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The effect of a gap between an inlet duct and a rotating impeller in a centrifugal fan is often neglected in the impeller design calculations or design-related computational fluid dynamics (CFD) analyses. This leads to an arbitrary determination of the gap size for the final fan configuration. Since the gap guides the volute flow back to the impeller flow field near the shroud high-curvature turning area, the low-momentum jet formed by the gap flow could prevent local flow from separation, reducing the local flow turning losses. However, this jet flow has enlarged flow separation in the blade passage, producing shedding vorticity in the downstream passage-flow. The passage-flow separation and the downstream volute flow, which is also affected by the passage-flow separation, have a higher impact on flow losses than the blade leading edge separation, lf the gap size is not selected carefully, the combined effect of the passage-flow separation and downstream volute flow losses reduces the fan's overall performance between 2% points and 5% points as demonstrated in the current study. In this paper, local impeller velocity distributions obtained from both design-CFD and analysis-CFD calculations are compared along the shroud from the gap to the blade trailing edge. The overall impeller flow fields with and without the gap and volute effects are also compared and discussed based on the CFD solutions. Finally, an example of controlling the gap effect is shown. [DOI: 10.1115/1.4002450]

The effects of excess feeding oxygen to the fuel-air mixture on air and fuel mass flow rates and also on air-fuel ratio are investigated here experimentally. This study concerned with the effects of injecting pure oxygen quantity to the mixture of fuel and air before interring the combustion chambers. It is found that the mass flow rate of fuel with the oxygen feeding is less than that of with no oxygen feeding at some specific values of engine speeds and the same thing was found for air mass flow rate. The air-fuel ratio also is less with considerable values in the case with oxygen feeding than that with no oxygen samples. Also this technique can be used partially, in some conditions oxygen can be feed into the combustion chambers to increase engine performance. Key words: Fuel-air mixture, fuel mass flow rate, excess oxygen feeding, air-fuel ratio, INTRODUCTION Today, decreasing fuel consumptions and exhaust emissions become a considerable matters because if these things are done then cost of energy or fuel will be decreased and an environmental [...]

The paper describes a study of an electrical discharge in a supersonic rarefied air flow. The discharge is created by applying negative dc potential difference between two electrodes flush mounted on the surface of a quartz flat plate placed in Mach 2 continuous airflow. The electrodes are arranged in spanwise direction. Two discharges are studied. In the first one, the negative potential is applied to the electrode placed close to the leading edge. In the second one the potential is applied to the electrode placed close to the trailing edge. Current-voltage characteristics, spectroscopic analyses, electrostatic probe and surface thermal measurement will be presented. Luminous emission pattern and potential measurements show that the plasma is similar to a low pressure tube discharge modified by a non uniform density field of the flow. A comparison between current voltage characteristics of the upstream and downstream discharge shows the effect of the density field on the discharge. Thermal and spectroscopic measurements also show that the plasma induces a surface and volume heatings. Index Terms--Supersonic, discharge, rarefied, plasma, heating.

Yam is one of the preferred staple foods in West Africa. The annual vegetative cycle of yam necessitates a long period of storage to make it available all year round. The major problems in yam tuber storage are sprouting, respiration and transpiration, which cause weight and quality losses. In this work, the effects of storage conditions and storage period on the nutritional and other qualities of stored yam tubers were investigated. Storage conditions used were two traditional yam barns, one with fan to aid air circulation and the other without. A total of 216 tubers of yam (Dioscorea roundata) 'Giwa' variety with 108 tubers in each barn, were stored in the barns. Parameters evaluated were temperature and relative humidity of the storage environment, signs of deterioration of the tuber such as sprouting, weight loss and rotting and some nutritional parameters (carbohydrate, calcium, phosphorus, crude fibre, crude fat, crude protein, ash and moisture contents). Temperature and relative humidity were measured three times a week and four times a day at 0800h, 1200h, 1600h and 2000h. The results showed that average temperature and relative humidity in the barn with fan were slightly lower than that of the barn without fan. These differences were statistically significant for April, May and June at P [less than or equal to] 0.1. Tubers stored in the barn with fan had the least sprout weight and least weight loss. At the end of a 3-month period, the tubers in the ventilated barn showed 4.7% less weight loss compared to the barn without fan. The difference in sprout weights and weight loss between the structures was statistically significant at P [less than or equal to] 0.05. Also, tubers stored in the barn with fan had the least percentage of rotten tubers (1.85% of stored tubers) compared to the tubers stored in the barn without fan (12.03%). A reduction of some of the nutritional content was also observed during the six months of storage period. From these results it can be concluded that intermittent air flow on stored yam tubers reduces sprouting, weight loss and rot development, thus reducing the overall loss in stored yam tuber. However, moisture, crude protein, carbohydrates, phosphorus and calcium contents of the tuber reduced significantly in the two barns studied. Key words: Yam, storage conditions, nutritional, content, INTRODUCTION Yam belongs to the genus Dioscorea (Family Dioscoreaceae) and is the second most important tropical root crop in West Africa, next to cassava, [1]. Yams originated in the Far [...]

