Showing papers on "Freestream published in 2019"

Cavity flameholding in an optical axisymmetric scramjet in Mach 4.5 flows

Abstract: The dynamics of an optical axisymmetric scramjet, preferred for generic combustion and flameholding studies without the corner boundary-layer effects that distort the freestream and flame propagation in rectangular scramjets, is investigated at Mach 45 Ethylene fuel is injected into supersonic flow via sixteen sonic fuel nozzles equally spaced in a circumferential direction and inclined at 45° to the freestream The gaseous fuel is auto-ignited by high-enthalpy flows that are compressed and decelerated by shockwaves and boundary layers while passing through the scramjet inlet and isolator An axisymmetric cavity located downstream of fuel injectors achieves flame holding by providing slow flow recirculation regions and inducing shockwaves at cavity leading edge and ramp to redirect and recompress flows Flow and flame behaviors are characterized by high speed flame chemiluminescence imaging and static pressure measurement, while the detailed flame structures are resolved by instantaneous ground-state hydroxyl radical (OH) distributions using planar laser-induced fluorescence (PLIF) Numerical simulation is employed to aid the inlet/isolator design to avoid unexpected unstart introduced by shock-boundary layer interactions in the shock train region The Mach number profiles in the radial direction are measured using a Pitot probe at the combustor exit We demonstrate supersonic flameholding with the presence of a cavity at ethylene fueled conditions where stable combustion is achieved without scramjet unstart under both mass and heat loading The heat addition from the cavity-stabilized flame mainly reduces the flow Mach number in the near-wall flow region In the core flow region, the flow Mach number is decreased by the jet- and cavity-induced shockwaves and minimally affected by heat addition

Aerodynamically Interacting Vertical-Axis Wind Turbines: Performance Enhancement and Three-Dimensional Flow

Abstract: This study examined three-dimensional, volumetric mean velocity fields and corresponding performance measurements for an isolated vertical-axis wind turbine (VAWT) and for co- and counter-rotating pairs of VAWTs with varying incident wind direction and turbine spacings. The purpose was to identify turbine configurations and flow mechanisms that can improve the power densities of VAWT arrays in wind farms. All experiments were conducted at a Reynolds number of R e D = 7.3 × 10 4 . In the paired arrays, performance enhancement was observed for both the upstream and downstream turbines. Increases in downstream turbine performance correlate with bluff–body accelerations around the upstream turbine, which increase the incident freestream velocity on the downstream turbine in certain positions. Decreases in downstream turbine performance are determined by its position in the upstream turbine’s wake. Changes in upstream turbine performance are related to variations in the surrounding flow field due to the presence of the downstream rotor. For the most robust array configuration studied, an average 14% increase in array performance over approximately a 50° range of wind direction was observed. Additionally, three-dimensional vortex interactions behind pairs of VAWT were observed that can replenish momentum in the wake by advection rather than turbulent diffusion. These effects and their implications for wind-farm design are discussed.

Dynamic Stall of a Finite-Aspect-Ratio Wing

Abstract: An investigation of dynamic stall on a pitching NACA 0012 aspect ratio 4 wing is performed by means of high-fidelity wall-resolved large-eddy simulations. The flow parameters are freestream Mach nu...

On the physical mechanism of tip vortex cavitation hysteresis

Abstract: Inception and desinence thresholds of tip vortex cavitation (TVC), generated by an elliptical NACA 16-020 hydrofoil, are measured at different flow conditions for various gas contents. It is observed that TVC often disappears at cavitation indices significantly higher than the inception thresholds introducing large hystereses. Our measurements reveal that TVC desinence pressure increases with gas content and, under specific flow conditions, may reach to atmospheric pressure. When the pressure of the cavitating core is below the initial saturation pressure of the dissolved gases, water flowing adjacent to the interface becomes supersaturated, which leads to the diffusion of air molecules into TVC. To estimate the outgassing rate, a simple diffusion model is proposed and analytically solved. In addition, we demonstrate that the extent of the delay in desinence due to outgassing is also dictated by the bulk flow parameters, i.e., the incidence angle and freestream velocity. Owing to flow visualizations, we assert that formation of a laminar separation bubble of appropriate size and shape at the hydrofoil tip is a necessary condition for a delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core by forcing the incoming flow to wrap around the axis. By roughening the hydrofoil tip, we demonstrate that the hysteresis is completely suppressed once the laminar separation bubble is destroyed. Moreover, our velocity measurements show that at near-wake, the incidence angle associated with delayed desinence is accompanied by a jet-like axial velocity profile while a wake-like profile is observed for the low-hysteresis case.

