Three-dimensional Separated Flow Research Articles

Hysteresis in slanted-base-cylinder afterbody flows

Afterbody flows such as those behind cargo aircraft type configurations display rich fluid dynamic behavior, including flow separation at the base of body, complex three-dimensional vortical structures and wake turbulence. A representative configuration for study is a cylinder with axis parallel to the freestream and an upswept base. At low upsweep angles, measured relative to the axis, the flow is characterized by a pair of streamwise vortices. As the angle is increased, the flow transitions to a wake closure. Experimental evidence shows that near the critical angle, (ϕ≃45°), either of the two patterns may be obtained, depending on the history of the flow i.e., the flow exhibits hysteresis. The features associated with this phenomenon are examined numerically using a validated Large-Eddy Simulation approach. A systematic campaign varying both Reynolds number and upsweep angle confirms the presence of hysteresis at ReD=25,000 and ϕ=45°. In the vortex regime, the three-dimensional separated flow from the sides rolls up into a horseshoe-type vortex with legs representing a streamwise oriented pair displaying coherent but counter-rotating vorticity field, while a diffused wake-like regime is obtained in the other. Sectional projected streamlines highlight the key kinematic differences between the two regimes. The change in base pressure due to very different separation patterns is analyzed to quantitatively examine the large drag disparity observed in experiments. Finally, instantaneous flowfields are used to obtain the shedding characteristics as well as the nature of wake turbulence in both regimes. The present study has relevance to flows in aerodynamic vehicles as well as in automobiles, and may aid in possible control strategies for drag reduction.

On the formation of three-dimensional separated flows over wings under tip effects

We perform DNS of flow over finite-aspect-ratio NACA 0015 wings to characterize the tip effects on the wake dynamics. This study focuses on the development of three-dimensional separated flow over the wing, and discuss flow structures formed on the wing surface as well as in the far field wake. Vorticity is introduced into the flow in a predominantly two-dimensional manner. The vortex sheet from the wing tip rolls up around the free end to form the tip vortex. At its inception, the tip vortex is weak and its effect is spatially confined. As the flow around the tip separates, the tip effects extend farther in the spanwise direction, generating noticeable three dimensionality in the wake. For low-aspect-ratio wings, the wake remains stable due to the strong downwash over the entire span. Increasing the aspect ratio allows unsteady vortical flow to emerge away from the tip at sufficiently high angles of attack. These unsteady vortices shed and form closed vortical loops. For higher-aspect-ratio wings, the tip effects retard the near-tip shedding process, which desynchronizes from the two-dimensional shedding over the mid-span region, yielding vortex dislocation. At high angles of attack, the tip vortex exhibits noticeable undulations due to the strong interaction with the unsteady shedding vortices. The spanwise distribution of force coefficients is related to the three-dimensional wake dynamics and the tip effects. Vortical elements in the wake that are responsible for the generation of lift and drag forces are identified through the force element analysis. We note that at high angles of attack, a stationary vortical structure forms at the leading edge near the tip, giving rise to locally high lift and drag forces. The analysis performed here reveals how the vortical flow around the tip influences the separation physics, the global wake dynamics, and the spanwise force distributions.

Investigation of a Dielectric Barrier Discharge Plasma Actuator to Control Turbulent Boundary Layer Separation

A numerical investigation was carried out to explore the effects of a dielectric barrier discharge (DBD) plasma actuator on a three-dimensional incompressible, separated flow. The test article selected for the simulations was the National Aeronautical and Space Administration (NASA) wall-mounted hump model. The simulations were run at a Reynolds number of 936,000, based on hump chord length, and a freestream Mach number of 0.1. Hybrid partially averaged Navier–Stokes/large-eddy simulations (PANS/LES) were completed using CALC-LES, a well-validated computational fluid dynamics (CFD) code, developed by Chalmers University of Technology. The baseline code was modified to simulate the effects of the actuator, which were modeled as source terms in the momentum equation and were assumed to be steady and constant in the span-wise direction. The numerical simulations were carried out for a baseline (no control) case and five plasma control cases. To optimize the performance of the actuator, the variation of actuator location and voltage frequency was investigated. For the baseline case, a comparison of time-averaged skin friction, the coefficient of pressure, and velocity profiles was made of the available experimental results. The results of the baseline case showed good agreement for a hybrid turbulence model, thus strengthening the solver’s ability to predict a three-dimensional separated flow with reasonable accuracy. The results with the plasma actuator turned on showed improved flow characteristics compared to the baseline simulations by reducing the region of separated flow. The actuator placed just downstream of the separation point at an operational frequency of 5kHz completely eliminated the separated flow for our test conditions.

