Fluid-Structure-Sound Interactions and Control

It is now well established that coherent structures exist in the majority of turbulent flows and can affect various aspects of the dynamics of these flows, such as the way energy is transferred over a range of scales as well as the departure from isotropy at the small scales. Reynolds and Hussain (J Fluid Mech 54:263–288, 1972) were first to derive one-point energy budgets for the coherent and random motions respectively. However, at least two points must be considered to define a scale and allow a description of the mechanisms involved in the energy budget at that scale. A transport equation for the second-order velocity structure function, equivalent to the Karman-Howarth (1938) equation for the two-point velocity correlation function, was written by Danaila et al. (1999) and tested in grid turbulence, which represents a reasonable approximation to (structureless) homogenous isotropic turbulence. The equation has since been extended to more complicated flows, for example the centreline of a fully developed channel flow and the axis of a self-preserving circular jet. More recently, we have turned our attention to the intermediate wake of a circular cylinder in order to assess the effect of the coherent motion on the scale-by-scale energy distribution. In particular, energy budget equations, based on phase-conditioned structure functions, have revealed additional forcing terms, the most important of which highlights an additional cascade mechanism associated with the coherent motion. In the intermediate wake, the magnitude of the maximum energy transfer clearly depends on the nature of the coherent motion.

In this paper we consider a strategy to manipulate the large-scale structures in wall-bounded turbulent flows, which have recently been shown to be a key mechanism for modulating levels of the skin-friction drag. For this, we use a rectangular wall-normal jet to target the large-scale structures as detected by an upstream spanwise array of skin-friction sensors. A second spanwise array of sensors, located downstream of the jet, records any modifications to the large-scale structure. In addition, a traversing hotwire probe is mounted above the second spanwise array of sensors to study the effects across the depth of boundary layer. It is found that the jet is able to create a low-speed region and when targeted on a high-speed structure changes the associated footprint at the wall.

The mixing promoting capability of right-angle triangular tab with sharp and truncated vertex has been investigated by placing two identical tabs at the extremities of an exit diameter of a Mach 2 axi-symmetric nozzle. The mixing promoting efficiency of these tabs have been quantified in the presence of adverse and almost zero pressure gradients. It is found that, at all levels of expansion of the present study though the core length reduction caused by both the tabs are appreciable, the mixing caused by the truncated tab is superior. The mixing promoting efficiency of the truncated tab is found to increase with increase of nozzle pressure ratio (that is, decrease of adverse pressure gradient). For all the nozzle pressure ratios of the present study, the core length reduction caused by the truncated tab is more than 95 %, with a maximum of 99 %, at NPRs 7 and 8. The present results clearly show that the mixing promoting capability of the tab is the best when the jet is almost correctly expanded (that is with almost zero pressure gradient).

This article experimentally investigates the self-excited impinging planar jet flow, specifically, the development and propagation of large-scale coherent flow structures convecting between the nozzle lip and the downstream impingement surface. The investigation uses phase-locked PIV measurements and a new structure-tracking scheme to measure convection velocity and characterize the impingement mechanism near the plate in order to develop a new feedback model that can be used to predict the oscillation frequency as a function of flow velocity (

U

o

), impingement distance (

x

o

) and nozzle thickness (

h

). The resulting model prediction shows a good agreement with experimental tone frequency data.

The self-excited oscillation generated by two opposing planar jets is investigated experimentally. Strong flow oscillations resulting in intense acoustic tone generation are observed for wide ranges of jet flow velocity and distance between the opposing jet exits. A study of phase-locked particle image velocimetry (PIV) complemented with sound measurements is performed to clarify the self-excitation mechanism and the oscillation pattern(s) of the opposing jets.

Extensive numerical computations have been carried out to understand the effects of a wall-mounted protuberance on the fluctuating pressure field on the surrounding surfaces. The Mach 1.6 supersonic turbulent boundary layer is modeled using a hybrid RANS-LES approach known as detached eddy simulation (DES). The effects of protuberance height to boundary layer thickness as well as surface curvature are investigated. Comparisons of the surface pressure coefficients to existing experimental data showed good agreement. In addition, it was found that increasing the protuberance height resulted in higher sound pressure levels on the surface. Surface curvature led to the spreading of the high pressure region in the spanwise direction upstream of the protuberance. Convection velocities of the turbulent structures increased downstream of the protuberance and were in good agreement with published literature.

We report an experimental study of the effects of polymer additives on the turbulent bulk flow. Our results confirm that both the acceleration fluctuation

a

and the velocity fluctuation

u

of the flow are suppressed when the polymer additives are present and the suppression effect on

a

is much stronger. We further found that polymer additives enhance the anisotropy of the flow at small scales, but do not affect the anisotropy at large scale very much. These results are qualitatively in agreement with a recent theory which predicts that only scales smaller than a critical scale are affected by the polymer additives.

The turbulent wake structures of various scales generated by an asymmetric bluff body have been experimentally investigated in this paper. Firstly, the instantaneous velocity and vorticity of turbulent wake were measured by the high-speed PIV technique at a Reynolds number of 8,960 in the circulating water channel. In order to decompose turbulent structures into a number of subsets based on their central frequencies, one- and two-dimensional wavelet multi-resolution technique is then used to analyze the instantaneous velocity and vorticity. It is found that the large-scale turbulent structure makes the largest contribution to the vorticity and Reynolds shear stresses. However, the small-scale structures make less contribution to the vorticity and Reynolds shear stresses.

