123 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
Bactericidal and Smear Layer Removal Efficacy of Herbal Alternatives Against Enterococcus Faecalis Dentinal Biofilm – An ex-vivo Study
Objective: To assess the antibacterial and smear layer removal ability of Trigonella foenum, Syzygium cumini, Terminalia chebula seed extracts against E. faecalis dentinal biofilm. Material and Methods: Agar well diffusion, micro broth dilution assay and time-kill curve assay were performed to determine the antibacterial activity. The ability of the herbal extracts to remove the smear layer on the root canal surface was assessed by scanning electron microscopy. Results: Antibacterial activity was observed for the extracts of S. cumini and T. chebula on E. faecalis dentinal biofilm and its planktonic counterparts. The smear layer was efficiently removed by the seed extracts of T. chebula alone. Seed extracts of T. foenum neither possessed antibacterial effect nor smear layer removal ability. Conclusion: The extracts of T. chebula seeds may replace conventional irrigant due to its antibacterial properties and smear layer removing the ability. The extracts of S. cumini may be used as an intracanal medicament as it exhibited a bactericidal effect against the E. faecalis dentinal biofilm following 18 hours of incubation
A Framework for Integrated Component and System Analyses of Instabilities
Instabilities associated with fluid handling and operation in liquid rocket propulsion systems and test facilities usually manifest themselves as structural vibrations or some form of structural damage. While the source of the instability is directly related to the performance of a component such as a turbopump, valve or a flow control element, the associated pressure fluctuations as they propagate through the system have the potential to amplify and resonate with natural modes of the structural elements and components of the system. In this paper, the authors have developed an innovative multi-level approach that involves analysis at the component and systems level. The primary source of the unsteadiness is modeled with a high-fidelity hybrid RANS/LES based CFD methodology that has been previously used to study instabilities in feed systems. This high fidelity approach is used to quantify the instability and understand the physics associated with the instability. System response to the driving instability is determined through a transfer matrix approach wherein the incoming and outgoing pressure and velocity fluctuations are related through a transfer (or transmission) matrix. The coefficients of the transfer matrix for each component (i.e. valve, pipe, orifice etc.) are individually derived from the flow physics associated with the component. A demonstration case representing a test loop/test facility comprised of a network of elements is constructed with the transfer matrix approach and the amplification of modes analyzed as the instability propagates through the test loop
Tuning a RANS k-e model for jet-in-crossflow simulations.
We develop a novel calibration approach to address the problem of predictive ke RANS simulations of jet-incrossflow. Our approach is based on the hypothesis that predictive ke parameters can be obtained by estimating them from a strongly vortical flow, specifically, flow over a square cylinder. In this study, we estimate three ke parameters, C%CE%BC, Ce2 and Ce1 by fitting 2D RANS simulations to experimental data. We use polynomial surrogates of 2D RANS for this purpose. We conduct an ensemble of 2D RANS runs using samples of (C%CE%BC;Ce2;Ce1) and regress Reynolds stresses to the samples using a simple polynomial. We then use this surrogate of the 2D RANS model to infer a joint distribution for the ke parameters by solving a Bayesian inverse problem, conditioned on the experimental data. The calibrated (C%CE%BC;Ce2;Ce1) distribution is used to seed an ensemble of 3D jet-in-crossflow simulations. We compare the ensemble's predictions of the flowfield, at two planes, to PIV measurements and estimate the predictive skill of the calibrated 3D RANS model. We also compare it against 3D RANS predictions using the nominal (uncalibrated) values of (C%CE%BC;Ce2;Ce1), and find that calibration delivers a significant improvement to the predictive skill of the 3D RANS model. We repeat the calibration using surrogate models based on kriging and find that the calibration, based on these more accurate models, is not much better that those obtained with simple polynomial surrogates. We discuss the reasons for this rather surprising outcome
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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