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Öğe A review of hydrodynamics and heat transfer through corrugated channels(Pergamon-Elsevier Science Ltd, 2019) Kurtulmus, Nazim; Sahin, BesirEver-increasing consumption of limited energy sources forces researchers and engineers to produce more efficient energy systems in order to use energy sources effectively. Enhancing the heat transfer rate with an acceptable pressure drop is an important parameter to produce more compact heat exchangers which are used in air-conditioning systems, chemical reactors, thermal power plants and others. Nowadays many engineering techniques such as surface modifications, swirl flow creators, flow conditioners, additives, etc. are implemented to enhance the heat transfer performance of energy systems. Researchers are still interested in the implementation of these techniques to improve the performance of energy systems further although the current literature has many experimental and numerical research data. Even though the corrugated channel is a passive technique to augment heat transfer, researchers have been insisting on trying to get further improvement by implementing one or more passive/active techniques to corrugated channels. The objective of the present work is to gather available research data which particularly focus on the flow characteristics and heat transfer rates through the corrugated channels. The design parameters, practical limitations and conclusions of energy systems, obtained previously are presented in tabular forms and the necessary discussion is also provided.Öğe Computational and experimental investigations of the vortical flow structures in the near wake region downstream of the Ahmed vehicle model(Elsevier, 2016) Tunay, Tural; Yaniktepe, Bulent; Sahin, BesirThe present study aims to investigate flow characteristics downstream of the Ahmed vehicle model using both experimental and computational methods. Ahmed vehicle model having 1/4 scale and 25 slant angle is employed at Reynolds number of Re-H=1.48x10(4). Investigations are conducted in two parts. In the first part, Large Eddy Simulation (LES) method is used to resolve the flow structures downstream of the Ahmed model, computationally. In the second part, the technique of the particle image velocimetry (PIV) is employed to obtain the flow fields downstream of the Ahmed model. The PIV and LES investigations provides time-averaged and instantaneous velocity field results, such as vorticity contours, streamline topology, velocity profiles and spectral analysis of the flow velocity. Flow features that have been predicted by computational study are in a good harmony with the results predicted by experimental studies both on the slanted surface and in the near wake region downstream of the Ahmed model. Results present that characteristics of flow features that exist on the rear slanted surface and in the near wake region of the Ahmed model exhibit great variations in a very short distance in both stream-wise and vertical direction of the flow.Öğe Effect of corrugated channel phase shifts on flow structures and heat transfer rate(Elsevier Science Inc, 2018) Tokgoz, Nehir; Tunay, Tural; Sahin, BesirThe study of quantitative visualization of flow hydrodynamics, turbulent flow characteristics and time-dependent development and disappearance of vortices in cavities of corrugated channel regarding enhancements of heat transfer is limited in the literature. For this reason, the aim of the work is to search the effects of two different phase shifts, phi (i.e. 90 degrees, 0 degrees) of corrugated channels on flow hydrodynamics and the rate of heat transfer for Reynolds numbers varying from 3 x 10(3) to 6 x 10(3). The technique of particle image velocimetry (PIV) which has the capability of observing turbulent unsteady flows and provide instantaneous velocity vectors covering defined flow fields were employed. An effective fluid mixture occurs between the core and the separated flow regions due to the occurrence of a rapid vortex shedding downstream of the entrance of cavities. Along the lower and upper sides of hollow spaces, these shedding vortices spin either counterclockwise or clockwise to stimulate entrainment. In addition, the effects of turbulent flow hydrodynamics on convective heat transfer were also investigated numerically. Numerical results of flow hydrodynamic were found to be consistent with the experimental results. Heat transfer augmentation was reported as a conclusion of different parameters including different phase shifts, phi, aspect ratios, s/H and Reynolds numbers, Re.