Unsteady Separated Flow Research Articles

Determination of unsteady wing loading using tuft visualization

Unsteady separated flow affects the aerodynamic performance of many large-scale objects, posing challenges for accurate assessment through low-fidelity simulations. Full-scale wind tunnel testing is often impractical due to the object’s physical scale. Small-scale wind tunnel tests can approximate the aerodynamic loading, with tufts providing qualitative validation of surface flow patterns. This investigation demonstrates that tufts can quantitatively estimate unsteady integral aerodynamic lift and pitching moment loading on a wing. We present computational and experimental data for a NACA0012 wing, capturing unsteady surface flow and force coefficients beyond stall. Computational data for varying angles of attack and Reynolds numbers contain the lift coefficient and surface flow. Experimental data, including lift and moment coefficients for a tuft-equipped NACA0012 wing, were obtained at multiple angles of attack and constant Reynolds number. Our results show that a data-driven surrogate model can predict lift and pitching moment fluctuations from visual tuft observations.

Influence of Time-Varying Freestream Conditions on the Dynamics of Unsteady Boundary-Layer Separation

Unsteady flow separation of a turbulent boundary layer under dynamic pressure gradients is investigated using the Large-Eddy Simulation technique. The unsteadiness is introduced by prescribing an oscillating freestream vertical-velocity profile at the top boundary of the domain. Although previous studies, including Ambrogi et al. (Journal of Fluid Mechanics, Vol. 945, Aug. 2022, p. A10) and Ambrogi et al. (Journal of Fluid Mechanics, Vol. 972, Oct. 2023, p. A36), focused on the kinematics of the flow and the effects of the oscillation frequency on flow separation, the goal of this paper is to analyze the effects of three time-varying freestream-forcing profiles while the oscillation frequency is kept the same for all Cases. Whereas in Case A the freestream-velocity profile changes from suction–blowing to blowing–suction in a complete cycle, Cases B and C are both suction–blowing only and the strength of the adverse pressure gradient is modulated in time. Moreover, the boundary layer in Case B never approaches a zero-pressure gradient condition. A closed separation bubble is formed for all Cases; however, its dimensions change depending on the far-field forcing. The time evolution of turbulent kinetic energy (TKE) reveals an advection mechanism of turbulent structures out of the domain for all Cases. Whereas in Case A and C the high-TKE region, generated in the separated shear layer, is washed out of the domain as a rigid body, in Case B the separation bubble remains present and the advection mechanism of TKE is characterized by a breathing pattern.

Analysis of coherent structures over interacting barchan dunes based on tomographic particle image velocimetry

The influence of barchan dune interaction upon unsteady flow separation and wake dynamics around the fixed-bed downstream barchan dune (DBD) model are experimentally investigated at a Reynolds number of 2640 based on the tomographic particle image velocimetry (PIV) system. The time-averaged statistics including the mean velocities, recirculation area, vortex spatial topology, Reynolds stress, and turbulent kinetic energy were used to characterize the flow field and large-scale anisotropy. It was found that arch-shaped vortex “chains” with strong spanwise coherence shedding from isolated barchan crestline populate the whole wake region, while elongated rod-shaped vortex structures with strong streamwise coherence induced by the up-downwash flow around the DBD were found to fill the whole measurement range, which is closely related to “sheltering” effect on the incoming flow acting at DBD due to the presence of upstream barchan dune (UBD). Additionally, in order to study the complex dynamic features of these predominated vortex structure transformations, time-resolved planar particle image velocimetry was applied. This technique allows for providing complementary insights into the temporal behavior of the unsteady coherent flow structures populating the wake field in different experimental configurations. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions including vortex pairing, merging, and breaking up can all be analyzed by dividing into certain scales with ease in a combination wavelet and Lagrangian framework.

A Field Test Study on the Instability of a Pump-Turbine under Turbine Mode

Abstract In the context of increasing demand for flexible operation of hydropower plants, this study analyses the factors leading to instability in a prototype pump turbine under turbine mode, based on experimental data collected on-site. Pressure sensors and accelerometers were installed to compare pressure signals and vibration acceleration signals. The test results show that the characteristics of pressure fluctuation are dominated by three specific factors under different output power. Vibration reaches its maximum value at approximately 30% of the rated output and its minimum value at around 70% of the rated output, and the causes of twice increases in vibration is not the same. These characteristics often indicate potential unsteady flow, vortex-induced vibration, and flow separation, subsequently affecting the stability of the pump turbine. Understanding the conditions and severity of the occurrence of instability characteristics in the unit is of great significance for power plant personnel to adjust the unit’s operation strategy and improve operational efficiency.

