General Flow Structure Research Articles

Detached eddy simulation of cyclic large scale fluctuations in a simplified engine setup

Computational Fluid Dynamics using RANS-based modelling approaches have become an important tool in the internal combustion engine development and optimization process. However, these models cannot resolve cycle to cycle variations, which are an important aspect in the design of new combustion systems. In this study the feasibility of using a Detached Eddy Simulation (DES) SST model, which is a hybrid RANS/LES model, to predict cycle to cycle variations is investigated. In the near wall region or in regions where the grid resolution is not sufficiently fine to resolve smaller structures, the two-equation RANS SST model is used. In the other regions with higher grid resolution an LES model is applied. The case considered is a geometrically simplified engine, for which detailed experimental data for the ensemble averaged and single cycle velocity field are available from Boreé et al. [Boreé, J., Maurel, S., Bazile, R., 2002. Disruption of a compressed vortex, Physics of Fluids 14 (7), 2543–2556]. The fluid flow shows a strong tumbling motion, which is a major characteristic for modern turbo-charged, direct-injection gasoline engines. The general flow structure is analyzed first and the extent of the LES region and the amount of resolved fluctuations are discussed. Multiple consecutive cycles are computed and turbulent statistics of DES SST, URANS and the measured velocity field are compared for different piston positions. Cycle to cycle variations of the velocity field are analyzed for both computation and experiment with a special emphasis on the useability of the DES SST model to predict cyclic variations.

Structure and mixing of a transient flow of helium injected into an established flow of nitrogen: two dimensional measurement and simulation

The transient injection and mixing between nitrogen and helium in a confined chamber at atmospheric pressure is studied experimentally. The 2D injector and mixing chamber contained a middle injection slot for nitrogen flanked by a pair of outer slots for helium. Experiments were conducted by introducing the helium streams into a previously established quasi-steady flow of nitrogen. The nitrogen stream was seeded with nitric oxide (NO) that served as a source for quantitative, planar laser-induced fluorescence (PLIF) imaging of the transient mixing process. PLIF images were acquired by triggering an Nd:YAG laser system at selected times following helium valve actuation. The observed flow structures and extent of mixing between the two streams proved to be highly unsteady and irregular with the helium/nitrogen jets frequently deviating from the centerline toward the confining walls. Representative unsteady CFD solutions also show this same absence of symmetry and the same general flow structures as the measurements, however, they predict somewhat higher helium concentration in recirculation regions than were observed in the measurements.

Effet du rapport de forme transverse sur l'écoulement tridimensionnel d'un fluide incompressible dans une cavité entraînée

A three-dimensional numerical study has been carried out to determine the effects of the aspect ratio, A y , on the general flow structure in a driven cavity. The numerical method is based on the Finite Volume formulism and multigrid acceleration. For a fixed Reynolds number, Re = 1000 , four span-wise aspect ratios, A y = 1 , 0.5, 0.25 and 0.125, have been considered. The stream lines in the central plane are displayed, relative to the different aspect ratios considered, and show the complexity of the flow when A y becomes low. One sees also that lowering the aspect ratio leads to a clear lowering of the kinetic energy transmission from the casing to the bottom of the cavity. To cite this article: N. Ben Cheikh et al., C. R. Mecanique 336 (2008).

Design, construction and performance of a large tornado simulator for wind engineering applications

