scholarly journals Laminarisation of flow at low Reynolds number due to streamwise body force

2016 ◽  
Vol 809 ◽  
pp. 31-71 ◽  
Author(s):  
S. He ◽  
K. He ◽  
M. Seddighi
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Body Force ◽  
Turbulent Viscosity ◽  
The Body ◽  
Wall Turbulence ◽  
Turbulence Structure ◽  
Pressure Force ◽  
Conventional View ◽  
Ejections And Sweeps

It is well established that when a turbulent flow is subjected to a non-uniform body force, the turbulence may be significantly suppressed in comparison with that of the flow of the same flow rate and hence the flow is said to be laminarised. This is the situation in buoyancy-aided mixed convection when severe heat transfer deterioration may occur. Here we report results of direct numerical simulations of flow with a linear or a step-change profile of body force. In contrast to the conventional view, we show that applying a body force to a turbulent flow while keeping the pressure force unchanged causes little changes to the key characteristics of the turbulence. In particular, the mixing characteristics of the turbulence represented by the turbulent viscosity remain largely unaffected. The so-called flow laminarisation due to a body force is in effect a reduction in the apparent Reynolds number of the flow, based on an apparent friction velocity associated with only the pressure force of the flow (i.e. excluding the contribution of the body force). The new understanding allows the level of the flow ‘laminarisation’ and when the full laminarisation occurs to be readily predicted. In terms of the near-wall turbulence structure, the numbers of ejections and sweeps are little influenced by the imposition of the body force, whereas the strength of each event may be enhanced if the coverage of the body force extends significantly away from the wall. The streamwise turbulent stress is usually increased in accordance with the observation of more and stronger elongated streaks, but the wall-normal and the circumferential turbulent stresses are largely unchanged.

A Study of Flow of Plastics Through Pipes

1946 ◽  
Vol 13 (2) ◽  
pp. A101-A105
Author(s):  
R. C. Binder ◽  
J. E. Busher
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Friction Coefficient ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Dynamic Viscosity ◽  
Apparent Viscosity ◽  
Turbulent Viscosity ◽  
Yield Value

Abstract The pipe friction coefficient for true fluids is usually expressed as a function of Reynolds number. This method of organizing data has been extended to tests on the flow of different suspensions which behaved as ideal plastics in the laminar-flow range and as true fluids in the turbulent-flow range. In the laminar-flow range, Reynolds number below about 2100, the denominator in Reynolds number is taken as the apparent viscosity. The apparent viscosity can be determined from the yield value and the coefficient of rigidity. In the turbulent-flow range, the denominator in Reynolds number is an equivalent or turbulent viscosity equal to the dynamic viscosity of a true fluid having the same friction coefficient, velocity, diameter, and density as that of the plastic. The various experimental data on plastics correlate well with this extension of the method for true fluids.

Effects of System Rotation on Disturbances Injected Into Laminar Flow

2015 ◽  
Author(s):  
Oaki Iida ◽  
Yosuke Aono
Keyword(s):  
Laminar Flow ◽  
Flow Field ◽  
Turbulent Flow ◽  
Turbulent Diffusion ◽  
Body Force ◽  
The Body ◽  
Rotational Axis ◽  
System Rotation ◽  
Inhomogeneous Flow ◽  
Turbulent Flow Field

Effects of system rotation are investigated on inhomogeneous flow where disturbances are transported from turbulent to non-turbulent flow field through advection and turbulent diffusion. With the body force on the fringe region, spectral method is used for inhomogeneous flow which is stirred at the bottom of the cuboid computational box. As a result, it is found that inertial waves with a constant helicity are transmitted in the direction perpendicular to the stirred surface, and parallel to rotational axis. In this study, the effects of system rotation on generation and propagation of wave are discussed.

Numerical Prediction of Secondary Flows in Complex Areas Using Concept of Local Turbulent Reynolds Number

2002 ◽  
Author(s):  
Vladimir Kriventsev ◽  
Hiroyuki Ohshima ◽  
Akira Yamaguchi ◽  
Hisashi Ninokata
Keyword(s):  
Reynolds Number ◽  
Secondary Flow ◽  
Axial Velocity ◽  
Complex Geometry ◽  
Turbulent Viscosity ◽  
Secondary Flows ◽  
Turbulence Structure ◽  
Empirical Parameter ◽  
Rod Bundle ◽  
Turbulent Reynolds Number