The current paper reports a large eddy simulation (LES) of turbulent flow over a finite-height square cylinder mounted normal to a ground plane. The cylinder aspect ratio is AR = 3 and the Reynolds number based on the cylinder width and inlet velocity is Re = 500. The flow field is complex, since the flow over the top of the cylinder interacts with the flow along the ground plane to create a complicated wake structure. The wake is characterized by a velocity field that changes rapidly in both direction and magnitude. Phase averaging based on the Strouhal number reveals a wake structure with quasi-periodic features that is much different from the structure suggested by the mean vorticity field.

The three-dimensional orthogonal wavelet multi-resolution technique was applied to analyze flow structures of various scales around an externally mounted vehicle mirror. Firstly, three-dimensional flow of the mirror wake was numerically analyzed by using the large eddy simulation (LES). Then, the instantaneous velocity and vorticity were decomposed into the large-, intermediate-, and relatively small-scale components by the wavelet multi-resolution technique. It was found that a three-dimensional large-scale vertical vortex dominates the mirror wake flow and makes a main contribution to vorticity. Some intermediate- and relatively small-scale vortices were extracted and were clearly identifiable.

Large-scale fluctuating flow structures are studied using snapshot proper orthogonal decomposition (POD) in a turbulent boundary layer over a realistic rough forward-facing step (FFS) at

Re

h

= 3,450 and

δ/h

= 8. It was observed that large-scale Q2 event induced by hairpin vortex packet and large-scale Q4 event are the dominant fluctuating flow structures that contribute significantly to the first POD mode for both smooth and rough steps. However, the flow structures close to the rough surface were affected differently by different roughness topographies.

We present multimaterial simulations using both Eulerian and Lagrangian schemes. The methods employed are based on classical Godunov-like methods that are adapted to treat the case of interfaces separating different materials. In the models considered, the gas, liquids, or elastic materials are described by specific constitutive laws, but the governing equations are the same. Examples of gas–gas and gas–elastic material interactions in one- and two-spatial dimensions are presented.

Direct numerical simulations were performed to study the effect of streamwise travelling waves of spanwise wall velocity on turbulent channel flow. Simulations with various control parameters, scaled by wall units, are performed at four Reynolds numbers, corresponding to

$$ {\text{Re}}_{\tau } = 200,\,400,\,800,\,1600 $$

. As the Reynolds number is increased the intensity of both the drag reduction and drag increase is reduced. This reduction does not scale universally, and the drag reduction deteriorates quickly with increased Reynolds number when the parameters used are close to optimal. The consequence of this variation in Reynolds scaling is that the value of the optimal forcing parameters change, even in wall units, with increased Reynolds number.

This work investigates experimentally active control of an air round jet using radial unsteady air microjets. Two microjets were placed at diametrically opposite locations upstream of the nozzle exit. The Reynolds number was 8,000. The flow was measured in two orthogonal diametrical planes using various techniques. Under the open-loop control, the jet centreline decay rate

K

exhibits a strong dependence on the mass flow ratio

C

m

of microjets to the primary jet and excitation frequency ratio

f

e

/f

0

of microjets, where

f

e

is the microjet excitation frequency and

f

0

the preferred mode frequency of the uncontrolled jet. A closed-loop control technique has been developed to achieve automatically and rapidly the optimal control performance and the maximum

K

, as obtained from the open-loop control.

An experimental investigation of airfoil aerodynamics control at a low Reynolds number of 5 × 10

4

was conducted within the attack angle

α

of 0–90

°

using a leading-edge protuberance technique. The essence of the technique is to manipulate flow around the airfoil through the effect of a humpback whale-like leading edge. Whereas the mean lift force, drag force, and lift-to-drag ratio were measured using a 3-component force balance, the flow was mainly documented using a particle image velocimetry (PIV). The sinusoidal protuberances effectively suppressed the airfoil stall, although the corresponding aerodynamic performances were impaired to some extent. Meanwhile, the control significantly improved the airfoil aerodynamics in the post-stall

α

region, i.e., 16

°

<

α

< 70

°

, leading to a maximum 25.0 and 39.2 % increase in lift coefficient and lift-to-drag ratio, respectively, and maximum 20.0 % decrease in drag coefficient. The flow physics behind the observations were discussed.

In this study, synthetic-jet-actuator (SJA) arrays were designed, implemented, and tested on a straight-wing model for flow separation control. First, the characteristics of a single SJA were determined. The jet velocity appears a peak between 400 and 500 Hz, which corresponds to the SJA’s Helmholtz resonance frequency. Second, two arrays of such SJAs were implemented at different chordwise locations on a straight-wing model. Force balance measurements and power spectrum analysis showed that both SJA arrays are able to effectively delay flow separation, with the front SJA array more effective than the rear one. For the front array, the improvement in

C

L

and

C

D

was 27.4 % and 19.6 %, respectively.

