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The present paper numerically investigates viscoelastic fluid flow in the developing flow regime through both straight and 90-degree curve ducts. The aim is to investigate the effects of the first and second normal-stress differences (N1 and N2) as well as the curvature ratio ( $$\kappa = D/R$$ , where R and D are the curvature radius and channel side, respectively) and Reynolds number on the secondary flow patterns. Simulations were carried out in developing flow conditions in curved ducts using the finite volume method. The Reynolds numbers were 10, 20, 30, 40, 50, and 100, and the curvature ratios were 0.05, 0.066, 0.1, and 0.2. The Giesekus constitutive equation was utilized to model the non-linear rheological behavior of a 5.0 wt.% solution of polyisobutylene (PIB) in tetradecane (C14H30). The results reveal that a secondary flow with eight corner vortices is generated in a fully developed flow of viscoelastic fluid through a straight duct. This behavior is attributed to the difference in the second normal stress in the flow field. The results of the current study confirm that the second normal stress difference and change in the sign of N2 around the cross section’s corners trigger these corner vortices. Furthermore, the effects of the curvature ratio on the distributions of first and second normal-stress differences in the developing Dean flow were studied for the first time. The vorticity vector method was used for mixing-enhancement assessment. The results show that the intensity of secondary flows was significantly higher in viscoelastic fluid at all curvature ratios and at all considered Reynolds numbers than in Newtonian fluid flow.

A continuous-curvature parameterization method was coupled with the genetic algorithm and a RANS flow solver to optimize the cascade of VKI and Aachen turbine blades. The main advantage of the method is to generate blades with a continuous curvature distribution, which results in a smooth distribution of pressure and velocity on the blade surface. The geometry of the blade cascade was parameterized by 33 variables, and two objective functions were considered for the optimization. The first cost function was to reduce the total pressure loss with the constraints of mass flow rate, blade loading, and outlet flow angle. At the second cost function, the constraint of constant cross-sectional area was added to the previous constraints to preserve the structural strength of the turbine blade. The total pressure loss for the VKI blade decreased by 14.7 % and 10.6 % for the first and second objective functions, respectively. The total pressure loss for the Aachen blade was also reduced by 9.5 % and 7.5 % for the first and second objective functions, respectively. Due to the efficient geometry parameterization, the proposed method quickly converged to a high-efficiency blade at the early generations. The proposed method can be developed for optimizing the different blades of turbine, compressor, and airfoil types.

Recently, an inverse design method called the ball spine algorithm (BSA) is introduced to design S-duct diffusers with elliptic cross-sections. The technique is developed for the 3-D design of S-sh...

The inverse design is one of the aerodynamic design methods, in which the pressure distribution along the wall is known, and the duct geometry is unknown. To obtain the best geometry by the inverse design, the target pressure distribution along the walls should be optimum. This paper presents an aerodynamic design of diffusers by optimizing the wall pressure distribution and applying it to the ball-spine inverse design method. A code was first developed to solve the boundary layer equations using the integration method, and then incorporated into a genetic algorithm to optimize the wall pressure distribution to achieve the maximum pressure recovery without separation occurrence. Depending on the type of the duct, a series of constraints was applied to the wall pressure distributions during the optimization process. The optimized pressure distribution was considered as the target pressure distribution for the inverse design problem. The duct geometry changes during the inverse design process to reach one satisfying the target pressure distribution. An offline connection was observed between the ball-spine inverse design method and the genetic algorithm. The boundary layer code was the medium for this offline connection. The optimized wall pressure distribution and inverse design process were evaluated for a straight diffuser and three S-shaped diffusers with different height to length ratios. The results revealed the robustness of the offline link of the inverse design and the genetic algorithm for the optimal aerodynamic design of ducts.

Elastic Surface Algorithm (ESA), which was proposed for the inverse design in external flows, substitutes the airfoil wall by an elastic curved beam that deforms due to a difference between the tar...

Achieving a unique solution for the 3D inverse design of a curved duct is a challenging problem in aerodynamic design. The centre-line curvature, and cross-sections’ area and shape of a 3D curved d...

