There has been increasing evidence that severe stenosis may cause artery compression and plaque cap rupture leading to heart attack and stroke. The physiological conditions under which that may occur and mechanisms involved are not well understood. It has been known that severe stenosis causes critical flow and wall mechanical conditions such as flow limitation, flow separation, low and oscillating shear stress distal to the stenosis, high shear stress and low or even negative flow pressure at the throat of stenosis, artery compression or even collapse. Those conditions are related to limitation of blood supply, intimal thickening and thrombosis formation, endothelism damage, platelet activation and aggregation, plaque cap rupture (for review, see [1,2]). Due to the complexity of the problem and lack of experimental data for mechanical properties of arteries under both expansion and compression, previous models were limited primarily to flow behaviors and with various limitations (axisymmetry, rigid wall, small strain, small pressure gradient). In this paper, experimental data for artery mechanical properties under physiological conditions were measured and a 3-d computational model is introduced to investigate flow behaviors and wall stress and strain distributions with fluid-structure interactions to better understand the mechanism involved in artery compression and plaque cap rupture.

Common heat transfer enhancement techniques are found in many of today’s processes. Devices such as inclined baffles, rib turbulators, jet impingement, etc are all used in order to augment the thermal transfer of a process. Inclined baffles have been studied along with the combination of jet impingement techniques. Inclined baffles enhance heat transfer by enhancing large-scale fluid bulk motion and through tripped boundary layer flow separation. Impinging jets, created by perforating the baffle, create additional heat transfer through flow impingement on the heated surface. Rib turbulators create heat transfer augmentation by disturbing the thermal boundary layer, unlike baffles; ribs enhance heat transfer in areas near the heat transfer surface leaving the core flow largely undisturbed. This investigation investigates experimental heat transfer of turbulent flow in a rectangular channel with inclined solid and perforated baffles combined with rib turbulators for isoflux heating at the top surface of the channel. Combining ribs with baffles yielded an increase in average Nusselt Number, albeit, with a pressure drop penalty. In situations where rate of heat transfer is critical to the performance of a device combining ribs with baffle is a viable solution.

A numerical study of rib augmented cooling of turbine blades is reported in this paper. The time-dependent velocity field around a pair of symmetrically placed ribs on the walls of a three-dimensional rectangular channel was studied by use of a modified version of Marker-And-Cell algorithm to solve the unsteady incompressible Navier-Stokes and energy equations. The flow structures are presented with the help of instantaneous velocity vector and vorticity fields, FFT and time averaged and rms values of components of velocity. The spanwise averaged Nusselt number is found to increase at the locations of reattachment. The numerical results are compared with available numerical and experimental results. The presence of ribs leads to complex flow fields with regions of flow separation before and after the ribs. Each interruption in the flow field due to the surface mounted rib enables the velocity distribution to be more homogeneous and a new boundary layer starts developing downstream of the rib. The heat transfer is primarily enhanced due to the decrease in the thermal resistance owing to the thinner boundary layers on the interrupted surfaces. Another reason for heat transfer enhancement can be attributed to the mixing induced by large-scale structures present downstream of the separation point.

A MEMS-based active system is currently under development for flow separation control in the transonic regime. The system consists of micro shear stress sensors for flow sensing and micro balloon actuators for separation control. We have successfully completed the first phase of the program in which the micro sensors and actuators were fabricated and tested in a wind tunnel facility. In the test, the sensors and actuators were flush mounted on a 3D model, which is representative of the upper surface of a wing with a deflected trailing edge flap. The model was installed in the wind tunnel and tested at a series of Mach numbers between 0.2 and 0.6. For all Mach numbers, the sensor output indicates that flow separates over the trailing edge when the micro balloons are in the ‘down’ position. When the micro balloons are inflated, the shear stress level on the trailing edge increases substantially, indicating an improvement of the separation characteristics. This result demonstrates the feasibility of using MEMS sensors and actuators to control flow separation. It is the first step toward the development of a revolutionary closed loop flow control system applicable to existing and future aircraft to enhance aerodynamic performance.

