Single Cylindrical Hole Research Articles

Numerical investigation of area ratio on sister hole applications at a constant exit area

A numerical study was performed to investigate the effect of the area ratio at sister hole applications on film cooling effectiveness and flow structure. Two secondary coolant holes are placed on both sides of the main hole at a lateral distance of one main hole diameter. Four different area ratios (AR = 2–5) between the main hole and sister hole were studied. A single cylindrical hole was also studied for comparison and baseline. The structured mesh was generated and realizable k-ε turbulence model with scalable wall function was used in these simulations. Blowing ratios of 0.25, 0.50, 0.80, and 1 were simulated to evaluate the effect of sister holes in practical applications. The centerline cooling effectiveness and laterally averaged cooling effectiveness are studied as well as temperature contours and streamlines at two different streamwise locations (x/D = 1 and x/D = 3). The area ratio on sister hole application has an important effect on cooling effectiveness as the sister hole exit area affects the Anti- Counter Rotating Vortex intensity (A-CRVP vorticity) and size. The positive effect of the sister hole is more pronounced with the increasing blowing ratio. Lower are ratios such as AR = 2 and AR = 3, decrease the vorticity of the CRVP around %30 which directly affects the cooling effectiveness. The results of this study showed area ratio, as well as lateral distance between main hole and sister hole centers heavily affects the cooling effectiveness, the lower area ratios are more efficient for film cooling.

Autogenous sealing of a cavity in bentonite clay: an x-ray computerised tomography study

Autogenous closure of cracks in clay during wet−dry cycles is usually observed through digital photography or x-ray, or inferred from changes in hydraulic conductivity. Samples in almost all experimental studies of clay self-sealing carry networks of cracks, making it challenging to control initial conditions of healing or to separate processes operating over a single crack from those pertaining to interaction between cracks. Furthermore, most observational studies are based on before-and-after images with little sense of the pace of crack closure. This paper reports a set of x-ray computerised tomography observations of a single artificial cylindrical hole, as a proxy for a crack, in consolidated, close-to-saturation bentonite samples, undergoing further hydration. The sample is scanned at different times and the scans analysed by image processing. The results show that water chemistry, pre-consolidation pressure and vertical boundary conditions all significantly affect the pace and extent of hole closure. The pace of hole closure is found to follow a logarithmic function when the sample is unconstrained vertically. Autogenous closure appears to be due mostly to osmotic swelling. This suggests that the latter may form the basis for developing constitutive relationships and evolution functions for sealing of cracks in swelling clay undergoing wet−dry cycles.

Experimental and numerical study on the effects of shaped multi-holes on the effectiveness of film cooling

In the present work, experimental and numerical study has been performed to investigate the effects of a novel film-cooling design with shaped multi-holes on the film-cooling effectiveness over a flat plate. A single cylindrical hole with 11.1 mm diameter has been replaced by multi-holes (14 holes with 2.97 mm diameter) with elliptical and fan-shaped configurations. The exit area, length to diameter ratio (L/D = 4.0) and injection angle (35°) of multi-holes are selected identical to the single cylindrical hole. The multi-holes were machined with a fixed spacing pitch (33.3 mm) between the centers of two adjacent holes. The surface temperature of the test plate was measured by an infrared camera. The experimental studies were performed at blowing ratios of 0.5 and 2. The numerical results based on the steady RANS with realizable k–e turbulence model and enhanced wall treatment were capable to predict the influence of the shape of multi-hole configurations on overall adiabatic film-cooling effectiveness. The experimental and numerical results showed that replacing a single hole with a shaped multi-hole leads to a considerable increase in film-cooling effectiveness in both axial and lateral directions. According to the results at blowing ratio of 2.0, the elliptical and fan-shaped configurations provide a higher area-averaged film-cooling effectiveness by 75% and 248.2% in comparison with the single hole, respectively.