In this study, the natural convective flow and heat transfer from a hot horizontal cylinder surrounded by two vertical plates with a negligible thickness was experimentally investigated. These plates act as a channel so that the natural convective flow of the hot cylinder passes through this channel. The flow field was visualized using particle image velocimetry technique to investigate the effect of changing, the width of the channel, and the height of the water free surface on the flow behavior. In each case, by changing the mentioned geometric parameters, flow velocity at the top of the cylinder, temperature of the cylinder surface, volume flow rate of the channel, the structure and location of vortices, and flow pat-tern inside the channel were studied. The results show that the flow is appropriately formed around the cylinder and inside the channel when both the free sur-face height above the channel and the channel width are twice the cylinder diameter. Also, the cylinder Nusselt number is increased for the channel with a width ratio of 2, relative to the case without channel around the cylinder.

Airfoils are mostly inefficient in their off-design conditions. In order to improve the aerodynamic performance of airfoils in these conditions, using an optimized cavity on airfoils as a passive method can be useful. In this study, a cavity on a Riso_B1_18 airfoil, which is used as a wind turbine airfoil, was optimized at an off-design angle of attack by incorporating a genetic algorithm into a RANS flow solver. For the cavity optimization, the geometry and downstream suction surface were defined by 16 parameters, and the lift-to-drag ratio was considered as the cost function at 14° angle of attack. The numerical solution showed that the optimized cavity traps a vortex, which postpones the stall. Due to the uncertainty of CFD especially at off-design conditions, it was necessary to evaluate the performance of the optimized cavity in a wide range of angles of attack. This study used the particle image velocimetry (PIV) measurement method to evaluate the improved flow structures over the optimized cavity. Two models of airfoils with and without the cavity were made of aluminum and installed inside the test section of an open-jet wind tunnel with an air speed of 30 m/s and a cross section of 30 × 30 cm2. The air flow on the suction side of the airfoils was measured at 7°–15° angles of attack by PIV. A comparison between the measured flow fields over the two airfoils showed that the optimized cavity postpones the stall angle by 3°. Furthermore, the cavity increases the momentum behind the airfoil at the angles of attack greater than 9°. After this angle, a further increase in the angle of attack increases the difference between the momentums behind the airfoils with and without cavity. The Riso_B1_18 airfoil with the optimized cavity can be used as a wind turbine airfoil at high angles of attack to increase the stall angle and decrease the instability and fluctuation at off-design conditions.

The present study introduces an innovative aerodynamic redesign of an axial flow fan based on constant diffusion factor and radial equilibrium. All input design parameters such as mass flow rate, hub to tip ratio, aspect ratio, tip diameter and angular velocity are taken from NASA Rotor 67 as a conventional axial flow fan. A computer program is developed to extract the three-dimensional geometry of a fan and to estimate the span-wise distribution of parameters. The new designed fan flow field is investigated in detail by CFD tool at both design and off design conditions. Finally, a turbofan cycle analysis is conducted based on thermodynamic and gas dynamic principles to evaluate the fan performance in a turbofan engine in comparison to NASA Rotor 67. Achieving a higher total pressure ratio, meeting the target pressure ratio in lower rotational speed with higher efficiency, delivering more bypass air in a constant diameter and less fuel consumption for the same specific thrust force are the main advantages for the new design strategy in comparison to the conventional designed fans such as Rotor 67. However, efficiency reduction in fan over speed is the main disadvantage.

Real-gas effects should be considered in ORC expanders, particularly in a supersonic flow. This study deals with the numerical evaluation of cubic real gas models in a supersonic R245fa flow over a wedge-shaped cascade. The NIST Refprop real gas model was selected as a benchmark model, and four other cubic real gas equations of state (EoS) were evaluated with respect to the reference model. The four cubic real gas EoS are studied. The results show that the Peng-Robison EoS is the most accurate model with an average error of less than 1.1 % for Mach number prediction for 3 different sections, and its computational cost is 39 % lower than NIST. However, the Aungier-Redlich-Kwong EoS model shows better performance accuracy and the computational cost with an average error of 1.7 % for Mach number prediction and a 17 % computational cost lower than NIST. Therefore, using this model is recommended for modeling R245fa in the supersonic regime.

