Ray Taghavi

Effect of initial swirl distribution on the evolution of a turbulent jet

An existing cold-jet facility at NASA Lewis Research Center was modified to produce swirling flows with controllable initial tangential velocity distribution. Distinctly different swirl velocity profiles were produced, and their effects on jet mixing characteristics were measured downstream of an 11.43 cm (4.5 in.) diameter convergent nozzle. It was experimentally shown that in the near field (i.e., x/D Q.6. This remarkable result leads to the conclusion that the integrated swirl effect, reflected in the swirl number, is inadequate in describing the mean swirling jet behavior in the near field. The relative size (i.e., diameter) of the vortex core emerging from the nozzle and the corresponding tangential velocity distribution are the controlling parameters influencing the swirling turbulent free-jet evolution.

Coupling of twin rectangular supersonic jets

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Resonant interaction of a linear array of supersonic rectangular jets: An experimental study

This paper examines a supersonic multi-jet interaction problem that we believe is likely to be important for mixing enhancement and noise reduction in supersonic mixer-ejector nozzles. We demonstrate that it is possible to synchronize the screech instability of four rectangular jets by precisely adjusting the inter-jet spacing. Our experimental data agree with a theory that assumes that the phase-locking of adjacent jets occurs through a coupling at the jet lip. Although synchronization does not change the frequency of the screech tone, its amplitude is augmented. The synchronized multi- jets exhibit higher spreading than the unsynchronized jets, with the single jet spreading the least. We compare the near-field noise of the four jets with synchronized screech to the noise of the sum of four jets operated individually. Our noise measurements reveal that the more rapid mixing of the synchronized multi-jets causes the peak jet noise source to move upstream and to radiate noise at larger angles to the flow direction. Based on our results, we have grounds to believe that screech synchronization is advantageous for noise reduction internal to a mixer-ejector nozzle, since the noise can now be suppressed by a shorter acoustically lined ejector.

Spatial instability of a swirling jet - Theory and experiment

The spatial instability of a swirling jet is investigated both experimentally and theoretically. A hydrodynamic stability analysis is applied to an inviscid incompressible top-hat jet, with a swirl distribution of solid-body rotation and free vortex in and outside the vortex core, respectively. Both plane and helical instability modes are examined. It is found that the top-hat jet with swirl distribution of Rankine vortex type is unstable in all the modes studied. The higher the positive helicity, the less spatially unstable the jet behavior; the higher the negative helicity, the more spatially unstable this behavior becomes. A comparison is made between theoretical results and experimental data on a low-intensity swirling jet. The trend of the initial growth of the instability waves in the near field is captured.

Controlled excitation of a cold turbulent swirling free jet

Experimental results from acoustic excitation of a cold free turbulent jet with and without swirl are presented. A flow with a swirl number of 0.35 (i.e., moderate swirl) is excited internally by plane acoustic waves at a constant sound pressure level and at various frequencies. It is observed that the cold swirling jet is excitable by plane waves, and that the instability waves grow about 50 percent less in peak rms amplitude, and saturate further upstream compared to corresponding waves in a jet without swirl having the same axial mass flux. The preferred Strouhal number based on the mass-averaged axial velocity and nozzle exit diameter for both swirling and nonswirling flows is 0.4. So far no change in the mean velocity components of the swirling jet is observed as a result of excitation.

Modern developments in shear flow control with swirl

Passive and active control of swirling turbulent jets is experimentally investigated. Initial swirl distribution is shown to dominate the free jet evolution in the passive mode. Vortex breakdown, a manifestation of high intensity swirl, was achieved at below critical swirl number (S = 0.48) by reducing the vortex core diameter. The response of a swirling turbulent jet to single frequency, plane wave acoustic excitation was shown to depend strongly on the swirl number, excitation Strouhal number, amplitude of the excitation wave, and core turbulence in a low speed cold jet. A 10 percent reduction of the mean centerline velocity at x/D = 9.0 (and a corresponding increase in the shear layer momentum thickness) was achieved by large amplitude internal plane wave acoustic excitation. Helical instability waves of negative azimuthal wave numbers exhibit larger amplification rates than the plane waves in swirling free jets, according to hydrodynamic stability theory. Consequently, an active swirling shear layer control is proposed to include the generation of helical instability waves of arbitrary helicity and the promotion of modal interaction, through multifrequency forcing.

