Natural Laminar Flow Research Articles

Numerical Investigations of Laminar Separation Bubbles Under the Influence of Periodic Gusts

Direct numerical simulations were performed to investigate the behavior of laminar separation bubbles subject to vertical gusts in an airfoil flow. Oscillating boundary-layer flows containing gusts—including unsteady pressure gradients—were prescribed via boundary conditions as well as forcing terms on a domain located in the upper rear section of a natural laminar flow airfoil. For this hybrid approach, unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of the entire flowfield were carried out in conjunction with the so-called disturbance velocity approach, providing transient boundary conditions for the direct numerical simulation. A steady-state reference case with a Reynolds number of is compared and validated with results from wind tunnel experiments. Results of simulations at four different gust frequencies at the same amplitude , each with and without additional disturbances to the boundary layer introducing oblique resonance, are presented and discussed in this paper. The time-dependent behavior of convective instability modes is evaluated by using the continuous wavelet transform and linear stability theory with an unsteady extension. Furthermore, the contribution of the absolute instability—which appears to be larger in the case of oscillating flow compared to the steady-state case—is discussed. Lock-in effects are identified at high gust frequencies.

Assessment of Potential Commercial Success of Business Jets with Natural Laminar Flow

The present research investigates the potential application of the natural laminar flow wing technology to business jet market segments from light to long-range jets. A database of existing business jets was generated to determine the range extension as a potential driver of customer interest. A conceptual design of several configurations for each market segment was performed to investigate potential improvements in the aircraft flight range, operating costs, and price changes using the technology. An initial sizing module and a low-fidelity multidisciplinary design optimization were used to size all aircraft. Results demonstrated a 13–30% increase in the flight range depending on the aircraft concept. Long-range jets with natural laminar flow could not achieve significant range extension due to a combination of the design airspeed and maintenance costs. A rapid increase in acquisition prices for all aircraft suggested that super-midsize and large jets that combine relatively low empty weight and high range extension could be more favorable options compared to other segments, but a significant reduction of acquisition costs and an increase in operating flight hours are required to make the technology implementation successful.

Numerical Investigation on the Transition Flow around NLF Airfoil

A natural laminar flow (NLF) airfoil is designed to reduce drag by expanding laminar flow areas. In-depth knowledge of transition performance is essential for its aerodynamic design. The k-ω-γ-Reθ framework, which consists of the SST k-ω turbulence model and γ-Reθ transition model, is employed to simulate transitional flows around an NLF wing RAE5243 airfoil. The transition performances of the RAE5243 airfoil under various values of turbulent intensity, temperature, angle of attack, and Mach number are simulated and compared. The results show that the rise of inflow turbulent intensity will promote an earlier transition on both the suction and pressure sides. The influence of wall temperature on transition is limited. The rise of angle of attack will lead to an earlier transition on the pressure side but a later transition on the suction side. With the rise of Mach number, the transition happens earlier under a zero and positive angle of attack but later under a negative angle of attack. In addition, the correlation of transition onset locations with respect to turbulent intensity, surface temperature, angle of attack, and Mach number is established based on numerical results.

Laminar–Turbulent Transition Prediction on Industrial Computational Fluid Dynamics Applications

This paper presents Ansys Fluent laminar–turbulent transition results using the shear stress transport model applied to the workshop cases of the First American Institute of Aeronautics and Astronautics Computational Fluid Dynamics (CFD) Transition Modeling Prediction Workshop. The key objectives of this workshop were to assess the current state-of-the-art laminar–turbulent transition models in an industrial Computational Fluid Dynamics environment and to determine and document the best practices to simulate laminar–turbulent transition flows. Sensitivity of the shear stress transport model to mesh refinement was established on a zero-pressure-gradient flat plate. Two other cases [a two-dimensional natural laminar flow (NLF) (1)-0416F airfoil, and a scaled Common Research Model (CRM)-NLF aircraft model] were selected as validation cases using a hierarchy of structured and unstructured meshes. Due to the complexity of the geometry and the airflow around the Common Research Model (CRM)- Natural laminar Flow (NLF) aircraft model, mesh adaptation cycles were also conducted to capture the shock, the wake, and the wing-tip vortices produced by the CRM-NLF. The accuracy of the model is evaluated using transition location measurements obtained with temperature-sensitive paint, pressure coefficient distributions at multiple wingspan stations, and aerodynamic coefficients at numerous angles of attack. The outcome of these comparisons will provide guidelines to conduct laminar–turbulent transition simulations with the model on simple and complex aerospace designs.

