Separated Flow Region Research Articles

Thermographic detection of turbulent flow separation on rotor blades of wind turbines in operation

An IR thermography-based detection and localization of turbulent flow separation at an operating wind turbine is presented and verified for the first time. Turbulent flow separation limits the efficiency of wind turbines and causes increased structural loads and acoustic emissions. IR thermography is an established measurement method for stall detection in wind tunnel experiments, however a transfer to operating wind turbines is an open research question. With respect to the state of the art for thermographic stall detection, a novel thermographic measurement approach for a feature-based stall detection is presented, verified and applied on wind turbines. The measurement approach evaluates the surface temperature response to unsteady inflow conditions and enables an unambiguous detection of flow separation by means of temperature fluctuation maxima in the regions of flow transition as well as an increasing temperature fluctuation within the separated flow region. Finally, the aim to obtain a non-invasive and in-process detection of flow separation on an operating wind turbine with IR thermography is achieved and verified using tufts flow visualization.

Experimental investigation of expansion effect on shock wave boundary layer interaction near a compression ramp

The experiment is conducted to investigate the effect of expansion on the shock wave boundary layer interaction near a compression ramp. The small-angle expansion with an angle degree of 5° occurs at different positions in front of the compression ramp. The particle image velocimetry and flow visualization technology show the flow structures, velocity field, and velocity fluctuation near the compression ramp. The mean pressure distribution, pressure fluctuation, and power spectral density are measured by high-frequency response pressure transducers. The experimental results indicate that the expansion before the compression ramp position affects the shock wave boundary layer interaction to induce a large-scale separation. But the velocity fluctuation and pressure fluctuation are attenuated near the large-scale flow separation region. When the expansion occurs closer to the compression ramp, the expansion has a more significant impact on the flow. The fluctuation of velocity and pressure is significantly attenuated, and the wall pressure rise of the separation point is reduced obviously. And the characteristic low-frequency spectrum signal related to the unsteadiness of the shock wave boundary layer interaction is significantly suppressed. In addition, variation of the separation region scale at different compression angle degrees is distinctive with the effect of expansion.

Detection and localization of flow separation on wind turbines by means of unsteady thermographic flow visualization

For the first time, a thermographic detection and localization of turbulent flow separation on an operating wind turbine is presented and verified. Flow separation on wind turbine rotor blades causes power reduction, structural loads and increased noise emissions. In contrast to established methods for stall detection, the presented infrared thermographic measurement approach is non-invasive, in-process capable and provides a high spatial resolution. With respect to the state of the art for thermographic stall detection in wind tunnel experiments, the thermal surface response to unsteady inflow conditions is evaluated for measurements on an operating wind turbine, in order to achieve unambiguous thermographic features for the detection of flow separation. The evaluation of the thermodynamic response behavior shows a clear detection of flow separation by means of temperature fluctuation maxima in the regions of flow transition as well as an increasing temperature fluctuation within the separated flow region. In addition, a geometric assignment is conducted which enables a localization of the separation point with an uncertainty of 0.6% of the chord length. The detection and localization of flow separation is verified by means of tufts visualization.

Direct numerical simulations of aerodynamic performance of wind turbine aerofoil by considering the blades active vibrations

In the present study, the aerodynamic performance of the horizontal-axis wind turbine blades by considering the flap-wise oscillations are numerically investigated by using direct numerical simulations (DNS). The details of flow structure can be analysed and predicted by performing DNS over an oscillating blade by considering the realistic behaviour of the wind turbine blade structure with natural vibration frequencies. In this study, the impact of vibrations on the flow separation point, laminar separation bubble (LSB) and stall over NACA-4412 aerofoil are investigated utilising the high-fidelity spectral-hp element methodology. The Reynolds number and angle of attack were selected in the range of 50,000≤Re≤75,000 and 8°≤AoA≤16°. It is found that the blade vibrations have a noticeable impact on the aerodynamic performance and delay the stall occurrence, and the lift remains high even at higher AoAs, in comparison with the stationary blade. The size of the flow separation is reduced by the blade oscillation and the vibration also affects the separation point. Due to the harmonic oscillation of the blade, the pressure signals are periodic, and the pressure fluctuations are amplified by the oscillations, especially in the flow separation region. The time-averaged lift coefficient is increased by 255.3% by raising the angle of attack, from 0° to 12° at Re = 75,000. Compared to Re = 50,000, the peak-to-peak amplitude for the angle of attack of 0° is higher, whereas that of 8° and 12° are slightly lower at Re = 75,000.

