Boundary Layer Separation Zone Research Articles

Study on the Design Method of a New Type of Compressor Slotted Blade

the compressor has been developing towards the direction of high pressure ratio, high efficiency and wide stability margin. The flow in the air compressor blade channel presents a strong inverse pressure gradient. In order to reduce losses and improve the stability of the pressure aircraft, this paper adopts a slot structure from the leading edge of the blade to the suction surface. Air is injected from the front edge of the blade and discharged from the suction surface. The automatic optimization method is used to design the slot. The results show that the blade profile with inlet Mach number 0.4 and the diffusion factor 0.6, the total pressure loss of slotted blade profile is lower than that of unslotted blade profile, and the airflow Angle is greater than that of unslotted blade profile in the whole range of low loss incident Angle. In the incidence of 0 degree, due to the control effect of slotted jet on the boundary layer of the blade suction surface, the boundary layer separation zones on the blade suction surface becomes smaller and the wake also decreases, so the total pressure loss of the slotted blade profile decreases by 37%. When the incidence angle is 4°, the separation area of the suction surface boundary layer is larger, and the slotted jet has a better control effect on the boundary layer of the blade suction surface. However, When the incidence angle is -6°, boundary layer separation zones on the suction surface becomes smaller, and the boundary layer separation zones appears on the pressure surface, so the slotting effect is poor.

Spatial coherence of low-frequency unsteadiness associated with a normal shock wave

The present data illustrate a unique analysis approach to explore the physics of shock-wave-flow interactions, based upon correlation and spectral analysis of digitized shadowgraph visualization time-sequence data. As such, demonstration is provided of the use of high resolution, flow visualization data for quantitative correlation and spectral analysis. A shadowgraph system is employed to visualize time-varying, shock wave flow features within the test section of one leg of the transonic/supersonic wind tunnel (which is also referred to as the SS/TS/WT or SuperSonic/TranSonic/WindTunnel), located within the Propulsion Research Center of the University of Alabama in Huntsville. Of interest is a flow field with a well-defined normal shock wave, lambda foot, and separated turbulent boundary layer near the entrance of the lower flow passage (which is part of the test section), produced with a test section inlet Mach number of 1.54. Shadowgraph flow visualization images are processed from different pixel locations to compute frequency spectra. Data associated with two separate pixel region locations, each from a different flow region, are employed to determine magnitude squared coherence values, and associated time lag magnitudes. The data associated with lower Strouhal numbers in vicinity of 0.0013 and 0.0039 (which correspond to respective frequencies of 5 Hz and 15 Hz) illustrate important and pronounced interactions between the normal shock wave and the boundary layer separation zone. As this zone breathes and oscillates, additional flow locations are affected in a subsequent and significant manner. With Strouhal numbers in the vicinity of 0.0091, 0.0104, and 0.0260 (which correspond to respective frequencies of 35, 40, and 100 Hz), the most pronounced and significant interactions between the normal shock wave occur with respect to the downstream boundary layer. When Str equals 0.0091, spatially-varying time lag data show events with significant coherence propagate from a range of locations within the downstream boundary layer to the normal shock wave location. As such, events which originate within the downstream boundary layer at these experimental conditions are far more important (compared to events which originate with the upstream boundary layer) in regard to their effects on shock wave unsteadiness and associated flow motions.

Change in Separation Flow Regimes over Obstacles in Subsonic Gas Flow as a Manifestation of Viscous Forces: Numerical Results

The unsteady subsonic viscous gas flows over forward- and backward-facing steps on a flat surface are numerically simulated at a free-stream Mach number of 0.1 by applying compact difference schemes of the 14th order of accuracy. The characteristics of the boundary layer separation zones are investigated at Reynolds numbers ranging from 103 to 107. The computations are based on the nonstationary Navier–Stokes equations supplemented, if necessary, with the formulas of the one-parameter differential SA model of turbulent viscosity. The conditions for pulsing separation zones arising in the boundary layer are determined, and the subsequent transition to a turbulent flow regime is physically justified.

Visualization of the shock wave diffraction on the wedge in a rectangular channel

The evolution of a shock wave interaction with a boundary layer on the surface of a shock tube channel after its reflection from a thin wedge is investigated. Visualization of the incident plane shock wave, the bow detached shock wave and flow behind the wedge in 48x24 mm channel was performed using the shadow method based on high-speed recording. The zone of boundary layer separation that occurs when the plane attached bow shock intersects with the channel wall is visualized on the basis of the pulse discharge glow recording. The flow evolution at incident shock wave Mach number of 2.3÷3.8 for the time interval up to 5 ms after the diffraction is investigated. The obtained data is compared with the corresponding CFD results: a numerical simulation based on the Euler equations of a nonstationary two-dimensional flow was performed under experimental conditions.