Tip shape effects on the axial-flow-induced vibration of a cantilever rod

The influence of tip shape on flow-induced vibration of a cantilever rod subjected to axial water flow is experimentally investigated, through optical tracking of the rod movement and mapping of the instantaneous flow field around the rod tip. The experimental setup consists of a vertical cantilever rod housed within a tube. The rod tip shapes considered in this study include a blunt tip and cones with height-to-diameter ratios of 0.5, 1, and 2. The experiments were conducted across a Reynolds number range between 20k to 100k in both clamped-free and free-clamped configurations, representing opposite flow directions. The rod tip motion was captured using fast video image tracking, whereas the flow field near the rod tip was obtained using particle image velocimetry (PIV). The rod vibration dynamics exhibited a primarily fuzzy period-1 behavior, characterized by a periodic motion with a chaotic component. Flutter-like oscillation and buckling were also observed at higher Reynolds numbers, depending on the flow direction. The mechanisms of fluid-structure interaction involved turbulent buffeting and movement-induced excitations. Unsteady flow separation around the rod tip was identified as a further contributing mechanism to flow excitation. In the clamped-free configuration, unsteady flow separation was more pronounced for the cone tips due to the increased rod surface area in the wake region, leading to larger vibration amplitude. In the free-clamped configuration, flow separation effects were more prominent for the blunt tip, as the streamlined cone shapes were less prone to flow separations. Overall, the rod with a blunt tip resulted in smaller displacement in the clamped-free configuration, while the rods with cone tips led to smaller displacement in the free-clamped configuration.

Large-Eddy vs. Reynolds-Averaged Navier–Stokes Simulations of Flow and Heat Transfer in a U-Duct with Unsteady Flow Separation

Large-eddy simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) equations were used to study incompressible flow and heat transfer in a U-duct with a high-aspect-ratio trapezoidal cross section. For the LES, the WALE subgrid-scale model was employed, and its inflow boundary condition was provided by a concurrent LES of incompressible fully-developed flow in a straight duct with the same cross section and flow conditions as the U-duct. LES results are presented for turbulent kinetic energy, Reynolds stresses, pressure–strain rate, turbulent diffusion, turbulent transport, and velocity–temperature correlations, with a focus on how they are affected by the U-turn region of the U-duct. The LES results were also used to assess three commonly used RANS models: the realizable k-ε with the two-layer model in the near-wall region, the two-equation shear-stress transport model, and the seven-equation stress-omega Reynolds stress model. Results obtained show steady and unsteady RANS to incorrectly predict the effects of unsteady flow separation. The results obtained also identified the terms in the RANS models that need to be modified and suggested how turbulent diffusion should be modeled when there is unsteady flow separation.

Investigation on unsteady characteristics of flow separation in a supercritical carbon dioxide centrifugal compressor

As one of the core components of supercritical carbon dioxide (S-CO2) closed Brayton cycle, the efficiency of S-CO2 centrifugal compressor plays a crucial role. Affected by the special thermophysical properties of S-CO2 fluid, the flow mechanism of S-CO2 centrifugal compressor is different from the centrifugal compressor of conventional fluid, such as air. Previous studies have found that the flow separation within the flow domain of this kind of compressor is easily to occur downstream the pressure surface. Many steady computational fluid dynamics (CFD) studies have been conducted on the S-CO2 centrifugal compressors, but few studies focused on the unsteady evolution of this flow separation. In this paper, the unsteady CFD simulation is carried out in the flow passage of a S-CO2 centrifugal compressor. The solution domain of CFD simulation includes compressor blades, diffuser and volute. The performance and the unsteady flow behavior of S-CO2 centrifugal compressor is obtained. Under low flow rate conditions, the flow separation on the pressure surface of the blade of S-CO2 centrifugal compressor is more likely to occur, causing the decrease of compressor performance. This flow separation has strong unsteady characteristics, which deteriorates several times during a rotation cycle, meanwhile the vortex shedding happens. At the time steps of vortex shedding, the pressure near the trailing edge of the impeller fluctuates greatly, indicating that this unsteady flow separation brings a large flow loss to the S-CO2 centrifugal compressor.

