Crossflow-induced Transition Research Articles

Crossflow-Induced Breakdown and Transition Correlation for a Hypersonic Swept Plate Flow

The crossflow-induced transition for a Mach 6 flow over a swept plate with a 35 mm nose radius and a sweep angle of 45 deg is investigated using direct numerical simulations (DNSs). In order to shed light on the complete transition process, the evolution of a stationary crossflow wave is first simulated. Then, unsteady wall blowing/suction perturbations are introduced to trigger transition to turbulence. The results show that both the type I and III secondary modes are excited, and they subsequently undergo a linear stage of amplification before breakdown. The type II mode is undetected despite its amplification predicted by the two-dimensional eigenvalue stability approach. Overall, the type I mode achieves a dominant amplitude and plays a key role in the transition. Furthermore, a transition correlation method is proposed based on the dominant secondary instability and threshold amplitude concept, and in order to verify and calibrate it, wall perturbations with different amplitudes are introduced to mimic the varying intensities of the background noise. The transition locations predicted by the most amplified secondary mode agree well with those by DNSs, validating the secondary-instability-based criterion for hypersonic crossflow transition.

Streamwise-Elongated Sinusoidal Roughness Elements with Enhanced Laminarizing Effect on Three-Dimensional Boundary Layer

As a laminar flow control device for delaying the crossflow-induced transition of a three-dimensional boundary layer, sinusoidal roughness elements (SREs) are placed in a Falkner–Skan–Cooke boundary layer, and the resultant laminarizing effect is numerically investigated in comparison with discrete roughness elements (DREs). Because SREs are elongated in the streamwise direction and designed to avoid flow tripping, the critical height of SREs is much higher than that of DREs. Moreover, the wake flow behind SREs efficiently generates and sustains crossflow vortices that are not dangerously unstable against secondary instabilities but able to strongly distort the mean crossflow profile into a less unstable one. By measuring this mean flow distortion by SREs and DREs, the laminarizing effect is compared among them. It is shown that the effect of SREs is higher than that of DREs and can be enhanced by choosing the appropriate height, angle, and wavelength depending on the local boundary-layer profile.

Physics-based model for boundary layer transition prediction in a wide speed range

A new physics-based model employing three transport equations is developed for the simulation of boundary layer transitions in a wide speed range. The laminar kinetic energy is used to represent pretransitional streamwise velocity fluctuations, taking account of different instability modes. The fluctuation velocity components normal to the streamwise direction are modeled by another transport equation. Transition is triggered automatically with the development of the pretransitional velocity fluctuations. In the fully turbulent region, the model reverts to the k-ω turbulence model. Different test cases, including subsonic, supersonic and hypersonic flows around flat plates, airfoils and straight cones, are numerically simulated to validate the performance of the model. The results demonstrate the excellent predictive capabilities of the model in different paths of transition. The model can serve as a basis for the extension of additional transition mechanisms, such as rotation and curvature effects, roughness-induced transition and crossflow-induced transition.

Local Correlation-Based Transition Model for High-Speed Flows

A local correlation-based transition model is developed for high-speed transitional flows. Through an analysis of self-similar solutions for compressible boundary layers, a new correlation for the maximum ratio between the vorticity Reynolds number and the momentum thickness Reynolds number is established. New empirical high-speed transition correlations are then developed that take account of streamwise instability, nose bluntness effects, and crossflow-induced transition. On this basis, an intermittency factor transport equation is constructed using strictly local variables and is coupled with the shear-stress transport turbulence model. The new model is validated through several basic test cases under a wide range of flow conditions, including high-speed flat plates, sharp/blunt cones, HIFiRE-5, and a circular cone at a small angle of attack. The results show that the proposed model is capable of predicting the transition behaviors induced by streamwise and crossflow instabilities, as well as the aeroheating environment on high-speed geometries with reasonable precision. Future work will focus on extending the model to pressure-gradient and wall-temperature effects and to roughness-induced transition, as well as to applications to complex high-speed vehicles.

Numerical investigation of effect of crossflow transition on rotor blade performance in hover

In the present study, the effect of crossflow transition on the rotor blade performance in hover was investigated numerically. The simulations were conducted for the Pressure Sensitive Paint (PSP) rotor in hover by using a Reynolds-averaged Navier-Stokes (RANS) solver based on unstructured meshes. The k−ωSST model was used for simulating the turbulent flow fields and for calculating the turbulent eddy viscosity. For predicting the laminar-turbulent onset phenomena involving crossflow-induced transition, the γ−Reθt−CF+ transition model was adopted. For the comparison of the transition locations without considering the crossflow transition effect, simulations were also carried out using the γ−Reθt transition model. The calculations were made for the PSP rotor at collective pitch angles from four to 12 degrees with an interval of two degrees. To investigate the effect of blade tip Mach number, two blade tip Mach numbers of 0.585 and 0.65 were tested. The predicted results such as the transition onset location and the rotor aerodynamic performance in terms of thrust coefficient, torque coefficient and figure of merit were compared with experimental data. It was found that proper consideration of the effect of crossflow transition is critical for the accurate prediction of the laminar-turbulent transition onset location on rotor blades. The figure of merit also compares better with the experimental data when the effect of laminar-turbulent transition is included. With the addition of the crossflow transition, the figure of merit is slightly decreased, particularly at low thrust levels.