Boundary layer turbulence and freestream turbulence interface, turbulent spot and freestream turbulence interface, laminar boundary layer and freestream turbulence interface

Abstract: We study the boundary-layer turbulence and freestream turbulence interface (BTFTI), the turbulent spot and freestream turbulence interface (TSFTI), and the laminar boundary-layer and freestream turbulence interface (LBFTI) using direct simulation. Grid spacings in the freestream are less than 1 Kolmogorov length scale during transition. Probability density functions of temperature and its derivatives are used to select the interface identification threshold, corroborated by a vorticity-based method. The interfaces so detected are confirmed to be physical a posteriori by the distinctive quasi-step-jump behavior in the swirling strength and temperature statistics along traverses normal to the BTFTI and TSFTI. No interface-normal inflection is detected across the LBFTI for either swirling strength, temperature, vorticity magnitude, Reynolds shear stress, streamwise velocity, normal velocity, or turbulence kinetic energy. The present direct numerical simulation data thus cast serious doubts on the shear-sheltering hypothesis/theory, which asserts that a subset of freestream fluctuations is blocked by the LBFTI. In the early stage of transition, quasi-spanwise structures exist on the LBFTI. The TSFTI shape is dominated by head prints of concentrated hairpin vortices. Further downstream, the BTFTI geometry is strongly modulated by groves of hairpin vortices (the boundary layer large-scale motions) with a distinct streamwise preferential orientation. Streamwise velocity and turbulence kinetic energy only exhibit minor plateaus (rather than quasi-step-jump) across the BTFTI and the TSFTI. We emphasize that it is more meaningful and important to acquire reproducible and reliable interface-normal statistics prior to considering any plausible substructures and elusive transient dynamics of the BTFTI, TSFTI, and LBFTI.

Görtler Vortices in Hypersonic Flow on Compression Ramps

Abstract: Direct numerical simulation of Gortler vortices in hypersonic compression ramp flow is performed. Three ramp angles of 15, 20, and 25 deg are considered for a freestream Mach number of 7.7 and a un...

Nonequilibrium Effects on Prediction of Aerothermodynamic Loading for a Double Cone

Abstract: There is a recent re-interest in hypersonic flight with special attention on the capability of numerical simulations. A recent set of experiments with equilibrium freestream conditions was performe...

Aerodynamic drag determination of a full-scale cyclist mannequin from large-scale PTV measurements

Abstract: The aerodynamic drag of a human-scale wind tunnel model is obtained from large-scale particle tracking velocimetry measurements invoking the conservation of momentum in a control volume surrounding the model. Lagrangian particle tracking is employed to obtain the velocity and static pressure statistics in a thin volume in the wake of a cyclist mannequin at freestream velocities between 12.5 and 15 m/s, corresponding to Reynolds numbers from 5 $$\times$$ 105 to 6 $$\times$$ 105 based on the torso length. The spatial distributions of the time-average streamwise velocity and pressure coefficient match well with previous works reported in literature. The streamwise velocity fluctuations in the wake of the cyclist’s model are presented, clearly demonstrating the unsteady nature of the main wake flow structures. Furthermore, the obtained aerodynamic drag follows the expected quadratic increase with increasing freestream velocity. The accuracy of this drag estimation is evaluated by comparison to force balance data and corresponds to 30 drag counts. The three terms composing the overall drag force, ascribed to the mean and fluctuating streamwise velocity and the mean pressure, are also evaluated separately, demonstrating that the resistive force is dominated by the contribution of the mean streamwise momentum deficit, whereas the contribution of the pressure term is negligible.

Flow control over a circular cylinder using virtual moving surface boundary layer control

Abstract: Flow control study of a circular cylinder is carried out using symmetric dielectric barrier discharge (DBD) plasma actuators at the Reynolds number of 10,000. Here, two symmetric DBD plasma actuators are located at the top and bottom of the circular cylinder, respectively, each of which induces pairs of counter-rotating starting vortices on both sides of exposed electrodes. The downstream starting vortices soon take the form of a wall jet along the freestream direction. On the other hand, the upstream starting vortices interact with the incoming flow remain for some time, bringing in high momentum from the freestream to near-wall region, enabling the boundary layer to withstand the adverse pressure gradient and suppressing the separation around the circular cylinder. The rotating vortical structures around the circular cylinder created by the plasma actuators lead to a reduction in the drag coefficient of up to 25%, providing a similar effect to moving surface boundary layer control (MSBLC). This configuration of symmetric DBD plasma actuator, which is studied for the first time in this investigation, is, therefore, called virtual MSBLC. Our results also indicated that the control effect of virtual MSBLC can be enhanced with an increase in the momentum coefficient of plasma jet. Unlike traditional MSBLC devices, the virtual MSBLC based on symmetric DBD plasma actuators do not have profile drag and are without complicated mechanical systems.

