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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Nozzle Modification Suppresses Flow Transients

Proposal for steadying flow from rocket nozzle on Space Shuttle main engine could be applied to other large-area-ratio contoured nozzles. Oscillations and pulsations in boundary-layer flow would be reduced by flaring nozzle exit. Transient side loads on nozzle would be suppressed. Large pressure gradients and eddies that can lead to fluctuations are suppressed. Exact radius of curvature of corner would depend on shear-layer thickness.

Rao, G. V. R.↗
NASA NTRS1954

Smoke Study of Nozzle Secondary Flows in a Low-speed Turbine

Still and motion pictures were made of boundary-layer and wake secondary-flow phenomena visualized by smoke. Two annular cascades of turbine nozzles were used, both designed for constant discharge angle but differing in blade shape and suction-surface velocity distribution. Flows were similar to those obtained with pressure and angle measurements at near-sonic airspeeds. Boundary-layer cross-channel and trailing-edge radial flows caused vortices and an accumulation of low-momentum air at the hub, which may affect flow in following blade rows. Motion of a downstream rotor blade row produced pulsations in trailing-edge radial flow. The motion-picture supplement may be obtained on loan from NACA Headquarters, Washington, D.C.

Kofskey, Milton G↗
NASA NTRS2004

A Resonant Pulse Detonation Actuator for High-Speed Boundary Layer Separation Control

A variety of different types of actuators have been previously investigated as flow control devices. Potential applications include the control of boundary layer separation in external flows, as well as jet engine inlet and diffuser flow control. The operating principles for such devices are typically based on either mechanical deflection of control surfaces (which include MEMS flap devices), mass injection (which includes combustion driven jet actuators), or through the use of synthetic jets (diaphragm devices which produce a pulsating jet with no net mass flow). This paper introduces some of the initial flow visualization work related to the development of a relatively new type of combustion-driven jet actuator that has been proposed based on a pulse detonation principle. The device is designed to utilize localized detonation of a premixed fuel (Hydrogen)-air mixture to periodically inject a jet of gas transversely into the primary flow. Initial testing with airflow successfully demonstrated resonant conditions within the range of acoustic frequencies expected for the design. Schlieren visualization of the pulsating air jet structure revealed axially symmetric vortex flow, along with the formation of shocks. Flow visualization of the first successful sustained oscillation condition is also demonstrated for one configuration of the current test section. Future testing will explore in more detail the onset of resonant combustion and the approach to conditions of sustained resonant detonation.

Beck, B. T.↗
NASA NTRS1999

The Three-D Flow Structures of Gas and Liquid Generated by a Spreading Flame Over Liquid Fuel

We developed a new experimental technique called: Combined laser sheet particle tracking (LSPT) and laser holographic interferometry (HI), which is capable of measuring the transient behavior of three dimensional structures of temperature and flow both in liquid and gas phases. We applied this technique to a pulsating flame spread over n-butanol. We found a twin vortex flow both on the liquid surface and deep in the liquid a few mm below the surface and a twin vortex flow in the gas phase. The first twin vortex flow at the liquid surface was observed previously by NASA Lewis researchers, while the last two observations are new. These observations revealed that the convective flow structure ahead of the flame leading edge is three dimensional in nature and the pulsating spread is controlled by the convective flow of both liquid and gas.

Tashtoush, G.↗
NASA NTRS1990

Boundary layer receptivity to unsteady free-stream pressure gradients

A linear triple-deck theory is applied to an examination of the response of a boundary layer to short-scale variations in an unsteady free-stream disturbance field. Two-dimensional incompressible flows are considered, and a locally-parallel Blasius mean flow is assumed. A simple pulsating pressure source and a traveling pressure field in the free-stream are modeled by introducing appropriate pressure sources in the upper deck of the triple-deck structure. The modification in unsteady thickness is obtained for these cases, the results are related to the Tollmien-Schlichting instability wave, and the generation of unstable Tollmien-Schlichting waves for both experiments modeled is confirmed.

