52 research outputs found
A Generalized Compressible Cavitation Model
A new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical headform and a NACA 66 hydrofoil
Control of Transonic Cavity Flow Instability by Streamwise Air Injection
A time-dependent numerical model of a turbulent
Mach 1.5 flow over a rectangular cavity has been developed,
to investigate suppression strategies for its
natural self-sustained instability. This instability adversely
affects the cavity form drag, it produces large-amplitude
pressure oscillations in the enclosure and it
is a source of far-field acoustic radiation.
To suppress the natural flow instability, the leading
edge of the two-dimensional cavity model is fitted with
a simulated air jet that discharges in the downstream
direction. The jet mass flow rate and nozzle depth are
adjusted to attenuate the instability while minimising
the control mass flow rate.
The numerical predictions indicate that, at the selected
inflow conditions, the configurations with the
deepest nozzle (0.75 of the cavity depth) give the most
attenuation of the modelled instability, which is dominated
by the cavity second mode. The jet affects both
the unsteady pressure field and the vorticity distribution
inside the enclosure, which are, together, key
determinants of the cavity leading instability mode
amplitude. The unsteadiness of the pressure field is reduced
by the lifting of the cavity shear layer at the rear
end above the trailing edge. This disrupts the formation
of upstream travelling feed-back pressure waves
and the generation of far-field noise. The deep nozzle
also promotes a downstream bulk flow in the enclosure,
running from the upstream vertical wall to the
downstream one. This flow attenuates the large-scale
clockwise recirculation that is present in the unsuppressed
cavity flow. The same flow alters the top shear
layer vorticity thickness and probably affects the convective
growth of the shear layer cavity second mode
POD Analysis of Cavity Flow Instability
A Mach 1.5 turbulent cavity flow develops large-amplitude
oscillations, pressure drag and noise. This
type of flow instability affects practical engineering applications,
such as aircraft store bays. A simple model
of the flow instability is sought towards developing a
real-time model-based active control system for simple
geometries, representative of open aircraft store bays.
An explicit time marching second-order accurate
finite-volume scheme has been used to generate time-dependent
benchmark cavity flow data. Then, a simpler
and leaner numerical predictor for the unsteady
cavity pressure was developed, based on a Proper Orthogonal
Decomposition of the benchmark data.
The low order predictor gives pressure oscillations
in good agreement with the benchmark CFD method.
This result highlights the importance of large-scale
phase-coherent structures in the Mach 1.5 turbulent
cavity flow. At the selected test conditions, the significant
pressure ‘energy’ content of these structures
enabled an effective reduced order model of the cavity
dynamic system. Directions and methods to further
streamline and simplify the unsteady pressure predictor
have been highlighted
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