The flow field due to a body in impulsive motion

1996 ◽  
Vol 325 ◽  
pp. 79-111 ◽  
Author(s):  
Renwei Mei ◽  
Christopher J. Lawrence
Keyword(s):  
Finite Difference ◽  
Flow Field ◽  
Transition Zone ◽  
Bluff Body ◽  
The Body ◽  
Matched Asymptotic Expansion ◽  
Volume Flux ◽  
Global Flow ◽  
Unsteady Wake ◽  
Long Time

An asymptotic analysis for the long-time unsteady laminar far wake of a bluff body due to a step change in its travelling velocity from U1 to U2 is presented. For U1 [ges ] 0 and U2 > 0, the laminar wake consists of a new wake of volume flux Q2 corresponding to the current velocity U2, an old wake of volume flux Q1 corresponding to the original velocity U1, and a transition zone that connects these two wakes. The transition zone acts as a sink (or a source) of volume flux (Q2 – Q1) and is moving away from the body at speed U2. Streamwise diffusion is negligible in the new and old wakes but a matched asymptotic expansion that retains the streamwise diffusion is required to determine the vorticity transport in the transition zone. A source of volume flux Q2 located near the body needs to be superposed on the unsteady wake to form the global flow field around the body. The asymptotic predictions for the unsteady wake velocity, unsteady wake vorticity, and the global flow field around the body agree well with finite difference solutions for flow over a sphere at finite Reynolds numbers. The long-time unsteady flow structures due to a sudden stop (U2 = 0) and an impulsive reverse (U1U2 < 0) of the body are analysed in detail based on the asymptotic solutions for the unsteady wakes and the finite difference solutions. The elucidation of the long-time behaviour of such unsteady flows provides a framework for understanding the long-time particle dynamics at finite Reynolds number.

Long-time behaviour of the drag on a body in impulsive motion

1995 ◽  
Vol 283 ◽  
pp. 307-327 ◽  
Author(s):  
Christopher J. Lawrence ◽  
Renwei Mei
Keyword(s):  
Reynolds Number ◽  
Transition Zone ◽  
Bluff Body ◽  
Transient Flow ◽  
The Body ◽  
Spherical Particles ◽  
Recent Analysis ◽  
Asymptotic Result ◽  
Hydrodynamic Drag ◽  
Rectilinear Motion

We consider the response of the hydrodynamic drag on a body in rectilinear motion to a change in the speed between two steady states, from U1 to U2 [ges ] 0. We consider situations where the body generates no lift, such as occur for bodies with an axis of symmetry aligned with the motion. At large times, the laminar wake consists of two quasi-steady regions – the new wake and the old wake – connected by a transition zone that is convected downstream with the mean speed U2. A global mass balance indicates the existence of a sink flow centred on the transition zone, and this is responsible for the leading-order behaviour of the unsteady force at long times. For the case of U1 [ges ] 0, the force is shown to decay algebraically with the inverse square of time for any finite Reynolds number (Re), and this result is also shown to hold for non-rectilinear motions. A recent analysis for small Reynolds number including terms to O(Re) (Lovalenti & Brady 1993 a) has indicated that the force decays as the inverse square of time for motion started from rest, but decays exponentially for a change between two positive velocities. The former result is found to be correct, but the exponential decay at O(Re) in the latter case is superseded at large times by the inverse-square time decay which is shifted to O(Re2) because the wake flux is nearly constant for small Re. The cases of reversed flow (U1 < 0) and stopped flow (U2 = 0) are treated separately, and it is shown that the transient force is dominated by the effects of the old wake, leading to a slower decay as the simple inverse of time. The force is determined by the far regions of the flow field and so the results are valid for any (symmetric) particle, bubble or drop and (in an average sense) for any Re, provided τ ma {Re, Re−1}, where the time τ is made dimensionless with the convection timescale. The analytical results are compared to detailed numerical calculations for transient flow over spherical particles and bubbles and compelling agreement is observed. These are believed to be the first calculations which adequately resolve the transient far wake behind a bluff body at long times. The asymptotic result for the force is applied to determine that the approach to terminal velocity of a body in free fall is also as the inverse square of time.

scholarly journals The disappearance of laminar and turbulent wakes in complex flows

