Flow Reattachment Research Articles

Control reattachment of backward-facing step flow using a row of mini-jets in recirculation bubble

In this experimental study, we introduce an open-loop control method for shortening the reattachment length in a backward-facing step flow using a row of pulsatile min-jets in the recirculation bubble. The mini-jet perturbations are imposed in two ways, i.e., in-phase perturbation and traveling-wave perturbation. Both types of perturbations are found to shorten the reattachment length, while the former is more effective and gives comparable extent of the length reduction as the mini-jets imposed at the step separation point. The change in the flow field by the perturbations is unraveled on embedded coherent flow structures.

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.

Current state and future trends in boundary layer control on lifting surfaces

Successful flow control may bring numerous benefits, such as flow stabilization, flow reattachment, separation delay, drag reduction, lift increase, aerodynamic performance improvement, energy efficiency increase, shock delay or weakening, noise reduction, etc. For these purposes, many different flow control devices, which can be classified as passive, semi-active and active, have been designed and tested. This review paper aims to highlight the most promising and commonly employed boundary layer control methods as well as outline their potential in specific applications in aerospace and energy engineering. Referenced studies, performed on various geometries (flat plates, channels, airfoils, wings, blades, cylinders), are primarily numerical or experimental. Although enhanced aerodynamic performance is achieved in many cases, further research is required to draw general conclusions. This paper aims to demonstrate that, in the future, we may expect further developments of flow control actuators, as well as their increased application.

Active flow control of a wing section in stall flutter by dielectric barrier discharge plasma actuators

This paper investigates the potential of using an active flow control technique to modify stall flutter oscillations of a NACA (National Advisory Committee for Aeronautics) 0015 wing section. Wind tunnel experiments have been performed with a test-rig that provides the elastic degree of freedom in pitch. Measurements of the clean airfoil are taken at preset angles of θ0=6°−12°, and for Reynolds numbers of Rec=6.2×104−1.25×105, which reveal the dependency of the stall flutter oscillations to Rec and θ0. Then, flow control experiments are carried out at θ0=10° and Rec=1.04×105. Two dielectric barrier discharge plasma actuators have been employed simultaneously to exert dual-point excitation to the baseline flow. It is shown that during the upstroke half-cycle, plasma actuation postpones the dynamic stall of the airfoil and increases the maximum pitch angle of the stall flutter cycle. On the downstroke, dual-point excitation effectively improves the rate of pitching moment recovery and leads to flow reattachment at a larger pitch angle. Normalized excitation frequencies Fex=fex/fw (where fw is the wake mode frequency of the stalled airfoil) ranging from 0.1 up to 3 have been examined. Among the controlled cases, excitation with Fex=0.6 and Fex=2.2 provides the largest and smallest pitch amplitude, respectively, and the case of Fex=3 demonstrates the most impact on flow reattachment. Finally, it has been concluded that the employed control strategy is effectively capable of modifying the dynamic stall process and associated pitching moment. However, a more sophisticated control strategy would be required to significantly mitigate the stall flutter oscillations.

Control of supersonic compression corner flow using a plasma actuator

The control performance of a pulsed nanosecond dielectric barrier discharge (NSDBD) plasma actuator with varying pulse voltages and locations on a supersonic compression corner is studied using experiments and numerical simulations. The compression corner with a flat plate length of 60 mm and a ramp angle of 10° under laminar flow separation is experimentally investigated in a Ludwieg wind tunnel under a unit Reynolds number of 7.8 × 106 m−1 and Mach number of 4. The plasma actuators are placed either upstream or downstream of the separation point, extending in the spanwise direction. The Schlieren technique is used to visualize the shock wave interaction and estimate the propagation speed of the induced shock by the plasma actuator. For the numerical simulations, a one-zone inhomogeneous phenomenological plasma model is adopted to predict key discharge parameters and simulate the fast-heating region. The results show that the reduction of separation bubble length is up to 17% and 45%, respectively, in the cases of upstream and downstream of the separation point under a high applied voltage of 50 kV. The evolution of the flow structures is examined to reveal the underlying control mechanism. The results indicate that the high-speed external fluid is entrained into the original separation region after NSDBD activation upstream of the separation point, resulting in flow reattachment upstream of the corner. The entrained fluid with high momentum compels the main separation to move downstream, accompanied by the fragmentation of the original shear layer. Furthermore, the suppression of the separation region is more effective when the plasma actuator is installed close to the separation region and in the first 200 μs during one pulse, providing a good suggestion for the actuation frequency and installed location.

