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Students’ emotional well-being and religiosity during the COVID-19 pandemic- an international study in 7 countries
- K. H. Karakula, A. Forma, R. Sitarz, J. Baj, D. Juchnowicz, J. Bogucki, W. Tuszyńska-Bogucka, M. L. Tee, C. A. Tee, J. T. Ly-Uson, M. S. Islam, M. T. Sikder, A. H. El-Monshed, A. Loutfy, M. F. Hussain Qureshi, M. Abbas, S. Taseen, M. Lakhani, S. Jayakumar, S. Ilango, S. Kumar, Á. A. Ruiz-Chow, A. Iturbide, D. D. González-Mille, H. Karakula-Juchnowicz
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- Journal:
- European Psychiatry / Volume 66 / Issue S1 / March 2023
- Published online by Cambridge University Press:
- 19 July 2023, p. S406
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Introduction
There are no conclusive findings about the possible protective role of religion on students’ mental health during the COVID-19 pandemic. Therefore, more research is needed.
ObjectivesThe purpose of this study was to assess the relationship between the level of emotional distress and religiosity among students from 7 different countries during the COVID-19 pandemic.
MethodsData were collected by an online cross-sectional survey that was distributed amongst Polish (N = 1196), Bengali (N = 1537), Indian (N = 483), Mexican (N = 231), Egyptian (N = 565), Philippine (N = 2062), and Pakistani (N = 506) students (N = 6642) from 12th April to 1st June 2021. The respondents were asked several questions regarding their religiosity which was measured by The Duke University Religion Index (DUREL), the emotional distress was measured by the Depression, Anxiety, and Stress Scale-21 (DASS-21).
ResultsEgypt with Islam as the dominant religion showed the greatest temple attendance (organizational religious activity: M=5.27±1.36) and spirituality (intrinsic religiosity: M=5.27±1.36), p<0.0001. On another hand, Egyptian students had the lowest emotional distress measured in all categories DASS-21 (depression: M=4.87±10.17, anxiety: M=4.78±10.13, stress: M=20.76±11.46). Two countries with the dominant Christian religion achieved the highest score for private religious activities (non-organizational religious activity; Mexico: M=3.94±0.94, Poland: M=3.63±1.20; p<0.0001) and experienced a moderate level of depressive symptoms, anxiety, and stress. Students from Mexico presented the lowest attendance to church (M=2.46±1,39) and spirituality (M=6.68± 3.41) and had the second highest level of depressive symptoms (M=19.13±13.03) and stress (M=20.27±1.98). Philippines students had the highest DASS-21 score (depression: M=22.77±12.58, anxiety: M=16.07±10.77, stress: M=4.87±10.08) and their level of religiosity reached average values in the whole group. The performed regression analysis confirmed the importance of the 3 dimensions (organizational religious activity, non-organizational religious activity, intrinsic religiosity) of religiosity for the well-being of students, except for the relationship between anxiety and private religious activities. The result was as presented for depression: R2=0.0398, F(3.664)=91.764, p<0.0001, SE of E: 12.88; anxiety: R2=0.0124, F(3.664)=27.683, p<0.0001, SE of E: 10,62; stress: R2= 0.0350, F(3.664)=80.363, p<0.0001, SE of E: 12.30.
ConclusionsThe higher commitment to organizational religious activity, non-organizational religious activity, and intrinsic religiositywas correlated with the lower level of depressive symptoms, stress, and anxiety among students during the COVID-19 pandemic, but taking into account factors related to religiosity explains the level of emotional well-being to a small extent.
Disclosure of InterestNone Declared
WHAT IS MORE DANGEROUS- SUICIDALITY IN EPILEPSY OR EPILEPSY AS A DISEASE? _A REVIEW
- H. Arshad, K. Hussain, M. Khalid, F. Arain, A. R. Khan, A. Arshad
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- European Psychiatry / Volume 66 / Issue S1 / March 2023
- Published online by Cambridge University Press:
- 19 July 2023, p. S918
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Introduction
Epilepsy is a neurologic condition characterized by spontaneous jerky body movements. It is a chronic morbid condition mostly diagnosed during childhood. Patients are maintained on long-term medications to prevent recurrent seizures that can damage the brain. Medications used for the management of epilepsy have several side effects and require proper monitoring. Patients with epilepsy are at increased risk of psychiatric comorbidities.
ObjectivesOur aim is to find factors responsible for causing suicidality in patients with epilepsy.
