Frequency Of Applied Force Research Articles

Impact of Turbulent Time Scales on Wake Recovery and Operation

Enhancing wake recovery behind wind turbines has the potential to significantly improve the power production and efficiency of large wind farms. Rather than investigating turbine control strategies, floater motion or global turbulent quantities such as turbulence intensity, this work aims to study wake stability and recovery through a focus on the turbulent scales of the inflow. Using Large Eddy Simulations of a single turbine, sinusoidal streamwise forcing is applied to the inflow with a constant amplitude and mean flow velocity, but differing time scales between 80s and 140s. For all applied time scales the turbine wake is characterised by the rolling-up of the near wake into the periodic shedding of vortex rings, and an excitation of the applied forcing frequency resulting in velocity fluctuations in the wake several times larger than that at the inflow. For shorter time scales (80s - 90s) a more aggressive and earlier wake roll-up led to a shorter near wake region, faster overall recovery and significantly improved the expected power output from 6R downstream onwards. An inflow time period of 80s gave rise to more than a 50% increase in power output of a fictive downstream turbine placed at 14R downstream, compared to an inflow time scale of 140s.

Shock Train Response to High-Frequency Backpressure Forcing

Numerical simulations are performed to study the effect of backpressure disturbances on a shock train. The basic problem consists of a shock train within a constant-area channel with an inflow and backpressure applied to the outlet. By subjecting the shock train to different fixed backpressures, it is shown that the shock train length varies linearly with backpressure, while the shock spacing is conserved. Through backpressure step forcing, it is shown that the response of the shocks, after an initial lag, is largely determined by the magnitude of the step change. Step increases in backpressure cause upstream shock movements with higher shock pressures. The opposite response occurs with backpressure decreases. With sinusoidal backpressure forcing, the shock train oscillates at the applied forcing frequency. The forcing propagates disturbances upstream, and therefore there is a noticeable forcing-response lag. Higher frequency disturbances are filtered as they travel upstream through the shock train while lower frequencies induce larger shock oscillations and produce upstream offsets in the shock positions. Further examination of the flow reveals a region of low-frequency motion below the leading shock, which becomes strongly amplified by the lowest backpressure forcing frequency.

Forcing frequency effects on turbulence dynamics in pulsatile pipe flow

The turbulence dynamics of pulsatile pipe flow are investigated using direct numerical simulation (DNS) at a mean friction Reynolds number of 180. Results are presented for a range of forcing frequencies at a fixed amplitude, which, based on existing classifications, corresponds to the current-dominated regime. This work directs attention towards the phase-variations of single and two-point turbulence statistics, with a particular emphasis on the response of the Reynolds shear stress to systematic changes in the applied forcing frequency. The study has yielded two key outcomes. (i) A new frequency classification procedure for pulsatile turbulent flows (at low-to-moderate friction Reynolds numbers), informed by the Reynolds shear stress frequency co-spectra and the value of the applied forcing frequency. (ii) A detailed account of single- and two-point Reynolds shear stress statistics, in response to high, very-high and ultra-high forcing frequencies in order to study turbulence dynamics in the physical and Fourier domains. Furthermore, the oscillatory velocity field obtained from the DNS data is compared against the laminar Womersley solution in order to assess the interaction (or lack thereof) between the oscillatory velocity field and phase-averaged Reynolds shear stress fluctuations. For the higher frequencies considered in this work, single- and two-point Reynolds shear statistics all enter the so-called “frozen” regime — which occurs as the forcing time-scale becomes smaller than that of the highest-frequency, energy-containing motions in the Reynolds shear stress co-spectra.

