Steady Jet Flow Research Articles

Experimental study on the control of temperature oscillation in power electronics via pulsating jet impingement

Thermal oscillations in power electronics due to transient load conditions present a significant challenge. This experimental study investigates the application of intermittent jet impingement, coupled with transient heat flux boundary conditions of 100 W, 200 W, and 300 W, to a thermal test chip at a frequency of 0.5 Hz (70 % duty cycle), to assess its effectiveness in minimizing junction temperature fluctuations. Temperature oscillations were recorded to compare the advantages of intermittent pulsation over steady jet flow. The results demonstrate reduced hot spot temperature oscillations of 10 %, 12.5 %, and 16.7 % at 100 W, 200 W, and 300 W, respectively, at a Reynolds number of 8850. Remarkably, even during the pulsation off-period, the minimum temperature limit was maintained at the inlet temperature of the jet, attributed to the formation of strong vorticity rings at the end of the jet pulse, inducing a cooling effect and reducing the junction temperature. Pulsating jets exhibit superior control over temperature fluctuation compared to steady jets, with the reduction being more pronounced at Reynolds numbers closer to 2000 and diminishing as Reynolds number increases to 17000. Moreover, the heat transfer coefficient and the Nusselt number enhanced by 18 % due to pulsating jet impingement. An enhancement factor is introduced to quantify the benefits of using pulsating jets, revealing a higher reduction in temperature oscillation compared to steady jets at the same time-averaged Reynolds number, valid within the Reynolds number range of 2000–14000. Additionally, a correlation is proposed to calculate the enhancement factor for pulsating jets. Finally, the pulsating jet impingement proves to reduce the exergy destruction by 30 % compared to steady jet.

Coupled Dynamics of Steady Jet Flow Control for Flexible Membrane Wings

We present a steady jet-flow-based flow control of flexible membrane wings for the adaptive and efficient motion of bat-inspired drones in complex flight environments. A body-fitted variational computational aeroelastic framework is adopted for the modeling of fluid–structure interactions. High-momentum jet flows are injected from the leading edge and transported to the wake flows to alter the aerodynamic performance and the membrane vibration. The coupled dynamic effect of active jet flow control on membrane performance is systematically explored. While the results indicate that the current active flow control strategy performs well at low angles of attack, its effectiveness degrades at high angles of attack with large flow separation. To understand the coupling mechanism, the variations of the vortex patterns are examined by the proper orthogonal decomposition modes, and the fluid transport process is studied by the Lagrangian coherent structures. Two scaling relations that quantitatively connect the membrane deformation with the aerodynamic loads presented in our previous work are verified even when active jet flow control is applied. A unifying feedback loop that reveals the fluid–membrane coupling mechanism is proposed. These findings can facilitate the development of next-generation bio-inspired drones that incorporate smart sensing and intelligent control.

Influences of the Braking Effect of Ruler EMBr on Molten Steel Flow and Steel–Slag Interface Fluctuation in a Continuous Casting Mold

Electromagnetic braking (EMBr) technology, as one of the most effective technologies in the continuous casting process, provides an effective tool for improving the internal and external defects of steel products. Specifically, the EMBr technology takes the benefit of the generation of Lorentz force to decrease flow instability, mold powder entrapment, and surface defects, if applied properly. For this purpose, to gain a clear understanding of the effect of EMBr technology on the continuous casting process, a commonly used EMBr technology, namely ruler EMBr technology, is applied in the current work to investigate the dynamic behaviors of molten steel flow and steel–slag interface fluctuation inside a slab mold. Furthermore, to obtain a desirable braking effect of the ruler EMBr technology, operational parameters including the magnetic flux density, submerged entry nozzle (SEN) depth, and magnetic pole location are numerically investigated. The results demonstrate that the braking effect exerted by the ruler EMBr device is favorable for suppressing the impact of upward stream on the steel–slag interface with the magnetic flux density exceeding 0.3 T. For the influence of the SEN depth and magnetic pole location on the effect of ruler EMBr mold, the results show that a steady jet flow pattern can be obtained through the adjustment of a location between the ruler EMBr device and the SEN depth. For instance, when the ruler EMBr device installation position of 225 mm corresponds to the SEN depth of 150 mm, the upward deflection of jet stream is suppressed and a stable interface fluctuation profile is formed. With this adjustment, the possibility of mold flux entrapment is decreased.

