O. Igra

Research and development for shrouded wind turbines

Abstract In order to exploit wind power as economically as possible, it was suggested that the wind turbine should be enclosed inside a specially designed shroud. Various geometries are discussed; it is shown that with a fairly compact shroud a significant power augmentation can be achieved. For improving the shroud performance, the use of a ring-shaped flap or boundary layer control technique is introduced. It is shown that up to 80% improvement in the shroud power augmentation can be obtained by the use of an appropriate ring-shaped flap while proper bleeding of the shrouds external flow into its inner rear part will increase its power augmentation by about 25%. The present review also discussed the design and performance of an axial flow turbine which is the most suitable for the proposed shrouds. It is shown that such a turbine produces a fairly stable output for varying wind speeds while exhibiting a fairly high efficiency. Based on the reported research with shrouds, a pilot plant producing 1 hp at 5 m/s with a 3 m dia. turbine was built. Its design and preliminary field test results are also included in the review.

Shock waves attenuation by granular filters

Abstract Proper design of protecting filters mitigates the effect of blast and shock waves and thereby makes such filters effective protection against both accidental and planned explosions. The main goal of the present study was to clarify the filter performance in reducing the loading on structures as well as reducing the strength of the transmitted shock. While most of the granular filters used for protection in the past were composed of sand or rock particles, in the present study the investigated granular filters were composed of small spherical particles. This was done in order to exclude the influence of the particle shape and to ease the numerical simulation of the filter performance. Moreover, in the simulations we neglected real effects such as particles movement and its rearrangement during the shock wave propagation and only discussion regarding the dependence of the granular filter performance on its length and composition is provided. Based on a comparison between experimental results and appropriate numerical simulations obtained for the pressure profiles inside and downstream of the filter it was found that the attenuation performance of the filter can be well predicted using a simple one-dimensional approach to the real, more complicated problems.

Shock wave interaction with cellular materials

The equations governing the head-on collision of a planar shock wave with a cellular material and a numerical scheme for solving the set of the governing equations were outlined. In addition, the condition for the transmitted compression waves to transform into a shock wave, inside the cellular material was introduced. It was proved analytically that a cellular material cannot be used as a means of reducing the pressure load acting on the end-wall of the shock tube. In subsequent papers, the interaction of planar shock waves with specific cellular materials, e.g., foams and honeycombs will be described in detail.

Shock wave interaction with cellular materials: Part II: open cell foams; experimental and numerical results

In the Part I of this study, namely the analytical part in Mazor et al. (1992), the governing equations of the phenomenon in which a planar shock wave collides head-on with a cellular material and interacts with it were developed using a Lagrangian approach. In addition, the numerical approach adopted by us during the numerical course of this study was briefly outlined there. The present part reports on experimental and numerical results of the head-on reflection of a planar shock wave with an open cell polyurethane foam. Foams as mentioned by Gibson and Ashby (1988) and summerized in Part I of this study by Mazor et al. (1992), are one of the two general types of cellular materials.

Experimental and numerical study of the interaction between a planar shock wave and a square cavity

The interaction of a planar shock wave with a square cavity is studied experimentally and numerically. It is shown that such a complex, time-dependent, process can be modelled in a relatively simple manner. The proposed physical model is the Euler equations which are solved numerically, using the second-order-accurate high-resolution GRP scheme, resulting in very good agreement with experimentally obtained findings. Specifically, the wave pattern is numerically simulated throughout the entire interaction process. Excellent agreement is found between the experimentally obtained shadowgraphs and numerical simulations of the various flow discontinuities inside and around the cavity at all times. As could be expected, it is confirmed that the highest pressure acts on the cavity wall which experiences a head-on collision with the incident shock wave while the lowest pressures are encountered on the wall along which the incident shock wave diffracts. The proposed physical model and the numerical simulation used in the present work can be employed in solving shock wave interactions with other complex boundaries.

Experimental and theoretical study of shock wave propagation through double-bend ducts

The complex flow and wave pattern following an initially planar shock wave transmitted through a double-bend duct is studied experimentally and theoretically/numerically. Several different double-bend duct geometries are investigated in order to assess their effects on the accompanying flow and shock wave attenuation while passing through these ducts. The effect of the duct wall roughness on the shock wave attenuation is also studied. The main flow diagnostic used in the experimental part is either an interferometric study or alternating shadow–schlieren diagnostics. The photos obtained provide a detailed description of the flow evolution inside the ducts investigated. Pressure measurements were also taken in some of the experiments. In the theoretical/numerical part the conservation equations for an inviscid, perfect gas were solved numerically. It is shown that the proposed physical model (Euler equations), which is solved by using the second-order-accurate, high-resolution GRP (generalized Riemann problem) scheme, can simulate such a complex, time-dependent process very accurately. Specifically, all wave patterns are numerically simulated throughout the entire interaction process. Excellent agreement is found between the numerical simulation and the experimental results. The efficiency of a double-bend duct in providing a shock wave attenuation is clearly demonstrated.

Drag coefficient of a sphere in a non-stationary flow: new results

The drag coefficient of a sphere placed in a non-stationary flow is studied experimentally over a wide range of Reynolds numbers in subsonic and supersonic flows. Experiments were conducted in a shock tube where the investigated balls were suspended, far from all the tube walls, on a very thin wire taken from a spider web. During each experiment, many shadowgraph photos were taken to enable an accurate construction of the spheres trajectory. Based on the spheres trajectory, its drag coefficient was evaluated. It was shown that a large difference exists between the sphere drag coefficient in steady and non-steady flows. In the investigated range of Reynolds numbers, the difference exceeds 50%. Based on the obtained results, a correlation for the non-stationary drag coefficient of a sphere is given. This correlation can be used safely in simulating two-phase flows composed of small spherical particles immersed in a gaseous medium.

Influence of surface roughness on the transition from regular to Mach reflection in pseudo-steady flows

The effect of surface roughness on the transition form regualr (RR) to Mach reflection (MR) over straight wedges in pseudo-steady flows was investigated both experimentally and analytically. A model for predicting the RR \rightleftarrows

Gas filtration during the impact of weak shock waves on granular layers

MR transition in the ( M i , θ w )-plane was developed ( M i is the incident shock wave Mach number and θ w is the reflecting wedge angle). Its valdity was checked agnainst experimental results. Since the experimental results are limited to the ranges 1 M i \rightleftarrows

Acceleration of a sphere behind planar shock waves

MR transition is related to the boundary - layer thickness which in turn depneds on the surface roughness.