Showing papers on "Strouhal number published in 2021"

Characteristic unsteady pressure field on a civil aircraft wing related to the onset of transonic buffet

Abstract: Unsteady pressure fields on a civil aircraft wing near the onset of transonic buffet have been investigated experimentally to identify flow unsteadiness that relates to the buffet onset determined by global criteria. An 80%-scale NASA Common Research Model was tested in the JAXA 2 m × 2 m Transonic Wind Tunnel at a Mach number of 0.85 and a chord Reynolds number of 2.27 × 106. The angle of attack was varied in small increments around the buffet onset angle determined by global criteria based on the lift curve and wing-root strain-gauge data. Unsteady pressure fields over the wing were measured using unsteady pressure-sensitive paint (PSP) with temperature-effect correction by temperature-sensitive paint (TSP). Characteristic pressure fluctuations, known as “buffet cells”, were observed under the off-design conditions at a bump Strouhal number of 0.2–0.5. The PSP results showed that the buffet cells arise at the mid-span wing at η ≈ 0.45, where a strong shock wave causes an initial boundary-layer separation. Phase shift distributions indicated that a pressure perturbation propagates from the inboard wing toward the outboard wing. The convection velocity and spanwise wavelength were approximately 0.5U∞ and 1.3cMAC, respectively. The angle of attack at which buffet cells first appear was found to be approximately equal to the buffet onset determined by the global criteria, indicating that the occurrence of the buffet cells is deeply related to the buffet onset for the present wing geometry.

Drag reduction of a D-shaped bluff-body using linear parameter varying control

Abstract: In this work, we report on a closed-loop flow control strategy that consistently reduces the drag of a D-shaped bluff body under variable freestream velocity conditions. The control strategy is guided by open-loop tests with pulsed Coanda blowing at two freestream velocities that yield optimal frequencies (Strouhal number of 0.33 and 1.3), which reduce the drag by up to 40%. The strong correlation between drag coefficient (Cd) and the wake fluctuations is exploited for the feedback signal, where a microphone signal is used to measure the pressure fluctuations at the model base. The results demonstrate the ability to perform accurate and robust H ∞-based control for drag reduction using solely the wake pressure fluctuations at the model base as feedback signal. The robust control strategy at constant freestream velocity is shown to improve output stability and enhance performance in terms of settling time, even when employing simple models of the flow response with large uncertainty. Building on that success, an H ∞-based linear parameter varying controller is designed and implemented to reduce drag under free stream variations and/or fluctuations. Similarly, the results demonstrate improved robustness and performance enhancements.

Flow measurements in the wake of an adhering and oscillating droplet using laser-Doppler velocity profile sensor

Abstract: The removal of droplets on surfaces by an (air-) flow is relevant, e.g., for cleaning processes or to prevent corrosion or damage of electronic devices. Still the condition for droplet movement is not fully understood. Droplets start to move downstream at a critical (air-) flow velocity vcrit. For increasing flow velocity, this process is related to a strong oscillation of the droplet. This oscillation is supposed to be a key mechanism for the onset of droplet movement in conjunction with the flow field around the droplet. We report on measurements in the wake of the adhering droplet by means of laser-Doppler velocity profile sensor and hot wire anemometry. Thanks to the excellent spatial and temporal resolution of laser-Doppler velocity profile sensor and its capability to measure bidirectional flows, a backflow region can be detected in the wake of the droplet. Therefore, it can be concluded that this backflow structure is the driving mechanism for the strong flow movement inside the droplet against channel flow direction found in previous work. Analyzing the frequency spectra of the flow velocity, it was found that the flow is also oscillating; frequency peaks are in the same range as for the contour oscillation. Based on frequency, diameter and flow velocity, a Strouhal number can be calculated. This Strouhal number is almost constant in the investigated regime of droplet volumes and is between 0.015 and 0.03. Therefore, it can be assumed that an aeroelastic self-excitation effect may be present that eventually leads to droplet movement.