The electrical efficiency of a photovoltaic system drops as its operating temperature rises and PV cooling is necessary. The photovoltaic/thermal (PV/T) system is a relatively recent type of solar collector where a circulating fluid of lower temperature than PV module extracts heat from it, cooling the module to improve its output power while the solar pre-heated fluid provides sensible heat. In the present work, air cooling of a commercial PV module configured as PV/T air solar collector by natural flow is presented, where two low cost modification techniques to enhance heat transfer to air stream in the air channel are studied. The considered methods consist of thin metal sheet suspended at the middle or fins attached to the back wall of the air-channel to improve heat extraction from the module. A numerical model was developed and validated against the experimental data obtained from outdoor test campaigns for both glazed and unglazed PV/T prototype models studied. The validation results show good agreement between predicted values and measured data and thus could be used to study analytically the performance of these PV/T air collectors with respect to several design and operating parameters. The modified systems present better performance than the usual type and will contribute to better performance of integrated PV systems for natural ventilation applications in buildings, both space cooling and heating. Keywords: PV/T collector; BIPV; Thermal performance; Natural flow; Modeling

This paper presents results of low-temperature plasma-assisted combustion experiments in premixed ethylene-air and methane--air flows. The plasma was generated by high-voltage, nanosecond pulse duration, high repetition rate pulses. The high reduced electric field during the pulse allows efficient electronic excitation and molecular dissociation, thereby generating a pool of chemically active radical species. The low duty cycle of the repetitively pulsed discharge improves the discharge stability and helps sustain diffuse, uniform, and volume filling nonequilibrium plasma. Plasma temperature was inferred from nitrogen second positive band system emission spectra and calibrated using thermocouple measurements in preheated flows (without plasma). The experiments showed that adding fuel to the air flow considerably increases the flow temperature in the plasma, up to [DELTA]T = 250 [degrees]C-350 [degrees]C. On the other hand, adding fuel to nitrogen flow at the same flow and discharge conditions resulted in a much less pronounced plasma temperature rise, only by about [DELTA]T : 50 [degrees]C. This shows that temperature rise in the air-fuel plasma is due to plasma chemical fuel oxidation reactions initiated by the radicals generated in the plasma. In a wide range of conditions, generating the plasma in air-fuel flows resulted in flow ignition, flameholding, and steady combustion downstream of the discharge. Plasma-assisted ignition occurred at low air plasma temperatures, 100 [degrees]C-200 [degrees]C, and low discharge powers, ~100 W (~1% of heat of reaction). At these conditions, the reacted fuel fraction is up to 85 %-95 %. The present results suggest that the flow temperature rise caused by plasma chemical fuel oxidation results in flow ignition downstream of the plasma. Index Terms--Absorption spectroscopy, emission spectroscopy, ignition, low-temperature plasma, nanosecond pulses.

This paper presents an adjoint method for the optimum shape design of unsteady three-dimensional viscous flows. The goal is to develop a set of discrete unsteady adjoint equations and the corresponding boundary condition for the nonlinear frequency-domain method. First, this paper presents the complete formulation of the time-dependent optimal design problem. Second, we present the nonlinear frequency-domain adjoint equations for three-dimensional viscous transonic flows. Third, we present results that demonstrate the application of the theory to a three-dimensional wing. DOI: 10.2514/1.27601

The effects of the blockage of a rectangular intake duct and impervious flow boundaries on the critical submergence of a rectangular intake are presented. The potential solution, based on the Rankine stagnation point, is determined to be another approximate method for the prediction of the critical submergence of this kind of intake. It is found that a critical cylindrical sink surface capped with two critical hemispherical sink surfaces at both ends with a radius equal to the radial distance of the stagnation point (which is 2/[pi] times the critical submergence of the rectangular intake) can also be used to predict critical submergence. Theoretical results and available experimental data are compared. The theory presented in this study acceptably (by about 1-20%) estimates the critical submergence for the cases where the distance (clearances) of the impervious solid boundaries are larger than 1/2 of the small inner dimension of the intake. On the other hand, the theory overestimates the critical submergence by about 80% for the cases where the distances of the solid boundaries (especially those cutting the free surface such as the dead-end wall) become zero. DOI: 10.1061/(ASCE)0733-9402(2007)133:2(91) CE Database subject headings: Intakes; Vortices; Air entrainment; Predictions; China.

