Large Separation Region Research Articles

On the investigation of the aerodynamics performance and associated flow physics of the optimized tubercle airfoil

In the present study, the evolutionary multi-objective optimization is employed to design the optimum amplitude and wavelength of sinusoidal leading-edge tubercles for the NACA0012 (National Advisory Committee for Aeronautics) airfoil to improve its aerodynamic performance at Reynolds number of 5.0 × 104 than that of the NACA0012 smooth leading-edge or baseline airfoil. Here, the optimum tubercle is found to have an amplitude of 11.71% and a wavelength of 25% of the baseline airfoil chord, respectively. Through a combination of in-house water tunnel experiments and numerical simulations, it is additionally established that the optimized tubercle airfoil exhibits superior lift and reduced drag characteristics compared to the baseline airfoil, particularly in the post-stall high angle of attack regime. Furthermore, it is noticed that the optimized tubercle design enhances the gap between large separation regions or stall cells along the tubercle airfoil span during the post-stall regime. Consequently, a more pronounced attached flow regime is developed between the consecutive stall cells, contributing to the tubercle airfoil's improved aerodynamic characteristics compared to the baseline airfoil. Our investigations also revealed that the formation and arrangement of the stall cells on the tubercle airfoil span are associated with a biased wake mechanism similar to the one observed in the wake of side-by-side arranged circular cylinders.

Transonic leading-edge stall flutter: modelling, simulations and experiments

This work is a numerical and experimental study of a rectangular thin plate undergoing stall flutter at Mach 0.8. This constitutes one of the first studies of this kind where three-dimensionality is fully implemented in a numerical simulation including the test-section effects characterizing wind-tunnel experiments. In order to break down the fluid–structure interaction to its main driving phenomena, an aerodynamic model is proposed that is based on computationally inexpensive steady-state simulations. Two types of dynamic instability are observed in the numerical simulations; Flutter by mode coalescence is promoted at zero flow incidence, however, high bending precludes this from happening for higher values of angle of attack. Stall flutter is instead a nonlinear one-degree type of instability. Both of these instability mechanisms can be explained in terms of hysteretic behaviour of the pressure distribution, which becomes more pronounced at high angles of attack, when a large separation region is formed. Tests were conducted employing titanium alloy plates in order to survive the aerodynamic loads characterizing the wind-tunnel initial transient. However, due to wall interference, high bending was promoted so that the internal stress exceeded the yield values before flutter could be measured. Numerical simulations were in general agreement with the experiment in terms of both amplitude and oscillation frequency.

Investigation of the aerodynamic effects of bio-inspired modifications on airfoil at low Reynolds number

A numerical study was performed to investigate flow behaviors around bio-inspired modified airfoils compared with NACA 4412 airfoil at Re=5.8x104 by solving the two-dimensional, RANS equations with k-ω STT turbulence model. The obtained results reveal a rather abrupt decrease of lift at stall for the NACA 4412 airfoil in contrast to the mild stall depicted by the top-modified airfoil. As compared to the experimental results of the profiled airfoil in the literature, the characteristic behavior of the variation in the lift coefficient shows resemblance. It is seen that from the velocity distribution results, fluid flowed smoothly along the streamlined nose of NACA 4412 airfoil until α=4⁰ and streamlines adhered well for both airfoils at low angles (0⁰, 2⁰). Smaller circulation bubbles were noticed to settle in the canyons of the corrugated cross-section of the top-modified airfoil. In the wake region of the modified airfoil, there is no obvious large flow separation or circulation region at low angles of attack. However, the blue regions of the dimensionless velocity over the NACA 4412 airfoil and bottom-modified airfoil were narrower than over the top-modified airfoil. The recirculation zone over the airfoil started to enlarge, and the rolling up of the trailing-edge vortex appeared. After α=12⁰, the adverse pressure gradient on the suction side of the airfoils became more intense. In the wake zones, it was seen that the circulation regions grew remarkably and became largest as the angle of attack rose to α=16⁰, which pointed out increased drag forces of airfoils.

