Formation Of Separation Bubble Research Articles

The use of patterned heating in controlling pressure losses within sloping channels

The effectiveness of utilizing heating patterns as a drag-reduction tool in sloping channels is analysed. The usefulness of heating is judged by determining the pressure gradient required to maintain the same flow rate as in the isothermal case. The key to reducing pressure loss is the formation of separation bubbles, although these bubbles are washed away at relatively large Reynolds numbers. The bubbles reduce the direct contact between the stream and the side walls, thereby reducing the friction experienced by the flow. Moreover, the fluid inside the bubbles tends to rotate, a motion provoked by longitudinal temperature gradients. This rotation also seems to reduce the resistance. On the other hand, the existence of the bubbles tends to obstruct the stream, increasing the flow resistance. In general, channels oriented close to horizontal experience a relatively small pressure loss, but this loss grows markedly as the channel inclines towards the vertical. When modest heating is applied, the pressure loss is approximately proportional to the square of the associated Rayleigh number. It is also shown that if the heating wavelength is too short or too long, the heating loses its effectiveness. In certain circumstances, it turns out that the theoretical pressure-gradient reduction achieved by judicious heating is so large that it exceeds the pressure gradient required to drive the flow in the isothermal problem. The conclusion is that in these instances, a pressure gradient of the opposite sign must be applied to prevent flow acceleration.

Laminar separation bubble formation and bursting on a finite wing

The transient processes of laminar separation bubble (LSB) formation and bursting on a rectangular NACA 0018 wing are studied experimentally. A two-dimensional airfoil model is used as a baseline for the assessment of finite wing effects. The models are subjected to ramp changes in free-stream velocity causing the flow to switch between a state where an LSB forms and a state without reattachment. Lift force and particle image velocimetry measurements are used to relate the flow development to the aerodynamic loading. The lift coefficient of the airfoil exhibits substantial hysteresis, and the duration of the lift transients range from $10$ to $22$ convective time scales for bubble formation and $22$ to $30$ convective time scales for bursting. In contrast, the transient lift coefficients of the wing change gradually, with less hysteresis. The wing tip causes greater three-dimensionality in the separation bubble, whose thickness increases near the midspan where bursting is initiated. During bubble formation, the region of separated flow contracts towards the midspan. The gradual change in lift of the wing is linked to slower spanwise expansion and contraction of the separated flow region relative to the airfoil. On both models, the wavenumbers and amplitudes of disturbances in the separated shear layer rapidly change when reattachment initiates or ceases. Applying the bursting criterion of Gaster (Tech. Rep. Reports and Memoranda 3595. Aeronautical Research Council, London, 1967) to the bubble on the wing shows that bursting of the bubble at a single spanwise location is insufficient to cause complete spanwise failure of reattachment, and that the relationship between bursting parameters depends on spanwise position.

Use of heated grooves for reduction in friction resistance

An analysis of the use of heated grooves for resistance reduction was carried out. The model problem consisted of two parallel plates in a relative motion. The stationary plate was equipped with grooves characterized by a sinusoidal pattern and exposed to a pattern of sinusoidal heating. The force required to maintain plate movement was used to judge changes in flow resistance. It was shown that isothermal grooves increase resistance, periodic heating of smooth plate reduces resistance, while periodic heating of grooved plate may either increase or decrease this resistance. The net effect depends on the relative position of the groove and heating patterns, which controls the strength and direction of the pattern interaction effect, and correct selection of this position may eliminate a significant portion of resistance. Increasing groove amplitude and heating intensity reduces resistance similarly to reducing the Prandtl number. The formation of separation bubbles is essential to resistance reduction; an increase in the plate velocity washes these bubbles away, thus eliminating the resistance-reducing effect.

