Showing papers on "Flow separation published in 2015"
TL;DR: In this paper, an infinite-span wing with an idealized representation of this geometry is reported at a Reynolds number of 1.2×10(to the power of 5) using large-eddy simulation, an adequate spatial resolution is first established before also examining the spanwise extent of the domain.
Abstract: The stall-delaying properties of the humpback whale flipper have been observed and quantified in recent years, through both experimental and numerical studies. In the present work, numerical simulations of an infinite-span wing with an idealized representation of this geometry are reported at a Reynolds number of 1.2×10(to the power of 5). Using large-eddy simulation, an adequate spatial resolution is first established before also examining the spanwise extent of the domain. These results are then analyzed to provide an explanation of the conditions that drive the lift observed beyond the conventional stall angle. The undulating leading-edge geometry gives rise to a spanwise pressure gradient that drives a secondary flow toward the regions of minimum chord. In turn, this leads to the entrainment of higher-momentum fluid into the region behind the maximum chord, which energizes the boundary layer and delays stall. Aside from demonstrating a significant poststall lift, the undulations also have the added benefit of substantially reducing lift fluctuations.
131 citations
TL;DR: In this article, the turbulent flow around a square cylinder at Reynolds number 22, 000 (based on the cylinder diameter and the inflow velocity) is studied by means of direct numerical simulation.
124 citations
TL;DR: In this article, the effects of VGs' size were analyzed from several perspectives, such as trailing-edge height, length, short and long spacing between an adjacent pair of generators, and flow field characteristics were further revealed by the analysis of streamlines and vortices in the wake region.
113 citations
TL;DR: In this paper, the authors presented a unique investigation of boundary layer flow at very high Reynolds numbers, including mean velocities, streamwise turbulence variances, and moments up to 10th order.
Abstract: Measurements are presented in zero-pressure-gradient, flat-plate, turbulent boundary layers for Reynolds numbers ranging from to ( ). The wind tunnel facility uses pressurized air as the working fluid, and in combination with MEMS-based sensors to resolve the small scales of motion allows for a unique investigation of boundary layer flow at very high Reynolds numbers. The data include mean velocities, streamwise turbulence variances, and moments up to 10th order. The results are compared to previously reported high Reynolds number pipe flow data. For , both flows display a logarithmic region in the profiles of the mean velocity and all even moments, suggesting the emergence of a universal behaviour in the statistics at these high Reynolds numbers.
111 citations
TL;DR: In this paper, the authors investigated the effect of super-hydrophobic (SH) surface micro-patterns on the dynamics of turbulent flow in the SH channel and found that between 80% and 100% of the DR in turbulent flow arises from the effective slip on the walls.
Abstract: The mechanism of turbulent drag reduction (DR) with super-hydrophobic (SH) surfaces is investigated by direct numerical simulation (DNS) and analysis of the governing equations in channel flow. The DNS studies were performed using lattice Boltzmann methods in channels with ‘idealized’ SH surfaces on both walls, comprised of longitudinal micro-grooves (MG), transverse MG, or micro-posts. DRs of to , to , and to were realized in DNS with longitudinal MG, transverse MG, and micro-posts, respectively. By mathematical analysis of the governing equations, it is shown that, in SH channel flows with any periodic SH micro-pattern on the walls, the magnitude of DR can be expressed as , where the first term represents the DR resulting from the effective slip on the walls, and the second term represents the DR or drag increase (DI) resulting from modifications to the turbulence dynamics and any secondary mean flows established in the SH channel compared to a channel flow with no-slip walls at the same bulk Reynolds number as the SH channel. Comparison of this expression to DNS results shows that, with all SH surface micro-patterns studied, between 80 % and 100 % of the DR in turbulent flow arises from the effective slip on the walls. Modifications to the turbulence dynamics contribute no more than 20 % of the total DR with longitudinal MG or micro-posts of high shear-free fraction (SFF), and a DI with transverse MG or micro-posts of moderate SFF. The effect of the SH surface on the normalized dynamics of turbulence is found to be small in all cases, and confined to additional production of turbulence kinetic energy (TKE) within a thin ‘surface layer’ of thickness of the order of the width of surface micro-indentations. Outside of this ‘surface layer’, the normalized dynamics of turbulence proceeds as in a turbulent channel flow with no-slip walls at the friction Reynolds number of the SH channel flow.
