Wake Generator Research Articles

Gudden’s dorsal tegmental nucleus is activated in carbachol-induced active (REM) sleep and active wakefulness

Previous studies have shown that GABAergic processes in the ponto-mesencephalic region of the brainstem are involved in the generation of wakefulness and active sleep (AS). The dorsal and ventral tegmental nuclei of Gudden (DTN and VTN, respectively) are known to contain a large population of GABAergic neurons. In the present study, utilizing Fos immunoreactivity as a marker of neuronal activity, it was determined that GABAergic DTN pars dorsalis neurons are active during active wakefulness and AS induced by carbachol, but not during quiet wakefulness or quiet sleep. In contrast, no differences in the number of Fos immunoreactive neurons were observed in the DTN pars ventralis and VTN across behavioral states.

GABAergic mechanisms in the pedunculopontine tegmental nucleus of the cat promote active (REM) sleep

The pedunculopontine tegmental nucleus (PPT) has been implicated in the generation and/or maintenance of both active sleep (AS) and wakefulness (W). GABAergic neurons are present within this nucleus and recent studies have shown that these neurons are active during AS [27,44]. In order to examine the role of mesopontine GABAergic processes in the generation of AS, the GABA A agonist muscimol and the GABA A antagonist bicuculline were microinjected into the PPT of chronic cats that were prepared for recording the states of sleep and wakefulness. Muscimol increased the time spent in AS by increasing the frequency and duration of AS episodes; this increase in AS was at the expense of the time spent in wakefulness. A decrease in PGO density during AS was also observed following the microinjection of muscimol. On the other hand, bicuculline decreased both AS and quiet sleep and increased the time spent in wakefulness. These data suggest that GABA acts on GABA A receptors within the PPT to facilitate the generation of AS by suppressing the activity of waking-related processes within this nucleus.

Rotor Blade-to-Blade Wake Variability and Effect on Downstream Vane Response

Experiments are performed in a high-speed fan stage to investigate multistage interaction effects on rotor blade-to-blade wake variability and the resulting downstream vane row unsteady aerodynamic response. Multistage effects on the rotor wake are quantified by acquiring data encompassing two complete rotor revolutions at each of several inlet guide vanes clocking positions relative to downstream stationary probes. The resulting downstream vane response data are acquired over a range of steady loading conditions, with both design and off-design operation of the rotor considered. Multistage interactions significantly affect the rotor wake characteristics and lead to the possible generation of rogue wakes. The fundamental periodicity of these interactions is one complete rotor revolution due to the unequal number of blades and vanes in the machine. The rotor wake and resultant vane row unsteady aerodynamic response variability are also quantified. Off-design rotor operation results in wakes with the largest variability, that is, on the order of the average velocity deficit. The variability of the vane response is even more pronounced, as large as 160% of the maximum average unsteady lift over one blade pass period.

Generation and instability of spiral wakes in sheared electron flows

Images of the electron density of a magnetized electron column illustrate the dynamics of intense vortices moving in a weaker background vorticity. A moving retrograde clump of vorticity generates a spiral wake that then evolves into many long-lived holes, contributing to the late time fluctuations.

Influence of unsteady wake on a turbulent separation bubble

An experimental study was made of turbulent separated and reattaching flow over a blunt body, where unsteady wake was generated by a spoked-wheel type of wake generator with cylindrical rods. The influence of unsteady wake was scrutinized by altering the rotation direction (clockwise and counter-clockwise) and the normalized passing frequency (0 ≤ StH ≤ 0.20). The Reynolds number based on the cylindrical rod was Red=375. A phase-averaging technique was employed to characterize the unsteady wake. The effect of different rotation directions, which gave a significant reduction of xR, was examined in detail. The wall pressure fluctuations on the blunt body were analyzed in terms of the spectrum and the coherence.

Wavelet Analysis of Turbulent Structures in Far Wake

Initial condition effects in a self-preserving plane wake were investigated for two wake generators, i.e. a triangular cylinder and a screen of 50% solidity. Two orthogonal arrays of sixteen X-wires, eight in the (x, y) -plane, i.e. the plane of mean shear, and eight in the (x, z) -plane, which is parallel to both the cylinder axis and the streamwise direction, are used to simultaneously obtain velocity data in the experimental investigation. Measurements were made at x/h (x is the streamwise distance downstream of the cylinder and h is the height of the wake generator) =330 for the triangular cylinder and 220 for the screen. A two-dimensional orthogonal wavelet transform is used to analyze the measured hot-wire data. This technique enables the turbulence structures of various scales to be separated and characterized. Discernible differences are observed between the two wake generators in the turbulence structures of large- down to intermediate-scales.

