Mainstream Velocity Research Articles

Study of premixed flame stabilization by using opposed jet

A preliminary study on the criterion for opposed jet flame stabilization is reported. The experimental investigation consists of the measurement of flame blowoff by means of premixed lean liquefied petroleum gas and the temperature distribution. The criterion for flame stabilization has been obtained. It is correlated with main stream velocity VPm, mass flux ratio of jet to main streamF, internal diameter of jet nozzled; and equivalent ratio of fuel-air ϕ. The stabilization character of opposed jet flame is described as a well-stirred reactor, and the critical zone acts as a strong ignition source to maintain flame stabilization. A criterion for flame stabilization by theoretical analyses is in good accordance with experimental results.

Application of the k—ε model to flow over a building placed in different roughness sublayers

Predictions of three-dimensional airflows over a building-like model with complex boundaries by solving the Reynolds averaged Navier-Strokes equations are presented. The obstacle is placed in boundary layers characterized by different roughness, and which have a main stream velocity component at different angles to the object. The pressure distributions are compared to wind-tunnel measurements. The effect of turbulent characteristics of the oncoming flow on pressure distribution is shown out-lined through the simulation of various roughness sublayers. The different results suggest that only the base pressure distribution is seriously affected by turbulence intensity and surface roughness. These two parameters are found to be of major importance in reproducing accurately a natural boundary layer. Additionally, the calculations have shown that the flow field is very sensitive to the specification of inflow conditions for the rate of dissipation of turbulent kinetic energy.

Effect of a Crossflow at the Entrance to a Film-Cooling Hole

Understanding the complex flow of jets issuing into a crossflow from an inclined hole that has a short length-to-diameter ration is relevant for film-cooling applications on gas turbine blades. In particular, this experimental study focused on the effect of different velocities in a coflowing channel at the cooling hole entrance. Flows on both sides of the cooling hole (entrance and exit) were parallel and in the same direction. With the blowing ratio and the mainstream velocity at the hole exit remaining fixed, only the flow velocity in the channel at the hole entrance was varied. The Mach number at the hole entrance was varied between 0 < Mac < 0.5, while the Mach number at the hole exit remained constant at Ma∞ = 0.25. The velocity ratio and density ratio of the jet were unity giving a blowing ratio and momentum flux ratio also of unity. The single, scaled-up film-cooling hole was inclined at 30 deg with respect to the mainstream and had a hole length-to-diameter ratio of L/D = 6. Flowfield measurements were made inside the hole, at the hole inlet and exit, and in the near-hole region where the jet interacted with the crossflow at the hole exit. The results show that for entrance crossflow Mach numbers of Mac = 0 and 0.5, a separation region occurs on the leeward and windward side of the cooling hole entrances, respectively. As a result of this separation region, the cooling jet exits in a skewed manner with very high turbulence levels.

FLOW VISUALIZATION AROUND SINGLE‐ AND MULTIPLE‐BLADED SEAWEEDS WITH VARIOUS MORPHOLOGIES1

ABSTRACTWater flow was visualized around 10 seaweeds with various morphologies to determine the onset of turbulence and to estimate the scales of motion generated by the seaweeds themselves. For single‐bladed specimens of the kelps Laminaria setchellii, Costaria costatum, Macrocystis integrifolia, and Alaria marginata, the transition from a laminar to turbulent velocity boundary layer occurred at mainstream velocities of 1.5 cm.s−1. Transition to turbulence fm multiple‐bladed specimens of M. integrifolia, Nereocystis luetkeana, Egregia menziesii, and Fucus gardneri occurred at 2.5–3 cm‐s−1 and at 5 cm.s−1 for the coarsely branched red seaweed, Gelidium coulteri. Flow features such as separation, recirculating eddies, and Von Kánncán vortex streets were observed around various morphologtcal features. We suggest that in the field, flow around larger macroalgae such as kelp is mostly turbulent and that many seaweeds will lie within the wakes of neighboring macroalgae. For small, branched seaweeds such as G. coulteri, however, the meshlike structure may damp turbulence within the thallus, thereby increasing the mainstream velocity at which the transition to turbulence occurs.

On pattern selection in mixed convection in rectangular ducts

Mixed convection in a horizontal rectangular duct has the same critical Rayleigh number as natural convection in a rectangular cavity for the onset of convection. The linear stability analysis predicts either an odd or an even number of convective rolls to appear depending on the aspect ratio of the cross section. However, it has been shown both experimentally and numerically that an even number of convective rolls appears under supercritical conditions for fully developed mixed convection. The paper first presents an analytical solution for the buoyancy-induced mainstream velocity, w b , at the onset of buoyancy-induced motion in a forced convective flow. Then, a comparison in the initial growth rate of w b is made between the case of an odd and an even number of rolls; which shows the selection of an even number of rolls over an odd number in mixed convection except for low aspect ratio ducts.