This paper presents a particle penetration model predicting particle penetration coefficient ([P.sub.p]) through a narrow crack of arbitrary incline angles ([theta]). The objective was to simulate [P.sub.p] for outdoor-to-indoor particle penetration for residential infiltration conditions. This model assumes laminar infiltration flow and considers particle deposition from both gravitational sedimentation and Brownian diffusion. For micron-sized particles, modeling results indicate that gravitational sedimentation is the major deposition mechanism. [P.sub.p] increases monotonically with [theta] because effective particle sedimentation velocity ([[upsilon].sub.s] cos[theta]) decreases monotonically with [theta]. For submicron-sized particles (0.1 [micro]m), Brownian diffusion is the major particle deposition mechanism. Because Brownian diffusion is a nondirectional deposition mechanism, crack inclination did not affect [P.sub.p]. This study applied this model to estimate [P.sub.p] for L-shaped cracks, and validated modeling results with experiments. Experimental results indicated that inertial impaction and crack entrance cutoff effects were not significant particle deposition mechanisms for the test micron-sized particles. Gravitational sedimentation was the major deposition mechanism. An L-shaped crack can be simulated as the combination of horizontal and vertical sections. This model agreed reasonably with experimental results. DOI: 10.106 I/(ASCE)0733-9372(2007)133:3(331) CE Database subject headings: Particles; Penetration; Indoor air pollution; Infiltration; Cracking.

A three-dimensional computational fluid dynamics (CFD) model is developed for investigating the air flow and heat transfer in louver-fin round-tube two-row two-pass cross-counterflow heat exchangers. Fin designs with different louver angles are analyzed to examine the effect of louver interaction, with flow patterns from pure tunnel flow to louver-directed flow.

This paper describes the development of a parametric mathematical model to predict wind-induced surface pressures on exterior vertical wall surfaces on a single model block in shielded environments. The first phase of the project focused on the development of a parametric model to determine the average pressure coefficients on exterior building surfaces shielded by a single windward obstruction model block. A pressure modification coefficient was defined to estimate the shielding effect based on the angular relationship between, the shielding block and the surface for which average pressure is to be computed, and the wind direction. The second phase involved expanding the scope of the prediction model to account for the various arrangement possibilities of the shielding blocks relative to the surfaces under investigation. The predicted pressure modification coefficient may be used to estimate average pressure coefficients taking into account the indoor velocity coefficients, surface porosity, window design, and interior partition types. The prediction model considers the effect of multiple shielding blocks, and the gaps between them on a vertical surface relative to wind direction. Wind tunnel verification of the model showed robust estimation of surface pressures in complex urban configurations. Keywords: Natural ventilation; Pressure coefficients; Shielding; Modeling; Urban; Airflow; Wind tunnel test

A study of the characteristics of turbulent lifted-jet flames in the hysteresis regime was performed using methane and ethylene fuels in laminar and turbulent air coflows. Reattachment velocities and lifted flame heights just prior to reattachment vary linearly as for laminar flames in coflow. The flow regime of the coflow (i.e., laminar or turbulent) did not appear to affect the behavior of these flames. These observations are of utility in designing maximum turndown burners in air coflow, especially for determining stability criteria in low fuel-flow applications. [DOI: 10.1115/1.2358147]

Mechanically driven orographic lifting is important for air pollution dispersion and weather prediction, but the small dimensions of mountain peaks often prevent numerical weather models from producing detailed forecasts. Mechanical lifting in stratified flow over mountains and associated thermodynamic processes were quantified and evaluated using Sheppard's model to estimate the dividing-streamline height [z.sub.t]. The model was based on numerical weather model profile data and was evaluated using ground-based measurements on a tall, axisymmetric mountaintop for which the nondimensional mountain height [h.sub.ND] = hN/[U.sub.[infinity]] is frequently between 1 and 10 (here h is mountain height, N is Brunt-Vaisala frequency, and [U.sub.[infinity]] is upstream horizontal wind speed). Sheppard's formula was successful in predicting water vapor saturation at the mountaintop, with a false-prediction rate of 14.5%. Wind speed was found to be strongly related to the likelihood of forecast errors, and wind direction, season, and stratification did not play significant roles. The potential temperature (water vapor mixing ratio) at [z.sub.t] in the sounding was found to be slightly smaller (larger) than at the mountaintop, on average, indicating less lifting than predicted and/or turbulent mixing with higher-altitude air during parcel ascent. Detailed analysis revealed that this difference is a result of less lifting than predicted for small [U.sub.[infinity]](Nh), whereas Sheppard's model predicts the relative increase in uplift with increasing [U.sub.[infinity]](Nh) correctly for [U.sub.[infinity]](Nh) > 0.2.

This paper presents new observations of deposition of heavy particles (glass beads of gravitational settling velocity 8.7 cm [s.sup.-1]) within an undisturbed flow and within a flow disturbed by a porous windbreak fence. These data are then used to diagnose the capability of a Lagrangian stochastic (LS) particle trajectory model, which simulates heavy particle dispersion. The model is based on existing parameterizations and is coupled to a wind model based on a Reynolds stress turbulence closure that provides computed fields of wind statistics. The deposition rates, as simulated by the model, match the observation within E = 30% of accuracy, with E being the root-mean-square error normalized by the peak value on the deposition swath. These results suggest that the LS model handles properly the heterogeneities of the flow and that the heuristic adjustments made to account for the inertia of heavy particles are useful approximations. The model consequently proves to be a valuable tool to investigate the patterns of dispersion about an obstacle.

