The aim of this experimental study is to investigate the diffusion of jets, with particular attention focused on the relationship between the static pressure and the stream wise mean velocity on the development of a two-dimensional air jet. A jet with three injection velocities (11.7, 23.3 and 35.0 m/s) was generated in a two-dimensional wind tunnel. The velocity distributions were measured by an X-type hot-wire anemometer. The static pressure distributions were measured by a static pressure probe developed in our laboratory, which incorporates a hot-wire anemometer. The probe is designed to be able to measure the mean static pressure and pressure fluctuation simultaneously. The sensitivity is 92.3 mV/Pa. The frequency response is flat from 16 Hz to 2.5 kHz. As a result of the experiment, it was found that negative static pressure exists in the turbulent shear layer. It is considered that the entrainment process from the negative static pressure by the vortex structure motion of the turbulent shear layer.
We present the flow characteristics of a plane jet from a nozzle with an extended lip-plate and serrated tabs. The interaction between spanwise and streamwise vortices in the plane jet was studied experimentally. The nozzle has an extended lip-plate and serrated tabs in order to enhance the spanwise and streamwise vortices generated in the mixing layer. The extended lip-plate length of the nozzle L was changed from 0 to 10H (H: height of plane nozzle). Serried isosceles triangle tabs were placed on the upper or lower side of the nozzle exit. It is found that the increment of entrainment for the jet from a nozzle of L/H =2 without tabs is related to the self-excited spanwise vortical structure. The tabs suppress the formation of self-excited vortices in shear layers of the plane jet from a nozzle with an extended lip-plate.
In order to improve the heat transfer on the wall, impinging jets are used in various industrial applications, and have been investigated experimentally and numerically so far. However, it is not enough to make clear the detail of vortical structure contributing to the heat transfer. In the present paper, direct numerical simulations (DNS) of the impinging jet are conducted through the control of vortical structure in order to investigate the heat transfer. The discretization in space is performed with a hybrid scheme in which Fourier spectral and 6th order compact scheme are adopted. As the control parameter, two cases of perturbations are superposed on the inflow boundary conditions. These excitations contribute to the generation of coherent vortical structures, resulting in the enhancement of mixing away from the impinging wall. However, the heat transfer at the wall is not vitalized in comparison to the no excitation case. The reason why no enhancement of the heat transfer occurs are considered, based on both the balance of the heat flux and the snapshot of flow. It is found that the excitation strengthens the upward flow, which disturbs the heat transfer, and that the upward lifting of coherent vortical structures make the inactive state in the vicinity of the impinging wall.
This paper describes the heat transfer enhancement by triangular tabs attached to the nozzle exit of the jet. Experiments on the impinging jet were performed to clarify the effect of the triangular tabs on the heat transfer. In particular, the attention is paid on the relationship between tab pitch angle θ and heat transfer enhancement ratio defined as NuTab/Nu0. Temperature distribution was measured by a thermosensitive liquid crystal sheet on the impingement plate of a glass coated by ITO (Indium Tin Oxide) film. In the case of θ =90 or 135 deg, the region of heat transfer enhancement is wider than the case of θ =45 deg. The velocity field was also measured by a hot-wire anemometer in the jet and over the impinging plate to investigate the relation between the flow field and the temperature field. It is found that the flow fluctuation by the tabs clearly contributes to the heat transfer promotion.
The experimental study of flow field and heat transfer characteristics of a submerged, rectangular jet impinging normally on a flat plate is presented. Four nozzle-to-surface spacing of H/w=3, 6, 10 and 20 with acoustic excitations was performed experimentally. The forcing Strouhal number is set from St=0.1 to 0.9 at 0.1 intervals. Velocity distributions with two excitation modes, symmetric and asymmetric excitation, are investigated by means of hot-wire measurement. It was found that the basic flow fields changed drastically, while there is little difference of heat transfer characteristic for both the excitation modes. For the short nozzle-to-plate distances, the heat transfer was enhanced in the wall jet region for the excitation of St=0.2. The maximum heat transfer ratio was obtained at the stagnation point for the middle spacing of H/w=6 for St=0.5. It was revealed that the non-periodic velocity component played the most important role in promotion of the heat transfer with acoustic excited impinging jet.
Flow management of a plane turbulent wall jet has been studied experimentally by utilizing a streamwise vortex pair with periodic variation in strength and radius. The magnitudes of the streamwise vorticity and secondary current induced by the vortex pair are weakened by the periodic variation. The integrated streamwise momentum flux is increased by the introduction of the streamwise vortex pair which consists of two counter-rotating longitudinal vortices into the shear layer. Despite the weakening of the vorticity and secondary current caused by the periodic variation, the increment rate of the momentum flux is slightly increased due to the periodic variation. In an analysis with triple velocity decomposition and phase averaging, it is evident that the periodic fluctuating velocity component contributes significantly to the production of the Reynolds shear stress.
