomposition was used to examine how changes in rib location modified the dynamics of the energetic modes towards the turbulent kinetic energy and Reynolds shear stress within the developing and self-similar regions.

Transition of a Steady to a Periodically Unsteady Flow for Various Jet Widths of a Combined Wall Jet and Offset Jet

In the present paper, a dual jet consisting of a wall jet and an offset jet has been numerically simulated using two-dimensional unsteady Reynolds-Averaged Navier–Stokes (RANS) equations to examine the effects of jet width (w) variation on the near flow field region. The Reynolds number based on the separation distance between the two jets (d) has been considered to be Re = 10,000. According to the computational results, three distinct flow regimes have been identified as a function of w/d. For w/d ≤ 0.5, the flow field remains to be always steady with two counter-rotating stable vortices in between the two jets. On the contrary, within the range of 0.6 ≤ w/d < 1.6, the flow field reveals a periodic vortex shedding phenomenon similar to what would be observed in the wake of a two-dimensional bluff body. In this flow regime, the Strouhal number of vortex shedding frequency decreases monotonically with the progressive increase in the jet width. For w/d ≥ 1.6, the periodic vortex shedding is still evident, but the Strouhal number becomes insensitive to the variation of jet width.

Effect of bottom wall proximity on the unsteady flow structures of a combined turbulent wall jet and offset jet flow

The present paper deals with a turbulent dual jet consisting of a wall jet and an offset jet. The flow field has been numerically simulated using two-dimensional unsteady RANS equations. The Reynolds number based on the separation distance between the two jets (s) is Re=10,000. The width of the offset jet nozzle equals the separation distance between the two jets. To examine the influence of the bottom wall, the width of the wall jet nozzle (h) has been varied in a range 0.2≤h/s≤2. According to the computational results, for h/s=0.2, the flow field remains to be always steady with two counter-rotating stable vortices in between the two jets. On the contrary, within the range of 0.3≤h/s≤1, the near flow field demonstrates a periodic vortex shedding phenomenon similar to what would be observed in the near wake region for flow over a two-dimensional bluff body. Within this flow regime, the Strouhal number based on the vortex shedding frequency gradually decreases with the progressive increase in h/s. However, for h/s>1, although the periodic vortex shedding phenomenon is still evident in the flow field, the Strouhal number becomes insensitive to the bottom wall similar to the flow behaviour for a flow over an unconfined cylinder.

Analysis of Conjugate Heat Transfer for a Combined Turbulent Wall Jet and Offset Jet

This paper presents a study of the conjugate heat transfer, involving conduction through a solid slab and turbulent convection in fluid, for a combined turbulent wall jet and offset jet flow using unsteady Reynolds averaged Navier–Stokes (URANS) equations. The conduction equation for the solid slab and convection equation for the fluid region are solved simultaneously satisfying the equality of temperature and heat flux at the solid–fluid interface. The fluid flow is complex because of the existence of periodically unsteady interaction between the two jets for the chosen ratio of jets separation distance to the jet width (i.e., d/w = 1). The heat transfer characteristics at the solid–fluid interface have been investigated by varying various important parameters within a feasible range: Reynolds number (Re = 10,000–20,000), Prandtl number (Pr = 1–4), solid-to-fluid thermal conductivity ratio (ks/kf = 1000–4000), and nondimensional solid slab thickness (s/w = 1–10). The bottom surface of the solid slab has been maintained at a constant temperature. The mean conjugate heat transfer characteristics indicate that the mean local Nusselt number along the interface is a function of flow (Re) as well as fluid (Pr) properties but is independent of solid properties (ks and s). However, the mean interface temperature and mean local heat flux along the interface always depend on all the aforementioned properties.

プラズマアクチュエータを用いた二次元オフセット噴流の 圧力特性と熱伝達特性

This study is concerned with the active flow control of two–dimension offset jet using a dielectric barrier discharge (DBD) plasma actuator (PA). The offset jet is produced by the flow of air which issues from the end of a long parallel channel, and the offset ratio H/h (H : step height, h : channel height) is 1.0. PA is installed on the lower wall of jet exit. The exit Reynolds number Re is changed in 1.5×103 to 6.0×103 . The wall static pressure and the heat transfer coefficient are measured. Flow visualization is carried out using a CCD camera and laser light. The results show that the pressure loss decreases and the heat transfer coefficient in the recirculating region increases with the induced flow due to the PA. These effects are remarkable at the low Reynolds number flow. In the downstream region, the heat transfer coefficient is smaller than without PA because the turbulent diffusion is promoted.

