| |
By Matevz Dular and Bernd Stoffel, Laboratory for Turbomachinery and Fluid Power, Darmstadt University of Technology, Darmstadt, Germany
View the pdf of this article
For water pumps, marine propellers, and other equipment
involving hydrofoils, cavitation can cause problems such as
vibration, increased hydrodynamic drag, pressure pulsation,
noise, and erosion on solid surfaces. Most of these problems are
related to the transient behavior of cavitation structures. To better
understand these phenomena, unsteady 3D simulations of
cavitating flow around single hydrofoils have been performed,
and the results have been compared to experiment. The symmetric
hydrofoil studied was 50 mm wide, 108 mm long, 16 mm
thick, and had a circular leading edge and parallel walls. It was
subjected to a steady incoming flow of 13 m/s at an incidence
angle of 5°. The highly turbulent flow was simulated using the
RNG k-ε model on a structured mesh of 360,000 cells.
The experimentally determined (top) and simulated
(bottom) “horseshoe” cavitation structure
Experimentally determined and
simulated velocity profiles
The cavitation model in FLUENT was used. For the first set of
simulations performed, the transient calculation predicted the
onset of cavitation, and revealed an initial fluctuation of the cavity
volume, with a subsequent quasi-steady stabilization of a cavitation
sheet. The overall length of the predicted cavity structure
was found to be about 50% too short, however, when compared
to the experimental results. The problem was due to the overprediction
of turbulent viscosity in the region of cavity closure.
To improve the agreement, a modification of the turbulence
model was applied using user-defined functions (UDFs). The turbulent
viscosity of the mixture was artificially reduced by using a power law function for density [1,2]. The modification simply
states that the turbulent viscosity falls faster with decreasing density
(increasing vapor volume fraction) than it would if no modification
were made. This modification limited the kinetic energy in
the region filled mainly by the vapor phase. The reduced kinetic
energy allowed the formation of a re-entrant jet and promoted
the separation of a cavitation cloud. A typical “horseshoe” vapor
structure – observed during the experiment and considered to be
the driving mechanism for cavitation erosion – was correctly predicted
by the simulation once the modifications were made.
Measurements [3] of the mean transverse velocity profiles at
several axial stations were compared to the CFD results. The best
agreement was found to be just past the leading edge of the
hydrofoil, where the vapor volume fraction is substantial. The
experiments and CFD simulations also captured a small region of
back flow. Downstream, the agreement was not as good, but the
same trends were still in evidence.
Additional simulations were done using a hydrofoil with a
swept leading edge. The results were found to be similar to the
experimental findings, including significant dynamic cavitation
near the front wall, with pulsations of the cavitation region and
the separation of a cavitation cloud. Steady, yet weaker cavitation
with no cloud separation was predicted (and measured)
near the rear wall.
Current work includes the development of a numerical cavitation
erosion model, with a future goal of being able to predict
dynamic cavitation effects, like cavitation erosion, using only
CFD.
References:
- O. Coutier-Delgosha, R. Fortes-Patella, J.L. Reboud, “Evaluation of the
Turbulence Model Influence on the Numerical Simulations of Unsteady
Cavitation,” Journal of Fluids Engineering, 125, 2003
- M. Dular, R. Bachert, B. Stoffel, B. Sirok, “Numerical and Experimental
Study of Cavitating Flow on 2D and 3D Hydrofoils,” Proceedings of the
Fifth International Symposium on Cavitation, Osaka, Japan, 2003.
- R. Bachert, G. Ludwig, B. Stoffel, M. Frobenius, R. Schilling, “Three-
Dimensional Unsteady Cavitation Effects on a Single Hydrofoil and in a
Radial Pump – Measurements and Numerical Simulations; Part One:
Experiments,” Proceedings of the Fifth International Symposium on
Cavitation, Osaka, Japan, 2003.
|
|
|