A laboratory tornado simulator was designed, constructed and tested to generate tornado-like vortices for the purpose of quantifying tornado-induced aerodynamic loads on civil engineering structures. This simulator generates a vortex that can translate along a ground plane to interact with models of structures on the ground. The simulator uses a “rotating forced downdraft” technique that loosely matches the rear flank downdraft (RFD) phenomenon often emphasized as important to tornadogenesis. The rotating forced downdraft is generated using a 1.83 m diameter fan and an annular duct suspended from an overhead crane that can translate along a 10.4 m ground plane. Measurements were conducted to quantify the simulator’s flow structures. An 18-hole pressure probe was used to traverse and map the vortex flow field. The flow structures of the vortices were validated by comparing with mobile Doppler radar observations of two major tornados. Results show that tornado vortex radii from 0.23 to 0.56 m can be generated. A wide range of maximum tangential velocities are also possible (from 6.9 to 14.5 m/s). The simulator generates a range of vortex structures from single-celled vortices to two-celled vortices with corresponding swirl ratios ranging from 0.08 to 1.14. Comparisons of general flow structure and normalized profiles of tangential velocity showed excellent match between simulator and radar data. The range of tornado sizes together with model structures of 1/100 to 1/500 scale will allow extensive examination of tornado-induced wind loads on terrestrial structures. The effects of a tornado’s size, translation speed and detailed flow structure on these loads can now be studied.

원주상으로 배열된 다중 원형 제트의 유동 특성

This paper describes the flow characteristics of circular multiple jet investigated by hot-wire anemometry. The nozzle arrays were classified into two cases; 6- or 7-nozzle located circumferentially in equal interval without or with a central jet. The flow field was measured according to the number of nozzles when the Reynolds number based on the nozzle exit is about 10⁴. Mean velocity, Reynolds shear stress and turbulent kinetic energy were investigated in the downstream of jets. The Tollmien’s theory holds far downstream only when a nozzle locates at the center. Jet interaction is influenced due to with or without a center nozzle. In addition, the two-dimensional numerical computation was conducted for 3-nozzle case to obtain the general flow structure near the nozzle exit, which verifies the formation of the recirculation region with captive vortices, that is, the evidence of the interaction between jets.

The Alpine Mountain–Plain Circulation: Airborne Doppler Lidar Measurements and Numerical Simulations

Abstract On summer days radiative heating of the Alps produces rising air above the mountains and a resulting inflow of air from the foreland. This leads to a horizontal transport of air from the foreland to the Alps, and a vertical transport from the boundary layer into the free troposphere above the mountains. The structure and the transports of this mountain–plain circulation in southern Germany (“Alpine pumping”) were investigated using an airborne 2-μm scanning Doppler lidar, a wind-temperature radar, dropsondes, rawinsondes, and numerical models. The measurements were part of the Vertical Transport and Orography (VERTIKATOR) campaign in summer 2002. Comparisons of dropsonde and lidar data proved that the lidar is capable of measuring the wind direction and wind speed of this weak flow toward the Alps (1–4 m s−1). The flow was up to 1500 m deep, and it extended ∼80 km into the Alpine foreland. Lidar data are volume measurements (horizontal resolution ∼5 km, vertical resolution 100 m). Therefore, they are ideal for the investigation of the flow structure and the comparison to numerical models. Even the vertical velocities measured by the lidar agreed with the mass budget calculations in terms of both sign and magnitude. The numerical simulations with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) (mesh size 2 and 6 km) and the Local Model (LM) of the German Weather Service (mesh size 2.8 and 7 km) reproduced the general flow structure and the mass fluxes toward the Alps within 86%–144% of the observations.

Near-wake flow structure of elliptic cylinders close to a free surface: effect of cylinder aspect ratio

The flow fields behind elliptic cylinders adjacent to a free surface were investigated experimentally in a circulating water channel. A range of cylinder aspect ratios (AR=2, 3, 4) were considered, while the cross-sectional area of the elliptical cylinder was kept constant. The main objective of this study was to investigate the effect of cylinder aspect ratio and a free surface on the flow structure in the near-wake behind elliptic cylinders. For each elliptic cylinder, the flow structure was analyzed for various values of the submergence depth of the cylinder beneath the free surface. The flow fields were measured using a single-frame double-exposure PIV (Particle Image Velocimetry) system. For each experimental condition, 350 instantaneous velocity fields were obtained and ensemble-averaged to obtain the mean velocity field and spatial distribution of the mean vorticity statistics. The results show that near-wake can be classified into three typical flow patterns: formation of a Coanda flow, generation of substantial jet-like flow, and attachment of this jet flow to the free surface. The general flow structure observed behind the elliptic cylinders resembles the structure previously reported for a circular cylinder submerged near a free surface. However, the wake width and the angle of downward deflection of the shear layer developed from the lower surface of the elliptic cylinder differ from those observed for a circular cylinder. These trends are enhanced as cylinder aspect ratio is increased. In addition, the free surface distortion is also discussed in the paper.

