Plane Jet Research Articles

Flow and heat transfer dynamics of an altered impinging plane jet using an insert

A technique that involves the use of a cylindrical insert to increase the heat-transfer rate in an impinging plane jet is studied using Large-Eddy Simulations. Results are presented for a plane impinging jet without an insert and for a jet with an insert, both at Reynolds number (based on the bulk inlet velocity and nozzle width) of 20,000 and with the target wall at a distance of four nozzle widths from the outlet. The secondary peak in the Nusselt number, which is characteristic of this flow, was obtained in the case where no insert was present, but was much diminished in the presence of the insert. Contours of the second invariant of the mean-velocity tensor are presented to better understand the role of coherent structures and their influence on heat transfer.

Cooling performance of the hot-rolled seamless steel tube with different jet forms

The jet form is closely related to the cooling strength and uniformity of steel tubes, which is the first consideration in the design of quenching equipment. A reasonable choice of jet form will improve product quality and reduce production costs. In this study, the heat transfer behavior of different jet forms is discussed. Two jet forms are considered: planar jet and annular jet. Besides, the size of the steel tube is also an important variable in this simulation. The remaining parameters are fixed values. The axial and circumferential temperature distributions of the steel tube were obtained. The results showed that the annular jet has better performance on cooling intensity and cooling uniformity. The cooling performance of the two jet forms are almost the same when the steel tube size is smaller than 50 mm. As the planar jet quenching equipment is simple to manufacture, low in cost, and convenient in site application, so the planar jet can be considered when the diameter is small.

Integral Turbulent Length and Time Scales of Higher Order Moments

Turbulent length and time scales represent a fundamental quantity for analysing and modelling turbulent flows. Although higher order statistical moments have been conveniently used for decades to describe the mean behaviour of turbulent fluid flow, the definition of the integral turbulent scales seems to be limited to the velocity or its fluctuation itself (i.e. the first moment). Higher order moments are characterized by smaller integral scales and a framework is proposed for estimating autocorrelation functions and integral turbulent length or time scales of higher order moments under the assumption that the probability distribution of the velocity field is Gaussian. The new relations are tested for synthetic turbulence as well as for DNS data of a turbulent plane jet at Reynolds number 10000. The present results in particular suggest that the length or time scales of higher order moments can be markedly smaller than those of the turbulent variable itself, which has implications for statistical uncertainty estimates of higher order moments.

Aerodynamic sealing using a segmented recirculating horizontal air curtain for buoyancy-driven flows in doorways

Dealing with buoyancy-driven flow to reduce infiltration heat is challenging for buildings such as cold storages where the indoor-outdoor temperature differences are significant. In this study, a segmented recirculating horizontal air curtain (SRHAC) is proposed to respond to the vertical thermal buoyancy profile in doorways and recover the cold air entrained by the air curtain. Specifically, SRHAC is a planar turbulent impingement jet discharged through a vertically mounted rectangular nozzle. The discharge nozzle is divided into segments and the return vent recirculates the discharged air to the rear of the discharge nozzle. In this study, the aerodynamic sealing performance of the SRHAC is investigated by considering structural, operational, and ambient parameters. Orthogonal analysis reveals the flow rate, nozzle width, and discharge angle as significant factors affecting the thermal separation efficiency of the SRHAC. Increasing the flow rate and decreasing the nozzle width leads to an increase in the jet momentum flux, thereby enhancing the thermal separation efficiency, with a maximum efficiency of 75.9 %. Setting the discharge parameters in the three sections along the height improves the thermal separation efficiency by 16.5% points. The maximum efficiencies of two-, three-, and five-segment air curtains are 83.6 %, 87.3 %, and 90.3 %, respectively. The separation efficiency of the SRHAC remains relatively stable even when the indoor-outdoor temperature difference varies between 32 and 57 K. These results establish that the SRHAC is an efficient energy-saving device, with the ratio of reduced infiltration load to energy consumption as high as 110.9.

