Showing papers on "Freestream published in 2003"

Flow past a rotating cylinder

Abstract: Flow past a spinning circular cylinder placed in a uniform stream is investigated via two-dimensional computations. A stabilized finite element method is utilized to solve the incompressible Navier–Stokes equations in the primitive variables formulation. The Reynolds number based on the cylinder diameter and free-stream speed of the flow is 200. The non-dimensional rotation rate, α (ratio of the surface speed and freestream speed), is varied between 0 and 5. The time integration of the flow equations is carried out for very large dimensionless time. Vortex shedding is observed for α < 1.91. For higher rotation rates the flow achieves a steady state except for 4.34 < α < 4:70 where the flow is unstable again. In the second region of instability, only one-sided vortex shedding takes place. To ascertain the instability of flow as a function of α a stabilized finite element formulation is proposed to carry out a global, non-parallel stability analysis of the two-dimensional steady-state flow for small disturbances. The formulation and its implementation are validated by predicting the Hopf bifurcation for flow past a non-rotating cylinder. The results from the stability analysis for the rotating cylinder are in very good agreement with those from direct numerical simulations. For large rotation rates, very large lift coefficients can be obtained via the Magnus effect. However, the power requirement for rotating the cylinder increases rapidly with rotation rate.

Flexible Flapping Airfoil Propulsion at Zero Freestream Velocity

Abstract: Thrust generation for an airfoil plunging at zero freestream velocity, the case relevant to hovering birds and insects, has been studied. The objective was to investigate the effect of airfoil stiffness. Particle image velocimetry and force measurements were taken for three airfoils of relative bending stiffnesses 1:8:512 in a water tank. The deformation of the flexible airfoils produces an angle of attack that varies periodically with a phase angle with respect to the plunging motion. Amplitude and phase of this combined plunging/pitching motion play a major role in the flowfield and thrust generation. Vortex pairs or alternating vortex streets were observed depending on the amplitude and phase lag of the trailing edge. The strength of the vortices, their lateral spacing, and the time-averaged velocity of the induced jet were found to depend on the airfoil flexibility, plunge frequency, and amplitude. Direct force measurements confirmed that at high plunge frequencies the thrust coefficient of the airfoil with intermediate stiffness was greatest, although the least stiff airfoil can generate larger thrust at low frequencies. It is suggested that there is an optimum airfoil stiffness for a given plunge frequency and amplitude. The thrust/input-power ratio was found to be greater for the flexible airfoils than for the rigid airfoil.

Aero-optical characteristics of compressible, subsonic turbulent boundary layers

Abstract: An extensive experimental study of optical aberrations due to propagation through fully-developed turbulent boundary layers at high subsonic Mach numbers was performed. Time-resolved, high- bandwidth, direct optical measurements of the dynamic aberrations were made using a Malley probe. The probe was used to obtain the convective speeds of the optically-significant turbulence structures and to measure the optical path differences. Measurements were made over a range of boundary layer thicknesses and Mach numbers. Optical distortions were found to scale linearly with boundary layer thickness and freestream density, and to go as the square of the freestream Mach number.

Unsteady Surface Pressures Due to Wake-Induced Transition in a Laminar Separation Bubble on a Low-Pressure Cascade

Abstract: This paper presents unsteady surface pressures measured on the suction surface of a LP turbine cascade that was subject to wake passing from a moving bar wake generator. The surface pressures measured under the laminar boundary layer upstream of the steady flow separation point were found to respond to the wake passing as expected from the kinematics of wake convection. In the region where a separation bubble formed in steady flow, the arrival of the convecting wake produced high frequency, short wavelength, fluctuations in the ensemble averaged blade surface pressure. The peak-to-peak magnitude was 30% of the exit dynamic head. The existence of fluctuations in the ensemble averaged pressure traces indicates that they are deterministic and that they are produced by coherent structures. The onset of the pressure fluctuations was found to lie beneath the convecting wake and the fluctuations were found to convect along the blade surface at half of the local freestream velocity. Measurements performed with the boundary layer tripped ahead of the separation point showed no oscillations in the ensemble average pressure traces indicating that a separating boundary layer is necessary for the generation of the pressure fluctuations. The coherent structures responsible for the large amplitude pressure fluctuations were identified using PIV to be vortices embedded in the boundary layer. It is proposed that these vortices form in the boundary layer as the wake passes over the inflexional velocity profiles of the separating boundary layer and that the rollup of the separated shear layer occurs by an inviscid Kelvin-Helmholtz mechanism.Copyright © 2003 by ASME

