M. Serdar Genç

Transient Temperature and Thermal Stress Distributions in a Hollow Disk Subjected to a Moving Uniform Heat Source

This study presents numerical analyses of transient temperature and thermally induced stress distributions in a stationary hollow steel disk partially heated by a moving uniform heat source from its outer surface under stagnant ambient conditions. The moving heat source applied on a certain angular segment of the processed surface rotates with a constant angular speed (ω). The peak levels of the temperature gradients and the thermal stress ratios at the heated segments do not rise very much after 2–3 cycles. When the value of ω is increased, the maximum effective thermal stress ratio can be decreased in a considerable amount.

Low Reynolds Number Flows and Transition

Due to the advances in unmanned aerial vehicles (UAV), micro air vehicles (MAV) and wind turbines, aerodynamics researches concentrated on low Reynolds number aerodynamics, transition and laminar separation bubble (LSB) and its effects on aerodynamic performance. In order to improve endurance, range, efficiency and payload capacity of UAVs, MAVs and wind turbines, the aerodynamic behaviors of these vehiclesmentioned should be investigated.

Progressive Damage Modeling of an Adhesively Bonded Unidirectional Composite Single-lap Joint in Tension at the Mesoscale Level

ABSTRACT: This study investigates initiation and propagation of damage zones in the composite plates of an adhesively bonded single-lap joint in tension at the mesoscale level. A set of material degradation rules is applied to the mechanical properties of each failed ply of the plates based on the Hashin failure criterion. The von Mises stress concentrates along the free edges of the adhesive layer and the corresponding lower and upper plate regions and peaks along the free edges of the adhesive–plate interfaces. Consequently, the damage initiates at the lower plate interface to the adhesive layer and propagates in the first ply along this adhesive free edge, and then spreads through the neighboring plies of the lower plate in a similar failure mechanism. The damage in the lower plate occurs in the matrix, delamination, and fiber–matrix failure modes, and the first ply-failure loads decrease significantly with increasing ply fiber angle. Tension tests showed that the adhesive joints with ply lay-ups between [0]10 and [15]10 fails through the adhesive layer, and the lower plates with larger fiber angles break along the free edge of the lower plate– adhesive interface whereas the adhesive layer is still intact. The adhesive failure is interfacial around the adhesive free edges and is through the adhesive in the middle of the overlap region in the joints with the ply lay-ups [0]10. Although a similar failure mechanism is observed in the ply lay-up [15]10, the damage zones in the first ply of the lower plate in which the matrix and fibers are damaged, are distributed in the overlap region. The damage is initiated in the first ply of the lower plate of the adhesive joints with the ply lay-ups [30–90]10 and propagated through the matrix in the fiber direction, and then the lower plate is broken. The present failure model predicts reasonably the initiation and propagation of the damage in the adhesively bonded single-lap joints in tension.

Progressive Damage Modeling of an Adhesively Bonded Composite Single Lap Joint Under Flexural Loads at the Mesoscale Level

This study investigates initiation and propagation of damage zones in the composite plates of an adhesively bonded single lap joint under flexural loads at the mesoscale level based on Tsai—Wu and Hashin failure criteria. The damage zones were evaluated in the unidirectional composite plates with ply lay-ups [0]10, [15]10, ... , [90]10. The mechanical properties of each failed ply of the upper and lower plates were modified based on a set of material degradation rules. Due to stress concentrations along the free edges of the adhesive layer and in the corresponding zones of the lower and upper plates, the peak stresses appeared along the left free edge of the upper plate—adhesive interface. Both failure criteria showed that the damage initiated in the first ply of the upper plate interface to the adhesive layer and propagated through this ply along this adhesive free edge, and then expanded through the neighboring plies in a similar failure mechanism. Hashin failure criterion predicted that the matrix and ...This study investigates initiation and propagation of damage zones in the composite plates of an adhesively bonded single lap joint under flexural loads at the mesoscale level based on Tsai—Wu and Hashin failure criteria. The damage zones were evaluated in the unidirectional composite plates with ply lay-ups [0]10, [15]10, ... , [90]10. The mechanical properties of each failed ply of the upper and lower plates were modified based on a set of material degradation rules. Due to stress concentrations along the free edges of the adhesive layer and in the corresponding zones of the lower and upper plates, the peak stresses appeared along the left free edge of the upper plate—adhesive interface. Both failure criteria showed that the damage initiated in the first ply of the upper plate interface to the adhesive layer and propagated through this ply along this adhesive free edge, and then expanded through the neighboring plies in a similar failure mechanism. Hashin failure criterion predicted that the matrix and delamination failure modes were dominant in the upper plate, and higher first ply-failure loads and joint failure loads decreased significantly with increased ply fibre angle for the Tsai—Wu failure criterion. The SEM photographs of the adhesive fracture surfaces due to three- and four-point bending tests showed that the local failure initiated inside the adhesive fillet and propagated along the upper plate—adhesive interface. In addition, the interfacial adhesive failure and the partial damage in the first ply of the upper plate appeared for the ply lay-ups between [0]10 and [30]10 whereas the upper plates with larger fiber angles failed through the matrix of the upper plate in the fiber direction and a triangular adhesive bonding region, which decreases with increasing fiber angle. The interfacial adhesive failure is more apparent along the left free edge of the upper plate—adhesive interface whereas the damage propagation is observed through both the adhesive layer and all plies from the interface to the top of the upper plate. Both failure criteria could predict reasonably the initiation and propagation of the damage zones in the composite plates of the adhesive joints, whereas the first ply failure and joint failure loads were underestimated since the non-linear adhesive model, non-linear matrix shear behavior, and geometrical non-linearity were not taken into account in the present analysis.

