An investigation to control base pressure in suddenly expanded flows

In suddenly expanded flows, due to the abrupt expansion of the flow from a nozzle into an enlarged duct, the pressure reduces in the base region of the enlarged duct which increases the base drag. The techniques used to increase the base pressure are namely passive control technique and active control technique. In passive control technique the geometrical modifications are employed by providing splitter plates, ribs, cavities etc. while in active control technique the secondary control jets are provided in the base region of an enlarged duct to increase base pressure up to atmospheric pressure. The air blowing pressure from secondary control jets should be optimum. This paper presents the computational fluid dynamic (CFD) analysis to optimize blowing pressure ratio i.e. the ratio of inlet pressure of control jets to the atmospheric pressure, to increase base pressure up to atmospheric pressure in the base region of an enlarged duct. In the present study, CFD analysis was carried out for different air blowing pressure ratios to optimize it. Flow and geometry parameters considered for the analysis are Mach number, area ratio, nozzle pressure ratio and blowing pressure ratio. Mach numbers considered for analysis are 1.5, 2.0 and 2.5. Area ratios and nozzle pressure ratios considered for analysis are 2, 5 and 8. The CFD analysis is done for different combinations of Mach numbers, area ratios, and the nozzle pressure ratios by varying blowing pressure ratio from 2 to 8 in step of 1. Based on analysis results anyone can select optimum value of blowing pressure ratio at a given Mach number, area ratio and nozzle pressure ratio to increase base pressure nearly up to atmospheric pressure

Effect of Nozzle Pressure Ratio and Control Jets Location to Control Base Pressure in Suddenly Expanded Flows

In this paper, computational fluid dynamic (CFD) analysis and experiments have been carried out to study the effect of nozzle pressure ratio, i.e. the ratio of inlet pressure to atmospheric pressure, and the pitch circle diameter of the control jets to regulate the base pressure. The variables considered for the analysis as well as the experiments are the nozzle pressure ratio (NPR), the Mach number (M) and the pitch circle diameter (PCD) of the control jets. The area ratio considered for the study is kept constant at 4.84 while the length to diameter (L/D) ratio of an enlarged duct is set constant at 5. The inertia parameter considered for the study is Mach number. The Mach numbers considered for study are 1.5, 2.0, and 2.5. The nozzle pressure ratio considered for study are 2, 5 and 8. Three different pitch circle diameters of control jets considered for study are 13.1 mm, 16.2 mm and 19.3 mm. From the numerical simulations and the results of the experimental tests, it is found that the control jets are very beneficial to increase the base pressure at higher NPR when the jets issuing from the nozzles are under-expanded. The control jets were able to increase the base pressure value from 160% to 400% at nozzle pressure ratio 8. It is concluded that the parameter D3 is the most effective pitch circle diameter of the control jets to increase the base pressure

CFD analysis of human powered submarine to minimize drag

This paper deals with finding the optimum fineness ratio, i.e. ratio of length to maximum diameter, of human-powered submarine of different shapes to reduce the drag force on the body using Computational Fluid Dynamics (CFD) analysis. These types of submarines are used in events like ISR and eISR. This paper focuses on finding the total drag force on submarine models with a constrained diameter and different fineness ratios. The analysis is done by using ANSYS Fluent. In this paper, only the fully submerged flow is considered on a hull without any appendages.The total drag on a body is caused in three different parts that are wave drag, skin friction drags and base drag.The analysis is done different shapes of submarines like Conic shape hull, Elliptical shape hull, Ogive shape hull and Parallel mid-body hull by flowing water at velocities of 3 m/s, 4m/s and 5 m/s. The fineness ratios at which the drag is minimum are found in all submarine shapes. The optimum value of fineness ratio, which gives minimum drag is obtained by the analysis is 6 for Conical shape hull, Elliptical shape hull and Ogive shape hull whereas for the submarine with Parallel mid-body hull shape the optimum fineness ratio is

