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Numerical study of the unsteadiness of a ground vortex

Nunes, Ricardo Bruno Freitas
Fonte: Universidade da Beira Interior Publicador: Universidade da Beira Interior
Tipo: Dissertação de Mestrado
Publicado em //2009 ENG
Relevância na Pesquisa
27.13%
Single impinging jets in a crossflow are typical in impingement cooling applications in industry, as well as of the flow beneath a V/STOL aircraft. In this latter application, a primary design consideration is the flow environment induced by the propulsion system during hover with zero or small forward momentum. Ground effect phenomena may occur and change the lift forces on the aircraft, cause re-ingestion of exhaust gases into the engine intake and raise fuselage skin temperatures. An important source of each is the ground vortex which forms far upstream of the impinging jet when the resulting radial wall jet meets a crossflow. Numerical and experimental studies have also been performed in this area. Some were dedicated to the study of the more fundamental configurations: single or multiple impinging jets through a crossflow. The present thesis extends the analysis of (Pandya, Murman, & Sankaran, 2003) to a wider range of velocity ratios, VR, from 0.065 to 0.2. The impact zone of a wall jet with a boundary layer was studied computationally using a Reynolds Averaged Navier-Stokes (RANS) approach with the “k-ε” turbulence model. The computational domain corresponds to complete experimental rig of (Cimbala, Billet, Gaublomme, & Oefelein...

The Epidemiology of Vertigo, Dizziness, and Unsteadiness and Its Links to Co-Morbidities

Bisdorff, Alexandre; Bosser, Gilles; Gueguen, René; Perrin, Philippe
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Publicado em 22/03/2013 EN
Relevância na Pesquisa
27.63%
Vertigo, dizziness, and unsteadiness (VDU) are common symptoms traditionally considered to result from different kinds of vestibular and non-vestibular dysfunctions. The epidemiology of each symptom and how they relate to each other and to migraine, agoraphobia, motion sickness susceptibility (MSS), vaso-vagal episodes (VVE), and anxiety-depression was the object of this population-based study in north-eastern France. A self-administered questionnaire was returned by 2987 adults (age span 18–86 years, 1471 women). The 1-year prevalence for vertigo was 48.3%, for unsteadiness 39.1%, and for dizziness 35.6%. The three symptoms were correlated with each other, occurred mostly (69.4%) in various combinations rather than in isolation, less than once per month, and 90% of episodes lasted ≤2 min. The three symptoms were similar in terms of female predominance, temporary profile of the episodes, and their link to falls and nausea. Symptom episodes of >1 h increase the risk of falls. VDU are much more common than the known prevalence of vestibular disorders. The number of drugs taken increase VDU even when controlling for age. Each VDU symptom was correlated with each co-morbidity in Chi-squared tests. The data suggest that the three symptoms are more likely to represent a spectrum resulting from a range of similar – rather than from different...

Displacement-Weighted Velocity Analysis of Gliding Assays Reveals that Chlamydomonas Axonemal Dynein Preferentially Moves Conspecific Microtubules

Alper, Joshua D.; Tovar, Miguel; Howard, Jonathon
Fonte: The Biophysical Society Publicador: The Biophysical Society
Tipo: Artigo de Revista Científica
Publicado em 07/05/2013 EN
Relevância na Pesquisa
17.75%
In vitro gliding assays, in which microtubules are observed to glide over surfaces coated with motor proteins, are important tools for studying the biophysics of motility. Gliding assays with axonemal dyneins have the unusual feature that the microtubules exhibit large variations in gliding speed despite measures taken to eliminate unsteadiness. Because axonemal dynein gliding assays are usually done using heterologous proteins, i.e., dynein and tubulin from different organisms, we asked whether the source of tubulin could underlie the unsteadiness. By comparing gliding assays with microtubules polymerized from Chlamydomonas axonemal tubulin with those from porcine brain tubulin, we found that the unsteadiness is present despite matching the source of tubulin to the source of dynein. We developed a novel, to our knowledge, displacement-weighted velocity analysis to quantify both the velocity and the unsteadiness of gliding assays systematically and without introducing bias toward low motility. We found that the quantified unsteadiness is independent of tubulin source. In addition, we found that the short Chlamydomonas microtubules translocate significantly faster than their porcine counterparts. By modeling the effect of length on velocity...

