Three-dimensional Separation Bubble Research Articles

Control of Reverse Flow over Cantilevered Swept Blades Using Passive Camber Morphing

High-speed rotorcraft experience a region of reversed flow over the retreating blade at high advance ratios. This reverse flow region results in multiple unfavorable effects, including negative lift, increased drag, and a large pitching moment impulse. The effect of the sweep angle on a rotor blade in reverse flow was explored, and it was demonstrated that the introduction of a trailing edge reflex camber could mitigate the adverse effects seen in this regime. A cantilevered, finite-span NACA 63-218 blade was examined at a Reynolds number Rec=2.38×105 both with and without a 10° trailing edge reflex camber. Three blades were tested: one with 20° forward sweep, one with 20° backward sweep, and one with no sweep. The experiments consisted of load measurements across a range of angles of attack, as well as volumetric flowfield measurements using stereoscopic particle image velocimetry on the two swept models at 10° angle of attack in reverse flow. The flowfields exhibited a highly three-dimensional separation bubble that existed on the blades in both backward and forward sweep conditions. Cambering of the trailing edge led to almost complete flow attachment on the backward swept model and a significant reduction in separation over the forward swept model. Cambering also led to a large reduction in drag in the entire reverse flow regime in both swept and unswept conditions. A large reduction in pitching moment and a smaller reduction in negative lift were also observed at moderate angles of attack.

Separation characteristics of shock wave/turbulent boundary layer interaction under the effect of a transverse jet

Shock wave/turbulent boundary layer interactions under the effect of a transverse jet at different distances are investigated via large eddy simulation. The instantaneous characteristics of the turbulent boundary layer affected by the jet wake, including near-wall streaks and large-scale vortices, are examined. Separation features are compared and analyzed through three-dimensional separation bubbles and skin frictions. Results show that a far jet can diminish separation and is mainly manifested as the reattachment point moving upstream. When the distance is long enough for the evolution of large-scale vortices, the effects on the spanwise direction are evenly distributed. A near jet has stronger suppression on separation, both the separation point and reattachment point have a significant movement. Due to the insufficient development of the jet wake, the separation bubble shows different characteristics along the spanwise direction. The streamwise Reynolds stress is calculated and investigated carefully. The jet enhances the turbulence intensity at the central line in both the inner and outer layers, while only affecting the intensity at the sideline in the out layer. A conclusion on the movements of the separation point and reattachment point is proposed. The location of the reattachment point is determined by the incident shock and near-wall characteristics of the boundary layer. The location of the separation point is dominated by the turbulent intensity in the outer layer while incident shock is fixed.

Wall-Modeled Large-Eddy Simulation of Turbulent Boundary Layers with Mean-Flow Three-Dimensionality

We examine the performance of wall-modeled large-eddy simulation (WMLES) to predict turbulent boundary layers (TBLs) with mean-flow three-dimensionality. The analysis is performed for an ordinary-differential-equation-based equilibrium wall model due to its widespread use and ease of implementation. Two test cases are considered for this purpose: a spatially developing TBL in a square duct with a 30 deg bend, and the flow behind a wall-mounted skewed bump with a three-dimensional separation bubble. In the duct simulation, WMLES is capable of predicting mean-velocity profiles and crossflow angles in the outer region of the flow to within 1–5% error using 10 points per boundary-layer thickness. The largest disagreement (20% error) is observed in the crossflow angles in the bend region, where three-dimensional effects are the most significant. In the skewed bump simulation, it is shown that the present equilibrium wall model with a grid resolution of about 40 points across the three-dimensional separation region predicts mean-velocity profiles and separation location to within 1–3% error. The bubble size and vortex structures in the bump wake are also correctly represented. It is demonstrated that WMLES is capable of achieving accurate results in the separated region and its vicinity, provided that the strong shear layer generated at the apex of the bump is well resolved.

Numerical research on the mixing mechanism of lobed mixer with inlet swirl in linear radial distribution

A detailed numerical simulation is presented to investigate the effects of inlet swirl and its radial distribution on the mixing mechanisms of a turbofan mixer with 12 lobes, by using the commercial ANSYS CFX solver and k–ω SST model. The core-to-bypass temperature ratio and pressure ratio were set to 2.59 and 0.97, respectively, giving the Mach number of 0.66 and bypass ratio of 2.65 at mixing nozzle outlet. In the core inlet, the swirl angle was raised from 0° to 30° in a uniform or linear radial distribution manner. The inlet swirl and its radial gradient did enhance the development, interaction, and dissipation of the vortices downstream of lobed mixer, resulting in accelerating the lobed jet mixing. When the inlet swirl was less than 20°, the total pressure and thrust loss increments of lobed jet were acceptable and no more than 0.26% and 1.57%, respectively, compared with the baseline case. The results also showed that the three-dimensional separation bubble on center-body and the backflows along jet axis at the rail of center-body, resulting from the swirling flow between lobes’ trough and center-body, were the dominant sources of total pressure and thrust losses for all cases with inlet swirl. And, reasonable radial distribution of inlet swirl could inhibit the aforementioned 3D separation and backflow, and thus limited the increment of jet mixing loss favorably.

