Two-dimensional Bump Research Articles

The Bump Suppression Strategy for the Transonic Buffet of the Supercritical Airfoil

Transonic buffet brings challenges to civil aircraft design, including but not limited to the structural fatigue and the handling quality degradation. Therefore, it is necessary to analyze the transonic buffet of the supercritical airfoil and propose the buffet suppression strategy. This paper first employs the delayed detached-eddy simulation method to simulate the unsteady buffet process of the OAT15A supercritical airfoil. Then, the temporal–spatial characteristics of the flowfield and the separation region dynamics are analyzed based on the time-domain analysis method and the dynamic mode decomposition method. After that, different two-dimensional bumps are arranged in the shock motion region to suppress the shock oscillation, and the suppression mechanism and suppression strategy are studied. The results show that the process of the developed transonic buffet is significantly influenced by the merging of the front and the rear separation regions. In case the bump obstructs this confluence phenomenon, the developed transonic buffet can be suppressed. Furthermore, it is also found that a two-dimensional bump with superior buffet performance should have the following features: The bump should be placed after the shock-induced boundary-layer separation point to make the shock-induced boundary-layer separation bubble generate on the bump ramp. Also, the height of the bump should be carefully designed to prevent the front and rear separation regions from merging and to make the front separation region disappear quickly. Finally, this strategy is successfully applied to another supercritical airfoil to confirm the universality of the controlling strategy proposed.

Direct numerical simulation of a turbulent boundary layer over a bump with strong pressure gradients

Abstract

Design and Evaluation of Generic Bump for Flow Control in a Supersonic Inlet Isolator

Abstract In this paper, the geometry of a supersonic inlet isolator is modified by the introduction of a two-dimensional (2D) bump to control the complex lip shock wave/boundary layer interaction (SWBLI). The bump is of general shape whose profile is designed primarily based on the inviscid theory of oblique shock waves, which accommodates the effect of freestream conditions; particularly, the flow Mach number. Further, the geometric constraints of the inlet are taken into consideration to generate a contoured bump. This well-designed generic bump is tested in the range of flight Mach number of 2.5 to 3.8 through numerical computations. The adopted computational methods are validated with the available experimental data. Results showed that the modified inlet using the present generic bump changes the internal shock structure, weakens the intensity of SWBLI, and subsequently reduces shock reflection phenomena which are prevalent in baseline inlet. The wall characteristics such as separation bubble (SB), skin friction, and total pressure loss are found to be reduced in inlet with bump. The SB in baseline inlet typically corresponds with the geometric profile of the bump. As a result, ramp of baseline inlet is apparently replaced by this generic bump, which eliminates the low momentum fluid adjacent to the wall and the passage of modified inlet is found to be mostly occupied by high momentum supersonic flow. The flow control and associated performance improvement are linked with this modification of supersonic inlet isolator.

Direct numerical simulation of stratified Ekman layers over a periodic rough surface

Abstract

Assessment of viscosity models that incorporate lag parameter scaling

Elliptic blending lag models combine a ‘stress-strain misalignment’ parameter with conventional k−ω or k−ε based linear eddy viscosity models. The lag parameter is effective in scaling the behavior of the eddy viscosity, particularly in the near wall region. This proves to be critical for prediction of non-equilibrium flows. Flow configurations demonstrating such regimes are evaluated with the new class of lag models and their performances are compared with frequently used RANS formulations (k−ωSST,k−ω-2006). A two-dimensional bump geometry that creates different pressure gradient regimes in the flow field was chosen as the first test case as RANS models often perform poorly in adverse pressure gradient flows. A swept configuration of similar bump geometry was also chosen to assess the performance of models for flows with transverse pressure gradients. Separation inside a 3D diffuser of varying inlet aspect ratios expanding in both wall-normal and spanwise directions was studied. Such a configuration incorporates the effect of multi-directional pressure gradient on a flow and usually proves challenging for linear eddy viscosity models. Finally, the models were tested on a complex 3D test case with a variation of NACA 0020 airfoil attached to a plate. This junction flow has been widely examined both experimentally and numerically in the past with emphasis being on the prediction of flow around the junction and subsequent downstream development. The lag models were observed to be successful in reproducing the mean flow quantities when compared to high fidelity/experimental solutions for the 2D and 3D flows.