The enhanced heat transfer behavior of laminar shear-thinning, power-law fluid flows in sinusoidal corrugated-plate channels is investigated. With duct plates at uniform wall temperature, periodically developed flows are considered for a wide range of channel corrugation aspect ratio (0 ≤ γ ≤ 1), flow rates (10 ≤ Re g ≤ 1500), and pseudo-plastic flow behavior indices ( n = 0.5, 0.8, and 1.0). Typical velocity and temperature distributions, along with extended results for isothermal friction factor f and Collburn factor j are presented. The enhanced forced convection is found to be strongly influenced by γ , and the flow field displays two distinct regimes: undisturbed laminar or no swirl, and swirl flow regimes. In the no-swirl regime, behavior similar to that in fully developed straight duct flows with no cross-stream disturbance is obtained. The shear-thinning nature of the fluid, however, decreases f and enhances j . In the swirl regime, flow separation and reattachment in the corrugation troughs generates transverse vortices that grow with Re g and γ . The transition to this regime is also seen to depend on Re g , γ , and n , and in shear-thinning flows, it occurs at a lower Re g . The combined effects of corrugated plate geometry and non-Newtonian fluid rheology produce a heat transfer enhancement, as measured by the factor j/f , of over 3.3 times that in a flat-plate channel depending upon γ , n , and Re g .

The heat transfer characteristics of an offset strip fin are investigated by the transient liquid crystal method. It is found that there are various flow patterns that may occur, such as separation, reattachment, flow acceleration, expansion, and trailing vortex, and that the heat transfer is directly influenced by them. These flow patterns vary depending on the fin geometry and Reynolds number. For a sparsely stacked fin array (strip thickness-to-fin pitch, t/D < 0.1), the heat transfer pattern is close to a flat plate in a free stream such that the heat transfer is high at the leading edge and decreases along the plate. A distinct pattern change occurs at Re D h ≈ 2300. The heat transfer is high at the leading edge, but decreases rapidly over the first 10–20% of the strip, then increases to a maximum at 25–30%, then decreases again over the rest of the strip. The maximum at 25–30% of the strip indicates flow reattachment there. The same trend is found in a densely stacked fin array ( t/D = 0.25) but the pattern change occurs at much lower Reynolds number (Re D h ≈ 800). The influence of flow reattachment is comparable at these Reynolds numbers in both of the fin arrays but it becomes more significant in the densely stacked fin array as Reynolds number increases. It appears that this is due to more dominant flow separation and reattachment, caused by an adverse pressure field near the leading edge of the strip in the densely stacked fin array. The results indicate the importance of the fin pitch in understanding the flow and heat transfer characteristics of offset strip fins.

The flow inside a centrifugal impeller has various complex three dimensional phenomena (flow separation, jet-wake structure, shock wave, etc.). In this study, the internal flow field calculation of Samsung, high pressure ratio, high speed, centrifugal impeller with splitter blades is obtained by commercially available CFX-Tascflow code with CFX-Turbogrid for grid generation. The results are compared to that obtained previously by Denton and Dawes codes. The impeller is used in the first stage centrifugal compressor of an industrial gas turbine. The CFX-Tascflow results showed some differences Mach number contours. Also, the calculations are performed for Krain’s backswept impeller and the results are compared to the experimental measurements. Simulation of tip clearance has been done and the results were in a good agreement with the previous experiments.

Initial development of the laminar thermal boundary-layer flow over an impulsively started translating and spinning axisymmetrical isothermal body with blowing and suction in the case of mixed convection is investigated. Velocity components and temperature are expanded in series in powers of the square root of time. Leading, first and second order functions are obtained analytically and the third, forth and fifth order functions are determined numerically. Application of the general results to a sphere shows that buoyancy driven aiding and opposing flows help surface suction in retarding and surface blowing in facilitating the onset of flow separation respectively. Surface heat flux is increased near the front stagnation point due to the axial inflow created by the body spin and enhanced by the circulating flow inside the separated region. Surface suction is found to increase the heat transfer while the surface blowing decreases it. Aiding and opposing flows increase the surface heat flux around the front and rear stagnation points respectively.