The similarity theory of free turbulent shear flows of viscoelastic fluids

A new theory is formulated for the description of the conformation state of the polymer chains in free turbulent shear flows of viscoelastic fluids. Using self-similarity arguments and new scaling relations for the turbulent flux of conformation tensor we show the existence of minimum and maximum solvent dissipation reduction asymptotes, and four different polymer deformation regimes. The similarities with the maximum drag reduction asymptote of turbulent pipe flow is discussed and new scaling laws are obtained for all components of the mean conformation tensor at each deformation regime. Analytical solutions for the self-similar transverse profiles of the conformation tensor components are also obtained, providing the complete solution for the mean flow problem at the far field. The analysis is developed for both planar jets and wakes and covers the two limits of shear flows, with large and small velocity differences, respectively. Comparisons of the new theoretical results with several direct numerical simulations employing the FENE-P rheological model show excellent agreement.

Experimental characterization of a modified high-speed plasma synthetic jet actuator with oblique-slot exit

In the field of flow control research, oblique jets are known to offer several advantages over vertical jets. To gain a comprehensive insight into the flow field characteristics of a plasma synthetic jet actuator with an oblique-slot exit, the related experiments are conducted. The experiment employed high-speed schlieren imaging techniques and electrical parameter measurements to acquire the flow field characteristics and discharge properties of the oblique-slot actuator, followed by a comparative analysis with a vertical circular orifice actuator. The oblique-slot plasma synthetic jet exhibits a wall-attaching effect and asymmetric flow characteristics, which differ from those of the vertical circular orifice actuator. The actuator generates a wall jet with an initial velocity of 389.5 ± 15.08 m/s, effectively propelling the fluid within the boundary layer. The Mach number of the precursor shock wave in the direction of the jet reaches 1.59, but decreases to just 1.02 in the opposite direction. Over a period in the range of 10–70 μs, the Froude number of the plasma jet decreases from 1841 to 238. The dominant role of the inertial force gradually weakens, while the influence of buoyancy increases, causing the jet boundary to move upward. The oblique-slot jet configuration represents a typical planar jet, exhibiting superior flow control uniformity compared with the vertical circular orifice jet. The results indicate that the high-speed oblique-slot plasma synthetic jet actuator designed in this study possesses distinct advantages over vertical circular orifice actuators for high-speed fluid flow control.

The mixing field and flame structure near the reaction zone of turbulent planar flames at different levels of mixture inhomogeneity

Turbulent flames with compositionally inhomogeneous mixtures are commonly used in many combustion systems. In this work, turbulent planar jet flames issued from a concentric flow slot burner, CFSB, were used to study the impact of mixture inhomogeneity near the flame sheet. The CFSB burner can control the mixing inhomogeneity by changing the mixing length “L” between the concentric fuel and air slot nozzles. At various levels of mixture inhomogeneity, the mixing field, presented by mixture fraction, Z, distribution, and the flame structure was investigated via simultaneous Rayleigh and OH-PLIF imaging technique. Our previous study investigated the mixing field immediately downstream of the burner exit in non-reacting conditions. The PDFs of Z showed that the mixing field covered a wide range of mixture fractions, where the high flame stabilization occurred when a large portion of the PDF(Z) was located within the fuel flammability limits. This work showed that further downstream, the highly stabilized flames were also obtained when the range of fluctuations in the mixture fraction was close to the stoichiometric mixture fraction. Moreover, plotting the mixing field using the scatter plot within the mixing regime diagram for various flame conditions showed that the mixing field downstream of the burner exit consistently follows the mixing diagram classification. Moreover, a close inspection of the flame structure showed that the flame sheet varies from thick, and corrugated at a low and high levels of mixing to thin flame sheet but less corrugated and that at a particular mixing normalized length L/D = 7. The flame corrugation data showed that the flame stability occurs at a minimum corrugation factor.

