Delaying Vortex Breakdown Research Articles

Mitigation of gas entrainment in swirling free surface flows

Gas entrainment presents a safety challenge within sodium-cooled fast reactors. This problem is similar to two-phase free surface interactions in a Taylor–Couette flow characterized by a large annulus gap (small radius ratio, η≈0.333) and highly turbulent swirling flows (Re≈105). The aim of the current work is to test strategies that could mitigate this phenomenon. A three-dimensional numerical model of the primary sodium pump annulus region involved in gas entrainment due to shaft rotation was developed, and turbulent swirling flow that occurs in the annular gap between the vessel and the shaft is investigated. The influence of baffle plates mounted on the stationary vessel with and without porosity was examined during the study. The simulation results revealed that submerged porous plates placed on the outer cylindrical wall both delayed vortex breakdown and suppressed shear waves near the surface. Both these effects substantially delayed gas entrainment into the liquid.

Effect of nature-inspired needle-shaped vortex generators on the aerodynamic features of a double-delta wing

The effects of using nature-inspired needle-shaped vortex generators (VGs) on the aerodynamic features of a 65°/35° double-delta wing were investigated experimentally in a wind tunnel. Particle Image Velocimetry (PIV) visualization was conducted on 7 chord-wise sections as well as 7 span-wise sections at an Angle of Attack (AOA) of 30° and on a plane adjacent to the wing surface at AOAs of 30° and 35° The experiments were carried out at Re = 2 × 105. The double-delta wing was equipped with a series of VGs at the leading edge of the apex and kink area. The effect of VGs on the time-averaged vortical flow features was investigated and then compared to those of the plain wing model. The assessment of the vortices’ diameter, vorticity and velocity distribution, streamline topology, circulation (Γ), and parameter Γ/Av showed that the use of the needle shaped VGs has an enhancing effect on the improvement in flow structure of the double-delta wing. The causes of the flow structure improvement were the increased flow momentum over and behind the wing, energized near-surface flow, a more concentrated vortex system, and delayed vortex breakdown.

Improved Simulation of the Antarctic Stratospheric Final Warming by Modifying the Orographic Gravity Wave Parameterization in the Beijing Climate Center Atmospheric General Circulation Model

The Antarctic stratospheric final warming (SFW) is usually simulated with a substantial delay in climate models, and the corresponding temperatures in austral spring are lower than observations, implying insufficient stratospheric wave drag. To investigate the role of orographic gravity wave drag (GWD) in modeling the Antarctic SFW, in this study the orographic GWD parameterization scheme is modified in the middle-atmosphere version of the Beijing Climate Center Atmospheric General Circulation Model. A pair of simulations are conducted to compare two orographic GWD schemes in simulating the breakdown of the stratospheric polar vortex over Antarctica. The control simulation with the default orographic GWD scheme exhibits delayed vortex breakdown and the cold-pole bias seen in most climate models. In the simulation with modified orographic GWD scheme, the simulated vortex breaks down earlier by 8 days, and the associated cold-pole bias is reduced by more than 2 K. The modified scheme provides stronger orographic GWD in the lower stratosphere, which drives an accelerated polar downwelling branch of the Brewer–Dobson circulation and, in turn, produces adiabatic warming. Our study suggests that modifying orographic GWD parameterizations in climate models would be a valid way of improving the SFW simulation over Antarctica.

Control effect of micro vortex generators on leading edge of attached cavitation

In this study, we investigate the effects of micro vortex generators (VGs) installed close to the leading edge of a quasi-two-dimensional NACA0015 hydrofoil under cavitating and non-cavitating conditions. Our aim is to improve physical insight into interaction mechanisms of the boundary layer with the formation and stability of partial cavities. Under non-cavitating conditions, the proposed micro VGs effectively suppress laminar separation. However, under cavitating conditions, even very small micro VGs within the boundary layer promote the formation of counter-rotating cavitating vortices. In comparison with the smooth hydrofoil surface (without micro VGs), the cavitation onset is shifted toward the leading edge. Additionally, classical “fingering structures” and Tollmien–Schlichting waves are no longer present. Since the onset of the cavity does no longer appear at (or close to) the laminar separation line, a novel onset mechanism is observed experimentally. The mechanism consists of stable vortex cavitation, followed by vortex break-down into bubbly structures that are finally accumulated into an attached cavity region. By reduction in the height of the micro VGs, a delayed vortex break-down is found, leading to an increase in the length of the cavitating vortex pattern. This allows for enhanced control on the cavity dynamics, especially with respect to the penetration depth of the re-entrant jet. As a result of our investigation, we conclude that well suited micro VGs show a high potential to manipulate and control cavity dynamics.

