Effect of compressibility, rotation and magnetic field on the drag on a sphere oscillating in a conducting, viscous medium
Effect of compressibility, rotation, and magnetic field on drag on sphere oscillating in conducting viscous medium
DRAG↗
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Effect of Compressibility on the Flow past a Two-dimensional Bump
An investigation has been conducted to determine experimentally the effects of compressibility on the flow past a bump and to compare the experimentally determined results with theory. Pressure measurements and Schlieren photographs were made of the flow past two bumps having thickness-chord ratios of 0.10 and 0.30 at Mach numbers between 0.20 and 0.84
FLOW, SUBSONIC↗
Similarity rules for effects of sidewall boundary layer in two-dimensional wind tunnels
A simple analysis of the interaction of the model pressure field with the boundary layer on an unventilated wind tunnel wall is presented. It is shown that the effects of this interaction are similar to compressibility effects for sidewall boundary layers in two dimensional wind tunnels. This similarity is used to derive modified forms of the Prandtl Glauert rule for subsonic flow and the von Karman rule for transonic flow which are validated by comparison with experimental data. The three dimensional interaction problem is discussed, and it is shown that model pressure field/wall boundary layer interaction effects are not similar to compressibility effects in three dimensional wind tunnels.
Barnwell, R. W.↗
Lead-Lag Control for Helicopter Vibration and Noise Reduction
As a helicopter transitions from hover to forward flight, the main rotor blades experience an asymmetry in flow field around the azimuth, with the blade section tangential velocities increasing on the advancing side and decreasing on the retreating side. To compensate for the reduced dynamic pressure on the retreating side, the blade pitch angles over this part of the rotor disk are increased. Eventually, a high enough forward speed is attained to produce compressibility effects on the advancing side of the rotor disk and stall on the retreating side. The onset of these two phenomena drastically increases the rotor vibratory loads and power requirements, thereby effectively establishing a limit on the maximum achievable forward speed. The alleviation of compressibility and stall (and the associated decrease in vibratory loads and power) would potentially result in an increased maximum forward speed. In the past, several methods have been examined and implemented to reduce the vibratory hub loads. Some of these methods are aimed specifically at alleviating vibration at very high flight speeds and increasing the maximum flight speed, while others focus on vibration reduction within the conventional flight envelope. Among the later are several types passive as well as active schemes. Passive schemes include a variety of vibration absorbers such as mechanical springs, pendulums, and bifilar absorbers. These mechanism are easy to design and maintain, but incur significant weight and drag penalties. Among the popular active control schemes in consideration are Higher Harmonic Control (HHC) and Individual Blade Control (IBC). HHC uses a conventional swash plate to generate a multi-cyclic pitch input to the blade. This requires actuators capable of sufficiently high power and bandwidth, increasing the cost and weight of the aircraft. IBC places actuators in the rotating reference frame, requiring the use of slip rings capable of transferring enough power to the actuators. Both schemes cause an increase in pitch link loads. Trailing Edge Flap (TEF) deployment can also used to generate unsteady aerodynamic forces and moments that counter the original vibratory loads, and thereby reduce rotor vibrations. While the vibrations absorbers, HHC, IBC, and TEF concepts discussed above attempt to reduce the vibratory loads, they do not specifically address the phenomena causing the vibrations at high advance ratios. One passive method that attempts to directly alleviate compressibility and stall, instead of reducing the ensuing vibrations, is the use of advanced tip designs. Taper, sweep, anhedral, and the manipulation of other geometric properties of the blade tips can reduce the severity of stall and compressibility effects , as well as reduce rotor power. A completely different approach to solve these problems is the tiltrotor configuration. As the forward velocity of the aircraft increases, the rotors, in this case, are tilted forward until they are perpendicular to the flow and act as propellers. This eliminates the edgewise flow encountered by conventional rotors and circumvents all the problems associated with flow asymmetry. However, the success involves a tremendous increase in cost and complexity of the aircraft. Another possible approach that has been proposed for the alleviation of vibratory loads at high forward flight speeds involves the use of controlled lead-lag motions to reduce the asymmetry in flow. A correctly phased 1/rev controlled lag motion could be introduced such that