Development and application of a novel technique for direct heat flux measurements in turbomachinery flows

The high performance and efficiency of modern gas turbines are only possible with temperatures inside the engine exceeding the allowed material temperatures in some areas by several hundred degrees. Therefore effective cooling methods are one of the key factors for the success of these engines. In order to achieve reliable predictions of the heat load of rotor or stator blades numerous research activities were performed to understand the nature of heat transfer in complex unsteady flows. Even numerical methods have made significant progress in recent years detailed experimental data are still necessary for validation and further development of the engines and the design tools. Here a new method to directly measure the heat flux $${\dot q}$$ at the material surface and accurately determine the heat transfer coefficienth is presented. The new sensor is based on the anisotropic characteristics of single crystals and allows the determination of the time varying heat flux on the surface of a model turbine airfoil. This feature is of special interest to study the influence of periodically disturbed flow conditions on the heat transfer characteristics of cooled turbine blades. The working principle of an anisotropic heat flux (AHF) sensor is briefly described together with the design of the actual sensor used in this study. Prior to the application of the sensor in a cascade test rig, comprehensive test of the sensor, the electronics and the data acquisition system were performed using a pulsed laser beam as heat source. To test the sensor under realistic conditions a large number of sensor was installed in a test blade and heat transfer measurements were performed in a cascade test rig equipped with a spoke-wheel wake generator. The results showed good agreement in the time mean results compared with standard techniques. Additionally time resolved data could be extracted from the sensor signals providing detailed information on the unsteady heat transfer characteristics and boundary layer development.

Experimental Investigation of Turbulence Influence of Wake Passing on the Boundary Layer Development of Highly Loaded Turbine Cascade Blades

Experimental investigations regarding the effects of wake passing on boundary layer development were performed on a highly loaded low pressure (LP) turbine cascade blade. Data was obtained for various flow conditions and is intended to be used for the validation of numerical models dealing with periodic unsteady transition. A phase shift between turbulence and velocity fluctuations in the wake path was observed in the near-wall boundary layer, which might have considerable impact on the use of integral-type transition criteria. The experimental data set is already publicly available for download on the web page of the institute. A moving bar type wake generator was employed to simulate the upstream blade row. Tests were performed at turbomachinery-like Mach and Reynolds number conditions in the High-Speed Cascade Wind Tunnel for different bar speeds, bar spacings and background turbulence levels. This paper focuses on the exemplary description of flow phenomena characteristic for unsteady flow for a single exit Reynolds number of Re2th = 200,000. Conventional measurement techniques like static pressure tappings on the suction and pressure side as well as five hole probe traverses were complemented with fast-response pressure transducers embedded into and hot film sensors glued onto the suction surface to study the time-resolved boundary layer development in detail. Additional studies using single hot wire traverses in the suction side boundary layer and triple hot wire probes upstream of the cascade inlet plane were deployed to complete the data sets. The results indicate that for the LP turbine the transition point moves periodically when subjected to wake passing, which greatly affects the loss generation in the suction side boundary layer. Laminar separation is suppressed or reduced whenever the wake is present.

Study of Wake-Blade Interactions in a Transonic Compressor Using Flow Visualization and DPIV

Flow-field interactions are studied in a high-through-flow, axial-flow transonic compressor using Digital Particle Image Velocimetry (DPIV). Measurement of instantaneous velocities in two-dimensional (2D) planes in the main flow direction allows characterization of the unsteadiness of spatial structures from an upstream blade row and their interaction with the downstream rotor. The measurement system is specially designed for a large transonic environment, which introduces conditions that differ from those generally encountered by traditional DPIV systems. Viewing windows on the compressor housing are used to allow optical access, and the design of a special optical probe permits laser-sheet delivery through one of the wake generators (WG). The system is synchronized with the blade passage and is remotely monitored and controlled. Through flow visualization and instantaneous and ensemble-averaged quantities, it clearly captures the interactions of the wake with the potential field of the rotor leading edge (LE) and its bow shock, vortex shedding, vortex-blade synchronization, wake chopping, and boundary-layer flow at the housing for several configurations.