The limits of β-plane turbulence

The quasigeostrophic shallow-water system on the mid-latitude $\beta$ plane with weak, small-scale turbulent forcing is explored in the limit of large energy. Forcing is weak in the sense that the energy input rate relative to the energy of the flow is very small, of the order of $10^{-5}$ – $10^{-10}$ , and the potential vorticity assumes an approximate staircase structure. The flow has large energy in the sense that the jet spacing is equal to the domain width so that no further jet mergers can occur. Quasi-stationary numerical experiments, in which the energy grows linearly, reveal late-time quasi-steady, translating solutions comprising a single jet and vortex dipole, with details of the jet-vortex configuration depending on the deformation radius. At a smaller deformation radius the jet may traverse the entire domain in the $y$ direction one or more times, giving a jet orientation that is predominantly north–south, rather than the usual east–west orientation characteristic of $\beta$ -plane jets at lower energy. In these meandering cases, a mode number is proposed that quantifies the degree of meandering relative to the vortices. Besides the steadily translating solutions, topological changes in the jet-vortex structure are identified that occur via a transient interaction of a meandering jet with a vortex. At high energy, these give rise to apparently periodic solutions of the system; at low energy, before a single, domain-wide jet is established, they indicate that jet merger may occur through more complicated processes than the simple merging of neighbouring jets.

Self-similarity of sand bed scour by an oblique impinging plane jet under different Reynolds numbers

The present study investigated the effects of varying Reynolds number (Re = 6,000, 8,000, 10,000 and 12,000) of a plane oblique impinging water jet on an erodible sand bed under a fixed impinging angle (θ = 40°) and a fixed impinging distance (h = 0.202 m). Time evolution of the scour hole profile was recorded using a video camera, and the flow fields in the scour hole at equilibrium state were measured using Particle Image Velocimetry (PIV) technique. The results reveal that both the scour hole profile and the flow structure in the scour hole under different Reynolds numbers are mostly self-similar. Specifically, the distributions of streamline, velocity, average kinetic energy and Reynolds shear stress at equilibrium state are analyzed in detail. It is found, among the first according to the authors' best knowledge, that all these flow quantities exhibit spatial self-similarity, when normalized with proper length and/or velocity scales. Furthermore, the Reynolds shear stress on the sand bed profile has two peak values. The positive peak near the impingement point is similar in magnitude (between 4 and 6 Pa for different Reynolds numbers), while the negative peak in the bed vortex region differs.

Non-Newtonian turbulent jets at low-Reynolds number

We perform direct numerical simulations of planar jets of non-Newtonian fluids at low Reynolds number, in typical laminar conditions for a Newtonian fluid. We select three different non-Newtonian fluid models mainly characterized by shear-thinning (Carreau), viscoelasticity (Oldroyd-B) and shear-thinning and viscoelasticity together (Giesekus), and perform a thorough analysis of the resulting flow statistics. We characterize the fluids using the parameter γ, defined as the ratio of the relevant non-Newtonian time scale over a flow time scale. We observe that, as γ is increased, the jet transitions from a laminar flow at low γ, to a turbulent flow at high γ. We show that the different non-Newtonian features and their combination give rise to rather different flowing regimes, originating from the competition of viscous, elastic and inertial effects. We observe that both viscoelasticity and shear-thinning can develop the instability and the consequent transition to a turbulent flowing regime; however, the purely viscoelastic Oldroyd-B fluid exhibits the onset of disordered fluid motions at a lower value of γ than what observed for the purely shear-thinning Carreau fluid. When the two effects are both present, an intermediate condition is found, suggesting that, in this case, the shear-thinning feature is acting against the fluid elasticity. Despite the qualitative differences observed in the flowing regime, the bulk statistics, namely the centerline velocity and jet thickness, follow almost the same power-law scalings obtained for laminar and turbulent Newtonian planar jets. The simulations reported here are, to the best of our knowledge, the first direct numerical simulations showing the appearance of turbulence at low Reynolds number in jets, with the turbulent motions fully induced by the non-Newtonian properties of the fluid, since the Newtonian case at the same Reynolds number is characterized by steady, laminar flow.

