Wake Strength Research Articles

Wake Sensing for Aircraft Formation Flight

It is well established that flying aircraft in formation can lead to improved aerodynamic efficiency. However, successfully doing so is predicated on having knowledge of the lead aircraft’s wake position. Here, a wake-sensing strategy for estimating the wake position and strength in a two-aircraft formation is explored in a simplified proof-of-concept setting. The wake estimator synthesizes wing-distributed pressure measurements, taken on the trailing aircraft, by making use of an augmented lifting-line model in conjunction with both Kalman-type and particle filters. Simple aerodynamic models are introduced in constructing the filter to enable fundamental wake-sensing challenges to be identified and reconciled. The various estimation algorithms are tested in a vortex lattice simulation environment, thus allowing the effects of modeling error to be analyzed. It is found that biases in the position estimates no longer arise if a particle filter is used in place of the Kalman-type filters. Filter divergence is observed when the relative aircraft separations are held fixed. This divergent behavior can be alleviated with the introduction of relative aircraft motions, for example in the form of a cross-track dither signal.

Outer-layer turbulence intensities in smooth- and rough-wall boundary layers

AbstractClear differences in turbulence intensity profiles in smooth, transitional and fully rough zero-pressure-gradient boundary layers are demonstrated, using the diagnostic plot introduced by Alfredsson, Segalini & Örlü (Phys. Fluids, vol. 23, 2011, p. 041702) – ${u}^{\prime } / U$ versus $U/ {U}_{e} $, where ${u}^{\prime } $ and $U$ are the local (root mean square) fluctuating and mean velocities and ${U}_{e} $ is the free stream velocity. A wide range of published data are considered and all zero-pressure-gradient boundary layers yield outer flow ${u}^{\prime } / U$ values that are roughly linearly related to $U/ {U}_{e} $, just as for smooth walls, but with a significantly higher slope which is completely independent of the roughness morphology. The difference in slope is due largely to the influence of the roughness parameter ($ \mathrm{\Delta} {U}^{+ } $ in the usual notation) and all the data can be fitted empirically by using a modified form of the scaling, dependent only on $ \mathrm{\Delta} U/ {U}_{e} $. The turbulence intensity, at a location in the outer layer where $U/ {U}_{e} $ is fixed, rises monotonically with increasing $ \mathrm{\Delta} U/ {U}_{e} $ which, however, remains of $O(1)$ for all possible zero-pressure-gradient rough-wall boundary layers even at the highest Reynolds numbers. A measurement of intensity at a point in the outer region of the boundary layer can provide an indication of whether the surface is aerodynamically fully rough, without having to determine the surface stress or effective roughness height. Discussion of the implication for smooth/rough flow universality of differences in outer-layer mean velocity wake strength is included.

Effect of the Axial Spacing between Vanes and Blades on a Transonic Gas Turbine Performance and Blade Loading

Unsteady numerical simulations have been conducted to investigate the effect of axial spacing between the stator vanes and the rotor blades on the performance of a transonic, single-stage, high-pressure, axial turbine. Three cases were considered, the normal case, which is based on the geometry of a commercial jet engine and has an axial spacing at 50% blade span equal to 42% of the vane axial chord, as well as two other cases with axial spacings equal to 31 and 52% vane axial chords, respectively. Present interest has focused on the effect of axial gap size on the instantaneous and time-averaged flows as well as on the blade loading and the turbine performance. Decreasing the gap size reduced the pressure and increased the Mach number in the core flows in the gap region. However, the flows near the two endwalls did not follow monotonic trends with the gap size change; instead, the Mach numbers for both the small gap and the large gap cases were lower than that for the normal case. This Mach number decrease was attributed to increased turbulence due to the increased wake strength for the small gap case and an increased wake width for the large gap case. In all considered cases, large pressure fluctuations were observed in the front region of the blade suction side. These pressure fluctuations were strongest for the smaller spacing. The turbine efficiencies of the cases with the larger and smaller spacings were essentially the same, but both were lower than that of the normal case. The stator loss for the smaller spacing case was lower than the one for the larger spacing case, whereas the opposite was true for the rotor loss.

Direct numerical simulation of axisymmetric wakes embedded in turbulence

AbstractDirect numerical simulation has been used to study the effects of external turbulence on axisymmetric wakes. In the absence of such turbulence, the time-developing axially homogeneous wake is found to have the self-similar properties expected whereas, in the absence of the wake, the turbulence fields had properties similar to Saffman-type turbulence. Merging of the two flows was undertaken for three different levels of external turbulence (relative to the wake strength) and it is shown that the presence of the external turbulence enhances the decay rate of the wake, with the new decay rates increasing with the relative strength of the initial external turbulence. The external turbulence is found to destroy any possibility of self-similarity within the developing wake, causing a significant transformation in the latter as it gradually evolves towards the former.