Laboratory Study of the Fracturing Process in Marble and Plaster Hollow Plates Subjected to Uniaxial Compression by Combined Acoustic Emission and Digital Image Correlation Techniques

Hollow plates of plaster and calcitic and dolomitic marbles, extracted from greek quarries, with a single pre-existing cylindrical hole of various diameters are subjected in uniaxial compression. The influence of the material and the hole’s diameter on the fracturing process are studied and compared to the literature. Full-field digital image correlation technique and acoustic emission analysis are employed for the monitoring of the fracturing process, the initiation of the main phenomena, their interaction, and the failure mode of the physical models. New precise techniques on the determination of the true time of the primary fracture and spalling initiation by digital image correlation are implemented and presented. The study highlights a new behavioural type of such physical models, with different fracture patterns than those presented in the literature. A comparison between the required applied axial stress for the primary fracture and spalling initiation of the studied materials and the previously published results is presented, showing an exponential relation to the hole’s diameter.

Effects of diameter ratio and inclination angle on flow and heat transfer characteristics of sister holes film cooling

In order to investigate the flow and heat transfer characteristics of sister holes cooling method, sister holes cooling structures have been applied on a flat plate with four primary to sister hole diameter ratios and three inclination angles under three blowing ratios. Single cylindrical hole film cooling structure is also established as a benchmark structure. The numerical results of the single cylindrical hole case and one sister holes case are validated with the experimental results available in literatures. The tangential velocity vectors and total vorticity distributions are studied and compared. The spanwise averaged film cooling effectiveness and the adiabatic temperature contours are studied and compared. Results show that the development of the kidney vortex induced by primary hole is impeded by the sister hole induced kidney vortex and thus the film cooling performance is promoted. The blowing ratio and diameter ratio heavily affect the flow and heat transfer characteristics of sister holes cooling cases, the influence of the inclination angle is much smaller. In this paper, the best film cooling performance of sister holes cooling cases is obtained under the smallest diameter ratio.

Numerical study on unsteady film cooling performance under the mainstream swing condition

Significant unsteady film cooling performance of a turbine shroud can be found under the periodic disturbance of rotor blades. The mainstream flow in film cooling on a turbine shroud is simplified as the periodic swing based on the alternate appearance of the cascade passage flow and the blade tip clearance flow. Three-dimensional unsteady numerical simulation was employed to analyze the film cooling effectiveness with a single cylindrical hole injection at mainstream swing frequencies of 100, 160 and 220 Hz, and at blowing ratios of 0.5, 0.8, 1.1 and 1.4, respectively. A steady simulation was also carried out as a comparison. The results show that mainstream swing provides instantaneous film spots. It is a novel phenomenon in film cooling. Spanwise coverage of film was more uniform compared with the steady case. There are considerable differences of film cooling effectiveness under the various mainstream swing frequencies. A larger swing frequency results in higher spanwise averaged time-averaged film cooling effectiveness.

Numerical investigation on the flow character and film cooling performance of novel merged holes structure

In order to investigate the flow characteristics and film cooling performance of novel merged holes cooling method, merged cylindrical holes structures have been applied on a flat plate with three merging ratios under three blowing ratios. Single cylindrical hole film cooling structure is also established as a benchmark structure. The numerical results of the single cylindrical hole are validated with the experimental results available in open literatures. The discharge coefficient, aerodynamic loss, tangential velocity vectors and total vorticity distributions are studied and compared. The spanwise averaged film cooling effectiveness and adiabatic temperature contours are studied and compared. Results show that the development of the kidney vortex is impeded and the film cooling performance is promoted in merged holes cooling cases. The blowing ratio and merging ratio both affect the cooling performance of merged holes cooling cases. In this paper, the best film cooling performance is obtained at the biggest merging ratio.

