Unsteady Separated Flow Research Articles

Numerical investigation on aerodynamic performance of a bionic flapping wing

This paper numerically studies the aerodynamic performance of a bird-like bionic flapping wing. The geometry and kinematics are designed based on a seagull wing, in which flapping, folding, swaying, and twisting are considered. An in-house unsteady flow solver based on hybrid moving grids is adopted for unsteady flow simulations. We focus on two main issues in this study, i.e., the influence of the proportion of down-stroke and the effect of span-wise twisting. Numerical results show that the proportion of down-stroke is closely related to the efficiency of the flapping process. The preferable proportion is about 0.7 by using the present geometry and kinematic model, which is very close to the observed data. Another finding is that the drag and the power consumption can be greatly reduced by the proper span-wise twisting. Two cases with different reduced frequencies are simulated and compared with each other. The numerical results show that the power consumption reduces by more than 20%, and the drag coefficient reduces by more than 60% through a proper twisting motion for both cases. The flow mechanism is mainly due to controlling of unsteady flow separation by adjusting the local effective angle of attack. These conclusions will be helpful for the high-performance micro air vehicle (MAV) design.

Numerical Investigation on Unsteady Separation Flow Control in an Axial Compressor Using Detached-Eddy Simulation

Unsteady excitation has proved its effectiveness in separation flow control and has been extensively studied. It is observed that disordered shedding vortices in compressors can be controlled by unsteady excitation, especially when the excitation frequency coincides with the frequency of the shedding vortex. Furthermore, former experimental results indicated that unsteady excitation at other frequencies also had an impact on the structure of shedding vortices. To investigate the impact of excitation frequency on vortex shedding structure, the Detached-Eddy Simulation (DES) method was applied in the simulation of shedding vortex structure under unsteady excitations at different frequencies in an axial compressor. Effectiveness of the DES method was proved by comparison with URANS results. The simulation results showed a good agreement with the former experiment. The numerical results indicated that the separation flow can be partly controlled when the excitation frequency coincided with the unsteady flow inherent frequency. It showed an increase in stage performance under the less-studied separation flow control by excitation at a certain frequency of pressure side shedding vortex. Compared with other frequencies of shedding vortices, the frequency of pressure side shedding vortex was less sensitive to mass-flow variation. Therefore, it has potential for easier application on flow control in industrial compressors.

Unsteady separated stagnation-point flow and heat transfer past a stretching/shrinking sheet in a copper-water nanofluid

Purpose The purpose of this paper is to theoretically investigate the unsteady separated stagnation-point flow and heat transfer past an impermeable stretching/shrinking sheet in a copper (Cu)-water nanofluid using the mathematical nanofluid model proposed by Tiwari and Das. Design/methodology/approach A similarity transformation is used to reduce the governing partial differential equations to a set of nonlinear ordinary (similarity) differential equations which are then solved numerically using the function bvp4c from Matlab for different values of the governing parameters. Findings It is found that the solution is unique for stretching case; however, multiple (dual) solutions exist for the shrinking case. Originality/value The authors believe that all numerical results are new and original, and have not been published elsewhere.

Leakage Noise and Related Flow Pattern in a Low-Speed Axial Fan with Rotating Shroud

The effect of rotational speed and pressure rise on the leakage flow noise radiated by a low-speed axial fan, provided with rotating shroud, has been systematically investigated. The flow in the gap region has been studied by means of particle image velocimetry (PIV) measurements taken in the meridional plane. At low blade loading, the leakage flow is restrained close to the rotor ring and, at higher loading, it forms a wide recirculation zone. In the latter conditions, an unsteady flow separation likely takes place in the blade tip region which may be observed in the instantaneous flow field only. The leakage flow noise generally increases with the blade loading, but is non-monotonic, as the overall sound pressure level (OASPL) growth is interrupted by local minima; such a trend is qualitatively independent of the rotational speed. As the loading increases, the sound pressure level (SPL) spectrum shows important modifications, since the characteristic frequency of the subharmonic narrowband humps related to the leakage noise decreases; furthermore, height and width of the humps vary non-monotonically. Such a complicated behavior is likely related to the modifications in the leakage flow pattern and also to the appearance of the flow separation at the blade tip.