Fluid mechanism analysis on the interaction between natural laminar flow nacelle and wingbody on a transonic aircraft

A transonic natural laminar flow (NLF) nacelle, which is a streamlined fairing used to contain a turbofan engine and mounted under the wing of civil aircraft, can reduce friction drag. Because the fluid mechanism of the interaction between an NLF nacelle and a wingbody is not clear, laminar flow at high Reynolds numbers in the transonic regime is maintained difficultly. In this work, such interaction on a civil aircraft is investigated. Three NLF nacelles with different pressure distribution characteristics and a baseline nacelle with the turbulent flow are examined. These are installed under the wing of a widebody aircraft to investigate the fluid mechanism between a natural laminar flow nacelle and wingbody. The results show that the influence of the wingbody on the fluid characteristics of the nacelle should be considered in the NLF nacelle design. A well-designed isolated NLF nacelle is different from the one that considers the effect of the wingbody. A favorable pressure gradient in the front part of the nacelle is a key factor in drag reduction. An installed NLF nacelle owning a large pressure peak with a weak favorable pressure gradient and a strong shock wave in front of the nacelle is recommended to be applied in civil aircraft.

In-flight measurement of free-stream turbulence in the convective boundary layer

Natural laminar flow airfoils have achieved such a level of refinement that further optimisation and subsequent wind tunnel testing need to regard the specific free-stream turbulence to be expected during operation. This requires the characterisation of this turbulence in terms of those properties which are relevant for boundary layer receptivity and subsequent transition. These parameters of turbulence change with environmental conditions and, in case of aircraft, along the flight profile. This study investigates the free-stream turbulence relevant for the case of sailplane airfoils. In-flight measurements with a constant temperature anemometer x-wire probe were conducted during cross-country flights in Central Europe and provided 22 h of flight data, covering thermalling phases as well as straight flight legs. Longitudinal and transversal velocity fluctuations were recorded well into the dissipation range. The special challenges of operating a constant temperature anemometer probe continuously for several hours are addressed. The permanent unsteadiness of the inflow poses challenges for the evaluation, but also provides a broad database of measured turbulence levels. The quality of the measurements is shown by verifying some of the predictions of Kolmogorov's inertial range theories. Free-stream turbulence in thermalling phases is sufficiently homogeneous to be described accurately, as the dissipation range fluctuates only in a limited range and follows a log-normal distribution. On the straight flight legs, the turbulence depends on the convective activity along the flight path. In general, within the convective part of the atmosphere, turbulence levels are found to be significantly larger than in low-turbulence wind tunnels.Graphical abstract

Numerical Simulation of the Common Research Model with Natural Laminar Flow

The Common Research Model with natural laminar flow (CRM-NLF) shows promising numerical and experimental results. This work aims at understanding the aerodynamic characteristics of the CRM-NLF wing under wind-tunnel conditions. The significant effects of turbulent wedges and static aeroelastic deformation are shown. Fluid–structure coupled simulations including laminar–turbulent transition with turbulent wedges show an improved agreement with wind-tunnel data made available by NASA when both effects are considered.

Robust Optimization of Natural Laminar Flow Airfoil Based on Random Surface Contamination

Natural laminar-flow (NLF) airfoils are one of the most promising technologies for extending the range and endurance of aircrafts. However, there is a lack of methods for the optimization of airfoils based on the surface contamination that destroys the laminar flow. In order to solve this problem, a robust optimization process is proposed using the Non-dominated Sorting genetic algorithm- II (NSGA-II) evolutionary algorithm, and Monte Carlo simulation combined with an aerodynamic calculation software Xfoil. Firstly, the airfoil is optimized normally and the aerodynamic performance of optimized airfoil under surface contamination is analyzed. Then, the original airfoil is robustly optimized under random surface contamination based on the assumption that its locations follow triangular and uniform probability distributions. Finally, all the optimized results and original airfoil are compared. It is found that robust optimization reduces the sensitivity of the airfoil to random surface contamination, hence, improving the robustness of the airfoil. The proposed methods make it possible to improve the aerodynamic performance of NLF airfoils considering surface contamination.

