Mitigation of Antisymmetric Mode and Drag with Base Cavities

2021 ◽  
Author(s):  
Viji M ◽  
Srinivas Nistala Vikramaditya
Keyword(s):  
Cavity Length ◽  
Pressure Fluctuations ◽  
Substantial Reduction ◽  
Base Pressure ◽  
Antisymmetric Mode ◽  
Azimuthal Mode ◽  
Total Drag ◽  
Base Cavity ◽  
Mach Numbers ◽  
The Impact

Abstract Experiments were carried out to examine the impact of base cavities on the base pressure fluctuations and total drag of a cylindrical afterbody for freestream Mach numbers 0.6-1.5. Significant improvement in the base pressure and a substantial reduction in the afterbody drag was noticed in the presence of a base cavity at subsonic Mach numbers. However, on increasing the cavity length beyond a certain value its performance deteriorates. At supersonic Mach numbers their effectiveness drops drastically. Tones in the spectra can be classified into two types depending on the dominant azimuthal mode which is either 0 or 1 and are referred to as symmetric and an antisymmetric mode, respectively. Spectra at subsonic Mach numbers exhibit tones which are related either to mode 0 or 1. However, at supersonic Mach numbers only tones related to mode 0 exist. The base cavity either, effectively suppress the antisymmetric mode or modify it into a symmetric mode resulting in mitigation of the tones related to antisymmetric mode.

Mach Number Effect on Symmetric and Antisymmetric Modes of Base Pressure Fluctuations

2018 ◽  
Vol 141 (2) ◽  
Author(s):  
N. S. Vikramaditya ◽  
M. Viji
Keyword(s):  
Mach Number ◽  
Vortex Shedding ◽  
Pressure Fluctuations ◽  
Base Pressure ◽  
Antisymmetric Mode ◽  
Recirculation Bubble ◽  
Pressure Drops ◽  
Wide Range ◽  
Convective Motions ◽  
Mach Numbers

An experimental study aimed at evaluating the influence of Mach number on the base pressure fluctuations of a cylindrical afterbody was performed over a wide range of Mach numbers from subsonic to supersonic speeds. Time-averaged results indicate that the coefficient of base pressure drops with the increase in the freestream Mach number at subsonic speeds and increases at supersonic Mach numbers. The coefficient of root-mean-square of the pressure fluctuations follows a decreasing trend with the increase in the Mach number. Examination of the spectra reveals different mechanisms dominate the pressure fluctuations from the center to the periphery of the base as well as with the change in the Mach number. Analysis of the azimuthal coherence indicates that all the dominant tones in the spectra can be classified either into a symmetric or an antisymmetric mode at subsonic Mach numbers. However, at supersonic Mach numbers, all the dominant tones in the spectra are symmetric in nature. The results from the cross-correlation suggest that two possible mechanisms of recirculation bubble pulsing and convective motions/vortex shedding are driving the dynamics on the base at subsonic Mach numbers. However, at supersonic Mach numbers, only single mechanism of the recirculation bubble pulsing dominates. Moreover, it indicates that the symmetric mode is associated with the dynamics of the recirculation bubble and the antisymmetric mode is related to the convective motions/vortex shedding.

Influence of base cavity on base pressure at subsonic and supersonic Mach numbers

2019 ◽  
Author(s):  
J. N. Murugan ◽  
Kiran Chutkey ◽  
S. B. Verma
Keyword(s):  
Base Pressure ◽  
Base Cavity ◽  
Mach Numbers

scholarly journals Base Pressure Fluctuations on Typical Missile Configuration in Presence of Base Cavity

2018 ◽  
Vol 55 (2) ◽  
pp. 335-345 ◽  
Author(s):  
N. S. Vikramaditya ◽  
M. Viji ◽  
S. B. Verma ◽  
Naveed Ali ◽  
D. N. Thakur
Keyword(s):  
Pressure Fluctuations ◽  
Base Pressure ◽  
Base Cavity

scholarly journals 47 COVID-19 Impact on Burn Care- A Summary of Weekly Bed Counts and Surge Capacity