Combustion stabilization modes in a hydrogen-fueled scramjet combustor at high stagnation temperature

Combustion stabilization modes were studied experimentally under a Mach 2.52 supersonic inflow at a high stagnation temperature of 1629 K. Optical observations including the high-speed flame luminosity and schlieren images were utilized to capture the combustion stabilization characteristics. Effects of the equivalence ratio and the injection distance on the combustion stabilization mode transition were discussed. Two supersonic combustion stabilization modes are identified clearly in the scramjet combustor, as the cavity stabilized mode and the jet-wake stabilized mode. At a relatively low equivalence ratio, the flame is stabilized in the cavity stabilized mode, and it will transfer to the jet-wake stabilized mode with the increase of the equivalence ratio. The flame stabilization mode is also affected by the injection distance. The critical equivalence ratio for mode transition reduces with the decrease of the injection distance. In the cavity stabilized mode, the shear layer of the cavity stabilizes the flame. In the jet-wake stabilized mode, the intense combustion induces high pressure, and leads to a large boundary-layer separation zone that exists in front of the flame. It contributes to the flame stabilization. In addition, a triple-reaction-zone mechanism is proposed to further explain the transition process from the cavity stabilized mode to the jet-wake stabilized mode, and the large boundary-layer separation zone also contributes to the transition process.

Investigation on axial effect of slot casing treatment in a transonic compressor

This paper reports the effect of the axial position of the slot casing treatment on the performance of transonic compressor NASA Rotor 67 by unsteady numerical simulation. The interaction of the recirculation in the slots and flow near the blade tip is analyzed to understand the flow mechanisms. The relative importance of the mechanisms of stall margin improvement due to the slot casing treatment is evaluated with the relative weight method. The results show that the bleeding and injecting effect caused by the recirculation is the most important factor that affects the blockage in the blade tip region, which determines the stall margin improvement. When the slots cover the initial position of the tip leakage vortex (TLV) and the boundary layer separation zone downstream the shock, the recirculation is stronger due to the larger pressure difference between the front and rear part of the slots. Consequently, the slot casing treatment can reduce the blockage near the casing more effectively, which results in a larger stall margin improvement. Shifting the slots upstream or downstream will reduce the driving force for the recirculation and the effect extent on the low energy fluid of the slots, which decreases the stall margin improvement.

Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part II: Numerical Modeling

Abstract Mountain waves and rotors in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part paper. Part I by French et al. delivers a detailed observational account of two rotor events: one displays characteristics of a hydraulic jump and the other displays characteristics of a classic lee-wave rotor. In Part II, presented here, results of high-resolution numerical simulations are conveyed and physical processes involved in the formation and dynamical evolution of these two rotor events are examined. The simulation results reveal that the origin of the observed rotors lies in boundary layer separation, induced by wave perturbations whose amplitudes reach maxima at or near the mountain top. An undular hydraulic jump that gave rise to a rotor in one of these events was found to be triggered by midtropospheric wave breaking and an ensuing strong downslope windstorm. Lee waves spawning rotors developed under conditions favoring wave energy trapping at low levels in different phases of these two events. The upstream shift of the boundary layer separation zone, documented to occur over a relatively short period of time in both events, is shown to be the manifestation of a transition in flow regimes, from downslope windstorms to trapped lee waves, in response to a rapid change in the upstream environment, related to the passage of a short-wave synoptic disturbance aloft. The model results also suggest that the secondary obstacles surrounding the Medicine Bow Mountains play a role in the dynamics of wave and rotor events by promoting lee-wave resonance in the complex terrain of southeastern Wyoming.

