Peristaltic flow of Walter’s B fluid in endoscope

The peristaltic flow of a Walter’s B fluid in an endoscope is studied.The problem is modeled in a cylindrical coordinate system.The main theme of the present analysis is to study the endoscopic effects on the peristaltic flow of the Walter’s B fluid.To the best of the authors’ knowledge,no investigation has been made so far in the literatures to study the Walter’s B fluid in an endoscope.Analytical solutions are obtained using the regular perturbation method by taking δ as a perturbation parameter.The appro...     

Numerical and analytical simulation of peristaltic flows of generalized Oldroyd‐B fluids

In this paper, we study the peristaltic flows of generalized Oldroyd‐B fluids through the gap between concentric uniform tubes under the assumption of large wavelength and low Reynolds number approximations. The inner tube is rigid and the outer tube has a sinusoidal wave travelling down its wall. Homotopy perturbation and variational iteration methods are used for solution of the problem. The obtained solution is then used to discuss various interesting features of peristalsis. The effects of relaxation time, retardation time and radii of the tubes on pressure rise and friction forces (per wavelength on the inner and outer tubes) are discussed with illustrations. It is found that pressure rise diminishes with increase in relaxation time or the ratio of radii of inner and outer tubes. It increases with increasing retardation time. The effects of both time parameters on friction forces have the opposite behavior to that of pressure rise. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparisons of rotordynamic coefficients in stepped labyrinth seals by using Colebrook-White friction factor model

The objective of this work is to observe the effects of friction factors for the stepped labyrinth seals. The gas flow through the seals creates net pressure and shear forces acting on the rotor. It is necessary to predict these forces for reliably operating turbomachinery. So we investigated the effect of shear forces on the calculation of rotordynamic coefficients by comparing the results in the case shear forces are considered and in the case they are neglected. We also compared our results, obtained with the Colebrook–White friction factor model, with some reference experimental and computational results.     

Exact and numerical simulation of peristaltic flow of a non‐Newtonian fluid with inclined magnetic field in an endoscope

In the present article, we have studied the effects of inclined magnetic field on the peristaltic flow of Jeffrey fluid through the gap between two coaxial inclined tubes. The inner tube is rigid, whereas the outer tube has sinusoidal wave traveling down its wall. The governing equations are simplified using long wave length and low Reynolds number approximations. Exact and numerical solutions have been derived for velocity profile. The expressions for pressure rise and friction force are calculated using numerical integration. Graphical results and trapping phenomenon is presented at the end of the article to see the physical behavior of different parameters. Copyright © 2010 John Wiley & Sons, Ltd.     

Unified semi‐analytical wall boundary conditions for inviscid,laminar or turbulent flows in the meshless SPH method

Wall boundary conditions in smoothed particle hydrodynamics (SPH) is a key issue to perform accurate simulations. We propose here a new approach based on a renormalising factor for writing all boundary terms. This factor depends on the local shape of a wall and on the position of a particle relative to the wall, which is described by segments (in two‐dimensions), instead of the cumbersome fictitious or ghost particles used in most existing SPH models. By solving a dynamic equation for the renormalising factor, we significantly improve traditional wall treatment in SPH, for pressure forces, wall friction and turbulent conditions. The new model is demonstrated for cases including hydrostatic conditions for still water in a tank of complex geometry and a dam break over triangular bed profile with sharp angle where significant improved behaviour is obtained in comparison with the conventional boundary techniques. The latter case is also compared with a finite volume and volume‐of‐fluid scheme. The performance of the model for a two‐dimensional laminar flow in a channel is demonstrated where the profiles of velocity are in agreement with the theoretical ones, demonstrating that the derived wall shear stress balances the pressure gradient. Finally, the performance of the model is demonstrated for flow in a schematic fish pass where both the velocity field and turbulent viscosity fields are satisfactorily reproduced compared with mesh‐based codes. Copyright © 2012 John Wiley & Sons, Ltd.     

Wall friction effects and viscosity reduction of gel propellants in conical extrusion

Influence of wall friction on mean apparent viscosity reduction of gel propellants in conical radial flow extrusion is investigated analytically and numerically. A parametric study has been conducted to evaluate the effects of injector geometry, rheological constants, wall friction factor and volumetric flow rate on fluid viscosity profile. Gel propellants were modeled by the non-Newtonian power law constitutive relation. The influence of wall friction on the mean apparent viscosity, on wall slip and on pressure gradient is found to be substantial. Central results are supported by asymptotic analysis for small cone angles and low wall friction. Contact is made with lubrication type flow pattern.     