2021 ◽  
Vol 42 (Supplement_1) ◽  
pp. S35-S35
Author(s):  
Jeffrey E Carter ◽  
Herbert Phelan ◽  
Colleen M Ryan ◽  
James C Jeng ◽  
Kathryn Mai ◽  
...  
Keyword(s):  
Substantial Reduction ◽  
Surge Capacity ◽  
Burn Centers ◽  
Resource Limitations ◽  
Burn Care ◽  
Regional Trends ◽  
Number Of Patients ◽  
Bed Capacity ◽  
Initial Survey ◽  
The Impact

Abstract Introduction The COVID-19 pandemic has raised global awareness of healthcare resource limitations. Specifically, the pandemic has demonstrated that burn disaster planning should involve non-burn disasters that threaten staff, supplies, or space. The ABA facilitated bed counts with the assistance of regional disaster coordinators from April through August of 2020. Our analysis examines the impact of the pandemic on burn surge and bed capacity in the U.S. Methods Bed availability was obtained by the ABA regional disaster coordinators through an initiative by the Organization and Delivery of Burn Care Committee. Bed availability was defined as immediately available burn beds and categorized as adult, pediatric, or flexible. Surge capacity was defined as the maximum number of patients that a burn center could admit in a surge situation. Data was deidentified by the central office with descriptive statistics to determine bed availability and surge capacity trends regionally and nationally. Results Bed counts were performed 6 times from 04/17/2020 through 08/14/2020. Response rates from the 137 North American burn centers varied from 86–96%. At least 6 burn centers (5%) were either closed or converted for COVID patients during the initial two bed counts. The total number of adult or pediatric burn beds was 2,082. Total bed availability decreased from 845 at the first survey down to 572 beds at the last survey. Surge capacity baseline was 1,668 beds and decreased from 1,132 beds in the initial survey down to 833 beds in the final survey. Conclusions Our study demonstrates a significant impact on burn bed availability due to the COVID-19 pandemic with a 37% reduction in available burn beds from April to August and a 26% reduction in surge capacity. This study demonstrates a substantial reduction in bed availability during the pandemic with additional analysis in process to examine regional trends.

Predicting the Influence of Damping Devices on the Stability Margin of an Annular Combustor

2017 ◽  
Author(s):  
Max Zahn ◽  
Michael Betz ◽  
Moritz Schulze ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Resonance Frequency ◽  
Stability Margin ◽  
Numerical Approach ◽  
Acoustic Mode ◽  
Acoustic Properties ◽  
Azimuthal Mode ◽  
Acoustic Damping ◽  
Linearized Euler Equations ◽  
Annular Combustor ◽  
The Impact

A numerical modeling approach based on linearized Euler equations is applied to predict the linear stability of an annular combustor with and without dampers. The acoustic properties of all relevant combustor components such as damping devices, swirl burner characteristics, swirl flame dynamics, and combustor exit are individually evaluated via experimental and numerical approaches. All of the components are incorporated subsequently into the combustor model using impedances and acoustic transfer matrices to obtain an efficient procedure. This study focuses on using this approach to predict an annular combustor’s stability margin and to assess how dampers influence the modal dynamics of the first azimuthal mode. Stability predictions are successfully validated with experimental data. Different combustor components’ contributions to the acoustic damping of the entire system is also determined based on that numerical approach. Damper application in combustors can engender uncertainties in resonance frequency in the case of hot-gas ingestion. The impact of “detuned” resonators on the predicted damping rates with respect to a deviation in the resonance frequency and the eigenfrequency of the attenuated acoustic mode is therefore evaluated. The influence of dampers on the annular combustor’s stability margin is also determined.

scholarly journals Strong Azimuthal Combustion Instabilities in a Spray Annular Chamber With Intermittent Partial Blow-Off