Comparison of turbulent flow through hexagram and hexagon orifices in circular pipes using large-eddy simulation

Characteristics of turbulent flow through a circular, a hexagon and a hexagram orifice with the same flow area in circular pipes are investigated using wall-modelled large-eddy simulation. Good agreements to available experimental data were obtained in both the mean velocity and turbulent kinetic energy. The hexagram orifice with alternating convex and concave corners introduces outwards radial velocity around the concave corners downstream of the orifice plate stronger than the hexagon orifice. The stronger outwards radial velocity transfers high momentum from the pipe centre towards the pipe wall to energize the orifice-forced vortex sheet rolling-up and leads to a delayed vortex break-down. Correspondingly, the hexagram has a more gradual flow recovery to a pipe flow and a reduced pressure drop than the hexagon orifice. Both the hexagon and hexagram orifices show an axis-switching phenomenon, which is observed from both the streamwise velocity and turbulent kinetic energy contours. To the best knowledge of the authors, this is the first comparison of orifice-forced turbulence development, mixing and flow dynamics between a regular and a fractal-based polygonal orifice.

Is Missing Orographic Gravity Wave Drag near 60°S the Cause of the Stratospheric Zonal Wind Biases in Chemistry–Climate Models?

Abstract Nearly all chemistry–climate models (CCMs) have a systematic bias of a delayed springtime breakdown of the Southern Hemisphere (SH) stratospheric polar vortex, implying insufficient stratospheric wave drag. In this study the Canadian Middle Atmosphere Model (CMAM) and the CMAM Data Assimilation System (CMAM-DAS) are used to investigate the cause of this bias. Zonal wind analysis increments from CMAM-DAS reveal systematic negative values in the stratosphere near 60°S in winter and early spring. These are interpreted as indicating a bias in the model physics, namely, missing gravity wave drag (GWD). The negative analysis increments remain at a nearly constant height during winter and descend as the vortex weakens, much like orographic GWD. This region is also where current orographic GWD parameterizations have a gap in wave drag, which is suggested to be unrealistic because of missing effects in those parameterizations. These findings motivate a pair of free-running CMAM simulations to assess the impact of extra orographic GWD at 60°S. The control simulation exhibits the cold-pole bias and delayed vortex breakdown seen in the CCMs. In the simulation with extra GWD, the cold-pole bias is significantly reduced and the vortex breaks down earlier. Changes in resolved wave drag in the stratosphere also occur in response to the extra GWD, which reduce stratospheric SH polar-cap temperature biases in late spring and early summer. Reducing the dynamical biases, however, results in degraded Antarctic column ozone. This suggests that CCMs that obtain realistic column ozone in the presence of an overly strong and persistent vortex may be doing so through compensating errors.

Numerical modeling of the vortex breakdown phenomenon on a delta wing with trailing-edge jet-flap

The flow development over delta wings is highly complicated since the interaction of the angle of attack with the delta-wing geometry leads to the appearance of a pair of well-organized counter-rotating leading-edge vortical structures. For relatively moderate angles of attack, these vortices remain robust and contribute to the enhancement of the overall lift performance. However, at higher angles of attack the vortices develop instabilities leading to the well-known vortex breakdown phenomenon, resulting in a deterioration of the aerodynamic properties. Thus, delaying vortex breakdown at higher angles of attack, is important and for this reason various techniques have been developed to control the breakdown mechanism. Such a technique is the use of trailing-edge jet-flaps. In the present work, an attempt to model the vortex breakdown together with its control, above a delta wing at high angles of attack, for cases with and without a trailing-edge jet-flap, is presented. To model the turbulent stresses, the low-Reynolds-number stress-omega model was used. The computational results were in good agreement with the available experimental data regarding the prediction of the onset of vortex breakdown and showed that the use of jet-flaps can lead to a significant delay of the breakdown process.