Finite Element Solution of Unsteady Mixed Convection Flow of Micropolar Fluid over a Porous Shrinking Sheet

Gupta, Diksha; Kumar, Lokendra; Singh, Bani
Fonte: Hindawi Publishing Corporation Publicador: Hindawi Publishing Corporation
Tipo: Artigo de Revista Científica
Publicado em 04/02/2014 EN
Relevância na Pesquisa
17.44%
The objective of this investigation is to analyze the effect of unsteadiness on the mixed convection boundary layer flow of micropolar fluid over a permeable shrinking sheet in the presence of viscous dissipation. At the sheet a variable distribution of suction is assumed. The unsteadiness in the flow and temperature fields is caused by the time dependence of the shrinking velocity and surface temperature. With the aid of similarity transformations, the governing partial differential equations are transformed into a set of nonlinear ordinary differential equations, which are solved numerically, using variational finite element method. The influence of important physical parameters, namely, suction parameter, unsteadiness parameter, buoyancy parameter and Eckert number on the velocity, microrotation, and temperature functions is investigated and analyzed with the help of their graphical representations. Additionally skin friction and the rate of heat transfer have also been computed. Under special conditions, an exact solution for the flow velocity is compared with the numerical results obtained by finite element method. An excellent agreement is observed for the two sets of solutions. Furthermore, to verify the convergence of numerical results...

Non-Hyperammonemic valproate encephalopathy

Farooq, Omar; Zunga, Pervaiz M.; Dar, Mohd I.; Rather, Abdul Q.; Rashid, Samia; Basu, Javid; Dar, Ishrat H.; Ashraf, Mohd
Fonte: Indian Academy of Neurosciences Publicador: Indian Academy of Neurosciences
Tipo: Artigo de Revista Científica
Publicado em /04/2014 EN
Relevância na Pesquisa
17.13%
A 21-year-old male known case of primary hypothyroidism, Seizure disorder sequelae of an old trauma receiving sodium valproate, clobazam and phenobarbitone for control of Generalized tonic clonic seizures reported to neurology OPD with history of altered sensorium and gait unsteadiness for 1 week with history of hike in valproate dose 2 weeks before. On examination he was drowsy. Neurological examination was unremarkable except for gait unsteadiness and ataxia. Patient was admitted and evaluated for acute worsening. All (the) biochemical parameters including complete blood count, liver function tests, kidney function tests, routine urine examination, arterial blood gas analysis, blood and urine culture tests were normal. CSF analysis was also normal. Repeat MRI brain was also done which depicted all old changes with no fresh changes which will account for worsening of his sensorium. EEG was suggestive of diffuse encephalopathy. Thyroid function tests were also normal. Valproate encephalopathy was suspected and Valproate was empirically stopped and he was put on levetiracetam and phenytoin. His sensorium improved rapidly after stoppage of valproate with normalization of EEG. Serum valproate Levels were high with serum ammonia levels were in the normal range. We made the inference of nonhyperammoneamic valproate encephalopathy. This case highlights the existence of non-hyperammonemic valproate induced encephalopathy...

Active control of tip clearance flow in axial compressors

Bae, Jinwoo W
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 93 p.; 6435890 bytes; 6435649 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
17.44%
Control of compressor tip clearance flows is explored in a linear cascade using three types of fluidic actuators; Normal Synthetic Jet (NSJ; unsteady jet normal to the mean flow with zero net mass flux), Directed Synthetic Jet (DSJ; injection roughly aligned with the mean flow), and Steady Directed Jet (SDJ), mounted on the casing wall. The objective is to affect the following measures: (1) reduction of tip leakage flow rate, (2) mixing enhancement between tip leakage and core flow, and (3) increase in streamwise momentum of the flow in the endwall region. The measurements show that the NSJ provides mixing enhancement only, or both mixing enhancement and leakage flow reduction, depending on its pitchwise location. The DSJ and SDJ actuators provide streamwise momentum enhancement with a consequent reduction of clearance-related blockage. The blockage reduction associated with the use of NSJ is sensitive to actuator frequency, whereas that with the use of DSJ is not. For a given actuation amplitude, DSJ and SDJ are about twice as effective as NSJ in reducing clearance-related blockage. Further the DSJ and SDJ can eliminate clearance-related blockage with a time-averaged momentum flux roughly 16% of the momentum flux of the leakage flow.; (cont.) However...