Effects of Nutrition on Lifelong Sleep and Activity Patterns in Drosophila melanogaster (Diptera: Drosophilidae)

Abstract Drosophila melanogaster (Meigen) (Diptera: Drosophilidae) exhibits several welldefined sleep parameters and a sleep:wake rhythm strength cycle that fragments and deteriorates with age. Because many aspects of functional senescence may be linked to the effects of accumulated oxidative damage, and as several lines of experimental evidence indicate that sleep parameters and sleep:wake rhythm strength may be modulated by oxidative stress in D. melanogaster, it has been postulated that sleep fragmentation is driven, at least in part, by accumulated oxidative damage in this fly. We investigated the contrasting effects of an enriched dietary regime that is likely to exacerbate processes associated with aging and oxidative damage accumulation, and a dietary restriction regime that has been demonstrated to delay the detectable effects of aging-related damage on lifelong sleep and activity patterns in male and female D. melanogaster. We found that female flies on a nutrient-rich dietary regime exhibited stronger sleep:wake cycles than females on a dietary restriction regime and that dietary restriction did not seem to alter the initiation of sleep fragmentation in female or male D. melanogaster.

SIMULATION OF WAKE VORTEX AIRCRAFT IN GROUND EFFECT

The problem developed in this paper is encountered in airplane aerodynamics and concerns the influence of long life longitudinal wake vortices generated by wing tips or by external obstacles such as reactors or landing gears. More generally it concerns 3D bodies of finite extension in cross flow. At the edge of such obstacles, longitudinal vortices are created by pressure differences inside the boundary layers and rotate in opposite senses. It can constitute a danger to another aircraft that flies in this wake, especially during takeoff and landing. In this case the wake vortex trajectories and strengths are altered by ground interactions. This study presents the results of a Large Eddy Simulation of wake vortex in ground effect providing the vorticity flux behavior.

Turbulence in rough-wall boundary layers: universality issues

Wind tunnel measurements of turbulent boundary layers over three-dimensional rough surfaces have been carried out to determine the critical roughness height beyond which the roughness affects the turbulence characteristics of the entire boundary layer. Experiments were performed on three types of surfaces, consisting of an urban type surface with square random height elements, a diamond-pattern wire mesh and a sand-paper type grit. The measurements were carried out over a momentum thickness Reynolds number (Re θ) range of 1,300–28,000 using two-component Laser Doppler anemometry (LDA) and hot-wire anemometry (HWA). A wide range of the ratio of roughness element height h to boundary layer thickness δ was covered (\(0.04 \leq h/\delta \leq 0.40\)). The results confirm that the mean profiles for all the surfaces collapse well in velocity defect form up to surprisingly large values of h/δ, perhaps as large as 0.2, but with a somewhat larger outer layer wake strength than for smooth-wall flows, as previously found. At lower h/δ, at least up to 0.15, the Reynolds stresses for all surfaces show good agreement throughout the boundary layer, collapsing with smooth-wall results outside the near-wall region. With increasing h/δ, however, the turbulence above the near-wall region is gradually modified until the entire flow is affected. Quadrant analysis confirms that changes in the rough-wall boundary layers certainly exist but are confined to the near-wall region at low h/δ; for h/δ beyond about 0.2 the quadrant events show that the structural changes extend throughout much of the boundary layer. Taken together, the data suggest that above h/δ ≈ 0.15, the details of the roughness have a weak effect on how quickly (with rising h/δ) the turbulence structure in the outer flow ceases to conform to the classical boundary layer behaviour. The present results provide support for Townsend’s wall similarity hypothesis at low h/δ and also suggest that a single critical roughness height beyond which it fails does not exist. For fully rough flows, the data also confirm that mean flow and turbulence quantities are essentially independent of Re θ; all the Reynolds stresses match those of smooth-wall flows at very high Re θ. Nonetheless, there is a noticeable increase in stress contributions from strong sweep events in the near-wall region, even at quite low h/δ.

