Aerodynamic Boundary Layer Research Articles

Flight Testing Of A Single Point Frequency Sweeping Filtered Rayleigh Scattering Probe

"We present a successful in-flight application of filtered Rayleigh scattering (FRS) for temperature measurement in close proximity to the air plane using a newly developed single point, frequency sweeping probe. The temperature measurement is of relevance in the quantification of the density required for the evaluation of velocity from Pitot-probe measurements to obtain the true air speed (TAS). Laser optical air data sensors have the potential for contactless measurements outside the aerodynamic boundary layer. They are capable of self-diagnosis of fault conditions (e.g. signal loss, increased measurement uncertainty or spectral distortion for FRS) thereby increasing the safety of the aircraft. Compared to classical probes, they are less susceptible potential icing and blockage issues. The FRS-results presented here, are the outcome of two separate flight campaigns, carried out in the Alpine region as well as in the North Sea area. Measurement data provided information on the functionality of the FRS-system under various flight and weather conditions. Furthermore, the implementation of the FRS measurement system in the aircraft is presented; application aspects and improvements are discussed. In particular, the measurement accuracy at the actually achievable measurement rate was analyzed. For straight ahead flights at constant height, temperature mean values were in good agreement with the flight data system of the DLR research aircraft Dassault Falcon 20 (D-CMET). Aiming at in-flight air data measurements, the optical FRS measurement system requires a real-time temperature sample rate of a few Hz. We demonstrate that 2 FRS spectra per second can be acquired by a newly developed method using frequency sweeps for temperature determination. The low signal intensity of cw-laser based FRS is countered using a combined transmitter/receiver optic that focusses the entire cw-laser power into one measuring point. The light backscattered from the probe volume is collected by the same optics, filtered and imaged in a confocal arrangement onto a photomultiplier. An improved and miniaturized version of the presented FRS system could be a part of a future optical air data system (OADS). However, the strength of FRS signal relative to bright ambient conditions (e.g. sunlight, cloud backscatter) must be further improved in order to be able to measure well above cloud cover. The potential for further development is foreseen even if the accuracy required for flight data systems (~0.5 K) has not yet been achieved at a temperature measurement rate of 2 Hz. "

Real Gas Effects on Receptivity to Roughness in Hypersonic Swept Blunt Flat-Plate Boundary Layers

Temperatures within the boundary layers of high-enthalpy hypersonic flows can soar to thousands or even tens of thousands of degrees, leading to significant real gas phenomena. Although there has been significant research on real gas effects on hypersonic boundary layer stability, their impact on the boundary layer’s receptive stage is still poorly understood. Most aerodynamic boundary layers in flight vehicles are three-dimensional. Because of complex geometry and significant crossflow effects, the crossflow mode in three-dimensional boundary layers is crucial in hypersonic vehicle design. In this study, a linear stability analysis (LST) accounting for chemical nonequilibrium effects (CNE) and its adjoint form (ALST) is developed to investigate the real gas effects on the stability and receptivity of stationary crossflow modes. The results indicate that real gas effects significantly influence the receptivity of stationary crossflow modes. Specifically, chemical nonequilibrium effects destabilize the crossflow modes but reduce the receptivity coefficients of the stationary crossflow modes. The Mach number effect was also investigated. It was found that increasing the Mach number stabilizes the stationary crossflow modes, but the receptivity coefficients increase. As the Mach number progressively rises, these effects alternately dominate, leading to a non-monotonic shift in the transition position.

Fluid Flow and Heat Transfer Characteristics of Clerestory-Shaped Attics Heated from Below

In this study, a finite volume analysis of the aerodynamics and heat transfer in attics of a clerestory roof design with pitch angles 14o, 18o, 30o and 45o and Rayleigh number range 3 x 105≤ Ra ≤ 2 x 107 is carried out. The shape of the enclosure has strong influence on the structure of the flow and temperature fields. The flow field is characterised by counter-rotating vortices enclosed by aerodynamic boundary layers. The size, strength and direction of rotation of the cells are controlled by the forces propelling the thermal plumes and the cold jets. The reduction of the number and size of the counter-rotating cells and their formation within the enclosures provide an analogous reduction of the total heat transfer rate as the roof pitch angle increases. The velocity and temperature profiles across midheight and midlength of the enclosures enable the prediction of appropriate position in the attic with the right condition to place sensitive items. On the heat transfer, the relationship between the mean Nusselt number and the Rayleigh number is presented in form of a correlation. Results obtained in the study are of significance to building engineers engaged in the analysis and design of building attics and tropical agriculturalists for the control of produce drying rates. Keywords: Heat transfer, heated below, natural convection, pitch roof, clerestory-shaped