Modulation of sound waves for flow past a rotary oscillating cylinder in a non-synchronous region

Modulation of sound waves for the laminar flow past a rotary oscillating circular cylinder has been studied for a free-stream Reynolds number Re = 150 and Mach number M = 0.2. Modulation of sound waves has been observed if the combination of applied rotary oscillation frequency and amplitude belongs to the nonsynchronous region where the hydrodynamic and acoustic quantities vary with the vortex shedding frequency as well as the applied forcing frequency. Two-dimensional direct numerical simulations (DNS) are carried out on a highly refined grid using high resolution physical dispersion relation preserving schemes for a nondimensional forcing frequency-ratio range 0.1 ≤ fr ≤ 2.0 at a nondimensional surface speed A1 = 0.1. Both the synchronous and the nonsynchronous zones are identified based on the time-varying fluctuations in the lift and the drag coefficients. In the nonsynchronous zone, modulation phenomena of the lift and the drag coefficients are explained by plotting the stream-function contours over multiple vortex shedding cycles. The modulation periods associated with the fluctuating lift and the drag coefficients are different for some cases. This particular observation is in contrast with the observation expressed in the previous studies investigating similar problems. Disturbance pressure fields obtained from the present DNS data are used to analyze the characteristics of radiated sound fields, especially in the nonsynchronous zone. Information related to aerodynamic sound sources has been obtained using approximated Lighthill’s stress tensor, and it is shown that the aerodynamic sound sources also display the modulation phenomenon similar to that observed in the vortex shedding process. Sound fields related to the nonsynchronous zone also exhibit the modulation phenomenon and are governed by the shedding frequency, the forcing frequency, and their linear combinations. Radiated sound field characteristics are further related to the time-varying fluctuations of the lift and the drag coefficients using Curle’s acoustic analogy. Modulated sound waves observed along the upstream and the transverse directions have similar time variation as that of the drag and the lift coefficients, respectively. The phenomenon of beat formation has been observed for the ranges 0.9 ≤ fr ≤ 0.99 and 1.2 ≤ fr ≤ 1.4. Although the observed modulation of sound waves varies significantly with the forcing frequency-ratio, the net radiated sound power has almost remained constant in the nonbeating, nonsynchronous zone. Furthermore, it is confirmed that the dominant sound modes obtained during the proper orthogonal decomposition of disturbance pressure fields in the nonsynchronous zone are related to the shedding frequency-ratio, the forcing frequency-ratio, and their linear combinations.

Forced synchronization and asynchronous quenching of periodic oscillations in a thermoacoustic system

We perform an experimental and theoretical study to investigate the interaction between an external harmonic excitation and a self-excited oscillatory mode ($f_{n0}$) of a prototypical thermoacoustic system, a horizontal Rijke tube. Such an interaction can lead to forced synchronization through the routes of phase locking or suppression. We characterize the transition in the synchronization behaviour of the forcing and the response signals of the acoustic pressure while the forcing parameters, i.e. amplitude ($A_{f}$) and frequency ($f_{f}$) of forcing are independently varied. Further, suppression is categorized into synchronous quenching and asynchronous quenching depending upon the value of frequency detuning ($|\,f_{n0}-f_{f}|$). When the applied forcing frequency is close to the natural frequency of the system, the suppression in the amplitude of the self-excited oscillation is known as synchronous quenching. However, this suppression is associated with resonant amplification of the forcing signal, leading to an overall increase in the response amplitude of oscillations. On the other hand, an almost 80 % reduction in the root mean square value of the response oscillation is observed when the system is forced for a sufficiently large value of the frequency detuning (only for$f_{f}<f_{n0}$). Such a reduction in amplitude occurs due to asynchronous quenching where resonant amplification of the forcing signal does not occur, as the frequency detuning is significantly high. Further, the results from a reduced-order model developed for a horizontal Rijke tube show a qualitative agreement with the dynamics observed in experiments. The relative phase between the acoustic pressure ($p^{\prime }$) and the heat release rate ($\dot{q}^{\prime }$) oscillations in the model explains the occurrence of maximum reduction in the pressure amplitude due to asynchronous quenching. Such a reduction occurs when the positive coupling between$p^{\prime }$and$\dot{q}^{\prime }$is disrupted and their interaction results in overall acoustic damping, although both of them oscillate at the forcing frequency. Our study on the phenomenon of asynchronous quenching thus presents new possibilities to suppress self-sustained oscillations in fluid systems in general.