Coherent Structure Interaction During Unsteady Separation

Unsteady flow separation in rotationally augmented flow fields plays a significant role in the aerodynamic performance of industrial rotors, including wind turbines. Current computational models underestimate the aerodynamic loads due to the inaccurate prediction of the emergence and severity of unsteady flow separation, especially in response to a sudden change in the effective angle of attack. Through the use of time-resolved particle image velocimetry (PIV), coherent structure formation during the unsteady separation over an experimental wind turbine blade is examined. Time-dependent empirical mode decomposition results during a dynamic pitching cycle give insight into the spatio-temporal scales that influence the transition from attached to separated flow. Empirical mode decomposition (EMD) modes are represented as two-dimensional fields and are analyzed together with Lagrangian coherent structures, the spatial distribution of vortices, the location of the separation point, and velocity contours focusing on the role of vortex shedding and shear-layer perturbation in unsteady flow separation, stall, and reattachment. The combination of these analytical techniques provides experimental evidence that the location of the separation point and the stability of the shear layer are directly influenced by the presence of vortices. Within this paper, each of the scales represented by the EMD are directly connected to the size of the vortices present, from the smallest representing a vortex radius to the largest reaching to two full vortex diameters. The velocity scales and spatial scales provided by the EMD modes are also found to supply valuable inputs into the identification of Lagrangian coherent structures within each of the PIV snapshots. This indicates that the scales captured by the EMD can be used to extract important turbulent scales present at the point of flow separation where the vortices are created, providing relenvant insight into the separation dynamics of the airfoil.

Structural bifurcation analysis of vortex shedding from shear flow past circular cylinder

In this paper, the unsteady flow separation of two-dimensional (2-D) incompressible shear flow past a circular cylinder is studied using theoretical structural bifurcation analysis based on topological equivalence. The stream function vorticity form of Navier–Stokes (N–S) equations in cylindrical polar coordinates are considered as the governing equations. Numerical simulations are performed, using higher-order compact finite difference scheme (Kalita and Ray J Comput Phys 228:5207–5236 2009), for the Reynolds number (Re) 100, 200 and shear parameter (K) 0.0, 0.05, 0.1 and 0.2. Through this structural bifurcation analysis, the exact location and time of occurrence of bifurcation points (flow separation points) associated with primary and secondary vortices are studied. In this process, the instantaneous vorticity contours and streakline patterns, center-line velocity fluctuation, lift and drag coefficients, phase diagram are also studied to confirm the theoretical results. Unlike the shear flow past a square cylinder, in this case, the structural bifurcation occurs from the downstream surface of the circular cylinder for the same Reynolds number. All the computed results very efficiently and very accurately reproduce the complex flow phenomena. Through this study, many noticeable and interesting results for this problem are reported for the first time.

Numerical investigation of highly unsteady accelerated/decelerated flows for blunt bodies experiencing impulsive motion

Scale-resolving simulations (SRSs) such as large-eddy simulation (LES) and hybrid LES/Reynolds-averaged Navier–Stokes are applied to analyze the unsteady characteristics of the drag and flow fields for blunt bodies subject to impulsive motion. These simulations reveal highly transient behaviors of the primary and secondary vortices triggered by inertial forces during impulsive motion. A distinctive characteristic of the transient drag, namely, the existence of a plateau region, induced by a circular cylinder during impulsive acceleration is also revealed. It is found that among the SRS methods, the LES one-equation eddy method is able to more precisely capture the intrinsic local pressure gradient arising from unsteadiness of the vortices together with the instantaneous vorticity magnitude. The secondary vortices created by inertial forces and shear stresses due to the impulsive motion and the interactions of these vortices with the primary vortices turn out to play a key role in the transient behaviors of the flow field and the drag. The mechanism causing the highly unsteady flow field and its relationship to the bluntness of the shape configuration are also explored using the LES one-equation eddy method. It is found from the range of retained local Cp values and the transient vorticity distribution along the cylinder surface that the mobility of the unsteady flow separation point depends on the bluntness of the configuration.