Large-eddy simulation of turbulent flow over a parametric set of bumps

Turbulent flow over a series of increasingly high, two-dimensional bumps is studied by well-resolved large-eddy simulation. The mean flow and Reynolds stresses for the lowest bump are in good agreement with experimental data. The flow encounters a favourable pressure gradient over the windward side of the bump, but does not relaminarize, as is evident from near-wall fluctuations. A patch of high turbulent kinetic energy forms in the lee of the bump and extends into the wake. It originates near the surface, before flow separation, and has a significant influence on flow development. The highest bumps create a small separation bubble, whereas flow over the lowest bump does not separate. The log law is absent over the entire bump, evidencing strong disequilibrium. This dataset was created for data-driven modelling. An optimization method is used to extract fields of variables that are used in turbulence closure models. From this, it is shown how these models fail to correctly predict the behaviour of these variables near to the surface. The discrepancies extend further away from the wall in the adverse pressure gradient and recovery regions than in the favourable pressure gradient region.

Investigation of the dynamics in separated turbulent flow

Abstract Dynamical behavior of the turbulent channel flow separation induced by a wall-mounted two-dimensional bump is studied, with an emphasis on unsteadiness characteristics of vortical motions evolving in the separated flow. The present investigations are based on an experimental approach and Direct Numerical Simulation ( Dns ). The main interests are devoted to give further insight on mean flow properties, characteristic scales and physical mechanisms of low-frequencies unsteadiness. The study also aims to clarify the Reynolds number effects. Results are presented for turbulent flows at moderate Reynolds-number R e τ ranging from 125 to 730 where R e τ is based on friction velocity and channel half-height. A large database of time-resolved two-dimensional Piv measurements is used to obtain the velocity distributions in a region covering the entire shear layer and the flow surrounding the bump. An examination of both high resolved velocity and wall-shear stress measurements showed that for moderate Reynolds numbers, a separated region exists until a critical value. Under this conditions, a thin region of reverse flow is formed above the bump and a large-scale vortical activity is clearly observed and analyzed. Three distinct self-sustained oscillations are identified in the separated zone. The investigation showed that the flow exhibits the shear-layer instability and vortex-shedding type instability of the bubble. A low-frequency self-sustained oscillation associated with a flapping phenomenon is also identified. The experimental results are further emphasized using post-processed data from Direct Numerical Simulations, such as flow statistics and Dynamic Mode Decomposition. Physical mechanisms associated with observed self-sustained oscillations are then suggested and results are discussed in the light of instabilities observed in a laminar regime for the same flow configuration.

Control of Transonic Buffet by Shock Control Bumps on Wing-Body Configuration

The aerodynamic shock buffet phenomenon limits the flight envelope of typical transonic aircraft at high Mach number or incidence. For future aircraft design, its correct prediction by numerical methods and control strategies are of great interest. In the present paper, simplified industrial applicable design guidelines for shock control bumps are summarized and assessed based on numerical (unsteady) Reynolds-averaged Navier–Stokes simulations of a wing-body configuration. The design guidelines cover both design objectives, wave drag reduction at dash flight, and buffet delay/alleviation. Three different shock control bumps have been designed: a two-dimensional bump for wave drag reduction at dash flight, a two-dimensional bump for delay of buffet onset, and a three-dimensional bump array with the intention of combining both design objectives. The two-dimensional bumps mark the maximum potential for each design objective with a drag reduction of 9 drag counts at dash flight and a delay of buffet onset by 4 lift counts. Even though the three-dimensional bump array combines both design objectives, it does not perform as well as expected due to massive crest-flow separation and a (so far) minor role of the vortical wake of three-dimensional bumps in delaying buffet onset.