Coherent Structures and Pressure Fluctuations over an Airfoil Using Time-Resolved Measurements

This study focused on the unsteady behaviors of large-scale vortical structures and wall pressure fluctuations and their coupling mechanism on a low-Reynolds-number airfoil. Three incidence angles were chosen for comparison: , and (representing two flow regimes, namely, the separation bubble formation and separation without subsequent reattachment). Simultaneous measurements of velocity fields and wall-pressure fluctuations were performed using a high-resolution time-resolved particle image velocimetry operating at 2 kHz and an array of flush-mounted microphones sampled at 10 kHz. The results provided a detailed description of the shear layer transition and the coherent structures development on the airfoil. The roll-up process, pairing process, spatial-temporal growth of the vortical structure, and vortex break-up were clarified through a long-period analysis. Spectral analysis demonstrated that vortical structures at the fundamental frequency were spatially and temporally localized and that their subharmonics were significantly enhanced. Frequency–wavenumber spectra revealed that the energy was dominantly concentrated along the convective ridge. With increasing incidence angle, the energy level of the inclined ridge increased, with a lower extension in the frequency domain and a higher dispersion in the wavenumber domain. The conditional averaging and quadrant analysis results demonstrated that strong events in the near-wall region were dominated by the sweep motion and ejection motion, which enhanced the interaction between the inner and outer layers. This also indicated the existence of a strong relationship between the wall-pressure peaks and turbulence production mechanisms.

Effects of blowing and suction jets on the aerodynamic performance of wind turbine airfoil

This work experimentally investigates the effects of blowing and suction jets on the aerodynamic performance of a NACA0012 airfoil at a moderate Reynolds number Re = 1.0 × 105. The performance of a single blowing jet is only examined first, and then combinations of blowing and suction are implemented. The loadcell and surface oil flow visualization techniques are respectively used to measure the aerodynamic forces and flow structures on the uncontrolled and controlled airfoil. Effects of momentum coefficient and location of blowing and suction jets on the lift, drag, stall angle, cut-in speed, flow separation, and flow reattachment are studied to find an optimum control case. The results prove that the single blowing jet located at 0.2c from the leading edge has the best performance in the lift enhancement, where c is the airfoil chord length. The maximum enhancements of peak lift force coefficient CL and lift-to-drag ratio CL/CD are achieved as 18.4% and 804%, respectively. The underlying physics of aerodynamic performance enhancement relies on suppression of the flow separation and formation of separation bubbles.

Structure and dynamics of a laminar separation bubble near a wingtip

The three-dimensional flow topology of a laminar separation bubble forming on the suction surface of a semispan wing with an aspect ratio of $2.5$ and NACA 0018 airfoil section is characterised experimentally using surface pressure measurements and particle image velocimetry at a chord Reynolds number of $125\ 000$ . In the inboard region of the wing, the separation bubble is essentially two-dimensional, and the transition process in the separated shear layer leads to periodic vortex shedding, which dominates the bubble dynamics, similar to two-dimensional separation bubbles. However, progressive spanwise changes in the mean structure and vortex dynamics occur near the wingtip, leading to an open separation and eventual suppression of the bubble. In the immediate proximity of the wingtip, the boundary layer remains attached, no vortex shedding occurs and the flow remains laminar, terminating separation bubble formation. Despite variations in the mean separation bubble topology and vortex dynamics along the span, the fundamental shedding characteristics remain nearly invariant across the portion of the wing where vortex shedding occurs, and the flow appears to lock onto a common instability mode across the span, leading to minimal changes in the mean bubble characteristics despite notable changes in the effective angle of attack along the span. A comparison with available surface flow visualisations from previous studies indicates that the observed changes to the mean bubble footprint along the span of the wing are similar across different geometries and flow characteristics, suggesting similarities in the three-dimensional bubble topology and dynamics on finite wings.

Investigation on Supersonic Flow Control Using Nanosecond Dielectric Barrier Discharge Plasma Actuators

In this paper, the effects of streamwise Nanosecond Dielectric Barrier Discharge (NS-DBD) actuators on Shock Wave/Boundary Layer Interaction (SWBLI) are investigated in a Mach 2.5 supersonic flow. In this regard, the numerical investigation of NS-DBD plasma actuator effects on unsteady supersonic flow passing a 14° shock wave generator is performed using simulation of Navier-Stokes equations for 3D-flow, unsteady, compressible, and k ‐ ω SST turbulent model. In order to evaluate plasma discharge capabilities, the effects of plasma discharge length on the flow behavior are studied by investigating the flow friction factor, the region of separation bubble formation, velocity, and temperature distribution fields in the SWBLI region. The numerical results showed that plasma discharge increased the temperature of the discharge region and boundary layer temperature in the vicinity of flow separation and consequently reduced the Mach number in the plasma discharge region. Plasma excitation to the separation bubbles shifted the separation region to the upstream around 6 mm, increased SWBLI height, and increased the angle of the separation shock wave. Besides, the investigations on the variations of pressure recovery coefficient illustrated that plasma discharge to the separation bubbles had no impressive effect and decreased pressure recovery coefficient. The numerical results showed that although the NS-DBD plasma actuator was not effective in reducing the separation area in SWBLI, they were capable of shifting the separation shock position upstream. This feature can be used to modify the structure of the shock wave in supersonic intakes in off-design conditions.