Control of Vortex Breakdown over a Delta Wing Using Forebody Spanwise Slot Blowing

In this paper we study the effectiveness of forebody slot blowing to control vortex breakdown over a generic delta wing-body configuration. The motivation is to exploit the benefits of both slot blowing and canards to control vortex breakdown over a delta wing. Parameters investigated include slot length, slot width, single and double-sided blowing, and the Reynolds number. Time-averaged flow images show that Reynolds number has little effect on vortex breakdown location, at least for the range of conditions studied here, and a single-sided blowing has favorable effects on the blowing side and unfavorable effects on the nonblowing side. It is postulated that when fluid is discharged from the slot at the forebody in the spanwise direction, it produces a vortex sheet that interacts with the freestream and is rolled up to form a trailing vortex further downstream. The rolled-up vortex sheet produces a downwash effect on the blowing side and a sideslip effect on the opposite side. The downwash modifies the flowfield around the leading edge of the wing, causing a delay in vortex breakdown. To compensate for the unfavorable effects of blowing on the opposite side, a much higher blowing momentum (more than two times the single-sided slot case) is needed to delay vortex breakdown on both sides of the wing when double-sided blowing is employed. Our results also show that for a given Reynolds number and angle of attack, increasing the blowing momentum leads to a substantial delay in the vortex breakdown position. In addition, for the same blowing momentum coefficient, varying the slot width has little effect on the breakdown location, whereas increasing the slot length has favorable effect, particularly at lower angles of attack.

Hingeless Flow Control over a Delta-Wing Planform

An experimental investigation of hingeless flow control by vortex breakdown manipulation over a delta wing is described. Employed as a method of longitudinal control for slender-wing planforms, the performance of this active flow control technique was evaluated as a possible replacement to conventional moving or hinged control surfaces. Testing was conducted with a 60-deg delta-wing model incorporating pneumatic vortex control (PVC) actuators. These actuators delivered high-pressure jets of varying parameters on the upper surface of the model. The jets were used to alter the flowfield over the wing caused by leading-edge vortices. The natural location of vortex breakdown was shifted by three sets of PVC actuators strategically positioned on the model. This effect was intended to control the chordwise lift distribution over the wing by inducing both premature and delayed vortex breakdown, resulting in nose-up and nose-down pitch responses, respectively. A comparative analysis of this hingeless flow control concept was conducted in Texas A&M University's 3 × 4 ft low-speed wind tunnel and 2 x 3 ft water tunnel

Quantitative visualization of the leading-edge vortices on a delta wing by using pressure-sensitive paint

Leading-edge vortices on a simple delta wing were visualized by using pressure-sensitive paint (PSP). PSP is an optical pressure measurement technique based on oxygen quenching of luminescent molecules. In the present study, we used PSP composed of platinum octaethylporphyrine (PtOEP) and fluoropolymer (poly-IBM-co-TFEM [Poly (isobutylmethacrtlate-co-trifluoroethylate)]). This new paint has higher sensitivity to pressure and lower sensitivity to temperature than previous ones, reducing an error due to temperature variation during a wind tunnel test. A thin coating of PSP was applied to a delta wing model with 70-degree leading-edge sweep. The coating was excited by Xenon light and emission from the coating was detected by a high-resolution CCD camera. Tests were done at subsonic speeds in the 0.2-m Supersonic Wind Tunnel at the National Aerospace Laboratory in Japan. Complicated flow structures on the delta wing including primary and secondary vortices were clearly visualized using pressure-sensitive paint. An a priori calibration technique was used to convert measured luminescent intensity into pressure. The obtained pressure distributions were in good agreement with pressure tap data. Pressure maps were obtained for various Mach numbers, Reynolds numbers and angles of attack. It was found that an increase in Mach number delayed vortex breakdown while Reynolds number had little effect on the vortex formation.

