Experimental and computational studies were performed on the 1.27m wide three-spindle lawn mower deck with side discharge arrangement. Laser Doppler Velocimetry was used to measure the air velocity at 12 different sections under the mower deck. The high-speed video camera test provided valuable visual evidence of airflow and grass discharge patterns. The strain gages were attached at several predetermined locations of the mower blades to measure the strain. In computational fluid dynamics work, computer based analytical studies were performed. During this phase of work, two different trials were attempted. First, two-dimensional blade shapes at several arbitrary radial sections were selected for flow computations around the blade model. Finally, a three-dimensional full deck model was developed and compared with the experimental results.
A two-dimensional analysis was carried out on the locomotion by tandem and parallel wings in relation to the free flight of dragonflies and beetles, remarking the mutual interference between fore and hind wings. The results obtained are summarized as follows: In the case of tandem wings, (1)High thrust and propulsive efficiency can be achieved when the forewing oscillates with a definite phase lag behind the hindwing, as in the case of real dragonflies, (2)Somewhat smaller amplitude of hindwing leads to optimum condition for work sharing of two wings, (3)The hard forewing does not serve for the thrust and propulsive efficiency, whereas the hard hindwing does for the augmentation of them; In the case of parallel wings, (4)The hard wing placed near the soft wing acts nearly as an infinite plate, as for the ground effect, increasing both thrust and propulsive efficiency.
Vortex shedding behind a square cylinder in an oscillatory incoming flow is studied at Reynolds numbers of 80, 100 and 200 by numerical solutions. The forcing frequencies investigated for the oscillatory incoming flow are ff/fso=0.4∼4.0 and the forcing amplitudes normalized by the mean incoming velocity are 0.1, 0.2 and 0.4, where fso is the vortex shedding frequency under steady incoming flow and ff is the forcing frequency of the oscillatory incoming flow. In the lock-on region, the flow is in a periodic state and there is a strong regularity between the drag and lift forces. The different phase diagram between the drag and lift forces is brought on by the different lift mode. The phase difference between the incoming velocity and the drag is nearly the same magnitude of 65∼70 degrees in the lock-on region. The time-averaged mean recirculation region in the oscillatory incoming flow is smaller than that for the steady one and is inversely proportional to the forcing amplitude or the Reynolds number. The time-averaged streamwise mean velocity in the oscillatory incoming flow recovers more quickly than that in the steady incoming flow.
Reliable design of space thermal management systems requires a through understanding of the hydrodynamic characteristics of two-phase flow influenced by the change in gravity. The data of flow patterns, void fraction, frictional pressure drop associated with its characteristics were obtained at normal gravity and in microgravity and hyper-gravity (2g) conditions aboard MU-300 aircraft capable of parabolic trajectory flying. Some experiments were performed for an air-water two-phase flow through 10mm diameter adiabatic test section with 600mm length of transparent acrylic resin horizontal tube. The results obtained at three gravity levels (µg, 1g and 2g) are compared with some of the existing flow pattern transition, void fraction and frictional pressure drop models and correlations. The gravity dependency of flow patterns was more clearly appeared with the decrease in gas and liquid flow rates. The effect of gravity on two-phase flow was insignificant for the turbulent flow regions.
This is an experimental study of a turbulent cylindrical wall jet that issues from an annular nozzle, flows along a cylindrical wall and finally impinges normally on a flat plate. In order to disturb the flow near the wall and to control the thickness of the wall shear layer, a front-facing step is equipped at an arbitrary location on the cylindrical wall. This type of jet flow has properties characterized by a pressure increase and boundary layer growth in the downstream direction, so that the flow separates from the cylindrical wall and reattaches to the impingement plate. This report mainly concerns the effects on the separation and reattachment properties of the flow with various nozzle-step distances and step-impingement distances. Mean velocity and static pressure on the cylindrical wall were measured. The flow patterns in the x-y plane normal to the cylindrical wall in the region near the step and before the impingement plate were obtained by the mean flow vectors and the flow visualization by an oil film method. The results show that the flow patterns can be classified into three main types.
Traffic tunnels built in recent years are equipped with traffic counters and pollution sensors. Utilizing these built-in sensors, it is possible to develop an algorithm to estimate the amount of pollutants exhausted from the various types of vehicles passing through the tunnel. Also, with this estimated data and the sensor outputs, more accurate pollution levels can be assessed utilizing a Kalman filter. The diffusion of pollutants in a tunnel can be described with a one-dimensional diffusion and advection equation. This equation is approximated with interpolation functions and a weighted residual method converts to an adequate form for standard state estimate algorithms. With this converted equation, a least square based algorithm is developed, whose outputs are the estimated amounts of pollutants emitted from each type of vehicles. Also, a Kalman filter is utilized to more accurately estimate the pollution levels of the tunnel under the existence of model and measurement uncertainties. In order to verify the feasibility of the developed algorithms, experiments and simulations are performed. The real data is acquisitioned from the Dunnae tunnel located in Young-Dong highway in Korea. The estimated emission rates and the computer simulated concentration levels agree reasonably to the measured values.