Drag reduction of a square-back bluff body under constant cross-wind conditions using asymmetric shear layer forcing

Highly resolved computations using Delayed Detached Eddy Simulations (DDES) of a canonical Windsor body are carried out. The flow around the car model, which is investigated imposing different yaw angles (up to 10°), is perturbed via a small square cylinder. This control element, whose length covers the whole height of the car model, is positioned at different streamwise locations on the windward face of the model. Depending on the location of the cylinder and the initial yaw angle of the model, drag increase or decrease up to 8% are observed. This sensitivity of the drag to the streamwise location of the cylinder is connected with the relative position of the flow reattachment on the side of the model after separation, which is caused by the cylinder. In the drag-optimal case where the flow reattachment occurs just at the streamwise location of the base, the asymmetry of the wake is reduced and the pressure in the wake is globally recovered. The flow mechanisms by which the drag reduction occur are twofold: (i) the flow is deviated inwards by the in-wash induced by the flow reattachment at the trailing-edge, thus acting as an inward-oriented flap, and (ii) the windward shear layer is forced by the turbulent field developing along the separated region of the perturbing cylinder.

Numerical Investigation of Thermal-Hydraulic Performance of U-Tubes Enhanced With Ellipsoidal 45 deg Dimples

Abstract This study aims to numerically investigate thermal–hydraulic performance augmentation of ellipsoidal 45 deg dimpled U-tubes with various bend curvatures subjected to constant external heat flux (q′′=10kW/m2) for a range of Reynolds numbers (5000 ≤ Re ≤ 30,000). Three smooth U bends with curvatures radii of 0.695Dh, 1.5Dh, and 2.0Dh, where Dh is the hydraulic diameter of the smooth tube, are used in both smooth and enhanced tubes. A comparison of thermal-hydraulic characteristics of dimpled and smooth U-tubes is carried out using steady-state Reynolds averaged Navier–Stokes simulations. The analysis shows that the performance of the dimpled U-tube is superior to the smooth tube for all bend curvatures. The swirl flow patterns generated by the dimples induce early flow reattachment in the postbend section of the tube, which enhances its heat transfer rate. The dimpled U-tube having the shortest curvature radius significantly alters Dean vortices, which leads to a substantial improvement in its heat and flow performances. The dimpled U-tube having the shortest curvature radius enhances the thermal–hydraulic performance by 21.4% while for other curvature radii (1.5Dh, and 2.0Dh), the performance augmentations are found to be 10.7% and 8.9%, respectively.

Secretory Vesicle and Glucoamylase Distribution in Aspergillus niger and Macromorphology in Regions of Varying Shear Stress.

In technical fermentations, filamentous microorganisms are exposed to different forms of mechanical stress, among which shear stress is prevalent in turbulent broths. Whereas small-scale bioreactors allow for realistic turbulent flow field conditions, they are not well-suited to investigate the fungal response to shear stress in more detail, as they only reveal the integral effect of a highly dynamic stress stimulus. Therefore, the widely used model system for producing constant, but rather low shear forces, the parallel plate flow chamber, is extended in this work by adding a backward-facing step (BFS). The BFS induces vortex shedding in the wake of the step and brings out distinct areas of different shear stress levels at the bottom of the chamber where mycelia grow. This allows for a stress-dependent analysis of growing cells using a confocal laser-scanning microscope. As the real stress cannot be measured in the experiment, the wall shear stress is estimated numerically using computational fluid dynamics (CFD). As a first application of the experimental setup, the relative biomass concentration, the relative amount of secretory vesicles and the relative amount of the chosen product glucoamylase produced by the filamentous fungus Aspergillus niger were measured. The obtained area scans show homogeneous mycelia growth in areas of low stress and cloud-like patterns downstream of the predicted flow reattachment length where high shear stress dominates. Quantitative analysis of the time course suggests that the amount of available secretory vesicles inside of A. niger decreases when the shear stress is increased, despite that no significant differences in biomass production could be found. In contrast, the highest level of glucoamylase was reached for intermediate volumetric flow rates, i.e., levels of shear stress.

