Low-speed Air Flow Research Articles

Heat Exchange Effectiveness and Pressure Drop for Air Flow Through Perforated Plates With and Without Crosswind

Low-porosity perforated plates are being used as absorbers for heating ambient air in a new type of unglazed solar collector. This paper investigates the convective heat transfer effectiveness for low-speed air flow through thin, isothermal perforated plates with and without a crosswind on the upstream face. The objective of this work is to provide information that will allow designers to optimize hole size and spacing. In order to obtain performance data, a wind tunnel and small lamp array were designed and built. Experimental data were taken for a range of plate porosities from 0.1 to 5 percent, hole Reynolds numbers from 100 to 2000, and wind speeds from 0 to 4 m/s. Correlations were developed for heat exchange effectiveness and also for pressure drop. Infrared thermography was used to visualize the heat transfer taking place at the surface.

6 濃度相関法による低速気流の画像解析

6 濃度相関法による低速気流の画像解析

The Roseville Bomb Disaster Simulated Train Braking System Tests and Boxcar Wood Floor Ignition Experiments

A massive chain of property damaging explosions involving an ammunition train occurred at the railroad yard, Roseville, California. The train had pulled into the yard after a night trip of some 100 miles across Donner Summit and down the extended Norden-Roseville grade. Physical evidence confirmed that first explosions were centered at a DODX type boxcar loaded with 250 lb. bombs. Further, bomb cook-off detonation tests established that the triggering bomb blast was not a result of shock loads but rather derived from an engulfing fire initiated in the boxcar wood plank floor under influence of extended heavy braking action on the mountain grade. It was also suspected that high friction composition brake shoes were fitted on the car as replacements for cast iron shoes but the brake mechanical linkage lever ratios had not been modified as required. Results of a comprehensive research program are presented within context of the explosion event, and include analytical computer simulation of train descent profiles on mountain grades through full scale dynamometer tests with actual rail wheels and ultimately more scientific scaled wood floor ignition experiments in the laboratory. The thermal response of a simulated DODX boxcar wood floor was studied through experiments, full scale at a rail wheel dynamometer test facility, and in the laboratory. Certain input data for the wood floor ignition test program were measured on an actual boxcar joined with a freight train consist in transit down the Norden-Roseville grade. Two series of scaled wood ignition experiments were conducted on simulated DODX boxcar floors. Objectives of these tests were to determine: Influence of a cooling air stream on the ignition behavior of radiantly heated wood surfaces, and effectiveness of DODX (stand-off) and AAR (flush) type spark shields in preventing ignition of wood surfaces under radiant heating. It was found that for radiant heat flux levels representative of high friction composition shoes under severe train braking conditions, low speed airflow (wind) exerts a dramatic influence on the wood ignition time. For example, average ignition time for a simulated boxcar floor at a heat flux level of 0.4 cal/cm2sec was determined to be 15.6 min. with a 5 mph wind as compared to 3.6 min. with no wind. In the spark shield effectiveness tests, conducted at heat flux levels representative of cast iron shoes under severe braking conditions, the DODX (stand-off) type spark shield failed to prevent spontaneous flaming ignition of a wood surface directly above it. Under identical conditions, no flaming ignition was encountered with the AAR (flush) type spark shield.

A new pulsed-wire probe for measuring the Reynolds stresses in the plane containing the main shear direction of a turbulent shear flow

The design, manufacturing procedures and results of testing a new pulsed-wire probe are presented. This probe is used for measuring Reynolds stresses in highly turbulent low speed air flows containing high mean shear gradients. Pulsed wire probes of the standard type suffer from measurement errors when the pulsed wire is oriented in or at 45 degrees to the main shear direction of such flows. This is due to the shearing of the thermal cloud giving the time of flight information which is used for measuring the instantaneous velocity. The new probe design proposed is designed in a fashion to avoid these 'gradient effects'; it is furthermore shown that aerodynamic interference effects have also been minimised. A detailed description of the manufacturing process is given.

A New Probe for Measurement of Velocity and Wall Shear Stress in Unsteady, Reversing Flow

Separated turbulent flows exhibit instantaneous reversals of flow direction which make measurement of the velocity field extremely difficult. A three-wire heat-tracer technique has been employed to measure streamwise velocity of a low-speed air flow very near a smooth, solid wall; the wall shear stress is calculated using a similarity hypothesis. Initial results presented show the evolution of average wall shear stress and rms fluctuation intensity of wall shear stress in a reattaching 2-D flow downstream of a backward-facing step.

