Inflow Disturbances Research Articles

Complex neurodegeneration in retina following moderate ischemia induced by bilateral common carotid artery occlusion in Wistar rats

Bilateral common carotid artery occlusion (BCCAO) produces moderate levels of ischemia in the retina of rats, which may simulate the inflow disturbances in severe carotid artery disease. ERG changes following acute BCCAO have been well described, but the effects of chronic BCCAO on the histopathology of the retina remain to be characterized in a reproducible model. Chronic BCCAO was induced in halothane-anaesthetized male Wistar rats and the retina fixed after 3, 6, or 24 hr, 1 week, and 2, 4, or 6 months. Cell counts and measurements of retinal layers were performed in H&E stained paraffin sections. Immunohistochemistry with a panel of fourteen antibodies served to examine the survival of different retinal cell class, astrocytic reactions and the expression of acute stress response proteins. A lectin method was used to label activated microglial cells. Microglial activation, heme oxygenase-1 upregulation and caspase-3 cleavage occurred during the first 24 hr in the absence of overt cell death of retinal ganglion cells (RGC). Three waves of neurodegeneration followed. RGCs were affected after 1 week, followed by neurons in the inner nuclear layer at 2 months, and finally photoreceptors at 4 months. Immunomarkers indicated acute damage to horizontal cells and prolonged survival of amacrine cells. In conclusion, chronic BCCAO produced delayed neuronal death in the retina of adult male Wistar rats. The window of moderate changes of at least 1 day may facilitate molecular studies on retinal ganglion cell loss.

On three-dimensionality and control of incompressible cavity flow

The incompressible large eddy simulation technique, coupled with the Lighthill-Curle acoustic analogy, is used to investigate the oscillation mechanism and sound source of a two-dimensional cavity with a length-to-depth ratio of L∕D=4 and Reynolds number of ReD=5000. It is demonstrated that the development of the three-dimensional flow field, initiated by the introduction of a random inflow disturbance, is eventually accompanied by transition from the wake to the shear layer oscillation mode, regardless of the amplitude and shape of the inflow disturbance. Once the transition to the shear layer mode is accomplished, the amplitude and frequency of oscillations are not very sensitive to the particular shape of the inflow disturbance. The effectiveness of controlling the flow oscillations by applying simultaneous steady injection and suction through the front and rear cavity walls, respectively, is demonstrated. The results show that, for injection levels exceeding a certain threshold value, the oscillations are quenched, and for levels below that value, the oscillation process is virtually unaffected. The major difference between the averaged uncontrolled and controlled velocity fields is the amount of reverse flow in the rear part of the cavity. With the aid of linear stability analysis, it is demonstrated that for injection levels leading to the quenching of the oscillations the mean velocity profiles in the cavity region are only convectively unstable, whereas for the uncontrolled case there is an absolutely unstable region. This suggests that, at least for incompressible flow, the reduction of the reverse flow inside the cavity can reduce or eliminate the oscillation process.

Inherently Linear Annular-Duct-Type Laminar Flowmeter

where: C is a constant depends on a duct cross-section shape , A is the duct cross-sectional area, Dh is the hydraulic diameter of the duct, K is the incremental pressure drop number in a fully developed flow region depends both on the duct cross-section shape and on the inflow conditions , L is the distance between the flowmeter pressure taps, is fluid density, and is fluid kinematic viscosity. The first component on the right-hand side of Eq. 1 is the pressure drop in the fully developed flow. The second one describes some additional pressure drop due to momentum change and accumulated increment in wall shear between developing flow and developed flow 1 . It is seen that the relationship between produced differential pressure P and flow rate Q is not linear. As a result, in the majority of commercial laminar flowmeters, to obtain precise Q values software corrections are required. In some of the commercial “laminar flow elements” the entrance effect is ignored as a relatively low one. In order to eliminate the necessity of reading corrections and to achieve inherent linearity the second term of Eq. 1 must be removed. It can be done by locating the first pressure tap P1 at the beginning of the fully developed flow region. As the entrance length Le increases with the Reynolds number Re=DhQ / A , the distance between the duct entrance and the pressure tap P1 should not be shorter than the entrance length for the maximum flow rate, i.e., for the laminar-turbulent transition Reynolds number. The main aim of this work was to determine the design recommendation for the inherently linear laminar annular-duct flowmeter. The pressure distribution along the annular duct was measured. The entrance lengths for various inlet conditions and Reynolds numbers were investigated.