A comparative study on post-stall flow separation control mechanism of steady and unsteady plasma actuators

In order to improve the aerodynamic performance of the wing at post-stall conditions, the experimental comparative investigations on the flow separation control over an ONERA 212 airfoil using steady and unsteady plasma actuators are carried out at Reynolds number of 3.1 × 105. The duty cycle ratio is fixed at 80%, and the non-dimensional unsteady frequency F+ is varied from 0.04 to 1. The lift coefficients are increased by 39.6% and 66%, respectively, after steady and unsteady operations (F+ = 0.08) at an angle of attack of 18°, which indicates that the unsteady actuation is more efficient than steady operation. Meanwhile, the study provides new insight into understanding the post-stall separation flow controlling mechanism. First, different from the general view that the injection of momentum is the controlling mechanism of steady operation, flow control using the steady actuation experiences four stages, namely, flow separation, promoting the instability of the separated shear layer to produce large-scale spanwise vortices, flow re-attachment, and the continuous generation of small-scale vortices in the separated shear layer. Second, flow control with the unsteady operation consists of several quasi-periodic flow processes. Each quasi-cycle is composed of three stages, namely, flow separation, promoting the separation of shear layer instability to produce large-scale spanwise vortices, and flow re-attachment. The off-time of the plasma actuator plays an important role in realizing the control effect of the unsteady actuation, and an effective strategy to promote the control effect of the unsteady operation is proposed based on the propagation time of the induced spanwise vortex.

Transition to turbulence in hypersonic flow over a compression ramp due to intrinsic instability

In this work, a transition process in a hypersonic flow over a cold-wall compression ramp is studied using direct numerical simulation (DNS) and global stability analysis (GSA). The free-stream Mach number and the Reynolds number based on the flat-plate length are 7.7 and $8.6 \times 10^5$ , respectively. The shock-induced pressure rise causes the boundary layer to separate on the flat plate, forming a separation bubble around the corner. Without introducing any external disturbances, the DNS captures the transition to turbulence downstream of flow reattachment. The DNS results agree well with the experimental data as well as theoretical predictions. To uncover the intrinsic instability in the flow system, GSA is employed to investigate the three-dimensionality of the two-dimensional base flow. Several stationary and oscillatory unstable modes are revealed, which result in spanwise periodicity inside and downstream of the separation bubble. The GSA and DNS results indicate that the intrinsic instability of the flow system triggers the formation of streamwise counter-rotating vortices and boundary-layer streaks near reattachment. The downstream transition to turbulence starts from the breakdown of the streamwise vortices and streaks. Moreover, the second harmonic of the most unstable global mode and a broadband low-frequency unsteadiness occur in the saturated flow, which has a significant influence on the transition process. In summary, the present study demonstrates a transition process in a hypersonic compression-ramp flow as a result of the intrinsic instability of the flow system.

Unsteady Aspects of Shock-Wave/Boundary-Layer Interaction Resulting from Control Surface Deflection

A computational investigation of a Mach 6.9 turbulent flow near the juncture (cove) of a wing’s trailing-edge and elevon was carried out. Turbulence was modeled using improved delayed detached eddy simulation (IDDES). The computational data were compared to the results of an experimental study, conducted by Deveikis and Bartlett, which simulated the flow over the windward surface of the NASA space shuttle. Flow visualization, surface variable comparison with experiment, and analysis of unsteady flow statistics were carried out. Reasonable agreement between prediction and measurement was obtained for time-averaged surface pressure and heat flux. Unsteady analysis of these quantities revealed spectra dominated by low-frequency energy content throughout the interaction region. A correlation analysis indicated coupling of the unsteady motions of the shear layer, shock wave, separation vortex, and flow reattachment. The results of the present study suggest that IDDES can be a valuable tool for the analysis of heating and unsteadiness in hypersonic gap flows.

