Showing papers on "Freestream published in 2022"

Low-frequency shock train oscillation control in a constant area duct

Abstract: The self-excited shock train oscillation control using partial removal of boundary layer flow in a constant area duct is studied numerically using unsteady Reynolds Averaged Navier–Stokes simulation. The effect of varying the suction flow ratio on the shock train oscillatory characteristics is analyzed using steady and unsteady statistics, space–time contour, power spectra, and cross correlation analysis. For the present study, a constant area duct of height 0.032 mm, and the aspect ratio of 25, at freestream Mach number of 2.0 and back pressure ratio of 0.52 is considered. The removal of the boundary layer through an angled suction slot with three different suction flow ratios is performed. Numerical results indicate that the removal boundary layer restricts the bifurcation process of the shock train and appears to be a single curved normal shock at a higher suction flow ratio. Also, the transition of regular reflection to the Mach reflection type is noted. The suction flow from the top and bottom slot tends to initiate a lateral oscillation that forms a wavy mixing flow region. The power spectral density contour suggests that the increase in suction flow ratio will tend to increase the dominant frequency ranges from (0.034 to 0.094). The cross correlation indicates the presence of downstream pressure disturbance that moves toward the upstream direction. With suction flow, a disturbance that emerges from the suction slot moves in the opposite direction and dominates at a lower suction flow ratio and these disturbances disappear at a large suction flow ratio.

Enhanced performance of tandem plunging airfoils with an asymmetric pitching motion

Abstract: The flapping wings or fins in an in-line arrangement are a common scene in flocks and schools, as well as flying creatures with multiple pairs of wings, e.g., dragonflies. Conventional studies on these topics are underpinned by tandem plunging airfoils in either a vertical or a declined stroke plane. The former model mostly considers a symmetrical pitching motion, and the latter model fails to separate the effect of the asymmetric pitching from that of the declined incoming flow. However, our study focuses on the tandem airfoils with vertical plunging and asymmetric pitching in a horizontal freestream and, therefore, explains the effects of asymmetric pitching on tandem plunging airfoils. Using numerical methods, the aerodynamic performance and vortical structures of the tandem airfoils are examined, and the effects of the non-zero geometric angle of attack (α0), phase angles in the plunging and pitching motion (φ and θ), and inter-foil spacing (G/c) are discussed. Our results show that the tandem arrangement is beneficial to enhance the propulsion thrust while retaining the lifting capacity of the airfoil at a non-zero α0. The effects of φ and G/c are coupled since they both determine the interaction between the hind airfoil and the leading-edge vortex in the wake and the out-of-phase mode is suggested for the tandem airfoils at G/c = 1 to enhance both lift and thrust. For a tandem airfoil with in-phase mode, the optimal G/c is around 1.5 to 2. Moreover, the asymmetric pitching of the in-phase plunging airfoils should be synchronized to retain the enhanced performance.

On the synchronisation of three-dimensional shock layer and laminar separation bubble instabilities in hypersonic flow over a double wedge

Abstract: Linear global instability of the three-dimensional (3-D), spanwise-homogeneous laminar separation bubble (LSB) induced by shock-wave/boundary-layer interaction (SBLI) in a Mach 7 flow of nitrogen over a $30^{\circ}-55^{\circ}$ double wedge is studied. At these conditions corresponding to a freestream unit Reynolds number, $Re_1=5.2\times 10^{4}$ m$^{-1}$, the flow exhibits rarefaction effects and comparable shock-thicknesses to the size of the boundary-layer at separation. This, in turn, requires the use of the high-fidelity Direct Simulation Monte Carlo (DSMC) method to accurately resolve unsteady flow features. We show for the first time that the LSB sustains self-excited, small-amplitude, 3-D perturbations that lead to spanwise-periodic flow structures not only in and downstream of the separated region, as seen in a multitude of experiments and numerical simulations, but also in the internal structure of the separation and detached shock layers. The spanwise-periodicity length and growth rate of the structures in the two zones are found to be identical. It is shown that the linear global instability leads to low-frequency unsteadiness of the triple point formed by the intersection of separation and detached shocks, corresponding to a Strouhal number of $St\sim0.02$. Linear superposition of the spanwise-homogeneous base flow and the leading 3-D flow eigenmode provides further evidence of the strong coupling between linear instability in the LSB and the shock layer.