Turbulence effects on the formation and growth of nano-particles in three-dimensional premixed and non-premixed flames

This study examines the impact of turbulence on particle-forming flames via three-dimensional direct numerical simulations. The simulations employ the finite rate chemistry approach to simulate both premixed and non-premixed methane-air turbulent planar jet flames. These flames are doped with titanium tetraisopropoxide (TTIP) to form titanium dioxide TiO2 nanoparticles. The sectional model is employed to solve the population balance equation governing particle dynamics. Through these simulations, a number of the Batchelor scales pertaining to the smaller nanoparticle structures are effectively captured. The analysis conducted on these simulations is to identify and quantify the respective influences of diffusion, coagulation, and inception on variations in particle concentration across different regions within the flame. Several regions of the computational domain with different turbulence intensities are analyzed. Results show that the physical mechanisms that contribute to particle growth are not negligible on the particle concentrations. The impact of normal aN and tangential aT strain rates, which govern the thickness of particle-loaded zones affected by turbulence effects, is also evaluated. Conditional mean values of the strain rates upon the particle number concentration fields and the PDFs of aN and aT confirm that on the iso-surfaces of the particle fields the compressive and stretching effects are predominant. The aforementioned information is used to gain a deeper understanding of the influence of turbulence on premixed and non-premixed particle-forming flames, which will help us to develop transferable models for the simulation of nanoparticle synthesis.

On the role of low frequency modes in the thermal and momentum mixing in parallel plane jets

Parallel plane jet mixing has been the subject of extensive study and its physics are characterized well in literature. However, the physics of parallel plane jet mixing in relatively small confined regions is not well known. In this study we investigate the effects of re-circulatory flow on thermal and momentum mixing in confined parallel plane jets. Studying confined jet interaction is important for a variety of engineering applications. One of these applications is sodium cooled fast reactors (SFR). It is common in SFRs to have coolant stream mixing in a confined domain, often the upper plenum. The Reactor Cavity Cooling System (RCCS) facility at the University of Michigan models this problem in a simplified enclosure with isothermal parallel plane jets as the incoming coolant streams. A Large Eddy Simulation (LES) of this simplified plenum was developed using optimal boundary conditions, proven mesh convergence, and consistent resolution. This model was then validated against experimental results from the RCCS facility. After model validation, flow structures in the jet interaction were discerned by means of Proper Orthogonal Decomposition (POD). Comparison of 3D POD to a clipped 2D POD region reveals that re-circulatory flow can lead to dominant oscillations within the jet mixing region. Subsequently, Power Spectrum Density (PSD) and Continuous Wavelet Transforms (CWT) were used to ascertain frequency-based phenomena that might occur in this mixing region. Frequency analysis uncovered a dominant low-frequency oscillation in both velocity and temperature PSDs. These frequencies fall within the known ranges of thermal striping in sodium. The data generated in this study can be used for the benchmarking of less computationally intensive approaches. This data will be made available upon request to ebm5351@psu.edu.

Length scales and the turbulent/non-turbulent interface of a temporally developing turbulent jet

The temporally developing self-similar turbulent jet is fundamentally different from its spatially developing namesake because the former conserves volume flux and has zero cross-stream mean flow velocity whereas the latter conserves momentum flux and does not have zero cross-stream mean flow velocity. It follows that, irrespective of the turbulent dissipation's power-law scalings, the time-local Reynolds number remains constant, and the jet half-width $\delta$ , the Kolmogorov length $\eta$ and the Taylor length $\lambda$ grow identically as the square root of time during the temporally developing self-similar planar jet's evolution. We predict theoretically and confirm numerically by direct numerical simulations that the mean centreline velocity, the Kolmogorov velocity and the mean propagation speed of the turbulent/non-turbulent interface (TNTI) of this planar jet decay identically as the inverse square root of time. The TNTI has an inner structure over a wide range of closely spatially packed iso-enstrophy surfaces with fractal dimensions that are well defined over a range of scales between $\lambda$ and $\delta$ , and that decrease with decreasing iso-enstrophy towards values close to $2$ at the viscous superlayer. The smallest scale on these isosurfaces is approximately $\eta$ , and the length scales between $\eta$ and $\lambda$ contribute significantly to the surface area of the iso-enstrophy surfaces without being characterised by a well-defined fractal dimension. A simple model is sketched for the mean propagation speeds of the iso-enstrophy surfaces within the TNTI of temporally developing self-similar turbulent planar jets. This model is based on a generalised Corrsin length, on the multiscale geometrical properties of the TNTI, and on a proportionality between the turbulent jet volume's growth rate and the growth rate of $\delta$ . A prediction of this model is that the mean propagation speed at the outer edge of the viscous superlayer is proportional to the Kolmogorov velocity multiplied by the $1/4$ th power of the global Reynolds number.