Numerical solution of incompressible Navier–Stokes equations using a fractional-step approach

A fractional step method for the solution of steady and unsteady incompressible Navier–Stokes equations is outlined. The method is based on a finite-volume formulation and uses the pressure in the cell center and the mass fluxes across the faces of each cell as dependent variables. Implicit treatment of convective and viscous terms in the momentum equations enables the numerical stability restrictions to be relaxed. The linearization error in the implicit solution of momentum equations is reduced by using three subiterations in order to achieve second order temporal accuracy for time-accurate calculations. In spatial discretizations of the momentum equations, a high-order (third and fifth) flux-difference splitting for the convective terms and a second-order central difference for the viscous terms are used. The resulting algebraic equations are solved with a line-relaxation scheme which allows the use of large time step. A four color ZEBRA scheme is employed after the line-relaxation procedure in the solution of the Poisson equation for pressure. This procedure is applied to a Couette flow problem using a distorted computational grid to show that the method minimizes grid effects. Additional benchmark cases include the unsteady laminar flow over a circular cylinder for Reynolds numbers of 200, and a 3-D, steady, turbulent wingtip vortex wake propagation study. The solution algorithm does a very good job in resolving the vortex core when fifth-order upwind differencing and a modified production term in the Baldwin–Barth one-equation turbulence model are used with adequate grid resolution.

Effect of Film-Hole Shape on Turbine-Blade Heat-Transfer Coefficient Distribution

Detailed heat-transfer coefe cient distributions on the suction side of a gas turbine blade were measured using a transient liquid crystal image method. The blade has only one row of e lm holes near the gill-hole portion on the suction side of the blade. Studies on three different kinds of e lm-cooling hole shapes were presented. The hole geometriesstudied includestandard cylindricalholesand holeswith adiffuser-shaped exitportion (i.e., fan-shaped holes and laidback fan-shaped holes ). Tests were performed on a e ve-blade linear cascade in a low-speed wind tunnel. The mainstream Reynolds number based on the cascade exit velocity was 5 :3 ££ 10 5 . Upstream unsteady wakes were simulated using a spoke-wheel-type wake generator. The wake Strouhal number was kept at 0 and 0.1. The coolant-to-mainstream blowing ratio was varied from 0.4 to 1.2. The results show that unsteady wake generally tends to induce earlier boundary-layer transition and enhance the surface heat-transfer coefe cients. When compared to the cylindrical hole case, both the expanded hole injections have much lower heat-transfer coefe cients over the surface downstream of the injection location, particularly at high blowing ratios. However, the expanded hole injections induce earlier boundary-layer transition to turbulence and enhance heat-transfer coefe cients at the latter part of the blade suction surface.

Effect of Film-Hole Shape on Turbine-Blade Film-Cooling Performance

The detailed heat transfer coefficient and film cooling effectiveness distributions as well as tile detailed coolant jet temperature profiles on the suction side of a gas turbine blade A,ere measured using a transient liquid crystal image method and a traversing cold wire and a traversing thermocouple probe, respectively. The blade has only one row of film holes near the gill hole portion on the suction side of the blade. The hole geometries studied include standard cylindrical holes and holes with diffuser shaped exit portion (i.e. fanshaped holes and laidback fanshaped holes). Tests were performed on a five-blade linear cascade in a low-speed wind tunnel. The mainstream Reynolds number based on cascade exit velocity was 5.3 x 10(exp 5). Upstream unsteady wakes were simulated using a spoke-wheel type wake generator. The wake Strouhal number was kept at 0 or 0.1. Coolant blowing ratio was varied from 0.4 to 1.2. Results show that both expanded holes have significantly improved thermal protection over the surface downstream of the ejection location, particularly at high blowing ratios. However, the expanded hole injections induce earlier boundary layer transition to turbulence and enhance heat transfer coefficients at the latter part of the blade suction surface. In general, the unsteady wake tends to reduce film cooling effectiveness.