Numerical modelling of airflow over an idealised transverse dune

The general flow structure over transverse aeolian dunes is now well documented through both field studies and wind tunnel experiments. Research on windward (stoss) slopes of dunes is extensive and has recently been complemented by research on the lee-side flow structure. However, a number of technical deficiencies in wind tunnel instrumentation and a lack of detailed resolution in and appropriate turbulence instrumentation for field research have resulted in an incomplete quantified characterisation of the flow structure over aeolian dunes. This study applies a two-dimensional numerical model with an RNG-modified κ- ϵ turbulence model to simulate the time-averaged flow field over an idealized aeolian dune. The model is successfully validated with wind tunnel experimental data. Results indicate that the model accurately predicts the flow patterns over the dune, producing regions of flow stagnation at the toe, acceleration up the stoss slope and a region of flow separation and reversal in the lee. Further development and application of this model will allow examination of flow-form interaction, the testing of more complex isolated dune morphologies, and characterisation of flow over multiple dunes.

Numerical calculation of electrostatic powder painting using the Euler/Lagrange approach

Numerical calculations of gas-particle flows involving an electrical field, as they are found in the powder painting process, are presented based on the Euler/Lagrange approach. The gas flow is calculated by solving the Reynolds-averaged Navier–Stokes equations including the k– ε turbulence model. In order to solve the Laplace equation for the electrostatic field, a finite-volume approach is applied. The particle phase is simulated by using a Lagrangian treatment where a large number of particles are tracked through the flow and the electrostatic field. In addition to the drag and gravity, also, the electric field force is considered in the equation of motion. Average properties of the particle phase are obtained by ensemble averaging. The method is applied to the powder painting process. Two types of powder guns were considered, a slit nozzle and a round nozzle with a dispersion cone. The calculated general flow structure in the vicinity of the painting gun and the large-scale flow within the painting booth agreed reasonably well with the experimental observations. Additionally, the coating layer thickness was used for comparison, which is also relevant to judge the quality of the coating process. The results showed a reasonable good agreement between calculations and measurements for the slit nozzle. However, the extent of the paint layer was remarkably under-predicted for the round nozzle, which is mainly caused by the complex flow in this case. Nevertheless, the results demonstrate that numerical calculations are useful and effective for supporting the design and optimization of powder painting booths.

Flow structure of the wake behind an elliptic cylinder close to a free surface

The flow fields around an elliptic cylinder of axis ratio AR=2 adjacent to a free surface were investigated experimentally using a water channel. The main objective is to understand the effect of the free surface on the flow structure in the near-wake. The flow fields were measured by varying the depth of cylinder submergence, for each experimental condition, 350 velocity fields were measured using a single-frame PIV system and ensemble-averaged to obtain the spatial distribution of turbulent statistics. For small submergence depths a large-scale eddy structure was observed in the near-wake, causing a reverse How near the free surface, downstream of the cylinder. As the depth of cylinder submergence was increased, the flow speed in the gap region between the upper surface of the cylinder and the free surface increased and formed a substantial jet flow. The general flow structure of the elliptic cylinder is similar to previous results for a circular cylinder submerged near to a free surface. However, the width of the wake and the angle of downward deflection of the shear layer developed from the lower surface of the elliptic cylinder are smaller than those for a circular cylinder.

