Aerodynamic Interference Research Articles

Numerical Investigation on Longitudinal Stage Separation of Spiked Two-Stage-to-Orbit Vehicle

A longitudinal stage separation (LSS) scheme for a parallel-arrangement two-stage-to-orbit (TSTO) vehicle is proposed and analyzed in detail, in which the orbiter moves along the upper surface of the booster. A TSTO concept comprising a waverider and a spaceplane was designed to numerically investigate the dynamic characteristics of the LSS at Mach 7. The influence of spike models assembled at the nose of the orbiter on the LSS at different angles of attack (AoA) (i.e., spike with half cone, spike with half cone-disk, and hemispheric spike) was explored. Moreover, the aerodynamic interference and characteristics were analyzed and compared for different spiked configurations. The aerodynamic interference during LSS is simple and weak, which is only associated with type VI shock/shock interaction, with rapid increases in the axial force when the shock waves of both stages converge. Furthermore, the model with a half cone spike has the best performance in drag reduction by 7%, whereas the model with a half cone-disk spike has the worst performance that increases axial force. The TSTO model with a half cone spike at and 5 deg is advantageous in LSS because of the high drag reduction and weak aerodynamic interference.

Fast prediction method for multirotor global tonal noise based on acoustic modal analysis

With the popularity of multirotor aircraft, the aerodynamic noise it generates has an increasingly serious impact on the environment. A fast method for globally predicting the tonal noise is proposed in this study. The goal of this method is to predict the global aerodynamic noise at a small computational cost, then providing a fast model for online noise prediction and control. Firstly, the global noise model of a single rotor in acoustic modal domain is established by using the near-field acoustic holography method. Then, the global noise model of a rotor with different configurations is obtained by using the mapping relationship of acoustic modal coefficients. Subsequently, the global noise of multirotor is predicted based on the configuration information of rotors by using the wave superposition principle. To validate the performance of this method, the predicted global noises of single rotor and multi rotors were compared with other numerical methods. It is revealed that the acoustic modal analysis can accurately predict global noise at a small computational cost. Moreover, a parametric study was conducted to evaluate the effect of different configurations on the global noise. The mechanism of phase synchronization method was analyzed comprehensively at the same time. Lastly, experiments were carried out in the anechoic chamber to validate the method proposed. When the separation distance between rotor shafts is greater than 2.8 R, the aerodynamic interference between the rotors can be ignored and the method can be well applied globally. Otherwise, the aerodynamic interference result in predictions that do not match the experiments at higher elevation angles, e.g. 60°. Meanwhile, as the elevation angle decreases, the prediction error decreases rapidly.

Experimental investigation on vortex-induced vibration of a long-span rail-cum-road bridge with twin separated parallel decks

Long-span rail-cum-road bridges with twin separated parallel decks have become a recommended structural form because of their superior traffic capacity. However, because of inconsistent dynamic characteristics of highway and railway bridges, different wake characteristics and special aerodynamic interference between the asymmetric twin decks, the vortex-induced vibration (VIV) characteristics for twin-deck bridges are more complicated. In this paper, wind tunnel tests were carried out to study the VIV characteristics of a long-span rail-cum-road bridge with twin separated parallel decks. Furthermore, fluctuating wind pressure of the highway and railway girders was measured to discuss the vibration phenomenon. The results indicated that the VIV of the railway girder only occurs upstream, while the VIV of the highway girder occurs both upstream and downstream. The aerodynamic interference has an impact on the VIV response of both upstream and downstream girders. 3° is still the most unfavorable wind incidence angle for the twin parallel deck bridges. Increasing structural damping helps to suppress VIV, but it cannot completely suppress the occurrence of VIV. Adjusting the wind fairing angle of the highway girder and sealing the handrail at intervals are both effective in controlling VIV, but the effects are different for upstream and down girders. For the twin separated parallel decks bridges, the VIV suppression effect of the aerodynamic measures needs to be comprehensively considered from the overall consideration of the twin decks under different incoming flow directions.