110 citations
TL;DR: In this paper, hydrodynamic characteristics of hydrofoils with leading-edge tubercles were experimentally investigated in a water tunnel at a Reynolds number of Re=1.4×104.
104 citations
TL;DR: In this paper, the feedback effects due to tonal noise emission in a laminar separation bubble (LSB) formed on the suction side of an airfoil in low Reynolds number flows were investigated.
Abstract: The subject of this experimental study is the feedback effects due to tonal noise emission in a laminar separation bubble (LSB) formed on the suction side of an airfoil in low Reynolds number flows. Experiments were performed on a NACA 0012 airfoil for a range of chord-based Reynolds numbers at angle of attack , where laminar boundary layer separation is encountered on both sides of the airfoil. Simultaneous time-resolved, two-component particle image velocimetry (PIV) measurements, unsteady surface pressure and far-field acoustic pressure measurements were employed to characterize flow development and acoustic emissions. Amplification of disturbances in separated shear layers on both the suction and pressure sides of the airfoil leads to shear layer roll-up and shedding of vortices from separation bubbles. When the vortices do not break up upstream of the trailing edge, the passage of these structures over the trailing edge generates tonal noise. Acoustic feedback between the trailing edge noise source and the upstream separation bubble narrows the frequency band of amplified disturbances, effectively locking onto a particular frequency. Acoustic excitation further results in notable changes to the overall separation bubble characteristics. Roll-up vortices forming on the pressure side, where the bubble is located closer to the trailing edge, are shown to define the characteristic frequency of pressure fluctuations, thereby affecting the disturbance spectrum on the suction side. However, when the bubble on the pressure side is suppressed via boundary layer tripping, a weaker feedback effect is also observed on the suction side. The results give a detailed quantitative description of the observed phenomenon and provide a new outlook on the role of coherent structures in separation bubble dynamics and trailing edge noise generation.
101 citations
TL;DR: In this article, a dielectric barrier discharge plasma actuator at a low Reynolds number is used for separation control of a NACA 0015 airfoil at a Reynolds number of 63,000.
Abstract: Large-eddy simulations have been conducted to investigate the mechanisms of separated-flow control using a dielectric barrier discharge plasma actuator at a low Reynolds number. In the present study, the mechanisms are classified according to the means of momentum injection to the boundary layer. The separated flow around the NACA 0015 airfoil at a Reynolds number of 63 000 is used as the base flow for separation control. Both normal and burst mode actuations are adopted in separation control. The burst frequency non-dimensionalized by the freestream velocity and the chord length (F+) is varied from 0.25 to 25, and we discuss the control mechanism through the comparison of the aerodynamic performance and controlled flow-fields in each normal and burst case. Lift and drag coefficients are significantly improved for the cases of F+ = 1, 5, and 15 due to flow reattachment associated with a laminar-separation bubble. Frequency and linear stability analyses indicate that the F+ = 5 and 15 cases effectively exc...
96 citations
TL;DR: In this paper, two approaches were evaluated: Response Surface Methodology (RSM) applying Neural Networking method, and Direct Optimization (DO) for heat transfer enhancement in a fin-tube heat exchanger.
87 citations
TL;DR: In this paper, a numerical investigation is performed to analyse the suppression of boundary layer separation first on a NACA 0015 airfoil and subsequently in a highly loaded subsonic compressor stator cascade using two different active flow control techniques: synthetic jet actuators (SJA) and continuous jet actuator (CJA).
82 citations
TL;DR: In this paper, the unsteady behavior of the flow around a symmetric airfoil is considered as incidence angle increases, and the flow patterns are presented for wide range of angles of attack values.
