Horizontal Air Velocity Research Articles

Case study of a multiple confluents jets system in a virtual chamber

This article presents a case study of a multiple confluents jets system in a virtual chamber. In this study, it is obtained the behavior of the airflow in an office, simulated by a virtual camera, whose ventilation system is of the horizontal confluent jets type. The ventilation system has an inlet system consisting of four vertical ducts mounted in the corners of the compartment. Each duct has four groups of 32 nozzles, aligned and equally spaced, positioned from the base of the duct close to the floor. Each two groups of nozzles are directed towards each of the walls adjacent to the duct The exhaust system consists of a quadrangular vertical duct mounted close to the ceiling and positioned in the center of the compartment. The study uses a numerical differential model in order to simulate the Computational Fluids Dynamics. Ventilation by horizontal confluent jets creates a horizontal air velocity close to the wall surface as well as the floor surface. With the increase in the exit velocity of the jets, the interactive effect of the confluence of the jets, coming from opposite ducts, in the central area of the wall becomes more evident. The airflow resulting from the interactions of the jets has an upward characteristic towards the exhaust system.

Airflow patterns and turbulence characteristics above the canopy of a tomato crop in a roof-ventilated insect-proof screenhouse

Airflow patterns and turbulence characteristics inside a naturally ventilated screenhouse with tomato plants were experimentally investigated using 3D sonic anemometers simultaneously measuring air velocities at six horizontal positions in the space between the screened roof and the top of the canopy. The screenhouse had a flat roof and was ventilated through the roof only. Testing was carried out during about 3 weeks of crop development, under variable external conditions. The mean horizontal air velocity component above the canopy was generally lower than 0.20 Uoh (where Uoh is the mean horizontal external wind speed) and mostly in a direction opposite to the external wind. At low external windspeed, the airflow direction was not necessarily opposite to the wind direction. The vertical air velocity component was generally lower than 0.07 Uoh. The root mean square of the air velocity components, the turbulence intensity and the turbulence kinetic energy were much larger during the day than at night mainly due to the higher daytime effect of buoyancy and to a lesser extent due to the higher external wind speed. Spectral energy slopes of the velocity components were close to −5/3 during the day when wind speed was high. However, at night, when wind speed was low, the spectral energy slope of the velocity vector increased significantly. The integral length scales of turbulent eddies were greater in the horizontal than the vertical direction which is not surprising given the volume under consideration is bounded by the top of the canopy, the roof and the sidewalls.

Experimental analysis of heat transfer and airflow in a closed refrigerated display cabinet

This study presents the experimental investigations on heat transfer and airflow in a closed refrigerated display cabinet. Air and product temperatures and air velocity were measured with thermocouples and a hot-wire anemometer, respectively. Temperature variation in the cabinet depends on the positions. The front areas contributed to higher temperature, whereas the rear areas were at a lower temperature. Benefits of doors were also examined by comparing the results of air and product temperatures with the case without doors. The cabinet with doors provided less temperature heterogeneity (ΔTmax= 2.1 °C) compared to the case without door (ΔTmax= 4.9 °C). The maximum air velocity in the air curtain of 0.6 m s−1 was observed at the discharge grille. The horizontal air velocity from the perforated plate was low (<0.2 m s−1) for all shelves. The loading percentage in the cabinet did not significantly affect the airflow rate through the perforated plate.

Two-dimensional analytical model of dry air thermal convection

In the present work, the steady-state stationary dry air thermal convection in a lower atmosphere has been studied theoretically. The thermal convection was considered without accounting for the Coriolis force, and with only the vertical temperature gradient. The stream function has been analytically obtained within the framework of two-dimensional thermal convection model in the Boussinesq approximation with velocity divergence taken as zero. It has been shown that the stream function is symmetrical about the horizontal and vertical. The expressions for the horizontal and vertical air velocity components have been obtained. The maximal vertical velocities level is in the center of the convective cell where the horizontal air velocity component is equal to zero. It has been shown that the air parcel’s rotation period during the thermal convection is determined by the Brunt–Vaisala frequency. The expression for the maximal air velocity vertical component has been found. The dependence of the maximal air velocity vertical component on the overheat function at ground surface and on the atmosphere instability has been demonstrated. The expression for the pressure disturbance has been obtained. It has been demonstrated that at the points with maximal pressure disturbance the vertical velocity is equal to zero and the horizontal velocity is maximal. It has been found that the convection cell size depends on the atmosphere stability state.