{tiThis study examines the effect of nature-inspired leading edge 3D serration on the aerodynamic performance of a 65/35-degree double delta wing. Force measurement experiments were conducted on a smooth-surface model and a model equipped with various combinations of serrated pieces in the leading edge area. Four symmetric different positions were examined in a low-speed wind tunnel. The transitional flow regime was applied with Reynolds numbers of 2×105 and angles of attack (AOA) of 0 to 36 degrees. Compared to the clean model, there is a notable enhancement in the maximum lift coefficient, especially for the pieces located at the apex and kink angle locations. However, when changing the combination of serrated leading edge (SLE) elements, a decline in CL can be seen at some AOAs, and adding more SLE pieces is not always useful. A drag reduction is also achieved by installing these elements, which provided a higher lif}t-to-drag ratio for all AOAs. Finally, the serrated leading edge postponed the stall angle of the double delta wing.

The current study measures the instantaneous velocity field inside and outside a new designed fluidic oscillator using planar time-resolved PIV technique on its symmetry plane in a water tank. The instantaneous velocity field showed that the interaction of four vortices with the jet was the main reason of jet oscillation inside the main chamber. Three-dimensional transient numerical solutions were also carried out using Ansys-Fluent 2020R2 software. Four turbulence models (k-e, k-e RNG, k-ω, and k-ω SST) were used for these numerical solutions, which their results were then compared with the PIV results, qualitatively and quantitatively. The comparison showed that the k-e and k-ω SST models were better for simulating the flow of the new oscillator. However, both models over-predicted the jet spreading angle and oscillation frequency compared to the PIV results. A similar PIV measurement was accomplished outside the conventional oscillator to compare the time-averaged velocity fields of the new and conventional oscillators. The comparison showed that the amplitude and frequency of the new oscillator were, respectively, lower and higher than those of the conventional one. The time-averaged velocity and momentum flux outside the new oscillator was significantly higher than for the conventional one.

The aim of an inverse design method is to calculate the geometry corresponding to a target pressure distribution along the walls. Elastic Surface Algorithm (ESA), as one of the newest resid...

This study numerically investigates a low Reynolds two-dimensional flow of a viscoelastic fluid over a circular cylinder using the finite volume method. The Phan-Thien–Tanner model, as one of the most accurate non-linear models, for the first time, describes the viscoelastic behavior of a high concentration polymer solution in the flow over a cylinder in high elastic regime in Re = 10. To avoid divergence and to stabilize the numerical process in high elastic cases, the log-conformation approach proposed by previous researchers is used. The convective terms of the equations are discretized using a high-resolution scheme. The physical instability, as observed by some researchers in the creeping regime of high De flow, is very weak in the cases of this research. The numerical results of this research show both drag reduction (for the elasticity number El 0.025). Compared with the dissipative nature of viscoelastic fluid flows, the drag coefficient, resulted from the polymeric portion of stress, in the high elastic flow regime (El > 10), approached to zero due to an increase in the storing nature of viscoelastic fluid. At these elasticity numbers, the drag coefficient remains constant with El, and the material behaves like a Newtonian fluid. Compared with the shear-thinning behavior and unlike the creeping flow, the elasticity is the main cause of drag variation in most cases. Shear stress affects the drag force directly, while normal stress influences the drag force by changing the pressure distribution over the cylinder. The maximum normal stress occurs at the back of the cylinder due to the wake behind it, while in creeping flow it occurs at its half-front. Finally, the effects of elasticity number, retardation ratio and model parameters on the distribution of stress components, drag coefficient, and vortex dimensions are investigated separately, and a physical discussion is presented.

This study qualitatively investigated the effects of riblets with circular cross section on the aerodynamic performance of a thick airfoil. Smoke lines inside a wind tunnel visualized the flow fields around the airfoils with and without riblets to compare the location of the separation point, the stability of the visualized smoke lines near the wake and the size of the wake region for 0°–20° angles of attack, at a Reynolds number of 2 × 104. Four sizes of riblets with the height-to-chord ratios of 0.002, 0.005, 0.008 and 0.01 were attached to the thick airfoil to prepare the textured models for the visualization tests. The sizes of the wake region were measured at four cross sections behind the models by image processing. Due to the fluctuations of the wake size, many consecutive images were considered for averaging the wake size at each section. The results showed the wake size to be changed as a result of using riblets depending on the size of riblets. The textured model with the height-to-chord ratio of 0.005, compared to the smooth model, significantly decreased the wake size for all the angles of attack.