Correlation Between Vortex Strength and Axial Velocity in a Trailing Vortex

Abstract: The vortices that trail from the wingtips of a large aircraft provide a significant hazard to an aircraft that follows in its wake. The objective is to contribute to the understanding of these vortices by identifying the conditions where an axial velocity in excess of the freestream value will be generated in the core of a trailing vortex. The axial velocity near the core of a trailing vortex was measured using a triple-sensor hot-wire probe and compared with measured values of vortex circulation strength. The vortex was generated in a wind tunnel using a NACA 0015 wing model with a semispan aspect ratio of 0.80. A linear relationship between the axial velocity and a nondimensional circulation parameter is indicated. For small values of the circulation parameter, the axial velocity shows a velocity deficit

Control of the flow past bodies using localized energy addition to the supersonic oncoming flow

Abstract: A possibility of controlling the supersonic flow past bodies of different shape by adding a small amount of energy to the freestream is studied experimentally. In the numerical calculations the model of a spatially distributed energy source based on the Euler equations for a perfect gas is used. The gasdynamic features of the flow around energy sources are studied, some new effects are revealed, and analytical models for their description are developed. It is shown that by optimizing the energy source parameters it is possible to initiate irregular regimes of flow past bodies characterized by radical changes in the bow shock structure with the formation of return flow zones. In this case an appreciable drag reduction can be achieved with high efficiency of energy expenditure.

Plasma Actuators for Separation Control of Low Pressure Turbine Blades

Abstract: This work involves the documentation and control of flow separation that occurs over turbine blades in the low-pressure-turbine (LPT) stage at the low Reynolds numbers typical of high-altitude cruise. We utilize a specially constructed linear cascade that is designed to study the flowfield over a generic LPT cascade consisting of Pratt and Whitney Pak B shaped blades. The center blade in the cascade is instrumented to measure the surface-pressure coefficient distribution. Optical access allows laser-Doppler-velocimetry measurements for boundary-layer profiles. Experimental conditions were chosen to give a range of chord Reynolds numbers from 10 4 to 10 5 , and a range of freestream turbulence levels from u'/U∞ = 0.08 to 2.85%. The surface-pressure measurements were used to define a region of separation and reattachment that depends on the freestream conditions

Aeroacoustic Properties of Supersonic Cavity Flows and Their Control

Abstract: A detailed experimental study of supersonic, Mach 2, flow over a 3D cavity was conducted using shadowgraph, particle image velocimetry (PIV), and unsteady surface pressure measurements. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were observed. The PIV data reveals the highly unsteady nature of the entire flowfield and the presence of a large recirculation zone with reverse velocities as high as 40 % of the freestream velocity. Supersonic microjets at the leading edge are used to control the cavity flow and suppress resonance in the cavity. Using minimal mass flux through the microjets, overall sound pressure level (OASPL) was reduced by greater than 9 dB with tonal reductions greater than 20 dB. The PIV data reveals that microjet injection modifies the cavity shear layer and results in a significant reduction in the unsteadiness of the cavity velocity-field.

Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions

Abstract: Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=(v/U 2 ∞)(dU∞/dx) as high as 9×10 -6 ) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts

Effects of Pulsing on a Vortex Generator Jet

Abstract: The evolution of a pulsed vortex generator jet embedded in a turbulent boundary layer was examined experimentally. The jet, which was pitched 45 deg and skewed 90 deg, had a velocity three times greater than the freestream. The velocity e eld in planes normal to the freestream was measured by the particle-image-velocimetry method at four stations downstream of the jet exit. The pulsed jet created a starting vortex ring followed by a pair of counter-rotating streamwise vortices, one of them being markedly stronger. Phase-averaged data indicate that the maximum circulation and peak vorticity of the stronger vortex are approximately 30% greater than the average values for a steady jet with the same velocity. However, circulation averaged over the entire pulse was less than that fora steady jet at the samelocation. Thecoreof theprimary streamwisevortex penetratesapproximately 50% farther into the boundary layer than a steady jet with the same velocity. The larger penetration takes place during the initial portion of the pulse and is caused by the jet starting vortex ring.