We analytically examine breakup phenomena of a compound liquid jet which consists of a gas or liquid core phase and a surrounding annular phase. Applying the long wave approximations to the basic equations and the boundary conditions for inviscid and incompressible fluids, simplified nonlinear equations are derived for large deformation of the jet. It is numerically shown for a doubly infinite jet that the core phase is periodically capsuled by the annular phase, whose profiles are largely affected by density ratios and velocity difference between the core and annular phases. On the other hand, for a semi-infinite jet emanating from a nozzle exit, breakup of the jet brings about encapsulation in the downstream, whose profile becomes more sensitive to input disturbances when the Weber number is small. For larger Weber number, however, the breakup profiles are almost the same as those in the doubly infinite jet. In order to see the validity of the present model, breakup profiles are also shown for the parameters used in the previous experiments.
An air jet, which remains laminar and axisymmetric in the single-phase flow condition, is simulated numerically in the particle-laden condition. The vortex method for particle-laden gas jet proposed by the authors is employed for the simulation. An air issues with velocity U0 from a round nozzle into the air co-flowing with velocity Ua. The Reynolds number based on U0 and the nozzle diameter is 1333, the velocity ratio Ua/U0 is 0.4. Spherical glass particles with diameter 65μm are loaded at the mass loading ratio 0.025. The particle velocity at the nozzle exit is 0.68U0. The particles impose disturbances on the air and induce the three-dimensional flow, resulting in the transition from the axisymmetric flow to the non-axisymmetric one. As the particles make the air velocity fluctuation increase, the air momentum diffuses more in the radial direction, and accordingly the spread of the jet becomes larger. The abovementioned results agree well with the trend of the existing experiments. The proposed vortex method can successfully capture the flow transition caused by the particles laden on an axisymmetric air jet.
This experimental research concerns the air-bubble-jet flow in water. We try to apply three-dimensional particle tracking velocimetry to the flow. Air bubbles are regarded as tracer particles. Then, we visualise the three-dimensional motions of the bubbles. As we record the stereo images using two high-speed video cameras, we can get timely-consecutive information of the bubbles' motions. As a result, the three-dimensional structure of two simple bubble-jet plumes with different nozzle's tilting angles has been quantitatively revealed near the nozzle exit.
Perforations caused by the laminar-turbulent transition of a radial liquid sheet are described. A liquid film discharged from a small gap between a disk and a nozzle flows radially outward on the disk and forms a radial liquid sheet beyond the disk edge. The radial liquid sheet is unstable because of the internal velocity profile with an inflexion point. The instability causes a laminar-turbulent transition just beyond the disk edge and the transition leads to strong turbulence. Flow patterns depend completely on the frequency of perforations produced by this turbulence; that is, a large number of perforations leads to atomization of the liquid sheet, whereas no perforations results in a smooth liquid sheet. In this paper, we clarified the dependence of perforation on the physical properties of test liquids.
A turbulent plane jet with a one-step reaction (R + B → S) in a liquid is investigated experimentally. The instantaneous concentrations of species R and S are measured simultaneously by using the light absorption spectrometric method, while the concentration of species B is determined by the conservation law. Statistics of reactive scalar field are compared with those of nonreactive scalar field. It is ascertained that, in comparison with the nonreactive case (frozen limit), the mean concentrations of reactants R and B decrease while the mean concentration of product S increases in the downstream direction because of the chemical reaction. With regard to scalar fluctuations, it is also observed that the change in the r.m.s. values of species R is small but these values become slightly small in comparison with those in the frozen limit, whereas the r.m.s. values of species B become apparently larger than those in the frozen limit. Furthermore, the segregation coefficients of species R and B in the frozen limit increase in the downstream direction because of the progress in mixing. In contrast, the segregation coefficients in the reactive case become slightly smaller than those in the frozen limit and lie between -0.14 and -0.02 in the measurement region. The present data show that the scalar covariance can not be neglected for modeling reactive flow, and provide very useful and important information for modeling concentration correlation and the chemical source term in a turbulent reactive flow.
A method for LES of ternary mixing in turbulent flows is presented and partly validated against DNS data. The subgrid-scale mixing state is characterized by joint presumed discrete distributions, i.e. discrete ensembles which approximate multivariate FDFs. A closure for the transport equations of the first and second order moments of mixture fraction fluctuations, including the covariance, is formulated in the LES context. Biased mixing models are employed to generate particle ensembles with prescribed first and second order moments. Filtered reaction rates, which are computed as an average over the distributions, can in this way be parameterized by the first and second order moments of the mixture fractions. The mixture state taken from a DNS, which has been filtered onto a coarser grid as suitable for LES, is then compared against particle ensembles generated with the mixing model, where good agreement has been found.
An unsteady flow in a low Reynolds number region attracts attention in recent years. The authors measured vortex flow in the wake of the rigid flat plate, rigid NACA0010, elastic flat plate and elastic NACA0010 with heaving motions and evaluated accelerating flows induced by the vortex flow patterns and vortex flows in the wake of the airfoils. Furthermore, we measured dynamic thrusts acting on these airfoils with heaving motions and clarified their relations with vortex flows. Thrust producing vortex streets same as those of the airfoils can be formed in the wake of the elastic flat plate with heaving motions by giving an elasticity to the latter half of the flat plate. The dynamic thrusts that act on the rigid NACA0010, elastic flat plate and elastic NACA0010 are strongly dependent on the Strouhal number based on the trailing edge amplitude regardless of heaving amplitudes, heaving frequencies, airfoil shapes and elastic deformations.