Supersonic Freestream Density Fluctuations from Focused Laser Differential Interferometry and Pitot-Probe Measurements

Abstract: Focused laser differential interferometry (FLDI) is used to make freestream density fluctuation measurements in the von Kármán Gas Dynamics Facility (VKF) Wind Tunnel D at Arnold Engineering Development Complex. A comparison is made between pitot-probe and FLDI measurements after converting both to freestream pressure fluctuation spectra. A modification of Stainback andWagner’s theory, incorporating recent numerical results from Chaudhry et al., is used to interpret the pitot data, while a new inversion algorithm is applied to the FLDI data. These methods of interpretationmake the assumption that the tunnel freestream disturbance environment is dominated by the acoustic Kovásznay mode, in line with the theoretical and experimental evidence. Close agreement is found between the two sets of spectra, showing that accurate quantitative data can be obtained with FLDI, and the instrument can be used to extend spectra beyond the pitot bandwidth.

Theoretical study of non-Newtonian micropolar nanofluid flow over an exponentially stretching surface with free stream velocity

Abstract: The computational analysis of the second-order micropolar stagnation point flow of nanofluid over an exponentially permeable stretching sheet is considered. The freestream velocity with the thermal slip effects is taken into account in this analysis. This model is developed on the basis of flow assumptions and reduced into partial differential equations before applying the boundary layer approximations. The governing equations as a mathematical model are simplified with the help of suitable transformations. The differential system is further solved by using the bvp4c. Both graphs and tables are used to report observations. The skin friction and Nusselt number are reported for both weak and strong concentrations. The magnitude of skin friction is noticed greater for strong concentration in comparison with weak concentration. Subject to couple stress, the values of skin friction are relatively high for the case of weak concentration in comparison with strong concentration. Micropolar profile admit the direct relation toward micropolar parameter and micro-gyration parameter. Both Sherwood number and Nusselt number admits higher values for strong concentration as compared to weak concentration.

The Effects of Nose Bluntness on Broadband Disturbance Receptivity in Hypersonic Flow

Abstract: While nose bluntness is known to have a large impact on the stability of hypersonic vehicles, its influence on the freestream receptivity process has not been fully characterized for a wide range of conditions. This paper investigates the effects of nose bluntness on the second mode receptivity coefficients and the development of boundary layer disturbances over two 7 degree half-angle circular blunt cones at mach 10 after perturbation with broadband freestream pulses of different types. The cones have nose radii of 9.525 mm (Case B) and 5.08 mm (Case I). Unsteady direct numerical simulation (DNS) and linear stability theory (LST) results compare well and predict stronger second mode growth for Case I in all pulse cases. Unsteady DNS also shows variations in extramodal excitation between the cones depending on freestream disturbance type. Spectral receptivity coefficients are generated by decomposing the unsteady DNS data into discrete frequency Fourier modes which are then corrected with LST N-factors. Fast acoustic disturbances demonstrate minimal variation in receptivity coefficients, while temperature and vorticity disturbances have much higher coefficients in Case I. Planar slow acoustic pulses induce stronger disturbances outside of the second mode in Case, resulting in higher peak receptivity coefficients. Results show significant variation in receptivity response based on nose bluntness, pulse geometry, and the type of the incident perturbation.

Flutter Predictions for Very Flexible Wing Wind Tunnel Test

Abstract: The stability boundaries of a very flexible wing are sought to inform a wind-tunnel flutter test campaign. The objective is twofold: to identify via simulation the relevant physical processes to be explored while ensuring safe and nondestructive experiments, and to provide a benchmark case for which computational models and test data are freely available. Analyses have been independently carried out using two geometrically nonlinear structural models coupled with potential flow aerodynamics. The models are based on a prototype of the wing for which static load and aeroelastic tests are available, and the experimental results have been successfully reproduced numerically. The wing displays strong geometrically nonlinear effects with static deformations as high as 50% of its span. This results in substantial changes to its structural dynamics, which display several mode crossings that cause the flutter mechanisms to change as a function of deformation. Stability characteristics depend on both the freestream velocity and the angle of attack. A fast drop of the flutter speed is observed as the wing deforms as the angle of attack is increased, whereas a large stable region is observed for wing displacements over 25%. The corresponding wind tunnel dynamic tests have validated these predictions.