Conical gas flows with shock waves and turbulent boundary layer separation

The results of a theoretical and experimental investigation of nonsymmetric flow around a V-wing with supersonic leading edges are presented. The range of the angles of attack and yaw, on which additional singular lines are formed on the windward cantilever of the wing, are experimentally determined using different techniques of flow diagnostics. These are one convergence line and two divergence lines in the transverse flow which were not previously observable in the calculations of ideal-gas flow around wings. It is established that the appearance of the three new alternating singular lines located between the central chord of the wing and a convergence line, exterior to them and occurring within the framework of the ideal gas model, is associated with the relation between the intensities of two contact discontinuities. One of these proceeds from the branching point of the bow shock above the leeward cantilever, while the second issues out of the triple point of a λ-shaped shock configuration accompanying developed turbulent-boundary-layer separation generated by an internal shock incident on the leeward cantilever surface. If the intensity of the contact discontinuity proceeding from the branching point of the bow shock is large as compared with that of the contact discontinuity of the λ-configuration, then the flow pattern realized on the windward cantilever is analogous to that obtained within the framework of the ideal gas model, that is, it includes one convergence line on the wing surface. Under these conditions, the results of the calculations within the framework of the ideal gas model are applicable for understanding the phenomena occurring in the wing shock layer in a considerable part of the control parameter range, including the regimes with intense internal shocks generating turbulent boundary layer separation from the leeward cantilever. Corrections should be made only for a carachteristic pressure distribution in the separation zone and, as a consequence of separation, for an elevated pressure level in the vicinity of the central chord which is the stagnation line of the transverse flow that has passed across the oblique and terminating shocks of the λ-configuration and possesses a higher stagnation pressure than the flow that has passed in an ideal gas across the internal shock incident normally on the leeward cantilever. This is possible only when the divergence line, at which the stream surface enclosing the turbulent boundary layer separation zone enters, does not go over from the leeward onto the windward cantilever.

CFD Simulation of a Novel Bileaflet Mechanical Heart Valve Prosthesis - An Estimation of the Venturi Passage Formed by the Leaflets

The aim of this study was to validate the flow characteristics of the novel Helmholtz-Institute Aachen Bileaflet (HIA-BL) heart valve prosthesis. The curved leaflets of the HIA-BL valve form a Venturi passage between the leaflets at peak systole. By narrowing the cross section the flow accelerates and the static pressure at the central passage decreases according to the Venturi effect. The low-pressure zone between the leaflets is expected to stabilize the leaflets in fully open position at peak systole. To investigate the Venturi passage, the flow fields of two valve geometries were investigated by CFD (Computational Fluid Dynamics): one geometry exhibits curved leaflets resulting in a Venturi passage; the other geometry features straight leaflets. The flow profiles, pressure distribution and resulting torque of both passages were compared and investigated. Although flow profiles downstream of both valves were similar, the flow passages between the leaflets were different for the investigated leaflet geometries. The straight leaflet passage showed a large boundary layer separation zone near the leaflets and the lowest pressure at the leading edge of the leaflet. The Venturi passage showed a reduction of the boundary layer separation zones and the lowest pressure between the leaflets could be found in the narrowest flow cross section of the Venturi passage. Additionally, the resulting torque showed that the Venturi passage produced an opening momentum. The results demonstrate that the Venturi passage stabilizes the leaflets in open position at peak systole.

New Timber Kiln Designs for Promoting Uniform Airflows Within the Wood Stack

Two geometrically innovative timber kilns have been designed, constructed, and subsequently tested for airflow uniformity within the stacked wood. The roofs of both kiln configurations were barrel-shaped rather than the traditional peak or flat-top shaped. The first configuration employed outward-swinging overhead baffles, which is the arrangement found within most commercial kilns. The second configuration had inward-swinging overhead baffles and contoured right-angled bends to the plenum chambers from the ceiling space, where the fans were located. Velocity measurements down the height of the timber stack showed that the uppermost packs in the kiln with outward-swinging baffles were starved of airflow. Flow visualizations indicated that the blocking action of a recirculation zone or a boundary-layer separation zone in the inlet plenum chamber was responsible for the reduction in flow through these packs. A similar phenomenon was shown to occur in a traditional kiln with a peak-shaped roof and outward-swinging baffles. The velocity range in the contoured kiln with inward-swinging baffles was reduced by a factor of three, and no recirculation zones appeared in the plenum chambers. Thus, the shape of the roof has very little influence on the uniformity of flow within the timber stack, although significant savings in pressure losses through the kiln are possible by employing a barrel-shaped roof. Airflow uniformity is improved by contouring of the inward edges of the right-angled bends from the ceiling space to the plenum chambers. The contoured configuration is currently operating commercially at a number of sites in New Zealand.

Steady flow visualization in a rigid model of the aortic bifurcation: application to atherosclerosis.