NUMERICAL ANALYSIS AND DIFFUSER VANE SHAPE OPTIMIZATION OF A RADIAL COMPRESSOR WITH THE OPEN-SOURCE SOFTWARE SU2

In recent years, the usage of open-source computational fluid dynamics tools is on a rise both in industry and academia. SU2 is one of these open-source tools. Unlike other open-source alternatives, SU2 is equipped with boundary condition types, solvers and methods that are especially developed for the analysis and design of turbomachinery. The aim of this work is to explore and investigate the capabilities of SU2 in the prediction of performance parameters of radial compressors. Two different single stage shrouded compressor geometries, one with a vaneless diffuser and the other with a vaned diffuser have been investigated with steady state CFD. The compressors were designed by MAN Energy Solutions Schweiz AG. Computational results with SU2 showed a satisfactory agreement with both the experimental data and reference CFD solutions obtained with Fidelity Flow, which is formerly known as Numeca Fine TURBO. Only at the relatively higher mass flow rates the difference between references and SU2 were higher compared to other operating points. After performance parameters were successfully calculated with SU2, the optimization tools that come with SU2 were also used. A 2D adjoint optimization study on the vane of the vaned diffuser was carried out. The study was carried out at a single operating point that is close to choke conditions. The loss generated by the large separated flow region at the suction side of the diffuser vane was reduced by 0.55 % in the optimized geometry using minimal modifications on the existing vane geometry to keep the performance of the compressor intact at other operating points. However, the resulting modification increased the total pressure loss by 0.86 % at one of the design operating points. This performance penalty could be due to the discontinuity in the vane geometry generated by the optimizer. Overall, the study shows that SU2 has the basic numerical schemes and models that are required for the analysis of radial turbomachinery flows and geometry optimization.

Pair interaction of localized topological structures in confined chiral media.

We study pairwise interactions between localized topological structures in chiral magnetic and cholesteric liquid crystal (CLC) systems confined in the planar geometry. Our calculations for magnetics are based on the lattice model that takes into account the bulk and surface anisotropies along with the exchange and the Dzyaloshinskii-Moriya interactions. In CLC cells, these anisotropies describe the energy of interaction with an external magnetic or electric field and the anchoring energy assuming that the magnetic or electric anisotropy is negative and the boundary conditions are homeotropic. We have selected the region of the phase diagram, where various localized solitonlike structures, including skyrmion tubes, torons, and leeches, embedded in the ground state of the z-cone (conical phase) coexist, and carried out numerical analysis of the distance dependencies of the effective intersoliton interaction potentials. For skyrmions and torons, the potentials are found to be attractive in the large separation region. It turned out that for these potentials, the effects of axial asymmetry are negligible. By contrast, it turned out that for the intermediate structures between the skyrmions and torons known as the leeches, the leech-leech potentials generally depend on the orientation of the intersoliton separation vector and their large distance parts may become repulsive at certain directions of the vector. All the potentials have the short distance repulsive parts and the local minima located at the equilibrium separations. It is found that the skyrmion-skyrmion potential has an additional metastable configuration shifted towards the short-distance region.

The investigation of a likely scenario for natural tornado genesis and evolution from an initial instability profile

A likely mechanism for the little-understood tornado genesis is proposed and its numerical implementation is presented. The Burgers-Rott vortex with its axis in the vertical direction is introduced as an instability mechanism, and the flow field then evolves under the influence of the atmospheric pressure, temperature and density variations with altitude. Buoyancy effects are implemented using the Boussinesq model. Results are presented and discussed for a set of conditions including mesh type and size, different turbulence models, and a few different boundary conditions. Post-processed results of the transient simulations including animations contain a wealth of information to help analyze tornado behavior. Velocity contours, pressure contours, vorticity contours, streamlines, and iso-surfaces show the evolution of a complex flow field possessing many characteristics of a tornado. At longer times from the start, the flow field becomes more asymmetric with the vortex core becoming more twisted, and the eye of the vortex drifting away from the axis of the computational domain. The single initial vortex then transitions into multiple vortices of varying size and orientation. These high Reynolds number (ReΓ ∼109) simulation results show flow fields that resemble highly unsteady, turbulent flows with large regions of flow separation, and large eddy size range.