Heinrich, Roland A. E.↗
NASA NTRS2003

Computational And Experimental Studies Of Three-Dimensional Flame Spread Over Liquid Fuel Pools

Schiller, Ross, and Sirignano (1996) studied ignition and flame spread above liquid fuels initially below the flashpoint temperature by using a two-dimensional computational fluid dynamics code that solves the coupled equations of both the gas and the liquid phases. Pulsating flame spread was attributed to the establishment of a gas-phase recirculation cell that forms just ahead of the flame leading edge because of the opposing effect of buoyancy-driven flow in the gas phase and the thermocapillary-driven flow in the liquid phase. Schiller and Sirignano (1996) extended the same study to include flame spread with forced opposed flow in the gas phase. A transitional flow velocity was found above which an originally uniform spreading flame pulsates. The same type of gas-phase recirculation cell caused by the combination of forced opposed flow, buoyancy-driven flow, and thermocapillary-driven concurrent flow was responsible for the pulsating flame spread. Ross and Miller (1998) and Miller and Ross (1998) performed experimental work that corroborates the computational findings of Schiller, Ross, and Sirignano (1996) and Schiller and Sirignano (1996). Cai, Liu, and Sirignano (2002) developed a more comprehensive three-dimensional model and computer code for the flame spread problem. Many improvements in modeling and numerical algorithms were incorporated in the three-dimensional model. Pools of finite width and length were studied in air channels of prescribed height and width. Significant three-dimensional effects around and along the pool edge were observed. The same three-dimensional code is used to study the detailed effects of pool depth, pool width, opposed air flow velocity, and different levels of air oxygen concentration (Cai, Liu, and Sirignano, 2003). Significant three-dimensional effects showing an unsteady wavy flame front for cases of wide pool width are found for the first time in computation, after being noted previously by experimental observers (Ross and Miller, 1999). Regions of uniform and pulsating flame spread are mapped for the flow conditions of pool depth, opposed flow velocity, initial pool temperature, and air oxygen concentration under both normal and microgravity conditions. Details can be found in Cai et al. (2002, 2003). Experimental results recently performed at NASA Glenn of flame spread across a wide, shallow pool as a function of liquid temperature are also presented here.

Ross, Howard D.↗
NASA NTRS1979

First results of a study on turbulent boundary layers in oscillating flow with a mean adverse pressure gradient

The development of a turbulent unsteady boundary layer with a mean pressure gradient strong enough to induce separation, in order to complete the extend results obtained for the flat plate configuration is presented. The longitudinal component of the velocity is measured using constant temperature hot wire anemometer. The region where negative velocities exist is investigated with a laser Doppler velocimeter system with BRAGG cells. The boundary layer responds by forced pulsation to the perturbation of potential flow. The unsteady effects observed are very important. The average location of the zero skin friction point moves periodically at the perturbation frequency. Average velocity profiles from different instants in the cycle are compared. The existence of a logarithmic region enables a simple calculation of the maximum phase shift of the velocity in the boundary layer. An attempt of calculation by an integral method of boundary layer development is presented, up to the point where reverse flow starts appearing.

Houdeville, R.↗
NASA NTRS1974

Study of the relation between Pc 3 micropulsations and magnetosheath fluctuations and for the multisatellite multimeasurement investigation of earths bow shock

Hourly averages of HEOS A interplanetary field and plasma parameters are compared with micropulsation spectrograms taken by auroral zone stations. Visual evaluation of tungsten induction coil records and a statistical summary indicate a class of pulsations sometimes in the Pc 3, sometimes in the Pc 4 range, whose appearance correlates with solar wind field flow alignment. It is concluded that there is a pulsation phenomenon of variable period strongly associated with certain interplanetary field directions.

Greenstadt, E. W.↗
NASA NTRS1982

A mathematical model of turbulence in flows with uniform stationary velocity gradients

Certain cases of turbulence as a postinstability state of a fluid in motion modeled by the introduction of multivalued velocity fields are examined. The turbulence is regarded as occurring in the form of random pulsations which grow until the external energy input in the average flow is balanced by the dissipated energy of pulsations by means of turbulent friction. Closed form analytic solutions are shown to be possible when the considered velocity fields, the pulsation velocity and the fluid velocity, are decoupled.