Turbulence structure in the vortex formation region behind a circular cylinder in lock-on conditions

The present experimental research has been undertaken in order to explain to what extent the turbulent structure of the flow around a stationary rigid cylinder may be affected by an oscillating approach flow. The general aim was to establish a direct link between the oscillating inflow, local time-resolved quantities like skin friction and surface pressure and near wake behaviour behind the circular cylinder. It has been confirmed that the lock-on synchronization between the periodicity of vortex shedding and free stream oscillations is an important factor affecting the formation mechanisms of the wake. In particular it was found that periodical inflow disturbances bring about a rapid growth of pressure and wall shear stress fluctuations near the separation point. The present results revealed also a tendency toward the shortening of vortex formation length and significant growth of the time and linear microscales of turbulence in lock-on conditions. Additional evidence concerning the role of inflow periodical disturbances in the near wake modification was obtained from analysis of turbulent kinetic energy transport in the flow.

Computational analysis of the two-dimensionalthree-dimensional transition in forward-facing step flow

Results are presented from a computational study of the flow over a forward-facing step in a plane channel. The aim of the study is to gain better insight into the three-dimensionality that is typically observed in the separation region of flows over steps and ribs, and in similar configurations. We consider laminar flow at a Reynolds number of 330, based on step height and bulk velocity of the oncoming flow, and the step is assumed to be infinitely extended in the spanwise direction. High-resolution simulations are undertaken using a mixed spectral/spectral-element code. Moreover, a linear stability study of the flow at the step is performed. The results show that, in the case considered, the three-dimensionality is not related to some absolute instability of the separation bubble in front of the step; rather, it is a sensitive reaction of the flow to three-dimensional perturbations present in the oncoming stream. It is demonstrated that disturbance amplitudes of less than 1% of the mean flow (at, say, 10 step heights ahead of the step) already suffice to produce a visibly three-dimensional structure of the separation zone. If the disturbance level is systematically decreased, the three-dimensional state evolves to an almost two-dimensional recirculation. Here, the key finding is that the intensity of the flow response is proportionate to the amplitude of the inflow disturbance, meaning that the breakup of the flow in the step region is a linear (i.e. small) perturbation of the two-dimensional base flow. A comparison of the present simulation results with experimental data shows close agreement concerning, for example, the flow topology in the step region, and the spanwise spacing of the characteristic streaks that form further downstream.

Direct numerical simulation of stagnation region flow and heat transfer with free-stream turbulence

A direct numerical simulation is performed for stagnation-region flow with free-stream turbulence. A fully implicit second-order time-advancement scheme with fourth-order finite differences and an optimized scheme are employed. The optimized scheme is developed to save computational cost. The free-stream turbulence is a precomputed field of isotropic turbulence. The present DNS results in the “damping” and “attached amplifying” regimes are found to be similar to those of the organized inflow disturbances. Emphasis is placed on the flow and temperature fields in the “detached amplifying” regime. The contours of instantaneous flow field illustrate that streamwise vortices are stretched in the streamwise direction by mean strain rate. The temperature field is also stretched in the streamwise direction near the wall. The surface contours reveal that the temperature field is influenced significantly by streamwise vorticity. Due to the dominance of the mean strain, the log-law region is not observed for ū and T̃, the inner scaling fails, but the outer scaling works. The single-point turbulence statistics and the turbulent statistics budgets are obtained. The flow statistics reflect the typical characteristics of stagnation-region flow which are generically different from those of other canonical shear flows. One of the typical features of the budgets is that the velocity pressure correlation and the turbulent transport play significant roles in the stagnation-region flow. Finally, the present simulation data are compared with experimental results. It is found that the effect of large-scale eddies on the enhancement of wall heat transfer is substantial in the turbulent stagnation-region heat transfer.