Airfoil Morphed Leading-Edge Design for High-Lift Applications Using Genetic Algorithm

A genetic algorithm was developed for the optimal design of morphing airfoil leading-edge shapes to maximize airfoil , while retaining a constant leading-edge arc length. Using a candidate slotted, natural laminar flow airfoil, an inviscid–viscous flow simulation program suite was used to evaluate a randomized population of airfoils during the design process at low-speed, high-lift flight conditions of a representative single-aisle commercial transport aircraft. The use of a continuous morphing strategy to perform treatment of the leading-edge geometry in high-lift scenarios, as opposed to leading-edge flap or slat elements, is envisaged as a means to mitigate surface discontinuities and retain laminar flow qualities of future airfoil sections in cruise conditions. By using different definitions of the objective cost function, multiple morphed airfoil designs were found through the genetic algorithm to create a library of possible designs aimed to purposefully alleviate the leading-edge stall tendencies of the baseline airfoil. Three morphed designs were validated through experimentation in a low-speed, open-return wind tunnel. Resulting data were used to verify that the leading-edge separation property of the baseline airfoil during stall was effectively alleviated with use of the morphed leading-edge geometries.

Climate Impact Mitigation Potential of Novel Aircraft Features

This work presents a transpacific airliner designed for minimal climate impact, incorporating several novel design features. These include open rotor engines, sustainable aviation fuels, natural laminar flow airfoils, and riblets. The design’s configuration and mission have been optimised simultaneously using a combination of standard preliminary techniques, experimental data, a multi-point mission analysis, and a model of average temperature response. It is demonstrated that, on an 8000 km mission, the design offers an 89.8% reduction in average temperature response relative to an Airbus A330-200, at the expense of a 7.3% increase in direct operating cost. The sensitivity of these results is investigated by comparing the performance over a range of operating conditions. In addition, several alternative designs incorporating only some of the above-mentioned features are analysed, allowing for an assessment of their individual contribution. Finally, a life-cycle average temperature response analysis is presented to place the climate impact of operation, manufacturing and end-of-life procedures in context.

Controlling transonic shock–boundary layer interactions over a natural laminar flow airfoil by vortical and thermal excitation

Investigations have been performed via implicit large eddy simulations to study the overall effects of exciting a flow field by thermal (wall-heating and wall-cooling) and vortical (with high and low frequencies) actuation. The actuator is placed on the suction surface of a natural laminar flow (SHM-1) airfoil having an angle of attack of α=0.38° (cruise setting). Oncoming flow has a Mach number of 0.72, and a Reynolds number based on a chord of Re=16.2×106, for which a complex shock system is formed on the suction surface. Vorticity dynamics of the flow is studied using time series of vorticity at different locations above the suction surface and instantaneous contour plots of vorticity in the domain. An inspection of the flow using snapshots of ∇ρ and ∇(ρT) is done to characterize the numerical schlieren. The comparative effects of the various forms of excitation on the shock–boundary layer interactions (SBLI) have been analyzed using time series of the magnitudes of ∇ρ across the identified shock structures from numerical schlieren snapshots. Also, the role of the frequency of imposed vortical actuation has been studied using vorticity and Mach contours for a comparative understanding of the control of the SBLI.