Flow separation causes aircraft to experience an increase in drag degrading their aviation performance. The current study aims to delay flow separation on an airfoil by embedding a high-frequency translational piezoelectric actuator along the surface of the airfoil. The actuators with two actuation surfaces were embedded on the suction surface of an Eppler 862 airfoil model and placed in a low-speed wind tunnel. Consecutive pictures of the flow fields with dry ice fogs around the airfoil were taken using a high speed camera in order to observe the flow separation phenomenon before and after turning on the high-frequency translational surface actuation. The effects of the actuation on the flow separation were observed at various actuation displacements, angles of attack, and free stream velocities. The operating frequency of the surface actuation was 565 Hz. The measured actuation mean-to-peak displacement ranged up to 0.12 mm at the maximum applied voltage of 150 V. The angle of attack of the airfoil varied from 6° to 24°. The chord Reynolds number was increased up to around 262,000. It was confirmed that the actuation had a very strong influence on the flow separation even at a very small displacement of 0.024 mm remaining significantly reduced separation bubble compared to the one before activating the actuators at 4.3 m/s of velocity and 14° of angle of attack. The flow separation was completely suppressed when the actuation displacement reached around 0.082 mm under the same conditions of flow velocity and angle of attack. This implied that the actuation should generate a strong enough momentum relative to the free stream in order to completely suppress the flow separation. In summary, the study confirmed that the employed high-frequency translational surface actuation had the obvious control authority on delaying or suppressing the flow separation over the airfoil depending on the parameters changed.

A new design of an intercept valve assembly of the intermediate-pressure turbine part of greater power output is investigated in terms of pressure losses and flow fluctuations by using measurement on an experimental valve model. In addition, numerical simulations are used to further clarify measured phenomena. For such valve assemblies, it is important to exactly predict pressure losses and avoid danger of vibrations, which are caused by undesirable flow fluctuations, in order to guarantee valve’s efficiency and operational reliability. For this type of valve, it is especially important for turbine operations in partial loads (off-design conditions). Measurements were carried out in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences (IT) in a modular aerodynamic tunnel. Numerical simulations were carried out in the Doosan Skoda Power Company (DSP) by using a package of ANSYS software tools. The experimental valve model is a scaled model of a real valve assembly. It consists of an inlet pipeline, a stop valve and a control valve including its diffuser and outlet pipeline. Measured regimes were defined by a mass flow rate and a control valve cone lift which can be precisely changed. In order to investigate pressure loses, total and static pressures at valve characteristic locations were measured by using Prandtl probes and wall static pressure taps. In order to measure pressure fluctuations, Kulite fast response pressure transducers were used. They were situated near the valve throat where the flow fluctuations, which are strongly related to a flow separation, are the most visible and influential. Measurement results are compared with numerical results and locations with a flow separation were found. In order to reduce this phenomenon, different valve seat angles were also tested. As a result, a valve design could be optimized and, for a pressure loss prediction, a pressure loss model for this new intercept valve assembly could be created. Therefore, pressure losses in similar valve assemblies can be predicted with required accuracy for each new turbine where modern intercept valves are used. This helps to increase steam turbine efficiency and reduce fuel consumption. Based on pressure fluctuations results, operating conditions at which dangerous flow instabilities occur were identified. It was concluded that there is an operating condition border where the flow field starts to be unstable. As a result, the areas of safe and dangerous operating conditions can be predicted so that the operational reliability of the valve can be guaranteed.

The energy harvesting performance of a flapping airfoil is studied through discrete vortex model. Results are obtained for a thin flat airfoil that undergoes a sinusoidal flapping motion for reduced frequencies of k = fC/U ∞ = 0.06–0.16 where f is the heaving frequency of the foil, C is the chord length and U ∞ is the freestream velocity. The airfoil pitches about the mid-chord and the heaving and pitching amplitudes of the airfoil are h 0 = 0.5C and θ 0 = 70° respectively, as these numbers have been shown to give optimal energy harvesting results for a rigid airfoil. The study applies a panel-based discrete vortex model that incorporates a leading edge suction parameter criterion to understand the flow behavior around the airfoil. The leading edge suction parameter is found from 2D CFD simulations (Navier-Stokes equations solved in Fluent) for all K values. A correlation between the critical leading edge suction parameter and reduced frequency is found from the identified critical LESP values. An empirical trailing edge separation correction is also applied to the transient force results since flow separation at the trailing edge is anticipated. The parameters of interest from the model are transient distributions of force, power output, and overall efficiency. Model results are then validated against 2D CFD simulations. The effect of reduced frequency on power production and overall efficiency is finally studied to identify the optimal reduced frequency for energy harvesting applications.