The dielectric barrier discharge plasma actuator (DBD PA) is applied to diffusion control of the jet flow issued at Reynolds numbers (based on a nozzle diameter) of Re = 1.0 × 10

3

and 2.0 × 10

3

from a nozzle of 10 mm in diameter. The detailed flow structure is documented using particle image velocimetry, hot-wire anemometry, and laser flow visualization. In the experiment of the present study, the jet flow was controlled using a coaxial DBD PA, which generates an induced flow in the same direction as the jet direction. We investigated the difference in the influence of the flow induced by DBD PA on the main jet flow while varying the voltage, frequency, and intermittency. The induced flow was found to become stronger as the frequency and voltage increased. The diffusion of the jet flow was controlled.

The end-effects of a finite synthetic jet used for controlling the flow around a two-dimensional circular cylinder are experimentally investigated in this study. The Reynolds number based on the cylinder diameter is Re = 800, and the corresponding natural vortex shedding frequency

f

0

is 0.24 Hz (St = 0.21). The synthetic jet is actuated at excitation frequency

f

e

/

f

0

= 2.08, with the equivalent momentum coefficient

C

μ

= 0.139. Six

x

–

y

planes of view from the mid-span to one of the slot ends with identical interval 5 mm are measured using two-dimensional time-resolved PIV system. It is found that the end-effects of the finite synthetic jet do not have a crucial influence on flow fields in the mid-span regions, which are independent of the slot length if it is longer than one cylinder diameter.

This paper reports the modifications of near-wall low-speed streaks by local surface oscillations generated by a spanwise-aligned actuator array in a turbulent boundary layer over a flat plate at Re

θ

= 1,000. The streaks were educed from PIV-measured fluctuating velocities in the viscous sublayer using a procedure proposed by Schoppa and Hussain (

2002

). The wall-based perturbations, corresponding to a large skin-friction drag reduction (about 50 % at 17 wall units downstream of the actuator array), modified greatly the low-speed streaks, leading to a reduction by over 15 % in both the averaged width and spacing while an increase by 17 % in the streak center number. The alterations of velocity streak distributions are consistent with results from other techniques such as smoke-wire flow visualization and two-point cross-correlation, where the breakup of large-scale coherent structures into small-scale ones was observed.

Experiments were carried out to study the effect of plasma flow control on airfoil in an open-circuit low-speed wind tunnel. Lift and drag were measured by a five-component strain gauge balance. Particle image velocimetry (PIV) technology was applied to visualize the flow field over the airfoil. Influence of plasma actuator voltage, electrode position, and control evolution on NACA0015 airfoil was investigated, and the control mechanism was preliminarily analyzed. Lift enhancement validation experiment on NACA23018 two-element airfoil was carried out. The results show that the plasma actuators can efficiently increase the maximum lift and stall angle on a symmetric airfoil and a high-lift airfoil.

This paper reports a large eddy simulation (LES) of the precession-like oscillation produced by partially confining a triangular-jet flow with a short cylindrical chamber. The present LES, which has been verified by previous experimental data, shows that there is a strong inward swirl around the jet near the inlet end of the chamber. At the center of the swirl, there is a cluster of three sink foci, where each focus is aligned midway between the corners of the triangular inlet orifice. In the time-averaged flow field, the vortices rising from the foci are helically twisted about the core of the jet. As the flow passes through the chamber, the foci merge to form a closed-loop “bifurcation line” which separates the inward swirl and the core flow. The core of the emerging jet is visible as a source node at the approximate centerline of the chamber. If the chamber is removed, a cluster of six counter-rotating foci is produced in the “free” jet. When this happens, the net swirl circulation is zero and there is no jet oscillation.

This paper reports a study that investigates the effects of the large-scale intermittency factor

γ

and the mean shear

S

on the inertial range spectral exponent

m

in the far field of a turbulent square jet (

γ

is defined as the fraction of time when the flow is turbulent at a specific location.). The exit Reynolds number is Re = 50,000. The turbulent energy recognition algorithm (TERA) method proposed by Falco and Gendrich (

1990

) is applied to estimate

γ

from velocity signals. It is found that

γ

has a strong impact on

m

while the influence of

S

is negligible. More specifically,

m

varies with

γ

following the relationship of

m

=

m

t

+ (ln

γ

−0.0173

)

1/2

, where

m

t

denotes the scaling exponent in full turbulence. Moreover, self-similarity of

γ

is observed in the far field of the jet.

The drag reduction phenomenon of a circular cylinder is investigated, using an X-wire probe, by placing a smaller diameter control cylinder of either circular or square cross-sectional upstream of the main circular cylinder. The separation between the two cylinders was chosen so as to minimise the overall drag of the main cylinder. Although both configurations resulted in drag reduction, the square control cylinder yielded a bigger reduction and had a greater impact on the mean velocity and turbulent velocity fluctuations in the wake. Both control cylinders led to a reduction in the wake half-width. Spectra in the near wake did not indicate any suppression in vortex shedding.

The flow above the free end of a surface-mounted finite-height circular cylinder was studied experimentally in a low-speed wind tunnel using particle image velocimetry. Velocity measurements were taken in horizontal and vertical planes above the free end at a Reynolds number of Re = 4.2 × 10

4

. Four cylinder aspect ratios, of AR = 9, 7, 5, and 3, were examined. The turbulent boundary layer on the ground plane had a thickness of

δ

/

D

= 1.6. The results revealed details of the mean recirculation zone, reattachment position, critical points, and vortex patterns in the flow field above the free end. The sensitivity of the free-end flow field to changes in AR was much less pronounced than what is observed for the near-wake region.