Natural convection in a partially open square cavity with internal heat source: An analysis of the opening mass flow

A steady buoyancy-driven flow of air in a partially open square 2D cavity with internal heat source, adiabatic bottom and top walls, and vertical walls maintained at different constant temperatures is investigated numerically in this work. A heat source with 1% of the cavity volume is present in the center of the bottom wall. The cold right wall contains a partial opening occupying 25%, 50% or 75% of the wall. The influence of the temperature gradient between the verticals walls was analyzed for Rae=103–105, while the influence of the heat source was evaluated through the relation R=Rai/Rae, investigated at between 400 and 2000. Interesting results were obtained. For a low Rayleigh number, it is found that the isotherm plots are smooth and follow a parabolic shape indicating the dominance of the heat source. But as the Rae increases, the flow slowly becomes dominated by the temperature difference between the walls. It is also observed that multiple strong secondary circulations are formed for fluids with a small Rae whereas these features are absent at higher Rae. The comprehensive analysis is concluded with horizontal air velocity and temperature plots for the opening. The numerical results show a significant influence of the opening on the heat transfer in the cavity.

Behaviour of a curtain of particles falling through a horizontally-flowing gas stream

With a view to an application in heat recovery from hot gases, the fluid dynamics of a curtain of particles falling in a horizontal gas stream was studied. Air with a uniform velocity profile flowed through a duct with cross-section of 0.15 × 0.60 m. A stream of sand particles, with a mean diameter of 204 μm and density of 2640 kg/m 3, was fed into the duct from above to form a curtain across the entire width of the duct. The particle flowrates ranged from 0.031 to 0.047 kg/s. The inlet particle curtain thickness in the direction of the gas flow ranged from 0.02 to 0.10 m, and the horizontal air velocity was varied from zero to 1.2 m/s. In an experimental study, the trajectory of the particle curtain was tracked using high-speed video camera and the velocity profile of the air downstream of the curtain was measured using a pitot tube. The fluid dynamic behaviour of the particle curtain was analysed using a Eulerian–Eulerian computational fluid dynamics (CFD) model that is capable of predicting the complete flow fields of the gas and particles. Also, a simple single particle model was used to predict the particle velocity at the centerline and trajectories of the leading and trailing edges of the particle curtains falling in a horizontal air flow. Good agreement was found between the experiments and the CFD model predictions. However, predictions from the simple single particle model were less satisfactory due primarily to the non-uniform gas flow found in practice, compared to the assumed uniform flow.

Lee-side Ventilation-induced Air Movement in a Large-scale Multi-span Greenhouse

Lee-side ventilation-induced air movement was experimentally investigated in a 1728 m2multi-span glasshouse. Air velocity measurements were performed by means of a customized multi-point two-dimensional sonic anemometer system which simultaneously measured the horizontal air velocity components parallel and normal to the ridge at 3 Hz frequency. The airflow pattern in the horizontal plane of a greenhouse was analysed qualitatively under different external wind conditions. The experimental results show both the vertical and the horizontal profiles of the normalized interior air speed as a function of the measurement height, the external wind speed and opening angle were acceptably presented by the simple linear functions. The airflow in the horizontal plane was of low turbulence with large eddies.

A study of a CASP storm: Analysis of radar data

Abstract The morphology and time evolution of a winter storm is studied using radar data taken during the Canadian Atlantic Storms Program (CASP). The vertical motions that generate the snow are derived from reflectivity measurements. The study reveals a banded structure in the precipitation pattern with ascending and descending air associated with the bands. Vertical velocities averaged over the radar‐covered area reach values close to 1 m s‐1 . The region of large‐scale uplifting moves horizontally at about half the horizontal air velocity. A persistent precipitation pattern results from the continuously renewed air within the uplifting region.

On the aerodynamics of animal flight in ground effect

Flight in ground effect above a flat, smooth surface may give an animal considerable performance advantages, including a reduction in cost of transport of up to 15%, and a reduction in mechanical flight power of as much as 35%, compared with values for flight out of ground effect. Previous theories modelling the phenomenon have either been incomplete or marred by typographical errors. A complete lifting line theory of flight in ground effect with a fixed wing is developed, and instructions are given so that it may be applied to animals such as skimmers, pelicans and myotid bats which fly and forage close above water. Several predictions are made about likely flight behaviour in ground effect, and about the appropriate flight morphology for taking advantage of the potential performance improvements. The most important conclusion, differing from previous analyses, is that slow flight performance in ground effect is very poor, owing to the horizontal air velocity induced around the wing in the presence of the ground.