Free Stream Air Velocity Research Articles

Turbulent mixed convection flow over a backward-facing step––the effect of the step heights

Effect of the backward-facing step heights on turbulent mixed convection flow along a vertical flat plate is examined experimentally. The step geometry consists of an adiabatic backward-facing step, an upstream wall and a downstream wall. Both the upstream and downstream walls are heated to a uniform and constant temperature. Laser–Doppler velocimeter and cold wire anemometer were used, respectively, to measure simultaneously the time-mean velocity and temperature distributions and their turbulent fluctuations. The experiment was carried out for step heights of 0, 11, and 22 mm, at a free stream air velocity, u ∞, of 0.41 m/s, and a temperature difference, Δ T, of 30 °C between the heated walls and the free stream air. The present results reveal that the turbulence intensity of the streamwise and transverse velocity fluctuations and the intensity of temperature fluctuations downstream of the step increase as the step height increases. Also, it was found that both the reattachment length and the heat transfer rate from the downstream heated wall increase with increasing step height.

Aspiration efficiency for thin-walled nozzles facing the wind and for very high velocity ratios

The study of thin-walled aerosol sampling probes facing the wind was originally driven by practical applications (e.g., stack sampling). But more recently it has provided useful scientific insights into the performances of more complicated blunt aerosol samplers. Previous work has been carried out for a relatively narrow range of conditions, especially for R (the ratio of freestream air velocity to probe suction velocity), leading to the widely accepted model of Belyaev and Levin (J. Aerosol Sci. 5 (1974) 325). This model has been shown to predict aspiration efficiencies (for sharp-edged, thin-walled samplers facing the wind) that agree well with experimental data, for R ranging from 0.03 to 11. Recently, there has been renewed interest in studying aspiration for a much wider range of R, driven primarily by the desire for a new generation of particle size-selective blunt samplers, including the need to sample at lower flowrates in order to use lighter sampling pumps for industrial hygiene applications. The study of thin-walled sampling for wider ranges of R, especially in the range above about 10 is the first step towards improved understanding of blunt sampling as it might be applied to practical samplers under such conditions. This is because much of what is currently known about the aspiration of aerosols into blunt samplers derives from what is known about the less complex thin-walled sampler configurations. New experiments have been conducted for thin-walled cylindrical sampling probes for R ranging from 0.5 to 50. The experiments were conducted in a small wind tunnel (cross-section 0.30 m×0.30 m) using narrowly graded fused alumina powders to produce aerosols with mass median particle aerodynamic diameters up to 90 μm . The external wind velocity was held constant at 1 m/ s , so that Stokes’ number ( St) ranged from about 0.05 to 3.7. An extensive, highly repeatable set of experimental results showed that, for R increasing above about 6, the Belyaev and Levin model increasingly overestimated aspiration efficiency compared to what was measured. An improved model for aspiration efficiency of thin walled sampling probes facing the wind is proposed that describes what happens over this much wider range of conditions.

Turbulent mixed convection flow over a backward-facing step

Measurements of turbulent mixed convection flow over a two-dimensional, vertical backward-facing step are reported. Laser-Doppler velocimeter (LDV) and cold wire anemometer were used, respectively, to measure simultaneously the time-mean velocity and temperature distributions and their turbulent fluctuations. The experiment was carried out for a range of free stream air velocities 0 m/s⩽ u ∞⩽0.41 m/s for a step height of 22 mm, and a temperature difference, ΔT, of 30°C between the heated walls and the free stream air. The step geometry consists of an adiabatic backward-facing step, an upstream wall and a downstream wall. Both the upstream and downstream walls are heated to a uniform and constant temperature. The present results reveal that the introduction of a small free stream velocity causes a decrease in the turbulence intensity of both the streamwise and transverse velocity and temperature fluctuations. Also, it was found that the reattachment length increases while the heat transfer rate from the downstream heated wall decreases as the free stream velocity increases.

Sensitivity of the food centre temperature with respect to the air velocity and the turbulence kinetic energy

The surface heat transfer coefficient is an important parameter to characterise forced convection heating and cooling processes. It expresses the influence of flow phenomena on the heat transfer rate to foods. For thermal process calculations, one often relies on dimensionless correlations that contain the flow properties, namely the free stream air velocity and the turbulence kinetic energy. This paper discusses the sensitivity of the food temperature with respect to these variables for different food geometries and conditions. The study is based on the finite element analysis of the product temperature deviations as a result from deviations of the surface heat transfer coefficient, combined with existing dimensionless correlations for the surface heat transfer coefficient. Sensitivity charts are constructed, which relate the temperature sensitivity with respect to the free stream air velocity and the turbulence kinetic energy to different air flow conditions as a function of time. By constructing sensitivity charts, it is shown that small velocity deviations can lead to large food temperature deviations. The effect of small deviations of the turbulence intensity is not as significant.