A Wind Tunnel Study of Airflow Near Model Swine Confinement Buildings

One of the most significant and persistent environmental concerns regarding swine production is the transport of odor constituents, trace gases, and particulates from animal production and manure storage facilities. The objectives of this study were to determine how swine housing unit orientation affects air velocity and turbulence and to assess the opportunities for reducing off-site transport of air quality constituents from manure storage facilities located downstream from confinement buildings. Measurements were made with 1:300 models of swine finisher buildings in a low-speed wind tunnel capable of producing air velocities up to 12 m s-1 (27 mph). Runs were completed with no building models, with one housing unit oriented parallel and perpendicular to airflow, and with four housing units oriented parallel, perpendicular, and at a 30° angle to airflow. Velocity and turbulence measurements were completed in a grid of 83 points within a 215 mm high × 400 mm wide (8.5 × 15.7 in.) vertical plane at separation distances 2H, 5H, and 10H downstream from the building model arrays (H = model height of 17.5 mm) using a constant temperature anemometer system with a 3-D hot-film probe. A large zone of reduced longitudinal velocity (u) and increased turbulence intensity (Iu) in the wake of model buildings oriented perpendicular to flow was observed and was still apparent 10H downstream. The size and strength of this turbulent wake is attributed to the sloping roofs of the building models that, with a frontal vortex under the upwind building eave, create an unexpectedly tall wake zone in the building lee. One or four parallel building model arrays exhibited the least influence on downstream velocities and turbulence intensities. One perpendicular and four 30° models produced intermediate effects. The observed reduction in air velocity up to 10H downstream from multiple buildings oriented perpendicular to the airflow suggests that entrainment and transport of air quality constituents from manure storage structures may be reduced when these structures are located in this zone. However, increased turbulence in the wake zone and field conditions with variable wind speed and direction and atmospheric stability may counter the effects of reduced velocity. Tracer studies, either in wind tunnel experiments or in the field, are needed to verify the potential for building orientation and spacing to reduce transport of air quality constituents from manure storage structures.

Similarity of the streamwise velocity component in very-rough-wall channel flow

The streamwise velocity component is studied in fully developed turbulent channel flow for two very rough surfaces and a smooth surface at comparable Reynolds numbers. One rough surface comprises sparse and isotropic grit with a highly non-Gaussian distribution. The other is a uniform mesh consisting of twisted rectangular elements which form a diamond pattern. The mean roughness heights (±) the standard deviation) are, respectively, about 76(±42) and 145(±150) wall units. The flow is shown to be two-dimensional and fully developed up to the fourth-order moment of velocity. The mean velocity profile over the grit surface exhibits self-similarity (in the form of a logarithmic law) within the limited range of 0.04≤y/h≤0.06, but the profile over the mesh surface does not, even though the mean velocity deficit and higher moments (up to the fourth order) all exhibit outer scaling over both surfaces. The distinction between self-similarity and outer similarity is clarified and the importance of the former is explained. The wake strength is shown to increase slightly over the grit surface but decrease over the mesh surface. The latter result is contrary to recent measurements in rough-wall boundary layers. Single- and two-point velocity correlations reveal the presence of large-scale streamwise structures with circulation in the plane orthogonal to the mean velocity. Spanwise correlation length scales are significantly larger than corresponding ones for both internal and external smooth-wall flows.

Far-Field Boundary Condition Effects of CFD and Free-Wake Coupling Analysis for Helicopter Rotor

Rotor aerodynamics is governed by wake geometry and strength. However, rotor wake characteristics computed by the rotor computational fluid dynamics are not clearly described due to numerical dissipation. To overcome this numerical problem, free-wake is used for wake simulation. The present free-wake describes the inboard vortices as well as the tip vortices of the blade. At each time step, the free-wake provides inflow and outflow conditions of the boundary of the Eulerian domain, and the Euler solver is used for solving the flow field near the rotor blade. Finally, the coupled method is compared with the conventional method and the experimental results.

Unsteady Analysis of Blade and Tip Heat Transfer as Influenced by the Upstream Momentum and Thermal Wakes

The effect of the upstream wake on the time averaged rotor blade heat transfer was numerically investigated. The geometry and flow conditions of the first stage turbine blade of GE’s E3 engine with a tip clearance equal to 2% of the span were utilized. The upstream wake had both a total pressure and temperature deficit. The rotor inlet conditions were determined from a steady analysis of the cooled upstream vane. Comparisons between the time average of the unsteady rotor blade heat transfer and the steady analysis, which used the average inlet conditions of unsteady cases, are made to illuminate the differences between the steady and unsteady calculations. To help in the understanding of the differences between steady and unsteady results on one hand and to evaluate the effect of the total temperature wake on the other, separate calculations were performed to obtain the rotor heat transfer and adiabatic wall temperatures. It was found that the Nusselt number distribution for the time average of unsteady heat transfer is invariant if normalized by the difference in the adiabatic and wall temperatures. It appeared though that near the endwalls the Nusselt number distribution did depend on the thermal wake strength. Differences between steady and time averaged unsteady heat transfer results of up to 20% were seen on the blade surface. Differences were less on the blade tip surface.