Hemodynamics have long been implicated in atherogenesis. The studiesreported here seek to explain the mechanisms for the formation ofatherosclerotic plaque in an aortic bifurcation. Flow studies were made ina model constructed from plexiglass to represent an aortic bifurcation. Under steady flow conditions at inflow Reynolds numbers of 80-1250,the streamline flow patterns and the boundary layer separation zones wereinvestigated in relation to the location of atherosclerotic plaques clinicallyfound at regions in the human aortic bifurcation. The streamline flowswere visualized by a slow injection of dye over the cross section of the tubeentrance and along the tube walls. The studies revealed a complex flowfield where secondary flows, induced by the centrifugal and viscous forces,cause the fluid to move towards the inner walls of the aortic bifurcation. The effect was more clearly seen with increasing Reynolds number. Boundary layer separation zones were observed to occur at the outercorners of the branching. The nature of the separation zone formed wasfound to be dependent on Reynolds number. The residence time of fluidparticles within such a separation zone was estimated by measuring thewashout time of a bolus of dye injected at strategic locations along the tubewalls. The residence time was found to decrease exponentially withincreasing Reynolds number. These observations provide strong support forthe role of flow separation in the accumulation of LDL and plateletaggregation within the aortic bifurcation.

A vorticity dynamics theory of three-dimensional flow separation

A theory of three-dimensional incompressible flow separation is presented in terms of the on-wall signatures of the flow. Some long-standing controversial issues are revisited and answers given, such as the inconsistency of the separation criteria based on the topological theory and “open separation,” and whether a separation line is an asymptote or envelope of neighboring skin-friction lines. General criteria for identifying an “open” or “closed” flow separation zone and separation line (including the initial point of the latter), steady and unsteady, are obtained, which apply to a generic smooth curved wall at any Reynolds numbers. The criteria are found to be most clearly given in terms of on-wall signatures of vorticity dynamics. These are then specified to steady boundary layer separation at large Reynolds numbers. A scale analysis under mild assumptions leads to a three-dimensional triple-deck structure near a generic boundary layer separation line. Criteria are presented for “separation watch,” which tells that a boundary-layer may soon break away, and for “separation warning,” which identifies the vorticity characteristics in an already formed boundary-layer separation zone and along a boundary-layer separation line. Flow behavior and its dependence on outer flow conditions are examined qualitatively. A numerical example is given which confirms the predictions of the theory.

Heat Transfer in the Zone of Boundary Layer Separation

Heat Transfer in the Zone of Boundary Layer Separation

A laboratory study of fluid flow and microhabitat selection by larvae of Simulium vittatum (Diptera: Simuliidae)

Microhabitat selection by Simulium vittatum Zetterstedt larvae in a flume was studied at different mainstream velocities on two substrates: a thin flat plate parallel to the flow and a cylinder in cross flow. The results do not support the generally accepted assumptions that simuliid larvae keep within the boundary layer to avoid the direct influence of mainstream current and that they select the fastest velocity available when offered a longitudinal velocity gradient within their tolerance range. Instead, larvae gathered along the zone of boundary layer separation and remained along the stagnation line at the leading point of the cylinder when artificially positioned there. Further, under most conditions, larvae avoided zones of maximum surface shear stress. Larval reaction to hydraulic changes was immediate. It is hypothesized that S. vittatum larvae first scan the velocity profile at the substrate, initially moving toward increasing flow velocity (or water acceleration). They then cue on a steep velocity gradient along the body as part of the processes involved in choosing a location for suspension feeding. Such conditions would maximize particle flux through the labral fans and minimize drag forces on the bulbous posterior abdomen. This study provides the first direct evidence for microhydraulic factors as causative agents in the formation of simuliid larval assemblages.

Flow instability in the laminar boundary layer separation zone created by a small roughness element

The development of disturbances of the laminar flow in the separation zone behind a surface projection in the boundary layer on a flat plate has been experimentally investigated. The linear instability characteristics of the separated flow are determined and the interaction between the oscillations growing in the separation zone and the average flow is studied.

Diagnostic significance of flow separation in the carotid bulb.

Pulsatile blood flow within the normal carotid sinus involves at least two distinct components. That near the flow divider is laminar and antegrade, whereas a boundary layer separation zone in the posterolateral aspect exhibits transient blood flow reversal. It is now possible to document these flow velocity components using pulsed Doppler ultrasound methods. When atherosclerosis develops, it preferentially involves the posterolateral bulb region, obliterating the normal configuration of the sinus with consequent loss of the flow separation zone. It was therefore hypothesized that if flow separation could be detected, it should be predictive of a normal angiogram. To assess this, we evaluated 20 symptomatic patients and two with only bruits found by duplex scanning to have flow separation in either one or both carotid bulbs and who also underwent cerebral angiography. Initial diagnoses were stroke in seven, reversible ischemic neurologic deficit in one, transient ischemic attack in 12, and bruit in two. Flow separation was bilateral in 13 patients (59%). There were 15 patients with symptoms in the territory of a carotid bulb exhibiting flow separation. By angiography, of the 35 bulbs with boundary layer separation, 27 (77%) were normal, with the remainder showing lesions that reduced the diameter of the vessel by 20% or less. Final diagnoses of the 15 patients with symptoms ipsilateral to a carotid sinus exhibiting flow separation were fibromuscular disease in two, lacunar stroke in three, dissection in two, subclavian steal in one, cardiogenic embolus in three, migraine in one, hyperventilation syndrome in one, kink of the mid-internal carotid artery in one, and no diagnosis in one.(ABSTRACT TRUNCATED AT 250 WORDS)