Abstract: Even for the stationary airfoils, due to the boundary and shear layer interactions of upper and lower surface of the airfoils, alternating vortex patterns form and the flow becomes time dependent. In the current study, the unsteady behavior of the flow around a symmetric airfoil is considered as incidence angle increases. The flow patterns are presented for wide range of angles of attack values. The vortex pattern generated is analyzed numerically for different angles of attack at Re=1000 around NACA 0012 airfoil. At this Reynolds number, the flow is laminar and boundary layers are quite thick. Flow separation and unsteady vortex shedding is observed even at low angles of attack. For NACA 0012 airfoil, the unsteady vortex pattern is observed at about 8° angle of attack for Re=1000. Spectral analysis is performed for angles of attack ranging from 0° to 90°. It is presented that amplitude spectrum of lift coefficient (C1) start to shows a peak at 8° for NACA 0012 and the aerodynamic forces presents oscillat...
TL;DR: In this paper, the authors evaluated the performance of shroud geometries for their augmentation of mass flow through the turbine. And they found that the Selig S1223 high-lift airfoil is the best airframe to best promote mass flow rate.
TL;DR: In this article, the utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally, and a NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105.
Abstract: The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic sta...
TL;DR: In this article, the authors investigated the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behavior over the airfoil and found that the recirculation region created by the DBD actuator over the pressure surface modifies the near-wake dynamics, leading to an increase in the lift coefficient.
Abstract: Flow control over a NACA 0012 airfoil is carried out using a dielectric barrier discharge (DBD) plasma actuator at the Reynolds number of 20 000. Here, the plasma actuator is placed over the pressure (lower) side of the airfoil near the trailing edge, which produces a wall jet against the free stream. This reverse flow creates a quasi-steady recirculation region, reducing the velocity over the pressure side of the airfoil. On the other hand, the air over the suction (upper) side of the airfoil is drawn by the recirculation, increasing its velocity. Measured phase-averaged vorticity and velocity fields also indicate that the recirculation region created by the plasma actuator over the pressure surface modifies the near-wake dynamics. These flow modifications around the airfoil lead to an increase in the lift coefficient, which is similar to the effect of a mechanical Gurney flap. This configuration of DBD plasma actuators, which is investigated for the first time in this study, is therefore called a virtual Gurney flap. The purpose of this investigation is to understand the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behaviour over the airfoil. First, the recirculation region draws the air from the suction surface around the trailing edge. The upper shear layer then interacts with the opposite-signed shear layer from the pressure surface, creating a stronger vortex shedding from the airfoil. Secondly, the recirculation region created by a DBD plasma actuator over the pressure surface displaces the positive shear layer away from the airfoil, thereby shifting the near-wake region downwards. The virtual Gurney flap also changes the dynamics of laminar separation bubbles and associated vortical structures by accelerating laminar-to-turbulent transition through the Kelvin–Helmholtz instability mechanism. In particular, the separation point and the start of transition are advanced. The reattachment point also moves upstream with plasma control, although it is slightly delayed at a large angle of attack.
TL;DR: In this article, the authors present a survey of the regimes of tonal noise generation for low to moderate chord-based Reynolds number between and and effective angle of attack between and for the NACA 0012 airfoil profile.
Abstract: Tonal noise generated by airfoils at low to moderate Reynolds number is relevant for applications in, for example, small-scale wind turbines, fans and unmanned aerial vehicles. Coherent and convected vortical structures scattering at the trailing edge from the pressure or suction sides of the airfoil have been identified to be responsible for such tonal noise generation. Controversy remains on the respective significance of pressure- and suction-side events, along with their interaction for tonal noise generation. The present study surveys the regimes of tonal noise generation for low to moderate chord-based Reynolds number between and and effective angle of attack between and for the NACA 0012 airfoil profile. Extensive acoustic measurements with smooth surface and with transition to turbulence forced by boundary layer tripping are presented. Results show that, at non-zero angle of attack, tonal noise generation is dominated by suction-side events at low Reynolds number and by pressure-side events at high Reynolds number. At smaller angle of attack, interaction between events on the two sides becomes increasingly important. Particle image velocimetry measurements complete the information on the flow field structure in the source region around the trailing edge. The influences of both angle of attack and Reynolds number on tonal noise generation are explained by changes in the mean flow topology, namely the presence and location of reverse flow regions on the two sides. Data gathered from experimental and numerical studies in the literature are reviewed and interpreted in view of the different regimes.
