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Journal ArticleDOI

Relationship between Shock-cell Length and Noise of Jets from Rectangular and Elliptic Disk Nozzles

01 Jan 2009-International Journal of Turbo & Jet-engines (Walter de Gruyter GmbH)-Vol. 26, Iss: 2, pp 145-153
Abstract: Jets issuing from rectangular and elliptic disk nozzles of various aspect ratios have been studied for their noise characteristics. The overall sound pressure level and directivity were measured for these jets. The effects of aspect ratio, measurement plane (majorvs. minor-) have been analyzed. The results show that the sound pressure levels are entirely different for higher and lower aspect ratio elliptic and rectangular jets. At lower aspect ratios (AR=2:1), the noise emissions from major axis plane dominate while for higher aspect ratios jets (AR>2), the noise from minor axis plane dominate. Further, the relationship between sound pressure level and average shock-cell length is observed to be quadratic at emission angles of 30° and 90° for both rectangular and elliptic jets.
Topics: Noise (radio) (66%), Shock (mechanics) (61%)
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Abstract: Experimental study is carried out to explore the influence of nozzle profile on heat transfer for underexpanded impinging jets. Circular and elliptical orifices are used to generate underexpanded jets for underexpantion ratio ranging from 1.25 to 2.67. The supply pressure maintained in the present study ranges from 2.36 to 5.08 times the ambient pressure. IR thermal imaging camera is used to measure surface temperature of thin foil at different nozzle to plate distances. Shadowgraph and pressure distribution are used to understand the flow structure and distribution of circular and elliptical nozzle. It is observed that plate shock and pressure distribution over the plate have significant influence on the local heat transfer. The performance of the circular orifice is far better at lower z/d. The axis switching is observed for an elliptical orifice. Correlation for local heat transfer predicts Nusselt number comparable within 15 % of experimental results.

5 citations

Journal Article
Abstract: This paper presents the influence of tabs on shock trains of Underexpanded jet issuing through a convergent nozzle. The shadowgraph of flow is captured to have an insight of shock structure prevailed in Underexpanded jet. Tabs were attached diametrically opposite. Tabs used in the study were elliptical and perforated elliptical. The blockage ratio was 5%. Shadowgraph pictures were captured for tabs along and normal to the flow for nozzle pressure ratios (NPRs) of 3, 4, 5, 6 and 7. It was observed that elliptical tabs, highly distorts the shock train compared to perforated elliptical tabs. However perforated elliptical tabs are more influential to reduce noise level as well as compensation for thrust loss.

Journal ArticleDOI
13 Sep 2021-Physics of Fluids
Abstract: A supersonic core length Lc and an average shock-cell length Ls¯ are experimentally investigated for the acoustic properties of supersonic jets. Following Kumar and Rathakrishnan [“Scaling law for supersonic core length in circular and elliptic free jets,” Phys. Fluids 33(5), 051707 (2021)], a proper scale for the average shock-cell length Ls¯ of circular and elliptic supersonic free jets is found as Ls*¯AR0.2e−0.15γMj2, where Ls*¯ is the average shock-cell length normalized by nozzle throat diameter Dth, AR is the nozzle aspect ratio, γ is the specific heat ratio, and Mj is the ideally expanded jet Mach number. The scaling law developed for Ls*¯ exhibits a close similarity with the scaling law for supersonic core length Lc*AR0.5e−0.11γMj2, where Lc* is the nozzle throat diameter normalized supersonic core length. The scaling laws Ls*¯AR0.2e−0.15γMj2 and Lc*AR0.5e−0.11γMj2 are valid across a wide range of parameters, such as γ from 1.28 to 1.6, the nozzle design Mach number Md from 1 to 2, AR from 1 to 6, and Mj corresponds to the absence of Mach reflection. Discussion conducted on the scaling laws provides important insight into the choice of AR. Furthermore, the relationship between Lc* and Ls*¯ with the intensity of shock-associated noise is also discussed; hence, new correlation functions are developed for Lc* and Ls*¯ that provide an analogous measure of the shock-associated noise.