Instability of Separated Shear Layer around Levitated Freestream-Aligned Circular Cylinder

Abstract: In the present study, the characteristics of the shear layer around a freestream-aligned circular cylinder and the relationship between the shear layer motion and the aerodynamic force were investigated under supportless condition. The 0.3-m magnetic suspension and balance system (MSBS) was employed and experiments were conducted without a mechanical supporting device. Velocity fields were measured using particle image velocimetry with a sufficient temporal and spatial resolution and high-frequency velocity fluctuations caused by small Kelvin-Helmholtz (KH) vortices were captured. The power spectral densities of velocity fluctuations represent phenomena such as KH vortex convection, vortex pairing, and convection of multiple vortices. Furthermore, fluctuations of the shear layer position were investigated. The results illustrates that the dominant frequency of the shear layer position is lower than the frequency of velocity and it shows good agreement with the characteristic frequency of lift force fluctuations. The present results together with the report in the previous study illustrate that the pressure fluctuations are considered to drive both fluctuations of the shear layer position and lateral aerodynamic force.

Review of Film Cooling in Gas Turbines with an Emphasis on Additive Manufacturing-Based Design Evolutions

Abstract: Film-cooling technology is used in high-temperature components of gas turbines to extend their service lives. Hot-gas path components are susceptible to damage or failure in the absence of film cooling. Much of the optimization research efforts have been focused on film hole shapes, heat/mass transfer measurement techniques, and film cooling performance under various mainstream and coolant side operating conditions. Due to recent rapid advancements in the areas of measurement techniques (e.g., pressure-sensitive paints and fast high-resolution imaging) and metal additive manufacturing (AM), film cooling technology has undergone significant changes and shows potential new development. In this review, a historical perspective is discussed covering over five decades of innovation: the geometrical effects from injection angle and hole shapes; flow effects from density ratio, momentum-flux ratio, blowing ratio, advective capacity ratio, and freestream conditions; and more items related to AM. The impact of AM on film hole design strategies, the challenges posed by state-of-the-art AM technology, and pathways for future research are discussed. A comparative analysis of AM assisted film hole fabrication and conventionally manufactured film holes is elaborated.

Velocity measurements in a hypersonic flow using acetone molecular tagging velocimetry.

Abstract: In the present work, a non-intrusive diagnostic technique known as molecular tagging velocimetry was used to collect quantitative freestream velocity measurements in the Mach 7 Ludwieg Tube Wind Tunnel located at The University of Texas at San Antonio. This laser-based diagnostic technique used a single Nd:YAG 4th harmonic laserline to excite acetone molecules seeded in the flow field. From the resulting emitted light, mean and instantaneous velocity profiles in the hypersonic freestream flow and facility boundary layer were measured. Uncertainty in the velocity measurements for individual test runs is estimated at ≤ 8% while overall 1D freestream mean velocity measurements were recorded with ±2.4% (± 21 m/s) accuracy. The effect of acetone seeding on the speed of sound was also quantified.

Mach 18 Flow Velocimetry with 100-kHz KTV and PLEET in AEDC Tunnel 9

Abstract: Krypton Tagging Velocimetry (KTV) and Picosecond Laser Electronic Excitation Tagging (PLEET) velocimetry at a 100-kHz rate were demonstrated in Mach 18 flow conditions at the Arnold Engineering Development Center (AEDC) Tunnel 9 employing a burst-mode laser system and a custom optical parametric oscillator (OPO). The measured freestream flow velocities from both KTV and PLEET agreed well with the theoretical calculation. The increase in repetition rate provides better capability to perform time-resolved velocimetry measurements in hypersonicflowenvironments.

