THE FLUCTUATION CHARACTERISTICS OF NATURAL AND VENTILATED CAVITIES ON AN AXISYMMETRIC BODY

Natural and ventilated cavitations generated on a smooth-nosed axisymmetric body were studied experimentally.The characteristics of small scale and localized fluctuations of “steady cavities” were measured by pressure transducers. Comparisons between natural and ventilated cavities at different measured points for several cavitation numbers were done. It was observed that the dominant fluctuations were concentrated in the frequency range of 0Hz-50Hz for all the cavitation cases, Similar shapes and magnitudes of the frequency spectra were detected for both natural and ventilated cavities. Much larger spectral amplitude in the cavity closure region suggested al fluctuations source. From partial cavitating flow to supercavitating flow, the dominant frequency and the corresponding amplitude decreased with decreasing cavitation number, which meant that cavity became more steady while developing.

Numerical Modeling and Analysis of Gas Entrainment for the Ventilated Cavity in Vertical Pipe

A semi-empirical gas entrainment model was proposed for the ventilated cavity in vertical pipe, based on which, a complete numerical scheme was established by coupling with the Eulerian-Eulerian two-fluid model to predict the multiscale flow field created by ventilated cavity. Model predictions were validated against experimental measurements on void fraction and bubble size distributions. Simulations were carried out to explore the effect of ventilation rate and inlet turbulence intensity on the macroscale cavity shape and the bubbly flow downstream of the ventilated cavity. As the ventilation rate increasing, a reverse trend was observed for the void fraction and bub-ble size distributions. It is concluded that the average void fraction in the pipe flow region is determined by the vo-lumetric ratio between liquid and gas. However, the bubble size evolution is dominated by the breakage effect induced by turbulence in the vortex region. Furthermore, simulations were conducted to analyze geometric scale effect based upon Froude similitude. The results imply that the velocity distributions were properly scaled. Slight scale effect was seen for the void fraction caused by faster dispersion of bubbles in the larger size model. The comparatively greater bubble size was predicted in the smaller model, implying significant scale effects in terms of tur-bulence and surface tension effect. It reveals that empirical correlations valid in wide range are required for the extrapolation from small-size laboratory models.

Computational Analysis of Nanofluid Mixed Convection in a Ventilated Enclosure with Linearly Varying Wall Temperature

The characteristic of flow and heat transfer of A12Oa-water nanofluids flowing through a horizontal ventilated cavity is investigated numerically. The bottom wall is subjected to a linearly varying increasing heating temperature profile, whereas the other boundaries are assumed to be thermally insulated. The enclosure is cooled by an injected or sucked imposed flow. The simulations are focused specifically on the effects of different key parameters such as Reynolds number, 200 〈 Re 〈 5,000, nanoparticles concentration, 0 〈 φ 〈 0.1, and mode of imposed flow, on the flow and thermal patterns and heat transfer performances. The findings demonstrate clearly the positive role of the nanoparticles addition on the improvement of the heat transfer rate and the mean temperature within the cavity. Also, the results presented show that, the suction mode is more favorable to the heat transfer in comparison with the case of the injection mode. The cooling efficiency is found to be more pronounced by applying the suction mode.

Experimental and numerical investigation on the gas leakage regime for ventilated tail cavity of an underwater vehicle

The objective of this paper is mainly to investigate the ventilated tail cavity flow of an underwater vehicle with focus on the gas leakage regime by experimental and numerical methods.A high-speed camera and pressure measurement system are used to record the cavity flow patterns and pressure behavior,respectively.The numerical simulation is carried out with volume of fluid(VOF)model and Filter-based turbulence model(FBM).Good agreement can be obtained between the experimental and numerical results.There are three gas leakage types in the evolution of the intact tail cavity,i.e.,twin-vortex tube entrainment(TVTE),toroidal vortex shedding(TVS)and hybrid twin-vortex tube entrainment and toroidal vortex shedding(TVTE-TVS).With the increase of Fr,the unsteady behavior of the cavity with different gas leakage types becomes more apparent.The internal flow characteristics revealed three distinct regions including the ventilation influence region,the reverse flow region and the high shear flow region and have an important effect on the transition of gas leakage regime.

