Effects of axisymmetric contractions on turbulence of various scales

Digitally acquired and processed results from an experimental investigation of grid generated turbulence of various scales through and downstream of nine matched cubic contour contractions ranging in area ratio from 2 to 36, and in length to inlet diameter ratio from 0.25 to 1.50 are reported. An additional contraction with a fifth order contour was also utilized for studying the shape effect. Thirteen homogeneous and nearly isotropic test flow conditions with a range of turbulence intensities, length scales and Reynolds numbers were generated and used to examine the sensitivity of the contractions to upstream turbulence. The extent to which the turbulence is altered by the contraction depends on the incoming turbulence scales, the total strain experienced by the fluid, as well as the contraction ratio and the strain rate. Varying the turbulence integral scale influences the transverse turbulence components more than the streamwise component. In general, the larger the turbulence scale, the lesser the reduction in the turbulence intensity of the transverse components. Best agreement with rapid distortion theory was obtained for large scale turbulence, where viscous decay over the contraction length was negligible, or when a first order correction for viscous decay was applied to the results

Experiments on identification and control of inflow disturbances in contracting streams

Vorticity from all surfaces and isolated objects in the vicinity of the fan intake, including the outside surfaces of the fan housing, were identified as the major sources for disturbances leading to blade passing frequency noise. The previously proposed mechanism based on atmospheric turbulence is refuted. Flow visualization and hot wire techniques were used in three different facilities to document the evolution of various types of disturbances, including the details of the mean flow and turbulence characteristics. The results suggest that special attention must be devoted to the design of the inlet and that geometric modeling may not lead to adequate simulation of the in flight characteristics. While honeycomb type flow manipulators appear to be effective in reducing some of the disturbances, higher pressure drop devices that generate adequate turbulence, for mixing of isolated nonuniformities, may be necessary to suppress the remaining disturbances. The results are also applicable to the design of inlets of open return wind tunnels and similar flow facilities

On the Interpretation of the Output of Hot-Film Anemometers and a Scheme of Dynamic Compensation for Water Temperature Variation

Using a special calibration tunnel developed during the course of this study, the static and dynamic response of several kinds of commercially available hot-film probes with single and multiple sensors of the cylindrical-fiber type are examined. The effects of different parameters, including those of the anemometer bridge, on the output and performance of the probes are evaluated. In particular, the consequences of variations in water temperature on the hot-film anemometer output are determined. The results reveal a large effect of the water temperature on the calibration curves (in an extreme case a change in temperature of only 5.5°F can result in a 100% error in the mean velocity reading). In general, the Fourier components are inclined to the wall - the lower frequencies making smaller angles with the wall than the higher frequencies. The higher frequency disturbances became more nearly perpendicular to the wall in the central region of the pipe. For points very near the wall the disturbances appear to be very obliquely inclined. A scheme which utilizes a temperature sensing probe immersed in the working fluid is used to compensate for the water temperature variation. Several possible circuit configurations for this scheme, including an optimum circuit design, are investigated and the results from some of them are presented and discussed. The circuit has a frequency response to temperature variations which depends on the thermal time constant of the temperature probe (up to several cycles per second can be obtained using commercially available probes) and can be used to compensate for temperature variations of more than 20°F with an accuracy better than + 0.2%. By using an effective value (much smaller than EQ) instead of the zero- velocity bridge voltage (E0) in exponential-type linearizers, a constant exponent is found useful in linearizing the anemometer output over a wider range of velocities, especially the very low ones. Finally, a linearized hot-film anemometer compensated for temperature variation by utilizing the present scheme is successfully used to obtain precision measurements in a standard laminar flow- field where the water temperature varied. The results compare favorably with classical theory which is quite encouraging in view of the low overheat ratio used with hot-films and the large effects of temperature on water density and viscosity

Interpretation of 2-probe turbulence measurements in an axisymmetric contraction

Simultaneous measurements of the streamwise and radial velocity components at two points, one on and one off the centerline with variable radial separation, were digitally recorded and processed at several stations along a four to one contraction with controlled upstream turbulence conditions. Various statistical quantities are presented including spectra and coherence functions. The integral L sub ux, L sub um, L sub vx, L sub vm were also estimated and their variation along the contraction is examined

Effects of obstacle separation distance on gas explosions: the influence of obstacle blockage ratio