it produces a backward velocity on the advancing side and a forward velocity on the retreating side, to delay compressibility effects and stall to a higher advance ratio. Using a large enough lead-lag amplitude, the tip velocities could be reduced to levels encountered in hover. This concept was examined by two groups in the 1950's and early 1960's. In the United States, the Research Labs Division of United Aircraft developed a large lead-lag motion rotor, meant to achieve lag motion amplitudes up to 45 degrees. In order to reduce the required actuation force, the blade hinges were moved to 40% of the blade radius to increase the rotating lag frequency to approximately 1/rev. The blade hinges were redesigned to produce a flap-lag coupling so the large flapwise aerodynamic loads could be exploited to actuate the blades in the lag direction. A wind tunnel test of this rotor concept revealed actuation and blade motion scheduling problems. The project was eventually discontinued due to these problems and high blade stresses. Around the same time, at Boelkow in Germany, a similar lead-lag rotor program was conducted under the leadership of Hans Derschmidt. Here, too, the blade hinges were moved outboard to 34% radius to reduce the actuation loads. The main difference between this and the United Aircraft program was the use of a mechanical actuation scheme with maximum lead-lag motions of 400. This program was also discontinued for unclear reasons. The present study is directed toward conducting a comprehensive analytical examination to evaluate the effectiveness of controlled lead-lag motions in reducing vibratory hub loads and increasing maximum flight speed. Since both previous studies on this subject were purely experimental, only a limited data set and physical understanding of the problem was obtained. With the currently available analytical models and computational resources, the present effort is geared toward developing an in-depth physical understanding of the precise underlying mechanisms by which vibration reduction may be achieved. Additionally, in recognition of the fact that large amplitude lead-lag motions would - (i) be difficult to implement, and (ii) produce very large blade stresses; the present study examines the potential of only moderate-to-small lead-lag motions for reduction of vibratory hub loads. Using such an approach, the emphasis is not on eliminating the periodic variations in tangential velocity at the blade tip, but at best reducing these variations slightly so that compressibility and stall are delayed to slightly higher advance ratios. This study was conducted in two steps. In the first step, a hingeless helicopter rotor was modeled using rigid blades undergoing flap-lag-torsion rotations about spring restrained hinges and bearings. This model was then modified by separating the lead-lag degree of freedom into two components, a free and a prescribed motion. Using this model, a parametric study of the effect of phase and amplitude of a prescribed lead-lag motion on hub vibration was conducted. The data gathered was analyzed to obtain an understanding of the basic physics of the problem and show the capability of this method to reduce vibration and expand the flight envelope. In the second half of the study, the similar analysis was conducted using an elastic blade model to confirm the effects predicted by the simpler model.
Gandhi, Farhan↗
Preliminary Investigation of the Effect of Compressibility on the Maximum Lift Coefficient, Special Report
Preliminary data are presented on the variation of the maximum lift coefficient with Mach number. The data were obtained from tests in the 8-foot high-speed tunnel of three NACA 16-series airfoils of 1-foot chord. Measurements consisted primarily of pressure-distribution measurements in order to illustrate the nature of the phenomena. It was found that the maximum lift coefficient of airfoils is markedly affected by compressibility even at Mach numbers as low as 0.2. At high Mach numbers pronounced decrease of the maximum lift coefficient was found. The magnitude of the effects of compressibility on the maximum lift coefficient and the low speeds at which these effects first appear indicate clearly that consideration of the take-off thrust for propellers will give results seriously in error if these considerations are based on the usual low-speed maximum-lift-coefficient data generally used.
Stack, John↗
The Effect of Compressibility on the Growth of the Laminar Boundary Layer on Low-Drag Wings and Bodies
The development of the laminar boundary layer in a compressible fluid is considered. Formulas are given for determining the boundary-layer thickness and the ratio of the boundary-layer Reynolds number to the body Reynolds number for airfoils and bodies of revolution. It i s shown that the effect of compressibility will profoundly alter the Reynolds number corresponding to the upper limit of the range of the low-drag coefficients . The available data indicate that for low-drag and high critical compressibility speed airfoils and bodies of revolution, this effect is favorable.