Influence of train roof boundary layer on the pantograph aerodynamic uplift: A proposal for a simplified evaluation method

The mean contact force between pantograph collectors and contact wire is affected by the aerodynamic uplift generated by aerodynamic forces acting on pantograph components. For a given pantograph geometry, orientation and working height, aerodynamic forces are strongly influenced by the position of the pantograph along the train roof, since an aerodynamic boundary layer grows along the train.This paper shows the experimental results of aerodynamic uplifts of full-scale pantographs located at four different positions along the roof of a high-speed train and adopts CFD simulations to examine the effect of the boundary layer velocity profile on the measured experimental forces. It is quantitatively demonstrated that the same pantograph located at different positions along the train roof can show relevant differences in the aerodynamic uplift, only due to the different flow characteristics.Moreover, a new simplified methodology is proposed to evaluate the aerodynamic uplifts at different positions of the pantograph along the train. Results of the proposed methodology are validated against full scale experimental results and full CFD simulations exploiting the complete model of the pantograph installed on the train roof.

A Review on Vibro-Acoustic Analysis of a Launch Vehicle Structure

Abstract: Space vehicles are subjected to significant dynamic pressure loads when their rocket propulsion systems are in use during flying missions. During the aerodynamic and launch phases, launch vehicles, payloads, and their parts are subjected to extremely high random acoustic loads. The noise from the engine exhaust gas, aerodynamic boundary layer noise, transonic buffering, structure-borne vibration, engine thrust fluctuation, etc. is the source of these loads, which also result in a secondary acoustic load. When the vehicle is lifting off and traveling at a speed greater than Mach number, acoustic stresses to the spacecraft and payload are very harsh and significant. This loading causes the structure to vibrate randomly, which could be dangerous for some vehicle parts, avionics, propulsion systems, and payloads like satellites. This paper discusses the vibration of the structure subjected to acoustic excitation on a diffuse acoustic field and the software used for the analysis.

ANA LYSIS OF THE FLOW AND THERMAL FIELDS IN BOBTAIL ROOFS HEATED FROM THE BASE WALL

Analysis of airflow and thermal characteristics of attics of bobtail-shaped pitched roofs heated through a horizontally suspended ceiling is numerically carried out in this study. Pitch angles of 14o, 18o, 30o, and 45o within the standard pitch roof range are selected. The configuration falls within Rayleigh number 3.19 x 105  Ra  2.04 x 107. A finite-volume CFD code was used to solve the mass, momentum, and energy conservation equations governing the problem. The results obtained indicate a strong influence on the shape and angle of the roof. At lower roof pitches, the flow field is characterized by multiple counter-rotating vortices asymmetrically arranged within the enclosures. Eight cells in the 14o enclosure were reduced to five in the 45o roof pitch. The size and rotating strength of a vortex increase from the left corner to the middle of the enclosures. At higher pitch angles, the vertical wall obstructed the flow leading to a number of distorted cells. The maximum velocity within the aerodynamic boundary layer along the base wall occurs at Y=0.02 with the values U=0.013 and U=0.028 in the 14o and 45o enclosures respectively. The thermal field portrays a convection system of rising hot plumes from the base wall and descending cold jets from the inclined walls; all enclosed by thin boundary layers along the walls. Graphical plots of velocity and temperature variations along some cross-sections within the enclosures enable the prediction of some important heat and flow parameters.

High Leaf Respiration Rates May Limit the Success of White Spruce Saplings Growing in the Kampfzone at the Arctic Treeline.

