Wall Interference Research Articles

Generalized, Three-Dimensional Wind Tunnel Wall Correction Using Nonlinear Least-Squares Optimization

A novel three-dimensional technique for correcting wind tunnel measurements for wall interference has been developed. The flow around the test body and its wake is approximated as a series of inviscid singularities, while the wind tunnel walls and model support are approximated using a panel method. A nonlinear least-squares approach with trust-region reflective optimization is then used to obtain the strength and locations of singularities together with the domain boundary panel strengths by fitting to measured wall pressures. The technique is demonstrated using computational fluid dynamics for the test case of a well-documented slender delta wing body with high blockage and validated against both experiments and legacy data. The nonlinear least-squares wall correction method is shown to demonstrate a significant improvement in wall corrections compared to the classical two-variable technique.

Feasibility study of data-driven wall interference correction framework for subsonic wind tunnel

Although the classical method is widely used for wall interference correction in wind tunnel testing, its reliability and accuracy for complex and unconventional geometries are rather limited. Studies on the evaluation of wall interference and the improvement of correction methods are desirable to enhance the reliability and generality for various geometric configurations. This study proposes a wall interference correction framework based on a deep neural network (DNN) ensemble using data obtained from the numerical panel method. The panel method is validated by comparing the results with those of Reynolds-averaged Navier-Stokes simulations. An automated process was established to generate a large amount of training data, and 600,000 datasets were generated based on the geometric parameters of the wind tunnel, test model, and angles of attack. The input variables of the DNN were determined through sensitivity analysis of the data. To alleviate the randomness of the initial weights and data distribution in the generation process of the DNN model, 20 DNNs with the same multi-layer perceptron structure were trained, and a DNN ensemble model was constructed using five ensemble members with high predictability. The accuracy of the DNN-ensemble based correction models were evaluated by comparing the correction results for the testing data.

Wind tunnel wall interference correction for transonic airfoils with data-reduced ensemble Kalman filter

The uncertain factors in wind tunnel experiments, especially the interference effects, will cause the flow around the test model to differ from that under the free flow condition. The wind tunnel wall interference effects are challenging to be eliminated, especially in transonic flow regions where classical linear correction methods are ineffective. To address this issue, an efficient wind tunnel wall interference correction framework based on data assimilation is proposed for steady and shock buffet flows around the airfoils in the transonic region in this work. The ensemble Kalman filter (EnKF) is used to find the combination of inflow conditions, which minimizes the differences between the pressure coefficients measured in the wind tunnel experiment and those estimated by computational fluid dynamics (CFD). The polynomial chaos expansion method is used to propagate uncertainty in the forecast step of the EnKF in order to improve efficiency. Typical wind tunnel wall interference correction problems are studied, including two steady cases: the transonic flow around the Royal Aircraft Establishment 2822 (RAE2822) and the National Aerospace Laboratory 7301 (NLR7301) airfoils, and one unsteady case: the transonic shock buffet on the National Aeronautics and Space Administration supercritical (phase 2)-0714 (NASA SC(2)-0714) airfoil. It is demonstrated that with the corrected Mach number and angle of attack, the estimated CFD results are in better agreement with the measured experimental data than traditional linear methods and the computational efficiency is improved compared to the original EnKF.

Electromagnetic wave absorption performance of porous geopolymer: Dual effects of fly ash on alkali activation and microwave dissipation