Numerical Study on the Ability of Shock Control Bumps for Buffet Control

In addition to efficient reduction of wave drag in transonic flight, shock control bumps also offer some potential for buffet alleviation. In the present paper, two different approaches for buffet control by shock control bumps are compared and assessed based on time-resolved (unsteady) Reynolds-averaged Navier–Stokes simulations: downstream positioning of two-dimensional bumps, necessitating an adaptive device for combining both features (wave drag reduction and buffet control), and vortex generation by three-dimensional bumps as “smart vortex generators”. With the intention of providing detailed insight into dominant flow features and linking geometrical bump characteristics to those flow structures, the effect of bump design condition, crest height, and streamwise positioning of two- and three-dimensional shock control bumps on buffet behavior and performance of a supercritical, unswept wing section has been analyzed. Two-dimensional shock control bumps improve buffet behavior by an efficient shock strength reduction in combination with positive effects on flow separation. For three-dimensional bumps, the same buffet-affecting mechanisms have been observed, only less dominant due to the finite spanwise extent. Furthermore, it has been demonstrated that the strength of three-dimensional bumps’ vortical wake can be tuned by appropriate bump shaping and that this strength positively correlates with delayed buffet onset.

Multiple solutions for granular flow over a smooth two-dimensional bump

Geophysical granular flows, such as avalanches, debris flows, lahars and pyroclastic flows, are always strongly influenced by the basal topography that they flow over. In particular, localised bumps or obstacles can generate rapid changes in the flow thickness and velocity, or shock waves, which dissipate significant amounts of energy. Understanding how a granular material is affected by the underlying topography is therefore crucial for hazard mitigation purposes, for example to improve the design of deflecting or catching dams for snow avalanches. Moreover, the interactions with solid boundaries can also have important applications in industrial processes. In this paper, small-scale experiments are performed to investigate the flow of a granular avalanche over a two-dimensional smooth symmetrical bump. The experiments show that, depending on the initial conditions, two different steady-state regimes can be observed: either the formation of a detached jet downstream of the bump, or a shock upstream of it. The transition between the two cases can be controlled by adding varying amounts of erodible particles in front of the obstacle. A depth-averaged terrain-following avalanche theory that is formulated in curvilinear coordinates is used to model the system. The results show good agreement with the experiments for both regimes. For the case of a shock, time-dependent numerical simulations of the full system show the evolution to the equilibrium state, as well as the deposition of particles upstream of the bump when the inflow ceases. The terrain-following theory is compared to a standard depth-averaged avalanche model in an aligned Cartesian coordinate system. For this very sensitive problem, it is shown that the steady-shock regime is captured significantly better by the terrain-following avalanche model, and that the standard theory is unable to predict the take-off point of the jet. To retain the practical simplicity of using Cartesian coordinates, but have the improved predictive power of the terrain-following model, a coordinate mapping is used to transform the terrain-following equations from curvilinear to Cartesian coordinates. The terrain-following model, in Cartesian coordinates, makes identical predictions to the original curvilinear formulation, but is much simpler to implement.

Japan Aerospace Exploration Agency Studies for the Second High-Lift Prediction Workshop

This paper summarizes Japan Aerospace Exploration Agency studies using a structured grid solver UPACS and an unstructured grid solver TAS-code for the second AIAA CFD High-Lift Prediction Workshop test cases and follow-on problems. Computations are performed for the grid convergence study using the configuration without any slat tracks and flap track fairings on a series of multi-block structured grids and a series of unstructured grids provided by the High Lift Prediction Workshop committee. In this paper, the following are mainly discussed: 1) grid convergence verification study of the turbulence models using a two-dimensional bump case, 2) grid convergence studies for Spalart–Allmaras and shear stress transport turbulence models using UPACS, 3) change of grid convergence and predicted flow fields with and without a nonlinear quadratic constitutive relation model for Spalart–Allmaras and shear stress transport turbulence models using UPACS including evaluation at higher angles of attack, and 4) comparison studies using different solvers UPACS and TAS on different grid systems with and without the quadratic constitutive relation model for the Spalart–Allmaras model.

