Prior Lake, MN, United States
Prior Lake, MN, United States

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Amromin E.L.,Mechmath LLC
Applied Ocean Research | Year: 2015

Bottom ventilated cavitation has been proven as a very effective drag reduction technology for river ships and planning boats. The ability of this technology to withstand the sea wave impact usual for seagoing ships depends on the ship bottom shape and could be enhanced by some active flow control devices. Therefore, there is the need in numerical tools to estimate the effects of bottom changes and to design such devices. The fundamentals of active flow control for the ship bottom ventilated cavitation are considered here on the basis of a special model of cavitating flows. This model takes into account the air compressibility in the cavity, as well as the multi-frequency nature of the incoming flow in wavy seas and of the cavity response on perturbations by incoming flow. The numerical method corresponding to this model was developed and widely manifested with an example of a ship model tested in a towing tank at Froude numbers between 0.4 and 0.7.The impact of waves in head seas and following seas on cavities has been studied in the range of wavelengths from 0.45 to 1.2 of the model (or ship) length. An oscillating cavitator-spoiler was considered as the flow controlling devices in this study. The oscillation magnitude and the phase shift between cavitator oscillation and the incoming waves have been varied to determine the best flow control parameters. The main results of the provided computational analysis include oscillations of cavity surface, of the pressure in cavity and of the moment of hydrodynamic load on the cavitator. The major part of computations has been carried out for the flap oscillating at the frequency coinciding with the wave frequency, but the effect of a frequency shift is also analyzed. © 2015 Elsevier Ltd.


Amromin E.,Mechmath LLC
Physics of Fluids | Year: 2016

Cavities behind a surface irregularity appear in vortices drifting downstream of it. Cavitation can occur there substantially earlier than over smooth surfaces of the same bodies. Cavitation inception and desinence behind surface irregularities have been intensively studied in the course of water tunnel experiments several decades ago, but no corresponding quantitative theoretical (numerical) analysis was reported. This numerical study is aimed at elaboration of a general approach to the prediction of cavitation desinence numbers for various irregularities over various surfaces and on determination of the major factors influencing these numbers in both computations and experiments. The developed multi-level computational method employs diverse models for flow zones of diverse scale. The viscous-inviscid interaction approach is used for the flow around irregularities submerged (or partially submerged) in the turbulent boundary layer. Combinations of the semi-empirical and asymptotic analyses are used for vortices and cavities in their cores. The computational method is validated with various known experimental data.


Amromin E.,Mechmath LLC
Journal of Fluids Engineering, Transactions of the ASME | Year: 2010

The effect of air flux from ventilated partial cavities on drag of bodies was studied. An integral equation method for estimation of air bubble effects on drag was employed and validated with earlier known experimental data for flat plates and bodies. The qualitative difference in the effects of flow speed and air supply rate on drag of flat plates and bodies was numerically confirmed and explained as a combined effect of the boundary layer density decrease and the increase in its displacement thickness. The numerical analysis shows reduction in the total drag of ventilated bodies with increasing air flux rate up to an optimum, but the drag rise for greater rates. A synergy of friction reduction under attached ventilated cavity and microbubble drag reduction downstream of it was shown. Copyright © 2010 by ASME.


Amromin E.L.,Mechmath LLC
Journal of Fluids Engineering, Transactions of the ASME | Year: 2013

A modification of the viscous-inviscid interaction concept with the employment of coupled vortices around the airfoil wake is introduced for analyzing the airfoil stall. The analyzed flow includes the laminar boundary layers, laminar separation bubble, laminar-turbulent transition zone, turbulent boundary layers, turbulent separation zone, wake, and outer inviscid flow. Integral methods are employed for the boundary layers. The boundaries of separation zones are analyzed as free surfaces, however, their lengths and shapes depend on the Reynolds number. The described modification is validated by a comparison of the numerical results with the previously published experimental data for various airfoils and Reynolds numbers at low Mach numbers. This modification achieves a reasonably good agreement of the computed lift and moment coefficients with their measured values. © 2013 by ASME.