05 Jan 2015
TL;DR: In this article, a full-scale Boeing 757 vertical tail model equipped with active flow control (AFC) was tested at the National Full-Scale Aerodynamics Complex 40- by 80-Foot Wind Tunnel at NASA Ames Research Center.
Abstract: This paper describes test results of a joint NASA/Boeing research effort to advance Active Flow Control (AFC) technology to enhance aerodynamic efficiency. A full-scale Boeing 757 vertical tail model equipped with sweeping jets AFC was tested at the National Full-Scale Aerodynamics Complex 40- by 80-Foot Wind Tunnel at NASA Ames Research Center. The flow separation control optimization was performed at 100 knots, a maximum rudder deflection of 30deg, and sideslip angles of 0deg and -7.5deg. Greater than 20% increments in side force were achieved at the two sideslip angles with a 31-actuator AFC configuration. Flow physics and flow separation control associated with the AFC are presented in detail. AFC caused significant increases in suction pressure on the actuator side and associated side force enhancement. The momentum coefficient (C sub mu) is shown to be a useful parameter to use for scaling-up sweeping jet AFC from sub-scale tests to full-scale applications. Reducing the number of actuators at a constant total C(sub mu) of approximately 0.5% and tripling the actuator spacing did not significantly affect the flow separation control effectiveness.
TL;DR: In this article, a large-eddy simulation (LES) is conducted on a compressor cascade configuration using NACA65 blade profiles (chord based Reynolds number: 3.82 × 105), in comparison with the previous obtained experimental data.
Abstract: The increase of the thrust/weight ratio of aircraft engines is extremely restricted by different 3-D flow loss mechanisms. One of them is the corner separation that can form at the junction between a blade suction side and a hub or shroud. In this paper, in order to further investigate the turbulent characteristics of corner separation, large-eddy simulation (LES) is conducted on a compressor cascade configuration using NACA65 blade profiles (chord based Reynolds number: 3.82 × 105), in comparison with the previous obtained experimental data. Using the shear-improved Smagorinsky model as subgrid-scale model, the LES gives a good description of the mean aerodynamics of the corner separation, especially for the blade surface static pressure coefficient and the total pressure losses. The turbulent dynamics is then analyzed in detail, in consideration of the turbulent structures, the one-point velocity spectra, and the turbulence anisotropy. Within the recirculation region, the energy appears to concentrate around the largest turbulent eddies, with fairly isotropic characteristics. Concerning the dynamics, an aperiodic shedding of hairpin vortices seems to induce an unsteadiness of the separation envelope.
TL;DR: In this paper, the authors present a number of parallel and spatially developing simulations of boundary layers to address the question of whether hairpin vortices are a dominant feature of near-wall turbul...
Abstract: The present work presents a number of parallel and spatially developing simulations of boundary layers to address the question of whether hairpin vortices are a dominant feature of near-wall turbul ...
TL;DR: In this article, an instability-sensitive, eddy-resolving turbulence model on the Second-Moment Closure level is proposed, which is based on the Unsteady RANS (Reynolds-Averaged Navier Stokes) framework.
TL;DR: In this paper, the results of eddy-resolving simulations and supporting flow visualizations are investigated for the turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel.
Abstract: The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).
TL;DR: In this article, a direct numerical simulation of a temporally developing mixing layer with a passive scalar transport is performed for various Schmidt numbers (Sc = 0.25, 1, 4, and 8).