Structural Characteristics of a Shock Train Flow Field in a Variable Cross-Section S-Shaped Isolator

Experiments were conducted in this study to reveal the flow characteristics of a variable cross-section S-shaped isolator, when applying the steady-state back pressure at the isolator outlet. The self-excited oscillation characteristics of the shock train generated under the influence of steady-state back pressure at an incoming flow speed of Mach 2, were also studied. The findings suggest that pressure oscillation within the area affected by the shock train’s flow field was significantly more potent than outside the affected area. Moreover, the forward movement velocity of the shock train in the variable cross-section S-shaped isolator was not uniform. The forward movement speed was slower when encountering sharp turns and faster during gentle turns. In the shock train flow field, high-frequency pressure oscillations, which mainly stemmed from the oscillations of the separated shock legs, propagated more readily within the flow field than low-frequency pressure oscillations. The significant separation of the shock train flow field will switch between the top and bottom walls, and the frequency of pressure oscillation in the large separation region is low. On another note, the closer the distance between two points is in the shock flow field, the stronger the coherence of pressure oscillations will be. In the distance upstream of the shock train flow field, the turbulent boundary layer (TBL) determines pressure oscillations instead of the shock train flow field, so the coherence was very high.

Bluntness and incidence effects in hypersonic flows with large separated regions

This paper describes a numerical investigation on the effects of small to large bluntness as well as the angle of incidence on the leading-edge separation in a laminar hypersonic flow. The results show that both bluntness and incidence strongly influence separation characteristics. A combined bluntness and incidence parameter is proposed and applied to plateau pressures in the separated region and is shown to demarcate effects of small and large bluntness at different angles of incidence. The investigation also showed profound changes in the internal structure of the separation bubble with changes in incidence and bluntness. The significance of the relationship between the two shear stress minima and the separation length is highlighted.

Numerical Modeling of Horizontal Axis Wind Turbine: Aerodynamic Performances Improvement Using an Efficient Passive Flow Control System

In this paper, we explore the improvement of the aerodynamic characteristics of wind turbine blades under stall conditions using passive flow control with slots. The National Renewable Energy Laboratory (NREL) Phase II rotor, for which detailed simulations and experimental data are available, served as a baseline for assessing the flow control system effects. The position and configuration of the slot used as a flow control system were determined using CFD analysis. The 3D-RANS equations are solved with ANSYS FLUENT using the k-ω SST turbulence closure model. The pressure coefficient for different wind speeds for the baseline configuration is compared to the available experimental data. The comparison shows that CFD results were better for the attached flow. The current work consists of a 3-D CFD modeling of a rotating blade equipped with different flow control systems: single-slot (S-S) and two-slots (T-S). The computation provides a better understanding of the influence of these flow control devices on the performance of wind turbine blades, the control of boundary layer separation, and the rotation effect. These control systems increase the power output by over 60% at high wind speeds with large separated boundary layer regions. For the configuration with the control system, the slot has shown its ability to delay the boundary layer separation. However, the improved aerodynamic performance has been proven for medium and high angles of attack where the flow is generally in the stall condition. The addition of the second slot changed the flow behavior, and an improvement was observed compared to the single slot configuration. The results are helpful for the design and development of a new generation of wind turbine blades.

Mass transport in binary TiO2:SiO2 and GeO2:SiO2 direct ink write glasses

AbstractThe mass transport mechanisms of Ti in TiO2:SiO2 and Ge in GeO2:SiO2 direct ink write, additively manufactured glasses were studied. Due to the low solubility of Ti in SiO2 and high melting point of TiO2 relative to SiO2, Ti transport was found to occur via solid state interdiffusion between adjoining SiO2 and TiO2 precursor particles. The diffusivity of titanium in SiO2 measured over typical sintering temperatures (1000–1300°C) using Rutherford backscattering spectrometry was . This provides an estimate of ∼30 nm for the diffusion length under typical sintering conditions (2 h at 1200°C). Although Ti and Ge have similar diffusivities in SiO2 glass at low concentrations, GeO2 was found to be much more mobile during the sintering of printed GeO2:SiO2 green bodies. This was evident in glasses with phase separated GeO2 regions over length scales of ∼10 µm and in experiments involving binary xerogel films in which GeO2 migrated over ∼10 µm through cracked, porous SiO2 layers. Large phase separated regions and long transport lengths in GeO2:SiO2 suggest that the transport of GeO2 occurs prior to the densification of the SiO2 matrix via an alternative mechanism such as capillary flow. These results inform important considerations in the design of index modifying inks for the direct ink write process, namely initial precursor phase, mutual solubility with the base SiO2 glass, and mass transport throughout the sintering process.

