Wall Of Test Section Research Articles

Supersonic Wind Tunnel Flow Characterization Using Planar Laser CO2 Rayleigh Scattering

A complementary experimental and computational study was performed to characterize the test section flowfield of a variable-Mach-number supersonic wind tunnel. A planar laser Rayleigh scattering technique using CO2 nanocrystals was used to capture cross-sectional flow visualizations at various test section streamwise locations. Reynolds-averaged Navier–Stokes simulations of the wind tunnel flow were performed using a k-ω shear stress transport turbulence model to compare against the flow visualizations. Mach 3.20 and 3.55 flow conditions were investigated in the wind tunnel with an empty test section. The Rayleigh signal profile near the wind tunnel walls was compared against boundary-layer thickness data from shadowgraph visualizations, pitot survey measurements, and numerical simulation predictions to guide the interpretation of the scattering results. A brief discussion regarding the nature and potential source of certain unexpected scattering patterns in the images was provided. The most distinguishable flowfield characteristics observed in the Rayleigh scattering visualizations were the asymmetric thickness of the boundary layers along the test section walls and the appearance of two floor lobes near the flow-path corners. The results of the numerical simulations showed favorable agreement with the experimental data and were used to provide insight into the mechanism that caused the boundary-layer features observed. This study demonstrated the usefulness of CO2 Rayleigh scattering to obtain flow visualization data of great value to better understand flow quality in supersonic wind tunnels with two-dimensional nozzles.

Aero-Optical Measurements of a Mach 8 Boundary Layer

Measurements are presented of the aero-optic distortion produced by a Mach 8 turbulent boundary layer in the Sandia Hypersonic Wind Tunnel. Flat optical windows installed in conformal test section walls enabled a double-pass arrangement of a collimated laser beam. The distortion of this beam was imaged by a high-speed Shack–Hartmann sensor using variable aperture sizes at a sampling rate of up to 1.75 MHz. Analysis is performed using two processing methods to extract the aero-optic distortion from the data: 1) a stitching method is applied to extract wavefronts without bias from a limited aperture size, and 2) a novel de-aliasing algorithm is proposed to extract convective-only deflection angle spectra and is demonstrated to correctly quantify the physical spectra even for relatively low sampling rates. Measurements of speed and size of large-scale convecting aero-optical structures are also presented. Overall levels of aero-optic distortions were estimated, and the results are compared with an existing theoretical model. It is shown that this model underpredicts the measured distortions regardless of the processing method used. Possible explanations for this discrepancy are presented. Finally, levels of the global streamwise jitter were estimated for different aperture sizes and compared with the results for the subsonic boundary layer. The results represent to-date the highest Mach number for which aero-optic boundary-layer distortion measurements are available.

Structure and dynamics of a laminar separation bubble near a wing root: towards reconstructing the complete LSB topology on a finite wing

The influence of the wing root junction on the laminar separation bubble forming on the suction surface of a semispan NACA 0018 wing cantilevered from the wind tunnel test section wall is studied using surface flow visualisations, particle image velocimetry and surface pressure measurements at a chord Reynolds number of 125 000 and an angle of attack of 6 $^\circ$ . The test section wall boundary layer upstream of the wing is turbulent, and the spanwise influence of the junction on the separation bubble extends well beyond the test section wall boundary layer thickness. Substantial three-dimensionality is seen in the separation bubble flowfield near the wing root, where earlier transition and a reduction in separation bubble thickness is observed. In contrast with the wing tip, earlier transition and a reduction in separation bubble length occurs near the wing root. Outside of the junction affected region, the separation bubble is similar to separation bubbles forming on two-dimensional geometries, and displays mild spanwise waviness. The transition process away from the end affected regions is characterised by the formation of spanwise roll-up vortices that are shed in a nearly two-dimensional manner across the span. The analysis of the results shows that, near the wing root, the increased level of perturbations leads to earlier vortex roll-up and spanwise flow contributes to more rapid vortex breakdown. The results in the wing root region are complemented by the analysis of data from Toppings and Yarusevych (J. Fluid Mech., vol. 929, 2021, A39) in the wing tip region to provide a more holistic outlook on the laminar separation bubble topology and dynamics on the entire finite wing.