An evaluation of linear instability waves as sources of sound in a supersonic turbulent jet

Mach wave radiation from supersonic jets is revisited to better quantify the extent to which linearized equations represent the details of the actual mechanism. To this end, we solve the linearized Navier–Stokes equations (LNS) with precisely the same mean flow and inflow disturbances as a previous direct numerical simulation (DNS) of a perfectly expanded turbulent M=1.92 jet [Freund et al., AIAA J. 38, 2023 (2000)]. We restrict our attention to the first two azimuthal modes, n=0 and n=1, which constitute most of the acoustic field. The direction of peak radiation and the peak Strouhal number matches the DNS reasonably well, which is in accord with previous experimental justification of the linear theory. However, it is found that the sound pressure level predicted by LNS is significantly lower than that from DNS. In order to investigate the discrepancy, individual frequency components of the solution are examined. These confirm that near the peak Strouhal number, particularly for the first helical mode n=1, the amplification of disturbances in the LNS closely matches the DNS. However, away from the peak frequency (and generally for the azimuthal mode n=0), modes in the LNS are damped while those in the DNS grow at rates comparable to those at the peak Strouhal number.

Breakdown of the Reynolds Analogy in a Stagnation Region Under Inflow Disturbances

A systematic analysis is performed for the Reynolds analogy breakdown at stagnation-region flow and heat transfer in the presence of inflow disturbances. The Reynolds analogy breakdown between momentum and energy transfers in a stagnation region is scrutinized by varying the Reynolds number (5000≤Re≤20000), the amplitude (0.00075≤A≤0.003) and the length scale (λ/δ=10.6). A spanwise sinusoidal variation is superimposed on the velocity component normal to the wall. Self-similarity solutions are obtained with trigonometric series expansions. The Reynolds analogy criterion demonstrates that the rate of change of skin friction is different from that of wall heat transfer. Different evolutions of the rates of skin friction and wall heat transfer are due to the difference between 〈s'v'〉 and 〈v'T'〉. An in-depth analysis on 〈s'v'〉 and 〈 v'T'〉 is performed by analysis using disturbance correlations based on the fluctuating velocity transport equations in vorticity form. It is found that the pressure fluctuations, the wall blocking and the Lamb vectors are responsible for the breakdown of the Reynolds analogy. A direct comparison is made between momentum and energy balances associated with the three responsible mechanisms. A common finding is that their profiles are changed significantly at a location where the evolution of the streamwise vortex is strong.

Natural Transition Phenomena on an Axial Compressor Blade

Data from a surface hot-film array on the outlet stator of a 1.5-stage axial compressor are analyzed to look for direct evidence of natural transition phenomena. An algorithm is developed to identify instability waves within the Tollmien–Schlichting (T–S) frequency range. The algorithm is combined with a turbulent intermittency detection routine to produce space-time diagrams showing the probability of instability wave occurrence prior to regions of turbulent flow. The paper compares these plots for a range of blade loading, with free-stream conditions corresponding to the maximum and minimum inflow disturbance periodicity produced by inlet guide vane clocking. Extensive regions of amplifying instability waves are identified in nearly all cases. The implications for transition prediction in decelerating flow regions on axial turbomachine blades are discussed.

Influence of Inflow Disturbances on Stagnation-Region Heat Transfer

Numerical simulations of laminar stagnation-region heat transfer in the presence of freestream disturbances are performed. The sensitivity of heat transfer in stagnation-region to freestream vorticity is scrutinized by varying the length scale, amplitude, and Reynolds number. As an organized inflow disturbance, a spanwise sinusoidal variation is superimposed on the velocity component normal to the wall. An accurate numerical scheme is employed to integrate the compressible Navier-Stokes equations and energy equation. The main emphasis is placed on the length scale of laminar inflow disturbances, which maximizes the heat transfer enhancement. Computational results are presented to disclose the detailed behavior of streamwise vortices. Three regimes of the behavior are found depending on the length scale: these are the “damping,” “attached amplifying,” and “detached amplifying” regimes, respectively. The simulation data are analyzed with an experimental correlation. It is found that the present laminar results follow a general trend of the correlation. [S0022-1481(00)01102-6]

