Motohiko Nohmi

Hydrodynamic Design System for Pumps Based on 3-D CAD, CFD, and Inverse Design Method

A computer-aided design system has been developed for hydraulic parts of pumps including impellers, bowl diffusers, volutes, and vaned return channels. The key technologies include three-dimensional (3-D) CAD modeling, automatic grid generation, CFD analysis, and a 3-D inverse design method. The design system is directly connected to a rapid prototyping production system and a flexible manufacturing system composed of a group of DNC machines. The use of this novel design system leads to a drastic reduction of the development time of pumps having high performance, high reliability, and innovative design concepts. The system structure and the design process of Blade Design System and Channel Design System are presented. Then the design examples are presented briefly based on the previous publications, which included a centrifugal impeller with suppressed secondary flows, a bowl diffuser with suppressed corner separation, a vaned return channel of a multistage pump, and a volute casing

Numerical Study of Sheet Cavitation Breakoff Phenomenon on a Cascade Hydrofoil

2-D unsteady cavity flows through hydrofoils in cascade which is the most fundamental element of turbomachinery are numerically calculated. In particular, attention was paid to instability phenomena of the sheet cavity in transient cavitation condition and the mechanism of break-off phenomenon was examined. A TVD MacCormacks scheme employing a locally homogeneous model of compressible gas-liquid two-phase media was applied to analyze above cavity flows. The present method permits us to treat the whole cavitating/noncavitating unsteady flow field. By analyzing numerical results in detail, it became clear that there are at least two mechanisms in the break-off phenomena of sheet cavity; one is that re-entrant jets play a dominant role in such a break-off phenomenon, and the other is that pressure waves propagating inside the cavity bring about an another type of break-off phenomenon accompanied with cavity surface waves.

Numerical Prediction Method of Cavitation Erosion

Bubble behavior in cavitating flow is analyzed for the development of practical erosion prediction method. CFD analysis with cavitation model is carried out for the flow field around a hydrofoil. Afterwards computation of bubble dynamics is carried out coupled with flow field CFD results by one way approach. For the bubble dynamic calculation, Rayleigh-Plesset equation is adopted. Bubble behaviors in the collapse of cloud cavitaion and in the break off of sheet cavity are analyzed. Bubble behavior at the trailing edge of sheet cavity is also calculated. It is observed that steep pressure change in the flow causes oscillation of the bubbles. Based on this qualitative information of bubble behaviors, numerical cavitation aggressiveness is simply defined. This numerical cavitation aggressiveness is a function of local void fraction and pressure over the solid surface and can be calculated directly from the cavitating flow field CFD results without concerning bubble dynamics.Copyright

Study of Cavitation Instabilities in Double- Suction Centrifugal Pump

In double-suction centrifugal pumps, it was found that cavitation instabilities occur with vibration and a periodic chugging noise. The present study attempts to identify cavitation instabilities in the double-suction centrifugal pump by the experiment and Computational Fluid Dynamics (CFD). Cavitation instabilities in the tested pump were classified into three types of instabilities. The first one, in a range of cavitation number higher than breakdown cavitation number, is cavitation surge with a violent pressure oscillation. The second one, in a range of cavitation number higher than the cavitation number of cavitation surge, is considered to be rotating cavitation and causes the pressure oscillation due to the interaction of rotating cavitation with the impeller. Last one, in a range of cavitation number higher than the cavitation number of rotating cavitation, is considered to be a surge type instability.

On Boundary Conditions for Cavitation CFD and System Dynamics of Closed Loop Channel

The unsteady two dimensional CFD for the cavitation in a orifice passage were carried out by using a commercial software ANSYS-CFX. The mass flow rate and the static pressure were controlled at the inlet and the outlet boundaries respectively. The CFD result showed periodic fluctuation of the cavitation flow. The time derivative of the cavitation volume showed relatively good agreement with the difference of the inlet and the outlet volume flow rate. The periodic fluctuation was ceased by the extension of the passage length downstream of the cavitation. The periodic fluctuation of cavitation flow was also induced by the fluctuation of the inlet volume flow rate or the fluctuation of the outlet static pressure. The dynamic flow behavior of the closed loop system including the orifice were analyzed by using the orifice cavitation CFD with the inlet and the outlet boundary conditions derived from the lumped parameter system dynamics for the tank, the pump and the pipes in the loop.© 2011 ASME

An Approximation Method for the Cavitation Eroded Surface

Flow characteristics of hydraulic machineries could be highly influenced by cavitation erosion. In order to evaluate the effect of cavitation erosion on the flow characteristics, an approximation method of the cavitation eroded surface is proposed. The cavitation eroded surface shows very complicated geometry with some patterned structure. In this study the particular eroded surface of metallic material with the sponge like porous structure often referred as orange peel is numerically approximated considering computer generated image technology. A solid model that is hollowed out with a great number of spherical surfaces, shows similar structure with the cavitation eroded surface. This approximated geometry can be used as the boundary condition of following CFD process.Copyright

Core-pressure alleviation for a wall-normal vortex by active flow control

We consider the application of active flow control to modify the radial pressure distribution of a single-phase wall-normal vortex. The present flow is based on the Burgers vortex model but with a no-slip boundary condition prescribed along its symmetry plane. The wall-normal vortex serves as a model for vortices that emerge upstream of turbomachinaries, such as pumps. This study characterizes the baseline vortex unsteadiness through numerical simulation and dynamic mode decomposition. The insights gained from the baseline flow are used to develop an active flow control technique with rotating zero-net-mass blowing and suction for the objective of modifying the core pressure distribution. The effectiveness of the control strategy is demonstrated by achieving a widened vortex core with increased pressure. Such change in the flow field weakens the local strength of the wall-normal vortex core, potentially inhibiting the formation of hollow-core vortices, commonly encountered in liquids.

A Detailed Observation of Hydrofoil Cavitation and a Proposal for Improving Cavitation Model

Cavitation of a hydrofoil is observed in detail by using a high speed video camera. A paint removal test is also carried out in order to evaluate cavitation aggressiveness for erosion. 2D hydrofoil profile is Clark Y 11.7% and its angle of attack is seven degrees. Cavitation number is σ = 1.08. The experimental results are compared with cavitation CFD. Numerous features of unsteady cavitation are observed such as cyclic fluctuation of the sheet cavity, existence of the glassy cones on a sheet cavity, generation of the cloud cavitation from the sheet cavity and the isolated bubbles traveling over the suction surface of the blade. The isolated traveling bubbles and their collapses are thought to be one of the main causes of the severe paint removals. The isolated traveling bubbles are derived from the flowing cavitation nucleus or from abrupt onset at the leading edge of the blade. For computing these complicated phenomena, combination of grid scale bubbles (GSB) and sub grid scale bubble model (SGSB) are proposed. GSB shall be computed by using the computational scheme for the free surface with phase change model. SGSB can be computed with conventional cavitation model. The breakup of GSB generates SGSB, and the coalescence of SGSB makes GSB. Upper limit of void fraction of SGSB is estimated in the range of five or ten percent from the simple speculation of the structure of packed spheres. The two types of cavitation bubble inception model are also discussed based on the generation of the isolated bubbles observed in the experiments. To verify the proposed concepts of cavitation model, a traveling air bubble over a hydrofoil is computed by using the free surface flow scheme of Volume of Fluid (VOF) approach. Cavitation on the hydrofoil is also computed by VOF approach with boiling model concerning the heat transfer. Both the computed results show qualitatively similar characteristics of the bubble dynamics to those in experimental results.Copyright