MethodsA review was conducted using Pubmed database with the search terms [epilepsy] OR [neurological conditions] OR [suicide] OR [suidical attempt] OR [suicidal ideation] OR [depression] OR [psychiatric diseases] OR [mood disorders] OR [anxiety] OR [sleeplessness] which yielded around 800 articles. The number was later reduced to be centered around the main area of interest and produced around 40 articles.
ResultsResults show that many contributing factors play an unavoidable role in promoting suicidal ideation that can lead to suicidal attempts in epilepsy. The stigma associated with epilepsy leads to social isolation, lack of opportunities, financial constraints, and impact on close relationships which can be the reasons for depression. According to the review, the foremost contributing factor is the underlying social, emotional, and economic condition of epilepsy patients. Depression and anxiety are the most prevalent psychiatric comorbidities in epileptic patients. Epileptic patients who develop psychiatric ailments show decreased adherence to medications that further worsen the problem.
ConclusionsThough, this area in neurology has started getting attention for further research and guidelines. But efforts are still inadequate for this to be put into clinical practice. More desperate actions needed to be taken for proper diagnosis and management of suicidal ideations by proper use of assessment tools so that timely actions are planned. This is a highly demanding area due to the impact of depressive symptoms on the prognosis of the chronic neurologic condition.
Keywords: Suicidality; Epilepsy; neurology.
Disclosure of InterestNone Declared
The ‘whistler-nozzle’ phenomenon
- A. K. M. F. Hussain, M. A. Z. Hasan
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- Journal of Fluid Mechanics / Volume 134 / September 1983
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- 20 April 2006, pp. 431-458
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The ‘whistler nozzle’ is a simple device which can induce jet self-excitations of controllable amplitudes and frequencies and appears highly promising for many applications involving turbulent transport, combustion and aerodynamic noise. This paper documents the characteristics of this curious phenomenon for different values of the controlling parameters and explains the phenomenon. It is shown that the whistler excitation results from the coupling of two independent resonance mechanisms: shear-layer tone resulting from the impingement of the pipe-exit shear layer on the collar lip, and organ-pipe resonance of the pipe nozzle. The crucial role of the shear-layer tone in driving the organ-pipe resonance is proven by reproducing the event in pipe–ring and pipe–hole configurations in the absence of the collar. It is also shown that this phenomenon is strongest when the self-excitation frequency matches the ‘preferred mode’ of the jet.
The ‘whistler-nozzle’ phenomenon occurs for both laminar and turbulent initial boundary layers; the excitation can be induced without the pipe nozzle (say, by ring or hole tone) when the exit flow is laminar but not when it is turbulent. Unlike the shear-layer tone and jet tone phenomena, where successive stages overlap, adjacent stages of the whistler-nozzle excitation are separated by ‘dead zones’ where the conditions for both resonance mechanisms cannot be simultaneously met. Also, unlike the shear-layer and jet tones, the whistler frequency cannot be varied continuously by changing the speed. Since the phenomenon is the coupling of two resonance mechanisms, the frequency data appear to defy a simple nondimensional representation for the entire range of its operation. Reasonable collapse of data is achieved, however, when the exit momentum thickness is used as a lengthscale, thus emphasizing the role of the shear-layer tone in the phenomenon.
Turbulence suppression in free shear flows by controlled excitation
- K. B. M. Q. Zaman, A. K. M. F. Hussain
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- Journal of Fluid Mechanics / Volume 103 / February 1981
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- 20 April 2006, pp. 133-159
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In an attempt to explain the mechanics of turbulence suppression previously observed by us in jets under controlled excitation, the near fields of four circular jets, a plane jet and a plane mixing layer, all with laminar efflux boundary layers, have been explored through hot-wire measurements. It is shown that controlled excitation, induced acoustically as well as by vibrating ribbons, can reduce turbulence intensities in all these flows. Reduction by as much as 80% is observed, the maximum decrease occurring at about 400θe downstream from the exit; θe is the initial shear-layer momentum thickness. The suppression effect is a maximum for excitation at the Strouhal number Stθ(≡ fθe/Ue) of about 0.017. In the jets, the turbulence suppression is observed over the range 0·75 ≤ x/D ≤ 8, while for the plane mixing layer it could be detected as far downstream as x ≅ 6000θe.
The flow-fields with and without excitation for a typical case of turbulence suppression have been studied in detail. Spectra of the u signal and time-averaged field data indicate that excitation at Stθ ≅ 0·017 suppresses the formation of naturally occurring energetic vortices - an observation confirmed by flow-visualization experiments and by study of the large-scale coherent structures of the shear layer, educed through conditional-sampling measurements. Excitation at Stθ ≅ 0·017 produces a rapid growth of the shear layer instability mode, and consequently, its saturation, roll-up and breakdown occurs much earlier in x than is found to occur naturally (at Stθ ≅; 0·012). The suppression effect is apparently a consequence of earlier transition of the shear layer vortices, which otherwise naturally grow to larger sizes and survive for larger x, as well as being due to the prevention of successive pairing of these structures.