Unsteady flows measurements using a calorimetric wall shear stress micro-sensor

A microscale low power high temperature gradient calorimetric (HTGC) sensor measuring both mean and fluctuating bidirectional wall shear stress is presented. The micromachined sensor is composed of three free-standing $$3\,\upmu \text {m}\times 1$$ mm micro-wires mechanically supported using perpendicular micro-bridges. The static and dynamic characterisations were performed in a turbulent boundary layer wind tunnel on a flat plate configuration, and compared to the one obtained with a conventional hot-film probe. The results demonstrated that the calorimetric sensor behaves similarly to the hot-film in constant temperature anemometry with nonetheless lower power consumption and better spatial resolution and temporal response. Additionally, its calorimetric measurement detected the direction of the wall shear stress component orthogonal to the wires, corresponding to the shear stress sign in 2D flows. The calibrated HTGC micro-sensor was then used for unsteady flow separation detection downstream a 2D square rib for $$Re_h= 2.56\times 10^4$$ . The calorimetric micro-sensor enabled self-correlated measurements and consequently successfully achieved the detection of flow separation and the reattachment point around $$x/h=10.7$$ .

Control of cylinder wake flow and noise through a downstream porous treatment

Various passive and active methods have been developed to control flow separation from bluff bodies. However, these methods require adjusting features of the solid surface, such as modifying its geometry or porosity, or applying external force or momentum. This paper develops an off-body-based method, without adjusting any features of the solid surface, for controlling the unsteady flow separation by fixing porous materials downstream of bluff bodies. Numerical study on flow past a circular cylinder at a subcritical Reynolds number is performed, and the result indicates that the added downstream porous material changes flow in the wake and re-laminarizes the turbulent flow around the curved cylinder surface, reducing the wall pressure fluctuation around the cylinder. Therefore, the associated aerodynamic noise is reduced greatly.

Evaluation of potential flow models for unsteady separated flow with respect to experimental data

Wake characteristics and force production of a flat plate wing starting from rest at high incidence are examined. Circulation production at the wing leading edge is shown to be coupled to wing kinematics and the wake state. Several potential flow models are compared to experiments.

Flow separation prevention around a NACA0012 profile through multivariable feedback controlled plasma actuators

In the present paper, the problem of controlling the unsteady flow separation over an aerofoil, using plasma actuators, is addressed. Although a plasma flow is essentially a two-phase flow, it has been proved that a local force term may be successfully considered within a flow solver in order to mimic the effect of a local acceleration (see [63]). By exploiting the Chimera Overlapping Grids technique to allow the inclusion of one or more actuators, accurate numerical simulations have been carried out with the in-house Finite Volume solver Xnavis. The effects of varying both number and position of the actuator/sensor pairs along the aerofoil are investigated by focusing on the dynamic control of the boundary layer separation. The capabilities of the proposed control strategy are also examined by showing how the practical flow separation control problem can be formulated as a simple output regulation problem, so that a simple control strategy may be used. Several numerical simulations of incompressible flows around a pitching NACA0012 at Reynolds Re=20,000 illustrate the effectiveness of the proposed approach, in the presence of time-varying angles of attack and complex non-linear dynamics, which are neglected in the control design. The present work extends the former paper of Broglia et al. [2], where the control algorithm was applied to the simple case of single input/single output regulation and implemented in a Finite Elements numerical code; in that work the boundary layer control of an airfoil at fixed angle of attack was addressed. In the present paper a multi input/multi output approach is theoretically developed and numerically implemented. Robust and fast flow reattachment is achieved, along with both stabilisation and increase/reduction of the lift/drag, respectively. A major advantage of the presented method is that the chosen controlled outputs can be easily measured in realistic applications.