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Journal ArticleDOI
Abstract: Experimental studies of incompressible elliptic jets of different aspect ratios and initial conditions are summarized along with the effects of excitations at selected frequencies and amplitudes. The experimental facilities and procedures are described and jet spread and decay are discussed. The instability of elliptic shear layers, the behavior of the jet column under controlled excitation, and the time-average measures of unexcited jets are addressed.

460 citations

Journal ArticleDOI
Abstract: The screech tone frequencies and shock cell structures of non-axisymmetric supersonic jets from convergent-divergent nozzles operating at off-design conditions are investigated. For the purpose of providing a simple yet fairly accurate first order estimate of the gross features of these shock structures and screech tone frequencies, a linear shock cell model is developed. In the model the mixing layer of the jet is approximated by a vortex sheet. It is shown that the model problem for a jet with arbitrary cross-sectional geometry can be solved by eigenfunction expansion. In this paper the associated eigenvalue problems for rectangular and elliptical jets are solved and explicit shock cell structure solutions for these jets are found. To test the validity of the model the theoretical shock cell spacings are compared with the experimental measurements of Powell [1] and Hammitt [2]. Good agreements are obtained over a wide range of Mach number. The shock cell spacing formula is then used to compute the screech tone frequencies of rectangular jets by incorporating it into the weakest link theory of Tam, Seiner and Yu [3]. The calculated tone frequencies are found to agree favorably with the measurements of Powell [4] and Krothapalli et al. [5] over the Mach number range of 1·15 to 1·80. The present theory is simple but admittedly crude. If a more accurate description of the shock cell structures of non-axisymmetric supersonic jets is desired a multiple-scales model following the recent work of Tam, Jackson and Seiner [24] may be constructed. In this case the present vortex sheet model solution could be used as the starting solution (at the nozzle exit) of such a multiple-scales expansion. At the nozzle exit region the mixing layers of the jet is usually fairly thin.

161 citations

Journal ArticleDOI
Abstract: An experimental investigation has been carried out on an underexpanded jet of air issuing from a converging rectangular nozzle of moderate aspect ratio. Schlieren pictures of the flow field along with hot-wire measurements in the jet were obtained at different pressure ratios. At the pressure ratio corresponding to the maximum screeching sound, schlieren photographs show a very strong organized cylindrical wave pattern on either side of the jet, with their respective sources being located at the end of the third shock cell. Associated with this wave pattern is a large increase in the angle of spread of the jet. It is shown that the self-excitation helps to induce large scale vortical motions in the jet both in the plane containing the small dimension of the nozzle and in the plane containing the long dimension of the nozzle. However, the locations of these structures are different in the two planes. Nevertheless, the characteristic Strouhal number corresponding to these large scale structures in both planes is the same and equal to 0·12. The influence of the self-excitation on the mean velocities and rms intensities has also been investigated. For the full range of pressure ratios studied, similarity was found both in the mean velocity and rms intensity profiles in the two central planes beyond 80 widths downstream of the nozzle exit. However, the shapes of the similarity profiles are different in the two planes. For the downstream distances studied, complete axisymmetry (identical mean velocity profiles in both planes) was not found, which suggests it may persis for a large distance downstream of the nozzle exit.

155 citations

Journal ArticleDOI
01 Jan 1983-Physics of Fluids
Abstract: Studies of unexcited and excited elliptic jets reveal their characteristics to be noticeably different from circular jets, suggesting applications of excited elliptic jets for enhanced mixing and chemical reaction, and control of aerodynamic noise. The near‐field turbulence characteristics, jet spread, and locations of switching of major and minor axes of the jet cross section can be drastically altered by forcing. The preferred mode and the stable pairing mode of an elliptic jet scale with the exit equivalent diameter.

126 citations

Journal ArticleDOI
Abstract: Sound generation by vortex pairing in circular and elliptic cold-air jets at Mach 015-035 is investigated experimentally, with a focus on the effects of initial conditions The results are presented in graphs and interpreted using the theory of vortex sound proposed by Moehring (1978) and vortex-filament models of jet coherent structure Tripping the nozzle boundary layer is shown to (1) preempt formation of shear-layer vortices, (2) remove the sound they produce in later pairing, and (3) increase the diffusion of coherent vorticity in the vortex rings Hence pairing noise should not be significant in practical jets, which are initially turbulent

126 citations

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