The flow around a finite-length square prism with aspect ratio of 5 is investigated using LES at Re

d

= 3,900. Two typical flow modes are found in its near wake. One is characterized by staggered arranged spanwise vortices, similar to that in 2D cylinder wake; on the other hand, the spanwise vortices are symmetrically arranged for the second mode. These two modes occur intermittently in the near wake. When the first mode occurs, the pressure on prism side surface fluctuates periodically, corresponding to large values of drag and fluctuating lift; for the second mode, there is no obvious pressure fluctuation, and the drag and fluctuation lift are significantly smaller than those for the first mode.

This work aims to gain relatively thorough understanding of unsteady flow structures around an Ahmed body and associated predominated Strouhal numbers

$$ {\text{St}}_{\sqrt A } $$

based on the square root of the frontal area

A.

Extensive hotwire and flow visualization measurements were conducted in a wind tunnel at

$$ \text{Re}_{\sqrt A } = 6.59 \times 10^{4} $$

over the roof, the rear window, and behind the vertical base of this body with a slant angle of 25°. Four distinct Strouhal numbers have been identified.

$$ {\text{St}}_{\sqrt A } = 0.196 $$

and 0.140 are detected above the roof.

$$ {\text{St}}_{\sqrt A } = 0.196 $$

is captured again over the rear window, along with another unsteady structure of

$$ {\text{St}}_{\sqrt A } = 0.265 $$

. Behind the vertical base, the wake is dominated by the quasi-periodic structures of

$$ {\text{St}}_{\sqrt A } = 0.442 $$

. A conceptual flow structure model is proposed based on the present data as well as those in the literature.

This work aims to investigate based on the Strouhal number St and the flow structure, the dependence of flow classification on the Reynolds number Re in the wake of two staggered cylinders, with Re varying from 1.5 × 10

3

to 2.0 × 10

4

. The cylinder center-to-center pitch,

P

*

=

P

/

d

examined is 1.2–6.0 (

d

is the cylinder diameter), and the angle (

α

) between the incident flow and the line through the cylinder centers is 0°–90°. Two single hotwires were used to measure simultaneously the St behind each of the two cylinders at streamwise positions from

x

*

=

x

/

d

= 2.5

–

15. While the present data reconfirms the flow structure modes previously reported, the dependence of the flow modes on

P

*

and

α

exhibits an appreciable dependence on Re. The observation is connected to the Re effect on the generic features of a two-cylinder wake such as flow separation, boundary layer thickness, gap flow deflection, and vortex formation length.

This work is an experimental study of the turbulent vortex structures and heat transport in the intermediate wake of a slightly heated cylinder. The three components of the vorticity vector were measured simultaneously with the fluctuating temperature at nominally the same point over

x/d

= 10–40 using a 3D vorticity probe and four cold wires. By using phase-averaging analysis, the contribution of coherent motion of various vorticity correlations such as vorticity-passive heat, as well as the velocity–vorticity correlations has been discussed, in conjunction with Reynolds shear stress and heat fluxes. The results indicate that coherent motion contributes significantly to the heat transport within the vortices while the incoherent motion mainly contributes between two vortices. The vorticity–velocity correlation involving spanwise vorticity component mainly contributes to the gradient of Reynolds shear stress.

The work investigates the effect of turbulent intensity (

T

u

) on the force and wake of a NACA0012 airfoil at chord Reynolds number Re

c

= 5.3 × 10

3

and 2 × 10

4

. Lift and drag coefficients (

C

L

and

C

D

) on and flow fields around the airfoil were measured with

T

u

varied from 0.6 to 6.0 %. Four Re

c

regimes are identified based on the characteristics of the maximum lift coefficient (

C

L,

max

), i.e., ultra-low (Re

c

< 10

4

), low (10

4

~ 3 × 10

5

), moderate (3 × 10

5

~ 5 × 10

6

), and high (> 5 × 10

6

). It is noted that at Re

c

= 5.3 × 10

3

(ultra-low Re

c

regime) the stall is absent for

T

u

= 0.6 % but occurs for

T

u

= 2.6 and 6.0 %. As Re

c

increases to low Re

c

regimes, the

T

u

influence decreases. So does the critical Re

c

, above which stall occurs. The effect of increasing

T

u

on flow bears similarity to that of increasing Re

c

, albeit with a difference; the flow separation point shifts upstream with increasing Re

c

but downstream with increasing

T

u

.

Measurements of the pressure waveforms along a grid in the far-field of an unheated Mach 3 jet were conducted in order to study crackle. A detection algorithm is introduced which isolates the shock-type structures in the temporal waveform that are responsible for crackle. Ensemble averages of the structures reveal symmetric shocks at shallow angles, while they appear to be asymmetric near the Mach wave angle. Spectral quantification is achieved through time–frequency analyses and shows that crackle causes the expected high-frequency energy gain. The increase in energy due to crackle is proposed as a more reliable metric for the perception of crackle, as opposed to the Skewness of the pressure or pressure derivative.