Computational Prediction of the Performance Map of a Transonic Axial Flow Compressor

Aviation fuel efficiency is an important target for aviation industry. Aircraft engine compression ratio is a key factor to improve fuel consumption. Compression ratio can be increased using transonic compressor. In this study, performance prediction of a transonic axial compressor at design and off-design operating conditions is investigated numerically using ANSYS-CFX software. The compressor is NASA Rotor 37. Firstly, the performance at design point is predicted, where mesh independence study is performed to determine suitable mesh size. Three-dimensional flow details for meridional plane, blade-to-blade plane and airfoil surface are explored. The design point study successfully captured flow features such as shock waves and flow separation regions. When compared with experimental data, the predicted compressor pressure ratio deviation error is less than 5%. 3D flow details show that shock wave strength increases from hub to tip. The shock wave moves backward as we move from hub to tip indicating that the flow separation covers lesser portion of the blade. Secondly, off-design performance is predicted for various rotational speeds. A simple procedure is utilized to predict surge and choke limits. The predicted compressor map is compared with experimental data and it shows overall root mean square error less than 5%. The success of the method developed in this research make it a viable method to be used in the design phase of transonic compressors to evaluate the effect of design modifications for both design and off-design operating conditions.

Investigation on shock-induced separation loss mitigation method considering radial equilibrium in a transonic compressor rotor

AbstractA shock-induced separation loss reduction method, using local blade suction surface shape modification (smooth ramp structure) with constant adverse pressure gradient with the consideration of radial equilibrium effect to split a single shock foot into multiple weaker shock wave configuration, is investigated on the NASA Rotor 37 for promoting aerodynamic performance of a transonic compressor rotor. Numerical investigation on baseline blade and improved one with blade modification on suction side has been conducted employing the Reynolds-averaged Navier–Stokes method to reveal flow physics of ramp structure. The results indicate that the passage shock foot of baseline is replaced with a family of compression waves and a weaker shock foot generating moderate adverse pressure gradient on ramp profile, which is beneficial for mitigating the shock foot and shrinking flow separation region as well. In addition, the radial secondary flow of low-momentum fluids within boundary layer is decreased significantly in the region of passage shock-wave/boundary-layer interaction on blade suction side, which mitigates the mass flow and mixing intensity of tip leakage flow. With the reduction of flow separation loss induced by passage shock, the adiabatic efficiency and total pressure ratio of improved rotor are superior to baseline model. This study herein implies a potential application of ramp profile in design method of transonic and supersonic compressors.

Aerodynamic Interactions of Side-by-Side Rotors in Ground Proximity

An experimental investigation is conducted to study the aerodynamic behavior of a two-rotor system in ground proximity. The counter-rotating rotors are placed side-by-side in the hovering condition. The time-averaged and unsteady flow behavior is studied when the rotor-to-rotor lateral distance and the distance between the rotors and the ground are varied. The experiments are performed using three-dimensional large-scale volumetric velocimetry with helium-filled soap bubbles as tracers, tracked by the particle motion analysis technique “Shake-The-Box.” The mean velocity field reveals the wake deflection due to the ground plane and the formation of toroidal-shape regions of separated flow below each rotor. The interaction of the wall jets formed by slipstream deflection results in a separation line with the flow emerging from the wall in a fountain-like pattern. Regimes of flow re-ingestion occur when the rotors are sufficiently far apart. The flowfield exhibits the tendency toward asymmetric states, during which the fountain flow column and the domain of re-ingestion shift closer to one of the rotors. A generic classification of flow regimes is proposed in relation to the behavior of two rotors in ground effect.