Piezoelectric patches are widely used on a micro scale energy harvesting due to their simplicity and high flexibility. In this study a fluidic oscillator was used to effectively convert the kinetic energy of a fluid into the strain energy of the piezoelectric structure. The relationship between the input velocity and the frequency of fluid fluctuations in the fluidic oscillator was obtained and different positions for the piezoelectric beam and the effect of the input velocity on the output voltage was examined. The optimum electrical resistance was finally calculated for the maximum harvested power and the pressure drop caused by the fluidic oscillator and piezoelectric beam was investigated. The results indicated when the free end of the beam was inside the main chamber of oscillator, the beam fluctuates with its natural frequency so that the fluid oscillations frequency is close to the natural frequency at different velocity. However, when the free end of the beam was outside the main chamber, the voltage and power were maximized at the frequency of fluid oscillation equal to the natural frequency of the piezoelectric beam.

This research investigates the aerodynamic performance and flow characteristics of a delta wing with 65° sweep angle and with coarse axial riblets, and then compares with that of a smooth-surface delta wing. Particle Image Velocimetry (PIV) were utilized to visualize the flow over the wing at 6 cross-sections upright to the wing surface and parallel to the wing span, as well as 3 longitudinal sections on the leading edge, symmetry plane, and a plane between them at Angles of Attack (AOA) = 20° and 30° and Re = 1.2 × 105, 2.4 × 105, and 3.6 × 105. The effects of the riblets were studied on the vortices diameter, vortex breakdown location, vortices distance from the wing surface, flow lines pattern nearby the wing, circulation distribution, and separation. The results show that the textured model has a positive effect on some of the parameters related to drag reduction and lift increase. The riblets increase the flow momentum near the wing’s upper surface except near the apex. They also increase the flow momentum behind the wing. Keywords: Aerodynamic performance, Coarse riblet, Delta wing, Flow structures, Particle image velocimetry, Vortex

This study investigates the flow structures and aerodynamic performance of a slender delta wing that has a sweep angle of 65 degrees and is equipped with acicular vortex generators. The results were compared with those of a smooth wing. Particle image velocimetry (PIV) was used to visualize the flow at six different sections of the delta wing in both the chord-wise and span-wise directions at an angle of attack (AOA) of 30 degrees and a Reynolds number of Re = 2.4 × 10 5 . The effect of the acicular vortex generators on the vortex diameters, locations of vortex breakdown, linear convolution integral patterns, circulation, and separation were studied. The Q-criterion was used for vortex boundaries for the span-wise sections. The results show the acicular vortex generators have positive effects on the aerodynamic performance of the wing. Furthermore, the acicular wing delays the vortex breakdown in comparison with a smooth wing and increases the flow momentum near the upper wing surface and behind the wing.

Many conventional airfoils, despite a good performance at their design points, get out of optimal conditions outside the design points. One passive way to enhance the airfoil performance is to use a cavity with an optimized shape. In this study, Riso_B1_18 airfoil, having a remarkable aerodynamic performance for wind turbine blades, is selected as a substrate for deploying an optimized cavity on the airfoil. For shape optimization of a cavity, its shape and downstream suction surface are parametrized to reach an optimum lift-to-drag ratio as the target function by using the genetic algorithm. The results of transient numerical solution indicate that the optimized cavity is well capable of draping vortex to control the stall margin, prevent flow fluctuations and significantly increase the lift-to-drag ratio at off-design conditions. To validate the performance improvement obtained from this numerical optimization, a force measurement setup is accomplished in a wind tunnel with 30 × 30 cm2 test section to measure the lift and drag forces of the Riso airfoil with and without optimized cavity. The experimental results shows that the lift-to-drag ratio increases 31% at AOA = 14° and 57% at AOA = 20° due to using the optimized cavity.