DYNAMICAL EFFECTS OF A HOT DISK ON THE DEVELOPMENT OF A NATURAL CONVECTION FLOW BETWEEN CYLINDRICAL WALLS

This study numerically investigates the problem of natural convection flow ina vertical cylinder. The importance of this issue mainly stems from thermal systems (e.g. chimneys, hot air generators, solar collectors, and many others.). Despite the numerous studies on the modeling of free convection flow between two perpendicular plates, there are very few studies that have investigated regarding the convective flow between two cylindrical walls. Therefore, the study is limited to vertical cylinders with different heating modes.The first configuration includes heated walls of thermosiphon flow in a cylindrical channel.The second configuration introduces a circular heat source at the channel entrance, and the third configuration includes an ended vertical cylinder with a heat source centered in the cylinder (zs = 0.04). The flow comparison from the three configurations demonstrates how the hot disk affects the flow from a dynamic point of view.Furthermore, details about the flow and dynamical fields can be obtained from the solution equation of the conservation of momentum and energy, considering the differences between the boundary conditions of the studied configurations. The study covers Rayleigh numbers and focuses on the effect of the cylinders geometry on the characteristic of the flow as well as dynamical and thermal field characteristics and variations in Nusselt number. The studied flow is natural convection, laminar, steady, two-dimensional, and incompressible flow. Solutions were obtained using a numerical model based on the finite volume method and the Navier–Stokes equations. The velocity–pressure coupling was resolved using the SIMPLER algorithm. The temperature and vertical velocity profiles of the flow showed the existence of a boundary layer regime along the heated channel wall without a heat source. The layer continued along the cylindrical wall with the introduction of the heat source at the channel inlet. However, introducing the heat source caused a considerable change in the flow structure at the channels central part.Calculations were performed for the channel aspect ratio R* = 0.2 and different Rayleigh numbers (Ra = 105, 107, 108, 109 and 1010). Numerical results included velocity, temperature, and Nusselt number profiles.

Transport-Based Transition Prediction for the Common Research Model Natural Laminar Flow Configuration

This paper presents the application of a new -based one-equation transition transport model to the Common Research Model Natural Laminar Flow Configuration (CRM-NLF), which was the central test case of the 1st AIAA CFD Transition Modeling and Prediction Workshop. The new model uses a simplified formulation of the Arnal, Habiballah, and Delcourt transition criterion that is stability based. The model is formulated in a grid-point-local manner and is coupled to the Menter shear stress transport eddy-viscosity turbulence model. The flow conditions applied are exactly those that were reported from the National Transonic Facility wind tunnel test campaign. The comparisons of the results yielded by the new model in terms of the transition fronts along the wing span show a very good match with the transition fronts derived from the temperature-sensitive paint images taken during the wind tunnel test and with computational result from an -based Two--factor strategy using critical -factors for Tollmien–Schlichting and crossflow instabilities and applying a local linear stability code.

Characterization of low levels of turbulence generated by grids in the settling chamber of a laminar wind tunnel

Wind tunnel investigations of how Natural Laminar Flow (NLF) airfoils respond to atmospheric turbulence require the generation of turbulence, whose relevant characteristics resemble those in the atmosphere. The lower, convective part of the atmospheric boundary layer is characterized by low to medium levels of turbulence. The current study focuses on the small scales of this turbulence. Detailed hot-wire measurements have been performed to characterize the properties of the turbulence generated by grids mounted in the settling chamber of the Laminar Wind Tunnel (LWT). In the test section, the very low base turbulence level of Tuu ≅ 0.02% (10 ≤ f ≤ 5000 Hz) is incrementally increased by the grids up to Tuu ≅ 0.5%. The turbulence spectrum in the u-direction shows the typical suppression of larger scales due to the contraction between grids and test section. Still, the generated turbulence provides a good mapping of the spectrum measured in flight for most of the frequency range 500 ≤ f ≤ 3000 Hz, where Tollmien-Schlichting (TS)-amplification occurs for typical NLF airfoils. The spectra in v and w-direction exhibit distinct inertial subranges with slopes being less steep compared to the − 5/3 slope of the Kolmogorov spectrum. The normalized spectra in u-direction collapse together well for all grids, whereas in v- and w-directions the inertial- and dissipative subranges are more clearly distinguished for the coarser grids. It is demonstrated that the dissipation rate ε is a suitable parameter for comparing the wind tunnel turbulence with the atmospheric turbulence in the frequency range of interest. By employing the grids, turbulence in the range 4.4 × 10–7 ≤ ε ≤ 0.40 m2/s3 at free-stream velocity U∞ = 40 m/s can be generated in the LWT, which covers representative dissipation rates of free flight NLF applications. In the x-direction, the spectra of the v and w-components develop progressively more pronounced inertial- and dissipative subranges, and the energy below f ≈ 400 Hz decreases. In contrast, the spectral energy of the u-component increases across the whole frequency range, when moving downstream. This behavior can be explained by the combination of energy transport along the Kolmogorov cascade and the incipient return to an isotropic state.Graphic