Modeling and identification of combined effects of pulsating inlet temperature and use of hybrid nanofluid on the forced convection in phase change material filled cylinder

Abstract: Effects of pulsating heat transfer fluid temperature and hybrid nano-additive inclusion in the base fluid are numerically studied for laminar forced convection through a phase change material embedded thermo-fluid system with finite element method Effects of different values of Reynolds number (between 250 and 1000), amplitude (between 0 and 005) and frequency (Strouhal number between 001 and 05) of pulsating inlet temperature, nanoparticle volume fraction of hybrid particles (between 0 and 002) on the dynamic features of the system with performance characteristics are analyzed It is observed that the phase change material onset temperature becomes oscillating with drastically reduction of full completion time as the Reynolds number and amplitude of pulsation are increased The amount of reduction in the full phase transition is 63 % when cases at Re=100 to Re=400 are compared When lowest and highest amplitude configurations are compared, 62 % reduction in the complete phase transition time is observed while the impact of frequency is marginal at higher frequencies When the hybrid nanoparticles are introduced in the base fluid, transition time and dynamic features of onset temperatures are affected Successful results that capture the dynamic behavior of the phase change embedded thermal system is achieved with a nonlinear dynamic system modeling approach

Lubricating hot stretching membrane with a thin hybrid nanofluid squeezed film under oscillatory compression

Abstract: The small-amplitude oscillatory compression is used in many micromachines to measure various properties of molten polymers and has several advantages over rotational devices due to a less sophisticated mechanism and the lower cost. In this article, an oscillatory squeezing flow of GO–MoS2 hybrid nanofluid mixed in C2H6O2–H2O is considered over a variably hot stretching elastic membrane. The two-dimensional unsteady heat convection problem is modeled by the Navier–Stokes and the energy conservation equations. The normalized governing equations are solved with an advanced fourth-order numerical finite-difference scheme. Various characteristics of the flow and the heat transfer phenomena, such as the velocity and the temperature profiles, the skin-friction drag and the heat transfer rate at lower wall, are calculated and analyzed through several graphs and tables. The results reveal that the forward and backward flow strongly depends upon the amplitude of compression. A 30% increase in the amplitude of compression oscillation results in 109% increase of heat transfer rate from fluid to stretching membrane without altering the frictional drag significantly. Further, adding 8% concentration of GO–MoS2 hybrid nanofluid enhances the heat transfer rate from fluid to membrane by 21.4%. Moreover, as the Strouhal number increases from 1 to 6 the heat transfer rate from fluid to membrane increases up to 77.6%.

Time-resolved wake dynamics of finite wall-mounted circular cylinders submerged in a turbulent boundary layer

Abstract: The unsteady flow separation and wake dynamics around finite wall-mounted circular cylinders fully immersed in a turbulent boundary layer (TBL) are investigated experimentally using a time-resolved particle image velocimetry (TR-PIV) system. The cylinder aspect ratios (h/d = 0.7–7.0, where h and d are the height and diameter of the cylinder, respectively) and the relative boundary layer thickness (δ/d = 8.7, where δ is the boundary layer thickness) were chosen to systematically investigate the effects of submergence ratio (δ/h = 1.2–12.4) using δ/h values much larger than that reported in the literature. With δ/h > 1.0, the cylinders encountered elevated turbulence levels (4%–10 %), reduced mean velocity and strong mean shear in the approach TBL which had profound effects on the attachment length and flapping motion of the reverse-flow region on the top surface of the cylinders. The time-averaged statistics including the mean velocities, Reynolds stresses and production terms were used to characterize the flow field and the large-scale anisotropy. The results showed that the wake structure of the submerged cylinders can be divided into dipoles and quadruples with a critical h/d = 3.5 and δ/h = 2.5. Both categories exhibited strong anisotropy, but the quadruples showed an interesting pattern where the streamwise Reynolds normal stress is less than the other components due to negative production in the wake region. Spectral analysis and joint-probability density functions are used to show that the reverse-flow region behind the cylinder is characterized by low-frequency flapping motions with a Strouhal number that decreases with increasing aspect ratio. The spatio-temporal evolution of the vortices also revealed the occurrence of cellular shedding behaviour where the vortices near the free end are shed discretely while those in the lower span are shed in the form of long streaky structures.