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Kevin Prieur ◽  
Daniel Durox ◽  
Thierry Schuller ◽  
Sébastien Candel
Keyword(s):  
Light Intensity ◽  
High Speed ◽  
Transverse Velocity ◽  
Pressure Fluctuations ◽  
Pressure Sensors ◽  
Nodal Line ◽  
Operating Conditions ◽  
Acoustic Energy ◽  
Transverse Motion ◽  
Azimuthal Mode

This article reports experiments carried out in the MICCA-spray combustor developed at EM2C laboratory. This system comprises 16 swirl spray injectors. Liquid n-heptane is injected by simplex atomizers. The combustion chamber is formed by two cylindrical quartz tubes allowing full optical access to the flame region and it is equipped with 12 pressure sensors recording signals in the plenum and chamber. A high-speed camera provides images of the flames and photomultipliers record the light intensity from different flames. For certain operating conditions, the system exhibits well defined instabilities coupled by the first azimuthal mode of the chamber at a frequency of 750 Hz. These instabilities occur in the form of bursts. Examination of the pressure and the light intensity signals gives access to the acoustic energy source term. Analysis of the phase fluctuations between the two signals is carried out using cross-spectral analysis. At limit cycle, large pressure fluctuations of 5000 Pa are reached, and these levels persist over a finite period of time. Analysis of the signals using the spin ratio indicates that the standing mode is predominant. Flame dynamics at the pressure antinodal line reveals a strong longitudinal pulsation with heat release rate oscillations in phase and increasing linearly with the acoustic pressure for every oscillation levels. At the pressure nodal line, the flames are subjected to large transverse velocity fluctuations leading to a transverse motion of the flames and partial blow-off. Scenarios and modeling elements are developed to interpret these features.

scholarly journals Computed Poststenotic Flow Instabilities Correlate Phenotypically With Vibrations Measured Using Laser Doppler Vibrometry: Perspectives for a Promising In Vivo Device for Early Detection of Moderate and Severe Carotid Stenosis

2020 ◽  
Vol 142 (9) ◽  
Author(s):  
Viviana Mancini ◽  
Aslak W. Bergersen ◽  
Kristian Valen-Sendstad ◽  
Patrick Segers
Keyword(s):  
Early Detection ◽  
Carotid Stenosis ◽  
Pressure Fluctuations ◽  
Laser Doppler Vibrometer ◽  
Power Spectra ◽  
Laser Doppler ◽  
Flow Instabilities ◽  
Promising Tool ◽  
The Impact

Abstract Early detection of asymptomatic carotid stenosis is crucial for treatment planning in the prevention of ischemic stroke. Auscultation, the current first-line screening methodology, comes with severe limitations that create urge for novel and robust techniques. Laser Doppler vibrometer (LDV) is a promising tool for inferring carotid stenosis by measuring stenosis-induced vibrations. The goal of the current study was to evaluate the feasibility of LDV for carotid stenosis detection. LDV measurements on a carotid phantom were used to validate our previously verified high-resolution computational fluid dynamics methodology, which was used to evaluate the impact of flowrate, flow split, and stenosis severity on the poststenotic intensity of flow instabilities (IFI). We evaluated sensitivity, specificity, and accuracy of using IFI for stenoses detection. Linear regression analyses showed that computationally derived pressure fluctuations correlated (R2 = 0.98) with LDV measurements of stenosis-induced vibrations. The flowrate of stenosed vessels correlated (R2 = 0.90) with the presence of poststenotic instabilities. Receiver operating characteristic analyses of power spectra revealed that the most relevant frequency bands for the detection of moderate (56–76%) and severe (86–96%) stenoses were 80–200 Hz and 0–40 Hz, respectively. Moderate stenosis was identified with sensitivity and specificity of 90%; values decreased to 70% for severe stenosis. The use of LDV as screening tool for asymptomatic stenosis can potentially provide improved accuracy of current screening methodologies for early detection. The applicability of this promising device for mass screening is currently being evaluated clinically.

