Three-dimensional Bumps Research Articles

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.

Superhydrophilicity/superhydrophobicity of nickel micro-arrays fabricated by electroless deposition on an etched porous aluminum template

Nickel nano-particles were deposited on an etched porous aluminum membrane template by electroless deposition without activation and sensitization to fabricate Ni micro-arrays. The increase in material surface roughness makes the surface of the Ni micro-arrays superhydrophilic with a water contact angle of 0°. Subsequently, the micro-arrays are made superhydrophobic with a water contact angle of 164±2° via modification by 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FAS-17). The superhydrophobic Ni micro-arrays have a very low surface energy of 1.62mJm−2 as compared to the 73.51mJm−2 of superhydrophilic Ni micro-arrays. The three-dimensional bump structure formed over the top edge of the pillars of Ni micro-arrays after modification with FAS-17 is hypothesized to be a nearly optimum superhydrophobic structure. Thus, the synthesized superhydrophobic Ni micro-arrays exhibit excellent self-cleaning properties.

Aerodynamic Design of a Supersonic Inlet with a Parametric Bump

Numerical investigations were performed with an external-compression inlet with a three-dimensional bump at Mach 2 to scrutinize the geometrical effects of the bump in controlling the interaction of a shock wave with a boundary layer. The inlet was designed for two oblique shock waves and a terminal normal shock wave followed by a subsonic diffuser, with a circular cross section throughout. The bump-type inlet that replaced the aft ramp of the conventional ramp-type inlet was optimized with respect to the inlet performance parameters as well as compared with the conventional ramp-type inlet. The current numerical simulations showed that a bump-type inlet can provide an improvement in the total pressure recovery downstream of the shock wave/boundary layer interaction over a conventional ramp-type inlet.

Numerical study on performance of supersonic inlets with various three-dimensional bumps

Numerical investigations were performed with a supersonic inlet system installed with a three-dimensional bump which was substituted for a diverter or conventional ramp-type compression systems at Mach 2. The modified inlets were designed to have two oblique shocks and a terminal normal shock followed by a subsonic diffuser, with a circular cross-section throughout. A numerical analysis was conducted to understand the three-dimensional flow field including shock/boundary layer interactions that occur around a three-dimensional bump and to evaluate the performance of the supersonic inlets. The current numerical simulations showed a bump-type inlet based on a conventional ramp-type inlet can provide an improvement in total pressure recovery downstream of the shock/boundary-layer interaction over a ramp-type inlet.

Shock-Wave/Boundary-Layer Interaction Control Using Three-Dimensional Bumps for Transonic Wings

Three-dimensional bumps have been developed and investigated on transonic wings, aiming to fulfill two major objectives of shock-wave/boundary-layer interaction control, that is, drag reduction and buffet delay. An experimental investigation has been conducted for a rounded bump in channel flow at the University of Cambridge and a computational study has been performed for a spanwise series of rounded bumps mounted on a transonic aerofoil at the University of Stuttgart. In both cases wave drag reduction and mild control effects on the boundary layer have been observed. Control effectiveness has been assessed for various bump configurations. A double configuration of narrow rounded bumps has been found to perform best, considerably reducing wave drag by means of a well-established X-shock structure with little viscous penalty and thus achieving a maximum overall drag reduction of about 30%, especially when significant wave drag is present. Counter-rotating streamwise vortex pairs have been produced by some configurations as a result of local flow separation. On the whole a large potential of three-dimensional control with discrete rounded bumps has been demonstrated both experimentally and numerically.

Three-dimensional contour bumps for transonic wing drag reduction

Two-dimensional and three-dimensional contour bumps are designed and optimized for substantial wave drag reduction for an un-swept natural laminar flow (NLF) wing (RAE5243 aerofoil section) at transonic speeds. An NLF aerofoil wing is chosen in this study, as shock control is more crucial for such wings due to the requirement of favourable pressure gradients on a substantial part of the wing. For the validation purpose and to focus on the wave drag issues, the boundary layer is assumed to be fully turbulent from the leading edge. Key bump geometrical parameters including the maximum height, the length, and the crest position have been chosen for the parameterization of the two-dimensional and three-dimensional shock control bumps. For the three-dimensional bumps, an array of the contour bumps is installed spanwise on the transonic wing and their width and spanwise spacing are chosen as additional design parameters. Both the two-dimensional and the three-dimensional bump shapes are optimized using a discrete adjoint-based optimization method. The performance of the three-dimensional contour bumps are compared in detail with the similarly optimized two-dimensional bumps both at and around the design point. The results show that, for the NLF wing studied, the optimized three-dimensional bumps are as effective as the optimized two-dimensional bump in terms of total drag reduction at the given design point, despite the significant difference in their geometrical shapes. More importantly, in terms of the operational range for varying lift conditions for practical applications, the three-dimensional bumps outperform the two-dimensional bump by a substantial margin.