The Effect of Aspect Ratio on Wall Pressure Fluctuations at a Wing-Plate Junction

AbstractMeasurements of the wall pressure fluctuations near a wing-plate junction were made for wings with three different aspect ratios (AR) of 0.2, 0.5, and 1.0 at several angles of attack. The chord-based Reynolds number for each wing was 274,000. The results show that the wall pressure fluctuations are a function of wing AR for cases where AR≤ 1.0. For each wing, the pressure fluctuations are highest upstream of the wing leading-edge due to three-dimensional flow separation; wings with AR = 1.0 and 0.5 show comparable levels, while those with AR = 0.2 show lower fluctuation levels over a wide frequency range. Downstream of the leading-edge, the pressure fluctuations decay rapidly on both sides of the wing until the maximum thickness location after which little variation is observed. The pressure fluctuations downstream of the leading-edge on the suction-side were observed to be comparable for AR = 0.2 and 0.5, while those for AR = 1.0 were higher in magnitude. On the pressure-side, the pressure fluctuations near the leading-edge are a weak function of AR; however, those further downstream remain independent of AR. The pressure fluctuations aft of the wing on the suction-side are more coherent for lower ARs and show higher convection velocity, possibly due to an interaction between the tip and the junction flows for lower ARs.

On the aerodynamic flow around a cyclist model at the hoods position

Aerodynamic flow around an 1/5 scale cyclist model was studied experimentally and numerically. First, measurements of drag force were performed for the model in a low-speed wind tunnel at Reynolds numbers from 5.5 times 10^{4} to 1.8 times 10^{5}. Meanwhile, numerical computation using a large eddy simulation method was performed at three Reynolds numbers of 1.1 times 10^{4}, 6.5 times 10^{4} and 1.5 times 10^{5} to obtain the drag coefficients for comparison. Second, flow visualization was made in a water channel and the wind tunnel mentioned to examine the three-dimensional flow separation pattern on the model surface, which could also be realized from the numerical results. Finally, a wake flow survey based on the hot-wire measurements in the wind tunnel showed that in the near-wake region, the flow was featured with the formation of multiple streamwise vortices. The numerical results further indicated that these vortices were evolved from the separated flows occurred on the model surface.Graphic

Formation and turbulent breakdown of large-scale vortical structures behind an obstacle in a channel at moderate Reynolds numbers

This paper deals with experimental investigation and direct numerical simulation of three-dimensional separated laminar and transitional flows behind a semicircular spanwise rib on a bottom wall of a rectangular channel at Reynolds numbers of up to 480. Particular emphasis is given to the formation mechanism of quasiperiodic large-scale vortex clouds in the mixing layer behind the rib. Vortical structures near the channel axis are formed due to pairing of spiral vortices emerging close to the vertical walls when the corner boundary layers impact on the rib. The effect of the Reynolds number and normalized channel size on the spiraling motion, generation, and shedding of large-scale vortex clouds has been estimated.

A hybrid RANS model of wing-body junction flow

Abstract The three-dimensional flow separation over the Rood wing-body junction is an exemplar application of separation affecting many important flows in turbomachinery and aerodynamics. Conventional Reynolds Averaged Navier Stokes (RANS) methods struggle to reproduce the complexity of this flow. In this paper, an unconventional use is made of a hybrid Reynolds Averaged Navier Stokes (RANS) model to tackle this challenge. The hybridization technique combines the Menter k − ω − S S T model with the one equation sub-grid-scale (SGS) model by Yoshizawa through a blending function, based on the wall-normal distance. The hybrid RANS turbulence closure captured most of the flow features reported in past experiments with reasonable accuracy. The model captured also small secondary vortices at the corner ahead of the wing nose and at the wing trailing edge. This feature is scarcely documented in the literature. The study highlights the importance of the spatial resolution near the wing leading edge, where this localized secondary recirculation was observed by the hybrid RANS model. It also provides evidence on the applicability of the hybrid Menter and Yoshizawa turbulence closure to the wing-body junction flows in aircraft and turbomachines, where these flows are characterized by a substantially time-invariant three-dimensional separation.