Use of Linearized Euler Equations for Supersonic Jet Noise Prediction

The use of linearized Euler equations for direct prediction of supersonic jet noise is explored. It is shown that a high-order numerical scheme coupled with proper boundary treatment can produce a stable solution nearly free from reflections. Results are verified against analytical results for sound radiated by instability waves. Applicability of this approach to real jets is explored by taking the inflow disturbances to be random in time and comparing the computed sound field to the experimentally measured one. Nomenclature D = jet exit nozzle diameter e = total energy per unit volume Me = jet exit Mach number p = pressure Ue = jet exit velocity u = axial velocity v = radial velocity w = azimuthal velocity p = density pe = jet exit density

Control of Large-Scale Structures in Spatially Developing Mixing Layers.

Direct numerical simulations of spatially developing mixing layers were conducted to clarify the effect of phase difference of inflow disturbances on the growth of mixing layers. The disturbances consist of the fundamental and subharmonic modes of the most unstable wave based on linear stability analysis, and the energy transfer between mean flow, fundamental and subharmonic modes was examined. The results show that it is possible to control the development of mixing layers by controling phase difference between two modes. The phase difference determines energy transfer in the regions downstream of the roll-up location where energy is transferred from fundamental to mean flow, and upstream of the pairing location where the energy is transferred from mean flow to subharmonic. This mechanism causes the enhancement and suppression of the growth rate of mixing layers. The phase difference which produces the slowest development of a mixing layer is about 11/16π.

Structure of supersonic jet flow and its radiated sound

The present paper explores the use of large-eddy simulations as a tool for predicting noise from first principles. A high-order numerical scheme is used to perform large-eddy simulations of a supersonic jet flow with emphasis on capturing the time-dependent flow structure representating the sound source. The wavelike nature of this structure under random inflow disturbances is demonstrated. This wavelike structure is then enhanced by taking the inflow disturbances to be purely harmonic. Application of Lighthill's theory to calculate the far-field noise, with the sound source obtained from the calculated time-dependent near field, is demonstrated. Alternative approaches to coupling the near-field sound source to the far-field sound are discussed.

Active aerodynamic control of wake-airfoil interaction noise - Experiment

A proof of concept experiment has been performed that shows the potential for active aerodynamic control of rotor wake/stator interaction noise in a simplified manner. A single airfoil model representing the stator was fitted with a movable trailing-edge flap controlled by a servomotor. This flap allowed control of the unsteady lift of the airfoil and acted as the active aerodynamic element in the system. Upstream of the airfoil, a disturbance generator created a periodic wake, similar to an actual rotor blade wake, which impinged on the airfoil. A control system moved the motor-driven flap in the correct angular displacement rate and phase to reduce the unsteady load on the airfoil during the wake interaction. The motion could be arbitrary, permitting the airfoil-flap combination to respond to generic inflow disturbances. Noise reduction results showed that this concept works very well in the bandwidth at which the servomotor drive actuated the flap (currently up to 100 Hz). The peak-to-peak acoustic interaction pulse was reduced by a factor of 2, whereas the corresponding noise was reduced by a significant 10 dB at some frequencies. A companion paper by £. J. Kerschen, entitled Active Aerodynamic Control of Wake-Airfoil Interaction Noise—Theory, demonstrates the ability to predict the required flap motion to achieve these dramatic noise reductions.

Diagnostic accuracy of Doppler ultrasound technique of the penile arteries in correlation to selective arteriography.