Dynamics of periodically-excited vortices in swirling flames

Abstract: This work investigates the dynamics of periodically-excited vortices in swirling cold flow and hot flame using high-speed particle image velocimetry, for three different Strouhal numbers, 0.68, 1.02, and 1.70. The periodic upstream perturbation induces coherent vortices in the shear layers both inside and outside of the flame surface. We find that the outer vortex rings (OVRs) play a dominant role in tuning the flame dynamics and heat release, whereas the inner vortex rings (IVRs) are related to the formation of the center recirculation zone (CRZ) and suffer from much stronger dissipation. The evolutions of core vorticity, circulation, trajectory, and convective velocity are quantitatively analyzed for the OVRs to understand two key mechanisms: vortex formation and vortex detachment. The results show that the growing and shedding of the OVRs are governed by the inlet shear layer as well as the variations in inlet velocity and acceleration. Furthermore, both the circulation increment during vortex formation and the vortex convection after detachment are qualitatively captured by existing vortex models, demonstrating the prospect of vortex dynamics in understanding general flame-vortex interaction problems.

On the effect of velvet structures on trailing edge noise: experimental investigation and theoretical analysis

Abstract: This study is inspired by the velvety structures on an owl's upper wing surface. Anechoic wind tunnel experiments were conducted to study the effect of the velvety structures on trailing edge noise as well as the boundary layer flow of a flat plate model. The tests were conducted in The Hong Kong University of Science and Technology low-speed wind tunnel, ultra-quiet noise injection test and evaluation device (UNITED). It was found that the trailing edge noise spectra are significantly modified by the velvety structures. In general, the velvety structures increase the low-frequency noise below a cross-over Strouhal number but reduce the spectral level at higher frequencies. The velvety surface also changes the boundary layer characteristics in terms of the boundary layer thickness, non-dimensional velocity distribution and turbulence distribution. Vortex shedding is suppressed by the velvety coating despite the blunt trailing edge. An analytic model is proposed for the trailing edge noise of a flat plate, including the effect of finite trailing edge thickness and velvety structures on the flat plate surface. The model uses the near wake distribution of the mean and fluctuating velocities in the streamwise direction as the input. The predictions, which require no empirical corrections, match well with the experiments for both the baseline and velvet-coated configurations. With a detailed non-dimensional analysis, this study proposes a potential aeroacoustic function of velvet structures, i.e. noise control through manipulation of boundary layer statistics.

Global hydrodynamic instability and blowoff dynamics of a bluff-body stabilized lean-premixed flame

Abstract: We experimentally study the hydrodynamic instability of a lean-premixed flame stabilized behind a circular cylinder. On reducing the equivalence ratio ( ϕ) at a fixed Reynolds number (ReD), we find that the flame transitions from a steady mode to a varicose mode and then to a sinuous mode. By examining time-resolved CH* chemiluminescence images and analyzing how the Strouhal number scales with ReD, we determine that the varicose mode is convectively unstable, maintained by the amplification of disturbances in the turbulent base flow, whereas the sinuous mode is globally unstable as a result of the constructive interaction between the two diametrically opposite shear layers (Benard–von Karman instability). We attribute the emergence of the sinuous global mode to the flame moving sufficiently far downstream with decreasing ϕ that it is out of the wavemaker region. Finally, we investigate the lean blowoff dynamics and find that local flame pinch-off, which occurs at the end of the recirculation zone, is a reliable precursor of global flame blowoff.