Direct numerical simulation of conical shock wave–turbulent boundary layer interaction

Direct numerical simulation of the Navier–Stokes equations is carried out to investigate the interaction of a conical shock wave with a turbulent boundary layer developing over a flat plate at free-stream Mach number $M_{\infty }=2.05$ and Reynolds number $Re_{\unicode[STIX]{x1D703}}\approx 630$, based on the upstream boundary layer momentum thickness. The shock is generated by a circular cone with half opening angle $\unicode[STIX]{x1D703}_{c}=25^{\circ }$. As found in experiments, the wall pressure exhibits a distinctive N-wave signature, with a sharp peak right past the precursor shock generated at the cone apex, followed by an extended zone with favourable pressure gradient, and terminated by the trailing shock associated with recompression in the wake of the cone. The boundary layer behaviour is strongly affected by the imposed pressure gradient. Streaks are suppressed in adverse pressure gradient (APG) zones, but re-form rapidly in downstream favourable pressure gradient (FPG) zones. Three-dimensional mean flow separation is only observed in the first APG region associated with the formation of a horseshoe vortex, whereas the second APG region features an incipient detachment state, with scattered spots of instantaneous reversed flow. As found in canonical geometrically two-dimensional wedge-generated shock–boundary layer interactions, different amplification of the turbulent stress components is observed through the interacting shock system, with approach to an isotropic state in APG regions, and to a two-component anisotropic state in FPG. The general adequacy of the Boussinesq hypothesis is found to predict the spatial organization of the turbulent shear stresses, although different eddy viscosities should be used for each component, as in tensor eddy-viscosity models, or in full Reynolds stress closures.

Impact of turbulence intensity on the unsteady flow characteristics of the integrated aggressive inter-turbine duct

Aggressive inter-turbine duct (AITD) which has ultra-short axial length and ultra-large area ratio is widely applied in the modern turbofan engine to meet the demand of higher efficiency and better low pressure rotor dynamic characteristics. However, the adverse stream-wise pressure gradient in this duct is enhanced, and there are complicate upstream and downstream flow conditions. The flow field of the AITD and its downstream low pressure turbine is seriously deteriorated and prone to separate, especially in high-altitude cruise state. The integrated AITD which integrates the AITD with strut had been proved to can restrain the traditional AITD's three-dimensional flow separation. Besides, periodic wake can suppress the downstream blade suction surface's flow separation by “calmed region” effect according to former studies. The AITD of this research taken from a real engine utilizes integrated design and inherent upstream wake to suppress the flow separation of the AITD's end wall and low pressure turbine nozzle's suction surface correspondingly. In this article, the computational simulation method is used to quantify the impact of turbulence intensity ( T u) on the integrated AITD's unsteady flow characteristics, especially the separation and transition mechanism of low pressure turbine nozzle's suction surface boundary layer. Research shows that the impact of T u on the integrated AITD's flow property depends on the balance of T u's dual effect.