Large amplitude acoustic excitation of swirling turbulent jets

A swirling jet with a swirl number of S = 0.12 is excited by plane acoustic waves at various Strouhal numbers (St = fD/U sub alpha). The maximum forcing amplitude of excitation was at 6.88 percent of the time-mean axial velocity at a Strouhal number of St = 0.39. The maximum time-mean tangential and axial velocities at the nozzle exit were 18 and 84 m/sec respectively. It was observed that the swirling jet was excitable by plane acoustic waves and the preferred Strouhal number based on the nozzle diameter and exit axial velocity of the jet was about 0.39. As a result of excitation at this frequency, the time-mean axial velocity decayed faster along the jet centerline, reaching about 89 percent of its unexcited value at x/D = 9. Also the half velocity radius and momentum thickness, at 7 nozzle diameters downstream, increased by 13.2 and 5.8 percent respectively, indicating more jet spread and enhanced mixing. To our knowledge, this is the first reported experimental data indicating any mixing enhancement of swirling jets by acoustic excitation.

Frequency-Domain Analysis of Fluctuating Pressure on a Pusher Propeller Blade Surface

Chordwise distribution of unsteady surface pressure is measured on a pylon-mounted pusher propeller in flight. Spectral decomposition of the fluctuating surface pressure signals reveals a strong presence of upstream wake interaction. The growth and decay behavior of the fundamental disturbance wave along the propeller chord exhibits the same characteristics as a separated, reattaching shear layer. Frequency-domain analysis further suggests a single or multiple vortex-shedding phenomenon from the pusher propeller trailing edge, per revolution, in an upstream wake interaction. The rms amplitude of higher harmonics (i.e., k = 3, 4, 5, and 6) along the propeller chord attains values corresponding to boundary-laye r random turbulence levels. Joint statistical properties between selected transducers on the propeller suction surface suggest a linear frequency response of the dynamical system to the fundamental and first harmonic disturbances, while higher-frequency Fourier components result in a nonlinear response behavior.

Mixer-Ejector Wall Pressure and Temperature Measurements Based on Photoluminescence

Ejector side-wall pressure distribution is a key indicator of supersonic jet-mixer ‐ejector performance. When documenting pressurepatterns on an ejectorwall using pressure-sensitivepaint (PSP), onehasto considertemperaturevariationscaused by the supersonicjet e owwithin theejector becausethesecan causesignie cantlocalerrors in the PSP results. If the temperature sensitivity of PSP is not corrected for in complex internal supersonic e ows, large localized errors could contaminate the results. In the present work, temperature-sensitive paint maps the temperature distribution on the ejector wall and corrects PSP results point-by-point for temperature sensitivity. The experiments were conducted on multijet supersonic mixer ‐ejector cone gurations with straight, convergent (6-deg), and divergent (6-deg) side walls. A comparison of corrected and uncorrected PSP readings shows that at Mj = 1:55, the error with respect to true data from static pressure ports can be reduced from 4.98 to 2.84% for the case of a simple ejector with parallel walls. For the complex 6-deg convergent ejector at Mj =1:39, the error reduces by almost an order of magnitude (from 20.83 to 2.66% ). Ourresults indicatethat the use of this correction technique can signie cantly reduce PSP errors in complex internal supersonic e ow situations.

Application of Pressure-Sensitive Paint in Supersonic Nozzle Research

An experimental study was conducted to evaluate the pressure-sensitive paint (PSP) technique as a means of obtaining global surface pressure distribution on the inside surface of a supersonic nozzle at NASA Langley Research Center. In this experiment, the PSP technique was used to measure the surface pressure distribution for several configurations of a convergent-divergent nozzle, which was also instrumented with a densely packed array of pressure taps. The PSP and pressure tap data were obtained simultaneously for each configuration of the nozzle, at several nozzle pressure ratios, to determine the effects of supersonic cavity-ramp vortex generators on the nozzle flow and performance. Results from the PSP data agreed very well with the pressure tap data and indicated that a pair of counter-rotating vortices were shed from the cavity vortex generator. The presence of these streamwise vortices caused a delay in the formation of the shock wave inside the nozzle.