Outer Flow Field Research Articles

Modeling of Heat Transfer Attenuation by Ablative Gases During the Stardust Reentry

Modern space vehicles designed for planetary exploration use ablative materials to protect the payload against the high heating environment experienced during reentry. To properly model and predict the aerothermal environment of the vehicle, it is imperative to account for the gases produced by ablation processes. The present study aims to examine the effects of the blowing of ablation gas in the outer flow field. Using six points on the Stardust entry trajectory at the beginning of the continuum regime, from 81 to 69 km, the various components of the heat flux are compared to air-only solutions. Although an additional component of the heat flux is introduced by mass diffusion, this additional term is mainly balanced by the fact that the translational–rotational component of the heat flux, the main contributor, is greatly reduced. Although a displacement of the shock is observed, it is believed that the most prominent effects are caused by a modification of the chemical composition of the boundary layer, which reduces the gas-phase thermal conductivity.

Study of Truck’s Aerodynamic Drag Reduction Based on Jet

Multiple schemes are adapted on truck's outer flow field based on numerical simulation. Comparative analysis with the state of air flow, the pressure distribution, the air movement between the cab and cargo is pursued, then obtain the effect of jet flow velocity to the truck Cd. With the increasing of the jet velocity, Cd increases first and then decreases. The maximum drag reduction can reaches 7.38%.

Numerical Simulation of Flow Field of Air-Jet in Quenching Process

The type of atomize-cool and air-cool are widely used in the world now. Air-jet is a very vital instrument in quenching by air-cooling. t is necessary to simulate the inner and outer flow-field of air-jet. By means of computational fluid dynamics soft system Fluent, numerical simulation of quenching air cooling process of air-jet was carried out. The work can support the actual production.

Preliminary Study on Streamlined Design of Longitudinal Profile of High-speed Train Head Shape

The effect of high-speed train head shape on its aerodynamic performance is very important. In order to prove the effect of longitudinal profile shape of train head on aerodynamic performance, using CFD method, a two dimension model is developed to investigate outer flow field of high speed train head. Three types of train head profile schemes are calculated. The effects of longitudinal profile shape of high speed train head on aerodynamic performance are analyzed. The results show that different head profile curves cause different surface pressure distributions of body, different airflow velocities of outer flow field and different turbulent distributions. If the longitudinal profile shape of train head is spindle and streamline length of train head is 8m, aerodynamic drag of the train will well reduce.

The impact of inlet angle and outlet angle of guide vane on pump in reversal based hydraulic turbine performance

In this paper, in order to research the impact of inlet angle and outlet angle of guide vane on hydraulic turbine performance, a centrifugal pump in reversal is adopted as turbine. A numerical simulation method is adopted for researching outer performance and flow field of turbine. The results show: inlet angle has a crucial role to turbine, to the same flow, there is a noticeable decline for the efficiency and head of turbine with the inlet angle increases. At the best efficiency point(EFP),to a same inlet angle, when the inlet angle greater than inlet angle, velocity circulation in guide vane outlet decreases, which lead the efficiency of turbine to reduce, Contrarily, the efficiency rises. With the increase of inlet angle and outlet angle, the EFP moves to the big flow area and the uniformity of pressure distribution becomes worse. The paper indicates that the inlet angle and outlet angle have great impact on the turbine performance, and the best combination exists for the inlet angle and outlet angle of the guide vane.

Simulation of tank sloshing based on OpenFOAM and coupling with ship motions in time domain

Tank sloshing in ship cargo is excited by ship motions, which induces impact load on tank wall and then affects the ship motion. Wave forces acting on ship hull and the retardation function are solved by using three-dimensional frequency domain theory and an impulse response function method based on the potential flow theory, and global ship motion is examined coupling with nonlinear tank sloshing which is simulated by viscous flow theory. Based on the open source Computational Fluid Dynamics (CFD) development platform Open Field Operation and Manipulation (OpenFOAM), numerical calculation of ship motion coupled with tank sloshing is achieved and the corresponding numerical simulation and validation are carried out. With this method, the interactions of wave, ship body and tank sloshing are completely taken into consideration. This method has quite high efficiency for it takes advantage of potential flow theory for outer flow field and viscous flow theory for inside tank sloshing respectively. The numerical and experimental results of the ship motion agree well with each other.