An investigation into the nonstationary characteristics of separation-bubble formation on a smooth circular cylinder in the critical transition range

Abstract To investigate the characteristics of the separation-bubble formation of flow over a smooth circular cylinder in the critical transition range, a sliding-window (SW) method and a peak-valley (PV) method were proposed to identify the intermittent jumps, named the characteristic events, in the real-time pressure signal obtained on both sides of the cylinder model. By evaluating the counts of the qualified events of the SW and PV methods, the PV method was found less sensitive to the small-scale disturbances in the pressure signal, therefore was adopted for later analysis. With the PV method, the characteristic events were identified from the pressure signal and categorized into two types: Type-1 is referred to the events of pressure descending and Type-2 is referred to the events of pressure ascending. Subsequently, the count per minute of the characteristic events was determined for describing the intermittency of the separation-bubble formation, and the time scale of each of the characteristic event was regarded as the time length of the separation-bubble formation. The count per minute of the characteristic events appeared to be the highest in the transitional regime. While the time scales of the characteristic events were varying with Reynolds number, the weighting-averaged normalized time scales in the transitional regime of the three cases studied were found comparable to the normalized time scale of the lift jump noted in the literature. Physically, the characteristic events found in the pressure signals in this study can be attributed to the three-dimensional aspect of separation-bubble formation.

Design and aerodynamic performance analysis of a finite span double-split S809 configuration for passive flow control in wind turbines and comparison with single-split geometries

With the previous knowledge of the effectiveness of single-split blades, questions arise about the effectiveness of a double-split configuration on horizontal axis wind turbine. To shed light on this question, a 3-D double-split finite-span configuration with S809 airfoil profile is designed based on the results of unsteady DES simulations of two families of single-split configurations. Firstly, the two 3-D single-split families with four different values of split width are simulated at angles of attack (AOA) up to 25°. Secondly, a finite span double-split model is designed based on the single-split geometries with the highest aerodynamic performances. It has been found that the double-split configuration had the poorest performance for 0°<AOA<17°; however, at 17°<AOA<22°, the double-split model showed the highest performance. Analyzing the time-averaged streamlines showed that excessive injected flow at low AOAs as well as the formation of separation bubbles inside the split channel were two main negative factors that severely reduced the performance of the double-split geometry. Also, findings revealed that using optimum cases of single-split geometry does not result in an optimum double-split geometry. Lastly, considering all findings, a conceptual selective (controlled) flow control method that led to a considerable increase in the aerodynamic performance of the double-split geometry has been proposed and discussed.

Flow over rectangular cylinder: Effects of cylinder aspect ratio and Reynolds number

Numerical simulations of two- and three-dimensional unconfined flows over rectangular cylinders are conducted at Reynolds number Re = 30 – 200. The cylinder cross-sectional aspect ratio AR (length-to-width) is varied as 0.25, 0.5, 1.0, 2.0, 3.0 and 4.0. The focus is given on how AR and Re influence the flow structure and associated aerodynamic parameters. The first critical Reynolds number (Recr1) associated with the transition from the steady flow to the two-dimensional unsteady flow is determined for each AR and found to linearly increase with increasing AR. The same observation is made for the second critical Reynolds number (Recr2) where the two-dimensional unsteady flow metamorphoses into the three-dimensional unsteady flow. A larger Re is required for a larger AR to have the onset of vortex shedding or the transition from two-dimensional flow to the three-dimensional. Three distinct scenarios of the flow separation and reattachment are identified in the ranges of Re and AR examined. The physical insight into the flow dependence on AR and Re is provided. Both AR and Re play a role in the formation of separation bubbles on the cylinder side surfaces. Two distinct mechanisms of the separation bubble formation are imparted. The detailed flow structures are linked to the mean and fluctuating forces and Strouhal numbers. The vortex strength beefs up with increasing Re but diminishes with AR, as do the normal and shear stresses and fluctuating forces.