Analysis of flow characteristics of two circular cylinders in cross-flow with varying Reynolds number: a review

Abstract: The present study reviews the physical attributes of the fluid behaviour in the flow regimes of steady cross-flow around two infinite circular cylinders. Some critical complexities in the wake interference of the flow across two cylinders for tandem, side-by-side and staggered arrangements have been described by different researchers in the past. In this aspect, the analyses as reported by the past researchers have been considered around the wake area. So far, the researchers have explored the effects of drag coefficient, vortex shedding frequencies, Strouhal number and different range of Reynolds number for bluff bodies. The real-time study to understand the fluid behaviour in the wake region is rarely carried out by any researcher in the past. In this study, an investigation of flow field around two circular cylinders with different geometrical configurations has been carried out. A broad range of Reynolds number and diameters of the cylinders have been used. With these two parameters in consideration, Strouhal number, near-wake flow interference and drag coefficient have been reviewed. The highlights of the flow characteristics in the wake of two cylinders have been evaluated and suitably outlined. The physical perspectives in each flow regimes have been discussed in detail as well. Experimental and numerical investigations as found in the literature have also been analysed.

New insights into numerical simulations of flow around two tandem square cylinders

Abstract: Flows around tandem square cylinders at Re = 100 are numerically studied using a characteristic-based penalty operator splitting finite element method based on a multistep algorithm. The validation of the code and numerical method is performed for flows around single square cylinders. L/D effects (with L being the center-to-center distance and D being the square cylinder width) on the flow structures and parameters are investigated for 1.5 ≤ L/D ≤ 9.0. The flow structures, especially far downstream of the square cylinder, indicate six distinct flow regimes. The two-layered vortex formation (TVF) and secondary vortices formation (SVF) modes are defined, and their mechanisms are analyzed. The evolution of TVF into SVF proceeds through the combination of single and binary vortex formations. Variations in the physical flow parameters, including the coefficients of fluctuating lift (CL′), time-averaged drag (CD), and amplitude lift (CLA), as well as the Strouhal number and phase lag (ϕ), are analyzed for various L/D values. Obvious jumps in the flow parameters occur at both square cylinders for L/D = 4.4–4.5 because of vortex impingement. Finally, insight into the physics underlying the relationship between CL−upA, CL−up′, and ϕ is derived from the simulation results. The local maximum and minimum of CL−upA and CL−up′ occur at ϕ = 2π and 3π, respectively, corresponding to in-phase and anti-phase vortex shedding by the cylinders. The pressures on the upper and lower sides of the upstream square cylinder decrease and increase, respectively, leading to reductions in CL−upA and CL−up′ as the flow pattern changes from in-phase to anti-phase.

The pulsatile flow of thermally developed non-Newtonian Casson fluid in a channel with constricted walls

Abstract: This article presents a numerical investigation of the pulsatile flow of non-Newtonian Casson fluid through a rectangular channel with symmetrical local constriction on the walls. The objective is to study the heat transfer characteristics of the said fluid flow under an applied magnetic field and thermal radiation. Such a study may find its application in devising treatments for stenosis in blood arteries, designing biomechanical devices, and controlling industrial processes with flow pulsation. Using the finite difference approach, the mathematical model is solved and is converted into the vorticity-stream function form. The impacts of the Hartman number, Strouhal number, Casson fluid parameter, porosity parameter, Prandtl number, and thermal radiation parameter on the flow profiles are argued. The effects on the axial velocity and temperature profiles are observed and argued. Some plots of the streamlines, vorticity, and temperature distribution are also shown. On increasing the values of the magnetic field parameter, the axial flow velocity increases, whereas the temperature decreases. The flow profiles for the Casson fluid parameter have a similar trend, and the profiles for the porosity parameter have an opposite trend to the flow profiles for the magnetic field parameter. The temperature decreases with an increase in the Prandtl number. The temperature increases with an increase in the thermal radiation parameter. The profile patterns are not perfectly uniform downstream of the constriction.