CFD analysis of effect of area ratio on suddenly expanded flows

This paper presents the Computational Fluid Dynamic (CFD) analysis to study the effect of area ratio i.e. the ratio of enlarged duct area to nozzle exit area on velocity distribution along axis by varying the nozzle pressure ratios. The convergent-divergent nozzle and suddenly expanded circular duct of larger cross-sectional area are used for analysis. The analysis is done for Mach number 2.0 by varying the area ratios and nozzle pressure ratios. The area ratios for the analysis are 1, 2, 4, 6, 8, 10 and 12. The values of nozzle pressure ratios considered are 2, 4, 6, 8, 10 and 12. The results are compared with the help of graphs and tables. By observing all the results it can be concluded that the flow field in the enlarged duct is strongly influenced by area ratios and Nozzle pressure ratios

CFD analysis of effect of Mach number, area ratio and nozzle pressure ratio on velocity for suddenly expanded flows

This paper presents the Computational Fluid Dynamic (CFD) analysis to study the effect of Mach numbers on velocity distribution at different area ratio and nozzle pressure ratio in the flow field from convergent-divergent nozzles to a suddenly expanded circular duct of larger cross-sectional area. The study is focusing on velocity distribution along axial length. The analysis is done for Mach numbers 1.4 and 1.8 by varying the area ratios and nozzle pressure ratios. The area ratios for the analysis are 1, 2, 4, 6, 8, 10 and 12. The values of nozzle pressure ratios considered for analysis are 2, 4, 6, 8, 10 and 12. The results are plotted with the help of graphs. By observing all the results it can be concluded that the flow field in the enlarged duct is strongly influenced by Mach numbers, area ratios and Nozzle pressure ratios

CFD analysis of effect of flow and geometry parameters on thrust force created by flow from nozzle

This paper presents the Computational Fluid Dynamic (CFD) analysis to study the effect geometry and flow parameters on thrust force created by the flow from convergent divergent nozzles to a suddenly expanded circular duct of larger cross-sectional area. The study is focusing on resultant thrust force. The nozzles are designed for Mach numbers 1.1, 1.4, 1.8 and 2.0. The CFD analysis is done by varying the area ratios and nozzle pressure ratios for all Mach numbers. The area ratios considered for the analysis are 1, 2, 4, 6, 8, 10 and 12. The values of nozzle pressure ratios considered in the analysis are 2, 4, 6, 8, 10 and 12. The results for different combinations of Mach number, area ratios and nozzle pressure ratios are compared with the help of tables. By observing all the results it can be concluded that the resultant thrust force created by flow through nozzle is strongly influenced by Mach number, area ratio and Nozzle pressure ratio

YASS: yet another steganographic scheme that resists blind steganalysis

Abstract. A new, simple, approach for active steganography is proposed in this paper that can successfully resist recent blind steganalysis methods, in addition to surviving distortion constrained attacks. We present Yet Another Steganographic Scheme (YASS), a method based on embedding data in randomized locations so as to disable the selfcalibration process (such as, by cropping a few pixel rows and/or columns to estimate the cover image features) popularly used by blind steganalysis schemes. The errors induced in the embedded data due to the fact that the stego signal must be advertised in a specific format such as JPEG, are dealt with by the use of erasure and error correcting codes. For the presented JPEG steganograhic scheme, it is shown that the detection rates of recent blind steganalysis schemes are close to random guessing, thus confirming the practical applicability of the proposed technique. We also note that the presented steganography framework, of hiding in randomized locations and using a coding framework to deal with errors, is quite simple yet very generalizable. Key words: data hiding, error correcting codes, steganalysis, steganography, supervised learning.

Assessment of steganalytic methods using multiple regression models

This paper proposes multiple regression models as a method for quantitative evaluation of the accuracy in steganalysis with respect to various moderating factors, such as parameter choice of the detector and properties of the carrier object. The case for multivariate statistical inference in steganalysis is particularly relevant: recent findings suggest that type and characteristics of carrier do matter, but the precise relations remain still opaque. In this paper we provide an exemplary comparison between two length-estimating attacks against LSB steganography. Extensions and applications for improved steganalysis are addressed