In this study, the flow features of vortex shedding from a pair of parallel arranged circular cylinders of unequal diameter oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/fK=0∼7, amplitude ratio 2a/d=0.125, 0.25, 0.5 and 0.75, gap ratio G/d=0.25, 0.75 and 1.75, diameter ratio d'/d=0.25 and 0.5 and Reynolds number Re=500. The variations of mean vortex shedding frequency from oscillating cylinder were investigated. As a result of the experiments, the occurrence of the lock-in phenomenon and its range were shown. It was obtained that the states of the interference flow at the time of lock-in by the cylinder oscillation. Some representative flow patterns were obtained. It was shown that the stage number of flow pattern was depended on the gap ratio G/d.
Unsteady, three-dimensional characteristics of vortex shedding were studied with self-made MEMS sensors situated spanwisely on a circular cylinder subjected to uniform incoming flow. Firstly, verification on the reliability of the MEMS sensor signals was made by comparing with the hot-wire signals obtained in the flow simultaneously. Subsequently, the MEMS sensor signals were analyzed with Wavelet and Hilbert transformations and 2D-FFT. By Wavelet and Hilbert transformations, the results obtained indicate that the strongly three-dimensional vortex shedding events are featured with pronounced spanwise variations in the instantaneous phases of vortex shedding, which are further identified as the occurrences of vortex dislocation. Furthermore, the results of 2D-FFT analysis reveal that the spanwise wave numbers of vortex shedding largely falls in a range between -2 to 2, for the MEMS sensors spanned over a spanwise region of 3 D in length.
For the Japan Sodium-cooled Fast Reactor, an experimental study on the fluctuating pressure of the hot legs was carried out with tests using a 1/3-scale model. The total resistance coefficient is consistent with the published data, and our research has given some additional data up to the Reynolds number of 8.0x106. The flow pattern in the postcritical regime is independent of a Reynolds number. The statistical examination revealed that fluctuating pressures on the pipe wall depend on the mean velocity but not on the viscosity of the fluid. Negative spikes of pressure appeared for high velocity. Based on these experimental data it is concluded that, there are similarity laws for the scale of a model and the property of fluid, but not for the velocity in the pipe. Theoretical considerations also gave a discussion how to extrapolate the fluctuating pressure to the actual hot-leg conditions.
The effects of bubble size on an upward gas-liquid (CO2-water) flow in a vertical pipe after an abrupt expansion is investigated visually and experimentally in the present work. The mean bubble size varies from small scale (db=0.3mm) to relatively large scale (db=4.5mm). Extensive visualization experiments and PIV analysis show different flow patterns downstream of the expansion of flows containing bubbles of different sizes. The effect of bubble size is also investigated measuring the pressure distribution along the pipe and the drag of the expansion and its difference under different bubble sizes is calculated and compared with that of single-phase flow. The fluctuation phenomena occurring downstream are also investigated. The experiments are conducted under constant Reynolds number (Re=1.0×104) and volumetric gas flow rate ratio (αv=0∼10%). The present work gives valuable information about how the bubble size affects the flow characteristics even under steady flow conditions, and explains the differences between results reported by other authors investigating under similar conditions.
Gas flows in micro-channels are, in general, theoretically treated with the Maxwell slip velocity as a boundary condition for the convection velocity at the wall. It is pointed out that wall slip is conventionally introduced in theoretical/numerical treatments of gas flows through micro-channels to obtain agreement with experimental results. In the present paper, we provide an alternative by solving the extended Navier-Stokes equations for compressible gas flows in micro-channels using the conventional no-slip velocity boundary condition for the convection velocity. Results obtained with this approach are presented and compared with experiments. It is also shown that the theoretical treatment of micro-channel gas flows using the “extended Navier-Stokes equations” also permits the phenomena such as the Knudsen paradox, to be treated in an analytical manner. Comparison with experimental data suggests that the derived analytical solution has excellent agreement up to Knudsen number of approximately 1, which shows the validity of extended Navier-Stokes equations with the conventional no-slip velocity boundary condition up to the early transition regime of micro-channel gas flows.
The flow from a wide space into a pipe has a large annular separated vortex region just after the inlet corner. This vortex region produces large flow resistance, or drag, in this kind of flow. To reduce the drag, a means to control flow in order to suppress the vortex region is needed. In this study, a simple method to reduce the drag of the pipe inlet flow by mounting a small ring-shaped obstacle instead of a bell-mouth has been proposed and examined. The effects of the side-walls and their offset distance on drag reduction were also examined. The small offset distance corresponds to the case in which the pipe inlet is placed near the bottom or corner of the tank. The distributions of pressure and velocity components in the axial direction at several cross-sections were measured, and a visualized flow pattern of the water flow just after the pipe inlet was examined. The effects of the small ring-shaped obstacle on drag reduction were also examined. It was clarified that the inlet loss (drag) coefficient was reduced by a maximum of about 90 percent by mounting the ring-shaped obstacle.