A Study on Turbulent Flow and Heat Transfer of an Off-set Jet

A Study on Turbulent Flow and Heat Transfer of an Off-set Jet

Experimental and Numerical Investigation of an Offset Jet Using Tangential Air Distribution System

In this paper an offset, attached jet is investigated both experimentally and numerically, applying the tangential room air distribution system. The attachment distance of the injected air jet, which influences the ventilation characteristics in the room, is investigated as a function of the inlet Reynolds-number. Results showed that the attachment distance is independent from the inlet Reynolds- number in the range of Re 0 = 2 ÷ 4 * 10 3 with fixed boundary conditions in a room. In the past decades several researchers have investigated offset jets. However, the tangential room air distribution as a type of comfort ventilation and lower Reynolds numbers have not been considered. Air velocity magnitude was measured in the injected air jet by a hot-wire anemometer. Three turbulence models were applied for the 3D numerical simulation: standard, RNG k–e and SST k–ω models. In the present work the SST k–ω model seemed to be the most accurate turbulence model. The numerical results showed a high similarity with the measured data.

Three-dimensional study of turbulent flow characteristics of an offset plane jet with variable density

An experimental and numerical investigation of the flow field of variable density turbulent offset jet is presented. The velocity measurements are performed using a Velocimetry Laser Doppler technique for an offset height h. Three cases of variable-density turbulent plane jets discharging from a rectangular nozzle into a quiescent medium are studied. The variation density jets considered were revealed at different Reynolds numbers. In the second step of this work, a numerical three-dimensional model of the problem is simulated through the resolution of the Navier–Stokes equations by means of the finite volume method and the Reynolds stress model second-order turbulent closure model. A non-uniform mesh system tightened close to the emitting nozzle and both the vertical and horizontal walls is also adopted. A good level of agreement was achieved, between the experiments and the calculations. Once the model validated, our model allowed the evaluation of the influence of the variation density on the characterizing features of the resulting flow filed. It is found that the centerline velocity and concentration of the heavier jet decays much faster than in the two other jets, and a similar behavior for the vertical profiles in the three variable-density jets is well reproduced in the simulation.

Insights derived from CFD studies on the evolution of planar wall jets

ABSTRACTThe main objective of this paper is to demonstrate that a simple and cost-effective 2D Reynolds-averaged Navier–Stokes (RANS) simulation approach can often be efficiently used in industrial design applications. We simplified the designing approach of a racetrack jet dryer to a problem involving the streamwise evolution of an offset wall jet. We compared our simple 2D RANS simulations with experiments and large eddy simulation (LES), and were encouraged to see that our simplified approach produced a better correlation with the experiments compared to LES, which is expected to be much more accurate (even though computationally orders of magnitude more expensive). We conclude that under certain circumstances a simple 2D approach can lead to a dependable solution. Additionally, we used the results from these simulations to enhance our understanding of the evolution of offset wall jets. The insights derived from these simulations suggest the existence of scaling parameters that can express offset wall jets as a family of self-similar flows.

Effect of Freestream Motion on Heat Transfer Characteristics of Turbulent Offset Jet

The numerical simulation of turbulent offset jet flow has been carried out using k–ω shear stress transport (SST) model. The simulations have been done for the offset jet flow in the quiescent medium and also in the presence of an external stream. The effect of freestream velocity on the flow and heat transfer characteristics of turbulent offset jet has been reported. The offset ratio and Reynolds number of flow considered are 5.7 and 16,000, respectively. The presence of coflow stream has been found to reduce the entrainment of surrounding fluid into the jet which in turn reduces the heat transfer from the jet to the surrounding medium. The effect of freestream velocity on the important parameters like decay of the local maximum streamwise velocity, jet spread, reattachment length, velocity logarithmic profile, velocity defect law profile, decay of the local maximum streamwise temperature, variation of wall temperature, temperature similarity profile, and Nusselt number distribution has been discussed.