Obstacle separation distance (pitch) has received little systematic study in the literature. Either too large or small spacing between obstacles would lead to lesser explosion severity. Therefore, an optimum value of the pitch that would produce the highest flame acceleration and hence overpressure is needed. It was the aim of this work to investigate the influence of obstacle blockage ratio on the obstacle spacing in gas explosions. The explosion tests were performed using methane-air (10% by vol.), in an elongated vented cylindrical vessel 162 mm internal diameter with an overall length-to-diameter, L/D of 27.7. Double 20-40% blockage ratio, BR orifice plates were used as obstacles. The spacing between the obstacles was systematically varied from 0.5 m to 2.75 m. The 40% BR produced the highest explosion severity in terms of overpressure and flame speeds when compared to 30% and 20% BR. However, the worst case obstacle spacing was found to be shorter with increase in obstacle blockage. In general, similar profiles of overpressures and flame speeds were obtained for all the obstacle blockages. This trend was equally observed in the cold flow turbulence intensity profile generated behind a grid plate by other researchers. In planning the layout of new installations, the worst case separation distance needs to be avoided but incorporated when assessing the risk to existing set-ups. The results clearly demonstrated that high congestion in a given layout does not necessarily imply higher explosion severity as traditionally assumed. Less congested but optimally separated obstructions can lead to higher overpressures

Sonic Anemometry to Measure Natural Ventilation in Greenhouses

The present work has developed a methodology for studying natural ventilation in Mediterranean greenhouses by means of sonic anemometry. In addition, specific calculation programmes have been designed to enable processing and analysis of the data recorded during the experiments. Sonic anemometry allows us to study the direction of the airflow at all the greenhouse vents. Knowing through which vents the air enters and leaves the greenhouse enables us to establish the airflow pattern of the greenhouse under natural ventilation conditions. In the greenhouse analysed in this work for Poniente wind (from the southwest), a roof vent designed to open towards the North (leeward) could allow a positive interaction between the wind and stack effects, improving the ventilation capacity of the greenhouse. The cooling effect produced by the mass of turbulent air oscillating between inside and outside the greenhouse at the side vents was limited to 2% (for high wind speed, uo ≥ 4 m s−1) reaching 36.3% when wind speed was lower (uo = 2 m s−1)

Behavior of VNC in high-latency environments and techniques for improvement

Thin-client computing offers many advantages over traditional PC computing including lower costs, ubiquitous access to resources, higher security, and easier maintenance. Unfortunately thin clients rely upon the services of other networked computers to properly function. Because of this tight-knit relationship, high-latency environments can render thin clients practically unusable. With the VNC thin-client system, performance can be capped at 1 frame per round-trip time (RTT) due to its client pull communication style. We extend VNC to build VNC-HL and show how to improve frame rate performance by employing PR, a pre-requesting technique, to periodically request updates, even with several previous requests pending. Experimental results demonstrate up to an order of magnitude of improvement. VNC-HL achieved 14 FPS of multimedia computing in a 500 ms latency network with negligible additional resource consumption. In this thesis we demonstrate that VNC can be improved to sustain high frame rate performance in high-latency environments by employing a periodic pre-request at the desired FPS rate

Behavior of VNC in high-latency environments and techniques for improvement

Thin-client computing offers many advantages over traditional PC computing including lower costs, ubiquitous access to resources, higher security, and easier maintenance. Unfortunately thin clients rely upon the services of other networked computers to properly function. Because of this tight-knit relationship, high- latency environments can render thin clients practically unusable. With the VNC thin-client system, performance can be capped at 1 frame per round-trip time (RTT) due to its client pull communication style. We extend VNC to build VNC-HL and show how to improve frame rate performance by employing PR, a pre-requesting technique, to periodically request updates, even with several previous requests pending. Experimental results demonstrate up to an order of magnitude of improvement. VNC-HL achieved 14 FPS of multimedia computing in a 500 ms latency network with negligible additional resource consumption. In this thesis we demonstrate that VNC can be improved to sustain high frame rate performance in high-latency environments by employing a periodic pre-request at the desired FPS rat

VNC in High-Latency Environments and Techniques for Improvement

Abstract—VNC (Virtual Network Computing), a form of thinclient computing, offers many advantages over traditional desktop computing including lower costs, higher security, ubiquitous access to resources, and easier maintenance. In this paper, we focus on the problems of using VNC to display video over a high-latency network, and how to solve them. VNC performance is inherently capped at 1 frame per round-trip time (between the client and server) due to its client-pull communication style. We describe VNC-HL, our extension of VNC, which improves frame rate performance by employing a pre-requesting technique to periodically request updates, even with several previous requests pending. Experimental results demonstrate up to an order of magnitude of improvement. VNC-HL achieved 14 FPS over a network with 500 ms of latency. I