Control of Laminar Separation Bubble on NACA 0009 Airfoil Using Electroactive Polymers

Control of a laminar separation bubble on a two-dimensional NACA 0009 was investigated experimentally in an open-return wind tunnel using particle image velocimetry measurements at a range of chord-based Reynolds numbers between and . In this study, flow control was accomplished through a row of surface-mounted electroactive polymers, centered at 20% of the airfoil chord. A three-dimensional separation bubble was seen to exist when the airfoil’s angle of attack was set at 5 deg, in agreement with the literature. Furthermore, the streamwise and cross-stream extents of the bubble decrease with the increase of the Reynolds number. To mitigate the separation bubble, the electroactive polymers were actuated at a frequency corresponding to the Kelvin–Helmholtz instability of the separated mixing layer. This frequency was found by applying the solution to the Michalke spatial stability analysis to the experimental data. Activation of the electroactive polymers resulted in mitigation of the separation bubble, as evidenced from suppression of reverse flow within the near-surface velocity profiles.

Flow around a hemisphere-cylinder at high angle of attack and low Reynolds number. Part I: Experimental and numerical investigation

Three-dimensional Direct Numerical Simulations combined with Particle Image Velocimetry experiments have been performed on a hemisphere-cylinder at Reynolds number 1000 and angle of attack 20°. At these flow conditions, a pair of vortices, so-called “horn” vortices, are found to be associated with flow separation. In order to understand the highly complex phenomena associated with this fully three-dimensional massively separated flow, different structural analysis techniques have been employed: Proper Orthogonal and Dynamic Mode Decompositions, POD and DMD, respectively, as well as critical-point theory. A single dominant frequency associated with the von Karman vortex shedding has been identified in both the experimental and the numerical results. POD and DMD modes associated with this frequency were recovered in the analysis. Flow separation was also found to be intrinsically linked to the observed modes. On the other hand, critical-point theory has been applied in order to highlight possible links of the topology patterns over the surface of the body with the computed modes. Critical points and separation lines on the body surface show in detail the presence of different flow patterns in the base flow: a three-dimensional separation bubble and two pairs of unsteady vortices systems, the horn vortices, mentioned before, and the so-called “leeward” vortices. The horn vortices emerge perpendicularly from the body surface at the separation region. On the other hand, the leeward vortices are originated downstream of the separation bubble, as a result of the boundary layer separation. The frequencies associated with these vortical structures have been quantified.

Effects of Scalloping on the Mixing Mechanisms of Forced Mixers With Highly Swirling Core Flow

This paper presents a detailed experimental and computational investigation of the effects of scalloping on the mixing mechanisms of a scaled 12-lobe turbofan mixer. Scalloping was achieved by eliminating approximately 70% of the lobe sidewall area. Measurements were made downstream of the mixer in a co-annular wind tunnel, and the simulations were carried out using an unstructured Reynolds averaged Navier–Stokes (RANS) solver, Numeca FINE/Hexa, with k-ω SST model. In the core flow, the swirl angle was varied from 0 deg to 30 deg. At high swirl angles, a three-dimensional separation bubble was formed on the lobe's suction surface penetration region and resulted in the generation of a vortex at the lobe valley. The valley vortex quickly dissipated downstream. The mixer lobes removed most of the swirl, but scalloped lobes removed less swirl in the region of the scalloped notch. The residual swirl downstream of the scalloped mixer interacted with the vortices and improved mixing rates compared to the unscalloped mixer. Core flow swirl up to 10 deg provided improved mixing rates and reduced pressure and thrust losses for both mixers. As core flow swirl increased beyond 10 deg, the mixing rate continued to improve, but pressure and thrust losses declined compared to the zero swirl case. Lobe scalloping, in high swirl conditions, resulted in better mixing and improved pressure loss over the unscalloped mixer but at the expense of reduced thrust.

Sensitivity of an asymmetric 3D diffuser to vortex-generator induced inlet condition perturbations

Modifications of the turbulent separated flow in an asymmetric three-dimensional diffuser due to inlet condition perturbations were investigated using conventional static pressure measurements and velocity data acquired using magnetic resonance velocimetry (MRV). Previous experiments and simulations revealed a strong sensitivity of the diffuser performance to weak secondary flows in the inlet. The present, more detailed experiments were conducted to obtain a better understanding of this sensitivity. Pressure data were acquired in an airflow apparatus at an inlet Reynolds number of 10,000. The diffuser pressure recovery was strongly affected by a pair of longitudinal vortices injected along one wall of the inlet channel using either dielectric barrier discharge plasma actuators or conventional half-delta wing vortex generators. MRV measurements were obtained in a water flow apparatus at matched Reynolds number for two different cases with passive vortex generators. The first case had a pair of counter-rotating longitudinal vortices embedded in the boundary layer near the center of the expanding wall of the diffuser such that the flow on the outsides of the vortices was directed toward the wall. The MRV data showed that the three-dimensional separation bubble initially grew much slower causing a rapid early reduction in the core flow velocity and a consequent reduction of total pressure losses due to turbulent mixing. This produced a 13% increase in the overall pressure recovery. For the second case, the vortices rotated in the opposite sense, and the image vortices pushed them into the corners. This led to a very rapid initial growth of the separation bubble and formation of strong swirl at the diffuser exit. These changes resulted in a 17% reduction in the overall pressure recovery for this case. The results emphasize the extreme sensitivity of 3D separated flows to weak perturbations.