NUMERICAL SIMULATION OF VENTILATED CAVITATING FLOW AROUND A 2D FOIL

By using a pressure-based method and the finite volume method in the framework of the time dependent Reynolds-averaged Navier-Stokes equations, the authors studied the unsteady process of ventilated cavities generated forcing air through an orifice in a 2D hydrofoil without natural cavitation physically. The computation was carried out with the Volume Of Fluid （VOF） technique to track the gas-liquid two-phase interface. The results of simulation indicate that the ventilation rate is an important parameter in determining the morphology of cavity. There exists a critical value to convert sheet cavity into supereavity. A high ventilation rate can induce a two phase interface fluctuation and enable the ventilated eavitating flow to present a characteristic of periodicity.

NUMERICAL SIMULATION OF ARTIFICIAL VENTILATED CAVITY

In order to determine artificial ventilated cavity shape and provide design reference for engineering applications, the shape of ventilated cavity and the drag of underwater body have been numerically simulated with commercial code in this work. Empirical formulas between ventilated cavity shape and cavitation number are obtained under the conditions of zero incidence and similarity shapes of ventilated cavity and vapor cavity have been validated under the same cavitation number. In addition, the relations between change of cavitator incidence and cavity unsymmetry and the relations of between drag and cavity shape have been primarily analysed. Furthermore, the numerical results fit well with the experimental results.

Experimental study on the characteristics of ventilated cavitation around an underwater navigating body influenced by turbulent drag-reducing additives

In this study, a new control strategy for turbulent drag reduction involving ventilated cavitation is proposed. The configurational and hydrodynamic characteristics of ventilated cavities influenced by turbulent drag-reducing additives were experimentally studied in water tunnel. The test model was fixed in the water tunnel by a strut in the aft-part. Aqueous solutions of CTAC/Na Sal(cetyltrimethyl ammonium chloride/sodium salicylate) with weight concentrations of 100, 200, 400 and 600 ppm(part per million), respectively, were injected into the ventilated air cavity from the edge of the cavitator with accurate control by an injection pump. The cavity configurations were recorded by a high-speed CCD camera. The hydrodynamic characteristics of the test model were measured by a six-component balance. Experimental results show that, within the presently tested cases, the lengths of cavity influenced by drag-reducing solution are smaller than normal condition(ventilated cavity) in water, but the asymmetry of the cavity is improved. The drag resisted by the test model is reduced dramatically(the maximum drag reduction can reach to 80%) and the re-entrant jet is more complex after the CTAC solution is injected into the cavity. Turbulent drag-reducing additives have the potential in enhancement of supercavitating asymmetry and further drag reduction.

Numerical investigation of turbulent bubbly wakes created by the ventilated partial cavity

This paper presents a numerical study on the turbulent bubbly wakes created by the ventilated partial cavity.A semi-empirical approach is introduced to model the discrete interface of the ventilated cavity and its complex gas leakage rate induced by the local turbulent shear stress.Based on the Eulerian-Eulerian two-fluid modeling framework,a population balance approach based on MUltiple-SIze-Group (MUSIG) model is incorporated to simulate the size evolution of the sheared off microbubbles and its complex interactions with the two-phase flow structure in the wake region.Numerical predictions at various axial locations downstream of the test body were in satisfactory agreement with the experimental measurements.The captured bubbly wake structure illustrates that the bubbles may disperse as a twin-vortex tube driven by gravity effect.The predicted Sauter mean bubble diameter has confirmed the dominance of the coleascense process in the axial direction.As the bubbles develop downstream,the coleascense and breakup rate gradually reach balance,resulting in the stable bubble diameter.A close examination of the flow structures,gas void fraction distributions and the bubble size evolution provides valuable insights into the complex physical phenomenon induced by ventilated cavity.