Experimental and numerical study of laminar separation bubble formation on low Reynolds number airfoil with leading-edge tubercles

The present work reports the effect of leading-edge tubercles on aerodynamic performance and flow features of a cambered airfoil E216 at a Reynolds number of 100,000 and at various angles of attack in the pre-stall regime. Amplitude values of 2 mm, 4 mm and 8 mm and wavelength values of 15.5 mm, 31 mm and 62 mm are used for both experimental and simulation studies. The Transition-SST RANS model is used to simulate transition phenomenon (laminar separation bubble) and three-dimensional flow features over the airfoil. Wind tunnel experimental results are used for the performance analysis and the validation of the simulation methodology. The experimental values of $$C_\mathrm{l}$$ and $$C_\mathrm{d}$$ are 1.37 and 0.081, respectively, at a stall angle of $$12^{\circ }$$ for the plain airfoil. The experimental results show that the lift generated by tubercled airfoils is higher than that produced by the plain airfoil in the pre-stall region but lower at the stall angle. A maximum benefit of 4.51% in $$C_\mathrm{l}$$ is obtained for the tubercled airfoil with the highest amplitude (8 mm) and wavelength (64 mm) at $$6^{\circ }$$ angle of attack. A higher $$C_\mathrm{d}$$ is observed for all the tubercled airfoils than for the plain one. The simulation is mainly carried out to study the flow structure. Simulation results indicate the presence of laminar separation bubbles on the plain airfoil with a straight separation and reattachment line parallel to the trailing edge. The tubercles considerably altered the laminar separation bubble formation and the flow structure. A sinusoidal laminar separation bubble is formed on the tubercled airfoils with reduced bubble length. The laminar separation bubble along the trough is formed ahead of that at peak. Two pairs of counter-rotating vortices are formed on the airfoil surface along the trough at two different chord-wise locations which strongly alter the flow pattern over it. Prandtl’s secondary flow of the first kind is the key reason for the vortex formation.

Perforated Wall in Controlling the Separation Bubble Due to Shock Wave –Boundary Layer Interaction

Abstract The shock wave boundary layer interaction (SWBLI) induced separation bubble formation (SB) and its control has been investigated numerically in the mixed compression type of intake in the scramjet engine. The external compression has occurred due to the three successive oblique shocks formed from the three successive ramps of the forebody with the semi-wedge angle of 7.6, 7.0, and 9.4 respectively. The intake is designed in such a way that all three shocks converge and impinge on the leading edge of the cowl lip for the operating Mach number of 5.0. The numerical simulation is carried out by solving steady, compressible 2-D RANS equations using transitional SST k-ω turbulence model to capture the influence of SWBLI in the performance of supersonic intake. The formation of SB and its control by establishing the perforated wall in its proximity are investigated for three different cases based on the perforation with respect to SSB. Findings of the numerical simulation have concluded that the size of the SB decreases to an acceptable level while establishing the perforation in the entire fore-body wall in the isolator region. The feedback loop established between the upstream and downstream of SB could be a possible reason for reducing its size.

Flow loss and structure of circular arc blades with different leading edges

Due to much lower cost of development and production, circular arc blades are often applied to axial flow fans. However, compared with NACA 65 profile, circular arc blades have higher losses, which are caused by the formation of separation bubbles or complete separation at the leading edge. Therefore, it is necessary to identify a specific geometry of leading edge, which can reduce the separation bubbles or complete separation. In this article, circular arc blades with different leading edges are examined by numerical method. The numerical investigations were performed with Reynolds numbers of [Formula: see text] and [Formula: see text]. All examinations were performed for different incidence angles. The influence of the sidewall on the flow loss and behavior is taken into account. In this article, the influence of leading edge geometry and Reynolds number on the flow losses is shown. The occurrence of flow behavior such as separation bubbles at the leading edge, secondary flow, and corner stall is shown and discussed. The flow structure on the blade and the corresponding sidewall region is illustrated with numerical streamline figures.