The drag on a flattened bubble moving across a plane substrate

AbstractThe equilibrium shape of an axisymmetric liquid drop or gas bubble in an immiscible supporting liquid held under gravity against a horizontal plane rigid surface is derived. A thin film of supporting liquid remains between the drop/bubble surface and the rigid substrate at equilibrium, of thickness ${h}_{0} $ determined by the balance of the disjoining pressure $\Pi (h)$ between the drop and the substrate and the internal Laplace pressure of the drop/bubble. The interface is macroscopically flat around the drop axis out to a radius $a$ but matches smoothly into the outer shape of radius $A$ through a boundary layer region of width $a{ H}_{0}^{1/ 2} $ where ${H}_{0} = 2{h}_{0} A/ {a}^{2} $ is a small parameter. The outer drop shape is determined by a balance of buoyancy forces and local Laplace pressure and is roughly spherical if $A/ \lambda \ll 1$, where $\lambda = \sqrt{\gamma / \mrm{\Delta} \rho g} $ is the capillary length in the interface with a logarithmic correction due to the action of the disjoining pressure across the flattened region. With the shape determined, we calculate the drag force on this flattened bubble to lowest order in the velocity as it moves across the rigid substrate using a lubrication approximation valid to terms of $O( \mathop{ ({h}_{0} / a)}\nolimits ^{2} )$ as an integral over the flattened bubble surface of the hydrodynamic pressure. The lubrication theory of itself is not sufficient to determine the drag due to the divergence of that integral if the outer flow field properties are neglected. By using the known exact result for the drag force on an undistorted bubble, the drag on the flattened bubble can be computed as an integral over the lubrication region alone. We derive the drag as a series expansion in the small parameter ${H}_{0} $ by means of a fairly intricate boundary layer analysis. The logarithmic divergence of the translational drag with film thickness ${h}_{0} $ for the undistorted bubble is replaced by the stronger divergence ${ h}_{0}^{\ensuremath{-} 1/ 2} $ to leading order for the flattened bubble case. We present explicit numerical results for the first few terms in the expansion for the case of an exponentially repulsive disjoining pressure, and analytic expressions for these terms in the limit of very short-range disjoining pressure forces. The results of this calculation are compared with recent work (Hodges, Jensen & Rallison, J. Fluid Mech., vol. 512, 2004, p. 95) where disjoining pressure is neglected and hydrodynamic pressure is balanced against buoyancy and Laplace forces. The limits of validity of this linear drag theory are also presented.

On the drag force of a viscous sphere with interfacial slip at small but finite Reynolds numbers

We investigate the hydrodynamic drag force on a viscous sphere in a fluid of different viscosities at small but finite Reynolds numbers when interfacial slip is present at the surface of the sphere. The sphere is small enough for it to retain its spherical shape, as is the case with most small droplets. By using a singular perturbation method, the exterior flow field of the droplet is decomposed into an inner region, where the viscous effects dominate, and an outer region, where the inertia is important. The interior flow of the viscous sphere is also solved analytically. By applying appropriate boundary conditions to the surface of the viscous sphere and matching the conditions between the inner and outer flow fields, stream functions up to the order of Re2 log Re for both the exterior and the interior flow are obtained. Thus, an analytical expression for the drag force coefficient of the viscous droplet is derived. This general expression yields, as special cases, several other expressions that are applicable to spheres that translate rectilinearly under more restrictive conditions. One of the practical conclusions from this study is that the presence of interfacial slip can significantly reduce the drag force on a droplet.

Investigation of a new flux scheme for the numerical simulation of the supersonic intake flow

A numerical code for supersonic intake design with a proper simulation of the normal and/or oblique shocks, boundary layer development, interaction of the shock and the boundary layer, as well as prediction of the flow separation is of great help to the designers. In this research, a numerical code is developed to solve the inner and outer flow fields of the intake and validated with various experimental tests. The intake is an axisymmetric external compression one. Roe scheme and new schemes, AUSM+-up (for all speed) and Advection Upstream Splitting Method with Pressure-Based Weight function (AUSMPW), are used to compute the convective fluxes. The original version of the AUSMPW scheme has serious problems concerning the stability and accuracy. In this investigation, several modifications are implemented to the original version of the AUSMPW scheme for the first time which resulted in a better stability and accuracy. Turbulent viscosity coefficient is computed using the Baldwin–Lomax algebraic model. The numerical results are compared well with the experimental data. It is further found that the Roe scheme has the best accuracy; however, the AUSM+-up (for all speed) scheme is numerically very efficient.