An experimental study of unsteady separation in a two-dimensional flow

Coral Pinto, Raul Javier
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 104 leaves; 6427232 bytes; 6431552 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
17.13%
An experimental study of flow separation in an incompressible two-dimensional unsteady flow was undertaken with the aim of validating recently developed flow separation criteria, which are based on kinematic principles. Specifically, the so-called rotor-oscillator arrangement was utilized to perform a series of experiments in steady, unsteady periodic, and aperiodic two- dimensional viscous flows at low Reynolds number. Flow separation under these conditions was investigated by means of flow visualization, shear stress sensors, and numerical simulation. The existence of fixed and moving separation structures, as predicted by the recent criteria, was verified in the experiments. Fixed separation structures were encountered in periodic flows and random flows, while moving separation was observed in a slow periodic flow and a random flow with linear drift. It was determined that separation in the rotor-oscillator experiment is strongly correlated to two factors: flow unsteadiness and flow ejection from the wall. The balance of the characteristic length and time scales of unsteadiness and ejection determines whether the separating structure is moving or is fixed.; (cont.) The experimental and numerical results strengthen the notion that the instantaneous zero skin friction point alone does not denote flow separation in unsteady flow. Rather...

Scaling of impeller response to impeller-diffuser interactions in centrifugal compressors

Tarr, David Landon
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 121 p.
ENG
Relevância na Pesquisa
17.13%
A numerical investigation has been conducted to quantify the effect of impeller-diffuser interaction on changes in impeller performance. An assessment is made of the hypothesis that the nondimensional parameter characterizing impeller performance change due to interaction is the ratio of the radial gap between the impeller trailing edge and the diffuser leading edge to the diffuser vane pitch. The time averaged results are found to show no measurable performance change with varying degrees of impeller-diffuser interaction, disproving the hypothesis. Analysis of the flow field shows that changes in loss and blockage due to interaction in the region of the blade leakage flow, which were expected to drive performance change, are negligible. The disproven hypothesis lacks parameters to relate the level of impeller passage unsteadiness to the level of performance change due to interaction, which are shown to be important. The ratio of the unsteady amplitude, of blade leakage velocity or blade loading, to the time average value appears to be a parameter capable of quantifying the impact of unsteadiness on impeller performance change.; by David Landon Tarr.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics...

The role of unsteadiness within a central jet on the structure of a combined central and co-annular jet flow

Smith, N.; Nathan, G.
Fonte: ADELAIDE UNIVERSITY; ADELAIDE UNIVERSITY, SOUTH AUSTRALIA 5005 Publicador: ADELAIDE UNIVERSITY; ADELAIDE UNIVERSITY, SOUTH AUSTRALIA 5005
Tipo: Conference paper
Publicado em //2001 EN
Relevância na Pesquisa
27.13%
Qualitative and quantitative assessments of the effect of the unsteadiness in a central precessing jet (PJ) on a co-annular jet were made using experimental methods. The central jets were an unsteady PJ, a simple axial jet, a simple jet directed at 45° to the nozzle axis and a conical jet directed at 45° to the nozzle axis. The central jets were compared on the basis of similar axial momentum. The simple directed jet corresponds to case in which the PJ flow has a precession frequency of zero and the conical jet corresponds to the case with a precession frequency of infinity. Experimental investigations of the near-nozzle region were performed in water using a two-colour planar laser induced fluorescence visualisation technique. The visualisations showed that, like the PJ flow, both the directed and the conical jets increased the initial spread of the combined flow. Nevertheless the PJ flow is fundamentally different from the steady analogues. The PJ flow was the only flow to increase the scale of the large-scale, visually coherent, motions in the combined flow. This observation was quantified by measurements of jet half-width. The central PJ flow increased the annular jet half width by 66%, while the steady jets reduced it by 7% - 32 %.

A 2-D DNS investigation of extinction and reignition dynamics in nonpremixed flame-vortex interactions

Venugopal, R.; Abraham, J.
Fonte: Elsevier Science Inc Publicador: Elsevier Science Inc
Tipo: Artigo de Revista Científica
Publicado em //2008 EN
Relevância na Pesquisa
27.44%
Two-dimensional (2-D) DNS investigations of extinction and reignition dynamics during interactions of laminar nonpremixed flames with counterrotating vortex pairs are performed. The length and velocity scales chosen for the vortices are representative of those in the near fields of high-Reynolds-number jets such as those occurring in Diesel engines. The governing equations are solved with sixth-order spatial discretization and fourth-order time integration. Chemistry is modeled as an irreversible single-step reaction. Local extinction along the symmetry axis, followed by reignition, is observed. The extinction is characterized by strong unsteady effects, which are captured well by 1-D transient diffusion flamelet libraries, provided the time-history of the instantaneous scalar dissipation rate is taken into account. On the other hand, reignition is essentially a 2-D phenomenon involving flame–flame interactions, which are favored for smaller vortices and increasing flame curvature. The effects of unsteadiness and curvature on extinction and reignition are carefully assessed through parametric studies involving a range of vortex and flame characteristics. The interaction outcomes are summarized on Reynolds–Damköhler number (Re–Da) diagrams...