Low Reynolds number aerodynamics of free-to-roll low aspect ratio wings

The aerodynamics of thin, flat-plate wings of various planforms (rectangular, elliptical and Zimmerman) have been studied in free-to-roll experiments in a wind tunnel. Non-zero trim angles at low angles of attack, self-induced roll oscillations with increasing angle of attack and even autorotation in some cases were observed. The rectangular wings with round leading-edge had non-zero trim angles at low incidences due to the asymmetric development of the three-dimensional separation bubble at these low Reynolds numbers. With increasing angle of attack, the bubble increases in length and once reattachment is lost, large amplitude roll oscillations develop. The Strouhal number of the roll oscillations is of the order of 10−2, which is in the same range as those expected for small aircraft experiencing atmospheric gusts. Velocity measurements revealed that variations in the strength of the vortices drove the rolling motion. At the mean roll angle, because of the time lag in the strength of the vortices, an asymmetric flow is generated, which results in a net rolling moment in the direction of the rolling motion.

The near-wall layer beneath a moderately converging three-dimensional turbulent separated and reattaching flow

Pulsed-wire mean velocity and surface shear stress measurements have been made in a three-dimensional separation bubble in which there is a mild lateral convergence, bounded by side regions of spanwise invariance. Even though the convergence is mild the bubble parameters change considerably with lateral position. Velocity measurements near the surface were made with a special through-wall pulsed-wire probe. The cross-flow layer is substantially thicker than the reverse-flow layer even in the invariant region. Cross-flow and reverse-flow velocity profiles are each remarkably close in shape, though probably not exactly self similar. Surface shear stresses in the cross- and reverse-flow directions conform to local scalings and Reynolds-number dependences based on thickness and `external' velocity. These scalings also apply (quantitatively) downstream of and, it appears, through attachment. The surface shear stress in the cross-flow direction is higher than the streamwise stress, consistent with a distinctly fuller mean velocity profile. There is a striking comparability with three-dimensional boundary layers once the flow directions are transposed, the cross flow taking the part of the primary flow, and the reverse flow the secondary flow.

LDA Investigation of the Flow Development Through Rotating U-Ducts

This paper reports results from the use of laser-Doppler anemometry (LDA) to measure the mean and fluctuating flow field in a U-bend of strong curvature, Rc/D = 0.65, that is either stationary or rotating in orthogonal mode (the axis of rotation being parallel to the axis of curvature). The data acquisition system enables a stationary optical fiber probe to collect flow data from a rotating U-bend sweeping past it. Three cases have been examined, all concerning a flow Reynolds number of 100,000; a stationary case, a case of positive rotation (the pressure side of the duct coincides with the outer side of the U-bend) at a rotational number (ΩD/Um) of 0.2, and a case of negative rotation at a rotational number of −0.2. Measurements have been obtained along the symmetry plane of the duct and also along a plane near the top wall. The most important influence on the development of the mean and turbulence flow fields is exerted by the streamwise pressure gradients that occur over the entry and exit regions of the U-bend. In the stationary case a three-dimensional separation bubble is formed along the inner wall at the 90 deg location and it extends to about two diameters downstream of the bend, causing the generation of high-turbulence levels. Along the outer side, opposite the separation bubble, turbulence levels are suppressed due to streamwise flow acceleration. For the rotation numbers examined, the Coriolis force also has a significant effect on the flow development. Positive rotation doubles the length of the separation bubble and generally suppresses turbulence levels. Negative rotation causes an extra separation bubble at the bend entry, raises turbulence levels within and downstream of the bend, increases velocity fluctuations in the cross-duct direction within the bend, and generates strong secondary motion after the bend exit. It is hoped that the detailed information produced in this study will assist in the development of turbulence models suitable for the numerical computation of flow and heat transfer inside blade-cooling passages.

Fluid flow behavior around a circular cylinder perpendicular arrangement of circular cylinders.

Fluid flow behavior around a circular cylinder placed downstream of and perpendicular to a single row of cylinders in a uniform flow was investigated varying the distance (L) between upstream and downstream cylinders. Flow visualization techniques (oil-film and smoke-wire methods) and a near-surface flow direction judgement (flow direction probe method) were used, and time-averaged and fluctuating pressure distributions on a circular cylinder were measured under the constant Reynolds number, Re=5×104. It was clarified that the flow structure could be divided as a border of L/D=2.5. In the connection with the vortex transfer, especially, the three-dimensional separation bubble formed at L/D≤2.5 was the result of the stretching of separating shear layers due to the horseshoe-like vortex and inner secondary vortex. At this distance, the span-averaged drag of the downstream cylinder was less than 1.15 times that of a single cylinder.