On the coherent structure of the axisymmetric mixing layer: a flow-visualization study
- A. K. M. F. Hussain, A. R. Clark
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- Journal of Fluid Mechanics / Volume 104 / March 1981
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- 20 April 2006, pp. 263-294
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In an effort to resolve some controversies regarding the turbulent mixing-layer structure, the near field of a large (18 cm diameter) air jet has been investigated for the jet exit speed of 30 m s−1. The smoke-laden axisymmetric mixing layer has been illuminated by a thin sheet of laser light in an azimuthal plane passing through the jet axis. High-speed visualization films of the mixing layer in the region of its self-preservation (of which a few picture sequences depicting space-time evolutions of the structure of the layer are presented) reveal that most of the time the mixing layer is in a state of disorganization, consisting of relatively smaller scale, random and diffuse turbulent motions; only occasionally are organized distinct large-scale coherent structures formed. The survival distances of the large-scale structures are found to be comparable to their average sizes. The survival time of these structures is about one ‘turnover’ time, each being roughly about five times the local characteristic time scale of the mixing layer. It is seen that tearing is as dominant a mode of large-scale interaction as pairing is; large-scale structures are continually sheared and typically fragmented due to a segment on the high-speed side being torn and swept away from the slower-moving outer portion. Evolution of the large structures occur not primarily through complete pairing as widely believed but quite frequently through ‘fractional pairing’ between segments which have been torn from different upstream large-scale coherent structures or through ‘partial pairing’ when one structure captures only a part of another. The movies show that along with entrainment of non-vortical ambient fluid, radially outward ejection of vortical fluid into the ambient is an important aspect of jet mixing. From aligned displays of ciné film frame sequences, space-time trajectories of identifiable vortical fluid elements have been traced. The convection velocity variation across the shear layer and even the overall structure convection velocity measured from these trajectories agree with those determined from the wave-number-celerity spectra, obtained from double-Fourier transformation of longitudinal velocity space-time correlation measurements with hot-wires.
The visualization films do not bear out the two-street vortex ring model recently propounded by Lau. Based on our observations, we propose that tearing, ‘slippage’ and fractional and partial pairings are responsible for the observed radial variation of structure passage frequency, and the causes of the different coherent structures educed by Bruun on the high- and low-speed sides of the mixing layer and for Yule's failure in educing a coherent structure on the low-speed side of the layer.
The self-excited axisymmetric jet
- M. A. Z. Hasan, A. K. M. F. Hussain
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- Journal of Fluid Mechanics / Volume 115 / February 1982
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- 20 April 2006, pp. 59-89
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The characteristics of a self-excited axisymmetric air jet driven by a whistler (i.e. pipe-collar) nozzle have been explored experimentally for various choices of the controlling parameters: namely, pipe length, jet diameter, collar length, step height, and jet speed. By appropriate choices of these parameters as well as the stage and mode (half- and full-wave), the self-sustained excitation has been induced at specific values of the excitation amplitude and the Reynolds number RD. The jet characteristics up to RD ≃ 3·1 × 105 and x/D ≃ 60 have been documented for both laminar and turbulent flows at the pipe exit. Comparison with corresponding unexcited-jet data reveals that self-excitation produces a large increase in the fluctuation intensity in the near field of the jet, while it increases the jet spread and decay rate for the entire x-range of measurement. The dependence of the jet structure on the initial condition is stronger when self-excited than when unexcited. The first stage of excitation always produces the highest turbulence augmentation and the spectral evolution is significantly modified by self-excitation up to x/D ≃ 6. The excitation produces a significant increase in the broad-band turbulence level over that of the unexcited jet. The broad-band amplification is maximized at x/D ≃ 4 and is the highest at the largest RD studied.
These data suggest interesting possibilities for the self-excited jet in the augmentation or control of entrainment, mixing and aerodynamic noise production.
A ‘turbulent spot’ in an axisymmetric free shear layer. Part 3. Azimuthal structure and initiation mechanism
- S. J. Kleis, A. K. M. F. Hussain, M. Sokolov
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- Journal of Fluid Mechanics / Volume 111 / October 1981
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- 20 April 2006, pp. 87-106
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The details of a spark-induced ‘spot’ in an axisymmetric mixing layer of a 12·7 cm diameter (D) free air jet have been educed in different azimuthal planes and at three streamwise stations corresponding to x/D = 1·5, 3·0, 4·5. The measurement technique and the spot properties in the plane of the spark at the three stations were discussed in parts 1 and 2 (Sokolov et al. 1980 and Hussain, Kleis & Sokolov 1980, hereinafter referred to as I and II, respectively). The present part describes the azimuthal structure of the spot and its initiation mechanism.