Flow separation is a major parameter affecting the compressor performance. It reduces the compressor efficiency, limits static pressure rise capability and contributes to instability in compressors. In applied research, there is a lack of understanding of the nature and mechanism of the three-dimensional (3-D) flow separation in the axial compressor especially on the juncture of the endwall and blade corner region. In the present study, the 3-D flow field in an axial compressor cascade has been studied experimentally as well as numerically. For the experimental study part, a linear compressor cascade has been installed in an open loop wind tunnel. The experimental data was acquired for a Reynolds number Re c = 2.98 × 10 5 based on the blade chord and the inlet flow conditions. The total pressure loss progress through the blade passage has been measured by using calibrated five and seven-hole pressure probes connected to ATX sensor module data acquisition system. The static pressure distribution on the endwall has been measured employing static pressure taps connected to digital micromanometers. To investigate the loss mechanism through the cascade, the total pressure loss coefficient has been calculated from the measured data. The computational fluid dynamics (CFD) study of the flow field was performed to gain a better understanding of the flow features. Two turbulence models, Spalart-Allmaras (S-A) and shear stress transport SST (k-ω) were used. From both parts of study, the flow field development and total pressure loss progress through the cascade have been investigated and compared. Moreover, the received data demonstrated a good agreement between the experimental and computational results. The predicted flow streamlines by numerical calculations showed regions characterized by flow separation and recirculation zones that could be used to enhance the understanding of the loss mechanism in compressor cascades. All measurements taken by 5-hole and 7-hole pressure probes have been analyzed and compared. It was found that their readings were almost the same and there are no excellences for using 7-hole probe. Furthermore S-A turbulence model calculations showed more consistencies with experimental results than SST (k-ω) model.

The unsteady airflow over automotive side-view mirrors is a typical source of turbulence which creates extra drag force, aerodynamic noise and vibration. A CFD analysis is presented for vortex generators (VGs) application on the vehicle side-view mirrors for the purpose of flow improvement. Vortex generators are used to delay flow separation and increase the control surfaces which affect the drag force and down force of the vehicle. Reduced drag force can potentially increase fuel economy, and an increased downforce will increase vehicle grip force and improve vehicle stability which is essential for racing cars. This paper presents practical solutions for mitigating flow turbulence and adjusting down force for existing side-view mirrors. Four VG configurations were designed and numerically analyzed in combination with the baseline model at air speeds ranged from 15 to 80 miles per hour. This research investigated the effect of each VG configuration on the side-view mirror’s aerodynamic performance. The turbulent flow through the side-view mirror were analyzed by using standard K-epsilon (K-ε) Reynolds-averaged Navier-Stokes method. The drag and down forces results were obtained and compared with the baseline model. The CFD analysis concluded the following: (1) Setting the VGs with a 5 degree attack angle on the upwind face of the mirror slightly reduced the drag force. (2) Setting the VGs at the top of the mirror surface greatly increased the downforce with a large drag force increase.