Optimal hierarchical trimming method for multi-lift system with helicopters considering aerodynamic interference

It is difficult to accurately trim the multi-lift system with helicopters due to complex aerodynamic interference and troublesome manipulation redundancy. To solve this problem, a mid-fidelity method is utilized to simulate the aerodynamic interference of the system. Particularly, the lifting surface method is applied to calculate the unsteady air loads of the rotors, and the viscous vortex particle method is used to model the wake. Moreover, an optimal cable-force allocation strategy based on the equilibrium optimizer is proposed to deal with the manipulation redundancy. On the basis, the load-leading optimal hierarchical trimming method for the multi-lift system is obtained by introducing the improved delta trimming algorithm and the hierarchical theory simultaneously. The trimming results for the multi-lift system at different forward speeds verify the effectiveness of the trimming method and indicate that: the optimal force allocation strategy is beneficial to reduce coupling and there are different characteristics of helicopters in the multi-lift system.

Aerodynamic interference effects between a triple-box girder and trains on aerodynamic forces and vortex-induced vibration

Aerodynamic interference effects between a triple-box girder and trains on aerodynamic forces and vortex-induced vibration

Study on Numerical Algorithm of the N-S Equation for Multi-Body Flows around Irregular Disintegrations in Near Space

There has been a concern that the accurate numerical simulation of multi-body flow, which is caused by the multiple disintegrations of expired spacecraft re-entering into the near space, has a critical bottleneck impact on the falling area of the disintegrated debris. To solve this problem, an O-type grid topology method has been designed for the multi-body flow field of irregular debris formed by multiple disintegrations in near space, and a finite-volume implicit numerical scheme has been constructed for the Navier-Stokes equations to solve the aerodynamic interference characteristics of irregular multi-body flow, and further the N-S equation numerical algorithm has been established for the irregular multi-body flows in near space. The reliability of the method has been verified by the comparison of the present computation and the experiment of the low-density wind tunnel for the two-body flow of sphere, cylinder and square scripts. The objects of this study are from the multiple disintegrations of the Tiangong-1 spacecraft during uncontrolled re-entry into the atmosphere, including propelling cylinders and low-temperature lock cabinets. A series of simulations of multi-body flow mechanisms around different combinations have been carried out with varied shapes and spacing. As a result, it is found that when the distance of irregular debris (e.g., two propelling cylinders) in the near space is in the range of Δy < 3D or Δx < D, there is an obvious multi-body interference between debris, and the flow characteristics are obviously changed. When the distance between the debris in near space reaches a certain level, the influence of mutual interference can be ignored. For example, when the y-direction distance between multiple bodies is greater than 3D, the flow interference tends to be small and can be ignored, and we can regard them as two separate pieces to be carried out by the numerical prediction of flight track and falling area in engineering application. The results provide a practical design criterion for the integrated simulation platform which is used to simulate the multi-physics complex aerodynamics of space vehicles from the free-molecule flow of the outer space to the near-ground continuum flow.

Research status and prospect of wind-vehicle-bridge coupling vibration system

To promote and develop the theoretical basis and application of the wind-vehicle-bridge coupling vibration system, the corresponding research status and prospects are reviewed and discussed from five aspects, i.e., the analytical framework, the aerodynamic interference, the evaluation criteria, the design loads of long-span bridge and the double-deck rail-cum-road bridge. The refining process of analysis system is reviewed from the aspects of simulation wind load, vehicle load and bridge structure, and the corresponding coupling relationship. For aerodynamic interference, the development process is summarized from the simulative precision of the elements (wind, vehicle and bridge), the load cases and the object of interference. For evaluation criteria, the corresponding development course is summarized from the certain evaluation method to uncertain one. For long-span bridge design load, the wind and vehicle loads are reviewed and summarized from current multinational codes and theoretical research. For double-deck rail-cum-road bridge, the mechanism of multi-element coupling relationship and corresponding aerodynamic interference are both reviewed. By comprehensive review and summary, the analytical framework is in the process from simplification to refinement. The simulation and consideration of the objects of structural interference gradually become complex. The corresponding simulation theory, wind tunnel scale, test equipment and technology are the key factors to limit its development. For systematic evaluation of vehicle and bridge, the structural and systemic security are the basis of the evaluation, and the auxiliary components and functional evaluation need to be paid more attention. The evaluation criterion will be developed from certain method to reliability assessment. For design load of long-span bridge, the vehicle load is gradually transferred from the simple application of the design load of small-medium span bridge into a complex model considering the load characteristics. For double-deck rail-cum-road bridge, the basic theory and experimental study on coupling mechanism and aerodynamic interference need to be developed.