Characterization of Pulsed Vortex Generator Jets for Active Flow Control

Abstract: : The flowfields produced by several active and passive flow control devices are being investigated experimentally and numerically as part of co-operative effort between the US Air Force Research Laboratory and the UK's Defense Evaluation & Research Agency (DERA). This manuscript reports the results of an experimental investigation of pulsed vortex generator jets (PVGJs) conducted at DERA's Boundary Layer Facility in Bedford. The focus of these tests was to investigate the influence of jet velocity, pulsing frequency, and duty cycle on the mean characteristics of the flowfield produced by a PVGJ in a turbulent boundary layer. The experiments were conducted in a zero-pressure- gradient flow at a freestream velocity of 32 m/s. The flowfield was explored using a three-component laser Doppler anemometry system, and the information is used to calculate local field properties such as velocity and velocity as well as global parameters like total circulation. The data give insight into the effectiveness of the VGs in terms of location. strength, and persistence of the generated vortices and their influence on the boundary layer. While the planned computational simulation effort is in its infancy, preliminary steady-jet computational results are compared with the flow field data that has been acquired in the boundary layer facility.

Effects of Concave Curvature on Boundary Layer Transition Under High Freestream Turbulence Conditions

Abstract: : An experimental investigation has been carried out on a transitional boundary layer subject to high (initially 9%) freestream turbulence, strong acceleration (K = (v/U(2)/w) x(dU(w)/dx) as high as 9 x 10(-6), and strong concave curvature (boundary layer thickness between 2% and 5% of the wall radius of curvature). Mean and fluctuating velocity as well as turbulent shear stress are documented and compared to results from equivalent cases on a flat wall and a wall with milder concave curvature. The data show that curvature does have a significant effect, moving the transition location upstream, increasing turbulent transport, and causing skin friction to rise by as much as 40%. Conditional sampling results are presented which show that the curvature effect is present in both the turbulent and nonturbulent zones of the transitional flow.

Effect of energy addition on MR → RR transition

Abstract: A combined computational and experimental study was performed to investigate the effect of a single laser energy pulse on the transition from a Mach Reflection (MR) to a Regular Reflection (RR) in the Dual Solution Domain (DSD). The freestream Mach number is 3.45 and two oblique shock waves are formed by two symmetric $22^\circ$ wedges. These conditions correspond to a point midway within the DSD wherein either an MR or an RR is possible. A steady MR was first obtained experimentally and numerically, then a single laser pulse was deposited above the horizontal center plane. In the experiment, the laser beam was focused resulting in a deposition volume of approximately 3 mm3, while in the simulation, the laser pulse was modeled as an initial variation of the temperature and pressure using Gaussian profile. A grid refinement study was conducted to assess the accuracy of the numerical simulations. For the steady MR, the simulation showed the variation of Mach stem height along the span due to side effects. The predicted spanwise averaged Mach stem height was 1.96 mm within 2% of the experimental value of 2 mm. The experiment showed that the Mach stem height decreased to 30% of its original height due to the interaction with the thermal spot generated by the laser pulse and then returned to its original height by $300\;\mu$ s. That the Mach stem returned to its original height was most likely due to freestream turbulence in the wind tunnel. The numerical simulation successfully predicted the reverse transition from a stable MR to a stable RR and the stable RR persisted across the span. This study showed the capability of a laser energy pulse to control the reverse transition of MR $\rightarrow$ RR within the Dual Solution Domain.

Development of a Steady Vortex Generator Jet in a Turbulent Boundary Layer

Abstract: The development of a vortex generator jet within a turbulent boundary layer was studied by the particle image velocimetry method. Jet velocities ranging from one to three times greater than the freestream velocity were examined. The jet was pitched 45 deg and skewed 90 deg with respect to the surface and flow direction, respectively. The velocity field in planes normal to the freestream was measured at four stations downstream of the jet exit. The jet created a pair of streamwise vortices, one of which was stronger and dominated the flow field

Aerodynamic Performance of a Very High Lift LP Turbine Blade With Emphasis on Separation Prediction

Abstract: The present paper is based on an experimental study of a front loaded very high lift low pressure turbine blade designed at the VKI. The experiments have been carried out in a low speed wind tunnel over a wide operating range of incidence and Reynolds number. The aim of the study is to characterize the flow through the cascade in terms of losses, mean outlet flow angle and secondary flows. At low inlet freestream turbulence intensity, a laminar separation bubble is present, and a prediction model for a separated flow mode of transition has been developed.Copyright © 2003 by ASME

Flows past a tiny circular cylinder at high temperature ratios and slight compressible effects on the vortex shedding

Abstract: The present study contributes to the numerical simulations of weak compressible effects for laminar flows around heated circular cylinders Our aim in this work is to understand the physics of the flow around a heated cylinder and the reasons for the decrease in the frequency of vortex shedding in the wake behind the cylinder as the ratio of surface temperature to freestream temperature increases for moderate Re numbers The previous achievements on this subject rely mainly on experimental investigations by using a “representative” or “film temperature” and therefore should be investigated more accurately by the numerical considerations In that context, numerical experiments are carried out A technique which was previously tested and validated on a variety of flow problems is utilized in the numerical experiments The results show that the temperature is not a passive contaminant especially for flows with temperature ratios T*=Tw/T∞ greater than 11 The temperature boundary layer surrounding the wall i