Cluster-based feedback control of turbulent post-stall separated flows

We propose a cluster-based control strategy for feedback control of post-stall separated flows over an airfoil. The present approach partitions the flow trajectories (force measurements) into clusters, which correspond to characteristic coarse-grained phases in a low-dimensional feature space. A feedback control law (using blowing/suction actuation) is then sought for each cluster state through iterative evaluation and downhill simplex search to minimize power consumption in aerodynamic flight. The optimized control laws re-route the flow trajectories to the aerodynamically favourable regions in the feature space in a model-free manner. Utilizing a limited number of sensor measurements for both clustering and optimization, these feedback laws were determined in only $O(10)$ iterations. The objective of the present work is not necessarily to suppress flow separation but to minimize the desired cost function to achieve enhanced aerodynamic performance. The present approach is applied to the control of two- and three-dimensional separated flows over a NACA 0012 airfoil in large-eddy simulations at an angle of attack of $9^{\circ }$, Reynolds number $Re=23\,000$ and free-stream Mach number $M_{\infty }=0.3$. The optimized control laws avoid the intermittent occurrence of long-period shedding associated with high-drag clusters, thus lowering the mean drag. The present work aims to address some of the challenges associated with feedback control design for turbulent separated flows at moderate Reynolds number.

Numerical Investigations on Oscillating Aspiration Control in High-Load Compressor Cascades

This current study presents a detailed analysis regarding three-dimensional separation flows controlled by steady and oscillating boundary-layer aspirations installed on suction surfaces of highly loaded planar compressor cascades. The influences of the oscillation parameters on the control effects are parametrically investigated with the objective of providing guidelines to determine the control parameters. The susceptibility to varying the aspiration locations as well as the control expenses of the steady boundary-layer suction methods are significantly improved due to the introduction of periodic oscillations into the aspirating flows. The effective excitation frequency spans a relatively wide bandwidth (from to ) only if the amplitude exceeds a threshold value (). Through a detailed comparative analysis on the steady and oscillating aspiration cases, the most prominent spatial change led by the oscillating aspiration is manifested by fully discretized separation vortices. These newly formed spanwise vortex tubes enhance the momentum exchange between the main flow and the recirculation zone, and hence alleviate the corner separation. Despite the increased local loss production caused by the increased vortical strength, the overall cascade performances are improved from a temporal average perspective. With the aid of the proper orthogonal decomposition method, the formation process of the discretized separation vortices is explored. It is found that the periodic disturbances originating from the suction slot are amplified, subject to the adverse pressure gradient and rollup, to form a series of vortical structures in the separated layers.

Assessment of Computational Fluid Dynamics Capabilities for the Prediction of Three-Dimensional Separated Flows: The DELFT 372 Catamaran in Static Drift Conditions

In this paper, capabilities of state-of-the-art computational fluid dynamics (CFD) tools in the prediction of the flow-field around a multihull catamaran advancing in straight ahead motion at nonzero drift angles are investigated. CFD estimations have been provided by three research institutes by using their in-house codes: CNR-INM using Xnavis, IIHR using CFDShip-Iowa, and CNRS/ECN using ISIS. These allowed an in-depth comparison between different methodologies, such as structured overlapping grids versus unstructured grid, different turbulence models and detached eddy simulations (DES) approaches, and level-set (LS) versus volume of fluid (VoF). The activities were pursued within the NATO AVT-183 group “reliable prediction of separated flow onset and progression for air and sea vehicles,” aimed at the assessment of CFD predictions of large three-dimensional separated flows. Comparison between estimations is provided for both integral and local quantities, and for wave-induced vortices. Validation is reported by comparison against the available experimental fluid dynamics (EFD) data. Generally, all the simulations are able to capture the main features of the flow field; grid resolution effects are dominant in the onset phase of coherent structures and turbulence model affects the dynamic of the vortices. Hydrodynamic loads are in agreement between the submissions with standard deviation of about 3.5% for the resistance prediction and about 7% for lateral force and yaw moment estimation. Wave-induced vortices are correctly captured by both LS and VoF approaches, even if some differences have been highlighted, LS showing well-defined and long life vortices.

Three-dimensional flow separations on a rolling sphere

Experiments are conducted to study the flow separations on a sphere rolling on an inclined plane submerged in water. These experiments are performed at Reynolds numbers Re between 1350 and 1550. The experiments show that the flow separations on the surface of a rolling sphere can be organized into four distinct regions: (i) region of primary separation-I on the front upper sphere and extending below the poles, (ii) region of viscous blockage at the crevice surrounding the point of contact and shear layer separation ahead and at the sides of the viscous blockage, (iii) region of primary separation-II on the rear lower sphere and (iv) secondary separations on the rear upper sphere surface. The ratio of the width of the viscous blockage to the diameter of the sphere is found to be 0.4. Primary separation-I surface is symmetrical about the equatorial plane of the rolling sphere. Primary separation-II from the rear lower sphere surface is asymmetrical about the equator and eddies are shed alternately on either side of the equator from this separation surface. These lower eddies are energetic and dominate the dynamics of the wake. The upper eddy shedding from the primary separation-I surface and the lower eddy shedding from the primary separation-II surface are synchronized.