Manifestations of the Black Sea undercurrent in the density and hydrostatic pressure fields

Vertical kinematic structure of the geostrophic currents has been found to be correlated to the distribution of seawater density and hydrostatic pressure over depth. The conditions necessary for the existence of the undercurrent have been defined. On this basis, hydrological survey data compiled by the Marine Hydrophysical Institute in the Black Sea have been treated. Examples are given, which support theoretical deductions about the possible existence of the undercurrent beneath the Black Sea rim current. The undercurrent's parameters have been determined, along with the depths of the upper and lower boundaries and the mainstream velocity.

Effect of Film Hole Row Location on Film Effectiveness on a Gas Turbine Blade

Experiments were performed to study the effect of film hole row location on local film effectiveness distribution of a turbine blade model with air or CO2 film injection. Tests were performed on a five-blade linear cascade at the chord Reynolds number of 3.0 × 105 at the cascade inlet. A combination of turbulence grid and unsteady wake was used to create a higher free-stream turbulence level. The test blade had three rows of film holes in the leading edge region and two rows each on the pressure and suction surfaces. Film hole row locations were set by leaving the film holes at that row location open and covering the remaining rows. In addition, the additive nature of film cooling on the turbine blade model was examined by comparing the measured film effectiveness with the predicted effectiveness from the superposition method. Results show that injection from a different film hole row location provides a different effectiveness distribution on pressure and suction surfaces depending on local mainstream velocity and blade curvature. In most cases, the superposition method holds downstream of the last film hole row.

Local size distributions of particles deposited by inertial impaction on a cylindrical target in dust-laden streams

We exploit recent developments on impinging single particle capture laws and rational correlations for inertial impaction on a circular cylinder in high Reynolds number crossflow [Israel and Rosner (1983) Aerosol Sci. Technol. 2, 45–51; Wessel and Righi (1988) Aerosol Sci. Technol. 9, 26–60] to predict the local size distribution of particles deposited by impaction on a cylindrical target when the mainstream particle suspension is “log-normal”. Because of both the aerodynamics of selective impingement, and the nature of the sticking/rebound law, we show that the granular deposit particle size distribution (hereafter abbreviated (PSD) w) is generally quite different from mainstream particle size distribution (PSD) ∞, by so much that (PSD) w generally cannot be characterized accurately by single-mode log-normal distribution parameters. Apart from its relevance in correcting for systematic errors in aerosol sampling from high-speed streams, this local variation of the “granular deposit” PSD along with information on deposit morphology, must be known (in addition to the total mass accumulated per unit area) to predict, say, the loss in convective heat transfer rate associated with the growth of a fouling layer. Three distinct classes of single solid particle capture laws are considered: constant capture fraction (independent of impinging particle velocity and angle of incidence), “on-off” capture behavior expected for impaction on a clean, particle-free, smooth solid surface, and particle capture on a dry, sufficiently thick, granular deposit. Our (PSD) w results are cast in terms of following accessible dimensionless parameters: sensitivity of capture fraction to particle incident velocity and angle, ratio of mainstream velocity to the critical (threshold) velocity for particle rebound (at, say, normal incidence), ratio of mean particle size in the mainstream to the critical size required for impaction on a cylindrical target in crossflow, spread of log-normal mainstream particle size distribution, and the characteristic “slip” Reynolds number for the critical size particle in the mainstream.

Rational prediction of inertially induced particle deposition rates for a cylindrical target in a dust-laden stream

We exploit recent developments on single-particle capture laws (Rosner et al., 1992, 1994) and rational correlations for inertial impaction on a circular cylinder (Israel and Rosner, 1983; Wessel and Righi, 1988) in high Reynolds number crossflow to predict local deposition rates on heat exchanger tubes immersed in polydispersed particle-laden streams of combustion products. Three distinct classes of single-particle capture laws are explicitly considered: constant capture fraction, s (independent of impinging particle velocity and angle of incidence); “on-off” capture behavior expected on a clean, particle-free smooth surface; and capture on a dry, sufficiently thick granular deposit. Our “universal” non-dimensional results are cast in terms of the following accessible parameters: sensitivity of capture fraction to particle incident velocity and angle, ratio of mainstream velocity to the threshold velocity for particle rebound, ratio of mainstream mean particle size to critical size required for impaction on a cylindrical target, spread of log-normal mainstream particle size distribution and the characteristic slip Reynolds number for the critical size particle (for impaction) in the mainstream. Applications of our results and their generalizations are discussed. We also predict the convective heat transfer reduction associated with local fouling layer growth on heat exchanger surfaces, as well as the sensitivity of the deposition rate to several system parameters (gas velocity U, target diameter d t ,…) under the control of the designer.