Amromin E.L.,Mechmath LLC
Applied Ocean Research | Year: 2016

The successful designs of hulls for ships employing drag reduction by air bottom cavitation have been based on solutions of inverse problems of the theory of ideal incompressible fluid. However, prediction of the drag reduction ratio, the air demand by ventilated cavities and the cavity impact on the hull–propeller interaction is impossible in the framework of this theory because all mentioned characteristics depend on interaction of air cavities with the ship boundary layers. Because the known CFD tools are not fitted to ventilated cavitation at low Froude numbers, an analysis of this interaction requires a novel flow model. This model includes the incompressible air flow in the ventilated cavity, the compressible flow of a water–air mixture in the boundary layer on cavities and downstream of them and the curl-free incompressible outer water flow. The provided 2D computations employing this model allows for explanations of the earlier observed effects and for prediction of the air demand by ventilated cavities. The computed velocity profiles downstream of cavities are in the accordance with the available experimental data. © 2016 Elsevier Ltd


Amromin E.,Mechmath LLC
Transactions of the Royal Institution of Naval Architects Part A: International Journal of Maritime Engineering | Year: 2016

Design of autonomous underwater vehicles (AUV) met the opposite challenges. Their achievable route can be enhanced with drag reduction due to an increase of AUV slenderness. However, blunt short AUV have others operational advantages. The possibility to design low-drag bodies for Reynolds numbers employed by contemporary AUV (2×106


Amromin E.L.,Mechmath LLC
Journal of Fluids and Structures | Year: 2014

Cavitation inception and growth on conventional shape hydrofoils and blades leads initially to a jump of their flow-induced noise, further to an amplification of flow-induced vibration with frequently assisted erosion and finally, to a lift/thrust decrease combined with the drag increase. These undesirable cavitation effects can be mitigated or even suppressed for stable partial cavities experiencing no tail pulsations. A design approach enhancing performance of cavitating hydrofoils/blades by maintaining stable partial cavities is described. Experimental data manifesting an increase of hydrofoil lift with reduction of its drag and of force pulsations by such design are provided. Application of this design approach to propeller/turbine blades and advantages of its employment for blades operating in non-uniform incoming flows are analyzed. The possibility of an increase of the lift to drag ratio and of a reduction of the cavity volume oscillation in gust flows for blade sections is numerically manifested. © 2013 Elsevier Ltd.


Amromin E.,Mechmath LLC
Journal of Fluids Engineering, Transactions of the ASME | Year: 2014

Various computational fluid dynamics (CFD) models employed for cavitating flows are substantially based on semi-empirical assumptions about cavitation forms and liquid flows around cavitating bodies. Therefore, the model applicability must be validated with experimental data. The stages of validation of the models are analyzed here with data on cavitating hydrofoils and axisymmetric bodies in water. Results of Reynolds-averaged Navier-Stokes (RANS), large-eddy simulation (LES), detached-eddy simulation (DES), and viscous-inviscid interaction methods are compared. The necessity of simultaneous validation of several flow parameters (as cavitation inception number and location of the appearing cavity) is pointed out. Typical uncertainties in water tunnel model test data (water quality, simplified account of wall effect) and possibilities to take them into account are also discussed. The provided comparison with experimental data manifests the impossibility to describe initial stages of cavitating flows using any single model and importance of employment of a combination of models for both the cavitation zones and the flow outside of cavities. Copyright © 2014 by ASME.


Amromin E.,Mechmath LLC
Journal of Physics: Conference Series | Year: 2015

Estimations of scale effects on blade cavitation require consideration of multiple models for both water flows and cavities. In particular, distinction of laminar and turbulent boundary layers is very important. A qualitative impact of selection of models is manifested for blade sheet cavitation. Its quantitative impact is shown for vortex cavitation inception.


Grant
Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase I | Award Amount: 79.94K | Year: 2012

The modern underwater vehicle hull is comprised of a fluid-loaded polymer coating attached to a rib-stiffened plate. Knowing the structural response of such a system is important for design and evaluation of hull coatings and embedded sensors in these coatings. The main goal of this project is a development of analytical methods and a software package for a transitioning model applicable for high frequency and wave number regimes for fully elastic structures. Our model is based on the development presented in the paper of Hull and Welch (J. of Sound and Vibration, Vol. 329, 20, p. 4192-4211) and in an essence is a three-dimensional analytical model of a fluid-loaded acoustic coating attached to a rib-stiffened plate. The project is aimed to develop a standard tool for evaluation of hull coatings and embedded sensors in these coatings. Validation problems to ensure the method and program are both working properly will be considered. The tools developed under this effort should find a wide variety of uses for modeling of fully elastic structures when conventional finite element methods fail.

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