Experimental Reduction of Transonic Fan Forced Response by Inlet Guide Vane Flow Control

The main contributor to the high cycle fatigue of compressor blades is the response to aerodynamic forcing functions generated by an upstream row of stators or inlet guide vanes. Resonant response to engine order excitation at certain rotor speeds can be especially damaging. Studies have shown that flow control by trailing edge blowing (TEB) can reduce stator wake strength and the amplitude of the downstream rotor blade vibrations generated by the unsteady stator-rotor interaction. In the present study, the effectiveness of TEB to reduce forced fan blade vibrations was evaluated in a modern single-stage transonic fan rig. Data were collected for multiple uniform full-span TEB conditions over a range of rotor speeds including multiple modal resonance crossings. Resonant response sensitivity was generally characterized by a robust region of strong attenuation. The baseline resonant amplitude of the first torsion mode, which exceeded the endurance limit on the critical blade, was reduced by more than 80% with TEB at 1.0% of the total rig flow. The technique was also found to be modally robust; similar reductions were achieved for all tested modal crossings, including more than 90% reduction in the second leading-edge bending response using 0.7% of the rig flow.

An Integral Solution for Heat Transfer in Accelerating Turbulent Boundary Layers

An equilibrium thermal wake strength parameter is developed for a two-dimensional turbulent boundary layer flow and is then used in the combined thermal law of the wall and the wake to give an approximate temperature profile to insert into the integral form of the thermal energy equation. After the solution of the integral x momentum equation, the integral thermal energy equation is solved for the local Stanton number as a function of position x for accelerating turbulent boundary layers. A simple temperature distribution in the thermal “superlayer” is part of the present modeling. The analysis includes a dependence of the hydrodynamic and thermal wake strengths on the momentum thickness and enthalpy thickness Reynolds numbers, respectively. An approximate dependence of the turbulent Prandtl number, in the “log” region, on the strength of the favorable pressure gradient is proposed and incorporated into the solution. The resultant solution for the Stanton number distribution in accelerated turbulent flows is compared with experimental data in the literature. A comparison of the present predictions is also made to a finite difference solution, which uses the turbulent kinetic energy—turbulent dissipation model of turbulence, for a few cases of accelerating flows.

Swimming performance of a biomimetic compliant fish-like robot

Digital particle image velocimetry and fluorescent dye visualization are used to characterize the performance of fish-like swimming robots. During nominal swimming, these robots produce a ‘V’-shaped double wake, with two reverse-Karman streets in the far wake. The Reynolds number based on swimming speed and body length is approximately 7500, and the Strouhal number based on flapping frequency, flapping amplitude, and swimming speed is 0.86. It is found that swimming speed scales with the strength and geometry of a composite wake, which is constructed by freezing each vortex at the location of its centroid at the time of shedding. Specifically, we find that swimming speed scales linearly with vortex circulation. Also, swimming speed scales linearly with flapping frequency and the width of the composite wake. The thrust produced by the swimming robot is estimated using a simple vortex dynamics model, and we find satisfactory agreement between this estimate and measurements made during static load tests.

Prediction of velocity profiles and Reynolds stress distributions in turbulent open-channel flows with adverse pressure gradient

Turbulence measurements in open-channel flows with adverse pressure gradient were conducted with an X-type hot-film anemometer. The velocity profiles in whole depth are described well by the log-wake law which involves a wake strength parameter π. The wake strength parameter π is controlled only by the pressure gradient parameter β in the equilibrium region, i.e., normalized velocity profiles are similar at all streamwise distances. A new relationship formula between the wake strength parameter π and the pressure gradient parameter β is suggested on the basis of experimental data. The equation of motion and continuity equation are transformed to predict the velocity profiles and also the Reynolds stress distributions under the conditions that the flow is in equilibrium and the coefficient of velocity is constant in the streamwise direction. The wake strength parameter π can be predicted with high accuracy from the obtained equations. The predicted Reynolds stress distributions correspond to the experimental data when the pressure gradient parameter β is low. On the other hand, the predicted ones do not correspond to the experimental data when the pressure gradient parameter β is high.