An experimental investigation of laminar separation bubble formation on flexible membrane wing

In this study, fluid–structure interaction over a flexible membrane wing with aspect ratio (AR) of 3 at low Reynolds numbers was investigated experimentally. Smoke wire flow visualization technique was performed for analyzing time-dependent behavior of flow over this flexible membrane wing. Furthermore, time dependent deformation of flexible membrane wing was measured. It was clearly seen that the value of membrane deformation increased with increasing angle of attack. Moreover, it was stated that the size of laminar separation bubble (LSB) changed with time due to the unsteady flow characteristics of membrane wing. The unsteady behavior over the flexible membrane wing caused different deformation modes to form at different angles of attack. For the flexible wings with higher aspect ratios, the LSB was more dominant in the membrane wings at low Re numbers, and caused the membrane vibration to increase based on the angle of attack which the LSB started to be overpowering.

Aerodynamic Optimization of a Winglet-Shroud Tip Geometry for a Linear Turbine Cascade

This paper presents a continued study on a previously investigated novel winglet-shroud (WS) (or partial shroud) geometry for a linear turbine cascade. Various widths of double-side winglets (DSW) and different locations of a partial shroud are considered. In addition, both a plain tip and a full shroud tip are applied as the datum cases which were examined experimentally and numerically. Total pressure loss and viscous loss coefficients are comparatively employed to execute a quantitative analysis of aerodynamic performance. The effectiveness of various widths (w) of DSW set at 3%, 5%, 7%, and 9% of the blade pitch (p) is numerically investigated. Skin-friction lines on the tip surface indicate that different DSW cases do not alter flow field features including the separation bubble and reattachment flow within the tip gap region, even for the case with the broadest width (w/p = 9%). However, the pressure side extension of the DSW exhibits the formation of separation bubble, while the suction side platform of the DSW turns the tip leakage vortex (TLV) away from the suction surface (SS). Meanwhile, the horse-shoe vortex (HV) near the casing is not generated even for the case with the smallest width (w/p = 3%). As a result, both the tip leakage and the upper passage vortices are weakened and further dissipated with wider w/p in the DSW cases. Larger width of the DSW geometry is indeed able to improve the aerodynamic performance, but only to a slight degree. With the w/p increasing from 3% to 9%, the mass-averaged total pressure loss coefficient over an exit plane is reduced by only 2.61%. Therefore, considering both the enlarged (or reduced) tip area and the enhanced (or deteriorated) performance compared to the datum cases, a favorable width of w/p = 5% is chosen to design the WS structure. Three locations for the partial shroud (linkage segment) are devised, locating them near the leading edge, in the middle and close to the trailing edge, respectively. Results demonstrate that all three cases of the WS design have advantages over the DSW arrangement in lessening the aerodynamic loss, with the middle linkage segment location producing the optimal effect. This conclusion verifies the feasibility of the previously studied WS configuration.

Numerical study on boundary layer control using CH4[sbnd]H2[sbnd]air Micro-reacting jet

The focus of present numerical study is on assessment of control of laminar separation bubble phenomenon using Micro-scale combustion actuators in an airfoil with low Reynolds number under surface effect and free flows. In this way, the characteristics of laminar separation bubble such as its formation, geometry, and transition from laminar to turbulent around airfoil SD8020 in attack angles of 5 and 8° are investigated. Following that, the new combustion actuators in Micro-scale, cold, and hot air-jet injection are introduced to control boundary layer flow in terms of eliminating the separation bubble. Some mechanisms are identified for improvement of methane-air premixed flame stabilization in a Micro-scale combustor, which includes effects of added hydrogen to methane as an additive, a central conductive wire insertion, and creating steps in the reactor. The finite volume method is used to solve the turbulent unsteady flow equations. The results are compared with experimental and numerical data obtained by other researchers and show good conformity for aerodynamic coefficient predicting. The SIMPLE-C method is used in numerical algorithm utilized to coupling pressure and velocity fields, the second order upwind method is used to discretization of momentum equation, and finally SST Transient k-ω equations is used for turbulent flow modeling. The results show that the selected numerical method is able to recognize reverse pressure gradient, laminar separation bubble, and flow transition from laminar to turbulent in boundary layer. The bubble formation location and flow transition are also inclined towards airfoil leading edge under surface effect, and pressure distribution makes a variation in laminar separation bubble formation location. The utilization of combustion actuators leads to a decrease in flow Reynolds number along with airfoil wall and a tendency of flow to relaminarization. This causes a larger increase in the lift coefficient than the hot-air-jet injection. But additionally the friction effects on the surface is increased by laminar flow, that leads to an increase in friction drag coefficient of reacting jet rather than the hot-air jet and generally an increase in drag coefficient. On the other hand, the stall phenomenon control by combustion actuator also leads to an increase in stall angle limit from 10 to 14°, and lift coefficient improvement to 26% than without jet injection, through flow separation delay.