Open-loop control of a separated boundary layer

Linear optimal gains Gopt(ω) are computed for the separated boundary-layer flow past a two-dimensional bump in the subcritical regime. Very large values are found, making it possible for small-amplitude noise to be strongly amplified and to destabilize the flow. Next, a variational technique is used to compute the sensitivity of Gopt(ω) to steady control (volume force in the flow, or blowing/suction at the wall). The bump summit is identified as the region the most sensitive to wall control. Based on these (linear) sensitivity results, a simple open-loop control strategy is designed, with steady wall suction at the bump summit. Calculations on non-linear base flows confirm that optimal gains can be significantly reduced at all frequencies using this control. Finally, sensitivity analysis is applied to the length of the recirculation region lc and reveals that the above control configuration is also the most efficient to shorten the recirculation region. This suggests that lc is a relevant macroscopic parameter to characterize wall-bounded separated flows, which could be used as a proxy for energy amplification when designing steady open-loop control.

Experimental investigation on laminar separation control for flow over a two-dimensional bump

The laminar boundary layer separation flow over a two-dimensional bump controlled by synthetic jets is experimentally investigated in a water channel with hydrogen-bubble visualisation and particle image velocimetry (PIV) techniques. The two-dimensional synthetic jet is applied near the separation point. Two Reynolds numbers (Re = 700 and 1120) based on the bump height and free-stream velocity are adopted in this experiment, and seven different excitation frequencies at each Reynolds number are considered, focusing on the separation control as well as the vortex dynamics. The experimental results show that the optimal control can only be achieved within some excitation frequencies at both Reynolds numbers. However, beyond this range, further increasing the excitation frequency leads to an increase in the separation region. The proper orthogonal decomposition (POD) technique and vortex identification by swirling strength (Λci) are applied for the deeper analysis of the separated flow. The reconstructed Λci field by the first four POD modes is used and vortex lock-on phenomenon is observed. It is found that the negative synthetic jet vortex with clockwise rotation draws the separated wake shear layer as it is convected downstream, and then they syncretise together. Thus, the new vortex is induced and shedding downstream periodically.

Control of Incident Shock/Boundary-Layer Interaction by a Two-Dimensional Bump

Separation due to the shock-wave/boundary-layer interaction in a supersonic/hypersonic inlet always brings negative effects on its performance. A new control method of the shock-wave/boundary-layer interaction based on a two-dimensional bump is brought forward and investigated by both experimental and computational methods. First, the uncontrolled case is studied. The flow turning angle of the incident shock is 12 deg, and the freestream Mach number is 3.5. The results show that the shock-wave/boundary-layer interaction generates complex three-dimensional flow structures with significant swirling nature, which will substantially deteriorate the performance of a supersonic/hypersonic inlet. Then, the controlled case demonstrates that the shock-induced flow separation can be effectively reduced by the coupling of the precompression effect and the acceleration effect of the bump with a height of 0.33 times of the boundary-layer thickness. In addition, the efficiency of the control method for different shock-impingement positions is obtained, indicating that the separation can be well controlled when the shock impinges on the convex surface of the bump or the concave surface on the leeward side of the bump.

Shifting media for carpet cloaks, antiobject independent illusion optics, and a restoring device

In traditional cases, the basic principle of carpet cloak is that a two-dimensional bump above ground plane is compressed into a one-dimensional line which is close to the ground plane. As a result, the bump can be hidden. In this paper, different from the traditional carpet cloak, we propose a shifting media to achieve the carpet cloak. By covering with the shifting media, an object floating on a ground plane can be directly shifted beneath the ground plane (rather than compressing it into a line), resulting in a floating carpet cloak. Antiobject independent illusion optics, i.e., turning an object into another one without any antiobjects, can be realized based on this kind of shifting media. As an application of the shifting media, a restoring device, by which the broken objects (such as antiques) can be perfectly restored, is also investigated.