Effects of Ag-water nanofluid on hydrodynamics and thermal behaviors of three-dimensional separated step flow

In this research, three-dimensional simulation of nanofluid flow over an inclined step is investigated numerically. The considered nanofluid in this study is a mixture composed by water as base fluid and nanoparticles of silver (Ag), such that the range of volume fraction of these nanoparticles has been changed from 0 to 0.2. The effects of these volume fractions on the hydrodynamics and thermal behaviors are analyzed with all details. Results show that the volume fraction of nanoparticles has a greater influence on the temperature distributions than velocity distributions. Also, the increase of nanoparticles volume fraction leads to a significant enhancement in the friction coefficient, mean bulk temperature and Nusselt number.

Effect of Thickness-to-Chord Ratio on Flow Structure of a Low Swept Delta Wing

The effect of thickness-to-chord () ratio on flow structure of a delta wing with sweep angle of 35 deg is characterized in a low-speed wind tunnel using laser-illuminated smoke visualization, particle image velocimetry, and surface pressure measurements. Four different ratios varying from 4.75 to 19% are tested at angles of attack, 4, 6, 8, and 10 deg, for Reynolds numbers and . The results indicate that the effect of thickness-to-chord ratio on flow structure is quite substantial, such that, as the wing thickness increases, the flow structure transforms from leading edge vortex to three-dimensional separated flow regime. The wing with lowest ratio of 4.75% has pronounced surface separation at significantly higher angle of attack compared with the wing with highest ratio, which indicates that lowest ratio wing might be more resistive to the stall condition. However, considering the low angles of attack where all wings experience leading edge vortex structure, the strength of the vortex structure increases as the ratio increases, which might suggest a better vortex-induced lift performance with high ratio wing at low angles of attack.

Skin-Friction and Surface-Pressure Structures in Near-Wall Flows

Skin-friction and surface-pressure in a viscous flow constitute a pair of the intrinsically coupled physical quantities denoted by the notation (, ), which determines the near-wall flow structures. The exact (, ) relation derived from the Navier–Stokes (NS) equations provides a rational foundation for reconstruction of the (, ) structures in complex flows. Based on this coupling relation, a field can be obtained from the perturbed field for the given boundary enstrophy flux field of a base flow as an inverse problem in the first-order approximation. Then, the corresponding near-wall velocity field is reconstructed by using the Taylor-series-expansion solution of the NS equations. Some elemental and complex (, ) structures in three-dimensional flow separations are reconstructed to demonstrate the applicability of the proposed method.

Numerical investigation of flow around one finite circular cylinder with two free ends

Flow around one finite circular cylinder with two free ends is of both fundamental and practical importance, without a detailed discussion yet. In this study a numerical investigation is performed to demonstrate its flow pattern and turbulent structure varying with the aspect ratio and Reynolds number. The results show that the drag coefficient decreases dramatically in comparison with the two-dimensional model and the horseshoe vortex around the mounted cylinder with one free end disappears. Instead, a pair of stationary recirculating eddies behind the cylinder is found in the axis direction at low Reynolds number. With the increasing of Reynolds number, four spiral vortices are formed by the interaction of the two three-dimensional separated flows from two free ends and the wake formed by the cylindrical surface. The separated flow recirculates to the backside of the cylinder and increases its local pressure. Additionally, an intersection zone and a backflow is found close to the end surface, caused by the up-wash flow from the sharp leading edge of the cylinder. Finally, a new relationship between the drag coefficient and the Reynolds number for one finite circular cylinder with two free ends is proposed.