2002 ◽  
Vol 457 ◽  
pp. 111-132 ◽  
Author(s):  
J. C. R. HUNT ◽  
I. EAMES
Keyword(s):  
Flow Field ◽  
Rigid Body ◽  
The Body ◽  
Viscous Drag ◽  
Total Force ◽  
Far Field ◽  
Volume Flux ◽  
Wake Region ◽  
Complex Flows ◽  
Turbulent Wakes

The singular effects of steady large-scale external strain on the viscous wake generated by a rigid body and the overall flow field are analysed. In an accelerating flow strained at a positive rate, the vorticity field is annihilated owing to positive and negative vorticity either side of the wake centreline diffusing into one another and the volume flux in the wake decreases with downwind distance. Since the wake disappears, the far-field flow changes from monopolar to dipolar. In this case, the force on the body is no longer proportional to the strength of the monopole, but is proportional to the strength of the far field dipole. These results are extended to the case of strained turbulent wakes and this is verified against experimental wind tunnel measurements of Keffer (1965) and Elliott & Townsend (1981) for positive and negative strains. The analysis demonstrates why the total force acting on a body may be estimated by adding the viscous drag and inviscid force due to the irrotational straining field.Applying the analysis to the wake region of a rigid body or a bubble shows that the wake volume flux decreases even in uniform flows owing to the local straining flow in the near-wake region. While the wake volume flux decreases by a small amount for the flow over streamline and bluff bodies, for the case of a clean bubble the decrease is so large as to render Betz's (1925) drag formula invalid.To show how these results may be applied to complex flows, the effects of a sequence of positive and negative strains on the wake are considered. The average wake width is much larger than in the absence of a strain field and this leads to diffusion of vorticity between wakes and the cancellation of vorticity. The latter mechanism leads to a net reduction in the volume flux deficit downstream which explains why in calculations of the flow through groups of moving or stationary bodies the wakes of upstream bodies may be ignored even though their drag and lift forces have a significant effect on the overall flow field.

Blockage Effects on Two-dimensional Bluff Body Flow

1975 ◽  
Vol 26 (4) ◽  
pp. 243-253 ◽  
Author(s):  
J E Fackrell
Keyword(s):  
Flow Field ◽  
Strouhal Number ◽  
Bluff Body ◽  
The Body ◽  
Base Pressure ◽  
Two Dimensional ◽  
Mean Flow ◽  
Free Air ◽  
Bluff Body Flow ◽  
Good Agreement

SummaryThe time mean flow past a two-dimensional bluff body in a wind-tunnel is modelled by an adaptation of the numerical free-streamline method of Bearman and Fackrell. Provided the base pressure and separation positions are specified in advance, the method allows the calculation of the flow field outside the wake and in particular of the pressures on the wetted surface of the body. Good agreement of the predicted pressures with experimental results is obtained for a flat plate, wedge and circular cylinder at various blockage ratios. In addition to predicting the confined flow, it is shown that a free-air base pressure can be simply deduced from the confined base pressure. With an assumption that the separation positions are unaffected by blockage, this allows the free-air flow field to be calculated. By using an analogy to Roshko’s Strouhal number, which has been found to be invariant under constraint, a free-air Strouhal number can also be deduced.

Effect of Body Aspect Ratio and Tank Size on the Hydrodynamics of a Rotating Bluff Body During the Initial Spin-Up Period

2001 ◽  
Vol 123 (3) ◽  
pp. 649-655
Author(s):  
D. Maynes ◽  
M. Butcher
Keyword(s):  
Aspect Ratio ◽  
Flow Field ◽  
Bluff Body ◽  
Initial Period ◽  
The Body ◽  
Bluff Bodies ◽  
Physical Description ◽  
Time Scaling ◽  
Rectangular Cylinders ◽  
Tank Geometry

Hydrodynamic torque measurements on rotating bluff bodies are presented for 32 different bodies and three different sized tanks for Reynolds numbers in the range 104-105. The present results focus on the initial period, build-up regime, where the torque remains constant before the tank walls have impacted the flow field in the vicinity of the body. The results show that during the build-up regime, the torque coefficient is a function only of the aspect ratio and increases to a maximum at an aspect ratio near unity, followed by a decrease for further increases in the aspect ratio. This behavior is similar to a uniform flow past rectangular cylinders of varied width and a physical description for the observed variation is proposed. A nondimensional time scale describing the time until the tank geometry impacts the flow field near the body is also presented. This time scaling is based on all of the measurements and appears to be quite general, predicting the spin-up time for bodies differing in volume by three orders of magnitude and tanks differing by two orders of magnitude.