Open-loop flow control design guided by the amplitude-frequency characteristics of the reduced-order model

Unsteady separated flow is a common flow condition causing many detrimental effects in aerospace and other fields. Open-loop control is a potential means to eliminate these drawbacks. At present, the unsatisfactory performance of open-loop control mainly attributes to the high-dimensional parameter optimization procedure and the lack of efficient knowledge-guided control law design methods. This paper proposes a method managing to directly obtain the anti-resonant frequency from the input and output amplitude-frequency characteristic curves of the reduced-order model of the flow-control coupled system. Simultaneously, a negative gain criterion is proposed to judge whether the target control effect can be achieved under the current parameter combination. For the control of low Reynolds number flow around a cylinder, the simulation results show that the optimal open-loop control frequency is 1.268 times the natural frequency of the flow, which is consistent with the anti-resonant frequency, and 26.8% of the lift fluctuation is suppressed. This paper also studies the influence of parameters such as flow frequency change, control start time, jet mass flow rate, and so on. Furthermore, control position is the key parameter affecting the amplitude-frequency characteristics. The anti-resonance points on the curves corresponding to different control positions can also guide the design of the optimal control frequency, and the negative gain criterion is still applicable. This method greatly reduces the time consumption in parameter optimization and improves the engineering application prospect of the open-loop control in unsteady separated flow control.

Characterisation of unsteady separation in a turbulent boundary layer: Reynolds stresses and flow dynamics

The large-eddy simulation technique was used to investigate the dynamics of unsteady flow separation on a flat-plate turbulent boundary layer. The unsteadiness was generated by imposing an oscillating, wall-normal velocity profile at the top of the computational domain, and a range of reduced frequencies ( $k$ ), from a very rapid flutter-like motion to a slow quasi-steady oscillation, was studied. Ambrogi et al. (J. Fluid Mech., vol. 945, 2022, A10) showed that the reduced frequency greatly affects the transient separation process, and at a frequency $k=1$ , the separation region became unstable and was advected periodically out of the domain. In this paper, we discuss the causes of the observed advection process and the effects of the unsteadiness on the second moments. The time evolution of turbulent kinetic energy, for instance, reveals that an advection-like phenomenon is also present at a very low reduced frequency, but its dynamic behaviour is completely different from that of the intermediate frequency ( $k=1$ ). At the intermediate frequency the entire recirculation region is advected downstream, keeping its shape. The advected structure is rotational in nature, and moves at constant speed. In contrast, in the low-frequency case the advected fluid originates at the reattachment point, and the structure is shear-dominated. Particle pathlines reflect the fact that the flow at the low frequency is quasi-steady-state, but show peculiar differences at the intermediate frequency, in which the flow response to the freestream forcing depends on the particle positions in the wall-normal direction.

Assessment of Turbulence Models for Unsteady Separated Flows Past an Oscillating NACA 0015 Airfoil in Deep Stall

This paper provides 2D Computational Fluid Dynamics (CFD) investigations, using OpenFOAM package, of the unsteady separated fully turbulent flows past a NACA 0015 airfoil undergoing sinusoidal pitching motion about its quarter-chord axis in deep stall regime at a reduced frequency of 0.1, a free stream Mach number of 0.278, and at a Reynolds number, based on the airfoil chord length, , of . First, eighteen 2D steady-state computations coupled with the SST model were carried out at various angles of attack to investigate the static stall. Then, the 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations of the flow around the oscillating airfoil about its quarter-chord axis were carried out. Three eddy viscosity turbulence models, namely the Spalart-Allmaras, Launder-Sharma , and SST were considered for turbulence closure. The results are compared with the experimental data where the boundary layer has been tripped at the airfoil’s leading-edge. The findings suggest that the SST performs best among the other two models to predict the unsteady aerodynamic forces and the main flow features characteristic of the deep stall regime. The influence of moving the pitching axis downstream at mid chord was also investigated using URANS simulations coupled with the SST model. It was found that this induces higher peaks in the nose-down pitching moment and delays the stall onset. However, the qualitative behavior of the unsteady flow in post-stall remains unchanged. The details of the flow development associated with dynamic stall were discussed

Large-Amplitude Gusts on the NASA Common Research Model

To date, there is limited knowledge on the topic of unsteady nonlinear transonic flow caused by large-amplitude excitations. The Certification Specifications for Large Aeroplanes, however, demand loads computations for large-amplitude excitations, e.g., for gust encounters of an aircraft. In this paper, numerical solutions to the unsteady Reynolds-averaged Navier–Stokes equations will be used to analyze gust responses of a transport aircraft configuration for a variety of sinusoidal gusts with different lengths and amplitudes at different Mach numbers. On the one hand, it is shown that unsteady nonlinear responses can lead to lower maximum lift values compared to their time-linearized reference due to unsteady shock-induced flow separation. This nonlinear effect dominates for large gust lengths and higher Mach numbers. On the other hand, for the lowest Mach number considered, the coalescence of a double shock at medium gust lengths introduces nonlinear effects leading to an overprediction of the time-linearized maximum lift by the nonlinear computations. Though the turbulence model of Spalart and Allmaras shows completely different separation patterns than a differential Reynolds stress model, the variation of the turbulence model does not alter the overall qualitative findings, but has an effect on the quantitative values.