Comparative study of transonic shock–boundary layer interactions due to surface heating and cooling on an airfoil

Implicit large eddy simulation results are compared to investigate the effects of wall-heating and wall-cooling on shock–boundary layer interaction over an airfoil. Heat flux is provided on the suction surface of the airfoil from x/c=0.40 to x/c=0.50 for a Mach number of 0.72 and a Reynolds number based on chord of Re=16.2×106. Flow quantities are compared for the effects of heating and cooling. Numerical Schlieren snapshots reveal an oscillation of the shock wave and its interaction with upstream propagating Kutta waves generated from the trailing edge of the airfoil. Quantitative data obtained from these Schlieren snapshots and the mean aerodynamic load values indicate a reduction in frequency of oscillation of shock wave and a decrease in shock strength for the case of heating. Flow control by heating shows higher fluctuations in flow features evident from instantaneous quantities. Both imposed excitations lead to a marginal increase in aerodynamic efficiency (lift/drag). We also compare the integral aerodynamic parameters, such as lift and drag coefficients, and their ratio, Cl, Cd,and Cl/Cd. The simulations reported here follow the techniques used in Sengupta et al. [“Thermal control of transonic shock–boundary layer interaction over a natural laminar flow airfoil,” Phys. Fluids 33(12), 126110 (2021)].

Stochastic Investigation on the Robustness of Laminar-Flow Wings for Flight Tests

Natural laminar-flow (NLF) wings and hybrid laminar-flow control (HLFC) wings are efficient drag reduction technologies. It is vital to develop robust laminar-flow wings to research the stochastic characteristics of both NLF and HLFC wings and reveal the generation mechanism for differences in their statistical responses. Wing-glove flight experiments and deterministic numerical simulations using the method with a suction-velocity model are performed, which are in good agreement. Uncertainty and correlation analyses based on nonintrusive polynomial chaos expansion are then performed to evaluate the influence of operational and geometric uncertainties. The simulations indicate that operational condition uncertainties have a greater impact on their statistical responses. Pressure coefficients near the leading edge and end region of the favorable pressure gradient are the most sensitive to disturbances in the angle of attack (AOA) and Mach number, respectively. Whereas HLFC wings significantly delay the laminar–turbulent transition, NLF wings may have better robustness against disturbances if the development of Tollmien–Schlichting (TS) waves is not completely suppressed in the suction region. This is because disturbances affect statistical responses of HLFC wings by changing both the local pressure gradients and suction coefficient distributions. Positive correlations between the AOA, local pressure gradients, and suction coefficients amplify the influence of AOA disturbances. Meanwhile, Mach-number disturbances have negative and positive correlations with local pressure gradients and suction coefficients, respectively. The opposite correlation weakens the influence of Mach-number disturbances.

Optimal shape design and transition uncertainty analysis of transonic axisymmetric natural laminar flow nacelle at high Reynolds number

Laminar flow drag reduction is an important way to improve the cost effectiveness of civil aircraft. In this paper, a hybrid game based optimization method is applied to optimize an axisymmetric natural laminar flow nacelle with inflow and geometric constraints at high Reynolds number. The effectiveness of the optimization algorithm and the ability to explore the solution of multi-objective optimization problem from engineering science are confirmed. In order to maintain the stable laminar flow performance and to achieve robust aerodynamic drag reduction in changing flight conditions, the global sensitivity and uncertainty analysis of the designed axisymmetric laminar nacelle with respect to the coupling of geometric parameters and the coupling of Mach number and turbulence intensity are finally performed, and the effects of the input parameters on the laminar flow range and the total drag coefficient are analyzed. The sensitivities of each parameter and its contribution to the laminar flow region were obtained. This lays the foundation for the subsequent development of a robust optimization modeling and the specification of its effective search space for the three-dimensional natural laminar flow nacelle.