Arctic Treeline is the transition from the boreal forest to the treeless tundra and may be determined by growing season temperatures. The physiological mechanisms involved in determining the relationship between the physical and biological environment and the location of treeline are not fully understood. In Northern Alaska, we studied the relationship between temperature and leaf respiration in 36 white spruce (Picea glauca) trees, sampling both the upper and lower canopy, to test two research hypotheses. The first hypothesis is that upper canopy leaves, which are more directly coupled to the atmosphere, will experience more challenging environmental conditions and thus have higher respiration rates to facilitate metabolic function. The second hypothesis is that saplings [stems that are 5–10cm DBH (diameter at breast height)] will have higher respiration rates than trees (stems ≥10cm DBH) since saplings represent the transition from seedlings growing in the more favorable aerodynamic boundary layer, to trees which are fully coupled to the atmosphere but of sufficient size to persist. Respiration did not change with canopy position, however respiration at 25°C was 42% higher in saplings compared to trees (3.43±0.19 vs. 2.41±0.14μmolm−2 s−1). Furthermore, there were significant differences in the temperature response of respiration, and seedlings reached their maximum respiration rates at 59°C, more than two degrees higher than trees. Our results demonstrate that the respiratory characteristics of white spruce saplings at treeline impose a significant carbon cost that may contribute to their lack of perseverance beyond treeline. In the absence of thermal acclimation, the rate of leaf respiration could increase by 57% by the end of the century, posing further challenges to the ecology of this massive ecotone.

Investigation on the aerodynamic performance and boundary layer characteristics of high deflection industrial turbine cascade at off-design conditions

This research leads to reach the effects of the off-design operation on the aerodynamic performance, boundary layer behavior, and associated loss occurring in a high deflection industrial turbine blade. Numerical simulations, proved opposed to experimental results, are used to achieve the detailed flow field of the cascade, incidence angels ranging from −15° to +10° at inlet chord Reynolds number varied from 9.63E+04 to 2.24E+05. Boundary layer specifications covering thickness, displacement thickness, and momentum thickness are achieved in the 15 points on the suction side and talk about detail on the authority of numerical simulations in various flow conditions. Results indicate that profile loss is forcefully increased by the rising up the incidence angle mostly in the positive range. Studying the effect of incidence angle on the boundary layer behavior indicates that as incidence angle varies from negative to positive, all three characteristics of the boundary layer increase gradually.

Comparative analysis of the boundary layer filtering of odor signals in the amblypygid (whip spider) species Paraphrynus laevifrons and Phrynus marginemaculatus

Amblypygids use a pair of modified walking legs (antenniform) as chemosensory and mechanosensory appendages. At the tip of these legs are covered in chemosensory sensilla, which the animals use to sample odor stimuli in their environment by moving the antenniform leg through the air. We designed a set of experiments to measure the filtering effect that aerodynamic boundary layers have on the temporal and spatial structure of chemical stimuli. In addition, two different species of amblypygids (Paraphrynus laevifrons and Phrynus marginemaculatus) that live in two distinct habitats were used for a comparative analysis. Pulses of a tracer molecule were quantified at different distances and flow velocities using an electrochemical detection system. Temporal attributes of the chemical pulses were extracted and were statistically compared across velocities, distances from the appendage, and the two species. Overall, the boundary layer significantly decreased the concentration and increased the duration of pulses for both species. This filtering effect was more pronounced for P. marginemaculatus than P. laevifrons, as the chemical signal was lower in concentration and longer in duration at any distance from the antenniform leg. It is speculated that the difference in boundary layer filtering, as a function of appendage morphology, is tuned to the different types of odor plumes in these animals’ native habitats.