The extinction of stretched premixed flames propagating in a stagnation-point flow under the influences of dilute fuel spray, preferential diffusion and upstream external heat loss is analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. The liquid fuel loading and the initial droplet size of the spray indicate the internal heat gain and heat loss for the lean and rich sprays, respectively. The flow stretch coupled with Lewis number (Le) intensifies the burning intensity of the Le>1 flame, but weakens that of the Le<1 flame. It is found that theLe>1 flame can be quenched with or without external heat loss, and that both the external heat loss and the flow stretch strongly dominate the trend for flame extinction characterized by a C-shaped extinction curve. In the absence of external heat loss, no extinction occurs under the influence of flow stretch for the Le<1 flame enduring completely prevaporized burning sprays. However, it can be extinguished by the external heat loss characterized by a C-shaped extinction curve if the flow stretch is small. Under the condition of Le<1 flames experiencing a small flow stretch and enduring a partially prevaporized spray composed of a large enough liquid loading and sufficiently large droplet size, a W-shaped extinction curve is obtained. The W-shaped extinction curve in distinction from the C-shaped one points out that the flame extinction is governed by the internal heat loss.
The contact between fin collar and tube surface of a fin-tube heat exchanger is secured through mechanical expansion of tubes. However, the characteristics of heat transfer through the interfaces between the tubes and fins have not been clearly understood because the interfaces consist partially of metal-to-metal contact and partially of air. The objective of the present study is to develop a new method utilizing an experimental-numerical method for the estimation of the thermal contact resistance between the fin collar and tube surface and to evaluate the factors affecting the thermal contact resistance in a fin-tube heat exchanger. In this study, heat transfer characteristics of actual heat exchanger assemblies have been tested in a vacuum chamber using water as an internal fluid, and a finite difference numerical scheme has been employed to reduce the experimental data for the evaluation of the thermal contact conductance. The present study has been conducted for fin-tube heat exchangers of tube diameter of 7mm with different tube expansion ratios, fin spacings, and fin types. The results show, with an appropriate error analysis, that these parameters as well as hydrophilic fin coating affect notably the thermal contact conductance. It has been found out that the thermal contact resistance takes fairly large portion of the total thermal resistance in a fin-tube heat exchanger and it turns out that careful consideration is needed in a manufacturing process of heat exchangers to reduce the thermal contact resistance.
A numerical study is performed to predict the thermal response of a card assembly during infrared reflow soldering to attach electronic components to a printed circuit board. The convective, radiative and conduction heat transfer within the reflow oven as well as within the card assembly are simulated. Parametric study is also performed to analyze the sensitivity of the thermal response of electronic components to various conditions such as conveyor speed, exhaust velocities and emissivities. The predictions of the detailed card assembly thermal response can be used to select the oven operating conditions such as infrared panel heater temperatures, conveyor speed, and exhaust velocity for each card assembly to ensure proper soldering and minimization of thermally induced card assembly stresses.
In this investigation, confined vortex shedding past a square cylinder with a planar jet is numerically studied. Flow and scalar-transport simulations are presented for various cases including both laminar and turbulent flow situations. It is shown that the ratio of jet velocity to uniform inlet velocity significantly affects the overall flow structures and thus scalar transport downstream of the cylinder. Especially, when the ratio is large enough, the jet penetrates the main vortices shed from the cylinder, resulting in significant changes in the flow and scalar fields. In the case of laminar flow, regions of intense scalar are formed along the streamlines from the jet exit, and the oscillation of the force on the cylinder eventually disappears as the jet velocity is close to the inlet velocity. Large Eddy Simulation of turbulent flow also reveals complex flow structures and intense mixing depending on the velocity ratio; regions of intense scalar coincide with those of high turbulence intensity. The results obtained exhibit fuel-air mixing characteristics observed in a planar combustor where the square cylinder plays the role of a flame-holder.
A combined experimental and computational investigation was performed in order to evaluate the effects of various design parameters of an in-line injection pump on the nozzle exit characteristics for DI diesel engines. Measurements of the pump chamber pressure and the delivery valve lift were included for validation by using specially designed transducers installed inside the pump. The results confirm that the simulation model is capable of predicting the pump operation for all the different designs investigated pump operating conditions. Following the successful validation of this model, parametric studies were performed which allow for improved fuel injection system design.
A three-dimensional simulation technique for the stratified combustion process in direct injection gasoline engines is developed. The effects of a widely distributed mixture equivalence ratio and a large amount of EGR on laminar flame speed are briefly modeled taking into account only the temperature of the unburned mixture and the flame temperature. The suggested laminar flame speed model is incorporated into a CFD code in combination with the coherent flame model. In burned gas, chemical equilibrium depending on the local equivalence ratio is assumed so that the post flame reaction upon mixing rich burned gas and lean burned gas or fresh air can be simply modeled as a change of the equilibrium. The calculated flame propagation process, heat release rate and exhaust emissions are confirmed by the results of measurements including the LIF technique. The good agreements obtained under various conditions indicate the applicability of this method.