Computation of shock wave/turbulent boundary layer interactions using a two-equation model with compressibility corrections

In this paper computational results for two different types of shock wave / turbulent boundary layer interaction flows are presented. It is shown that upstream effects of the shock induced separation cannot be reproduced by Wilcox's (1991) k--model, whereas downstream of the interaction, predictions of pressure distribution and skin friction are acceptable. The inclusion of the compressible part of the dissipation rate and the pressure dilatation in the model has noticeable, but not dramatic effects on wall pressure and skin friction in the selected flow cases.     

A note on velocity-profile similarity of turbulent boundary layers with negligible wall stress

Summary The velocity profiles of a turbulent boundary layer with zero skin friction throughout its region of pressure rise, measured by Stratford in 1959, are analyzed in terms of a law of the wall and a velocity-defect law with a common velocity scale and a logarithmic velocity profile in the region of overlap. The analysis deviates from earlier work by Stratford and Townsend. It is shown that the flow in Stratford's boundary layer, even at the largest value of x ₁ at which measurements were taken, is not yet in a state of equilibrium. The velocity scale for turbulent boundary layers with zero skin friction is proportional to the cube root of the pressure gradient.     

Experimental study on influence of microscale effects on liquid flow characteristic in microtubes

Using deioned water as a working fluid, the influence of the microscale effects on liquid flow resistance in microtubes with inner diameters of 19.6 and 44.2 μm, respectively, is experimentally studied. The temperature rise resulted from the microscale effects, such as viscous dissipation, electric double layer, wall rough on the wall surface, etc., is obtained by an IR camera with a special magnified lens adopting micro-area thermal image technology and the corresponding pressure drop and the flux are also measured, so the relationship among friction factor, temperature rise and Reynolds number is obtained. Investigation shows that experimental data are almost equal to those of Hagen–Poiseuille when Reynolds number is low. With the increase of Reynolds number, the values of the friction factor depart from that of classical theory due to the microscale effects. Moreover, the values of the experimental friction factor considering various microscale effects is the maximal 10–15% deviation from that of friction factor without considering various microscale effects with further increase of Reynolds number.     

Die turbulente Grenzschicht an einer stark geheizten ebenen Platte bei Unterschallströmung

Experiments for air flowing over a flat plate heated up to 250°C with velocities of 10 to 30 m/s, which have been made at the DFVLR-AVA, are briefly reviewed and a new analysis of the data is given. The analysis is based on an analytical representation of the velocity and temperature profiles. Close to the wall, a law of the wall approximation is used, which includes the effect of density and viscosity variation. The whole velocity profile is constructed by adding Coles' law of the wake to the law of the wall. In a similar way, the temperature profile is obtained from the law of the wall and an auxiliary distribution. The integrals of momentum and heat flux for two-dimensional flow are used in conjunction with a similarity assumption, to derive a relation between rate of heat transfer from the plate and skin friction. A maximum likelihood procedure has been applied to determine skin friction and rate of heat transfer from the measured dynamic pressure profiles.—The analytical velocity and temperature profiles are found in good agreement with the experimental data, except for the stations near the leading edge of plate. The skin friction coefficients and the Stanton numbers decrease slightly in downstream direction as a consequence of growing local Reynolds number, and decrease with increasing ratio of plate to free stream temperature. The latter fact is in qualitative agreement with the behavior of turbulent boundary layers in supersonic flow. The ratio of Stanton number to half of skin friction coefficient (Reynolds analogy factor) varies with increasing local boundary layer Reynolds number from 1.23 to 1.16.     

A method for estimating wall friction in turbulent wall-bounded flows

We describe a simple method for estimating turbulent boundary layer wall friction using the fit of measured velocity data to a boundary layer model profile that extends the logarithmic profile all the way to the wall. Two models for the boundary layer profile are examined, the power-series interpolation scheme of Spalding and the Musker profile which is based on the eddy viscosity concept. The performance of the method is quantified using recent experimental data in zero pressure gradient flat-plate turbulent boundary layers, and favorable pressure gradient turbulent boundary layers in a pipe, for which independent measurements of wall shear are also available. Between the two model profiles tested, the Musker profile performs much better than the Spalding profile. Results show that the new procedure can provide highly accurate estimates of wall shear with a mean error of about 0.5% in friction velocity, or 1% in shear stress, an accuracy that is comparable to that from independent direct measurements of wall shear stress. An important advantage of the method is its ability to provide accurate estimates of wall shear not only based on many data points in a velocity profile but also very sparse data points in the velocity profile, including only a single data point such as that originating from a near-wall probe.     