마이크로 가스터빈 엔진에 적용된 압축기 공력성능 특징

A centrifugal compressor is essentially applicable to a micro gas turbine because it can produce a high-pressure ratio over than 3.0 within a limited geometrical allowance. In this paper, aerodynamic design and internal flow characteristics of the centrifugal compressor were described in detail by comparing them with those of turbocharger compressors. Then a centrifugal compressor design procedure with a pressure ratio of 5.0 at the design point for a 1,000 N thrust micro gas turbine has been introduced and the internal flow field at the design point has been further investigated with CFD methods. High impeller pressure ratio led to high absolute Mach number at the impeller outlet, however, small gap between the impeller outlet and diffuser inlet did not sufficiently recover static pressure and it caused large flow separation region and corresponding total pressure at the diffuser hub. For this reason, it requires sophisticated matching between impeller and diffuser stages in micro gas turbines. Based on the internal flow analysis, ideas for further performance improvements have been proposed while maintaining essential compressor performances.

The Effects of Reynolds Number on the Stall and Pre-Stall Behavior of Compact Axial Compressors

Abstract Compact axial compression systems are of interest to the domestic appliance industry. The associated low Reynolds number leads to high losses compared to large-scale compressors due to a transitional flow field with large regions of separation. This paper investigates how Reynolds number variations affect the three-dimensional and unsteady flow field in a compact compressor both pre-stall and in stall. An experimental study has been conducted using a scaled-up single-stage axial compressor across a Reynolds number range of 104–105. Steady and unsteady casing static pressure measurements, along with rotor upstream and downstream unsteady velocity measurements, have been used to observe the rotor flow field. As the Reynolds number is reduced below a critical value, 60,000 in the case of the compressor studied, the pressure rise coefficient of the compressor rapidly decreases. The exact value of the critical Reynolds number is expected to vary with the compressor geometry. This fall-off in performance corresponds to an increase in the compressor rotor secondary flows. Prior to stall, a broadband hump at around 50% of the blade passing frequency is present in the near-field casing static pressure spectra. At Reynolds numbers below the critical value, multiple equally spaced peaks also appear around the peak of the broadband hump. The spacing of these peaks has been found to be exactly equal to the measured stall cell speed once rotating stall is established. When operating in stall, the stall cell is found to increase in circumferential size and slow down as Reynolds number decreases. The measured spectra and observed flow structures show that disturbances exist prior to stall at frequencies consistent with the frequencies within stall. The size and shape of the stall cells that form are related to the extent of the three-dimensional flow field present prior to stall. Below a critical value, all of these flow features are highly sensitive to Reynolds number.

Modes of unsteadiness in shock wave and separation region interaction in hypersonic flow over a double wedge geometry

This study investigates different modes of unsteadiness that could be seen in a hypersonic flow over a double wedge configuration with a large separation region size. In the previous studies with a double wedge or a double cone configuration, small turn angles of the surfaces are seen to produce a small separation region that does not interact with the outside shock structures and produce a steady flow. However, a large separation region can be observed with large wedge angles that could interact with the outside shock waves and produce different types of unsteady flows. Some recent numerical studies on the double wedge configuration with a large fore-wedge angle (θ1) of 30° and various aft-wedge angles (θ2) at the free-stream flow Mach number (M∞) of 7 have shown that the flow exhibits periodically oscillating states. The current study investigates this geometric configuration over the parameter space of wedge length ratios (L1/L2) of 0.5, 1.0, 1.5, and 2.0 and aft-wedge angles (θ2) of 45°, 50°, 55°, and 60°. Three modes of unsteadiness are observed, namely, vibration mode, oscillation mode, and pulsation mode. The effect of the variation in geometric parameters on switching between different modes of unsteadiness is assessed, and it is shown that the unsteady characteristics of the flow are determined by the size of the separation region and location of the shock waves with respect to it.