Pulsation Characteristics of Ventilated Supercavitation on a 2D Hydrofoil

Based on a suite of computational fluid dynamics code, the pulsation characteristics are studied for turbulent supercavitating flows over a 2D base-vented symmetric hydrofoil using a pressure-based Navier-Stokes solver coupled with a phase mass fraction transport cavitation model and local linear low-Reynolds-number k-ε turbulence model. It is found that there exists a critical air supply flow under certain inflow condition. When the gas supply flow exceeds the critical value, the cavity begins to pulsate. Pulsating cavity has a visually wavy surface, and air leakage shows as a mass of air-water mixture detaching from the rear part of the cavity periodically.

Double-skin façade simulation with computational fluid dynamics:A review of simulation trends,validation methods and research gaps

Dynamic simulation of a double-skin façade(DSF)with computational fluid dynamics(CFD)can be challenging due to the lack of validated models and benchmarking datasets.Furthermore,there is a lack of consensus in the scientific community on what constitutes a successfully validated DSF model.The present review study identifies simulation trends and research gaps for DSFs simulated with CFD.Additionally,this article presents a series of CFD simulations in which key aspects of the DSF modelling are varied:2D or 3D modelling approaches,turbulence viscosity models(TVMs),radiation models,and wall function.These simulation results are compared to the empirical data(both temperature and velocity fields)of a benchmark test with laboratory-controlled boundary conditions.This analysis shows that using the k-εRNG model with enhanced wall treatment and surface-to-surface(S2S)radiation model yields the best results for the 2D case of natural convection flow.Moreover,it is shown that accounting for the velocity field in the validation process is essential to ensure the suitability of a model.Finally,the authors advocate for the use of selected dimensionless numbers to improve the comparability of the different DSF scientific studies.This would also help to identify relevant experimental datasets for validation and suitable CFD simulation settings for specific DSF cases.

CFD simulation of wind and thermal-induced ventilation flow of a roof cavity

The hygrothermal performance of a ventilated roof cavity is greatly affected by the airflow passing through it.This ventilation flow is mainly driven by the wind pressure difference between openings and the thermal-induced buoyancy.However,the wind effect is not well understood as it is often neglected in previous studies.The present study investigates the properties of such airflows,including the flow pattern,flow regime,and flow rate,using a CFD method.The target building is a large-span commercial building with a low-pitched roof.To study the wind-induced airflows,the onset atmospheric boundary layer wind flow was modelled,and the results were compared with the site-measured data recorded in the literature.To study the thermal-induced buoyancy effects,a roof cavity model found in the literature with experimental data was adopted.The findings show that the flow pattern in the roof cavity varied with the airflow driven factors.The flow separation at the windward eave inlet of the thermally induced flows are more pronounced compared with those of the wind-induced flows.Furthermore,the wind-induced airflows can generate around two times more ventilation flow rate through the roof cavity compared to the thermal-induced airflow.The findings indicate that wind-induced ventilation flows are the dominant factor of the roof cavity ventilation in a large-span,low-pitched building.

Preliminary Clinical Observation on Treatment of Chronic Simple and Hypertrophic Rhinitis with Rhinitis Spray

Objective: To investigate the clinical effect of Rhinitis Spray (RS) in treating chronic simple and hypertrophic rhinitis.Methods: Eighty patients with chronic rhinitis were divided into three groups and treated with RS, ephedrine and normal saline respectively.Results: The short-term effective rate in the three groups was 89.3%, 66.7% and 8.0% respectively. RS was effective in alleviating symptoms, increasing IgG level in nasal discharge, improving ventilatory function of nasal cavity and transfer function of nasal muosa cilia obviously.Conclusion: The therapeutic effect of RS in treating chronic rhinitis is satisfactory.