It is shown that the distributions of phase-average longitudinal and lateral velocities, the intermittency and the coherent Reynolds stress within the spot are essentially the same in various azimuthal planes at each streamwise location. The spark induces a local boundary-layer spot on the nozzle wall and simultaneously triggers the instability of the free shear layer downstream from the lip. The boundarylayer spot persists initially in the free shear layer but decays downstream due to the lack of a sustaining mechanism. The mixing-layer spot – the result of a roll-up of a natural instability mode triggered in the free shear layer by the acoustic disturbance radiated from the spark – grows downstream and undergoes intense interactions, remaining essentially axisymmetric and travelling at about 60% of the core fluid velocity. Velocity signals in different azimuthal planes of the free shear layer show that the natural instability of the jet occurs axisymmetrically on an instantaneous basis even though the jet diameter is considerably larger than the instability wavelength. The natural instability is amplitude-modulated in a random manner; this modulation is also essentially axisymmetric.
The large-scale coherent structures produced by the intense localized spark are not only axisymmetric on the phase-average basis, but are also individually axisymmetric in the laminar instability region.
The ‘preferred mode’ of the axisymmetric jet
- A. K. M. F. Hussain, K. B. M. Q. Zaman
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- Journal of Fluid Mechanics / Volume 110 / September 1981
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- 20 April 2006, pp. 39-71
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The ‘preferred mode’ of an incompressible axisymmetric free jet has been organized through controlled perturbation, and spatial distributions of time-average as well as phase-average flow properties in the near field are documented. The excitation produces noticeable changes in the time-average measures of the jet, although these changes are less dramatic than those for the excitation producing stable vortex pairing. For different stages in the evolution of the preferred-mode coherent structure, the phase-average vorticity, coherent Reynolds stress, and incoherent turbulence intensities and Reynolds stress have been educed through phase-locked hot-wire measurements, over the spatial extent of the structure and without invoking the Taylor hypothesis. For a particular stage of the evolution (i.e. when the structure is centred at x/D ≃ 3) the distributions of these quantities have been compared for both initially laminar and fully turbulent exit boundary layers, and for four jet Reynolds numbers. The relative merits of the coherent structure streamline and pseudo-stream-function patterns, as compared with phase-average velocity contours, for structure boundary identification have been discussed. The structure shape and size agree closely with those inferred from the average streamline pattern of the natural structure educed by Yule (1978).
These data as well as τ-spectra show that even excitation at the preferred mode cannot sustain the initially organized large-scale coherent structure beyond eight diameters from the jet exit. The background turbulence is organized by the coherent motions in such a way that the maximum rate of decrease of the coherent vorticity occurs at the structure centres which are the saddle points of the background-turbulence Reynolds-stress distributions. The structure centres are also the locations of peak phase-average turbulence intensities. The evolving shape of the structure as it travels downstream helps explain the transverse variations of the wavelength and convection velocity across the mixing layer. The coherent structure characteristics are found to be independent of whether the initial boundary layer is laminar or turbulent, but depend somewhat on the jet Reynolds number. With increasing Reynolds number, the structure decreases in the streamwise length and increases in the radial width and becomes relatively more energetic, and more efficient in the production of coherent Reynolds stress.
Statistics of fine-scale velocity in turbulent plane and circular jets
- R. A. Antonia, B. R. Satyaprakash, A. K. M. F. Hussain
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- Journal of Fluid Mechanics / Volume 119 / June 1982
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- 20 April 2006, pp. 55-89
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Higher-order statistics of the streamwise velocity derivative have been measured on the centre-line of turbulent plane and circular jets. The instrumentation and sources of error are discussed to establish the accuracy of the data and convergence of statistics. The optimum setting for the low-pass filter cut-off was found to be 1·75 times the Kolmogorov frequency fK, in contrast with the majority of previous investigations where it was set equal to fK. The magnitude of the constant μ in Kolmogorov's revised hypothesis is obtained using statistics derived from the instantaneous velocity derivative or its squared value. The correlation and spectrum of fluctuations of the squared velocity derivative and the Reynolds-number variation of the skewness and flatness factors of the velocity derivative are consistent with μ ≃ 0·2, while the most popular value used is 0·5. Second-order moments of the locally averaged dissipation, assumed proportional to the squared streamwise velocity derivative, and breakdown coefficients also suggest a value of μ of about 0·2. Higher-order correlations and spectra of the dissipation are in closer agreement with the Novikov-Stewart or β-model than with Kolmogorov's lognormal model. Higher-order moments of locally averaged values of the dissipation rate are more closely represented by the lognormal than the β-model.