Effect of void fraction and friction factor models on the prediction of pressure drop of R134a during downward condensation in a vertical tube

The theoretical flow models of homogeneous and separated flow are applied to in-tube condensation to predict the pressure drop characteristics of R134a. The homogeneous flow model is modified by ten different dynamic viscosity correlations and various alternative correlations of total, frictional and momentum pressure drops to take account of the partial condensation inside the tube. Numerical analyses were performed to determine the average and local homogeneous wall shear stresses and friction factors by means of a CFD program. The equivalent Reynolds number model is modified by six different two-phase friction factors to determine the total condensation pressure drop in the separated flow model. The refrigerant side total pressure drops, frictional pressure drops, friction factors and wall shear stresses are determined within a ±30% error band. The importance of using the alternative total, momentum and frictional pressure drop correlations for the homogeneous flow model is also shown.     

IMPROVEMENT OF THE METHOD OF DETERMINING THE HEIGHT OF ACTION POINT OF COULOMB ACTIVE EARTH PRESSURE

库伦主动土压力作用点位置高度确定为墙高的1/3 处，分析表明这一结论及其推求过程不尽合理.为解决此问题，针对墙后滑动土楔体的受力情况，根据三力汇交原理，考虑墙背土压力的分布与土楔体对其下稳定土体的压力分布的相关关系，采取几何推导方法，得出了主动土压力作用点位置高度表达式；进一步分析了作用点高度对影响因素的敏感性，结果表明其敏感性从大至小的排序为：墙背倾角、内摩擦角、外摩擦角、填土坡角.     

Turbulent Drag Reduction by Uniform Blowing Over a Two-dimensional Roughness

Direct numerical simulation (DNS) of turbulent channel flow over a two-dimensional irregular rough wall with uniform blowing (UB) was performed. The main objective is to investigate the drag reduction effectiveness of UB on a rough-wall turbulent boundary layer toward its practical application. The DNS was performed under a constant flow rate at the bulk Reynolds number values of 5600 and 14000, which correspond to the friction Reynolds numbers of about 180 and 400 in the smooth-wall case, respectively. Based upon the decomposition of drag into the friction and pressure contributions, the present flow is considered to belong to the transitionally-rough regime. Unlike recent experimental results, it turns out that the drag reduction effect of UB on the present two-dimensional rough wall is similar to that for a smooth wall. The friction drag is reduced similarly to the smooth-wall case by the displacement of the mean velocity profile. Besides, the pressure drag, which does not exist in the smooth-wall case, is also reduced; namely, UB makes the rough wall aerodynamically smoother. Examination of turbulence statistics suggests that the effects of roughness and UB are relatively independent to each other in the outer layer, which suggests that Stevenson’s formula can be modified so as to account for the roughness effect by simply adding the roughness function term.     

Isolating curvature effects in computing wall-bounded turbulent flows

An adjoint optimization method is utilized to design an inviscid outer wall shape required for a turbulent flow field solution of the So–Mellor convex curved wall experiment using the Navier–Stokes equations. The associated cost function is the desired pressure distribution on the inner wall. Using this optimized wall shape with a Navier–Stokes method, the abilities of various turbulence models to simulate the effects of curvature without the complicating factor of streamwise pressure gradient are evaluated. The one-equation Spalart–Allmaras (SA) turbulence model overpredicts eddy viscosity, and its boundary layer profiles are too full. A curvature-corrected version of this model improves results, which are sensitive to the choice of a particular constant. An explicit algebraic stress model does a reasonable job predicting this flow field. However, results can be slightly improved by modifying the assumption on anisotropy equilibrium in the model's derivation. The resulting curvature-corrected explicit algebraic stress model (EASM) possesses no heuristic functions or additional constants. It slightly lowers the computed skin friction coefficient and the turbulent stress levels for this case, in better agreement with experiment. The effect on computed velocity profiles is minimal.     