Numerical study of boundary layer transition using intermittency model

Purpose In this paper, the applicability of shear stress transport k-ω model along with the intermittency concept has been investigated over pitching airfoils to capture the laminar separation bubble (LSB) position and the boundary layer transition movement. The effect of reduced frequency of oscillations on boundary layer response is also examined. Design/methodology/approach A two-dimensional computational fluid dynamic code was developed to compute the effects of unsteadiness on LSB formation, transition point movement, pressure distribution and lift force over an oscillating airfoil using transport equation of intermittency accompanied by the k-ω model. Findings The results indicate that increasing the angle of attack over the stationary airfoil causes the LSB size to shorten, leading to a rise in wall shear stress and pressure suction peak. In unsteady cases, both three- and four-equation models are capable of capturing the experimentally measured transition point well. The transition is delayed for an unsteady boundary layer in comparison with that for a static airfoil at the same angle of attack. Increasing the unsteadiness of flow, i.e. reduced frequency, moves the transition point toward the trailing edge of the airfoil. This increment also results in lower static pressure suction peak and hence lower lift produced by the airfoil. It was also found that the fully turbulent k-ω shear–stress transport (SST) model cannot capture the so-called figure-of-eight region in lift coefficient and the employment of intermittency transport equation is essential. Practical implications Boundary layer transition and unsteady flow characteristics owing to airfoil motion are both important for many engineering applications including micro air vehicles as well as helicopter blade, wind turbine and aircraft maneuvers. In this paper, the accuracy of transition modeling based on intermittency transport concept and the response of boundary layer to unsteadiness are investigated. Originality/value As a conclusion, the contribution of this paper is to assess the ability of intermittency transport models to predict LSB and transition point movements, static pressure distribution and aerodynamic lift variations and boundary layer flow pattern over dynamic pitching airfoils with regard to oscillation frequency effects for engineering problems.

Simulations of Airfoil Suction Side Flow Separation Control Using Plasma Aerodynamic Actuation

Plasma flow control, as a novel active flow control technique, has become a newly-rising focus of international aerodynamics and aerothermodynamics fields. Surface DBD plasma aerodynamic actuation is widely studied in subsonic flow separation control. Numerical simulation investigation of flow stall separation suppression of NACA 0015 airfoil by plasma aerodynamic actuation was performed. The plasma aerodynamic actuation was simplified as momentum and thermal source terms. The numerical model of airfoil flow separation control by plasma aerodynamic actuation was constructed. The simulation results showed that when the inflow velocity was 100m/s and angle of attack was 20 degree, the flow separates at the leading edge of the suction side of the airfoil. There is a 50.3% lift augmentation and 42.0% drag reduction after the plasma aerodynamic actuation was on. The best position of plasma aerodynamic actuation is just near the separation point. The simulation results were basically consistent with the test results, which demonstrated that the simulation model was correct. The results are useful to reveal the mechanism of the correlation of plasma aerodynamic actuation and boundary layer.

Development of a novel anemometry technique for velocity and temperature measurement

Hot-wire anemometry is a widely used measurement technique for obtaining local velocities or temperatures in a flow-field. Operated under constant temperature or constant current the small diameter wires can resolve high-frequency velocity or temperature fluctuations with a high degree of spatial and temporal resolution. Therefore, more or less sophisticated hardware, e.g. using a Wheatstone bridge for constant temperature anemometry (CTA), is used with relatively high acquisition costs. In this paper a new technique is used, where the wire temperature or current is digitally controlled via a FPGA unit. The electric circuit is simpler than the usual set-ups and the hardware used is very robust, allowing to operate the control unit under harsh environments. This allows also to reduce the cable lengths, and therefore, eventually to increase the frequency response of the hot wire. The application of this technique for measurements of aerodynamic and thermal boundary layers under varying Reynolds numbers for laminar and turbulent boundary layers is demonstrated. The results are compared to measurements with standard CTA system and show that similar results are obtained with this new technique.

On the transient response of the turbulent boundary layer inception in compressible flows