The simultaneous occurrence of aggregation reactions in fly ash (FA) facilitates the uniform dispersion of internal ferrite into the geopolymer. Considering the excellent impedance matching and multiple reflection effects of foam concrete, this paper develops a ‘zero-absorber’ foam geopolymer-based electromagnetic absorbing material. The effects of different densities (400, 600, 800, 1000, 1200 kg/m3) and FA contents (40 %, 50 %, 60 %, 70 %, 80 %) on the electromagnetic absorption performance of foam geopolymer are investigated, respectively. Combined with the iron element distribution and 3D pore structure by SEM and X-CT analysis, the dissipation mechanism of electromagnetic energy by FA and pore structure in foam geopolymer are further explored. Experimental results show that FA can uniformly disperse inherent ferric ions from the matrix into the geopolymer during the polymerization reaction, leading to excellent magnetic losses through magnetic moment orientation adjustment and intermolecular friction interactions without adding external absorber. Additionally, the uniformly dispersed porous structure consumes incident electromagnetic energy through reflection, scattering, coherent attenuation, and interference resonance loss effects. It shows multi-scale electromagnetic absorption effects in the ‘zero-absorber’ foam geopolymer-based electromagnetic absorbing material, including pore structure multiple reflections (millimeter scale), inter-pore wall interference resonance (micrometer scale), and ferric ion magnetic loss (nanometer scale). For the foam geopolymer with 1000 kg/m3 density and 50 % FA, it achieves the optimal electromagnetic absorbing performance with the minimum reflection loss (RL) of −34.9 dB and the effective absorption bandwidth of 14.1 GHz (RL below −10 dB) in 2–18 GHz range.

MAP kinase MKK: A central regulator in the development, toxigenic potential, and pathogenesis of Penicillium expansum infecting pears

Penicillium expansum is one of the primary pathogens causing postharvest blue mold of pears, which leads to the spoiling of pulp and produces patulin, endangering food security. Mitogen-activated protein kinase (MAPK) signaling pathway regulates the growth and development, cell wall integrity, and pathogenicity of fungi. Mitogen-activated protein kinase kinase (MKK) in the MAPK signaling pathway has been reflected to be associated with fungal cell wall integrity. Nevertheless, the pivotal function of MKK in the process of P. expansum infection remained unexplored. In this study, MKK was knocked out and repaired by the Agrobacterium-mediated homologous recombination technology to explore the function of MKK in P. expansum infection process. It has been found that the deletion of MKK induces morphological changes in the mutant strain and delayed mycelium and spore growth. Further studies showed that the mutant strain was susceptible to cell wall interference compounds and showed a significant decrease in decay diameters and virulence during the infection of the pears. RT-qPCR analysis showed that the deletion of MKK affected the relative expression levels of its upstream and downstream genes. Our results suggest that MKK is essential in the growth and development, cell wall integrity, pathogenicity, and toxigenic capacity of P. expansum.

Transonic leading-edge stall flutter: modelling, simulations and experiments

This work is a numerical and experimental study of a rectangular thin plate undergoing stall flutter at Mach 0.8. This constitutes one of the first studies of this kind where three-dimensionality is fully implemented in a numerical simulation including the test-section effects characterizing wind-tunnel experiments. In order to break down the fluid–structure interaction to its main driving phenomena, an aerodynamic model is proposed that is based on computationally inexpensive steady-state simulations. Two types of dynamic instability are observed in the numerical simulations; Flutter by mode coalescence is promoted at zero flow incidence, however, high bending precludes this from happening for higher values of angle of attack. Stall flutter is instead a nonlinear one-degree type of instability. Both of these instability mechanisms can be explained in terms of hysteretic behaviour of the pressure distribution, which becomes more pronounced at high angles of attack, when a large separation region is formed. Tests were conducted employing titanium alloy plates in order to survive the aerodynamic loads characterizing the wind-tunnel initial transient. However, due to wall interference, high bending was promoted so that the internal stress exceeded the yield values before flutter could be measured. Numerical simulations were in general agreement with the experiment in terms of both amplitude and oscillation frequency.

Evaluation of wind tunnel wall interference using homogeneous and measured boundary conditions

The experimental results obtained in a wind tunnel must be subjected to a correction process which aims to eliminate the influence of the limited dimensions of the flow field around the model. This is necessary because the results must be independent of the characteristics of the laboratory where they were obtained, in order to ensure the quality of the parameters.The wall corrections are applied to the global quantities that characterize the undisturbed flow, such as the Mach number or the dynamic pressure, but they are also applied to the quantities related to the model, namely the global aerodynamic coefficients.Consequently the corrections will be applied to the global quantities of the undisturbed flow and therefore they will be transmitted to the aerodynamic quantities related to the model.