A combined experimental and numerical study of flow structures over three-dimensional shock control bumps

A detailed study of transonic flow over three-dimensional bumps has been conducted using experimental measurements and computational simulation. The aim of the investigation is to determine the flow characteristics over these potentially effective flow control devices for wave drag reduction for transonic aircraft wings. Careful qualitative and quantitative matching of the simulation and experimental conditions has considerably improved the agreement between them. In both the experiments and the computation, the shock position in the working section was found to be sensitive not only to the back pressure and the sidewall effects, but also to the incoming boundary layer characteristics. Two turbulence models, a modified Baldwin–Lomax model with enhanced performance in separated flow (curvature model) and a two-equation model ( k – ω ) have been implemented in the numerical simulation. Overall, both turbulence models gave reasonable results for the uncontrolled and controlled cases regarding the inviscid flowfield and the shock structure. In particular, a pair of streamwise vortices embedded in the boundary layer was captured by the two models. The traces in the surface oil flow pictures from the experiment also suggested the existence of the streamwise vortices. The combined surface and flowfield data provide some further insight into the flow physics on the shock control ramp bump, which is discussed in the paper. In addition, the study also demonstrates the enhanced capability of the algebraic model in capturing separated flow features with lower computational cost as compared to the k – ω model.

A numerical analysis on three-dimensional flow field in a supersonic bump-type inlet

The characteristic of a supersonic inlet system with three-dimensional bump which is substituted for the diverter or conventional ramp-type compression systems has been studied numerically. A comprehensive numerical analysis has been performed to understand the three-dimensional flow field including shock/boundary layer interaction and growth of turbulent boundary layer that might occur around a three-dimensional bump in a supersonic inlet. The current numerical simulations showed the supersonic bump-type inlet which is modified from a conventional ramp-type inlet to control shock/boundary layer interaction effectively and evolved to maximize inlet performance.

Stokes flow through a tube with bumpy wall

The Stokes flow through a tube with a bumpy wall is solved through a perturbation in the small amplitude of the three-dimensional bumps. The solution can be expressed in terms of modified Bessel functions. For given area or volume of a bump, there exists an optimum circumferential wave number and an optimum aspect ratio for which the flow rate is maximized.

Various types of growth instabilities in coherent epitaxy

The instability of coherent epitaxial systems Ge/Si(0 0 1) is investigated. The qualitatively different features observed at low misfit (1 or 2%) and at larger misfit (more than 3%) are explained by a different type of nucleation of new terraces. At high misfit they are nucleated on the upper edge of existing steps and this triggers the sudden formation of three-dimensional bumps. At lower misfits, new terraces are nucleated near the centres of existing terraces and growth proceeds as in a pure crystal. The instability takes place at later times as an effect of kinetic roughness.

A ghost-cell immersed boundary method for flow in complex geometry

An efficient ghost-cell immersed boundary method (GCIBM) for simulating turbulent flows in complex geometries is presented. A boundary condition is enforced through a ghost cell method. The reconstruction procedure allows systematic development of numerical schemes for treating the immersed boundary while preserving the overall second-order accuracy of the base solver. Both Dirichlet and Neumann boundary conditions can be treated. The current ghost cell treatment is both suitable for staggered and non-staggered Cartesian grids. The accuracy of the current method is validated using flow past a circular cylinder and large eddy simulation of turbulent flow over a wavy surface. Numerical results are compared with experimental data and boundary-fitted grid results. The method is further extended to an existing ocean model (MITGCM) to simulate geophysical flow over a three-dimensional bump. The method is easily implemented as evidenced by our use of several existing codes.

Multiple scale modeling of turbulent nonequilibrium boundary layer flows

Multiple scale models are investigated in order to improve modeling of turbulent nonequilibrium boundary layers. The model is first calibrated on the standard equilibrium boundary layer and then applied to several nonequilibrium flows. Applications include the flow around an airfoil, the shock–boundary layer interaction, the transonic bump flow, and a three-dimensional bump. The multiple scale model predictions are compared with experimental results and with standard one-scale models. Several improvements are obtained that make the multiple scale model an interesting tool for tackling the prediction of nonequilibrium turbulent flows.

Secondary instability modes generated by a Tollmien-Schlichting wave scattering from a bump

The plane finite-amplitude Tollmien-Schlichting wave interaction with a three-dimensional bump on a wall is considered for plane channel flow. The scattering of this wave leads to the production of unsteady three-dimensional disturbances which transform into growing secondary instability modes. The generation of such modes is studied assuming the three-dimensional disturbances to be small in comparison with the primary plane instability wave. The solution predicts that secondary disturbance amplification takes place only within a narrow wedge downstream of the bump. The qualitative comparison of results with experiments on turbulent wedge origination at an isolated roughness in a boundary layer is presented.

The velocity field created by a shallow bump in a boundary layer

The results of measurements of the disturbance velocity field generated in a boundary layer by a shallow three-dimensional bump oscillating at a very low frequency on the surface of a flat plate are reported. The disturbance was entirely confined to the boundary layer. Profiles of the mean velocity, the disturbance velocity at the fundamental frequency, and at the first harmonic are presented. These profiles were measured both upstream and downstream of the oscillating bump. About 100 boundary layer thicknesses downstream of the bump the intensity had a maximum near η=2, with a magnitude of about 0.2%. Measurements of the disturbance velocity were also made at various spanwise and downstream locations at a fixed distance from the boundary of one displacement thickness. The spanwise profiles of the disturbance field changed dramatically from the near-field region of the bump to the far downstream region. Finally, the spanwise spectrum of the disturbances at three locations downstream of the bump are presented. The spanwise spectrum of the disturbance showed a primary peak consistent with the motion and geometry of the driver. This peak decreased slowly with downstream distance. In addition, a small secondary peak in the spanwise spectrum was apparent near the bump. This peak increased rapidly with downstream distance.