First Principles‐Based Approach for 3D Scour Processes Under Variable Jet Discharge

AbstractScour processes caused by plunging jets are particularly important in hydraulic engineering because of the connected risk of structural failure. Therefore, many investigations have been conducted on this topic, focusing on jet‐induced scour under constant flow discharge (i.e., steady case). Yet, to the best of the authors' knowledge, no studies have analyzed the scour mechanism under variable jet discharge (i.e., unsteady case) and clear‐water conditions. Following recent theoretical advancements on scour phenomena for steady cases based on the phenomenological theory of turbulence, we address in this study the general validity of such an approach for time‐dependent (i.e., unsteady) jet inflow conditions, in non‐cohesive soils. Results show that scour evolution is consistent with that obtained for steady jet flows. In particular, they confirm the existence of two distinct phases—developing and developed—and indicate that the transition time between the two phases does not depend on the jet inflow characteristics. Once again, this study confirms that the phenomenological theory of turbulence is a valid tool for all jet‐driven scour problems.

Experimental study of vortex formation in pulsating jet flow by time-resolved particle image velocimetry

We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.

The steady incompressible jet flows issuing from a finitely long nozzle

This paper mainly involves with the well-posedness of two-dimensional irrotational, non-viscous, incompressible jet flows through a finitely long nozzle. In comparison with the significant results on incompressible jet flows issuing from an infinitely long nozzle in [2] and [3], we propose a set of physically reasonable boundary conditions on the inlet and the outlet of the finitely long nozzle and establish the existence and uniqueness of the incompressible jet issuing from the finite nozzle. More precisely, given a horizontal velocity in the inlet and the atmospheric pressure patm, there exists a unique incompressible jet flow issuing from the given finite nozzle.

Universal scaling parameter for a counter jet drag reduction technique in supersonic flows

The reduction in aerodynamic drag by injecting a gaseous jet from the nose of a blunt body into a supersonic stream is investigated numerically. The penetration of the jet into the supersonic flow modifies the shock structure around the body and creates a low pressure recirculation zone, thereby decreasing the wave drag significantly. Combining various theoretical estimates of different flow features and numerical simulations, we identify a universal parameter, called the jet to freestream momentum ratio (RmA), which uniquely governs the drag on the blunt body. The momentum ratio fundamentally decides the penetration of the jet as well as the extent of low pressure envelope around the body. In addition, various influencing parameters reported in the literature are reviewed for different steady jet flow conditions. Furthermore, their limitations in regulating the flowfield are explained by correlating the facts with the jet to freestream momentum ratio. We perform the simulations for various combinations of physical and flow parameters of the jet and the freestream to show a universal dependence of drag on the momentum ratio.

Gaussian models for late-time evolution of two-dimensional shock–light cylindrical bubble interaction

Two-dimensional shock–bubble interaction is an analogy of the steady three-dimensional jet flow in a scramjet. On the basis of Navier–Stokes simulations, a cylindrical bubble embedded with hydrogen surrounded by air was accelerated by a shock. The evolution can be divided into the lobe-emergence stage, the back-lobe suction stage, and the equilibrium stage. Based on the inhomogeneity between the hydrogen mass fraction and the vorticity field, a correlation coefficient is proposed to quantitatively determine the starting moment of the equilibrium stage. In the equilibrium stage, quasi-Gaussian distributions are modeled for the mass fraction and the vorticity. Surface integrals are performed to derive corresponding mixedness and circulation models, both controlled by two statistical parameters (standard deviation and peak value). Such Gaussian integrated models are universal for different cylindrical bubble aspect ratios ($$\mathrm {AR}=0.5$$–2) and shock Mach numbers ($$M=1.22$$–2). They provide a statistical perspective of late-time SBI evolution in addition to the description from certain physical quantities and help better understand the compressible mixing of scramjet combustors.