Dynamic characterization of wind pressure fluctuations in separated and reattaching flows

Flow separation and reattachment over bluff bodies has been a primary concern in wind engineering community as it may generate large suction pressure. This study re-visits the characteristics of wind pressure underneath the separated shear layer, with emphasis on diagnosing the underlying dynamic properties. Nonlinear dynamic analysis techniques, namely recurrence plot and recurrence quantification analysis are applied to reveal and compare the wind pressure dynamics associated with different test conditions. The results indicate that the dynamic behaviour of wind pressure is closely tied with leading edge shape, turbulence intensity, as well as turbulence scale. Generally, the underlying dynamics of wind pressure become less deterministic as the measurement position moves from leading edge towards downstream. Such pattern is reversed once the descriptive indicator reaches its minimum. The position of such minimum value corresponds well to the downstream boundary of the reverse flow region. Moreover, the minimum value of dynamic descriptive indicator, as well as the reverse position in its distribution pattern, is correlated with the leading edge shape. On the other hand, the minimum value of dynamic descriptive indicator is also dependent on turbulence scale, while the reverse position in the distribution pattern of dynamic descriptive factor is dependent on turbulence intensity.

Investigation of local severe suction on the side walls of a high-rise building by standard, spectral and conditional POD

Local severe suction greatly determines the safe and economic wind-resistant design of cladding system in the façade of buildings. This study focuses on two localized regions with severe suction of a building, i.e., the lower windward corner of side wall and the region immediately upstream of trailing edge. The aspect ratio of building is 3. The characteristics and mechanism of wind peak pressures are investigated by standard, spectral and conditional proper orthogonal decomposition (POD). Spectral POD extracts the space–time modes which oscillate at the single frequency of the anti-symmetric vortex shedding, where the pressure distributions have the shape of ‘dragonfly’ wing and opposite signs on two side walls. Conditional POD is demonstrated to be a powerful tool to study the average evolution of peak pressures. By inspection of conditional POD modes, high positive fluctuating pressure is observed in the trailing portion of the lower side wall, accompanied by the minimum pressure in the windward corner. The identified pressure behaviors provide the statistical evidences for flow reattachment and shear-layer roll-up which were observed by instantaneous flow visualization in the literature. The trailing-edge peak pressure in the middle-height region is induced by trailing-edge vortex of great vorticity concentration. The flow mechanism and occurrence phase of trailing-edge peak pressures are similar to those of infinite-length cylinders.

Effects of jet-to-target spacing on primary and secondary peak heat transfer for an obliquely impinging jet

This paper presents an experimental investigation on the primary and secondary peak heat transfer characteristics of an obliquely (45°) impinging jet on a flat and smooth target surface. Local convective heat transfer coefficients have been calculated through a transient liquid crystal thermography technique employing one-dimensional semi-infinite conduction model for the target surface. The effects of jet Reynolds number (Re) and jet-to-target spacings (z/d) have been investigated where Re was varied from 9,000 to 18,000 and z/d was varied from 2.5 to 6. Local heat transfer measurements reveal several important features of oblique jet impingement, e.g., the location of the primary and secondary peaks with respect to the geometric center (GC), its dependence on Re and z/d, and the shape of enhanced heat transfer zone with stagnation, separation, and reattachment features. The two-dimensional heat transfer map is presented in a normalized form Nu/Numax, where Numax is the primary peak heat transfer. It has been found that for a certain z/d, the primary peak occurs at a fixed distance from the geometric center for the range of Re investigated, while the secondary peak location varies with Re. For the smallest z/d, the primary peak heat transfer location was offset by one jet diameter from the GC in the uphill direction, whereas for the moderate z/d = 4, the primary peak occurred near the GC. For the highest z/d, the primary peak was again in the vicinity of GC, however, slight offset on the downhill side. The spent flow reattachment (post the first impingement) which also determined the secondary peak location and heat transfer level was strongly dependent upon the Re. The oblique jet impingement radial variation is also compared against 90° (orthogonal) jet impingement at z/d = 2.5 and 6 at Re = 18,000. It has been observed that the primary peak for above two z/d configurations were sharper for the 45° case when compared to 90°, and the secondary peak was further offset in the downhill direction with higher heat transfer levels for the 45° impingement case compared to 90°.