Zak, M. A.↗
NASA NTRS1987

Preliminary results of geologic and remote sensing studies of Rima Mozart

In order to better understand the processes responsible for the formation of lunar sinuous rilles, a study of Rima Mozart was conducted using a variety of geologic, photographic, and remote sensing data. The apparent source of this rille is located in a highlands unit of known composition and it is hypothesized that thermal and mechanical erosion played an important role in the formation of Rima Mozart. Excellent photographic, topographic, multispectral, and radar data exist for this rille. The preliminary results of an analysis of this data are presented. Photographic data indicates the presence of two volcanic source vents for Rima Mozart: Kathleen and Ann. It is suggested that Rima Mozart, like many other lunar sinuous rilles, was most likely formed by a combination of events. Rima Mozart does follow a pre-existing, dominant NW/SE structural trend suggesting the influence of structural features on the rille, however, the tectonic influence is not the sole source for the formation of the rille, as suggested by the presence of the two source vents and the spatter around Ann. It is suggested that the rille formation began with an explosive eruption at Kathleen which later calmed down to a pulsating, high volume, low-viscosity lava flow. The rapid effusion rate of the magma as well as its high temperature and turbid nature helped carve the sinuous rille into the fractured and structurally weak Apennine Bench Formation underneath. Similar eruptions and subsequent flows were also created at Ann and joined to the main channel by a NE-trending secondary rille.

Coombs, C. R.↗
NASA NTRS1990

Leading-edge effects on boundary-layer receptivity

Numerical calculations are presented for the incompressible flow over a parabolic cylinder. The computational domain extends from a region upstream of the body downstream to the region where the Blasius boundary-layer solution holds. A steady mean flow solution is computed and the results for the scaled surface vorticity, surface pressure and displacement thickness are compared to previous studies. The unsteady problem is then formulated as a perturbation solution starting with and evolving from the mean flow. The response to irrotational time harmonic pulsation of the free-stream is examined. Results for the initial development of the velocity profile and displacement thickness are presented. These calculations will be extended to later times to investigate the initiation of instability waves within the boundary-layer.

Gatski, Thomas B.↗
NASA NTRSDate not supplied

Extinction of Solid Diffusion Flame in Microgravity: Details of Quenching and Blowoff Processes

Long duration microgravity experiments aboard the International Space Station determine diffusion flame extinction limits of PMMA spheres. Upon ignition from an electrically-heated coil, the 4-cm-diameter samples are exposed to forced flows ranging from 0.2 to 80 cm/s and an oxygen ranging from 13 to 28% in one atmosphere total pressure. Extinction is reached as oxygen concentration gradually decreases by natural depletion. Five extinction tests are presented at different flow velocities and oxygen concentrations. Quenching at low velocity is observed with the flame tip shrinking upstream and blowoff is observed when a hole forms in the flame at the forward flow stagnation point. However, these processes are not quasistatic. The quenching motion involves periodic flame tip pulsating toward downstream and shrinking upstream with a continually decreasing flame size at the end of each cycle. In blowoff, the flame base pulsates between the flame hole and the downstream location, although with far fewer cycles compared to quenching. The pulsations appear to be the result of a premixed flame front spreading into a combustible mixture. Two specific cases are discussed. In the first, at very low flow velocity (~ 0.4 cm/s) and elevated oxygen, self-sustained flame tip cyclic pulsations are observed for a lengthy period (~15 minutes). In the second case with a higher flow velocity (50 cm/s), the diffusion flame is stabilized at the shoulder of the spherical sample after local stagnation point blowoff. With steady decrease in ambient oxygen due to depletion, spinning flamelets are formed. The long-duration microgravity environment makes it possible to observe. these interesting and detailed extinction processes.

Flame quenching↗
NASA NTRS1956

A Theory for Stability and Buzz Pulsation Amplitude in Ram Jets and an Experimental Investigation Including Scale Effects