Numerical investigation of the aerodynamic interference in 2-aircraft formation flight

Due to obvious reduction in drag, formation flight attracts wide attention of major powers in the world. Based on wild geese flying in a ‘V’ formation, a formation composed of two flying-wing aircrafts is designed. The aerodynamic interference in 2-aircraft formation flight is studied by CFD. Results show that 2-aircraft formation flight has little impact on leading aircraft, but has an evident impact on following aircraft. At the same AoA, the effect of increment in lift and reduction in drag of following aircraft is large. As considering the balance of gravity and lift, the reduction in drag of following aircraft is much larger, that is up to 25.1%. Under the aerodynamic interference effect, the following aircraft has a nosedown pitching moment increment, and becomes lateral static instability, which is opposite to the characteristic of solo flying-wing aircraft. The aerodynamic interference mainly comes from upwash flow induced by wingtip vortex of leading aircraft, which reduces the actual AoA and induced drag of following aircraft, and changes the lift distribution of both sides of following aircraft.

Aerodynamic interference and unsteady loads for a hovering intermeshing rotor

Synchropters are increasingly applied for precision work in placing suspended loads due to their unique flying characteristics. However, the special intermeshing rotor structural helicopter easily introduces rotor-and-rotor interference, leading to unsteady aerodynamic loads on the rotor blades in mission flight. Hence, we use the unsteady vortex lattice method to validate and analyze the aerodynamic characteristics of the intermeshing rotor in hover. In this method, a circulation suppression technique on the blade root is introduced to enhance the stability of the rotor wake in the steady-state hover. Results show that the meniscus truncated regions on the intermeshing rotor disks formed by the blade–vortex impact are nearly unchanged to collective pitches. The low-order frequency harmonic components and phase lag phenomenon universally found in dual rotor loads and additional oscillating roll moment become inherent characteristics of the intermeshing rotor in hover, especially for kΩ caused by interference of rotor downwash flow and 2 kΩ produced by the blade–vortex interaction at 10° and 190° azimuth. Finally, the left/right rotor loads exist harmonic components of 1, 2, and 3 kΩ. In contrast, the total thrust of the intermeshing rotor system eliminates the odd-order harmonic components but aggravates the even-order harmonic components.

Numerical Simulation Method with Engine Power and Analysis of Jet Interaction

To explore the effects of engine jet on overall aircraft aerodynamic performance, an aerodynamic numerical simulation method and the interference effects of aircraft with wing-mounted nacelle were studied. In aspect of numerical simulation, based on isentropic flow relationship and automatic mass-flow matching method, a high-precision / high-efficiency numerical simulation of inlet and exhaust boundary conditions is achieved. The calculation result is in good agreement with the experimental values, which validates the calculation method and boundary condition treatment. Through numerical simulation, it is found that the ejection of engine jet can accelerate the air flow, reduce the lower wing pressure, enhance the upper wing shock wave and increase the pitch down moment. At the same time, the effect of jet brings blowing drag and loses the cruise efficiency. The airflow acceleration caused by the narrow channel composed of engine, wing and pylon may cause strong aerodynamic interference. It can be effectively weakened by adjusting the pylon contraction shape based on the near-field pressure distribution. In this way, the shock in the inner side of the pylon can also be eliminated.