Numerical investigation of supersonic flow past blunt bodies of intricate shape at an angle of attack and slip angle

A supersonic flow of viscous homogeneous gas past blunt bodies of intricate shape at an angle of attack and slip angle is investigated numerically within the model of complete three-dimensional viscous shock layer using the time relaxation method. The main regularities are studied of the general structure of flow and of the distribution of pressure and heat flux along the surface. An analysis is performed of their dependence on the shape of the body, angle of attack and slip angle, Mach and Reynolds numbers, and on other determining parameters of the problem. The accuracy and range of validity of a number of approximate approaches to the solution of the problem are estimated.

Further results relating to axi-symmetric rotational flows of elastico-viscous fluids

Additional results concerning the effect of uniform suction on the axi-symmetric rotational (elasticoviscous) flows caused by an infinite disk (with the fluid at infinity in a state of solid rotation) are presented. 1. In a previous communication of the same main title (Jones and Thomas (1990)) (hereafter referred to as I ) the motion of an elastico-viscous fluid occupying the halfspace z > 0 due to a disk z = 0 rotating steadily about the z-axis with uniform suction applied across its surface was investigated. The results presented in I are now generalized so as to take-in the B6dewadt inverse problem of an infinite (plane) disk at rest in a fluid rotating with uniform angular velocity at large distance z from the disk: in the present regime the disk is supposed to rotate with angular velocity f2 and the fluid at infinity with uniform angular velocity s~2. The purely viscous B6dewadt-type regime has been studied by several authors, notably by Rogers and Lance (1960), and later generalized by Evans (1969) to allow for uniform suction at the disk surface. Their investigations are primarily concerned with the nature of the flow at large negtive values of s (i.e. with the counter-rotation regime) and on how the flow is affected by the suction mechanism.. What is shown is that for sufficiently small (negative) values of s the disk acts as a centrifugal fan, the flow being directed outward away from the disk axis and downward toward the disk (i.e. a flow having positive radial velocity and negative vertical velocity). As s increases negatively some radial flow reversal begins to showup in the far-flow field, whilst near the disk the flow, although radially outward, becomes progressively weaker. Eventually, the centripetal radial pressure gradient appropriate to the far-flow field becomes the dominant flow mechanism and results in a radial inflow near the disk (i.e. a flow with negative radial velocity and positive vertical velocity): above a critical negative value s = s o the high shear boundary layer that is adjacent to the disk disattaches itself from the disk. The effect of suction applied at the surface of the disk is found to counterbalance somewhat the radially induced pressure gradients, reducing substantially the radial and circumferential velocity components and postponing boundary-layer disattachment to higher (negative) values of s. The purpose of the present note is to ascertain how the elasticity of the fluid, as characterized by the Oldroyd number, affects this general flow structure. It is no more than an exploratory analysis, and may, possibly, make a case for a more detailed numerical study of the problem. 2. The analysis follows closely that of L Again the working elastico-viscous prototype the generalized Maxwell fluid is of the class for which the yon Kfirm~m rotational similarity-type flows are possible, the governing equations reducing in this case to a set of ordinary, non-linear differential equations*). The appropriate kinematic and dynamic variables are non-dimensionalized as in I(6)**), the angular velocity f2 of the disk being chosen as the characteristic velocity. It then follows that the equation of continuity I (7) together with the rheological equations of state I (8) remain unaltered. But to accommodate the changed conditions in the far-flow field, the equations of motion and the associated boundary conditions, viz. Eqs. I(9), I(10), are to replaced by

Flow measurements in a model ramjet secondary combustion chamber

Experimental studies were conducted on a typical secondary combustion chamber of a ramjet to understand the influence of various inlet parameters such as primary nozzle configuration, secondary air injection angle, and flow Reynolds numbers on the secondary combustion chamber (SCC) performance. Cold flow studies were made with air as the flow medium for both primary and secondary jets followed by similar studies with hot primary jets. The general flow structure in the SCC obtained from surface oil film technique showed recirculation zones near the head end. The combustor length required for jet mixing was found to be unrelated to recirculation zone length confirmed by selective temperature and total pressure profile measurements. The calculated fFictional loss from the momentum balance consideration was found to be small. That significant improvement in mixing can be achieved by a choice of multiple-hole primary nozzle configuration has been demonstrated.