Architects and engineers working in different climatic conditions have come up with many new ideas to adapt with nature. In so doing, they have managed to construct environmentally friendly buildings that can provide the required cooling/heating without using nonrenewable energies. Shavadoon is one such building. Constructed at a depth of 5–12 m below ground level, this structure takes advantage of the high thermal capacity of soil to provide thermal comfort conditions, thus helping the inhabitants of the building to escape the exhausting heat during summer. Tals (structural elements used in Shavadoons) act as communication bridges for connecting neighboring Shavadoons. As a first step in this study, two separate Shavadoons were simulated. Then, the effect of Tals and the effect of communication methods on ventilation velocity were studied. The results showed that adding a Tal could reduce the existing vortices around the stairway, thus increasing the inlet flow rate by 57%. Adding a second Tal and connecting the two Shavadoons via two Tals, while not affecting the overall flow rate, created an even flow rate distribution through the Tals. Tal length also influenced the flow rate: increasing the length of the Tals caused the flow rate through them to decrease.

The upgraded elastic surface algorithm (UESA) is a physical inverse design method that was recently developed for a compressor cascade with double-circular-arc blades. In this method, the blade walls are modeled as elastic Timoshenko beams that smoothly deform because of the difference between the target and current pressure distributions. Nevertheless, the UESA is completely unstable for a compressor cascade with an intense normal shock, which causes a divergence due to the high pressure difference near the shock and the displacement of shock during the geometry corrections. In this study, the UESA was stabilized for the inverse design of a compressor cascade with normal shock, with no geometrical filtration. In the new version of this method, a distribution for the elastic modulus along the Timoshenko beam was chosen to increase its stiffness near the normal shock and to control the high deformations and oscillations in this region. Furthermore, to prevent surface oscillations, nodes need to be constrained to move perpendicularly to the chord line. With these modifications, the instability and oscillation were removed through the shape modification process. Two design cases were examined to evaluate the method for a transonic cascade with normal shock. The method was also capable of finding a physical pressure distribution that was nearest to the target one.

In this research, interactions between a solitary wave and two submerged rectangular obstacles with different distances were studied experimentally. The particle image velocimetry (PIV) technique was used to measure the velocity field on the weather and lee sides of the two obstacles. Two submerged rectangular obstacles with 12-cm height and 8-cm length were installed in an open water channel with 7-m length. The velocity fields were obtained with 5 fields of view (FOVs) and then synchronized together. The vortices that formed on the weather and lee sides of the rear and front obstacles were compared to each other. It was found that the vortex on the weather side of the rear obstacle was flatter and smaller than that on the weather side of the front obstacle. Moreover, the weather-side vortex of the front obstacle grew more. After the wave passes over the two obstacles, the vortices on the weather and lee sides of the front obstacle are merged together and form a larger vortex. A higher vorticity at the center of the weather side vortex of the front obstacle was observed. By increasing the distance between the obstacles, the vortex between the obstacles expands more, and the interaction of the flow with the obstacles is more intensive so that more and larger vortices are formed. These vortices depreciate the wave energy. The measurement of the wave height before and after the obstacles showed that the larger the distance between the obstacles, the higher wave energy loss will be.

In this study, a new inverse design method is proposed for the full 3-D inverse design of S-ducts using curvature-based dimensionless pressure distribution as a target function. The wall pressure distribution in a 3-D curved duct is a function of the centerline curvature and the cross-sectional profile and area. A dimensionless pressure parameter was obtained as a function of the duct curvature and height of the cross-sections based on the normal pressure gradient equation. The dimensionless pressure parameter was used to eliminate the effect of the cross-sectional area on the wall pressure distribution. Full 3-D inverse design of an S-shaped duct was carried out by substituting the 3-D duct with a large number of 2-D planar ducts. The ball-spine inverse design method with vertical spins was coupled with the dimensionless pressure parameter as a target function for the design of the planar ducts. The inverse design process was performed in two steps. First, the height of each cross-section was considered constant, and only the duct centerline was allowed to be deformed by applying the difference between the dimensionless pressure on the upper and lower lines of symmetry plane. Then, a constant curvature was considered for each centerline in the equation, and the difference between the current and the target dimensionless pressure was applied to each upper and lower line of the planar sections to correct the heights of the 2-D planar sections, separately. The method was validated by choosing a straight duct as an initial guess, which converges to the target S-shaped duct. The results showed that the method is an efficient physical-based residual-correction method with low computational cost and good convergence rate. The 3-D wall pressure distribution of a high-deflected 3-D S-shaped diffuser was modified to eliminate the separation, secondary flow, and outlet distortion. Finally, the geometry corresponding to the modified pressure was obtained by the proposed 3-D inverse design method, which revealed higher pressure recovery, lower total pressure loss, and lower outlet flow distortion and swirl angle.