A numerical investigation on flow past skewed vortex generators ahead of a backward facing ramp

Flow past a backward facing ramp (BFR) with rectangular vane-type vortex generators (VGs) located upstream has been studied numerically using OpenFOAM based steady-state RANS simulations. In particular, single and multiple pair(s) of boundary layer height VGs are skewed at 10° and 30° to study their effects on the flow separation behaviour at Re=3×106. Streamwise and cross-stream results show that single VGs produce counter-rotating streamwise vortices with increasingly different vortex-core strengths and vortical interactions when skewness angle increases. At 30° however, co-rotating vortices are formed instead with significantly heightened vortical interaction levels, leading to asymmetric flow separation and reattachment behaviour. In particular, the use of multiple VGs under the same condition further accentuate these behaviour and results in significant changes to the wall shear stress distribution. Clarifications on how the flow separation region is distorted by the symmetric/asymmetric streamwise vortices based on velocity component analysis are also provided. Lastly, trajectories of the streamwise vortices and vortex-core characteristics support the notion that the streamwise vortices behave significantly more non-linearly at 30° skewness angle here, and that skewing the present VGs such that they produce co-rotating vortices instead of counter-rotating ones leads to very different flow separation control characteristics.

The double backward-facing step: interaction of multiple separated flow regions

The backward-facing step is perhaps the quintessential geometry used to study separated flow. Extensive previous research has quantified its detailed flow characteristics. However, often regions of separated flow do not exist in isolation; rather, interaction occurs between multiple regions. This motivated an experimental investigation into the time-averaged and dynamic flow features of a double backward-facing step, covering separations of zero to eight step heights between equal-height steps. Three flow regimes are identified. A single reattachment regime occurs for separations of less than four step heights, perhaps remarkable for the lack of variation in key flow characteristics from a single backward-facing step response. Next, an intermediate regime is identified for a separation of four step heights. In this case, the flow does not yet reattach on the first step, although significant differences in reattachment length, surface pressure on the vertical step faces and turbulence statistics occur. Finally, for greater step separations, a double reattachment regime, with reattachment on both steps, is identified. Downwash, induced by the first recirculation zone, reduces the reattachment length and turbulent fluctuations of the second recirculation zone. The surface pressure on the first-step vertical face is reduced, seemingly a result of an upstream influence due to the low pressure in the second-step recirculation zone. Detailed characterisation of the regimes offers insight into the fundamental interaction of regions of separated flow, revealing aspects of complex dynamics relevant to a broad range of practical scenarios.

Three-Dimensional Analysis of the Influence of the Magnetic Flux Density on Minimum PR in a Faraday-Type MHD Channel

The magnetohydrodynamic (MHD) behavior and performance characteristic have been revealed by 3-D numerical simulation of the Faraday-type MHD generator. Numerical results show that the generator inlet–outlet pressure ratio (PR) will affect the plasma behaviors and generator performance in the channel. At high PR, the plasma flows at supersonic, and higher electric power [enthalpy extraction rate (EE)] can be obtained. However, at low PR, boundary layer separation occurs with deflected flow of velocity streamlines and vortex in the separated flow region, owing to the oblique shock wave. Therefore, there exists a minimum PR to maximize power generation performance. Under the strong MHD interaction, the Lorentz force causes the loss of Mach number and the increase of static pressure, but it can alleviate the effect of oblique shock for a lower PR. Furthermore, suppressing the boundary layer separation effect gradually evolves into a normal shock wave. When PR is higher, the thermoelectric conversion efficiency increases, and then a weak shock wave will be generated in the channel, which will propagate upstream and evolve into a normal shock wave. Consequently, under a higher magnetic flux density, the minimum PR can be effectively reduced, and the best generator performance can be obtained. In addition, an “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> ”-shaped velocity profile will be generated due to the significantly increased MHD interaction. The results calculated in the present simulation are valuable and important for setting working gas conditions and evaluating generator performance.