Toward Designing with Three-Dimensional Bumps for Lift/Drag Improvement and Buffet Alleviation

The desire to design more efficient transport aircraft has led to many different attempts to minimize drag. One approach is the use of three-dimensional shock control bumps, which have gained popularity in the research community as simple, efficient and robust devices capable of reducing the wave drag of transonic wings. This paper presents a computational study of the performance of three-dimensional bumps, relating key bump design variables to the overall wing aerodynamic performance. An efficient parameterization scheme allows three-dimensional bumps to be directly compared to two-dimensional designs, indicating that two-dimensional bumps are capable of greater design point aerodynamic performance in the transonic regime. An advantage of three-dimensional bumps lies in the production of streamwise vortices, such that, while two-dimensional bumps are capable of superior performance near the design point, three-dimensional bumps are capable of breakingup regions of separated flow at high Mach numbers, suggesting improvement in terms of buffet margin. A range of bump designs are developed that exhibit a tradeoff between design point aerodynamic efficiency and improvementinbuffet margin, indicating the potential for bespoke designs to be generated for different sections of a wing based on its flow characteristics. Copyright © 2012 by Jeremy Eastwood and Jerome Jarrett.

Transverse instability and low-frequency flapping in incompressible separated boundary layer flows: an experimental study

AbstractThe unstable dynamics of a transitional laminar separation bubble behind a two-dimensional bump geometry is investigated experimentally using dye visualizations and particle image velocimetry measurements. For Reynolds numbers above a critical value, the initially two-dimensional recirculation bubble is subject to modulations in the spanwise direction which can trigger vortex shedding. Increasing the Reynolds number further, the unstable behaviour is dominated by a low-frequency flapping motion, well known in transonic flows, and here investigated for the first time experimentally in an incompressible flow regime. These phenomena are characterized by non-intrusive measurements of the spatial structure and the frequencies of the unsteady motion. The results are in excellent agreement with previous numerical and theoretical predictions for the same geometry.

Two-Dimensional Bumps in Piecewise Smooth Neural Fields with Synaptic Depression

We analyze radially symmetric bumps in a two-dimensional piecewise-smooth neural field model with synaptic depression. The continuum dynamics is described in terms of a nonlocal integrodifferential equation, in which the integral kernel represents the spatial distribution of synaptic weights between populations of neurons whose mean firing rate is taken to be a Heaviside function of local activity. Synaptic depression dynamically reduces the strength of synaptic weights in response to increases in activity. We show that in the case of a Mexican hat weight distribution, sufficiently strong synaptic depression can destabilize a stationary bump solution that would be stable in the absence of depression. Numerically it is found that the resulting instability leads to the formation of a traveling spot. The local stability of a bump is determined by solutions to a system of pseudolinear equations that take into account the sign of perturbations around the circular bump boundary.

Investigation of large scale shock movement in transonic flow

Large eddy simulations were made of transonic flow over a two-dimensional bump where shock wave/turbulent boundary layer interaction takes place. Different flow conditions were investigated to find conditions for large scale shock movement. The innermost part of the shock was found to be moving for sufficiently strong shocks. None of the cases display large scale movement of the whole shock.

Atomic force microscopy of the mirror cleavage surface of defect TGS crystals

The nanorelief of the mirror cleavage surface of triglycine sulfate crystals with various defect densities has been studied. Typical nanorelief features of both defect and clean (without artificial impurity) crystals are two-dimensional rounded bumps (dips) of equal height (depth) of about half the lattice parameter and sub-micrometer lateral sizes. The density, lateral size, and scatter of such 2D structures are several times larger for defect crystals than for clean ones. The correlation between the crystal defect density and the density and lateral size of 2D structures on the cleavage surface has been revealed. Conclusions are made about the defect origin of the typical nanorelief on the mirror cleavage.