Extraction and analysis of flow features in planar synthetic jets using different machine learning techniques

Synthetic jets are useful fluid devices with several industrial applications. In this study, we use the flow fields generated by two synchronously operating synthetic jets and simulated using direct numerical simulations. These flow fields are characterized by a jet Reynolds number, Re=100, 150, and 200, and a Strouhal number, St=0.03. We benchmark four different dimensionality reduction techniques: (1) higher-order dynamic mode decomposition (HODMD), (2) proper orthogonal decomposition, (3) vector quantization via principal component analysis (VQPCA), and (4) linear autoencoders. These techniques are often used in generating reduced-order models (ROMs). The performances of these techniques are compared (i) in terms of their ability to accurately reconstruct the high-dimensional flow fields from their low-dimensional manifolds and (ii) in terms of their ability to extract meaningful low-dimensional patterns/features/structures that best describe the main dynamics of the synthetic jets. The similarity between the extracted features is also quantitatively assessed with the help of Procrustes analysis, showing how manifolds from different techniques become more similar when a larger number of modes are retained. Accurate reconstruction and model complexity (or interpretability) are often two counter-balancing objectives. In this comparative study, we found that among the four techniques, VQPCA has clear advantages for developing accurate ROMs, while HODMD is useful for understanding the dynamics of synthetic jets, providing additional information that is not readily available with other methods.

Characteristics of Initial Development of Plane Jet Scour

Characteristics of Initial Development of Plane Jet Scour

Stability of a particle-laden planar jet in the dilute suspension limit

Particle laden flows are commonly seen in many industrial applications, such as fluidized beds in process industry, air laden with abrasive particles in abrasive machining, and particle laden plumes in chemical industries. In the present work, we perform local analysis of a particle laden planar jet in the dilute suspension regime. Unladen parallel planar jets have been extensively studied using normal modes and are shown to have two unstable modes, namely, sinuous and varicose modes. Sinuous modes are found to be more unstable compared to the varicose modes. In the present study, we investigate the effect of particles on the stability of planar jets. Addition of particles at low Stokes numbers (St) (fine particles) results in higher growth rates than that of the unladen jet. In the intermediate Stokes number regime, addition of particles has a stabilizing effect on both the sinuous and the varicose modes. Interestingly, for St∼10, the unstable varicose mode is completely damped. Increasing the Stokes number by increasing the particle size, both sinuous and varicose modes, shows increasing growth rates, while increasing the density ratio has a stabilizing effect on the flow. For non-uniform particle loading, additional modes apart from the sinuous and varicose modes are observed. These modes suggest the occurrence of compositional instability with an increased particle accumulation in the shear layer that is an order of magnitude higher compared to that of the sinuous and varicose modes.

Dispersion of expectorated cough droplets with seasonal influenza in an office

We do not fully grasp viral droplet transmission processes in ventilated interior environments. The core focus of our research is to investigate the effectiveness of the protected occupied zone ventilation (POV) through computational fluid dynamics models in a simplified office setting. The large-eddy simulation technique with the Eulerian–Lagrangian model was implemented to address complicated indoor processes such as turbulence, flow–aerosol interaction, and ventilation impact. We computationally investigated the effects of desk partitions and the POV on cough droplets in an office. The ventilation approach was tested using two distinct exhaust layouts and four different ventilation rates (1.0, 1.2, 1.5, and 1.8 m/s). A comparative analysis of the ventilation flow fields, topologies, and particle directions has been studied. The findings indicate that the plane jet's ventilation rates influence the protected occupied zone ventilation performance. The ventilation rates distributed the virus droplets around the room, but compared to the up-exhaust cases, the down-exhaust cases appeared to have better shielded the healthy person. This pattern could indicate that the placement of the exhaust outlet in ventilation systems significantly influences indoor aerosol dispersion. The results also show that substantial flow streams may carry tiny particles (≤70 μm) throughout their path. Large particles (≥100 μm) could not go far in cough gas clouds. Most viral particles deposit on solid surfaces in various work sites per specific ventilation rates. Office workers need to be very cautious around these hazardous areas.