Aerodynamic Uncertainty Quantification for Tiltrotor Aircraft

The tiltrotor has unique flight dynamics due to the aerodynamic interference characteristics. Multiple aerodynamics calculation approaches, such as the CFD method, are utilised to characterise this feature. The calculation process is usually time-consuming, and the obtained results are generally varied from each other. Thus, the uncertainty quantification (UQ) method will be utilised in this research to identify the aerodynamic inaccuracy effect on the handling qualities of the tiltrotor aircraft. The study aims to quantify the influence of the aerodynamic interference on the tiltrotor flight dynamics in different flight states, such as forward speeds and nacelle tilting angles, which can guide the flight dynamics modelling simplification to improve the simulation efficiency. Therefore, uncertainty identification and full factorial numerical integration (FFNI) methods are introduced to scale these aerodynamic uncertainties. The eigenvalue and bandwidth and phase delay requirements are presented as the failure criteria. The UQ calculation indicates that the uncertainties of the aerodynamic calculation significantly affect the handling quality ratings in two flight ranges: the helicopter mode and the conversion and aeroplane modes with higher forward speed (close to the conversion envelope). Furthermore, a sensitivity analysis is performed to identify the mechanism behind these influences. The results demonstrate that aerodynamics affect the pitching attitude, the pitching damping, and the velocity and incidence stability derivatives. However, the effects of the velocity stability and the incidence stability are the reason causing the handling qualities’ degradation in the helicopter mode and high-speed mode, respectively.

Aerodynamic Interference on Trim Characteristics of Quad-Tiltrotor Aircraft

The aerodynamic interference between the different components of quad-tiltrotor (QTR) aircraft were considered to analyze its influence on trim characteristics. A comprehensive method with the fixed-wake model was developed for multiple aerodynamic interactions, improving the accuracy of the flight dynamics analysis. Additionally, a more general control strategy was developed to tackle the redundant control issue of the QTR, improving its control efficiency by coordinating the authority relationship of various control surfaces across the flight range. Then, the trim features were calculated in the helicopter mode, conversion mode, and airplane mode, and the relevant results with and without interaction were compared. The results show that the aerodynamic interaction mainly influences the body’s vertical force, longitudinal force, and pitching moment. Furthermore, there are significant differences between collective and longitudinal sticks and pitch attitudes. The interference plays a major role in helicopter and conversion modes with a less-than-30-degree tilt angle.

Influence of Fluid Viscosity and Compressibility on Nonlinearities in Generalized Aerodynamic Forces for T-Tail Flutter

The numerical assessment of T-tail flutter requires a nonlinear description of the structural deformations when the unsteady aerodynamic forces comprise terms from lifting surface roll motion. For linear flutter, a linear deformation description of the vertical tail plane (VTP) out-of-plane bending results in a spurious stiffening proportional to the steady lift forces, which is corrected by incorporating second-order deformation terms in the equations of motion. While the effect of these nonlinear deformation components on the stiffness of the VTP out-of-plane bending mode shape is known from the literature, their impact on the aerodynamic coupling terms involved in T-tail flutter has not been studied so far, especially regarding amplitude-dependent characteristics. This term affects numerical results targeting common flutter analysis, as well as the study of amplitude-dependent dynamic aeroelastic stability phenomena, e.g., Limit Cycle Oscillations (LCOs). As LCOs might occur below the linear flutter boundary, fundamental knowledge about the structural and aerodynamic nonlinearities occurring in the dynamical system is essential. This paper gives an insight into the aerodynamic nonlinearities for representative structural deformations usually encountered in T-tail flutter mechanisms using a CFD approach in the time domain. It further outlines the impact of geometrically nonlinear deformations on the aerodynamic nonlinearities. For this, the horizontal tail plane (HTP) is considered in isolated form to exclude aerodynamic interference effects from the studies and subjected to rigid body roll and yaw motion as an approximation to the structural mode shapes. The complexity of the aerodynamics is increased successively from subsonic inviscid flow to transonic viscous flow. At a subsonic Mach number, a distinct aerodynamic nonlinearity in stiffness and damping in the aerodynamic coupling term HTP roll on yaw is shown. Geometric nonlinearities result in an almost entire cancellation of the stiffness nonlinearity and an increase in damping nonlinearity. The viscous forces result in a stiffness offset with respect to the inviscid results, but do not alter the observed nonlinearities, as well as the impact of geometric nonlinearities. At a transonic Mach number, the aerodynamic stiffness nonlinearity is amplified further and the damping nonlinearity is reduced considerably. Here, the geometrically nonlinear motion description reduces the aerodynamic stiffness nonlinearity as well, but does not cancel it.