Öğe Effect of ground on flow characteristics and aerodynamic performance of a non-slender delta wing(Elsevier France-Editions Scientifiques Medicales Elsevier, 2021) Tumse, Sergen; Tasci, Mehmet Oguz; Karasu, Ilyas; Sahin, BesirThe aerodynamic performance and the structure of vortical flow on a delta wing are affected by the influence of the ground in take-off and landing stages. In this context, the effect of the ground on a delta wing having a sweep angle of 40 degrees was investigated by employing Particle Image Velocimetry (PIV), aerodynamic force measuring system and the dye flow visualization technique. Flow characteristics of delta wing were examined under two different angles of attack, alpha = 8 degrees and 11 degrees and variation of the distance between trailing edge of the wing and ground, h normalized with the root chord length, c of the wing. It was observed that the existence of the ground attenuates the magnitude of peak values of primary and secondary vortices due to incomplete development of vortices. The ground effect caused the outboard movement of the leading-edge vortex in a spanwise direction as well as an increase in the size of vortices. Furthermore, the presence of the ground induced a decrease in Strouhal number, St due to the slowing down of vortex formation. Lift and drag coefficient, C-L, and C-D of the delta wing were observed to increase with descending from unbounded flight region into ground effect region. Finally, it was found that C-L/C-D increases by reducing the distance between ground and wing, h/c, and a rise of C-L/C-D is much more effective under the lower angles of attack, alpha. (C) 2020 Elsevier Masson SAS. All rights reserved.Öğe Effect of Yaw Angles on Aerodynamics of a Slender Delta Wing(Asce-Amer Soc Civil Engineers, 2019) Karasu, Ilyas; Sahin, Besir; Tasci, M. Oguz; Akilli, HuseyinIn the present investigation, flow structures over a slender delta wing with a 70 degrees sweep angle, ? at an angle of attack, alpha of 30 degrees, and under the variation of yaw angles within the range of 0 degrees <=beta <= 20 degrees were investigated experimentally. To analyze the flow structure qualitatively and quantitatively, dye visualizations, surface oil visualizations, surface pressure measurements, and velocity measurements using the stereo particle image velocimetry (PIV) were conducted at Reynolds numbers of Re=2x10(4) and Re=1x10(5). Instantaneous and time-averaged flow data were analyzed to demonstrate the effect of yaw angle (beta) on the alterations of leading-edge vortex formation, vortex breakdown, and also vortical flow structures over the wing surface. Time-averaged distributions of streamwise velocity component, u over bar /U, transverse velocity component v over bar /U, patterns of streamline, psi, and distributions of turbulent kinetic energy TKE/U2 were presented to reveal the physics of flow structures under yaw angle (beta) variations. Distribution of pressure coefficients, (-Cp) at various x/c and y/c locations were also presented. The experimental results demonstrated that the variation of yaw angles (beta) has a strong effect on the alterations of the flow structure, the leading-edge vortex formation, and its breakdown. An increase of yaw angle (beta) causes asymmetrical flow structures and hence expands this asymmetrical flow domain further. (c) 2019 American Society of Civil Engineers.Öğe Effects of rear slant angles on the flow characteristics of Ahmed body(Elsevier Inc., 2014) Tunay, Tural; Sahin, Besir; Ozbolat, VeliAhmed body is a simplified vehicle model which retains important features of real ground vehicles. Present study aims to investigate effects of rear slant angles (?=25°, 30° and 35°) on time-averaged and instantaneous flow characteristics downstream of the Ahmed body because angle of the rear slanted surface considerably affects the flow characteristics. The particle image velocimetry (PIV) technique is employed to measure the flow field in the symmetry plane z=0 downstream of the Ahmed body. Demonstration and detailed discussions of the flow features are provided using time-averaged velocity vectors, streamlines, vorticity and TKE contours. Three critical flow points are revealed in the wake downstream of the Ahmed body. Namely, they are focus point 1 (F1), focus point 2 (F2) and saddle point (S). Variations in slant angle ? cause shift in the locations of these critical flow points. Effects of change of slant angle ? on each critical flow point vary according to their locations in the wake. Instantaneous vorticity contours and spectral analysis of the velocity field are also provided. As a result of the spectral analysis, a single dominant frequency location is only detected for ?