Time-Resolved Ship Airwake Measurements in a Simulated Atmospheric Boundary Layer

The unsteady flows produced over the stern of a Simple Frigate Shape 2 ship model are studied in a low-speed wind tunnel. Time-resolved particle image velocimetry (TR-PIV) measurements were performed in several streamwise and cross planes. Two grids of Cowdrey rods were used to simulate the basic characteristics of an atmospheric boundary layer in terms of upstream velocity profile and turbulence intensity. The TR-PIV measurements showed large regions of unsteady flow separation, dominant vortical flows, and significant wall-normal flows over the flight deck regions caused, in part, by the irregular shedding of vorticity and turbulence from the upstream funnel and superstructure of the ship. Turbulence intensities were found to be particularly high over the flight deck. The measured flows also suggested the existence of asymmetric, intermittent flow in the near-wall regions of the deck, and time-varying fluctuations in the recirculation region behind the hangar. The crossplane measurements showed the development of shear layers at the corners of the flight deck on both the port and starboard sides, and sets of counterrotating vortices at the edges of the flight deck. An energy spectrum analysis showed dominant frequencies in the regions where the shear layer was developed behind the funnel and above the flight deck. Spectral proper orthogonal decomposition revealed that the physically relevant coherent structures in the airwake were low-frequency modes near the flight deck and at large-scale of the order of the length of the deck.

Droplet coalescence and phase separation in a topical ointment: Effects of fluid shear and temperature

The physical stability of a prototypical pharmaceutical topical ointment, consisting primarily of an emulsion of propylene glycol droplets dispersed in a continuous white petrolatum medium, was studied with regard to droplet size growth and phase separation when the ointment undergoes heating or fluid shear. To investigate the effects of shear, the ointment at 32°C was sheared using a transparent, narrow-gap, temperature-controlled Taylor-Couette flow apparatus operated under laminar flow conditions which provided approximately uniform shear rates. Optical methods based on microscopy were used to obtain in-situ, time-dependent propylene glycol droplet size distributions, while a wide-field lens and camera were simultaneously used to detect gross phase separation as the ointment was sheared. Microscopy was also used to observe and quantify ointment stability via analysis of droplet size evolution in the absence of fluid shear for a range of elevated temperatures. For a quiescent ointment, it was observed that the dispersed propylene glycol droplets do not exhibit any appreciable growth over a period of one month and temperatures as high as 45°C. In contrast, fluid shear imposed at 32°C was observed to cause rapid growth of dispersed phase droplets and the onset of large phase separated regions on time scales ranging between a few minutes to approximately half an hour for fluid strain rates ranging between 5.5 and 50s-1, respectively. The experimental results from the lab-scale Couette flow apparatus were used to evaluate the risk of phase separation during commercial-scale manufacturing.

The flow around a surface mounted cube: a characterization by time-resolved PIV, 3D Shake-The-Box and LBM simulation

The flow around a surface mounted cube with incoming turbulent or laminar boundary layer has been topic of many experimental and numerical investigations in the past decades. Despite its simple geometry the flow generates a set of complex vortical structures in front and around the cube, includes flow separation at the three front plane edges with corresponding subsequent shear layer dynamics enveloping recirculation zones. Downstream of the cube a large unsteady flow separation region is present which is associated with typical quasi-periodic bluff-body wake dynamics. Therefore the flow configuration is well suited to enhance the understanding of similar unsteady and separated flow phenomena in many aerodynamic and engineering applications. In the present experimental investigation we aim at resolving a large spectrum of spatial and temporal scales in the flow around a cube with incoming laminar and turbulent boundary layers by using the most recent developments of dense 3D Lagrangian particle tracking (LPT) and high resolution TR-PIV for Reynolds numbers based on cube size in the range text {Re}_H = U_infty ,H,nu ^{-1} = 2000 - 8000. The results documented in the present paper consist of snapshots and the analysis of long time-series of highly resolved 3D and 2D velocity fields suited to enhance the understanding of coherent structure dynamics and of corresponding statistical Lagrangian and Eulerian flow properties. Premultiplied velocity spectra and 3D pressure distributions are calculated and discussed as well. Finally, the measurement data is compared to results obtained with a simulation based on the lattice Boltzmann method (LBM).Graphic abstract

Experimental study and analysis of shock train self-excited oscillation in an isolator with background waves