On the possibility of blowing a gas jet into a supersonic flow without formation of a three-dimensional boundary-layer separation zone

We consider the flow formed by the interaction of a supersonic flow and a transverse sonic or supersonic jet blown at right angles to the direction of the main flow through a nozzle whose exit section is in a flat wall. When a gas jet is blown through a circular opening [1] the pressure rises in front of the jet because of the stagnation of the oncoming flow. This leads to separation of the boundary layer formed on the wall in front of the blowing nozzle. The resulting three-dimensional separation zone leads to a sharp increase in the pressure and the heat fluxes to the wall in front of the blowing nozzle, which is undesirable in many modern applications. The aim of the present investigation was to find a shape of the exit section of the blowing nozzle for which there is no three-dimensional separation zone of the boundary layer in front of the blowing nozzle.

Effect of Injector Shape on Penetration and Spread of Liquid Jets

L injection into a supersonic air stream finds applications in supersonic combustion ramjets (scramjets), transpiration cooling of re-entry bodies and thrust vector control of rockets. The injector geometry will have a significant role in liquid injection applications. The injection of liquid into a supersonic air stream produces an interaction shock and a freestream boundary-layer separation zone upstream of the injector. The separation zone plays an important role during combustion due to the high rate of heat transfer to the wall in this region. The shock system associated with each injector in a practical supersonic combustor has two important effects: 1) it reduces the total pressure of the freestream and thus adversely affects the overall performance of the engine; 2) static temperature and pressure of the freestream rise through the injector shock system thus creating better conditions from the viewpoint of chemical reaction rates. The shape and strength of the shock also affect the forces on the liquid column and thus penetration. In general, the shock system is a strong function of injector geometry. The influence of injector geometry on penetration and spread of the jet were given major emphasis in the present investigation. The purpose was to obtain experimental penetration and spread data suitable for engineering use and to seek theoretical correlations incorporating and governing parameters. The motivation for the present work comes from the work of Kush and Schetz who observed that a liquid jet through a rectangular slot aligned with the flow gives significantly higher penetration than through a circular hole of the same area.

Experimental investigation of the heat exchange in separation zones ahead of cylindrical obstacles

Results of an experimental investigation of the heat exchange in turbulent boundary layer separation zones ahead of cylindrical obstacles at a subsonic air stream velocity are elucidated. The investigation was conducted for changes in the ratio between the obstacle diameter and altitude between 0.25 and 4, between the boundary layer thickness at the separation point and the obstacle altitude between 0.09 and 0.7, between the Reynolds number computed by means of the free stream parameters and the obstacle height between 104 and 4·105. The Mach number reached 0.85. The temperature factor was 0.7. It is shown that the distribution of the heat transfer coefficients in the separation zone depends on the Reynolds and Euler numbers, the ratio between the boundary layer displacement thickness and the diameter (or altitude) of the obstacle, and the ratio between the diameter and the altitude. Criterial dependences are obtained which extend the heat-exchange results at characteristic points of the separation zones, as are also dimensionless distributions of the heat transfer coefficients to determine the heat fluxes on a plate in the plane of symmetry of the separation zone ahead of obstacles.

Effect of three-dimensional flow on limiting pressure differential for interaction of boundary layer with shock waves

In the interaction of shock waves with a boundary layer there is a limiting value of the shock wave intensity which may be sustained by the boundary layer without separation. If the intensity of the shock wave is greater than the limiting value, a boundary layer separation zone is formed, and the flow rearranges itself so that the intensity of the shock wave 2 ahead of the separation zone (Fig. 1) is equal to the limiting value. In the study of the separation of the two-dimensional boundary layer the relations $$Cp = \frac{{p_2 - p_1 }}{{{\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}\rho _1 u_1 ^2 }} = /\left( {R, M} \right)$$ are obtained for the laminar and turbulent boundary layers. Here p1 and p2 are the static pressures ahead of and behind the shock, ρ1 and u1 are the density and velocity in the approaching flow, M is the Mach number, R is the Reynolds number. In the present paper we consider the effect of flow three-dimensionality on the value of Cp. We shall limit ourselves to consideration of three-dimensional flow on lines of convergence and lines of divergence, although the conclusions are qualitatively valid in the more general case.