Effect of Jet Oscillation on the Maximum Impingement Plate Skin Friction

The maximum impingement plate skin friction and flow field is measured for an acoustically forced planar impinging gas jet using oil film interferometry (OFI) and particle image velocimetry (PIV), respectively. The study is performed at a jet Reynolds number of Rejet = 11,000 and an impingement distance H, which is set to eight times the nozzle width W. The planar impinging gas jet is forced at the jet nozzle exit using Strouhal numbers StH = 0.39, 0.76, and 1.1, which are similar to those associated with the jet-plate tones measured in air-knife wiping experiments. The flow-field measurements indicate that the jet column oscillates at the applied forcing frequency, and depending on the forcing frequency, organized vortex structures can be identified in the shear layers that impinge on the plate surface. Both of these jet oscillation features result in a reduction in the time-averaged maximum impingement plate skin friction. This skin friction reduction is attributed to momentum loss at the jet centerline caused by increased levels of fluid entrainment and mixing of the surrounding quiescent fluid.

Soot formation in unsteady counterflow diffusion flames

The formation of pollutant species in turbulent diffusion flames is strongly affected by the coupling between the highly non-linear chemical kinetics with three-dimensional, unsteady hydrodynamics. It is necessary to better understand this interdependency of transport and kinetic mechanisms, in order to accurately predict non-equilibrium effects in the numerical modeling of pollutant formation (especially PAH and soot). Unsteady counterflow diffusion flames can be conveniently used to address the effects of hydrodynamic unsteadiness on the pollutant chemistry, because they posses much of the physics postulated for diffusion flamelets and exhibit a large range of combustion conditions with respect to steady flames. Thus, these flames give insights into a variety of chemistry-flow field interactions important in turbulent combustion.In this paper, spatially resolved concentrations of PAH and soot are presented for counterflow diffusion flames fed with methane, propane and ethylene at several strain rates. Unsteady effects on the formation of PAH and soot are investigated for the propane flame by imposing harmonic oscillations in the strain rate with frequencies between 0.1 and 100Hz. Numerical results reveal a net increase in the concentration of aromatic species and soot when the strain rate oscillations are imposed to the flame. The response of the flame in terms of soot and PAH concentrations appears strongly dependent on the applied forcing frequency. PAH and soot exhibit specific behavior according to their characteristic chemical time scales which are longer than those of the main combustion process. The PAH and soot formation is found more sensitive to velocity fluctuations for flames with large initial strain rate. At low frequencies of imposed oscillations the structure of the soot profile shows strong deviations from the steady-state profile. At large frequencies a decoupling between the soot concentration and the velocity field is evident.

Vibratory Plate Loading of Compacted and Instrumented Field Soil Beds

Abstract Field scale studies of foundation and plate vibrations, particularly in the nonlinear domain, are vital to improving understanding and validating analytical and numerical efforts. This paper describes the development of an experimental program to investigate the dynamic signature of a rigid plate and of the soil during surface plate vibration of compacted soil beds. Nineteen testing sequences were performed on 1.2-m deep prepared soil test beds embedded with miniature triaxial accelerometers. Various vibratory loading frequencies (10–60 Hz) and amplitudes (up to 220 kPa contact stress) were applied via a 0.45-m-diameter plate to the prepared test beds. The paper describes the load testing frame, the preparation of free field partially-saturated sand test beds, and the placement of triaxial accelerometers within the soil. Plate and soil acceleration profiles are presented for the range of applied forcing frequency and amplitude levels involved. The nonlinear soil-compactor system response is analyzed and presented in both the time and frequency domains to present key features observed during testing.