A new model of turbulence is proposed for the estimation of Reynolds stresses in turbulent fully-developed flow in a wall-bounded straight channel of an arbitrary shape. Ensemble-averaged Navier-Stokes, or Reynolds, equations are considered to be sufficient and practical enough to describe the turbulent flow in complex geometry of rod bundle array. We suggest the turbulence is a process of developing of external perturbations due to wall roughness, inlet conditions and other factors. We also assume that real flows are always affected by perturbations of any possible scale lower than the size of the channel. Thus, turbulence can be modeled in the form of internal or “turbulent” viscosity. The main idea of a Multi-Scale Viscosity (MSV) model can be expressed in the following phenomenological rule: A local deformation of axial velocity can generate the turbulence with the intensity that keeps the value of the local turbulent Reynolds number below some critical one. Therefore, in MSV, the only empirical parameter is the critical Reynolds number. From analysis of dimensions, some physical explanations of Reynolds number are possible. We can define the local turbulent Reynolds number in two ways: i) simply as Re = ul/v, where u is a local velocity deformation within the local scale l and v is total accumulated molecular and turbulent viscosity of all scales lower then 1. ii) Re = K/W, where K is kinetic energy and W is work of friction/dissipation forces. Both definitions above have been implemented in the calculation of samples of basic fully-developed turbulent flows in straight channels such as a circular tube and annular channel. MSV has been also applied to prediction of turbulence-driven secondary flow in elementary cell of the infinitive hexagonal rod array. It is known that the nature of these turbulence-driven motions is originated in anisotropy of turbulence structure. Due to the lack of experimental data up to date, numerical analysis seems to be the only way to estimate intensity of the secondary flows in hexagonal fuel assemblies of fast breeder reactors (FBR). Since MSV can naturally predict turbulent viscosity anisotropy in directions normal and parallel to the wall, it is capable to calculate secondary flows in the cross-section of the rod bundle. Calculations have shown that maximal intensity of secondary flow is about 1% of the mean axial velocity for the low-Re flows (Re = 8170), while for higher Reynolds number (Re = 160,100) the intensity of secondary flow is as negligible as 0.2%.

Parametric Numerical Study on the Self-Propulsive Performance of The DARPA Suboff Submarine via the Body-Force Propeller Method

2021 ◽  
Author(s):  
Kenshiro Takahashi ◽  
Takayuki Mori
Keyword(s):  
Reynolds Number ◽  
Free Surface ◽  
Body Force ◽  
Surface Effect ◽  
Numerical Study ◽  
The Body ◽  
The Self ◽  
Navier Stokes ◽  
Suction Force ◽  
Propulsive Performance

Abstract This study is based on previous works in a series of numerical studies on submarine hydrodynamics, which involved developing a computational fluid dynamics method to estimate the self-propulsive performance of underwater vehicles. Herein, the Defense Advanced Research Projects Agency SUBOFF submarine model was adopted as a benchmark. The computational modeling applied was based on the Reynolds-averaged Navier-Stokes turbulence model. A body-force propeller method was adopted to model the propulsion. The self-propulsive performance was verified via mesh refinement and validated by comparing the computational solutions with the results obtained from the experiments. The effect of the Reynolds number on the self-propulsive performance was investigated by varying the positions of the stern planes, while the free surface effect was determined by varying the Froude number (Fr) via the volume of fluid method. The computed Taylor wake fraction (w) and hull efficiency (ηH) depended on the Reynolds number as it decreased monotonically. The w and thrust deduction fraction (t) for the model of aft-fitted stern planes were approximately 3–7% and 8–10% higher than those of the baseline and fore-fitted stern planes, respectively. The differences in ηH between the models were insignificant. Regarding the free surface effects, the computations of w, t, and ηH generally decreased with Fr, thus exhibiting several humps and hollows. The computed upward suction force and pitching moment varied from negative to positive and vice versa, depending on Fr.

The transverse force on a drop in an unbounded parabolic flow

1974 ◽  
Vol 62 (1) ◽  
pp. 185-207 ◽  
Author(s):  
Philip R. Wohl ◽  
S. I. Rubinow
Keyword(s):  
Reynolds Number ◽  
Poiseuille Flow ◽  
Weber Number ◽  
Lift Force ◽  
Body Force ◽  
The Body ◽  
Flow Profile ◽  
External Fluid ◽  
Parabolic Flow ◽  
Undisturbed Flow