Hypersonic Boundary-Layer Instabilities over Ogive-Cylinder Models

Abstract: Computational investigations of a tangent ogive-cylinder geometry with varying forebodies at zero degrees angle of attack are presented. The model geometry and conditions are selected to match experiments conducted in the Air Force Research Laboratory (AFRL) Mach 6 Ludwieg Tube. Five forebodies of interest were selected for the computational studies herein: two sharp ogives, two blunt ogives, and one hemispherical forebody. Computations are performed at a freestream unit Reynolds number of 7.01x10^6 m-1. Each sharp and blunt tip ogive had a 14 and 28 degree version for the tip angles providing a 4 caliber and 2 caliber ogive forebody, respectively. A cylindrical section follows the forebody, resulting in a meter long model matching the experiments. The laminar flow solutions are analyzed. The boundary layer-edge properties and the velocity and temperature profiles are compared across streamwise locations aft of the ogive-cylinder junction. The blunt forebodies induce an entropy layer that envelopes the boundary-layer profiles. Modal stability analysis identifies most amplified frequencies corresponding to Mack’s second modes that agree with experimental results for the sharp forebodies. Similar to the experimental measurements based on wall-mounted pressure sensors, no unstable modes are found for the blunt models. Nonmodal analysis revealed a broadband set of disturbances present for the blunter forebodies, in agreement with experimental observations. Flow perturbation contours of most amplified planar and oblique disturbances are shown to qualitatively match wind tunnel schlieren images, with a switch from rope-like to elongated structures, i.e., from high frequency Mack’s second modes to low frequency Mack’s first modes, as the forebody angle is increased for the sharp tip, and from boundary-layer to entropy-layer disturbances as the bluntness is increased.

Linear Instabilities over Ogive-Cylinder Models at Mach 6

Abstract: Computational investigations of a tangent ogive-cylinder geometry with varying forebodies at zero degrees angle of attack are presented. The model geometry and conditions are selected to match experiments conducted in the Air Force Research Laboratory (AFRL) Mach 6 Ludwieg Tube. Specifically, five forebodies of interest consisting of sharp and blunt ogives of 4 and 2 caliber and a hemispherical shape are studied at a freestream unit Reynolds number of . Boundary-layer-edge properties and wall-normal profiles of velocity and temperature are compared across streamwise locations past the ogive-cylinder junction. Modal stability analysis is used to characterize the most amplified frequencies corresponding to Mack’s first and second modes and those are found to agree with experimental results for the sharp forebodies. The entropy layer induced by blunt forebodies envelopes the boundary layer and stabilizes modal disturbances. Nonmodal analysis revealed a broadband set of disturbances present for the blunter forebodies, in agreement with experimental observations. Flow perturbation contours of most amplified planar and oblique disturbances are shown to qualitatively match wind tunnel schlieren images, with a switch from rope-like to elongated structures, i.e., from high-frequency Mack’s second modes to low-frequency Mack’s first modes, as the forebody angle for the sharp tip is increased, and from boundary-layer to entropy-layer disturbances as the bluntness is increased.

Aerodynamics of a wing in turbulent bluff body wakes

Abstract: Abstract The aerodynamics of a stationary wing in a turbulent wake are investigated. Force and velocity measurements are used to describe the unsteady flow. Various wakes are studied with different dominant frequencies and length scales. In contrast to the pre-stall angles of attack, the time-averaged lift increases substantially at post-stall angles of attack as the wing interacts with the von Kármán vortex street and experiences temporal variations of the effective angle of attack. At an optimal offset distance from the wake centreline, the time-averaged lift becomes maximum despite of small amplitude oscillations in the effective angle of attack. The stall angle of attack can reach 20° and the maximum lift coefficient can reach 64 % higher than that in the freestream. Whereas large velocity fluctuations at the wake centreline cause excursions into the fully attached and separated flows during the cycle, small-amplitude oscillations at the optimal location result in periodic shedding of leading edge vortices. These vortices may produce large separation bubbles with reattachment near the trailing-edge. Vorticity roll-up, strength and size of the vortices increase with increasing wavelength and period of the von Kármán vortex street, which also coincides with an increase in the spanwise length scale of the incident wake, and all contribute to the remarkable increase in lift.

Unsteady pulsating flowfield over spiked axisymmetric forebody at hypersonic flows