We study the acoustic signature of a rigid wing, equipped with a movable downstream flap and interacting with a line vortex, in a two-dimensional low-Mach number flow. The flap is attached to the airfoil via a torsion spring, and the coupled nonlinear fluid–structure interaction problem is analyzed using thin-airfoil methodology and the Brown and Michael equation. Passage of the incident vortex above the airfoil initiates flap oscillations at the system natural frequency, amplified over all other frequencies excited by the vortex. Far-field sound radiation is analyzed, yielding the leading order dipole-type signature of the system. The acoustic radiation is dominated by vortex sound, consisting of relatively strong leading and trailing edge interactions of the airfoil with the incident vortex, together with late-time wake sound resulting from induced flap oscillations. In comparison with counterpart rigid (non-flapped) configuration, we find that the flap may act as sound amplifier or absorber, depending on the value of flap-fluid natural frequency.

Numerical simulations were conducted to investigate the pressure fluctuations (PF) around a leading-edge slat, which was assumed to have close relationship with the far-field slat noise. The PF predicted that not only the reattachment region of the free shear layer shedding from the slat cusp, but the region close to the leading edge of main element were the crucial regions that radiated noise into far field. Proper orthogonal decomposition (POD) was applied to the PF around the slat. By checking the temporal sequence of vorticity distribution around the slat, the crucial flow features corresponding to the first four POD modes were examined, in particular the first mode was induced by the vortical structures convecting through the slat gap.

This paper presents results of a study on the flow and noise generated by two-dimensional step elements attached to a flat plate. The step elements used in this study are equivalent to a forward–backward facing step pair and have length to height ratio of

l

/

h

= 8, 4 and 2.7. Aerodynamic and acoustic measurements have been taken in an anechoic wind tunnel at the University of Adelaide. Numerical simulations of turbulent boundary layer flow over the step with

l

/

h

= 2.7 are also presented. The data given in this paper provide fundamental information on how a sub-boundary layer step element affects the noise and flow field.

In this paper, a prediction method of hydrofoil broadband noise is developed based on large eddy simulation (LES) methods and Ffowcs Williams- Hawkings (FW-H) equations. The broadband noise of an airfoil is calculated and the comparison of computed and measured sound pressure spectra shows good agreement over approximately a decade of frequency. The broadband noise of five different hydrofoils are calculated and presented in this paper. The relationship between the broadband noise of hydrofoils and the hydrofoil thickness and camber distributions are discussed. An optimized hydrofoil is presented in this paper for which the broadband noise is about 4 dB lower than NACA-66mod foil.

In this paper, we design a class of linear compact schemes based on the cell-centered compact scheme of ((Lele, J Comput Phys 103:16–42, 1992). These schemes equate a weighted sum of the nodal derivatives of a smooth function to a weighted sum of the function on both the grid points and the cell-centers. Through systematic Fourier analysis and numerical tests, we observe that the schemes have good properties of high order, high resolution, and low dissipation. It is an ideal class of schemes for the simulation of multiscale problems such as aeroacoustics and turbulence.

Active control of rotor–stator interaction noise is studied numerically based on the surface oscillation of stator vanes. The problem is formulated in terms of 3D Euler equations linearized around uniform steady flow. Governing equations are solved with an explicit numerical technique based on a high-order approximation of spacial derivatives. It is found that the acoustic response with actuation on the suction side of the vanes differs considerably from that on the pressure side. The noise radiation also exhibits a strong dependence on the actuation frequency and position on the vane. Noise reduction of up to 80 % can be achieved.

A new experimental study, aimed at investigating the coupling between the flow in a corrugated pipe, the acoustically generated flow oscillations, and the emitted resulting noise is carried out. Hot-wire anemometry, Particle Image Velocimetry, and microphone measurements are associated to characterize the flow. The flow response to the corrugation is shown to fit to the sixth to ninth acoustic modes of the pipe according to the flow rate. When low frequency acoustically generated oscillations interfere with this, one checks that they either significantly reduce the noise level or modify the peak frequencies. In addition, theoretical/numerical works are also performed, in order to provide an analytical framework describing the acoustical properties of such corrugated pipe flows.

In this work, the problem of underwater noise generated during the offshore installation of steel monopiles is addressed. The monopiles are driven into place with the help of hydraulic hammers. During installation, the underwater noise levels generated can be very high and harmful for the marine life. A linear semi-analytical model is developed which is able to represent the dynamics of the coupled vibro-acoustic system. The pile is modelled using a high order thin shell theory whereas both water and soil are modelled as three-dimensional continua. The results indicate that the near-field response in the water column consists of pressure conical waves due to the the supersonic compressional waves in the pile generated by the impact hammer. The soil response is dominated by vertically polarised shear waves. Scholte waves are also generated at the water-seabed interface and can produce pressure fluctuations in the water column that are particularly significant close to the sea floor.

The free stress and pressure waves in fluid filled piping are investigated numerically. The motivation of this work is to find measures and rules for the low frequency vibration and noise reduction for piping designers. The Timoshenko beam equation and extended water hammer equation are used to calculate the transfer matrix of two piping configurations. And the wave propagation constants are then obtained by get the eigenvalue of the transfer matrix. It’s found that when the support stiffness reaches a critical value

k

c

the piping will be a long bend wave filter. It’s the same as obtained by increasing the support distance. The flexible hose shows good performance in controlling both stress and pressure waves, but the support shows insignificant capability in pressure wave reduction.