A novel wind-tunnel facility has been designed for measurement of lift and drag on micromachined airfoils. The tunnel is designed to operate with pressures ranging from 0.15 to 1.0 atmosphere, over a velocity range of 30 to 100 m/s, allowing for independent control of Reynolds and Knudsen number. The tunnel is designed for testing of airfoils with chords of 10 to 100 microns, giving a range of Reynolds numbers from 10 to 600, with Knudsen numbers reaching 0.01. Due to the structural constraints of the airfoils being tested, the wind tunnel has a 1 cm cross-section. This small size allows the use of a 100-1 contraction area, and extremely fine turbulence screens, creating a low turbulence facility. Computational fluid dynamics is used to show that an ultra-short 100-1 contraction provides uniform flow without separation, or corner vortices. Velocity data obtained with impact and hot-wire probes indicate uniform flow and turbulence intensities below 0.5%. [DOI: 10.1115/1.2236128]

Wind-tunnel steady-state unidirectional pressure-drop measurements for airflow through nine compressed and uncompressed isotropic open-cell aluminum foam samples, having different porosities and pore densities, were undertaken. The compressed foam produced significantly higher pressure drop. which increased with increasing Darcian velocity following the quadratic Forchheimer equation. The permeability and the inertia coefficient data for the compressed for showed less scatter compared to those for the uncompressed foam. Both were correlated using an Ergun-like equation, with the correlation being better for the permeability. The permeability correlation predicted the results of some previous studies very well. The friction factor correlated well with the Reynolds number. [DOI: 10.1115/1.2236132] Keywords: pressure drop, permeability, metal foam, porous media

1. Introduction An indirect comparison was made between the air-kerma primary standards for medium-energy x-rays of the National Institute of Standards and Technology (NIST) and the Bureau International des Poids […]

Municipal wastewater collection systems, due to the nature of their functions, carry varying concentrations of odorous gases. The production rate and transport of these gases within and out of sewer systems depend on air flow rate in the system piping. However, municipal sewers are generally designed to only transport sewage flow without giving consideration to the air flow field. As a consequence, the movement of air into, along, and out of collection systems is for the most part uncontrolled. The purpose of this paper therefore is to provide a new design protocol based on system theoretic techniques to be used by municipal engineers and environmentalists involved in odor control and sewer foul air transport studies. The modeling formulation accounts for combined wastewater drag and pressure-induced air flows, and manhole pressurization. The developed framework is applied to both hypothetical and real sewer systems to only illustrate the applicability of the modeling formulation. CE Database subject headings: Air flow; Drop structures; Engineering education; Odors; Sewers; Ventilation; Municipal engineering; Waste management.

We report on large eddy simulations of the turbulent flow of air in hard disk drives (HDDs) using a commercial CFD code. In particular, we focus on HDD casings in which flow-induced vibrations are reportedly reduced by small geometrical modifications. The modifications investigated are M1: a blocking plate situated between the disks, M2: a spoiler (or deflector) located behind (downstream of) the actuator arm, and M3: a similar deflector upstream of the arm. We observed that Ml, M2, and M3 significantly modify the mean flow patterns in the drives. Ml reduces velocity magnitudes in most parts of the drive, the modification of M2 causes flow reversal in regions close to the hub, while M3 causes the shedding of vortices upstream of the actuator arm. Our analysis points to Ml as the best candidate for mitigating the effects of turbulent airflow. This is because M1 is more effective in reducing the root-mean-square velocity fluctuations near the suspension. Ml is also more effective in reducing the pressure fluctuations near the base-plate and suspension region. This reduction, however, is at the cost of approximately 20% higher windage. Finally, we note that M3 has the adverse effects of increasing velocity and pressure fluctuations and hence is not the ideal candidate for mitigating airflow effects, among the modifications considered here. Index Terms--Airflow, blocking plate, large eddy simulation, spoiler.

Aeroelastic control of wings using distributed, trailing-edge control surfaces is of interest for maneuvers, gust alleviation, and flutter suppression. The use of high-energy-density, piezoelectric materials as motors provides an appealing solution to the problem of flutter suppression. A new piezoelectric actuator, the V-stack piezoelectric actuator, was designed and bench tested at Duke University. This actuator meets the requirements for trailing-edge flap actuation in both stroke and force. It is compact, simple, and sturdy and leverages stroke geometrically with minimum force penalties while displaying linearity over a wide range of stroke. Integration of the actuator inside a structure requires minimal modifications. The shape of the actuator makes it extremely suitable for trailing-edge flap actuation, eliminating the need for a push rod. A typical section prototype was constructed and tested experimentally in the wind tunnel at Duke University. This experiment was designed for preliminary evaluation of the actuation concept. During bench tests the desired flap deflection of +5 deg was obtained. Wind-tunnel experiments showed that air flow has little influence on flap deflection, suggesting good actuation authority. Actuator-flap frequency bandwidth achievable for this experiment, in the context of -4-5 deg flap deflection, was sufficient and facilitated control design. Positive position feedback (PPF) control was used to add damping to the unstable flutter mode. Operating in closed loop, the flutter was suppressed at the speed at which the flutter occurred open loop, and the flutter speed was increased by more than 30%.