Unsteady flow control mechanisms of a bio-inspired flexible flap with the fluid–structure interaction

Recently, the development of bio-inspired aircrafts has broad application prospects. However, the flow separation in the boundary layer of the bio-inspired wing under low Reynolds number becomes a great challenge for the design of a novel bio-inspired aircraft. It is worth noting that birds in nature can easily control flow separation, thanks to the flap-like flexible plumes attached to their wing surfaces. In this paper, the unsteady flow control of the flexible flap is studied by the immersed boundary-lattice Boltzmann-finite element method (IB-LB-FEM). The mechanism of flow separation on the airfoil surface at a bio-inspired large angle of attack (AOA) is suggested. The effects of the flexible flap position and its material properties on the unsteady flow control of the airfoil at large AOA are systematically discussed. The deformation law of the flexible flap with fluid–structure interaction (FSI) is revealed, and its influence on unsteady aerodynamics of the airfoil is discussed. The results show that with the increase in the AOA, the aerodynamic characteristics of the airfoil change with time from “periodic state” to “chaotic state” to “quasi-periodic state,” which is closely related to the unsteady flow separation on the airfoil upper surface. The new induced vortex is formed at the end of the flexible flap because of the FSI, which enhances or weakens the strength of vortices on the airfoil surface, affecting the aerodynamics of the airfoil. The flow control mechanism of the flexible flap proposed in this paper will provide a new design idea for the novel bio-inspired aircraft.

Airfoil-based convolutional autoencoder and long short-term memory neural network for predicting coherent structures evolution around an airfoil

Airfoil stall induced by unsteady flow separation is a major inhibitor that constrains the aircraft design. Hence, it is critical to have a better understanding of the flow dynamics around the airfoil near stall conditions. Recently, machine learning (ML) has been widely used to predict the spatiotemporal evolutions of coherent structures over the airfoil. Nonetheless, due to the existence of strong nonlinearity introduced by turbulence, most of the existing approaches focus on prediction of the flow fields in the laminar flow regime. In this work, we seek to examine the applicability of deep neural network (DNN) to prediction of turbulent flows around a NACA (National Advisory Committee for Aeronautics) 4412 airfoil. Large eddy simulations (LES) are performed to prepare the training dataset. Both proper orthogonal decomposition (POD) and DNN-based reduced-order models (ROM) are employed for reconstructing the flow fields. A convolutional autoencoder (CAE) is applied to map the high-dimensional flow dynamics into the low-dimensional nonlinear manifold. The geometric features are considered in the input matrix of the CAE network by setting physical quantities inside the airfoil to zero to reflect the physics. Then, the multi-head attention-strengthened long short-term memory (MH-LSTM) is utilized to predict the evolutions of latent vectors. Finally, the high-dimensional flow dynamics can be reconstructed by utilizing the decoder part of CAE. We believe that the MH-LSTM methodology provides a promising path towards predicting the unsteady flow field over the airfoil by capitalizing on the capability of multi-head attention allowing for attending to distinct parts of input sequence.

Time-Accurate Solution of Unsteady Flows in an Implicit Solver Using Block LUSGS Method

Implicit solvers based on the lower-upper symmetric Gauss–Seidel (LUSGS) method are widely used in Computational Fluid Dynamics (CFD) codes to obtain steady-state solutions using large false-transient time steps. However, the method destroys time accuracy and thus cannot be directly used for true transient problems. For such problems, the LUSGS method has to be used with a dual time-stepping algorithm. In this paper, a modified method that preserves time accuracy and is called the block LUSGS (B-LUSGS) solver is employed to compute unsteady flows using larger time steps. This method is compared with the mentioned dual time-stepping technique used for transient problems. Numerical solutions obtained for Stokes' second problem simulation underline the superior temporal accuracy of the block LUSGS method in computing unsteady flows, even at CFL numbers close to 30. Simulations of 2D laminar and 3D turbulent vortex shedding downstream of a circular cylinder further validate the modified method in accurately predicting unsteady separated flows at a CFL number of 100. Comparisons of overall simulation CPU times show that the B-LUSGS method is at least six times computationally cheaper than the dual time-stepping algorithm that uses the traditional LUSGS solver. Abbreviations: BDF, Backward differentiation formula; CFD, Computational fluid dynamics; CFL Courant–Friedrichs–Lewy; CPU, Central processing unit; DTS, Dual time-stepping; LES, Large-eddy simulation; LUSGS, Lower-upper symmetric Gauss–Seidel; NASA, National Aeronautics and Space Administration; PDE, Partial differential equation; RAM, Random access memory; RANS, Reynolds-averaged Navier–Stokes; SA, Spalart–Allmaras; SST, Shear stress transport.