Incompressible Jet Flows in a de Laval Nozzle with Smooth Detachment

In this paper, we are concerned with the well-posedness theory of steady incompressible jet flow in a de Laval type nozzle with given end pressure at the outlet. The main results show that for any given incoming mass flux Q > 0 in the upstream and end pressure at the outlet, there exists an admissible interval to the Bernoulli’s constant, if the Bernoulli’s constant lies in the interval, there exists a unique smooth incompressible jet flow issuing from the nozzle. Moreover, the free boundary of the jet flow initiates smoothly from the surface of the divergent area of the de Laval nozzle. In particular, it is shown that the initial point of the free boundary lies behind the throat of the de Laval nozzle wall and varies continuously and monotonically with respect to the Bernoulli’s constant. As a direct corollary, imposing initial point of the free boundary on the divergent part of nozzle wall, there exists a unique incompressible jet for given incoming mass flux Q > 0 and pressure Pe at the outlet. This work is inspired by the significant works (Alt et al. in Commun Pure Appl Math 35:29–68, 1982; Arch Rational Mech Anal 81:97–149, 1983) for the incompressible jet flow imposing the initial point of the free boundary at the endpoint of the nozzle.

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.

Effect of turbulence on zonal jet flows in equivalent-barotropic quasi-geostrophic model

The linear stability of zonal flows superposed upon a sinusoidal, slightly supercritical background flow in an equivalent-barotropic quasi-geostrophic model on a \(\beta\) plane is investigated using asymptotic analysis. Consistent with results of full numerical simulations, all the steady isolated zonal jet solutions were found to be linearly unstable, and specifically, the mode independent of longitudinal direction was found to be most unstable, even when the amplitude equation of the zonal flows has longitudinal dependence. References K. Obuse, S. Takehiro and M. Yamada, Long-time asymptotic states of forced two-dimensional barotropic incompressible flows on a rotating sphere . Phys. Fluids 22 :156601, 2010. doi:10.1063/1.3407652 R. K. Scott and L. M. Polvani, Forced-dissipative shallow-water turbulence on the sphere and the atmospheric circulation of the giant planets . J. Atmos. Sci. 64 :3158–3176, 2007. doi:10.1175/JAS4003.1 A. J. Manfroi and W. R. Young, Slow evolution of zonal jets on the beta plane . J. Atmos. Sci. 56 :784–800, 1999. 2.0.CO;2>doi:10.1175/1520-0469(1999)056 2.0.CO;2 K. Obuse, S. Takehiro and M. Yamada, Linear stability of steady zonal jet flows induced by a small-scale forcing on a \(\beta \) plane . Physica D 240 :1825–1834, 2011. doi:10.1016/j.physd.2011.08.009 S. Takehiro, M. Odaka, K. Ishioka, M. Ishiwatari, Y.-Y. Hayashi and SPMODEL Development Group, A series of hierarchical spectral models for geophysical fluid dynamics . Nagare Multimedia, 2006. http://www.nagare.or.jp/mm/2006/spmodel/

Numerical simulation for optimizing the nozzle of moist-mix shotcrete based on orthogonal test

The nozzles of moist-mix shotcrete are the key parts of forming a steady jet flow field and ensuring the uniform mixing of water and other ingredients. In this paper, for optimizing the nozzle of moist-mix shotcrete, both the internal and external field of a variety of spray nozzles were simulated and analyzed by adopting orthogonal test method with Fluent simulation software combined. Then the phase volume fraction and single-phase velocity of the outlet section of flow field inside the nozzles and cloud pictures including single-phase velocity and volume of different sections lengthways in the external flow field of nozzle were obtained. The results demonstrated that the change of different factors and different levels of the same factor affected the shotcreting performance of spray nozzle, but the effect degree is different. Additionally, compared with the traditional nozzle, the rationality of new-type nozzle structure was verified, which provided a basis for the improvement and optimization of the nozzle structure in the future.