Taylor hypothesis and large-scale coherent structures
- K. B. M. Q. Zaman, A. K. M. F. Hussain
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- Journal of Fluid Mechanics / Volume 112 / November 1981
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- 20 April 2006, pp. 379-396
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The applicability of the Taylor hypothesis to large-scale coherent structures in turbulent shear flows has been evaluated by comparing the actual spatial distributions of the structure properties with those deduced through the use of the hypothesis. This study has been carried out in the near field of a 7[sdot ]62 cm circular air jet at a jet Reynolds number of 3[sdot ]2 x 104, where the coherent structures and their interactions have been organized through controlled excitation. Actual distributions of the structure properties have been obtained through phase-average hot-wire data, the measurements having been repeated at different spatial points over the extents of the structure crosssections at a fixed phase. The corresponding ‘spatial’ distributions of these properties obtained (by using the Taylor hypothesis) from the temporal data at appropriate phases and locations, show that the hypothesis works quite well for an isolated coherent structure if a constant convection velocity, equal to the structure centre velocity, is used in the hypothesis everywhere across the shear flow. The popular use of the local time-average or even the instantaneous streamwise velocity produces unacceptably large distortions. When structure interactions like pairing are involved, no convection velocity can be found with which the hypothesis works. Distributions of the terms in the Navier–Stokes equation contributing to the phase-average vorticity, but neglected by the hypothesis, have been quantitatively determined. These show that the terms associated with the background turbulence field, but not those associated with the coherent motion field, can be neglected. In particular, the pressure term due to the coherent motion field is large and cannot be neglected.
Turbulence suppression in free turbulent shear flows under controlled excitation. Part 2. Jet-noise reduction
- A. K. M. F. Hussain, M. A. Z. Hasan
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- Journal of Fluid Mechanics / Volume 150 / January 1985
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- 20 April 2006, pp. 159-168
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It is shown that reduction of broadband (even total) far-field jet noise can be achieved via controlled excitation of a jet at a frequency in the range 0.01 < Stθ < 0.02, where Stθ is the Strouhal number based on the exit momentum thickness of the shear layer. Hot-wire measurements in the noise-producing region of the jet reveal that the noise suppression is a direct consequence of turbulence suppression, produced by early saturation and breakdown of maximally growing instability modes.
An experimental study of organized motions in the turbulent plane mixing layer
- A. K. M. F. Hussain, K. B. M. Q. Zaman
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- Journal of Fluid Mechanics / Volume 159 / October 1985
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- 20 April 2006, pp. 85-104
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Large-scale coherent structures in a large, single-stream plane mixing layer of air have been investigated experimentally. The unforced, initially fully turbulent mixing layer rolls up into organized structures whose average passage frequency fm at any downstream distance x from the lip depends on x. These structures are detected for the entire length of the measurement, i.e. up to x = 3 m or 5000θe. The Strouhal number Stθ (= fm θ/Ue) is observed to be a constant (≈ 0.024) at all x. θe and θ are, respectively, the exit and local momentum thicknesses of the mixing layer, and Ue is the free-stream velocity. (The entrainment velocity on the zero-speed side is found to be 0.032 Ue.) The coherent-structure properties are educed in the developing and self-preserving regions of the mixing layer using an optimized conditional-sampling method, triggered on the peaks of a local reference ũ-signal obtained from the high-speed edge of the layer. Sectional-plane contours of the properties of the structure such as coherent vorticity, Reynolds stress and production reveal that the structure formation and evolution are complete by x ≅ 500θe, beyond which the structure achieves an ‘equilibrium’ state as defined by the structure properties.