Unsteady separated stagnation point flow due to an EMHD Riga plate with heat generation in hybrid nanofluid

• The unsteady separated stagnation point (USSP) flow of Cu-Al 2 O 3 /H 2 O flow is analyzed. • The effects of heat generation and EMHD conditions are also examined. • The dual solutions are obtained for the USSP flow case. • The streamlines for the second solution splits into two regions (reverse flow). • The heat generation reduces the thermal rate of the Cu-Al 2 O 3 /H 2 O. This paper examines the unsteady separated stagnation point (USSP) flow on a Riga plate subject to the heat generation and electro-magnetohydrodynamic (EMHD) conditions. The fluid is consisting of two different nanoparticles which are Cu (copper) and Al 2 O 3 (alumina). The model of the flow includes the Navier-Stokes and energy equations. These equations are then simplified with the aids of the similarity variables. The numerical results are generated by the bvp4c function and then, presented in graphs and tables. Outcomes from the computations reveal that the electro-magnetohydrodynamic parameter boosted the magnitude of the skin friction and the heat transfer coefficients. However, the retardation of the heat transfer is observed when the heat generation is imposed on the boundary layer. Besides, the domain of the solutions is expanded for larger value of electro-magnetohydrodynamic parameter, and these solutions are terminated at certain points of the unsteadiness parameter. It should be noted that the streamlines for the first solution acts as a normal stagnation point flow whereas for the second solution splits into two region which proves the occurrence of the reverse flow. Lastly, by using the stability analysis, the second solution is found to be unstable while the first solution is stable as time evolves.

Insight into Unsteady Separated Stagnation Point Flow of Hybrid Nanofluids Subjected to an Electro-Magnetohydrodynamics Riga Plate

The main objective of this work is to analyze and compare the numerical solutions of an unsteady separated stagnation point flow due to a Riga plate using copper–alumina/water and graphene–alumina/water hybrid nanofluids. The Riga plate generates electro-magnetohydrodynamics (EMHD) which is expected to delay the boundary layer separation. The flow and energy equations are mathematically developed based on the boundary layer assumptions. These equations are then simplified with the aid of the similarity variables. The numerical results are generated by the bvp4c function and then presented in graphs and tables. The limitation of this model is the use of a Riga plate as the testing surface and water as the base fluid. The results may differ if another wall surfaces or base fluids are considered. Another limitation is the Takabi and Salehi’s correlation of hybrid nanofluid is used for the computational part. The findings reveal that dual solutions exist where the first solution is stable using the validation from stability analysis. Graphene–alumina/water has the maximum skin friction coefficient while copper–alumina/water has the maximum thermal coefficient for larger acceleration parameter. Besides, the single nanofluids (copper–water, graphene–water and alumina–water) are also tested and compared with the hybrid nanofluids. Surprisingly, graphene–water has the maximum skin friction coefficient while alumina–water has the maximum heat transfer rate. The findings are only conclusive and limited to the comparison between graphene–alumina and copper–alumina with water base fluid. The result may differ if another base fluid is used. Hence, future study is necessary to investigate the thermal progress of these hybrid nanofluids.