A study of boundary layer-wake interaction in shallow open channel flows

This paper describes an experimental investigation of the interaction between the boundary layer on a horizontal floor of a shallow open channel flow and the wake of a thin flat plate mounted vertically on the floor of the channel. The nominal thickness of the flat plate was limited to 2 mm in order to minimize the effect of the flume side walls on the generated wake. Two flat plate chord to thickness ratios (10 and 25) and two depths of flow (50 and 80 mm) were considered. The boundary layer thickness of the approaching flow was comparable with the depth of flow. The recovery of the boundary layer is then studied by observing the characteristics of the velocity profile downstream of the flat plate along the wake axis. The results indicate that the recovery process is slow, and that it is clearly non-monotonic. When compared with the approaching flow, the streamwise turbulence intensity values increase in the near-wake region followed by a gradual but slow decrease with increasing axial distance. Neither mean nor higher-order moments indicate a complete recovery even at large distances from the wake generator. The present results also indicate that the inner region appears to develop more quickly than the outer flow. Since the development of the quasi-two-dimensional wake is also of interest, velocity measurements are also presented along the wake cross-section. These velocity profiles indicate that the wake effects are still prevalent at 200 plate widths downstream of the wake generator. Through a proper choice of scaling variables, the mean velocity profiles across the wake can be collapsed onto a single curve, indicating a sense of similarity.

Shallow wakes generated on smooth and rough surfaces

This study deals with the behavior of shallow turbulent wakes generated on smooth and rough surfaces. The wake generator used is a flat plate placed normal to the flow. Experiments were conducted at flow depths of 40 and 80 mm. The boundary layer thickness in the approaching flow occupies 60–75% of the flow depth. The Reynolds number based on the plate width and approaching freestream velocity varies from 13.0 × 103 to 14.5 × 103. Velocity measurements were carried out in the near-wake region (1–10 plate widths) using a laser-Doppler anemometer. The mean velocity distributions at various axial stations collapse onto a single curve by a proper choice of the length and velocity scales. It is important to note that a sense of self-similarity is attained even in the near-wake region. Attempts were made to clarify the relative effects of the transverse shear and bed friction in shallow open channel wakes.

Microinjection of glutamate into the pedunculopontine tegmentum induces REM sleep and wakefulness in the rat.

The aim of this study was to test the hypothesis that the cells in the brain stem pedunculopontine tegmentum (PPT) are critically involved in the normal regulation of wakefulness and rapid eye movement (REM) sleep. To test this hypothesis, one of four different doses of the excitatory amino acid L-glutamate (15, 30, 60, and 90 ng) or saline (control vehicle) was microinjected unilaterally into the PPT while the effects on wakefulness and sleep were quantified in freely moving chronically instrumented rats. All microinjections were made during wakefulness and were followed by 6 h of polygraphic recording. Microinjection of 15- ng (0.08 nmol) and 30-ng (0.16 nmol) doses of L-glutamate into the PPT increased the total amount of REM sleep. Both doses of L-glutamate increased REM sleep at the expense of slow-wave sleep (SWS) but not wakefulness. Interestingly, the 60-ng (0.32 nmol) dose of L-glutamate increased both REM sleep and wakefulness. The total increase in REM sleep after the 60-ng dose of L-glutamate was significantly less than the increase from the 30-ng dose. The 90-ng (0.48 nmol) dose of L-glutamate kept animals awake for 2-3 h by eliminating both SWS and REM sleep. These results show that the L-glutamate microinjection into the PPT can increase wakefulness and/or REM sleep depending on the dosage. These findings support the hypothesis that excitation of the PPT cells is causal to the generation of wakefulness and REM sleep in the rat. In addition, the results of this study led to the identification of the PPT dosage of L-glutamate that optimally induces wakefulness and REM sleep. The knowledge of this optimal dose will be useful in future studies investigating the second messenger systems involved in the regulation of wakefulness and REM sleep.

New aspects of wake field generation in plasmas

We consider various properties of wake field generation in plasmas. In particular the influence of an external magnetic field is studied. It turns out that the magnetic field makes it possible for the wake field to propagate out of the plasma, and thereby provide information about the electron density profile. Furthermore, an example where a dust-acoustic wake field is excited by an ion-acoustic pulse is considered, as a means to illustrate general principles for wake field generation.