=25°, on the other hand two dominant frequency locations are detected for ?=30° and 35° in the wake. Mean stream-wise and vertical velocity components are compared with available data in the literature. Although there are some discrepancies between the present and previous results at certain locations in the flow field, general agreement between these results is reasonably good when taking the difference between Reynolds numbers of the two studies into account. © 2014 Elsevier Inc.Öğe Experimental Investigation of Ground Effect on the Vortical Flow Structure of a 40° Swept Delta Wing(Asce-Amer Soc Civil Engineers, 2022) Tumse, Sergen; Karasu, Ilyas; Sahin, BesirThe ground effect influences the flow structure on a delta wing during landing and take-off processes. In this regard, comprehensive instantaneous velocity measurements and flow visualizations were carried out by particle image velocimetry and dye flow visualization techniques to reveal the ground effect on leading-edge vortex characteristics of a 40 degrees swept delta wing. The flow behaviors on the delta wing under the impact of the ground were analyzed at two angles of attack, 8 degrees and 11 degrees, and the space between the ground and lower surface of the wing was nondimensionalized with the wing's chord length. It was found that the presence of the ground caused premature leading-edge vortex breakdown due to the increasing adverse pressure gradient on the wing's suction side along the chord direction. The ground effect caused an increase in peak value and distributions of turbulent kinetic energy on the wing surface that depended on the earlier leading-edge vortex bursting and complex and disorganized flow structures. The value of time-averaged vertical velocity was lower when the delta wing descended from the free-stream flow zone into the ground effect zone because of the blocking of fluid flow in the gap between the ground and pressure surface of the wing. Thus, it can be concluded that the ground effect is very influential on the change of vortical flow characteristics of nonslender delta wings.Öğe Experimental investigation of pulsating flow structures and heat transfer characteristics in sinusoidal channels(Pergamon-Elsevier Science Ltd, 2020) Kurtulmus, Nazim; Sahin, BesirIn the present work, hydrodynamics and heat transfer characteristics in the sinusoidal channel are investigated experimentally for both steady and pulsating flow conditions. The experiments for heat transfer investigations were performed under a constant heat flux in the range of Strouhal number, St 0.11 <= St <= 2.07for the Reynolds number, in the range of 4 x 10(3) <= Re <= 7 x 10(3). After seeing the improvement of heat transfer with employing pulsation to the working fluid the hydrodynamics of pulsating flow was analyzed by considering the pulsating flow characteristics such as the time-averaged streamlines topology, (Psi), streamwise velocity distribution, < u >, cross-streamwise velocity distribution, < v >, and turbulent Reynolds stress, (u'v') over bar /U-2 using instantaneous flow data measured by the Particle image velocimetry (PIV) system. The results revealed that pulsating flow is highly effective for the lower turbulent flow case in the sinusoidal channel. As the Reynolds number increases, the effect of Strouhal number, St becomes less effective. There is an optimum Strouhal number,St value for different Reynolds numbers, Re to reach the maximum enhancement compared to steady flow cases.. The entrainment between the core flow and recirculating flow enhances the heat transfer rates in a steady flow. But the pulsating flow forces the recirculating flow zones in the diverging-converging section of the channel wave to exchange the flud from the core flow region further and that is an additional mechanism to upgrade the rate of heat transfer comparing to the steady flow cases.