scholarly journals A Hybrid Data-Driven Deep Learning Technique for Fluid-Structure Interaction

2019 ◽  
Author(s):  
T. P. Miyanawala ◽  
Rajeev K. Jaiman
Keyword(s):  
Time Series ◽  
Deep Learning ◽  
Flow Field ◽  
Bluff Body ◽  
Dimensional Subspace ◽  
Data Driven ◽  
Long Time ◽  
Hybrid Data ◽  
Low Dimensional Approximations ◽  
Low Dimensional

Abstract This paper is concerned with the development of a hybrid data-driven technique for unsteady fluid-structure interaction systems. The proposed data-driven technique combines the deep learning framework with a projection-based low-order modeling. While the deep learning provides low-dimensional approximations from datasets arising from black-box solvers, the projection-based model constructs the low-dimensional approximations by projecting the original high-dimensional model onto a low-dimensional subspace. Of particular interest of this paper is to predict the long time series of unsteady flow fields of a freely vibrating bluff-body subjected to wake-body synchronization. We consider convolutional neural networks (CNN) for the learning dynamics of wake-body interaction, which assemble layers of linear convolutions with nonlinear activations to automatically extract the low-dimensional flow features. Using the high-fidelity time series data from the stabilized finite element Navier-Stokes solver, we first project the dataset to a low-dimensional subspace by proper orthogonal decomposition (POD) technique. The time-dependent coefficients of the POD subspace are mapped to the flow field via a CNN with nonlinear rectification, and the CNN is iteratively trained using the stochastic gradient descent method to predict the POD time coefficient when a new flow field is fed to it. The time-averaged flow field, the POD basis vectors, and the trained CNN are used to predict the long time series of the flow fields and the flow predictions are quantitatively assessed with the full-order (high-dimensional) simulation data. The proposed POD-CNN model based on the data-driven approximation has a remarkable accuracy in the entire fluid domain including the highly nonlinear near wake region behind a freely vibrating bluff body.

scholarly journals Development of Novel Oral Formulations of Disulfide Antioxidants Based on Porous Silica for Controlled Release of the Drugs

Materials ◽  
2021 ◽  
Vol 14 (4) ◽  
pp. 963
Author(s):  
Ekaterina S. Dolinina ◽  
Elena V. Parfenyuk
Keyword(s):  
Antioxidant Activity ◽  
Surface Chemistry ◽  
Sol Gel ◽  
Porous Silica ◽  
The Body ◽  
Oral Drug ◽  
Therapeutic Concentration ◽  
Release Process ◽  
Zero Order Kinetics ◽  
Long Time

Powerful antioxidant α-lipoic acid (LA) exhibits limited therapeutic efficiency due to its pharmacokinetic properties. Therefore, the purpose of this work was to evaluate the ability of silica-based composites of LA as well as its amide (lipoamide, LM), as new oral drug formulations, to control their release and maintain their therapeutic concentration and antioxidant activity in the body over a long time. The composites synthesized at different sol–gel synthesis pH and based on silica matrixes with various surface chemistry were investigated. The release behavior of the composites in media mimicking pH of digestive fluids (pH 1.6, 6.8, and 7.4) was revealed. The effects of chemical structure of the antioxidants, synthesis pH, surface chemistry of the silica matrixes in the composites as well as the pH of release medium on kinetic parameters of the drug release and mechanisms of the process were discussed. The comparative analysis of the obtained data allowed the determination of the most promising composites. Using these composites, modeling of the release process of the antioxidants in accordance with transit conditions of the drugs in stomach, proximal, and distal parts of small intestine and colon was carried out. The composites exhibited the release close to the zero order kinetics and maintained the therapeutic concentration of the drugs and antioxidant effect in all parts of the intestine for up to 24 h. The obtained results showed that encapsulation of LA and LM in the silica matrixes is a promising way to improve their bioavailability and antioxidant activity.

scholarly journals Mangiferin as New Potential Anti-Cancer Agent and Mangiferin-Integrated Polymer Systems—A Novel Research Direction