Numerical Study of the Fracturing Process in Marble and Plaster Hollow Plate Specimens Subjected to Uniaxial Compression

Physical models of plaster, calcitic and dolomitic marbles from greek quarries with a single pre-existing cylindrical hole of various diameters, subjected in uniaxial compression are simulated numerically with a bonded particle model by employing the two- and the three-dimensional versions of the Particle Flow Code. The linear parallel bond model is employed for the plaster’s simulation, and respectively the flat-joint model for the marbles. The calibration procedure and results are presented, as well as the simulation results of the hollow specimens. The micro-cracking, the fracturing process and the sequence of their appearance during the numerical tests of the hollow plates are in good agreement with the laboratory tested physical models. Each fracture pattern is similar to the one of the physical models. Also, the regions of micro-cracking in the numerical models is quite similar to the regions of intense deformation observed from digital image correlation analysis on the respective physical models. The followed methodology for the determination of the phenomena’s onset is presented as well. A comparison between the peak strength and the required applied axial stress for the primary fracture and spalling initiation of the numerical and the physical models is presented along with the accompanied scale effect. Additional numerical investigation is performed, quantifying the stress distribution along and normal to the primary fracture’s path during the numerical test. The current study reveals the potential of the bonded particle model to reliably simulate laboratory and field structures, at least for the studied materials.

Simulation of fracture mechanism of pre-holed concrete model under Brazilian test using PFC3D

In the previous studies on the porous rock strength the effect of pore number and its diameter is not explicitly defined. In this paper crack initiation, propagation and coalescence in Brazilian model disc containing a single cylindrical hole and or multiple holes have been studied numerically using PFC3D. In model with internal hole, the ratio of hole diameter to model diameter was varied between 0.03, 0.17, 0.25, 0.33, and 0.42. In model with multiple hole number of holes was different in various model, i.e., one hole, two holes, three holes, four holes, five holes, six holes, seven holes, eight holes and nine holes. Diameter of these holes was 5 mm, 10 mm and 12 mm. The pre-holed Brazilian discs are numerically tested under Brazilian test. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured. The mechanism of cracks propagation in the wall of the ring type specimens is also studied. In the case of multi-hole Brazilian disc, the cracks propagation and b cracks coalescence are also investigated. The results shows that breaking of the pre-holed disc specimens is due to the propagation of radially induced tensile cracks initiated from the surface of the central hole and propagating toward the direction of diametrical loading. In the case of disc specimens with multiple holes, the cracks propagation and cracks coalescence may occur simultaneously in the breaking process of model under diametrical compressive loading. Finally the results shows that the failure stress and crack initiation stress decreases by increasing the hole diameter. Also, the failure stress decreases by increasing the number of hole which mobilized in failure. The results of these simulations were comprised with other experimental and numerical test results. It has been shown that the numerical and experimental results are in good agreement with each other.

A fracture mechanics simulation of the pre-holed concrete Brazilian discs

Brazilian disc test is one of the most widely used experiments in the literature of geo-mechanics. In this work, the pre-holed concrete Brazilian disc specimens are numerically modelled by a two-dimensional discrete element approach. The cracks initiations, propagations and coalescences in the numerically simulated Brazilian discs (each containing a single cylindrical hole and or multiple holes) are studied. The pre-holed Brazilian discs are numerically tested under Brazilian test conditions. The single-holed Brazilian discs with different ratios of the diameter of the holes to that of the disc radius are modelled first. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured and the crack propagation mechanism around the wall of the ring is investigated. The crack propagation and coalescence mechanisms are also studied for the case of multi-holes\' concrete Brazilian discs. The numerical and experimental results show that the breaking mechanism of the pre-holed disc specimens is mainly due to the initiation of the radially induced tensile cracks which are growth from the surface of the central hole. Radially cracks propagated toward the direction of diametrical loading. It has been observed that for the case of disc specimens with multiple holes under diametrical compressive loading, the breaking process of the modelled specimens may occur due to the simultaneous cracks propagation and cracks coalescence phenomena. These results also show that as the hole diameter and the number of the holes increases both the failure stress and the crack initiation stress decreases. The experimental results already exist in the literature are quit agree with the proposed numerical simulation results which validates this simulation procedure.