Allen, H. Julian↗
The effects of compressibility on dynamic stall
In this paper, typical computational predictions and experimental measurements of compressible flow past an airfoil at dynamic stall conditions are studied and compared to develop an insight into the effect of compressibility on dynamic stall. The dependency of the critical Mach number on airfoil leading edge curvature, camber, and angle of attack is investigated. Evidence is presented to show that a local region of supersonic flow occurs on an oscillating airfoil, even for a freestream Mach number as low as 0.2, if the boundary layer remains attached and the angle of attack is sufficiently high; that a shock terminates this local supersonic bubble; and that the vorticity that this shock generates grows rapidly and becomes very unstable as the angle of attack increases beyond the value at which the maximum local flow speed first exceeds the speed of sound. It is suggested that these shock-induced effects compete with the dynamic viscous effects occurring in the boundary layer in determining the onset of separation, which can lead to premature dynamic stall and can significantly reduce the maximum dynamic lift that can otherwise be obtained.
Fung, K.-Y.↗
Effects Of Compressibility On Dynamic Stall
Report presents results of computations and measurements of compressible flow about an airfoil, angle of attack of which oscillates about static-stall angle. Study focuses on effects of compressibility on dynamic stall. Of particular interest are conditions determining onset of separation of flow, which leads to premature dynamic stall and consequent significant reduction of lift.
Carr, L. W.↗
Effects of compressibility on the performance of two full-scale helicopter rotors
Report presents the results of an investigation conducted on the Langley helicopter test tower to determine experimentally the effects of compressibility on the performance and blade pitching moments of two full-scale helicopter rotors. Two sets of rotor blades were tested which differed only in that the blades of one set incorporated -8 degrees of linear twist, whereas the blades of the other set were untwisted. The tests covered a range of tip speeds from 350 to 770 feet per second and a range of pitch angles from 0 degrees to the limit imposed by extreme vibration.
Carpenter, Paul J↗
Effect of compressibility on the nonlinear prediction of the aerodynamic loads on lifting surfaces
The vortex-lattice technique for incompressible flow which accounts for separation at sharp edges is modified to account for compressibility. This is accomplished by extending the Prandtl-Glauert transformation to moderate angles of attack. Thus, the aerodynamic characteristics for the compressible case are obtained from the solution of an equivalent incompressible problem. Numerical results are presented for parallelogram and delta wings to assess the effects of compressibility. The results are in good agreement with available experimental data.
Kandil, O. A.↗
Reynolds number effects on the turbulence field in compressible boundary layers
Detailed experiments were conducted in a zero pressure gradient, supersonic turbulent boundary layer, including measurements of the three components of velocity fluctuations and the turbulent shear stress, for Reynolds numbers ranging from 11.7 million to 105 million at a freestream Mach number of 2.3. The mean flow measurements established the fully developed and equilibrium nature of the boundary layer. Measurements of the turbulence field show that the vertical and transverse fluctuations are essentially equal throughout the boundary layer at all Reynolds numbers, a feature that is different from observations in incompressible flows. The data show that the boundary layer exhibits similarity in the turbulence profiles for the entire Reynolds number range and agrees with previous compressible and incompressible data using Morkovin's scaling to account for compressibility effects.