The behavioural characteristics of thermal boundary layer inception dictate the efficiency of heat exchangers and the operational limits of fluid machinery. The specific time required by the thermal boundary layer to be established is vital to optimize flow control strategies, as well as the thermal management of systems exposed to ephemeral phenomena, typically on the millisecond scale. This paper presents the time characterization of the momentum and thermal boundary layer development in transient turbulent compressible air flows. We present a new framework to perform such estimations based on detailed unsteady Reynolds averaged Navier–Stokes simulations that may be extended to higher fidelity simulations. First of all, the aerodynamic boundary layer initiation is described using adiabatic simulations. Additional numerical calculations were then performed by setting the isothermal wall condition to evaluate the additional time required by the thermal boundary layer to establish after the aerodynamic boundary layer reaches its steady state. Finally, full conjugate simulations were executed to compute the warm up effect of the solid during the blowdown of a hot fluid over a colder metallic test model. The transient performance of the turbulent thermal and momentum boundary layers is quantified through numerical simulations of air blowdown over a flat plate for different mainstream flow conditions. The effects of Reynolds number, free stream velocity, transient duration, test article length and free stream temperature were independently assessed, to then define a mathematical expression of the momentum boundary layer settlement. This paper presents a novel numerical correlation of the additional time required by the thermal boundary layer to be stablished after the settlement of the momentum boundary layer. The time scales of the aerodynamic and thermal boundary layers are presented as a function of relevant non-dimensional numbers, as well as the description of the response of the near wall flow to sudden free stream changes. The characterization of the boundary layer mechanisms discussed in this paper contribute to the establishment of an evidence-based foundation for advances in the field of flow control.

Development of Aerodynamic Design of Hypersonic Quiet Nozzles

The hypersonic quiet nozzle has to be carefully designed by the aerodynamic techniques to obtain the quiet-flow test region with low noise levels comparable to flight. Two conceptual aerodynamic techniques tolaminarizethe nozzle wall boundary layer in the past quiet nozzle design, such as the use of the slower expansion nozzle and the bleed slot in the subsonic region upstream of the throat, are summarized. For the design of the expansion contour of quiet nozzles, the analysis based on the favorable pressure gradient effect is performed and relative results seem to be helpful to theoretically understand the quiet nozzle's evolution from rapid expansion to slow expansion. On the other hand, the bleed slot for the suction of the subsonic boundary layer in the contraction is also required to be analyzed in details. The distance from lip to throat and the maximum width of bleed slot could affect the suction of the slot to the boundary layer by changing the separation location and would be also analyzed.

A fast and accurate method to predict 2D and 3D aerodynamic boundary layer flows

A quasi-simultaneous interaction method is applied to predict 2D and 3D aerodynamic flows. This method is suitable for offshore wind turbine design software as it is a very accurate and computationally reasonably cheap method.This study shows the results for a NACA 0012 airfoil. The two applied solvers converge to the experimental values when the grid is refined. We also show that in separation the eigenvalues remain positive thus avoiding the Goldstein singularity at separation.In 3D we show a flow over a dent in which separation occurs. A rotating flat plat is used to show the applicability of the method for rotating flows.The shown capabilities of the method indicate that the quasi-simultaneous interaction method is suitable for design methods for offshore wind turbine blades.

Pressure and hot-film measurements on a wind turbine blade operating in the atmosphere

In the present study the aerodynamic boundary layer at a rotor blade is investigated while the turbine is working under real operating conditions in the atmosphere. Owing to the complexity of the experimental set-up, up to now most research on transition is conducted in wind tunnels and field measurements are rare. Hence important effects such as the unsteady behavior of the inflow is not taken into account. For the current measurements the blade is equipped with a hot film at the most interesting part of the upper side midspan of the blade in order to detect non-laminar structures in the boundary layer. Furthermore, 34 pressure tubes are installed along the chord length in order to gain information about the flow field. A preliminary analysis of the hot-film measurements combined with a CFD calculation and a stability analysis based on the eN method leads to two results. Firstly it is possible to determine the state of the boundary layer (laminar or turbulent) and secondly we propose to discuss our findings in case of medium rotational speed within so called Tollmien-Schlichting scenario.

Effect of the charge surface distribution on the flow field induced by a dielectric barrier discharge actuator