Abstract: The paper gives experimental observations on the hypersonic flow past an axisymmetric flat-face cylinder with a protruding sharp-tip spike at a freestream Mach number of $M_\infty = 8.16$ at two different freestream Reynolds numbers based on the base body diameter ($Re_D = 0.76 \times 10^6$, and $3.05 \times 10^6$). Furthermore, modal analysis is done on schlieren images to understand the flow dynamics parallel with the unsteady pressure measurements. The protruding spike of length to base body diameter ratio of $[l/D]=1$ creates a familiar unsteady flowfield called 'pulsation.' Pressure loading and fluctuation intensity at two different $Re_D$ cases are calculated. A maximum drop of 98.24\% is observed in both parameters between the high and low ReD cases. Based on the analysis, a difference in the pulsation characteristics are noticed, which arise from two vortical zones, each from a system of two `$\lambda$' shocks formed during the `collapse' phase ahead of the base body. The interaction of shedding vortices from the $\lambda$-shocks' triple-points, along with the rotating stationary waves, contributes to the asymmetric high-pressure loading and the observation of shock pulsation on the flat-face cylinder. The vortical interactions form the second dominant spatial mode with a temporal mode carrying a dimensionless frequency ($f_2D/u_\infty \approx 0.34$) almost twice that of the fundamental frequency ($f_1D/u_\infty \approx 0.17$). The observed frequencies are invariant irrespective of the ReD cases. However, for the high-frequency range, the spectral pressure decay is observed to follow an inverse and -7/3 law for the low and high $Re_D$ cases, respectively.

Concave Bump for Impinging-Shock Control in Supersonic Flows

Abstract: In the present study, a novel concave bump for impinging-shock control in two-dimensional supersonic flows is investigated. An analytical method for preliminary bump design based on a generalized shape of a shock-canceling bump has been developed and verified numerically. An extensive proof-of-concept study was performed at a freestream Mach number ranging from 2.5 to 5.0 for shock-generator angles varying from 6 to 12 degrees. It could be demonstrated that a concave bump designed for a given flow-deflection angle is capable of significantly reducing the size of the separation bubble as well as the total pressure losses throughout the Mach number range investigated. The achievable gains depend on the Mach number, the flow-deflection angle, and the relative impingement position of the incident shock front on the bump. The highest values of separation-length reduction (up to 100%), momentum thickness reduction (up to 31%), and pressure recovery factor increase (up to 33%) were obtained at the optimum shock impingement position for the largest deflection angle studied. The concave bump is less effective, and in some cases even disadvantageous, when the incident shock wave does not optimally strike the bump crest.

Large eddy simulation of a thermal impinging jet using the lattice Boltzmann method

Abstract: A compressible Hybrid Lattice Boltzmann Method solver is used to perform a wall-resolved Large eddy simulation of an isothermal axisymmetric jet issuing from a pipe and impinging on a heated flat plate at a Reynolds number of 23 000, a Mach number of 0.1, and an impingement distance of two jet diameters. The jet flow field statistics, Nusselt number profile (including the secondary peak), and shear stress profile were well reproduced. The azimuthal coherence of the primary vortical structures was relatively low, leading to no discernible temporal periodicity of the azimuthally averaged Nusselt number at the location of the secondary peak. While local unsteady near-wall flow separation was observed in the wall jet, this flow separation did not exhibit azimuthal coherence and was not found to be the only cause of the thermal spots blue, which lead to the secondary peak in the Nusselt number, as stream-wise oriented structures also played a significant role in increasing the local heat transfer.

Wake flow effects on the energy harvesting characteristics of piezoelectric tandem flags

Abstract: In this study, we experimentally investigate the effects of the interaction of piezoelectric flags in the tandem configuration on energy harvesting efficiency. The flags are placed in wake flow behind the bluff body and their flapping behaviors are examined. The experiments are performed in a low-speed water tunnel by varying the flow velocity and streamwise gap behind an inverted C-shape cylinder to analyze the effect of wake flow on amplitude, flapping frequency, and harvested power by the piezoelectric flags. Threshold values for energy harvesting of the streamwise gap and freestream velocity are found to be the same for both flags i.e. 1.5 and 0.18m/s, respectively. While analyzing the dynamical behaviors of the flags, inverted drafting phenomenon is observed in flags: the flapping amplitude of the rear flag is increased by excitation from the vortices and wake of the front flag. This kind of interaction helps out in boosting the energy harvesting efficiency based on the random excitations with high amplitude of rear flag. Results show, as the streamwise gap in-between the flags changes, the influence of the front flag on downstream flag alters and dynamical behavior of front flag show variation when the distance between bluff body and front flag changes. The highest power is also obtained for the rear flag at streamwise gap equals to 1.75 and freestream velocity of 0.26m/s. The tandem configuration produces 216% more power and remarkably improved the energy harvesting efficiency as compared to the single flag energy harvester.