In 63% of 265 patients with erectile dysfunction a relevant arterial inflow disturbance was found by Doppler ultrasound examination. Correlation between Doppler and arteriography in 58 patients showed an accuracy of 95% in detecting penile arteries and an accuracy of 91% in discovering a pathological arterial pattern (arterial anomaly or arteriosclerotic obstruction). In 15 patients the arterial inflow was measured additionally by Doppler ultrasound technique after intracavernosal injection of vasoactive drugs (IIVD) (7.5 mg papaverine and 0.25 mg phentolamine). This technique proved to be more reliable than in the flaccid state and markedly facilitated localization and assessment of pathological changes of the cavernosal arteries.

Tone generation by rotor-downstream strut interaction

A JT15D fan stage was acoustically tested in the NASA Lewis anechoic chamber as part of the joint Lewis/Langley Research Center investigation of flight simulation techniques and flight effects using the JT15D engine as a common test vehicle. Suspected rotor-downstream support strut interaction was confirmed through the use of simulated support struts, which were tested at three axial rotor-strut spacings. Tests were also per- formed with the struts removed. Inlet boundary layer suction in conjunction with an inflow control device was also explored. The removal of the boundary layer reduced the fan fundamental tone levels, suggesting that the mounting and mating of such a device to the nacelle requires careful attention. With the same inflow control device installed, good acoustic agreement was shown between the engine on an outdoor test stand and the fan in the anechoic chamber. HE development of effective inflow control devices (ICD's) makes it possible to study noise generation mechanisms, such as rotor-stator interaction, with reduced masking effects of inflow disturbances. Modern turbofan engines are often designed with blade/vane numbers selected to prevent propagation of the fundamental rotor-stator in- teraction tone. However, less consideration has been given to possible rotor interactions with engine support struts. These struts are either located downstream of the stator row or are integrated into the stator as large cross-section vanes.! This paper presents results for a JT15D fan stage which was acoustically tested in the NASA Lewis Research Center anechoic chamber2 as part of a joint NASA Lewis/Langley investigation of flight simulation techniques and flight effects using the JT15D-1 engine as a common test vehicle.37 The engines used in these studies were instrumented with blade and vane pressure transducers to assist in isolating noise generation mechanisms. Although the primary goal of this study was to evaluate inflow control techniques, the results revealed that for the JT15D-1 engine in particular speed ranges the fundamental tone was controlled by the presence of six engine support struts located downstream of the stator. Blade pressure results showing a strong six per revolution disturbance pointed to these struts as the probable noise source. The interaction between the 28-blade rotor of the JT15D and the six support struts would result in a m-22 acoustic spinning mode having 22 circumferential lobes. This mode was shown to exist in the inlet duct of a JT15D engine when the results from two pressure sensors located in the duct so as to allow spining mode identification by signal phase relationship were used.3 However, it was not possible to alter the support struts in the engine to establish the behavior of this apparent noise source. Downstream support struts were not required for the JT15D fan installation in the anechoic chamber. Six simulated support struts were fabricated and installed in the test fan stage to simulate the actual engine support strut installation. These simulated struts were located at three axial spacings from the stator trailing edge. Thus, in the present study results were obtained for the spacing effect of downstream support struts as well as for fan stage alone with no downstream struts.

Changes in spontaneous and evoked release of transmitter induced by the crotoxin complex and its component phospholipase A 2 at the frog neuromuscular junction