Design and Parametric Analysis of a Mach 4 Wind Tunnel for SSLCs Studies

Abstract The use of shear sensitive liquid crystals (SSLCs) provides a nonintrusive technique for accurate visualization and quantification of shear stress on surfaces under investigation. However, investigations using liquid crystal involving supersonic and hypersonic flows poses specific requirements for the design of wind tunnel. These constraints are pertaining to time response, erosion, and repeated application of the layer of SSLCs on the model surface as well as its sensitivity to wall temperature under the influence of high-speed flows. In this research, the Mach 4 wind tunnel facility is designed using existing empirical, semi-empirical methods and detailed literature survey and is analyzed numerically in ansysfluent 19.0 environment using transient k–ω shear stress transport Reynolds-averaged Navier–Stokes turbulence model. The influence of geometric contouring of converging section of the supersonic nozzle on the reservoir pressure requirement to establish a M = 4 flow condition in test section is also analyzed. The iterative design method is adopted for the supersonic diffuser section of the wind tunnel where different geometric parameters are varied to achieve higher diffuser efficiency as well as minimum startup time requirement. The parametric study is carried out to ascertain the factors influencing test section wall shear stress during wind tunnel startup process and a novel approach is explored in which the Coanda effect is generated to keep the jet emanating from the supersonic nozzle away from the applied SSLC layer, thereby reducing its erosion effect. Furthermore, the influence of the location of a diaphragm for the partially ruptured case on the wind tunnel startup is also critically analyzed.

Oblique shock wave reflection at plasma array presence

This work discusses the effect of a filamentary plasma array on shock wave (SW) reflection pattern and on a shock-induced separation zone geometry. It includes experimental and computational components both. The experimentation was performed in the supersonic blowdown test rig SBR-50 at the University of Notre Dame at flow Mach number M=2, stagnation pressure P0=1.7-2.7 bar and stagnation temperature T0=300 K. Oblique shock wave generator composed of a symmetric solid wedge was installed on the top wall of test section while the filamentary plasma generator was arranged on the opposite wall. Thus, the main SW originating from the wedge impinged the plasma area. As a result of the SW-plasma interaction, the flowfield was significantly modified, including a shift of the main SW upstream and redistribution of wall pressure over the test section. The computational analysis allowed a 3D reconstruction of the SW interaction with the plasma array. The physics of SW-plasma array interaction are also discussed.

Numerical study on the supersonic near-wake flow structures of a cylinder at an angle of attack

The supersonic near-wake flow experiment conducted by Dutton on a cylinder with 10 degrees angle of attack was studied numerically with Delayed Detached Eddy Simulation (DDES) method. The wind tunnel test section were included in the geometry model to examine if its omission led to the unsuccessful prediction of several important flow features in previous studies. Flow structures were compared to experiment measurements, Gaitonde's k-ε model results and our previous Monotonically Integrated Large Eddy Simulation (MILES) results. It was revealed that with present geometry model and numerical method, good agreements with wind tunnel experiment result were obtained, including the wave structures, density fields and velocity fields. In particular, the inclusion of wind tunnel test section wall in geometry model reproduces the finite area nozzle exit flow and the expansion wave behind the recirculation zone in the actual experiment setup, which lead to correct prediction of the position of maximum streamwise velocity point. The present simulation also reproduced the uniform pressure profile and value observed in the experiment and the deviation between computation and experiment was within 10%. The flow field data obtained was further analyzed to investigate the vortex surface shape and recirculation region shape in near wake flow with non-zero angle of attack and significant differences were found compared to the zero angle of attack case. The vortex surfaces show a non-axisymmetric slitting movement in the near wake region, which gives an explanation to the non-circular density contour pattern on streamwise flow field slices. The recirculation region shape was identified with zero isosurface of streamwise velocity and was found to be of complex three-dimensional shape with maximum length beside the symmetry plane and a local minimum at windward side of the symmetry plane, which is significantly different from that of zero angle of attack case. The observation that the length of the recirculation zone was greatly reduced on the symmetry plane at non-zero angle of attack in previous study is due to the aforementioned local minimum on the symmetry plane. The reduction of recirculation length is much less if we consider the three-dimensional shape of the region.