Öğe Experimental investigation of the flow around a simplified ground vehicle under effects of the steady crosswind(Elsevier Science Inc, 2018) Tunay, Tural; Firat, Erhan; Sahin, BesirCrosswinds which stem from on-road flow conditions can deteriorate a ground vehicle's driving performance. In spite of the numerous studies investigating aerodynamics of the ground vehicles in the previous literature, there is still a need for thorough understanding of the underlying flow phenomena related to crosswind sensitivity of the ground vehicles. Therefore, the present study is aimed to investigate characteristics of the flow features around the ground vehicles under steady crosswind conditions within a range of yaw angles 0 degrees <= beta <= 15 degrees using the Ahmed vehicle model with rear slant angles of alpha = 25 degrees and 35 degrees. Throughout the investigations, flow visualizations using dye injection technique and velocity measurements using particle image velocimetry PIV technique are conducted. The Reynolds number of the flow which is based on the free stream flow velocity U = 200 mm/s and height of the model H = 72 mm is Re-H = 1.4 x 10(4). Results are presented in terms of time averaged vorticity contours and streamline patterns together with velocity magnitude ((< u >(2)+ < w >(2))(1/2)/U) of the flow fields. Additionally, turbulence quantities such as Reynolds stresses < u'w' >/U-2 and turbulent kinetic energy TKE are presented and discussed in details. Results of the present study show that characteristics of the flow features in the near wake region downstream of the Ahmed model are very sensitive to the effects of the crosswind.Öğe Heat transfer and flow characteristics in a sinusoidally curved converging-diverging channel(Elsevier France-Editions Scientifiques Medicales Elsevier, 2020) Kurtulmus, Nazim; Zontul, Harun; Sahin, BesirThe objective of this work is to present the thermal performance characteristics and to examine the hydrodynamic structure of the fluid which improves the rate of heat transfer in parallel with the penalty of pressure drop by means of the time-averaged streamlines topology, , streamwise velocity distribution, , vorticity concentration, and turbulent Reynolds stress, (u'v') over bar /U-2 for the sinusoidal wavy channel. A wide range of experiments were performed for Reynolds numbers, Re ranging from 4 x 10(3) to 1 x 10(4) in order to determine the heat transfer rate and the friction factor, f with varying the channel height expansion/contraction ratio, M = H-min/H-max such as 0.5, 0.35 and 0.28. The results revealed that a significant heat transfer enhancement was achieved with a considerable penalty of pressure drop. The highest thermal performance factor, TPF was obtained as 1.46 for M = 0.5. Numerical simulations were conducted to confirm the experimental results for the same parameters. The Shear Stress Transport k-w (SST k-w) turbulence model was used to perform numerical analyses. After ensuring the consistency of experimental thermal performance results with numerical predictions, the Particle image velocimetry (PIV) system was utilized for investigating the flow physics in the sinusoidally curved converging-diverging channel for all M values at Re = 4 x 10(3) where the TPF is maximum.Öğe Investigation of convective heat transfer and flow hydrodynamics in rectangular grooved channels(Pergamon-Elsevier Science Ltd, 2021) Zontul, Harun; Hamzah, Hudhaifah; Kurtulmus, Nazim; Sahin, BesirAn experimental and numerical investigation is conducted for convective heat transfer and flow characteristics in a channel with rectangular grooved top and bottom walls. The study is performed between 2 x 103and 6.5 x 103 of Reynolds numbers. Heat transfer experiments are conducted by applying constant heat flux to the channel upper and lower walls. The averaged Nusselt number, Nu, is presented as an indicator of heat transfer. In the experiments, 1.9-2.4 times larger Nu values were obtained by using grooved channels instead of straight channels. Pressure measurements show that the corrugated channel causes an increase in pressure loss. The Particle Image Velocimetry, PIV method is employed to reveal the flow hydrodynamics and its relation with convective heat transfer. Using the ability of the PIV method to provide instantaneous flow data, interactions between the recirculation flow bubbles in the grooves and the mainstream are observed. In the numerical part of the study, the k-epsilon turbulence model is used, and predicted results are found to be consistent with experiments. The validated numerical method is used to investigate the effect of groove aspect ratio on heat transfer and pressure drop.