Evolution of wake structures behind oscillating hydrofoils with combined heaving and pitching motion

Abstract: The wake of an oscillating teardrop hydrofoil with combined heaving and pitching motion was studied numerically at Reynolds number of 8000 and Strouhal numbers of , the wake exhibited conjoint hairpin-horseshoe vortex structures that led to stronger deformations on the coupled vortex rollers. The statistical characteristics of secondary structures resembled the long wavelength mode and mode A identified previously for purely pitching and heaving foils, respectively. They also mimicked mode B for stationary cylinders. Novel wake models are introduced based on a complete vivid three-dimensional depiction of coherent wake structures.

Time-Resolved Ship Airwake Measurements in a Simulated Atmospheric Boundary Layer

Abstract: The unsteady flows produced over the stern of a Simple Frigate Shape 2 ship model are studied in a low-speed wind tunnel. Time-resolved particle image velocimetry (TR-PIV) measurements were perform...

Influence of the pivot location on the thrust and propulsive efficiency performance of a two-dimensional flapping elliptic airfoil in a forward flight

Abstract: In this article, the effect of the pivot point location on the thrust performance of a two-dimensional sinusoidal flapping elliptic airfoil in a forward flight condition is investigated using numerical simulations and in-house water tunnel experiments. On the chord line, three different pivot locations at a distance of 0.25c, 0.5c, and 0.75c from the leading edge of the airfoil are considered, where c is the chord length of the airfoil. The flapping frequency and effective angle of attack are varied to investigate the propulsive performance of the airfoil at a Reynolds number of 5000. It is noticed that a modification in the pivot location significantly influences the linear velocity distribution, the evolution of the leading-edge vortex, and the near wake region on the airfoil. Consequently, both the transient and time-averaged thrust coefficient of the flapping airfoil is considerably affected. In addition, we have observed when the flapping frequency is increased, the time-averaged thrust coefficient of the airfoil tends to increase up to a critical Strouhal number and deteriorates thereafter. The same trend of time-averaged thrust coefficient is seen at all considered pivot locations and effective angle of attacks. Our finding suggests, at the high flapping frequency, the formation of rotation induced adverse suction region around the airfoil and delay in the shedding of the leading edge vortex developed in the previous flapping stroke are the primary sources, attributing to the thrust deterioration of the flapping airfoil with symmetric pivot location 0.5c. On the other hand, the thrust degrading effects at the two asymmetric pivot locations, 0.25c and 0.75c, are triggered by the adverse suction regions induced by asymmetric airfoil-surface velocity distribution as well as airfoil-wake vortices interaction. Moreover, the thrust degradation can be postponed to a higher critical Strouhal number if the airfoil pivot location is set near the leading edge and higher amplitude of effective angle of attack is followed. Besides, we found that the airfoil propulsive efficiency is affected due to a change in the aerodynamic power co-efficient with the modification of the pivot location. Furthermore, our observation concludes that the pivot location at 0.25c from the leading edge has maximum time-averaged thrust and propulsive efficiency performances at least for the range of pivot locations, flapping frequencies, and effective angle of attacks examined here.

An LES study of secondary motion and wall shear stresses in a pipe bend

Abstract: Large-eddy simulations (LES) of a single-phase, turbulent flow in a 90° pipe bend are performed at three Reynolds numbers (5300, 27 000, and 45 000) to investigate the correlation between secondary flow motion and wall shear stresses, which is suspected to be a potential mechanism responsible for material erosion. The isothermal flows are validated against available experimental and numerical data first. The snapshot proper orthogonal decomposition (POD) is applied for the medium and high Reynolds number flows to identify the secondary flow motions and the oscillation of the Dean vortices that are found to cause swirl-switching. Distinguished frequencies of the POD time coefficients at Strouhal numbers of 0.25 and 0.28 are identified for Reynolds numbers at 27 000 and 45 000, respectively. Moreover, shear stress on the pipe wall and the associated power spectral density are obtained and shown to have the same oscillating frequency as the swirl-switching.