Jet Flow Configuration Research Articles

On the role of aortic valve architecture for physiological hemodynamics and valve replacement, Part I: Flow configuration and vortex dynamics

Aortic valve replacement has become an increasing concern due to the rising prevalence of aortic stenosis in an ageing population. Existing replacement options have limitations, necessitating the development of improved prosthetic aortic valves. In this study, flow characteristics during systole in a stenotic aortic valve case are compared with those downstream of two newly designed surgical bioprosthetic aortic valves (BioAVs). To do so, advanced three-dimensional fluid–structure interaction simulations are conducted and dedicated analysis methods to investigate jet flow configuration and vortex dynamics are developed. Our findings reveal that the stenotic case maintains a high jet flow eccentricity due to a fixed orifice geometry, resulting in flow separation and increased vortex stretching and tilting in the commissural low-flow regions. One BioAV design introduces non-axisymmetric leaflet motion, which reduces the maximum jet velocity and forms more vortical structures. The other BioAV design produces a fixed symmetric triangular jet shape due to non-moving leaflets and exhibits favourable vorticity attenuation, revealed by negative temporally and spatially averaged projected vortex stretching values, and significantly reduced drag. Therefore, this study highlights the benefits of custom-designed aortic valves in the context of their replacement through comprehensive and novel flow analyses. The results emphasise the importance of analysing jet flow, vortical structures, momentum balance and vorticity transport for thoroughly evaluating aortic valve performance.

Analysis of Thermal and Flow Characteristics in a Combined Cross-Flow and Jet-Flow Configuration with Flow-Guiding Fins

This study investigated the enhanced cooling of electronic components at high temperatures with cross-flow and jet-flow combinations. The cooling performance of four different model geometries (Models 1, 2, 3, and 4) of an electronic component was analysed by considering different jet-to-channel inlet velocity ratios (Vj/Vc) and ratios of the distance between the jet and impinging surface to jet diameter (H/D). The Vj/Vc and H/D ratios were varied in the 0–3 and 2–4 ranges, respectively, in the computational fluid dynamics analysis. The thermal and flow characteristics were revealed through a comparative result analysis, also considering results from the literature. The heat transfer improved, the Nusselt number increased, and the electronic surface temperature decreased with an increase in the Vj/Vc ratio. However, the Nusselt number decreased with an increase in the H/D ratio. Models 2 and 4 had higher heat transfer from the electronic component than the other models. A low H/D ratio and low Vj/Vc ratio yielded higher heat transfer in Model 3 than in Model 1.

Research on the flow field and film cooling effectiveness of the endwall with swirling film cooling

In order to obtain the effect of endwall secondary flow on the swirling film cooling, a geometric model of cascade is established to research the endwall swirling film cooling and swirling flow induced by prismatic jet impingement configurations. Numerical simulation is applied to investigate three jet flow configurations on the endwall film cooling performance at the compound angles of film hole γ = 0° and 30° and blowing ratios M = 0.5–2.0. The influence of complex vortex structures near endwall for jet flow is analyzed in detail; the strong transverse cross flow near the endwall is the main reason affecting the film cooling effectiveness. The variation laws of endwall film cooling effectiveness with the compound angle of film hole, jet flow configuration, and the blowing ratio are obtained. As the blowing ratio increases, the spanwise average film cooling effectiveness increases first and then decreases. While the blowing ratio is M =1.0, the endwall film cooling effectiveness is the best. Increasing the compound angle of the film hole leads to a decrease in the endwall cooling effectiveness. The spanwise average cooling effectiveness of γ = 30° decreases by 35% compared to the γ = 0°.

Comparative study and hybrid modeling approach with POD for convective drying performance of porous moist object with multi-impinging jet and channel flow configurations

Comparative studies for convective drying performance of a porous moist object in channel flow and with multi-impinging jet flow configuration are performed with finite element method. The analysis is conducted for different Re numbers (between 100 and 300), hot inlet temperature (between 50 and 80 °C), porous moist object size (between w and 3.5w) and jet flow arrangement parameters in unsteady configuration for times up to 1000 min. Flow re-circulations and isotherms within the channel and jet flow arrangement vary with Re number, size of the object and jet flow parameters. The moisture reduction (MR) becomes higher for the jet flow arrangement and higher Re number while at Re = 300, its value is 59.66%. Increasing the hot air inlet temperate resulted in increment of MR by about 17.5% for multi-impinging jet flow configuration while it is only 10% for channel flow arrangement. There is nearly 20% deviation in the MR value when cases with porous moist object with the lowest and highest size are compared. The vertical distance between the inlet jets to the porous object becomes more effective on the convective drying performance when compared to horizontal distance in between the jets while the MR values reduces from 61% to 48.5% when configurations with lowest and highest vertical distances are considered. An efficient hybrid model with proper orthogonal decomposition (POD) is proposed for efficient computations of convective drying. The computational cost for one case is reduced from 1.3 h for fully coupled high fidelity computational fluid dynamics simulations to 0.1 h with the proposed hybrid approach. The method is also flexible to handle complex external flow configurations and will be more efficient in 3D simulation case.