Computational study of periodically unsteady interaction between a wall jet and an offset jet for various velocity ratios

The interaction between a turbulent wall jet and a turbulent offset jet for various velocity ratios is numerically investigated using two-dimensional unsteady RANS equations. Throughout the study, the ratio of jets separation distance (d) and jet width (w) is kept at d/w=1, and the Reynolds number based on jet width is considered as Re = 10,000. The velocity ratio, Vr = uw/uo, is varied by varying the inlet velocity of the wall jet (uw) consistently while the inlet velocity of the offset jet (uo) is retained as a reference velocity. Computational results reveal that when the velocity ratio lies in the range 0.78 ≤ Vr ≤ 1.34, a periodic vortex shedding phenomenon occurs close to the nozzle plate similar to what would be expected in the region behind a two-dimensional bluff body immersed in a fluid. On the contrary, this periodic phenomenon ceases if Vr ≤ 0.77 and Vr ≥ 1.35. Unsteady time-marching simulations show that for Vr = 0.77 and 1.35, two stable counter rotating vortices form close to the nozzle plate. The velocity signals, within the range for which the periodic vortex shedding occurs, exhibit a trend of pure sinusoidal oscillation. Fast Fourier transform (FFT) of the velocity signals provides the vortex shedding frequency corresponding to the Strouhal number that gradually increases with the progressive increase in velocity ratio.

Experimental study of the flow structures of 3D turbulent offset jets

ABSTRACTThree-dimensional turbulent offset jets were investigated using a particle image velocimetry technique. Detailed measurements were performed for offset height ratios of 0, 2 and 4. The presence of backflow influenced the distribution of the mean velocity and Reynolds stresses. A two-point correlation analysis was used to investigate the spatial distribution of large-scale structures within the inner shear layer of the flow domain. The results revealed that large-scale structures dominate the inner layer of the self-similarity region. Proper orthogonal decomposition was performed on the fluctuating velocity field within the symmetry and lateral planes using the snapshot approach. Results from the reconstructed field provided insight into the contributions of the most energetic structures to the turbulence statistics. The energetic structures contributed more to the Reynolds shear stress and streamwise turbulence intensity, while contributing less to the wall-normal turbulence intensity.

Mean flow characteristics of a turbulent dual jet consisting of a plane wall jet and a parallel offset jet

An exhaustive study is carried out to highlight the flow features arising due to a dual jet consisting of a plane wall turbulent jet and a parallel turbulent offset jet (hereafter this combination is termed as a dual jet). The standard high Reynolds number two equation k–ε model is used to solve the two-dimensional, incompressible, turbulent flow. A detailed analysis is carried out to compare the flow characteristics of the offset jet and the dual jet for an offset ratio between 3 and 15 at an interval of 2. It is noticed that the presence of wall jet in addition to the parallel offset jet has a tremendous effect on the flow characteristics. In the present study, the location of reattachment point and vortex centre of an offset jet and merge point, combined point, and the vortex centres of a dual jet are identified and expressed by correlations which depend on the offset ratio. It is interesting to observe that the cross-stream movement of vortex centre is linear with the change in offset ratio for both kind of jets either an offset jet or a dual jet. However, the streamwise movement of vortex centre follows a parabolic trend with respect to the offset ratio with an exception of combined point which is weakly dependent on the cubic power of offset ratio. The jet half width of a dual jet is compared for different offset ratios by a unique scaling method for which the correlation is also proposed; it is observed that the variation is parabolic with respect to the axial location. The total momentum flux of a dual jet is found to decrease with the increase in the offset ratio as opposed to almost insignificant change in the value for an offset jet. And, the value of integral constant is on the lower side as compared to the corresponding case of either an offset jet or a parallel jet.

Mean Flow and Thermal Characteristics of a Turbulent Dual Jet Consisting of a Plane Wall Jet and a Parallel Offset Jet

The standard high-Reynolds number two-equation k − ϵ model is used to study the flow and thermal characteristics of a dual jet consisting of a plain wall turbulent jet and a parallel turbulent offset jet (hereafter, dual jet). The flow and thermal characteristics are presented in the form of streamlines, mean velocity vector, turbulent kinetic energy, dissipation of turbulent energy, Reynolds stresses, and isothermal contour plots. The variation in local heat flux and local Nusselt number on the bottom wall is also presented. The finite-volume-method-based SIMPLE algorithm is utilized for understanding the complex nature of flow arising due to a dual jet. The convective flux is discretized using the power-law upwind scheme, while the diffusive term is discretized using the central difference scheme. To study the effect of offset ratio, which is defined as the ratio of height of the jet from the horizontal wall to the width of the jet (nozzle), it is varied between 3 and 15 at an interval of 2. It is noted that the presence of a wall jet in addition to the parallel offset jet has a significant effect on flow and thermal characteristics.