Numerical investigation of double sided plasma vortex generator in separation control

Control of flow separation is a great issue to deal with a moving body to ensure its proper aerodynamic characteristics. To achieve this, various methods including active and passive control are suggested depends upon the flow characteristics and the surface in which control is necessary. To make the better use of both active and passive method of flow control this article proposed a new type of double sided plasma actuator (DSPVG) to overcome the drag penalty of conventional vortex generators (VGs) that commonly used in controlling flow and to use actively control. In this regard, the effectiveness of DSPVG has been numerically and experimentally investigated in a separated flow region of a 20° diffuser of an open type tunnel. DSPVG is placed at the upstream of separation location normal to the surface as like as conventional VG except zero angle with flow direction. Both numerical and experimental results of DSPVG are compared with that of VG and baseline flow and better agreements are found. Moreover, DSPVG has shown better separation suppression ability than conventional VGs due to its dual vortices. It is found that DSPVG significantly delay the separation. A freestream flow of 4 m s−1 is used for experiments and numerical computations.

Numerical investigation of a Mach 6 hypersonic laminar flow on two-dimensional cold-wall compression corners with controlled surface roughness

Numerical simulations are performed in order to investigate the physics of a Mach 6 hypersonic, laminar flow on different configurations of a two-dimensional compression corner. For this geometry, typically characterising control surfaces of re-entry vehicles, the interaction between the incoming boundary layer and the ramp-induced shock wave can lead to the formation of a separated-flow region. The numerical experiments, carried out under adiabatic and a cold-wall conditions, include the presence of a two-dimensional sinusoidal roughness patch, which is placed at various locations upstream of the ramp, in order to evaluate the effects of the surface waviness on the separation bubble. When the roughness elements are placed upstream of the smooth separation point, the recirculation bubble stretches towards the roughness position. This behaviour is induced by the thickening of the boundary layer downstream of the roughness patches, which cause a displacement of the laminar flow. No transition is observed downstream of the patches under the investigated geometrical and flow conditions. For a patch downstream of the smooth separation point, instead, no appreciable effects on the bubble length are noticed. However, the surface roughness inside the bubble can alter the recirculation pattern within the reverse-flow region. Hence, together with the length of the separated flow region, also the heat transfer at the wall is affected by the roughness because of the different recirculation patterns inside the bubble. In particular, the Stanton number highlights a redistribution of the heat-flux intensity between the main peak at reattachment and the secondary peaks inside the separated flow. This effect is more evident for higher ramp angles. These results highlight the importance of roughness-induced effects on the complex phenomenon that shock-wave/boundary-layer interaction is.

Wind load evaluation on storm shelters using wind tunnel testing and North American design codes

In Canada, the number of extreme wind events is increasing due to climate change. Accordingly, storm shelters are becoming more popular, as it provides a life-saving solution during wind events. Currently, the National Building code of Canada (NBCC 2015) and the American Society of Civil Engineers (ASCE 7-16) codes consider storm shelters as low-rise buildings with high importance factor with no specific guidelines for their smaller scale. The current study aims at comparing North American codes for designing wind shelters to a detailed experimental wind simulation using boundary layer wind tunnel testing under the synoptic wind. Wind tunnel tests were conducted at Ryerson University for three commonly used shapes of storm shelters. The governing wind loads from the wind tunnel results were compared to the design codes for both structural forces and cladding pressures. The study also illustrates and quantifies critical zones that may exceed the codes provided values due to being flow separation regions.

마이크로 가스터빈 엔진에 적용된 압축기 공력성능 특징

A centrifugal compressor is essentially applicable to a micro gas turbine because it can produce a high-pressure ratio over than 3.0 within a limited geometrical allowance. In this paper, aerodynamic design and internal flow characteristics of the centrifugal compressor were described in detail by comparing them with those of turbocharger compressors. Then a centrifugal compressor design procedure with a pressure ratio of 5.0 at the design point for a 1,000 N thrust micro gas turbine has been introduced and the internal flow field at the design point has been further investigated with CFD methods. High impeller pressure ratio led to high absolute Mach number at the impeller outlet, however, small gap between the impeller outlet and diffuser inlet did not sufficiently recover static pressure and it caused large flow separation region and corresponding total pressure at the diffuser hub. For this reason, it requires sophisticated matching between impeller and diffuser stages in micro gas turbines. Based on the internal flow analysis, ideas for further performance improvements have been proposed while maintaining essential compressor performances.