The steady flow in and around a deformable liquid sphere moving in an unbounded viscous parabolic flow and subject to an external body force is calculated for small values of the ratio of the Weber number to the Reynolds number in the creeping-flow regime. It is found that, in addition to the drag force, the drop experiences a force orthogonal to the undisturbed flow direction. When the body force is absent (neutrally buoyant drop), this lift force tends to drive the drop inwards to the axis, where the undisturbed flow velocity is maximum, i.e., towards a position of lower velocity gradient. In the case for which the parabolic flow profile is a Poiseuille flow profile, the lift force is given by the expression. \[ {\bf F}_1 =-6\pi\mu\epsilon U_0\frac{\alpha +\frac{2}{3}}{\alpha + 1}\bigg(\frac{a}{R_0}\bigg)^4{\bf b}F[1+o(\epsilon)]. \] Here a is the radius of the undeformed sphere, R0 is the radial distance from the position of maximum undisturbed flow U0 at the profile axis to the position of zero flow, ε is the ratio of the Weber number to the Reynolds number, given by ε=μU0T−1, where μ is the external fluid viscosity and T is the surface tension of the drop, α is the ratio of the drop and external fluid viscosities, b is the radial vector from the flow axis to the centre of mass of the drop, and F is a function of α and a dimensionless parameter dependent on the body force that is determined in the analysis. Reasonable agreement is found between the observations by Goldsmith & Mason (1962) of the axial drift of liquid drops in Poiseuille flow and the predictions of the theory herein.

Boundary-layer transition triggered by hairpin eddies at subcritical Reynolds numbers

1995 ◽  
Vol 297 ◽  
pp. 101-122 ◽  
Author(s):  
Masahito Asai ◽  
Michio Nishioka
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Leading Edge ◽  
Shear Layers ◽  
Boundary Layer Transition ◽  
Wall Turbulence ◽  
Turbulence Structure ◽  
Layer Transition ◽  
Wall Shear ◽  
Near Wall

Subcritical transition in a flat-plate boundary layer is examined experimentally through observing its nonlinear response to energetic hairpin eddies acoustically excited at the leading edge of the boundary-layer plate. When disturbed by the hairpin eddies convecting from the leading edge, the near-wall flow develops local three-dimensional wall shear layers with streamwise vortices. Such local wall shear layers also evolve into hairpin eddies in succession to lead to the subcritical transition beyond the x-Reynolds number Rx = 3.9 × 104, where the momentum thickness Reynolds number Rθ is 127 for laminar Blasius flow without excitation, and is about 150 under the excitation of energetic hairpin eddies. It is found that in terms of u- and v-fluctuations, the intensity of the near-wall activity at this critical station is of almost the same order as or slightly less than that of the developed wall turbulence. The development of wall turbulence structure in this transition is also examined.

Energy constraints in forced recirculating MHD flows

1998 ◽  
Vol 375 ◽  
pp. 319-343 ◽  
Author(s):  
D. KINNEAR ◽  
P. A. DAVIDSON
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
High Reynolds Number ◽  
Body Force ◽  
Model Problem ◽  
Shear Layers ◽  
The Body ◽  
Strong Constraint ◽  
High Reynolds ◽  
Stream Transport

We are concerned here with forced steady recirculating flows which are laminar, two-dimensional and have a high Reynolds number. The body force is considered to be prescribed and independent of the flow, a situation which arises frequently in magnetohydrodynamics. Such flows are subject to a strong constraint. Specifically, the body force generates kinetic energy throughout the flow field, yet dissipation is confined to narrow singular regions such as boundary layers. If the flow is to achieve a steady state, then the kinetic energy which is continually generated within the bulk of the flow must find its way to the dissipative regions. Now the distribution of u2/2 is governed by a transport equation, in which the only cross-stream transport of energy is diffusion, v∇2 (u2/2). It follows that there are only two possible candidates for the transport of energy to the dissipative regions: the energy could be diffused to the shear layers, or else it could be convected to the shear layers through entrainment of the streamlines. We investigate both options and show that neither is a likely candidate at high Reynolds number. We then describe numerical experiments for a model problem designed to resolve these issues. We show that, at least for our model problem, no stable steady solution exists at high Reynolds number. Rather, as soon as the Reynolds number exceeds a modest value of around 10, the flow becomes unstable via a supercritical Hopf bifurcation.