Large-scale flow structures in a planar offset attaching jet with a co-flowing wall jet

Conditional averaging techniques were used to examine the periodic motions that were observed in flows consisting of an offset planar jet and a co-flowing planar wall jet. The offset jet is one jet height (Hj) away from the wall and has a Reynolds number of approximately 40, 000, based on Hj and flow-rate averaged velocity Uo; for the co-flowing jets, different heights (i.e., 0.18Hj and 0.5Hj) and velocities (i.e., 0.56Uo and 0.36Uo) were considered. The flows had periodic motions with frequencies fHj/Uo = 0.28 and 0.49 or fHc/Uo = 0.23 and 0.25, where Hc is the distance between the jets. The periodic motions were present in both the inner shear layer of the offset jet above the re-circulation region and the outer shear layer of the wall jet below the re-circulation region. The motions from the inner shear layer of the offset jet persisted in the shear layer that formed downstream of the re-circulating region. There were periodic motions in the outer shear layer of the offset jet particularly in the flow with the smaller wall jet. The relative contribution of the motions to the total fluctuations increased as the flow evolved downstream reaching a maximum approximately 4Hc downstream of the flow exit. The relative contribution of the periodic motions to the turbulent fluctuations was similar in the two flows but the periodic motions had a much larger impact on the near-wall velocity and pressure fluctuations in the flow with the smaller wall jet due to the trajectory of the periodic structures, the distance of the periodic structures to the wall and the size of these structures.

二次元オフセット噴流の流動特性と熱伝達

The flow field of an offset jet is complex and is frequently found in many engineering devices. In this paper, the fluid flow and heat transfer characteristics of two-dimensional offset jet are investigated experimentally. The offset jet is produced by the flow of air which issue from the end of a long parallel channel. The exit Reynolds number Re is changed in 1.5×103 to 7.0×103, and the offset ratio H/h (H : step height, h : channel height) is changed in 0.5, 1.0 and 1.5. The wall shear stress is measured using a micro flow sensor. Flow visualization is carried out using a CCD camera and laser light. The velocity profiles are measured by a PIV system. When the flow at the channel exit is laminar, fluid in the outer region has a counter-clockwise vorticity and fluid in the inner region has a clockwise vorticity, and a lifting-off of the wall jet flow into the ambient fluid is occurred. The Strouhal number of the vortex frequency is almost constant against the offset ratio. The formation of those vortices is not seen at the turbulent flow. Two peaks exist in the wall static pressure, Nusselt number and skin friction coefficient distributions of the laminar flow with H/h = 0.5 and 1.0. The 1st peak appears near the reattachment point, and the 2nd peak appears in the neighborhood of the lifting-off of the wall jet flow. The two peaks coalesce into one at H/h = 1.5. As the offset ratio increases, the reattachment point moves to downstream direction and Nu becomes large both the laminar and turbulent flows. The reattachment point of the turbulent flow moves to upstream direction compared with the laminar flow because Coanda-effect is strengthen.

Experimental and numerical study of an offset jet with different velocity and offset ratios

The present study examines the configuration of an offset jet issuing into either a quiescent medium or a moving stream (co-flowing). The mean velocity and turbulence characteristics of the turbulent offset jet have been investigated using a particle image velocimetry technique at three velocity ratios and for two offset ratios. A numerical simulation of a three-dimensional offset jet has also been carried out by comparing the corresponding results with previous experimental data and our measurements. The numerical investigation was performed by means of the finite volume method together with a second-order turbulent closure model – the Reynolds stress model (RSM) – to examine the behavior of the flow for different velocity ratio and offset ratios. Results give a satisfactory agreement between the experimental data and the calculations. Data from the early flow region clearly show a significant influence of the velocity ratio and the offset ratio on the mean flow and turbulence characteristics.