Measurementimprovement of flow quality of slotted test section in transonic wind tunnel

Experimental studies were carried out in the 0.6 m×0.6 m continuous transonic wind tunnel of CARDC in order to investigate the flow characteristics of the slotted test section. Experimental results show that the root-mean-square deviation of axial Mach number in the model area is above 0.01 when the test section Mach number is above 1.0.Numerical simulation under the same conditions to investigate the flow characteristics of the slotted section, together with the experimental studies indicate tow phenomena may directly cause the Mach number fluctuation. Firstly, a straight section was installed to connect the nozzle and the test section in the wind tunnel. Weak shock waves due to the curvature discontinuity at the joint of the test section and the straight section contribute to Mach number fluctuation. Secondly, the open-area ratio of both the upper and lower wall of test section, each with 8 slots, is of 10%. The larger porosity leads to stronger expansion waves in the acceleration zone located at the inlet of the test section. The flow was over accelerated because of the stronger expansion wave and thus fluctuate the flow field severely. Two measures were taken to improve the flow quality of the slotted test section based on the above-mentioned analysis: ①Flexible plate instead of solid straight plate was installed to bridge nozzle and test section to eliminate the curvature discontinuity; ②Decreasing the open-area ratio of the upper and lower test section wall to 6% and the number of slots to 6. Numerical and experimental results show that the Mach number fluctuation in the model area was suppressed to a satisfactory degree.

Thermal characterization of a heating cylinder under ultrasonic effects

Ultrasonic effects were investigated on the heat transfer characteristics of a vertical cylinder releasing a constant heat flux of 14,150 W/m2 in distilled water. A triple-frequency ultrasonic transducer was installed at the center of the lateral wall of the cubic test section to emit 40, 80, and 120 kHz ultrasonic waves. The thermal characterization was investigated by varying the distance (5, 10, and 15 cm) between the ultrasonic transducer and the heating surface. The surface temperature of the heater, detected using 12 thin-leaf thermocouples, decreased rapidly because of the ultrasonic effect, leading to an increase in the Nusselt number. An infrared camera and a video camera were used for the visualization and investigation of acoustic streaming and acoustic cavitation. The results showed that ultrasound enhanced heat transfer to the maximum level of 82.4% using 40 kHz waves at a distance of 5 cm. At distances of 10 and 15 cm, 120 kHz ultrasonic waves produced the highest heat transfer enhancement levels of 74.2% and 59.7%, respectively. The heat transfer enhancement ratio decreased with increasing incident angle of the wave beam as well as with the distance between the ultrasonic transducer and the heating surface. The key mechanisms leading to the enhancement of heat transfer in the near and far fields were identified and discussed. Finally, an accurate prediction formula was proposed for the overall heat transfer enhancement ratio, depending on the ultrasonic frequency and distance.

Lift Interference in Wind Tunnels with Perforated and Solid Walls

In order to obtain accurate results in wind tunnel testing it is necessary to determine the effect of interaction between the flow around the model and the test section walls. In this paper, the classical theory for wind tunnel wall corrections assessment is used to evaluate the wall induced change in the circulation caused by the presence of the test article in the wind tunnel. This primary correction, also known as lift interference is based on the test section geometry and it is applied to the test article angle of attack. The computations performed in this paper employ the assumption of the potential linearized flow between the test section walls and the model. As well, the principle of superposition is a key element in this analysis.

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.

X-Ray Imaging Calibration for Fuel-Coolant Interaction Experimental Facilities

During a severe accident in either sodium-cooled or water-cooled nuclear reactors, jets of molten nuclear fuel may impinge on the coolant resulting in fuel-coolant interactions (FCI). Experimental programs are being conducted to study this phenomenology and to support the development of severe accident models. Due to the optical opacity of the test section walls, sodium coolant, and the apparent optical opacity of water in the presence of intense ebullition, high-speed X-ray imaging is the preferred technique for FCI visualization. The configuration of these X-ray imaging systems, whereby the test section is installed between a fan-beam X-ray source and a scintillator-image intensifier projecting an image in the visual spectrum onto a high-speed camera, entails certain imaging artefacts and uncertainties. The X-ray imaging configuration requires precise calibration to enable detailed quantitative characterization of the FCI. To this end, ‘phantom’ models have been fabricated using polyethylene, either steel or hafnia powder, and empty cavities to represent sodium, molten fuel and sodium vapor phases respectively. A checkerboard configuration of the phantom enables calibration and correction for lens distortion artefacts which magnify features towards the edge of the field of view. Polydisperse steel ball configurations enable precise determination of the lower limit of detection and the estimation of parallax errors which introduce uncertainty in an object’s silhouette dimensions. Calibration experiments at the MELT facility determined lower limits of detection in the order of 4 mm for steel spheres, and 1.7-3.75 mm for vapor films around a molten jet.