On the scalability of CFD tool for supersonic jet flow configurations

New regulations are imposing noise emissions limitations for the aviation industry which are pushing researchers and engineers to invest efforts in studying the aeroacoustics phenomena. Following this trend, an in-house computational fluid dynamics tool is build to reproduce high fidelity results of supersonic jet flows for aeroacoustic analogy applications. The solver is written using the large eddy simulation formulation that is discretized using a finite difference approach and an explicit time integration. Numerical simulations of supersonic jet flows are very expensive and demand efficient high-performance computing. Therefore, non-blocking message passage interface protocols and parallel Input/Output features are implemented into the code in order to perform simulations which demand up to one billion grid points. The present work addresses the evaluation of code improvements along with the computational performance of the solver running on a computer with maximum theoretical peak of 2.727 PFlops. Different mesh configurations, whose size varies from a few hundred thousand to approximately one billion grid points, are evaluated in the present paper. Calculations are performed using different workloads in order to assess the strong and weak scalability of the parallel computational tool. Moreover, validation results of a realistic flow condition are also presented in the current work.

Strong scaling of numerical solver for supersonic jet flow configurations

Acoustics loads are rocket design constraints which push researches and engineers to invest efforts in the aeroacoustics phenomena which is present on launch vehicles. Therefore, an in-house computational fluid dynamics tool is developed in order to reproduce high-fidelity results of supersonic jet flows for aeroacoustic analogy applications. The solver is written using the large eddy simulation formulation that is discretized using a finite-difference approach and an explicit time integration. Numerical simulations of supersonic jet flows are very expensive and demand efficient high-performance computing. Therefore, non-blocking message passage interface protocols and parallel input/output features are implemented into the code in order to perform simulations which demand up to one billion degrees of freedom. The present work evaluates the parallel efficiency of the solver when running on a supercomputer with a maximum theoretical peak of 127.4 TFLOPS. Speedup curves are generated using nine different workloads. Moreover, the validation results of a realistic flow condition are also presented in the current work.

Subgrid-scale scalar flux modelling based on optimal estimation theory and machine-learning procedures

ABSTRACTNew procedures are explored for the development of models in the context of large eddy simulation (LES) of a passive scalar. They rely on the combination of the optimal estimator theory with machine-learning algorithms. The concept of optimal estimator allows to identify the most accurate set of parameters to be used when deriving a model. The model itself can then be defined by training an artificial neural network (ANN) on a database derived from the filtering of direct numerical simulation (DNS) results. This procedure leads to a subgrid scale model displaying good structural performance, which allows to perform LESs very close to the filtered DNS results. However, this first procedure does not control the functional performance so that the model can fail when the flow configuration differs from the training database. Another procedure is then proposed, where the model functional form is imposed and the ANN used only to define the model coefficients. The training step is a bi-objective optimisation in order to control both structural and functional performances. The model derived from this second procedure proves to be more robust. It also provides stable LESs for a turbulent plane jet flow configuration very far from the training database but over-estimates the mixing process in that case.

Evaluation of stochastic particle dispersion modeling in turbulent round jets

ODT (one-dimensional turbulence) simulations of particle-carrier gas interactions are performed in the jet flow configuration. Particles with different diameters are injected onto the centerline of a turbulent air jet. The particles are passive and do not impact the fluid phase. Their radial dispersion and axial velocities are obtained as functions of axial position. The time and length scales of the jet are varied through control of the jet exit velocity and nozzle diameter. Dispersion data at long times of flight for the nozzle diameter (7 mm), particle diameters (60 and 90 µm), and Reynolds numbers (10, 000–30, 000) are analyzed to obtain the Lagrangian particle dispersivity. Flow statistics of the ODT particle model are compared to experimental measurements. It is shown that the particle tracking method is capable of yielding Lagrangian prediction of the dispersive transport of particles in a round jet. In this paper, three particle-eddy interaction models (Type-I, -C, and -IC) are presented to examine the details of particle dispersion and particle-eddy interaction in jet flow.