Acoustic characteristics of supersonic planar impinging jets

The present work aims to study the tonal and broadband noise associated with a supersonic planar jet, of Mach number 1.28 and Reynolds number 50000, impinging on a flat plate. Five different cases are considered corresponding to different plate distances and angles of impingement. The near-field noise is analyzed using Large Eddy Simulation (LES). For this, we employ a low dispersive and dissipative Finite Volume solver using AUSM for inviscid fluxes and third-order Runge–Kutta method for temporal discretization. The far-field noise is calculated using a hybrid approach in which the acoustic sources and mean flow are provided by the fluid solver while the far-field acoustic pressure is calculated by an acoustic solver. It solves Acoustic Perturbation Equations using a spectral/hp element method with a Discontinuous Galerkin projection. The present work highlights many important acoustics studies of impinging jets. Firstly, the far-field sound with the effect of non-uniform base fields is obtained which is important as the far-field acoustic pressure in most of the literature is reported either without any base fields or by the application of uniform base fields. Secondly, a correlation between the different modes of oscillations of the impinging jet and the production of impinging tones is studied using Modal Decomposition techniques. This analysis also sheds some light on the number of concurrent cycles for the different nozzle-to-plate distances. Lastly, the relation between the phase lag term and the distance between the plate and the nozzle exit is highlighted using a vortex tracking method. These observations can serve to provide a better understanding of the sound generation process.

The velocity and thermal fields of two parallel plane jets using simultaneous PIV and two-color LIF measurements

This paper presents the results of simultaneous measurements of the velocity and thermal fields of two parallel plane jets. Jets with two different distance ratios having Reynolds numbers of 850 and 910 are reported, with distance ratios selected based on whether or not they have a periodic vortex shedding phenomenon, and were experimented using simultaneous particle imaging velocimetry and two-color laser induced fluorescence. Results showed that jets with a distance ratio of 1.4 have a periodic vortex shedding phenomenon, which are absent in jets with a distance ratio of 2.0. The overall structure of the temperature fields are dissimilar to the velocity fields and major differences in Reynolds shear stress and turbulent heat flux were also apparent, showing that transfer processes of momentum and heat flux differ greatly. The spreading of turbulence statistics showed the mixing capabilities of the periodic vortex shedding phenomenon.

Reduced-order variational mode decomposition to reveal transient and non-stationary dynamics in fluid flows

A novel data-driven modal analysis method, reduced-order variational mode decomposition (RVMD), is proposed, inspired by the Hilbert–Huang transform and variational mode decomposition (VMD), to resolve transient or statistically non-stationary flow dynamics. First, the form of RVMD modes (referred to as an ‘elementary low-order dynamic process’, ELD) is constructed by combining low-order representation and the idea of intrinsic mode function, which enables the computed modes to characterize the non-stationary properties of space–time fluid flows. Then, the RVMD algorithm is designed based on VMD to achieve a low-redundant adaptive extraction of ELDs in flow data, with the modes computed by solving an elaborate optimization problem. Further, a combination of RVMD and Hilbert spectral analysis leads to a modal-based time-frequency analysis framework in the Hilbert view, providing a potentially powerful tool to discover, quantify and analyse the transient and non-stationary dynamics in complex flow problems. To provide a comprehensive evaluation, the computational cost and parameter dependence of RVMD are discussed, as well as the relations between RVMD and some classic modal decomposition methods. Finally, the virtues and utility of RVMD and the modal-based time-frequency analysis framework are well demonstrated via two canonical problems: the transient cylinder wake and the planar supersonic screeching jet.