This paper presents a complete study about three-dimensional (3-D) flow field development in a linear compressor cascade where flow field in the blade passage has been studied experimentally as well as numerically. In the experimental work, a linear compressor cascade test section was installed in an open loop wind tunnel. The experimental data was acquired for a Reynolds number of 2.98 × 10 5 based on the blade chord and the inlet flow conditions. The flow field characteristics in blade passage including 3-D flow velocity and velocity magnitude have been measured by using calibrated five and seven-hole pressure probes connected to ATX sensor module data acquisition system (DAQ). To investigate flow development in the blade passage, velocity coefficient through streamwise planes has been calculated from the measured data. The computational fluid dynamics (CFD) study of the flow field was performed to gain a better understanding of the flow features. Present computational study was first validated with previous experimental and numerical work to check mesh accuracy and give confidence for computational results. Then, two turbulence models, Spalart-Allmaras (S-A) and shear stress transport SST (k-ω) were used for the present work. From both parts of study, the flow field development through the cascade have been investigated and compared. Moreover, the received data demonstrated a good agreement between the experimental and computational results. The predicted flow streamlines by numerical calculations showed regions characterized by flow separation and recirculation zones such as corner separation that could be used to enhance the understanding of the loss mechanism in compressor cascades. All measurements taken by the two probes, 5 and 7-hole pressure probes, have been analyzed and compared. The 5-hole pressure probe measurements have showed more agreements with computational results than 7-hole probe. Furthermore S-A turbulence model calculations showed more consistencies with experimental results than SST (k-ω) model.

The Stirling engine is a high efficiency and high reliability energy converter which is expected to be a solution for future space power generation and commercial applications. One of the key components in a Stirling engine, to keep high efficiency, is the regenerator. Currently, woven screens or random fibers are mostly used as the regenerator material. However, since both woven screen and random fiber regenerators are composed of wires, the flow across the wires is similar to cylinders in cross flows. As a result, flow separation occurs and the regenerator results in high friction losses and thermal dispersion. In the previous study, a robust foil type regenerator is designed and CFD analysis of the regenerator was conducted to predict the friction coefficient and the thermal efficiency under oscillating flow conditions. In this research, a regenerator test bench was designed and constructed to investigate the friction coefficient and thermal efficiency of the regenerator. The experiment conditions are decided based on the one-dimension thermodynamic modeling software SAGE. The experiment result shows that the friction coefficient of the experiment is close to CFD prediction at high Reynolds number.

Passive vortex generators are widely used for heat and mass transfer enhancement in static mixers and heat exchangers. Trapezoidal vortex generators are used in the high efficiency vortex static mixer (HEV) because they generate a complex flow structure enhancing the transport phenomena. Moreover, vortex generators are used on airfoils and cars to delay or suppress flow separation. The flow past triangular and rectangular winglets was studied in the open literature showing good performance in enhancing the lift and drag coefficients. In the present study, a non-conventional vortex generator is proposed consisting on an inclined trapezoidal tab similar to that used in the HEV static mixer. In addition, the tab is perforated at its center (circular perforation). Inline array of several vortex generators are fixed on an airfoil and the drag and lift coefficients are analyzed for different geometries using computational fluid dynamics. Different cases are analyzed where the inclination angles of the vortex generators are changed and their effect is investigated. Furthermore, the effect of changing the size of the vortex generator is also assessed. The results are then compared to conventional vortex generators, mainly triangular winglets. The present results are validated against experimental and numerical data from the literature. The results show that the drag coefficient can be reduced with such vortex generators. They also show good agreement with experimental results for the lift coefficient.

The global power potential of the waves that hit all the coasts worldwide has been estimated to be in the order of 1 TW. On an average, each wave crest transmits 10–50 kW/m of energy and this corresponds to 15 to 20 times more energy per meter than wind or solar energies. Wave energy is environmentally friendly and is the most consistent of all the intermittent sources. While wind, solar and wave are all intermittent, wave is the most consistent. Availability of waves is 90% compared to 30% for wind and solar energy. The oscillating water column (OWC) is the most investigated wave energy converter (WEC). OWC is a partially submerged hollow structure positioned, either vertically or at an angle. The bidirectional flow of air above the water column is used to drive a turbine. Majority of the OWC devices have chambers which are perpendicular to the incident waves. These conventional OWCs suffer severely from flow separation that occurs at the sharp corners of the chamber. In order to address this issue, researchers have proposed inclining the chamber at an angle with respect to the incident waves. This improves the flow characteristics. In addition to this, the flow in the chamber which ultimately decides the turbine performance, also increases. In the present study, the effect of OWC inclination on rotor performance was numerically studied using commercial computational fluid dynamics (CFD) code ANSYS CFX. The results highlight that the 55° inclined OWC showed improved performance when compared to the conventional OWC and current OWC. The maximum power for the inclined OWC was 13% higher than that recorded for the rotor in the current OWC and 28% than that recorded in the conventional OWC at mean wave condition. The 55° inclined OWC recorded peak rotor power of 23.2 kW which corresponded to an efficiency of 27.6% at the mean sea state. The peak power and efficiency at maximum sea state was 26.5 kW and 21.5% respectively. Higher oscillation was observed in the 55° inclined OWC. The combination of increased flow rate and energy in the flow lead to better performance of the 55° inclined OWC.