A self-seeding particle method is proposed for particle image velocimetry measurements in closed cycles such as Organic Rankine Cycles. Condensed droplets of vapor are used as tracers in a closed cycle for both subsonic and supersonic regimes. A free jet of R245fa in the vapor phase is examined in a case study with two different nozzle pressure ratios of 5.1 and 2.1 to evaluate the tracer particles in both supersonic and subsonic conditions. A simple turbulent jet in subsonic conditions and an under-expanded jet are observed in high supersonic conditions. The flow structures of the under-expanded jet are captured using the proposed method, and vivid images of the Mach disk and shock cells are obtained. A series of Schlieren photography experiments are performed to validate the proposed method. The results show that the method can be a good candidate for tracer particles in the closed cycles where condensation of the working fluid is possible.

Due to many applications in engineering, the study of the vortex-induced vibration is of great importance. In the present study, a passive heat transfer enhancement technique for turbulent flow through the channel with constant wall temperature at Re D = 1000 (based on the cylinder diameter) is studied. A flexible plate is attached to the stationary cylinder and the numerical analysis are carried out over a wide range of solid and fluid parameters such as density ratio (2.54-127), dimensionless Young’s modulus (1.1-11 *106), Reynolds number (300–1000), Prandtl Number (6.8 and 68), velocity and temperature profiles. As a result, although the variation of natural frequency by density and Young’s modulus of the plate alters the frequency and amplitude of oscillation, the performance evaluation criteria (PEC) number does not change. Velocity profile has a significant effect on increasing heat transfer rate and PEC number while temperature profile only increases heat transfer without any change in PEC number. Similar numerical analysis for the case without the flexible plate is also carried out to be compared to that with flexible plate. The results show that adding the flexible plate behind the stationary cylinder decreases the amplitude of lift coefficient 86%, the frequency of vortex shedding 35.8%, and total pressure loss 31.1%. The average Nusselt number is also improved by 2.3% and eventually, PEC increases by 23%.

A real dense gas such as R245fa is mostly used in organic-Rankine-cycle turbine expanders. The dense gas effects should be taken into account, especially in the transonic and supersonic flow regimes. Oblique shock and the interaction of shock and separation on the turbine blades are phenomena that have little deviation between a real gas and an ideal gas. This research numerically simulated the flow passing through a cascade of simple straight blades with keen edges considered for an ideal gas (air) and dense R245fa gas in a supersonic flow regime. The blade geometry was selected so that the deviations between the dense and ideal gas flows would be clearer than that with actual blades. The AUSM density-based method and NIST real gas model were used to model the ideal and dense gas, respectively. A second-order scheme was used for discretization, and the shear stress transport (SST) model was for the turbulence. The results show that an oblique shock is created on the leading edge when the inlet Mach number is 2.18 in dense gas. In ideal gas, a bow shock is created at the front of the leading edge. Moreover, for a wall pressure coefficient distribution, the separation point in dense gas is posterior than that in ideal gas.

Jet engine manufacturers and designers are seeking for lighter and smaller type of axial compressors. Improving the aerodynamic characteristics of blades is carried out by controlling the boundary layer. One way to control the boundary layer is using tandem blades. Tandem-blade cascades are capable of using highly loaded stages for axial compressors because they provide more works than single-blade cascades. In other words, tandem blades help to achieve a specified total pressure ratio with less number of stages. Therefore, one of the most important problems for researchers is to optimize the aerodynamic parameters of tandem blades. Changing the geometrical parameters of blades is a method to achieve this purpose.In this work, the stagger and camber angle of each blade are first changed while the other geometrical parameters such as overall camber, total stagger angle, the axial overlap, percent pitch and chord ratio are fixed. Secondly, the overall camber angle of tandem blade is changed by increasing the difference between the stagger angle of the first and second blade while the type of two airfoils, axial overlap and percent pitch, overall chord length and overall stagger angle are fixed.The aerodynamic performances of the generated tandem-blade cascades are obtained using two-dimensional numerical solution of flow. For this, a viscous turbulent flow solver is used for solving the Navier-Stokes equations. In these simulations, inlet Mach number is fixed to 0.6. Keywords: Tandem blade, Axial compressors, Cascade, Camber angle, Stagger angle, Flow deflection, Loss coefficient

