Yasuyuki Oguma

Numerical analysis of cavitation cloud shedding in a submerged water jet

Focused on the unsteady behavior of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a practical compressible mixture flow bubble cavitation model with a simplified estimation of bubble radius. The mean flow of two-phase mixture is calculated by unsteady Reynolds averaged Navier-Stokes (URANS) for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas bubbles whose radius is estimated with a simplified Rayleigh- Plesset equation according to pressure variation of the mean flow field. High-speed submerged water jet issuing from a sheathed sharp-edge orifice nozzle is treated. The periodically shedding of cavitation clouds is captured in a certain reliability compared to experiment data of visualization observation and the capability to capture the unsteadily shedding of cavitation clouds is demon- strated. The results demonstrate that cavitation takes place near the entrance of nozzle throat and cavitation cloud expands conseque- ntially while flowing downstream. Developed bubble clouds break up near the nozzle exit and shed downstream periodically along the shear layer. Under the effect of cavitation bubbles the decay of core velocity is delayed compared to the case of no-cavitation jet.

Measurements of wall-shear-stress distribution on an NACA0018 airfoil by liquid-crystal coating and near-wall particle image velocimetry (PIV)

Measurements of wall-shear-stress distributions along curved surfaces are carried out using non-intrusive experimental methods, such as liquid-crystal coating and near-wall particle image velocimetry (PIV). The former method relies on the color change of the liquid-crystal coating sensitive to the wall shear stress, while the latter is based on the direct evaluation of shear stresses through the near-wall PIV measurement in combination with the image deformation technique. These experimental methods are applied to the measurement of wall-shear-stress distributions of air flow at a free-stream velocity of 15 m s−1 on a flat plate and an NACA0018 airfoil. The experiments are carried out at zero angle of attack for the flat plate and at 0° and ±6° angles of attack for the airfoil, and then the variations of shear-stress distribution along these surfaces are studied. These measurements in wall shear stresses agree with each other within their experimental uncertainties, suggesting the validity of experimental methods for non-intrusive shear-stress measurements. It is found that the wall-shear-stress distribution shows a small negative value upstream of the reattachment point on the NACA0018 airfoil, which is followed by an increase in shear stresses downstream due to laminar–turbulent transition of boundary layers. Such behavior of wall-shear-stress distribution is well correlated with the mean flow and turbulence characteristics along the airfoil surfaces, which are measured by PIV.

Numerical Simulation of Unsteady Cavitation in a High-speed Water Jet

Abstract Concerning the numerical simulation of high-speed water jet with intensive cavitation this paper presents a practical compressible mixture flow method by coupling a simplified estimation of bubble cavitation and a compressible mixture flow computation. The mean flow of two-phase mixture is calculated by RANS for compressible fluidU he intensit. T y of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the mean flow, and the effect of cavitation on the flow turbulence is considered by applying a ion to density correctthe evaluation of eddy viscosity. High-speed submerged water jets issuing from a sheathed sharp-edge orifice nozzle are treated when the cavitation number, σ = 0.1, and the computation result is compared with experimental data The result reveals that cavitation occurs initially at the entrance of orifice and bubble cloud develops gradually while flowing downstream along the shear layer. Developed bubble cloudbreaks up and then sheds downstream periodically near the sheath exit. The pattern of cavitation cloud shedding evaluated by simulation agrees experimental one, and the possibility to capture the unsteadily shedding of cavitation clouds is demonstrated. The decay of core velocity in cavitating jetis delayed greatly compared to that in no-activation jet, and the effect of the nozzle sheath is demonstrated.

Measurement of Pressure Field around a NACA0018 Airfoil from PIV Velocity Data

In solving these equations numerically, Eqs. (1) and (2) are transformed into general coordinates and they are solved by SOR scheme to obtain the time-averaged static pressure field around the airfoil. Note that the numerical grids are given by O type in the present study and the grid spacing is controlled to be fine near the airfoil and outer boundary, which is set at 5 times of the chord length C of the airfoil from the airfoil center. The number of grids is 600 x 300 in circumferential and radial direction, respectively. In solving these equations, all velocities on the right hand side of Eq. (1) are evaluated from the experimental velocity data measured by PIV. 3. Experiment The velocity field around a NACA0018 airfoil is measured using a standard PIV system, which consists of Nd:YAG laser (532 nm, 30 mJ/pulse), CCD camera with frame straddling function (1280 x 1024 pixels with 12 bits in gray level) and the pulse controller. The experiment is carried out in an { } ( ) j i j u u

Visualization Observation of Cavitation Cloud Shedding in a Submerged Water Jet

An experiment investigation on the behaviour of cavitation cloud caused in submerged water jets issuing from a sheathed sharp edge orifice nozzle was carried out by high-speed camera visualizing observation. It is demonstrated that cavitation bubble cloud appears when the cavitation number σ decreases to the level of 0.6–0.7. The dominant frequencies of bubble cloud expanding and contracting in the axial direction is closely related to the pressure pulsation of plunger pump, which is often employed in industry application of water jets. However, the dominant frequencies of jet width oscillation in the radial direction mainly depend on the shedding of shear vortexes as well as the collapsing of cavitation bubble clouds.

Numerical simulation of unsteady cavitating jet by a compressible bubbly mixture flow method

Focused on the unsteady behaviour of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a compressible mixture flow method combined with a simplified bubble cavitation model. The mean flow of two-phase mixture is calculated by URANS for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the expansion and contraction of bubbles the mean flow field. High-speed submerged water jet issuing from a sheathed nozzle is treated when ? = 0.1 and Re = 4.5x105 . The distribution of cavitation clouds predicted approximately agrees with experiment data of visualization observation and the unsteady behaviour of cavitation cloud shedding is captured acceptably. Computational results reveal that cavitation occurs at the entrance of nozzle throat and bubble clouds expand and develop while flowing downstream along the boundary layer. Developed bubble clouds split into small blocks and sheds downstream periodically near the exit of sheath.

Measurement of Velocity Field of an Abrasive Fan Jet by PIV

Water jets issuing from a fan jet nozzle (Fan Jets: FJs) are widely used in cleaning, decontamination of radiological substances, and removal of plasma spray coating and asbestos. The material removal performance of abrasive suspension jets issuing from a fan jet nozzle (Abrasive Fan Jets: AFJs) are much higher than that of FJs. In the present study, flow structure and velocity distribution of AFJs are investigated by using PIV method to clarify the material removal characteristics of AFJs.

Effect of Ventilation on the Velocity Decay of Cavitating Submerged Water Jet

High-speed water jet shows a peculiar processing property in cutting of submerged objects but its processing ability decreases quickly with the increase of standoff distance. Aiming at to improve the performance of submerged water jet an experiment investigation on the velocity distribution of cavitating submerged jet is carried out by PIV method where micro bubbles are used as flow tracers. Further, a sheathed orifice nozzle with ventilation hole is developed and the effect of air ventilation is investigated. The result reveals that the core velocity of no-ventilation cavitating jet is higher than ventilated ones near the sheath exit. However it becomes reverse at the downstream when x/d ≥ 40. The core velocity of ventilated jets decays much slowly compared to the case of no-ventilation jet. Air ventilation is demonstrated to be an effective way to enhance the perframnce of submerged water jets.