Experimental investigation on the performance of compressor cascade using blended-blade-end-wall contouring technology

Three-dimensional flow separations commonly occur in the corner region formed by the blade suction surface and end wall in compressors. How to control or reduce these separations is a vital problem for aerodynamic designers all the time. Blended blade and end wall contouring technology has been proposed to control flow separation for several years and validated in many cases using the numerical method, but experimental data was not obtained so far. So in this paper, the baseline cascade scaling from the NACA65 airfoil with 42° turning angle is designed, tested, and analyzed firstly. Then, based on the experimental results of the baseline cascade, blended blade and end wall contouring is applied to the suction surface and hub corner region of the baseline cascade and the detailed experiment is carried out. The results show that the blended blade and end wall contouring technology can decrease the total pressure loss by 8% and 7% at 0° and +10° incidence angles separately. The improved span range mainly focuses on the 10–25% span height. The rolling change of the passage vortex influenced by the accumulation of low energy fluid driven by cross flow in the hub corner should be the main reason for the performance improvement.

Analysis of vortical flow field related to aero-acoustic sound in an air conditioning system by wall pressure measurement and CFD (2nd Report, blade passing frequency in a blower)

This paper presents an elucidation of three-dimensional flow phenomena that induce an aero-acoustic sound of Blade passing frequency (BPF). Three-dimensional separated and vortical flow fields in an air conditioning system have been investigated by Experimental fluid dynamics (EFD) analysis using an unsteady wall pressure measurement system and by Computational fluid dynamics (CFD) analysis using a Reynolds-averaged Navier-Stokes (RANS) simulation as described in the first report of the present study. Generally, the aero-acoustic sound of BPF is one of the main aero-acoustic sounds in air conditioning systems. Based on the EFD analysis, the wall pressure fluctuation of BPF is found to be not only located near the scroll tongue region but also widely located around the scroll casing wall. It was found that there is no correlation between the sensor positions in the circumferential direction and the pressure fluctuation level of BPF. Based on the CFD analysis results, it was found that an attachment line on the center of the scroll casing wall develops by interacting with secondary flow from the multi-blade fan. A pair of secondary vortex structures in the scroll casing longitudinally develops in the circumferential direction. As the pair of secondary flow structures with high vorticity develops in the circumferential direction and interacts with the scroll casing, wall pressure fluctuation of BPF is generated even though it is located far away from the scroll tongue.

Analysis of vortical flow field related to aero-acoustic sound in an air conditioning system by wall pressure measurement and CFD (1st report, resonance in a duct)

This paper elucidates three-dimensional flow phenomena that induce aero-acoustic noise at certain frequencies. Three-dimensional separated and vortical flow fields in an air conditioning system are investigated by Experimental fluid dynamics (EFD) analysis using an unsteady wall pressure measurement system with 30 high-respond pressure transducers and by Computational fluid dynamics (CFD) analysis using a Reynolds-averaged Navier-Stokes (RANS) simulation with a k-e turbulence model. The EFD investigation revealed that the regions with high wall pressure fluctuation are located near the scroll tongue and on the bottom of the diffuser casing wall. A wall pressure fluctuation of 0.056 Blade passing frequency (BPF) was observed at sensor A near the scroll tongue, and a wall pressure fluctuation of 0.173 BPF was observed on part of the evaporator casing wall. The CFD investigation revealed that the reverse flow is generated by interaction between the scroll tongue and multi-blade fan. The reverse flow develops strongly over a wide range in the multi-blade fan near the scroll tongue. Furthermore, there is also a longitudinally separated vortex near the bottom of the diffuser casing. The reverse flow and longitudinally separated vortex interact with the wall casing. For this reason, the scroll tongue and the bottom of the diffuser casing had high pressure fluctuations in the EFD investigation. By considering aero-acoustic factors, an aerodynamic sound of 0.056 BPF can be explained by Helmholtz resonance phenomenon, which is generated by high pressure fluctuation due to the reverse flow near the scroll tongue. An aerodynamic sound of 0.173 BPF can be explained by the standing wave phenomenon, which is generated by high pressure fluctuation due to the longitudinally separated vortex on the bottom of the diffuser casing wall.