Conversion of a general three-dimensional linear unsteady aerodynamic software system to a configuration shape sensitivity code for planar-lifting-surface configurations at subsonic, sonic, supersonic, and hypersonic flows is described. This was achieved by the accomplishment of the following: A global shape design variable sensitivity scheme was formulated based on the complex variable differentiation (CVD) technique. The CVD technique was incorporated into the software system for shape sensitivity analysis. The new capability was validated with the finite difference method and parametric studies on an AGARD swept wing, AGARD wing--tail configuration, a 60-deg swept wing, and an F-5 wing with a control surface. This selection of test cases covers subsonic to hypersonic planar cases in which singularities in the integrals involved are strong, with aerodynamic interference between lifting surfaces, and including supersonic cases with discontinuous pressure distributions. The most challenging aspects of shape sensitivity analysis for linear unsteady aerodynamics have, thus, been addressed.

This study proposes a new concept for quantifying the energy of flowing compressed air, called air power. Air power is defined as the work-producing potential of compressed air, and its definition and general equation are presented. The properties of air power are also discussed. Air power is comprised of two components, transmission power and expansion power, while air temperature and kinetic energy can generally be neglected. [DOI: 10.1115/1.2170129]

This computational study evaluated lateral instabilities observed during developmental testing of the preproduction F/A-18E/F in the power-approach configuration. These instabilities, described here as abrupt wing stall (AWS), occurred whenever the airplane exceeded 12-deg angle of attack. The AWS was quickly corrected in flight test by the closure of a vent at the wing root, without understanding its physical cause or cure. Computational solutions with both the vent open and closed provide insight into the likely cause of the instability, revealing two distinctive flow topologies. Specifically, all F/A-18 Hornet models depend upon strong vortex flows from the leading-edge extension (LEX) to provide lift at elevated angle of attack. Flow through the open vent displaces the LEX vortex core inboard and up off the wing's upper surface. This displacement dramatically weakens the flow stability over the wing's entire upper surface. Wind-tunnel studies performed in parallel by another researcher provide quantitative corroboration of the computational solutions. Finally, computational-fluid-dynamics metrics developed within the NASA/Navy/Industry AWS program are applied to determine their validity at this low-speed flight condition and modified to enhance their viability as predictive tools for AWS prone configurations.

Zero-net mass-flux periodic excitation was applied at the leading-edge flap shoulder of a simplified high-lift airfoil to delay flow separation. The term simplified infers that no slat or Fowler flaps are used. The NASA energy efficient transport supercritical airfoil was fitted with a 15% chord simply hinged leading-edge flap and a 25% chord simply hinged trailing-edge flap. Initially, the cruise configuration data from previous experiments were reproduced. The effects of leading- and trailing-edge flap deflections on the airfoil integral parameters were quantified. Detailed flow features were measured to identify optimal actuator placement. The measurements included steady and unsteady model and tunnel wall pressures, wake surveys, arrays of surface hot films, flow visualization, and particle image velocimetry. Eventually, high-frequency periodic excitation was applied to delay the occurrence of leading-edge flap shoulder stall and increased the maximum lift by 10-15%. Low-frequency amplitude modulation was used to reduce the oscillatory momentum coefficient by roughly 50% with similar aerodynamic performance gains. It is demonstrated that the efficacy of the amplitude-modulated excitation is due to the generation of low-frequency motion, which is amplified by the separating shear layer.

This paper describes a study of the topology of the skin-friction line pattern on a sharp-edged 65-deg swept delta wing. Special attention is paid to the flow pattern on the apex. Because the pattern on this part of the wing is poorly resolved in experiments, a large-scale model of the apex was constructed to obtain more insight into the flow topology. The angle of attack is shown to have a strong effect on the apex flow topology. The topology shows evidence of primary and secondary separation.

Recent physical model studies have refined designs for high-flow air diffusers for managing accumulations of broken ice at navigation projects. Although these solutions are successful in the model, implementing them in the field can be difficult because of uncertainties in airflow scaling. This study uses field and laboratory data to test theoretical relationships between airflow from the diffuser and the resulting near-surface water velocity. In the experiments, water velocities were measured adjacent to bubbler plumes for depths ranging from 0.52 to 6.5 m and airflow rates ranging from 0.015 to 0.68 standard cubic meters per minute/meter. The observed vertical and horizontal water velocity data compared moderately well to theoretical curves based on the equations of Kobus and Ashton. In addition, a reasonably linear relationship was found between the average velocity of the horizontal, near-surface flow field V and unit airflow from the diffuser [Q.sub.a] CE Database subject headings: Validation; Bubbles; Diffusion; Velocity; Air flow.