A major barrier to the acceptance of small wind turbines is that they are perceived to be noisy particularly when mounted on monopole towers rather than traditional guy-wired ones. Noise emission from a 2.4 kW downwind turbine due to its 10.2 m monopole tower was investigated. Tower vibration was measured using 24 accelerometers. A finite-element tower model combined with simple assumptions for the turbine and wind loads allowed the noise to be obtained from solution of the wave equation. The measured vibration levels were matched to the tower model amplitudes. Sound pressure level produced by the fluid–structure interaction reached 30 dB at about 11 m from the tower and decreased to 5 dB 1 km away. Propagation switched from cylindrical to hemispherical when the distance was about 200 times larger than the tower height.

The Spherical Nearfield Acoustic Holography—Sound Quality (SNAH-SQ) hybrid methodology is proposed in this paper. This methodology builds the mapping model from distribution of sound pressure to sound quality parameters on basis of reconstruction of the three-dimensional distribution of interior acoustic parameters reconstructed by SNAH. The three-dimensional distribution of sound quality parameters related to the human’s subjective auditory experience can be obtained. The sound field which contained two monopole sound sources is used to conduct numerical simulation study. The distribution of sound quality parameters and the sound source localization results varied with the sound field reconstruction distance, sound frequency, and intersection angle between two sound sources. The advantages of the traditional sound source localization by nearfield acoustic holography and the sound quality sound field evaluation are adopted in the SNAH-SQ method which could provide an approach to revealing the three-dimensional inherent correlation between noise source locations and sound quality parameters distribution, and locate the noise sources with most annoyance.

A novel acoustic sensor has been developed which is capable of remotely monitoring the free surface ‘fingerprint’ of shallow flows. The temporal and spatial properties of this fingerprint are shown to contain a wealth of information regarding the nature of the flow itself. The remote measurement can thereby be used to infer the bulk flow properties such as depth, velocity, and hydraulic roughness to within 8 % accuracy. The instrument is totally non-invasive and as such is low cost, low maintenance, and low power. Such a device will allow for widespread monitoring of flow conditions in drainage and river networks, informing flood models, and facilitating pro-active maintenance and real time control.

Manta rays propel themselves by combining oscillating and undulating motions of flexible surfaces. We describe two experiments to study the effects of excitation and flexibility on the wake flowfield: experiments on undulating and flapping three-dimensional fins of elliptical planform, and experiments on pitching two-dimensional panels of rectangular planform with varying flexibility. To interpret the results on thrust and efficiency, we propose scalings for aspect ratio and flexibility, and develop a stability analysis called wake resonance theory. Here we focus on the insights provided by wake resonance theory.

We present a broad review of our laboratory’s contributions to the understanding of flow generated sounds which emanate from the lung and are used by clinicians to ascertain the status of health of the respiratory system. Important sounds from diseased lungs include wheezing, which occurs primarily during expiration, and crackles which occur primarily during inspiration. We have analyzed flow-induced flutter of flexible tubes as the mechanism of wheezing sounds and connected its appearance to the onset of flow limitation in a lung. Wheezes and difficulty expiring air are common to asthma and emphysema. The flutter theory is for both linear and nonlinear wall properties, while using potential flow with a friction factor as well as a coupled Orr-Sommerfeld system using the Navier–Stokes equations coupled to the wall. Both the value of the critical flow velocity that instigates flutter, as well as the flutter frequency, compares well with experiments on isolated flexible tubes as well as an excised lung. We also studied the propagation and rupture of simulated mucus plugs using carbopol 940 gels to investigate the source of crackling sounds which occur in lungs with an abnormal amount of liquid in the airways, congestive heart failure, or small airway inflammation. The non-Newtonian properties of yield stress, storage modulus, and loss modulus were matched well with normal and abnormal mucus. A collapsed airway of the 12th generation was modeled using a quasi-two-dimensional polydimethylsiloxane channel. The plug yields at a plane about one-third of the half channel width away from the lateral channel walls and then ruptures, creating a crackle sound, and reopening the model airway to gas flow. The relatively high shear stresses during rupture, and crackle sound production, may increase epithelial cell damage.

A new system in fluid-structure interaction (FSI) is studied wherein a cantilevered thin flexible plate is aligned with a uniform flow with the upstream end of the plate attached to a spring-mass system. This allows the entire system to oscillate in a direction perpendicular to that of the flow as a result of the dynamic interaction of the mounting with the flow-induced oscillations, or flutter, of the flexible plate. While a fundamental problem in FSI, the study of this variation on classical plate flutter is also motivated by its potential as an energy-harvesting system in which the reciprocating motion of the support system would be tapped for energy production. In this paper, we formulate and deploy a hybrid of theoretical and computational models for the fluid-structure system and map out its linear stability characteristics. The computational model detailed is a novel fully implicit solution that is robust to spatial and temporal discretization. Compared to a fixed cantilever, the introduction of the dynamic support system is shown to yield lower flutter-onset flow speeds and a reduction of the order of the mode that yields the critical flow speed; these effects would be desirable for energy-harvesting applications.