Flow Characteristics of Gas Permeation through a PVF Porous Tube

AbstractPVF (polyvinyl formal) porous materials have attractive properties, such as noise attenuation, good structural integrity, thermal and chemical stability, high permeability and large specific surface area, for many flow‐through applications. Several characteristics of the porous material will have an impact on the permeability, and gas flow and diffusion. However, the shape and the design of the device may also have significant impact on the gas flow. A porous media model and Darcy‐Forchheimer principle were used as the basic theoretical frame. The unified governing equations were used to describe the compressible flow in and out of a PVF porous tube. A robust NND numerical scheme was used to discretize the equations and the TDBC (time‐dependent boundary conditions) were used to treat the nonreflective boundaries. Numerical simulations of an interior and exterior flow field of a PVF porous tube were completed. The detailed flow characteristics of the inner and outer flow fields of the tube were obtained. The velocity distribution of the outside of the tube compare very well with the experimental data.

Numerical simulation of the flow field of a diffused pneumatic silencer

A diffused pneumatic silencer had been widely used in the pneumatic fields due to its small dimensions and high level of performance in noise reduction. A numerical simulation of its interior and exterior flow field was important for studying the gas flow in the silencer and the flow structure outside the silencer, as well as for understanding the mechanism of the silencer’s noise reduction. A porous media model and the Darcy–Forchheimer principle were used as the basic theoretical models in this paper. The unified governing equations were used here to describe the compressible flow in and out of the silencer. A robust numerical scheme was used to discritize the equations, and the TDBC (Time-dependent boundary conditions) was used to treat the non-reflecting boundaries. The detailed structures of the inner and outer flow fields of the diffused pneumatic silencer were obtained. The simulation results displayed the characteristics of the flow in the silencer. The nature of the flow outside the silencer, comparable with the experimental data, was also obtained.

Hydraulic Darrieus turbines efficiency for free fluid flow conditions versus power farms conditions

The present study deals with the efficiency of cross flow water current turbine for free stream conditions versus power farm conditions. In the first part, a single turbine for free fluid flow conditions is considered. The simulations are carried out with a new in house code which couples a Navier–Stokes computation of the outer flow field with a description of the inner flow field around the turbine. The latter is based on experimental results of a Darrieus wind turbine in an unbounded domain. This code is applied for the description of a hydraulic turbine. In the second part, the interest of piling up several turbines on the same axis of rotation to make a tower is investigated. Not only is it profitable because only one alternator is needed but the simulations demonstrate the advantage of the tower configuration for the efficiency. The tower is then inserted into a cluster of several lined up towers which makes a barge. Simulations show that the average barge efficiency rises as the distance between towers is decreased and as the number of towers is increased within the row. Thereby, the efficiency of a single isolated turbine is greatly increased when set both into a tower and into a cluster of several towers corresponding to possible power farm arrangements.

Viscid contributions to the hydrodynamic flows past a rising spherical gas bubble with a time-dependent radius

We study the effect of the viscosity on the hydrodynamic flow fields past the interface of a spherical deforming gas bubble impulsively started at a constant velocity in a viscous liquid of large extent at rest. Exact solutions for the unsteady inner and outer flow fields within the boundary layers are obtained making appropriate scalings on the position, velocity and time variables in the non-linear Navier–Stokes equations. These theoretical results apply to any slowly deforming fluid sphere, whatever the time-dependence of its radius, provided that the internal circulation is complete, the flow separation is negligible, the Reynolds numbers are large and the bubble retains its spherical shape. A numerical application to the case of deforming air bubble in water is discussed.

Calculation of Flow Field of Diffused Pneumatic Silencer

Diffused pneumatic silencer has been widely used in pneumatic fields because of its small dimensions and high performance on noise reduction. Numerical simulation of its interior and exterior flow field is important to studying the gas flow in the silencer and the flow structure outside the silencer, understanding the mechanism of its noise reduction. Porous media model and Darcy-Forchheimer principle are used as the basic theoretical frame in this paper. The unified governing equations are used to describe the compressible flow in and out of the silencer. Robust numerical scheme is used to discritize the equations and the TDBC(Time-dependent boundary conditions) is used to treat the non-reflect boundaries. The detailed structures of the inner and outer flow field of the diffused pneumatic silencer are gotten. The results of the simulation display the characteristics of the flow in the silencer. And the structure of the flow outside the silencer that can be compared with the experimental data is gotten.