NUMERICAL SIMULATION OF HIGH REYNOLDS NUMBER TURBULENT FLOW IN A VANE PASSAGE

2012 ◽  
Vol 19 ◽  
pp. 83-89
Author(s):  
J. F. HUANG ◽  
L. X. ZHANG ◽  
Y. K. GUO
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
High Reynolds Number ◽  
Moving Boundary ◽  
Guide Vane ◽  
Turbulent Viscosity ◽  
Ansys Fluent ◽  
Mesh Model ◽  
Simplec Algorithm ◽  
High Reynolds

The numerical simulations of two-dimensional high Reynolds number turbulent flow in a guide vane passage of a Francis hydro turbine are performed successfully by using the unstructured dynamic mesh model for moving body. The standard K – ε turbulence model is employed for the simulation of turbulence. The pressure-velocity coupling method is realized with the SIMPLEC algorithm. The temporal distributions of the pressure and turbulent viscosity in a passage were obtained in the closing of wicket gate on a platform of the software ANSYS FLUENT. The results show that the evolution of the flow field is unsteady with decrease of the geometrical opening of the gate. The details of the flow changes are obtained in the moving rigid body domain. The method can be used to simulate the high Reynolds number incompressible turbulent flow with moving boundary. The calculation provides some references for vortex-induced vibration of the structure in a complex turbulent flow.

scholarly journals Statistical Structure and Deviations from Equilibrium in Wavy Channel Turbulence

Fluids ◽  
2019 ◽  
Vol 4 (3) ◽  
pp. 161 ◽  
Author(s):  
Saadbin Khan ◽  
Balaji Jayaraman
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Outer Layer ◽  
Strong Dependence ◽  
Surface Topology ◽  
Intermittent Flow ◽  
Turbulence Structure ◽  
Near Surface ◽  
Practical Applications ◽  
Major Interest

The structure of turbulent flow over non-flat surfaces is a topic of major interest in practical applications in both engineering and geophysical settings. A lot of work has been done in the fully rough regime at high Reynolds numbers where the effect on the outer layer turbulence structure and the resulting friction drag is well documented. It turns out that surface topology plays a significant role on the flow drag especially in the transitional roughness regime and therefore, is hard to characterize. Survey of literature shows that roughness function depends on the interaction of roughness height, flow Reynolds number, and topology shape. In addition, if the surface topology contains large enough scales then it can impact the outer layer dynamics and in turn modulate the total frictional force. Therefore, it is important to understand the mechanisms underlying drag increase from systematically varied surface undulations in order to better interpret quantifications based on mean statistics such as roughness function. In this study, we explore the mechanisms that modulate the turbulence structure over a two-dimensional (2D) sinusoidal wavy surface with a fixed amplitude, but varying slopes that are sufficiently small to generate only intermittent flow separation. To accomplish this, we perform a set of highly resolved direct numerical simulations (DNS) to model the turbulent flow between two infinitely wide 2D wavy plates at a friction Reynolds number, R e τ = 180 , which represents modest scale separation. We pursue two different but related flavors of analysis. The first one adopts a roughness characterization flavor of such wavy surfaces. The second one focuses on understanding the nonequilibrium near-surface turbulence structure and their impact on roughness characterization. Analysis of the different statistical quantifications show strong dependence on wave slope for the roughness function indicating drag increase due to enhanced turbulent stresses resulting from increased production of vertical velocity variance from the surface undulations.

scholarly journals Statistical Structure and Deviations from Equilibrium in Wavy Channel Turbulence

2019 ◽  
Author(s):  
Saadbin Khan ◽  
Balaji Jayaraman
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Outer Layer ◽  
Strong Dependence ◽  
Surface Topology ◽  
Turbulence Structure ◽  
Near Surface ◽  
Practical Applications ◽  
Flat Surfaces ◽  
Major Interest

The structure of turbulent flow over non-flat surfaces is a topic of major interest in practical applications in both engineering and geophysical settings. A lot of work has been done in the fully rough regime at high Reynolds numbers where the effect on the outer layer turbulence structure and the resulting friction drag is well documented. It turns out that surface topology plays a significant role on the flow drag especially in the transitional roughness regime and therefore, hard to characterize. Survey of literature shows that roughness function depends on the interaction of roughness height, flow Reynolds number and topology shape. In addition, if the surface topology contains large enough scales then it can impact the outer layer dynamics and in turn modulate the total frictional force. Therefore, it is important to understand the mechanisms underlying drag increase from systematically varied surface undulations in order to better interpret quantifications based on mean statistics such as roughness function. In this study, we explore the mechanisms that modulate the turbulence structure over a two-dimensional (2D) sinusoidal wavy surface with a fixed amplitude, but varying slope. To accomplish this, we model the turbulent flow between two infinitely wide 2D wavy plates at a friction Reynolds number, $Re_{\tau}=180$. We pursue two different but related flavors of analysis. The first one adopts a roughness characterization flavor of such wavy surfaces. The second one focuses on understanding the non-equilibrium near surface turbulence structure and their impact on roughness characterization. Analysis of the different statistical quantifications show strong dependence on wave slope for the roughness function indicating drag increase due to enhanced turbulent stresses resulting from increased production of vertical velocity variance from the surface undulations.

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