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Khairul Zaman

Publications and source records attributed to Khairul Zaman.

10 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2024

‘Trapped Wave’ Resonance Tones Radiated in the Forward Arc of High-Speed Jets

An experimental study is conducted investigating the characteristics of ‘trapped wave’ resonance tones in the forward arc of jet noise radiation fields. While these spectral peaks disappear in the far acoustic field in the aft and sideline directions, which is why they went unnoticed in decades of jet noise measurements, the present study clearly shows that they radiate in the forward arc (shallow upstream polar locations in the approximate range of θ < 45°). Far-field jet noise spectra in the forward arc, data on which had been lacking in the literature, are not smooth but are characterized by these peaks. This is found for high subsonic to supersonic jets up to the highest jet Mach number covered in the experiment (MJ ≈ 1.9), for round, rectangular as well as convergent-divergent nozzles. With heated jets these spectral peaks are weakened especially around transonic conditions, however, they persist in supersonic conditions. Note that digital files containing all spectral data shown in the following are in a supplemental file to this NASA Technical Memorandum (TM-20240009873-SUPPL). These files can be obtained from the NASA Technical Report Server (NTRS) as a separate download from this report.

jets↗
NASA NTRSDate not supplied

Experiments to Challenge Jet Noise Prediction

Recent fundamental jet noise tests were conducted with the objective of providing data that challenges jet noise prediction methods. Three cases have been extracted to pose as challenges to the jet noise prediction community. Case 1 targets efforts in LES to appropriately mimic unresolveable turbulent boundary layers in nozzles, and to demonstrate where they are, and are not, important for noise. Case 2 targets acoustic analogy methods, providing cardinal data for the question of azimuthal localization and the broader issue of designing nozzles with favorable azimuthal directivity. Case 3 is an example where nozzle systems exhibit global resonances, which cannot be attacked by steady-state calculations. Given that it is very difficult to foresee such unsteady behavior (speaking from experience), it is crucial that low-cost methods be developed to catch these during the design process. Analysis and documentation of the test data is ongoing.

jet noise↗
NASA NTRSDate not supplied

Basics of Mixer-Ejectors for Quiet Propulsion

A series of numerical and experimental studies were undertaken to take a fresh look at how a mixer-ejector exhaust system might provide noise reduction for the LTO operation of a commercial supersonic vehicle. Historical understandings of aero and acoustic performance were challenged and new details of noise reduction mechanisms were uncovered. New CFD-based noise prediction tools, applied to conceptual designs, predicted noise reductions of 3-5EPNdB for a few percent thrust loss. Recommendations from the study include validation of the acoustic prediction method, and using it to refine variants of practical nozzle systems for low noise exhaust systems that perform well both acoustically and aerodynamically.

jet noise↗
NASA NTRSDate not supplied

Flow and noise from supersonic plug nozzles

Nozzles with external plugs are candidates for propulsion of future supersonic aircraft. A model-scale experimental study is conducted exploring the flow and noise characteristics of various plug nozzles. For a given outer nozzle, the plug geometry is varied from conic with various half-angles (lengths) to plugs designed by the method of characteristics (MoC) as well as truncated and porous plugs. So far, noise characteristics and schlieren flow visualization data have been acquired. Limited numerical simulations have also been done addressing thrust performance. The simulations show that the MoC plugs out-perform the conic cases, especially around high Mach number cruise conditions. The experimental data, however, show that the MoC plugs are noisy relative to the conic cases at low Mach number landing and takeoff (LTO) conditions. The increased noise (with MoC at LTO) is accompanied by unsteady turbulent structures and increased jet spreading, as seen in schlieren pictures. At LTO, the longest conic plug is found to be the least noisy. Furthermore, a porous surface on the long conic plug effectively suppresses broadband shock associated noise (BBSN) at higher Mach numbers. Thus, a dilemma remains. MoC design gives best thrust performance at cruise but it is noisy at LTO. A long conic plug, on the other hand, is quieter at LTO but may have unacceptable thrust at cruise.