Active flutter control of a bridge-flap system considering aerodynamic interferences in practical considerations

Designing bridges with spans larger than 2000 m is challenging. One critical problem is that the flutter stability of such bridges is extremely weak. In past studies, actively controlled flaps were found to improve the flutter stability performance. However, in those studies, the deck was always assumed to be an ideal streamlined plate to simplify the aerodynamic interference effect in the deck-flap system. This simplification limits the application of this technique on real projects. To improve the applicability, we adopt a typical bluff body deck to investigate the effect of the active flaps under different wind attack angles. The novelty of this work is the analysis of the aerodynamic interference and resulting control impact considering the application scenario. The tests are conducted through the computational fluid dynamic method. To give intuitive explanations of the aerodynamic interference, the distributed aerodynamic characteristics method is innovatively introduced to quantify the contribution of flap on the flutter stability during the control process. Our results show that the leading flap has a dominant contribution to the flutter stability by inducing the beneficial aerodynamic damping on the deck’s leading surfaces. Its optimal phase should be about π/2 lagging from the deck’s torsional motion. In comparison, the effect of the trailing flap is unstable under different wind attack angles. To guarantee a robust control effect, it is suggested that it remains stationary.

Numerical simulation of hovering flow field and interference characteristics of rotor system

In order to investigate the aerodynamic interference characteristics of tandem rotor, this paper simulates the hovering flow field of single rotor and tandem rotor based on the dynamic structure overset grid method and unsteady Navier-Stokes equations in computational fluid dynamics (CFD). In the study of hover state of single rotor with twist, the trajectory of the blade vortex and the change pattern are briefly analyzed. Subsequently, the flow field disturbance of tandem rotor, the vortex structure, the pull coefficient and the downwash flow are investigated. The results show that there is obvious blade vortex interference in the overlapping area of the tandem rotor, where the downwash flow reduces the effective angle of attack of the blade and thus has a significantly effect on the pull coefficient. The rotor is less disturbed when it is far from the overlapping area, and the tension coefficient does not change significantly. After integrating the pull coefficient of the upper and lower rotors in one cycle, it was found that the loss of pull coefficient of the two rotors reached 5.7% and 10.7% respectively, which proved that the aerodynamic interference between the tandem rotor in hovering condition had an important effect on the aerodynamic performance of the vehicle.

Low-Order Aerodynamic Model for Interference in Multirotor Systems

The paper proposes an aerodynamic model capable of capturing interference effects in arbitrary multirotor systems. The current analysis is limited for nonoverlapping rotors. The model is based on a combined inflow theory approach, where self-induced and influence-induced velocities are obtained using different inflow models. The generalized dynamic wake model was used to obtain the self-induced velocity distribution, whereas the influence-induced velocity field was computed using Beddoes’s generalized wake model. The results of the combined inflow model were compared against free-vortex wake simulations of a twin-rotor system in edgewise flight. Furthermore, the study examined the effects of tip-to-tip separation distances and freestream orientation angles on overall system performance in terms of thrust and power ratios. The comparison indicated that the results of the proposed model agreed within around 5% with the free-vortex wake analysis over all operating conditions. The computational time of the combined inflow model was two to three orders of magnitude lower than the free-vortex wake approach, depending on the code architecture and domain discretization. The analysis shows that the proposed approach is adequate to model primary aerodynamic interference effects in multirotor systems operating at advance ratios greater than or equal to 0.15.

Effects of High-Speed Trains on Trucks Running on a Road–Rail Dual-Use Bridge Under Crosswind

The road–rail dual-use bridge can simultaneously meet the requirements of both the highway and railway transportation owing to its unique bridge type, which makes it environmentally friendly and cost-effective when compared to independent railway and road bridges. This study establishes a dynamic model of a wind-road vehicle–bridge system considering the aerodynamic sheltering effect of trains based on the coupling vibration method of wind-road vehicle–bridge systems. A container truck and a CRH2 high-speed train are considered as the research objectives, and a series of wind tunnel tests are conducted for the train and truck running simultaneously on a road–rail dual-use bridge to determine the effect of the aerodynamic interference of trains on the aerodynamic characteristics and dynamic behavior of vehicles under crosswinds. The entire process of driving the truck through the cable-stayed bridge under crosswinds and the interference of stationary trains is subsequently simulated to generate the time histories of the dynamic displacements of the truck. Consequently, the effect of the stationary trains is separately compared at different locations, such as on the track of the bridge deck and on the truck placed upwind and downwind. Furthermore, the interference effect of stationary trains on the truck placed upwind and downwind is analyzed corresponding to different wind yaw angles. The results demonstrate that the presence of trains on the bridge significantly affects the aerodynamic coefficients and dynamic responses of the truck placed downwind when compared to the truck placed upwind. Additionally, the presence of the trains on the bridge deck has a different effect on the aerodynamic coefficients and dynamic responses of the truck placed downwind. The effect of the trains on the truck at different wind yaw angles presents different patterns of change for different locations of the trains on the bridge deck.