Determination of the effect due wall interference on the flow around airfoil in the T-128 wind tunnel

Determination of the effect due wall interference on the flow around airfoil in the T-128 wind tunnel

Wall-proximity effects on vortex-induced vibrations of a circular cylinder

Vortex-induced vibrations (VIV) of an elastically supported circular cylinder (of diameter D) undergoing a wall interference are investigated for a mass-damping ratio 0.31, Reynolds numbers 1.77 × 103–1.24 × 104, and wall-cylinder gap height ratios e* = e/D = 0.1–1.6. Measurements of vibration responses and vortex shedding frequencies are done using a laser vibrometer and a hotwire anemometer, respectively, while particle image velocimetry techniques are employed to capture flow fields. This investigation aims to assimilate the effect of e* on vibration response, frequency response, VIV range, quality factor of VIV, added mass, added damping, lift force, and flow structure. A smaller e* is observed to increase the added mass, reduce the maximum vibration amplitude, and shorten the VIV range while enhancing the quality factor of VIV. The increase in added mass and the decrease in the maximum vibration amplitude in the VIV range are dramatic when e* is decreased from 0.6. Decreasing e* significantly impacts the flow structures and vortex shedding modes, resulting in intensified asymmetry in pressure distributions. The interference between the wall and the cylinder is further reflected by increasing time-mean lift forces and decreasing fluctuating lift forces with decreasing e* < 0.6.

Advances in Wall Interference Corrections for Two-Dimensional Lifting Bodies

A novel approach to correcting the effects of wall interference has been developed for low-speed closed-wall wind tunnels using a nonlinear least-squares optimization process independent of the model geometry. Potential flow singularities are used to represent the test object, and the method of images is used to represent the tunnel walls. Trust-region reflective optimization enables the locations and strengths of singularities to be determined iteratively with only wall pressure measurements as independent inputs. Results are obtained from a range of two-dimensional and three-dimensional airfoil test cases using both computational fluid dynamics simulations and wind tunnel experiments, including a NACA 0012 with deployed spoiler, a NACA 0015, a NACA 4415, and a McDonnell Douglas 30P30N high-lift configuration. The nonlinear least-squares wall-correction method is shown to demonstrate a considerable improvement over the conventional two-variable technique.

The boundary acoustic impedance effects of a towing tank underwater sound radiation induced by a steel cylinder continuous vibration

In this study, it was found the characteristics of the boundary was an important factor to the accuracy of measurements and simulations for underwater radiation in a towing tank. Measurements and FEM simulations of the radiation sound field were conducted. In the measurements, two types of continuous vibration were performed in the towing tank which are: A 250 Hz/500 Hz speaker continuous noise and a 45 Hz machine continuous running were used to generate vibrations in a submerged steel circular cylinder. The experiment was conducted to vibrate the watertight cylinder and to measure the underwater sound field, which was significantly impacted by reflections from the tank walls. FEM simulations of radiation sound field were evaluated to compare with the experimental results.The experimental measurements and underwater sound field simulations were consistent with each other at 45 Hz and 250 Hz. The simulation and experiment was slightly larger in the 500 Hz case. The quantitative result was the verification of the measurement and simulation of radiated sound in water. Furthermore, the qualitative result was the simulations investigated the factors affecting the towing tank’s boundary effects. Tests of alternative impedances of the walls showed that the wall impedance had strong effects on the underwater sound field. Another boundary effects were investigated by changing the source(the cylinder) direction showed that the wall interference effect was stronger when the cylinder was oriented towards the wall than towards the watercourse. Finally, tests of the source frequencies from 20 Hz to 500 Hz showed that higher frequencies generated stronger boundary interference effects in the underwater sound field than did lower frequencies.

An analytical blockage correction model for high-solidity turbines

A significant challenge in the experimental or computational characterisation of porous bodies and wind turbines is the correction of the obtained flow quantities for wall interference effects. Conventional corrective models are based on the Rankine–Froude theory, which is valid when the body solidity, or turbine induction factor, is sufficiently low. To resolve this issue, this work presents a new corrective model that builds on an extension of the Rankine–Froude theory, valid at arbitrary solidities, coupled with the method of mirror images to account for the existence of channel walls. The predictions of the new model are validated using laboratory and numerical experiments of porous plates and wind turbines. The results show that the new model performs equally as well as conventional ones when the solidity is low, but becomes increasingly more accurate as the latter grows.