Biomolecules ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 79
Author(s):  
Svetlana N. Morozkina ◽  
Thi Hong Nhung Vu ◽  
Yuliya E. Generalova ◽  
Petr P. Snetkov ◽  
Mayya V. Uspenskaya
Keyword(s):  
Molecular Mechanisms ◽  
Biological Activities ◽  
Research Direction ◽  
Biological Action ◽  
The Body ◽  
Biologically Active ◽  
Polymer Systems ◽  
Anti Cancer ◽  
Long Time ◽  
Cancer Agent

For a long time, the pharmaceutical industry focused on natural biologically active molecules due to their unique properties, availability and significantly less side-effects. Mangiferin is a naturally occurring C-glucosylxantone that has substantial potential for the treatment of various diseases thanks to its numerous biological activities. Many research studies have proven that mangiferin possesses antioxidant, anti-infection, anti-cancer, anti-diabetic, cardiovascular, neuroprotective properties and it also increases immunity. It is especially important that it has no toxicity. However, mangiferin is not being currently applied to clinical use because its oral bioavailability as well as its absorption in the body are too low. To improve the solubility, enhance the biological action and bioavailability, mangiferin integrated polymer systems have been developed. In this paper, we review molecular mechanisms of anti-cancer action as well as a number of designed polymer-mangiferin systems. Taking together, mangiferin is a very promising anti-cancer molecule with excellent properties and the absence of toxicity.

scholarly journals Petiole-Lamina Transition Zone: A Functionally Crucial but Often Overlooked Leaf Trait

Plants ◽  
2021 ◽  
Vol 10 (4) ◽  
pp. 774
Author(s):  
Max Langer ◽  
Thomas Speck ◽  
Olga Speck
Keyword(s):  
Transition Zone ◽  
Three Dimensional ◽  
Spatial Arrangement ◽  
Body Plan ◽  
The Body ◽  
Vascular Bundles ◽  
Cross Sectional ◽  
Thin Sections ◽  
Transition Zones ◽  
The Cross

Although both the petiole and lamina of foliage leaves have been thoroughly studied, the transition zone between them has often been overlooked. We aimed to identify objectively measurable morphological and anatomical criteria for a generally valid definition of the petiole–lamina transition zone by comparing foliage leaves with various body plans (monocotyledons vs. dicotyledons) and spatial arrangements of petiole and lamina (two-dimensional vs. three-dimensional configurations). Cross-sectional geometry and tissue arrangement of petioles and transition zones were investigated via serial thin-sections and µCT. The changes in the cross-sectional geometries from the petiole to the transition zone and the course of the vascular bundles in the transition zone apparently depend on the spatial arrangement, while the arrangement of the vascular bundles in the petioles depends on the body plan. We found an exponential acropetal increase in the cross-sectional area and axial and polar second moments of area to be the defining characteristic of all transition zones studied, regardless of body plan or spatial arrangement. In conclusion, a variety of terms is used in the literature for describing the region between petiole and lamina. We prefer the term “petiole–lamina transition zone” to underline its three-dimensional nature and the integration of multiple gradients of geometry, shape, and size.

Survey on the Indirect Methods of Potential Flow Used for the Optimization of Airships Profile

2014 ◽  
Vol 554 ◽  
pp. 717-723
Author(s):  
Reza Abbasabadi Hassanzadeh ◽  
Shahab Shariatmadari ◽  
Ali Chegeni ◽  
Seyed Alireza Ghazanfari ◽  
Mahdi Nakisa
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Field ◽  
Drag Coefficient ◽  
Body Surface ◽  
Potential Flow ◽  
The Body ◽  
Indirect Methods ◽  
Singularity Method ◽  
Singularity Distribution

The present study aims to investigate the optimized profile of the body through minimizing the Drag coefficient in certain Reynolds regime. For this purpose, effective aerodynamic computations are required to find the Drag coefficient. Then, the computations should be coupled thorough an optimization process to obtain the optimized profile. The aerodynamic computations include calculating the surrounding potential flow field of an object, calculating the laminar and turbulent boundary layer close to the object, and calculating the Drag coefficient of the object’s body surface. To optimize the profile, indirect methods are used to calculate the potential flow since the object profile is initially amorphous. In addition to the indirect methods, the present study has also used axial singularity method which is more precise and efficient compared to other methods. In this method, the body profile is not optimized directly. Instead, a sink-and-source singularity distribution is used on the axis to model the body profile and calculate the relevant viscose flow field.

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