The measurement of high-speed unlubricated metallic friction : K. Williams, Second Convention on Lubrication and Wear, 1964, Paper 1. To be published in the Proc. Inst. Mech. Engrs., London, 1964/1965; 7 figs., 13 refs.

In-cylinder air flow structures are known to play a major role in mixture preparation and engine operating limits for spark-ignition engines. In this paper PIV measurements were undertaken in an optical spark-ignition at 1500 RPM, part-load with 0.5 bar intake plenum pressure. One of the PIV planes was vertical, cutting through the centrally located spark plug (tumble plane). The other plane was horizontal 1 mm below the spark plug ground electrode (swirl plane). The effect of engine head temperature was also examined by using engine-head coolant temperatures of 20 °C and 80 °C. The flow field was examined late in the compression stroke at typical ignition timing. The study was conducted under air-only motoring engine conditions but also under fuelled conditions in the early intake stroke using direct injection of gasoline, iso-octane, ethanol and butanol fuels. The flow field under air-only motored conditions showed velocities between 3 and 5 m/s predominantly from intake to exhaust. Little differences were observed between hot and cold engine-head temperature; typically ∼10% larger mean velocity and turbulent kinetic energy was seen on the intake side. Integral length scales were on the tumble plane between 2 and 4.5 mm in the vertical and 4–7 mm in the horizontal direction. The swirl view showed scales between 4 and 10 mm, larger at cold than at hot engine-head conditions. The vertical length scales appeared to be limited by the clearance height, scaling typically by about 10–15%. The horizontal components scaled to the cylinder bore diameter by about 5–12%. The fuel injection process in the early intake stroke led to little differences in the general mean flow structure at ignition timing, except for a small increase in the maximum velocities, ∼10%. The turbulent kinetic on the tumble plane was highest towards the exhaust side of the engine under non-fuelled engine conditions but fuel injection resulted in highest values on the intake side. The integral length scales with fuel injection were of the same order of magnitude to those of air only measurements on the tumble plane, but showed distinctly larger areas with length scales up to 9 mm on the swirl plane. Differences between fuels for their fuel specific injection duration were small, with average length scales between 4 and 6 mm. Ethanol exhibited typically largest scales and butanol smallest.

On certain points in the construction of marine boilers

A laboratory tornado simulator was designed, constructed and tested to generate tornado-like vortices for the purpose of quantifying tornado-induced aerodynamic loads on civil engineering structures. This simulator generates a vortex that can translate along a ground plane to interact with models of structures on the ground. The simulator uses a “rotating forced downdraft” technique that loosely matches the rear flank downdraft (RFD) phenomenon often emphasized as important to tornadogenesis. The rotating forced downdraft is generated using a 1.83 m diameter fan and an annular duct suspended from an overhead crane that can translate along a 10.4 m ground plane. Measurements were conducted to quantify the simulator’s flow structures. An 18-hole pressure probe was used to traverse and map the vortex flow field. The flow structures of the vortices were validated by comparing with mobile Doppler radar observations of two major tornados. Results show that tornado vortex radii from 0.23 to 0.56 m can be generated. A wide range of maximum tangential velocities are also possible (from 6.9 to 14.5 m/s). The simulator generates a range of vortex structures from single-celled vortices to two-celled vortices with corresponding swirl ratios ranging from 0.08 to 1.14. Comparisons of general flow structure and normalized profiles of tangential velocity showed excellent match between simulator and radar data. The range of tornado sizes together with model structures of 1/100 to 1/500 scale will allow extensive examination of tornado-induced wind loads on terrestrial structures. The effects of a tornado’s size, translation speed and detailed flow structure on these loads can now be studied.