Interaction of synthetic jets with a massively separated three-dimensional flow field

The interaction of an array of finite span synthetic jet actuators with a massively separated flow was explored experimentally over a cantilevered, swept, and tapered model having a deflected control surface. The interaction of one synthetic jet with the local flow led to a flow field associated with formation and advection of vortex rings that bent into the flow, resulting in flow reattachment in their trajectory. Under specific conditions, when multiple jets were actuated, their interaction was either constructive or distractive, depending on the separation severity and spanwise flow magnitude. This work improves control of three-dimensional flow separation using distributed actuators.

Combined effects of bleed hole extraction and rotation on internal heat transfer in a ribbed two-pass channel

Serpentine passages arranged with ribs, are commonly used to guide the cooling air and remove heat transferred from hot gas. In this study, the effects of bleed hole extraction on internal heat transfer of ribbed channel are numerically studied in a rotating frame. Three positions of bleed hole are studied as rotation number ranges from 0 to 0.2. The flow structure and heat transfer features are depicted and evaluated through qualitative and quantitative analysis. Numerical results reveal that heat transfer distribution features are quite different for channel with bleed holes at different locations. The allocation of bleed hole extraction flowrate varies distinctively under rotating condition. Rotation is responsible for affecting the flow reattachment and corresponding secondary vortex structures, and consequently heat transfer features. Channels rotated in clockwise and counter-clockwise direction share opposite Coriolis force acting direction, thus effects of heat transfer augmentation are different. The effects of bleed hole position on local heat transfer are more pronounced on the endwall opposite to the endwall where the Coriolis force acts. Channel with bleed holes at upstream position shows the poorest heat transfer enhancement at inter-rib regions, with maximum reduction by 11.4% under stationary condition and 13.2% under rotating condition.

Numerical Investigation of Plasma Actuator Effects on Flow Control Over a Three-Dimensional Airfoil With a Sinusoidal Leading Edge

Abstract The impetus of the present bio-inspired work is to investigate the impact of simultaneously using wavy leading-edge (WLE) airfoils in combination with curved multidielectric barrier discharge (DBD) plasma actuators as hybrid passive and active flow control mechanisms, respectively. A precise distinction of the produced frequency and noise signals, altogether with the acoustic effect of using WLE and DBD plasma actuators, is herein analyzed with precision. Two specific DBD plasma actuators are designed to actuate at x/C = 3% and x/C = 30% on a NACA 634-021 airfoil with sinusoidal WLE that bears a wavelength of 25% and an amplitude of 5% of the mean chord length and straight-leading-edge (SLE). A large eddy simulation (LES) turbulence model was used. This includes the dynamic control of unsteady flow separation, the three-dimensional vortical structure and induced trains of vortices, the aerodynamic forces, the velocity variation, and also the spanwise flow. The momentum transfer between the main flow and boundary layer was improved by the DBDs-induced vortices train and formed streamwise counter-rotating pair-of-vortices over the tubercle. Also, both the continuous wavelet transform (CWT) and fast Fourier transform (FFT) methods were used to investigate the induced plasma flow spectral content for the WLE and SLE geometries. We witnessed an optimized flow control, by using DBD plasma actuators with the WLE airfoil, that resulted in less massive flow separation, faster turbulent transition, and a robust earlier flow reattachment. This modification was beneficial in increasing the efficiency and decreasing the noise for low Reynolds number operational conditions.