Öğe Investigation of crossflow features of a slender delta wing(Techno-Press, 2020) Tasci, Mehmet O.; Karasu, Ilyas; Sahin, Besir; Akilli, HuseyinIn the present work, the main features of primary vortices and the vorticity concentrations downstream of vortex bursting in crossflow plane of a delta wing with a sweep angle of Lambda=70 degrees were investigated under the variation of the sideslip angles, beta. For the pre-review of flow structures, dye visualization was conducted. In connection with a qualitative observation, a quantitative flow analysis was performed by employing Particle Image Velocimetry (PIV). The sideslip angles, beta were varied with four different angles, such as 0 degrees, 4 degrees, 12 degrees, and 20 degrees while angles of attack, alpha were altered between 25 degrees and 35 degrees. This study mainly focused on the instantaneous flow features sequentially located at different crossflow planes such as x/C=0.6, 0.8 and 1.0. As a summary, time-averaged and instantaneous non-uniformity of turbulent flow structures are altered considerably resulting in non-homogeneous delta wing surface loading as a function of the sideslip angle. The vortex bursting location on the windward side of the delta wing advances towards the leading-edge point of the delta wing. The trajectory of the primary vortex on the leeward side slides towards sideways along the span of the delta wing. Besides, the uniformity of the lift coefficient, C-L over the delta wing plane was severely affected due to unbalanced distribution of buffet loading over the same plane caused by the variation of the sideslip angle, beta. Consequently, dissimilarities of the leading-edge vortices result in deterioration of the mean value of the lift coefficient, C-L.Öğe Near-surface particle image velocimetry measurements over a yawed slender delta wing(Sage Publications Ltd, 2021) Karasu, Ilyas; Tumse, Sergen; Tasci, Mehmat O.; Sahin, Besir; Akilli, HuseyinIn this study, extensive instantaneous velocity measurements were conducted within a flow area by stereo particle image velocimetry (SPIV) to investigate the influence of the yaw angle, beta, on the vortical flow structure formed on a slender delta wing. This sideslip angle, beta, in the yaw plane was varied from 4 degrees up to 20 degrees with an interval of 4 degrees at two critical angles of attack, alpha = 25 degrees and 35 degrees, respectively. In order to reveal the influence of the yaw angle, beta over the flow structure of the delta wing, time-averaged flow statistics, and instantaneous flow data obtained by the SPIV technique in the plan-view plane close to the suction surface of the delta wing were presented. It was observed that even a low yaw angle, for instance beta = 8 degrees, becomes to be effective on the flow characteristics of the delta wing, and this effect was augmented with increasing beta. The influence of beta is quite high on the vortical flow structure at alpha= 35 degrees compared to the angle of attack of alpha = 25 degrees. The flow structure that is symmetrical with respect to the centerline of the wing in the case of no yaw has disrupted with the existence beta. Furthermore, the extent of the asymmetry enlarges with increasing beta. The leading-edge vortex (LEV) on the windward side broken earlier and dominated the flow on the wing surface. It is concluded that this asymmetric flow structure can deteriorate the aerodynamic performance and cause other adverse effects such as unsteady loading.Öğe The impact of the pitching motion on the structure of the vortical flow over a slender delta wing under sideslip angle(Springer, 2021) Tasci, Mehmet Oguz; Pektas, Mehmet Can; Tumse, Sergen; Karasu, Ilyas; Sahin, Besir; Akilli, HuseyinThe primary purpose of this investigation is to observe the effect of the pitching motion on the vortical flow structure and bursting of leading-edge vortices over a delta wing under the sideslip angle, beta using a dye visualization technique. In the current work, a delta wing with a sweep angle of ? = 70 degrees was oscillated in upstroke and downstroke direction to be able to discover the influence of pitching motion on the flow characteristics of the delta wing. The values of mean angles of attack were selected as alpha(m) = 25 degrees and alpha(m) = 35 degrees, and the sideslip angle was altered from beta = 0 to 16 degrees. The delta wing oscillated with the various periods of T-e = 5 s, 20 s, and 60 s, respectively. Amplitude of motion was adjusted as alpha(o) = +/- 5 degrees. It is found that the pitching motion of the delta wing under the sideslip angle beta varies the location of the vortex bursting and vortical flow structure substantially.