Rotational effects on the blade flow of a horizontal axis wind turbine under axial and yawed inflow conditions

Unsteady aerodynamic loads and fluctuating output power of wind turbines remain challenging to predict accurately due to the incomplete understanding of blade flow mechanisms. Rotational effects can significantly impact the wind turbine aerodynamic performance. Therefore, this paper presents a careful investigation into rotational flow characteristics of the NREL Phase VI blade under axial and yawed inflow conditions based on the DDES method. Rotational effects change the flow regime from the massive 2D trailing-edge separation into the moderate 3D leading-edge separation on the NREL Phase VI blade. Flow separation is found sufficient but not necessary for the radial flow. Under the axial inflow, rotational effects can effectively suppress the flow separation and increase the nonlinear aerodynamic loads. Under the yawed inflow, rotational effects also effectively suppress the unsteady separated flow and accelerate the flow reattachment, particularly on the downwind side. Sectional lift coefficient is therefore greatly increased at the high angles of attack due to the pronounced rotational effects. Increasing the yaw angle and wind speed may also strengthen the rotational effects and widen the difference between the 2D airfoil flow and 3D sectional flow. These findings imply that flow regime and aerodynamic hysteresis on the inboard blade can be substantially changed in comparison with the 2D airfoil flow due to strong rotational effects. This study might deepen the understanding of rotational effects on the near-wall flow of wind turbine blades and also improve the aerodynamic modelling.

Control of bow shock induced three-dimensional separation using bleed through holes

The unsteady three-dimensional separated flow on a wall induced by a square protrusion (approximately twice the local boundary layer thickness in width and height), and its control by means of passive suction through holes, is investigated using wind tunnel experiments at Mach 2.87. The baseline flow without any control was characterized and compared against the cases with bleed. A bow-shaped separation line on the wall with a mid-span separation length of 5.57δ from protrusion face was traced from the oil-flow visualization. The averaged pressure distribution surveyed using static pressure ports placed on the wall has mapped plateau, high-pressure, and low-pressure regions in the separated flow, distinctive to three-dimensional interactions. Ten control configurations were tested with suction holes placed along mid-span in the different pressure regions. Significant spanwise “Mean Reduction in Separation Length” of up to 0.93δ was observed from oil-flow visualization. A comparison of observations from various control configurations suggested that bleeding the flow from the high-pressure region could delay the separation and reduce the bubble size in general. Furthermore, time-resolved Schlieren visualizations have confirmed reduction in both “mid-span separation length” and “shock-intermittent-region” with the introduction of suction in high-pressure region. Fourier and Proper Orthogonal Decomposition analysis done on the Schlieren data has confirmed the presence of low-frequency separation-shock oscillations at Strouhal Numbers of order 10−2, both with and without control. Furthermore, the amplitudes of separation-shock oscillations in the spectrum were reduced with the introduction of suction simultaneously from two holes placed in high- and low-pressure regions.

Effects of the height and chordwise installation of the vane-type vortex generators on the unsteady aerodynamics of a wind turbine airfoil undergoing dynamic stall

Dynamic stall can produce nonlinear and unsteady aerodynamic loads on wind turbines. It has attracted considerable attention recently by introducing vortex generators (VGs) to suppress the dynamic stall, although the effects of decisive VG parameters have not yet been investigated adequately. This paper aims to provide insights into the impacts of vane height and chordwise installation on the dynamic stall phenomenon of an NREL S809 airfoil equipped with VGs. Unsteady flow characteristics are identified through fully resolved URANS simulations. Unsteady flow separation can be suppressed effectively via the VG-induced streamwise vortices. Consequently, the delayed onset of dynamic stall leads to greatly increased maximum lift and accelerated flow reattachment in the downstroke process. Low-profile VGs can cause an early abrupt dynamic stall and remarkable aerodynamic hysteresis, thereby losing their effectiveness. Moreover, positioning VGs too far downstream can cause significant increases in both drag and hysteresis intensity, because the streamwise vortices decay rapidly under the high adverse pressure gradient. Increasing VG size and utilizing double-row VGs simultaneously can strengthen the VG-induced streamwise vortices and hence the boundary layer momentum transfer, resulting in effectively controlled local flow structures. Furthermore, double-row VGs perform efficiently in improving unsteady aerodynamics due to low drag penalty.

Mesh Adaptation for Simulating Lateral Jet Interaction Flow

Under the condition of supersonic incoming flow, a missile lateral jet flow field has complex flow structures, such as a strong shock wave, an unsteady vortex and flow separation. In order to improve ability to capture complex flow structures in numerical simulation of lateral jets, this paper proposes a combined-grid adaptive method. When combined with finite volume approximation of second-order and h-type adaptive technology, our method was verified by numerical experiments, which shows that wave structure and vortex structure in the jet flow field can be effectively captured at the same time. In comparison of uniformly refined mesh results, it was found that accuracy of computed results and resolution of characteristic flow structures were significantly improved after mesh adaptation. In comparison of the pressure coefficient, it was found that the error between the adaptive mesh and the uniformly refined mesh was smaller, and the maximum errors of the base grid, adaptive grid and uniformly refined grid were 92.1% and 12.3%.