This work presents the mathematical model of a complex flow network containing short metering orifices, compressible flow, and two-phase flow in small diameter pipes. It has been developed to study the steady and dynamic flows in a carburetor for small engines. It extends the previously published models by incorporating a detailed review of two-phase flow pressure drop, the effect of the fuel well on the control of air-bleed flow, and dynamic flow. The homogeneous two-phase flow model, which is commonly used in previous models, was compared to an empirical correlation derived from experiments in small pipes and found to be in poor agreement. In order to assess dynamic flow conditions, the model was extended by solving instantaneous one-dimensional Navier-Stokes equations in single-phase pipes. This strategy proved successful in explaining the mixture enrichment seen under pulsating flow conditions. The model was also used to derive a sensitivity analysis of geometries and physical properties of air and fuel. [DOI: 10.1115/1.1949644]

A multigrid numerical method has been applied to a three dimensional, high-resolution diagnostic model for flow over complex terrain using a mass-consistent approach. The theoretical background for the model is based on a variational analysis using mass conservation as a constraint. The model was designed for diagnostic wind simulation at the microscale in complex terrain and in urban areas. The numerical implementation takes advantage of a multigrid method that greatly improves the computation speed. Three preliminary test cases for the model's numerical efficiency and its accuracy are given. The model results are compared with an analytical solution for flow over a hemisphere. Flow over a bell-shaped hill is computed to demonstrate that the numerical method is applicable in the case of parameterized lee vortices. A simulation of the mean wind field in an urban domain has also been carried out and compared with observational data. The comparison indicated that the multigrid method takes only 3%-5% of the time that is required by the traditional Gauss-Seidel method.

Separation of stratified flow over a two-dimensional hill is inhibited or facilitated by acceleration or deceleration of the flow just outside the attached boundary layer. In this note, an expression is derived for this acceleration or deceleration in terms of streamline curvature and stratification. The expression is valid for linear as well as nonlinear deformation of the flow. For hills of vanishing aspect ratio a linear theory can be derived and a full regime diagram for separation can be constructed. For hills of finite aspect ratio scaling relationships can be derived that indicate the presence of a critical aspect ratio, proportional to the stratification, above which separation will occur as well as a second critical aspect ratio above which separation will always occur irrespective of stratification.

The study of the atmospheric boundary layer flow over two-dimensional low-sloped hills under a neutral atmosphere finds numerous applications in meteorology and engineering, such as the development of large-scale atmospheric models, the siting of wind turbines, and the estimation of wind loads on transmission towers and antennas. In this paper, the intermediate variable technique is applied to the momentum equations in streamline coordinates to divide the flow into regions, with each characterized by the dominance of different terms. Using a simple mixing-length turbulence closure, a simplified form of the x momentum equation is solved for the fully turbulent region, resulting in a modified logarithmic law. The solution is expressed as a power series correction to the classical logarithmic law that is valid for flat terrain. A new parameter appears: the effective radius of curvature of the hill. The modified logarithmic law is used to obtain new equations for the speedup, the relative speedup, the maximum speedup, and the height at which it occurs. A new speedup ratio is proposed to calculate the relative speedup at specific heights. The results are in very good agreement with the Askervein and Black Mountain field data.

Large-eddy simulations (LES) have been carried out in order to investigate the structure and development of organized mesoscale cellular convection (MCC) during cold air outbreaks (CAOB) and to reevaluate results by MOiler and Chlond. Some limitations of this and other earlier LES studies of CAOBs have been removed by using a parallelized model with both a large horizontal domain and a fine grid resolution. These model simulations reveal a hitherto undiscovered insight into the development of MCC. It is shown that MCC with aspect ratios larger than 10 only develop in the presence of diabatic heat sources, that is, latent heat release within the clouds and cloud-top radiative cooling, which confirms results from previous studies. Simulated cells are of closed type. The wind field is seen to be correlated with the liquid water field, and thus, dynamic variables are organized on the mesoscale as well. Updrafts predominantly occur in regions with high liquid water content. Two-dimensional spectral analysis confirms clear peaks at the wavelength of the MCC. The dynamic variables are characterized by an organized conglomeration of randomly distributed up- and downdrafts, where each convective cell can hardly be detected by eye. Whereas scalar variables like temperature and the liquid water path field are characterized by large scales with aspect ratios of 9 to 11, in the vertical velocity field multiple scales are present simultaneously, with small scales having aspect ratios between 1 and 3 contributing most to the total energy, but also large scales correlated with scales seen in the thermodynamic variables.

