Browsing by Subject "Turbulence"

Now showing 1 - 4 of 4

Open Access
Derivation of Rytov variance for jet engine-induced turbulence
(Institute of Physics Publishing Ltd., 2024-07-15) Oktay, Semih; Bayraktar, Mert; Tabaru, Timuçin Emre; Chatzinotas, Symeon
In this study, we analytically derived the Rytov variance and scintillation index value of turbulence caused by jet engines. In addition, we analyzed the variation characteristics of the Rytov variance and scintillation index values numerically depending on the variations in turbulence strength, experimental data, and wavelength. We observe that Rytov variance reaches up to high values due to the strong turbulence resulting from high refractive index fluctuations. This result brings high-intensity fluctuations regardless of the anisotropy of the turbulence. Rytov variance is directly proportional to turbulence strength. We present scintillation index curves considering the aperture averaged case. We plot our results considering the variations in the operating wavelength, turbulence strength, and the scaling parameter. According to our results, we think that it will be useful for a system such as directed infrared countermeasure (DIRCM), which is highly sensitive and should be exposed to minimum turbulence in the field of use. Since DIRCMs transmit codes to paralyze the missile's seeker, intensity fluctuations play a vital role during this transmission. It could reduce the performance of these systems when intensity fluctuations are high.
Open Access
Linear and nonlinear stability of a quasigeostrophic mixing layer subject to a uniform background shear
(American Physical Society, 2019) Biancofiore, Luca; Umurhan, O. M.
The aim of this work is to shed light by revisiting, from the kernel-wave (KW) perspective, the breakdown of a quasigeostrophic (QG) mixing layer (or vortex strip or filament) in atmosphere under the influence of a background shear. The QG mixing layer is modeled with a family of quasi-Rayleigh velocity profiles in which the potential vorticity (PV) is constant in patches. From the KW perspective, a counterpropagating Rossby wave (CRW) is created at each PV edge, i.e., the edge where a PV jump is located. The important parameters of our study are (i) the vorticity of the uniform shear m and (ii) the Rossby deformation radius Ld, which indicates how far the pressure perturbations can vertically propagate. While an adverse shear (m<0) stabilizes the system, a favorable shear (m>0) strengthens the instability. This is due to how the background shear affects the two uncoupled CRWs by shifting the optimal phase difference towards large (small) wave number when m<0 (m>0). As a finite Ld is introduced, a general weakening of the instability is noticed, particularly for m>0. This is mainly due to the reduced interaction between the two CRWs when Ld is finite. Furthermore, nonlinear pseudospectral simulations in the nominally infinite-Reynolds-number limit were conducted using as the initial base flow the same quasi-Rayleigh profiles analyzed in the linear analysis. The growth of the mixing layer is obstructed by introducing a background shear, especially if adverse, since the vortex pairing, which is the main growth mechanism in mixing layers, is strongly hindered. Interestingly, the most energetic configuration is for m=0, which differs from the linear analyses for which the largest growth rates were found for a positive m. In the absence of a background shear additional modes are subharmonically triggered by the initial disturbance enhancing the turbulent character of the flow. We also confirm energy spectrum trends for broken-down mixing layers reported in the literature. We interpret the character of mixing-layer breakdown as being a phenomenological hybrid of Kraichnan's [R. H. Kraichnan, Phys. Fluids 10, 1417 (1967)] direct enstrophy cascade picture and the picture of self-similar vortex production.
Open Access
Navigating the Covid 19 turbulence in higher education: Evidence from Turkish faculty members
(Dokuz Eylul University, 2022-09-30) Örücü, Deniz; Kutlugün, Habibe Elif
Covid19 was the first pandemic of the modern era to strike with such virulence. We sought to understand this recent phenomenon and contribute to the empirical findings on the expectations from HEI leadership and management in Turkey. Drawing on the Turbulence Theory, we explored how the academic staff experienced the initial phase of the pandemic in Turkey and how they perceived the HE leaders’ navigation of the crisis at the selected universities. Within qualitative phenomonology, data from semi-structured interviews with a convenient sample of 10 academic staff in five public and five private universities in Turkey, was analysed through content analysis. Findings highlighted the opportunities and challenges of the pandemic for the faculty at personal and organizational level in an intersectional pattern. Moreover, the ways HEI leaders navigated the crisis created binaries in the form of experience vs. inexperience and trust vs. distrust. The challenges derived from the rapid but ineffective decision-making processes and the heightened surveillance mechanisms over the academic staff; which in some cases resulted in lack of trust. Hence, the turbulence level was shaped by how the universities and their leaders addressed it. In such cases, practices of building trustworthy connections, more distributive forms of leadership and robust communication; which would help the leaders to navigate the turbulence at times of crises are significant. Further recommendations are provided for research, policy and practice.
Open Access
A spectral vanishing viscosity method for large-eddy simulations of two-fluid flow
(2020-12) Khoshavaz, Solmaz
DNS studies of turbulent flows have proved to be inefficient in terms of time and computational resources. On the other hand, Large-eddy simulation (LES) is an effective approach towards modeling turbulence. The current research applies an extension of the Spectral Vanishing Viscosity (SVV) method to finite differences. This straight-forward LES technique allows turbulence modeling without the need for filtering or upwinding. The result is a hybrid DNS/LES Solver. The solver is applied to the two-fluid problem of falling liquid film in the presence of turbulent gas. Numerical simulation of falling liquid films requires a mathematical representation of the multiphase flow. A Direct Numerical Simulation (DNS) solver implementing finite volumes is used to solve the Navier-Stokes equations for the liquid phase. The Front Tracking method is used to model the moving gas-liquid interface. Gravity-driven falling liquid films are commonplace in engineering applications. Perturbed falling films dramatically increase the heat/mass transport across the interface compared to flat films, which highlights the significance of studying interfacial flows. The present research aims to develop a numerical tool, which will be used to further investigate falling liquid film phenomena.