Numerical studies of the response of flamelets to unsteadiness in the near-field of jets under diesel conditions

Venugopal, R.; Abraham, J.
Fonte: Taylor & Francis Ltd Publicador: Taylor & Francis Ltd
Tipo: Artigo de Revista Científica
Publicado em //2010 EN
Relevância na Pesquisa
27.13%
Turbulent time records of scalar dissipation rates are numerically generated in the near-field of a high Reynolds number turbulent jet. The response of one-dimensional unsteady diffusion flamelets to these records were studied under pressure and temperature conditions representative of those in diesel engines. n-Heptane was chosen as the diesel fuel surrogate, its oxidation chemistry modeled by a 1,540-step mechanism comprising 159 species. Unsteadiness in the scalar dissipation rates was observed to impact autoignition, extinction, and reignition in the near-field. Possible linkages between these observations and flame liftoff, as well as its prediction, in diesel engines are discussed. Steady flamelet models were shown to not capture unsteady extinction–reignition events and associated chemical phase-lag effects. Unsteady flamelet–progress variable (UFPV) models in which the progress variable uniquely identifies all the flame states corresponding to a given value of χ are more suitable for representing the physics.; Rishikesh Venugopal and John Abraham

Experimental validation of unsteady models for fluid structure interaction: Application to yacht sails and rigs

AUGIER, Benoit; BOT, Patrick; HAUVILLE, Frédéric; DURAND, Mathieu
Fonte: Elsevier Publicador: Elsevier
EN
Relevância na Pesquisa
17.13%
• We present full scale measurements on a yacht sailing upwind in moderate head swell • We highlight the unsteadiness of the loads : peaks may be twice the mean value • The dynamic behavior differs from the steady state : hysteresis loop • Experimental results compare well with an original FSI model; This work presents a full scale experimental study on the aero-elastic wind/sails/rig interaction in real navigation conditions with the aim to give an experimental validation of unsteady fluid structure interaction (FSI) models applied to yacht sails. An inboard instrumentation system has been developed on a J80 yacht to simultaneously and dynamically measure the navigation parameters, yacht’s motion, and sails flying shape and loads in the standing and running rigging. The first results recorded while sailing upwind in head waves are shown. Variations of the measured parameters are characterized and related to the yacht motion (trim mainly). Correlations between the different parameters are examined. In the system’s response to the dynamic forcing (pitching motion) we attempt to distinguish between the aerodynamic effect of varying apparent wind induced by the motion and the structural effect of varying stresses and strains due to the motion and inertia. The dynamic full scale measurements presented underline the necessity of considering the unsteadiness of phenomena to correctly simulate a yacht’s behavior in actual sailing conditions. The simulation results from the FSI model compare very well with the experimental data for steady sailing conditions. For the unsteady conditions obtained in head waves...

Experimental and Numerical Investigation of Unforced unsteadiness in a Vaneless Radial Diffuser

PAVESI, Giorgio; DAZIN, Antoine; CAVAZZINI, Giovanna; CAIGNAERT, Guy; BOIS, Gérard; ARDIZZON, Guido
Fonte: EUROTURBO Publicador: EUROTURBO
EN
Relevância na Pesquisa
37.44%
The paper reports combined experimental and numerical investigations of unforced un- steadiness in a vaneless radial diffuser. Experimental data were obtained within the diffuser using stereoscopic time resolved Particle Image Velocimetry (PIV) recording three velocity components in a plane (2D/3C), coupled with unsteady pressure transducers. To characterize the inception and the evolution of the unsteady phenomena, spectral analyses of the pressure signals were carried out both in frequency and time-frequency domains and the PIV results were post processed by an original averaging method. Two partial flow rates were investigated in detail in this paper. A single unforced unsteadiness was identified for the lowest flow rate, whereas, two competitive intermittent modes were recognized for the higher mass flow. Numerical analyses were carried out on the same pump by the commercial code CFX. All the computations were performed using the unsteady transient model and the turbulence was modelled by the Scale-Adaptive Simulation (SAS) model. Numerical pressure signals were compared with the experimental data to verify the development of the same pressure fluctua- tions.