The Oseen Drag at Infinite Reynolds Number

It is shown that the Oseen drag on an arbitrarily shaped object at infinite Reynolds number is the sum of two contributions. The first contribution is the product of the object’s cross section perpendicular to the main stream direction and $\rho U^2 $, where $\rho $ is the constant fluid density and U the main stream velocity. The second contribution is the pressure drag on the rearward side of the object. The pressure distribution can be obtained by a generalization of Stewartson’s calculational scheme for the sphere.

An experimental study on the two-dimensional opposed wall jet in a turbulent boundary layer: Change in the flow pattern with velocity ratio

Results of the measurement of flow properties in a two-dimensional turbulent wall jet which is injected into the turbulent boundary layer in the direction opposite to that of the boundary layer flow are presented by varying the ratio of the jet issuing velocity to the mainstream velocity λ. This flow includes the flow separation and the recirculating flow, and so it requires the magnitude and direction of instantaneous velocity be measured. In the present work, a tandem hot-wire probe is manufactured and its characteristics are examined experimentally. With the use of this probe the change in the penetration length of the jet with the velocity ratio is investigated. It is clarified that two regimes of flow patterns exist: in the low velocity ratio the penetration length of the jet changes little with λ, and in the high velocity ratio it goes far from the nozzle with increasing λ. Streamlines, turbulence intensity contours and static pressure contours are presented in the two typical velocity ratios corresponding to each of two flow patterns, and they are compared.

Lifting-Line Theory of a Supercavitating Hydrofoil in Both Vertical and Spanwise Shear Flow.

A supercavitating hydrofoil placed in vertical and spanwise shear flow between two parallel planes is approximated to a lifting-line. By introducing two kinds of separation of variables into both a fundamental solution and a main stream velocity distribution, an equation of motion with respect to disturbance pressure is reduced to two ordinary differential equations. The lifting-line equation is obtained from up-wash velocity distribution along a lifting-line derived from solutions of ordinary differential equations. Hydrodynamic force characteristics are expressed and calculated by series summation, whose spectra are solutions of the lifting-line equation, and then cavity distributions are obtained by applying the method proposed by the authors.

NUMERICAL PREDICTION OF A RECTANGULAR TURBULENT JET IN A CROSS FLOW

The trajectories and decay of the maximum velocity for turbulent jets in a cross flow were simulated by means of the κ — ϵ two-equation turbulence model in which differential equations were solved for the kinetic energy of turbulence and for the rate of its dissipation. The solution procedure employed an elliptic finite-difference scheme with the three velocity components and the pressure as the main dependent variables. The essential properties of the jet were discussed in the case of different ratios of the rectangular orifice length to the width H/B, ratios of the jet injected velocity to the main stream velocity R; and different injection angles α. The numerical prediction results were presented in the form of equivalent diameter. Lastly, two correlation equations were obtained and the numerical predictions are shown to agree well with the experimental data.

A laboratory study of fluid flow and microhabitat selection by larvae of Simulium vittatum (Diptera: Simuliidae)

Microhabitat selection by Simulium vittatum Zetterstedt larvae in a flume was studied at different mainstream velocities on two substrates: a thin flat plate parallel to the flow and a cylinder in cross flow. The results do not support the generally accepted assumptions that simuliid larvae keep within the boundary layer to avoid the direct influence of mainstream current and that they select the fastest velocity available when offered a longitudinal velocity gradient within their tolerance range. Instead, larvae gathered along the zone of boundary layer separation and remained along the stagnation line at the leading point of the cylinder when artificially positioned there. Further, under most conditions, larvae avoided zones of maximum surface shear stress. Larval reaction to hydraulic changes was immediate. It is hypothesized that S. vittatum larvae first scan the velocity profile at the substrate, initially moving toward increasing flow velocity (or water acceleration). They then cue on a steep velocity gradient along the body as part of the processes involved in choosing a location for suspension feeding. Such conditions would maximize particle flux through the labral fans and minimize drag forces on the bulbous posterior abdomen. This study provides the first direct evidence for microhydraulic factors as causative agents in the formation of simuliid larval assemblages.

Experimental study of particle deposition onto a circular cylinder in high-temperature particle-laden flows

Deposition (transport) of particles onto a cold circular cylindrical surface in high-temperature particle-laden cross flows was investigated experimentally. Local particle deposition efficiencies for two Reynolds number (Re = 36, 54) and three wall/gas temperature ratio parameters ( T w / T ∞ = 0.731, 0.853, 0.987) were obtained by using scanning electron microscopy (SEM) techniques. Solid TiO 2 particles, generated by a fluidized-bed particle generator, ranged in diameter from 0.065 to 20 μm. Mainstream particle number density and velocity fields were measured by laser-Doppler anemometry (LDA). Calibration for particle concentration was accomplished with the particle size and number density data obtained from inertial impaction and filtration methods. Thermophoresis and inertial impaction are the important deposition mechanisms for these experimental conditions. The effects of particle size, target temperature ratio parameter, and flow Reynolds number on the particle deposition rates at each local angular position were estimated. The experimental particle deposition rates for small particles are compared to the theoretical thermophoresis correlation.