From a theory developed on a quasi-one-dimensional-flow basis, it is found that the stability of the ram jet is dependent upon the instantaneous values of mass flow and total pressure recovery of the supersonic diffuser and immediate neighboring subsonic diffuser. Conditions for stable and unstable flow are presented. The theory developed in the report is in agreement with the experimental data of NACA-TN-3506 and NACA-RM-L50K30. A simple theory for predicting the approximate amplitude of small pressure pulsation in terms of mass-flow decrement from minimum-stable mass flow is developed and found to agree with experiments. Cold-flow tests at a Mach number of 1.94 of ram-jet models having scale factors of 3.15:1 and Reynolds number ratios of 4.75:1 with several supersonic diffuser configurations showed only small variations in performance between geometrically similar models. The predominant variation in steady-flow performance resulted from the larger boundary layer in the combustion chamber of the low Reynolds number models. The conditions at which buzz originated were nearly the same for the same supersonic diffuser (cowling-position angle) configurations in both large and small diameter models. There was no appreciable variation in stability limits of any of the models when the combustion-chamber length was increased by a factor of three. The unsteady-flow performance and wave patterns were also similar when considered on a reduced-frequency basis determined from the relative lengths of the model. The negligible effect of Reynolds number on stability of the off-design configurations was not anticipated in view of the importance of boundary layer to stability, and this result should not be construed to be generally applicable. (author)

Trimpi, Robert L↗
NASA NTRS1972

Effects of rotating flows on combustion and jet noise.

Experimental investigations of combustion in rotating (swirling) flow have shown that the mixing and combustion processes were accelerated, flame length and noise levels significantly decreased, and flame stability increased relative to that obtained without rotation. Unsteady burning accompanied by a pulsating flame, violent fluctuating jet, and intense noise present in straight flow burning were not present in rotating flow burning. Correlations between theory and experiment show good agreement. Such effects due to rotating flows could lead to suppressing jet noise, improving combustion, reducing pollution, and decreasing aircraft engine size. Quantitative analysis of the aero-acoustic relationship and noise source characteristics are needed.-

Schwartz, I. R.↗
NASA NTRS1984

Chronology, morphology and stratigraphy of pumiceous pyroclastic-flow (ignimbrite) deposits from the eruption of Mount St. Helens on 18 May 1983

Between 1217 and 1620 hours (PDT), on May 18, 1980, the magmatic eruption column of Mount St. Helens formed an ash fountain and pyroclastic flows dominated the eruption process over tephra ejection. Eurption-rate pulsations generally increased to a maximum at 1600 to 1700 hrs. After 1620 hrs, the eruption assumed an open-vent discharge with strong, vertical ejection of tephra. Relative eruption rates (relative mass flux rates) of the pyroclastic flows were determined by correlating sequential photographs and SLAR images, obtained during the eruption, with stratigraphy and surface morphology of the deposits.

Criswell, C. W.↗
NASA NTRS2008

Flow Instabilities in Feather Seals due to Upstream Harmonic Pressure Fluctuations

Feather seals (also called slot seals) typically found in turbine stators limit leakage from the platform into the core cavities and from the shroud to the case. They are of various geometric shapes, yet all are contoured to fit the aerodynamic shape of the stator and placed as close as thermomechanically reasonable the powerstream flow passage. Oscillations engendered in the compressor or combustor alter the steady leakage characteristics of these sealing elements and in some instances generate flow instabilities downstream of the seal interface. In this study, a generic feather seal geometry was studied numerically by imposing an upstream harmonic pressure disturbance on the simulated stator-blade gap. The flow and thermal characteristics were determined; it was found that for high pressure drops, large fluctuations in flows in the downstream blade-stator gap can occur. These leakages and pulsations in themselves are not all that significant, yet if coupled with cavity parameters, they could set up resonance events. Computationally generated time-dependent flow fields are captured in sequence video streaming.

Deng, D.↗
NASA NTRS1993

Computational predictions of flame spread over alcohol pools

The effects of buoyancy and thermocapillarity on pulsating and uniform flame spread above n-propanol fuel pools have been studied using a numerical model. Data obtained indicate that the existence of pulsating flame spread is dependent upon the formation of a gas-phase recirculation cell which entrains evaporating fuel vapor in front of the leading edge of the flame. The size of the recirculation cell which is affected by the extent of liquid motion ahead of the flame, is shown to dictate whether flame spread is uniform or pulsating. The amplitude and period of the flame pulsations are found to be proportional to the maximum extent of the flow head. Under conditions considered, liquid motion was not affected appreciably by buoyancy. Horizontal convection in the liquid is the dominant mechanism for transporting heat ahead of the flame for both the pulsating and uniform regimes.

Schiller, D. N.↗