Initiating ECF bleaching sequences of eucalyptus kraft pulps with Z/D and Z/E stages

Holzforschung ◽  
2010 ◽  
Vol 64 (1) ◽  
Author(s):  
Juan C. García ◽  
Francisco López ◽  
Antonio Pérez ◽  
M. Angels Pèlach ◽  
Pere Mutjé ◽  
...  
Keyword(s):  
Hydrogen Peroxide ◽  
Chlorine Dioxide ◽  
Ecological Impact ◽  
Substantial Reduction ◽  
Reference Sequence ◽  
Kraft Pulps ◽  
Eucalyptus Kraft Pulp ◽  
The Impact ◽  
Ecf Bleaching ◽  
Ozone Bleaching

Abstract Ozone bleaching is a common practice in pulping, and also of eucalyptus, where it is usually applied in combination with bleaching sequences based on oxygen, hydrogen peroxide, or chlorine dioxide. Ozone has been proven to be a highly efficient and competitive bleaching chemical in terms of delignification efficiency, low costs, and reducing ecological impact. The objective of the present work was to evaluate technology with ozone/alkaline extraction (Z/E) and ozone/chlorine dioxide (Z/D) for bleaching of eucalyptus kraft pulp. Primarily, the impact of these bleaching steps on refinability and quality of pulp should be investigated. As reference, the sequence D*(EP)D (hot chlorine dioxide, extraction in presence of hydrogen peroxide, chlorine dioxide) was selected, which is considered as the state-of-the-art bleaching in elemental chlorine free (ECF) bleaching technology. Various bleaching sequences with ozone in their first step (Z/D(EP)DP, Z/D(EP)DD, Z/EDP, Z/EDD and A*Z/EDP) were found to provide kraft pulps of similar brightness and in similar yield as the reference sequence D*(EP)D. The kappa number, viscosity, and the contents of glucose and xylose, and hexenuronic acid of the pulps were also similar. In addition, the Z sequences resulted in a substantial reduction of the total chlorine dioxide consumption (more than 30.3% in all cases). The A*Z/EDP sequence, which proved to be the most efficient, yielded 87.5% ClO2 reduction. The studied bleaching sequences also resulted in substantially improved brightness reversal with regard to the reference sequence. The sequence A*Z/EDP was also the most efficient as regards the removal or organochlorines (OX) from the pulp and their reduction in the effluents (AOX). Ozone bleaching sequences improved paper strength, especially with the A*Z/EDP sequence.

A Numerical Optimization Technique Applied to the Design of Two-Dimensional Cavitating Hydrofoil Sections

1996 ◽  
Vol 40 (01) ◽  
pp. 28-38
Author(s):  
Shigenori Mishima ◽  
Spyros A. Kinnas
Keyword(s):  
Numerical Optimization ◽  
Cavity Length ◽  
Optimization Technique ◽  
Cavitation Number ◽  
Quadratic Functions ◽  
Two Dimensional ◽  
Systematic Design ◽  
Hydrodynamic Analysis ◽  
Total Drag ◽  
Cavitating Hydrofoil

A numerical nonlinear optimization technique is applied to the systematic design of two-dimensional partially or supercavitating hydrofoil sections. The design objective is to minimize the hydrofoil drag for given lift and cavitation number. The hydrodynamic analysis of the cavitating hydrofoil is performed in nonlinear theory, via a low-order potential-based panel method. The effects of viscosity are taken into account via a uniform friction coefficient applied on the wetted foil surface. The total drag, lift, cavitation number, and other quantities involved in the imposed constraints, are expressed in terms of quadratic functions of the main parameters of the hydrofoil geometry, angle of attack, and the cavity length. The optimization is based on the method of multipliers by coupling the Lagrange multiplier terms and the penalty function terms. The robustness and convergence of the method are extensively investigated, and the results are compared with those from applying other design methods.

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