The paper presents a new solution for the wind turbine profile shape modeling based on the concept of the maximum lift force, capable to be produced at different values of the wind velocities. The profile is designed and realized in accordance with the new concept emerged in the last decade, on the operation of the wind turbines with maximum lifting force. The purpose is to provide a low-noise during operation because a negative effect on the medium and long-term operation of the wind turbines (wind farms) is the noise that affects the flight of birds, terrestrial animal life, and especially human communities. Various sources generate independent acoustic emissions on wind profiles, such as the turbulent flow, the interaction of the turbulent boundary layer area of the trailing edge, the flow separation, and the boundary layer separation of vortices formed in the zone of the trailing edge. There is also considered the influence of the apparent wind on the incidence variation of the profile. In order to maintain an optimum angle of attack relative to the wind velocity, a fixed blade inclination must increase its speed to be proportional to the wind. Thus, to maximize the aerodynamic performance, the rotor must spin faster when the wind intensity increases. Measurement of the acoustic signal requires electronic devices that operate on electric signals obtained from the conversion of the pressure variations in voltage or variations in electrical current. The noise caused by the turbulent flow is generated primarily by the sharply pointed leading edge and cannot be diminished. There are presented some numerical results correlated with the measurements made in the field.

The flow characteristics of spherical bodies, arising in a variety of important engineering and environmental problems, range from laminar to turbulent flow. Turbulent flows are predominantly studied using the models based on Reynolds-averaged Navier-Stokes (RANS) equations. Especially, in case of flows around bluff bodies RANS models have limitations in capturing flow separation and other characteristic flow properties. Hence, the use of high-fidelity turbulent models is required to investigate the physics of these types of flow in detail. This study aims to compare and analyze the results of an incompressible turbulent flow around a sphere with additional geometric detail, like a trip wire, using different simulation techniques: Large Eddy Simulation (LES) and RANS. Modeling bodies with different characteristic geometric scales may require high-performance computing (HPC) resources due to the need to include accurate spatial and temporal resolution using unstructured mesh generation. This may be under circumstances additional criterion for decision which simulation approach is to be adopted.

In this work the influence of different radial work distributions on the performance of low pressure axial fans for automotive cooling purposes was investigated. The general standard solution in the design of axial fans is to assume constant work distribution (free vortex design). This also leads to a constant meridional flow velocity and thus makes the calculations for the design rather simple. To fulfill the constant work assumption, however, a high swirl component of the absolute outlet flow velocity results near the hub. Assuming the incoming airflow to be swirl-free, this means that the flow near the hub must be strongly deflected, leading to a very high blade load in this area and to very long chords. Thus, the assumption of constant radial work distribution leads to a high risk of flow separation near the hub, especially for low pressure cooling fans, where a small hub to shroud diameter ratio is needed in order to achieve higher flow rates. Furthermore, in fan applications, often the total-to-static pressure and total-to-static efficiency are the relevant design parameters, and not the total-to-total pressure and the total-to-total efficiency. In order to address these issues, linear and parabolic non-constant work distributions were investigated. These distributions were parametrized and for each work distribution a series of designs was created with the airfoil theory method. These designs were computed with the commercial Navier-Stokes-Solver STAR CCM+ and the results were analyzed in detail. As an application example a series of fans for a Formula Student racing cars cooling applications was developed. With this method, it was possible to achieve smaller hub to shroud diameter ratios, higher flow rates and better total-to-static efficiencies. The result was a new series of fans with improved cooling properties for automotive applications. These new fans, the design method and the results are presented in detail in this work.