Round-robin tests of porous disc models

Abstract: Nine research teams organized a round-robin measurement campaign of the wake of two porous discs in a homogeneous and “low-turbulent’ flow. Mean streamwise velocity and turbulence intensity profiles at four diameters downstream of the discs were measured and compared through such metrics as the maximum of velocity deficit, the maximum of turbulence intensity, the wake width and the thrust coefficient. The dependence of these metrics onthe inflow conditions (freestream turbulence intensity and Reynolds number based on the disc diameter) is discussed.

Experimental Model-Based Estimation and Control of Natural Tollmien–Schlichting Waves

Abstract: A compensator strategy based on the linearized Navier–Stokes equations, aimed to suppress Tollmien–Schlichting waves naturally occurring in low freestream turbulence conditions, is experimentally d...

Instability Wave–Streak Interactions in a High Mach Number Boundary Layer at Flight Conditions

Abstract: The interaction of stationary streaks undergoing nonmodal growth with modally unstable instability waves in a hypersonic boundary-layer flow is studied using numerical computations. The geometry and flow conditions are selected to match a relevant trajectory location from the ascent phase of the HIFiRE-1 flight experiment; namely, a 7 degree half-angle, circular cone with 2.5 mm nose radius, freestream Mach number equal to 5.30, unit Reynolds number equal to 13.42 m-1, and wall-to-adiabatic temperature ratio of approximately 0.35 over most of the vehicle. This paper investigates the nonlinear evolution of initially linear optimal disturbances that evolve into finite-amplitude streaks, followed by an analysis of the modal instability characteristics of the perturbed, streaky boundary-layer flow. The investigation is performed with stationary direct numerical simulations (DNS) and plane-marching parabolized stability equations (PSE), in conjunction with partial-differential-equation-based planar eigenvalue analysis. The overall effect of streaks is to reduce the peak amplification factors of instability waves, indicating a possible downstream shift in the onset of laminar-turbulent transition. The present study confirms previous findings that the mean flow distorsion of the nonlinear streak perturbation reduces the amplification rates of the Mack-mode instability. More importantly, however, the present results demonstrate that the spanwise varying component of the streak can produce a larger effect on the Mack-mode amplification. The study with selected azimuthal wavenumbers for the stationary streaks reveals that a wavenumber of approximately 1.4 times larger than the optimal wavenumber is more effective in stabilizing the planar Mack-mode instabilities. In the absence of unstable first-mode waves for the present cold-wall condition, transition onset is expected to be delayed until the peak streak amplitude increases to nearly 35 percent of the freestream velocity, when intrinsic instabilities of the boundary-layer streaks begin to dominate the transition process. For streak amplitudes below that limit a significant net stabilization is achieved, yielding a potential transition delay that can exceed 100 percent of the length of the laminar region in the uncontrolled case.

Stochastic Store Trajectory of Ice Models from a Cavity into Supersonic Flow

Abstract: Store separation of prolate spheroid ice models with fins from a 4.5∶1 length-to-depth cavity into Mach=2.22 freestream was conducted experimentally to statistically investigate the existence of a ...

Influence of Nose-Tip Bluntness on Conical Boundary-Layer Instabilities at Mach 10

Abstract: Hypersonic boundary-layer transition experiments are performed at the von Karman Institute for Fluid Dynamics in the low-enthalpy Longshot wind tunnel Freestream Mach numbers are within the range

Synthetic Freestream Disturbance for the Numerical Reproduction of Experimental Zero-Pressure-Gradient Bypass Transition Test Cases

Abstract: The present work introduces a means of forcing bypass transition within a zero-pressure-gradient smooth flat-plate boundary layer (ZPGSFPBL), suitable for DNS or LES computations for reproducing experimental datasets. In this type of bypass transition, one of the means of forcing transition within the boundary layer is via the introduction of a specific disturbance along the inflow boundary. Following this principle, the current method, introduces synthetic turbulence, generated by the method of Klein et al. (J. Comput. Phys. 186(2), 652–665, 2003), at the inflow, a certain height above the boundary layer, thereby confining it within the freestream. The principle parameter which dictates the transition behaviour is the height above the boundary layer at which the freestream turbulence is injected. By adjusting this parameter, as well as the integral length-scale and the intensity (either estimated or known a priori from experiments), ILES computations providing good agreement with experimental data sets can be achieved upon a variety of grids. This procedure has been validated upon the ERCOFTAC T3A experimental test case (freestream turbulent intensity of 3%), where good matching is achieved on streamwise quantities like skin-friction coefficient, shape factor, boundary layer thickness and fluctuating velocity growth rates as well as for profiles of mean velocity and fluctuating velocities in the wall-normal direction.