Flow instability in laminar jet flames driven by alternating current electric fields

The effect of electric fields on the instability of laminar nonpremixed jet flames was investigated experimentally by applying the alternating current (AC) to a jet nozzle. We aimed to elucidate the origin of the occurrence of twin-lifted jet flames in laminar jet flow configurations, which occurred when AC electric fields were applied. The results indicated that a twin-lifted jet flame originated from cold jet instability, caused by interactions between negative ions in the jet flow via electron attachment as O2+e→O2– when AC electric fields were applied. This was confirmed by conducting systematic, parametric experiment, which included changing gaseous component in jets and applying different polarity of direct current (DC) to the nozzle. Using two deflection plates installed in parallel with the jet stream, we found that only negative DC on the nozzle could charge oxygen molecules negatively. Meanwhile, the cold jet instability occurred only for oxygen-containing jets. A shedding frequency of jet stream due to AC driven instability showed a good correlation with applied AC frequency exhibiting a frequency doubling. However, for the applied AC frequencies over 80Hz, the jet did not respond to the AC, indicating an existence of a minimum flow induction time in a dynamic response of negative ions to external AC fields. Detailed regime of the instability in terms of jet velocity, AC voltage and frequency was presented and discussed. Hypothesized mechanism to explain the instability was also proposed.

층류제트유동 불안정성에 미치는 교류 전기장 효과

The effect of applied electric fields on jet flow instability was investigated experimentally by varying the direct current (DC) voltage and the alternating current (AC) frequency and voltage applied to a jet nozzle. We aimed to elucidate the origin of the occurrence of twin-lifted jet flames in laminar jet flow configuration, which occur when AC electric fields are applied. The results indicate that a twin-lifted jet flames originates from cold jet instability, caused by interactions between negative ions in the jet flow via electron attachment as <TEX>$O_2+e{\rightarrow}O_2{^-}$</TEX> when AC electric fields are applied. This was confirmed by experiments in which a variety of gaseous jets were ejected from a nozzle to which DC voltages and AC frequencies and voltages were applied, with ambient air between two deflection plates connected to a DC power source. Experiments in which jet flows of several gases were ejected from a nozzle and AC electric fields were applied in coflow-nitrogen provided further evidence. The flow instability occurred only for oxygen and air jets. Additionally, jet instability occurred when the applied frequency was less than 80 Hz, corresponding to the characteristic collision response time. The effect of AC electric fields on the overall structure of the jet flows is also reported. Based on these results, we propose a mechanism to reduce jet flow instability when AC electric fields are applied to the nozzle.

Simulation of Subsonic Turbulent Nozzle Jet Flow and Its Near-Field Sound

A direct numerical simulation framework is developed and validated for investigating a jet-flow configuration in which a short cylindrical nozzle and the acoustic near field are included in the simulation domain. The nozzle flow is modeled by a potential flow core and a developing turbulent wall boundary layer, which is numerically resolved. The setup allows to create well-controlled physical nozzle-exit flow conditions and to examine their impact on near-nozzle flow dynamics, jet-flow development, and the near-field sound. Turbulence at the nozzle inflow is generated by the synthetic-eddy method using flat-plate boundary-layer direct numerical simulation data and imposed softly in a sponge layer. The jet Mach number in the present investigation is , the diameter-based jet Reynolds number is , and the maximum axial rms fluctuations attain 13% at the nozzle exit. The accuracy of the numerical results is checked by varying grid resolution and computational domain size. The rapid flow development in the changeover region from wall turbulence to the turbulent free shear layer within about one nozzle diameter is documented in detail. Near-field sound pressure levels compare favorably with experimental reference data obtained at the much higher Reynolds number of 780,000. This agreement is essentially attributed to a compensation of the effects of Reynolds number and turbulence level on the noise for which an empirical scaling is derived from published data. A brief comparison is also made to the jet sound field arising from a laminar nozzle-exit boundary layer.

Simulation of a Nozzle‐Jet Configuration Including the Acoustic Near‐Field

AbstractA numerical simulation setup is presented which allows to study a circular jet flow configuration in which the nozzle is included in the simulation domain. Direct Numerical Simulations (DNS) are performed using up to 10th order compact finite‐difference schemes which are stabilized by applying a mild low‐pass filter. A parallelization approach has been implemented which shows good weak and strong scaling behavior. At the inflow the Synthetic Eddy Method is employed to generate turbulent fluctuations in the nozzle boundary layer with prescribed statistics, which are imposed by a sponge (forcing) layer technique. Simulation results for the jet flow field obtained at Reynolds number ReD = 19000 and a Mach number Ma = 0.9 as well as for the acoustic near‐field are found to be in good agreement with recent nozzle‐jet simulation results as well as experimental findings. (© 2012 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Flow visualization in an annulus between co-axis rotating cylinders with a circular jet on stationary outer cylinder