Experimental study of post-CHF heat transfer in a vertical tubular test section

In this paper, post-CHF (critical heat flux) experiments are performed in a vertical tubular test section with water as the working fluid to study the heat transfer characteristics during the process of formation, growth, and propagation of dry patches. The formation, growth, and propagation of the dry patches are indicated from the measured wall temperatures of the test section. The so-called dry temperature and quench temperature, corresponding to the formation and departure of dry patches, are obtained by analyzing the first-order derivative of the measured wall temperatures. The temperature gradient along the axial direction of the test section wall is also calculated, up to 15 °C/mm. Large temperature gradients result in axial heat conduction as well as thermal stresses in the test section. The convective heat transfer coefficient for the dry patches is calculated based on the saturation temperature of the fluid and compared with three heat transfer correlations in the literature. The computed convective heat transfer coefficient for the dry patches in our experiments is larger than the calculated results of the correlations, especially for conditions of relatively low wall superheats.

Reynolds number and wind tunnel wall effects on the flow field around a generic UHBR engine high-lift configuration

RANS simulations of a generic ultra-high bypass ratio engine high-lift configuration were conducted in three different environments. The purpose of this study is to assess small scale tests in an atmospheric closed test section wind tunnel regarding transferability to large scale tests in an open-jet wind tunnel. Special emphasis was placed on the flow field in the separation prone region downstream from the extended slat cut-out. Validation with wind tunnel test data shows an adequate agreement with CFD results. The cross-comparison of the three sets of simulations allowed to identify the effects of the Reynolds number and the wind tunnel walls on the flow field separately. The simulations reveal significant blockage effects and corner flow separation induced by the test section walls. By comparison, the Reynolds number effects are negligible. A decrease of the incidence angle for the small scale model allows to successfully reproduce the flow field of the large scale model despite severe wind tunnel wall effects.

Modal Parameter Estimation of a Compliant Panel Using Phase-based Motion Magnification and Stereoscopic Digital Image Correlation

This paper demonstrates the use of broad-band phase based motion magnification (PMM) to improve the modal parameter estimation from high-speed stereoscopic digital image correlation (DIC). PMM is used as a diagnostic technique to investigate the free vibration response of a panel. The compliant panel, consisting of a thin polycarbonate sheet, forms a test section wall in a supersonic blow-down wind tunnel, where it is used to investigate supersonic fluid-structure interaction in the presence of shock wave boundary layer interaction. The panel is excited by an impact hammer and the transient deformation is captured using high-speed cameras. The original and motion-magnified images are input to a digital image correlation algorithm to calculate the out-of-plane deformation of the panel. The measured deformation is used to extract the modal parameters of the compliant panel. By using PMM as a preprocessing tool in a broad frequency band containing multiple structural modes, the signal to noise ratio of the measured deformation is improved. The use of PMM improves the estimated mode shapes, increasing the MAC value of the first mode compared to FEM predictions from 0.29 to 0.99. Motion magnification also improves the coherence between measured input force and panel deformation by up to 13% if suitable parameters are chosen.

On the need for conjugate heat transfer modeling in transient CFD simulations

Within the European SESAME project, system thermal-hydraulics (STH), computational fluid dynamics (CFD) and coupled 1D-3D thermal-hydraulic simulations were carried out for Generation IV nuclear reactor systems. The Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS) gGmbH participated in the project with activities related to the development and validation of coupled STH-CFD codes. The TALL-3D facility, operated by the Royal Institute of Technology (KTH) in Stockholm, is designed for thermal-hydraulic experiments with lead-bismuth eutectic (LBE) coolant. A well-instrumented, partially heated test section with cylindrical form is installed in the primary circuit. It is the domain of complex 3D flow and heat transfer phenomena. The CFD analyses showed that only the consideration of the fluid domain is not sufficient for the correct prediction of the thermal-hydraulic transient dynamics. Therefore, solid structures like the test section walls, inner circular plate, test section heater, and the insulation were explicitly included in the CFD model. This allowed the consideration in the simulations of the heat conduction and the thermal inertia of these components. The paper focuses on the analysis of the observed thermal-hydraulic flow phenomena in two experimental cases, and the comparison between CFD predictions and data. Both deal with the transition from forced to natural circulation. In the first experiment the test section heater is switched on, while in the second the heat is generated only in the main heater component. The influence of conjugate heat transfer (CHT) on the numerical results is investigated and discussed in detail.