jets↗
NASA NTRSDate not supplied

Pressure Fluctuations due to ‘Trapped Waves’ in Heated Jets

An experimental study was conducted to investigate the presence and characteristics of ‘trapped waves’ in the near field of a small axisymmetric jet at heated conditions. Trapped waves occur within the potential core of compressible jets and manifest as distinct peaks in the unsteady pressure spectra measured just outside the jet near the nozzle exit. The existence and properties of the trapped wave spectral peaks in round and rectangular jets of various sizes in subsonic and supersonic conditions were studied in our recent experiments, at cold conditions [7]. The current work extends the study to heated jets. Pressure fluctuation spectra near the nozzle exit are analyzed for a small (D = 6.35 mm) axisymmetric jet at stagnation temperature ratios up to 1.6. It is shown that trapped waves continue to persist in heated jets. At sufficiently high Mach numbers, one of the trapped wave harmonics emerges as screech tone, a behavior consistent with that observed in cold jets. For a given Mach number, the frequencies of both trapped waves and screech increase with jet temperature with no significant change occurring in their amplitudes. The Strouhal numbers based on jet diameter for the trapped waves and screech continue to follow similar scaling laws as observed previously with cold jets.

aeroacoustics↗
NASA NTRSDate not supplied

Pressure Fluctuation Spectral Peaks Due to ‘Guided Waves’ in Regions Upstream of the Jet Exit

Pressure fluctuation spectral peaks due to ‘guided’ (or ‘trapped’) waves in compressible jets in regions upstream of the nozzle exit are investigated experimentally. These spectral peaks are known to disappear in the far acoustic field on the sideline and in the downstream direction, where noise measurements are done typically. The data in this paper show that they are prevalent in the far upstream regions of the ‘forward arc’, at shallow angles relative to the jet axis. This is noted with a round as well as a rectangular jet. The result is significant since such spectral behavior is not predicted by existing noise prediction codes. The experimental results not only expand our understanding of the trapped wave phenomenon but may also aid in refining noise prediction codes for basic as well as application-oriented propulsion systems.

Jets↗
NASA NTRSDate not supplied

Study of a Plug Nozzle for Supersonic Aircraft Concepts

A plug nozzle with a cylindrical cowl is studied. The geometry is adopted following commercial supersonic aircraft concepts considered in an ongoing NASA program. For the given cowl, the plug geometry is varied that includes shapes developed by a numerical optimization study as well as porous plugs. Experimental data on noise and flow field are discussed with an eye for minimum noise at low nozzle pressure ratios (NPR2) representing landing and takeoff (LTO) conditions. NPR up to about 4.7 is covered in the experiments. Limited thrust data with accompanying numerical simulation results are also presented. Results show that a plug designed for optimum thrust at cruise is noisier while a plug designed for optimum thrust at LTO has less noise, throughout the NPR range covered. The results also show that a plug nozzle can generate transonic tones and excess broadband noise (EBBN) at lower NPR, apart from well-known screech tones and broadband shock associated noise (BBSN) at higher NPR. A long porous plug is found to effectively suppress all these aberrant noise components while suffering less than 1% loss in thrust coefficient.

Jets↗
NASA NTRSDate not supplied

Jet Noise Radiation in the Forward Arc Due to ‘Trapped Waves’ AAPL Test (6/17 - 7/11, 2024)

An experimental study is conducted investigating the characteristics of ‘trapped wave’ resonance tones in the forward arc of jet noise radiation fields. The results show that they indeed radiate in the forward arc (shallow upstream polar locations in the approximate range of <50°). This is found for high subsonic to supersonic jets up to the highest jet Mach number covered in the experiment (MJ1.9). With heated jets these spectral peaks get weakened especially around transonic conditions, however, they persist in supersonic conditions. Key results from the experiment are included in these presentation slides.

jets↗
NASA NTRSDate not supplied

Investigation of Physical Mechanisms for Jet Noise Reduction by Plug Nozzle Porosity

This paper presents the results of a computational study on porous-plug nozzles for their eventual application in reducing takeoff noise levels for supersonic civil transport. The study explores a wide range of porous-plug geometries, encompassing variations in hole size, plug length and porosity. Computational fluid dynamics (CFD) analyses are employed in concert with an advanced automated mesh refinement (AMR) scheme, Sketch-to-Solution (S2S), to investigate porous plug nozzle design strategies that may diminish shock strength, broad-band shock noise (BBSN), flow separation, and other undesirable characteristics associated with high takeoff noise levels. A selected subset of the porous-plug nozzles that were analyzed computationally were manufactured at subscale using stereolithography 3D printing. These nozzles were tested experimentally, obtaining far-field noise spectral data and schlieren flow visualization pictures. Detailed comparisons between CFD predictions and experimental results were conducted. These comparisons focus on gaining a better understanding of the underlying physical mechanisms responsible for noise reduction with the porous plugs relative to corresponding solid plugs.

Nozzles↗