The three-dimensional stability of a fluid-loaded flexible panel is studied to determine the effectiveness of adding localised stiffening to control or postpone instability. A hybrid of computational and theoretical modelling is used to cast an eigenvalue problem for the fluid-structure system. It is shown that the addition of each of transverse and streamwise stiffening strips postpones divergence onset but for the former there is a threshold strip stiffness above which no further postponement is possible. Streamwise stiffening is additionally shown to be effective for increasing post-divergence flutter-onset flow speeds while in aero-elastic applications a transverse stiffening strip can be used to replace flutter instability with divergence. The present results suggest a relatively economical and practicable way to ameliorate panel instability in both hydro- and aero-elastic applications.

A state-space method is used to investigate the surface instabilities of a flexible plate comprising one wall of an inviscid channel flow computationally modelled with a finite-difference method coupled with a boundary-element method. Simple elastic and spring-backed plates are considered and in both cases it is found that reducing the height of the channel causes divergence and modal-coalescence to occur at lower flow velocities. An analytical prediction for an infinitely long plate is also developed and the divergence-onset predictions are compared with those obtained by the state-space method for a spring-backed plate.

This paper describes a study of the flow instability over a plate hinged at its leading edge by the pseudospectral numerical method for fluid loading and the Galerkin method for the eigen-value problem. The mechanism of modal coupling for the plate flutter is illustrated. It is found that flutter arises from the coupling between the first and second in-vacuo modes, with flow-to-structure energy transfer. The fluid loading on the second in-vacuo mode is found to be the dominant source of instability. Compared with a cantilever plate with the same material property, the plate with a simply supported leading edge has similar threshold of flutter velocity, which suggests that the bending stiffness of the plate is crucial for the stability instead of the structural boundary condition at the leading edge. This conclusion is also validated by the analytical study for a simplified model in which the flexible plate is replaced by two rigid plates connected by a hinge.

The numerical simulation associated with fluid–structure interaction problems is very complicated for the grid regeneration in the traditional method. In the present work, a fast explicit numerical method is established to solve the unsteady flow with oscillation of a rotor blade on the basis of the immersed boundary method. The governing equations are discretized on simple Cartesian meshes by using the immersed boundary method and the blade can move arbitrarily in the computational domain. It is found that the oscillation of rotor blades is influenced greatly by the reduced velocity and cascade solidity.

High Altitude, Long Endurance (HALE) aircraft can achieve sustained uninterrupted flight time if they use solar power. Wing morphing of solar powered HALE aircraft can significantly increase solar energy absorbency. An example of the kind of morphing considered in this paper requires the wings to fold so as to orient a solar panel to be hit more directly by the sun’s rays at specific times of the day. In this paper solar powered HALE flying wing aircraft are modeled with three beams with lockable hinge connections. Such aircraft are shown to be capable of morphing passively, following the sun by means of aerodynamic forces and engine thrusts. The analysis underlying Nonlinear Aeroelastic Trim And Stability of HALE Aircraft (NATASHA), a computer program that benefits from geometrically exact, fully intrinsic beam equations, and a finite-state-induced flow model was extended to include the ability to simulate morphing of the aircraft into a “

Z

” configuration. Because of the “long endurance” feature of HALE aircraft, such morphing needs to be done without relying on actuators and as near zero energy cost as possible. The emphasis of this study is to substantially demonstrate the processes required to passively morph a flying into a Z- shaped configuration and back again.

Large amplitude roll oscillations are inherent to fixed-wing MAVs. This paper reviews our recent works on the suppression of the self-induced roll oscillations of LAR rectangular wings using active flow control techniques. Both acoustic excitation and synthetic jet blowing have been used to attenuate the self-excited roll oscillations. Three rectangular wings, of flat plate, NACA0012 and SD7003-085-88 profiles were tested. It was found that roll oscillations can be completely suppressed and the onset of the roll oscillations can be delayed by active flow control approaches. PIV measurements indicated that the excitations could restore a symmetric vortex flow over free-to-roll wings thus stabilize the roll oscillations.

Aeroelastic analysis of wind turbine blade is one of the most important studies, which is a typical phenomenon of the fluid–structure interaction. In order to increase the accuracy of the aeroelastic analysis, in this paper, we developed a new three-dimensional (3D) stall delay model for horizontal axis wind turbine (HAWT), which is inviscid stall delay model (ISDM). The model is derived from the Navier–Stokes equations, in which we treat the stall delay effects differently by the delay of the separation point on the airfoil, and aim to capture the further negative pressure reduction in the separation area due to the span wise flow driven by the centrifugal force on the rotating blade. Based on the analytical solution, the ISDM is created and the correction factor

S

is analyzed. In order to validate ISDM, the model is applied to the NREL Phase VI wind turbine blade. Both the corrected lift and drag coefficients and the power/torque results are compared with the experimental data. From the comparison it can be concluded that ISDM gives reasonable predictions of the 3D lift and drag coefficients as well as the corresponding power and thrust force obtained from blade element and moment (BEM) computation.

In this study, a suction flow control method was employed to suppress the vortex shedding of a circular cylinder based on a multi-point suction type. A digital particle image velocimetry (PIV) system was used to conduct detailed flow field measurements under different suction flow control conditions with simultaneously measuring the pressure distribution on the test models. Five steady suction flow rates of 20, 40, 60, 80, and 95 L/min were employed in the testing. The suction control effects were investigated under an angle of 90.0° of the suction holes; the effects of the suction flow control along the axial direction of the test model were investigated. The mean and fluctuating pressure coefficients, the lift and drag coefficients, the mean velocity fields of the circular cylinder, and the mean velocity profiles in the wake under the suction flow control were then analyzed. The results indicated that the steady suctions exhibited excellent control effects and could dramatically reduce the fluctuations of lift coefficients and the averages of the drag coefficients and distinctly suppress the alternating vortex shedding (changing into symmetric mode). The control effects of the suction along the axial direction of the model are not uniform and the control effects are improved from the section without suction to the suction section.