The effects of free stream air velocity on water droplet size and distribution for an impaction spray nozzle

Fog generation for ingestion by gas turbine engines is a method by which the amount of work done during compression can be reduced by evaporative cooling of the inlet air and can improve the engine's efficiency by as much as 15 per cent. Generating fog produces problems if the droplet size is too high which can result in erosion and possibly corrosion damage to the compressor and stator blades. This study examines the water droplet spray characteristics of an impaction-type atomizer over a range of free stream air velocities, water pressures and fluid viscosities.

On the Retention of Inertialess Particles in the Near Wake Region of a Two-Dimensional Flat Plate Placed Normal to an Aerosol-Laden Freestream

ABSTRACT The dispersion of inertialess particles in the recirculation zone immediately behind long, two-dimensional flat plates positioned perpendicular to an aerosol-laden freestream was investigated both experimentally and numerically. The physical situation corresponds to the release of particles from a two-dimensional line source positioned upstream of the flat and consequent entrainment of those particles into the near wake region. After the sudden interruption of the aerosol generation process, the particles persist in the near wake, with the number concentration decaying exponentially with time. The experiments were carried out in a low-turbulence wind tunnel. An optical detection system was developed for the fast observation and recording of changes in the particle concentration in the near wake of the plate. The dimensionless residence time for fine inertialess smoke particles in the near wake region H(= τU/D, where τ is the time constant for the exponential concentration decay, U is the undisturbed freestream air velocity, and D is the width of the obstruction) was measured in the range of Reynolds numbers (Re) from about 1000 to 10,000. H was found to be approximately constant at about 9.5 for Re above about 4000. These results were in agreement with a simple model and with numerical calculations based on a two-equation, steady-flow, k — ε approach with coefficients chosen to reflect to some degree the actual physical situation which also includes vortex shedding.

Dispersion of large particles in the near wake of a disc in a turbulent free stream

The dispersion of particles in the near wake of a flat disc placed perpendicular to a turbulent free stream was studied by optically measuring the dimensionless near-wake residence time of particles, H ( = t r U D , where t r is the time constant of the decay of the near-wake concentration following removal of the source, U the free-stream air velocity, and D the disc diameter). The dependence of H on the Stokes number (St, expressed in terms of U and D) was investigated for St up to about 8. The experiments were conducted for aerosols which were generated from narrowly graded fused alumina dust and glass beads, whose surface median aerodynamic diameter (SMAD) ranged from 13 to 96 μm, introduced directly into the near-wake region at the base of the disc. The results showed H decreasing steadily and uniquely with increasing St, leveling off for St > 1. This behavior is well explained by simple physical models which take account of the centrifugal forces exerted on a particle during its motion inside the recirculating body of fluid enclosed in the near-wake region, supplementing the turbulent exchange dispersion processes which occur for particles of all sizes.

Aerodynamic characterisation of model tree canopies in a wind tunnel

The aerodynamic characteristics of ‘model’ tree canopies within a wind tunnel are reported. Wind speed profiles above and within canopies comprising Norway spruce (Picea abies) and Holm oak (Quercus ilex) saplings were determined by hot wire and hot film anemometry at a free-stream air velocity subsequently used for experiments on the dry deposition of aerosol particles to both canopies. Friction velocities (U∗) of 128 and 104 cm s−1 were observed for Norway spruce and Holm oak, respectively. Vertical distributions of wind speed (U) and turbulence intensity (i) within the canopies were similar in form and magnitude to those reported in the literature for full-sized forest canopies, suggesting that the sapling canopies provide a realistic ‘model’ with which to investigate the process of dry deposition.

Aerodynamic characterisation of model tree canopies in a wind tunnel

The aerodynamic characteristics of ‘model’ tree canopies within a wind tunnel are reported. Wind speed profiles above and within canopies comprising Norway spruce ( Picea abies) and Holm oak ( Quercus ilex) saplings were determined by hot wire and hot film anemometry at a free-stream air velocity subsequently used for experiments on the dry deposition of aerosol particles to both canopies. Friction velocities ( U∗) of 128 and 104 cm s −1 were observed for Norway spruce and Holm oak, respectively. Vertical distributions of wind speed ( U) and turbulence intensity ( i) within the canopies were similar in form and magnitude to those reported in the literature for full-sized forest canopies, suggesting that the sapling canopies provide a realistic ‘model’ with which to investigate the process of dry deposition.