High-fidelity numerical investigations of rotor-propeller aerodynamic interactions

This paper presents high-fidelity numerical investigations of rotor/propeller aerodynamic interactions related to thrust-augmented compound rotorcraft. High-fidelity CFD simulations of a generalised rotor/propeller integration, as studied both numerically and experimentally by ONERA, were performed using the HMB3 solver within the GARTEUR AG25. Simulations of the baseline isolated propulsors were first carried out and validated against experimental measurements. Simulations of the rotor/propeller integration were then performed and compared with ONERA numerical results. Evaluations of the spatial/temporal resolutions were first carried out at low-speed forward. The actuator disk modelling of the main rotor was shown to be an effective and efficient simulation method compared to blade-resolved results. Simulations of the installed propeller were also performed in hover, and the aerodynamic interference was found to be considerably exacerbated. A ducted propeller configuration was also proposed to study how ducting the propeller would change the aerodynamic interference. The duct offloaded the propeller and produced extra thrust, and the blades suffered less from the main rotor downwash. However, the duct generated a strong downwards force and a nose-down pitching moment because of its flow blockage. The loading variations induced by the aerodynamic interactions were found to be strong at low advance ratios but ducting provided significant alleviation.

Faster-than-realtime inverse simulation method for tiltrotor handling qualities investigation

The tiltrotor aircraft has unique flight dynamics characteristics because of the extensive aerodynamic interference and the unique control strategy. Inverse simulation offers an opportunity to study a vehicle's performance during manoeuvring flights. In this paper, an improved inverse simulation method is developed with an Automatic Differentiation (AD) approach embedded in the code based on the verified flight dynamics model of the tiltrotor aircraft. The AD algorithm would accelerate the computational rate of the inverse simulation process and make it achieve faster-than-realtime capability. Then, the XV-15 tiltrotor's control inputs and flight states encountered during a pop-up manoeuvre are investigated using this AD-augmented inverse simulation method, and the real-time capability of this method is also evaluated. The results indicate that the proposed method guarantees both accuracy and faster-than-realtime calculation performance. Lastly, the tiltrotor's manoeuvrability is assessed by executing this manoeuvre in different flight states and manoeuvre settings. Lastly, an envelope involving the velocity and nacelle incidence angle is calculated to indicate the safety region to achieve this pop-up manoeuvre.

Distribution pattern of fluctuating wind pressures on cooling towers in grouped rectangular arrangement

The cooling towers in this study are in a typical grouped arrangement. Traditional research on grouped-tower interference effects usually emphasizes equivalent static wind loads and ignores quantitative definition of interference effects under aerodynamic load, especially those for fluctuating wind pressure distribution patterns. An eight-tower rectangular arrangement with various aerodynamic interference characteristics was selected as the research object on which a rigid-model synchronous-pressure-measurement wind-tunnel test was performed. Fluctuating time-variant pressures on the tower surfaces for different wind directions was obtained. In three typical incoming flow modes: “windward flow”, “crosswind flow” and “leeward flow”, the fluctuating wind pressure distribution around circumferential directions can be summarized as “V” type, “M” type, and half “V” + half “M” type. The interference patterns can be defined as “weak interference”, “asymmetric strong interference” or “symmetric strong interference”, according to which a 9-item fitting formula of fluctuating wind pressure is proposed and a recommended distribution is given. Interference involving aerodynamic correlation and spectra characters of distributed wind loads are further discussed, and a fitting expression of a specific point spectrum is given. The research strategies and conclusions on fluctuating wind pressure interference provide a loading mode reference for evaluating wind resistance design of similar groups of structures.