Experimental study of flow unsteadiness in a Mach 9 compression ramp interaction using a laser schlieren system

Abstract: This paper demonstrates the use of a simple laser schlieren technique to obtain flow information in a two-dimensional separated compression ramp-induced shock-wave boundary-layer interaction. Tests were made at a freestream Mach number of 9 and freestream Reynolds number per unit length of 2.078 × 105 m−1. The importance of this technique in studying hypersonic flows is unique since the run times of hypersonic wind tunnels are of very short duration. The method is based on the schlieren principle and uses a parallel sheet of a low-power (15 W) diode laser and an array of very fast response (4 ns) photodiodes. Although the arrangement detects an integral part of the signal from the fluctuating density gradients across the span of the flow, it yields significant insights into the details of the flow structure. The details of the flow field are discussed using time-dependent fluctuating density gradient profiles, the related power spectra and autocorrelation and cross-correlation functions in the interaction region.

Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors—Part II: Near-Wall Reaction Effects on Film-Cooled Heat Transfer

Abstract: As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions on a film-cooled surface is enhanced. Currently, there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. A shock tube experiment was employed to generate short duration, high temperature (1000-2800 K) and pressure (6 atm) flows over a film-cooled flat plate. The test plate contained two sets of 35 deg film cooling holes that could be supplied with different gases, one side using air and the other nitrogen. A mixture of ethylene and argon provided a fuel rich freestream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5-2.0), and Damkohler numbers (ratio of flow to chemical time scales) from near zero to 30. For high Damkohler numbers, reactions had sufficient time to occur and increased the surface heat flux by 30 percent over the inert cooling side. When these results are appropriately scaled, it is shown that in some situations of interest for gas turbine engine environments significant increases in surface heat flux can be produced due to chemical reactions in the film-cooling layer. It is also shown that the non-dimensional parameters Damkohler number (Da), blowing ratio (B), heat release potential (H*), and scaled heat flux (Q s ) are the appropriate quantities to predict the augmentation in surface heat flux that arises due to secondary reactions.

Velocity Measurements in a Three-Dimensional Compressible Base Flow

Abstract: The velocity field in the base region of a circular cylinder with a length-to-radius ratio of 3.0 aligned at a 10-deg angle of attack to a nominal Mach 2.5 freestream has been investigated experimentally. The objective is to better understand the mechanisms that govern the characteristics and development of three-dimensional, compressible base flows. Laser Doppler velocimetry was used to measure both mean velocity components and turbulence statistics

Inlet/Engine Interactions in an Axisymmetric Pulse Detonation Engine System

Abstract: The effects of oscillatory backpressure on the air induction system for pulse detonation engines were examined for an axisymmetric, external compression cone guration at a freestream Mach number of 2.1. The pressure perturbations at the diffuser exit were produced by a rotating mechanism simulating a valve, which periodically opened and closed the detonation tubes. Theoscillation frequency wasvaried from 30 to 100 Hzfor each individual tube. Through varying the downstream blockage, the spillage was altered, and different mass e ows wereobtained. In all casestheresults indicated that e uctuations in thestatic and stagnation pressures in the inletwere within 3%, without e ow instability noted near the inlet capture.

Characteristics of Compressible Rectangular Cavity Flows

Abstract: Experiments are performed to study the effect of cavity geometry and Mach number on the characteristics of compressible rectangular cavity flows. The study indicates that the corresponding length-to-depth ratio for the open- and transitional-type cavities increases with higher freestream Mach number. The depth-to-incoming boundary-layer thickness ratio is another important parameter to define the type of the cavity flow. The upstream influence region is minimized with the presence of a cavity, and larger downstream influence region is observed for the transitional-closed- and closed-type cavities. The distributions of surface pressure fluctuations show similar trend as those of static pressure distributions. The amplitude of surface pressure fluctuations increases toward the rear face for an open-type cavity, whereas a minor peak near the middle of the cavity floor is observed for a closed-type cavity. A transitional-type cavity induces more intense surface pressure fluctuations at the cavity floor. Higher levels of pressure fluctuations near the rear face are observed at higher Mach numbers for the transitional-and open-type cavities

Supersonic Flow Past a Thermal Source

Abstract: The results of an experimental investigation and numerical simulation of a gasdynamic structure formed as a result of supersonic flow past a pulsating thermal source are presented. Heat was supplied to the flow by producing a limited plasma volume as a result of the breakdown of the focused radiation of a CO2 laser operating in the pulse periodic regime. On the basis of the experimental data obtained, a thermal source model was developed and accepted for further numerical calculations. The calculations were carried out within the framework of the inviscid gas model using the TVD scheme and nonreflecting boundary conditions. The effect of the relevant gasdynamic and energetic parameters on the flow pattern associated with the studied phenomenon is established. Data on the flow parameter distributions in the wake of the thermal source are obtained as a function of the freestream Mach number.