Heat-flux measurements on a hypersonic inlet ramp using fast-response temperature-sensitive paint

Abstract: Global heat-flux measurements are performed using a newly developed temperature-sensitive paint (TSP) on an inclined ramp with sidewalls in a hypersonic shock tunnel. The paint response and image acquisition rate are sufficiently fast to allow flow phenomena on timescales of around $$100\,\upmu \mathrm{{s}}$$ to be resolved. Although a priori calibration of the new TSP proves inaccurate, in situ calibration allows the recovery of heat fluxes that agree well with embedded thermocouple measurements on both short and long timescales. At low unit Reynolds numbers, the flow on the main ramp surface is entirely laminar, but transition occurs in the corner-flow regions, causing a turbulent region to spread inwards from each sidewall and producing weak, unsteady features in the heat-flux distribution of the main laminar region. Within this laminar region, roughly steady streamwise streaks with a period of approximately ten times the boundary-layer thickness are also observed. At higher unit Reynolds numbers, the boundary layer on the main ramp surface transitions to turbulence. The fast-response TSP allows tracking of the time-resolved transition front: significant unsteadiness is observed, which appears to be only weakly correlated to unsteadiness in the freestream flow conditions. Based on the heat-flux signature in the transition region, the breakdown mechanism seems to be quite different from that observed in earlier measurements on a slender cone at similar conditions.

Wind loads on a PV array

Abstract: This study experimentally determines the wind loads on a stand-alone solar array (length-to-width ratio of 0.19; 1/10-scale commercial modules). The freestream velocity in a uniform flow is 14.5 ± 0.1 m/s, and the turbulence intensity is 0.3%. The angle of tilt ranges from 10° to 80° and the wind is incident at angle of 0°–180°. Mean surface pressure measurements on the upper and the lower surface of the inclined solar panels are used to calculate the lift coefficient. For the angle of incidence of 0°–60° for the wind, the variation in the lift coefficient with the angle of tilt is U-shaped. The formation of a strong windward corner vortex results in greater lift force on the right half of the inclined plate for the angle of incidence of 30°–45° for the wind.

Volumetric Velocimetry of Complex Geometry Effects on Transonic Flow over Cavities

Abstract: Volumetric measurements of the flow within four cavities were made by scanning stereoscopic particle image velocimetry at a freestream Mach number of 0.8. The cavities have a length-to-depth ratio ...

Study of Conjugate Heat Transfer from Heated Plate by Turbulent Offset Jet in Presence of Freestream Motion using Low-Reynolds Number Modeling

Abstract: The present study deals with conjugate heat transfer from a heated flat plate by a turbulent offset jet in presence of freestream motion. The turbulent convection in fluid and conduction in solid is solved in a coupled manner by simultaneously satisfying the equality of temperature and heat flux at the solid-fluid interface. The computations have been carried out using low-Reynolds number (LRN) k −ω SST model in the fluid region. The capability of LRN modeling have enabled to solve the entire boundary layer including the thin viscous sublayer due to which Moffatt vortices (secondary recirculation regions) have been captured near the corner of the wall where the turbulence Reynolds number is low. The bottom surface of solid plate is maintained at a constant temperature higher than the jet inlet temperature whereas the jet inlet temperature is same as that of the ambient. The present investigation reports the effects of offset ratio of jet (OR), Reynolds number of flow (Re), solid to fluid thermal conductivity ratio (K), solid slab thickness (S) and freestream velocity (U∞) on conjugate heat transfer arises due to solid and fluid interaction. The offset ratio is varied in the range OR = 3 − 11, Reynolds number in the range Re = 10000 − 25000, solid to fluid thermal conductivity ratio in the range K = 1 − 2000, solid slab thickness in the range S = 1−20 and freestream velocity in the range U∞ = 0.1 − 0.25. The effects of various parameters on the near-wall velocity profile, solid-fluid interface temperature, local Nusselt number variation along the plate, heat flux variation along the plate, etc. have been discussed in detail.