Interpretation of Four Unique Phenomena and the Mechanism in Unsteady Flow Separation Controls

Unsteady flow separation controls are effective in suppressing flow separations. However, the unique phenomena in unsteady separation control, including frequency-dependent, threshold, location-dependent, and lock-on effects, are not fully understood. Furthermore, the mechanism of the effectiveness that lies in unsteady flow controls remains unclear. Thus, this study aims to interpret further the unique phenomena and mechanism in unsteady flow separation controls. First, numerical simulation and some experimental results of a separated curved diffuser using pulsed jet flow control are discussed to show the four unique phenomena. Second, the bases of unsteady flow control, flow instability, and free shear flow theories are introduced to elucidate the unique phenomena and mechanism in unsteady flow separation controls. Subsequently, with the support of these theories, the unique phenomena of unsteady flow control are interpreted, and the mechanisms hidden in the phenomena are revealed.

Vortex formation and evolution for flow over a circular cylinder excited by symmetric synthetic jets

The flow around a circular cylinder is controlled by a pair of synthetic jets symmetrically positioned near the separation points at a Reynolds number of 1200. The flow field in the near wake is measured by a collaboration of the flow visualization and the particle image velocimetry (PIV) techniques. The experiment is conducted under six different momentum coefficients Cμ, ranging from 0.007 to 0.421, while seven excitation frequencies are selected as harmonics of the dominant frequency. Various wake patterns appeared during this study, among which are the 2S asymmetric mode, the 2(P + S) asymmetric mode, the 2P symmetric mode, the 2S symmetric mode and the bistable state where vortices shed with both symmetric and asymmetric modes. The phase averaging method, the proper orthogonal decomposition (POD) based vorticity reconstruction and Fourier analysis are adopted to study the vortex dynamics. Particular attention is given to the vortex-formation mechanism for different modes under Cμ = 0.187, when all of the typical modes emerged. Vortex behaviors under symmetric excitation can be interpreted by looking into the processes of flow separation and vortex shedding. It is shown that synthetic jets can lead to unsteady flow separation and various vortex shedding modes are influenced by the periodic vorticity concertation induced by the synthetic jets.

Wake dynamics of low-Reynolds-number flow around a two-dimensional airfoil

We perform two-dimensional simulations of unsteady flow separation around a NACA0015 airfoil. We consider five different angles of attack, 10°, 12.5°, 15°, 17.5°, and 20°, with the Reynolds number varying from 100 to 1300. The central aim is to study the wake dynamics under the low-Reynolds-number condition, when the flow is most likely experiencing transitions between different states. For the low angles of attack of α = 10° and α = 12.5°, we find different branches on the Strouhal-Reynolds number relationship curves. Specifically, for α = 10°, we identify four distinct branches, marked with L1 to L4, in which the second discontinuity between L3 and L4 is proved to be hysteretic, characterized by the wake deflection. The branches L1 and L2 differ from the other two in the near-body flow topology, with the “trailing-edge vortex” mode for the first two, while the “separation vortex” mode for the last two. For the high angles of attack, α = 15°, 17.5°, and 20°, we cannot find marked discontinuity on the Strouhal-Reynolds number relationship curve. We further demonstrate that the flow transits to a chaotic state by a sequence of successive period-doubling bifurcations at α = 20°. Our results suggest that the operation of micro unmanned air vehicles in very low Reynolds numbers should be treated with caution because the flow is very sensitive to the Reynolds number, as well as the angle of attack.