막대형 표면조도가 난류경계층에 미치는 영향

Abstract The effects of surface roughness on a turbulent boundary layer (TBL) were investigated using particle image velocimetry (PIV). The roughness elements used were periodically arranged two-dimensional spanwise rods, and the roughness height was . Introduction of the roughness elements increased the wake strength and the turbulent stress not only in the roughness sublayer but also in the outer layer. This indicates the existence of interaction between inner and outer layers for 2D rod-roughened wall. Roughness effects on a turbulence structure near the wall were obtained by PIV measurements. Iso-contours of mean velocities and Reynolds stresses in the roughness sublayer showed a very good agreement with previous DNS results. † 책임저자, 회원, 한국과학기술원 기계공학과 E-mail : hjsung@kaist.ac.kr TEL : (042)869-3027 FAX : (042)869-5027 * 한국과학기술원 기계공학과 ** 현대중공업 선박저항추진 연구실 *** 한국해양대학교 기계정보공학부 기호설명 : 표면조도 높이  : 유효사조도높이  : 레이놀즈수   ∞    : 주유동방향 평균속도  : 무차원 속도

Dynamics of Vortical Flows Induced by Swept-Notched Wings in Aircraft Wake Simulations

The motivation behind the present investigations is the alleviation of vortex wake hazard. The influence of wing-tip geometry modifications on the near-field wake behind swept-notched wings is investigated up to 24 span lengths, using computational fluid dynamics. Three-dimensional parallel simulations are run at a chord-based Reynolds number of 5.34 x 10 4 . The effectiveness of notched wing modifications on wake properties is assessed via comparison with conventional and original notched wings. Current results are promising: the tip and flap vortices produced by the swept-notched wings remain distinct and unmerged, unlike those associated with the conventional wing. In all cases, the wakes of the novel wings have circulations and vorticity values that are substantially lower than the original notched wing. Numerical data clearly indicate that such geometrical combinations break apart the concentrated area of streamwise vorticity shed by the wing into three or more less intense regions, significantly reducing the wake strength and induced rolling moment, and affecting the location and merging process of the overall wake system.

Modern Integral Method Calculation of Turbulent Boundary Layers

Heat transfer in turbulent boundary layers, with zero or adverse pressure gradients, is calculated using modern integral methods. Inner variables are used with the velocity profile taken to be Coles's combined law of the wall and wake. The temperature profile used is the thermal law of wall and wake with an equilibrium wake strength function. The modeling employed takes into approximate account the dependence of velocity wake strength and thermal wake strength on the momentum thickness Reynolds number and the enthalpy thickness Reynolds number, respectively. Also, an attempt is made to incorporate the dependence of turbulent Prandtl number on pressure gradient into the model. Calculations are started at the leading edge, or beginning, of the surface with very small boundary-layer thicknesses, rather than matching the measured values of Stanton number and skin-friction coefficient at the location of the first experimental data point. Predictions are compared to experimental data which include both constant flux and constant temperature surfaces, some of which may have unheated starting lengths. There is reasonably good agreement between them for all but the case of very severe adverse pressure gradient.

Self-preservation of a Turbulent Boundary Layer over d-type Roughness

Using a direct drag balance measurement for the local wall shear stress, self-preserving development of a turbulent boundary layer was achieved experimentally over a d-type rough surface without pressure gradients. The wall shear stress and mean velocity measurements confirmed the requirements for exact self-similarity and highly similar Reynolds stress profiles within the Reynolds number range for a constant skin friction coefficient. Under the condition that the boundary layer flow is completely independent of Reynolds number, the effect of wall roughness was investigated with respect to the similarity laws for the wall layer as well as the outer layer. Experimental observation reveals the wall similarity y u y U κ τ = ∂ ∂ to be applicable to the present rough wall boundary layer remaining the accepted value of Karman constant to be κ = 0.41. Otherwise, investigation of the wake strength in the mean velocity and Reynolds stress profiles reveals that the wall roughness does affect the outer layer structure. Reynolds stress measurements indicate that the primary effect of wall roughness on turbulence properties is in the component normal to the wall.

Modified log-wake law for zero-pressure-gradient turbulent boundary layers

This paper shows that the turbulent velocity profile for zero-pressure-gradient boundary layers is affected by the wall shear stress and convective inertia. The effect of the wall shear stress is dominant in the so-called overlap region and can be described by a logarithmic law in which the von Karman constant is about 0.4 while the additive constant depends on a Reynolds number. The effect of the convective inertia can be described by the Coles wake law with a constant wake strength about 0.76. A cubic correction term is introduced to satisfy the zero velocity gradient requirement at the boundary layer edge. Combining the logarithmic law, the wake law and the cubic correction produces a modified log-wake law, which is in excellent agreement with experimental profiles. The proposed velocity profile law is independent of Reynolds number in terms of its defect form, while it is Reynolds number dependent in terms of the inner variables. The modified log-wake law can also provide an accurate equation for skin friction in terms of the momentum thickness. Finally, by replacing the logarithmic law with van Driest's mixing-length model in which the damping factor varies with Reynolds number, the modified log-wake law can be extended to the entire boundary layer flow.