Mixing enhancement and penetration improvement induced by pulsed gaseous jet and a vortex generator in supersonic flows

A good injection strategy with high penetration and mixing efficiency relates to the overall performance of the airbreathing hypersonic propulsion system. In the current study, the transverse injection flow field with a vortex generator placed in front of jet has been investigated, and the jet mixing enhancement and penetration improvement induced by the pulsed jet has been evaluated as well. The obtained results predicted by the three-dimensional Reynolds-average Navier – Stokes (RANS) equations coupled with the two equation k-ω shear stress transport (SST) turbulence model show that the jet-to-crossflow pressure ratio has an great impact on penetration depth and mixing efficiency in the steady jet flow field. In the case of lower jet-to-crossflow pressure ratio, higher mixing efficiency but lower penetration depth are shown. On the other hand, compared with the steady jet flow field, the pulsed jet flow field with the periodic variation of jet-to-crossflow pressure ratio of the fuel injection has better comprehensive performance, and the penetration depth and mixing efficiency are improved simultaneously.

Steady Vortex-Generator Jet Flow Control on a Highly Loaded Transonic Low-Pressure Turbine Cascade: Effects of Compressibility and Roughness

A new high-speed linear cascade has been developed for low-pressure turbine (LPT) studies at The Ohio State University. A compressible LPT profile is tested in the facility and its baseline performance at different operating conditions is assessed by means of isentropic Mach number distribution and wake total pressure losses. Active flow control is implemented through a spanwise row of vortex-generator jets (VGJs) located at 60% chord on the suction surface. The purpose of the study is to document the effectiveness of VGJ flow control in high-speed compressible flow. The effect on shock-induced separation is assessed by Mach number distribution, wake loss surveys and shadowgraph. Pressure sensitive paint (PSP) is applied to understand the three dimensional flow and shock pattern developing from the interaction of the skewed jets and the main flow. Data show that with increasing blowing ratio, the losses are first decreased due to separation reduction, but losses connected to compressibility effects become stronger due to increased passage shock strength and jet orifice choking; therefore, the optimum blowing ratio is a tradeoff between these counteracting effects. The effect of added surface roughness on the uncontrolled flow and on flow control behavior is also investigated. At lower Mach number, turbulent separation develops on the rough surface and a different flow control performance is observed. Steady VGJs appear to have control authority even on a turbulent separation but higher blowing ratios are required compared to incompressible flow experiments reported elsewhere. Overall, the results show a high sensitivity of steady VGJs control performance and optimum blowing ratio to compressibility and surface roughness.

Investigation into Flow Field in Relation to Laser Gas Assisted Processing: Influence of Assisting Gas Velocity on the Flow Field

A transiently developing jet finds application in metal processing, such as a vapor jet produced during laser ablation of solid surfaces. In the present study, laser gas-assisted processing is considered and vapor ejection from the solid surface during the ablation process is modeled. In this case, a transiently developing jet emerging from the solid wall and steady jet opposing (impinging onto) the transiently developing jet situation is studied. Since the thermal properties of vapor ejecting from the wall is not known, transiently developing helium jet is considered to resemble the vapor emanating from the wall. An air jet is employed to impinge onto the developing jet. The transient flow properties of the developing jet are obtained from the previous study. The flow and temperature fields are predicted numerically using a control volume approach. A low Reynolds number k–ϵ turbulence model is used to account for the turbulence. In order to investigate the influence of mean velocity of the steady jet on the flow field near the transiently developing jet, four mean velocities are considered. It is found that mean velocity of the steady jet has significant effect on the flow field in the region close to the transiently developing jet. In this case, a circulation cell is developed next to the steady jet boundary and the orientation of cell is influenced by the steady jet flow conditions.

Steady and Unsteady Air Impingement Heat Transfer for Electronics Cooling Applications

This paper focuses on two forced convection methods—steady jet flow and pulsating flow by synthetic jets—that can be used in applications requiring significant amounts of heat removal from electronics components. Given the dearth of available data, we have experimentally investigated steady jets and piezoelectrically driven synthetic jets that provide pulsating flow of air at a high coefficient of performance. To mimic a typical electronics component, a 25.4-mm × 25.4-mm vertical heated surface was used for heat removal. The impingement heat transfer, in the form of Nusselt number, is reported for both steady and unsteady jets over Reynolds numbers from 100 to 3000. The effect of jet-to-plate surface distance on the impingement heat transfer is also investigated. Our results show that synthetic jets can provide significantly higher cooling than steady jets in the Reynolds number range of 100 to 3000. We attribute the superior performance of synthetic jets to vortex shedding associated with the unsteady flow.