Scale effect during the dynamic stall of dynamic airfoil

Studying the scale effect of dynamic airfoil is helpful for understanding the dynamic stall phenomenon and unsteady wall interference. In this paper, the scale effect of the dynamic stall characteristics of the dynamic National Advisory Committee for Aeronautics 0012 airfoil was studied using time-resolved pressure measurements for the first time, which is different from the Reynolds number effect as the experimental Reynolds number was not varied. The evolution of the pressure field and vortex development characteristics on different scaled models was investigated. Significant scale effects were observed on the dynamic airfoil during its dynamic stall. With the model scale increases from c = 500 to 900 mm, the shear layer vortex and the secondary separation were observed, indicating that vortices on the suction surface were enriched. Furthermore, the vortex-induced suction and its occurrence time were significantly affected. The vortex-induced suction varied from Cp ≈ −4.25 to −1.5. With the model scale increases from c = 500 to 900 mm, the vortex structure occurrence angle of attack was delayed by Δα ≈ 0.5°. Further analyses showed that the scale effects of the dynamic airfoil are attributed to the flow transition and unsteady wall interference interactions. Thus, two physical quantities, Sm and Sw, were innovatively defined to associate the vortex complexity, model scale, and wind tunnel wall pressure. This study helps deepen the understanding of the dynamic stall phenomenon and provides guidance for the unsteady wall interference correction.

Histone Methylation Is Required for Virulence, Conidiation, and Multi-Stress Resistance of Alternaria alternata

Histone methylation, which is critical for transcriptional regulation and various biological processes in eukaryotes, is a reversible dynamic process regulated by histone methyltransferases (HMTs) and histone demethylases (HDMs). This study determined the function of 5 HMTs (AaDot1, AaHMT1, AaHnrnp, AaSet1, and AaSet2) and 1 HDMs (AaGhd2) in the phytopathogenic fungus Alternaria alternata by analyzing targeted gene deletion mutants. The vegetative growth, conidiation, and pathogenicity of ∆AaSet1 and ∆AaSet2 were severely inhibited indicating that AaSet1 and AaSet2 play critical roles in cell development in A. alternata. Multiple stresses analysis revealed that both AaSet1 and AaSet2 were involved in the adaptation to cell wall interference agents and osmotic stress. Meanwhile, ∆AaSet1 and ∆AaSet2 displayed serious vegetative growth defects in sole carbon source medium, indicating that AaSet1 and AaSet2 play an important role in carbon source utilization. In addition, ∆AaSet2 colony displayed white in color, while the wild-type colony was dark brown, indicating AaSet2 is an essential gene for melanin biosynthesis in A. alternata. AaSet2 was required for the resistance to oxidative stress. On the other hand, all of ∆AaDot1, ∆AaHMT1, and ∆AaGhd2 mutants displayed wild-type phenotype in vegetative growth, multi-stress resistance, pathogenicity, carbon source utilization, and melanin biosynthesis. To explore the regulatory mechanism of AaSet1 and AaSet2, RNA-seq of these mutants and wild-type strain was performed. Phenotypes mentioned above correlated well with the differentially expressed genes in ∆AaSet1 and ∆AaSet2 according to the KEGG and GO enrichment results. Overall, our study provides genetic evidence that defines the central role of HMTs and HDMs in the pathological and biological functions of A. alternata.

Growth Process of Vortex Structure Formed by a Sweeping jet in Cross Flow near a Wall

The Fluidic Oscillator can permanently generate a high-velocity oscillating jet (Sweeping jet) and give a large fluctuation to the flow field without driving parts. Therefore, it is attracting attention as a new active flow control device. However, the flow characteristics that directly contribute to flow separation control have not been sufficiently clarified, and their optimization and practical application have not progressed. Therefore, in order to clarify the details of the flow characteristics that contribute to the flow separation control, it is necessary to clarify the flow structure generated by the sweeping jet that blows into the main flow from the wall surface．In this study, we clarify the growth process of the vortex structure generated by the interaction of the sweeping jet, which blows out from the wall at 30 degrees to the main flow, by numerical simulation. It is conducted to understand the complex and large-scale flow structure and its generation process generated by a sweeping jet in the main flow. The vortex structure forms one flow tube at z/d < 4.5 and splits into two at z/d > 4.5. Shear layer vortices with the same frequency as the oscillation frequency are generated in the stream tube, and after splitting into two, it is generated alternately to the left and right. After that, at z/d > 6.0, the shear layer vortex begins to collapse due to wall interference, and at z/d > 7.5 it transforms into discrete vortices.