New drag reduction methods have received much attention due to the importance of drag reduction in airplanes and wind turbines. One of the ways for drag reduction is the use of riblets. We investigated the effects of riblets on the aerodynamic performance of the Riso airfoil quantitatively. By installing a load cell and using the one-sided force measurement method, the drag and lift coefficients of the Riso airfoil were measured in two modes: With and without riblets at three different arrangements. The shape of riblets is a circularcross- section and the ratio of riblets’ diameter to the airfoil chord is equal to 0.005. The tests were carried out in transient flow regime (Two Reynolds numbers of 2.02×105 and 1.4×105), and at attack angles from 0 to 20 degrees. The results indicate that the extent of the riblets effect on the aerodynamic performance of the airfoil depends on the angle of attack, Reynolds number, and arrangement of the riblets on the airfoil. The maximum drag reduction at the Reynolds numbers of 2.02×105 and 1.4×105 is about 29.7 % and 54 %, respectively, that occurs at an attack angle of 7 degrees for both two Reynolds numbers.

Riblets are effective approach to improve the aerodynamic performance of low speed wing configurations. For a delta type wing, it is important to alleviate the flow features such as large rollup and tip vortices in low Reynolds number. In this paper, for first time, the effect of the square riblets on delta wings is studied. In the first step, flow visualization is accomplished to find out the effects of riblets on the flow pattern. For this purpose, two slender delta wings with and without riblet are examined in a vertical wind tunnel in which smoke stream lines are passed over the delta wings to visualize the flow pattern. The visualization tests are performed at five angles of attack; i.e., 0, 10, 20, 30 and 35 degrees. The results of flow visualization indicate that considerable improvements are obtained because of delaying flow separation at high attack angles. In the second step, the aerodynamic performance of the two delta wings are quantitatively evaluated by measuring the lift and drag forces using two load cells in another wind tunnel. The force tests are performed at attack angles of 5–65 degrees with 5-degree step sizes. The measured forces show that the riblets decreases the drag coefficient for all attack angles and increase the lift to drag ratio more than 12 percent for 5–20 degrees attack angles.

The present study investigates the effects of riblet on the aerodynamic performance and flow characteristics of a delta wing. Flow visualizations and measurements of the aerodynamic forces are performed on a smooth-surface as well as few textured-surface models of the delta wing. Flow visualizations were undertaken at flow speed of 2.5 m/s and various angles of attack in a vertical wind tunnel. The effects of riblet are investigated on the diameter of the vortices, location of the vortex breakdown, distance of the vortices from the wing surface, pattern of flow lines around the wing, and separation phenomenon. The results show that the riblet with a height-to-chord ratio of 0.013 has the most positive effect on the aforementioned parameters as far as drag reduction is concerned. In the second part of the study, lift and drag forces of the models are measured in a horizontal wind tunnel for a smooth-surface model as well as the other two textured models with height-to-chord ratios of 0.006 and 0.013, height-to-distance ratio of 1, sweep angle of 63.5°, and at Reynolds number of 2 × 105 and 0 to 35° angles of attack. The riblet-surface delta-wing model with height-to-chord ratio of 0.013 shows an increased lift-to-drag ratio at the whole range of angles of attack from 0 to 35°.