MODELING A WORKER'S EXPOSURE FROM A HAND-HELD SOURCE IN A UNIFORM FREESTREAM

The phenomenon of boundary layer separation can be an important factor in determining a worker's exposure to toxic airborne pollutants. A conceptual model was developed to understand this phenomenon and to predict the average concentration in the reverse flow region downstream of a worker in a uniform freestream. Subsequently, the assumptions of this model were tested experimentally in wind tunnel studies. On the basis of these results, a revised model is presented and validated by using a tracer gas method. The revised model provides a reasonable estimate of the average concentration in the reverse flow region of the mannequin. Empirical models are presented that relate both the average concentration in the reverse flow region and the breathing zone concentration to the body dimensions and the freestream air velocity. Applications and limitations of the results are discussed.

The nature of air flow near the inlets of blunt dust sampling probes

This paper sets out to describe the nature of air flow near blunt dust samplers in a way which allows a relatively simple assessment of their performances for collecting dust particles. Of particular importance is the shape of the limiting stream surface which divides the sampled air from that which passes outside the sampler, and how this is affected by the free-stream air velocity, the sampling flow rate, and the shape of the sampler body. This was investigated for two-dimensional and axially-symmetric sampler systems by means of complementary experiments using electrolytic tank potential flow analogues and a wind tunnel respectively. For extreme conditions the flow of air entering the sampling orifice may be wholly divergent or wholly convergent. For a wide range of intermediate conditions, however, the flow first diverges then converges, exhibiting a so-called “spring onion effect”. Whichever of these applies for a particular situation, the flow may be considered to consist of two parts, the outer one dominated by the flow about the sampler body and the inner one dominated by the flow into the sampling orifice. Particle transport in this two-part flow may be assessed using ideas borrowed from thin-walled probe theory.

Analytical Investigation of the Characteristics of the Co-Flowing Velocity Sensor

An analytical finite-difference method is used to predict the linearity and sensitivity of a co-flowing air velocity sensor. The sensor, conceived, developed and tested at the National Research Council consists of a 0.023-in. inside diameter nozzle and receiver assembly which, when placed in a free-stream of unknown velocity, can be used to measure that free-stream velocity. For a certain range of jet and stream velocities the measured pressure from the receiver is linearly related to the free-stream velocity. With known jet supply pressures the analysis predicts jet exit velocities and the dynamic recovery pressure in the receiver for different values of free-stream velocities. The predictions compare well with variable density wind tunnel experimental results covering free-stream air velocities, temperatures and pressures of 50-720 ft/sec, −40°C to +20°C and 400-750 mm. Hg abs., respectively, and sensor jet nozzle exit velocities of 50-500 ft/sec.

Mechanisms of small particle re-entrainment from flat surfaces

The potential of a flat surface to act as a source of air-borne particles was investigated by exposing deposited particles of known size to measured air flow in a low speed wind tunnel. An hypothesis for the mechanisms involved in particle re-entrainment was proposed and verified. The two-part mechanism includes: (a) initiation of particle movement when shear stresses on the particles exceed friction forces acting on the particles and (b) movement of particles downstream from the point of initiation caused by momentum transferred from initially moving particles. The hypothesis was verified by comparison of prediction equations and experimental results. The experimental results were obtained by measurement of the distance downstream from the leading edge of the surface at which re-entrainment began. This distance was found to be a function of free-stream air velocity, turbulence of the free-stream and particle size. The results confirm that flat surfaces of sufficient size can be sources of air-borne particles even when air velocities are magnitudes normally existing in enclosed spaces.

Geometrical considerations in the burning of liquid drops

Previous experimental work has demonstrated the existence of two régimes of burning on liquid fuel spheres placed in a moving air stream: envelope flames and wake flames. In the present study, a third régime of burning is distinguished wherein a flame is stabilized in the boundary layer at the side of a liquid sphere. The angular position which the side flame assumes when measured from the forward stagnation point of the sphere is shown to increase with free stream air velocity. It is also demonstrated that the angular position depends on the sphere size as well as on the orientation of the air flow with respect to the gravity force thus proving natural convection to be an important factor. Boundary layer theory is employed and provides a quantitative basis for the above phenomena.

Determination of Air Velocity by Ion Transit-Time Measurements

A method is described for measuring the velocity of an air stream by producing positive ions at one point in the flow and measuring the time required for the ions to be carried to an induction-type detector located a known distance (0.5 to 2.0 inches) downstream. Positive ions are produced intermittently near the tip of a positive corona point by burst discharges which are initiated by stray ions in the air stream or by irradiating the air in the neighborhood of the positive corona point with polonium alpha-particles. The ion transit-time is measured by observing the induced pulses from the detector on a cathode-ray oscillograph sweep which is triggered by the burst discharges of the corona point. The accuracy of velocity measurement using a measuring length of one inch with a free-stream air velocity of 1730 feet per second (based on pressure and temperature measurements) was better than two percent. It is shown that the direction of streamlines in a supersonic flow can be determined approximately by aligning the corona point and the induction detector probe to give the maximum amplitude of induced pulses. It is believed that this method may be applied to map velocity fields in supersonic wind tunnel experiments.