Turbulence Model Selection for Low Reynolds Number Flows.

One of the major flow phenomena associated with low Reynolds number flow is the formation of separation bubbles on an airfoil’s surface. NACA4415 airfoil is commonly used in wind turbines and UAV applications. The stall characteristics are gradual compared to thin airfoils. The primary criterion set for this work is the capture of laminar separation bubble. Flow is simulated for a Reynolds number of 120,000. The numerical analysis carried out shows the advantages and disadvantages of a few turbulence models. The turbulence models tested were: one equation Spallart Allmars (S-A), two equation SST K-ω, three equation Intermittency (γ) SST, k-kl-ω and finally, the four equation transition γ-Reθ SST. However, the variation in flow physics differs between these turbulence models. Procedure to establish the accuracy of the simulation, in accord with previous experimental results, has been discussed in detail.

Drag reduction in a thermally modulated channel

Flow in a horizontal channel exposed to external heating which results in sinusoidal temperature variations along the upper and lower walls with a phase shift between them has been studied using a combination of analytical and numerical methods. The most intense convection is observed when the upper and lower hot spots are located above each other. It has been demonstrated that the heating results in a significant reduction of the pressure gradient required to drive the flow when compared to a similar flow in an isothermal channel. The drag reduction is associated with the formation of separation bubbles which insulate the stream from direct contact with the bounding walls. The fluid inside of the bubbles rotates due to horizontal density gradients, which further reduces the required pressure gradient. The magnitude of the drag reduction depends on the phase shift between the heating patterns and can increase by up to threefold when compared to the drag reduction which can be achieved by heating only one wall. A detailed analysis of the associated heat fluxes has been presented.

Drag reduction in heated channels

AbstractIt is known that the drag for flows driven by a pressure gradient in heated channels can be reduced below the level found in isothermal channels. This reduction occurs for spatially modulated heating and is associated with the formation of separation bubbles which isolate the main stream from direct contact with the solid wall. It is demonstrated that the use of a proper combination of spatially distributed and spatially uniform heating components results in an increase in the horizontal and vertical temperature gradients which lead to an intensification of convection which, in turn, significantly increases the drag reduction. An excessive increase of the uniform heating leads to breakup of the bubbles and the formation of complex secondary states, resulting in a deterioration of the system performance. This performance may, under certain conditions, still be better than that achieved using only spatially distributed heating. Detailed calculations have been carried out for the Prandtl number $\mathit{Pr}=0.71$ and demonstrate that this technique is effective for flows with a Reynolds number $\mathit{Re}&lt;10$; faster flows wash away separation bubbles. The question of net gain remains to be settled as it depends on the method used to achieve the desired wall temperature and on the cost of the required energy. The presented results provide a basis for the design of passive flow control techniques utilizing heating patterns as controlling agents.

STUDY OF ENTRANCE CONFIGURATION EFFECT ON STREAMWISE VORTICES IN WAVY CHANNEL

Two different entrance configurations of a channel with wavy (sinusoidal) surface, namely cosine entrance and negative cosine entrance configuration, were numerically and experimentally investigated to study the downstream development of the induced counterrotating streamwise vortices. The study is limited to the laminar boundary layer flow. The negative cosine entrance configuration has a “blockage effect” which subjects to large pressure penalty. However, under a certain amplitude and wavelength of the sinusoidal surface, this configuration might have lower hydraulic channel gap than the other configuration due to the formation of separation bubbles at the valley of the sinusoidal surface which makes the stream lifted-up and behaves like a flow in a smooth channel. As such, the flow in the negative cosine entrance configuration is more stable than that in the cosine configuration. This is indicated by the evolution of streamwise vortices at Re = 2700 that are preserved farther downstream prior to its breakdown into turbulence.