Аtmospheric wind flow around the mountain landscape in the vicinity of Danang airport and flight safety issues

Wind boundary layer flow over the mountain landscape and large structures located around runways (RWs) creates coherent vortex structures (CVSs) that can cross a glideslope and airspace in the vicinity of an airport. The aircraft, encountering a vortex structure, experiences significant changes of the aerodynamic forces and moments, what is especially hazardous due to proximity to terrain. From a mathematical point of view, the solution of this problem presents a challenge due to extremely large space – time scale of the phenomenon, the lack of relevant atmospheric models, as well as comprehensive initial – boundary conditions in numerical modeling. In this paper, a composite solution is constructed: the CVSs area generation is computed in sufficient details within the framework of the grid method. Based on the data obtained in the approximation of analytical functions, an initial vortex structure is formed, the evolution and stochastics of which are modeled within the potential approximation by means of Rankine vortices. The evaluation of the forces and moments increment from the impact of vortex structures on the aircraft was carried out by the panel method using the engineering approach. As an example, the CVSs, resulting from wind flow around the mountainous area of the Son Tra Peninsula, that is located short of RWs 35R-17L and 35L-17R of Da Nang airport, are investigated. To improve the computational grids quality and verify the method of solving the boundary value problem for the Reynolds-averaged Navier-Stokes equations, we used the criteria based on the principle of maximum pressure, requiring Q-parameter positivity property in the vortices cores and flow separation regions. A CVS related aviation event, involving a passenger aircraft MC-21, is studied. The aircraft, after takeoff from RW 35R-17L setting the course close to the direction of the vortex wind structure axis from the Son Tra Peninsula, encountered the mountainous area CVS.

The impacts of the free-surface and angle of attack on the flow structures around a torpedo-like geometry

This study presents the hydrodynamic characteristics around a torpedo-like geometry under the free-surface effect at different angles of attack using preliminary dye visualization and Particle Image Velocimetry (PIV). The optimized torpedo-like geometry is placed at submersion ratios between 0.5≤h/D≤3.50 where h is the distance to the free-surface from its centerline and D is the diameter. Throughout the experiments, angles of attack were taken as 0°≤α≤12° for two Reynolds numbers, Re = 2.0 × 104 and 4.0 × 104. The PIV method provided instantaneous vorticity and time-averaged velocity components, vorticity, streamline topology, fluctuating velocity components, Reynolds stress correlation, and the turbulent kinetic energy. This study focused on the stern section and the wake structures at h/D=1.0 . It is demonstrated that a jet-like flow region occurred between the model and the free-surface for all angles of attack at small submersion ratios of 0.5≤h/D≤1.0 while it is observed at h/D=1.5 for α=8°, a jet-like flow region occurred between the model and the free-surface. The impact of the jet-like flow was more noticeable for α=4° and 8° with velocity fluctuations in lower magnitudes. The nose section partially pierced the free-surface for h/D = 0.5, 0.75, and 1.0 in the range of 4°≤α≤12° and it prevented the wake region from connecting with the free-surface by directing the separated flow region toward the stern section. Pointwise variations of the turbulence data extracted from vertical lines within the wake region for all cases revealed that the effect of the free-surface on the turbulence statistics was negligible beyond h/D=2.0.

Numerical Investigation of Sweep Effect on Turbulent Shock-Wave Boundary-Layer Interaction

Implicit large-eddy simulations of reflecting shock-wave turbulent boundary-layer interactions at a freestream Mach number of 2.05 were carried out for sweep angles of 0, 20, and 40 deg. The simulation for the unswept interaction reveals the shedding of spanwise coherent structures as well as a low-frequency unsteadiness at separation. Intermittent spanwise deformations of the separation line, also referred to as rippling, are observed with a frequency similar to that of the low-frequency unsteadiness. Interactions with 20 and 40 deg sweep are generated by adding a spanwise approach flow component. Because the inviscid shock system is not affected by the spanwise velocity component, the pressure rise across the impinging shock is the same as for the unswept interaction, and the resulting mean flowfields are similar. This approach, which enforces spanwise periodicity, can be understood as a model for interactions with perfect cylindrical similarity. For the swept interactions, the separated flow regions are enlarged, and the overall level of the pressure fluctuations is increased, although the relative increase of the pressure fluctuations over the interaction is reduced. For the swept cases, the low-frequency unsteadiness is shifted to higher frequencies, and the low-frequency amplitude is reduced compared to the unswept case. For 40 deg sweep, the rippling of the separation line is diminished, and oblique structures are observed downstream of the interaction.