Numerical study of the unsteadiness of a ground vortex

Nunes, Ricardo Bruno Freitas
Fonte: Universidade da Beira Interior Publicador: Universidade da Beira Interior
Tipo: Dissertação de Mestrado
Publicado em //2009 ENG
Relevância na Pesquisa
27.13%
Single impinging jets in a crossflow are typical in impingement cooling applications in industry, as well as of the flow beneath a V/STOL aircraft. In this latter application, a primary design consideration is the flow environment induced by the propulsion system during hover with zero or small forward momentum. Ground effect phenomena may occur and change the lift forces on the aircraft, cause re-ingestion of exhaust gases into the engine intake and raise fuselage skin temperatures. An important source of each is the ground vortex which forms far upstream of the impinging jet when the resulting radial wall jet meets a crossflow. Numerical and experimental studies have also been performed in this area. Some were dedicated to the study of the more fundamental configurations: single or multiple impinging jets through a crossflow. The present thesis extends the analysis of (Pandya, Murman, & Sankaran, 2003) to a wider range of velocity ratios, VR, from 0.065 to 0.2. The impact zone of a wall jet with a boundary layer was studied computationally using a Reynolds Averaged Navier-Stokes (RANS) approach with the “k-ε” turbulence model. The computational domain corresponds to complete experimental rig of (Cimbala, Billet, Gaublomme, & Oefelein...

Gait Unsteadiness and Fall Risk in Two Affective Disorders: A Preliminary Study

Stoll, Andrew L; Hausdorff, Jeffrey M.; Peng, Chung-Kang; Goldberger, Ary Louis
Fonte: BioMed Central Publicador: BioMed Central
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
27.44%
Background: In older adults, depression has been associated with increased fall risk, but the reasons for this link are not fully clear. Given parallels between major depression and Parkinson's disease, we hypothesized that major depression and related affective disorders would be associated with impairment in the ability to regulate the stride-to-stride fluctuations in gait cycle timing. Methods: We measured stride-to-stride fluctuations of patients with two forms of mood disorders, unipolar major depressive disorder (MDD) and bipolar disorder, and compared their gait to that of a healthy control group. The primary outcomes were two measures of gait unsteadiness that have been associated with fall risk: stride time variability and swing time variability. Results: Compared to the control group, the two patient groups tended to walk more slowly and with decreased swing time and increased stride time. However, none of these differences was statistically significant. Compared to the control group, swing time variability was significantly larger in the subjects with bipolar disorder (p < 0.0001) and in the subjects with MDD (p < 0.0004). Conclusions: Patients with MDD and patients with bipolar disorder display gait unsteadiness. This perturbation in gait may provide a mechanistic link connecting depression and falls. The present findings also suggest the possibility that measurement of variability of gait may provide a readily quantifiable objective approach to monitoring depression and related affective disorders.

Change in symptoms of hypertensive patients after referral to hospital clinic.

Bulpitt, C J; Dollery, C T; Carne, S
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /02/1976 EN
Relevância na Pesquisa
17.13%
At their first visit to a hospital clinic 178 patients referred with a diagnosis of hypertension were given a self-administered questionnaire. They received a similar questionnaire 12 months later. Of the 178 patients 99 were not initially on treatment. Similarly 78 normotensive subjects were drawn randomly from the local population and sent a second questionnaire 10 months later. The symptoms at the first visit of the normotensive controls, the untreated hypertensive patients, and 477 patients on long-term treatment in the hypertension clinic were compared. Treated and untreated hypertensive patients complained more of nocturia and also of unsteadiness either on standing or in the morning. Treated hypertensives complained more of sleepiness, dry mouth, diarrhoea, and, in men, impotence and failure of ejaculation. Similarly, untreated hypertensives complained of excessive depression, blurred vision, and waking headache. Fifty-five of the normotensive subjects and 110 of the newly referred hypertensive patients responded to the second questionnaire. The proportions losing and gaining symptoms were calculated together with the proportions always complaining and never complaining of a symptom. Hypertensive patients tended to lose the complaints of unsteadiness and headache but to gain the symptoms of vivid dreams...