Observations show that regions of anomalous deep convective El Nino-Southern Oscillation (ENSO) heating tend to be balanced by anomalous ENSO cooling elsewhere so that, averaged around the globe from (say) 10[degrees]S to 10[degrees]N, the net anomalous heating is nearly zero. The zonally symmetric heating is weak because it is approximately proportional to vertical velocity that, when averaged over a constant pressure surface S around the earth from 10[degrees]S to 10[degrees]N, is nearly zero. The horizontally averaged vertical velocity over S is small because the net horizontal geostrophic convergent flow across 10[degrees]S and 10[degrees]N is zero. Although the zonally symmetric ENSO heating is weak, the observed ENSO tropospheric air temperature anomaly has a large zonally symmetric component. Past work has shown that with weak momentum and thermal damping, Kelvin and Rossby waves can travel around the earth without significant loss of amplitude so that a zonally symmetric response is favored. This physical interpretation depends on knowing temperature and momentum anomaly damping times over the depth of the troposphere. Such times are not well known. Here a Gill tropical atmospheric model is generalized to include realistic surface friction and so theoretically estimate a frictional spindown time. Using this spindown time (approximately 3 weeks), together with an estimate of the Newtonian cooling time (1 month) the authors show, in agreement with observations, that the extremely weak zonally symmetric heating anomaly generates a symmetric air temperature anomaly comparable to the asymmetric one.

The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction--Department of Energy (NCEP-DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.

Air-sea transfer processes over the ocean strongly affect how hurricanes develop. High winds generate large amounts of sea spray, which can modify the transfer of momentum, heat, and moisture across the air-sea interface. However, the extent to which sea spray can modify extratropical or midlatitude hurricanes and intense cyclones has not been resolved. This paper reports simulations of extratropical Hurricanes Earl (1998) and Danielle (1998) and an intense winter cyclone from January 2000 using a mesoscale atmospheric model and a recent sea spray parameterization. These simulations show that sea spray can increase the sea surface heat flux, especially the latent heat flux, in a midlatitude cyclone and that sea spray's impact on cyclone intensity depends on the storm structure and development and is strongest for cyclones with high winds.

The dynamical origin of midlatitude zonal-jet variability is examined in a thermally forced, quasigeostrophic, two-layer channel model on a [beta] plane. The model's behavior is studied as a function of the bottom-friction strength. Two distinct zonal-flow states exist at realistic, low, and intermediate values of the bottom drag; these two states are maintained by the eddies and differ mainly in terms of the meridional position of their climatological jets. The system's low-frequency evolution is characterized by irregular transitions between the two states. For a given branch of model solutions, the leading stationary and propagating empirical orthogonal functions are related to eigenmodes of the model's dynamical operator, linearized about the climatological state on this branch. Nonlinear interactions between these modes are instrumental in determining their relative energy level. In particular, the stationary modes' self-interaction is shown to vanish. Thus, these modes do not exchange energy with the mean flow and, consequently, dominate the lowest-frequency behavior in the model. The leading stationary mode resembles the observed annular mode in the Southern Hemisphere. The bimodality is due to nonlinear interactions between nearly equivalent barotropic, stationary, and propagating modes, while the synoptic eddies play a modest role in determining the relative persistence of the two states. The role of synoptic eddies is very substantial only at unrealistically high values of the bottom drag, where they give rise to ultralow frequency variability by modifying the jet in a way that reinforces generation of the eddy field. This type of behavior is related to the presence of a homoclinic orbit in the model's phase space and is not apparent for more realistic, lower values of the bottom drag.

A multiscale model of the MJO is developed here that accounts, in a simplified fashion, for both the upscale transfer from synoptic to planetary scales of momentum and temperature from wave trains of thermally driven equatorial synoptic-scale circulations in a moving convective envelope as well as direct mean heating on planetary scales. This model involves idealized thermally driven congestus synoptic-scale fluctuations in the eastern part of the moving wave envelope and convective superclusters in the western part of the envelope. The model self-consistently reproduces qualitatively many of the detailed structural features of the planetary circulation in the observations of the MJO, including the vertical structure in both the westerly onset region and the strong westerly wind burst region, as well as the horizontal quadrupole planetary vortex structure. The westerly midlevel inflow in the strong westerly region and the quadrupole vortex are largely produced in the model by the upscale transport of momentum to the planetary scales, while the midlevel easterly jet in the westerly onset region is substantially strengthened by this process. The role of wave trains of tilted organized synoptic-scale circulations is crucial for this fidelity with observations. The appeal of the multiscale models developed below is their firm mathematical underpinnings, simplicity, and analytic tractability while remaining self-consistent with many of the features of the observational record.

A field program to study atmospheric releases around a complex building was performed in the summers of 1999 and 2000. The focus of this paper is to compare field data with a large-eddy simulation (LES) code to assess the ability of the LES approach to yield additional insight into atmospheric release scenarios. In particular, transient aspects of the velocity and concentration signals are studied. The simulation utilized the finite-element method with a high-fidelity representation of the complex building. Trees were represented with a canopy term in the momentum equation. Inflow and outflow conditions were used. The upwind velocity was constructed from a logarithmic law fitted to velocities obtained on two levels from a tower equipped with a 2D sonic anemometer. A number of different kinds of comparisons of the transient velocity and concentration signals are presented--direct signal versus time, spectral, Reynolds stresses, turbulent kinetic energy signals, and autocorrelations. It is concluded that the LES approach does provide additional insight, but the authors argue that the proper use of LES should include consideration of cost and may require an increased connection to field sensors; that is, higher-resolution boundary and initial conditions need to be provided to realize the full potential of LES.