用管轴流速确定动脉中血液振荡流的速度剖面

本文通过求解圆管内血液振荡流的基本方程，求得圆管内血液流的速度与压力梯度之间的关系式，文章提出一种利用管轴外流速计算管内压力梯度，进而确定血液振荡流动速度分布的方法，该方法用于检测活体血管内血液振荡流的速度剖面，具有操作简单，精度较高的优点，最后，以人体颈动脉为例，讨论血液周期振荡流的速度分布特征，发现在任意时刻，除了邻近管壁速度迅速降为零之外，沿管截面速度分布相当均匀，呈现出与定常流不同的速度分布特征。     

Fluid mechanics of microvascular vasomotion and the effects of blood viscoelasticity

This paper deals with blood fiow caused by microvascular vasomotion with the focus on the effects of blood viscoelasticity on the pressure rise and wall resistance. It is shown that rnicrovascular vasomotion plays a role of the "second heart" of the body which is of importance in conveying blood, and that the effects of blood viscoelasticity greatly depend on the Weissenberg number and mean flow rate.     

Boundary layer on the moving surface of a cylinder

A study is made of the problem of the boundary layer on a cylinder with a moving surface when the cylinder moves with constant velocity in an incompressible fluid. Expressions are obtained for the distributions of the frictional stress on the surface of the cylinder and the coordinate of the singular point in the solution of the boundary layer equations that indicates the appearance of a region of reverse flow for different values of the relative velocity of the motion of the surface of the cylinder. Numerical calculations have been made of the work of the force of friction associated with displacement of the cylinder, the work expended on the motion of its surface, and, in the case of flow separation, the work of the pressure forces (it being assumed here that the pressure and friction on the wall behind the singular point are constant and equal to the pressure and friction at the singular point).     

Numerical analysis of turbulent flow separation in a rectangular duct with a sharp 180‐degree turn by algebraic Reynolds stress model

Turbulent flow in a rectangular duct with a sharp 180‐degree turn is difficult to predict numerically because the flow behavior is influenced by several types of forces, including centrifugal force, pressure‐driven force, and shear stress generated by anisotropic turbulence. In particular, this type of flow is characterized by a large‐scale separated flow, and it is difficult to predict the reattachment point of a separated flow. Numerical analysis has been performed for a turbulent flow in a rectangular duct with a sharp 180‐degree turn using the algebraic Reynolds stress model. A boundary‐fitted coordinate system is introduced as a method for coordinate transformation to set the boundary conditions next to complicated shapes. The calculated results are compared with the experimental data, as measured by a laser‐Doppler anemometer, in order to examine the validity of the proposed numerical method and turbulent model. In addition, the possibility of improving the wall function method in the separated flow region is examined by replacing the log‐law velocity profile for a smooth wall with that for a rough wall. The analysis results indicated that the proposed algebraic Reynolds stress model can be used to reasonably predict the turbulent flow in a rectangular duct with a sharp 180‐degree turn. In particular, the calculated reattachment point of a separated flow, which is difficult to predict in a turbulent flow, agrees well with the experimental results. In addition, the calculation results suggest that the wall function method using the log‐law velocity profile for a rough wall over a separated flow region has some potential for improving the prediction accuracy. Copyright © 2007 John Wiley & Sons, Ltd.     

FINITE ELEMENT SURFACE MODEL FOR FLOW AROUND VERTICAL WALL ABUTMENTS

A two-dimensional finite element surface model is developed to determine velocities, depths, and turning angles around vertical wall abutments. The model solves the Reynolds-averaged turbulent flow equations along a horizontal plane passing through the average water surface. This approach is an improvement over the depth-averaged flow models where dispersion terms reflecting vertical effects are neglected. In the model, vertical gradient effects are accounted for through the use of power law for the vertical distribution of the longitudinal velocity; a similar treatment is applied to lateral turbulent shear stresses. The model is capable of computing the dynamic pressure distribution, which in turn is converted to water elevation values. The model, being two dimensional, is computationally efficient and practical to use. The numerical model was successfully verified using experimental data from vertical wall abutments and groins with protrusion ratios (ratio of protrusion length perpendicular to direction of flow to total channel width) of 0·1, 0·2 and 0·3. The results show the occurrence of a high intensity velocity zone close to the upstream abutment nose similar to those observed experimentally. The effects of roughness, depth, and energy slope on the intensity of flow field is investigated and an analytical expression is developed. Numerical experiments indicate that grain roughness affects flow field around the abutment nose by controlling the magnitude of the lateral velocity component and by controlling the lateral extent of the affected zone. Velocity amplification at the abutment nose is found to be mainly related to the protrusion ratio and to the friction factor, and can be up to 1·75 times the approach velocities for protrusion ratios of 0·3. For a protrusion ratio of 0·3, for a typical range of roughness values the increase in nose velocities due to friction factor alone was found to be up to 20 percent.