On the Late-Time Behaviour of a Bounded, Inviscid Two-Dimensional Flow

Dritschel, David G.; Qi, Wanming; Marston, J. B.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/06/2015
Relevância na Pesquisa
17.13%
Using complementary numerical approaches at high resolution, we study the late-time behaviour of an inviscid, incompressible two-dimensional flow on the surface of a sphere. Starting from a random initial vorticity field comprised of a small set of intermediate wavenumber spherical harmonics, we find that -- contrary to the predictions of equilibrium statistical mechanics -- the flow does not evolve into a large-scale steady state. Instead, significant unsteadiness persists, characterised by a population of persistent small-scale vortices interacting with a large-scale oscillating quadrupolar vorticity field. Moreover, the vorticity develops a stepped, staircase distribution, consisting of nearly homogeneous regions separated by sharp gradients. The persistence of unsteadiness is explained by a simple point vortex model characterising the interactions between the four main vortices which emerge.; Comment: 21 pages, 14 figures, submitted to JFM

Universal Approach to Overcoming Nonstationarity, Unsteadiness and Non-Markovity of Stochastic Processes in Complex Systems

Yulmetyev, Renat M.; Mokshin, Anatolii V.; Hänggi, Peter
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/06/2005
Relevância na Pesquisa
27.13%
In present paper we suggest a new universal approach to study complex systems by microscopic, mesoscopic and macroscopic methods. We discuss new possibilities of extracting information on nonstationarity, unsteadiness and non-Markovity of discrete stochastic processes in complex systems. We consider statistical properties of the fast, intermediate and slow components of the investigated processes in complex systems within the framework of microscopic, mesoscopic and macroscopic approaches separately. Among them theoretical analysis is carried out by means of local noisy time-dependent parameters and the conception of a quasi-Brownian particle (QBP) (mesoscopic approach) as well as the use of wavelet transformation of the initial row time series. As a concrete example we examine the seismic time series data for strong and weak earthquakes in Turkey ($1998,1999$) in detail, as well as technogenic explosions. We propose a new way of possible solution to the problem of forecasting strong earthquakes forecasting. Besides we have found out that an unexpected restoration of the first two local noisy parameters in weak earthquakes and technogenic explosions is determined by exponential law. In this paper we have also carried out the comparison and have discussed the received time dependence of the local parameters for various seismic phenomena.

A model of dispersion in the unsteady separated shear flow past complex geometries

Franzese, P.; Zannetti, L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/07/1999
Relevância na Pesquisa
17.13%
Separated flows past complex geometries are modelled by discrete vortex techniques. The flows are assumed to be rotational and inviscid, and a new technique is described to determine the streamfunctions for linear shear profiles. The geometries considered are the snow cornice and the backward-facing step, whose edges allow for the separation of the flow and reattachment downstream of the recirculation regions. A point vortex has been added to the flows in order to constrain the separation points to be located at the edges, while the conformal mappings have been modified in order to smooth the sharp edges and let the separation points be free to oscillate around the points of maximum curvature. Unsteadiness is imposed on the flow by perturbing the vortex location, either by displacing the vortex from equilibrium, or by imposing a random perturbation with zero mean on the vortex in equilibrium. The trajectories of passive scalars continuously released upwind of the separation point and trapped by the recirculating bubble are numerically integrated, and concentration time series are calculated at fixed locations downwind of the reattachment points. This model proves to be capable of reproducing the trapping and intermittent release of scalars...

The role of unsteadiness in direct initiation of gaseous detonations

Eckett, Chris A.; Quirk, James J.; Shepherd, Joseph E.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 25/10/2000
Relevância na Pesquisa
27.63%
An analytical model is presented for the direct initiation of gaseous detonations by a blast wave. For stable or weakly unstable mixtures, numerical simulations of the spherical direct initiation event and local analysis of the one-dimensional unsteady reaction zone structure identify a competition between heat release, wave front curvature and unsteadiness. The primary failure mechanism is found to be unsteadiness in the induction zone arising from the deceleration of the wave front. The quasi-steady assumption is thus shown to be incorrect for direct initiation. The numerical simulations also suggest a non-uniqueness of critical energy in some cases, and the model developed here is an attempt to explain the lower critical energy only. A critical shock decay rate is determined in terms of the other fundamental dynamic parameters of the detonation wave, and hence this model is referred to as the critical decay rate (CDR) model. The local analysis is validated by integration of reaction-zone structure equations with real gas kinetics and prescribed unsteadiness. The CDR model is then applied to the global initiation problem to produce an analytical equation for the critical energy. Unlike previous phenomenological models of the critical energy...