An experimental and computational study to determine the effects of riblets on the performance of the Supersonic Throughflow Fan (STF) cascade blades was performed. The cascade was tested in the Virginia Tech intermittent wind tunnel facility, where the Mach and Reynolds (based on chord) numbers were 2.36 and 4.8 x [10.sup.6], respectively. The riblet sheets were symmetric v-grooved type and were applied onto the blade surfaces. Three different riblet heights were tested: 0.023, 0.033, and 0.051 mm. Riblet testing was conducted at design incidence as well as at off-design conditions (incidence angles: +5, -10 deg). Loss coefficients were measured and compared with a control test case where an equivalent thickness of smooth material was applied to the blade. Results show that at the design incidence, the riblet sheet with a height of 0.033 mm provides the optimal benefit, with a reduction of 8.5% in loss coefficient compared to the control case. Smaller effects were measured at the off-design conditions. In addition to the experimental study, a numerical investigation of the riblet effect on the STF cascade was conducted at design incidence. A simple method was developed to model riblet effects due to decrease in turbulent viscous drag and the delay of turbulent transition on the blades. Conclusions from numerical study indicate the 2/3 of the total decrease in losses are the result of delaying the transition location. The final 1/3 decrease in loss coefficient comes from the decrease in turbulent viscous losses. [DOI: 10.1115/1.1667883]

We present new experimental results on the interfacial instabilities and breakup of Newtonian liquid drops suddenly exposed to rarefied, high-speed (Mach 3) air flows. The experimental approach allows for the first time detailed observation of interfacial phenomena and mixing throughout the breakup cycle over a wide range of Weber numbers. Key findings are that Rayleigh-Taylor instability alone is the active mechanism for freestream Weber numbers as low as 28 for low viscosity liquids and that stripping rather than piercing is the asymptotic regime as We [right arrow] [infinity]. This and other detailed visual evidence over 26< We

The vertical co-current downward air-water two-phase flow was studied under adiabatic condition in round tube test sections of 25.4-mm and 50.8-mm ID. In flow regime identification, a new approach was employed to minimize the subjective judgment. It was found that the flow regimes in the co-current downward flow strongly depend on the channel size. In addition, various local two-phase flow parameters were acquired by the multisensor miniaturized conductivity probe in bubbly flow. Furthermore, the area-averaged data acquired by the impedance void meter were analyzed using the drift flux model. Three different distributions parameters were developed for different ranges of nondimensional superficial velocity, defined by the ration of total superficial velocity to the drift velocity.

Byline: M. A. Pogodin (1), E. V. Bobrova (1), M. P. Granstrem (1) Abstract: To clarify the mechanisms of interaction between voluntary and involuntary control of respiratory movements in a waking human, respiratory patterns were studied during self-controlled artificial ventilation used in place of natural breathing. Seven subjects controlled both the duration of artificial inhalations and the flow rate of air at excess pressure, continuously adjusting their actions to obtain the sensation of comfortable breathing. At rest, pulmonary ventilation was higher during self-controlled artificial breathing than during natural breathing. This trend was also noted during exercise. A correlation was observed between the velocity of the movement that started air flow and the artificial ventilation volume (r = 0.91). During self-controlled artificial breathing, the subjects sometimes took natural breaths. Natural inhalations did not influence the beginning or end of an artificial inhalation. Information received from respiratory receptors was assumed to play a certain role in the self-control of artificial breathing. Author Affiliation: (1) Pavlov Institute of Physiology, Russian Academy of Sciences, St. Petersburg, 199034, Russia Article History: Registration Date: 06/10/2004

'Supply Air' windows work as a heat reclaim device by intercepting a substantial proportion of incident solar energy as well as entraining heat escaping from rooms. This series of experiments [...]

Numerical works and experimental studies on upstream airflow around a cube reported in the literature will be reviewed. The 'bubble length' on the top of a rib or a cube [...]

Ability of a dc electrical discharge to control low-velocity airflow along a flat plate is analyzed. Specifically, the electrodes are flush mounted on the insulating surface of the plate creating a tangential corona discharge at close vicinity of the wall. In this paper, visualizations of the low-velocity airflow (up to 1.4 m/s corresponding to Re = 16 000) along the flat plate are presented. They show that the ionic wind induced by the corona discharge modifies the original airflow considerably, resulting in the airflow reattachment to the wall and reduction of the wake size. Velocity measurements by particle imaging velocimetry and by Pitot tube are conducted in a wind-tunnel loop for higher airflow velocities (up to 11 m/s corresponding to Re = 117 800). Results show that the corona discharge at such high airflow velocities does affect significantly the velocity profile within the viscous boundary layer. Index Terms--Airflow, corona discharge, electrohydrodynamic, flat plate, flow control.