Fully Local Transition Closure Model for Hypersonic Boundary Layers Considering Crossflow Effects

A computational fluid dynamics (CFD) compatible and fully local transition closure model is proposed for three-dimensional hypersonic boundary layers considering crossflow effects. The transition model is constructed with transport equations for laminar fluctuation viscosity and intermittency factor and coupled with Menter’s shear stress transport turbulence model. Based on the parameter analyses of the compressible Falkner–Skan–Cooke similarity solutions, the key factors for crossflow transition modeling, namely, the maximum crossflow velocity and the crossflow Reynolds number, are evaluated through new formulations based on local variables only. The new transition model is first tested against two axisymmetric cones to verify that the first and second modes are well modeled and not influenced by the newly established formulations for crossflow instability. Then, the HIFiRE-5 cases in quiet and noisy conditions and a circular cone at an angle of attack of 3 deg are selected to assess the performance of the present model for crossflow-induced transition. The numerical results show that the transition onset locations predicted by the present transition model are quite consistent with the experimental results, which indicates that the present transition model is capable of predicting crossflow-induced transition with a reasonable degree of accuracy and good compatibility with modern parallel CFD codes using local variables.

Control of crossflow-vortex-induced transition by unsteady control vortices

The fundamental mechanisms of a hitherto unstudied approach to control the crossflow-induced transition in a three-dimensional boundary layer employing unsteady control vortices are investigated by means of direct numerical simulations. Using a spanwise row of blowing/suction or volume-force actuators, subcritical travelling crossflow vortex modes are excited to impose a stabilizing (upstream) flow deformation (UFD). Volume forcing mimics the effects of alternating current plasma actuators driven by a low-frequency sinusoidal signal. In this case the axes of the actuators are aligned with the wave crests of the desired travelling mode to maximize receptivity and abate the influence of other unwanted, misaligned modes. The resulting travelling crossflow vortices generate a beneficial mean-flow distortion reducing the amplification rate of naturally occurring steady or unsteady crossflow modes without invoking significant secondary instabilities. It is found that the stabilizing effect achieved by travelling control modes is somewhat weaker than that achieved by the steady modes in the classical UFD method. However, the energy requirements for unsteady-UFD plasma actuators would be significantly lower than for steady UFD because the approach makes full use of the inherent unsteadiness of the plasma-induced volume force with alternating-current-driven actuators. Also, the input control amplitude can be lower since unsteady crossflow vortex modes grow stronger in the flow.

Active control of crossflow-induced transition by means of in-line pneumatic actuator orifices

The possibility of a pneumatic actuator system for controlling the crossflow vortex-induced laminar breakdown is investigated by means of hot-wire measurements. Steady blowing or suction through a spanwise row of periodically arranged orifices initiates a system of longitudinal vortices which reduces the amplitude of the most amplified stationary crossflow vortices. Thus, the onset of high-frequency secondary instability and the following laminar–turbulent transition was shifted farther downstream. All experiments were conducted at the redesigned DLR swept flat plate experiment in the open test section of the 1 m wind tunnel at the DLR in Gottingen.

Enhancement of a Correlation-Based Transition Turbulence Model for Simulating Crossflow Instability

A new laminar–turbulence transition model for capturing three-dimensional boundary layers involving crossflow-induced transition has been developed by extending the existing transition model. In the present transition model, to resolve transition due to crossflow instability, the C1-criterion was evaluated locally using the Falkner–Skan–Cooke velocity profiles. Thus, only local flow quantities are used in determining the onset of three-dimensional transition. As a result, the extended transition model is compatible and works well with modern computational fluid dyanmics flow solvers, particularly for those based on unstructured meshes, under massive parallel environment. The new transition model was implemented in a three-dimensional incompressible unstructured mesh flow solver. Validations of the new transition model were made for an infinite swept-wing configuration and an inclined prolate spheroid by comparing the results with those of the baseline model and experiment. It was found that the new transition model works well by capturing the crossflow-induced transition inside three-dimensional boundary layers more accurately than the baseline model.

G0500605 横流れ遷移現象を含む流れ場の数値解析

G0500605 横流れ遷移現象を含む流れ場の数値解析

Development of square nozzles for supersonic low-disturbance wind tunnels

Two Mach 2.4 nozzles with square test sections have been designed and analyzed, as part of an effort to develop low-disturbance facilities for laminar-flow control for high-speed civil transport. The mean flows have been simulated using a finite volume, central-differencing scheme to solve the thin-layer NavierStokes equations. Substantial crossflow exists in the boundary layers of both nozzles. The crossflow changes direction about halfway between the throat and exit, because of a change in the sign of the crossflow pressure gradient. S-shaped crossflow profiles are present in this region, just as on a swept wing. Both the standard crossflow Reynolds number and the new Reed and Haynes transition estimator predict transition to occur in the throat as well as near the exit. An analysis of the crossflow pressure gradients in the transonic throat region indicates that streamwise curvature has only a weak effect on the crossflow pressure gradient. Further research on crossflow-induced transition will have to be carried out before these nozzles will be suitable for low Mach number quiet-tunnel designs.

Design of a wing shape for study of hypersonic crossflow transition in flight

Computational fluid dynamics methods were used in the design of a wing shape for study of hypersonic crossflow transition in flight. The flight experiment is to be performed on the delta wing of the first stage of a Pegasus launch vehicle as a piggy-back experiment to support boundary-layer stability code development and validation. The design goal is to obtain crossflow-induced transition at 20–40% of the chord for a flight Mach number of approximately six. The present paper describes the design and analysis process utilized to obtain desired glove shape. A variety of schemes were used in the design, ranging from simple empirical crossflow correlations to three-dimensional Navier-Stokes codes in conjunction with linear stability N- factor computations. The sensitivity to various parameters, such as trajectory variations, allowable wing thickness, leading-edge radius and surface temperature, is also discussed.