The performance of coastal breakwaters in reducing wave height and energy is an important problem. This paper presents an experimental and numerical investigation of solitary wave interaction with two submerged rectangular obstacles. White light and particle image velocimetry (PIV) techniques were utilized to study the free surface profile of a solitary wave and flow field in an experimental procedure. The PIV test results revealed that two clockwise vortices are generated between and after the two obstacles, and the white light test results showed that three phenomena of wave breaking, crest-crest exchange, and air-water mixing occur in the solitary wave passage over the two obstacles. A transient two-dimensional numerical model was used to study solitary wave interaction with two rectangular obstacles. The numerical model was validated with experimental results in terms of free surface profile, velocity fields, and velocity profiles. Using this model, the effects of obstacles height and distance were investigated. The numerical results showed that when increasing the height of the obstacles, the drag force applied on the obstacles, the strength of vortices, the energy loss, and the height reduction of the solitary wave increased. The presence of the second obstacle and wave breaking occurrence constrain the movement of vortices and cause a negative drag coefficient on the obstacles in some cases. The energy loss, the height reduction of the solitary wave, and the strength of the vortex generated between the obstacles increased as the distance of the obstacles increased up to S/L = 1.5. In contrast, the strength of the vortex generated after the second obstacle was decreased by increasing the distance of two obstacles.

In the present paper, flow characteristics of an oscillating jet in a cross-flow (OJICF) emitted from a double feedback fluidic oscillator were investigated. The results were compared with a steady jet in a cross-flow (JICF) from a jet nozzle with length and diameter equal to those of the fluidic oscillator. Smoke visualization in a low-speed wind tunnel was used. The measurements were done in the symmetry plane with 3 cross sections for 4 different blowing ratios (BR) of 1 to 3.33. The OJICF shows high potential to enhance the effectiveness of film cooling since it penetrates to a lesser height and leads to considerable spanwise penetration. Then, heat transfer behaviors of an impinging OJICF (IOJICF) and impinging JICF (IJICF) were compared via thermographic phosphor thermometry (TPT). The cross-flow Reynolds number based on the hydraulic diameter of the jet exit and cross-flow velocity was varied between 510 and 2,700, and the spacing between the jet and target surface was 3 times the jet throat's hydraulic diameter. The results show that for the IOJICF, heat transfer is dominated by the cross flow, and changing the BR does not have a significant effect. However, the IJICF promotes heat transfer around 2 times more than the IOJICF, and the more the BR is, the greater the Nu number. Therefore, the OJICF could be applied for film cooling rather than impinging applications.

The effects of using nature-inspired needle-shaped vortex generators (VGs) on the aerodynamic features of a 65°/35° double-delta wing were investigated experimentally in a wind tunnel. Particle Image Velocimetry (PIV) visualization was conducted on 7 chord-wise sections as well as 7 span-wise sections at an Angle of Attack (AOA) of 30° and on a plane adjacent to the wing surface at AOAs of 30° and 35° The experiments were carried out at Re = 2 × 105. The double-delta wing was equipped with a series of VGs at the leading edge of the apex and kink area. The effect of VGs on the time-averaged vortical flow features was investigated and then compared to those of the plain wing model. The assessment of the vortices’ diameter, vorticity and velocity distribution, streamline topology, circulation (Γ), and parameter Γ/Av showed that the use of the needle shaped VGs has an enhancing effect on the improvement in flow structure of the double-delta wing. The causes of the flow structure improvement were the increased flow momentum over and behind the wing, energized near-surface flow, a more concentrated vortex system, and delayed vortex breakdown.

In the present study, numerical solution of an inverse conjugate heat transfer problem including conduction and forced convection is carried out via the ball spine algorithm (BSA). The BSA inverse method is originally developed for inverse shape design of pure fluid flow problems with a desired pressure distribution along an unknown surface but in this paper it is shown how proposing a novel remedy enables the BSA method of solving inverse heat transfer problems as well. The proposed method has a truly low computational cost accompanied by high convergence rate. The inverse problem is defined as heat source surrounded by a solid medium exposed to free stream in external flow. Heat is generated by a heat source and spreads all over the medium by conduction and then is transferred to free stream by forced convection. The objective is finding a configuration for the solid body to satisfy a prescribed uniform temperature along its surface. Solution of the mentioned inverse problem is dependent on different combinations of the dominant parameters including Reynolds number ( Re ), thermal conductivity ratio ( k f / k s ), desired outer surface temperature ( θ s ) and also Prandtl number ( Pr ).