Flow separation inside a supersonic nozzle exhausting into a subsonic compressible crossflow

Abstract: Surface pressure data have been acquired along the nozzle wall and surrounding the exit plane for an axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible crossflow. These measurements have shown that the backpressure is sufficient to instigate nozzle flow separation under flowfield conditions that may be found in flight. The separation line has been found to be axially asymmetric, which results from the angular variation in the backpressure on the nozzle generated by the jet's interaction with the freestream. As either the jet-to-freestream momentum ratio or the crossflow freestream Mach number is independently reduced, the size of the separated flow region becomes larger because the backpressure on the nozzle is increased relative to the jet stagnation pressure. Schlieren imaging is consistent with these observations and provides further elucidation of the resulting jet shock wave structure. Comparison of the data to correlations derived from freejet separation data is possible by employment of these predictions in a piecewise fashion around the perimeter of the nozzle.

Comparison of Experimental and Numerical Studies of Ionizing Flow over a Cylinder

Abstract: Comparisons are made between experimental measurements and numerical simulations of ionizing flows generated in a superorbital facility. Nitrogen, with a freestream velocity of around 10 km/s, was passed over a cylindrical model, and images were recorded using two-wavelength holographic interferometry. The resulting density, electron concentration, and temperature maps were compared with numerical simulations from the Langley Research Center aerothermodynamic upwind relaxation algorithm. The results showed generally good agreement in shock location and density distributions. Some discrepancies were observed for the electron concentration, possibly, because simulations were of a two-dimensional flow, whereas the experiments were likely to have small three-dimensional effects.

Prediction of Turbulence Statistics Behind a Square Cylinder Using Neural Networks and Fuzzy Logic

Abstract: Experiments are carried out behind a square cylinder mounted in the freestream of a wind tunnel, and hot-wire anemometry is used to determine the profiles of the mean and statistical turbulence quantities. Artificial neural networks and fuzzy-logic models successfully predict the statistical quantities like mean velocity profiles and Reynolds stresses. The fuzzy-logic modeling is more convenient to use, is less computationally intensive, and gives a higher correlation coefficient in comparison to the neural network

Effects of High Intensity, Large-Scale Freestream Combustor Turbulence On Heat Transfer in Transonic Turbine Blades

Abstract: : The influence of freestream turbulence representative of the flow downstream of a modem gas turbine combustor and the first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. Measurements were performed on a high turning, transonic turbine blade. The facility is capable of heated flow with inlet total temperature of 120 degrees C and inlet total pressure of 10 psig. The Reynolds number based on blade chord and exit conditions (5x10(exp 6)) and the inlet and exit Mach numbers (0.4 and 1.2, respectively) are representative of conditions in a modem gas turbine engine. High intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modem combustors after it has passed through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10-12% and an integral length scale of 2 cm near the entrance of the cascade passages, which is believed to be representative of the core flow entering a first stage gas turbine rotor blade row. Mean heat transfer results showed an increase in heat transfer coefficient of approximately 8% on the suction surface of the blade, with increases on the pressure surface on the order of two times higher than on the suction surface (approximately 17%). This corresponds to increases in blade surface temperature of 5- 10%, which can significantly reduce the life of a turbine blade. The heat transfer data were compared with correlations from published literature with good agreement.

Cartesian-Grid Simulations of a Canard-Controlled Missile with a Free-Spinning Tail

Abstract: The proposed paper presents a series of simulations of a geometrically complex, canard-controlled, supersonic missile with free-spinning tail fins. Time-dependent simulations were performed using an inviscid Cartesian-grid-based method with results compared to both experimental data and high-resolution Navier-Stokes computations. At fixed free stream conditions and canard deflections, the tail spin rate was iteratively determined such that the net rolling moment on the empennage is zero. This rate corresponds to the time-asymptotic rate of the free-to-spin fin system. After obtaining spin-averaged aerodynamic coefficients for the missile, the investigation seeks a fixed-tail approximation to the spin-averaged aerodynamic coefficients, and examines the validity of this approximation over a variety of freestream conditions.