The time course and characteristics of the changes induced by the action of the crotoxin complex and of its component phospholipase A 2 on transmission at the frog neuromuscular junction was studied in vitro at single endplates using intracellular recording techniques. Both the crotoxin complex and phospholipase A 2 induced variable changes in spontaneous transmitter release in which bursts of miniature endplate potentials alternated with periods when the frequency of miniature endplate potentials was stable but their amplitudes were markedly heterogeneous. The bursts of miniature endplate potentials were of sudden onset and of a high initial frequency, and their duration and incidence were highly variable both at one endplate and between endplates. These changes in the frequency and amplitude of miniature potentials occurred both before and after the abolition of neurally evoked endplate potentials and in the presence of tetrodotoxin. At a late stage in crotoxin intoxication, potassium depolarization of the motor nerve terminals failed to increase spontaneous transmitter release. Both the crotoxin complex and phospholipase A 2 reduced the amplitude of endplate potentials although phospholipase A 2 was less effective. The course of depression of the quantal content of evoked transmitter release induced by the crotoxin complex was occasionally interrupted by a brief facilitation of response; these irregularities in the rate of depression were more clearly seen when endplate potentials were recorded at partially curarized endplates exposed to either the crotoxin complex or to phospholipase A 2. At the stage of abolition of endplate potentials an increase in the external concentration of Ca 2 + enabled the evoked response to be restored temporarily. The crotoxin complex and phospholipase A 2 had no effect on the input resistance of the sarcolemmal membrane, but raised the threshold stimulus required to initiate an action potential at concentrations which affected transmitter release. Crotapotin did not affect any of the measured parameters even at high concentrations. The diversity of changes observed in the properties of spontaneous and evoked transmitter release, suggests that the crotoxin complex and phospholipase A 2 may induce two kinds of alterations in the structure of the presynaptic membrane as a result of the action of phospholipase A 2, viz ( a) the formation of transient instabilities with an increased Ca 2 + inflow and ( b) prolonged disturbance of the calcium channel and/or the synchronized release of subunits at the active zone.

Reduction of fan noise in an anechoic chamber by reducing chamber wall induced inlet flow disturbances

The difference between flight and ground static noise data has arisen in the past few years as a significant problem in fan jet engine noise testing. The additional noise for static testing has been attributed to inlet flow disturbances or turbulence interacting with the fan rotor. In an attempt to determine a possible source of inflow disturbances entering fans tested in the Lewis Research Center anechoic chamber the inflow field was studied using potential flow analyses. These potential flow calculations indicated that there was substantial flow over the wall directly behind the fan inlet. This flow near the wall anechoic wedges could produce significant inflow disturbances. Fan noise tests were run with various extensions added to the fan inlet ducting to move the inlet away from this back wall and thereby reduce the inlet flow disturbances. Significant noise reductions were observed with increased inlet length. Over 5‐dB reduction of the blade passage tone sound power level was observed between the shortest and longest inlet at 90% fan speed and the first overtone was reduced 9 dB. High‐frequency reductions in the broadband noise were also observed.

Computational experiments on the effects of inlet turbulence and inflow distortion on fan noise

Existing solutions to the problem of sound production by an axial-flow fan interacting with inlet turbulence and inflow distortion (Marvin E. Goldstein et al., NASA TN D-7676, May 1974 and NASA TN D-7667, June 1974) were evaluated over a range of inflow parameters. Sound power in the fan fundamental and second harmonic tones was computed and the modal power distributions contributing to the total tone power were identified. Comparisons and contrasts between the relations of input parameters to sound power produced were made for the two forms of inflow disturbance. It was found that the bulk of the modal power was carried by modes near cutoff for both forms of inflow disturbance when described by parameter values typical of those expected in practical fan test situations. The implications of the numerical results for the design of passive inlet flow control devices proposed to simulate flight conditions during static fan tests are discussed.

Effects of nonuniform inflow on fan noise

A prominant source of turbomachinery noise is due to high-speed unsteady flows interacting with blades and vanes. In particular, atmospheric turbulence, cross-wind effects, and the “ground vortex” apparent during most static test conditions can interact with the fan on turbofan engines to produce significant levels of noise. An analysis is presented that models this noise-generating mechanism and is capable of predicting propagating noise, accepting as input various inlet turbulence structures, temperature fluctuations, and flow distortions. Because of the random uature of most inflow disturbances, a statistical approach was used to develop the model which is a modification and extension of a previous analysis presented by Mani [“Noise Due to Interaction of Inlet Turbulence with Isolated Stators and Rotors,” J.S.V. 19 (1971)]. The analysis shows that the level of fan blade passing tone and harmonics and their relative forward and rearward radiated intensities are critically dependent on both the inlet turbulence intensity and correlation length scales in the axial, circumferential, and spanwise directions. This analysis is used in conjunction with measured fan noise data to infer the turbulence properties that would be required to account for levels of observed blade passage tone noise.