Abstract: A direct numerical simulation of a temporally developing mixing layer with a passive scalar transport is performed for various Schmidt numbers (Sc = 0.25, 1, 4, and 8). Turbulent mixing is investigated near the turbulent/non-turbulent interface (TNTI), which is a layer consisting of the turbulent sublayer (TSL) and viscous superlayer (VSL). The irrotational boundary, which is close to the outer edge of the TNTI layer, is detected as the isosurface of small vorticity magnitude. The movement of fluid elements relative to the irrotational boundary movement is analyzed. Once the non-turbulent fluid is entrained into the VSL across the irrotational boundary by the viscous diffusion of vorticity, the fluid moves away from the irrotational boundary in the VSL in the normal direction of the irrotational boundary. After the fluid reaches the TSL, it is transported in the tangential direction of the irrotational boundary and is mixed with the fluid coming from the turbulent core (TC) region. The boundary between the TSL and VSL roughly separates the region (VSL) mostly consisting of the fluid entrained from the non-turbulent flow from the region (TSL) where the fluids from both the TC and non-turbulent regions coexist. Therefore, the scalar value in the VSL is close to the non-turbulent value especially for high Sc cases. Because of a large difference in the scalar between the TSL and VSL, a peak value of the conditional mean scalar dissipation rate appears near the boundary between the TSL and VSL independently of Sc.
TL;DR: In this paper, the potential of vortex generators to mitigate shock-induced boundary layer separation is discussed, and it is shown that their potential and efficiency vary considerably in practical applications, such as transonic and supersonic inlets.
Abstract: This article reviews research into the potential of vortex generators to mitigate shock-induced separation. Studies ranging from those conducted in the early post-war era to those performed recently are discussed. On the basis of the investigations described in this report, it is clear that vortex generators can alleviate shock-induced boundary layer separation. Yet, it will be shown that their potential and efficiency varies considerably in practical applications. Much more success is reported in transonic test cases compared to separation induced in purely supersonic interactions. Under a variety of flow conditions, the best performance is achieved with vortex generators with a height of roughly half the boundary layer thickness and a shape similar to a swept vane. Notwithstanding this, vortex generator performance is not as consistent as it is in low-speed applications. Further work is required before vortex generators can be implemented into the design process for eliminating shock-induced separation on transonic wings and in supersonic inlets.
TL;DR: In this paper, a comparison between classical opposition control applied in the configuration of a fully developed turbulent channel flow and applied locally in a spatially developing turbulent boundary layer is presented, and it is found that the control scheme yields similar drag reduction rates if compared at the same friction Reynolds numbers.
Abstract: A comparison between classical opposition control applied in the configuration of a fully developed turbulent channel flow and applied locally in a spatially developing turbulent boundary layer is presented. It is found that the control scheme yields similar drag reduction rates if compared at the same friction Reynolds numbers. However, a detailed analysis of the dynamical contributions to the skin friction coefficient reveals significant differences in the mechanism behind the drag reduction. While drag reduction in turbulent channel flow is entirely based on the attenuation of the Reynolds shear stress, the modification of the spatial flow development is essential for the turbulent boundary layer in terms of achievable drag reduction. It is shown that drag reduction due to this spatial development contribution becomes more pronounced with increasing Reynolds number (up to Reτ = 660, based on friction velocity and boundary layer thickness) and even exceeds drag reduction due to attenuation of the Reynol...
TL;DR: The fundamental flow physics of the interaction between an array of fuel jets and a hypersonic boundary layer is investigated in this article, where hydrogen is injected at jet-to-freestream dynamic pressure ratios ranging from 0.350 to 0.875 on a flat plate into a Mach 4.5 crossflow.