Bubble jet flow interaction during pool boiling of subcooled water on horizontal thin wires

Both visual experiments and numerical analyses were conducted to investigate the interaction between bubble jet flows during pool boiling of subcooled water on horizontal thin wires. The bubble jet flows nearby attracted each other, and they can combine into one jet flow under strong interaction. As the adjacent bubble departs, the bubble jet flow would experience an unsteady evolution process with the jet flow interaction weakening. Since the unsymmetrical thermocapillary force at the bubble interface was induced by the adjacent bubble as a cold source, the bubble jet flow would trend to the adjacent bubble, and the mechanism based on thermocapillary force and cold source can explain the bubble jet flow interaction very well. The steady bubble jet flow interaction phenomena were further simulated by laminar model, and the calculated jet flow interaction phenomena were in a good agreement with the experimental results.

Gas Phase Pressure Drop of a Water-Sparged Aerocyclone Reactor

Air stripping of ammonia is a widely used process for the pretreatment of wastewater. Scaling and fouling on the packing surface in packed towers and a lower stripping efficiency are the two major problems in this process. New patented equipment that is suitable for the air stripping of wastewater with suspended solids has been developed. Air stripping of ammonia from water with Ca(OH)2, was performed in the newly designed gas-liquid contactor, a water-sparged aerocyclone (WSA). The pressure drop of gas phase was investigated in order to know the momentum transfer characteristic of the WSA. It was found that the gas phase pressure drop exhibited a different change tendency compared with the traditional aerocyclone, and could be divided into three characteristic stages, including low pressure drop, pressure drop jump and high pressure drop phases. The three different pressure drop areas corresponds to steady jet, spiral and fogged flow patterns of the liquid phase. The critical value for the air flow rate which caused the mass transfer coefficient to increase rapidly took place at the point where the steady jet was transformed into the spiral flow pattern.

Linear stability of steady zonal jet flows induced by a small-scale forcing on a [formula omitted] plane

We analytically obtain steady isolated zonal jet solutions of the evolution equation of zonal flows on a β plane with a homogeneous zonal flow and a small-scale sinusoidal transversal flow in the background, derived by Manfroi and Young (1999) [9]. It is shown that these steady zonal jet solutions are all linearly unstable. Numerical time integrations of the evolution equation also confirm that the perturbed unstable steady solution becomes a uniform flow in the long run. These results suggest that mergers/disappearances of zonal jets superposed upon background forced two-dimensional turbulence on a β plane or a rotating sphere might be due to the intrinsic instability of the zonal jets.

On the Mach number and temperature dependence of jet noise: Results from a simplified numerical model

Numerical simulations of sound radiation from perturbed round jets are used, firstly to explore the structure of the sound sources and then to carry out a parametric study of the effect of jet Mach number and jet temperature. The simplified model problem includes a steady base jet flow, maintained in the absence of disturbances, superimposed with instability waves that are free to interact nonlinearly. Simulations over a range of subsonic jet Mach numbers show that a nonlinear mechanism dominates over a linear mechanism for low-frequency sound radiation, while for supersonic Mach numbers the linear mechanism is dominant. Additional insight is gained from a frequency-wavenumber analysis, including a transformation in the radial direction. With this decomposition, the acoustic field is located by the arc of a circle in plots of radial against streamwise wavenumber for discrete frequencies. The transformation is applied to both the pressure field, showing the sound directivity, and to selected source terms, showing characteristic directivity patterns for the streamwise and radial quadrupole terms. Decreasing the Mach number leads to a reduction in amplitude of the sources and of the sound radiation. Simulations with broadband forcing show that the qualitative effects of Mach number and jet heating are captured by this approach, which requires less resolution than a direct numerical simulation. A significant increase in the strength of the acoustic radiation for cold jets is observed, which is worthy of further investigation.