This work experimentally investigated the effects of jet flow and flow outlet configuration on the fluid flow in an annulus between co-axis rotating cylinders. By using the incense and the laser light, smoke flow visualization in a rotating annulus can be obtained. Firstly, the flow behavior in a rotating annular without jet flow and flow outlet was investigated. When Taylor number (Ta) exceeded 1708, the well-known Taylor vortices were successfully observed. Subsequently, the flow characteristics in a rotating annular with a jet flow and various flow outlet configurations were investigated. The circle jet nozzle was located at the middle position of the stationary outer cylinder. In addition, two flow outlet configurations were employed. One was the model of single outlet at the right side of the annulus, the other was the model of double outlets at both side of the annulus. The jet Reynolds number (Re) was 1351. The Taylor number (Ta) varied from 545 to 24,217. When the impinging force and the inertia force from the jet flow interacted with the Coriolis force and the centrifugal force due to rotation, the fluid flow should become very complicated. The experimental results indicated that the rapid rotation broke the original stream line of the jet flow. On the other hand, when the jet flow turned 90° to be the axial flow, it would suppress the onset of Taylor vortices. Finally, fixing the Re and Ta, the model of single outlet more suppressed the onset of Taylor vortices than the model of double outlets did.

Metal On‐Line Voltammetric Analyzer for On‐Site Monitoring of Iron Speciation in Acid Mine Drainage Waters: Development and Characterization

AbstractThe development of a simple, low cost, portable metal on‐line voltammetric analyzer (MOVA) for the iron analysis, as well as other species, in mining effluents is presented. It consists of a voltammetric cell based on an impinging jet flow configuration, a fluidic system controlled by gravity and electronics. Laboratory tests were performed to optimize the system as well as the analytical conditions for Fe speciation measurements. MOVA was then tested in the laboratory, in samples of both mining effluents. The results showed that iron in both mining effluents, as well as copper and arsenic, can be measured in oxygenated samples.

Experimental Determination of the Sub-grid Scale Scalar Flux in a Non-Reacting Jet-Flow

This work presents an experimental approach to the direct determination of the sub-grid scale scalar flux used in Large Eddy Simulation (LES). Two non-reacting jet-flow configurations were examined, one with the jet temperature similar to the ambient air temperature and one preheated jet. Hence data were obtained for varying viscosities. Each case was investigated for two different, high Reynolds numbers of 17,471 and 31,714. For the measurements of mixture fraction and flow velocity field a finest resolution of almost 100 μm was achieved. Thus both the sub-grid scale scalar flux as well as the spatially filtered quantities can be obtained from the same data set. The data were acquired by the simultaneous application of planar laser-induced fluorescence (LIF) and particle image velocimetry (PIV).

An experimental investigation of free and submerged miniature liquid jet array impingement heat transfer

Liquid water jet impingement cooling was investigated experimentally for both free-surface jet arrays and confined submerged jet arrays. The jet arrays consisted of straight holes of 1.0 mm diameter arranged in rectangular arrays with spacings of 3, 5 and 7 jet diameters between adjacent jets. For the impingement surface area of 780 mm 2, these jet array configurations can be considered well populated, with a total of 21, 45 and 121 jets impinging on the surface. Average heat transfer and pressure drop measurements are presented for volumetric flow rates in the range of 2 L / min ⩽ V ˙ ⩽ 9 L / min and dimensionless jet-to-target spacings between 2 ⩽ H/ d n ⩽ 30. For the submerged jet arrays a strong dependence on both jet-to-target and jet-to-jet spacing is observed and correlations are presented that adequately predict the experimental measurements. The free-surface jets show a non-monotonic change with jet-to-target spacing with a local minimum in the heat transfer coefficient at approximately H/ d n = 10. Here a transition from a submerged to a free jet flow configuration occurs. Once again, a correlating equation is presented that adequately predicts the free-surface jet array heat transfer data. The pumping power required to form the submerged and free jet flows show a different relationship to the heat transfer coefficient. Generally, submerged jets have a higher heat transfer coefficient for a given pumping power requirement.

Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows

Stabilization mechanisms of lifted laminar propane flames are investigated in an axisymmetric jet flow configuration. Detailed mixing and flow fields upstream of the flame lift-off heights measured by Chung and coworkers are calculated on a nonreacting flow basis. Jet exit velocity, nozzle diameter, coflow air velocity as well as partial jet premixing with air are varied to obtain the local stoichiometric axial velocity, U st , and scalar dissipation rate, χ st , at points that are upstream of the flame stabilization locations by a redirection region length. It is found that U st decreases consistently with increasing χ st when the centerline mixture fraction is higher than that of the rich flammability limit. Beyond this threshold at downstream locations, a rich-joined flame is formed with U st independent of the local scalar dissipation rate. Instead, U st decreases with increasing ξ st , the stoichiometric mixture fraction of the jet flow. For lifted flames stabilized close to the burner exit with an edge-flame appearance, the maximum attainable mixture fraction gradient approaches that calculated for extinction of stretched diffusion flames in the counterflow geometry. The flame propagation velocity remains positive. The trend of decreasing U st with increasing χ st is also found for numerical simulations with a unity Lewis number and for experimental data of lifted methane flames. These results corroborate the triple flame stabilization concept. An empirical formula for the triple flame propagation velocity is proposed with a nonlinear dependence on local scalar dissipation rate. In addition, stabilization mechanisms previously proposed for lifted turbulent diffusion flames are reconciled for lifted laminar flames, depending on the local flow/mixing conditions. Appropriate flame stability and blow-out criteria are derived and these predict the flame lift-off height and blow-out velocity accurately. Implications for flame stabilization in turbulent jet flows with such triple flame structures are also discussed.

Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows

Stabilization mechanisms of lifted laminar propane flames are investigated in an axisymmetric jet flow configuration. Detailed mixing and flow fields upstream of the flame lift-off heights measured by Chung and coworkers [28–30] are calculated on a nonreacting flow basis. The local stoichiometric axial velocity, U st , and scalar dissipation rate, χ st , are obtained at points that are upstream of the stabilization locations by a redirection region length. Variation is investigated with jet exit velocity, nozzle diameter, coflow air velocity as well as partial jet premixing with air. It is found that U st decreases consistently with increasing χ st when the centerline mixture fraction is higher than that of the rich flammability limit. Beyond this threshold, a rich-joined flame is formed at downstream locations with U st independent of the local scalar dissipation rate. Instead, U st decreases with increasing ξ st , the stoichiometric mixture fraction of the jet flow. For lifted flames stabilized close to the burner exit with an edge-flame appearance, the maximum attainable mixture fraction gradient approaches that calculated for extinction of stretched diffusion flames in the counterflow geometry. The flame propagation velocity remains positive. The trend of decreasing U st with increasing χ st is also found for numerical simulations with a unity Lewis number [18] and for experimental data of lifted methane flames [19]. These results corroborate the triple flame stabilization concept. An empirical formula for the triple flame propagation velocity is proposed with a nonlinear dependency on local scalar dissipation rate. In addition, different flame stabilization mechanisms previously proposed for lifted turbulent diffusion flames are reconciled for lifted laminar flames, depending on the local flow/mixing conditions. Appropriate flame stability and blow-out criteria are derived and these predict the flame lift-off height and blow-out velocity accurately. Implications for flame stabilization in turbulent jet flows with such triple flame structures are also discussed.

Slot jet impingement heat transfer from circular cylinders

The heat transfer characteristics of circular cylinders exposed to slot jet impingement of air has been studied experimentally. The study focused on Reynolds numbers (based on nozzle average exit velocity and cylinder diameter) in the range 600–8000. Both contoured orifice and sharp-edged orifice jet configurations were investigated for cylinder diameter-to-jet width spacings of 0.66, 1.0 and 2.0, and for jet exit-to-nozzle width spacings in the range 1< z/ w<11. Reference measurements were made with the same cylinder in infinite parallel flow in a wind tunnel. The results reveal that the slot jet yields considerably higher average heat transfer from the cylinder when compared to the infinite parallel flow case on the basis of identical average velocity in the slot jet and infinite flow configurations. As with slot jet impingement heat transfer from flat surfaces, there appears to be a nozzle-to-cylinder spacing, scaled by nozzle width, which exhibits a maximum in heat transfer. The average heat transfer characteristics of the cylinder are correlated empirically.

平板に衝突する超音速噴流に関する数値解析及び実験

An impingement of two-dimensional axisymmetric supersonic jet on a plate is simulated numerically to evaluate the jet flow configuration on the plate with a real gas assumption. An explicit TVD-Upwind scheme is used for the Euler equations to evaluate interaction between shock waves. An experimental study of the underexpanded axisymmetric supersonic N2 jets impinging on a plate is also discussed. Pressure measurements on the plate surface and Schlieren photometries are performed to investigate the impinging jets. Numerical results with the Euler equations have a good agreement with the experimental results at the shock structure and pressure distribution on the plate. The results show that three types for complicated shock-shock interactions are revealed and that a stagnation bubble is formed due to triple point near the plate.