Wall Interference Correction Method for Kevlar Wall Test Section

This Paper describes a new method of correcting the wind tunnel wall interference of permeable Kevlar test section walls. The classical three-dimensional panel method is extended to flows in a Kevlar-walled test section and modified to improve its general predictability. The results computed using this enhanced panel method are in good agreement with experimental results with regard to the pressure distributions on a wind tunnel model and Kevlar wall surfaces and the Kevlar wall deformation. With this new tool, which can compute flows in a Kevlar-walled wind tunnel precisely, it is possible to compute the disturbance flows due to the walls correctly and separately from those due to the wind tunnel model. The effects of the wind tunnel walls on blockage and downwash of the flow in the wind tunnel are estimated by calculating the disturbance flow velocity due to the walls at the model reference point. Wall interference corrections using this new method are applied to a high-lift semispan wing model installed in a Kevlar-walled test section, and the results demonstrate that the method is practicable and reliable.

Influence of T-128 wind tunnel perforated walls on aerodynamic characteristics of reentry vehicles at transonic speed

The paper is devoted to the study of the influence of wind tunnel perforated walls on the aerodynamic characteristics of the reentry vehicle model at transonic speed. An experimental investigations were conducted with a model at zero angle of attack in the T-128 TsAGI transonic wind tunnel within the range of Mach numbers M = 0.9–1.1 at three perforation ratios of test section walls F = 2; 6; 10%. Aerodynamic forces and moments acting on the model were measured with a six-component internal strain-gauge balance. A distribution of static pressure on the model surface and on the test section perforated walls was measured along with static pressure fluctuations. Integral parameters of the boundary layer on the perforated wall were measured by means of the rake with total pressure probes. Numerical investigations of the flow around the model in the test section were performed by solving Reynolds Averaged Navier–Stocks (RANS) equations using the Electronic Wind Tunnel software (EWT-TsAGI). Satisfactory agreement was obtained between the computational and experimental data on the effect of the test section perforated walls on the aerodynamic characteristics of the model and the flow parameters on the wall. The influence of the perforation coefficient on the model drag coefficient in the calculations and in the experiment had the same trend. The maximum error of the drag coefficient at a simplified computational model is estimated to be 0.05. The highest effect of perforation on the drag coefficient of reentry vehicle is comprised if 1.05 < M < 1.1 and makes ΔCD ≈ 0.03–0.04.

The experimental study of the weak shock wave action on the boundary layer of the sweep flat plate

The weak shock waves impact on the flow in the boundary layer of the sweep flat plate at Mach number 2 is studied experimentally. The model was installed at a zero angle of attack. Shock waves in the oncoming flow were generated by a two-dimensional roughness on the side wall of the wind tunnel test section. The data were obtained with a two-dimensional roughness on the wall of the wind tunnel and with a smooth wall at sweep angles (35 and 40 degrees) of the leading edge of the flat blunt plate. The measurements were carried out with a constant temperature anemometer.

Prediction of Noise from Turbulent Boundary Layers Attached to Porous Media

Aerospace vehicles, wind-tunnel test section walls, and other structures often contain porosity that alters the turbulent boundary layer and radiated noise. A semi-empirical mathematical model is developed to predict and analyze the acoustic radiation from turbulent boundary layers over porous media. The model is an acoustic analogy that depends on local flowfield statistics. These statistics are calculated through a steady Reynolds-averaged Navier–Stokes computational-fluid-dynamics solver that includes porous material. Acoustic predictions are conducted for four subsonic Mach numbers without a pressure gradient. At each Mach number, four porosities with constant liner depth and porous turbulent length scale are examined along with the nonporous solution. The flowfield is validated through comparison with acoustic measurement. Predictions are conducted to ascertain changes in acoustic radiation with varying porosity. It is found that noise is amplified or reduced in a nonintuitive way with the introduction of porosity, variation of frequency, and increase of Mach number.

Exploitation of hybrid anechoic wind tunnels for aeroacoustic and aerodynamic measurements

The last 10 years have seen the establishment of the hybrid anechoic wind tunnel as a significant tool for experimental aeroacoustics research. The hybrid anechoic wind tunnel features a test section which is effectively closed in the aerodynamic sense but open in the acoustic sense, this unique behavior being achieved by stretching fabric of high strength-to-weight ratio (such as Kevlar) over windows in the test section walls which open to anechoic chambers. The hybrid anechoic arrangement offers a number of advantages over the traditional open-jet test section, including low interference corrections that allow for testing of large models at higher Reynolds numbers—advantages well suited to aircraft noise source research. In this article, we review recent applications of hybrid anechoic tunnels for aircraft noise source studies. We discuss the established practice for acoustic and aerodynamic correction of measurements performed in these facilities, and assess some of the latest findings in aerodynamic behavior of hybrid anechoic tests sections.