Effects of side hole position and blowing ratio on sister hole film cooling performance in a flat plate

Three kinds of sister hole and one single cylindrical hole were experimentally studied in order to analyze the effects of side hole position and blowing ratio on film cooling performance in a flat plate model by applying Thermochromic Liquid Crystal (TLC) technique. The blowing ratio varied from 0.3 to 2.5 and the density ratio of coolant to mainstream was 1.05. The Reynolds number based on the velocity of mainstream and the diameter of the main hole was fixed at 3400. Compared to the base hole, the film cooling performance of all sister holes showed obvious improvement at all blowing ratios. The middle stream sister hole and downstream sister hole each demonstrated good film cooling performance at all blowing ratios, while the upstream sister hole performs well only at a lower blowing ratio. A numerical calculation is also performed to better analyze the experimental results and determine the relationships among CRVP intensity, flow structure, and film cooling effectiveness at a low blowing ratio of M = 0.5 and high blowing ratio of M = 2.0. The presence of side holes can restrain the CRVP intensity of the main hole and reduce the coolant lift-off, improving the film coverage and film cooling effectiveness. The downstream sister hole can perform best in reducing the CRVP intensity.

Fracture analyses of different pre-holed concrete specimens under compression

Crack propagation processes in specially prepared concrete discs and rectangular specimens containing a single cylindrical hole or multiple holes of varying diameters have been studied both experimentally and numerically. In this research, the cracks coalescence paths in Brazilian disc and rectangular specimens made from rock-like material containing multi-holes are investigated. These concrete specimens are specially prepared from an appropriate mixture of Portland Pozzolana Cement (PPC), fine sands, and water. The pre-holed Brazilian discs and rectangular specimens are experimentally tested under compression. The breakage load in the ring type disc specimens containing an axial hole with varying diameters is measured and the distribution of the induced lateral stress is obtained. The mechanism of cracks propagation in the wall of the ring type specimens is also studied. In the case of multi-hole Brazilian disc and rectangular specimens, the cracks propagation and cracks coalescence are also investigated. These experiments are numerically modeled by a modified higher order displacement discontinuity method. It has been shown that the corresponding experimental and numerical results are in good agreement with each other. The results presented in this research validate the accuracy and applicability of these crack analyses procedures.

Experimental time-resolved study of the interaction between a pulsating injectant and a steady cross-flow: aerodynamics of film cooling

A test rig incorporating the injection from a single cylindrical hole with an inclination of 30° to a thermally uniform mainstream flow was used for determining variations in flow structures due to injectant pulsation. The average blowing ratios (\( \overline{M} \)) were 0.65, 1, and 1.25. The periodic variations in injectant flow were rendered by a loudspeaker-based pulsation system to nondimensionalized excitation frequency (\( St \)) of 0, 0.2, 0.3, and 0.5. Pulsation resulting in a close-wall orientation of injectant fluid compared with steady blowing bearing outward orientation was only observed in few cases. At \( \overline{M} \) = 0.65, jet fluid remains aligned and covers a significant part of the wall under steady blowing. At higher blowing ratios, pulsation induces large spatial variations in the jet trajectory, collapsing of the jet body, and the shedding of wake structures due to the periodic variation of injection flow rate. It was found that the pulsation improves wall coverage of the injectant fluid under low frequency excitation as the separation of the jet from the wall becomes evident (\( \overline{M} \) = 1 and 1.25).

Transmission of light by a single subwavelength cylindrical hole in metallic films

This letter presents theory and finite-difference time-domain calculations in excellent agreement with experiments for the transmission of a single subwavelength cylindrical hole in Ag metallic films. The authors discuss the very large enhancement in transmission by single holes in terms of surface plasmons at the interface of the hole. These are similar to the surface plasmons in metallic cylinders reported by Economou and co-workers [Phys. Rev. B 10, 3038 (1974); 24, 6908 (1981)]. The peak structures observed in experiments with arrays of holes are a consequence of interference of the beams transmitted by each hole and are determined by the hole periodicities. The role of the arrays in enhancement of the transmission due to surface plasmon polaritons appears to be marginal.