Acoustic Control of Flow over NACA 2415 Airfoil at Low Reynolds Numbers

Within the concept of this study, first low Reynolds number flow phenomena , including laminar separation bubble (LSB) and stall were explained. Then a literature review on the acoustic flow control was presented in three categories: flow control with constant frequency, flow control with constant amplitude, and flow control with variable frequency and amplitude. Aside from the review part, results of a comprehensive experimental study on the effects of acoustic flow control at low Reynolds numbers were presented. Within the scope of this experimental study, the effects of both parallel and perpendicular acoustic flow control were examined by means of pressure measurements, force measurements, hot-wire anemometry, flow visualization, and particle image velocimetry (PIV). In order to establish a baseline data, all of the measurements were first applied for the no control case. The effects of acoustic control were examined especially on the characteristics of LSB and stall. It was found that the acoustic excitation can be employed to decrease the height and length of LSB leading to increased lift coefficient and decreased drag coefficient. Also, acoustic flow control increased the angle of stall. It was also seen that the effective excitation frequency increases also, but the range of Zaman number (St/Re 0.5) based on effective frequency still is approximately same with increasing Reynolds numbers. Finally, it was shown that the general aerodynamics performance of an aerofoil at low Reynolds numbers can easily be enhanced by employing acoustic flow control, and the parameters of the acoustic flow control can easily be adjusted in order to keep up with the changing conditions of flow.

An Experimental Study of Perpendicular Acoustic Disturbances Effect on Flow Over an Aerofoil at Low Reynolds Numbers

In this study, the effect of perpendicular acoustic excitation on laminar separation bubble over a NACA2415 aerofoil and stall characteristics of the aerofoil was investigated experimentally at low Reynolds numbers (50000 and 75000) and various angles of attack on the aerofoil using force measurement, pressure measurement and smoke-wire flow visualization techniques. The experiments of resonance characteristics of test section of the wind tunnel showed that the frequencies used for acoustic control were not equal to the peak (resonance) frequencies of the wind tunnel test section. The experimental results of acoustic control showed that when acoustic excitation of a certain frequency was applied, the length of the laminar separation bubble was shortened owing to the energy added to the flow by acoustic excitation. Because of the shortened laminar separation bubble, coefficient of lift was increased. Furthermore, at the stall angles the separated flow was forced to reattach to the surface of the aerofoil, so the stall angle was increased from 12° to 17° and maximum coefficient of lift was increased from 1.16 to 1.24 at Re = 50000 and from 13° to 18° and maximum coefficient of lift was increased from 1.18 to 1.34 at Re = 75000.Copyright

Control of Flow Separation and Transition Point over an Aerofoil at Low Re Number using Simultaneous Blowing and Suction

Control of flow over a NACA2415 aerofoil which experiences a laminar separation bubble for a transitional Reynolds number of 2x10 is numerically simulated under the effects of blowing and suction. An earlier experimental study using hot-wire anemometry for a clean (no jet) NACA 2415 aerofoil at α = 8° shows a laminar separation bubble over the one-third of the airfoil upper surface. In the no jet case, the recently developed k-kL-ω and k-ω SST transition models accurately predict the location and extent of the separation bubble. Later, single or multiple jets with a width of 2.5% the chord length are placed on the aerofoil upper surface for simulating the isolated or simultaneous blowing and suction jets. For single jets with blowing or suction, whereas the k-ω SST model suppress the separation bubble in both cases, the k-kL-ω transition model doesn’t completely eliminate the separation bubble but moves it downstream in the suction case. Overall, the blowing/suction control mechanism appears to be suppression of the separation bubble and reduction of the upper surface pressure to increase the lift and decrease the drag. For simultaneous blowing and suction, firstly, a blowing jet at 10%c and a suction jet at 36%c are placed at the beginning and end of the separation bubble based on the best results of the single jets. Then, the locations of the blowing and suction jets are reversed. In both cases, both transition models eliminate the separation bubble resulting in increase in the lift and decrease in the drag. However, the best results in terms of the L/D ratios are still obtained with the single suction jets.

Hydrogen Production Potential and Cost of Wind-Hydrogen Hybrid Energy System

Main aim of this study is to investigate annual wind power output, annual hydrogen production quantity and hydrogen production cost of wind-hydrogen energy system in the Pinarbasi-Kayseri region in Turkey which has remarkable wind potential in the central region of Turkey. Turkish State Meteorological Service (TSMS) measured the value of mean wind speed of Pinarbasi as 3.67 m/s above 10m ground between 2000 and 2006 years. In this study, three different hub heights (50m, 80m and 100m) were considered, and so the measured mean wind speed at 10m was extrapolated to considered heights and annual wind power output was calculated. Four different turbine rated powers (800kW, 900kW, 2000kW and 3000 kW) and two different electrolyser powers (120kW and 40kW) for hydrogen production were assumed. Levelised cost of electricity method was used in order to determine the cost analysis of wind energy and hydrogen production. The results of this study bring out clearly the variation of potential of hydrogen production and cost with wind speed, wind turbine hub height and wind turbine rated power and electrolyser power.Copyright