Flow Separation Control in a Curved Diffuser with Rigid Traveling Wave Wall and Its Mechanism

Traveling wave wall is a useful method to suppress flow separation. However, the interaction between the traveling wave wall and unsteady separation flow is complex, which causes difficulty in discovering the corresponding mechanism. To reveal the mechanism of traveling wave wall control, numerical simulation of a separated curved diffuser using rigid traveling wave wall flow control is performed, which shows some unique characteristics. Then, a nonlinear simplified model is used to explain this phenomenon in flow control in consideration of nonlinear dynamics and order of degree. Flow field data from the numerical simulation are further analyzed using fast Fourier transform analysis, linear stability theory of free shear layers, and the nonlinear simplified model to reveal the control mechanism of traveling wave wall.

A modified discrete-vortex method algorithm with shedding criterion for aerodynamic coefficients prediction at high angle of attack

• New algorithm of the LDVM with a gain of a factor 2 in computing time. • Transient pitching motion: the leading edge vortex formation is two-dimensional. • Merging of vortices: decrease of the computing cost for long time simulations. • Evidence of the development of the lift coefficient between 30° and 90 ∘ . Low-order methods require less computing power than classical computational fluid dynamics and can be implemented on a laptop computer, which is needed for engineering tasks. Discrete vortex methods are such low order methods that can describe the unsteady separated flow around an airfoil . After a presentation of the leading edge suction parameter discrete vortex method, a modified algorithm is proposed, in order to reduce the computing cost, and compared with the previous one. Several reference unsteady airfoil motions are discussed in terms of gain in the computation time with comparisons between the previous scheme and the present one. The accuracy of the new method is demonstrated through aerodynamic coefficients . The application of the present discrete vortex method to a transient pitching motion of an airfoil is also presented, in order to understand the leading edge vortex formation, and its implication in terms of lift and drag coefficients. The method is not limited to unsteady or transient motions but can also simulate the flow around a constant angle of attack airfoil. In that case, an original method of fast summation of the vortices located far away from the airfoil, allows a linear dependence of the computation time versus the number of vortices shed, which is a great improvement over the quadratic dependence observed in the classical discrete vortex methods. The development of the aerodynamic coefficients with angle of attack, from values ranging between −10° and 90°, is obtained for a purely two-dimensional flow. In particular, the shape of the lift coefficient of the airfoil in the fully detached flow region is established. Comparisons with relevant experimental or computational fluid dynamics data are discussed in order to grasp the influence of upstream turbulence level and three-dimensional effects in the measured data in the fully detached flow region.

Experimental investigation of the aerodynamics of a freight train passing through a tunnel using a moving model

The objective of this study was to investigate the aerodynamic effects of a freight train passing through a tunnel. The nose entry generates a complex pattern of reflective pressure waves (piston effect) which can lead to intense aerodynamic forces. Previous research on the topic has focused on passenger trains because of higher speeds. The experiments of this study use a 1/25th scaled moving model at the TRAIN Rig at a speed of 33.5 m/s with a blockage ratio of 0.202. The monitored pressure along the tunnel wall can increase up to almost 1000 Pa because of the initial compression wave, while it drops when an expansion wave or the tail passes by. The maximum pressure is observed at the train nose due to air stagnation (1500 Pa) where the flow is steady, while the roof and sides experience negative pressures due to unsteady flow separation. The effect of loading configuration is significant as partially loaded trains can create a second pressure peak on the tunnel walls (after the initial compression wave) and affect the flow at the tunnel entrance wall. Under the current testing conditions, the results indicated compliance with the requirements of the Technical Specification for Interoperability and a constant pressure gradient of the initial compression wave which is in contrast with the passenger train's two-part gradient. Further work on the topic could provide visual information about the exiting jet towards the portal and the separation bubble around the train.