Abstract: The fundamental flow physics of the interaction between an array of fuel jets and a hypersonic boundary layer is investigated. Hydrogen is injected at jet-to-freestream dynamic pressure ratios ranging from 0.350 to 0.875 on a flat plate into a Mach 4.5 crossflow. The injection array consists of four streamwise-aligned flush circular portholes. Both the streamwise spacing and jet-to-freestream dynamic pressure ratio are varied in a parametric study. The injection was performed completely within the boundary layer, with the intention of application to film-cooling drag reduction and boundary-layer combustion. Numerical simulations of four streamwise-aligned transverse sonic injectors in a fully turbulent hypersonic boundary layer revealed a very complex jet interaction flowfield. Variation of the streamwise injector port spacing, along with the jet-to-freestream dynamic pressure ratio, induced a variety of flow structures in the cases investigated. For all downstream interactions, the associated flow behavior was found to be a direct result of both the various upstream effects and interactions between adjacent injectors. Variations in the jet-to-freestream dynamic pressure ratio had a strong effect on the flow behavior. At low injection mass flow rates, coupling of adjacent injectors was small, whereas high mass flow rates increased the effect of jet-to-jet coupling. Variations in the streamwise jet-to-jet spacing were also found to play a critical role in the flow behavior. At very close spacings, intense interactions coupled the behavior of the individual jets; however, at increased spacings, the larger spatial freedom allowed individual jets to develop more naturally, leading to less jet-to-jet interactions. At the maximum spacing investigated, the jet interactions behaved more like discrete jets in crossflow.
TL;DR: In this paper, the authors present results of direct numerical simulations of a synthetic jet (SJ) based separation control of flow past a NACA-0018 (National Advisory Committee for Aeronautics) airfoil, at 10° angle of attack and Reynolds number 104 based on the air-foil chord length C and uniform inflow velocity U0.
Abstract: We present results of direct numerical simulations of a synthetic jet (SJ) based separation control of flow past a NACA-0018 (National Advisory Committee for Aeronautics) airfoil, at 10° angle of attack and Reynolds number 104 based on the airfoil chord length C and uniform inflow velocity U0 The actuator of the SJ is modeled as a spanwise slot on the airfoil leeward surface and is placed just upstream of the leading edge separation position of the uncontrolled flow The momentum coefficient of the SJ is chosen at a small value 213 × 10−4 normalized by that of the inflow Three forcing frequencies are chosen for the present investigation: the low frequency (LF) F+ = feC/U0 = 05, the medium frequency (MF) F+ = 10, and the high frequency (HF) F+ = 40 We quantify the effects of forcing frequency for each case on the separation control and related vortex dynamics patterns The simulations are performed using an energy conservative fourth-order parallel code Numerical results reveal that the geometric v
TL;DR: An optimization design method of supercritical natural laminar flow airfoil based on Genetic Algorithm and Computational Fluid Dynamics is tested in this paper, where class shape transformation method is adopted as geometry parameterization method Constraints on pressure distribution are applied to gain appropriate flow field in addition to the L / D performance.
TL;DR: In this article, the force and moment histories during an extended surge motion over several chord-lengths of travel were investigated under several different velocity profiles and incidence angles, and it was determined that the formation and subsequent shedding of a leading edge vortex correspond to oscillations in force coefficients for wings at high angle of attack.
Abstract: Rectilinearly surging wings are investigated under several different velocity profiles and incidence angles. The primary wing studied here was an aspect ratio 4 rectangular flat plate. Studies on acceleration distance, ranging from 0.125c to 6c, and incidence angles 5°–45° were performed to obtain a better understanding of the force and moment histories during an extended surge motion over several chord-lengths of travel. Flow visualization and particle image velocimetry were performed to show the flow structures responsible for variations in force and moment coefficients. It was determined that the formation and subsequent shedding of a leading edge vortex correspond to oscillations in force coefficients for wings at high angle of attack. Comparing unsteady lift results to static force measurements, it was determined that for cases with large flow separation, even after 14 chords traveled at a constant velocity, the unsteady forces do not converge to the fully developed values. Forces were then broken up...
TL;DR: In this article, the authors studied the physics of convex bank flow separation occurring in a variety of channel configurations, including confluences and bifurcations, in an open channel bend and showed that the boundary of the zone of flow separation is characterized by a shear layer, enhanced velocity gradients, tke, turbulent shear stresses and reversal of the streamwise vorticity and vertical velocity.