The interaction of near-critical swirling flows in a pipe with inlet azimuthal vorticity perturbations

The interaction of a near-critical axisymmetric incompressible swirling flow in a straight pipe with small inlet azimuthal vorticity perturbations is studied. Certain flow conditions that may reflect the physical situation are prescribed along the pipe inlet and outlet. It is first demonstrated that under these conditions a regular-expansion solution in terms of the small azimuthal vorticity perturbations has a singular behavior around the critical swirl. This singularity infers that large-amplitude disturbances may be induced by the small perturbations when the incoming flow to the pipe has a swirl level around the critical swirl. In order to understand the nature of flows in this swirl range, a small-disturbance analysis is developed. It shows that under the prescribed inlet/outlet conditions, a small but finite inlet azimuthal vorticity perturbation breaks the transcritical bifurcation of solutions of the Euler equations at the critical swirl into two branches of perturbed solutions. When the azimuthal vorticity perturbations are positive these branches show a regular behavior. However, when they are negative, the perturbed branches fold at limit points near the critical swirl, with a finite gap between the two branches, and no near-columnar equilibrium state can exist for an incoming flow with swirl close to the critical level. The flow must develop large disturbances in this swirl range. Beyond this range, two equilibrium states may exist under the same inlet/outlet conditions. When the negative inlet vorticity perturbations become larger in their size, this special behavior uniformly changes into a branch of single equilibrium state for each incoming swirl. The relevance of the results to the appearance of the axisymmetric vortex breakdown in a pipe and the control of this phenomenon using inlet vorticity perturbations is also discussed. The results suggest that, in general, positive inlet azimuthal vorticity perturbations may be used to delay vortex breakdown to higher swirl levels whereas negative perturbations induce the appearance of vortex breakdown at levels below the critical swirl.

Control of Leading-Edge Vortices with Suction

Abstract : Several control techniques have been applied in order to control leading-edge vortices at high angle of attack. The purpose of these control techniques is to influence the strength and structure of the vortices, to generate rolling moment, and to delay vortex breakdown. Since the vorticity of the leading-edge vortices originates from the separation point along the leading-edge, control of development of the shear layer has been chosen as a control strategy in several investigations: blowing and suction in the tangential direction along a rounded leading-edge.

Flow control over a maneuvering delta wing at high angles of attack

The possibility of delaying vortex breakdown to higher angles of attack by employing control surfaces is studied experimentally. The effect of an apex flap is tested for fixed and dynamically pitching delta wings. Flow visualization, surface pressure measurements, and laser-Doppler velocimetry are employed to map out pressure, velocity, and vorticity fields. It is found that a drooping apex flap can delay vortex breakdown by an angle of 8 deg beyond the steady flow breakdown angle of attack. The apex flap effect is equally pronounced in dynamic maneuvers.

Delta wing vortex control via recessed angled spanwise blowing

A novel vortex control technique, known as recessed angled span wise blowing, has been investigated in a water tunnel on a beveled 60-deg delta wing. Three pairs of blowing ports, located on the suction side of the wing and beneath the vortex core, were canted upward in the spanwise direction such that the injected flow is parallel to the beveled edge. Flow visualization experiments were performed using dye injected into the leadingedge vortices near the wing apex with the vortex burst location visually observed. Blowing coefficient values up to 0.1 were used in the experiments, although the effectiveness of blowing at a single chord location remained the same above blowing coefficients of 0.05. Maximum improvement in vortex breakdown location of 15% chord was obtained at an angle of attack of 22 deg and blowing coefficient of 0.05. Blowing from a port downstream of the natural burst location delayed vortex breakdown, whereas blowing upstream of the natural burst prompted vortex breakdown at the blowing port. Blowing ports at 41 % chord had the greatest effect in delaying vortex breakdown. These observations are explained via Rayleigh's stability criterion for circular flow.