Natural large-scale structures in the axisymmetric mixing layer
- K. B. M. Q. Zaman, A. K. M. F. Hussain
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- Journal:
- Journal of Fluid Mechanics / Volume 138 / January 1984
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- 20 April 2006, pp. 325-351
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This paper summarizes results of our investigations on: optimization of conditional sampling technique for eduction of naturally occurring large-scale structures in an axisymmetric mixing layer, comparison of the natural structure with that induced via controlled excitation, and the sensitivity of the educed structure to the excitation amplitude and of the natural coherent structure to Reynolds number and initial condition. Measurements include sectional-plane contours of various structure properties; however, coherent vorticity is the principal measure used in these considerations. All plausible alternative triggering criteria, based on reference velocity signals from fixed probes, were considered in order to arrive at the best practical eduction technique. It is shown that the simple criterion of triggering on the positive peaks of the longitudinal velocity signal derived from the high-speed edge of the mixing layer results in the optimum eduction. The characteristics of the natural structures, educed by the optimum detection criterion, are found to be independent of ReD over the measurement range 5.5 × 104−8 × 105. A mild dependence on the initial condition (viz laminar vs. turbulent) is observed, the structures being more disorganized for the initially laminar boundary-layer case. The educed natural structures agree well with those induced by controlled sinusoidal excitation at low excitation levels; higher levels, however, produce considerably stronger structures.
A ‘turbulent spot’ in an axisymmetric free shear layer. Part 2
- A. K. M. F. Hussain, S. J. Kleis, M. Sokolov
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- Journal:
- Journal of Fluid Mechanics / Volume 98 / Issue 1 / 15 May 1980
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- 19 April 2006, pp. 97-135
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The mechanics of a spark-induced coherent structure (called a ‘spot’) in the turbulent mixing layer of a 12.7 cm diameter incompressible air jet has been investigated through phase-locked measurements at three streamwise stations. Phase averages have been obtained from 200 realizations of X-wire (time-series) data after these are optimally time-aligned with respect to one another through an iterative process of maximization of cross-correlation of individual realizations with the ensemble average. Realizations that are grossly out of alignment owing to turbulence-induced distortions have been rejected; the rejection ratio increases with increasing radial position. Data include phase-average time series of background turbulence intensities, coherent and background Reynolds stresses, vorticity and intermittency at different transverse positions. Spatial distributions of these properties over the extent of the spot have been presented as contour maps. The computed pseudo-stream-functions have been compared with the phase-average streamlines inferred from the measured distributions of the velocity vector. Comparison with the phase-average intermittency contours show that the pseudo-stream-functions are reliable and, even though the integration involved produces smoothed-out stream functions, are most useful in deducing the structure dynamics and its convection velocity.
The spark-induced spot is an elongated large-scale coherent vortical structure spanning the entire thickness of the mixing layer, which moves downstream at a convection velocity of about 0.68Ue. The dynamics of the turbulent mixing layer spot, whose signature is buried in the large-amplitude background fluctuations, is much more complicated than that of the boundary-layer spot. The spot transports jet-core fluid outwards at its front and entrains ambient fluid primarily at its back; the outward-momentum transport dominates the inward transport. The Reynolds stress contribution by the spot structure is noticeably larger than that due to the background turbulence. The coherent structure vorticity is significantly modified by the structure-induced organization of the background Reynolds stress at the locations of ‘saddle points’ of the latter's distribution. The vorticity, intermittency and other turbulence measures, zone averaged over the extent of the spot, compare well with the time-average values, thus suggesting that the spark-induced ‘spot’ is probably not different from a naturally occurring large-scale coherent structure.
Vortex pairing in a circular jet under controlled excitation. Part 2. Coherent structure dynamics
- A. K. M. F. Hussain, K. B. M. Q. Zaman
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- Journal:
- Journal of Fluid Mechanics / Volume 101 / Issue 3 / 11 December 1980
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- 19 April 2006, pp. 493-544
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The coherent structure dynamics in the near field of a circular jet has been experimentally explored by inducing ‘stable’ vortex pairing through controlled excitation (see Zaman & Hussain 1980) and applying phase-averaging techniques. Hot-wire measurements were made in a 7·62 cm air jet with laminar exit boundary layer at the Reynolds number ReD = 3·2 × 104, excited at the Strouhal number StD = 0·85. At a particular phase during the pairing process, spatial distributions of the phase-average longitudinal and lateral velocity perturbations (〈u)〉, 〈v〉), vorticity, streamlines, the coherent and background Reynolds stresses and turbulence intensities have been educed. These data have been obtained for four different locations occupied by the vortices at the same phase (preceding, during, and following the pairing event), in the region 0 < x/D < 5. Spatial distributions of these measures at four successive phases during the pairing process are also educed in an attempt to further understand the vortex-pairing dynamics. The flow physics is discussed on the basis of measurements over the physical extent of the vortical structures, phase-locked to specific phases of the pairing event and thus do not involve use of the Taylor hypothesis.