Abstract: Laboratory experiments in an open channel bend provide insight into the physics of convex bank flow separation occurring in a variety of channel configurations, including confluences and bifurcations. The edge of the zone of flow separation is characterized by a shear layer, enhanced velocity gradients, tke, turbulent shear stresses and reversal of the streamwise vorticity and vertical velocity. The latter result from turbulence-induced secondary flow near the convex bank. When bankline curvature abruptly increases, flow tends to move away from the convex bank along a straight path, as represented by the inertial forces including the centrifugal force in the transverse momentum equation written in curvilinear coordinates. Mass accumulation at the opposite bank leads to a transverse tilting of the water surface, and a pressure gradient towards the convex bank that causes the flow to change direction. The pressure gradient force lags spatially behind the inertial forces, which promotes flow separation. How separation typically occurs downstream of the location of maximum change in the bankline curvature, because an abrupt increase in bankline curvature also leads to water surface gradients that cause local flow redistribution towards the convex bank that opposes flow separation. The zone of convex bank flow separation is shaped by the secondary flow induced by streamline curvature and turbulence. The latter is conditioned by the production rate of tke, which crucially depends on the accurate description of the Reynolds stresses. Hydrodynamic, geometric and sedimentologic control parameters of convex bank flow separation are identified and discussed.
TL;DR: In this paper, an analysis of 3D-dimples with depth to diameter ratios of 1.5% and 5% was performed in a turbulent channel flow at Reynolds numbers between 5000 and 35 000.
Abstract: Arrays of shallow dimples with depth to diameter ratios of 1.5% and 5% are studied in a turbulent channel flow at Reynolds numbers between 5000 and 35 000. Pressure measurements show that drag reduction of up to 3% is possible. The mechanism of skin friction drag reduction with dimples is the same as that observed for flat surfaces using active methods such as spanwise wall motions or transverse wall jets. The three dimensional dimples introduce streamwise vorticity into the flow which results in spanwise flow components near the wall. The result is that the normal energy cascade to the smaller scales is suppressed, which leads to a reduction in turbulent skin friction drag because of the stabilized flow. Increasing the dimple depth from 1.5% to 5% of its diameter increases the streamwise vorticity introduced, which leads to a greater reduction in skin friction. However, increasing the dimple depth also results in flow separation which increases form drag. The net effect to the total drag depends on the relative dominance between the drag reducing streamwise vorticity and the drag increasing flow separation region. As the Reynolds number increases, the region of flow separation can shrink and result in increasing drag reduction. By understanding the flow physics of drag reduction in dimples, there is opportunity to minimize the form drag by passive contouring of the dimples using non-spherical shapes to optimize the dimple performance.
TL;DR: In this paper, a joint experiment within the European FP7 project AFDAR and particle tracking velocimetry (PTV) has been successfully applied to measure the flow within the ERCOFTAC test case Nr. 81 "periodic hill" (PH) water tunnel at TU Munich delivering field data with high spatial and temporal resolution.
Abstract: In order to increase the prediction capabilities of advanced numerical methods for turbulent wall bounded flows at relatively high Reynolds numbers accurate experimental validation data-sets including the full Reynolds stress tensor at high spatial resolution are strongly required. In particular the influence of pressure gradients and wall curvatures up to flow separation and the development of related shear layers need to be investigated experimentally in order to provide reliable data for the validation process but also to prove scaling laws, sub-grid- and turbulence-models. Furthermore, for advanced unsteady flow simulation methods (LES, DES, DNS etc.) the integration times and domains which are necessary for resolving flow features with very low spatial or temporal frequencies are often not sufficient for a fully converged solution. Consequently, the used experimental methods have to be able to resolve a large range of spatial and temporal scales for serving the code validation process. In a joint experiment within the European FP7 project AFDAR several advanced particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods have been successively applied to measure the flow within the ERCOFTAC test case Nr. 81 ”periodic hill” (PH) water tunnel at TU Munich delivering field data with high spatial and temporal resolution.