Flight Tests of Passive Flow Control for Suppression of Cavity Aeroacoustics

This research focused on furthering the current understanding of the flowfield inside a cavity under typical flight conditions. The flight envelope included altitudes of up to 40,000 ft and maximum Mach number of 1.28 and included several test points between Mach 0.9 and 1.1, which are challenging to isolate in wind tunnel testing due to tunnel wall interference. The pod was instrumented to collect aeroacoustic pressures and qualitative flowfield imaging for a cavity with a length-to-depth ratio of four. Test missions were flown with several different passive flow control configurations, and their respective effect on acoustic properties and flow patterns were collected. Flow control comprising a rod-in-a-crossflow or a traditional sawtooth spoiler was generally found to reduce sound pressure level, though the extent of effectiveness varied with flight condition. The flow control devices installed at or below the cavity exit plane were found to have little influence. Tuft-based flow visualization complemented the effort, for one example, by demonstrating the outward flow near the cavity lip for the sawtooth spoiler.

基于线性涡模型的风洞孔壁干扰特性分析

An estimation and correction method of perforated wall-induced interference characteristics for 2D airfoils was proposed. Based on the Prandtl-Glauert velocity potential equation and linear vortices arranged on the surfaces of the airfoil and tunnel walls, the effects of the perforated wind tunnel wall on pressure distributions on the airfoil surface were solved iteratively to analyze influences of air permeability parameters of the perforated wall. Previous reference data and wind tunnel test results were used for comparison to validate the accuracy of the proposed calculation method. The comparison shows that, the effects of the perforated tunnel walls on the flow around the airfoil mainly act on the upper surface between the suction peak and the maximum thickness, and lead to lower pressure coefficients and decreased integrated lift coefficients. With the increase of the air permeability parameter of the perforated wall, the wall interference characteristics develop from the solid wall to the open boundary sharply. With good efficiency and reliability, the proposed method applies to quick estimation of the pressure on the airfoil surface in the subcritical range, and evaluation of the wall interference in the 2D airfoil tests.

Methods of the wall interference reduction at low supersonic velocities in wind tunnels

The paper describes the research aimed at reducing wall interference in wind tunnels at low supersonic velocities. The results of such wind tunnel tests are often affected by wave perturbations generated by the model and reflected from the test section walls. This research describes the methods that can effectively reduce the wave reflection from the wall. A numerical investigation is presented that includes a novel boundary condition that eliminates disadvantages of previously considered methods. The numerical study was conducted in the frame of electronic wind tunnel concept and ANSYS CFX software. The numerical investigation showed that the effect of shock wave reflection could be reduced using a new type of boundary condition that is a combination of perforated walls and a controlled boundary layer.

Jet-perforated boundaries as an effective method to reduce wall interference for airfoil tests in a transonic wind tunnel

Jet-perforated boundaries as an effective method to reduce wall interference for airfoil tests in a transonic wind tunnel

Nonlinear Wind-Tunnel Wall-Interference Corrections Using Data Assimilation

This paper presents a novel approach for correcting wind-tunnel wall interference in the nonlinear flow regime, that is, in the presence of phenomena such as flow separation and shocks. The methodology uses a gradient-based optimization to minimize the difference between experimental measurements and a Favre-averaged Navier–Stokes (FANS) simulation. The aim is to exploit the high-fidelity experimental data to correct turbulence-modeling errors in the FANS simulations, as well as to use the accurate angle of attack and Mach number from the FANS simulations to correct the in-tunnel flow conditions. The optimization is carried out directly in free air, thus avoiding the requirement to mesh the wind-tunnel walls and/or to model the ventilated-wall boundary condition. A byproduct of this method is the availability of flow information everywhere around the test object, which augments and complements the experimental data. The methodology is tested on two-dimensional and three-dimensional flow cases, demonstrating a significant improvement in the agreement between experimental and numerical data.