The Electro-Hydro-Dynamics (EHD) interaction induced by a surface dielectric barrier discharge in the aerodynamic boundary layer at one atmosphere still air has been investigated. Three different geometrical configurations of the actuator have been utilized. In the first configuration, an electrode pair separated by a 2 mm dielectric sheet has been used. The second and the third configurations have been obtained by adding a third electrode on the upper side of the dielectric surface. This electrode has been placed downstream of the upper electrode and has been connected to ground or has been left floating. Three different dielectric materials have been utilized. The high voltage upper electrode was fed by an a.c. electric tension. Measurements of the dielectric surface potential generated by the charge deposition have been done. The discharge has been switched off after positive and negative phases of the plasma current (the current phase was characterized by a positive or a negative value, respectively). The measurements have been carried out after both phases. The charge distribution strongly depended on the switching off phase and was heavily affected by the geometrical configuration. A remarkable decrease of the charge deposited on the dielectric surface has been detected when the third electrode was connected to ground. Velocity profiles were obtained by using a Pitot probe. They showed that the presence of the third electrode limits the fluid dynamics performance of the actuator. A relation between the charge surface distribution and the EHD interaction phenomenon has been found. Imaging of the plasma has been done to evaluate the discharge structure and the extension of the plasma in the configurations investigated.

Experimental Results in DBD Plasma Actuators for Air Flow Control

In this paper, several planar dielectric barrier discharge fluid-dynamics actuators have been experimentally investigated. The actuator is made by an electrode pair separated by a dielectric. It is characterized by a planar geometry, and it induces a volume force on the gas above it which modifies the fluid dynamics within the aerodynamic boundary layer. Several actuator geometries, dielectric materials, ac supply voltages, and supply frequencies have been used. Rotational and vibrational temperatures have been determined by means of spectroscopic measurements. Vibrational temperatures have been found to be one order of magnitude higher than the rotational ones, confirming the nonthermal behavior of the discharge. The translational temperature has been evaluated by thermographic observations of the dielectric surface heated by the plasma. The values of the translational temperature are in a very good agreement with those of the rotational temperature. Electric measurements have been carried out to determine the average power delivered to the discharge and to estimate the average electron number density. In the plasma induced by the discharge above the actuator, an average electron density of has been determined. The gas speed of the induced wall jet has been measured with a Pitot tube. The average power delivered to the electrodes increases with a power law of the applied voltage and linearly with the supply frequency. The induced speed increases with the increase of the average power until a plateau is reached. The efficiencies of the different actuators have been compared by evaluating the relation between average power and induced gas speed.

Engineering the race car wing: application of the vortex panel numerical method

This study examined aerodynamic properties and boundary layer stability in five cambered airfoils operating at the low Reynolds numbers encountered in motor racing. Numerical modelling was carried out in the flow regime characterised by Reynolds numbers 0.82–1.29 × 106. The design Reynolds number of 3 × 106 was used as a reference. Aerodynamics variables were computed using AeroFoil 2.2 software, which uses the vortex panel method and integral boundary layer equations. Validation of AeroFoil 2.2 software showed very good agreement between calculated aerodynamic coefficients and wind tunnel experimental data. Drag polars, lift/drag ratio, pitching moment coefficient, chordwise distributions (surface velocity ratio, pressure coefficient and boundary layer thickness), stagnation point, and boundary layer transition and separation were obtained at angles of attack from −4° to 12°. The NASA NLF(1)-0414F airfoil offers versatility for motor racing with a wide low-drag bucket, low minimum profile drag, high lift/drag ratio, laminar flow up to 0.7 chord, rapid concave pressure recovery, high resultant pressure coefficient and stall resistance at low Reynolds numbers. The findings have implications for the design of race car wings.

Hurricane boundary-layer theory

In the light of the plethora of definitions for the hurricane boundary layer, we advocate a dynamical definition based on the distribution of agradient flow. We seek also to clarify the fundamental role of the boundary layer in the hurricane intensification process. In particular, we contrast the differences between unsteady boundary layers that are able to facilitate the spin-up of the vortex above and steady boundary layers that cannot. If slaved to the time-dependent vortex aloft, the latter can spin up the interior vortex only indirectly by changing its thermodynamic properties through vertical advection of these from below and adjustment to thermal wind balance. These differences are highlighted by an analytical demonstration that the application of a zero-vertical-gradient condition on velocity above a steady boundary layer does not provide a direct means of allowing the boundary layer to determine the flow in the interior vortex. This result assumes that frictional forces are negligible at this boundary. Finally, echoing a few previous insights, we question the applicability of conventional boundary-layer theory at radii of strong ascent into the eyewall, where the flow is akin to that of separation in aerodynamic boundary layers. Copyright © 2010 Royal Meteorological Society