Breakup in stochastic Stokes flows: sub-Kolmogorov drops in isotropic turbulence

Deformation and breakup of drops in an isotropic turbulent flow has been studied by numerical simulation. The numerical method involves a pseudospectral representation of the turbulent outer flow field coupled to three-dimensional boundary integral simulations of the local drop dynamics. A statistical analysis based on an ensemble of drop trajectories is presented; results include breakup rates, the distribution of primary daughter drops produced by breakup events, and stationary distributions for drop deformation and orientation. Depending on the local flow history, drops may break at modest length or become highly elongated and relax without breaking. Drop deformation is the dominant mechanism of drop reorientation. The volume of the primary daughter drops, produced by a given fluctuation in flow strength, scales with the volume of the corresponding critical drop size for the fluctuation. A simplified description for the evolution of the drop size distribution, based on this scaling, is presented.

Heat transfer and thermal behaviour of a rotating disk passed by a planar air stream

The aerodynamics of a thin, but finite, free rotating disk in an outer forced flow field with constant mean velocity parallel to the disk plane are investigated numerically. The heat transfer coefficients and their distributions on the isothermal disk surface are calculated for several velocities in case of air. Appropriate correlations are derived for the Nusselt number as function of the involved similarity numbers. The idealised model is faced also with the heat transfer problem of an actual motorbike disk brake. The transient heat conduction within rotating disks are treated analytically using the integral-transform technique. This enables even for heat sources which are very short in space a certain estimation of the azimuthal variations of the temperature field due to the rotation.

Wave propagation in a mixed convection boundary-layer over a horizontal surface

The propagation of disturbances in a mixed convection boundary-layer flow over a horizontal plate is described by a triple deck problem in the case of the buoyancy parameter being small. The pressure correction in the lower deck consists of two parts: One due to the buoyancy effects in the main deck and one due to the displacement of the outer flow field. The response of the boundary layer flow to an oscillator of frequency ω will be computed and upstream travelling waves will be identified.

Analytical study for hypersonic flow past an elliptic cone with longitudinal curvature

For hypersonic flow past an elliptic cone with longitudinal curvature, the outer-expansion analysis of shock layer and the inner-expansion analhsis of vortical layer are two major subjects in this research. The zeroth-order approximation of hypersonic conical flow obtained by nonlinear asymptotic theory is chosen as the basic-cone solution for the outer and inner expansions. In the outer analysis of shock layer, the complicated governing equation of outer flowfield can be simplified by an appropriate approximation scheme, and the first-order approximations of properties are derived. In sequence, to study the phenomenon in vortical layer, the inner expansion based on the outer solutions is proceeded near the cone surface. Thereafter, in accordance with the asymptotic matching principle, the uniformlyvalid approximation can be obtained and expressed in closed form. Moreover, it is verified that the perturbation pressure and azimuthal velocity from outer solutions are actually the perturbation expansion values from inner solutions. However, the analysis of inner solutions illustrates apparently the rapid variations on entropy and density of the vortical layer. These results enable us to understand well about the flow characteristics in the entire shock layer, and may provide an important guide for grid arrangement to accomodate the dramatic adjustment occurring in the vortical layer.

The influence of higher-order effects on the linear instability of thermal boundary layer flow in porous media

We examine the instability of free convective boundary layer flows in porous media. The medium is bounded by two semi-infinite plane surfaces forming a wedge of angle α. One of the surfaces is heated uniformly and the other is either cold or insulated. The basic flow used in the analysis is the most accurate obtainable by means of higher order boundary layer theory. In general, the critical distance from the leading edge beyond which disturbances grow is found to be strongly dependent on the outer flow field. The only exception to this arises when the heated surface is close to the vertical and if the wedge is not too close to either 0° or 360°. The main implication of this paper is that instability occurs too close to the leading edge for the basic flow to be represented adequately either by the leading order boundary layer theory used in previous papers, or by even the most accurate higher order theory obtained using matched asymptotic expansions.

The inner and outer flow fields due to a pair of spheres moving unsteadily through a compressible fluid: rectilinear motion

Two rigid spheres with equal radii a 0 move with variable speed along their line of centres, through an inviscid compressible fluid. The acoustic velocity potential is determined asymptotically when a typical speed u 0 of the spheres is small compared with the mean sound speed c 0 and the separation of the two spheres varies on a time scale t 0 = a 0 /u 0 . Inner and outer asymptotic approximations are matched and the distant sound field is expressed in terms of point sources of monopole, dipole and quadrupole type.