An experimental and analytical study of a new flap valve for generating simulated sonic booms

The design, operation, and performance of a sonic boom simulator, featuring a radically new dual-flap valve and electromechanical control system, are described. This new flap valve with its large maximum throat area (160 cm 2) was designed to regulate the air flow from a low pressure reservoir (up to 0·2 atm overpressure) into the apex of a large pyramidal horn (25 m long, 3 m × 3 m base), where the incoming low speed air flow (up to 150 m/s) produces a travelling simulated sonic boom or N-wave with relatively little superposed high frequency noise. As a consequence, the full scale simulated sonic boom is virtually free of superposed jet noise, a major advance over past work with such horn-type simulators. Additionally, an advanced gasdynamic analysis of the reservoir coupled with an advanced acoustic analysis of the wave motion in the horn is presented to predict the characteristics of the simulated sonic boom—wave form, amplitude, duration, and rise time. Predicted and measured overpressure signatures are shown to be in excellent agreement.

On Fiber Alignment Using Fluid-Dynamic Forces

A simple approach to the alignment of short fibers moving in an air stream was explored, in which transverse air jets acted to alter main stream velocity and direction in a desired manner. An approximate analysis was developed for the effects of aerodynamic forces on fiber motion and was combined with a theory for the influence of transverse air jets on the main flow. Experiments on fiber alignment using this approach were performed, the principal data being multiflash photographs of the fiber motion. The approximate theory is in qualitative agreement with the experimental results. These results showed that fiber alignment parallel to the main stream can be achieved for a wide range of initial fiber angles. Further, this alignment can be performed with low-speed air flows, which may facilitate fiber handling in many applications.

Pressure and force distributions on a sharp-nosed circular cylinder at large angles of inclination to a uniform subsonic stream

This paper reports an experimental investigation of pressure and force distributions on a sharp-nosed circular cylinder inclined to a uniform low-speed air flow under conditions of laminar separation of the boundary layer. The main concern is with the out-of-plane force (i.e. the side force if the body is at incidence). The experimental model consisted of an extensively pressure-tapped cylinder to which four different noses were fitted. The results show that there is an oscillatory distribution of out-of-plane force along the cylinder for most of the inclination range 0-90°. The amplitude of this distribution is strongly affected by nose shape in conditions where the out-of-plane force extends onto the nose. At very high angles of inclination the oscillatory distribution disappears and is replaced by a vortex pattern like that found on an infinite yawed cylinder. The general nature of the out-of-plane force is found to be consistent with the impulsively started flow analogy. Unsteadiness in the flow was found to cause a serious reduction in many of the time-averaged values. The unsteadiness is ascribed to the switching of the flow pattern due to free-stream turbulence. Measurements of the time histories of certain pressures enabled values of the force in the unswitched state to be calculated. The Reynolds number was found to have an important influence at inclinations above 55°. However, it was also found that the range of Reynolds numbers over which this effect occurs can depend on the scale of the model.

Combined forced and natural convection with low-speed air flow over horizontal cylinders

This article describes experimental work on the mixed convection régime with flow normal to electrically heated cylinders. The forcing velocities used were in the range 0·0085–3 ft./sec (i.e. 10−2 < Ref < 45) and temperature differences in the range 30°C to 200°C (i.e. 10−3 < Ra < 10) were covered.Correlations are proposed for the forced convection and natural convection conditions. A correlation is also developed for the combined forced and natural convection region by a vectorial addition of the flow parameters, which gives good agreement with the experiments except over a limited range in the contraflow régime.

Effects of air Movement at the Hive Entrance on the Flight Activities of Honeybees

SummaryLow- and high-speed airflows were forced through and across hive entrances to observe changes in the flight activity of honeybees. With hives having two entrances, more bees consistently entered their hives where a low-speed airflow was moving outwards, and departed where the airflow was inwards. A high-speed airflow parallel to the hive entrance hindered most bees from leaving, and stopped all bees from returning.

Traversing orifice hot-wire probe pair for use in wall pressure correlation measurements

Tests are described on an orifice hot-wire probe suitable for wall pressure fluctuation measurements. The probe was found to respond to the true pressure for both acoustic and turbulent flow excitation, with a useful frequency response up to 5 kHz in air at atmospheric pressure. A traversing pair of probes proved to be accurate for the measurement of wall pressure correlations in lowspeed air flows.