The computed pseudostream functions at particular phases are compared with the corresponding streamlines drawn by the method of isoclines. Transition of the vortices is examined on the basis of vorticity diffusion, the superimposed random fluctuation field intensities and Reynolds stress and phase-locked circumferential correlation measurements. The peak vorticity drops rapidly owing to transition and interaction of the vortices during pairing but, farther downstream, the decay can be attributed to destruction of the coherent vorticity by the background turbulence Reynolds stress, especially at the locations of the latter's ‘saddle points’. Controlled excitation enhances the initial circumferential coherence of the vortical structures, but is ineffective in delaying turbulent breakdown near the end of the potential core; the breakdown appears to occur through evolution of the circumferential lobe structures. The coherent structure Reynolds stress is found to be much larger than the background turbulence Reynolds stress for 0 < x/D [lsim ] 3, but these two are comparable near the end of the jet potential core. The zone average of the coherent structure Reynolds stress over the cross-section of the merging vortex pair is much larger than that over a single vortical structure either before or after the completion of pairing. During the pairing process, such average correlations are found to be the largest at an early phase of the process while entrainment, turbulent breakdown as well as rapid diffusion of vorticity occur at a later phase. The regions of alternate positive and negative coherent Reynolds stresses associated with the structures and their interactions help explain ‘negative production’.
A ‘turbulent spot’ in an axisymmetric free shear layer. Part 1
- M. Sokolov, A. K. M. F. Hussain, S. J. Kleis, Z. D. Husain
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- Journal of Fluid Mechanics / Volume 98 / Issue 1 / 15 May 1980
- Published online by Cambridge University Press:
- 19 April 2006, pp. 65-95
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A three-dimensional ‘turbulent spot’ has been induced in the axisymmetric free mixing layer of a 12.7 cm diameter air jet by a spark generated at the nozzle boundary layer upstream of the exit. The spot coherent-structure signature, buried in the large-amplitude random fluctuating signal, has been educed at three downstream stations within the apparent self-preserving region of the mixing layer (i.e. x/D = 1.5, 3.0 and 4.5) at the jet exit speed of 20 ms−1. The eduction has been performed through digital phase averaging of the spot signature from 200 realizations. In order to reduce the effect of the turbulence-induced jitter on the phase average, individual filtered signal arrays were optimally time-aligned through an iterative process of cross-correlation of each realization with the ensemble average. Further signal enhancement was achieved through rejection of realizations requiring excessive time shifts for alignment. The number of iterations required and the fraction of realizations rejected progressively increase with the downstream distance and the radial position.
The mixing-layer spot is a large-scale elongated structure spanning the entire width of the layer but does not appear to exhibit a self-similar shape. The dynamics of the mixing-layer spot and its eduction are more complicated than those of the boundary-layer spot. The spot initially moves downstream essentially at a uniform speed across the mixing layer, but further downstream it accelerates on the high-speed side and decelerates on the low-speed side. This paper discusses the data acquisition and processing techniques and the results based on the streamwise velocity signals. Phase average distributions of vorticity, pseudo-streamlines, coherent and background Reynolds stresses and further dynamics of the spot are presented in part 2 (Hussain, Kleis & Sokolov 1980).
Controlled symmetric perturbation of the plane jet: an experimental study in the initial region
- A. K. M. F. Hussain, C. A. Thompson
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- Journal:
- Journal of Fluid Mechanics / Volume 100 / Issue 2 / 25 September 1980
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- 19 April 2006, pp. 397-431
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The response of the near field of a free, plane air jet (aspect ratio 44:1) to a controlled, sinusoidal perturbation was investigated by hot-wire measurements. The experiments were carried out at an exit excitation amplitude of 1·4 % for the Strouhal number range 0·15 [les ] StH [les ] 0·6 and the Reynolds-number range 8 × 103 [les ] ReH [les ] 3·1 × 104. The influence of the excitation, introduced with a loudspeaker attached to the jet settling chamber, on the mean and fluctuating velocity fields is much weaker than that in the circular jet. The amplitude and phase profiles of the fundamental, educed through phase-locked measurements, show that the induced symmetric mode remains symmetric as it travels downstream. The wave growth rate is much higher and the wavelength much smaller in the shear layer than on the centre-line of the jet. The wave fundamental attains its maximum amplitude at StH ≃ 0·18 on the jet centre-line and at StH ≃ 0·45 in the shear layer. The amplitude profiles of the fundamental in the shear layer agree quite well with the spatial stability theory of Michalke (1965b); however, the phase data do not agree well with the theoretical predictions. The growth rate and the disturbance wavenumber increase monotonically with the StH both in the shear layer and on the centre-line but tend to approach constant values at higher StH. The phase velocity data show that, in the lower Strouhal-number range, the plane jet acts as a non-dispersive waveguide.