Film cooling characteristics of rows of round holes at various streamwise angles in a crossflow: Part I. Effectiveness

Film cooling effectiveness were studied experimentally on rows of cylindrical holes with streamwise angles of 30°, 60° and 90°, in a flat plate test facility with a zero pressure gradient. Detailed effectiveness and heat transfer results for a single cylindrical hole at the same inclinations have been presented in Yuen and Martinez-Botas [C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of a single hole at various streamwise angles: Part I. Effectiveness, Int. J. Heat Mass Transfer 46 (2003) 221–235; C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of a single hole at various streamwise angles: Part II. Heat transfer coefficient, Int. J. Heat Mass Transfer 46 (2003) 237–249] with the same test facility and measurement technique. This present investigation commenced with a single row of holes with two pitch-to-diameter ratios ( p/D) of 3 and 6. It then presents and discusses the effects of introducing inline and staggered rows for each streamwise angle and pitch-to-diameter ratio. The row spacing in the inline and staggered rows is 12.5 diameters in the streamwise direction. The short but engine representative hole length ( L/D = 4) is constant for all geometries. The blowing ratio ranges from 0.33 to 2, and the freestream Reynolds number based on the freestream velocity and hole diameter ( Re D) was 8563. Both local values and laterally averaged ones are presented, the latter refers to the averaged value across the central hole. The current results are compared with the experimental results obtained by other researchers, the effects of the additional inline and staggered rows, and of the variations in injection angle, pitch-to-diameter ratio are described. The objectives of the present study are to provide a consistent set of measurements in terms of effectiveness and heat transfer coefficients presented in the companion paper [C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of rows of round holes at various streamwise angles: Part II. Heat transfer coefficient, Int. J. Heat Mass Transfer, in press], obtained systematically with the same test facility, and to deliver a better understanding of film cooling performance. The present results also serve as a database with 105 test cases, in addition to the 21 cases presented in [C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of a single hole at various streamwise angles: Part I. Effectiveness, Int. J. Heat Mass Transfer 46 (2003) 221–235], for future numerical modelling.

Light transmission through a single cylindrical hole in a metallic film

The transmission of light through a subwavelength hole drilled in a metallic thin .lm is calculated by numerically solving Maxwell's equations both for a simple hole and for a hole with additional structure. A maximum in the transmission cross section is observed for hole diameters of the order of but smaller than the wavelength. Transmission cross sections well above the hole area are shown to be attainable by filing the hole with a high-index material. The effect of adding a small particle inside the hole is also analyzed.

Effect of constraint on void growth near a blunt crack tip

The growth of a single cylindrical hole ahead of a blunt crack tip was studied using large deformation finite element analysis in three-point bend specimens with different precrack depth. The effect of small second phase particles was taken into account by incorporating Gurson's constitutive equation. The effects of strain hardening and the initial distance from the hole to the crack tip were also investigated. The results show that the variation of crack tip opening displacement with load is not sensitive to constraint level. The effects of constraint on the growth of hole and ductile initiation toughness are diminished with decreasing initial distance from the hole to the blunt crack tip.

A Computer Method to Model the Dose Distribution of High Energy Photon Grid Therapy in Three Dimensions

High energy photon grid therapy is the application of multiple circular divergent photon beams to the palliative care of patients. The photon beams are defined by cylindrical holes in a large attenuator placed in the path of an open radiation beam. We have developed a computer method to model the three-dimensional dose distribution of high energy photon grid therapy. The semiempirical method requires the measured data from a single cylindrical hole of the size to be modeled. For a given hole diameter, any arbitrary geometry can be calculated. This information is important when investigating the clinical implications of grid therapy for patient care.