Unsteady separated stagnation-point flow over a permeable surface

In this paper, we have investigated numerically the unsteady separated stagnation-point flow of an incompressible viscous fluid over a porous flat plate subjected to continuous suction or blowing. The analysis covers the complete range of the suction/blowing parameter d, including $$d = 0$$ and $$d \rightarrow \pm \infty $$ , in conjunction with the flow strength parameter $$a (> 0)$$ and the unsteadiness parameter $$\beta $$ . Two types of solutions, namely attached flow solution (AFS) and reverse flow solution (RFS), have been found for a negative value of $$\beta $$ . A novel result that emerges from our analysis is the characteristic features of the boundary layer flows which firmly depend on the sum values of $$(a + \beta )$$ in case of massive blowing $$(d \ll 0)$$ given at the wall. In fact, a negative value of $$(a + \beta )$$ opposes the flow like an adverse pressure gradient and the solution domain ends off with a RFS. On the other hand, a positive value of $$(a + \beta )$$ assists the flow like a favourable pressure gradient for which the solution continues with an AFS. An interesting result of this analysis is that after a certain negative value of $$(a + \beta )$$ , dependent on a and $$\beta $$ , the solution of this flow problem does not exist for blowing, and this trend persists even for suction $$(d>0)$$ . On the other hand, for large positive values of $$(a + \beta )$$ , the attached flow solution exists in case of both strong suction and massive blowing, whereas for small positive values of $$(a + \beta )$$ and especially in case of hard blowing, the solution of the governing boundary layer equation does not appear to have the boundary layer character. Furthermore, when $$(a + \beta ) = 0$$ , the velocity gradient at the wall is zero for both AFS and RFS flows, and ultimately, the governing boundary layer equation produces a trivial solution which does not satisfy the outer boundary condition.

Numerical study of pore-scale flow and noise of an open cell metal foam

This paper studies numerically the three-dimensional pore-scale flow inside a single cell structure of an open cell metal foam and its aeroacoustic features. Since the Reynolds number based on the pore diameter is very low, the Navier–Stokes equations are solved directly to simulate the unsteady pore-scale flow. The permeability and pressure drop obtained from numerical simulations are compared with existing reference results. Numerical results reveal that the flow drag of the pore-scale structure is dominated by the pressure drag which is mainly caused by the flow separation, while the friction drag is much smaller than the pressure drag despite a very high surface area–volume ratio of the metal foam. The unsteady flow separation contributes primarily to the pressure drag and causes the self-noise of the metal foam, therefore suppressing the unsteady flow separation, e.g., by optimizing the cell structure of the metal foam, would reduce the drag and aerodynamic noise. The aeroacoustic features, such as noise sources, spectra and the directivity pattern, are investigated, and the results reveal a high correlation between the noise generation and the flow separation. The direction of the maximum sound pressure for the studied cell is parallel to the flow, which is different to flow separation from a cylinder where the direction of maximum radiation is perpendicular to the flow.

An Unsteady Separated Stagnation-Point Flow Towards a Rigid Flat Plate

This paper discusses an unsteady separated stagnation-point flow of a viscous fluid over a flat plate covering the complete range of the unsteadiness parameter β in combination with the flow strength parameter a (>0). Here, β varies from zero, Hiemenz's steady stagnation-point flow, to large β-limit, for which the governing boundary layer equation reduces to an approximate one in which the convective inertial effects are negligible. An important finding of this study is that the governing boundary layer equation conceives an analytic solution for the specific relation β = 2a. It is found that for a given value of β (≥0) the present flow problem always provides a unique attached flow solution (AFS), whereas for a negative value of β the self-similar boundary layer solution may or may not exist that depends completely on the values of a and β (<0). If the solution exists, it may either be unique or dual or multiple in nature. According to the characteristic features of these solutions, they have been categorized into two classes—one which is AFS and the other is reverse flow solution (RFS). Another interesting finding of this analysis is the asymptotic solution which is more practical than the numerical solutions for large values of β (>0) depending upon the values of a. A novel result which arises from the pressure distribution is that for a positive value of β the pressure is nonmonotonic along the stagnation-point streamline as there is a pressure minimum which moves toward the stagnation-point with an increasing value of β > 0.