Acharya, M.↗
Effect of compressibility at high subsonic velocities on the moment acting on an elliptic cylinder
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FLOW - CYLINDERS, ELLIPTICAL↗
Simple curves for determining the effects of compressibility on pressure drop through radiators
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EFFECT OF COMPRESSIBILITY↗
Effects of Compressibility on the Flow Past Thick Airfoil Sections
Six, 3-inch-chord symmetrical airfoil sections having systematic variations in thickness and thickness location were tested at Mach numbers near flight values for propeller-shank sections. The tests, the results of which are presented in the form of schlieren photographs of the flow past each model and pressure-distribution charts for two of the model, were performed to illustrate the effects of compressibility on the flow past thick symmetrical airfoil sections. Representative flow photographs indicated that at Mach numbers approximately 0.05 above the critical Mach number a speed region was reached in which the flow oscillated rapidly and the separation point and the location of the shock wave were unstable. Fixing the transition on both surfaces of the airfoil was effective in reducing these rapid oscillations. The pressure distributions showed that the section normal-force coefficients for thick airfoils were very erratic at subcritical speeds; at supercritical speeds the section normal-force coefficients for the thick airfoils became more regular. Drag coefficients showed that considerable drag decreases can be expected by decreasing the model thickness ratio.
Daley, Bernard N.↗
Effects of compressibility on boundary-layer turbulence
A series of turbulence measurements in a subsonic compressible turbulent boundary-layer flow in the Mach number range of 0.1 to 0.7 is described. Measurements include detailed surveys of the turbulence intensities and Reynolds shear stresses, and other quantities such as the turbulent kinetic energy. These data are examined to bring out the effects of compressibility and show that the stream-wise and transverse fluctuations and the turbulent shear stress follow a universal scaling law. A preliminary attempt is made to examine some of the assumptions made in turbulence models commonly used in numerical codes for the calculation of compressible flows.
Acharya, M.↗
Effect of Compressive Stresses on Leakage Currents in Microchip Tantalum Capacitors
Microchip tantalum capacitors are manufactured using new technologies that allow for production of small size capacitors (down to EIA case size 0402) with volumetric efficiency much greater than for regular chip capacitors. Due to a small size of the parts and leadless design they might be more sensitive to mechanical stresses that develop after soldering onto printed wiring boards (PWB) compared to standard chip capacitors. In this work, the effect of compressive stresses on leakage currents in capacitors has been investigated in the range of stresses up to 200 MPa. Significant, up to three orders of magnitude, variations of currents were observed after the stress exceeds a certain critical level that varied from 10 MPa to 180 MPa for capacitors used in this study. A stress-induced generation of electron traps in tantalum pentoxide dielectric is suggested to explain reversible variations of leakage currents in tantalum capacitors. Thermo-mechanical characteristics of microchip capacitors have been studied to estimate the level of stresses caused by assembly onto PWB and assess the risk of stress-related degradation and failures. Keywords: tantalum capacitors, leakage current, soldering, reliability, mechanical stress.
Teverovsky, Alexander↗
Effects of compressibility on design of subsonic fuselages for natural laminar flow
Compressible linear boundary-layer stability analyses of two representative axisymmetric fuselage geometries indicate that a favorable effect will be exerted on the characteristics of a fuselage's axisymmetric boundary layer by compressibility. A freestream Mach number increase from 0.6 to 0.8 significantly reduces TS wave growth rates in the laminar boundary layer of the fuselages analyzed. The generally destabilizing effect of increasing length Re number on boundary layer stability can be overpowered by the favorable effects of compressibility on the fluid.
Vijgen, P. M. H. W.↗
Progress in turbulence modeling for complex flow fields including effects of compressibility
Two second-order-closure turbulence models were devised that are suitable for predicting properties of complex turbulent flow fields in both incompressible and compressible fluids. One model is of the "two-equation" variety in which closure is accomplished by introducing an eddy viscosity which depends on both a turbulent mixing energy and a dissipation rate per unit energy, that is, a specific dissipation rate. The other model is a "Reynolds stress equation" (RSE) formulation in which all components of the Reynolds stress tensor and turbulent heat-flux vector are computed directly and are scaled by the specific dissipation rate. Computations based on these models are compared with measurements for the following flow fields: (a) low speed, high Reynolds number channel flows with plane strain or uniform shear; (b) equilibrium turbulent boundary layers with and without pressure gradients or effects of compressibility; and (c) flow over a convex surface with and without a pressure gradient.
Wilcox, D. C.↗