Vortex pairing in a circular jet under controlled excitation. Part 1. General jet response
- K. B. M. Q. Zaman, A. K. M. F. Hussain
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- Journal of Fluid Mechanics / Volume 101 / Issue 3 / 11 December 1980
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- 19 April 2006, pp. 449-491
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Hot-wire and flow-visualization studies have been carried out in three air jets subjected to pure-tone acoustic excitation, and the instability, vortex roll-up and transition as well as jet response to the controlled excitation have been investigated. The centreline fluctuation intensity can be enhanced by inducing stable vortex pairing to a level much higher than even that at the ‘preferred mode’, but can also be suppressed below the unexcited level under certain conditions of excitation. The conditions most favourable to vortex pairing were determined as a function of the excitation Strouhal number, the Reynolds number (ReD), and the initial shear-layer state, i.e. laminar or turbulent. It is shown that the rolled-up vortex rings undergo pairing under two distinct conditions of excitation: ‘the shear layer mode’ when the Strouhal number based on the initial shear-layer momentum thickness (Stθ) is about 0·012, and ‘the jet column mode’ when the Strouhal number based on the jet diameter (StD) is about 0·85. The former involves pairing of the near-exit thin vortex rings when the initial boundary layer is laminar, irrespective of the value of StD. The latter involves pairing of the thick vortex rings at x/D ≅ 1·75, irrespective of Stθ or whether the initial boundary layer is laminar or turbulent. For laminar exit boundary layer, pairing is found to be stable, i.e., occurring regularly in space and time, for ReD < 5 × 104, but becomes intermittent with increasing ReD or fluctuation intensity in the initial boundary layer.
The trajectories of the vortex centres and their convection velocities during a pairing event have been recorded through phase-locked measurements. In the presence of stable vortex pairing, the time average profiles of fluctuation intensities and Reynolds stress show noticeable deviations from those in the unexcited jet. The vortex pairing phenomenon produce considerably larger excursions of the $\widetilde{uv}(t)$ signal than the time-average Reynolds stress reveals, suggesting that only certain phases of the pairing process may be important in entrainment, and production of Reynolds stress and jet noise.
The free shear layer tone phenomenon and probe interference
- A. K. M. F. Hussain, K. B. M. Q. Zaman
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- Journal of Fluid Mechanics / Volume 87 / Issue 2 / 26 July 1978
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- 12 April 2006, pp. 349-383
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Free shear layer stability measurements with a hot wire revealed that the probe itself can trigger and sustain upstream instability modes like the slit jet-wedge edge tones. The flow fields associated with the free shear layer tones induced in axisym-metric and plane air shear layers by a hot-wire probe and by a plane wedge were then explored experimentally, and found to be different in many ways from the widely investigated jet edge tone phenomenon.
As many as four frequency stages have been identified, there being a fifth stage associated with the subharmonic attributed to vortex pairing in the free shear layer. No evidence of hysteresis could be found in the shear layer tone. In the interstage jump (i.e. bimodal) regions, the tone occurred in only one mode at a time while intermittently switching from one to the other. Frequency variations in each stage are shown to collapse on a single curve when non-dimensionalized with the initial momentum thickness θe or with the lip-wedge distance h, and plotted as a function of h/θe.
Phase average measurements locked onto the tone fundamental show that the phase velocity and wavelength of the tone-induced velocity fluctuation are essentially independent of the stage of tone generation; in each stage, both phase velocity and wavelength decrease with increasing frequency but undergo jumps at starts of new stages. The measured amplitude and phase profiles, as well as the variations of the shear tone wavenumber and phase velocity with the Strouhal number, show reasonable agreement with the predictions of the spatial stability theory. The wavelength λ bears a unique relation to h, this h, δ relation being different from the Brown-Curle equation for the jet edge tone.
Shear layer tones would be typically induced in near-field shear layer measurements involving invasive probes, and can produce misleading results. A method for determining the true free shear layer natural instability frequency is recommended.
The mechanics of an organized wave in turbulent shear flow
- A. K. M. F. Hussain, W. C. Reynolds
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- Journal of Fluid Mechanics / Volume 41 / Issue 2 / 13 April 1970
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- 29 March 2006, pp. 241-258
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Some preliminary results on the behaviour of controlled wave disturbances introduced artificially into turbulent channel flow are reported. Weak plane-wave disturbances are introduced by vibrating ribbons near each wall. The amplitude and relative phase of the streamwise component of the induced wave is educed from a hot wire signal, allowing the wave speed and attenuation characteristics and the wave shape to be traced downstream. The normal component and wave Reynolds stress have been inferred from these data. It appears that Orr–Sommerfeld theories attempted to date are inadequate for description of these waves.