Flow patterns and velocities around a suspension-feeding gorgonian polyp: evidence from physical models

The influence of polyp posture on local velocity profiles around the suspension-feeding tentacles was determined using a scale model of a polyp of the tropical gorgonian Pseudopterogorgia acerosa (Pallas). Polyp flexion was found to be an important mechanism for maintaining reduced and less variable flow velocities at the level of the polyp: local velocities were reduced by 40% in mainsstream velocities of 5 cm·s −1, by 80% in mainstream velocities of 10 cm·s −1, and by 50% in mainstream velocities of 20 cm·s −1. Flow patterns indicated a recirculation of water behind the polyp at intermediate mainstream velocities that may further enhance particle capture. Polyp flexion may act in conjuction with colony flexibility to increase the range of current velocities over which suspension-feeding is successful.

Drag-induced deformation: a functional feeding strategy in two species of gorgonians

The Jamaican gorgonians Pseudoptorogorgia acerosa (Pallas) and P. americana (Gmelin) exhibit whole colony flexibility in regions of moderate to high water movement. Juvenile colonies were tested under laboratory conditions to delinate the functional consequences of flexibility. Comparison of the drag forces exerted on flexible and artificially stiffened colonies in different flows to the forces required to detach colonies in the field suggested that the reduction in drag afforded by flexible colony deformation is not critical to preventing dislodgement. A further consequence of colony deformation was the reduction in the range of flow velocities encountered by the polyps relative to mainstream current velocity. Colonies appeared to feed most successfully when exposed to intermediate mainstream velocities of 10–15 cm·s −1. Therefore, the reduction of flow velocities at the level of the polyp broadens the range of ambient velocities over which successful feeding can occur, and as such, may be the most important consequences of flexibility.

Effects of Obstacle on Film Combustion of Methanol in a Parallel Air Stream.

The term film means the diffusive combustion of fuel gas evaporated from a condensed fuel surface with an oxidizer gas flowing along the surface. The present study aims to gain a fundamental understanding of the effects of separated flow created by an obstacle on stabilization and structure of the diffusion flame. As an obstacle, a rod with diameter d is placed upstream of the fuel surface with its axis perpendicular to the flow direction. The distance L from the fuel surface to the axis of the rod as well as the main stream velocity U∞ is varied to change the effects of the separated flow on the fuel surface. It is found that under appropriate conditions, a flame moves forward from the fuel surface and is stabilized behind the rod. A map is obtained which describes the state of flame stabilization in the U∞ plane. Distributions of temperature and main species concentration are measured to elucidate the heat and mass transfer processes in the flame.

Calculation of partially cavitating flow by singularity method. 7th Report, Wing in spanwise shear flow.

An analytical method for the partially cavitating wing in spanwise shear flow between two parallel plane walls is proposed. An equation of motion with respect to disturbance pressure is transformed into two problems by separation of variables. One of them is that of the spanwise direction and the other is that of the wing sectional plane. For the former, an existing analytical solution is adopted and the latter is reduced to a simultaneous integral equation with respect to singularity distributions and solved by introducing an analytical method previously proposed by the author for the noncavitating wing in spanwise shear flow. These two kinds of solutions corresponding to the individual eigenvalues are combined linearly, and the expressions for the wing and cavity characteristics are obtained. Numerical calculations are carried out to clarify the notable effects of cavity length, main stream velocity profile, and aspect ratio on the cavity and wing characteristics of the partially cavitating wing in spanwise shear flow between two parallel plane walls.

Lifting-Line Theory for Supercavitating Hydrofoil in Shear Flow. (Continuing Report, Application to Partial Cavitation).

An existing lifting-line theory for a supercavitating hydrofoil in spanwise shear flow between parallel plane walls is applied to partial cavitation in the same condition. A lifting-line equation is derived from an assumption holding two-dimensional at any spanwise position, and the method of solution of the lifting-line equation is applied, in which an eigenvalue problem of the Sturm-Liouville type should be solved. The method to obtain the cavity spanwise distribution, in which the solution of type lifting-line equation is used, is also applied. The wing for the numerical example is flat in section, rectangular in planform, and 5 degrees in attack angle. Effects of the main stream velocity profile, aspect ratio and cavitation number on local lift coefficient, total lift coefficient and induced drag coefficient for force characteristics, and on spanwise distribution in cavity length and its three-dimensional effect for cavity characteristics are clarified.