Aerodynamic Control of a Dynamically Pitching Airfoil Using Pulsed Actuation

Transitory control and regulation of the aerodynamic loads on a static and dynamically pitching VR-12 airfoil is investigated in wind tunnel experiments. Actuation is enabled by a spanwise array of integrated combustion-based actuators that are triggered intermittently relative to the airfoil’s motion, producing high-impulse actuation jets on characteristic time scales that are an order of magnitude shorter than the airfoil’s convective time scale. Poststall transitory actuation leads to a momentary reattachment of the flow over the airfoil, and the effects of sequencing of actuation pulses on the aerodynamic characteristics of the airfoil are assessed using time-resolved measurements of the lift force and pitching moment coupled with particle image velocimetry that is acquired phased-locked to the airfoil motion. The present investigations show that the aerodynamic loading can be significantly altered by a number of actuation programs using strategically timed bursts during the pitch cycle. While actuation during the upstroke primarily affects the formation, evolution, and advection of the dynamic stall vortex, actuation during the downstroke accelerates flow reattachment. Superposition of such actuation programs can lead to enhancement of cycle lift and pitch stability, and reduced cycle hysteresis.

Uncertainty and sensitivity analysis of heat transfer in hypersonic three-dimensional shock waves/turbulent boundary layer interaction flows

Hypersonic shock waves/turbulent boundary layer interaction flow can cause severe localized thermal loads, while the fluctuations in freestream conditions and inevitable mechanical vibrations can strongly influence the flow structures and heat transfer. This study aims to quantify the uncertainties in heat transfer and pressure of fin-plate configurations and find their main contributors. Freestream velocity, freestream static pressure, freestream static temperature, wall temperature, and fin angles are chosen as uncertainty sources. The point-collocation non-intrusive polynomial chaos method is employed to propagate the uncertainties. Subsequently, Sobol indices are utilized to denote the contribution of each input parameter. The uncertainty and sensitivity results are plotted along representative lines and on the entire bottom surface to obtain the key parameters of uncertainties. As a consequence, the uncertainty in heat transfer can exceed 30%, which is higher than that in pressure. Freestream velocity, freestream static pressure, and fin angles are the key parameters of heat transfer. Freestream velocity is the biggest contributor to the uncertainty in heat flux near the heat flux peak by influencing flow reattachment. Fin angles dominate the developments of primary vortexes and are the source of the uncertainties near the leading edge of fins. The Sobol index of freestream static pressure is relatively high at the separation line, which reveals that the boundary of flow separation is sensitive to static pressure.

Topology of flow and heat transfer from prisms in square array

Structures in an array with multiple rows and columns may undergo both inline and side-by-side interferences. A system of nine square prisms in a 3 × 3 square array is numerically investigated for pitch ratio L/D = 1.2 – 7.0 at a Reynolds number of 150, where L is the spacing between the centers of two adjacent prisms, and D is the side width of a square prism. The focus is given on investigating the effect of L/D on flow topology, fluid forces, heat transfer, vortex shedding, and recirculation bubbles. An increase in L/D from 1.2 to 7.0 leads to evolutions of five distinct flows: single bluff body flow (L/D < 1.6), reattachment flow (1.6 < L/D < 3.3), lateral-interaction-induced coshedding flow (3.3 < L/D < 4.1), mixed flow (4.1 < L/D < 4.6) and free coshedding flow (4.6 < L/D < 7.0). The reattachment flow corresponds to small fluid forces while the lateral-interaction-induced coshedding flow induces large fluid forces. The time-mean drag force coefficient of the center prism drastically increases from 0.09 to 1.0 as the flow evolves from the reattachment flow to the lateral-interaction-induced coshedding flow. The maximum heat transfer from the center prism, 10% higher than that from a standalone prism, occurs for the lateral-interaction-induced coshedding flow where the flow around the center prism resembles creeping flow, no recirculation bubble forming on the front or rear surface of the prism. The coherence between heat transfer and flow patterns is discussed, including the impacts of shear layer reattachment, flow recirculation, and vortex shedding on heat transfer.