Studies on Development of Wake Flow and Generation of Unsteady Hydrodynamic Force

Studies on Development of Wake Flow and Generation of Unsteady Hydrodynamic Force

Detailed Heat Transfer Coefficient Distributions on a Large-Scale Gas Turbine Blade Tip

Measurements of detailed heat transfer coefficient distributions on a turbine blade tip were performed in a large-scale, low-speed wind tunnel facility. Tests were made on a five-blade linear cascade. The low-speed wind tunnel is designed to accommodate the 107.49 deg turn of the blade cascade. The mainstream Reynolds number based on cascade exit velocity was 5.3×105. Upstream unsteady wakes were simulated using a spoke-wheel type wake generator. The wake Strouhal number was kept at 0 or 0.1. The central blade had a variable tip gap clearance. Measurements were made at three different tip gap clearances of about 1.1 percent, 2.1 percent, and 3 percent of the blade span. Static pressure distributions were measured in the blade mid-span and on the shroud surface. Detailed heat transfer coefficient distributions were measured on the blade tip surface using a transient liquid crystal technique. Results show that reduced tip clearance leads to reduced heat transfer coefficient over the blade tip surface. Results also show that reduced tip clearance tends to weaken the unsteady wake effect on blade tip heat transfer.

Acceleration and compression of charged particle bunches using counterpropagating laser beams

The nonlinear interaction between counterpropagating laser beams in a plasma results in the generation of large (enhanced) plasma wakes. The two beams need to be slightly detuned in frequency, and one of them has to be ultrashort (shorter than a plasma period). Thus produced wakes have a phase velocity close to the speed of light and can be used for acceleration and compression of charged bunches, The physical mechanism responsible for the enhanced wake generation is qualitatively described and compared with the conventional laser wakefield mechanism. We also demonstrate that depending on the sign of the frequency difference between the lasers, the enhanced wake can be used as a "snow-plow" to accelerate and compress either positively or negatively charged bunches. This ability can be used in an electron-positron injector.

The Use of Hot-Wire Anemometry to Investigate Unsteady Wake-Induced Boundary-Layer Development on a High-Lift LP Turbine Cascade

Recent research has revealed positive effects of unsteady flow on the development of boundary layers in turbine cascades, especially at conditions with a laminar suction side separation bubble at low Reynolds numbers. Compared to steady flow, a reduction of total pressure loss coefficient over a broad range of Reynolds numbers has been shown. Taking into account the positive effects of wake-induced transition already during the design process, new high lift bladings with nearly the same low losses at unsteady inlet flow conditions could be achieved. This leads to a reduction of weight and cost of the whole turbine module for a constant stage loading. Unsteady flow in turbomachines is caused by the relative motion of rotor and stator rows. For simulating a moving blade row upstream of a linear cascade in the High-Speed Cascade Wind Tunnel of the Universita¨t der Bundeswehr Mu¨nchen, a wake generator has been designed and built. The wakes are generated with bars, moving with a velocity of up to 40 m/s in the test section upstream of the cascade inlet plane. Unsteady flow causes the transition on the surface of the suction side of a low-pressure turbine blade to move upstream whenever an incoming wake is present on the surface; moreover, a laminar separation bubble can be diminished or even suppressed. In order to detect the effects of wakes on the boundary layer development a new hot wire data acquisition system is required. Due to the fact that hot wires give a good insight into boundary layer development, a new hot-wire data acquisition system has been set up. The anemometry system can acquire four channels simultaneously, therefore being capable of logging a triple hot-wire sensor and a bar trigger simultaneously. One further channel is utilized for a once-per-revolution trigger. The once-per-revolution trigger is used to start the measurement of one data block. Using the well-established ensemble-averaging technique, 300 ensembles each consisting of five wake passing periods have been acquired. Ensemble averaging can be directly performed without any data reduction. The adaptation of this new hot-wire anemometry data acquisition system to the High-Speed Cascade Wind Tunnel of the Universita¨t der Bundeswehr Mu¨nchen is pointed out. First, results on unsteady periodic boundary layer development of a highly loaded low-pressure turbine cascade under unsteady inlet flow conditions are presented. During the present investigation four boundary layer traverses, ranging from x/lax=0.82 to x/lax=0.99 (suction side), at steady and unsteady inlet flow conditions Ubar=10 m/s at an outlet Reynolds number of Re2th=100,000 have been conducted. [S0889-504X(00)00204-X]