A study of shock train self-excited oscillation in an isolator with background waves was implemented through a wind tunnel experiment. Dynamic pressure data were captured by high-frequency pressure measurements and the flow field was recorded by the high-speed Schlieren technique. The shock train structure was mostly asymmetrical during self-excited oscillation, regardless of its oscillation mode. We found that the pressure discontinuity caused by background waves was responsible for the asymmetry. On the wall where the pressure at the leading edge of the shock train was lower, a large separation region formed and the shock train deflected toward to the other wall. The oscillation mode of the shock train was related to the change of wall pressure in the oscillation range of its leading edge. The oscillation range and oscillation intensity of the shock train leading edge were affected by the wall pressure gradient induced by background waves. When located in a negative pressure gradient region, the oscillation of the leading edge strengthened; when located in a positive pressure gradient region, the oscillation weakened. To find out the cause of self-excited oscillation, correlation and phase analyses were performed. The results indicated that the instability of the separation region induced by the leading shock was the source of perturbation that caused self-excited oscillation, regardless of the oscillation mode of the shock train.

Balance measurements on a frigate type ship model

Balance measurements performed by testing sub-scaled ship models determine the global forces and moments acting on the ship, which allows knowing the power required for the ship's movement and provides insight to be applied in the design of the control systems used to steer the ship and to avoid instabilities while sailing.The ship superstructure may produce large separated regions and high air wake turbulence levels resulting in a set of fluctuations of the flow parameters usually determined by measuring velocity or pressure. This paper presents the balance measurement of the aerodynamic forces acting on the ship hull. Aerodynamic forces and moments produced on the ship can be interpreted as an integration of the flow parameters (velocity and pressure distributions) over the ship surface wetted by the air. Balance method provides averaged values and fluctuations of forces coefficients.Aerodynamic environment in the vicinity of a ship is influenced by a large number of factors (atmospheric wind, sea state, ship superstructure, masts, stacks, antennas …) affecting helicopter operations on board ships and their safety during the take-off and landing manoeuvres.

Analysis of the ground effect on development of flow structures around an inclined solar panel

The complex three-dimensional flow that develops around an inclined flat solar panel near the ground is investigated using Computational Fluid Dynamics. The early stage evolution of the flow and the interaction of the shear layers emanating from the sides of the panel, the large separation region behind the panel and the boundary layers on the panel and ground are captured using Delayed Detached-Eddy Simulation to model the turbulence. The mean analysis shows that a small clearance produces a wall-jet like flow in the gap region between the panel and the ground, which tends to elongate the wake region in the downstream direction. On the other hand, a strong upwash is observed for a larger gap, reducing the length of the wake. Transient three-dimensional flow structures are captured using vorticity contours and the λ2-criterion. The early stage development of flow around the panel shows inverted hairpin-like vortices that are shed from the leading edge, touch down on the ground, generate a counter-rotating sheared vortex and a pair of vertical vortex tubes that extend from the ground and curl up into the wake. This pair of vortex tubes appears to be the source of the meandering structures reported in the literature. When the flow reaches a quasi-steady state, there is an asymmetric distorted flow for the smaller gap, whereas there is a nearly symmetric wake pattern for the larger gap.

Numerical simulation of inlet buzz

Comprehensive numerical analyses are conducted to simulate and capture “Buzz” phenomenon in a supersonic mixed compression air inlet. The buzz is an unsteady self-sustained feature that occurs in supersonic inlets, especially when operating in their subcritical condition. In such a situation, the shock waves oscillate along the inlet and cause mass flow fluctuations inside the inlet that will deteriorate the engine performance significantly. An axisymmetric unsteady numerical simulation was used to solve the Navier–Stokes equations combined with the URANS SST k–ω turbulence model. The simulations for two different free-stream Mach numbers of M∞=2.0 and 2.2 and at two specific subcritical conditions of the inlet operation, where the buzz was experimentally detected, were performed. The numerical solution was able to accurately capture the buzz and its oscillation which is seen to be periodic for this specific inlet. The results show that a large separation region on the compression ramp blocks the duct entry and causes conical and lambda shocks, which are formed on the compression ramp, to move upstream and creates the self-sustained oscillation. The predicted buzz frequencies are in a good agreement with the experimental frequencies calculated based on the surface pressure data with a discrepancy of less than 2%. Further, the peak and trough of both total and static pressure fluctuations and, as a result, the amplitude of buzz all are accurately predicted. The inlet performance curve during the buzz cycle indicates that both TPR and MFR values fluctuate about 70% and 60% of their maximum possible values, respectively. In addition, the influence of the Hysteresis-Effect on the inlet operation and its performance curve is beheld clearly.