Step-induced transition in compressible high Reynolds number flow

The effect of sharp forward-facing steps on boundary-layer transition is systematically investigated in this work in combination with the influence of variations in Mach number, Reynolds number and streamwise pressure gradient. Experiments have been conducted in a quasi-two-dimensional flow at Mach numbers up to 0.77 and chord Reynolds numbers up to 13 million in the Cryogenic Ludwieg-Tube Göttingen. The adopted experimental set-up allows an independent variation of the aforementioned parameters and enables a decoupling of their respective effects on the boundary-layer transition, which has been measured accurately and non-intrusively by means of a temperature-sensitive paint. The functional relations determined between a non-dimensional transition parameter and the non-dimensional step parameters allow the step effect on transition to be isolated from the influence of variations in Mach number, Reynolds number and pressure gradient. Criteria for acceptable heights of forward-facing steps on natural laminar flow surfaces for the examined test conditions are derived from the present functional relations. The measured transition locations are also correlated with the results of linear, local stability analysis for the smooth configuration, enabling the estimation of the step-induced increment of the amplification factor ΔN of Tollmien–Schlichting waves, which can be incorporated in the eN transition prediction method.

Thermal control of transonic shock-boundary layer interaction over a natural laminar flow airfoil

Implicit large eddy simulations are performed to show control of shock and boundary layer interaction over a natural laminar flow airfoil by harmonic heat transfer on the suction surface at a free-stream Mach number M∞=0.72 and angle of attack of α=0.38°, for which experimental/flight test results exist. Surface heat flux is added in a time-periodic manner with the exciter strip located at different locations in the vicinity of the shock location at x/c≈0.49 for the flow without any control. Four cases of localized excitation with Gaussian distribution centered at x/c=0.45, 0.48, 0.50, and 0.55 on the airfoil surface are considered, with a width of 10% chord. The effects of unsteady heat flux on shock strength, location, and modification of its structure are demonstrated by instantaneous and time-averaged flow quantities. Detailed vortical and entropy contour plots and numerical Schlieren indicate flow features, such as creation and propagation of Kutta waves and their interactions with the shock wave. Time-averaged load distributions reveal a shift in the location and strength of the shock, with altered lift and drag time histories. The Fourier transforms of lift and drag coefficients help explain the alteration of the strength and structures of the shock-induced unsteadiness. The imposed excitation results in an improvement of aerodynamic efficiency (lift to drag ratio). The numerical simulations follow the algorithm developed in Sengupta et al. [Comput. Fluids 88, 19–37 (2013)].

Robust Design of Transonic Natural Laminar Flow Wings Under Environmental and Operational Uncertainties

The robustness of laminar wings is critical, both against instabilities that can unexpectedly trigger transition and against off-design conditions outside the cruise point. However, current inverse design methodologies not only provide suboptimal configurations but are unable to come up with robust configurations. The objective of this paper is the development and demonstration of a framework for the robust direct design of transonic natural laminar flow wings using state-of-the-art industrial tools such as computational fluid dynamics, linear stability theory, and surrogate models. The deterministic optimization problem, which serves as a baseline, searches for the optimum shape that minimizes drag applying a surrogate based optimization strategy. In that case, crossflow and Tollmien–Schlichting critical factors are fixed according to calibration data. For the robust approach, uncertainties in these critical factors as well as operational conditions are considered to account for situations that could prematurely trigger transition. The framework is able to come up with state-of-the-art natural laminar shapes for a short-haul civil aircraft configuration. The robust configurations are more balanced than the deterministic, as the transition location smoothly moves upstream as the critical factors are reduced. The pressure profiles are more resistant against instabilities, extending the current design envelope of natural laminar flow wings.