Flow separation control in S-shaped ∼inlet with a nanosecond pulsed surface dielectric barrier discharge plasma actuator

A nanosecond pulsed surface dielectric barrier discharge (nSDBD) plasma actuator is proposed to eliminate the complex 3D flow separation in an S-shaped ∼engine inlet. A set of wind tunnel experiments based on pressure measurements are conducted to verify the flow control effectiveness and obtain the influence laws of actuation voltage, pulse frequency, actuator layout and coverage area. The results show that nSDBD can effectively eliminate flow separation, leading to a noticeable improvement in the total pressure distribution at the outlet, manifested as a reduction in the low-pressure zone and an extension of the high-pressure zone. The spanwise layout nSDBD delivers a better flow control effect compared to the streamwise layout. For the same geometrical layout, the flow control effect improves with increasing voltage, and the optimal actuation frequency corresponds to a Strouhal number of approximately F + = 0.5. In particular, with a spanwise array of nSDBD plasma actuators operating at V p-p = 10 kV and F + = 0.5, the flow separation region is completely suppressed. The mechanism of flow separation with nSDBD can be ascribed to the mixing enhancement between the boundary layer and the main flow, which improves the boundary layer’s ability to resist the adverse pressure gradient.

Flow structure and surface heat transfer from numerical predictions for a double wall effusion plate with impingement jet array cooling

To provide additional understanding of double wall cooling arrangements, especially local distributions of flow properties which are responsible for hot-side surface and cold-side surface heat transfer variations, investigated are numerically-simulated distributions of turbulent flow structural characteristics. Also considered are numerically-simulated surface heat transfer characteristics, including comparisons with experimentally-measured distributions. The numerical results are obtained using the ANSYS FLUENT Version 19.1 numerical code, with a k-ω SST turbulence model. The present arrangement includes a full-coverage effusion cooling plate, with coolant initially supplied by an impingement jet array. Considered are the effects of effusion blowing ratio, impingement jet Reynolds number, and streamwise development on flow structure, and on hot-side and cold-side surface heat transfer characteristics. Data are obtained for an approximately constant main flow Reynolds number of 138000 to 159000, with blowing ratios from 2.2 to 8.5, which correspond to impingement jet Reynolds numbers from 7360 to 25200. Of particular importance are horseshoe-shaped vortices, which form upstream and around each effusion jet, which then result in two horseshoe-originated vortex legs which advect downstream along coolant trajectories. Also important are secondary flows, local flow separation regions, and local velocity regions, which are evident within the effusion hole passages near entrance locations. Within the cross flow passage, locally increased flow velocities, associated with impingement jet locations, are apparent, especially near the lower surface of the cross flow passage. These are associated with regions of augmented streamwise vorticity, which are present around the circumference of each impingement jet.

Experimental Study of the Leakage Flow in an Axial-Flow Fan at Variable Loading

The present paper reports a 2D-PIV (particle image velocimetry) study of the effect of the operating point on the leakage flow in a low-speed ring fan. First, the flow pattern has been studied at 12 operating points covering the whole characteristic curve. At very low loading, the leakage flow streams along the rotor ring and is directly reingested; then, a separation bubble attached to the ring forms that, approaching the design point, modifies in a flow streaming radially outward. As the loading further increases, a separated flow region appears in the blade tip region that finally merges with the leakage flow. A further, more detailed study has been performed at eight operating points in the neighborhood of the design one. Very small loading variations may yield the leakage flow pattern modification, but no intermittence is present during the transition, as instantaneous flow patterns of any intermediate type continuously alternate. These results provide a consistent explanation for the ones of previous acoustic measurements.