The vortex-induced vibration of two elastic cylinders in tandem has been numerically simulated based on the Galerkin finite element method and dynamic mesh technique. Influences of the reduced velocity to the amplitude of vibration displacement of the cylinders was investigated, with cylinder spacing of

L/D

= 5 and a Reynolds number of 1,000. In the simulation, several classic vortex-induced vibration modes have been obtained, such as beat vibration and resonance. It was found that vortices shed periodically from both the upstream and downstream cylinders. The downstream cylinder undergoes larger amplitude of oscillations in both transverse and streamwise directions than those of the upstream cylinder. Variations in the maximum amplitudes of the oscillation displacement were found to agree with the “three-branch” model.

The work investigates flow-induced response of a circular cylinder interacting with another smaller diameter cylinder placed upstream. The upstream cylinder diameter

d

was varied from 0.24 to 1.00 times the diameter

D

of the downstream cylinder, which was cantilever-supported. Experimental observation was made at a spacing ratio,

L

/

d,

of 1–2, where

L

is the center of the upstream cylinder to the forward stagnation point of the downstream. A violent vibration of the cylinder occurred at

d

/

D

= 0.24–0.8 for

L

/

d

= 1 or

d

/

D

= 0.24–0.6 for

L

/

d

= 2, but not at

d

/

D

= 1. The violent vibration occurs at a reduced velocity

U

r

= 13–22.5, depending on

d

/

D

and

L

/

d

, and increases rapidly, along with the fluctuating lift, for a higher

U

r

. At a small

d

/

D

, the upstream cylinder wake narrows, hence the high-speed slice of the shear layer could flow alternately along the two different sides of this cylinder, thus exciting the downstream cylinder.

In the present paper, a numerical study is performed on the vortex-induced vibrations of three elastically mounted cylinders in a regular triangle arrangement at low Reynolds number. The motion of every single cylinder, which is free to oscillate in two degrees-of-freedom in an uniform flow and has the same mass and natural frequency in both inflow and cross flow directions, is modeled by a mass-spring-damper system. The vibrating displacement, mean and fluctuating aerodynamic forces, Strouhal number (St) and vortex shedding pattern in the wake of each cylinder are analyzed with seven spacing ratios

L

/

D

changing from 2.0 to 6.0. The results indicate that the simultaneous resonance may occur and the frequency of inflow vibration keeps consistent with that of cross flow vibration for the downstream cylinders. The cross flow oscillation amplitude of three cylinders significantly increased compared with the flow-induced vibration of a single elastically mounted cylinder and the cross oscillation of downstream cylinder is non-negligible for vortex-induced vibration of multi-cylinder system with low mass and damping.

A cylinder/cluster subjected to axial flow is the most fundamental and revealing problem in the general subject of fluid-structure interaction (FSI). In this paper, the FSI of a system in which a simple cluster consisting of four cylinders is subjected to an axial flow is studied numerically with an explicit partitioned scheme. The cylinders are modeled by Euler–Bernoulli beams and the flow is solved based on Navier–Stokes equations with LES turbulence model. The effect of the dimensionless flow velocity on the dynamics of the cylinders is investigated in detail. For small dimensionless flow velocity, the initial strong vibration of the cylinder is damped and the buckling instability may occur if the dimensionless velocity is large enough.

High speed flows in working hard disk drives (HDDs) can induce off-track vibrations on a head gimbals assembly (HGA), which limit positioning accuracy of the slider magnetic head on the tip of the HGA for high magnetic storage density in disks. This paper presents experimental studies and numerical simulations on the flow-induced vibrations (FIV) of the HGA inside an HDD and active control of such vibrations. First, the HGA off-track vibration and the airflow pressure fluctuations around the HGA are measured to characterise the FIV of the HGA. Second, we propose an active control strategy for such FIV of the HGA, in which feedback acoustic pressures are employed to suppress pressure fluctuations in turbulence around the HGA. Numerical simulations have been carried out on this issue by introducing virtual sensors into the close regions around the HGA. Finally, by using the FIV on the HGA as feedback error signals through the laser Doppler vibrometer, the feedback control of FIV on the HGA has also been demonstrated.

Two finite element models are coupled, with the aim of computing the mutual interactions between fluids and floating solids. The fluid and solid domains are discretised differently in space and time, and at every time step, the solid mesh is mapped onto the fluid mesh. The effect of the solid on the fluid dynamics, and vice versa, is modelled through a volumetric penalty force added to the momentum balances of the fluids and solids. A novel algorithm ensures that the action-reaction principle is satisfied at the discrete level. The coupled models are used to simulate uniform flow past a wind turbine, which is represented as a fixed actuator disc. Preliminary results on a floating pile also demonstrate the applicability of the models to fully coupled simulation of a floating spar. This work is a first-step towards the fully coupled modelling of floating wind turbines.